Last December, when the ATLAS and CMS experiments gave two bacl-to-back talks at the end-of-the-year LHC "physics jamboree" in the CERN main auditorium, the whole world of particle physics was confronted with a new question nobody had seen coming: could a 750 GeV particle be there, decaying a sizable fraction of the time into pairs of energetic photons? What new physics could account for it? And how to search for an experimental confirmation in other channels or phenomena?

As those of you who have followed the proceedings know, the answer on the existence of the phantomatic 750 GeV boson was given at ICHEP this August, when larger datasets by the LHC experiments excluded the earlier claimed signal. Mind you, the fact that one dataset hints at the existence of a particle and a larger one disproves it does not mean we should be absolutely sure of the spurious nature of the signal. The particle could still exist, but with a much smaller production and decay rate than earlier claimed; what is most relevant for a physicist is that there is absolutely no evidence for it, once the full information is considered together.

Because of the above point, the additional information on the existence of the boson coming from ancillary analyses targeting potential additional signatures is still interesting in the post-mortem phase we are in. Indeed, since last December the LHC researchers did not sit still, and they considered many possible complementary decay channels and search strategies that could confirm or disprove the December 2015 claim, by mining the data already in their possession and planning more searches on the data about to pour in. So it was not just phenomenologists spewing out interpretations at an accelerated pace (about half a thousand papers!): also the ATLAS and CMS experimentalists busied themselves in the chase of the 750 GeV phantom, during the past few months.

The result of ancillary searches that could be sensitive to the 750 GeV boson are now coming out, maybe less promptly than they would have if there had been some extraordinary confirmation to publish. Also, the fact that a good part of the original motivation for those studies has now been disproven causes the title and abstract of those publications to be devoid of a direct mention of the diphotonium. if you had no clue about what physics a 750 GeV particle might contribute to, you would have a hart time choosing which paper to read to learn about further evidence for or against its existence.

This is indeed the case of one of the latest public results produced by CMS, a search for Z+gamma resonances. Granted, it takes little fantasy to realize that a resonance decaying to a Z boson (a spin-one, neutral particle belonging to the electroweak sector) and a photon (a spin-one, neutral particle belonging to the electroweak sector) might also decay to two photons (spin one, neutral particles belonging to the electroweak sector). But still, I think it's peculiar to note that the article makes no mention whatsoever of the 750 GeV particle in the text, relegating a hint of the motivations to four references:

In the context of the wider search for new resonances in the diphoton final state [7–9], information from the Zγ channel provides important complementary information. For example, an extended SM incorporating a scalar (or pseudoscalar) decaying to two photons would imply that Zγ decays should be observed as well [10].

As an author of CMS papers I support the above, but since as a science popularization agent I am also always looking at particle physics publications with the eye of the bystander, I have the feeling that an explanation of the connections of this physics result with the search for the 750 GeV boson is due, at least in the context of commentaries such as this article.
So now let's see what the search provides. The Z boson signature is sought in decays to electron-positron or muon-antimuon pairs, which offer a very clean starting point, with almost no background once the two leptons are required to be isolated from any hadronic activity (background leptons from B decays might otherwise provide an annoying extra source of events). Once one searches for an additional photon, the data is 80 to 90 percent pure of real Z plus photon events, with a minor contamination from Z plus jet events where the jet fragmented to a leading neutral particle. 

The three-body (lepton-lepton-photon) mass distribution that is derived is shown below, separately for the electron and muon channels (first two plots, respectively left and right) in 19.7 inverse femtobarns of 8-TeV data, and for the same signatures in 2.7 inverse femtobarns of 13-TeV collisions acquired in 2015 (second set of plots. Do you see a peak?

Of course there's no peak. The data is compatible with the background model, shown as a red line and grey band in the graphs. From the agreement of data and model, CMS extracts limits on the production rate of such a resonance. This is shown below.

It is of course possible to invoke some arcane mechanism that would allow a heavy boson to decay to photon pairs but not to Z+photon pairs, but it is clear that the absence of a signal in the above spectra is a further evidence of the spurious nature of the 750 GeV particle - but as I said before, this was not really much needed at this point. Still, it is of course extremely important to continue searching for new physics with the LHC data in all possible thinkable ways. Every corner of the dark cavern we are exploring with the powerful lamp in our possession needs to be carefully examined!

The CMS preliminary result is available at this link.


About the author

Tommaso Dorigo is a INFN researcher, a member of the CMS experiment at the CERN LHC, and the scientific coordinator of the AMVA4NewPhysics network. He is active in science outreach and he recently published the book "Anomaly! Collider Physics and the Quest for New Phenomena at Fermilab", available for purchase on Amazon.