LUX and High Energy Muon Induced Backgrounds in DUSEL
Project Description
I. Introduction
As part of the effort of pursuing large deep underground facilities by the particle physics
community to solve some fundamental problems in physics, several experiments have been
planned or proposed in the Deep Underground Science and Engineering Lab (DUSEL, [1]) in
Homestake, Lead, South Dakota to address topics such as dark matter [2, 3], neutrino-less double
beta decay [4,5], neutrino physics, and proton decay [6,7] (such as the Long-Baseline Neutrino
Experiment, LBNE). All these underground experiments will benefit from the low background
environment in deep underground caverns in DUSEL. Nevertheless, various backgrounds, such as
gamma rays, neutrons from fissions, cosmic ray induced high energy muons and particles
produced by them when they interact in the rocks and the detector materials still exist at a level
that affects not only the experiment design but also the explanation of the experimental data. For
most underground experiments that run at very low energy threshold, these particles form a
complicated radiation environment for which we have to fit experimental data taken at the
particular experiment sites in order to give precise predictions and estimate the backgrounds in
the experiments.
Being the first dark matter experiment currently scheduled in DUSEL, the LUX experiment
is primarily designed to look for signals from dark matter particles, such as the weakly interacting
massive particles (WIMPs) predicted in supersymmetric and extra-dimensional extensions of the
standard model [8]. The LUX detector consists of a 350 kg liquid xenon detector that is
submerged in a 300-m3 water tank, see Figure 1. The water tank is designed to shield gamma rays
and attenuate neutrons that are produced in the rocks and surrounding materials. In addition, the
water inside the tank is monitored by 20 photomultiplier tubes (PMTs) so that muons and other
charged particles with tracks of sufficient length in the water can be tagged. Due to its low
threshold, capability of electron and nuclear recoil discrimination and very low internal
radioactivity level in the LUX inner detector, systematic study of the data from the liquid Xenon
(LXe) detector and the water tank will provide a benchmark in describing and characterizing
various backgrounds in DUSEL, which will further benefit all other future experiments in
DUSEL. The LUX collaboration is currently assembling its inner detector in the surface lab at
Homestake Lead. Underground deployment to Davis Cavern at 4850 ft level (~ 4300 m.w.e.
meter-water-equivalent overburden) is scheduled in the summer of 2011. A 1.5-ton scale twophase Xenon dark matter experiment LZS using the same water shield technique is under
planning, as is a 20-ton experiment, LZD.
Muons with energy above several TeV on the surface can reach labs at 4850 ft level in
DUSEL. These multi-TeV muons originate from the interaction of high-energy cosmic ray
particles in the Earth’s atmosphere. Our understanding of their production and propagation plays
a critical role in the underground background estimation, and its association with other large scale
environmental conditions, such as the seasonal changes in the Earth’s atmosphere, the position
and moving direction of the Earth in space, etc. The Comic-ray Working Group in the IceCube
Collaboration has a long history in cosmic ray physics, and it is currently studying high-energy
muon signals using data collected with IceTop and the in-ice array in coincidence. Such data
provides unique insight into not only cosmic rays from 300 TeV (1 TeV=1012 eV) to 1 EeV (1
EeV=1018 eV), but also high-energy muon and muon bundle production in the atmosphere [9, 10,
11, 12, 13]. Rapid progress on ultra-high energy cosmic rays and muon production are being
made in Pierre Auger Project [14] as well. Since the muons or muon bundles produced by comic
rays in this energy range are the dominant muon component at the DUSEL depths, to apply the
expertise and progress accumulated in IceCube to the study of underground muons at DUSEL is
of great interest to both projects.
The proposed research work in this proposal will focus on the LUX experiment and the use
of its data for a systematic study of underground muons and muon-induced background signals in
LUX and High Energy Muon Induced Backgrounds in DUSEL
Davis Cavern. Being the first dark matter experiment at DUSEL, LUX will provide a benchmark
for improving and verifying the simulation of various backgrounds in DUSEL. The research also
includes building a simulation scheme to describe the production of high-energy muons and the
backgrounds they induce in DUSEL. Such a full Monte Carlo will not only produce more realistic
fluctuations in muon-induced backgrounds, but also enable us to study possible correlations
between muon-induced background effects and large-scale/long-term modulations in the Earth
atmosphere or in cosmic ray flux. These two elements are essential in the explanation of
experimental data; however, both are ignored to a large extent in the data analysis of many
underground experiments.
Figure 1. LUX water shield conceptual
diagram. The water shield is a cylinder,
8 meters in diameter and 6 meters high,
filled with purified water. 20 PMTs
around the sidewall and at the bottom
monitor Cherenkov light produced by
relativistic charged particles in the tank.
The cylinder hung on the frame at the
center is the two-phase Xenon detector.
The cryogenic and circulating systems
are on the upper level that is not shown
in this diagram.
The PI on this proposal, Xinhua Bai, became a faculty member in the physics department at
South Dakota School of Mines and Technology (SDSMT) in August of 2009. Since then,
SDSMT has become an institutional member in the LUX Collaboration and an associate
institutional member in the IceCube Collaboration. As a new member in LUX, SDSMT missed
all project funding opportunities for the LUX S4 experiment R&D, hardware design, and
fabrication. Funding requested in this proposal will support our work with LUX water shield
calibration and simulation, LUX deployment, operation, data analysis and other service work
including education outreach activities in South Dakota. The PI anticipates hiring one
postdoctoral research fellow and supporting two master graduate students and one undergraduate
summer student in the next three years. Funds for adding a computer cluster at SDSMT will
enable the group to carry out the proposed analysis and simulation work more effectively. Since
the LUX Collaboration does not have a centralized computing facility for data storage and
analysis, the cluster will also be shared with the other LUX groups and visitors from IceCube. A
new digital oscilloscope, a fast laser diode pulser (two wavelengths), an optical attenuator, two
beam splitters and seven multimode fibers are needed in the PI’s lab and in Davis Cavern for
forthcoming testing, calibration, and education outreach (EO) activities. More details about the
personnel plan and the usage of these equipments/devices are given in the “Budget Justification”.
Current resources in PI’s lab are summarized in “Facilities, Equipment, and Other Resources”. In
addition to research work, the PI’s lab will become the only base on the SDSMT campus for
physics major student’s training and education outreach activities related to DUSEL physics
programs.
II. Results From Prior NSF Supported Research
While working as a post doctoral research fellow and then as a research scientist in
Fermilab, University of Wisconsin-Madison, Bartol Research Institute and in the Department of
Physics and Astronomy at the University of Delaware, the PI, Dr. Xinhua Bai was supported by
several NSF grants listed in Table 1; however, being a non-faculty member, Dr. Bai was not a PI
or Co-PI on these prior grants. This section only summarizes the major results of these grants in
which the PI of current proposal played a significant role.
LUX and High Energy Muon Induced Backgrounds in DUSEL
Table 1. Projects the PI of this proposal joined since 1998. Other projects funded by the
Chinese National Science Foundation the PI participated before 1998 are not listed.
Title & Grant No. of Support Period Total Grant PI/Award No.
Co-PIs
the Projects
South Pole Air Shower
Experiment -2
No. 9615101
Continued Operation of
the South Pole Air
Shower Experiment -2
No. 9980801
IceCube Startup and
Construction Project
No. 0236449
Air Showers in IceCube
No. 0602679
May 01, 1997 January 31, 2001
$665,000
Thomas Gaisser
Todor Stanev,
Paul Evenson
July 01, 2001 June 30, 2004
$719,855
Thomas Gaisser
Todor Stanev
August 1, 2002 March 31, 2011
$201,914,198
Francis Halzen
June 01, 2006 May 31, 2010
$750,000
Thomas Gaisser
Todor Stanev,
David Seckel
(1) SPASE2 experiment (Grants No. 9615101, No. 9980801): The SPASE2 experiment [15]
was an air shower experiment located at the South Pole. Dr. Bai first worked as a winter-over
scientist at the South Pole for this experiment and AMANDA (Antarctica Muon and Neutrino
Detector Array) during 1998-1999. In later years, he worked in almost all aspects in this
experiment, including detector calibration, experiment operation, improving air shower
reconstruction techniques, experiment simulation and physics analysis. He did the muon survey
for AMANDA optical modules (OMs) with SPASE2-AMANDA coincident events [16]. In the
cosmic ray composition study [17] and point source search work [18] with SPASE2 data, he
made significant contributions by reconstructing air shower events and building up a Monte Carlo
library at Bartol Research Institute during those two funding period.
(2) IceCube experiment (Grants No. 0236449, No. 0602679): Dr. Bai joined IceCube Project in
its conceptual design phase. By carrying out tests at the South Pole and in the lab he set up on the
campus at the University of Delaware, Dr. Bai made important contributions in the IceTop ice
Cherenkov detector design, early R&D and simulation work [19] and IceCube DOM calibration
work [20]. After the first IceCube string was successfully deployed, Dr. Bai carried out and led
on various calibrations and performance verifications using IceTop and in-ice coincident data.
Some of the results were reported in journal publications or international professional conference
proceedings [10, 21, 22, 23]. Particularly and more relevant to the research work in this proposal,
Dr. Bai was the first who carefully measured the muon flux on the surface at the South Pole [24].
He did the measurement using scintillate detectors together with a large ice Cherenkov detector.
The Cherenkov detector was used as an absorber and as a detector in coincidence. The
experiment measured muons with zenith angle from vertical to nearly horizontal. To eliminate the
severe background for the horizontal events, various techniques were used, such as coincidence
and anti-coincidence, time-of-flight (TOF), and waveform discrimination. In this work, the
measured flux was also compared with muon flux from Monte Carlo simulations. Recently, in
order to understand the energy loss of high-energy muons or muon bundles in deep under ice, Dr.
Bai and his colleagues carried out a Monte Carlo study [25], in which several questions regarding
muon bundle energy losses through different interaction channels and the reconstruction of muon
bundle energy loss were addressed.
Dr. Bai is also well experienced in using computer clusters. He has been using clusters for
his data analysis and simulation work for many years in IceCube. He helped manage the cluster
from the science side at Bartol Research Institute for nearly two years. He also worked closely
with the computer system administer at Bartol Research Institute in the extension and upgrade of
the Bartol IceCube cluster in 2008.
In addition to his research activities, Dr. Bai has participated and led several educational
outreach activities for SPASE2, AMANDA and IceCube Project; for example, the presentation
LUX and High Energy Muon Induced Backgrounds in DUSEL
for IceCube at the “Antarctic Treaty Meeting Displays” at the Maryland Science Center in April
2009. At “2010 Engineer's Week” on February 19th in Rapid City, SD, the PI led the physics
department effort introducing dark matter detection in DUSEL in five sessions to an audience of
about 80 students from regional middle schools including Spearfish Middle School, Newell
Middle School and Dakota Middle School. Dr. Bai has also served as a member on the IceCube
publication committee between 2007 and early 2010.
Located in Rapid City, 50 miles from the DUSEL, SDSMT is expanding its research
interest into experimental astro-particle physics, dark matter search, and neutrino physics. Using
the startup fund (total $100K) the PI received from SDSMT, the group has transformed three
storage rooms into a particle physics lab that consists of an analysis room, an optical lab and a
general electronics/assembly room. We are playing an active role in LUX, such as LXe PMT
internal review, water shield PMT unit assembly, testing and calibration. In addition to a master
degree student, Mark Hanhardt, currently dedicated to the LUX experiment, the PI is also tutoring
an undergraduate student, Douglas Tiedt, in GEANT4 detector simulation using IceTop ice
Cherenkov detector simulation package as an example. This student is planning to participate in
LUX water shield simulations and to become a graduate student here. The PI and the current
graduate student also support and participate in the LUX detector integration in the surface lab.
The requested funding in this proposal will allow this local group to continue all the work already
started with our university funds and to make more contributions to the LUX experiment
deployment, operation and physics analysis.
III. Proposed Work
The proposed work for this project includes: A) LUX water shield calibration, monitoring,
simulation and operation; B) analysis of the data from LUX experiment to systematically
characterize underground muons and muon-induced background signals in the Davis Cavern; C)
building up a full Monte Carlo scheme to simulate muon induced backgrounds in DUSEL starting
from cosmic ray primary fluxes, and improving the simulation by applying progress in high
energy muon production anticipated in IceCube. As part of this proposal, we will also participate
in educational outreach programs through the administration of the Education Outreach (EO)
Office at Sanford Lab.
A. Work toward the first dark matter experiment at DUSEL: LUX water shield calibration,
simulation and operation.
The water shield used in the LUX experiment is a 300-ton water Cherenkov detector that is
designed to shield gamma rays and attenuated neutrons in the Davis Cavern. With the water
volume monitored by 20 PMTs (10” Hamamatsu R7081), the water shield can also provide a veto
to the LUX inner detector to be free from particles (and their secondary particles) that trigger the
water shield. In order to estimate the precise veto efficiency and extract more physics results
using the signal in the water shield, one has to carry out very careful calibration and simulation of
the PMTs and the integrated water Cherenkov detector. Given the similarity between the LUX
water shield and the Ice Cherenkov detector of IceTop and the water Cherenkov detector in Pierre
Auger surface array, the experience and knowledge the PI has with these two cosmic ray
experiments are unique among LUX member institutions in developing a research and service
plan for the LUX experiment.
Currently, we are doing the PMT testing and calibration in the PI’s lab at SDSMT. This
work includes taking the single photon electron (SPE) spectrum and waveform data for all 20
PMTs to be used in the water shield, the calibration of their gains, linearity, timing and dark noise
rates in the PI’s Particle Physics Lab at SDSMT. The PI’s lab now has the equipment needed to
perform most of the calibration work: a data acquisition system, a dark box that can test three
PMTs at a time, high voltage supplies, necessary NIM electronics. The water shield signal/ HV
splitter box and pre-amplifier box were designed and fabricated by another LUX collaborator UC
Davis. Supported by the PI’s start-up fund, a programmable LED pulser recently fabricated at
LUX and High Energy Muon Induced Backgrounds in DUSEL
SDSMT (modified from the design used in IceCube PMT calibration work [26]) is under testing.
It can be used to study the PMT response to extremely bright light pulses. A fast (~ tens of picosecond) laser diode (such as Hamamatsu PLP-10 or PicoQuant PDL-800 plus diode heads) is
requested in the proposal for the timing calibration.
At the time of proposal writing, one Ph.D. student, Nick Walsh, from UC Davis works part
time with one master degree student, Mark Handardt, of SDSMT on the PMT calibration in the
PI’s lab. A calibration database will be created for each of the PMTs. This database is crucial in
signal simulation, veto trigger and data acquisition (DAQ) system design, and the estimation of
the energy loss by particles in the water. To optimize the operation of the water shield and use the
data from it for physics analysis described in Section B below, a lot more calibration and
simulation work are required in the lab and at the experiment site in Davis Cavern. Having a local
team will provide great convenience during the experiment. Since the funding for the physics
Ph.D. program at SDSMT was delayed, both UC Davis and SDSMT agreed to cooperate on this
project closely (letter of agreement from LUX group at UC Davis is attached). In the following
years, Mark Hanhardt will continue working together with Nick Walsh from UC Davis on the
LUX water shield calibration, monitoring and operation, with Douglas Tiedt to join in the LUX
water shield simulation and analysis in year 2011.
A-1: Water shield calibration, simulation, and monitoring using various sources:
In the design, the water shield should have 100% trigger efficiency for particles that have
their Cherenkov radiation path length greater than one meter in water [27]. This trigger can be
used as a veto to the background produced by particles such as muons that hit the water shield.
Signals from the water shield can also be used to veto muons that stop and decay in the water
volume (see analysis in Section B-1). To determine the background in the final dark matter search
results, one has to quantitize the actual veto efficiency and its dependence on the energy and the
type of the incoming particles. This cannot be done without precise Monte Carlo simulations.
Along with the PMT testing, the SDSMT group will participate in building the water shield
simulation package.
At present, many other parameters in the simulation, such as the reflectivity of the Tyvek
liner and water property are taken from published results [28]. In reality, even with the properties
of individual detector components measured and incorporated in the simulation, the integrated
detector’s performance may still be different from what the simulation predicts. One main reason
for the discrepancy is the chance in the interface between the detector components after they are
integrated into a system under operation condition. A practical way to overcome this difficulty is
to calibrate the detector during its operation. Unlike water Cherenkov detectors in air shower
experiment on the surface where background muons suffice for effective calibration and
monitoring purposes [19, 29], the water shield for LUX has to be calibrated with artificial sources
due to the very low muon flux at 4850 ft level. We plan to calibrate and monitor the water shield
with optical light. The optical light will be produced with a LED pulser (large pulses for linearity,
saturation and after-pulse study) or laser diode (fast pulses for timing calibration) and guided into
the tank through several optical fibers (to be added under this proposal). We can make several
measurements at different wavelengths by choosing different LEDs or laser diodes. Calibration
run using the optical light can happen periodically to monitor the long-term stability of the water
shield without interrupting LUX dark matter data taking.
An additional R&D project with the large water shield is to study how to use it for the
study of muon-generated small showers in Davis Cavern. The physics of those sub-MeV showers
is poorly known due to the lack of our knowledge about muon photonuclear cross sections in the
range of materials constituting the overburden in underground laboratories. Gamma ray with
energy higher than several MeV (like those in muon induced local showers) can also be detected
after Compton scattering or pair production in the water. In the present design, since the water
shield is one continuous volume, except the pulse size and time, little other information can be
used to identify different particles. On the other hand, since the water volume is a lot larger than
the radiation length (6 m – 8 m versus ~ 40 cm) and Moliere radius (6 m - 8 m versus ~10 cm),
LUX and High Energy Muon Induced Backgrounds in DUSEL
small electromagnetic showers can be well contained in a small portion of the water volume. One
improvement in the study of muon-induced underground showers may be achieved by adding
several more layers or sections in the water volume [30] in future experiments. The R&D work
during this funding period will only focus on optimizing the design with full Monte Carlo
simulation.
A-2: Work toward the first dark matter search experiment in DUSEL and prepared to make
more contributions in the future:
Since the LUX integration campaign started in Lead at the end of 2009, SDSMT group has
contributed shifts and provided support to the LUX surface integration with resources based on
SDSMT campus, such as sharing electronics and equipment in the PI’s lab, and cleaning LUX
internal pieces using facilities in our chemistry department. The SDSMT group will participate in
the LUX detector deployment to the underground lab, the water shield maintenance, operation,
monitoring, and LUX physics data analysis.
After reliable calibration and simulation tools are developed for the water shield, we will
carry out measurements using this 300-ton water shield alone and in coincidence with the inner
LXe detector (see proposed research work in Section B below). As described in the “LZ
Governance Structure” [31], SDSMT is expected to take the responsibility for the water shield in
LUX future development such as LZS (1.5 or 3.0-ton liquid Xenon) and LZD (20-ton liquid
Xenon). It also became clear at the last “Fall Workshop on DUSEL Science and Development of
the MREFC” that the water shield at present size or bigger will be used in all dark matter
experiments being planned for DUSEL [32]. Systematic analysis and simulations of the water
shield and LUX experiment fits in DUSEL dark matter search plan. To grow a strong local group
through the actual research activities with LUX will benefit the forthcoming projects in DUSEL.
B. Study of the muons and muon induced backgrounds in DUSEL using LUX data.
In addition to the low energy radiation from surrounding rocks and the construction
material, high-energy muons produced by cosmic rays are an important background source at
4850 ft level. The muon flux at 4850 ft level was estimated to be ~ 4.4×10-9 cm-2sec-1, with an
expected mean energy of 320 GeV [33] on a spectrum spread over several orders of magnitude.
These muons can create complicated backgrounds that may affect the experiment design, data
acquisition design, detector performance, and data analysis. For example, high-energy neutrons
produced by these muons can penetrate the attenuator and mimic dark matter signal in the LXe
detector. Muons with kinetic energy less than 55 MeV (muon Cherenkov threshold energy in
water) can be stopped in the water shield without producing Cherenkov light to trigger the water
shield. Depending on their incident angle and position, muons with energy up to 400 MeV ~ 600
MeV can also stop in the inner detector after losing their energy in the water shield around it.
These stopped muons can either decay (μ+/μ- to positron/electron and neutrinos) or be captured
through the semileptonic weak interaction by a proton μ- + p → νμ + n or by a nucleus μ- + (Z, N)
→ νμ + (Z−1, N+1)*. Since the μ- -capture cross-section increases rapidly with Z of the target
elements [34], besides neutrons produced in the water, over long time μ--capture processes can
make more significant contribution in cosmogenic production [35] in the detector construction
materials and in Xenon than in water and scintillation materials used for active vetoing.
For many years, different experiments have been done in major underground laboratories to
measure muon and neutron flux and/or spectrum [see references in 33, 36, 37]. With these data,
great progress has also been made in the flux parameterization and Monte Carlo simulation [see
for example 36, 38, 39, 40, 41]. For the muon flux calculation, it is important to know the average
rock composition and density between the lab and the surface, while for evaluation of background
from radioactivity and muons one has to know the rock composition around the lab, which can be
very different for different underground sites. At present, for certain experiment sites, assuming
that the knowledge of rock composition and surface muon profile gives accurate predictions for
muon flux and spectrum, reports show Monte Carlo simulations of muon-induced neutron
background using GEANT4 and FLUKA can predict the neutron event rate with an accuracy of
LUX and High Energy Muon Induced Backgrounds in DUSEL
about a factor of two [ex. 39, 40, 41, 42, 43]. Nevertheless, big uncertainties still exist at higher
energies. For example, at 3650 m.w.e. level, the calculated neutron yield starts to be lower than
the LVD data at about 100 MeV. It becomes lower by nearly one order of magnitude at about 400
MeV [39]. Ideally any simulation of muon propagation (with MUSIC, MMC, etc.) should be
normalized using experimental data; however, comparing with other major underground sites,
few measurements of muon and neutron fluxes were done at DUSEL site except several early
measurements done in the Davis Cavern [44, 45].
With its sensitivity goal of a cross-section of 7×10-46 cm2 (for WIMPs with mass M=100
GeV), LUX is expected to be sensitive to WIMP signals at a rate of about 6.5×10-6 drur (1 drur=1
evt/keVr/kg/day). To reach this goal, given the identification capability between electron recoils
and nuclear recoils, the LUX inner detector is designed to ensure background level at or below
8.3×10-4 druee for gammas and 3.7×10-6 drur for neutrons prior to applying the charge to light
ratio-based discrimination. Water radioactivity goal of U/Th/K less than 2 ppt/3 ppt/4 ppb (~106
lower than the rock) is attained by a commercial purifier. Radon in the purified water will be at
the level of about 2 mBq/m3, and will be further reduced with an N2 purge blanket over time [46].
Given such low internal backgrounds and the LXe (350 kg) and water (300 Ton) mass in a single
detector system, the LUX experiment is also an ideal instrument to study in-situ backgrounds in
addition to its primary goal of dark matter search. The background results may affect all forthcoming projects in DUSEL. In this proposal we will focus on the following three different, but
closely related, measurements and analysis:
B-1. Full particle spectrum, through-going and decay muons in the water shield
Muon flux and multiplicity were measured in several early experiments in the Davis
Cavern [44, 45], with a focus on their implications for cosmic ray physics and high-energy
interactions. In this proposal we will measure the signal in the water shield with a focus on their
connection with the backgrounds in a modern dark matter search experiments.
One interesting measurement is the energy deposition in the water shield together with
identified muon component. Based upon the absolute calibration proposed in Section A above,
together with detailed simulation of water shield response function to different particles, one can
measure the energy deposition of local sub-MeV showers that hit the water shield. By separating
different particles (only possible to some extent and on a statistical basis with the current water
shield configuration), one can estimate the contributions from muons and other electromagnetic
particles on the spectrum. This can be used to further estimate the spectrum and properties of
local small showers in the Davis Cavern. In order to identify the muon component from the
electromagnetic component in water shield signal, we propose the following two steps:
(1) Add a layer of plastic scintillator at the bottom of the water tank to identify muons that can
penetrate the 6-meter water depth from those stopped in the water volume or small shower events.
The electronics and eight plastic scintillator detectors (0.2 m2 each) are from the South Pole Air
Shower Experiment II (SPASE2) [15], which was decommissioned in 2006. Nearly vertical
muons of energy Eμ≥~1.6 GeV passing through these scintillator detectors will trigger them at an
estimated rate of about 1~5 events per day according to a previous measurement [45] and
estimation [33]. To reject the dark noise rate in a single scintillate detector, a coincidence with
signal by energy deposition between 1.6 GeV±2σ in the water shield will be first tried. Detailed
trigger criteria will be determined by more study of the actual data.
(2) Identify low energy muons by looking for muon decay signature in the water. The energy loss
of muon in water can be described as dE/dX ≈ - a - b×E, with a ~ 0.260 GeV/m.w.e. and b ~
0.360×10-3/m.w.e. [47]. Local muons with energy up to ~1.5 GeV may stop in the water shield.
The maximum and average energies of Michel electron from muon decay are 53 MeV and 37
MeV. Given the experience in Cherenkov detectors used in IceTop and Pierre Auger projects
summarized in Table 2 on the next page, we expect to see an average of no less than 10 PEs for
Michel electrons on each individual PMT in the LUX water shield. Using data from other
experiments [36, p. 558 and 48] and the parameterization in [38, p.80], the estimated muon decay
event rate is between 1 and10 per day; however, with unknown systematic uncertainties related to
LUX and High Energy Muon Induced Backgrounds in DUSEL
the local rock composition and other environmental parameters. This measurement is very
interesting because the stopped μ- can be captured by the nuclei of the detector materials and
produces various isotopes, some of which are radioactive [35]. Since the μ- capture cross-section
depends on the atomic mass of the target elements, successfully tagging muon decay events in
water, together with detailed detector simulation, one can estimate the μ- -capture rate in the dark
matter detector construction materials and liquid Xenon that have much larger atomic numbers.
The through-going and decay muons provide an alternative ways to calibrate the water
shield and crosscheck the simulation. If the water shield is stable enough (monitored by the
periodic runs using optical pulses as described in Section A), after 1~2 months the penetrating
muons will be statistically enough to provide an additional calibration point at a mean energy
deposition of about 1.5 GeV in the water shield and with decay muons of about 37 MeV. Based
on the results, together with measurements done with different techniques, such as the recent
gamma spectrum measurement [51], we can further check our understanding and the accuracy of
simulations of local muon-induced backgrounds in DUSEL.
Table 2. Summary of the Cherenkov detectors used in IceTop, Auger and LUX water shield.
IceTop and Auger calibration results are also included [49, 50]. For IceTop and Auger tanks,
photoelectron (PE) numbers in the last column are the mean values from the fit to the observed
spectrum. They are listed here for the estimation of the signal size in LUX water shield (in
italic). The lower PE yield (150PEs) in some IceTop tanks is due to the lower reflectivity of the
Zirconium coating used in those tanks. “M.E.” stands for Michel electron from muon decay.
“VEM” stands for Vertical Equivalent Muon.
Project
IceTop
Auger
LUX
Tank Size (m),
Target Medium,
Liner
Φ2.0-H0.9,
Clear ice,
Tyvek
(Zirconium)
Φ3.6-H1.2,
Filtered Water,
Tyvek
Φ8.0-H6.0,
Filtered Water,
Tyvek (?, TBD)
PMT
N. of
PMTs
Photocathode
Coverage (%)
10”
Hamamats
u R7081-02
2
0.80
240PEs/PMT/VEM
(150PEs/ PMT/VEM)
45PEs/PMT/M.E.
9”
Photonis
XP1805
10”
Hamamats
u R7081
3
0.36
90PEs/ PMT/VEM
11PEs/PMT/M.E.
20
0.40
~100 PEs/ PMT/VEM
~12PEs/PMT/M.E.
Muon Cal.
Michel ele. Cal.
B-2. Neutrons produced by high-energy muons
Neutrons are probably among the most important backgrounds that affect almost all
underground experiments. For double-beta decay experiments, high-energy neutrons (a few MeV
and above) produce background gamma rays via inelastic scattering while thermal neutrons
contribute to the gamma ray background through neutron capture. Neutrons at several MeV and
above also pose a threat to neutrino detection in low-energy neutrino experiments via inverse beta
decay. Neutrons at sub-GeV and GeV energies, although rare, constitute a background for proton
decay and atmospheric neutrino experiments. In WIMP dark matter detectors, nuclear recoils of
several keV energies by neutron elastic scattering mimic WIMP-nucleus interactions.
The LUX experiment provides an opportunity to study the muon-induced neutrons around
and in a dark matter detector. Immersed in the water shield, the LXe detector is a two-phase
liquid xenon detector. It can achieve high sensitivity, low threshold and electron recoil versus
nucleon recoil discrimination down to a few keV recoil energy. The way it works is shown in
Figure 2. Events in the liquid Xenon target create direct scintillation light (“S1”, measured largely
by the bottom PMT array). Electrons that survive electron-ion recombination and are extracted
into the gas phase by the electric fields (~ 5-10 kV/cm) will create scintillation light (“S2”,
measured largely by the top PMT array). Since the top PMT array images the x-y location of the
LUX and High Energy Muon Induced Backgrounds in DUSEL
S2 signal while the drift time between S1 and S2 gives the depth of the recoil, this technique
provides a 3-dimensional imaging of the recoil location. The position resolution is expected to be
better than 5 mm in the x-y plane and 2 mm in the z-direction for events down to several keVee
(keVee: energy of electron recoil event). For a given event in the liquid Xenon, the nuclear or
electron recoil energy can be determined based on the scintillation signal S1. In addition, this
technique also provides the discrimination of electron recoils (caused by gammas and betas) from
nuclear recoils from neutrons or WIMPs. This can be explained by an example shown in Figure 3
[52], in which the ratio of charge to scintillation light (S2/S1) versus S1-based energy is shown
for electron recoils and nuclear recoils. The separation between these two distributions is clearly
visible. The electron recoil events in LUX may be rejected at a level higher than 99% above the
analysis threshold of 5 KeVr (keVr: energy of electron recoil event). In a typical two-phase
Xenon experiment, the electron recoils are expected to yield a detectable signal of about 5
PEs/keVee and the nuclear recoils of about 2 PEs/keVr [3, 46]. With the PMTs sensitive to single
photoelectrons, the threshold can be in the range of a few keVr, which is the crucial energy region
in many underground experiments.
Figure 2. Signals of interactions in the
LUX detector. Color circles at the top
and bottom of the volume represent two
arrays of PMTs (Hamamatsu R8778).
The hit pattern in the bottom array
provides the x-y localization of an event,
while the time between the primary (S1)
and the secondary (S2) scintillation
signals provides the z-localization of the
collision.
Figure 3. Calibration data for the LUX prototype detector in Case [52] showing the ratio of
charge to light (S2/S1) versus S1-based energy for electron recoils (left) and nuclear recoils
(right). In both plots, the red line and the green line represent the mean of distribution, along
with 99% and 99.9% ER discrimination levels.
EDELWEISS-II (4,800 m.w.e. level, Frejus site, muon flux ~4×10-6 /m2/d, fast neutron flux
~1.6×10-6 /cm2/s [53]) for the first time claimed several coincidences between its muon veto and
muon-induced neutrons in its Ge crystals (consisting of 330 g Ge/NTD, 400 g Ge/NbSi and new
400 g Ge/NTD) [54]. However, the rather small rate of ~0.04 coincidences/kg/d for both E≤250
keV neutron- and electron-type recoils limits a detailed investigation of μ-induced neutrons in
this experiment. The measurement of muon-induced neutrons reported by the ZEPLIN-II group
[55] has not detected any coincidences between low-energy (< 100 keV) events in its xenon
vessel and high-energy events (> 0.5 MeV) in its liquid scintillator [43]. Comparing to the 730 kg
liquid scintillator shield and 7.2 kg of liquid xenon used in ZEPLIN-II, LUX (300-tons of water
and 350 kg of LXe) should have a better chance to see the coincidence between high-energy
muons (triggering the water shield) and nuclear recoil in the inner LXe detector. Successful
observation of the coincidence between muons and the nuclear recoil signals in the LUX inner
detector will give us enormous confidence on muon-induced neutron level in the LUX
experiment, which will further provide an anchoring point in the simulation of neutron
background level in the Davis Cavern. Accumulating more coincidence events will eventually
help to characterize the true fluctuations of muon-induced backgrounds in a real detector, which
cannot be done by either calibration with radioactive sources or in present simulation.
Given the importance of neutrons on all underground experiments, we also propose to have
more study on muon-induced neutrons in the LUX water shield. Two approaches for tagging
neutrons in a water Cherenkov detector were proposed. One is to use the 2.2 MeV gamma from
the n + p  d + γ reaction [56]. The second is by adding gadolinium trichloride GdCl3, which is
highly soluble and transparent in solution [57]. Neutron capture on gadolinium yields a 7.9 MeV
(80.5%) and an 8.5 MeV (19.3%) gamma cascade. Using these techniques, progress was made
recently in measuring high-energy muon induced neutrons in the ZEPLIN-II liquid scintillator
[55] and at Super-Kamiokande [58]. The maximum kinetic energy of a Compton-scattered
electron by a 2.2 MeV gamma is 1.97 MeV, the so-called “Compton edge”. Super-Kamiokande
reported the 8 MeV gamma cascades from neutron capture on gadolinium to have a mean
measurable energy of 4.3±0.1 MeV. Taking those numbers in Table 2 for vertical equivalent
muons (VEM) or Michel electrons in the Auger tank for example, one expects an average of 0.5
PEs and 1.1 PEs for a 1.97 MeV electron and a 4.3 MeV cascade in the LUX water shield.
Without changing its current configuration, tagging neutrons in the LUX water shield by either of
these two methods seem marginally doable.
Because of the very low light yield in either of the two processes, neutron tagging
efficiency strongly depends on Cherenkov light collection. We will use the verified water shield
simulation package to investigate how to improve neutron tagging techniques and Cherenkov
light collection efficiency by optimizing PMT locations and using various reflective liners and/or
wavelength shifter additives. We may propose, for example, a fully instrumented water shield
with greater PMT cathode coverage and/or Gd doping for next generation dark matter
experiments. We will also simulate neutron-tagging efficiency in the liquid scintillator proposed
for active vetoing in our next projects LZS and LZD and compare all these techniques to optimize
the design for next generation dark matter experiment.
B-3. Study the long-term behavior in LUX data and compare with IceCube results
Because the expected event rate is extremely low, most underground experiments have to
take years of data to have enough statistics to reach their physics goals. Therefore, monitoring the
long-term behavior of their signals (both background signals and “event” signals) is essential in
pursuing any physics results. Despite the anisotropy of cosmic ray arrival directions observed by
surface array [59] and deep under-ice muon detector [60], one well-known long-term behavior is
the annual modulations observed in different experiments at different depths. Two representatives
are, (a) the rate in the DAMA dark matter search experiment, (b) the underground muon rate.
(a) DAMA: The DAMA Collaboration reported preliminary results of a positive annual
modulation indication in late 1997 [61]. The modulation was later confirmed by the DAMA
experiment that consists of an array of nine NaI(Tl) crystals with a total mass of 100 kg operated
for a continuous seven-year period that ended in July 2002 [62]. The annual modulated variation
in the signal was reported at 6.3σ CL. The newly combined 11 years from data of both
experiments (with an exposure of 0.83 ton×year) show an 8.2σ annual modulation signal [63],
Figure 4.
DAMA is the only experiment that has claimed to observe an annual modulation in their
data compatible with the signal expected from dark matter particles bound to our galactic halo,
contrary to the negative results of all other direct DM searches. DAMA statement of detection of
dark matter signals has raised many discussions on the WIMP scattering mechanism with nuclei
and various halo models [3, 63, 64]. Nevertheless, given various assumptions in the explanation
of their signal, it is clear that more analysis is needed to reach a consensus.
Figure 4. Model-independent residual rate of the single-hit scintillation events, measured by
the DAMA/LIBRA experiment in the 2-6 keV energy intervals as a function of time, from
[63].
Figure 5. Top: Annual muon flux modulation for energies great than 1.3 TeV observed by
the LVD experiment [69]. Bottom: The superposition of the mean monthly variations in the
muon rate (in percentage on left Y axis), and the mean monthly variation in the effective
temperature (in percentage on right Y axis), from [65]. A total of 5.33×106 single muons
collected in 1992-1994 were used.
(b) Annual variation of muon flux: The muon flux modulation has been reported by several
underground or under-ice experiments, such as the MACRO experiment [65], MINOS [66, 67],
IceCube [68] and, very recently, LVD [69, see the top entry in Figure 5]. The muon flux
measured by MACRO shows a nearly sinusoidal time behavior with one-year period and a
maximum in summer, (see the bottom entry in Figure 5). The plotted effective temperature
depends on both the atmosphere temperature profile at different depths and the atmospheric
attenuation length for pions and nucleons as described by the formula below,
in which the Λπ and ΛN are the atmosphere attenuation lengths for pions and nucleons. The
integral is from the surface to the top of the atmosphere. This analysis also showed that muons
from the decay of pions and kaons are 77% and 23% in MACRO data. The annual modulation in
the IceCube in-ice muon rate was associated with the temperature profile of the stratosphere at
pressure layers from 20 hPa to 100 hPa where the first cosmic ray interactions happen [68].
Authors in IceCube also explained the modulation as the result of the seasonal change of the
Antarctic atmosphere and the characteristics of cosmic ray interactions in the atmosphere. Given
the strong evidence of the correlation of muon flux with the atmospheric temperature, the scale of
the modulation also depends on the depth of the experiment and the threshold in the experiment.
Studying these effects in LUX data is important because it helps determine the background due to
non-dark matter contributions.
It is very much worth pointing out that, using the details provided in [63], the annual
modulation amplitude in DAMA experiments with recoil energy between 2-6 keV is
(0.0129±0.0016) cpd/kg/keV. This corresponds to relative amplitude of about 1.3±0.1 % against
an overall background counting rate of about 1 cpd/kg/keV. The phase is 144±8 days, with the
maximum in early June. Meanwhile, in the LVD muon result, the amplitude is 1.5±0.1 % with a
phase of 185±15 days and the maximum in early July [69]. Before any solid conclusion can be
made whether the similarity is a true effect or just a coincidence, detailed studies with both
simulation and new experiment data are needed, especially using data from different experiment
such as LUX.
One important lesson from the arguments around the DAMA results is that eliminating any
modulation due to cosmic rays or other terrestrial sources by normal matter or processes, annual
or not, is essential to understand any observed modulation in dark matter data. In this proposal,
we will carry out the following systematic study of modulations in the water shield and LXe data:
(1) Look for modulations in signals in the water shield and in the LXe detector. The total muon
rate in the LUX water shield is high enough to see monthly modulation of several percent after
2~3 years of data taking.
(2) Estimate the annual modulation amplitude in muon flux at the 4850 ft level in the Davis
Cavern by simulation. In order to include the annual change in the atmosphere, the simulation
will be done using cosmic ray primary particle flux. We will also update the atmospheric
parameters to include the local atmosphere monitoring data. See more details about this work in
Section C below.
(3) If any modulations are measured in the LUX water shield, we will compare them with
simulations to cross check the expectations. We will compare them with the muon modulation
recorded in IceCube data taken during the same time period to look for any anti-correlations
between them. Since LUX and IceCube have different overburdens and are located in different
hemispheres and under very different weather and environmental conditions, such combined
analysis will provide a unique view in the sense that the two data sets represent different overburden depths, different locations on the Earth and different view directions in space.
C. Building a full Monte Carlo simulation of muon induced backgrounds in DUSEL,
starting from cosmic ray primary particles and improving the simulation by applying upto-date progress on high-energy muons or muon bundles made in IceCube and Auger.
Most current underground background simulations start from an average muon flux on the
surface. The advantage of this approach is that one can alter parameters and make sure the surface
muon profile gives accurate predictions for muon flux and spectrum. Nevertheless, this approach
does not include several important features that are equally important for underground
experiments searching for exotic events. One of them is the fluctuation (in both energy and
multiplicity) related to the muon production in air shower development. This fluctuation is much
larger than the fluctuations introduced in the muon propagation through the overburden, and can
affect the dispersion of muon-induced backgrounds in the LXe detector. The other is the
modulation in air shower development in the Earth atmosphere. Both of these have features
visible in underground labs at DUSEL level.
The IceCube and Auger collaborations are making significant effort in the study of highenergy muon production by high-energy cosmic rays. In particular, the IceCube Cosmic-ray
Working Group led by Bartol Research Institute and the University of Delaware is working on
several topics using IceTop and in-ice coincident events that may eventually improve the
calculation of muon production in the TeV region and above. One topic is the production of
muons above 100 GeV. Despite the progress made through many years of both experimental and
theoretical work, discrepancies still exist among different model predictions. See summary in
Figure 6 [70, 71].
Figure 6. Left: Comparison of inclusive muon flux predictions to L3 data [70]. Shown are
calculations using QGSJET, SIBYLL and TARGET as high-energy hadronic interaction
model. Right: Vertical atmospheric muon (and neutrino) fluxes. Conventional muon and
neutrino fluxes by solid and dashed lines marked “conven.” Other curves and shaded areas are
the prompt muon flux predictions from different model calculations together with two
experimental bounds from LVD and AKENO. See [71] and references therein for more details.
Another relevant topic is the high-energy muon bundle structure. For a long time, people
have known high-energy air showers can produce multiple high energy muons in the form of
muon bundle in which muons are highly collimated and close to each other in space [38]. An
empirical description of the integral muon energy spectrum in air shower was given by the Elbert
formula [72]:
in which A, E0 and  are the mass, total energy, and zenith angle of the primary nucleus. p1=0.757
and p2=5.25 [38]. Nevertheless, most underground muon flux measurements have assumed single,
uncorrelated muons. In the LUX water shield, the ratio of multiple-muon events to single muon
events is about few percent. Study has shown that giving the energy carried by all muons in the
bundle to a single muon makes a huge difference in IceCube [73].
Some efforts have been made to improve the IceTop-InIce coincidence data analysis [25,
74]. As IceCube completes its deployment in 2011, within several years, the 1-km2 IceTop array
will accumulate significant amount of air shower data from several hundred TeV up to ~ 1 EeV,
among which there will be unprecedented coincident events between IceTop and the in-ice array.
Progress is expected in the calculation of muon production and muon bundle characteristics in the
energy region that dominates the muon flux in DUSEL. The PI of this proposal has been joining
the cosmic-ray working group phone conferences and will follow the progress in IceCube and
manage to apply it in the development of a full Monte Carlo simulation scheme to simulate
muon-induced backgrounds in DUSEL.
With the advantage of working on LUX experiment, we will be able to cross check the
simulation to great detail with the systematic measurements done with LUX data described in
Section B, which is the only reliable way of developing a precise background models for a certain
site.
IV.
Broader Impacts
Benefits to science: The results of the proposed work will provide the underground physics
community with the first systematic experimental muon background profile at the 4850 ft level in
DUSEL and their signatures in a sensitive underground dark matter detector, which will provide a
benchmark for all forth-coming underground experiments in DUSEL. The modulation study will
help the physics community understand better this long debated phenomenon in dark matter
experiment and its influence in the search for extremely rare events. By developing a full Monte
Carlo background simulation scheme starting from cosmic ray flux, we expect to eventually build
up a full Monte Carlo background library for DUSEL that also includes the fluctuations in air
shower development and seasonal effect in the Earth atmosphere. After being improved over time,
such a library can eventually serve all planned underground experiments in DUSEL. Before dark
matter particles are detected, the search for them is basically to increase the detector sensitivity by
increasing the detector size and understand/eliminate the backgrounds. It is worth to point out that
significant uncertainties still exist in our understanding of various backgrounds in present dark
matter search experiments [75]. Based on new results and ideas that may come out of our
research, with the full Monte Carlo background simulation to be built, we will be able to reevaluate more quantitatively the necessity of having an appropriate surface array for DUSEL
[76].
Prepare for future DUSEL programs by integrating research and education at SDSMT:
During the course of the proposed work, the postdoctoral fellow and graduate students will be
expected to play major roles in all areas of the research, and will have the chance to be trained in
many aspects of research including data analysis, simulation and some service tasks. Through
participation in the large international collaborations LUX and IceCube, they will also have the
opportunity to experience working with large teams of diverse scientists, helping them to develop
important skills for their future careers. All group members will be encouraged to attend
professional conferences to present their work, and to mentor undergraduate interns, thus
improving their own communication skills.
Similarly, advanced undergraduate students will gain valuable research experience by
working closely with the postdoctoral fellow and graduate students on subtasks such as
maintaining and upgrading the software and the study of simpler problems in major tasks. They
will also accompany the group to the DUSEL site to participate in experiment deployment,
calibration, maintenance and operation. They will be encouraged to present research results at
undergraduate research symposia at professional conferences.
Moreover, since water Cherenkov detector is one of the favored options in the LongBaseline Neutrino Experiment design, to grow a local group with experience on the LUX 300-ton
water shield will benefit DUSEL science programs in the future.
Formal education: The Education Department at Sanford Laboratory is in the planning stages
for a major science education center, the Sanford Center for Science Education (SCSE), to be
built as part of the DUSEL Laboratory. The mission for the SCSE is, in part, to draw upon the
science and engineering to be pursued at DUSEL to inspire and prepare the next generation of
scientists, engineers and educators. They are in the process of developing prototype programs that
would transition to and build capacity for the programs to be run by the SCSE, both onsite, offsite
and virtual. The programs will be grounded in education research best practices and employ
rigorous evaluation.
The planning team for the SCSE has need of content experts in designing these programs.
From scientific point of view, the detection of dark matter would open a new window to the
hidden portion in the Universe that is nearly ten times heavier than what we know today. It would
be a major scientific discovery with wide-ranging implications for all of particle physics,
cosmology and astrophysics. The search for dark matter has gained broad public attention. To
help impose more positive repercussions for the role of science in society, the PI will work in
partnership with the planning team for the SCSE to help devise and prototype models for handson or virtual delivery of astroparticle physics content. The PI is beginning that partnership in the
summer of 2010 by being a lecturer for the Davis-Bahcall and Fermilab-BNL-Homestake
summer scholars programs, which will bring twenty of the brightest science students (rising
freshmen and sophomore undergraduates) in South Dakota to Sanford Lab for one week each.
While there, the students will learn modern physics, participate in tours and hand-on activities,
and perform an experiment of their choosing underground. These students will be tracked and
mentored through the rest of their undergraduate careers and will be prime candidates for
internships at SDSMT, Sanford Lab and elsewhere (students from 2009 have secured DUSELrelated internships for 2010 at Princeton University, Brookhaven National Laboratory, Colorado
School of Mines and Sanford Lab)
Involvement of underrepresented groups: Located in the Great Plains, residents in South
Dakota have a long tradition of mining and farming, the population of 800,000 being 67% rural.
South Dakota is also the home to nine Native American tribes, comprising 9% of the population.
From family structures to spirituality, people in South Dakota have a rich and colorful culture. In
general the perception among all groups is that if one is interested in a career in Science,
Technology, Engineering or Math (STEM) fields, one must move out of the state.
The reservations located in South Dakota are among the poorest counties in the country.
American Indian students have high drop-out rates and only small numbers typically attend
college; even fewer finish college, and only a very few pursue degrees in STEM fields. The
presence of DUSEL in western South Dakota provides an opportunity to involve more American
Indians in the design, construction, science and engineering of the facility.
SDSMT only has about 2.5% Native Americans on campus and many of them are the first
generation to attend college in their family history. In order to improve this, SDSMT hosts
several programs, which include NSF Tiospaye in Engineering Program, and the SD GEAR UP
Honors Program, etc. Some of those programs (such as the SD GEAR UP Honors Program [77])
are designed to prepare Native American students to be successful in the science and college
setting. Being the first astroparticle physicist in the history of the SDSMT, the PI will participate
in local educational outreach programs for all groups with different cultural backgrounds. The PI
and other group members will develop mini-lectures related to the research work with the
IceCube and LUX experiments for interested SD GEAR UP students during their six week
residential program on campus every summer. Tours will be provided of the PI’s lab on
campus. This work will be coordinated with the work that Sanford Lab Education Department is
also doing with GEAR-UP students, in order to provide an integrated approach to activities and
lectures based on DUSEL and the science and engineering to take place there. This integrated
approach will touch the GEAR-UP students at every grade level, and prepare them to graduate
into other enrichment activities such as the Davis-Bahcall Scholars program, which would give
the students an opportunity to visit Gran Sasso Laboratory in Italy and to study physics at
Princeton for three weeks during the summer after their senior year of high school.