Higgs boson observed decaying in b quarks

The Brout-Englert-Higgs mechanism solves the apparent theoretical impossibility of weak vector bosons (W and Z) to have a mass. The discovery of the Higgs boson in 2012 was a triumph of the standard model. The Higgs field can also be used elegantly to provide mass to charged fermions (quarks and leptons) through interactions involving Yukawa couplings with a force proportional to the mass of particles. The observation of the Higgs boson disintegrating in pairs of τ leptons provided the first direct evidence of this type of interaction

Six years after its discovery, the ATLAS experiment at CERN observed about 30% of the Higgs boson decays provided for in the standard. Model. However, the favored disintegration of the Higgs boson into a pair of b (H → bb) quarks, which should account for nearly 60% of all possible decays, has remained elusive up to now. Observing this mode of decay and measuring its rate is a necessary step to confirm or refute mass generation for fermions via Yukawa interactions, as predicted in the standard model

at the 2018 International Conference on High Physics energies (ICHEP) in Seoul (South Korea), the ATLAS experiment reported a preliminary result establishing observation of the decay of the Higgs boson in b-quark pairs at a rate consistent with model prediction standard. It is necessary to exclude at a level of one million three million the probability that decay detection results from a fluctuation of background noise that could mimic the process. When such a probability is at only one in 1000, the detection is called "proof". The proof of H → bb decay was first provided to Tevatron in 2012, and a year ago by the ATLAS and CMS Collaborations, independently

Combing through the b quarks haystack

Given the abundance of the H → bb decay, and how many more rare decay modes such as H → γγ had already been observed at the time of discovery, why

The main reason is that the process of producing the Higgs boson in proton-proton interactions leads to a single pair of particle jets from fragmentation of b quarks (b-jets) . These are almost indistinguishable from the overwhelming background of b quark pairs produced by strong interaction (quantum chromodynamics or QCD). To overcome this challenge, it was necessary to consider less productive production processes, but with characteristics not present in QCD. The most efficient is the associated production of the Higgs boson with a vector boson, W or Z. The leptonic decays, W →, ν, Z → ℓℓ and Z → νν (where ℓ represents an electron or a muon) provide signatures.

However, the Higgs boson signal remains an order of magnitude lower than the remaining backgrounds from the production of top quarks or vector bosons, leading to similar signatures. For example, a pair of upper quarks can disintegrate with tt → [(W→ℓν)b][(W→qq)b] with a final state containing an electron or a muon and two b quarks, just like the signal (W → ℓν) (H → bb). manipulating to discriminate the signal of such backgrounds is the invariant mass, m _bb of pairs of b-jets identified by sophisticated algorithms of "b-tagging". An example of such a mass distribution is shown in Figure 1, where the sum of the signal and background components is confronted with the data

When all the channels WH and ZH are combined and the backgrounds (except production WZ and ZZ) are subtracted from the data, the distribution shown in Figure 2 shows a clear peak resulting from the decays of Z bosons in pairs. -quark, which validates the procedure of analysis. The shoulder of the upper face is of constant shape and cadence while waiting for the production of the Higgs boson

However, this is not enough to reach the level of detection that the Can be described as observation. To this end, the mass of the b-jet pair is combined with other kinematic variables which show distinct differences between the signal and the different backgrounds, for example the angular separation between the two b-jets or the Transverse momentum of the associated vector boson. This combination of several variables is performed using the technique of boosted decision trees (BDT). A combination of the BDT outputs of all channels, reorganized in terms of signal-to-noise ratio, is shown here. It can be seen that the signal closely follows the expected distribution of the standard model. BDT outputs are subjected to sophisticated statistical analysis to extract the "meaning" of the signal. This is another way of measuring the probability of a false observation in terms of standard deviations, σ, from a Gaussian distribution. The magic number corresponding to the observation of a signal is 5σ

The analysis of 13 TeV data collected by ATLAS during run 2 of the LHC in 2015, 2016 and 2017 leads to a significance of 4.9σ – almost enough to claim the observation. This result was combined with those of a similar analysis of Run 1 data and other research by ATLAS for the H → bb decay mode, ie when the Higgs boson is produced in combination with a pair of top quark or via a process called vector boson. merger (VBF). In addition, the combination of the present analysis with others that target Higgs boson decays to photon pairs and Z bosons measured at 13 TeV provides 5,3σ observation of the associated VH. (V = Z). or W) production, in accordance with the prediction of the Standard Model. The four main modes of Higgs boson production in hadron colliders have now been observed, only two of them this year. In order of discovery: (1) fusion of gluons with a Higgs boson, (2) fusion of weak bosons with a Higgs boson, (3) associated production of a Higgs boson with two top quarks, and (2) 4) Associated production of a Higgs boson. The Higgs boson with a weak boson

With these observations, a new era of detailed measurements in the Higgs sector opens up, through which the standard model will still be challenged.

Learn more:
Who receives his Higgs mass?

More information:
Observation of H → bb decays and VH production with the ATLAS detector: atlas.web.cern.ch/Atlas/GROUPS … ATLAS-CONF-2018-036 /