The latest data from the Large Hadron Collider has now been released, and it contains a most interesting new result. Perhaps not on par with the Higgs boson discovery, or any of the hundreds of predictions of the theoretical physics community, but still enough to make the research community sit up and take notice.

While most of the data is little more than a confirmation of existing knowledge, there is a new result at 750 GeV. The two detectors at the LHC - CMS and Atlas - have both found an excess of diphoton signals at that energy level. (A diphoton signal represents the production of two high energy photons, without anything else significant being produced). And the two independent teams have both put the significance at around 3-sigma, which suggests it is probably not a statistical fluctuation.

So what is this new signal?

As expected, my colleagues in the theoretical physics community are working night and day to bombard the pre-print servers with predictions. Every researcher has their own favorite models, and they are all attempting to make this new result fit their own preferences. And so at the risk of offending many of these theorists by omitting their theories, here are some of the most general possibilities.

A sudden excess at a specific energy is almost always a signal of a new particle. The simplest explanation is that there is a heavier bound state of three quarks (essentially a heavier version of the neutron) or of a quark-antiquark pair which for some as yet unknown reason likes to decay to photons. If it is three quarks, then that becomes very interesting because these bound states cannot decay to photons without producing other particles - so either the two teams have missed something in the background noise or it is a new type of decay that has never been seen before. A quark-antiquark pair could produce this signal, but it would have to be very heavy and have some other unusual properties.

Another very interesting explanation would be a heavy lepton. This is the family of particles that includes the electron, the muon, and the tau. However this new particle is close to 500 times heavier than the heaviest known lepton, and furthermore leptons also produce other particles in their decays - specifically neutrinos. So it is a possibility, but one that requires several new properties that have not been seen before.

A more interesting option is a non-Standard Model particle, and this is the one the theory community is really focused on. We know that 95% of the energy in the Universe is not described by the Standard Model, and so this could be a window on the dark part of the Universe. The LHC could have produced very heavy Weakly Interacting Massive Particles (WIMPs) which compose the dark matter in the Universe. The only problem is that dark matter should not decay this quickly, and definitely should not be decaying to photons! But there again, perhaps this is some excited state of dark matter that decays to the normal form that fills the Universe. We just do not know.

Another interesting option is that it could be a hint of higher dimensions in our Universe. The Randall-Sundrum model, first proposed in 1998, predicts that there exists a fifth dimension which manifests itself in our Universe in the form of a very heavy particle called a radion. There are also models that contain five or more dimensions, which have been studied by theorists since the 1920s, and which predict that all of the Standard Model particles will have heavier copies of themselves due to interactions with the hidden dimensions. This could possibly be one of those copies.

There are also technical reasons to believe that every particle species in the Standard Model is paired with a heavier particle known as a superpartner, in which case the diphoton excess could be the first experimental evidence of supersymmetry.

Or it could be a gauge boson from a fifth force. The Standard Model predicts three forces of nature, each of which corresponds to a specific symmetry of one, two, or three dimensions. It has long been speculated that there could be another force that is generated by a four-dimensional symmetry, known as either SU(4) or technicolour, and which would have 15 new, heavy particles, any of which could be the cause of the diphoton excess through some new interaction.

And those are just the most popular candidates. At this point, we know nothing of this new particle except that it is at least four times heavier than anything we have seen before, and it really likes to decay to two photons and nothing else that is easy to detect. And now we can await the onslaught of explanations from the theorists...

Note: This article was originally posted on December 15, 2015 but due to a server error it was deleted.