CMS AN AN-10-192

CMS Analysis NoteThe content of this note is intended for CMS internal use and distribution only

2010/07/02

Tau production from Z decays in pp collision at√

s = 7 TeV

M. Bachtis1, G. Bagliesi2, E. Friis3, S. Gennai4, A. Gurrola5, A. Kalinowski6, L. Lusito7,A. Savin1, M. Vazquez Acosta8, and C. Veelken3

1 University of Wisconsin2 INFN - Sezione Pisa

3 University of California Davis4 CERN

5 Texas A&M University6 Laboratoire Leprince-Ringuet, Ecole Polytechnique

7 University and INFN Bari, also at CERN8 Imperial College London

Abstract

The reconstruction of tau leptons in a hadronic environment can be challenging, yetis important for many searches for new particles as well as studies of Standard Modelprocesses. The production of Z bosons decaying subsequently into tau pairs servesas an important benchmark for tau reconstruction. The first search of tau productionfrom Z decays with the CMS detector at the LHC at a

√s = 7 TeV is presented.

1

1 IntroductionTau leptons are an excellent signature to probe new physics at the LHC. As the heaviest of theleptons they have the largest coupling to the Higgs boson both in the Standard Model and in theMinimal Supersymmetric Model. Tau leptons can also be an important signature for searchesof Supersymmetry and extra dimensions. Channels involving taus are crucial for discoveryphysics but also provide the possibility to measure the mass, polarization and couplings ofnew heavy decaying particles.

Taus can decay into purely leptonic states (τlep) or semileptonic states (τhad), with a hadronicsystem and one neutrino. Since it is not possible to discriminate between prompt light leptonsand leptons from tau decays, the hadronic decay modes serve as signature of tau production.Constrained by the tau mass, the hadronic system is characterized by a low particle multiplicityand highly collimated jet-like signature which allows their separation from the large QCD jetbackground.

The largest source of isolated taus at the LHC comes from W and Z boson production chan-nels: W → τν and Z → ττ. The W channel has the largest production rate but requires agood understanding of the tau identification and missing transverse energy from the unde-tected neutrino. The study of Z → ττ production in the τlepτhad final state will be a powerfulcommissioning tool for tau physics, setting the grounds for searches for new physics in the di-τmass spectrum.

In this study a search for Z → ττ in the µτhad final state has been performed, using 13 nb−1 ofproton-proton collision data at

√s = 7 TeV from the 2010 LHC run.

2 Trigger SelectionCollision events have been selected with the muon High Level Trigger. All events pass theHLT Mu9 trigger, which requires at least one muon candidate with pT > 9 GeV and |η| < 2.1,which is reconstructed using both the inner tracker and the muon detector information. Noisolation is required at the trigger level.

3 Data and Monte Carlo SamplesTo analyze the CMS collision data based on the events triggered by the muon HLT algorithmsthe Muon Primary/Secondary datasets have been used as described in Table 1.

Samples of electroweak processes with Z and W production and a sample of QCD with a muonin the final state have been produced with the PYTHIA [1] Monte Carlo (MC) program. Asample of tt produced with MADGRAPH and interfaced to the PYTHIA parton-shower hasalso been analyzed. The details of the MC samples can be seen in Table 2.

Table 1: CMS datasets used.Collision Dataset Run Range

/MinimumBias/Commissioning10-May27thSkim SD Mu-v2/RECO 131511-135802/Mu/Run2010A-May27thReReco v2/RECO 135821-136587

/Mu/Run2010A-PromptReco-v2/RECO 136087-137028

2 5 Tau Identification

Table 2: MC samples used in this study. Cross sections are NLO except the QCD sample wherethe QCD LO cross section is used.

MC Dataset σ ∗ εfilter (pb)/Ztautau/Spring10-START3X V26 S09-v1/ 1667/Wmunu/Spring10-START3X V26 S09-v1/ 7696/Zmumu/Spring10-START3X V26 S09-v1/ 1667/Wtaunu/Spring10-START3X V26 S09-v1/ 10312

/TtbarJets Tauola-madgraph/Spring10-START3X V26 S09-v1/ 165/QCD Pt-20 MuEnrichedPt10 7TeV-pythia6/Spring10-START3X V26 AODSIM-v1/ 343940

4 Muon IndentificationA cut based muon identification has been used with the following selection:

• The muon is identified as a tracker or global muon. This selection is effective againstdecays-in-flight, punch-through and accidental matching with noisy or backgroundtracks or segments.

• The muon is required to have a pT > 10 GeV and |η| < 2.1.

• The number of hits in the tracker track part of the muon has to be larger than 10.

• There should be at least one pixel hit in the tracker track part of the muon.

• The muon track has to have at least two chambers in different stations with “match-ing” segments, that is the propagated track to the muon chamber is consistent.

• Bad fits are rejected by requiring that the the global muon fit quality χ2/nd f < 10,where ndf is the number of degrees of freedom.

• The muon has to contain at least one “valid” hit.

• The impact parameter dxy with respect to the beam spot is required to be < 2 mm.

• The mass of the muon and any opposite-sign (OS) track in the event does not lie inthe mass window 80-100 GeV.

5 Tau IdentificationThe identification of hadronic tau has been carried out with two independent reconstructionalgorithms.

5.1 Particle-flow Tau Algorithm

The particle Flow (PF) technique allows to improve the τ identification efficiency. In a first stepall tracks and energy clusters are indepently reconstructed in each subdetector. Then all the PFCandidates (muons, electrons, charged and neutral hadrons and photons) are associated in anoptimal combination to one or more of these subdetector signals, if they are compatible withthe physics properties of each particle, and reconstructed in the event. The final set of particlesis used to derive composite physics objects such as jets which are clusterized using the standardjet algorithm, anti-kt with R=0.5.

Once the PF jets have been identified, a requirement of at least one charged hadron with pTlocated at a distance 0.1 in η − φ space from the jet axis is required. The highest momentumcharged hadron that satisfies this requirement is referred to as “leading track” of the jet, which

5.2 Hadrons Plus Strips Algorithm 3

is required to have pT > 5 GeV. Around the leading track a signal cone, which is expected tocontain all τ decay products, and an isolation annulus, in which little activity is expected dueto the isolation characteristics of the τhad, are defined. A signal cone of size 0.07 and an isolationcone of 0.45 has been used in this study. An alternative approach to the use of a fixed signalcone is the shrinking cone, in which the size scales as 5/ET with minimum and maximumvalues set to 0.07 and 0.15. This relies on the fact that the τhad is more collimated are higherenergies. Results using a shrinking signal cone are shown in Appendix A.1. A discriminationagainst muons is also applied.

The performance of the PF Tau based algorithm on signal and background MC with a fixedand shrinking signal cone is described in [2].

5.2 Hadrons Plus Strips Algorithm

The Hadrons Plus Strips Algorithm (HPS) Tau ID algorithm uses Particle Flow and in similarway to the standard PF Tau ID algorithm [2] HPS Tau algorithm starts from a Particle Flow Jetand reconstructs the possible tau decays inside the jet.

Hadronic taus decay in modes involving neutral pions most of the time so a robust π0 recon-struction is essential. Photon conversions in the CMS tracker material often result in broadercalorimeter signatures for neutral pions. For this reason , a strip reconstruction is proposed forthe electromagnetic fraction of the taus. Starting from the highest EM PF candidate inside thejet, the photon (and/or PF electron) candidates are clustered in strips that are narrow in eta andare dynamically growing in phi to include the converted photon energy. The association dis-tance for η is 0.05 and for φ is 0.2 radians. A minimum threshold on the transverse momentumof the strip is applied and the strips are then combined with the charged hadrons to reconstructthe tau decays. The decay modes that are currently reconstructed with the HPS algorithm arethe following

1. Single Hadron: This decay mode reconstructs one prong tau decays or decays of typeτ− → h−π0 when the π0 has very low energy.

2. Hadron Plus Strip: This type is aiming to reconstruct one prong taus that are producedin association with a π0 where the photons from the pi0 decay are very close in the cal-orimeter surface. The strip takes care of the possibility that one or both of those photonshave converted.

3. Hadron Plus Two Strips: This type is aiming to reconstruct one prong taus that are pro-duced in association with a π0 where the photons from the π0 decay are well separatedin the calorimeter surface.

4. Three Hadrons: This type is aiming to reconstruct the three prong decays of taus. ThreeHadrons are required with a compatible charge (|q| = 1) coming from a common vertexestimated by the Kalman vertex fit algorithm.

The tau decay modes of τ− → h−h+h−π0ντ and τ− → h−π0π0ντ have not been introduced tothe algorithm yet but can be supported in the near future to increase efficiency.

The hadron+strip decay mode requires a strip with a minimum ET > 1 GeV and a chargedhadron. A constraint is applied on the strip mass to match the π0 mass and the invariant massof the hadron plus strip is calculated. This invariant mass should be compatible with the massof the ρ(770) resonance.

The performance of the HPS algorithm on signal and background MC is described in [3].

4 7 Event Selection

6 Event Pre-selectionTo probe the two τ reconstruction algorithms described in Sections 5.1 and 5.2, two differentpre-selections have been considered in this study.

• Pre-Selection I

• at least one identified µ with pT > 10 GeV and |η| < 2.1, with the identi-fication criteria in Sect. 4

• at least one PF identified τ with pT > 20 GeV and |η| < 2.4• ∆R(µ, τ) > 0.2

• Pre-Selection II

• at least one identified µ with pT > 10 GeV and |η| < 2.1• at least one HPS identified τ with pT > 15 GeV and |η| < 2.4• ∆R(µ, τ) > 0.2.

7 Event SelectionFor the final event selection the muon is required to be isolated. The isolation of the muon iscalculated as a relative combined isolation:

Irelcomb = ∑ (ptracks

T + EEMT + EHAD

T )/pµT (1)

in a ∆R < 0.5 cone around the muon direction. The muon is considered to be isolated ifIrelcomb < 0.1. A cone size of ∆R < 0.3 has also been studied and have shown to give a large

QCD background contribution after all selections.

The reconstructed tau isolation is calculated in a solid cone of ∆R around the reconstructed taudecay mode axis. Isolation can use the energy sum of the candidates or counting of candidatesabove a certain threshold. All the candidates in the cone are calculated in the isolation andthen the constituents that correspond to the reconstructed tau are subtracted. This procedure isexpected to provide better separation than the standard annulus isolation since it can separatetaus from narrow jets with high electromagnetic, or hadronic energy fractions. For the selectionof isolated PF (HPS) taus, it is required that there are no PF charged candidates with pT > 1(0.8) GeV and no PF gamma candidates with ET > 1.5 (0.8) GeV in a cone of ∆R of 0.45 (0.5),respectively.

To reject the W background a requirement of the transverse mass of the muon and the missing-transverse energy, MET, MT(µ, MET) of the event has been applied: MT(µ, MET) < 40 GeV.Two different MET reconstruction methods, track-corrected MET (tcMET) [4] and particle-flowMET (pfMET) [5] have been tested which give similar event selection.

The two event selections considered in this study are the following:

• Selection I

• at least one identified µ with pT > 15 GeV and |η| < 2.1• at least one PF identified τ with pT > 20 GeV and |η| < 2.4• ∆R(µ, τ) > 0.2• µ and τ are of opposite-sign (OS)• Irel

comb < 0.1• τ-isolation: no charged PF candidates with pT > 1 GeV and no PF gamma

candidates with ET > 1.5 GeV, in a cone ∆R=0.45

5

• MT(µ, MET) < 40 GeV

• Selection II

• at least one identified µ with pT > 15 GeV and |η| < 2.1• at least one HPS identified τ with pT > 15 (20) GeV and |η| < 2.4• ∆R(µ, τ) > 0.2• µ and τ are of opposite-sign (OS)• Irel

comb < 0.1• τ isolation: no charged PF candidates with pT > 0.8 GeV and no PF

gamma candidates with ET > 0.8 GeV, in a cone ∆R=0.5• MT(µ, MET) < 40 GeV.

8 Results8.1 PF Tau based Reconstruction

Muon-τhad non-isolated pairs have been selected with Pre-Selection I as described in Sect. 6,which is based on PF tau reconstruction. The distribution of the muon pT and combined rela-tive isolation can be seen in Fig. 1. The distribution of the PF recontructed hadronic tau pT andleading track pT are shown in Fig. 2. The measured quantities are compared to the MC simu-lation events normalized using the experimentally measured luminosity and the cross sectionsin Table 2. The distributions are dominated by the QCD contribution as no isolation require-ments have been applied and are well described by the MC predictions. The transverse massdistribution of the muon and the MET for tcMET and pfMET are shown in Fig. 3. Both METreconstruction methods give similar MT distributions. This observable is powerful in rejectingthe contribution of the W background. The reconstructed visible Muon-τhad mass can be seenin Fig. 4 (left). The contribution of same-sign (SS) and opposite-sign (OS) events to the visi-ble mass can be seen in Fig. 4 (right). The OS mass shape is in good agreement with SS massshape. The ratio of isolated and non-isolated events will be used to determine the contribu-tion of QCD background in the final signal selection [3, 6]. The SS/OS ratio with no isolationapplied is measured to be: (NSS/NOS)non−isolated = 0.94± 0.15 with this pre-selection.

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Figure 1: Muon pT (left) and muon relative combined isolation with an isolation cone of R=0.5(right). The selection applied is described in Sect. 6 (Pre-Selection I).

6 8 Results

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Figure 3: Transverse mass of the Muon and MET for tcMET (left) and pfMET (right). Theselection applied is described in Sect. 6 (Pre-Selection I).

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8.2 HPS Tau based Reconstruction 7

Finally, isolation requirements have been applied on the Muon-τhad pairs following the Selec-tion I as described in Sect. 7. The pairs have been required to be of OS and MT(µ, MET) < 40GeV. No data events survive these selections. The expected visible mass distribution after allselections is shown in Fig. 5. 0.8 events are expected to be observed with 100 nb−1 of collectedCMS data with a selection purity (Signal/Signal+Background) of 64%. An alternative approachbased on the use of PF-based muon isolation is described in the Appendix A.2.

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Figure 5: Expected visible Mass for events with an isolated PFtau and an isolated muon for aluminosity of 100 nb−1. The full selection applied is described in Sect. 7 (Selection I).

8.2 HPS Tau based Reconstruction

Muon-τhad non-isolated pairs have been selected with Pre-Selection II as described in Sect. 6,which is based on HPS tau reconstruction. The distribution of the muon pT and combinedrelative isolation can be seen in Fig. 6. The distribution of the HPS recontructed hadronic taupT and the charged and neutral isolation pT are shown in Fig. 7. The measured quantitiesare compared to the MC simulation events normalized using the experimentally measuredluminosity and the cross sections in Table 2. The distributions are dominated by the QCDcontribution as no isolation requirements have been applied and are well described by the MCpredictions.

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8 8 Results

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Figure 8: Transverse mass of the Muon and MET for tcMET (left) and pfMET (right). Theselection applied is described in Sect. 6 (Pre-Selection II).

8.2 HPS Tau based Reconstruction 9

The transverse mass distribution of the muon and the MET for tcMET and pfMET are shownin Fig. 8. Both MET reconstruction methods give similar MT distributions. This observable ispowerful in rejecting the contribution of the W background. The reconstructed visible Muon-τhad mass can be seen in Fig. 9 (left). The contribution of same-sign (SS) and opposite-sign(OS) events to the visible mass can be seen in Fig. 9 (right). The OS mass shape is in goodagreement with SS mass shape. The ratio of isolated and non-isolated events will be used todetermine the contribution of QCD background in the final signal selection. The SS/OS ratiowith no-isolation applied is measured to be: (NSS/NOS)non−isolated = 0.99± 0.08(stat.).

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Figure 9: Visible Muon-τhad Mass (left) and comparison of the same-sign and opposite signvisible Mass (right). The selection applied is described in Sect. 6 (Pre-Selection II).

Finally, isolation requirements have been applied on the Muon-τhad pairs following the Selec-tion II as described in Sect. 7. The pairs have been required to be of OS and MT(µ, MET) < 40GeV. No data events survive these selections. The expected visible mass distribution after allselections is shown in Fig. 10 for events with a HPS identified tau with pT > 15 GeV (left) andpT > 20 GeV (right). 0.8 (0.7) events are expected with 100 nb−1 of collected CMS data with aselection purity (S/S+B) of 77 (85) %.

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Figure 10: Expected visible Mass for events with an isolated HPS identified tau, with pT > 15GeV (left) and pT > 20 GeV (right), and an isolated muon for a luminosity of 100 nb−1. The fullselection applied is described in Sect. 7 (Selection II).

10 A Further reconstruction method checks

9 SummaryA search for Z → ττ in the µτhad final state has been performed, using 13 nb−1 of proton-protoncollision data at

√s = 7 TeV from the 2010 LHC run. Two different hadronic tau recontruction

algorithms have been tested, Particle Flow Tau and Hadron Plus Strip algorithms. With loosepreselection, the muon and hadronic quantities measured are well described by the MC simu-lation. After requiring muon and tau isolation no events survive all selections. With 100 nb−1

of CMS data, 1 high purity event is expected.

A Further reconstruction method checksA.1 PF Tau Reconstruction: muon isolation cone and tau signal cone definition

The same analysis discussed in this note has been carried out with the muon isolation cone sizeof R = 0.3, which is the value used in the W → µν analysis. After all selections, the signalpurity drops from 64% to 55% if one uses a cone of R = 0.5 and R = 0.3, respectively. Theexpected visible mass distribution can be seen in Fig. 11.

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Figure 11: Expected visible Mass for events with an isolated PFtau and an isolated muon, witha cone of R=0.5 (left) and R=0.3 (right) for a luminosity of 100 nb−1. The full selection appliedis described in Sect. 7 (Selection I).

The definition of the PF Tau signal cone has also been varied from a fixed signal cone of R =0.07 to a shrinking cone size that varies as 5/ET, with minimum and maximum values set to0.07 and 0.15. After all selections, the signal purity varies from 64% to 48%, when using a fixedor shrinking signal cone. The expected visible mass distribution can be seen in Fig. 12.

A.2 HPS Tau Reconstruction: particle flow muon isolation

The same analysis discussed with this note has been carried out with a different muon isolationtechnique. Instead of the combined relative muon isolation defined in Eq. 1 with R = 0.5, theisolation is calculated using particles reconstructed with the particle flow algorithm in a coneof R = 0.4. The variable used for isolation is defined as

IrelPF−isol = ΣEcharged hadron

T + ΣEneutral hadronT + ΣEphoton

T /pµT, (2)

where the sum runs over all the PF charged and neutral hadrons and the PF gamma candidatesin a specific cone around the reconstructed muon. The sum of the photon and charged hadron

A.2 HPS Tau Reconstruction: particle flow muon isolation 11

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Figure 12: Expected visible Mass for events with an isolated PFtau, with a fixed (left) andshrinking (right) signal cone, and an isolated muon for a luminosity of 100 nb−1. The fullselection applied is described in Sect. 7 (Selection I).

ET is more efficient than sequential isolation requirements on hadron and photon candidatesbecause the Particle Flow algorithm associates the calorimetric energy to the tracks while creat-ing charged hadronic candidates, therefore there is no double counting of the energy betweenthe hadron and the photon collections. Neutral hadronic energy that comes from HCAL clus-ters not associated to tracks can also be used in the isolation sum.

The distribution of the PF based muon isolation is shown in Fig. 13.

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Figure 13: Muon particle-flow based isolation with an isolation cone of R=0.4.

With a selection of the particle based muon isolation of: IrelPF−isol 0.1 (Eq. 2) a similar selection

purity can be achieved. Fig. 14 shows the expected distributions after all selections with stan-dard and particle-flow based muon isolation. For a HPS tau of pT > 15 GeV (left) and pT > 20GeV (right), a signal purity of 77 % and 85 % with both methods.

12 A Further reconstruction method checks

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Figure 14: Visible Mass for events with an isolated tau and an isolated muon with a HPS identi-fied tau with pT > 15 GeV (left) and pT > 20 GeV (right). The relative combined muon isolationhas been used in the top and the particle-flow based isolation in the bottom.

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