First Evidence the Higgs Talks to Other Generations

Article Titles: “Measurement of Higgs boson decay to a pair of muons in proton-proton collisions at sqrt(S) = 13 TeV” and “A search for the dimuon decay of the Standard Model Higgs boson with the ATLAS detector”

Authors: The CMS Collaboration and The ATLAS Collaboration, respectively

References: CDS: CMS-PAS-HIG-19-006 and arxiv:2007.07830, respectively

Like parents who wonder if millennials have ever read a book by someone outside their generation, physicists have been wondering if the Higgs communicates with matter particles outside the 3rd generation. Since its discovery in 2012, phycists at the LHC experiments have been studying the Higgs in a variety of ways. However despite the fact that matter seems to be structured into 3 distinct ‘generations’ we have so far only seen the Higgs talking to the 3rd generation. In the Standard Model, the different generations of matter are 3 identical copies of the same kinds of particles, just with each generation having heavier masses. Due to the fact that the Higgs interacts with particles in proportion to their mass, this means it has been much easier to measure the Higgs talking to the third and heaviest generation of mater particles. But in order to test whether the Higgs boson really behaves exactly like the Standard Model predicts or has slight deviations -(indicating new physics), it is important to measure its interactions with particles from the other generations too. The 2nd generation particle the Higgs decays most often to is the charm quark, but the experimental difficulty of identifying charm quarks makes this an extremely difficult channel to probe (though it is being tried).

The best candidate for spotting the Higgs talking to the 2nd generation is by looking for the Higgs decaying to two muons which is exactly what ATLAS and CMS both did in their recent publications. However this is no easy task. Besides being notoriously difficult to produce, the Higgs only decays to dimuons two out of every 10,000 times it is produced. Additionally, there is a much larger background of Z bosons decaying to dimuon pairs that further hides the signal.

The branching ratio (fraction of decays to a given final state) of the Higgs boson as a function of its mass (the measured Higgs mass is around 125 GeV). The decay to a pair of muons is shown in gold, much below the other decays that have been observed.

CMS and ATLAS try to make the most of their data by splitting up events into multiple categories by applying cuts that target different the different ways Higgs bosons are produced: the fusion of two gluons, two vector bosons, two top quarks or radiated from a vector boson. Some of these categories are then further sub-divided to try and squeeze out as much signal as possible. Gluon fusion produces the most Higgs bosons, but it also the hardest to distinguish from the Z boson production background. The vector boson fusion process produces the 2nd most Higgs and is a more distinctive signature so it contributes the most to the overall measurement. In each of these sub-categories a separate machine learning classifier is trained to distinguish Higgs boson decays from background events. All together CMS uses 14 different categories of events and ATLAS uses 20. Backgrounds are estimated using both simulation and data-driven techniques, with slightly different methods in each category. To extract the overall amount of signal present, both CMS and ATLAS fit all of their respective categories at once with a single parameter controlling the strength of a Higgs boson signal.

At the end of the day, CMS and ATLAS are able to report evidence of Higgs decay to dimuons with a significance of 3-sigma and 2-sigma respectively (chalk up 1 point for CMS in their eternal rivalry!). Both of them find an amount of signal in agreement with the Standard Model prediction.

Combination of all the events used in the CMS (left) and ATLAS (right) searches for a Higgs decaying to dimuons. Events are weighted by the amount of expected signal in that bin. Despite this trick, the small evidence for a signal can be seen only be seen in the bottom panels showing the number of data events minus the predicted amount of background around 125 GeV.

CMS’s first evidence of this decay allows them to measuring the strength of the Higgs coupling to muons as compared to the Standard Model prediction. One can see this latest muon measurement sits right on the Standard Model prediction, and probes the Higgs’ coupling to a particle with much smaller mass than any of the other measurements.

CMS’s latest summary of Higgs couplings as a function of particle mass. This newest edition of the coupling to muons is shown in green. One can see that so far there is impressive agreement with the Standard Model across a mass range spanning 3 orders of magnitude!

As CMS and ATLAS collect more data and refine their techniques, they will certainly try to push their precision up to the 5-sigma level needed to claim discovery of the Higgs’s interaction with the 2nd generation. They will be on the lookout for any deviations from the expected behavior of the SM Higgs, which could indicate new physics!

Further Reading:

Older ATLAS Press Release “ATLAS searches for rare Higgs boson decays into muon pairs

Cern Courier Article “The Higgs adventure: five years in

Particle Bites Post “Studying the Higgs via Top Quark Couplings

Blog Post from Matt Strassler on “How the Higgs Field Works

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