ATLAS experiment takes its first glimpse of the Higgs boson in its favourite decay

ATLAS Experiment takes its first glimpse of the Higgs boson in its favourite decay
ATLAS event display of a Higgs boson decaying to two b-quarks. Credit: ATLAS Collaboration/CERN

Previously, the Higgs boson has been observed decaying to photons, tau-leptons, and W and Z bosons. However, these impressive achievements represent only 30 percent of Higgs boson decays. The Higgs boson's favoured decay to a pair of b-quarks (H→bb) was predicted to happen around 58 percent of the time, thus driving the short lifetime of the Higgs boson, and thus remained elusive. Observing this decay would fill in one of the big missing pieces of our knowledge of the Higgs sector and confirm that the Higgs mechanism is responsible for the masses of quarks; additionally, it might also provide hints of new physics beyond our current theories. All in all, it is a vital missing piece of the Higgs boson puzzle.

But after over 1 million H→bb decays in the ATLAS Experiment alone, why haven't researchers seen it yet? This seems especially strange considering that less frequent Higgs boson decays have been observed.

The answer lies in the abundance of b-quarks created in the ATLAS detector due to strong interactions. We create pairs of b-quarks 10 million times more frequently than we create a H→bb decay, which makes picking them out against that large background an extremely challenging task. We therefore look for H→bb decays when they are produced in association with another particle—in this case, a vector boson (W or Z). The more distinctive decays of vector bosons provide a way to reduce the large background. This leads to a much lower production rate – we expect to have created only 30,000 H→bb decays this way, but it provides an opportunity to spot this elusive decay.

Nevertheless, even in this condition, the background processes that mimic the H→bb signal are still large, complex and difficult to model. The ATLAS collaborators made a major effort to isolate the small H→bb signal from the large background. After selecting the collisions of interest, they were left with the expected number of around 300 H→bb events compared to 70,000 background events. Ultimately, they were hoping to see an excess of collision events over our background prediction (a bump) that appears at the mass of the Higgs boson.

ATLAS Experiment takes its first glimpse of the Higgs boson in its favourite decay
A comparison of the excess of collision data (black points) over the background processes (which have been subtracted from the data), which clearly shows the H→bb decays (filled red area) and the well understood diboson Z→bb decay (grey area) used to validate the result. (Image:) Credit: ATLAS Collaboration/CERN

After analysing all the data ATLAS collected in 2015 and 2016, the researchers have finally achieved the level of precision to confirm evidence for H→bb with an observed significance of 3.6 σ when combining the Run 1 and Run 2 datasets. As shown in the figure, a bump is observed that is highly consistent with expectations, confirming many key aspects of the Higgs bosons behaviour. Next to the bump, there is a decay of a Z boson (mass of 91 GeV) to a b-quark pair, produced in a similar way as the Higgs , but more abundantly. It serves as a powerful validation of the analysis.

Spotting H→bb is just the beginning. Studies of this new decay will open a whole new window onto the Higgs, and may also provide hints of beyond our current theories. Stay tuned to this channel.


Explore further

New ATLAS precision measurements of the Higgs Boson in the 'golden channel'

More information: Evidence for the H→bb decay with the ATLAS detector: atlas.web.cern.ch/Atlas/GROUPS … ATLAS-CONF-2017-041/
Provided by ATLAS Experiment
Citation: ATLAS experiment takes its first glimpse of the Higgs boson in its favourite decay (2017, July 11) retrieved 20 May 2019 from https://phys.org/news/2017-07-atlas-glimpse-higgs-boson-favourite.html
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Jul 11, 2017
Wow! You guys are really smart! What's a particle? What is your basis function? Have you identified anything that is axiomatic? Maybe you are wrong, I think you are only measuring field events. Everything else is nonsense. I think we know the source of the field. No? Ask the guy who built your instrumentation. You do acknowledge your instrumentation. You are not that guy that identified a 2ns delay in an optical coupler are you? Juz say'n clear signal an all, delay? I think either you no idea of calibration, or ... Dr.

Jul 11, 2017
Not even a coupler, a connector! Guys? There's a hole in your theory.

Basis: What holds the nucleus together? Not a theory, a question!

Not gluons, neutrons!

Jul 12, 2017
May be it would be useful having a look at the graph:

"On the progression of the mass of elementary particles" at:

https://www.resea...articles

Jul 12, 2017
Why don't you try to analyse your experimental results in the framework of the QOD (Quantum Orbital Dynamics) in addition to that of QCD. QOD is conceptually much handier: neutral particles are considered to be composed of two electric charges of opposite sign in the quantum state . In their dissociation the two electric charges get apart and then acquire the charged quantum states , which can be a pair e-, e+, or µ-, µ+, or π-, π+, or K-, K+, etc.

The electric charge q- must not be assimilated to an electron, as it is commonly done, since it can acquire diverse quantum states such as that of the electron, the muon, the negative pion, the negative kaon, and so on. What defines the particle is just the quantum state of the electric charge q.

Jul 12, 2017
If interested in this viewpoint, for further information you may read the following articles:

Space, this great unknown

and:

Fundamentals of the Orbital Conception of Elementary Particles and of Their Application to the Neutron and Nuclear Structure

and still more at:

https://www.resea...ibutions

Jul 12, 2017
If interested in this viewpoint, for further information you may read the following articles:

Space, this great unknown

and:

Fundamentals of the Orbital Conception of Elementary Particles and of Their Application to the Neutron and Nuclear Structure

and still more at:

https://www.resea...ibutions

I prefer Maxwell!

Jul 14, 2017
if charge exist then we may exist. First master this domain!

Jul 14, 2017
We only see the superposition and position of a field center in time and space. This could be a delusion! Think there only exist the field, we see as spherical and diametrical, i.e. the existence of it's center. So if we want to understand that, don't look for a beginning or an end. We created that. Just stack the centers at a point in time and call that the beginning. It's still part of a continuum! Space and time, those are our ideas.

You are looking at it all wrong!

You may simulate any with random numbers, Monte Carlo, self assimilation by using a space defined by think lamda in space and time, however you visualize it, C = 1. Dimensionally we see at any chosen point, the update of all fields, as per initial conditions,

If you want to assume at first there is nothing and now there is us then you start with an infinite stack of pairs, within an infinite space, and an infinite time, probability of instability? The field from each point and timing and harmonics,

Jul 14, 2017
...
If you want to assume at first there is nothing and now there is us then you start with an infinite stack of pairs, within an infinite space, and an infinite time, probability of instability? The field from each point and timing and harmonics,

or see things based upon any time scale or spatial scale, without blowing up the world.

Jul 14, 2017
...
If you want to assume at first there is nothing and now there is us then you start with an infinite stack of pairs, within an infinite space, and an infinite time, probability of instability? The field from each point and timing and harmonics,

or see things based upon any time scale or spatial scale, without blowing up the world.

Make this tool available available to kids. That's how we discover our world. Now what mathematical models can we build for control?

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