Evidence for the H → b b ¯ $$ H\to b\overline{b} $$ decay with the ATLAS detector

Abstract

A search for the decay of the Standard Model Higgs boson into a bb pair when produced in association with a W or Z boson is performed with the ATLAS detector. The analysed data, corresponding to an integrated luminosity of 36.1 fb −1 , were collected in proton-proton collisions in Run 2 of the Large Hadron Collider at a centre-of-mass energy of 13 TeV. Final states containing zero, one and two charged leptons (electrons or muons) are considered, targeting the decays Z → νν, W → ν and Z → . For a Higgs boson mass of 125 GeV, an excess of events over the expected background from other Standard Model processes is found with an observed significance of 3.5 standard deviations, compared to an expectation of 3.0 standard deviations. This excess provides evidence for the Higgs boson decay into b-quarks and for its production in association with a vector boson. The combination of this result with that of the Run 1 analysis yields a ratio of the measured signal events to the Standard Model expectation equal to 0. [7]. The properties of the discovered particle have been measured using the Run 1 dataset, collected at centre-of-mass energies of 7 TeV and 8 TeV, and were found to be compatible with those predicted by the Standard Model (SM) within uncertainties, typically of the order of ±20% [8-11]. The Run 2 dataset at an energy of 13 TeV provides an opportunity to increase the precision of such measurements, and to challenge theory predictions further. While analyses of Higgs bosons decaying into vector bosons are entering an era of detailed differential measurements, direct evidence for the coupling of the Higgs boson to fermions was established only via the observation of the decay into τ -leptons through the combination of ATLAS and CMS Run 1 results [11], and, more recently, through the combination of CMS Run 1 and Run 2 results [12]. Although the gluon-gluon fusion production mode provides indirect evidence for the coupling of the Higgs boson to top quarks, there is currently no direct observation of the coupling of the Higgs boson to quarks. The decay of the SM Higgs boson into pairs of b-quarks is expected to have a branching ratio of 58% for m H = 125 GeV [13], the largest among all decay modes. Accessing H → bb decays is therefore crucial for constraining, under fairly general assumptions [14,15], the overall Higgs boson decay width. At the LHC, the very large backgrounds arising from multi-jet production make an inclusive search extremely challenging. The most sensitive production modes for probing H → bb decays are those where the Higgs boson is produced in association with a W or Z boson [16]; their leptonic decay modes lead to clean signatures that can be efficiently triggered on, while rejecting most of the multi-jet backgrounds.Searches for a Higgs boson in the bb decay mode were conducted at the Tevatron by the CDF and D0 Collaborations. They reported an excess of events in V H associated production (where V is used to denote W or Z) in the mass range of 120 GeV to 135 GeV, with a global signifi...

Off-Shell Higgs Probe of Naturalness

“…Thus, for example, a modification of the on-shell Higgs signal strengths at the LHC due to the presence of the singlet-Higgs coupling can be compensated for by a simultaneous but independent modification on Γ h . In this context, we note that the current uncertainties in the Higgs signal strength measurements in various channels, especially in the bottom pair final state [20], allow for a significant downward shift of the total Γ h compared to its SM value. In order to not induce extra model dependency from assumptions about the Higgs boson width, and to capture the momentum dependence from the new physics, we entirely focus on the off-shell region in this study, with m 4l > 150 GeV.…”

Hierarchical fermions and detectable Z′ from effective two-Higgs-triplet 3-3-1 model

“…The measurements of the Higgs boson properties and their actual agreement with the Standard Model predictions [1][2][3][4] have corroborated the simplest implementation of the Higgs mechanism as the source of electroweak symmetry breaking. Although the CERN Large Hadron Collider (LHC) has not yet provided clear evidence for new physics, the Standard Model (SM) consolidation has helped us to put in a firmer footing a series of its theoretical problems such as the severe hierarchy of the Yukawa couplings (the flavor problem), including the neutrino masses and mixing problem; the number of fermion generations; the chiral nature of the electroweak interaction; matter-antimatter asymmetry of the Universe; the strong CP problem; the dark matter content of the Universe; and the vacuum stability.…”

“…If they were complex a more general expression can be written reparametrized by SUð2Þ rep . 4 It is also clear that Uð1Þ G would be generally broken if there is an additional scalar triplet that can acquire a vev in its second component.…”

“…(8) and (9) when we write it explicitly for χ and ρ: GðχÞ ¼ diagð1; −1; 0Þ and GðρÞ ¼ diagð0; −2; −1Þ. 4 Thus, two independent generators out of the ten initial remain unbroken after spontaneous symmetry breaking. The eight would-be Goldstone bosons associated with the broken generators are all absorbed to form the longitudinal degrees of freedom of the massive vector bosons: Z, W AE , Z 0 , V AE , and the neutral non-Hermitian V 0 and V 0 † , with G charges 0, AE2, 0, AE1, −1 and þ1, respectively; see Eq.…”

Connecting Electroweak Symmetry Breaking and Flavor: A Light Dilaton D and a Sequential Heavy Quark Doublet Q

“…The large bb cross section from QCD implies jet-tag-assistance would not work, and measurements are based on V H associated production, where both experiments use VZ production for validation. By combining the 2016 data with Run 1, ATLAS [48] and CMS [49] experiments find evidence at 3.6σ (4.0σ) and 3.8σ (3.8σ), respectively. Both experiments find excess events in m bb above the Z pole.…”