Boosted searches for WW/WZ resonances in the
ℓνJ final state
Columbia University REU 2016
Billie Lubis
Department of Physics and Astronomy, University of Kansas
3 August 2016
Abstract
This project examines the selection and optimization criteria for the
backgrounds for VV resonance decays from WW/WZ to a final state of a
lepton, neutrino, and two quarks. We used 13.2 f b−1 data at 13TeV center
of mass energy, and compared it with simulation backgrounds composed of
W+ jets, Z+ jets, tt̄, and standard model dibosons. Our results describe
the data well but we observed an overall normalization problem between
the data and Monte Carlo.
1
Contents
1 Introduction
1.1 CERN . . . . . . . . . . . . . . . .
1.2 LHC . . . . . . . . . . . . . . . . .
1.3 ATLAS . . . . . . . . . . . . . . .
1.4 Particles and the Standard Model .
1.4.1 Fermions . . . . . . . . . .
1.4.2 Bosons . . . . . . . . . . . .
1.5 Particle Identification . . . . . . .
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2 VV Resonances
2.1 Decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2.2 Objects Selection . . . . . . . . . . . . . . . . . . . . . . . . . . .
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3 Analysis
3.1 Description of Background and Data .
3.2 Description of Signal/Control Regions
3.3 Signal Region Results . . . . . . . . .
3.3.1 High Purity Signal Region . . .
3.3.2 Low Purity Signal Region . . .
3.4 W+ Jets Control Region Results . . .
3.4.1 High Purity WCR . . . . . . .
3.4.2 Low Purity WCR . . . . . . . .
3.5 Top Control Region Results . . . . . .
3.5.1 High Purity TCR . . . . . . . .
3.5.2 Low Purity TCR . . . . . . . .
3.6 Data/MC Comparison Summary . . .
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4 Conclusions
38
5 Acknowledgements
38
2
Figure 1: Map of the CERN accelerator complex in Geneva [1].
1
1.1
Introduction
CERN
The European Organization for Nuclear Research (CERN) was founded in 1954
by twelve western European countries. Their aim was to conduct innovative particle physics research in a state-of-the-art facility, and also to facilitate scientific
collaboration between member countries. These twelve founding countries have
since been joined by other European nations to bring the total number of CERN
member countries to twenty-two. Additionally, CERN has two associate member countries, as well as several countries applying for membership and several
observers to council. The several thousand scientists at CERN are responsible for some of the most important scientific accomplishments over the past 60
years, including the development of the World Wide Web, the creation of the
first antihydrogen atoms, and most recently, the discovery of the Higgs Boson
[13]. Currently, CERN has many active accelerator experiments, as shown in
Figure 1.
1.2
LHC
The Large Hadron Collider (LHC) was built by CERN from 1998 to 2008, and
is the largest and most powerful particle collider in the world. It is located in
Geneva, Switzerland. The LHC is housed 175 meters underground in a tunnel
with a circumference of 27 kilometers. The LHC collides proton beams at an
energy of up to 14 TeV at several points along the circumference of the tunnel,
where detectors collect data from the collisions. There are several detectors at
the LHC, but the most notable of these are ATLAS, CMS, ALICE, and LHCb
[14].
3
Figure 2: Map of LHC ring with location of main experiments shown [2].
The two general purpose detectors, ATLAS and CMS, investigate several
different physics topics, including the search for the Higgs boson, dark matter,
and extra dimensions. The ALICE experiment studies heavy ion collisions in
the hope to produce quark-gluon plasma, which is believed to have existed
right after the Big Bang. The LHCb is attempting to investigate antimatter
through studying the interactions of b-hadrons. There are also several smaller
experiments at the LHC. The first run of the LHC began in 2009, and was
followed by a long shutdown period in 2013 to prepare the collider for higher
luminosity and energy. The second run of the LHC began in 2015. The LHC is
presently running and taking new data.
1.3
ATLAS
The ATLAS (“A Toroidal LHC ApparatuS”) detector is a general purpose detector used to study collisions of high-energy protons. It is one of several international research collaborations at the LHC. The ATLAS collaboration itself is
comprised of more than 3000 physicists from 38 countries [15].
The detector has several components able to detect particles from these collisions. The innermost detector is comprised of a pixel detector, strip detector,
and tracker. A magnetic field surrounds the inner detector, so it can be used
to track charged particles resulting from the collision. From a particle’s track,
we can determine information about both its charge and momentum. Outside
the inner detector are the electromagnetic calorimeter, which absorbs energy
from charged particles (such as electrons) and photons that interact via electromagnetism, and the hadronic calorimeter, which absorbs energy from hadrons
and jets that interact via the strong force. The outermost detecting component
of ATLAS is the muon detector. The muon detector measures the charge and
momentum of muons by tracking their path in a uniform magnetic field. A cross
4
Figure 3: A cross section of the ATLAS detector showing the inner components
[3].
section of the ATLAS detector can be seen in Figure 3.
Due to the high number of collisions happening per second in the detector, a
large data collection system is needed to process the incoming data. The trigger
system is used to select events to record. A data acquisition (DAQ) system is
used to measure the data from the detector and transfer it to the computing
system, where it can then be analyzed. The computing system analyzes the
nearly one billion events recorded by the detector per year during a data run.
1.4
Particles and the Standard Model
The Standard Model (SM) of particle physics is a theory that explains how
particles interact via the electromagnetic, weak, and strong nuclear forces. The
Standard Model divides elementary particles into two classes: fermions and
bosons.
1.4.1
Fermions
Fermions are particles with 1/2 integer spin, including protons, neutrons, and
electrons. The Standard Model includes 12 fermions, which are classified by
how they interact. Leptons are the lightest particles in the SM, and include the
electron, muon, and tau particles, and the electron, muon, and tau neutrinos.
The electron, muon, and tau particles have charge -1 and interact electromagnetically, while the neutrinos have charge 0, and interact via the weak force.
The neutrinos are the only particles in the SM that ATLAS does not detect.
However, when reconstructing an event, we can generally assume that the miss5
Figure 4: Leptons in the Standard Model [4].
ing energy is due to neutrinos. A table of SM leptons and their corresponding
masses and charges are shown in Figure 4.
Quarks are fermions that carry a color charge and interact via the strong
force. Quarks are very strongly bound to one another, and combine to form
neutral particles called hadrons. Hadrons can be made up of either three quarks
(baryons) or a quark-antiquark pair (mesons). Examples of baryons include
protons and neutrons, while examples of mesons include kaons and pions. There
are six quarks: up, down, top, bottom, charm, and strange. The charge and
mass of each quark is shown in Figure 5.
1.4.2
Bosons
The last group of particles in the SM are bosons, which are the force carriers of
each of the different types of interactions. Photons mediate the electromagnetic
interaction, while gluons mediate the strong interaction, and the W and Z bosons
mediate the weak interaction. The Higgs boson is also included in the Standard
Model, and in theory, the existence of the Higgs boson explains why the other
elementary particles have certain masses. Of particular interest to us is the decay
of W and Z bosons either hadronically or leptonically, which will be explained
in the next section.
1.5
Particle Identification
The ATLAS detector was built to identify elementary particles, and the structure of the detector is designed in such a way that we do not misidentify particles.
We can determine identity of a particle based on its charge and energy. The
first detector that a particle comes in contact with after a collision is the inner
detector. The inner detector is surrounded by a uniform magnetic field, so if
6
Figure 5: Quarks in the Standard Model [4].
we measure a particle with a curved track, we can conclude that the particle
is charged. Similarly, if we have no track in the inner detector, but we pick up
a signal in the calorimeter, we can conclude that we have a neutral particle.
The energy of a particle determines how far into the ATLAS detector it travels
before decaying or being absorbed. Relatively low energy particles such as photons or electrons interact in the electromagnetic calorimeter. Again, we don’t
confuse the two because one has no track in the inner detector and the other
has a curved track. Higher energy hadrons like protons and neutrons make it to
the hadronic calorimeter before decaying. Muons have a sufficiently high energy
to travel through both calorimeters, so we can then follow their path through
the muon detector. As mentioned earlier, the only particles we don’t detect in
ATLAS are neutrinos. A representation of the tracks certain particles would
make when traveling through the ATLAS detector is shown in Figure 6.
2
2.1
VV Resonances
Decay
Our search is for VV resonances in semileptonic decay modes, specifically V V →
ℓνqq, with exactly one lepton in the final state. We are looking at events
with either WW or WZ resonances, where the W boson decays leptonically to
a lepton-neutrino pair, and the W/Z boson decays hadronically to a quarkantiquark pair. This is shown in Figure 7.
From each decay, we can reconstruct the products, and calculate the VV
mass of the event. We would expect to see this signal mass as a peak above the
Standard Model background. We reconstruct the hadronic decay in a large-R
jet by using an anti-kt algorithm [7]. For the leptonic decay, we reconstruct
7
Figure 6: Particles interacting with layers of ATLAS detector [5].
Figure 7: VV resonance and WW/WZ decay [5]
8
Figure 8: Loose vs. tight electron selection cuts
Figure 9: Loose vs. tight muon selection cuts
the W boson by adding the lepton and neutrino vectors [6]. The signals we use
in our search are either heavy vector triplets (HVT) or gravitons. We use the
HVT parameterization, which predicts W’ → WZ and Z’ → WW processes, for
spin-1 resonances. The bulk Randall-Sundrum graviton (RS G*) model features
a spin-2 graviton decaying to a WW or ZZ boson pair. The RS model predicts
Kaluza-Klein (KK) gravitons with a warped extra dimension [9].
2.2
Objects Selection
We have several requirements regarding the objects selection for each event.
For the leptonic decay, we require a final state with exactly one lepton. We
classify each lepton as loose or tight based on preselection cuts. The cuts for
both electrons and muons are shown in Figures 8 and 9, respectively. This
categorization is based on different signal efficiencies. We veto events with more
than one loose lepton while keeping signal events with one tight lepton.
Additionally, we have applied a missing ET trigger to the muon selection
and an electron trigger to the electron selection. A table of these triggers is
shown in Figure 10.
We calculate the missing transverse energy (MET) using reconstructed electrons, muons, and jets.
The small-R jets have several mass and transverse momentum requirements.
9
Figure 10: Electron and muon triggers for 2015 and 2016 data
Figure 11: Small-R jet cuts
Specifically, we keep the jet if PT > 20 GeV, -2.5 <η< 2.5, ∆R > 1 (between
the small and large R jets), and either η> 2.4 or PT > 60 GeV or JVT (Jet
Vertex Tagging) > 0.59. We use JVT in order to suppress pile-up. We classify
a small-R jet as a b-jet if the b-tagging variable MV2c10> 0.1758. The cuts for
the small-R jets are summarized in Figure 11.
For the large-R jets, we require a ∆R > 1 between the jet and lepton, |η|
< 2.0, mJ > 50 GeV, and PT > 200 GeV. We also require that the transverse
momentum PT > 0.4 for both the W boson and the large-R jet in the event. We
have removed any overlap between the leptons and the large-R jets, meaning
no events have muons or electrons inside the large-R jet. We have two main
cuts to identify jets originating from hadronic W/Z decays, the mass cut and
the substructure cut. Additionally, we use these cuts to distinguish different
regions within our event. An event that passes the mass cut will have a large-R
jet mass from approximately 65 to 120 GeV. The substructure cut is used to
differentiate W and Z jets from quantum chromodynamics (QCD) jets. The
substructure variable is defined as
(β)
D2
(β)
=
e3
(β)
(e2 )3
(β)
, and e2
=
ECF 2 (β)
ECF 3 (β)
(β)
, e3 =
.
2
ECF 1 (β)
ECF 1 (β)3
where ECF 1 (β), ECF 2 (β) and ECF 3 (β) are 1-point, 2-point, and 3-point
energy function correlation functions of the jet, given by
10
ECF 1 (β)
=
X
pT i ,
i∈J
ECF 2 (β)
=
X
pT i pT j (∆Rij )β ,
i<j∈J
ECF 3 (β)
=
X
pT i pT j pT k (∆Rij ∆Rik ∆Rjk )β ,
i<j<k∈J
However, the substructure cut has a fairly low efficiency (50%). Because of
this low efficiency, we have introduced a low purity region in order to recover
efficiency loss, especially in the high mass region where the SM background is
reduced.
3
3.1
Analysis
Description of Background and Data
Our background is composed primarily of W+ jets and tt̄, with small contributions from Z+ jets and SM dibosons. The background contribution from QCD
is neglible. We have generators for each background type. The generators are
listed as follows: for V+ jets, Sherpa v22 &Nj ets correction applied [10]; for tt̄,
Powheg-Pythia [11]; for SM dibosons: Sherpa [10]. The signal generators are,
for both HVT and gravitons, MadGraph+Pythia8, with a mass width of 6%
[8,9, 12].
We used data from both 2015 and 2016, with a luminosity of 13.2 f b−1 and
a center-of-mass energy of 13 TeV.
The signal normalization is such that it appears on each plot, and the MC
backgrounds are each scaled to their cross sections.
3.2
Description of Signal/Control Regions
We have several regions with different b-jet, mass cut, and substructure cut
requirements so we can better study the various backgrounds that go along
with our signal. A summary and illustration of cuts for these different regions
are shown in Figures 12 and 13.
In the signal region, we are looking at events with one lepton, and we require
PT > 200 GeV and PT /MV V > 0.4 for both the leptonically decaying W boson
and the hadronically decaying W/Z boson, and MET > 100 GeV. We also veto
any events that have b-jets outside the large-R jet. We apply this b-veto in order
to suppress tt̄ background, illustrated in Figure 14. The high purity regions are
made up of events that pass the substructure cut while the low purity regions
are made up of events that fail the substructure cut.
For W+ jets control region (WCR), we require the event to have no b-jets
and to fail the mass cut.The W+ jets background behaves in the same manner
11
Figure 12: Summary of cuts for WCR, TCR, SR
Figure 13: Illustration of WCR, TCR, SR
Figure 14: ttbar topology
12
inside and outside the mass window, so we study the W background outside so
it doesn’t overlap with the signal region. We have both high purity and low
purity WCRs.
For the top control region, we require the event to have at least one b-jet and
to pass the mass cut. We have an inverted b-jet cut in the tt̄ regions because we
expect to find a b-jet outside the large-R jet. The TCR region has high purity
and low purity regions that follow the same substructure cut guidelines as the
SRs.
3.3
3.3.1
Signal Region Results
High Purity Signal Region
Plots from the high purity signal region are shown in Figures 15-18. The transverse momentum, η, and φ of the lepton vector are shown in Figure 15. The
MET plot and PT /MV V for both the W boson and large-R jet are shown in
Figure 16. The plots for η, φ, PT , mass, D2, and number of jets, for the large-R
jets, are shown in Figure 17. The reconstructed VV mass, mℓνJ , is shown in
Figure 18. We use the ℓνJ mass as our final discriminant to say whether or not
we have a signal.
13
(a)
(b)
(c)
Figure 15: Lepton η, φ, and pT for high purity SR
14
(a)
(b)
(c)
Figure 16: MET, ratio of Jet and ℓν pT to VV mass for high purity SR
15
(a)
(b)
(c)
(d)
(e)
(f)
16
Figure 17: η, φ, pT , mass, D2, number of jets for high purity SR
Figure 18: ℓνJ mass for high purity SR
3.3.2
Low Purity Signal Region
Plots from the low purity signal region are shown in Figures 19-22.
17
(a)
(b)
(c)
Figure 19: Lepton η, φ, and pT for low purity SR
18
(a)
(b)
(c)
Figure 20: MET, ratio of Jet and ℓν pT to VV mass for low purity SR
19
(a)
(b)
(c)
(d)
(e)
(f)
20
Figure 21: η, φ, pT , mass, D2, number of jets for low purity SR
Figure 22: ℓνJ mass for low purity SR
3.4
3.4.1
W+ Jets Control Region Results
High Purity WCR
The plots for the high purity W+ jets control region are shown in Figures 23-26.
21
(a)
(b)
(c)
Figure 23: Lepton η, φ, and pT for high purity WCR
22
(a)
(b)
(c)
Figure 24: MET, ratio of Jet and ℓν pT to VV mass for high purity WCR
23
(a)
(b)
(c)
(d)
(e)
(f)
24
Figure 25: η, φ, pT , mass, D2, number of jets for high purity WCR
Figure 26: ℓνJ mass for high purity WCR
3.4.2
Low Purity WCR
The plots for the low purity W+ jets control region are shown in Figures 27-30.
25
(a)
(b)
(c)
Figure 27: Lepton η, φ, and pT for low purity WCR
26
(a)
(b)
(c)
Figure 28: MET, ratio of Jet and ℓν pT to VV mass for low purity WCR
27
(a)
(b)
(c)
(d)
(e)
(f)
28
Figure 29: η, φ, pT , mass, D2, number of jets for low purity WCR
Figure 30: ℓνJ mass for low purity WCR
3.5
3.5.1
Top Control Region Results
High Purity TCR
The high purity top control region plots are shown in Figures 31-34.
29
(a)
(b)
(c)
Figure 31: Lepton η, φ, and pT for high purity TCR
30
(a)
(b)
(c)
Figure 32: MET, ratio of Jet and ℓν pT to VV mass for high purity TCR
31
(a)
(b)
(c)
(d)
(e)
(f)
32
Figure 33: η, φ, pT , mass, D2, number of jets for high purity TCR
Figure 34: ℓνJ mass for high purity TCR
3.5.2
Low Purity TCR
The low purity top control region plots are shown in Figures 35-38.
33
(a)
(b)
(c)
Figure 35: Lepton η, φ, and pT for low purity TCR
34
(a)
(b)
(c)
Figure 36: MET, ratio of Jet and ℓν pT to VV mass for low purity TCR
35
(a)
(b)
(c)
(d)
(e)
(f)
36
Figure 37: η, φ, pT , mass, D2, number of jets for low purity TCR
Figure 38: ℓνJ mass for low purity TCR
3.6
Data/MC Comparison Summary
The data and background yields for the high purity regions are shown in Figure
39, and the yields for the low purity regions are shown in Figure 40.
Figure 39: Yields for high purity WCR, TCR, SR
Figure 40: Yields for low purity WCR, TCR, SR
37
4
Conclusions
We were able to develop techniques in order to study specific backgrounds for
the WW/WZ → ℓνJ final state. Particulary, we reverted the signal region cuts
in order to study the W+ jets and tt̄ background. We added a low purity region
of events that failed the substructure cut for both control and signal regions in
order to increase sensitivity, especially in the high mass region. In examining
the plots for each control and signal region, we noticed an overall normalization
problem between the data and the MC background. However, the background
fits the shape of the data fairly well.
5
Acknowledgements
I would like to thank Dr. John Parsons for this REU opportunity, and Dr.
Kalliopi Iordanidou for her help and mentorship throughout this experience. I
would also like to acknowledge the Columbia ATLAS group for their support.
This REU was made possible by Columbia University with funding from the
National Science Foundation.
38
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39