So What Is It? That’s the question one hears in all the bars and on all the street corners and on every Twitter feed and in the whispering of the wind. Everybody wants to know. That bump seen on the ATLAS and CMS two-photon plots! What… IS… it…?
Well, to be honest, probably it’s just that: a bump on a plot. But just in case it’s not — just in case it really is the sign of a new particle in Large Hadron Collider [LHC] data — let me (start to) address the question.
First: what it isn’t. It can’t just be a second Higgs particle (a heavier version of the one found in 2012) that is just appended to the known particles, with no other particles added in. If you try to put that idea into the equations, you immediately find the new particle’s mass is so large that it (or anything similar) would decay very often to other known particles, and very rarely to photons.
[Like the Higgs particle, it would only interact with photons indirectly, via direct interactions with the other known particles (see figure 3 of this post); but unlike the Higgs particle, it is heavy enough that it can decay to any particle/anti-particle pair with which it has direct interactions (see figure 2 of the same post); and the direct effect beats any indirect effect.]
Therefore, if this idea were correct, either
- no two-photon events would be observed yet, making the bump impossible, or
- there would be tens of thousands of these new particles decaying to pairs of jets (from quark/anti-quark pairs), or to top quark/anti-quark pairs, or to charged lepton/anti-lepton pairs… any of which would have been easily seen in studies done by ATLAS and CMS using the 2011-2012 LHC data.
Now, this has a really interesting implication! If this bump is a real particle, there must be at least one other particle that makes such a two-photon bump possible. That new particle (maybe heavy, or maybe lightweight) hasn’t yet been found by the ATLAS or CMS (or LHCb!) experiments, but is almost certainly accessible to them. In fact, signs of an additional particle or particles may already be obvious (or almost obvious) in their existing data. It might just be a question of looking in the right place and of asking the right question.
This is what a lot of theoretical work this week has been about: considering the options for what the two-photon bump’s source might be, and thinking about further consequences of its possible existence, as far as other particles and forces with which it might be associated.
(And yes, I admit it; I chased this ambulance too. Took the week off from work. Didn’t sleep much.)
So what is it? There are a lot of possibilities. It could be a simple elementary particle like the electron, interacting with known particles indirectly, as a result of direct interactions with some heavy, as yet unknown particles. It could be a composite object like a proton, made from objects bound together by a new force that we are about to discover. It might be that the photons to which it seems to decay aren’t photons after all, but are new particles that mimic photons in some way (an idea that goes back at least to 2000.) I certainly don’t know what it is, and neither does anyone else yet. But each of the many different ideas comes with predictions — predictions which in many cases lead to LHC signals that can be looked for NOW!
We’ll have to wait quite a while, six months to a year, before we have enough new data to see the bump either disappear or become a completely convincing signal of a new particle. However, perhaps the bump is a clue … a clue that can lead us to a discovery that can happen much sooner.
BUT WHAT IS IT???? Sometime in the next few weeks I’ll write a non-expert’s guide to the various options. Probably this won’t happen before the holidays, but stay tuned in the New Year.