Saturday, July 31, 2021

Both ATLAS, CMS: it may look like a new \(1\TeV\) particle

One week ago, Tošihide Maskawa (81) died of acncer; see Physics World. He co-authored the CKM matrix with Kobajaši and got the 2008 Nobel Prize in Physics with Jojčiro Nambu (with the symmetry breaking as a theme).
The excesses are 3.00-3.12 sigma

I have never stopped going through all the ATLAS and CMS (LHC) papers on the arXiv plus the CERN preprint server (which alerts me through Feedly), looking for signs of new physics. The number of people who do this job in the world may be counted on the fingers of two hands; it is more or less true that the LHC collider was built almost just for me and I deserve it. An hour ago, a brand new ATLAS preprint
Search for resonant and non-resonant Higgs boson pair production in the \(b\bar b\,\tau^+\tau^-\) decay channel using \(13\TeV\) \(pp\)-collision data from the ATLAS detector
had to attract my attention because the mandatory search for the word "excess" has produced nice whole promising sentences, not to mention the Figure 8 with the excess.



Excess in what? Even the abstract of the ATLAS paper is enough to answer this question.
The largest excess in the resonant search is observed at a resonance mass of \(1\TeV\), with a local (global) significance of \(3.0\,\sigma\) (\(2.0^{+0.4}_{−0.2}\sigma\)).
OK, if it is more than a fluke, a \(1\TeV\) boson decays to \(HH\) and those Higgs bosons (regular 2012 Higgs bosons at \(125\GeV\)) decay to bottom pairs and tau pairs, respectively (third generation; the decay to top pairs is impossible on-shell, tops are too heavy; third generation neutrinos could be possible products as well but they are almost impossible to be detected).



That's cute, probably a cute fluke, but I felt deja vu. More precisely, I knew with quite some certainty that I had to see another paper just a day or two earlier – clearly not quite the same search but a rather similar one – that also reported a 3-sigma excess around \(1\TeV\). A few minutes were enough to find that paper (out of a dozen of candidates) and happily enough, it came from the ATLAS' competitors at CMS:
Search for new particles in an extended Higgs sector in the four \(b\)-quark final state at \(\sqrt s = 13\TeV\)
These CMS physicists have searched for a resonance \(X\), a new boson, that decays to a pair of bosons \(X\to aa\), and those continue to decay to \((b\bar b) (b \bar b)\). It is almost the same thing as the ATLAS search except that the intermediate bosons are assumed to be more general, \(aa\) instead of \(HH\) (but the Higgs pair would probably lead, anyway), and one of the Higgses also decays to a bottom pair instead of a tau pair. The most eye-catching sentence says
The largest excess of events, with a local significance of \(3.12\sigma\) is around a dijet mass of \(1\TeV\) and average jet mass of \(70\GeV\).
Cute. The brutal combination of the excesses would raise the significance of the \(1\TeV\) signal above 4 standard deviations but that's only the CERN's local score, the global significance could be close to 3 sigma from the combination. A new \(1\TeV\) boson wouldn't confirm any "truly promising" theorist's extension of the Standard Model that I can think of but obviously, it may play a role in numerous theories beyond the Standard Model. My moral addition has to be added: the first interpretation has to be the heavier new Higgs boson in an extended Higgs sector, most sanely the Higgs sector of a (not necessarily minimal) supersymmetric standard model.



You may also look at another paper, a CMS search for higgsinos (search for the words "single bin" on that page) where a single bin with 3 bottom quarks, MET between \(300\) and \(400\GeV\), and \(\Delta R_{\rm max}\lt 1.1\) contains 4 events instead of 0.07 predicted ones. Because of a heavily asymmetric distribution, this seemingly huge amplification ends up being equivalent to a 3.2-sigma deviation.

You can see that in all these three papers' excesses, at least three fermions from the heaviest generation appear in the final state.

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