Higgs boson observed decaying to b quarks

July 10, 2018, ATLAS Experiment
Event display for the H→bb decay analysis with the ATLAS detector. Credit: ATLAS Collaboration/CERN

On 9 July, at the 2018 International Conference on High Energy Physics (ICHEP) in Seoul (South Korea), the ATLAS experiment reported a preliminary result establishing the observation of the Higgs boson decaying into pairs of b quarks, furthermore at a rate consistent with the Standard Model prediction.

The Brout-Englert-Higgs mechanism solves the apparent theoretical impossibility of weak vector bosons (W and Z) to have mass. The discovery of the Higgs in 2012 was a triumph of the Standard Model. The Higgs field can also be used in an elegant way to provide mass to charged fermions (quarks and leptons) through interactions involving Yukawa couplings with strength proportional to the particle mass. The observation of the Higgs boson decaying into 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 percent of the Higgs boson decays predicted in the Standard Model. However, the favoured decay of the Higgs boson into a pair of b quarks (H→bb), which is expected to account for almost 60 percent of all possible decays, has remained elusive up to now. Observing this decay mode and measuring its rate is a mandatory step to confirm or disconfirm the mass generation for fermions via Yukawa interactions, as predicted in the Standard Model.

At the 2018 International Conference on High Energy Physics (ICHEP) in Seoul (South Korea), the ATLAS experiment reported a preliminary result establishing the observation of the Higgs boson decaying into pairs of b quarks at a rate consistent with the Standard Model prediction. It is necessary to exclude at a level of one in 3 million 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 level of only one in 1000, the detection is qualified as "evidence." Evidence of the H→bb decay was first provided at the Tevatron in 2012, and a year ago by the ATLAS and CMS Collaborations, independently.

Figure 1: Distribution of mbb in the (W→ℓν)(H→bb) search channel. The signal is shown in red, the different backgrounds in various colours. The data are shown as points with error bars. Credit: ATLAS Collaboration/CERN
Combing through the haystack of b quarks

Given the abundance of the H→bb decay, and how much rarer decay modes such as H→γγ had already been observed at the time of discovery, why did it take so long to achieve this observation?

The main reason is that the production process for the Higgs boson in proton-proton interactions leads to a single pair 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 strong interaction (quantum chromodynamics or QCD). To overcome this challenge, it was necessary to consider production processes that are less copious, but exhibit features not present in QCD. The most effective of these is the associated production of the Higgs boson with a vector boson, W or Z. The leptonic decays, W→ℓν, Z→ℓℓ and Z→νν (where ℓ stands for an electron or a muon) provide signatures that allow for efficient triggering and powerful QCD background reduction.

However, the Higgs boson signal remains orders of magnitude smaller than the remaining backgrounds arising from top quark or vector boson production, which lead to similar signatures. For instance, a top quark pair can decay as tt→[(W→ℓν)b][(W→qq)b] with a final state containing an electron or a muon and two b quarks, exactly as the (W→ℓν)(H→bb) signal.

The main handle to discriminate the signal from such backgrounds is the invariant mass, mbb, of pairs of b-jets identified by sophisticated "b-tagging" algorithms. An example of such a mass distribution is shown in Figure 1, where the sum of the signal and background components is confronted to the data.

Figure 2: Distribution of mbb from all search channels combined after subtraction of all backgrounds except for WZ and ZZ production. The data (points with error bars) are compared to the expectations from the production of WZ and ZZ (in grey) and of WH and ZH (in red). Credit: ATLAS Collaboration/CERN

When all WH and ZH channels are combined and the backgrounds (apart from WZ and ZZ production) subtracted from the data, the distribution shown in 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 in shape and rate with the expectation from Higgs boson production.

This is, however, not sufficient to reach the level of detection that can be qualified as observation. To this end, the mass of the b-jet pair is combined with other kinematic variables that show distinct differences between the signal and the various backgrounds, for instance the angular separation between the two b-jets, or the transverse momentum of the associated vector boson. This combination of multiple variables is performed using the technique of boosted decision trees (BDTs). A combination of the BDT outputs from all channels, reordered in terms of signal-to-background ratio, is shown in here. It can be 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 another way to measure the probability of a fake observation in terms of standard deviations, σ, of 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 sufficient to claim observation. This result was combined with those from a similar analysis of Run 1 data and from other searches by ATLAS for the H→bb decay mode, namely where the Higgs boson is produced in association with a top quark pair or via a process known as vector boson fusion (VBF). The significance achieved by this combination is 5.4σ.

Furthermore, combining the present analysis with others that target Higgs boson decays to pairs of photons and Z bosons measured at 13 TeV provides the observation at 5.3σ of associated VH (V = Z or W) production, in agreement with the Standard Model prediction. All four primary Higgs boson production modes at hadron colliders have now been observed, of which two only this year. In order 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 with two top quarks, and (4) associated production of a 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 be further challenged.

Explore further: Who gets their mass from the Higgs?

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

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15 comments

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holoman
1 / 5 (7) Jul 10, 2018
This is the beginning of a miracle. Energy to Mass, WOW !
antialias_physorg
4.6 / 5 (10) Jul 10, 2018
This is the beginning of a miracle. Energy to Mass, WOW !

Erm...particle pair production ("mass from energy") has been observed since the late 1940's.
https://en.wikipe...oduction

Mimath224
5 / 5 (1) Jul 10, 2018
@antialias_physorg
This is the beginning of a miracle. Energy to Mass, WOW !

Erm...particle pair production ("mass from energy") has been observed since the late 1940's.
https://en.wikipe...oduction

I admire very much such research but I am also wary, being a layman, of being drawn into results that might be explained (by someone else, not me of course) as being a rushed interpretation before duplication. Have you any thoughts on this? If you see these results as being correct by how much would you suggest they add confirmation to the St.M.? I freely admit that doubt is probably due to my lack of understanding or knowledge...The Higgs at 126 Mev and b quarks at about 4.2 Mev for example. Are they saying here that the vBosons are produced besides bb or totally multiple bb (say 30b for 1H) Sorry to ask but some days I can't get my head the interpretations...maybe I should re-read tomorrow, Ha! Thanks in advance.
antialias_physorg
4.4 / 5 (7) Jul 11, 2018
as being a rushed interpretation before duplication.

The experiment in question that shows matter from energy (photons) is so simple that you can probably set it up at home. Here's one of the original papers from 1933 describing teh setup and the measurements.
www.physics.princ...6_33.pdf

As for the Higgs decaying in to two b Quarks: This effect was produced at the LHC. There is no other machine like the LHC (if you mean this by duplication - by that 'logic' no one could ever report anything).
But they did replicate at the LHC (of course). We're not dealing with a singular event, here but with a statistical significance over many, many events that satisfies the criteria for reporting a detection.
Mimath224
5 / 5 (3) Jul 11, 2018
@antialias_physorg
as being a rushed interpretation before duplication.

The experiment in question that shows matter from energy (photons) is so simple that....
http://www.physic...6_33.pdf
As for the Higgs decaying in to two b Quarks: This effect was produced at the LHC. There is no other machine like the LHC (if you mean this by duplication - by that 'logic' no one could ever report anything).
But they did replicate at the LHC (of course). We're not dealing with a singular event, here but with a statistical significance over many, many events that satisfies the criteria for reporting a detection.

Thank you very much. Yes I was aware of the 1933 data but not by that specific paper. See, I told you I wasn't feeling very bright today. Of course, duplication somewhere else is not yet possible, Ha! Yes, wasn't the replication mentioned here on Physorg? Anyway, again thanks for pointing it out. Much obliged.
granville583762
5 / 5 (1) Jul 12, 2018
Quarks and their elementary relationships
phys.org> the observation of the Higgs boson decaying into pairs of b quarks

There appears some relationship between Higgs bosons and quarks as being essentially similar in structure enables higgs bosons ability to decay into quarks as bottom quarks
To decay it has to be dropping to a lower energy level becoming a stable elementary element.
Essentially a Higgs boson becomes a quark.
Kron
not rated yet Jul 13, 2018
Keep in mind that the Higgs boson (whose mass is 125 GeV) in the LHC is produced through the collision of 2 protons (whose rest masses are 938 MeV or 0.938 GeV each). It is the kinetic energy the protons carry as they are accelerated to relativistic speeds that allow them to produce the Higgs boson upon collision. When accelerated to the top speed reachable at the LHC, a proton weighs in at a whopping 450 GeV (500 times! its restmass).

Keeping that in mind it might be easier to comprehend how the Higgs boson (a particle of 125 GeV) can decay into 2 bottom quarks (weighing in at 4.2 GeV each). The bottom quarks produced are not at rest. They jet out of the collision with a lot of energy, and this energy has a mass equivalent.
Kron
not rated yet Jul 13, 2018
At 7 TeV protons weigh in at 7460 times their restmass. Sorry about misinfo but this showcases the point ever more so.
katesisco
1 / 5 (1) Jul 13, 2018
Awaiting Miles Mathis interpretation of this claim, and the clarity he brings.
granville583762
5 / 5 (1) Jul 13, 2018
Protonic Quarks in collision
Kron> Keep in mind that the Higgs boson (whose mass is 125 GeV) in the LHC is produced through the collision of 2 protons (whose rest masses are 938 MeV or 0.938 GeV each). It is the kinetic energy the protons carry as they are accelerated to relativistic speeds that allow them to produce the Higgs boson upon collision.

How long before the 6 quarks in collision on producing a higgs boson before it decays back to quarks
TimLong2001
not rated yet Jul 16, 2018
Matter in the universe came from pair-formation (supersymetrically) from micro-charges originating at much smaller scales, and the gauge boson (the photon of electromagnetic energy) propagates by utilizing a small amount of energy from the opposite charges from which it is composed. This results in a gradual lengthening of wavelength since the rotational radius of the binary photon increases with loss of energy. This redshift is not a Doppler shift, and distances determined by assuming it was are necessarily in error. The photon was assumed to have zero mass when calculating atomic mass transitions in order to avoid overly complex calculations for electron shell transitions that really weren't large enough to have a substantial affect on atomic mass. See "The Mass of the Photon," by Alfred Goldhaber and Michael Nieto in the May 1976 issue of Scientific American.
savvys84
1 / 5 (2) Jul 16, 2018
Lol. Folks get over it.
There ain't no bloody higgs boson. It doesn't and cannot exist as you cannot create a particle out of time itself
milnik
not rated yet Jul 16, 2018
This field of science is an interesting stage for "scientists" showing "clown products" in these adult and super smart toys.
Do these scientists know what's in those pipes, except for the protons, and they think the pipes are empty. When the bone falls apart, about 600 protons can be obtained from it. Why this technique is not applied to crash airplanes. In this way, 600 aircraft can get out of the plane. Or take two eggs.
All these particles, acquired by the suicide of "certain" sentenced to death ", are just abortions of" dead-borns "derived from something that science knows nothing.
These particle collars are similar toys for making bubbles of soap.
Anonym642864
not rated yet Jul 16, 2018
It is truth that energy is converted into matter and Antimatter. Einstein greatest scientist till now gave the last formula E= MC (square).
savvys84
1 / 5 (1) Jul 17, 2018
It is truth that energy is converted into matter and Antimatter. Einstein greatest scientist till now gave the last formula E= MC (square).

FYI Einstein did not give that formula E = Mc2

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