Test of Lepton Flavour Universality using B → D*τν decays at LHCb

Stage numéro : Doctorat-2124-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
Directeur de thèse :Olivier Leroy - 04 91 82 76 05 - Olivier.Leroy@in2p3.fr

Thématique : Physique des particules

Applications are invited for a PhD student position at the Centre de Physique des Particules de Marseille (CPPM), France. Applicants must hold a Master degree (or equivalent) in Particle Physics, or expect to obtain such a degree by September 2021. The PhD contract will start on October 1st, 2021, for 3 years. The deadline for application is March 15th, 2021, but early applications are encouraged.

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 → sll flavour changing neutral current and another one in the b → cτν charged current. Both effects are  ∼ 4σ 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 3.8σ 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 τ lepton is reconstructed into its 3 pions final state (π+ππ+ν). In addition to a measurement of the branching ratio, the student will perform an angular analysis of the decay products which will bring new sensitivity to BSM effects. She/he will use the full run1 and run2 data sets (2011-2018), to achieve an un-precedented precision and hopefully clarify the current situation.

The student will often travel to Cern, to participate to run3 data taking and to present her/his results.

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. Applications with a detailed CV, a motivation letter and 2 recommendation letters 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