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Reference : UMR5814-CLABOM-049
Workplace : ANNECY LE VIEUX
Date of publication : Tuesday, February 16, 2021
Type of Contract : FTC Scientist
Contract Period : 24 months
Expected date of employment : 1 June 2021
Proportion of work : Full time
Remuneration : between 2648 euros and 3053 euros monthly gross, according to work experience
Desired level of education : PhD
Experience required : 1 to 4 years
The LHCb group at LAPP is inviting applications for a two-year term postdoctoral position to work on radiative B decays.
In the standard model (SM) of particle physics, the b → sγ transition proceeds via loop Feynman diagrams. The small size of the SM amplitude makes such a process sensitive to the contribution of possible new particles. The emitted photons are produced predominantly with left-handed helicity in the SM due to parity violation in the weak interaction, with a small relative right-handed component proportional to the ratio of s- to b-quark masses. In many extensions of the SM, the right-handed component can be enhanced, leading to observable effects in mixing-induced CP asymmetries and time-dependent decay rates of radiative Bd and Bs decays. In the time dependent decay rates of Bs/Bs → φγ where the φ meson decays to a pair of charged kaons, the coefficients S and H in front of the sin(Δms.t) and the sinh(ΔΓs/2.t) terms are sensitive to those right-handed currents. As compared to the Bd system there is a sizable width difference ΔΓ in Bs mesons which leads to the additional measurable observable H, accessible without knowledge of the Bs flavour.
A first measurement of CP-violating observables S and C in Bs → φγ decays was published by the LHCb collaboration in 2019  using LHC Run1 data, which also contains an update of the H observable. In an effective field theory approach, these results are combined to constraint the left- and right-handed currents and to spot eventual deviations from SM expectations (expressed as the complex Wilson coefficients C7 and C7' of the effective theory). Today's constraints are compatible with the SM and dominated by the recent angular analysis of the Bd → K*ee decay at very low invariant mass of the electron pair which uses the full LHCb dataset . The extrapolated precision in Bs → φγ using the full dataset should reach similar precision and thus substantially improve the sensitivity to new physics in b → sγ currents.
 Phys. Rev. Lett. 123, 081802 (2019).
 Journal of High Energy Physics volume 12, 81 (2020).
The LAPP group has a long-standing expertise in flavour physics and calorimeter objects and plan to engage in the update of the time-dependent CP violation measurement of the Bs → φγ decay. The successful candidate will lead the physics analysis. Using the full Run1 and Run2 LHCb dataset, he/she will extract a high purity sample of this decay by optimising the offline selections and control the simulation efficiencies using abundant Bd → K*γ decays. The expertise necessary for a time-dependent analysis should be developed, from the mastering of the flavour tagging tools to the strategies for calibration of the time resolution and acceptance for this particular decay topology. Finally, the CP violating parameters should be extracted from a fit to background-subtracted data and used to constraint contributions from right-handed currents and potential new physics in the (C7,C7') plane.
The candidate should also study the performance of the LHCb detector to measure this channel during Run3 and propose solutions, at the rneutral econstruction or PID level, to mitigate the impact of the five-fold increase in pile-up. He/she could also participate to simulation studies for a new electromagnetic calorimeter (ECAL) optimised for an eventual running of LHCb at very high luminosity after the second upgrade of the experiment. The Bs → φγ decay can be used to benchmark the performance of this new ECAL as new handles such as increased granularity and precise timing become available. Several aspects of photon reconstruction and PID can be revisited and classical tools enhanced by deep learning algorithms.
- Education: PhD in experimental particle physics.
- Physics: Acquaintance with flavour physics is preferred though not mandatory.
- Programming: Skills in C++, ROOT and python, acquaintance with the LHCb software is preferred though not mandatory.
- Language: Fluency in spoken and written English.
- Good communication skills and ability to work in a team.
Applicants must submit a cover letter explaining their interest in the position and a detailed curriculum vitae (containing a description of their research experience with a list of publications highlighting their personal contributions).
Have referrals to send at least 2 letters of recommendation directly to firstname.lastname@example.org
LAPP is a combined CNRS and Université de Savoie Mont-Blanc unit. Close to 150 people are working at LAPP: researchers, professors, engineers and support staff, students and visiting scientists. The laboratory is very actively involved in several large international collaborations working on accelerators (ATLAS and LHCb on the LHC at CERN, BaBar at SLAC in California, OPERA in Gran Sasso) and astroparticle experiments (Virgo, AMS, H.E.S.S. and CTA). Involved in many R&D and construction programs for large research infrastructures, the laboratory's skills cover a large scope of domains involving innovative technologies in micro-electronics, complex mechanical structure design and optimization, grid computing and scientific software developments. The location of LAPP, 50 km from CERN, and the presence of the theory laboratory LAPTh in the same premises, make the LAPP campus a very attractive research place for scientists contributing to particle and astroparticle physics. LAPP hosts the MUST mid-range data storage and computing centre - widely open to distributed computing infrastructure supporting research and academic projects.
LHCb is one of the four main experiments at LHC (with ATLAS, CMS and ALICE) and is dedicated to the study of heavy flavour physics which is essential to the indirect search for physics beyond the standard model by means of by means of precision measurement of CP violation and studies of rare decays in beauty and charm hadrons. The LAPP group has a long-standing expertise in flavour physics and calorimetry since the beginning of the data taking of the experiment. It contributed significantly to the migration of the calorimeter calibration to the online trigger system for the start of data taking in 2015. Several members had convenership responsibility in the calorimeter objects and particle identification working group and in the physics sub-working group on B-radiative decays. The group also published several studies on the Dalitz plot analysis of open-charm decays as well as on quarkonia spectroscopy. Finally, the group takes a leading role in the development of firmware and of the novel 40 MHz data acquisition system for the upcoming LHCb upgrade.
Constraints and risks
Some short trips have to be planned in France and abroad
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