At the LHC, new particles can be expected that decay to final states
involving taus. Examples are given from simulations by the ATLAS experiment
showing how such final states can be exploited.Comment: Invited talk at the Seventh International Workshop on Tau Lepton
  Physics (TAU02), Santa Cruz, Ca, USA, Sept 2002, 6 pages, LaTeX, 4 eps
  figure

Assamagan

Cavalli

Djouadi

Ellis

Feng

Hinchliffe

Hisano

Ian Hinchliffe

Kodama

Serin

English

arXiv

Hinchliffe, Ian

eScholarship - University of California

Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryTitleUse of Taus in ATLASPermalinkhttps://escholarship.org/uc/item/8w71297fAuthorHinchliffe, IanPublication Date2002-10-24eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaarXiv:hep-ph/0210340 v1   24 Oct 20021Use of Taus in ATLASIan Hinchliffea ∗a Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USAAt the LHC, new particles can be expected that decay to final states involving taus. Examples are given fromsimulations by the ATLAS experiment showing how such final states can be exploited.1. INTRODUCTIONThe total production rate for taus at a hadroncollider is not a useful quantity. Taus must havesignificant transverse momentum (pT ) in order tobe observable. Leptonic decays of taus will yieldisolated electrons or muons that can be detectedbut these can also be directly produced so dis-criminating their origin can be difficult. Hadronicdecays of taus result in jets that must be distin-guished from jets arising from QCD processes us-ing the particle multiplicity and invariant mass.2. PROPERTIES OF TAUS AT LHCThe dominant standard model production thatresults in an observable sample is W → τν whichproduces ∼ 1.5 × 108 events per 10fb−1. Giventhis large sample it is reasonable to ask if any use-ful measurements of tau properties can be made.Measurement of the lifetime is difficult as it re-quires a determination of the decay length usingthe vertex tracking system and knowledge of thetau momentum. In the case of Z → ττ the mo-mentum can be reconstructed using a constrainedfit involving the Z mass (see Section 3) . Theprocess is statistics limited to a precision of ap-proximately 1.8 fs for 30fb−1 of data ([1] vol Ip. 305). Use of the W decay which has a muchlarger rate is more difficult as there are two neu-trinos [2]. Two methods were attempted involv-ing using the observed tau decay products and thetau mass constraint to determine the momentum∗This work was supported in part by the Director, Officeof Science, Office of Basic Energy Research, Division ofHigh Energy Physics of the U.S. Department of Energyunder Contract DE-AC03-76SF00098of the neutrino in the tau decay and an estima-tor method [3]. In this case the statistical errorcould be less due to the larger event sample pro-vided that the QCD background can be rejected.However, the systematic uncertainties from align-ment and other sources are difficult to estimate.Rate decays of the tau can provide a probe ofnew physics. Lepton number is known to be vi-olated in the neutrino sector and the rare decaysτ → µγ, τ → 3µ or τ → µe+e− can be expectedto occur. In many models [4] [5], [6] the first ofthese is expected to be the largest and a simula-tion will be discussed here [7]. The signal is anisolated µ and photon whose invariant mass re-constructs to the tau. There are two sources ofbackground τ → µννγ and W → γτ → γµνν; thelatter dominates. After cuts to reduce the back-ground, the signal acceptance is approximately0.5% and the mass resolution is 20 MeV resultingin a background of approximately 17 events per10fb−1. The resulting limit is not competitivewith what is possible at Belle and BaBar unlessthe full luminosity of the LHC can be exploited.A study in this environment where the pile-upof minimum bias events degrades resolution andincreases background has not been undertaken.3. RECONSTRUCTION OF Z → ττThe H → ττ process is important as a tool forsearching for Higgs bosons at LHC. As a preludeto this and to illustrate the technique, I will firstdiscuss Z → ττ . Missing neutrinos imply thattau energy cannot be measured directly. How-ever the direction of tau can be obtained fromobserved decay products as the energy of the pro-2duced taus is large compared to the mass. IfZ has signifificant transverse momentum so thattau’s are not back to back in phi and the onlymissing PT in the event arises from tau decay,then the tau momenta and invariant mass of theττ system can be reconstructed by assuming thatthe neutrinos follow the direction of the observeddecay products. These events have no other fea-tures so that the selection must be based on thepair of taus. There is a very large di-jet back-ground from QCD events which must be over-come and the events require a trigger. Events areselected by requiring an isolated electron or muonwith PT > 25 GeV and | η |< 2.5 and hadronicjet with ET > 30GeV , | η |< 2.5, and a number ofassociated reconstructed tracks Ntrack = 1 or 3,The jet is also required to be narrow, having itsenergy deposits in the electromagnetic calorime-ter confined with a cone of radius Rem = 0.07 inη − φ space. Figure 1 shows the reconstructedZ mass with the peak at the correct value. Thefigure shows the mass resolution of ∼ 10% whichis dominated by the missing ET resolution of thedetector. The small residual QCD background isalso shown. The subset of events with Ntrack = 3can be used to determine the tau decay vertex forthe lifetime measurement discussed above.4. NEUTRAL HIGGS SEARCHThe decay H → ττ can be reconstructed usingthe technique described in the previous sectionand the mass of the Higgs measured. The pro-duction rates and branching ratios of the stan-dard model higgs are too small for the signal tobe seen above the QCD background using thedominant production process gg → H . How-ever the lower rate process qq → qqH → qqττwhile it may not enable a discovery to be made,will give information on the Higgs couplings. Thetwo jets arising from the quarks in the productionprocess are at large rapidity and can be used toreject background. Final states ττ → ℓ±h∓ℓpTand ττ →→ e±µ∓ are used and the ττ invari-ant mass reconstructed as above with a resolutionσ ∼ 0.1M . The table shows the expected signaland background [8]. It is important to emphasizethat this result is based on a parametrized simu-020400 50 100 150 200mττ (GeV)Events / 2 GeVFigure 1. Reconstructed mass of the Z → ττ sig-nal for 30fb−1. The dashed line shows the back-ground. From [1], figure 9.54Table 1Number of expected signal and backgroundevents for the qq → qqH → ττjj channel, for100 fb−1 and two detectors. Cross sections areadded for ττ → ℓ±h∓ℓpT and ττ → e±µ∓mH 100 110 120 130 140 150Signal 211 197 169 128 79 38Background 305 127 51 32 27 24lation that assumes that the tau tagging and jettagging is not degraded in the presence of pile-up.Nevertheless it indicates a viable signal for massrange 110 GeV < MH < 150 GeV if the Higgsmass were already known from another process.Note that signal to background ratio is such thatstructure would clearly be seen at the Higgs mass.In extensions to the standard model more higgsbosons are expected. The minimal supersymmet-ric model (MSSM) has three neutral (h, A, H)and one charged (H±) particles. Their proper-ties are determined by two parameters which canbe taken to be the mass of A and tanβ, which3controls the coupling to tau’s. A and H can beobserved over certain regions of parameter space.The technique adopted is the same as that usedfor Z → ττ . Figure 2 shows three possible signalsand the background which arises from QCD jets.This signal covers a large fraction of MSSM pa-rameter space and is one of the most importantprobes of the MSSM Higgs sector.0 100 200 300 400 500 600020406080100m(A)=150 GeVm(A)=300 GeVm(A)=450 GeV Back-groundMττ (GeV)Events/bin/30 fb-1Figure 2. Reconstructed mass of the A → ττsignal for 30fb−1 showing three possible masses.The background is shown as the unfilled his-togram. From [1].5. CHARGED HIGGSA charged Higgs boson is expected to decayH+ → τν or H+ → qq. The latter decay in-volves the reconstruction of jets and the QCDbackground in the di-jet mass distribution is a se-rious difficulty to observation of this mode. Theformer decay is more promising. Production ofa heavy H± via qq → H+H− is too small, theproduction via gb → H−t has σ(H+t) ∼ 1pb for a mass of H± of 400 GeV. The decayH+ → τν will result in either an isolated leptonor jet. The former is not useful so the latter isexploited. The final state Ht → τνWb is identi-fied by requiring three jets one of which is taggedas containing a b-hadron from the vertex systemand requiring that these three jets are consistentwith t → bW → bqq (m(jj) = MW ± 25 andm(jjb) = Mt ± 25). Another jet with only oneassociated charged track and pT > 100 GeV is re-quired along with missing ET (> 100 GeV). Thesignal is in transverse mass of “τ” and missingET which is shown in Figure 3. The peak is dis-placed below the Higgs mass as the missing EThas contributions from both neutrinos in the de-cay H+ → τν → πνν and these tend to cancel.gb→tH+, t→jjb, H+→τν012345678910100 200 300 400 500 600Signal Backgroundsb-jet vetopTτ-jet > 100 GeVpTmiss > 100 GeV∆φ > 1.0 radmT (GeV)Events / 12 GeVmH+=250 GeVtanβ=40mH+=500 GeVtanβ=50Figure 3. Transverse mass of the Missing ET andthe jet identified as a tau. The signal from H →τν shows two possible masses. The backgroundis also shown. From [10].6. TAUS IN SUPERSYMMETRYEVENTSIf supersymmetry (SUSY) is relevant to theproblems of electroweak symmetry breaking,SUSY particles will be sufficiently light that theLHC will be a SUSY factory. Third genera-tion sparticles are particularly interesting as theycarry information about how supersymmetry isbroken. In the case of the first two generationsthe mixing between the supersymmetric partnersof the left and right handed charged leptons isexpected to be small. However the two τ̃ masseigenstates are mixtures of τ̃L and τ̃R and weneed to measure masses and mixings. The direct4production of τ̃ via electroweak processes suchas qq → τ+τ− is small. In many cases manymore τ̃ ’s are produced from decays of stronglyproduced squarks and gluinos via processes suchas q̃ → χ̃2q → qτ̃+τ− → qχ̃01τ+τ− where χ̃2 andχ̃01 are weak gauginos and the latter is stable andexits the detector contributing to missing ET . Itis possible that taus may be the only leptons pro-duced in gaugino decay.Leptonic tau decays are of limited use as theorigin of the lepton is unknown; one must searchfor hadronic tau decays that give rise to charac-teristic jets. SUSY events are expected to be verycomplex and one can rely on jet and /ET cuts toget rid of standard model background and ob-tain clean SUSY sample. This sample can thenbe studied in detail. The background to hadronictau decays then arises from QCD jets in the SUSYevent and a rejection factor of order 10 (the num-ber of jets in a such an event) rather than order100 which is needed in the analysis described insection 4.A particular model is used in the example il-lustrated here. ([1] vol II p. and [11]). Eventsare selected as follows: ≥ 4 jets, one of whichhas pt > 100 GeV and the rest have pt > 50GeV, there are no isolated electrons or muonswith pt > 10 GeV and /ET > 100 GeV. The jetsare examined and “tau” candidates extracted byselecting jets with low track multiplicity and in-variant mass [12] using an algorithm based on astudy using Z decays to generate taus in the ap-propriate kinematic regime. The invariant massof observed “tau” pairs is then shown in Figure 4.A clear peak can be seen above the backgroundwhich is dominated by events where one of thetau jet candidates is a misidentified QCD jet. Ifall the tau energy had been detected, the distri-bution would have had a sharp cut-off atMmaxττ = Mχ̃02√√√√1 −M2τ̃1M2χ̃02√√√√1 −M2χ̃01M2τ̃1which evaluates to 61 GeV in this example. Thepeak is below this due to the energy carried of byneutrinos in the tau decays.The situation is rather more complicated thanthis analysis shows. The energy distribution of05001000Events/2.4 GeV/10 fb-125 50 75 100Mττ (GeV)0Figure 4. Invariant mass of the two tau candi-dates in SUSY events; the solid histogram showsthe signal while the dashed shows the backgroundfrom the combination of real tau and misiden-tified QCD jet, the small solid shows the back-ground from QCD jets were both are misidenti-fied. [1], Figure 20-42.the visible tau decay products depends on thetau polarization and the dependence is largest inthe τ → πν final state and smallest in the stateswith large hadron multiplicity. A separation ofthese states could therefore yield information onthe polarization of the taus. This in turn gives in-formation on the τ̃ composition. Simulation workon this difficult topic is underway.REFERENCES1. ATLAS Collaboration, “ATLAS Physics andDetector Performance Technical Design Re-port,” LHCC 99-14/15.2. D. Cavalli and B. Osculati, Atlas physics noteATL-PHYS-2000-014 (2000)3. K. Kodama et al. [Fermilab E653 Collabora-tion], Phys. Lett. B 303, 359 (1993).4. J. Ellis, M. E. Gomez, G. K. Leontaris, S. Lolaand D. V. Nanopoulos, hep-ph/9911459.5. J. L. Feng, Y. Nir and Y. Shadmi, Phys. Rev.D61, 113005 (2000).6. J. Hisano, T. Moroi, K. Tobe and M. Yam-5aguchi, Phys. Rev. D53, 2442 (1996) [hep-ph/9510309].7. L. Serin and R. Stroynowski, Atlas physicsnote, ATL-PHYS-97-114 (1997).8. A. Djouadi et al., arXiv:hep-ph/0002258.9. D. Cavalli and S. Resconi, Atlas physics note,ATL-PHYS-2000-005 (2000)10. K. A. Assamagan, Y. Coadou and A. De-andrea, Eur. Phys. J. directC 9, 1 (2002)[arXiv:hep-ph/0203121].11. I. Hinchliffe and F. E. Paige, Phys. Rev. D61, 095011 (2000) [arXiv:hep-ph/9907519].12. Y. Coadou et al., ATLAS Internal Note ATL-PHYS-98-126.

Use of Taus in ATLAS

The total production rate for taus at a hadron collider is not a useful quantity. Taus must have significant transverse momentum (p{sub T}) in order to be observable. Leptonic decays of taus will yield isolated electrons or muons that can be detected but these can also be directly produced so discriminating their origin can be difficult. Hadronic decays of taus result in jets that must be distinguished from jets arising from QCD processes using the particle multiplicity and invariant mass. At the LHC, new particles can be expected that decay to final states involving taus. Examples are given from simulations by the ATLAS experiment showing how such final states can be exploited

UNT (University of North Texas) Digital Library

arXiv:hep-ph/0210340 v1   24 Oct 20021Use of Taus in ATLASIan Hinchliffea ∗a Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USAAt the LHC, new particles can be expected that decay to final states involving taus. Examples are given fromsimulations by the ATLAS experiment showing how such final states can be exploited.1. INTRODUCTIONThe total production rate for taus at a hadroncollider is not a useful quantity. Taus must havesignificant transverse momentum (pT ) in order tobe observable. Leptonic decays of taus will yieldisolated electrons or muons that can be detectedbut these can also be directly produced so dis-criminating their origin can be difficult. Hadronicdecays of taus result in jets that must be distin-guished from jets arising from QCD processes us-ing the particle multiplicity and invariant mass.2. PROPERTIES OF TAUS AT LHCThe dominant standard model production thatresults in an observable sample is W → τν whichproduces ∼ 1.5 × 108 events per 10fb−1. Giventhis large sample it is reasonable to ask if any use-ful measurements of tau properties can be made.Measurement of the lifetime is difficult as it re-quires a determination of the decay length usingthe vertex tracking system and knowledge of thetau momentum. In the case of Z → ττ the mo-mentum can be reconstructed using a constrainedfit involving the Z mass (see Section 3) . Theprocess is statistics limited to a precision of ap-proximately 1.8 fs for 30fb−1 of data ([1] vol Ip. 305). Use of the W decay which has a muchlarger rate is more difficult as there are two neu-trinos [2]. Two methods were attempted involv-ing using the observed tau decay products and thetau mass constraint to determine the momentum∗This work was supported in part by the Director, Officeof Science, Office of Basic Energy Research, Division ofHigh Energy Physics of the U.S. Department of Energyunder Contract DE-AC03-76SF00098of the neutrino in the tau decay and an estima-tor method [3]. In this case the statistical errorcould be less due to the larger event sample pro-vided that the QCD background can be rejected.However, the systematic uncertainties from align-ment and other sources are difficult to estimate.Rate decays of the tau can provide a probe ofnew physics. Lepton number is known to be vi-olated in the neutrino sector and the rare decaysτ → µγ, τ → 3µ or τ → µe+e− can be expectedto occur. In many models [4] [5], [6] the first ofthese is expected to be the largest and a simula-tion will be discussed here [7]. The signal is anisolated µ and photon whose invariant mass re-constructs to the tau. There are two sources ofbackground τ → µννγ and W → γτ → γµνν; thelatter dominates. After cuts to reduce the back-ground, the signal acceptance is approximately0.5% and the mass resolution is 20 MeV resultingin a background of approximately 17 events per10fb−1. The resulting limit is not competitivewith what is possible at Belle and BaBar unlessthe full luminosity of the LHC can be exploited.A study in this environment where the pile-upof minimum bias events degrades resolution andincreases background has not been undertaken.3. RECONSTRUCTION OF Z → ττThe H → ττ process is important as a tool forsearching for Higgs bosons at LHC. As a preludeto this and to illustrate the technique, I will firstdiscuss Z → ττ . Missing neutrinos imply thattau energy cannot be measured directly. How-ever the direction of tau can be obtained fromobserved decay products as the energy of the pro-2duced taus is large compared to the mass. IfZ has signifificant transverse momentum so thattau’s are not back to back in phi and the onlymissing PT in the event arises from tau decay,then the tau momenta and invariant mass of theττ system can be reconstructed by assuming thatthe neutrinos follow the direction of the observeddecay products. These events have no other fea-tures so that the selection must be based on thepair of taus. There is a very large di-jet back-ground from QCD events which must be over-come and the events require a trigger. Events areselected by requiring an isolated electron or muonwith PT > 25 GeV and | η |< 2.5 and hadronicjet with ET > 30GeV , | η |< 2.5, and a number ofassociated reconstructed tracks Ntrack = 1 or 3,The jet is also required to be narrow, having itsenergy deposits in the electromagnetic calorime-ter confined with a cone of radius Rem = 0.07 inη − φ space. Figure 1 shows the reconstructedZ mass with the peak at the correct value. Thefigure shows the mass resolution of ∼ 10% whichis dominated by the missing ET resolution of thedetector. The small residual QCD background isalso shown. The subset of events with Ntrack = 3can be used to determine the tau decay vertex forthe lifetime measurement discussed above.4. NEUTRAL HIGGS SEARCHThe decay H → ττ can be reconstructed usingthe technique described in the previous sectionand the mass of the Higgs measured. The pro-duction rates and branching ratios of the stan-dard model higgs are too small for the signal tobe seen above the QCD background using thedominant production process gg → H . How-ever the lower rate process qq → qqH → qqττwhile it may not enable a discovery to be made,will give information on the Higgs couplings. Thetwo jets arising from the quarks in the productionprocess are at large rapidity and can be used toreject background. Final states ττ → ℓ±h∓ℓpTand ττ →→ e±µ∓ are used and the ττ invari-ant mass reconstructed as above with a resolutionσ ∼ 0.1M . The table shows the expected signaland background [8]. It is important to emphasizethat this result is based on a parametrized simu-020400 50 100 150 200mττ (GeV)Events / 2 GeVFigure 1. Reconstructed mass of the Z → ττ sig-nal for 30fb−1. The dashed line shows the back-ground. From [1], figure 9.54Table 1Number of expected signal and backgroundevents for the qq → qqH → ττjj channel, for100 fb−1 and two detectors. Cross sections areadded for ττ → ℓ±h∓ℓpT and ττ → e±µ∓mH 100 110 120 130 140 150Signal 211 197 169 128 79 38Background 305 127 51 32 27 24lation that assumes that the tau tagging and jettagging is not degraded in the presence of pile-up.Nevertheless it indicates a viable signal for massrange 110 GeV < MH < 150 GeV if the Higgsmass were already known from another process.Note that signal to background ratio is such thatstructure would clearly be seen at the Higgs mass.In extensions to the standard model more higgsbosons are expected. The minimal supersymmet-ric model (MSSM) has three neutral (h, A, H)and one charged (H±) particles. Their proper-ties are determined by two parameters which canbe taken to be the mass of A and tanβ, which3controls the coupling to tau’s. A and H can beobserved over certain regions of parameter space.The technique adopted is the same as that usedfor Z → ττ . Figure 2 shows three possible signalsand the background which arises from QCD jets.This signal covers a large fraction of MSSM pa-rameter space and is one of the most importantprobes of the MSSM Higgs sector.0 100 200 300 400 500 600020406080100m(A)=150 GeVm(A)=300 GeVm(A)=450 GeV Back-groundMττ (GeV)Events/bin/30 fb-1Figure 2. Reconstructed mass of the A → ττsignal for 30fb−1 showing three possible masses.The background is shown as the unfilled his-togram. From [1].5. CHARGED HIGGSA charged Higgs boson is expected to decayH+ → τν or H+ → qq. The latter decay in-volves the reconstruction of jets and the QCDbackground in the di-jet mass distribution is a se-rious difficulty to observation of this mode. Theformer decay is more promising. Production ofa heavy H± via qq → H+H− is too small, theproduction via gb → H−t has σ(H+t) ∼ 1pb for a mass of H± of 400 GeV. The decayH+ → τν will result in either an isolated leptonor jet. The former is not useful so the latter isexploited. The final state Ht → τνWb is identi-fied by requiring three jets one of which is taggedas containing a b-hadron from the vertex systemand requiring that these three jets are consistentwith t → bW → bqq (m(jj) = MW ± 25 andm(jjb) = Mt ± 25). Another jet with only oneassociated charged track and pT > 100 GeV is re-quired along with missing ET (> 100 GeV). Thesignal is in transverse mass of “τ” and missingET which is shown in Figure 3. The peak is dis-placed below the Higgs mass as the missing EThas contributions from both neutrinos in the de-cay H+ → τν → πνν and these tend to cancel.gb→tH+, t→jjb, H+→τν012345678910100 200 300 400 500 600Signal Backgroundsb-jet vetopTτ-jet > 100 GeVpTmiss > 100 GeV∆φ > 1.0 radmT (GeV)Events / 12 GeVmH+=250 GeVtanβ=40mH+=500 GeVtanβ=50Figure 3. Transverse mass of the Missing ET andthe jet identified as a tau. The signal from H →τν shows two possible masses. The backgroundis also shown. From [10].6. TAUS IN SUPERSYMMETRYEVENTSIf supersymmetry (SUSY) is relevant to theproblems of electroweak symmetry breaking,SUSY particles will be sufficiently light that theLHC will be a SUSY factory. Third genera-tion sparticles are particularly interesting as theycarry information about how supersymmetry isbroken. In the case of the first two generationsthe mixing between the supersymmetric partnersof the left and right handed charged leptons isexpected to be small. However the two τ̃ masseigenstates are mixtures of τ̃L and τ̃R and weneed to measure masses and mixings. The direct4production of τ̃ via electroweak processes suchas qq → τ+τ− is small. In many cases manymore τ̃ ’s are produced from decays of stronglyproduced squarks and gluinos via processes suchas q̃ → χ̃2q → qτ̃+τ− → qχ̃01τ+τ− where χ̃2 andχ̃01 are weak gauginos and the latter is stable andexits the detector contributing to missing ET . Itis possible that taus may be the only leptons pro-duced in gaugino decay.Leptonic tau decays are of limited use as theorigin of the lepton is unknown; one must searchfor hadronic tau decays that give rise to charac-teristic jets. SUSY events are expected to be verycomplex and one can rely on jet and /ET cuts toget rid of standard model background and ob-tain clean SUSY sample. This sample can thenbe studied in detail. The background to hadronictau decays then arises from QCD jets in the SUSYevent and a rejection factor of order 10 (the num-ber of jets in a such an event) rather than order100 which is needed in the analysis described insection 4.A particular model is used in the example il-lustrated here. ([1] vol II p. and [11]). Eventsare selected as follows: ≥ 4 jets, one of whichhas pt > 100 GeV and the rest have pt > 50GeV, there are no isolated electrons or muonswith pt > 10 GeV and /ET > 100 GeV. The jetsare examined and “tau” candidates extracted byselecting jets with low track multiplicity and in-variant mass [12] using an algorithm based on astudy using Z decays to generate taus in the ap-propriate kinematic regime. The invariant massof observed “tau” pairs is then shown in Figure 4.A clear peak can be seen above the backgroundwhich is dominated by events where one of thetau jet candidates is a misidentified QCD jet. Ifall the tau energy had been detected, the distri-bution would have had a sharp cut-off atMmaxττ = Mχ̃02√√√√1 −M2τ̃1M2χ̃02√√√√1 −M2χ̃01M2τ̃1which evaluates to 61 GeV in this example. Thepeak is below this due to the energy carried of byneutrinos in the tau decays.The situation is rather more complicated thanthis analysis shows. The energy distribution of05001000Events/2.4 GeV/10 fb-125 50 75 100Mττ (GeV)0Figure 4. Invariant mass of the two tau candi-dates in SUSY events; the solid histogram showsthe signal while the dashed shows the backgroundfrom the combination of real tau and misiden-tified QCD jet, the small solid shows the back-ground from QCD jets were both are misidenti-fied. [1], Figure 20-42.the visible tau decay products depends on thetau polarization and the dependence is largest inthe τ → πν final state and smallest in the stateswith large hadron multiplicity. A separation ofthese states could therefore yield information onthe polarization of the taus. This in turn gives in-formation on the τ̃ composition. Simulation workon this difficult topic is underway.REFERENCES1. ATLAS Collaboration, “ATLAS Physics andDetector Performance Technical Design Re-port,” LHCC 99-14/15.2. D. Cavalli and B. Osculati, Atlas physics noteATL-PHYS-2000-014 (2000)3. K. Kodama et al. [Fermilab E653 Collabora-tion], Phys. Lett. B 303, 359 (1993).4. J. Ellis, M. E. Gomez, G. K. Leontaris, S. Lolaand D. V. Nanopoulos, hep-ph/9911459.5. J. L. Feng, Y. Nir and Y. Shadmi, Phys. Rev.D61, 113005 (2000).6. J. Hisano, T. Moroi, K. Tobe and M. Yam-5aguchi, Phys. Rev. D53, 2442 (1996) [hep-ph/9510309].7. L. Serin and R. Stroynowski, Atlas physicsnote, ATL-PHYS-97-114 (1997).8. A. Djouadi et al., arXiv:hep-ph/0002258.9. D. Cavalli and S. Resconi, Atlas physics note,ATL-PHYS-2000-005 (2000)10. K. A. Assamagan, Y. Coadou and A. De-andrea, Eur. Phys. J. directC 9, 1 (2002)[arXiv:hep-ph/0203121].11. I. Hinchliffe and F. E. Paige, Phys. Rev. D61, 095011 (2000) [arXiv:hep-ph/9907519].12. Y. Coadou et al., ATLAS Internal Note ATL-PHYS-98-126.

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At the LHC, new particles can be expected that decay to final states involving taus. Examples are given from simulations by the ATLAS experiment showing how such final states can be exploited

CERN Document Server

1Use of Taus in ATLASIan HinchliffeaMCSD] Lawrence Berkeley National Laboratory, Berkeley, CA 94720 USA a[At the LHC, new particles can be expected that decay to final states involving taus. Examples are given fromsimulations by the ATLAS experiment showing how such final states can be exploited.1. INTRODUCTIONThe total production rate for taus at a hadroncollider is not a useful quantity. Taus must havesignificant transverse momentum (pT ) in order tobe observable. Leptonic decays of taus will yieldisolated electrons or muons that can be detectedbut these can also be directly produced so dis-criminating their origin can be difficult. Hadronicdecays of taus result in jets that must be distin-guished from jets arising from QCD processes us-ing the particle multiplicity and invariant mass.2. PROPERTIES OF TAUS AT LHCThe dominant standard model production thatresults in an observable sample is W !  whichproduces  1:5  108 events per 10fb−1. Giventhis large sample it is reasonable to ask if any use-ful measurements of tau properties can be made.Measurement of the lifetime is difficult as it re-quires a determination of the decay length usingthe vertex tracking system and knowledge of thetau momentum. In the case of Z !  the mo-mentum can be reconstructed using a constrainedfit involving the Z mass (see Section 3) . Theprocess is statistics limited to a precision of ap-proximately 1.8 fs for 30fb−1 of data ([1] vol Ip. 305). Use of the W decay which has a muchlarger rate is more difficult as there are two neu-trinos [2]. Two methods were attempted involv-ing using the observed tau decay products and the∗This work was supported in part by the Director, Officeof Science, Office of Basic Energy Research, Division ofHigh Energy Physics of the U.S. Department of Energyunder Contract DE-AC03-76SF00098tau mass constraint to determine the momentumof the neutrino in the tau decay and an estima-tor method [3]. In this case the statistical errorcould be less due to the larger event sample pro-vided that the QCD background can be rejected.However, the systematic uncertainties from align-ment and other sources are difficult to estimate.Rate decays of the tau can provide a probe ofnew physics. Lepton number is known to be vi-olated in the neutrino sector and the rare decays ! γ,  ! 3 or  ! e+e− can be expectedto occur. In many models [4] [5], [6] the first ofthese is expected to be the largest and a simula-tion will be discussed here [7]. The signal is anisolated  and photon whose invariant mass re-constructs to the tau. There are two sources ofbackground  ! γ and W ! γ ! γ; thelatter dominates. After cuts to reduce the back-ground, the signal acceptance is approximately0:5% and the mass resolution is 20 MeV resultingin a background of approximately 17 events per10fb−1. The resulting limit is not competitivewith what is possible at Belle and BaBar unlessthe full luminosity of the LHC can be exploited.A study in this environment where the pile-upof minimum bias events degrades resolution andincreases background has not been undertaken.3. RECONSTRUCTION OF Z ! The H !  process is important as a tool forsearching for Higgs bosons at LHC. As a preludeto this and to illustrate the technique, I will firstdiscuss Z !  . Missing neutrinos imply thattau energy cannot be measured directly. How-ever the direction of tau can be obtained from2observed decay products as the energy of the pro-duced taus is large compared to the mass. IfZ has signifificant transverse momentum so thattau’s are not back to back in phi and the onlymissing PT in the event arises from tau decay,then the tau momenta and invariant mass of the system can be reconstructed by assuming thatthe neutrinos follow the direction of the observeddecay products. These events have no other fea-tures so that the selection must be based on thepair of taus. There is a very large di-jet back-ground from QCD events which must be over-come and the events require a trigger. Events areselected by requiring an isolated electron or muonwith PT > 25 GeV and j  j< 2:5 and hadronicjet with ET > 30GeV , j  j< 2:5, and a number ofassociated reconstructed tracks Ntrack = 1 or 3,The jet is also required to be narrow, having itsenergy deposits in the electromagnetic calorime-ter confined with a cone of radius Rem = 0:07 in −  space. Figure 1 shows the reconstructedZ mass with the peak at the correct value. Thefigure shows the mass resolution of  10% whichis dominated by the missing ET resolution of thedetector. The small residual QCD background isalso shown. The subset of events with Ntrack = 3can be used to determine the tau decay vertex forthe lifetime measurement discussed above.4. NEUTRAL HIGGS SEARCHThe decay H !  can be reconstructed usingthe technique described in the previous sectionand the mass of the Higgs measured. The pro-duction rates and branching ratios of the stan-dard model higgs are too small for the signal tobe seen above the QCD background using thedominant production process gg ! H . How-ever the lower rate process qq ! qqH ! qqwhile it may not enable a discovery to be made,will give information on the Higgs couplings. Thetwo jets arising from the quarks in the productionprocess are at large rapidity and can be used toreject background. Final states  ! ‘h‘pTand  !! e are used and the  invari-ant mass reconstructed as above with a resolution  0:1M . The table shows the expected signaland background [8]. It is important to emphasize020400 50 100 150 200mττ (GeV)Events / 2 GeVFigure 1. Reconstructed mass of the Z !  sig-nal for 30fb−1. The dashed line shows the back-ground. From [1], figure 9.54Table 1Number of expected signal and backgroundevents for the qq ! qqH ! jj channel, for100 fb−1 and two detectors. Cross sections areadded for  ! ‘h‘pT and  ! emH 100 110 120 130 140 150Signal 211 197 169 128 79 38Background 305 127 51 32 27 24that this result is based on a parametrized simu-lation that assumes that the tau tagging and jettagging is not degraded in the presence of pile-up.Nevertheless it indicates a viable signal for massrange 110 GeV < MH < 150 GeV if the Higgsmass were already known from another process.Note that signal to background ratio is such thatstructure would clearly be seen at the Higgs mass.In extensions to the standard model more higgsbosons are expected. The minimal supersymmet-ric model (MSSM) has three neutral (h; A; H)and one charged (H) particles. Their proper-ties are determined by two parameters which can3be taken to be the mass of A and tan, whichcontrols the coupling to tau’s. A and H can beobserved over certain regions of parameter space.The technique adopted is the same as that usedfor Z !  . Figure 2 shows three possible signalsand the background which arises from QCD jets.This signal covers a large fraction of MSSM pa-rameter space and is one of the most importantprobes of the MSSM Higgs sector.0 100 200 300 400 500 600020406080100m(A)=150 GeVm(A)=300 GeVm(A)=450 GeV Back-groundMττ (GeV)Events/bin/30 fb-1Figure 2. Reconstructed mass of the A ! signal for 30fb−1 showing three possible masses.The background is shown as the unfilled his-togram. From [1].5. CHARGED HIGGSA charged Higgs boson is expected to decayH+ !  or H+ ! qq. The latter decay in-volves the reconstruction of jets and the QCDbackground in the di-jet mass distribution is a se-rious difficulty to observation of this mode. Theformer decay is more promising. Production ofa heavy H via qq ! H+H− is too small, theproduction via gb ! H−t has (H+t)  1pb for a mass of H of 400 GeV. The decayH+ !  will result in either an isolated leptonor jet. The former is not useful so the latter isexploited. The final state Ht ! Wb is identi-fied by requiring three jets one of which is taggedas containing a b-hadron from the vertex systemand requiring that these three jets are consistentwith t ! bW ! bqq (m(jj) = MW  25 andm(jjb) = Mt  25). Another jet with only oneassociated charged track and pT > 100 GeV is re-quired along with missing ET (> 100 GeV). Thesignal is in transverse mass of “” and missingET which is shown in Figure 3. The peak is dis-placed below the Higgs mass as the missing EThas contributions from both neutrinos in the de-cay H+ !  !  and these tend to cancel.gb→tH+, t→jjb, H+→τν012345678910100 200 300 400 500 600Signal Backgroundsb-jet vetopTτ-jet > 100 GeVpTmiss > 100 GeV∆φ > 1.0 radmT (GeV)Events / 12 GeVmH+=250 GeVtanβ=40mH+=500 GeVtanβ=50Figure 3. Transverse mass of the Missing ET andthe jet identified as a tau. The signal from H ! shows two possible masses. The backgroundis also shown. From [10].6. TAUS IN SUPERSYMMETRYEVENTSIf supersymmetry (SUSY) is relevant to theproblems of electroweak symmetry breaking,SUSY particles will be sufficiently light that theLHC will be a SUSY factory. Third genera-tion sparticles are particularly interesting as theycarry information about how supersymmetry isbroken. In the case of the first two generationsthe mixing between the supersymmetric partnersof the left and right handed charged leptons isexpected to be small. However the two ˜ masseigenstates are mixtures of ˜L and ˜R and we4need to measure masses and mixings. The directproduction of ˜ via electroweak processes suchas qq ! +− is small. In many cases manymore ˜ ’s are produced from decays of stronglyproduced squarks and gluinos via processes suchas q˜ ! ˜2q ! q˜+− ! q˜01+− where ˜2 and˜01 are weak gauginos and the latter is stable andexits the detector contributing to missing ET . Itis possible that taus may be the only leptons pro-duced in gaugino decay.Leptonic tau decays are of limited use as theorigin of the lepton is unknown; one must searchfor hadronic tau decays that give rise to charac-teristic jets. SUSY events are expected to be verycomplex and one can rely on jet and /ET cuts toget rid of standard model background and ob-tain clean SUSY sample. This sample can thenbe studied in detail. The background to hadronictau decays then arises from QCD jets in the SUSYevent and a rejection factor of order 10 (the num-ber of jets in a such an event) rather than order100 which is needed in the analysis described insection 4.A particular model is used in the example il-lustrated here. ([1] vol II p. and [11]). Eventsare selected as follows:  4 jets, one of whichhas pt > 100 GeV and the rest have pt > 50GeV, there are no isolated electrons or muonswith pt > 10 GeV and /ET > 100 GeV. The jetsare examined and “tau” candidates extracted byselecting jets with low track multiplicity and in-variant mass [12] using an algorithm based on astudy using Z decays to generate taus in the ap-propriate kinematic regime. The invariant massof observed “tau” pairs is then shown in Figure 4.A clear peak can be seen above the backgroundwhich is dominated by events where one of thetau jet candidates is a misidentified QCD jet. Ifall the tau energy had been detected, the distri-bution would have had a sharp cut-off atMmaxττ = Mχ˜02√√√√1− M2τ˜1M2χ˜02√√√√1− M2χ˜01M2τ˜1which evaluates to 61 GeV in this example. Thepeak is below this due to the energy carried of byneutrinos in the tau decays.05001000Events/2.4 GeV/10 fb-125 50 75 100Mττ (GeV)0Figure 4. Invariant mass of the two tau candi-dates in SUSY events; the solid histogram showsthe signal while the dashed shows the backgroundfrom the combination of real tau and misiden-tified QCD jet, the small solid shows the back-ground from QCD jets were both are misidenti-fied. [1], Figure 20-42.The situation is rather more complicated thanthis analysis shows. The energy distribution ofthe visible tau decay products depends on thetau polarization and the dependence is largest inthe  !  final state and smallest in the stateswith large hadron multiplicity. A separation ofthese states could therefore yield information onthe polarization of the taus. This in turn gives in-formation on the ˜ composition. Simulation workon this difficult topic is underway.REFERENCES1. ATLAS Collaboration, “ATLAS Physics andDetector Performance Technical Design Re-port,” LHCC 99-14/15.2. D. Cavalli and B. Osculati, Atlas physics noteATL-PHYS-2000-014 (2000)3. K. Kodama et al. [Fermilab E653 Collabora-tion], Phys. Lett. B 303, 359 (1993).4. J. Ellis, M. E. Gomez, G. K. Leontaris, S. Lolaand D. V. Nanopoulos, hep-ph/9911459.5. J. L. Feng, Y. Nir and Y. Shadmi, Phys. Rev.5D61, 113005 (2000).6. J. Hisano, T. Moroi, K. Tobe and M. Yam-aguchi, Phys. Rev. D53, 2442 (1996) [hep-ph/9510309].7. L. Serin and R. Stroynowski, Atlas physicsnote, ATL-PHYS-97-114 (1997).8. A. Djouadi et al., arXiv:hep-ph/0002258.9. D. Cavalli and S. Resconi, Atlas physics note,ATL-PHYS-2000-005 (2000)10. K. A. Assamagan, Y. Coadou and A. De-andrea, Eur. Phys. J. directC 9, 1 (2002)[arXiv:hep-ph/0203121].11. I. Hinchliffe and F. E. Paige, Phys. Rev. D61, 095011 (2000) [arXiv:hep-ph/9907519].12. Y. Coadou et al., ATLAS Internal Note ATL-PHYS-98-126.

Use of Taus in ATLAS

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