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High Energy Physics



Val Gibson

Group Members

Paula Alvarez Cartelle, Fionn Bishop, Michele Blago, Harry Cliff, Blaise Delaney, Daniel Foulds-Holt, Val Gibson, Chris Jones, George Lovell, John Smeaton, Yuan Wang, Ifan Williams, Stephen Wotton

We are members of the CERN LHCb collaboration. LHCb is an experiment at the CERN LHC collider, which has been taking data since 2009. LHCb was primarily designed to investigate the decays of B-particles (particles containing b-quarks) and so provide an insight into the phenomenon of CP-violation. The collaboration comprises almost 1000 physicists and engineers from more than 65 institutes from all over the world.

The Big Bang

What is CP violation?

One of the outstanding puzzles in particle physics and cosmology is the dominance of matter over antimatter in the observable universe. In the Big Bang, matter and antimatter would have been produced in equal amounts. However, a minute flaw in the symmetries of nature resulted in the matter-dominated universe we know today. The phenomenon that distinguishes matter from antimatter is called CP violation and, although it is now well established, its origin remains one of the most compelling issues in particle physics.

What do we do?

Physics Analysis

We are engaged in an active experimental program in which we are investigating the properties of B-particle decays and how they can best be used to measure the CP-violation parameters. We also study rare B decays, which open a window onto new physics beyond the Standard Model. In addition we are testing the Standard Model through studies of electroweak physics and QCD at LHCb.

CP-violation is embodied in the Standard Model through four parameters which characterise the "CKM mixing matrix". In order to test whether the Standard Model provides an adequate description, it is important to measure CP violation accurately in many different B-particle and charm decays, so as to test whether the same set of parameters can model all the data. We are involved in several such analyses, which can be interpreted as measurements of the same angle γ.

LHC provides very large numbers of B-hadrons decaying in the LHCb detector, making it possible to observe very rare decay processes. These are all decay modes which are predicted to be highly suppressed in the Standard Model. For example the decay Bs→μ+μ- is predicted to have a branching fraction (3.56±0.30)x10-9. However, if new particles exist beyond the Standard Model, these could modify the prediction. LHCb (with strong Cambridge involvement) recently measured (2.9±0.7)x10-9, a measurement which rules out many New Physics scenarios. Another rare decay mode in which Cambridge physicists are centrally involved is K(*) μ+μ- (see, for example, here ).

LHCb provides high precision detectors at angles within ~15o of the collision axis. This complements the other big LHC detectors, ATLAS and CMS, which have their most precise detectors close to 90o. Therefore many LHCb measurements complement those of the other experiments. For example we study the Standard Model processes in which W± and Z bosons are produced, decaying to electrons or muons, possibly accompanied by jets (See papers here and here for Cambridge work). Since the underlying electroweak production process is already well understood in the Standard Model, these measurements provide a sensitive probe of the quarks and gluon in the proton, in a region not studied by previous experiments.

HPD Photon Detectors

Detector responsibilities

Before LHCb started up, we collaborated closely with other UK institutes and CERN to build the Ring Imaging Cherenkov (RICH) detectors for LHCb. These detectors allow us to distinguish between the different types of particles produced in high energy proton-proton collisions. To do this, we exploit the Cherenkov effect in which photons are emitted by charged particles when the particle speed exceeds the local speed of light. These photons from this exceedingly feeble source are detected by extremely sensitive and high speed electronic detection systems which we have helped to design, test and build. The Cambridge LHCb group tested hybrid photon detectors (HPDs) and Multi-anode Photomultipliers (MaPMTs) (see doi:10.1016/j.nima.2009.02.011, doi:10.1016/j.nima.2007.01.100, doi:10.1016/j.nima.2003.09.062, doi:10.1016/S0168-9002(98)00291-5). We also designed and built the L1 readout electronics for the RICHs. In addition we had central responsilities for software development, for event simulation and for RICH pattern recognition for particle ID.

Now that LHCb is operating, we and our colleagues work together to ensure smooth operation and calibration of the detector. We are also still centrally involved in particle identification, mainly using the RICH detectors.

Detector upgrade

Now that LHCb has been successfully operating for more than four years, is is necessary to consider upgrades to the detector. It is clear that much interesting physics can be addressed so long we can increase the rate of data collection by an order of magnitude. This necessitates improvements to several of the detector systems, where we can also take advantage of newer technologies. Our involvement is again with the upgrades to the RICH. This involves some reconfiguration of the detector geometry in order to reduce confusion when more particles are passing through. The HPD photon detectors will be replaced by multianode photomultipliers (MAPMTs) and the readout electronics needs to be redesigned to handle the new photodetectors and increased data rates. The intention is to run with the updated detector from ~2019 onwards.

Interested in joining the Cambridge LHCb group?

General information about applying to do a Ph.D. in the Cambridge group and typical projects available .