Mapping uncommon Higgs-boson decays – CERN Courier

Rare, unobserved decays of the Higgs boson are pure locations to seek for new physics. At the EPS-HEP convention, the ATLAS collaboration offered new improved measurements of two extremely suppressed Higgs decays: right into a pair of muons; and right into a Z boson accompanied by a photon. Producing a single occasion of both H → μμ or H → Zγ → (ee/μμ) γ on the LHC requires, on common, round 10 trillion proton–proton collisions. The H → μμ and H → Zγ alerts seem as slim resonances within the dimuon and Zγ invariant mass spectra, atop backgrounds some three orders of magnitude bigger.

In the Standard Model, the Brout–Englert–Higgs mechanism provides mass to the muon by means of its Yukawa coupling to the Higgs area, which will be examined through the uncommon H → μμ decay. An oblique comparability with the well-known muon mass, decided to 22 components per billion, offers a stringent take a look at of the mechanism within the second fermion technology and is a robust probe of recent physics. With a branching ratio of simply 0.02%, and a big background dominated by the Drell–Yan manufacturing of muon pairs by means of digital photons or Z bosons, the inclusive signal-over-background ratio plunges to the extent of 1 half in a thousand. To single out its decay signature, the ATLAS collaboration employed machine-learning methods for background suppression and generated over 5 billion Drell–Yan Monte Carlo occasions at next-to-leading-order accuracy in QCD, all handed by means of the total detector simulation. This high-precision pattern offers templates to refine the background mannequin and minimise bias on the tiny H → μμ sign.

The Higgs boson can decay right into a Z boson and a photon through loop diagrams involving W bosons and heavy charged fermions, like the highest quark. Detecting this uncommon course of would full the suite of established decays into electroweak boson pairs and supply a window on physics past the Standard Model. To cut back QCD background and enhance sensitivity, the ATLAS evaluation targeted on Z bosons additional decaying into electron or muon pairs, with an general branching fraction of seven%. This further choice reduces the occasion fee to about one in 10,000 Higgs decays, with an inclusive signal-over-background ratio on the per-mille degree. The low momenta of final-state particles, mixed with the high-luminosity situations of LHC Run 3, pose further challenges for sign extraction and suppression of Z + jets backgrounds. To improve sign significance, the ATLAS collaboration improved background modelling methods, optimised occasion categorisation by Higgs manufacturing mode, and employed machine studying to spice up sensitivity.

The two ATLAS searches are primarily based on 165 fb^–1 of LHC Run 3 proton–proton collision information collected between 2022 and 2024 at √s = 13.6 TeV, with a rigorous blinding process in place to stop biases. Both channels present excesses on the Higgs-boson mass of 125.09 GeV, with noticed (anticipated) 2.8σ (1.8σ) significance for H to μμ and 1.4σ (1.5σ) for H to Zγ. These outcomes are strengthened by combining them with 140 fb^–1 of Run-2 information collected at √s = 13 TeV, updating the H → μμ and H → Zγ noticed (anticipated) significances to three.4σ (2.5σ) and a pair of.5σ (1.9σ), respectively (see determine 1). The measured sign strengths are per the Standard Model inside uncertainties.

These outcomes mark the ATLAS collaboration’s first proof for the H → μμ decay, following the sooner declare by CMS primarily based on Run-2 information (see CERN Courier September/October 2020 p7). Meanwhile, the H → Zγ search achieves a 19% improve in anticipated significance with respect to the mixed ATLAS–CMS Run-2 evaluation, which first reported proof for this course of. As Run 3 data-taking continues, the LHC experiments are closing in on establishing these two uncommon Higgs decay channels. Both will stay statistically restricted all through the LHC’s lifetime, with ample room for discovery within the high-luminosity section.