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A Challenge to Lepton Universality in B-Meson Decays

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A Challenge to Lepton Universality in B-Meson Decays A Challenge to Lepton Universality in B-meson Decays Talk by Kla Karava Original Paper by Gregory Ciezarek et al (Nature22346) 23/04/18 University of Oxford Lepton Universality SM assumes Lepton Universality—same weak coupling constant for all 3 generations of leptons. Precision test 1: comparison of Kaon leptonic decay rates: and − − − − → � → Precision test 2: comparison of heavy lepton decay rates: and − − − − → � → � No definite lepton universality violation has been observed. Challenge to Lepton Universality Leptonic B-meson decay: − − → � Semi-leptonic B-meson decay: ( ) ∗ − � → � BaBar at SLAC, USA Belle at KEK, Japan LHCb at CERN, Switzerland (Credit: SLAC) (Credit: KEK) (Credit: CERN) SM Predictions of B-meson Decay Rates Leptonic decay: Semi-leptonic decay: ( ) − − ∗ − → � � → � B-meson Production BaBar and Belle: = 10. 579 GeV (1999-2010) + − � decays exclusively (>96%)→ Υ 4 to → (51.4%) or (48.6%). + − 0 0 Υ 4 LHCb: pp inelastic collisions = 7 and 8 TeV (2008-2012) Production of a pair of B hadrons via interactions of gluons in roughly 1 in every 100 pp collisions Measurements of (BaBar and Belle) − − • At BaBar and Belle, production via: → � + − → Υ 4 → � • A BB pair can be tagged by the reconstruction of a hadronic or semi- leptonic decay of one of the two B-mesons, referred to as Btag. • All remaining particles in the event originate from the other B decay, either a signal leptonic or semi-leptonic decay (the one we are interested in this experiment), or another B decay passing the selection criteria. Measurements of (BaBar and Belle) − − Two tagging algorithms: → � 1. Hadronic Tag (HT) • Search for the best match between one of more than a thousand possible decay chains and a subset of all detected particles in the event • Low efficiency of 0.3% 2. Semi-leptonic Tag (ST) • Exploit the large branching fraction for ( ) with = ∗ + + , → + + • Efficiency of 1% Measurements of (BaBar and Belle) − − • Tau decay modes: → � − − − − → � → − − − − 0 • The signature for signal events is a→ single charged particle, → either a charged lepton, a or a accompanied by a , plus a Btag. − − 0 • Combining the fitted signal yields for the 4 modes of Tau decay, corrected for the efficiency of the tag and signal B decays, the branching fraction can be determined. Measurements of ( ) (BaBar, Belle, and LHCb) ∗ − ( ) • Measure the ratio of branching fractions �( ) →between � ( ) ∗ ∗ − (signal) and (normalization)ℛ � → � ∗ − • BaBar and Belle� → events � consist of a hadronic Btag, a D or D* meson, and a charged lepton = , . − − − • D mesons are reconstructed from pions and kaons with invariant mass compatible with the D meson mass. • The higher mass D* mesons are identified by their and ∗ ∗ decays. → → • At LHCb, only decays of mesons producing a and a meson 0 − ∗+ are selected. Muons are chosen over electrons because of higher detection efficiency. Results (Blue = SM expectation, Red = Experimental average) Results 1-sigma contours Possible Explanations New vector boson —similar to SM but with larger mass and varying coupling strengths′− for quarks and leptons;− no change in kinematics. Charged Higgs boson —part of MSSM; mediates weak interaction but different coupling strengths− for leptons of different mass; q2 and angular distribution affected due to its spin-0. , ′− − , ′− − Possible Explanations Leptoquarks (LQ)—particles with both electric and colour (strong) charge; allow transitions from quarks to leptons and vice versa; various properties depending on different theories. Conclusions and Outlook Enhancements of leptonic and semi-leptonic B-meson decay rates have been observed but their significance is not sufficient to declare new discovery. The measurements are limited by the size of data samples and uncertainties in the reconstruction efficiencies and background estimates. Future improvements include increasing the number of data samples, improving the tagging algorithms, adding additional decay modes, etc. BACKUP Measurements of (BaBar and Belle) − − → � Two tagging algorithms: 1. Hadronic Tag (HT) • Search for the best match between one of more than a thousand possible decay chains and a subset of all detected particles in the event • MET and pT of all undetected particles can be inferred. • Events with one neutrino have 0. 2 • Events with multiple neutrinos or other≈ undetected particles have > 0. 2 • Low efficiency of 0.3% Measurements of (BaBar and Belle) − − → � Two tagging algorithms: 2. Semi-leptonic Tag (ST) • Large branching fraction for ( ) with = , ∗ + + + + • Search for these decay products → to tag the B meson • Weaker constraints on the Btag and signal B decay • Efficiency of 1%.
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