Intellectual Development Statement
Ashutosh V. Kotwal
Duke University
April 26, 2009
I joined the Physics Department at Duke University in January 1999 as an Assistant
Professor in the field of experimental particle physics. I submitted my tenure dossier in
October 2004, and was promoted to Associate Professor with tenure in July 2005. In
this statement I will describe the research I have conducted and and leadership responsibilities that I have held in my field, since the submission of my tenure dossier.
1
Introduction to Experimental Particle Physics
Particle physics involves the study of fundamental building blocks of matter and
their interactions. Experimental particle physicists seek to make measurements that
will reveal the physical properties of nature at its most elementary level.
Experimental physicists have successfully used the technique of scattering particles
off a target to study the structure of the target. The theory of Quantum Mechanics
dictates that studying physical properties at smaller distances requires probe particles
of increasing energy. A hundred years ago, Rutherford studied the pattern of α particles
(later understood to be the nuclei of Helium atoms) scattering off gold atoms to conclude
that atoms consisted of a very small, dense, charged nucleus, surrounded by a cloud of
electrons. Using higher energy collisions of nuclei, the nucleus was found to be a tightly
bound state of protons and neutrons. Yet higher energy collisions in the 1970’s led to
the discovery that protons, neutrons and eventually a host of other similar particles were
bound states of more fundamental particles called quarks.
At the same time, a deeper understanding of the forces acting on matter was gained.
In addition to the electric and magnetic forces and gravity, two new interactions were
1
discovered. The “weak” interaction, initially hypothesized to be responsible for radioactivity, and has since manifested itself in many other phenomena. The “strong” interaction is responsible for binding protons and neutrons in nuclei and for binding quarks in
protons and neutrons. The theories of quarks, electrons and other electron-like particles,
and their non-gravitational interactions have merged into an elegant theory called the
Standard Model (SM). The Standard Model provides a coherent explanation for a large
number of measurements of the behavior of elementary particles.
While the Standard Model has been very successful, it leaves some important questions
unanswered:
1. Is there an underlying structure or symmetry to what we consider today to be the
fundamental building blocks of matter, such as quarks and electrons?
2. Are the four known forces different manifestations of a single unified interaction?
3. What is the origin of the mass of particles?
4. What is the structure of space and time? Are there additional dimensions at small
distances?
Answering these questions is one of the highest priorities of the field. In my research
as an experimental particle physicist, I have chosen to pursue measurements that could
provide evidence of new physical phenomena which cannot be explained by the Standard
Model. The questions mentioned above have inspired alternate theories, and by testing
their predictions, I hope to shed light on some of these mysteries of nature.
Rutherford’s pioneering technique of scattering particles to understand their properties
is still the mainstay of the particle physics. The particle accelerator with the highest
energy in the world is called the Tevatron, located at the Fermi National Accelerator
Laboratory (Fermilab) near Chicago, Illinois. The energy of the particle collisions at the
Tevatron is available to create new forms of matter that may interact via new forces,
due to the equivalence of matter and energy given by Einstein’s equation E = mc2 . Two
large detectors, called CDF and DØ respectively, have been built by teams of particle
physicists to record electronically the properties of the outgoing particles produced in
these collisions.
I obtained my Ph.D degree in experimental particle physics, making precise measurements of the internal structure of the proton and neutron in the highest-energy muon
scattering experiment at Fermilab. Since then, I have worked over the last 14 years on
the design and construction of the CDF and DØ experiments, and on the computer programs used to analyze the electronic data and produce physical interpretations. Due to
2
the enormous complexity of these modern experiments at the frontier of research, each
of the CDF and DØ collaborations consist of ≈ 700 physicists from U.S. universities,
national laboratories such as Fermilab, and a large number of international institutions.
Together they include about half of the U.S. particle physics community and about
one-fifth of the world-wide community.
The questions posed above, strongly motivate new experiments at higher energies than
the Tevatron can provide. The Large Hadron Collider (LHC) has been built at CERN,
Switzerland (the European laboratory for particle physics) to generate particle collisions
with seven times more energy than the Tevatron. The LHC will start collecting data in
the near future, and it is anticipated that it will run for 10-20 years. My future research
program will focus on physics at the LHC.
2
Executive Summary of Research Program
Possible extensions of the Standard Model include theories of compositeness (i.e.
substructure) and additional symmetries relating the known particles to hypothetical,
new particles. I have chosen to test these theories using two complementary strategies:
precision measurements to test for deviations from the Standard Model predictions, and
direct searches for new particle production.
The mass of the W boson, a mediator of the weak force, is one of the most precise
and important measurements at the Tevatron. It is influenced by the existence of new
particles via quantum mechanical corrections, making it a sensitive observable to probe
new physics. I have therefore chosen to measure the W boson mass using the CDF and
the DØ experimental data.
I strategized the current CDF W mass analysis from its conception, created the software infrastructure for it, and made precise determinations of the leptonic and hadronic
detector response. Under my leadership, we published the first measurement of the
W boson mass from Run II of the Tevatron, using the data from the ungraded CDF
detector. At the time of publication, this measurement was the single most precise
measurement of this quantity in the world, with an uncertainty of 48 MeV. The impact
of this measurement, via the precision electroweak fits, was to lower the inferred value
of the SM Higgs boson mass by 6 GeV. The best-fit value of the (undiscovered) Higgs
mass is lower than the direct exclusion lower bound from the European large electron
positron (LEP) experiments by 38 GeV, hinting (though not conclusively due to comparable uncertainties) with an incompatibility with the SM and thus pointing to possible
new physics beyond the SM. This result has been published in Physical Review Let3
ters (CV # 5, TT # 1) and Physical Review D (CV # 4, TT # 7), and is still
the best published measurement of the W boson mass.
I have co-authored a comprehensive review article describing the status of current W
boson mass measurements, and prospects and challenges for further improvements, in
an invited article for the Annual Reviews of Nuclear and Particle Science (CV
# 3, TT # 9).
The mass of the heaviest quark, the top quark, is another very important parameter
in the SM. The quantum mechanical radiative corrections to the W boson mass, as well
as to another precision parameter called sin2 θW (which is measured at LEP and the
Stanford Linear Collider), receive contributions from the top quark. In order to extract
the most sensitivity to Higgs and other new physics, the top quark contribution has
to be subtracted. For this calculation the top quark mass is needed. I have published
in Physical Review D, Rapid Communications (CV # 7, TT # 8) the most
precise measurement of the top quark mass in the dilepton channel, using multivariate
matrix element technique to extract the most information per data event. Following this
publication, the technique has been augmented with a new method of event selection,
based on evolving neural networks (modelled on genetic evolution in biology). This novel
method, applied for the first time in high energy physics, is used to find a neural network that minimizes the top quark mass uncertainty directly, rather than classification
accuracy or other secondary criteria. Also doubling the size of the data set, the new
technique and result has been published in Physical Review Letters (CV # 1, TT
# 2).
The SM fit to the values of the W boson mass, top quark mass and sin2 θW strongly
favor a relatively light Higgs boson, with the preferred value of its mass below 200
GeV. Building on the techniques I am using for the top quark mass measurement in
the dilepton channel (with the final state containing two leptons and two b quarks), I
have initiated a direct search for the Higgs boson in this preferred mass range. In the
Higgsstrahlung process, a Higgs boson is radiated off a Z boson, with the Higgs decaying
preferably to two b quarks if it is light. This search for associated Higgs production,
ZH → l¯lbb̄, is based on a per-event likelihood estimator using multivariate matrix
elements. The result will be submitted for publication in summer 2009.
My other research on direct new particle production has centered on testing models
of compositeness and extended symmetries. These models explore different mechanisms
of explaining the structure of the SM and creating a more unified description of nature.
My work in these areas has also been at the research frontier.
Based on my research on compositeness, I have produced new results on excited electron states (CV # 11, TT # 6) and excited muon states (CV # 8, TT # 4),
4
leading to two publications in Physical Review Letters. The excited electron search
was the first of its kind at the Tevatron.
Pursuing the possibility of additional symmetries, I have tested the extension of the
left-handed weak interaction to the Left-Right Symmetric model [6], which is motivated
by recent results on neutrino oscillations. I have pursued the extended Higgs sector
predicted by the Left-Right model, choosing the doubly-charged Higgs boson as the
ideal testing ground. Following on a previous publication of a doubly-charged Higgs
boson into electrons and muons, I have also obtained the most stringent direct limit on
the production of a long-lived doubly-charged Higgs boson, which has been published in
Physical Review Letters (CV # 10, TT # 5).
Many grand unified theories attempting to unify the different particles and forces at
high energies using a larger symmetry group, predict that at lower energies the broken
symmetry group contains an additional, heavy neutral boson. This particle would appear
as a narrow resonance decaying to two leptons, and is often called the Z ′ boson. Narrow
dilepton resonances are also predicted in theories of warped extra dimensions, wherein
the extremely weak force of gravity is predicted to become strong at the energy scale
accessed by current particle colliers. I have completed an extensive search for these
narrow resonances decaying in the muon channel, and the results have been published
in Physical Review Letters (CV # 2, TT # 3). These are the most stringent mass
limits on supersymmetric neutrinos, Z ′ bosons and Randall-Sundrum [1] gravitons for
certain values of model parameters.
I have led large teams of particle physicists on several projects. I served as co-leader
of the CDF Offline Software and Computing Project for two and half years. I supervised
all the analysis software and computing activities of the experiment. This is one of the
seven most important positions in the collaboration. I was responsible for all reconstruction and simulation software, the processing of all data and simulation events, the
final detector calibrations, as well as all computing infrastructure, budget and personnel
issues. I conceived of and implemented a rapid data processing scheme, and deployed improved reconstuction software and distributed (GRID) computing infrastructure. Under
my leadership, these developments resulted in a substantial increase in the experiment’s
physics productivity. Some of the new methodology, processes and technical designs are
described in two publications in Nuclear Instrumentation and Methods (CV #
6) and IEEE Transactions on Nuclear Science (CV # 9).
I have given nine major presentations in international conferences and workshops in
the last four years, all upon direct invitation and in the plenary session. I have also
given thirteen invited seminars at research universities, international institutes and laboratories.
5
I was elected as Chairperson of the Fermilab Users Executive Committee. Currently
serving in this role, I am the primary contact person between the Fermilab user community and the Fermilab management, and the U.S. Congress. I have also served on the
international advisory committee of the Hadron Collider Physics Symposium, the joint
NSF/DoE review panel of the US LHC project, the DoE Outstanding Junior Investigator
Award selection committee.
I chaired and organized the Hadron Collider Physics conference on the Duke campus in
2006. This is an international conference, and the special theme of this conference was to
bring the Fermilab and Large Hadron Collider (LHC at CERN in Europe) communities
together. The conference was highly successful and well-appreciated by the participants.
I have been elected Fellow of the American Physical Society in 2008, with the citation
to my W boson mass measurements.
I have supervised four post-doctoral research associates, six graduate students and
three undergraduate students. I have been successful in securing funding to fully support
my research group. Since receiving tenure I have supervised two Ph.D. theses, two
Masters theses and one undergraduate Honors thesis.
3
Research Accomplishments
In this section I will describe my research achievements in the area of precision
electroweak measurements, followed by direct searches for new particle production.
Tevatron experiments have collected data during 1992–1995 and again since 2001. In
the following, I refer to the former as “Run I” data and the latter as “Run II” data.
I am a co-author of over 200 publications from the CDF, DØ and the E665 Collaborations at Fermilab. The convention in these collaborations is that all collaboration
members are listed as authors on every publication. I describe below my own research
since receiving tenure.
3.1
Precision Electroweak Measurements
In the Standard Model, the gauge symmetry that predicts massless W and Z bosons
as mediators of the weak force is broken to impart masses to these particles. This
6
“electroweak symmetry breaking” is induced through the “Higgs” mechanism where a
hypothetical Higgs field acquires a non-zero vacuum expectation value. The coupling of
the Higgs condensate to other particles imparts them their mass.
The underlying dynamics causing the formation of the Higgs condensate is not known.
A precise measurement of the W boson mass helps to uncover the mechanism of electroweak symmetry breaking. Radiative corrections to the W mass due to quantum loops
in the W boson propagator depend on the spectrum of the particles in the loops, including the Higgs boson. A measurement of the W mass implies a measurement of these
radiative corrections, which can be converted into a Higgs mass constraint.
Various extensions of the Standard Model predict new particles coupling to the W boson, such as Supersymmetric (SUSY) particles. The theory of Supersymmetry predicts a
duality between particles of matter and particles that mediate interactions. In the Minimal Supersymmetric extension of the Standard Model (MSSM), for example, additional
corrections can increase the predicted W boson mass by up to 250 MeV [3]. Therefore
a precise measurement of the W mass provides a sensitive search of new physics beyond
the SM. It is complementary to direct searches and is sensitive to new particles that are
heavier than the mass reach of direct searches.
3.1.1
CDF W Mass Analysis
I conceived all aspects of the Run II W mass measurement at CDF. I developed the
analysis strategy to perform the electron and muon channel analyses in a common
framework. I developed a fast detector simulation for generating W , Z, J/ψ, Υ and
E/p spectrum lineshapes, and the template fitter for performing maximum-likelihood
fits to data. The detector simulation is based on a hit-level simulation of the COT
drift chamber, in which I incorporated a detailed simulation of multiple scattering, ionization energy loss, bremsstrahlung, photon conversion, electromagnetic and hadronic
calorimetry, and electron and muon acceptance.
An important success of my analysis is the determination of a consistent detector
energy and momentum calibration using the experimental data, with a precision of
0.03%. The momentum scale of the tracker is set using the precisely known masses of
the J/ψ → µµ and Υ → µµ resonances. The muon momentum dependence of the J/ψ
mass is used to tune the passive material map in terms of the ionization energy loss,
such that the momentum dependence is eliminated. The Υ → µµ mass fit provided a
consistent measurement of the momentum scale, as well as confirming the correctness of
the beam-constraining procedure.
7
The tracker momentum scale is transferred to the electromagnetic calorimeter for the
electron channel measurement, by performing a template fit to the Ecal /ptrack spectrum
of electrons from W → eν decays. The position of the peak in the Ecal /ptrack spectrum
is sensitive to the calorimeter energy scale, and also sensitive to the bremsstrahlung
spectrum (and at second order to the conversion of the radiated photons). The fraction
of events in the tail of the Ecal /ptrack spectrum is fitted as a function of a passive material
tune factor, which is then propagated into the peak fit for extracting the energy scale.
The non-linearity of the calorimeter response is measured by repeating the energy scale
fit in bins of ET .
A key success and confirmation of these calibration procedures was that the measured
Z boson masses in both the electron and muon channels were consistent with the precisely known world average of the Z boson mass. This validation was a challenge for an
earlier, Run I analysis of the W boson mass.
The Z → µµ mass fit is used to set the tracker momentum resolution, by tuning the hit
resolution and hit efficiency in the simulation. The calorimeter resolution is measured
from the observed width of the E/p peak, and from the observed width of the Z → ee
mass peak.
I developed and tuned the parametric model for the calorimeter response to hadronic
particles accompanying W and Z boson production. The model includes the contributions from the spectator interaction, instantaneous luminosity-dependent additional pp̄
interactions, and the hard recoil. The model is tuned using pT -balance in Z → ee, µµ
events.
As a result of my methods to use all available data for the various calibrations, most
of the systematic uncertainty was determined from the statistics of the control datasets.
This had a number of positive consequences. First, the measurement was more precise
than what one would expect extrapolating from the Run I statistics. Second, I established the methods to be used for future, even more precise measurements of the W
boson mass, using larger datasets.
Examples of maximum-likelihood template fits to the W transverse mass and lepton
pT fits are shown in Figs. 1-2. The simulation gives a good description of the data in all
the lineshapes. A total of six separate fits for the W boson mass were performed: the
distributions of the transverse mass, charged-lepton transverse momentum and neutrino
transverse momentum, for the electron and muon channels respectively. They produce
consistent measurements and cross-check each other since they have different systematic
uncertainties, and they are combined to obtain the final result:
MW = 80.413 ± 48 MeV,
8
W → eν
mW = (80451 ± 58) MeV
χ2/dof = 63 / 62
1000
Events / (0.5 GeV)
Events / (0.25 GeV)
1500
W → µν
1000
500
500
mW = (80349 ± 54) MeV
χ2/dof = 59 / 48
0
30
40
0
60
50
p T (GeV)
Figure 1: The W mass fit for electron
channel, using the lepton transverse momentum.
70
80
90
mT (GeV)
Figure 2: The W transverse mass fit for
muon channel.
the most precise published single measurement of the W boson mass in the world. Table 1
summarizes the uncertainties in the various W mass fits, along with the correlations
between the electron and muon channels. The analysis was performed using a “blind”
technique, so that the value was unknown until the analysis was final, and no changes
were made after the value was unmasked. The result was published in Physical Review
Letters (CV # 5, TT # 1) and Physical Review D (CV # 4, TT # 7).
In addition to the elements of the W mass analysis, my work on the drift chamber
tracker software, alignment and calibration were cornerstones of this success.
My collaborators in this work were the following colleagues from CDF: Prof. William
Trischuk (University of Toronto), Oliver Stelzer-Chilton and Ian Vollrath, both graduate
students from University of Toronto, my post-doc Christopher Hays, and Larry Nodulman (Argonne National Laboratory). I continued to work with Oliver Stelzer-Chilton
when he became a post-doc at University of Oxford, as well as with Christopher Hays
when he became a Lecturer at Oxford.
3.1.2
Review Article on W Mass Measurement
The W boson mass measurement is reaching the same level of precision as the measurement of sin2 θW from the LEP and SLC experiments, in the ability to provide a
stringent test of the SM, including the ability to constrain the Higgs boson mass and
other new physics.
9
100
Systematic
W → eν
W → µν
Common
pT (W ) model
QED radiation
Parton distributions
Lepton energy scale
Lepton energy resolution
Recoil energy scale
Recoil energy resolution
u|| efficiency
Lepton removal
Backgrounds
Total systematic
Total uncertainty
3
11
11
30
9
9
7
3
8
8
39
62
3
12
11
17
3
9
7
1
5
9
27
60
3
11
11
17
0
9
7
0
5
0
26
26
Table 1: Systematic and total uncertainties in MeV/c2 for the transverse mass fits, which
are the most precise. The last column shows the correlated uncertainties.
I was invited to co-author a comprehensive review article on the W boson mass measurement at the Tevatron. In this paper we discuss the general techniques, the final
Tevatron Run I measurements and their uncertainties, my Run II measurement from
CDF data and the DØ experiment’s Run II analysis (in progress at the time), as well
as what we have learnt about the Higgs. Finally, we discuss how the W boson mass
measurement may be further improved at the Tevatron. The article was published in
Annual Reviews of Nuclear and Particle Science (CV # 3, TT # 9).
My collaborator on this paper was my co-author, Dr. Jan Stark (Universite Joseph
Fourier Grenoble).
3.1.3
Top Quark Mass Measurement in the Dilepton Channel
The top quark mass mt is another key parameter of the Standard Model. The mass
value is needed to calculate various radiative corrections in the SM, in order to compare
values of precision observables to predictions and hence test the theory.
In the dilepton decay channel, top quark pair production is followed by the quarks
decaying into W bosons and b quarks, and both W bosons decay into leptons (electrons
or muons): tt̄ → W b + W b̄ → lν̄b + ¯l′ ν ′ b̄. Due to the presence of two neutrinos in the
final state, the mass fit is under-constrained on an event by event basis. In spite of
10
this challenge, it is possible to perform an event-by-event analysis by assigning a mt dependent probability to each event, and obtaining a best-fit mt value from the ensemble
of event probabilities.
I have worked with my student Ravi Shekhar, post-doc Bodhitha Jayatilaka and CDF
collaborator Prof. Daniel Whiteson (UC Irvine, previously postdoc at University of
Pennsylvania) to extract the top quark mass from dilepton events using such a per-event
technique. More information is extracted from each event, in the form of a posterior
probability curve P(mt ), as compared to the use of a single number to characterize the
event. We use a multivariate method to extract all the kinematic information in the
event in order to construct P(mt ). This method uses the full vector of kinematics ~x
from the event, ie. the momentum 3-vectors of the two leptons and the two b-quark jets,
and information on the remaining transverse energy flow in the event, as input to the SM
matrix element M(~x; mt ) for top quark pair production. We then use Bayes Theorem
to obtain the posterior probability P(mt ), from the SM prediction of the probability for
producing an event with kinematics ~x.
This technique has the benefits that the full event information is combined with the
complete SM prediction of top quark production and decay, in order to constrain the
measured top quark mass. We published in Physical Review D Rapid Communications (CV # 7, TT # 8) the world’s then best measurement of the top quark mass
in the dilepton channel:
mt = 164.5 ± 3.9(stat) ± 3.9(syst.) GeV,
using 1 fb−1 of CDF Run II data.
Following this publication, we have augmented this multivariate “matrix-elements”
technique with a novel method of optimizing the event selection. The matrix-elements
technique extracts the most information about the measured parameter for a given
sample of candidate events. However the event selection criteria are not defined by the
method. Ideally, one wants to select the sample (containing an admixture of signal
and background events with a kinematics-dependent signal efficiency and background
contamination), that maximizes the sensitivity to the measured parameter. We solve this
difficult heuristic problem by using neural networks in a novel application for high energy
physics. In a method modelled on biological evolution, we create a set of randomlygenerated neural networks, whose inputs are the event kinematics. Each network’s
output is used to provide a binary cut to select/reject a candidate event. The optimum
network is defined as the one whose selected candidate sample would provide the smallest
statistical uncertainty on the mt measurement. This optimum network is created as
follows: all networks in the initial set are tested on simulated samples of signal and
background events (pseudo-experiments). A subset of networks that predict the smallest
mt uncertainty is “bred”, ie. copied and randomly modified to create a new full set of
11
networks. In this way the “fittest” networks are used to derive the next generation
of networks, and this “breeding of the fittest” process is iterated until the generations
asymptote to an optimal performance. The final event selection on the data sample is
performed using the “best” network from all generations, which is typically from the
final generation.
Our analysis of 2 fb−1 of CDF Run II data, using the above amalgamation of matrixelement and evolutionary neural network techniques, has been published in Physical
Review Letters (CV # 1, TT # 2). The use of the evolutionary neural network
technique for event selection resulted in a 20% improvement in the statistical uncertainty,
over the use of likelihood fitting using matrix elements alone. The result is
mt = 171.2 ± 2.7(stat) ± 2.9(syst) GeV,
again the world’s best measurement of the top quark mass in the dilepton channel.
3.2
Higgs Search in the ZH Associated Production Mode
In addition to precise measurements of the W boson and top quark masses, the
direct search for the Higgs boson is one of the highest priorities of the CDF experiment.
The electroweak measurements, via SM fits, prefer a low value of the SM Higgs mass,
making the search for a low-mass Higgs boson a very interesting topic. In this mass
range (mH < 130 GeV), the principle modes of sensitivity for the Tevatron are the
associated production modes of Higgs boson along with a W or Z boson, where the
Higgs is radiated off the latter. The Higgs then decays predominantly into a pair of b
quarks.
One of the final states of the process Z + H → l¯lbb̄ leads to a very similar final
state as the top mass analysis discussed above, i.e. two charged leptons and two b
quarks. Exploiting this similarity, I am pursuing with my student Ravi Shekhar, post-doc
Bodhitha Jayatilaka and collaborator Daniel Whiteson, a search for the SM Higgs boson
in this mode. This is Ravi Shekhar’s Masters thesis topic. To obtain as high a sensitivity
as possible, we are using the per-event likelihood technique, where the likelihood is
constructed from SM matrix elements for the signal and background processes. We
choose the measurement parameter to be the fractional ZH content of the data, for a
given value of the Higgs boson mass. As with the top mass measurement, we exploit
the full kinematic information in the data events, including all momentum and angular
correlations such as those due to the Higgs being a scalar particle.
The paper describing this analysis is being reviewed for publication in CDF, and will
be submitted for publication in summer 2009.
12
3.3
Searches for New Phenomena
The CDF detector sits at the energy frontier and has sensitivity to many of the new
physics phenomena that potentially underlie the Standard Model. The direct discovery
of new physics through the identification of its signature(s) in particle production and
decay is a primary goal of research in particle physics.
3.3.1
Exotic and Excited Leptons
The Standard Model of particle physics describes the non-gravitational interactions
using the SU(3)C × SU(2)L × U(1)Y gauge group. The particle content of the model is
given by three generations of quarks and leptons, each containing an SU(2)L doublet.
This fermion multiplicity motivates a description in terms of underlying substructure, in
which all quarks and leptons consist of fewer, more elementary particles bound by a new
strong interaction [5]. In this compositeness (CI) model, quark-antiquark annihilations
may result in the production of excited lepton states, such as the excited electron, e∗
and the excited muon µ∗ . The SM gauge group may be embedded in larger gauge groups
such as SO(10) or E(6), motivated by grand unified theories or string theory. These
embeddings also predict additional, exotic fermions such as the e∗ and the µ∗ , which can
be produced via their gauge interactions [5] (GM model). I have published two analyses
using the CDF Run II data, searching for the e∗ and the µ∗ respectively.
The Ph.D. thesis topic of my graduate student Heather Gerberich was the search for
associated ee∗ production followed by the radiative decay e∗ → eγ. This mode yields the
distinctive eeγ final state, which is fully reconstructable with high efficiency and good
mass resolution, and has small backgrounds. The evidence for e∗ production would be
the observation of a narrow resonance in the eγ invariant mass distribution.
Since no signal above background predictions was observed, limits were set on the e∗
production cross section and mass. Figure 3 shows the limits in the parameter space
of f /Λ vs Me∗ for the GM model, and Me∗ /Λ vs Me∗ for the CI model. In the gaugemediated model, we exclude Me∗ < 430 GeV for f /Λ ≈ 0.01 GeV−1 at the 95% confidence level (C.L.), extending the exclusion region well beyond other limits. We have
also presented the first e∗ limits in the CI model as a function of Me∗ and Λ, excluding
100 < Me∗ < 906 GeV for Me∗ = Λ. These search results for excited and exotic electrons were the first at a hadron collider. The results have been published in Physical
Review Letters (CV # 11, TT # 6).
We performed a similar search for excited and exotic muon production, using a sig13
Me* ⁄ Λ
-1
f/ Λ (GeV )
10
-1
1
0.8
95% C.L. Exclusion Region
10
10
10
95% C.L.
Exclusion Region
-2
0.6
CDF
ZEUS
H1
L3
Γe* = 2 Me*
-3
0.4
0.2
0
-4
100
200
300
100 200 300 400 500 600 700 800 900
400
200
Me* (GeV)
400
600
800
Me* (GeV)
Figure 3: The 2-D parameter space regions excluded by this analysis for the GM model
(left) and the CI model (right), along with other limits.
nificantly larger dataset. The search was significantly more sensitive than LEP for high
mass values, and HERA has no sensitivity for µ∗ production, making this search quite
unique. This was the first hadron-collider search in the context of the GM model, and
extended previous mass limits in both the GM and CI models. In the GM model, we
exclude Mµ∗ < 400 GeV/c2 for 10−3 GeV−1 < f /Λ < 10−1 GeV−1 at the 95% C.L., well
beyond previous limits. We have also presented the first µ∗ limits in the CI model as
a function of Mµ∗ and Λ, excluding Mµ∗ < 853 GeV/c2 for Λ = Mµ∗ . These results,
shown in Fig. 4, have been published in Physical Review Letters (CV # 8, TT #
4).
I supervised the senior honors thesis of Edward Daverman, an undergraduate at Duke,
on this topic. I collaborated with Heather Gerberich on this paper (then post-doc at
University of Illinois, Urbana-Champaign).
3.3.2
Doubly-Charged Higgs Bosons
In the standard Model, the Higgs field is postulated to break the SU(2)L × U(1)Y
electroweak gauge symmetry to U(1)EM . The Higgs boson is eagerly sought after but
has not yet been observed. Extensions of the SM predict larger Higgs sectors. For in14
10
10
10
-2
Λ
95% C.L. Exclusion Region
1
Mµ* ⁄
-1
f/ Λ (GeV )
10
(a)
-1
0.8
0.6
0.4
CDF Run II
Γµ* = 2 Mµ*
OPAL
-3
0.2
-4
100
200
300
400
95% C.L.
Exclusion Region
0
2
Mµ* (GeV/c )
200
400
600
(b)
800 2
Mµ* (GeV/c )
Figure 4: The 2-D parameter space regions excluded by this excited/exotic muon analysis
for (a) the GM model, along with other limits, and (b) the CI model.
stance, the introduction of a Higgs triplet containing neutral, singly, and doubly-charged
members is required in the Left-Right Symmetric Model, which is well-motivated by the
recent discovery of neutrino oscillations. The Supersymmetric Left-Right Model [6] further implies a relatively light doubly-charged Higgs boson (H ±± ), motivating its search
at the Tevatron.
The observation of any Higgs particle would be an important step toward understanding the physics of the electroweak scale. The H ±± boson is particularly fascinating
because of its simultanous implications for two theories beyond the Standard Model,
in addition to exposing one of the untested foundations of the Standard Model. Furthermore, the experimental signature is clean with high efficiency and low background,
making the search for the (H ±± ) an ideal hunting ground for new physics.
By charge conservation, the doubly-charged Higgs boson can only decay to like-sign
leptons, W bosons and W ± H ± . For light H ±± bosons, the branching ratio to W ± W ±
and W ± H ± is small. Hence either leptonic decays will prevail or the boson may have a
sufficiently long life-time to be detected as a doubly-charged particle.
The dominant H ±± production mode at a hadron collider is in pairs via Z/γ exchange.
The production cross section is O(0.1 pb) for H ±± mass around 100 GeV. We exploited
the quadrupled ionization of the H ±± in this search, which was the Masters thesis topic
15
of my graduate student Joshua Tuttle. We use an a-priori “blind-to-data” strategy to
define two separate sets of ionization cuts. The loose cuts, to be used exclusively for
setting a mass limit, seek to maximize efficiency. The tight cuts, to be used only to
quote observation of H ±± signal, further suppress the mis-identification backgrounds.
The tight cuts were optimized for single-event sensitivity by using ionization information
from the calorimeters and the drift chamber. Loose cuts use drift chamber ionization
only.
My cosmic-ray finder was particularly useful in obtaining a clean sample of cosmic-rays
for efficiency and background studies. Experimental backgrounds from muons, electrons,
hadronic τ ’s, and jets were estimated using fake rates from either the data or appropriate
simulation samples and were shown to be extremely small, giving this search single-event
sensitivity.
Cross Section (pb)
Upon unblinding the signal data sample, we found no H ±± candidates in either category. Figure 5 shows the theoretical and 95% C.L. cross section limits for our doublycharged Higgs search. We set a lower mass limit of 133 GeV/c2 (109 GeV/c2 ) for the
quasi-stable HL±± (HR±± ) boson, which is much more stringent than the previous best
limit of 97.3 GeV/c2 from the DELPHI collaboration. The paper describing this result
has been published in Physical Review Letters (CV # 10, TT # 5). I collaborated
with my post-doc Christopher Hays on this publication.
H±L±
H±R±
0.2
±±
±±
degenerate HL & HR
experimental limit
(95% C.L.)
0.1
0
90
100
110
120
130
140
150
160
H±± Mass (GeV/c )
2
Figure 5: The theoretical and experimental H ±± cross section limits for the loose ionization cuts.
16
3.3.3
Z ′ Boson, Graviton and Heavy Dimuon Resonance
The Standard Model (SM) is usually viewed as an effective theory, expected to be
modified at higher energies. Larger symmetry groups, eg. those motivated above, may
undergo spontaneous symmetry breaking such that a broken U(1) gauge symmetry may
appear. Associated with it would appear a new, neutral heavy boson, called the Z ′
boson. Like the SM Z boson, the decay Z ′ → l¯l provides an excellent experimental
signature, due to the excellent efficiency and momentum resolution of the leptons.
Additional spatial dimensions are a possible explanation for the gap between the electroweak symmetry-breaking scale and the gravitational energy scale MPlanck [1, 2]. In
the Randall-Sundrum (RS) scenario [1], the space-time metric varies exponentially in a
fourth spatial dimension, corresponding to the ground-state wave function of the graviton which is localized on another brane in this dimension. The wave function overlap
with the SM brane is exponentially suppressed, thus explaining the apparent weakness of
gravity and the large value of MPlanck . This model predicts excited Kaluza Klein modes
of the graviton, which are localized on the SM brane and therefore couple with SM
particles with electroweak strength. Such Randall-Sundrum gravitons G∗ would appear
as spin-2 resonances in the process q q̄ → G∗ → µµ̄, with a narrow intrinsic width when
k/MPlanck < 0.1, where k 2 is the spacetime curvature in the extra dimension. Finally,
spin-0 resonances such as sneutrinos, q q̄ → ν̃ → µµ̄ are predicted by supersymmetric
theories with R-parity violation, in addition to the scalar Higgs bosons in the SM and
its extensions.
I have designed and performed an analysis in the dimuon channel, to search 2.3 fb−1
of CDF Run II data for evidence of the production and decay process B → µµ̄, where B
denotes a boson with spin-0, 1 or 2. I have performed a precise alignment and calibration
of the CDF drift chamber using cosmic rays, for this dataset, in order to achieve the
best momentum resolution possible. This work results in the narrowest possible dimuon
mass peak, giving the best search sensitivity. Through my work on the W boson mass
measurement, I have developed a deep understanding of muon tracking, which is the
key aspect of this search. The cosmic ray tagger I developed has achieved very good
performance, making the cosmic ray background negligible. The improvements I developed in the drift chamber track reconstruction and fitting have allowed us to suppress
“ghost” muons from π, K decays in flight to a very low level. I have also developed the
statistical methods to search the data for all possible mass values of a heavy dimuon
resonance, and to quantify the significance of a potential signal. The binning of the data
was optimized using the momentum-dependent resolution, and full simulated lineshapes
were used to extract the most information from the data.
The search was designed as a “blind” analysis, i.e. the entire procedure was developed
17
without access to the collider data. The resulting publication in Physical Review
Letters (CV # 2, TT # 3) provides some of the world’s most stringent mass limits
on supersymmetric neutrinos, Z ′ bosons and RS Gravitons, as shown in Table 2.
Z′
model
ZI′
′
Zsec
ZN′
Zψ′
Zχ′
Zη′
′
ZSM
Z′
mass limit
789
821
861
878
892
904
1030
RS graviton graviton
k/MPlanck
mass limit
0.01
293
0.015
409
0.025
493
0.035
651
0.05
746
0.07
824
0.1
921
ν̃
λ2 · BR
0.0001
0.0002
0.0005
0.001
0.002
0.005
0.01
ν̃
mass limit
397
441
541
662
731
810
866
Table 2: 95% C.L. lower limits on Z ′ , graviton, and sneutrino masses (in GeV) for
various model parameters [1, 7]. For the R-parity-violating sneutrino model, λ is the
¯ coupling and BR denotes the ν̃, ν̄˜ → µµ̄ branching ratio.
ddν̃
I collaborated with Christopher Hays and Oliver Stelzer-Chilton on this paper.
4
Experimental Project Leadership
I served as the co-leader of the CDF Offline Analysis Project for a period of two
and half years, from July 2004 through December 2006. This is one of the seven most
important positions in the 700-member collaboration.
The Offline Analysis group is responsible for all CDF activity related to reconstruction
and simulation software, offline operations related to software releases, calibrations, processing of data and simulation, and computing resources. The goal is to ensure that the
physics potential of the experiment is realized, by providing core analysis software and
resources to the physicists. My strong background in physics analyses and convenership,
stood me in good stead when leading the Offline Analysis Project.
I was responsible for management and coordination of ongoing operations, and for
strategizing future development of the offline analysis infrastructure. The data recorded
by CDF grew by a factor of four during my term. Further increases by a factor of 4-5
18
were anticipated in future running of the experiment. It was my responsibility to (a)
understand and predict the physics analysis, software and computing needs of the experiment, (b) provide the best possible software for data reconstruction and simulation,
(c) manage the day-to-day operations of the experiment related to detector calibrations,
databases, timely data reconstruction and all the computing, (d) strategize, develop and
deploy new computing technology and infrastructure to meet the ever growing needs,
and (5) develop and defend the computing budget of about $1.5 million per year.
In addition to these duties, I implemented some new initiatives of my own. I present
some highlights and achievements during my term as Offline Project co-leader.
• Single-pass data reconstruction plan: Before I started my term, the prevailing
mode of data processing was such there is a delay of about six months between
recording the raw data and availability of analysis-quality data. This caused a long
delay in the publication of results from the data, hurting the physics productivity of
the experiment. The reason for the delay was the “double-processing”, where data
collected over 6-8 months were processed, then calibrated, and finally reprocessed.
I had the idea that this delay could be significantly reduced by moving to a new
scheme of “single-pass” data processing. I developed the details of the new scheme,
which I presented to the CDF collaboration and built a consensus to implement it.
In this scheme, this latency was reduced to about 6 weeks. The idea was to preprocess small, well-defined calibration datasets, from which detector calibrations
could be speedily extracted and verified. These calibrations would be applied in
the reconstruction of collider data, segmented in small time periods. The whole
process would be repeated for sequential data-taking periods. This scheme increases the time available for analysis and also requires less effort and computing
for processing the data. The scheme was very successful and has been in use ever
since I implemented it. The time to publication for CDF physicists has reduced
dramatically as a result. Furthermore, the process of detector calibration was automated significantly, increasing the reliability and effectiveness of the calibrations
and hence the data quality.
• New reconstruction software package: Many detector upgrades were performed in the time leading up to my term as Offline Project co-leader. These
upgrades required new software to process the data, incorporating the detector
changes. The Offline Project produced a new software package on schedule. The
software also incorporated significant improvements to reconstruction algorithms,
which had a positive impact on the physics capability of the experiment. Finally,
a number of technical improvements in software and computing technology were
also incorporated into the package. This package has been used by the experiment
for the last four years, since it was released under my leadership. Improvements to
drift chamber track reconstruction that resulted in increased acceptance at large
19
rapidity, which were developed by my research group, are described in a publication
in Nuclear Instrumentation and Methods ( CV # 12, TT # 10).
• Upgrade of data processing platform: Motivated by the growing computing
needs for timely data processing, I initiated a project to create a new computing platform for data processing. The goal was to create a scalable and more
maintainable platform which would also require less human effort to operate. We
were successful in creating this platform, which is still in use after four years since
deployment. This infrastructure has frequently set new records in demonstrated
processing power. This project is described in two publications in Nuclear Instrumentation and Methods (CV # 6) and IEEE Transactions on Nuclear Science (CV # 9).
• Distributed computing: The computing needs of a large collider experiment like
CDF are so large and growing so rapidly, that it cannot be satisfied by the resources
at Fermilab. It became vital to use offsite computing facilities, including facilities
in Canada, Europe and Asia, for CDF needs. This created a whole new challenge
with technical, organizational and budgetary challenges. During my term, we dealt
with all three challenges. We developed and deployed a new distributed computing
technology, which makes the remote computing facility appear dynamically (ie at
run-time) as if it is an extension of a local facility. Users are then able to run their
programs on the remote facility in a transparent fashion, as if they are running on
a local facility. This technology was therefore extremely convenient for the CDF
physicists, minimizing their time and effort spent on computing issues and allowing
them to focus on physics. Also, the dynamic nature of this technology, called
“condor glide-in” technology, allows the computing facility to be shared easilly
between users, with efficient resource allocation. As a result, I was able to negotiate
the organizational and budgetary issues between collaborating institutions and
international funding agencies.
Due to CDF’s success in this arena under my leadership, CDF has become a
leading player in the multi-disciplinary “Open Science Grid” project of the US
government, in which distributed computing has been identified as a major tool
for scientific research. Distributed computing technologies such as the “glide-in”
technology and network-accessible virtual file-servers with global namespaces that
I pioneered at CDF have since become integral components of the Open Science
Grid.
I have organized several reviews of the CDF Offline Project, including annual and biannual reviews of progress, plans and budgets by Fermilab and international funding
agencies.
20
5
Grants and Funding
Prior to receiving tenure, I received the Outstanding Junior Investigator (OJI) Award
from the Department of Energy, with an annual grant of $60,000. This OJI award was
independent of the HEP group umbrella grant, and is funded in perpetuity.
I was selected to lead the Software and Computing Project of the CDF experiment
starting June 2004. My deputation to Fermilab for 2 12 years as CDF project leader
generated revenue for Duke University. My academic salary during this period was
provided by Fermilab, releasing equivalent funds for Duke University.
My leadership of this CDF project opened a new area of experimental activity for the
Duke HEP group. I got Douglas Benjamin, a senior scientist supported by the DOE
grant, involved in the CDF Software and Computing Project in 2005. This was a new
area of activity for him, but under my supervision he gained valuable computing skills
and became a valued member of the CDF Software and Computing Project. He has held
numerous positions of responsibility in this project, including head of the data-handling
group and head of the distributed computing group in CDF. As a result, we have been
able to justify and obtain funding to support Douglas Benjamin as a senior scientist.
Secondly, I negotiated partial salary support for Dr. Benjamin from Fermilab, as
compensation for his work on CDF computing. This support from Fermilab released
funds from the DOE grant for use towards other research efforts.
Dr. Benjamin has now transitioned his computing support work from CDF to ATLAS,
which is welcomed by the ATLAS management due to the experience he gained on
CDF with me. The Duke group’s involvement that I initiated, has thus proved quite
fruitful for the long term. It continues to generate new funding opportunities: we now
receive partial salary support for Dr. Benjamin from Brookhaven National Laboratory,
in compensation of his work on ATLAS computing.
I am co-principal investigator of the Duke HEP grant from the Department of Energy.
This umbrella grant funds the research of Duke professors Al Goshaw, Mark Kruse, Seog
Oh, Kate Scholberg, Chris Walter and myself on the CDF and ATLAS experiments at
the energy frontier, and neutrino physics. I will be the principal investigator/program
director of this grant starting June 2009. This grant is renewable every three years.
21
6
Future Research Plans
The CDF experiment will continue to record data for the next two years, bringing a
substantial increase in the precision of key measurements.
In the near term, I have planned an improved measurement of the W boson mass,
increasing the precision by another factor of two beyond my published analysis. In the
context of the Standard Model, the Higgs boson mass will be constrained to 30%, or an
inconsistency with direct search limits will point to new physics.
The full Tevatron dataset offers the opportunity to either exclude or see 3σ evidence
of the SM Higgs boson if its mass is less than 200 GeV, the range preferred by the
precision electroweak fit. Certain extensions of the SM also motivate the mass of the
lightest Higgs boson in this range. I plan to continue the SM Higgs boson search by
building on the tools I have recently developed.
I am developing my research program at the Large Hadron Collider (LHC), the new
accelerator being built in Europe which will produce particle collisions with 7× the
energy of the Tevatron, or 14 TeV. The energy scale of electroweak symmetry breaking
is O(1 TeV), within the reach of the LHC. It is likely that a new and rich dynamics at
this energy will be revealed. Research at the ATLAS experiment is going to be my focus
for the forseeable future.
I plan to continue searches for new phenomena using novel methods to maximize the
discovery potential of the LHC. This research will build on my work on excited/exotic
leptons, extended Higgs sectors and gauge symmetries at the Tevatron, as well as my
experience with advanced analysis methods that I have developed and used for the W
mass and top mass measurements and the SM Higgs search.
The task of reconstructing and analysing the LHC data will be extremely challenging,
due to the large number of detector elements and the high luminosity environment. My
Tevatron experience with software, algorithms and computing will help me make rapid
progress at the LHC, on the ATLAS experiment.
I am developing the tools to pursue two analyses of ATLAS data. A powerful signature
for new physics is the presence of same-sign leptons in the event, for which the SM
backgrounds are small. As charge misidentification is an issue with tracking, I will work
on tracking software at ATLAS in order to gain an in-depth understanding of tracking
performance and momentum/charge measurement at high momentum. Second, the top
quark mass measurement has excellent potential for improvement at ATLAS, given the
large top pair production cross section and luminosity and hence very high statistics.
22
The dilepton channel has the advantage that only the two b quark jets need calibration.
I propose to use the SM process ZZ → l¯lbb̄ for b quark jet calibration: the reconstruction
of the Z → bb̄ decay can provide a precise in − situ calibration, in a very similar final
state as top dilepton decays. Thus, a very precise top quark mass measurement should
be possible.
The LHC will be upgraded for higher luminosity around 2017, and this will require
upgrades to the ATLAS experiment to handle the increased data rate. The replacement being considered for the charged particle tracking detector is a detector comprised
completely of silicon sensors. I have a strong interest in this ATLAS upgrade project,
because the tracking performance is the key to the success of the ATLAS physics program in the 2020’s. I have experience in electronics, and I plan to build the necessary
infrastructure using the clean-room facilities in the Fitzpatrick Center at Duke.
7
Summary
My research focusses on physics beyond the Standard Model through the precision
measurement of the W boson mass and direct searches for new particle production. My
CDF W mass measurement was published as the most precise single measurement. I have
co-authored a comprehensive review article on W mass measurement at the Tevatron. I
have also published the most precise measurements of the top quark mass in the dilepton
channel. My search for associated Z+Higgs production will be submitted for publication
in summer 2009.
My research on new particle production has centered on testing models of compositeness and extended symmetries. I have published the best limits on excited electrons
and excited muon states. I have also published the best limits on the doubly-charged
Higgs boson in the long-lived mode. My search for heavy dimuon resonances has been
published, with some of the most stringent mass limits.
Since receiving tenure, I have six publications in Physical Review Letters, one each
in Physical Review D and Physical Review D Rapid Communications, one in Annual
Reviews of Nuclear and Particle Science, two in Nuclear Instrumentation and Methods,
and one in IEEE Transactions on Nuclear Science.
I have held several high-level leadership positions on the CDF experiment at Fermilab,
including co-leadership of the CDF Software and Computing Project. I have recently
given nine invited plenary talks, and thirteen invited seminars at research universities
and international institutes and laboratories. I am currently chairing the Fermilab Users
23
Organization.
I have supervised four post-doctoral research associates, six graduate students and
three undergraduate students. I have supervised two Ph.D. theses, two Masters theses
and one undergraduate Honors thesis.
My work on the W boson mass measurements was cited for my election as Fellow of
the American Physical Society.
References
[1] L. Randall and R. Sundrum, Phys. Rev. Lett. 83, 4690 (1999); L. Randall and R.
Sundrum, Phys. Rev. Lett. 83, 3370 (1999).
[2] N. Arkani-Hamed, S. Dimopoulos, and G. Dvali, Phys. Lett. B 429, 263 (1998).
[3] P. Chankowski et al., Nucl. Phys. B417, 101 (1994); D. Garcia and J. Sola, Mod.
Phys. Lett. A 9, 211 (1994); A. Dabelstein, W. Hollik and W. Mosle, in Perspectives for Electroweak Interactions in e+ e− Collisions, ed. by B. A. Kniehl (World
Scientific, Singapore, 1995) p. 345; D. Pierce et al., Nucl. Phys. B491, 3 (1997).
[4] G. Degrassi et al. Phys. Lett. B 418, 209 (1998); G. Degrassi, P. Gambino, and
A. Sirlin, Phys. Lett. B 394, 188 (1997).
[5] U. Baur, M. Spira and P. M. Zerwas, Phys. Rev. D 42, 815 (1990), and references
therein; E. Boos et al., Phys. Rev. D 66, 013011 (2002), and references therein.
[6] R. N. Mohapatra, Unification and Supersymmetry (Springer, New York, 1992), and
references therein.
[7] D. Choudhury, S. Majhi, and V. Ravindran, Nucl. Phys. B 660, 343 (2003). We
assume the ν̃ and ν̄˜ have equal masses and couplings and contribute equally to a
potential signal, as in W. Shao-Ming, H. Liang, M. Wen-Gan, Z. Ren-You and J. Yi,
Phys. Rev. D 74, 057902 (2006).
24