BaBar Tests the Standard Model

Constraints on tanbeta divided by the charged Higgs mass for type-II two higgs doublet models.  These results exclude the model at a 99.8% confidence level.

BaBar recently unveiled two results that test the limits of the Standard Model of Particle Physics.  Both of these were shown at the Flavor Physics and CP Violation (FPCP 2012) conference in China during May.  The first of these was a measurement of B mesons decaying into a final state containing either a Dτν or a D*τν.  Too many events were found, and the combination of these measurements is in tension with the Standard Model, the measured result is 3.4σ from what was expected, with a p value of 6.9×10-4

While this is not enough to categorically say that there is new physics, this result is incompatible with some models proposed by theorists.  In particular this result essentially rules out the so-called Two Higgs Doublet Model type II, which introduces charged Higgs particles that mediate quark flavor changing transitions.  The exclusion of this model is evident as the ratio of tanbeta vs charged Higgs mass is found to be different for the D and D* final states (see figure).  This result has been submitted to Phys. Rev. Lett. and is described in more detail on the BaBar web site.

Testing fundamental symmetries:  There are three fundamental discrete symmetries that play a crucial role in particle physics, these are parity P, charge conjugation C, and time reversal T.  It is well known that parity is violated, and BaBar was built to test the standard model description of CP violation.  In 2001 BaBar discovered CP violation in B meson decays and the result was just as predicted.   The combined symmetry CPT is conserved in the standard model, and that too has been tested (although the most recent result from BaBar showed a deviation of 2.8σ from the Standard Model).  The one thing that had not been tested in a stand alone way before was the T symmetry.  We expect that this was violated, as the consequences of that not being the case would be far reaching for physics, but the problem is how do you reverse the arrow of time to test T?  Finally it was realised that one could look at T conjugate processes, pairs of related decays that are created in e+e- collisions at BaBar to test T.  The result is rather striking and was also unveiled at FPCP 2012: the observation of a 14σ signal for T violation and a paper on this discovery will be submitted soon.