A couple of months ago I wrote about an anomalous bump in data from the Large Hadron Collider at CERN. This is one of a few little indicators we have from the data recorded over the past years, straws in the wind which, if we are lucky, are the first signs of a new breakthrough. Another such teaser comes from the LHCb experiment at CERN, as well as the Babar experiment in California. It is a rather different kind of hint than the bump. Before describing that though, there is also a minor update on the bump.
The bump is in the “diboson mass distribution”, and can be interpreted as tentative evidence for a new fundamental particle, beyond the Standard Model, with a mass about two thousand times that of the proton.
“Diboson” in the name of the distribution means that it appears to come from a pair of W or Z bosons (or one of each). The W and Z bosons are the carriers of the weak force, which operates at very short distances, deep inside the atomic nucleus.
These particular W and Z bosons are created in proton-proton collisions at the LHC, and they decay almost immediately. They decay most often to jets of hadrons, and it is in these decays that the bump appears most strongly. However, they sometimes decay to leptons – electrons, muons or neutrinos. This means that with a few assumptions about the mix of W and Z bosons in the data (which implies assuming something about the speculative new physics) the hadronic jet data can be combined with the lepton data, to see what that might tell us. The ATLAS collaboration (of which I am a member) have just done this with our data.
The main result of this combination is below, showing the probability that the Standard Model (with statistical fluctuations) might produce the bump on its own. The lower the dip, the less likely that is, and therefore the stronger the evidence for new physics:
Note that the long-dashed lines, which exclude the hadronic jet data, show almost no dip. The dip is strongest in the short-dashed lines, which include only the jet data. The combination (solid lines) still shows a dip, but somewhere in the middle (at about 2.5 sigma, compared to 3.4 sigma for the jet data alone).
The red and blue sets of lines are for different theoretical assumptions as to what the new physics might be, if there is any. This affects which decay modes should be included, and so gives different results.
There are also hints from the CMS experiment which are not combined here. Also, the experiments should be able to tell something from possible signatures where a Z boson decays to neutrinos, but that data hasn’t been used here either.
None of this is very surprising, but it is good to see the combination done formally. It doesn’t really change my earlier opinion, that is:
If you ask me, I take this bump seriously enough to be very interested, bordering on excited, but if I had to bet I would bet against it being a new particle.
Ultimately, the new data being taken now will decide, though it will take a while. The cross section for these kind of events is about 0.01 picobarns¹, which means that to get just one more event we need about a hundred inverse picobarns of data. Currently we have about 300 inverse picobarns (see here for the latest – plots will updated as the data arrive). A handful of events isn’t enough to say much, we could do with a factor ten to a hundred more data before they start to make a difference. But that should happen over the next few months. Continue to watch this space.
Over to LHCb
Meanwhile, another experiment on the LHC has published some intriguing straws of its own, which are blowing in the same direction as some results from previous experiments.
The particles of the Standard Model come in threes, which we refer to as “generations”. There are three types of neutrino. The electron has two heavier relatives, the muon and the tau. And then there are the quarks; the up, charm and top quarks belong together, and the down, strange and bottom complete the set.
In the Standard Model, the only difference between the different generations is the mass of the particles. So a tau is supposed to be the same as a muon in every way, except that is is more massive. And muons are likewise supposed to be the same as electrons. This is a postulate know as “lepton universality”, and in the Standard Model at least, it holds. The question is, does it hold in nature?
Some measurements indicate not. The Babar and Belle experiments made measurements of hadrons containing bottom quarks, decaying to muons or to taus. Babar in particular saw some evidence that the tau decay happens more often than the muon, violating lepton universality.
If true this would be extraordinary. Particle decays are affected by quantum loops in which different particles can exist briefly. Therefore, unexpected decay rates can be a sign that new, unexpected particles are zipping around those loops.
Like the top quark (the heaviest Standard Model particle), the bottom quark and the tau belong to the third generation, and in many ideas about physics beyond the Standard Model, the third generation plays a special role, simply because it is more massive. For example, if there are extra Higgs bosons out there, they might be expected to interact more with heavy particles because of the role the Higgs plays in determining mass. This could change the decay rates. There are other possibilities.
The experiment at the LHC which specialises in measuring such things is LHCb (the b is for bottom, or beauty – the other name sometimes associated with that quark). LHCb results on this kind of bottom-quark decay were eagerly awaited, and, as quite widely reported, they seem to confirm the evidence from Babar that decays to taus happen more often than expected, and specifically, more often than those to muons.
The evidence is, as usual, not quite five sigma. It is 2.1 sigma from LHCb and 3.4 sigma from Babar. Three sigma is conventionally defined as evidence, five sigma as a discovery. As usual we want more data. But it is another reason to be cheerful.
Perhaps there is even a common cause – a new 2 TeV particle being responsible both for the bump and, via a quantum loop, for the decay anomaly. Some theorist somewhere is bound to be working on it.
Or perhaps the straws will blow away in the wind of the new LHC collisions. Time, and data, will tell.
¹ See here for some discussion of what these units mean. Also, (added 15/9/15) I meant to say at this point that the increased beam energies will increase the cross section somewhat, which helps too. But we still need to wait a while.