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Is the Higgs Boson Acting Weird Again at the LHC?

A new signal excess is probably a statistical fluke, but, if not, we could again be talking about New Physics.

Though no one could really say what it was specifically, the Large Hadron Collider's 750 GeV diphoton bump registered at least one unambiguous conclusion for physicists: they'd found something new. In the showers of proton collision byproducts that occurred during the 2015 run of CERN's ATLAS and CMS experiments, it seemed there was a new particle.

2016 data, however, failed to replicate the bump, indicating that the earlier observations were just statistical fluctuations. This has resulted in a generally grim attitude shared by many researchers in high-energy physics: The LHC managed to bag the Higgs boson, yes. But bagging New Physics, the presence of a particle or interaction so-far unknown? Not so much.

Yet, just as the diphoton bump was being kicked to the curb, a potential new strangeness emerged at the LHC, albeit one that's less plainly seen. This has to do with a process known as tth (top-top-Higgs), which is an alternative mode of Higgs boson production that results in the creation of Higgs particles alongside pairs of top quarks, the heaviest known fundamental particles.

The process is exceedingly rare, occurring about once every four minutes while the LHC is operating at normal luminosities. Many of the events are simply missed, while others are obscured by background processes mimicking them. tth is important because observing the process by which these superheavy particles acquire mass from (presumably) the Higgs boson will tell us some important things about how the whole mass-conferring process works. Measurements of tth events can be compared to other known Higgs mechanisms to help determine if they help form a "coherent picture" of Higgs interactions. If tth events, as identified by the decay products of the Higgs boson, are screwy in relation to other mechanisms, it could point again to New Physics.

Observations made during the LHC's current run indicate an excess in tth signals recorded at the ATLAS experiment, according to CERN data presented last month. As physicist Adam Falkowski (aka Jester) explains at Résonaances, these signals correspond to a parameter known as Yukawa coupling, which mathematically describes the interaction between the Higgs field and massless particles that results in, well, particles that have mass.

This value is well defined by the Standard Model, and so if we were to observe variations in it beyond that definition, we might be onto New Physics.

The signal difference you can see below. The red dashed line is what the Standard Model predicts and the black line is obviously what was observed IRL at the LHC. They definitely don't match, with the difference being somewhere around a factor of two.

It's important to note that the signals above are actually sitting on top of an absolute mountain of background data, which makes it difficult to say that we've even observed tth events at all, let alone superweird tth events indicating New Physics. The findings indicating tth events (of any sort) are at a 2.8-sigma level, which is really promising but by no means a discovery. Jester thinks there's a proper tth discovery forthcoming once the LHC has bagged some more data.

As for the signal strength discrepancy, he's not enormously hopeful, writing, "Should we get excited that the measured tth rate is significantly larger than Standard Model one? Assuming that the current central value remains, it would mean that the top Yukawa coupling is 40 percent larger than that predicted by the Standard Model. This is not impossible, but very unlikely in practice. The reason is that the top Yukawa coupling also controls the gluon fusion—the main Higgs production channel at the LHC—whose rate is measured to be in perfect agreement with the Standard Model."

So, OK, it's probably a statistical fluctuation. But the discovery of the tth production mode, whether it aligns with the Standard Model or not, is one of the few remaining goals of the LHC. It seems that soon we'll be able to cross it off our high-energy list.