Cross-section measurements of the Higgs boson decaying into a pair of τ -leptons in proton-proton collisions at s=13  TeV with the ATLAS detector

Abstract

ggF H→ττ = 3.1 ± 1.0 (stat.) +1.6 −1.3 (syst.) pb, respectively. Similarly, results of a fit are reported in the framework of simplified template cross sections. All measurements are in agreement with Standard Model expectations.The ATLAS and CMS collaborations discovered [1, 2] a particle consistent with the Standard Model (SM) [3][4][5] Higgs boson [6][7][8][9][10] in 2012. Several properties of this particle, such as its coupling strengths, spin and charge-parity (CP) quantum numbers, were studied with 7 and 8 TeV center-of-mass energy ( √ s) proton-proton collision data delivered by the Large Hadron Collider (LHC) in 2011 and 2012, respectively, referred to as 'Run 1'. These results rely predominantly on studies of the bosonic decay modes [11][12][13][14] and have not shown any significant deviations from the SM expectations.The coupling of the Higgs boson to the fermionic sector has been established with the observation of the H → ττ decay mode with a signal significance of 5.5σ from a combination of ATLAS and CMS results [15][16][17] using LHC Run-1 data. A measurement performed by the CMS Collaboration with Run-2 data at √ s = 13 TeV reached a significance of 4.9σ using 35.9 fb −1 of integrated luminosity and 5.9σ combined with data from Run 1 [18]. While the Higgs-boson coupling to other fermions such as top quarks [19, 20] and bottom quarks [21, 22] have been observed, only upper limits exist on its coupling to muons [23, 24] and the H → ττ decay mode has been the only accessible leptonic decay mode. It was also used to constrain CP violation in the production via vector-boson fusion (VBF) [25] and is unique in that it provides sensitivity to CP violation in the Higgs-boson coupling to leptons [26]. This paper presents cross-section times branching-fraction measurements of Higgs bosons that decay into a pair of τ-leptons in proton-proton (pp) collisions at √ s = 13 TeV using data collected by the ATLAS experiment in 2015 and 2016, corresponding to an integrated luminosity of 36.1 fb −1 . All combinations of leptonic (τ → νν with = e, µ) and hadronic (τ → hadrons ν) τ decays are considered.1 The corresponding three analysis channels are denoted by τ lep τ lep , τ lep τ had and τ had τ had and are composed of different dominant backgrounds. While Z → ττ is a dominant background in all channels, the relative contributions from other backgrounds from top-quark and other vector-boson decays, as well as from misidentified leptonic or hadronic τ decays, vary considerably between the channels. Two analysis categories are defined that are predominantly sensitive to Higgs bosons produced via VBF and gluon-gluon fusion (ggF). A maximum-likelihood fit is performed on data using distributions of the reconstructed di-τ mass in signal regions (SRs), simultaneously with event yields from control regions (CRs) that are included to constrain normalizations of major backgrounds estimated from simulation. The dominant and irreducible Z → ττ background is estimated from simulation. This is different from the search for H...

Non-standard interactions in radiative neutrino mass models

“…The individual analysis by each experiment examines a specific Higgs boson decay mode corresponding to various production processes. We use the measured signal strengths in the following dominant decay modes for our numerical analysis: h → γγ [121][122][123][124], h → ZZ [125,126], h → W W [127][128][129], h → τ τ [130,131] and h → bb [132][133][134].…”

Higgs bosons with large couplings to light quarks

“…Only the Yukawa couplings to third generation quarks have been measured at the LHC thus far [114][115][116][117]. However, limits on Higgs Yukawas to light quarks at current and future colliders have been discussed in refs.…”

Multistep strongly first order phase transitions from new fermions at the TeV scale

“…As explained in the previous paragraph, the new Yukawa coupling has to be large, which would result in a strong mixing between the VLL and τ and hence a significant departure from the SM prediction of the τ couplings. This forces us to discard such a scenario, as the τ couplings are tightly constrained to be SM-like by experimental measurements such as Z → τ τ decays at LEP [42] or h → τ τ decays at LHC [43,44]. Therefore, throughout this section, we neglect the mixing between VLLs and the SM fermions 3 , as the corresponding Yukawa couplings would have an insignificant effect on the phase structure of the Universe.…”