Wednesday 25 February 2015

Persistent trouble with bees

No, I still have nothing to say about colony collapse disorder... this blog will stick to physics for at least 2 more years. This is an update on the anomalies in B decays reported by the LHCbee experiment. The two most important ones are:

  1. The  3.7 sigma deviation from standard model predictions in the differential distribution of the B➝K*μ+μ- decay products.
  2.  The 2.6 sigma violation of lepton flavor universality in B+→K+l+l- decays. 

 The first anomaly is statistically more significant. However, the theoretical error of the standard model prediction is not trivial to estimate and the significance of the anomaly is subject to fierce discussions. Estimates in the literature range from 4.5 sigma to 1 sigma, depending on what is assumed about QCD uncertainties. For this reason, the second anomaly made this story much more intriguing.  In that case, LHCb measures the ratio of the decay with muons and with electrons:  B+→K+μ+μ- vs B+→K+e+e-. This observable is theoretically clean, as large QCD uncertainties cancel in the ratio. Of course, 2.7 sigma significance is not too impressive; LHCb once had a bigger anomaly (remember CP violation in D meson decays?)  that is now long gone. But it's fair to say that the two anomalies together are marginally interesting.      

One nice thing is that both anomalies can be explained at the same time by a simple modification of the standard model. Namely, one needs to add the 4-fermion coupling between a b-quark, an s-quark, and two muons:

with Λ of order 30 TeV. Just this one extra coupling greatly improves a fit to the data, though other similar couplings could be simultaneously present. The 4-fermion operators can be an effective description of new heavy particles coupled to quarks and leptons.  For example, a leptoquark (scalar particle with a non-zero color charge and lepton number) or a Z'  (neutral U(1) vector boson) with mass in a few TeV range have been proposed. These are of course simple models created ad-hoc. Attempts to put these particles in a bigger picture of physics beyond  the standard model have not been very convincing so far, which may be one reason why the anomalies are viewed a bit skeptically. The flip side is that, if the anomalies turn out to be real, this will point to unexpected symmetry structures around the corner.

Another nice element of this story is that it will be possible to acquire additional relevant information in the near future. The first anomaly is based on just 1 fb-1 of LHCb data, and it will be updated to full 3 fb-1 some time this year. Furthermore, there are literally dozens of other B decays where the 4-fermion operators responsible for the anomalies could  also show up. In fact, there may already be some hints that this is happening. In the table borrowed from this paper we can see that there are several other  2-sigmish anomalies in B-decays that may possibly have the same origin. More data and measurements in  more decay channels should clarify the picture. In particular, violation of lepton flavor universality may come together with lepton flavor violation.  Observation of decays forbidden in the standard model, such as B→Keμ or  B→Kμτ, would be a spectacular and unequivocal signal of new physics.


Anonymous said...

Did you put a vectorial muon current just for simplicity, but it could be left-handed?

Anonymous said...

That is interesting. It is amazing that std model makes all these predictions. I guess these experiments are also testing the "renormalizable" assumption of the std model.

Jester said...

Yes, the left-handed muon current would also work (just slightly worse than the vectorial one according to 1411.3161).

Anonymous said...

"These are of course simple models created ad-hoc. Attempts to put these particles in a bigger picture of physics beyond the standard model have not been very convincing so far, which may be one reason why the anomalies are viewed a bit skeptically"

These kind of statements are arrogantly funny. May be in your thinking Fermi should have not published his model before getting the full Standard Model?

andrew said...

Given the across the board pull of excessively high B to Kaon-mu-mubar events by about two SDs, one wonders if there isn't just some systemic error across the board at LHCb and CDF, for example, in a trigger threshold that underreports electron pairs and hence makes the cross section of muon pairs look higher than it should be comparison, or something in the event analysis code that misdescribes high energy electrons as muons because the data for B decays is outside of the realm of applicability that the code was designed for.

Alternately, perhaps D to Kaon-mu-mubars are being miscoded as B to Kaon-mu-mubars somehow and inflating the B source cross section.

One could also imagine that the theoretical prediction is light because it incorrectly omitted loop diagrams that one would naively expect to be immaterial, that actually work out to contribute almost as much to the total cross section as the diagrams that were included in the SM estimate. For example, b->-c->s with two W boson decays is one way to get from a B meson to a Kaon, but b->u->s and b->t->s paths are also possible and might have been neglected yet contribute more to the final cross-section than expected (e.g. due to a failure to consider virtual particle paths).

BSM theory would be interesting, but a long and hard look for potential sources of error in either the theoretical expectation or the experimental measurement, or both, should still be to preferred resolution if at all possible.

Xezlec said...

The cool stuff is always at about 2 or 3 sigma. Clearly, the way to make physics more fun and exciting is just to redefine "discovery" from 5 sigma to 2! New physics for everybody!

Chris said...

if you want to have this kind of fun, just follow medical research :-)