Tuesday, August 2, 2011

Another Standard Model Deviation Looking Like An Experimental Fluke

Something called the diject mass bump observed by the CDF high energy physics experiment has not been replicated by its fellow experiment at the same facility, DZero, and has also not been replicated (yet, the data are too thin so far to be definitive) at the ATLAS experiment at the large hadron collider (LHC).

This adds to the ever growing list of hints of beyond the standard model physics that appeared in early data sets in the past few years but have later proven to be statistical flukes or experimental error. Mass asymmetries between the top and anti-top quark, and the anomalous magnetic moment of the muon (muon g-2), also appear to be headed that way, as do a number of ressonances that might have indicated beyond the standard model particles. Efforts to probe for a composite nature of the quark have pushed previous boundaries on quark size down by a factor of three or four this year making the quark appear more and more point-like.

For that matter, there has also been no success in identifying a ressonance of the within the standard model Higgs boson, although there is a narrow mass window where experiments can't definitively rule it out.

Rather than finding experiments that point to more than the Standard Model, we are finding less physics than the Standard Model allows.

One area of beyond the standard model physics supported by experimental data that does seem to be holding up in the face of growing data sets is the existence of larger than predicted CP violation in heavy flavor quark and lepton decay, experimental results that have pushed the ability of a parameterized matrix of weak force transitions to the breaking point or beyond consistent with the margins of error of the various experiments done to date. Prior readers of this blog will know that one of the least radical ways to resolve this would be to discover another very high mass generation of fermions that otherwise are precisely like their existing Standard Model counterparts.

Another, which is a single data point, but a potentially important one, is that the measured radius of muonic hydrogen is not in line with theoretical predictions to the tune of a five sigma deviation from theoretical expectation, although this could have some subtle sources of error in the constants that go into making the theoretical prediction. This is a result that basically no one expected, which makes it particularly interesting. The absence of any unexpected finding in experiments with "anti-protonic helium" which has a similar methodology is discouraging to the cause of making sense of the muonic hydrogen results with any coherent theory, as have been suggestions that there may have been minor calculation errors in the first estimate. Some fumblings in the dark to look for a cause of this effect were described in a February 2011 workshop and another one in March of this year and also here. A paper proposing new physics to explain the result (not very convincingly) was published in May.

FWIW, the most plausible resolutions of this quandry in my mind are the possibility that the uncertainty in the component constants of the calculation are off in some way, leading to an overestimation of the precision of the theoretical calculation.

Another possiblity that comes to mind is that the amplitude for a muon to become a tau (third generation electron) is underestimated (a 2003 paper suggested that it might be higher than naiively expected, with the best available data coming from tauon decay), and that the measured result represents a superposition of muon and tau states cause the effects of a more massive negatively charged lepton circling the proton (a tau is almost double the mass of a proton) to be exaggerated. The error is about 4% from the predicted value, the tau is much heavier than the muon, and the PMNS matrix values aren't known to terribly great accuracy, so what looks like it should be a disregardable term might be materially greater than expected, although I am not competent to do even a back of napkin numerical impact of the transition probability that would be needed. A 2010 paper has looked at the impact of a fourth generation of leptons on these values. The amplitude for electrons to remain electrons so dominates the PMNS coeffecients that flavor changing phenomena are probably virtually irrelevant in hydrogen radius calculations because it has a more than 99% factor to stay an electron. But, coefficients for muons and taus are much more similar to each other (within a factor of two or so) than the electron coefficient is to either (at least an order of magnitude larger), so this factor which is immaterial in the hydrogen radius calculation for ordinary hydrogen may matter in the muonic hydrogen radius calculation. (See also here for the state of estimates of PMNS coefficient values.)

(This is probably as good a place as any to mention as a footnote that we still don't understand the cause of the Koide formula, which states that the sum of the charged lepton masses divided by the square of the sum of their square roots is equal to precisely two-thirds, something that has been shown empirically to be "0.666659(10)" and was used to predict the tau's mass. Some interesting speculation on what this could mean and how a similar formula might be applied to neutrino masses is found here. Carl Brannen has a discussion linked to in that post from 2006, regarding the neutrino generalization, which received a little bit of academic notice including papers by none other than Koide himself, that is also interesting as is a late 2007 blog post of his in the same vein. Whether or not Brannen is right, he is asking the kinds of questions that more academic theoretical physicists should be asking and making plausible suggestions that derve fuller airing and investigation.)

A third possiblity is that we are seeing a quantum gravity effect, something that was proposed as a possibility back in 2006, long before this experiment was conducted.

Implications For Dark Matter Theories

In addition to leaving high energy physicists looking like i dotting, t crossing technicians who write boring papers that tell us nothing new, this also impacts astronomers, cosmologists and gravity theorists, because one after another potential dark matter candidate is being ruled out by their failure to appear in direct searchers. Prevailing dark matter theories require immensive amounts of stable non-baryonic matter with very low interaction cross-sections (rather akin to neutrinos in that regard). But, the only meta-stable candidates with less than about 800 GeV of mass that is seriously in the running at this point would be some form of right handed or sterile fourth generation neutrino. Moreover, most of the theoretically conceptualized greater than 800 GeV mass, low interaction cross-section particles would rapidly decay. Even if SUSY is correct, most predictions assume that only a single kind of SUSY particle (the lighest non-charged supersymmetric particle) would be stable. Yet, a non-charged lightest supersymmetric particle, like a neutralino, now apparently constrained by LHC to be 800 GeV of mass or more by LHC, might be ruled out as a dark matter candidate by direct experiments even if we could find that one existed.

Direct astronomy oriented searches for dark matter particles have also largely come up empty handed although one did hint at a possible seasonal variation that could be a signal of a dark matter particle (although this is not being confirmed by similar experiments of the same kind that saw a signal) and there have been some less than statistically significant possible hints in the data that dark matter might be interacting weakly and leaving a trace. Their parameter space exclusions seem to naiively favor mass ranges that existing atom smashers should have discovered already, a seven or more times lighter than a top quark, which has been discovered, and to rule out dark matter particles of 800 GeV of mass or more.

Of course, charged fermions are easier to spot in a detector than those that are only weakly interacting; a slow moving heavy particle with neutral charge can masquerade as one or more fast moving neutrinos or might not be produced in those experiments at all because they don't interact with the weak force that is associated with the decay chains seen in these experiments.

Combining the direct dark matter searches of astronomers with the higher energy physics findings produces overlapping exclusion zones for dark matter candidates that both would have any possibility of observing, something that makes previously out of left field concepts, like a massive graviton or a dark matter candidate that doesn't interact with the weak force, look more attractive. It also puts pressure on scientists to identify gravity modifications or improved general relativity calculations that can fit the data that gave rise to dark matter theory (or at least enough of a modification to end the need to find a dark matter particle that has been ruled out).

From a theoretical physics perspective, the mere experimental determination that any beyond the standard physics particles that could be dark matter candidates are excluded, even if a resolution of the dark matter issue is not reached, undercuts one of the major motivations for beyond the standard model physics. The less there is for beyond the standard model physics to explain, there more doubt is cast of the need for it at all, and the more likely it is that whatever is needed to modify the standard model is going to be subtle and boring.

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