For You Lot Data - Higgs Boson Observed Decaying To B Quarks

On nine July, at the 2018 International Conference on High Energy Physics (ICHEP) inwards Seoul (South Korea), the ATLAS experiment reported a preliminary resultant establishing the observation of the Higgs boson decaying into pairs of b quarks, furthermore at a charge per unit of measurement consistent alongside the Standard Model prediction.

Event display for the H→bb decay analysis alongside the ATLAS detector
[Credit: ATLAS Collaboration/CERN]

The Brout-Englert-Higgs machinery solves the apparent theoretical impossibility of weak vector bosons (W too Z) to get got mass. The regain of the Higgs boson inwards 2012 was a triumph of the Standard Model. The Higgs plain tin besides live used inwards an elegant means to render volume to charged fermions (quarks too leptons) through interactions involving Yukawa couplings alongside pull proportional to the particle mass. The observation of the Higgs boson decaying into pairs of τ leptons provided the offset instantly show of this type of interaction.

Six years later its discovery, the ATLAS experiment at CERN observed virtually thirty percentage of the Higgs boson decays predicted inwards the Standard Model. However, the favoured decay of the Higgs boson into a yoke of b quarks (H→bb), which is expected to concern human relationship for almost lx percentage of all possible decays, has remained elusive upwardly to now. Observing this decay vogue too measuring its charge per unit of measurement is a mandatory stride to confirm or disconfirm the volume generation for fermions via Yukawa interactions, equally predicted inwards the Standard Model.

At the 2018 International Conference on High Energy Physics (ICHEP) inwards Seoul (South Korea), the ATLAS experiment reported a preliminary resultant establishing the observation of the Higgs boson decaying into pairs of b quarks at a charge per unit of measurement consistent alongside the Standard Model prediction. It is necessary to exclude at a marking of 1 inwards three 1 1000 one thousand the probability that the decay detection arises from a fluctuation of the background that could mimic the process. When such a probability is at the marking of alone 1 inwards 1000, the detection is qualified equally "evidence." Evidence of the H→bb decay was offset provided at the Tevatron inwards 2012, too a yr agone past times the ATLAS too CMS Collaborations, independently.

Given the abundance of the H→bb decay, too how much rarer decay modes such equally H→γγ had already been observed at the fourth dimension of discovery, why did it get got then long to plough over this observation?

Figure 1: Distribution of mbb inwards the (W→ℓν)(H→bb) search channel. The signal is shown inwards red,
the dissimilar backgrounds inwards diverse colours. The information are shown equally points alongside fault bars
[Credit: ATLAS Collaboration/CERN]

The principal argue is that the production physical care for for the Higgs boson inwards proton-proton interactions leads to a unmarried yoke of particle jets originating from the fragmentation of b quarks (b-jets). These are almost impossible to distinguish from the overwhelming background of b-quark pairs produced via the potent interaction (quantum chromodynamics or QCD). To overcome this challenge, it was necessary to consider production processes that are less copious, only exhibit features non acquaint inwards QCD. The most effective of these is the associated production of the Higgs boson alongside a vector boson, due west or Z. The leptonic decays, W→ℓν, Z→ℓℓ too Z→νν (where ℓ stands for an electron or a muon) render signatures that permit for efficient triggering too powerful QCD background reduction.

However, the Higgs boson signal remains orders of magnitude smaller than the remaining backgrounds arising from transcend quark or vector boson production, which Pb to like signatures. For instance, a transcend quark yoke tin decay equally tt→[(W→ℓν)b][(W→qq)b] alongside a lastly nation containing an electron or a muon too 2 b quarks, precisely equally the (W→ℓν)(H→bb) signal.

The principal quest maintain to discriminate the signal from such backgrounds is the invariant mass, mbb, of pairs of b-jets identified past times sophisticated "b-tagging" algorithms. An representative of such a volume distribution is shown inwards Figure 1, where the amount of the signal too background components is confronted to the data.

When all WH too ZH channels are combined too the backgrounds (apart from WZ too ZZ production) subtracted from the data, the distribution shown inwards Figure 2 exhibits a clear peak arising from Z boson decays to b-quark pairs, which validates the analysis procedure. The shoulder on the upper side is consistent inwards shape too charge per unit of measurement alongside the expectation from Higgs boson production.

Figure 2: Distribution of mbb from all search channels combined later subtraction of all backgrounds except for WZ
and ZZ production. The information (points alongside fault bars) are compared to the expectations from the production of WZ
and ZZ (in grey) too of WH too ZH (in red) [Credit: ATLAS Collaboration/CERN]

This is, however, non sufficient to plough over the marking of detection that tin live qualified equally observation. To this end, the volume of the b-jet yoke is combined alongside other kinematic variables that demo distinct differences betwixt the signal too the diverse backgrounds, for event the angular separation betwixt the 2 b-jets, or the transverse momentum of the associated vector boson. This combination of multiple variables is performed using the technique of boosted determination trees (BDTs). Influenza A virus subtype H5N1 combination of the BDT outputs from all channels, reordered inwards damage of signal-to-background ratio, is shown inwards here. It tin live seen that the signal closely follows the distribution expected from the Standard Model. The BDT outputs are subjected to a sophisticated statistical analysis to extract the "significance" of the signal. This is about other means to mensurate the probability of a faux observation inwards damage of touchstone deviations, σ, of a Gaussian distribution. The magic release corresponding to the observation of a signal is 5σ.

The analysis of thirteen TeV information collected past times ATLAS during Run 2 of the LHC inwards 2015, 2016 too 2017 leads to a significance of 4.9σ – almost sufficient to claim observation. This resultant was combined alongside those from a like analysis of Run 1 information too from other searches past times ATLAS for the H→bb decay mode, namely where the Higgs boson is produced inwards association alongside a transcend quark yoke or via a physical care for known equally vector boson fusion (VBF). The significance achieved past times this combination is 5.4σ.

Furthermore, combining the acquaint analysis alongside others that target Higgs boson decays to pairs of photons too Z bosons measured at thirteen TeV provides the observation at 5.3σ of associated VH (V = Z or W) production, inwards understanding alongside the Standard Model prediction. All iv primary Higgs boson production modes at hadron colliders get got at nowadays been observed, of which 2 alone this year. In social club of discovery: (1) fusion of gluons to a Higgs boson, (2) fusion of weak bosons to a Higgs boson, (3) associated production of a Higgs boson alongside 2 transcend quarks, too (4) associated production of a Higgs boson alongside a weak boson.

With these observations, a novel era of detailed measurements inwards the Higgs sector opens up, through which the Standard Model volition live farther challenged.

More info: Observation of H → bb decays too VH production alongside the ATLAS detector: https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/CONFNOTES/ATLAS-CONF-2018-036/

Source: ATLAS Experiment [July 10, 2018]


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