Identification and energy calibration of hadronically decaying tau leptons with the ATLAS experiment in pp collisions at [Formula: see text][Formula: see text]

Abstract

This paper describes the trigger and offline reconstruction, identification and energy calibration algorithms for hadronic decays of tau leptons employed for the data collected from pp collisions in 2012 with the ATLAS detector at the LHC center-of-mass energy [Formula: see text] [Formula: see text]. The performance of these algorithms is measured in most cases with [Formula: see text] decays to tau leptons using the full 2012 dataset, corresponding to an integrated luminosity of 20.3 fb[Formula: see text]. An uncertainty on the offline reconstructed tau energy scale of 2-4 %, depending on transverse energy and pseudorapidity, is achieved using two independent methods. The offline tau identification efficiency is measured with a precision of 2.5 % for hadronically decaying tau leptons with one associated track, and of 4 % for the case of three associated tracks, inclusive in pseudorapidity and for a visible transverse energy greater than 20 [Formula: see text]. For hadronic tau lepton decays selected by offline algorithms, the tau trigger identification efficiency is measured with a precision of 2-8 %, depending on the transverse energy. The performance of the tau algorithms, both offline and at the trigger level, is found to be stable with respect to the number of concurrent proton-proton interactions and has supported a variety of physics results using hadronically decaying tau leptons at ATLAS.

Fig. 1

Tau trigger rates at a Level 1 and b Event Filter as a function of the instantaneous luminosity for $\sqrt{s}=8$ $\text{TeV}$. The triggers shown are described in Table 1, with the ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$+${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$being the rate for the lowest threshold trigger reported in the table. The rates for the higher threshold triggers are approximately three and five times lower at L1 and HLT, respectively, and are partially included in the rate of the lowest threshold item

Fig. 2

Signal and background distribution for the 1-track ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$decay offline tau identification variables a $f_{\mathrm{cent}}$ and b $N_{\mathrm{track}}^{\mathrm{iso}}$. For signal distributions, 1-track ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$decays are matched to true generator-level ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$in simulated events, while the multi-jet events are obtained from the data

Fig. 3

Signal and background distribution for the 3-track ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$decay offline tau identification variables a $R_{\mathrm{track}}$ and b $m_{{\mathit{\pi }}^0+\mathrm{track}}$. For signal distributions, 3-track ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$decays are matched to true generator-level ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$in simulated events, while the multi-jet events are obtained from data

Fig. 4

Offline tau identification efficiency dependence on the number of reconstructed interaction vertices, for a 1-track and b 3-track ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$decays matched to true ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$(with corresponding number of charged decay products) from SM and exotic processes in simulated data. Three working points, corresponding to different tau identification efficiency values, are shown

Fig. 5

Inverse background efficiency versus signal efficiency for the offline tau identification, for a a low-$p_{\mathrm{T}}$ and b a high-$p_{\mathrm{T}}$ ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$range. Simulation samples for signal include a mixture of Z, W and $Z^{\prime }$ production processes, while data from multi-jet events is used for background. The red markers correspond to the three working points mentioned in the text. The signal efficiency shown corresponds to the total efficiency of ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$decays to be reconstructed as 1-track or 3-track and pass tau identification selection

Fig. 6

Distribution of $\mathrm{\Delta }{z}_0$ for the tau trigger at L2 in simulated $Z\to \mathit{\tau }\mathit{\tau }$ events with an average of eight interactions per bunch crossing. The wide Gaussian distribution corresponds to pile-up tracks while the central peak, displayed in the upper-right corner, corresponds to the main interaction tracks. A Breit–Wigner function is fitted to the central peak and 68 % of the signal events are found within a distance $\mathit{\sigma }$ = 0.32 mm from the peak

Fig. 7

Inverse background efficiency versus signal efficiency for the tau trigger at the EF level, for ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$candidates which have satisfied the L1 requirements. The signal efficiency is defined with respect to offline medium tau identification ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$candidates matched at generator level, and the inverse background efficiency is calculated in a multi-jet sample

Fig. 8

Signal and background distribution for two of the electron veto variables, a $f_{\mathrm{HT}}$ and b $f_{\mathrm{EM}}$. Candidate 1-track ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$decays are required to not overlap with a reconstructed electron candidate which passes tight electron identification [23]. For signal distributions, 1-track ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$decays are matched to true generator-level ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$in simulated $Z\to \mathit{\tau }\mathit{\tau }$ events, while the electron contribution is obtained from simulated $Z\to ee$ events where 1-track ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$decays are matched to true generator-level electrons

Fig. 9

Electron veto inverse background efficiency versus signal efficiency in simulated samples, for 1-track ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$candidates. The background efficiency is determined using simulated $Z\to ee$ events

Fig. 10

Template fit result in the muon channel, inclusive in $\mathit{\eta }$ and $p_{\mathrm{T}}$ for $p_{\mathrm{T}}>20$  $\text{GeV}$ for the offline ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$candidates a before the requirement of tau identification, and b fulfilling the medium tau identification requirement

Fig. 11

Correction factors needed to bring the offline tau identification efficiency in simulation to the level observed in data, for all tau identification working points as a function of $\mathit{\eta }$. The combinations of the muon and electron channels are also shown, and the results are displayed separately for a 1-track and b 3-track ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$candidates with $p_{\mathrm{T}}>20$ $\text{GeV}$. The combined systematic and statistical uncertainties are shown

Fig. 12

The tau trigger efficiency for ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$candidates identified by the offline medium tau identification, as a function of a the offline ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$transverse energy and b the number of primary vertices. The error bars correspond to the statistical uncertainty in the efficiency

Fig. 13

The measured tau trigger efficiency in data and simulation, for the offline ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$candidates passing the medium tau identification, as a function of offline ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$transverse energy. The expected background contribution has been subtracted from the data. The uncertainty band on the ratio reflects the statistical uncertainties associated with data and simulation and the systematic uncertainty associated with the background subtraction in data

Fig. 14

a Visible mass of electron–positron pairs for the offline electron veto efficiency measurement, after tag-and-probe selection, where the probe lepton passes medium tau identification and does not overlap with loose electrons, before the electron veto is applied. b $\mathit{\eta }$ distribution for ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$candidates (electrons misidentified as hadronic tau decays) after applying a loose electron veto. Uncertainties shown are only statistical

Fig. 15

Offline ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$energy response curves as a function of the reconstructed ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$energy $E_{\mathrm{LC}}^{\mathit{\tau }}$ for hadronic tau decays with a one and b more than one associated tracks. One curve per pseudorapidity region $|{\mathit{\eta }}^{\mathrm{LC}}|$ is shown. The region where markers are shown corresponds approximately to a transverse energy $E_{\mathrm{T},\mathrm{LC}}^{\mathit{\tau }}>15$ $\text{GeV}$. For very low and very high energies, the response curves are assumed to be constant. Uncertainties are statistical only

Fig. 16

Offline energy resolution for hadronically decaying tau leptons, separately for a one and b three associated tracks and for different pseudorapidity regions. The resolution shown is the standard deviation of a Gaussian function fit to the distribution of $(E_{\mathrm{reco}}-E_{\mathrm{vis}}^{\mathrm{true}})/E_{\mathrm{vis}}^{\mathrm{true}}$ in a given range of $E_{\mathrm{vis}}^{\mathrm{true}}$ and $|{\mathit{\eta }}_{\mathrm{vis}}^{\mathrm{true}}|$

Fig. 17

The $m_{\mathrm{vis}}$ distribution used for the in-situ offline TES measurement. Shown is the comparison between data and simulation for ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$with a one or b three associated tracks

Fig. 18

The measured tau trigger transverse energy resolution for the offline ${\mathit{\tau }}_{\mathrm{had}\text{-}\mathrm{vis}}$candidates passing medium tau identification at a L1, b L2 and c the EF. The grey hashed area reflects the statistical uncertainties on the sum of the expected signal and background