A New Particle at LHC for Christmas??

Hello particle gobblers and happy new year from my new location at the University of Granada.

In between presents and feasting, you may have heard rumblings over the holidays that the LHC could be seeing hints of a new and very massive particle. The rumors began even before the ATLAS and CMS experiments announced results from analyzing the brand new 13 TeV (in particle physics units!) data which was collected in 2015. At 13 TeV we are now probing higher energy scales of nature than ever before. These are truly uncharted waters where high energy physicists basically have no idea what to expect. So there was a lot of anticipation for the first release of new data from the LHC in early December and it appears a tantalizing hint of new physics may have been left there dangling for us, like a just out of reach Christmas cookie.

Since the announcement, a feeding frenzy of theoretical work has ensued as theorists, drunk from the possibilities of new physics and too much holiday food, race to put forth their favorite (or any) explanation (including yours truly I must confess:/). The reason for such excitement is an apparent excess seen by both CMS and ATLAS of events in which two very energetic photons (particles of light) are observed in tandem. By `excess’ I basically mean a `bump‘ on what should be a `smooth‘ background exactly as discussed previously for the Higgs boson at 125 GeV. This can be seen in the CMS (Figure 1) and ATLAS (Figure 2) results for the observed number of events involving pairs of photons versus the sum of their energies.

Figure 1: CMS results for searches of pairs of photons at 13 TeV.
Figure 1: CMS results for searches of pairs of photons at 13 TeV.
Figure 2: ATLAS results for searches of pairs of photons at 13 TeV.
Figure 2: ATLAS results for searches of pairs of photons at 13 TeV.













The bump in the ATLAS plot is easier to see (and not coincidentally has a higher statistical significance) than the CMS bump which is somewhat smaller. What has physicists excited is that these bumps appear to be at the same place at around 750 GeV^1. This means two independent data sets both show (small) excesses in the same location making it less likely to be simply a statistical fluctuation. Conservation of energy and momentum tells us that the bump should correspond to the mass of a new particle decaying to two photons. At 750 GeV this mass would be much higher than the mass of the heaviest known particle in the Standard Model; the top quark, which is around 174 GeV while the Higgs boson you will remember is about 125 GeV.

It is of course statistically very possible (some might say probable) that these are just random fluctuations of the data conspiring to torture us over the holidays. Should the excess persist and grow however, this would be the first clear sign of physics beyond the Standard Model and the implications would be both staggering and overwhelming. Simply put, the number of possibilities of what it could be are countless as evidenced by the downpour of papers which came out just in the past two weeks and still coming out daily.

A simple and generic explanation which has been proposed by many theorists is that the excess indicates the presence of a new, electrically neutral, spin-0 scalar boson (call it \varphi) which is produced from the fusion of two gluons and which then decays to two photons (see Figure 3) very much like our earlier discussion of the Higgs boson. So at first appearance this just looks like a heavy version of the Higgs boson discovered at 125 GeV. Crucially however, the new potential scalar at 750 GeV has nothing (or atleast very little) to do with generating mass for the W and Z bosons of the Standard Model which is the role of the Higgs boson. I will save details about the many possibilities for a future post^2, but essentially the many models put forth attempt to explain what occurs inside the gray `blobs’ in order to generate an interaction between \varphi with gluons and photons.

Figure 3: Production of a new scalar particle via gluon fusion followed by decay into photons.
Figure 3: Production of a new scalar particle via gluon fusion followed by decay into photons.

It will of course take more data to confirm or deny the excess and the possible existence of a new particle. Furthermore, if the excess is real and there is indeed a new scalar particle at 750 GeV, a host of other new signals are expected in the near future. As more data is collected in the next year the answers to these questions will begin to emerge. In the meantime, theorists will daydream of the possibilities hoping that this holiday gift was not just a sick joke perpetrated by Santa.


1. It is a bit difficult to tell by eye because the ATLAS plot axis is linear while that for CMS is logarithmic. A nice discussion of the two bumps and their location can be found here.

2. For those feeling more brave, a great discussion about the excess and its implications can be found here and here.

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Roberto Vega-Morales is currently a post-doctoral researcher in high energy theory at the University of Granada in Spain. Previously he was at the Laboratoire de Physique Thèorique in Paris France. He conducted his Ph.D studies at Northwestern University as well as Fermilab and was awarded the 2014 J.J. and Noriko Sakurai Dissertation Award in Theoretical Particle Physics. His research focuses on the phenomenology of the Higgs boson at the LHC as well as models of Supersymmetry and extended Higgs sectors. He struggled mightily with French and is happy to be speaking Spanish nowadays.

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One Reply to “A New Particle at LHC for Christmas??”

  1. The theoretical prceduije on the second question is much weaker, however; we really have no idea whether the Higgs is of Standard Model type or not. But it seems likely we’ll need the full year’s data set before we start making much progress on this crucial issue (though surprises are possible, if the Higgs is sufficiently different from a Standard Model-type Higgs). In what respects might another year of data distinguish a 125 GeV +/- particle that is merely impersonating a Standard Model-type Higgs from the real McCoy?Presumably we would know, almost immediately, given the nature of the searches that are showing some kind of signal at that mass range already that: (1) this particle has no electromagnetic charge, (2) that this particle experiences weak force decays that include the predicted channels from the Standard Model, (3) this particle has an even integer spin (which follows from diphoton production), (4) this particle lacks QCD color charge (this follows from both diphoton productioon and some of the other decay chains attributed to the Higgs boson signal, and (5) that the mass of this particle is not inconsistent with constraints on the mass of a Standard Model Higgs boson derived from electroweak data, vacuum stability considerations, or unitarity considerations.One can imagine that an imposter Higgs boson would have different couplings to some fermions than the Standard Model predicts, I suppose. But, if this were the case, would another year of LHC data really reveal that? I support one could imagine a Higgs boson that interacts equally with both right handed and left handed fermions in a way that resembles weak force interactions, even though W and Z bosons only appear to interact with left handed fermions (potentially providing a bridge to a sterile fermion sector), but again, would another year of LHC data reveal that?I guess I could imagine CP violation with the Higgs boson that differs from the Standard Model prediction value, but even then, a change of a parameter like that, unless it was extremely striking, would seem on a par with the addition to the Standard Model of mass for neutrinos. Nobody talks about neutrino oscillation which supports the existence of neutrino mass as evidence of non-Standard Model neutrinos. The only result I can imagine that would be consistent with data so far, that would be revealed by another year of LHC data but inconsistent with a SM Higgs, would be non-SM branching fractions in its weak force decays. Am I missing any interesting possibilities?

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