CMS Physics Analysis Summary

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Available on the CERN CDS information server CMS PAS EXO-16-015 CMS Physics Analysis Summary Contact: [email protected] 2016/08/04 Search for excited quarks in the γ + jet final state in proton proton collisions at s = 13 TeV The CMS Collaboration Abstract The study presents a search for excited quarks (q ? ) decaying into a γ + jet final state at s = 13TeV with the CMS experiment, using the dataset corresponding to an integrated luminosity of 2.7 fb -1 collected during 2015 data taking at the LHC. High transverse momentum photons and jets are selected to search for a resonance peak in the continuous invariant mass distribution of γ + jet. The 95% confidence level upper limits on cross section times branching ratio are evaluated as a function of excited quark mass ( M q ? ). We exclude at 95% CL excited quarks with mass 1.0 < M q ? < 4.37 TeV and coupling strength f = 1.0, and present exclusions of excited quark mass as a function of coupling strength.

Transcript of CMS Physics Analysis Summary

Page 1: CMS Physics Analysis Summary

Available on the CERN CDS information server CMS PAS EXO-16-015

CMS Physics Analysis Summary

Contact: [email protected] 2016/08/04

Search for excited quarks in the γ + jet final state in protonproton collisions at

√s = 13 TeV

The CMS Collaboration

Abstract

The study presents a search for excited quarks (q?) decaying into a γ + jet final stateat√

s = 13 TeV with the CMS experiment, using the dataset corresponding to anintegrated luminosity of 2.7 fb−1 collected during 2015 data taking at the LHC. Hightransverse momentum photons and jets are selected to search for a resonance peak inthe continuous invariant mass distribution of γ + jet. The 95% confidence level upperlimits on cross section times branching ratio are evaluated as a function of excitedquark mass (Mq?). We exclude at 95% CL excited quarks with mass 1.0 < Mq? <4.37 TeV and coupling strength f = 1.0, and present exclusions of excited quark massas a function of coupling strength.

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1 IntroductionThe standard model (SM) of strong and electroweak interactions has been tested for the lastfew decades at many experiments with unprecedented accuracy. Although the SM has been re-markably successful in describing experimental observations so far, there are still fundamentalquestions which it leaves unanswered, and thus the SM may well be an effective Lagrangian toa more fundamental theory. The increase in colliding energy at the LHC and improvement indetector technology make it possible to search for extensions of the SM. Many such extensionsof the SM predict quark substructure or composite quarks [1–3].

A compelling signature for substructure of quarks would be the discovery of an excited stateof a quark (q?). Excited quarks may couple to the ground state quark via gauge transitionsdescribed by the Lagrangian [1],

Lint =1

2 Λq̄∗R σµν

[gs fs

λa

2Ga

µν + g fτ

2Wµν + g′ f ′

Y2

Bµν

]qL + h.c., (1)

where Gaµν, Wµν and Bµν are the field-strength tensors of the SU(3), SU(2) and U(1) gauge

fields; λa, τ, Y are the corresponding gauge structure constants and gs, g, g′ are the gaugecoupling constants; Λ denotes the typical scale of these interactions, and fs, f , f ′ are unknowndimensionless constants determined by the compositeness dynamics, representing the cou-pling strength and naively assumed to be of order unity. In proton-proton collisions excitedquarks would be produced predominantly by quark-gluon annihilation (qg), and would thendecay into a quark and a gauge boson (g, W, Z, γ). Many searches for their existence have beenperformed in different decay channels previously [4–9], and no evidence of their existence hasbeen found. This analysis searches for the single process qg → q? → qγ shown in Fig. 1. Thesignal model considered includes the dominant production modes of excited quarks (u? andd?) with spin- 1

2 [2, 3]. The compositeness scale and mass of the excited quarks are set to beequal in the signal model, i.e., Λ = Mq? .

g

q

q*

γ

q

Figure 1: Signal for production of an excited quark in the s−channel and subsequent decay intoa quark and a photon.

This paper presents the first results of a search for resonances in the γ+ jet final state at√

s = 13TeV at CMS. The dataset used in this study corresponds to an integrated luminosity of 2.7 fb−1

collected by the CMS experiment during the pp collision run at the LHC in 2015.

A final state with a photon and a jet are produced in the SM mainly by qg→ qγ, qq→ gγ, QCDmulti-jet, and W/Z +γ. Among these the main irreducible backgrounds are the quark-gluonCompton scattering (qg → qγ) and quark-antiquark annihilation (qq → gγ). Although theQCD multi-jet production is two to three orders of magnitude higher, the probability for a jet tofake and be reconstructed as a photon is about 10−3, making it the second largest background.

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2 3 Event Reconstruction and Selection

Electroweak production (W/Z+ γ) where W/Z decays to jets, contributes only a small fractionbecause of low production cross section.

2 The CMS DetectorThe central feature of the Compact Muon Solenoid (CMS) apparatus is a superconductingsolenoid of 6 m internal diameter, providing a magnetic field of 3.8 T. Within the supercon-ducting solenoid volume are a silicon pixel and strip tracker, a lead tungstate crystal electro-magnetic calorimeter (ECAL), and a brass and scintillator hadron calorimeter (HCAL), eachcomposed of a barrel and two endcap sections. Forward calorimeters extend the pseudora-pidity [10] coverage provided by the barrel and endcap detectors. Muons are measured ingas-ionization detectors embedded in the steel flux-return yoke outside the solenoid. In theregion |η| < 1.74, the HCAL cells have widths of 0.087 in pseudorapidity and 0.087 in azimuth(φ). In the η-φ plane, and for |η| < 1.48, the HCAL cells map on to 5× 5 ECAL crystals arraysto form calorimeter towers projecting radially outwards from close to the nominal interactionpoint. At larger values of |η|, the size of the towers increases and the matching ECAL arrayscontain fewer crystals. Within each tower, the energy deposits in ECAL and HCAL cells aresummed to define the calorimeter tower energies, subsequently used to provide the energiesand directions of hadronic jets. A more detailed description of the CMS detector, together witha definition of the coordinate system used and the relevant kinematic variables, can be foundin Ref. [10].

The CMS experiment keeps interesting events using a two-tier trigger system, the Level-1 (L1)and High Level Trigger (HLT). The L1 trigger selects events of interest using information fromthe calorimeters and the muon system only, and reduces the readout rate from ∼ 40MHz ofbunch crossing frequency to below 100 kHz. The HLT is a software-based trigger system andfurther decreases the event rate to an average of a few 100 Hz. Events are kept if they passboth L1 and HLT level triggers. The data sample used in this analysis consists of those eventsthat either pass a trigger with an Eγ

T threshold of 165 GeV and an additional photon isolationrequirement, or pass a trigger with Eγ

T threshold of 175 GeV and no additional isolation require-ment. The combined trigger efficiency is 100% for pγ

T > 190 GeV.

3 Event Reconstruction and SelectionEvent reconstruction at the CMS experiment is performed using a particle-flow (PF) algo-rithm [11, 12], which reconstructs and identifies individual particles using an optimized combi-nation of all sub-detector information. Photon candidates are identified as energy depositionsin the ECAL called superclusters. The energy of charged hadrons is estimated using trackmomentum and the corresponding ECAL and HCAL energy depositions. These energies arecalibrated for the nonlinear response of the calorimeters and zero suppression. The energy ofneutral hadrons is obtained from the corresponding calibrated ECAL and HCAL energy.

The jets in each event are formed from PF based reconstructed particles with the infrared- andcollinear-safe anti-kT algorithm [13], operated with distance parameter R = 0.4. Jet energiesare corrected to establish a uniform calorimetric response in η and an absolute response inpT at the particle level. Jet energy scale (JES) corrections are derived from Monte Carlo (MC)simulations, and a residual correction is derived from the data by measuring the pT balance inγ + jet events and other processes [14].

In offline selection each event is required to have at least one good primary vertex reconstructed

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within |z| < 24 cm from the center of the detector and with transverse distance less than 2 cmfrom the z axis.

In each event photons with pγT > 190 GeV and |ηγ| < 1.4442 (i.e., barrel region), are selected

in the list of the possible photon candidates. The photon with highest transverse momen-tum (leading) amongst the ones which satisfy the photon identification and isolation require-ments [15], is used as the final photon candidate in the event. To select this photon candidatethe following PF based isolation and identification requirements are used:

• the energy deposited in the HCAL tower closest to the supercluster position insidea cone of ∆R = 0.15 centered on the photon direction must be less than 5% of theenergy deposited in ECAL for that supercluster, where ∆R ≡

√∆φ2 + ∆η2.

• the scalar sum ET of photons within a cone of ∆R = 0.3 around the superclustermust be less than 0.81 GeV + 0.0053pγ

T.

• the sum pT of charged hadrons associated to the primary vertex within a cone of∆R = 0.3 around the supercluster must be less than 3.32 GeV.

• the sum ET of all neutral hadrons within a cone of ∆R = 0.3 around the superclustermust be less than 1.92 GeV + 0.014pγ

T + 0.000019(pγT)

2.

• the photon candidate should not to be matched with any of the charged tracks iden-tified by the Gaussian Sum Filter algorithm [16] that has no missing hits on the in-nermost tracker layers.

• the photon candidates are required to have an ECAL shower energy profile consis-tent with that of a photon.

The isolation quantities are corrected for effects from overlapping proton-proton interactions(pileup) in the same bunch crossing by subtracting energy calculated from the mean energydensity in the event, as computed using the FastJet package [17]. All these photon identifica-tion selection criteria are measured to provide ∼ 82% signal efficiency. A selection is appliedon the shower shape variables to remove photon candidates from anomalous calorimeter sig-nals [18].

The jets separated from the selected photon candidate by ∆R > 0.5 and satisfying the tightjet identification criteria [19] are selected as the possible jet candidates. The jet identificationcriteria have requirements on the number of constituents, and on the fraction of the jet energyheld by each constituent type. The jet is required to be within the pseudorapidity region |ηjet| <2.4 and must have a transverse momentum pjet

T > 190 GeV. If more than one photon or jetcandidates exist in the event, the γ + jet invariant mass is calculated using the leading photoncandidate and leading jet candidate.

The production of excited quarks considered in this study is an s-channel production whichgives an isotropic distribution of final photon and jet. This feature of signal provides a handleto reduce the backgrounds as all backgrounds are predominantly t-channel processes and havean angular distribution peaking in the forward direction. Therefore the |∆η(γ, jet)| < 1.8 selec-tion is imposed between the selected photon and the jet candidate to reduce the backgroundwhile keeping large acceptance for the signal. We keep only events with the photon and jetback-to-back in φ, |∆φ(γ, jet)| > 1.5, because this is the expected event topology of both thesignal and background at lowest order. These selections are optimized to get the best signalsignificance. The invariant mass of the γ + jet system is required to be Mγ,jet > 695 GeV tohave full kinematic acceptance for events with the photon and jet pT, η and |∆η| requirements.The cumulative efficiency for signal events after various selection criteria are given in Table 1.

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4 4 Resonance Shape and Background Fit

Selection 2 TeV 4 TeV(%) (%)

Photon ID 81.5 82.0pγ

T > 190 GeV,|ηγ| < 1.4442 73.5 75.4Jet ID 73.4 75.4

pjetT > 190 GeV, |ηjet| < 2.4 71.2 74.9

Mγ,jet > 695 GeV 71.0 74.8|∆φ(γ, jet)| > 1.5 70.8 74.8|∆η(γ, jet)| < 1.8 58.3 60.1

Table 1: The cumulative efficiency for signal event selection for q? resonance mass of 2 TeV and4 TeV and coupling multiplier f = 1.0.

The signal efficiency goes from about 49% at Mq? = 1 TeV to 58% at Mq? = 2 TeV and 60% atMq? = 5 TeV

4 Resonance Shape and Background FitThe invariant mass distribution of the γ + jet events in the collected data and MC simula-tion after applying full selection is shown in Fig. 2. The γ + jet and W/Z + γ backgroundMC predictions are generated using MADGRAPH aMC@NLO program [20] at leading order(LO), with the showering and hadronization carried out by the PYTHIA8 program [21]. TheQCD dijet MC predictions are generated using PYTHIA8 event generator. Generated eventsare processed through a full CMS detector simulation based on GEANT4 [22]. The simulationuses CUETP8M1 underlying event tune [23, 24] and a renormalization and factorization scaleµ = pT of the hard-scattered partons and NNPDF2.3LO parton distribution functions [25]. Thesame event reconstruction is employed both in data and MC simulations. The SM γ + jet pre-dictions from MADGRAPH were corrected in shape using the simulation of the next-to-leadingorder SM γ + jet from JETPHOX program [26, 27]. The JETPHOX version 1.3.2 is interfaced toLHAPDF 5.8.7 in order to select the CT10 parton distribution function for protons. The BFG-IIset of parton-to-parton fragmentation functions are used, and default values of the renormal-ization (µR), initial-state factorization and final-state factorization scales are all set to the pT ofthe photon. The invariant mass distribution of the γ + jet events at the generator level fromMADGRAPH event generator is compared to the invariant mass spectrum from the JETPHOX

program. The reconstructed simulation of γ + jet mass spectrum is then reweighted by massdependent scale factors, varying between 1.1 at Mγ+jet = 700 GeV and 0.8 for Mγ+jet = 1.1 TeV,derived from comparison of generator level MADGRAPH spectrum and the one obtained usingJETPHOX program.

After applying all the selection criteria, the background at these values of γ + jet mass is es-timated via MC simulation to originate predominantly from direct photon production (70%),a small contribution of dijet production where one jet fakes a photon (29%), and a negligiblecontribution of W/Z + γ production where the W/Z decays hadronically and is reconstructedas a single jet (1%).

The mass distribution shown in Fig. 2 is binned with bin-width approximately equal to theexpected γ + jet mass resolution, which varies from about 4.5% at a mass of 1 TeV to 3.6% at4 TeV. The highest mass event observed in data is at 3.29 TeV. A 3-dimensional view of thisevent is shown in Fig. 3.

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1000 1500 2000 2500 3000

Eve

nts

per

bin

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10

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310

q* (1.0 TeV)

q* (2.0 TeV)

q* (3.0 TeV)

Data

Simulated background

Background-only fit to data

(13 TeV)-12.7 fb

CMSPreliminary

γ q→q*

[GeV] jetγM1000 1500 2000 2500 3000

Dat

(Dat

a-F

it)

-2

0

2

4

Figure 2: The γ+ jet invariant mass distribution in data (points) and MC (histogram) predictionafter full selection. The result of the fit to the data using the parameterization of Eq. (2) isshown with the red dotted curve. The bin-by-bin pull, (data-fit)/σdata, is shown at the bottom.Simulations of excited quark signals (q?) for coupling multiplier f = 1.0 are shown at massesof 1, 2 and 3 TeV (dashed curves).

The expected signal from excited quarks produced via qg fusion is simulated with PYTHIA8event generator. The reconstructed γ + jet mass distributions for q? model using PYTHIA8,Tune CUETP8M1, and the CMS detector simulation are shown in Fig. 4. The natural width ofthe resonance peak, at parton level before experimental reconstruction, can be approximatedas Γ ∼ 0.03 f 2Mq? . The production cross section is also proportional to f 2.

To search for γ+ jet resonances, the mass distribution of a photon and a jet from the SM sourcesis modelled using a smooth parameterization:

dm=

P0(1−m/√

s)P1

(m/√

s)P2+P3 ln(m/√

s)(2)

where√

s = 13 TeV and P0, P1, P2, and P3 are four parameters used to describe the shape.This functional form has been widely used in similar previous searches [7–9, 28]. We fit theparameterization to the data after final selection. The fit to the data has a χ2/ndf = 35.9/33,and is shown in Fig. 2. The pull (data - fit/σdata) for each mass bin is shown at the bottom ofFig. 2. The largest differences between the data and the background-only fit are seen at a massof ∼ 2.0 TeV, where 14 events are expected from the background prediction and 34 data eventswere observed in a single bin.

In order to examine the presence of a possible systematic bias due to the chosen parametricfunction, tests are performed using alternate functional forms. For these tests, the backgroundevents are estimated using MC simulation and quantified based on the difference between

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6 5 Results

Figure 3: Event display of the highest invariant mass γ + jet candidate with mass of 3.29 TeVobserved in data in the 3-dimensional plane. The pT, η, and φ values of the photon and jet areindicated.

the true and predicted number of background events in different γ + jet mass windows. Theinvariant mass distribution obtained from the MC simulation is fitted with alternate test func-tions and the result of the fit, considered as truth model, is used to generate large number ofpseudo-data following Poisson distribution with mean as numbers of events observed in data.The pseudo-data are then fitted with the chosen parameteric function, and for each γ+ jet masswindow, pull distributions are obtained (where pulls are defined as the difference between thetrue and predicted number of events divided by the estimated statistical uncertainty). The ab-solute value of the median of this distribution is found to be below 0.5 in the full search regionand we conclude that the possible statistical bias from the choice of parameteric function issmall as compared to the statistical uncertainty of the fit.

5 ResultsA Bayesian approach [4] with uniform prior is used for estimating the upper limit on the pro-duction cross section of excited quarks. The data is fitted to the background function plus signalline shape with the signal cross section treated as parameter of interest. A binned likelihood,L, of Poisson type is used and is written as a function of a constant α as

L = ∏i

µnii e−µi

ni!(3)

whereµi = αNi(S) + Ni(B), (4)

where ni is the number of events observed in the ith γ + jet invariant mass bin, Ni(S) is thenumber of signal events in ith γ + jet invariant mass bin taken from the q? signal shapes, α is

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[GeV] jetγM500 1000 1500 2000 2500 3000 3500 4000 4500

Nor

mal

ized

Yie

ld/2

0 G

eV

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14 f = 0.1

f = 1.0

(13 TeV)

CMSSimulation Preliminary

γ q→q*

q* (1.0 TeV)

q* (2.0 TeV)

q* (3.0 TeV)

q* (4.0 TeV)

Figure 4: The reconstructed resonance mass spectrum generated using PYTHIA8 simulation forγ + jet resonances modeled by qg→ qγ for coupling multipliers f = 1.0 (filled histogram) andf = 0.1 (dotted histogram) having resonance masses ranging from 1.0 to 4.0 TeV.

a constant to scale the signal amplitude, Ni(B) is the number of events expected from back-ground in the same γ + jet invariant mass bin.

The local significance for the largest excess in data is estimated using a likelihood-based signif-icance estimator defined as:

Sig = sgn(S)

√−2 ln

(LB

LS+B

)(5)

where LB and LS+B are the maximum likelihoods from the best background-only and sig-nal+background fits to the data, respectively. The likelihood function is the same as the onedefined in Eq. 3. The local significance does not include the look-elsewhere effect or systematicuncertainties. A fit to the hypothesis of a q? resonance with a mass of 2.0 TeV has a local signif-icance of 3.2σ for f = 1.0 and 3.7σ for f = 0.1, and is sensitive to the choice of f because theexcess in data is observed in a single bin. The global significance over a mass range from 1 TeVto 2.5 TeV is found to be 2.36σ for f = 1.0 and 2.84σ for f = 0.1.

Log-normal priors are used to model systematic uncertainties, which are marginalized as nui-sance parameters in the limit setting procedure. We calculate the posterior probability densityas a function of signal cross section for various resonance masses. The 95% confidence level(CL) upper limit is calculated from the posterior probability density.

The major sources of systematic uncertainty are jet energy scale (JES) [15], jet energy resolution(JER) [15], photon energy scale (PES) [15], photon energy resolution (PER) [29], backgroundparameters, and luminosity. The uncertainties on the JES and PES are estimated to be 0.5%−0.8% and 1.0% respectively (as a function of γ+ jet mass). These uncertainties are accounted forby shifting the reconstructed signal mass conservatively by 1.5%. The systematic uncertaintyon the JER and PER are estimated to be 10% and 0.5%. The systematic uncertainties from PERand JER translate into a 5% relative uncertainty on the resolution of invariant mass of the γ+ jet

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8 5 Results

and is propagated into the results by increasing and decreasing the width of reconstructed massshape of signal. The uncertainty in photon identification is found to be 2% for photons withpT upto 200 GeV and within 4% in a pT range from 200 GeV to 1 TeV. The estimated systematicuncertainty on the luminosity for the early Run 2 dataset is 2.7% [30], and is propagated to thenormalization of the signal cross section. The uncertainty on the signal acceptance due to thechoice of PDF [31] is found to be about 2%. For background fit-parameters, the variations ofparameters from the best-fit values introduce a systematic uncertainty on the signal strengthmodelled using flat priors.

The systematic uncertainties in JER, PER, JES, PES, and luminosity are used in the limit settingprocedure as nuisance parameters and affect only the signal. The 95% CL upper limit on σ×Bas a function of Mq? is shown in Fig. 5. At a mass of 2.0 TeV the observed cross section limit is45 fb and the expected cross section limit is 19 fb.

The observed limits are compared to the LO theoretical predictions to estimate the lower massbound on the excited quarks, which is shown in Fig. 5 for f = 1.0, 0.5, and 0.1. A lower boundof 4.37 TeV is obtained for f = 1.0. The corresponding expected limit is found to be 4.33 TeV.The dependence of the σ × B upper limit on f is found to be negligible for f ≤ 1. Using thetheoretical predictions for different coupling strengths from 0.1 to 1.0 and the observed limits,a mass region as a function of the coupling strength is excluded as shown in Fig. 6.

q* Mass [TeV]1 2 3 4 5 6

B [f

b]× σ

1

10

21095% CL upper limits

Observed limitExpected limit

σ 1±Expected limit σ 2±Expected limit

Excited quark (f = 1.0)Excited quark (f = 0.5)Excited quark (f = 0.1)

(13 TeV)-12.7 fb

CMSPreliminary

γ q→q*

Figure 5: The expected and observed 95% CL upper limits on σ × B for excited quark searchin γ + jet final state corresponding to coupling f = 1.0. The limits are also compared withthe theoretical predictions for excited quark production, shown for coupling values f = 1.0,0.5, and 0.1. The uncertainty at 1σ and 2σ levels are shown as green and yellow shaded bandsaround expected limit.

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q* Mass [TeV]1 1.5 2 2.5 3 3.5 4 4.5 5 5.5 6

= f'

]s

Cou

plin

gs [f

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Expected 95% CL Exclusion = 13 TeVs = 8 TeVs

(13 TeV)-1 (8TeV) 2.7 fb-119.7 fb

CMSPreliminary

γ q→q*

Figure 6: The observed (yellow filled) and expected (blue dashed line) excluded regions at 95%CL as a function of excited quark mass and the coupling strength for Λ = Mq? (left axis) or themass divided by compositeness scale for coupling strength of f = 1 (right axis). The excludedregion from Run-I is also shown (red shaded).

6 SummaryWe have presented a search for excited quarks in the γ + jet final state. The data set collectedat√

s = 13 TeV corresponds to an integrated luminosity of 2.7 fb−1. The data are found to beconsistent with the predictions of the standard model and no evidence is found for an excitedquark resonance. The largest excess of events in data is seen at a γ + jet invariant mass of2.0 TeV.

A 95% CL upper limit is placed on σ×B for q? production in the γ+ jet final state. Comparingthese upper limits with the theoretical predictions, an excited quark state with mass 1.0 <Mq? < 4.37 TeV is excluded at 95% CL for f = 1.0. Also, excited quarks with masses 1.0 <Mq? < 3.64 (1.36)TeV are excluded for f = 0.5 (0.1). We also present the excluded mass as afunction of coupling strength in Fig. 6.

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