Friday, September 4, 2015

Waiting For The Second One

What did they see?

The big buzz in physics land is that the CMS experiment at the Large Hadron Collider (LHC) has finally seen something (other than the Higgs boson) that might be interesting in the very early going of the higher energy Run II of the experiment, which began a few months ago after a long lull of no new data from Run I while equipment was upgraded and data was analyzed.

What they saw was an electron and a positron (i.e. anti-electron) moving with almost identical kinetic energies in opposite directions from each other at approximately right angles to the direction of the beam (which is called "high rapidity" in particle physics talk).

The combined energy of the two particles was 2.9 TeV, which has a bit less than a one in five hundred probability of happening at this point according to the Standard Model of Particle Physics as interpreted by CMS scientists (who could have, of course, simply overlooked some subtle part of the Standard Model theoretical prediction that makes this event more likely than it seems in the Standard Model).

Should I get excited yet?

No.  Don't get excited yet.  But, a bit of heightened awareness of what is going on at the LHC over the next few months or year or two may be in order.

In and of itself, this event could still be a fluke which is really less unlikely than it seems due to look elsewhere effects (i.e. considering all of the other events which could have happened which would have been equally notable over all the times other events could have been observed).  The rule of thumb in physics is that three sigma events (ignoring look elsewhere effects) have about a 50-50 chance of being interesting as opposed to being mere statistical flukes or experimental mistakes.

For example, there is some probability that a J/psi meson (made up of a charm quark and an anti-charm quark) could have been produced and then lead to a quite similar decay.  Almost 6% of J/psi (1S) mesons decay in this way, and there is a small probability that any given proton-proton collision at the LHC will produce a J/psi meson with the right energy (one would normally expect 1000 times less energy, however, which is one of the reasons the SM probability is so low). The point is that while the event that was seen is mildly improbable in the Standard Model, it is certainly not impossible to explain without BSM physics.  This event can easily be explained away with Standard Model physics if it doesn't happen again for another decade or so (depending upon how often the LHC runs and at what energies in that time period, of course).

Researchers recently discovered that the OZI rule of QCD is violated to an unexpected degree in charmonium decays, for reasons that aren't entirely clear.  A violation of the OZI rule means that decays involving quarkonia, such as charmonium mesons like the J/psi (1S) meson described above, are much more common that would be expected in certain circumstances.  This recently discovered violation of the OZI rule, if not taken into account in the theoretical prediction, would be just the sort of thing that could produce an unexpectedly high number of mesons that would be likely to decay as the boson in this event did, making events like this much more common than expected at the energies involved. (Also, to be clear, a violation of the OZI rule does not imply that QCD is wrong and that New Physics are necessary.  The OZI rule is particular to perturbative QCD approximation methods, and the violations observed can be demonstrated to arise empirically using means of approximating QCD that are non-perturbative such as lattice QCD.  The discovery of an OZI rule violation in a particular situation merely means that you have identified a systemic error in the methods used by perturbative QCD together with the OZI rule to approximate the actual equations of QCD which are too hard to calculate with analytically and exactly, except in the most simplified circumstances, using existing mathematical tools.)

Another likely place for the Standard Model prediction to be off, in any case, would be for there to be an inaccuracy in the QCD estimates of the PDF functions of the protons that are colliding which governs the probability that they would generate a J/psi meson or some other meson with similar decays.  Determining the PDF functions of protons at high energies is almost as much an art as it is a science.

This doesn't mean that I've proved that this is not new physics.  But, I am familiar enough with what goes into the theoretical predictions to understand some of the kinds of things that could go wrong and produce a false alarm.  Admittedly, CERN scientists are very, very careful.  But, they also do an immense number of calculations in a high pressure environment, not always knowing which factors will be most important, but they aren't omnipotent to the point of being familiar with every single relevant publication ever printed, and everybody makes mistakes sometimes as well.

While just one such event could easily be a fluke, however, even one or two more such events with the same energy would almost surely not be a fluke and would count as the discovery of a new particle, if careful analysis of all relevant calculations showed that there was no mistake in the theoretically predicted probability of this kind of event occurring.

This would mean that there is more to the world than the Standard Model.

So far, I'm skeptical, but, as I say, just one or two more of these events (particularly if one of the events is of the same energy and at the ATLAS experiment, ruling out a CMS specific systemic error) would change everything.

This would be a far bigger deal than the Higgs boson, which had been predicted and fully described except its mass for half a century.  This would be not quite a "who ordered that?" moment, hundreds if not thousands of theoretical physics have predicted particles like this, but certainly an intriguing first glimpse of some previously unrecognized part of the nature of the universe.

What if it's real?

Certainly any particle that creates an electron-positron pair would have to have an integer spin (to get combinations of +/- 1/2), and hence would have to be a boson, would have to have zero electric charge, and would have to couple to electrons and positrons either via the weak nuclear force, or some new undiscovered force (since it has no net electric charge and since the electrons and positrons don't couple to the strong force) that couples (at least) to electrons and positrons.

The high mass of the resonances, if it is a New Physics resonance, suggests that it, like the W and Z bosons, would operate only at short ranges and would probably have a very short mean lifetime.  This particle itself, if it is a particle, does not look like a likely dark matter candidate, although it may have decay products that are dark matter candidates or carry a force important to dark matter self-interactions (although it would be much heavier than the predicted mass of a boson that had that role - the expected dark matter self-interaction boson mass is on the order of hundreds of MeV, similar to muons or strange quarks at rest).

There have been predictions made regarding a possible new particle that would produce a decay like that called a Z' boson (a more technical review from 2009 is here) which would have zero electric charge, a mass of about 2.9 TeV according to some predictions (although gillions of predictions have been made regarding Z' masses, so that isn't very impressive), no strong force interactions, and an association with a new force associated with the same kind of U(1) group that describes the electromagnetic force.

The most boring possibility, in contrast, (other than a mere fluke) would be that it has exactly the same weak force couplings as the Z boson itself and is merely an excited state of the Z boson (the Standard Model doesn't predict such excited states, but excited states are observed in all composite bosons we have seen in nature), rather than the carrier of some new force.  This might imply, however, that the Z boson itself is actually composite rather than fundamental.  This possibility is called a Sequential Standard Model Z' boson (SSM) or in a more generalized description a Generalized Sequential Model (GSM) Z' boson.  It turns out that if this event is a Z' boson, that the SSM is the best fit to the part of the parameter space where this event would be found (see the original Z' boson link above).

I think it could possibly also be an A boson, the spin-0 odd parity (i.e. pseudo-scalar) companion of the Higgs boson in a scenario where there are two Higgs doublets instead of one, which would imply the a second scalar Higgs boson (another possibility perhaps) and a pair of charged Higgs bosons were likely future discoveries.  I don't know enough about spin statistics and this particular event to know that for sure.

Certainly, even one or two more observations of a similar resonance would dramatically strengthen the case for a new, more powerful collider on the model of the LHC.

If there is a Z' or something like it at 2.9 TeV, there is almost certainly new physics out there to be discovered than that.  At least a W' boson, or a Z'' boson, and perhaps more, at higher energies.


This particle, if discovered at the LHC, would also continue the folk hypothesis that bosons are discovered in Europe, while fermions are discovered in America.

1 comment:

andrew said...

Lubos has fun with numerology interpreting various recent LHC bumps of modest statistical significant as right handed W', Z' and Higgs boson prime particles in the context of a heuristic outline of a left-right symmetric GUT.

Needless to say, I am very skeptical.