# Studying rare b- and c-decays with the LHCb experiment

The results of current experiments in particle physics are described by a theory called the Standard Model (SM). The SM describes data astonishingly well, without significant discrepancies. Despite its success at describing the wealth of particle physics data, there are several known issues with the SM. Attempts to solve the issues with the SM often do so by extending it to include new particles with masses that are heavier than the known SM particles. Many of these extensions have consequences that should be measurable at the LHC experiments.

Amongst the many ways of searching for experimental evidence of these new particles, searches involving decays of hadrons containing a bottom or charm quark play a prominent role. In these searches, the new heavy particles appear as virtual particles inside Feynman diagrams and influence how the bottom and charm hadrons decay. One interesting class of decays involves transitions where the b- or c-hadron decays to a lighter hadron and a pair of leptons. These processes are rare in the SM and include the decays $B^0 \to K^{*0}\mu^+\mu^-$, $B_{s}^{0} \to \phi\mu^+\mu^-$ and $\Lambda_{b} \to \Lambda \mu^+\mu^-$. The decay distributions of the particles coming from these decays provides a wide sensitivity to different new physics models and importantly can be used to distinguish between different types of model. The LHCb experiment is dedicated to studying b- and c- hadron decays, providing large samples of even these rare processes, which will allow us in the coming years to test the SM to unprecedented precision and hopefully find evidence for something beyond it.

The project will involve two main aspects: you will contribute to the data taking and quality assurance of the LHCb experiment and to the development of software that could enhance the capabilities of the experiment; and you will carry out a study of one (or more) rare b- or c-hadron decays, using the LHCb dataset, with the aim of establishing evidence for physics beyond the SM. Both aspects will be achieved using modern computing techniques and courses in these techniques will be included as a part of the elementary particle physics group graduate training curriculum. The project will require some international travel, including visits to CERN in Switzerland.

## Particle identification

Charged-hadron separation in particle physics experiments is typically performed using either energy loss information (dE/dx), time-of-flight or Cherenkov emission. For example, the LHCb experiment at CERN currently exploits the coherent emission of Cherenkov radiation that occurs when a relativistic charged-particle passes through a pair of fluorocarbon gasses. The photons of Cherenkov light are emitted in a cone whose opening angle is related to the particles velocity. This can then be combined with an independent estimate of the particles momentum to identify if the particle is a charged pion, kaon or proton. This technique works well if the particle has a momentum in the range 10–100GeV/c but below 10GeV/c cannot be used to separate protons from kaons. Improved particle identification in this lower momentum range could significantly boost the capability of an upgraded LHCb experiment.

The prompt nature of Cherenkov emission can also be exploited in a novel way to determine the arrival time of particle in a detector. This is the premise behind the TORCH project, which aims to use Cherenkov emission in a quartz bar and fast timing to identify charged particles based on their propagation time to the detector. The fast-timing from the optical photons is provided by micro-channel-plate detectors, which are being developed in conjugation with an industrial partner in the UK.

The student will take part in the characterisation of the micro-channel-plate devices at CERN and will develop reconstruction algorithms that are able to exploit the timing and spatial information from the detector. These are both areas where extra effort could make a big difference to the project. The development of the reconstruction presents several interesting challenges due to the large data rates and large particle multiplicity of pp collisions at the Large Hadron Collider.

## Contact

Enquiries should be sent to:

Michal Kreps (M.Kreps@warwick.ac.uk)

Thomas Blake (T.Blake.1@warwick.ac.uk).