Centre national de la recherche scientifique Commissariat à l'énergie atomique et aux énergies alternatives
Université Savoie Mont Blanc Université Joseph Fourier


On this website

On the whole CNRS Web

Home page > PhD fellowships > Proposed PhD subjects 2016-2019 > LHCb : Measurement of the CP violating phase γ in open-charm B decays at the LHCb experiment

LHCb : Measurement of the CP violating phase γ in open-charm B decays at the LHCb experiment

by Luc Frappat - 17 December 2015

Topics:  LHC Physics, LHCb collaboration

Proponents:  Vincent Tisserand

Address:  LAPP - 9, chemin de Bellevue - BP 110 - 74941 ANNECY-LE-VIEUX CEDEX

Phone:  + 33 (0)450 09 17 81

Contact Email:

Experimental status & importance of the CKM phase γ

The LHCb experiment at the LHC (CERN) was designed to continue the study of heavy flavours in the sector of beauty and charmed quarks, after the huge success of the B-factories (KEK/Japan and SLAC/USA), recognised by the 2008 Nobel Prize to Kobayashi and Maskawa, and those of the TeVatron (CDF/D0 at FNAL/USA). The years 2009-2012 (LHC Run 1) were even more successful than expected for the LHCb experiment, since it shifted from the phase of detector commissioning to that of massive data collection, with performances often exceeding those of the original design. After slightly more than 2 years, the LHCb has become the main player in the physics of heavy flavours as a strategy to indirectly search for physics beyond the Standard Model (SM). Nearly 3/fb of data was collected at proton-proton collision energies of respectively 7 and 8 TeV. After a stop of about 2 years to improve the accelerator and the experiments, the LHC has restarted data taking in Spring 2015 at collision energy of 13 TeV (Run 2), with a data sample expectedly doubled by the end of 2016 already. In addition, starting from 2017 the Belle-II experiment at KEK will take over the B-factories thanks to the super-KEKB accelerator, providing an instantaneous luminosity 40 times larger than the previous machine.

The LHCb physics program is mostly concerned with the measurement of the CKM-matrix entries that includes the violation of the CP symmetry and with the study of rare decays of beauty hadrons and charmed mesons. Precision studies of these observables allow to test the SM and look for signs of New Physics indirectly, namely via the accurate analysis of quantum effects. This strategy is complementary to that pursued by the ATLAS and CMS experiments. Specifically, the data collected at LHCb provide access to rare or extremely rare decays, such as Bs,d ® μ⁺μ⁻, B0® K*0 μ+μ, Bs ® J/ψϕ or B ® Xsγ, decays characterized by a rich phenomenology and very sensitive to beyond-SM effects. In fact, these measurements already allow to strongly constraining many possible models of New Physics.

One of the measurements of critical importance for this program is that of the angle γ of the Unitary Triangle. The analyses performed at LHCb have already reached an unprecedented accuracy on γ, which allows probing more and more severely the overall consistency of the Kobayashi-Maskawa (KM) mechanism. This angle is directly associated with the complex phase of the CKM matrix, responsible for CP violation. In addition, it serves as a reference ("standard candle") for tests of overall consistency of the KM mechanism as it can be fully measured through tree-level decays, which are expected to be very little polluted by New-Physics effects. It is worthwhile to stress that the theoretical uncertainties on γ using tree-level decays of B-hadrons to charmed mesons are totally negligible, having been estimated at the level of 10-7 (cf. Brod & Zupan JHEP 1401(2014)051).

To the present day, not all of the LHCb Run1 data and not all of the known theoretical methods for measuring γ have been exploited yet. Nonetheless, at the recent CKM2014 conference in Vienna (September 2014), the LHCb experiment has announced an accuracy better than 10° on the measurement of this phase (6.5° for the CKMfitter combination that includes the corresponding BaBar and Belle measurements). The limiting factor for this measurement remains by far the size of the samples available for the deployment of the various theoretical methods. In ten years from now LHCb and Belle II should allow a γ measurement to an accuracy of less than 1°. This will test the CKM mechanism within the SM up to scales of a few 10²-103 TeV, without model assumptions (Charles et al. Phys. Rev. D 89 (2014) 033016) . These scales are inaccessible to direct searches of new particles by ATLAS & CMS. Already with the first data from the LHC Run 2, corresponding to the period 2015 to 2017 of the proposed PhD thesis, LHCb should be able to achieve an accuracy of about 4°. This assumes full exploitation of the existing methods for γ extraction and inclusion of additional ones. As indeed not a single channel, nor any single method, allows to obtain this ultimate precision, therefore it’s highly necessary to multiply the methods and to include original and new channels to fully exploit the potential of the LHCb data. 

Description of the PhD thesis subject
In the LAPP LHCb group, in collaboration with Diego Guadagnoli of the LAPTh, and thanks to the support of physicists from the CKMfitter group and their tools (theorists and experimentalists from BaBar, Belle (II) and LHCb), we have all the required skills and means to conduct a study providing a complete overview of the measurement of the CP phase γ of the CKM matrix.
For the PhD thesis subject, we study the open-charm B decays and their phenomenology. Due to our long expertise on the BaBar experiment and our involvement in the LHCb electromagnetic calorimeter, we work at LAPP on γ measurements from more original channels than those previously studied at LHCb (B-®D0[K--]). These are the complementary modes B-®D*0[K--] using ADS and GLW methods, where the D*0 decays to D0 π0/γ. A preliminary study using 2011-2012 data indicates a data sample that is already significantly larger than each of those from BaBar and Belle. The PhD student will perform the measurement in these two modes and with the mentioned two methods on the data to be collected by LHCb until the end of 2017. This allows an unprecedented dataset size for those channels. A complementary method employing a partial reconstruction technique of the D*0 mesons is underway together with our colleagues of the LHCb group of the Oxford University (Malcom John and his students Donal Hill and Alex Rollings). We are yet collaborating closely, the successful applicant to that PhD subject would contribute efficiently to that enterprise.  We will also work on the decays B-®D0K*- [K-π0/Ksp-], with the GLW and ADS methods, in collaboration with the LHCb groups in Oxford (Sneha Malde (B2OC LHCb working group convenor) and her student Anita Nandi). These decays are likewise original with respect to LHCb’s physics program and have not been reused since our 2009 study within BaBar.

In addition, since 2011, the LHCb group at LAPP works in close collaboration with the University of Warwick on the decays B⁰(s) ® D̅(*)⁰h1⁺h2⁻ (where h1 and h2 are respectively a kaon and/or a pion). These modes offer new perspectives for the measurement of γ. Prerequisite work is the observation of the decay modes B⁰(s) ® D̅(*)⁰K⁺K⁻ and the measurement of their branching ratio. This undertaking is currently being performed by N. Déléage, who defended his PhD on Oct 14th 2015. We are being preparing the publication of this foundation work.  Furthermore, a Dalitz analysis of the decay B⁰® D̅⁰ π⁺π⁻performed with S. T’Jampens and W. Qian, who will end his postdoc on Nov 1st 2015 is now published in Phys Rev D . After their departure, thanks to the expertise acquired by the Annecy group, a study of the Dalitz structure of the decays B⁰(s) ® D̅⁰K⁺K⁻ will become possible, as well as a time-dependent CP analysis of the decay B⁰® D̅⁰ π⁺π⁻ and together with time-independent measurements based on the sub-decay B⁰s ® D̅⁰φ. Those preliminary studies, in addition to the understanding of their spectroscopy, are essential to separate the CP-violation CKM phase from the corresponding strong phases, which vary all over the Dalitz plane. Those studies may as well be part of the PhD thesis proposed, depending on the speed of the above works.
In short, the PhD thesis subject concerns one of the key measurements in the physics program of the LHCb experiment and at LAPP. We have the expertise to supervise such a project. The membership of Vincent Tisserand to the worldwide known CKMfitter group allows a close collaboration with French Heavy Flavor theorists, in addition to D. Guadagnoli from LAPTh: S. Descotes-Genon (LPT Orsay) and J. Charles (CPT Marseille). This gives an access to rigorous statistical treatment of the experimental data to perform in particular combinations of the CKM angle γ. Other CKMfitter colleagues, originated from the largest heavy flavour experiments, contribute to the vigour of this project. Among those worldwide recognized experts, let’s mention the collaboration with K. Trabelsi (Belle KEK et LHCb EPFL Lausanne).