Saturday, June 07, 2014

New physics? LHCb insists on a flavor anomaly in \(B\) decays

The Symmetry Magazine has told us about intriguing claims at an ongoing conference in the New York City:
LHCb glimpses possible sign of new physics

Electroweak penguins at LHCb (slides from the talk)
The LHCb experiment is smaller than the two LHC bulls, ATLAS and CMS, but it is more careful when it comes to the analyses of particles including \(b\)-quarks. These quarks incorporate themselves into hadrons – most typically, the \(B\)-mesons. The latter are analogous to pions.

Because the \(b\)-quarks belong to the third generation of the Standard Model quarks, all three generations are involved in their life stories. It follows that processes where \(B\)-mesons do something interesting are also sensitive to the CP-violating phase of the CKM matrix. The phase is inconsequential for all phenomena that only involve at most two generations of fermions. In other words, the LHCb experiment is particularly sensitive to violations of the CP symmetry, the symmetry with respect to the transformation placing all particles in the mirror and replacing them by antiparticles at the same moment.

The Standard Model violates the CP symmetry by the CKM phase only. Experiments even imply that the \(\theta\)-angle in front of the QCD \(F\wedge F\) pretty much vanishes – a puzzing result known as the strong CP-problem whose resolution requires axions, according to most phenomenologists. All older experiments are compatible with the assumption that this CKM CP-violating phase is indeed the only "CP offender" in Nature. However, LHCb seems to be carefully coming with a paradigm shift by daring to suggest that they sometimes see new sources of CP violation.

Generic theories of new physics, such as "not carefully selected" models of supersymmetry, predict many new sources of CP-violation. The number of types of these new CP-violating interactions is so high that low-energy supersymmetry is almost certainly not "generic" in this sense. It must be close to some symmetry points where the preservation of CP is approximately favored.

However, the talk at the New York City conference isn't about CP at all but about another potential signal, a flavor asymmetry.

In the Standard Model, there are three generations of quarks and leptons. Let's talk about the leptons, for example. They are charged leptons \[

e^\pm, \quad \mu^\pm, \quad \tau^\pm

\] along with their neutrinos \(\nu_e,\nu_\mu,\nu_\tau\) – which are either identical to the antineutrinos (with bars) or different from them, depending on whether neutrinos are Majorana or Dirac particles. If we return to the charged leptons, how do the electron, the muon, and the tau differ?

According to the Standard Model, they only differ by the mass – and by the strength of the corresponding Yukawa interaction with the Higgs boson. In all other respects, they behave like the same particle with an additional flavor label \(f=1,2,3\) that doesn't seem to play a role. Moreover, at high enough energies, much higher than the \(\tau\) mass, the masses can be neglected and the close relationship between the three charged leptons – almost an equivalence – becomes an even more striking.

Just like in the case of the CP symmetry, models of new physics – and supersymmetry seems to be representative of all the possibilities again – may violate this "democracy between flavors". Supersymmetric and other theories of new physics provide us with many new fields and interactions that may violate the "democracy between flavors" in many ways. Again, it's pretty much clear that in Nature, assuming that there's any LHC-scale new physics at all, the values of these interactions can't be generic. Too many flavor-democracy-violating interactions that would be too strong would cause new, unobserved decays, among other problematic processes.

At any rate, new flavor-democracy-violating processes may exist to a certain extent and their proof would be a proof of new physics. At the ongoing conference in the New York City, the LHCb folks talked about the unexpected and exciting flavor democracy violation in a decay of the \(B\)-mesons. I suppose it is the very same anomaly I was discussing in July 2013 but there is one difference here: one extra year of hindsight. And maybe the significance has grown, too. The article in the Symmetry Magazine didn't tell us about any numbers.

Because of the extra year from that blog post, the probability that the excess is due to some error that good experimenters could easily find has decreased. More data – probably the 2015 data – will be needed to decide whether the deviation from the Standard Model is real and due to new physics.


  1. Hm, what are the echo comments anyway? Is that what the default (non-disqus) commenting feature of blogspot is called?

  2. Eche was the previous comment feature (it unfortunately stopped its service) we had before Discus.