A challenge to lepton universality in B meson decays
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A challenge to lepton universality in B meson decays ¨ 1 Vera Luth Published online: 26 November 2018 © Springer Nature Switzerland AG 2018
Abstract One of the key assumptions of the Standard Model of fundamental particles is that the interactions of the charged leptons, namely electrons, muons, and taus, differ only because of their different masses. While precision tests have not revealed any definite violation of this assumption, recent studies of B meson decays involving the higher-mass tau lepton have resulted in observations that challenge lepton universality at the level of four standard deviations. A confirmation of these results would point to new particles or interactions, and could have profound implications for our understanding of particle physics. Keywords Lepton universality · Flavor physics · BABAR · Belle · LHCb
1 Motivation More than 70 years of particle physics research have led to an elegant and concise theory of particle interactions at the sub-nuclear level, commonly referred to as the Standard Model (SM) [1, 2]. In the framework of the SM of particle physics the fundamental building blocks, quarks and leptons, are each grouped in three generations of two members each. The three charged leptons, the electron (e− ), the muon (μ− ) and the tau (τ − ) are each paired with a very low mass, electrically neutral neutrino, νe , νμ ,and ντ . The three generations are ordered by the mass m of the charged lepton ranging from 0.511MeV for e± to 105MeV for μ± , and 1,777MeV for τ ± [3]. Charged leptons participate in electromagnetic and weak, whereas neutrinos only undergo weak interaction. The SM assumes that these interactions of the charged and neutral leptons are universal, i.e., the same for the three generations. Precision tests of lepton universality have been performed by many experiments. To date no definite violation of lepton universality has been observed. Among the most precise tests is a comparison of decay rates of K mesons, K − → e− ν e versus K − → μ− ν μ [4, 5]. Furthermore, taking into account precision measurements of the tau and muon masses and
This article is part of the Topical Collection on Proceedings of the 7th Symposium on Symmetries in Subatomic Physics (SSP 2018), Aachen, Germany, 10-15 June 2018 Edited by Hans Str¨oher, J¨org Pretz, Livia Ludhova and Achim Stahl Vera L¨uth
[email protected] 1
SLAC National Accelerator Laboratory, Stanford, CA 94309, USA
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Hyperfine Interact (2018) 239: 59
Fig. 1 Diagrams for decay process for B → D (∗) − ν ) decays: a for a tree level process mediated either by a vector boson (W − ) or a hypothetical spin-0 charged Higgs boson (H − ), or b couplings to a hypothetical lepto-quark (LQ)
lifetimes, the measured decay rates τ − → e− ν e ντ and μ− → e− ν e νμ , have confirmed the equality of the weak coupling strengths of the tau and muon [3]. However, recent studies of semileptonic decays of B mesons of the form B → D (∗) − ν , with = e, μ,or τ , have resulted in observations that seem to challenge lepton univ
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