Measurement of τ Polarization in W → τν Decays with the ATLAS Detector inpp Collisions at

√s = 7 TeV

Zofia Czyczula on behalf of the ATLAS collaboration.

Yale University, New Haven, USA

Abstract

In this proceedings, the first measurement of τ polarization at a hadron collider is presented. The method forextracting the τ polarization is independent of the mode of τ production and can be applied to the characterization ofnew phenomena at the Large Hadron Collider. The method has been applied to W → τν decays and the measuredvalue of Pτ = −1.06 ± 0.04 (stat) +0.05

−0.07 (syst) is in agreement with the Standard Model prediction.

Keywords: LHC, ATLAS, tau, polarization.

1. Introduction

The τ lepton, as the most massive lepton and a thirdgeneration particle, provides an excellent signature toprobe new physics at the Large Hadron Collider (LHC).In particular, the measurement of the branching frac-tion of the Standard Model (SM) Higgs boson candi-date [1] to τ pairs will provide constraints to the natureof electroweak symmetry breaking. Furthermore, de-cay chains with τ leptons are discovery channels, forexample in the case of a supersymmetric charged Higgsboson [2] or a Beyond the Standard Model (BSM) ex-tra gauge boson that couples preferentially to third gen-eration fermions [4]. Due to the short-enough lifetimeof τ leptons and their parity-violating decays, τ leptonsare the only leptons that offer the potential for measure-ments of the polarization and parity of decaying heavierobjects [3] at the LHC. Hence, in the event of a discov-ery of a new particle BSM, they are uniquely suited forinvestigating the origin of the new resonance.

The τ polarization, Pτ, is a measure of the asymme-try of the cross-section for left-handed and right-handedτ production, defined by

Pτ =σR − σL

σR + σL(1)

for the production of τ−. In W → τν decays, the W− is

expected to couple exclusively to a left-handed τ− cor-responding to a τ polarization of Pτ = −1. By con-vention, the sign of the polarization follows the helicityin the anti-particle case, not the handedness. A parity-conserving decay results in a value of Pτ = 0. This is thecase for the decay of a SM scalar Higgs boson to τ lep-ton pairs. On the other hand, a supersymmetric chargedscalar Higgs boson decaying via H → τν is expectedto produce Pτ = +1. Tau polarization may thereforebe used as a discriminating variable in searches for thisparticle. For hypothetical extra gauge bosons postulatedin theories BSM, the value of tau polarization dependson the extension of the SM. If an extra gauge boson isdiscovered at the LHC, measurement of Pτ will provideadditional information on its couplings and in some sce-narios it will allow stringent constraints of new theories,such as the mixing angle α in the E6 model [5].

The method outlined in this proceedings for extract-ing the τ polarization is independent of the mode ofτ production and can be applied to the characterizationof new phenomena at the LHC.

2. Measurement of tau polarization

The description of the measurement follows closelythe original publication in [6].

Available online at www.sciencedirect.com

Nuclear Physics B (Proc. Suppl.) 253–255 (2014) 180–183

0920-5632/© 2014 Elsevier B.V. All rights reserved.

www.elsevier.com/locate/npbps

http://dx.doi.org/10.1016/j.nuclphysbps.2014.09.044

2.1. Data and Simulation SamplesThe data for the measurement were collected during

2010 operation, when the LHC provided proton-protoncollisions with a center of mass energy of 7 TeV. Theintegrated luminosity of the data sample used for themeasurement is 24 pb−1. The sample was recordedin the ATLAS detector [7] using a trigger designed tocollect events with a hadronically decaying τ leptonwith transverse momentum of the visible decay prod-ucts pT > 16 GeV along with missing transverse mo-mentum Emiss

T > 22 GeV [8]. Two W → τν MonteCarlo (MC) simulation samples were produced withτ leptons decaying hadronically as left-handed τ lep-tons in one sample and as right-handed τ leptons in theother. The following MC samples were prepared to sim-ulate the electroweak processes that may contribute tothe background: leptonic W boson decays to muons andelectrons, W → τν decays in which the τ decays lep-tonically, Z boson decays to lepton pairs, and top-quarkpair (tt̄) production. All details related to the simulatedsamples can be found in [6].

2.2. Sample compositionThe main selection requirements were comprised of

an identified single-track τ candidate with pT between20 GeV and 60 GeV and |η| < 2.5 together with Emiss

T >30 GeV. To further suppress the multijet backgrounds, aseparation of |Δφ(jet, Emiss

T )| > 0.5 radians was requiredfor jets with pT > 20 GeV. Multijet events were effec-tively suppressed with the requirement that S Emiss

T> 6,

with S EmissT

defined as:

S EmissT

=Emiss

T

σ(EmissT )

=Emiss

T

0.5√

GeV√∑

ET. (2)

Finally, events with an identified electron or muon withpT > 15 GeV were rejected to suppress the backgroundcontributions from leptonic decays of W and Z bosons.

The electroweak backgrounds were estimated by ap-plying the event selection to fully simulated samples ofthe relevant background processes and normalizing thecontributions to the integrated luminosity of the datasample with the corresponding cross-sections measuredby ATLAS [9, 10]. The multijet background was esti-mated from data through a comparison of the numberof events passing the full selection in four statisticallyindependent regions (three control regions and the sig-nal region), and was corrected for contamination fromelectroweak events in the control regions which rangesfrom approximately 1% to 35%. The expected numbersof signal and background events passing the event selec-tion are summarized in Table 1, along with the number

Table 1: Number of events passing the full event selection for data,the Monte Carlo electroweak background estimate, the Monte CarloW → τν signal for left-handed and right-handed polarization, andthe multijet background estimated from data. The errors denote thestatistical uncertainty from the Monte Carlo samples.

Sample Number of Events

Data 1136Electroweak Background 138 ± 4

Left-Handed SignalW → τLν 1002 ± 16Multijet Background 69 ± 6

Right-Handed SignalW → τRν 1523 ± 22Multijet Background 79 ± 4

of selected events in data. The overall multijet back-ground normalization depends on the τ polarization, dueto signal contamination in the control regions.

2.3. Tau Polarization ObservablesThe angle θ between the τ direction of flight and the

visible decay products in the τ rest frame is one of themain observables sensitive to τ polarization [11, 12]. Inthe relativistic limit, E � mτ, the angle θ is related tothe ratio of the energy of the hadronic decay products tothe τ energy in the laboratory frame. The reconstructionof the τ energy in W → τν decays, however, is exper-imentally not possible due to the multiple unobservedneutrinos in the final state. For τ lepton decays via anintermediate ρ meson, an additional observable cosψ isdefined in the ρ rest frame, where ψ is the angle betweenthe flight direction of the ρ meson and the charged pion.This observable is related to the kinematics of the finalstate charged and neutral pions, which are experimen-tally accessible, as follows:

cosψ =mρ√

m2ρ − 4m2

π

Eπ− − Eπ0

|pπ− + pπ0 | , (3)

where the particle energies and momenta are measuredin the laboratory frame. In τ → ρν decays, to conserveangular momentum, transversely polarized ρ mesonsare favored in left-handed τ decays while longitudinallypolarized ρ mesons are favored in right-handed τ de-cays. The transversely polarized ρ decays to chargedand neutral pions with comparable energies while thelongitudinal ρ results in an asymmetry in the energysharing.

From Equation (3), an observable, referred to as the“charged asymmetry,” is derived. Experimentally, the

Z. Czyczula / Nuclear Physics B (Proc. Suppl.) 253–255 (2014) 180–183 181

energy associated with the charged pion is given bythe transverse momentum of the single track associatedwith the τ candidate. The energy ascribed to the neutralpion(s) is calculated as the difference between the τ lep-ton pT measured in the calorimeter and the track pT ofthe τ candidate. The charged asymmetry Υ is calculatedas follows:

Eπ−T − Eπ0

T

pT≈ 2

ptrkT

pT− 1 = Υ. (4)

Although the observable in Equation (3) is definedfor τ → ρν decays with branching fraction of about25% [13], in this analysis the charged asymmetry ismeasured in all of the decay modes to a single chargedmeson inclusively, which comprise roughly 50% of allτ decays. After the event selection, the τ decay modeswith a single final state neutral pion account for 60%and of the left-handed and 53% of the right-handed sim-ulated samples. Figure 1 shows the distribution of thecharged asymmetry observable in the left-handed andright-handed Monte Carlo signal samples plus the esti-mated electroweak and multijet backgrounds along withthe observed distribution in data. These distributions il-lustrate the power of the charged asymmetry observablein distinguishing between left-handed and right-handedτ leptons.

The analyzing power [14] of the charged asymmetryΥ in a measurement of τ polarization is evaluated af-ter reconstruction and the full event selection. Table 2summarizes the results for assumed τ polarization val-ues of −1, 0, and +1, with calculations at the generatorlevel included for reference. The event selection im-proves the sensitivity of Υ by suppressing events fromτ → πν decays, which exhibit softer spectra of bothEmiss

T and τ lepton pT than those of the τ → ρν de-cays. This suppression is enhanced for left-handed τleptons from τ → πν decays, which account for 7%of the simulated signal compared to 27% in the simu-lated right-handed signal, after the full event selection.Detector effects lead to a loss in sensitivity of approxi-mately 20%.

2.4. Extracting the τ Polarization

The τ polarization is measured by fitting the observedcharged asymmetry distribution of single-track hadron-ically decaying τ candidates satisfying the full W → τνevent selection to a linear combination of templatesprepared with the left-handed and right-handed MonteCarlo samples. The right-handed (left-handed) templateconsists of the charged asymmetry distribution in the

ϒ

-1 -0.5 0 0.5 1 1.5 2 2.5 3

Entri

es/0

.1

0

20

40

60

80

100

Data 2010 νLτ →W

MultijetsEW

ATLAS

= 7 TeVs-1 L dt = 24 pb∫

(a) Left-handed

ϒ

-1 -0.5 0 0.5 1 1.5 2 2.5 3

Entri

es/0

.1

0

20

40

60

80

100

120

140

Data 2010 νRτ →W

MultijetsEW

ATLAS

= 7 TeVs-1 L dt = 24 pb∫

(b) Right-handed

Figure 1: Charged asymmetry distributions. The combined statisticaland energy scale systematic uncertainties are overlaid on the stackedsignal and background distributions [6].

right-handed (left-handed) signal in addition to the es-timated contributions from the electroweak and multi-jet backgrounds. The electroweak background distri-butions are effectively independent of the τ polariza-tion in W → τν decays and are therefore commonto both templates. Since the estimated multijet back-ground depends on the τ polarization, rather than fixingthe multijet normalization in the templates, the normal-ization is included as a parameter in the fit. The τ po-larization, Pτ, is extracted from the fit by maximizinga binned log-likelihood function. Figure 2 shows theleft-handed and right-handed templates plotted togetherwith the observed charged asymmetry distribution indata along with the resulting fit. The fitted value of theτ polarization and its associated statistical uncertainty isPτ = −1.06 ± 0.04(stat).

Systematic uncertainties are evaluated for their ef-fect on the shape of the Υ distribution, as well as forany changes in the relative acceptance of the signal andbackground events. The dominant source of systematic

Z. Czyczula / Nuclear Physics B (Proc. Suppl.) 253–255 (2014) 180–183182

Table 2: Analyzing power of the charged asymmetry observable at various stages in the simulation process. Pτ denotes the assumed polarization.Symbols: pτ,vis

T and |ητ,vis | denote truth level visible momentum and pseudorapidity of the tau while (∑

pν)T denotes the sum of transverse momentaof the neutrinos.

Stage of Simulation Pτ = –1 Pτ = 0 Pτ = +1

Generator Level, No Selection 0.32 0.25 0.26Generator Level, pτ,vis

T > 20 GeV, |ητ,vis| < 2.5, (∑

pν)T > 30 GeV 0.57 0.45 0.53Reconstruction and Full Event Selection 0.46 0.37 0.40

ϒ

-1 -0.5 0 0.5 1 1.5 2 2.5 3

Entri

es/0

.1

0

20

40

60

80

100

Data 2010 FitLeft-handedRight-handed

ATLAS

= 7 TeVs-1 L dt = 24 pb∫

Figure 2: Simulated signal and background templates for left-handedand right-handed τ decays along with the observed charged asymme-try distribution in data. The best fit resulting from maximizing thelikelihood is plotted in bold [6].

uncertainty arises from the calibration of energy scalesused to make measurements of the τ candidate and clus-ter pT, and the event Emiss

T and S EmissT

and accounts for+0.04−0.06. The other sources of systematics are: the resolu-tion of Emiss

T components, the identification and recon-struction efficiencies for τ candidates, the efficiency ofthe combined τ and Emiss

T trigger as well as the uncer-tainty due to Monte Carlo modeling of the signal andnormalization of the signal and backgrounds.

The result of the τ polarization measurement in theselected sample of W → τν decays is:

Pτ = −1.06 ± 0.04 (stat) +0.05−0.07 (syst).

Pτ is found to lie within the 95% credibility interval [–1,–0.91].

3. Summary and discussion

The τ polarization has been measured in the single-track hadronic decays of τ leptons in W → τν eventswith data collected in 2010 with the ATLAS detectorcorresponding to an integrated luminosity of 24 pb−1.The measured value is consistent with the Standard

Model prediction within the statistical and systematicuncertainties. This result marks the first measurementof τ polarization in a hadron collider and the first suchmeasurement with τ leptons in W boson decays.

The developed method relies upon a single polariza-tion observable, cosψ, sensitive to the helicity of an in-termediate ρ meson in τ → ρν decays. The analyzingpower could therefore be enhanced by selecting eventswith one charged and one neutral pion in the final state.This is especially important if the measurement is car-ried in a sample of a heavy resonance decaying to τ-pair, where the event selection (Emiss

T and τ lepton pTrequirements) provides smaller suppression of τ → πνdecays. Furthermore for the τ-pair final state, the massof the parent resonance provides enough constraints toreconstruct the τ energy in the laboratory frame [15]. Inthis case, the primary observable cos θ is experimentallyaccessible and full analyzing power can be achieved us-ing “the optimal method” described in [14]. An exten-sion of the method to the three-track hadronic τ decaysis straightforward with the definition of the angle cosψprovided in [12].

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