Two papers say the SM is doomed. But is it ? March 7, 2008Posted by dorigo in news, physics, science.
A colleague pointed me today to two papers recently appeared in the Arxiv: this and that. First signs of physics beyond the Standard Model ? Not if you ask me. Let me describe in short -because I am about to go to sleep!- what these new interesting works are about.
The first paper (0803.0512) discusses leptonic decays of mesons: ones involving a charged lepton and a neutrino. An average of different determination finds that they are more frequent than what theory predicts, and the effect is quoted at 3.8 standard deviations. Hmmm. What is it, however, that the experimental average is comparing to, to see that significant discrepancy ?
It is a computation from lattice QCD. The paper reviews “critically” two different computations of the factor , which is the quantity measured at by BaBar, Belle, and CLEO. One computation finds , another finds . There are reasons, according to the authors, to prefer the former to the latter, and there you go, a discrepancy of arises.
In my deep ignorance of lattice QCD, I spent some time pondering on the claims in the paper on why we should prefer the measurement which disagrees most with experiment, and I came to the conclusion that they are insufficient. You can go through the same process and come to your own conclusion, and of course it might differ from mine: the fact remains, however, that if you take the determination that has the larger, more conservative error bar, you get a disagreement with experiment only at 1.5 standard deviations. If I have to choose which one is the most likely, I am sorry, but I will prefer to buy the smaller discrepancy and cast doubt on the more “precise” lattice calculation, rather than going screaming we have seen the first evidence of charged higgs decays in D_s mesons….
And actually, there is a sentence that to me is a give-away of the happiness of the hunt for discrepancy in the paper. Here it is:
“The only other modern lattice-QCD calculation agrees, , but its quoted error is five times larger and would not influence a weighted average with [the more precise result]”.
Weighted average ? A weighted average of two lattice-QCD calculations, using poorly understood uncertainties and similar methodologies ? What about correlations ? I guess I understand too little of lattice-QCD, but to me this sounds like an alarm bell: a metallic voice is whispering in my ears… do-not-trust-these-figures-they-are-just-an-exercise-yet. I am sorry if I sound disrespectful: these are esteemed theorists and I do respect them and their work. Only, I am used to be wary of lattice QCD calculations, especially if these are used to claim for new physics. Hell, they go straight at claiming for charged higgs and leptoquarks as the possible source of discrepancy…
Paper number 2 (0803.0659) is titled even more boldly “First evidence of new physics in b<->s transitions“. What is this about ? It is a combined fit of quantities measured at the Tevatron on the sector, which finds a disagreement with the standard model at more than 3-sigma level on the phase of mixing. I must say I am not an expert on B mixing analyses any more than I am of lattice QCD, so I have to keep my criticism of the method to a minimum level. The analysis combines CDF and D0 data which have different theoretical assumptions on the strong-CP phases. I was not able to fully understand what it is exactly that they do to combine results, but they do make an attempt at explaining it, noting that “Hopefully D0 will present results without assumptions on the strong phases in the future, allowing for a more straightforward combination”. What I get is just that I am not alone in finding rather un-straightforward their combined result.
The authors come to the conclusion that the phase is different from zero at about 3 standard deviations, and say:
“We conclude that the combined analysis gives a stable evidence for new physics, although the precise number of standard deviations depends on the procedure followed to combine presently available data.”
To me, that sounds like saying, “we wish this were evidence for new physics, but we do not really know whether it is solid enough. But the deviation, if there is any, can be explained well by models of new physics beyond the SM”. I cannot help remembering the warnings of Michelangelo Mangano in a recent paper, where he says that the establishment of a deviation from standard model and the interpretation in terms of new physics of this or that kind should be kept separate….
One last remark about the paper. They admit that it was triggered by input from a CDF member, an italian colleague of mine of whom I have a high esteem, Marco Rescigno. Despite the esteem, though, I have to note that Marco’s prodding means the analysis was not blind, but triggered by a discrepancy. I cannot help thinking that it would be really remarkable if among all the beautiful measurements that CDF is making these days, one were unable to squeeze out a 3-sigma discrepancy with standard model expectations. Sure, it is a discrepancy that fits well with some model-independent new physics scenario. But is that enough to get hyper ? Here is what the authors have to say in their conclusions:
“With the procedure we followed to combine the available data, we obtain evidence for NP at more than . […] We are eager to see updated measurements using larger data sets from both the Tevatron experiments in order to strenghten the present evidence, waiting for the advent of LHCb for a high-precision measurement of the NP phase”.
What can I say. Good luck. I would be so damn happy if you were right… But I bet you are just being optimistic.