# Project descriptions of example LHCb projects

## Testing the Standard Model with the LHCb experiment

Contact: Tim Gershon (t.j.gershon@warwick.ac.uk) and Matt Kenzie (matt.kenzie@warwick.ac.uk)

Recent experiments have established that CP violation in the quark sector of the Standard Model (SM) can be accommodated by the Cabibbo-Kobayashi-Maskawa (CKM) mechanism. These results led to the recent award of the 2008 Nobel Prize in Physics to Kobayashi and Maskawa. However, the measurements also confirm that the level of CP violation in the SM is too small to explain the matter-antimatter asymmetry of the Universe. Therefore, there must be other sources from physics beyond the SM. An exciting possibility is that these new sources of CP violation are related to the as-yet unobserved particles that are predicted to be discovered at the CERN Large Hadron Collider (LHC) in models (such as supersymmetry) that attempt to address some of the theoretical shortcomings of the Standard Model. In order to experimentally test these possible mechanisms of physics beyond the SM, it is vitally important to make precise measurements of CP-violating phases in as many different processes, governed by as many different quark-level transitions, as possible.

In any case, if new sources of CP violation exist in nature, as they must, they should produce effects that can be seen in measurements as deviations from the Standard Model prediction for the pattern of CP violation effects. Among existing and approved experiments, the LHCb experiment is uniquely capable of reaching the high precision that is needed to resolve the situation. Over the timescale of this project, results from LHCb could dramatically redefine the direction of research in the field of particle physics, either by observing new physics (NP) effects, or by setting strong constraints on their possible contributions.

The project will involve (at least) two aspects. The first part is to take a prominent role in the data quality assurance of the LHCb experiment, or to develop software to enhance the physics capabilities of the experiment and maximise the sensitivity to interesting physics signals. The second part will be to carry out measurements of (one or more) observables sensitive to NP effects. Both aspects will be achieved using modern computing techniques. Courses in these techniques are included as a part of the elementary particle physics group’s graduate training curriculum. The project will require some international travel, including visits to CERN, and has the potential for a long term attachment (1 year) at CERN.

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

Contact: Tom Blake (T.Blake.1@warwick.ac.uk) and Michal Kreps (m.kreps@warwick.ac.uk)

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.

## Improving the particle identification capabilities of the LHCb experiment

Contact: Tom Blake (T.Blake.1@warwick.ac.uk) and Michal Kreps (m.kreps@warwick.ac.uk)

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.

## Measuring the W boson mass with the LHCb experiment

Contact: Mika Vesterinen (Mika.Vesterinen@Warwick.ac.uk)

In the standard model the weak-nuclear and electromagnetic interactions are described by the unified electroweak theory. At tree-level (neglecting quantum loop corrections) the interactions are controlled by three parameters, namely two gauge coupling constants and the vacuum expectation value of the Higgs field. These three parameters are precisely determined by measurements of the fine structure constant, Fermi constant, and the Z boson mass. This allows, for example, the mass of the W boson (roughly 80 GeV) to be precisely predicted. The W mass is directly related to the product of one of the gauge couplings and the energy of the Higgs field. Quantum loop corrections have been calculated including contributions from known standard model fields. The dominant loop correction to the W mass, involving third generation quarks, is at the percent-level in relative terms. Measuring the W mass to much higher precision we can test for possible loop corrections from fields beyond those of the standard model. The most precise measurement to date has an uncertainty of 19 MeV. The standard model prediction has an uncertainty of 7 MeV so the precision of the test of the standard model is limited by the experiments (mostly hadron collider experiments).

The W mass is notoriously difficult to measure because in the leptonic decay channels only the charged lepton can be reconstructed. It is therefore not possible to determine an invariant mass of a candidate W boson decay at a hadron collider where the momentum of the W is unknown. Instead we need to precisely model the kinematics of W boson production such that the W mass can be statistically inferred from the shape of the charged lepton transverse momentum distribution. The challenge of modelling the W boson kinematics is exacerbated by the restricted angular coverage of the typical hadron collider experiments, such as ATLAS and CMS at the LHC. A novel idea is to exploit the LHCb experiment, which uniquely instruments the small-angle (with respect to the colliding beams), to complement ATLAS and CMS. LHCb has excellent muon identification and charged particle tracking and is therefore well suited to muonic W decays.

The student would contribute to the measurement of the W mass with the LHCb Run-II (2011-2018) dataset, which should be published by the end of the first year. The student would simultaneously work on the commissioning of the LHCb Upgrade detector, which should begin data taking in 2022. This work would focus on the real time analysis system of the experiment. The student would then develop a more precise measurement of the W mass using the first (Run-III) data from the Upgraded detector.