Tuesday, June 21, 2011

Coming Attractions in Physics

There have been several "false alarms" this year with data showing beyond the Standard Model results from experiments only to be promptly contradicted with additional data or other experiments.

Make or break point for Higgs should be in time for Oscars

But, at the rate the Large Hadron Collider is going, we can expect the big event of big events for high energy physics within the next six months. The Standard Model Higgs boson should be either discovered or ruled out by then. We should know if there is a Higgs before we learn who the best actor was for 2011. Finding a Standard Model Higgs boson would complete the Standard Model's predictions, while not finding it would open up a whole new can of worms as theorists scramble to find the most plausible alternative.

The Higgs, recall, is a theoretical particle that imparts mass to the massive particles in the Standard Model (and also has some W particle-like transformative effects that blend different quantum components of ordinary particles), with zero intrinsic spin, a significant vaccuum expection value (something that is zero for almost all other particles in almost all places), has no charge, interacts via the weak force, and has a mass on the order of 120 GeV/c^2 (the exact potential mass range varies from month to month).

Key experiment driving SUSY particle mass predictions may be wrong

Another interesting paper makes an contribution to the long standing effort to confirm or deny the extension of the Standard Model called supersymmetry (SUSY for short) which is a necessary component of any string theory.

There are lots of moving parts in SUSY, but most versions of it have predicted at least one particle (the "lightest supersymmetric particle") to have a mass of 1 TeV or less, which is bad news for SUSY fans because LHC has largely ruled this out for all but one candidate particle and has ruled out that candidate particle for masses of less than 750 GeV and closing. But, the theoretical "preference of the global fits for light superparticle masses is driven by one measurement: the long standing 3 sigma excess in the muon anomalous magnetic moment." (See, e.g., here (2008), here (2007), here (2003), and with an early version that seems to impose mass limits long since exceeded here (2001).)

This gap between the Standard Model prediction and the experimental result was big news in 2001. The new paper suggests that the apparent excess of the experimental value of the muon anomalous magnetic moment ("muon g-2") over the theoretically predicted value in the Standard Model is a product of inaccurate theoretical calculations, which, when calculated properly confirm the Standard Model prediction. If you look at the raw data this is less surprising, it doesn't take a huge nudge in the theoretical value (which in turn depends upon other experimentally measured quantities) to make a big difference in the discrepency, which is on the order of one part per million.

The experimental result from 2001 is aμ+ = 11659202(14)(6)×10^-10 (1.3 ppm).

The current theoretical value from the standard model from 2001 is aμ(SM) = 11659159.6(6.7)×10^-10 (0.57 ppm).

The difference, aμ(exp)-aμ(SM) = 43(16)×10^-10.

A 2006 experiment found a discrepency of 2.2 to 2.7 standard deviations and included theoretical uncertainty of about the same order of magnitude at the experimental uncertainty. Concerns that inaccuricies theoretical estimate were the source of the discrepency were already being raised in 2004 and repeated with more force indicating that the discrepency could be as small as one sigma with certain experiments used to set Standard Model constants in a 2009 publication. On the other hand, the Standard Model prediction is one of ongoing research with a 2007 estimate of it arguing that the discrepency was actually understated, and a 2009 paper came to a similar conclusion.

If the new calculation by a novel method is correct, the gap between the theoretical prediction of the Standard Model and the experiment result would be roughly 1% of the previous estimate - bringing it from a 2-3 sigma gap to a 0.02 to 0.03 sigma gap, which would normally be treated as an extremely clear confirmation of the theory by the experiment.

The good news for SUSY fans is that such an error would make higher masses for the lighter supersymmetric particles plausible, thereby preventing SUSY (and with it string theory) from being ruled out by LHC experiments in the near future due to an absence of light supersymmetric particles (concerns that a large muon g-2 were inconsistent with SUSY were being raised years before the LHC came online).

The bad news for SUSY fans is that the fewer deviations from the Standard Model we observe, the less we have an empirical data driven need for any kind of Standard Model extension. While a value for muon g-2 that is too high implies that SUSY particles are too light to be consistent with experiment, a value for muon g-2 that is extremely low suggests that SUSY particles may be predicted to be too heavy to make sense given other experimental constraints on the model.

Low energy QCD calculations close to confirmation

Preliminary results from the RHIC experiment tend to confirm a "phase change" in low energy quark behavior as a result of the strong force equations at a critical boundary that is a function of temperature and baryon chemical potential. Up until you hit the critical boundary, you expect norm "hadrons" made of quarks collected together in lumps like protons, and neutrons and mesons. Beyond it, you expect a quark-gluon plasma which should look different in the experiment (or perhaps a "color superconductor" in a special part of the beyond the boundary region of parameter space found in places like neutron stars that isn't within the scope of these experiements).

But, the preliminary experimental results from RHIC have an error bar too wide to confirm the results because there is too much statistical uncertainty in the small data set collected to date. Enough data to confirm or deny the QCD equation predictions should be available from this experiment by year end. If the predicted results are still observed when the data are all in, it will be a major confirmation that the QCD equations are experimentally accurate, and that the two camps of people doing the insanely difficult math that the QCD equations imply (the discrete mathematicians who use a lattice approximation to exactly calculate the full equations at discrete points, and the perturbative mathematicians who start with simplified QCD equations and then estimate how much different the real results are from the predicted ones used advanced calculus methods that are continuous) are on the right track. Both camps predicted essentially the same critical point in recent theoretical work.

Now, "experiments confirm that forty year old equations still work" won't make the same kind of headlines that the Higgs will, but, it still matters quite a bit because QCD has some of the biggest holes in the program of experimental confirmation of the Standard Model, in part, because it is so hard to figure out what the Standard Model predicts because the math is so difficult and in part because the experiments themselves aren't easy to devise or cheap to conduct.

Proving a theoretical result like this is a bit like flying completely blind from New York to Los Angeles and coming to a safe landing anyway. There is a very long chain of mathematical reasoning that goes from the experiments that caused us to propose the QCD equations of the Standard Model (which govern the forces that hold atomic nuclei together) to this experimental confirm of those equations where almost none of the experimental conditions that the equations were designed in are similar.

Even more importantly, the phase change that RHIC seems to be observing and that is predicted is the first real evidence of chiral symmetry breaking in QCD. There are natural terms in the QCD equations that were used to make these predictions that suggest that the strong force distinguishes between particles with left handed spin, and particles with right handed spin, something that has so far been observed only in weak force interactions. But, so far, no distinction has been observed and instead we have seen "chiral symmetry" in the strong force. This has cast doubt on whether the chiral symmetry breaking term should even be in the Standard Model QCD equations.

But, if this experiment's implications are correct, the chiral term does belong in the QCD equations and hasn't been observed to date because we haven't observed quarks and gluons in conditions where term has the effect of breaking the chiral symmetry. Also, if chiral symmetry breaking is observed, it may shed light on the "strong CP problem," one of the big unsolved questions in fundamental physics (the absence of CP violations in the strong force).

This result are also important because they establish that theoretical physicists are finally reaching a point where they are able to solve equations that have to date been a question mark. Accurate QCD calculations are the background against which all other high energy physics phenomena are calculated, and the more confidence that we can have in their accuracy, there more statistical significance experimental results, past and present, have in relation to the theoretical predictions.

The muon g-2 reanalysis mentioned above is just one example of the kind of result that we can expect to appear over and over again in the entire range of quantum physics phenomenology as the theoretical prediction calculations grow more precise and reliable. Old results that seemed to show a discrepency between experiment and theory may be reconciled in some cases, and anomalous results that weren't statistically significant enough given the undercertainty in the underlying theoretical prediction in the past may rise to statistically significant levels that call for further investigation in other cases.

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