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A search for high-mass resonances decaying to $\tau^{+}\tau^{-}$ in $pp$ collisions at $\sqrt{s}=8$ TeV with the ATLAS detector

The ATLAS collaboration
JHEP 1507 (2015) 157, 2015

Abstract (data abstract)
CERN-LHC. A search for high-mass resonances decaying into $\tau^{+}\tau^{-}$ final states using proton-proton collisions at $\sqrt{s}= 8$ TeV produced by the Large Hadron Collider is presented. The data were recorded with the ATLAS detector and correspond to an integrated luminosity of 19.5-20.3 fb$^{-1}$. No statistically significant excess above the Standard Model expectation is observed; 95% credibility upper limits are set on the cross section times branching fraction of $Z^{\prime}$ resonances decaying into $\tau^+\tau^-$ pairs as a function of the resonance mass. As a result, $Z^{\prime}$ bosons of the Sequential Standard Model with masses less than 2.02 TeV are excluded at 95% credibility. The impact of the fermionic couplings on the $Z^{\prime}$ acceptance is investigated and limits are also placed on a $Z^{\prime}$ model that exhibits enhanced couplings to third-generation fermions All events meet the following requirements: - At least one reconstructed primary vertex with at least 3 associated tracks having pT > 0.4 GeV. - For data, events occurred during periods of stable beam with all relevant detector sub-systems operational. Events are triggered by one of the following trigger selections: - A trigger requiring the presence of at least one energy deposit in the calorimeter with pT > 125 GeV that has 1-3 associated tracks and passes strict hadronic tau identification criteria based on tracking and calorimetric information (had-had channel). - A trigger requiring the presence of at least one energy deposit in the electromagnetic calorimeter either with pT > 24 GeV and tight isolation or with pT > 60 GeV without isolation. The shower profiles are required to be consistent with those expected for electromagnetic showers (lep-had). - A trigger requiring the presence of at least one isolated muon with pT > 24 GeV or at least one muon with pT > 36 GeV (lep-had). The had-had channel is defined as: - No electrons with pT > 15 GeV in the pseudorapidity range [0,1.37] or [1.52,2.47] - No muons with pT > 10 GeV in the pseudorapidity range [0,2.5] - At least 2 hadronic tau decays with visible pT > 50 GeV - The "leading" hadronic tau decay has visible pT > 150 GeV - Hadronic tau decays are in the pseudorapidity range [0,1.37] or [1.52,2.47] - Leading 2 hadronic tau decays pass tau identification - Leading 2 hadronic tau decays have opposite electric charge sign - Angle between the leading 2 hadronic tau decays in the transverse plane > 2.7 radians - Total transverse mass of leading two hadronic tau decays and missing transverse momentum is above a threshold defined for each Z' mass hypothesis The lep-had channel is defined as: - 1 isolated muon OR 1 isolated electron; pT > 30 GeV - Electrons are in the pseudorapidity range [0,1.37] or [1.52,2.47] - Electron track and cluster satisfy criteria requiring shower profiles consistent with those expected - Electrons are considered isolated if the summed pT of tracks in dR<0.3 is less than 5% of the electron pT and the summed energy in dR<0.2 is less than (5 GeV + 0.7% of the electron pT) - Muons are in the pseudorapidity range [0,2.5] - Muons are considered isolated if the summed pT of tracks in dR<0.3 is less than 5% of the muon pT - No additional electrons with pT > 15 GeV - No additional muons with pT > 10 GeV - At least 1 hadronic tau decay with visible pT > 30 GeV - Hadronic tau decays are in the pseudorapidity range [0,1.37] or [1.52,2.47] - Leading hadronic tau decay passes tau identification - Isolated lepton and leading hadronic tau decay have opposite electric charge sign - Angle between isolated lepton and leading hadronic tau decay in the transverse plane > 2.7 radians - Total transverse mass of isolated lepton, leading hadronic tau decay and missing transverse momentum is above a threshold defined for each Z' mass hypothesis The transverse mass is defined as: \begin{equation*} m_{\rm T}(p^{A},p^{B})=\sqrt{2p_{\rm T}^{A}p_{\rm T}^{B} \left ( 1-\cos\Delta\phi(p^{A},p^{B}) \right )} \, , \end{equation*} where $p^{A}$ and $p^{B}$ are two reconstructed physics objects with transverse momenta $p_{\rm T}^{A}$ and $p_{\rm T}^{B}$, respectively, which subtend an angle of $\Delta\phi(p^{A},p^{B})$ in the transverse plane. The total transverse mass is defined as: \begin{equation*} m_{\rm T}^{\rm tot}(\tau_1,\tau_2,E_{\rm T}^{\rm miss}) = \sqrt{m_{\rm T}^2(\tau_1,\tau_2) + m_{\rm T}^2(\tau_1,E_{\rm T}^{\rm miss}) + m_{\rm T}^2(\tau_2,E_{\rm T}^{\rm miss})}\, , \end{equation*} where $\tau_1$ and $\tau_2$ denote the reconstructed visible decay products of the two tau leptons ($e$, $\mu$ or $\tau_{\rm had-vis}$).

  • Table 1

    Data from Figure 4

    10.17182/hepdata.68362.v1/t1

    Signal acceptance times efficiency (ACC*EFF) for Z'L, Z'R, Z'narrow and Z'wide divided by ACC*EFF for Z'SSM as a function of...

  • Table 2

    Data from Figure 5A

    10.17182/hepdata.68362.v1/t2

    Ratio of the Z'NU to Z'SSM cross section times tau+tau- branching fraction (SIG*BR) as a function of sin^2phi and the...

  • Table 3

    Data from Figure 5B

    10.17182/hepdata.68362.v1/t3

    Ratio of the Z'NU to Z'SSM acceptance times efficiency (ACC*EFF) in the had-had channel as a function of sin^2phi and...

  • Table 4

    Data from Figure 5C

    10.17182/hepdata.68362.v1/t4

    Ratio of the Z'NU to Z'SSM acceptance times efficiency (ACC*EFF) in the lep-had channel as a function of sin^2phi and...

  • Table 5

    Data from Figure 7

    10.17182/hepdata.68362.v1/t5

    Z' SSM acceptance (ACC) and product of acceptance and efficiency (ACC*EFF) for the had-had and lep-had selections as a function...

  • Table 6

    Data from Table 4

    10.17182/hepdata.68362.v1/t6

    Number of expected Z'SSM signal (NS), background (NB) and observed (N) events passing the total transverse mass ($m_{\rm T}^{\rm tot}$)...

  • Table 7

    Data from Figure 8A

    10.17182/hepdata.68362.v1/t7

    Median expected and observed 95% CL upper limits on the cross-section times branching fraction to tau+tau- for Z'SSM production for...

  • Table 8

    Data from Figure 8B

    10.17182/hepdata.68362.v1/t8

    Observed 95% CL upper limits on the cross-section times branching fraction to tau+tau- for Z'L and Z'R production for the...

  • Table 9

    Data from Auxiliary Figure 15

    10.17182/hepdata.68362.v1/t9

    Observed 95% CL lower limit on the Z'NU mass as a function of sin^2(phi).

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