Search for New Physics at LHCb using Bd → D* τν decays

Stage numéro : M2-2021-LH-01
Laboratoire :Centre de Physique des Particules de Marseille Case 902
 163 avenue de Luminy - 13288 Marseille Cedex 9
Directeur :Cristinel Diaconu - 04.91.82.72.01 - diaconu@cppm.in2p3.fr
Correspondant :William Gillard - 04.91.82.72.67 - gillard@cppm.in2p3.fr
Groupe d'accueil :LHCb
Chef de groupe :Olivier leroy - 04.91.82.76.05 - oleroy@cppm.in2p3.fr
Responsable de stage :Olivier Leroy - 04 91 82 76 05 - Olivier.Leroy@in2p3.fr

Thématique : Physique des particules

LHCb is one of the four major experiments installed at the largest proton-proton collider ever built, the LHC, at Cern, Geneva, Switzerland. The experiment is dedicated to the search for Physics Beyond the Standard Model (BSM) studying beauty and charm hadrons. Since the beginning of data taking in 2010, LHCb has accumulated the largest sample of b-hadrons ever collected and published hundreds of world-leading measurements. The CPPM LHCb group is deeply involved in the experiment since its beginning.

In the past few years, two intriguing anomalies have shown up in the flavour sector. One related to the b → s ll flavour changing neutral current and another one in the b → c τν charged current. Both effects are  ∼ 4 σ away from the Standard Model (SM) expectation.

Concerning the second effect, the measured quantities are the ratios of branching fractions R(D(*)) = BR(B → D(*) τν) / BR(B → D(*) lν) , ( l = μ,e) . In the Standard Model, the only difference between the numerator and the denominator is the lepton mass. When combining the results of the BaBar, Belle and LHCb experiments, the measurements appear to be 3.8 σ away from their SM expectation. This is a striking hint of violation of the lepton flavour universality which clearly needs to be checked by all means.

The student will study the B → D(*) τν channel, where the tau lepton is reconstructed into its 3 pions final state ( π+ π π0 ν).

She/he will use for the first time the data collected by the LHCb experiment during the years 2017-2018, to achieve an un-precedented precision and hopefully clarify the current situation.

Applicant profile: The ideal candidate will have some experience in experimental particle physics, a strong interest in data analysis and good software skills (C++, python, ROOT).

She/he should have excellent academic background. There is the possibily to continue for a 3-years PhD after the master internship period.

Applications should be sent to: Olivier.Leroy@in2p3.fr

Keywords: Experimental High Energy Physics, LHCb, Flavour physics, New Physics, Beyond the Standard Model searches. LFU, Lepton Flavour Universality, Flavour anomalies, Data analysis.

References:


- Measurement of B → D* τν branching fraction using three-prong τ decays, Phys. Rev. D97, 072013 (2018).


- Model-independent method for measuring the angular coefficients of B → D* τν decays, JHEP 11 (2019) 133