BABAR Data in Tension with the Standard Model

The BABAR collaboration* reported on new measurements of decays of B mesons into final states containing a tau lepton, τ, the heavy third-generation cousin of the electron and muon. The decay branching fractions are higher than predicted by the Standard Model with a 3.4σ level of significance.

The decays Bντ and BD*τντ provide an interesting probe of possible new physics at high mass scales. While the related decays BD(*)lνl (where l represents an electron or muon and D(*) is a D or D* meson (which contain a charm quark) have long been used to measure properties of the Standard Model, the decays BD(*)lνl are unique: the large mass of the tau implies an affinity to Higgs bosons or possibly non-Standard Model processes. Within the Standard Model, the Higgs boson is related to the mechanism by which the quarks, leptons and force-carrying gauge bosons acquire mass. Consequently, the Higgs is predicted to couple more strongly to heavier particles than to lighter ones. In extensions of the Standard Model, such as Supersymmetry (SUSY), there can be multiple electrically neutral and charged Higgs bosons, but the property of preferentially coupling to more massive particles remains in these models. In BD(*)τντ, the initial B meson and the final-state D(*) and tau all contain massive quarks or leptons, making it a very sensitive probe of possible non-Standard Model Higgs effects. However, because the tau rapidly decays into a variety of lighter particles, including neutrinos which pass undetected through particle detectors, the process BD(*)τντ is difficult to detect.

BABAR measures the ratios of branching fractions R(D) = Br(Bντ)/B(BDlνl) and R(D*) = B(BD*τντ)/B(BD*lνl) to be R(D)=0.4400.0580.042 and R(D*) = 0.3320.0240.018, which exceed the Standard Model expectations by 2.0 and 2.7 standard deviations, respectively. Taken together, and assuming the Standard Model, the probability of obtaining these results or results further from the Standard Model is 0.069%, which for a one-dimensional Gaussian-distributed observable is equivalent to a significance of 3.4 standard deviations. It is also worth noting that this excess cannot be explained in a minimal extension of the Standard Model known as the Type II Two Higgs Doublet Model. By measuring the ratios R(D) and R(D*), many systematic uncertainties cancel, including much of the uncertainty associated with theoretical input, including notoriously difficult to calculate non-perturbative QCD contributions. Restricting the analysis to leptonic decays of the tau further minimizes experimental systematic errors, as the same types of particles are detected in both the numerator and denominator of the ratios R(D) and R(D*).

The results are based on the full BABAR experiment data set and are significantly more sensitive than previously published studies of these decays. BABAR recorded more than 470 million B meson pairs, produced in the process e+e→Upsilon(4S) at the SLAC B-factory between 1999 and 2008. To deal with the fact that neutrinos from the tau decays in BD(*)τντ are not detected, BABAR identified and reconstructed all of the decay products associated with the second B meson in each Upsilon(4S) decay. This enabled the presence of the undetected neutrinos to be inferred from conservation of energy and momentum. Advanced multivariate techniques were used to distinguish signal events from possible backgrounds from other B decays or non-Upsilon(4S) events. BABAR observed BD(*)τντ in each of four individual decay modes, involving the four combinations of charged or neutral B and D(*) mesons. They reported the first observations of the Bντ mode with a significance of 6.8σ.

The paper ( describing these results is being submitted to Physical Review Letters, and the results were presented at the Flavor Physics and CP (FPCP) conference in Hefei, China on 24 May 2012.

BABAR is an international collaboration of approximately 400 physicists from Canada, France, Germany, India, Italy, Israel, Netherlands, Norway, Russia, Spain, the U.K., and the U.S., which is based at the SLAC National Accelerator Laboratory, which is operated by Stanford University for the U.S. Department of Energy Office of Science. The Collaboration operated the BABAR experiment, which recorded the collisions of more than 9 billion pairs of electrons and positrons between 1999 and 2008, The BABAR experiment was originally constructed to answer completely different questions driven by the desire to understand why the universe contains matter with no antimatter. BABAR data helped confirm the matter-antimatter theory for which two researchers won the 2008 Nobel Prize in Physics. The BABAR data continue to be analyzed for various studies—including this one.

This work is supported by DOE and NSF (USA), NSERC (Canada), CEA and CNRS-IN2P3 (France), BMBF and DFG (Germany), INFN (Italy), FOM (The Netherlands), NFR (Norway), MES (Russia), MICIIN (Spain) and STFC (United Kingdom). Individuals have received support from the Marie Curie EIF (European Union) and the A. P. Sloan Foundation (USA).

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Last modified: Thu May 31 19:33:42 PDT 2012