This Letter presents direct searches for lepton flavour violation in Higgs boson decays, H→eτand H→μτ, performed with the ATLAS detector at the LHC. The searches are based on a data sample of proton–proton collisions at a centre-of-mass energy √s=13 TeV, corresponding to an integrated luminosity of 36.1fb−1. No significant excess is observed above the expected background from Standard Model processes. The observed (median expected) 95% confidence-level upper limits on the lepton-flavour-violating branching ratios are 0.47% (0.34+0.13−0.10%) and 0.28% (0.37+0.14−0.10%) for H→eτand H→μτ, respectively.©2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.1. IntroductionThe search for processes beyond the Standard Model (SM) is one of the main goals of the Large Hadron Collider (LHC) pro-gramme at CERN. A possible sign of such processes is lepton flavour violation (LFV) in decays of the Higgs boson [1,2]. Many beyond-SM theories predict LFV decays of the Higgs boson, such as supersymmetry [3,4], other models with more than one Higgs doublet [5,6], composite Higgs models [7], models with flavour symmetries [8]or warped extra dimensions [9–11]models and others [12, 13].In this Letter, searches for LFV decays of the Higgs boson, H→eτand H→μτ, at the LHC with the ATLAS experiment are presented. Studies are based on proton–proton (pp) collision data recorded in 2015–2016 at a centre-of-mass energy √s=13 TeV. The dataset corresponds to an integrated luminosity of 36.1fb−1.Previous ATLAS searches [14, 15]placed an upper limit of 1.04% (1.43%) on the H→eτ(H→μτ) branching ratio (B) with a 95% confidence level (CL) using Run 1 data collected at √s=8TeV, cor-responding to an integrated luminosity of 20.3fb−1. The CMS Col-laboration recently provided 95% CL upper limits on these branch-ing ratios of 0.61% and 0.25%, respectively, using data collected at √s=13 TeV, with an integrated luminosity of 35.9fb−1[16].The searches presented here involve both leptonic (τ→′ν ̄ν1) and hadronic (τ→hadrons+ν) decays of τ-leptons, denoted τ′and τhadrespectively. The dilepton final state τ′only consid-ers pairs of different-flavour leptons. Same-flavour lepton pairs are E-mail address:atlas .publications @cern .ch .1Unless explicitly mentioned otherwise, leptons (denoted by or ′) refer to electrons or muons.rejected due to the large lepton pair-production Drell-Yan back-ground. Two channels are considered for each of the two searches: eτμand eτhadfor the H→eτsearch, μτeand μτhadfor the H→μτsearch. The analysis is designed such that any potential LFV signal overlap between the H→eτand H→μτsearches is negligible. Many methods are reused from the measurement of the Higgs boson cross-section in the H→ττfinal state [17].The ATLAS detector2is described in Refs. [18– 20]. It con-sists of an inner tracking detector covering the range |η| <2.5, surrounded by a superconducting solenoid providing a 2 Tax-ial magnetic field, high-granularity electromagnetic (|η| <3.2) and hadronic calorimeters (|η| <4.9), and a muon spectrometer (MS) which covers the range |η| <2.7 and includes fast trigger cham-bers (|η| <2.4) and superconducting toroidal magnets.2. Simulation samplesSamples of Monte Carlo (MC) simulated events are used to op-timize the event selection, and to model the signal and several of the background processes. The samples were produced with the ATLAS simulation infrastructure [21]using the full detector simu-lation performed by theGeant4[22] toolkit. The Higgs boson mass was set to mH=125 GeV [23]. The four leading Higgs boson pro-duction mechanisms are considered: the gluon–gluon fusion (ggF), vector-boson fusion (VBF) and two associated production modes 2ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point in the centre of the detector and the z-axis along the beam pipe. The azimuthal angle φruns around the beam pipe, the pseudorapidity is defined in terms of the polar angle θas η≡− lntan(θ/2). Angular distance in the η–φspace is defined as R ≡√( η)2+( φ)2

Aad G., A.B. (2020). Searches for lepton-flavour-violating decays of the Higgs boson in √s = 13 TeV pp collisions with the ATLAS detector. PHYSICS LETTERS. SECTION B, 800, 1-23 [10.1016/j.physletb.2019.135069].

Searches for lepton-flavour-violating decays of the Higgs boson in √s = 13 TeV pp collisions with the ATLAS detector

Alberghi G. L.;Bellagamba L.;Bindi M.;Boscherini D.;Cabras G.;Caforio D.;De Castro S.;Di Sipio R.;Fabbri L.;Franchini M.;Gabrielli A.;Giacobbe B.;Lasagni Manghi F.;Macchiolo A.;Massa L.;Monzani S.;Polini A.;Rinaldi L.;Romano M.;Sbarra C.;Sbrizzi A.;Semprini-Cesari N.;Sioli M.;Todome K.;Ucchielli G.;Valentinetti S.;Villa M.;Vittori C.;Vivarelli I.;Zoccoli A.;
2020

Abstract

This Letter presents direct searches for lepton flavour violation in Higgs boson decays, H→eτand H→μτ, performed with the ATLAS detector at the LHC. The searches are based on a data sample of proton–proton collisions at a centre-of-mass energy √s=13 TeV, corresponding to an integrated luminosity of 36.1fb−1. No significant excess is observed above the expected background from Standard Model processes. The observed (median expected) 95% confidence-level upper limits on the lepton-flavour-violating branching ratios are 0.47% (0.34+0.13−0.10%) and 0.28% (0.37+0.14−0.10%) for H→eτand H→μτ, respectively.©2019 The Author(s). Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). Funded by SCOAP3.1. IntroductionThe search for processes beyond the Standard Model (SM) is one of the main goals of the Large Hadron Collider (LHC) pro-gramme at CERN. A possible sign of such processes is lepton flavour violation (LFV) in decays of the Higgs boson [1,2]. Many beyond-SM theories predict LFV decays of the Higgs boson, such as supersymmetry [3,4], other models with more than one Higgs doublet [5,6], composite Higgs models [7], models with flavour symmetries [8]or warped extra dimensions [9–11]models and others [12, 13].In this Letter, searches for LFV decays of the Higgs boson, H→eτand H→μτ, at the LHC with the ATLAS experiment are presented. Studies are based on proton–proton (pp) collision data recorded in 2015–2016 at a centre-of-mass energy √s=13 TeV. The dataset corresponds to an integrated luminosity of 36.1fb−1.Previous ATLAS searches [14, 15]placed an upper limit of 1.04% (1.43%) on the H→eτ(H→μτ) branching ratio (B) with a 95% confidence level (CL) using Run 1 data collected at √s=8TeV, cor-responding to an integrated luminosity of 20.3fb−1. The CMS Col-laboration recently provided 95% CL upper limits on these branch-ing ratios of 0.61% and 0.25%, respectively, using data collected at √s=13 TeV, with an integrated luminosity of 35.9fb−1[16].The searches presented here involve both leptonic (τ→′ν ̄ν1) and hadronic (τ→hadrons+ν) decays of τ-leptons, denoted τ′and τhadrespectively. The dilepton final state τ′only consid-ers pairs of different-flavour leptons. Same-flavour lepton pairs are E-mail address:atlas .publications @cern .ch .1Unless explicitly mentioned otherwise, leptons (denoted by or ′) refer to electrons or muons.rejected due to the large lepton pair-production Drell-Yan back-ground. Two channels are considered for each of the two searches: eτμand eτhadfor the H→eτsearch, μτeand μτhadfor the H→μτsearch. The analysis is designed such that any potential LFV signal overlap between the H→eτand H→μτsearches is negligible. Many methods are reused from the measurement of the Higgs boson cross-section in the H→ττfinal state [17].The ATLAS detector2is described in Refs. [18– 20]. It con-sists of an inner tracking detector covering the range |η| <2.5, surrounded by a superconducting solenoid providing a 2 Tax-ial magnetic field, high-granularity electromagnetic (|η| <3.2) and hadronic calorimeters (|η| <4.9), and a muon spectrometer (MS) which covers the range |η| <2.7 and includes fast trigger cham-bers (|η| <2.4) and superconducting toroidal magnets.2. Simulation samplesSamples of Monte Carlo (MC) simulated events are used to op-timize the event selection, and to model the signal and several of the background processes. The samples were produced with the ATLAS simulation infrastructure [21]using the full detector simu-lation performed by theGeant4[22] toolkit. The Higgs boson mass was set to mH=125 GeV [23]. The four leading Higgs boson pro-duction mechanisms are considered: the gluon–gluon fusion (ggF), vector-boson fusion (VBF) and two associated production modes 2ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point in the centre of the detector and the z-axis along the beam pipe. The azimuthal angle φruns around the beam pipe, the pseudorapidity is defined in terms of the polar angle θas η≡− lntan(θ/2). Angular distance in the η–φspace is defined as R ≡√( η)2+( φ)2
2020
Aad G., A.B. (2020). Searches for lepton-flavour-violating decays of the Higgs boson in √s = 13 TeV pp collisions with the ATLAS detector. PHYSICS LETTERS. SECTION B, 800, 1-23 [10.1016/j.physletb.2019.135069].
Aad G., Abbott B., Abbott D.C., Abed Abud A., Abeling K., Abhayasinghe D.K., Abidi S.H., Abouzeid O.S., Abraham N.L., Abramowicz H., Abreu H., Abulait...espandi
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