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 − − − − 𝐾𝐾 → 𝑒𝑒 𝜈𝜈�𝑒𝑒 𝐾𝐾 → 𝜇𝜇 𝜈𝜈𝜇𝜇 Precision test 2: comparison of heavy lepton decay rates: and − − − − 𝜇𝜇 → 𝑒𝑒 𝜈𝜈�𝑒𝑒𝜈𝜈𝜇𝜇 𝜏𝜏 → 𝑒𝑒 𝜈𝜈�𝑒𝑒𝜈𝜈𝜏𝜏 No definite lepton universality violation has been observed. Challenge to Lepton Universality
Leptonic B-meson decay: − − 𝐵𝐵 → 𝜏𝜏 𝜈𝜈�𝜏𝜏
Semi-leptonic B-meson decay: ( ) ∗ − 𝐵𝐵� → 𝐷𝐷 𝑙𝑙 𝜈𝜈�𝑙𝑙
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: ( ) − − ∗ − 𝐵𝐵 → 𝑙𝑙 𝜈𝜈�𝑙𝑙 𝐵𝐵� → 𝐷𝐷 𝑙𝑙 𝜈𝜈�𝑙𝑙 B-meson Production
BaBar and Belle: = 10. 579 GeV (1999-2010) + − � decays exclusively𝑒𝑒 𝑒𝑒 (>96%)→ Υ 4𝑆𝑆 to → 𝐵𝐵𝐵𝐵 (51.4%)𝑠𝑠 or (48.6%). + − 0 0 Υ 4𝑆𝑆 𝐵𝐵 𝐵𝐵 𝐵𝐵 𝐵𝐵
LHCb: pp inelastic collisions = 7 and 8 TeV (2008-2012)
Production of a pair of B hadrons via interactions𝑠𝑠 of gluons in roughly 1 in every 100 pp collisions Measurements of (BaBar and Belle) − − 𝜏𝜏 • At BaBar and Belle, production via: 𝐵𝐵 → 𝜏𝜏 𝜈𝜈� + − 𝑒𝑒 𝑒𝑒 → Υ 4𝑆𝑆 → 𝐵𝐵𝐵𝐵� • 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) − − 𝜏𝜏 Two tagging algorithms: 𝐵𝐵 → 𝜏𝜏 𝜈𝜈� 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 = ∗ + + , 𝐵𝐵 → 𝐷𝐷 𝑙𝑙 𝜈𝜈𝑙𝑙 𝑙𝑙 + + • Efficiency𝑒𝑒 𝜇𝜇 of 1% Measurements of (BaBar and Belle) − − 𝜏𝜏 • Tau decay modes: 𝐵𝐵 → 𝜏𝜏 𝜈𝜈� − − − − 𝜏𝜏 → 𝑒𝑒 𝜈𝜈�𝑒𝑒𝜈𝜈𝜏𝜏 𝜏𝜏 → 𝜇𝜇 𝜈𝜈𝜇𝜇𝜈𝜈𝜏𝜏 − − − − 0 • The signature for signal events is𝜏𝜏 a→ single𝜋𝜋 𝜈𝜈𝜏𝜏 charged particle,𝜏𝜏 → 𝜋𝜋 either𝜋𝜋 𝜈𝜈𝜏𝜏 a
charged lepton, a or a accompanied by a , plus a Btag. − − 0 • Combining the fitted𝜋𝜋 signal𝜋𝜋 yields for the 4 modes𝜋𝜋 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) ∗ − ( ) 𝜏𝜏 • Measure the ratio of branching fractions 𝐵𝐵�( ) →between𝐷𝐷 𝜏𝜏 𝜈𝜈� ( ) ∗ ∗ − (signal) and (normalization)ℛ𝐷𝐷 𝐵𝐵� → 𝐷𝐷 𝜏𝜏 𝜈𝜈�𝜏𝜏 ∗ − • BaBar and Belle𝐵𝐵� → 𝐷𝐷events𝑙𝑙 𝜈𝜈�𝑙𝑙 consist of a hadronic Btag, a D or D* meson, and a charged lepton = , . − − − • D mesons are reconstructed𝑙𝑙 𝑒𝑒 from𝜇𝜇 pions and kaons with invariant mass compatible with the D meson mass. • The higher mass D* mesons are identified by their and ∗ ∗ decays. 𝐷𝐷 → 𝐷𝐷𝐷𝐷 𝐷𝐷 → • At𝐷𝐷𝛾𝛾 LHCb, only decays of mesons producing a and a meson 0 − ∗+ are selected. Muons are chosen𝐵𝐵 over electrons because𝜇𝜇 of 𝐷𝐷higher 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. 𝑾𝑾 𝑾𝑾
Charged Higgs boson —part of MSSM; mediates weak interaction but different coupling strengths− for leptons of different mass; q2 and angular distribution affected due 𝑯𝑯to its spin-0.
, ′− − , 𝑾𝑾 𝑯𝑯 ′− − 𝑾𝑾 𝑯𝑯 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) − − 𝐵𝐵 → 𝜏𝜏 𝜈𝜈�𝜏𝜏 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 or𝑚𝑚 𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚other≈ undetected particles have > 0. 2 • 𝑚𝑚Low𝑚𝑚𝑚𝑚𝑚𝑚𝑚𝑚 efficiency of 0.3% Measurements of (BaBar and Belle) − − 𝐵𝐵 → 𝜏𝜏 𝜈𝜈�𝜏𝜏 Two tagging algorithms: 2. Semi-leptonic Tag (ST) • Large branching fraction for ( ) with = , ∗ + + + + • Search for these decay products𝐵𝐵 → 𝐷𝐷to tag𝑙𝑙 𝜈𝜈the𝑙𝑙 B meson𝑙𝑙 𝑒𝑒 𝜇𝜇
• Weaker constraints on the Btag and signal B decay • Efficiency of 1%