Wednesday, September 08, 2021

An informal ATLAS, CMS combo: a new \(151.5\GeV\) scalar at 5 sigma

Five weeks ago, I discussed three possible values of a mass of a new Higgs-like scalar that may be seen by both ATLAS and CMS experiments at the LHC: \(28\GeV\), \(400\GeV\), \(1000\GeV\).

In this very short linker-not-thinker memo, I have to mention the first hep-ph preprint today (submitted one second after the previous deadline, 18:00:01 UTC),
Accumulating Evidence for the Associate Production of a Neutral Scalar with Mass around \(151 \GeV\)
by Crivellin at al. (10 co-authors from Switzerland, China, U.K., South Africa). The previous three new-scalar-like excesses were reported in experimental papers. This decuplet of physicists is made of phenomenologists.

OK, they have picked some experimental papers that looked for new Higgs bosons and they combined the curves for the number of events as a function of the mass of a new Higgs boson. As nicely summarized by their Figure 3, there is a nice peak with the center at \(m_S=151.5\GeV\) whose width is around \(2\GeV\). A nice new Higgs-like scalar, the same kind of a particle as the previous three.

This new scalar resonance isn't produced directly but instead, it seems to be a product of a decay of a heavier particle, arguably yet another and heavy Higgs, perhaps one at \(400\GeV\) or \(1000\GeV\) mentioned previously.

Locally, the excess is as strong as 5.1 sigma (nominally a discovery!); it gets reduced to 4.8 sigma globally, after we acknowledge that we are scanning over the interval \(130\)-\(160\GeV\) or so. I think that a further reduction should be applied to acknowledge that they are scanning over different ways how to pick the papers that are being combined but that reduction could be rather modest because 5 sigma really is rather strong.

The dominant decay of this new scalar particle \(S\) seems to be \(S\to \gamma\gamma\) while \(S\to Z\gamma\) also contributes. The decay \(S\to b\bar b\dots\) seems to be there but it is weak, not very important for the statistical significance, and it may be absent, too. \(S\to ZZ^*\) seems to be surprisingly absent. The authors also mention that a \(96\GeV\) scalar-like hint from LEP and CMS could be justified by a better story if that particle resulted from the decay of this \(151.5\GeV\) pal.

A new scalar particle, if real, may be interpreted as a part of the extended supersymmetric Higgs sector, it may play other roles, too. It may also be a mirage. In particular, I am a bit worried that this mass is close to twice W-boson mass or something like that. Maybe in the calculation of the cross section, some events are incorrectly assumed to be identified as a \(W^+ W^-\) final state and subtracted from new events, and because they see more of these events, they are seen as an excess. I don't claim to fully understand this vague alternative and boring explanation, however.

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