by- 25 February 2013
Topics : Study of Radiative decays of beautiful hadrons with the LHCb detector
Proponents : LAPP : Edwige Tournefier - Marie-Noëlle Minard LAPTH: Diego Guadagnoli
Address : LAPP and LAPTH - 9 chemin de Bellevue - BP110 - 74941 Annecy-le-Vieux Cedex
Phone : +33 4 50 09 16 62
Contact Email : firstname.lastname@example.org; email@example.com; firstname.lastname@example.org
Very impressive experimental progress has been made in ﬂavour physics over the past decade. This progress is testified, among the other achievements, by the quite precise determination of the parameters of the Cabibbo-Kobayashi-Maskawa (CKM) matrix achieved from the data recorded at the Beauty factories in the US (Babar) and in Japan (Belle) as well as from the Fermilab experiments (CDF, D0). These achievements are now being completed by the physics program at the LHCb experiment.
The proposed subject aims at studying the exclusive b → s gamma radiative transition, already observed for the first time about 20 years ago by the CLEO collaboration as one of the earliest proofs for the observation of flavour-changing neutral current processes. The study of the b → s gamma and b → d gamma radiative transitions in neutral B meson decays is still very interesting because, within the Standard Model, contributions to the amplitude tend to be very suppressed, as they can come only at the “loop” level. As such these transitions can be very sensitive to New Physics (that is Physics Beyond the Standard Model), in particular, they can prove indirectly the presence of new heavy particles virtually propagating in the loops. We note incidentally that the New Physics scales probed by flavour experiments are typically much higher than those testable via direct production of new particles. In flavour experiments, the key to such sensitivity is precision. The precise determination of the branching ratios, asymmetries or angular distributions of the radiative decays could then reveal unexpected effects. For example, these measurements may be used in a model-independent analysis of New Physics in b → s gamma transitions, based on the formalism of effective Hamiltonians, which provides an elegant separation of long distances (hadronic effects encoded in form factors, accessible through lattice simulations) and short distances (perturbative dynamics from SM and New Physics heavy particles, encoded in Wilson coefficients, that are the quantities constrained in practice). This formalism allows a robust, model-dependent assessment of new effects.
In the framework of the LHCb radiative group, it is proposed to measure in a first stage the branching ratio of rare radiative decays namely B+—>phiK+gamma ,Bs->phi phi gamma, Bd->phi K*gamma, Bs->phi pi0gamma , for which, after developing the selection, branching ratios will be measured. The work will be based on the 2011-2012 (2.5fb-1 at 7 TeV) and 2015 ( 2fb-1 at 14 TeV) statistics. Technical work on particle identification of photon and pi0background rejection will have to be mastered. The final goals of this analysis will be the measurements of asymmetries for charged modes, and of the photon polarization for neutral modes. These measurements will be exploited in a model-independent analysis of new physics effects. A potential spin-off of great theoretical interest would be other radiative decay channels such as D—>Vgamma (V= vector meson): in view of the recent LHCb measurement of CP asymmetries in the D system, for which enhancement due to new physics is a possibility, a promising way is to study new D-meson decay channels, such that the new physics contribution would yield maximal effect. In this perspective CP asymmetry measurement of D—>Vgamma becomes a challenging decay. Such measurements would bring important constraints on new physics in otherwise incalculable observables. This possibility will be investigated from both the experimental and phenomenological point of view.
The candidate is also expected to contribute to the common effort of the LHCb running.
Groups: LHCb group at LAPP and Particle Physics group at LAPTH
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