I’m here again at the Rencontres de Moriond conference in Italy. Some of you might remember an update from last year from the same conference on a signal in data taken during 2015 at the Large Hadron Collider (LHC), hinting at a new particle that weighed as much as 750 protons and decayed into two particles of light. This signal wasn’t present in fresh data last year, so it was dismissed - we suppose that it was just a chance fluctuation.
This conference has a history of releasing some exciting experimental results from colliders, so I’ve been eagerly awaiting the experimental analyses of the searches for new physics. While there are – disappointingly – no significant direct signals of new particles from the collisions, evidence is mounting in the decays of some composite particles that have bottom quarks stuck together with another quark (or anti-quark): “bottom mesons”.
The calculation of Standard Model predictions for these decays is tricky and subject to large uncertainties, so the interpretation is a little murky. But measurements from LHCb and other experiments show that bottom mesons are decaying in funny ways (with the wrong probability) compared to the standard predictions.
The simplest decays to calculate involve decays to an electron, muon or tau (each with other particles). By taking ratios of the probabilities, a lot of the theoretical murkiness can be removed. Doing a fit to these and other interesting measurements yields a discrepancy with the Standard Model at around the “4 sigma” level: a result that tells you that it’s unlikely to be due to chance fluctuations in the data alone. So the first question is: is it due to a combination of the theoretical murkiness and chance fluctuations, or is it really due to the effect of new physics?
This is a difficult one to answer, but even if you convince yourself through other measurements that it really is new physics, the interpretation is difficult: the bottom mesons would be decaying wrongly because of quantum fluctuations of new particles, which are (so far) unseen. So it’s quite an indirect probe: to be really sure of what’s going on, we’d like to produce the new particles directly in LHC collisions and measure directly how they behave. Still, these are fascinating hints from very pretty data and we shall be closely watching developments in this area.
There is a new theoretical idea that is getting quite a bit of attention at the conference: “A Clockwork Theory”, invented by Matthew McCullough and Gian Giudice at CERN. It is an idea that explains how very small effective interaction strengths can be generated in models of particle physics that start with normal interaction strengths. It’s like a new toy because you can use it in lots of different ways to solve various mysteries.
If the theory ends up being used by nature, it can for example explain why the Higgs boson mass is so light (only 125 times the mass of the proton) – a problem which is difficult to solve. Calculations involving quantum fluctuations of other particles tend to drag the Higgs mass up to the heaviest mass scale that you have in the theory. The heaviest mass scale we have is associated with gravity - this is some billion billion times heavier than the proton. So-called clockwork models give indirect interactions between the Higgs boson and gravity, helping to effectively screen its mass and keep it light. Each indirect interaction is of a normal strength, but just a little bit weak. These are imagined to be like the gears of a clockwork mechanism, hence the name of the model. The whole interaction is described by many of these gears, each one cranking down the effect of gravity on the Higgs boson.
Matthew Mc Cullough gave a talk about his theory, and showed a nice way that you could verify one realisation of it at the LHC: you sieve out all of the collisions that give you two particles of light (photons) at high energies, then look at the relative probabilities of their energies¹. If the theory is correct, you should see lots of characteristic wiggles in this probability as a function of the energy. There are lots of nice mathematical techniques you could use to analyse such data: first, subtracting a smooth background and then use Fourier analysis to get information about the characteristic frequency, for example.
There have been several other talks about Clockwork Theory, which was released recently in October 2016, and it looks to be new, fertile ground.
¹ For experts, it’s really in the invariant mass of the photon pair.
