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CMS in \(ZZ\) channel: a 3-4 sigma evidence in favor of a \(650\GeV\) boson

Today, the CMS collaboration has revealed one of the strongest deviations from the Standard Model in quite some time in the paper

Search for diboson resonances in the semileptonic \(X \to ZV \to \ell^+\ell^- q\bar q\) final state at \(\sqrt{s} = 13\TeV\) with CMS
On page 21, Figure 12, you see the Brazilian charts.




In the channel where a resonance decays to a \(ZZ\) pair and one \(Z\) decays to a quark-antiquark pair and the other \(Z\)-boson to a lepton-antilepton pair (semileptonic decays), CMS folks used two different methods to search for low-mass and high-mass particles.




In the high-mass search – which contributed to the bottom part of Figure 12 – they saw a locally 2-sigma excess indicating a resonance around \(1000\GeV\), possibly compatible with the new \(\gamma\gamma\) resonance near \(975\GeV\) that appeared in some new rumors about the 2016 data.

More impressively, the low-mass search revealed a locally 3.4 or 3.9 sigma excess in the search for a Randall-Sundrum or "bulk graviton" (I won't explain the differences between the two models because I don't know the details and I don't think one should take a particular interpretation too seriously) of mass \(650\GeV\). The "bulk graviton" excess is the stronger one.

Even when the look-elsewhere effect (over the \(550-1400\GeV\) range) is taken into account, as conclusions on Page 22 point out, the deviation is still 2.9 or 3.5 sigma, respectively. That's pretty strong.

We may wait for ATLAS whether they see something. (Update: In comments, a paper with a disappointing 2-sigma deficit on that place is shown instead.) CMS has only used 2.7 inverse femtobarns of the 2015 data in this analysis. Obviously, if the excess at \(650\GeV\) were real, the particle would already be safely discovered in the data that have already been collected in 2015-2016 (by CMS separately) – about 18 inverse femtobarns (CMS).

Concerning the \(650\GeV\) mass, in 2014, CMS also saw a 2.5-sigma hint of a leptoquark of that mass (more on those particles). Note that the leptoquarks carry very different charges (quantum numbers) than the "bulk graviton".

The previous CMS papers on the same channel but based on the 2012 data were these two: low-mass, high-mass strategies. I think that there was no evidence in favor of a similar hypothesis in those older papers. That also seems to be true for the analogous ATLAS paper using the 2012 data.

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