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Flavour Physics
The detailed study of quark transitions provides insights into the structure of the Standard Model, and allows the discovery and study of physics beyond the standard model.
Precision flavour physics is sensitive to loop effects caused by particles that can be far heavier than those directly produced at colliders - it can see beyond the energy frontier. In the past, this approach resulted in the prediction of the charm quark, and a heavy top quark, well before colliders could produce these particles directly. The observation of CP violation led to the prediction of the entire third generation of particles even before the 2nd one had been fully discovered, an achievement that has recently been rewarded with the 2008 Nobel Prize.
The Bristol Flavour Physics Group has a comprehensive flavour physics programme, covering B physics (LHCb), charm physics (CLEO-c) and kaon physics (NA62.) These areas complement each other in their sensitivity to New Physics, providing an unprecedented experimental precision where a whole host of New Physics models could become apparent. Bristol are the only group in the UK with such a far-reaching programme.
With the next generation of flavour physics experiments, we enter a completely new realm of New Physics sensitivity. We do not know which approach will discover evidence of New Physics first, the highly sensitive search for deviations from SM predictions in precision flavour physics at LHCb and NA62, or direct observation of new particles at CMS. However, it is certain that the input from both will be needed if we wish not merely to destroy the Standard Model, but to understand the physics that lies beyond it. The discovery and analysis of directly produced new, massive particles is the domain of CMS. The measurement of coupling constants, and access to mass-scales beyond those produced at the LHC, are the domain of flavour physics.
The group is at the forefront of analyses at LHCb. One of the most powerful ways to increase the New Physics sensitivity of the quark flavour sector is, paradoxically, to measure precisely the value of the Standard Model parameter γ using B±→DK± or B0→DK* decays.
The NA62 experiment at CERN will deliver physics that will complement the LHC data. The Bristol group is studying the rare decay K+→π+νν. By measuring this branching ratio to 10% the Standard Model will be tested decisively, or New Physics discovered. The large number of kaons at NA62 will allow other rare decays to be studied with unprecedented precision and searches for lepton flavour violation.
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The CMS Experiment at CERN
The Compact Muon Solenoid detector is one of two general purpose experiments at the LHC. It has a broad programme of physics including direct searches for the Higgs boson, Supersymmetric particles and other new phenomena. The CMS detector is built around a high-field solenoid magnet which allows precision measurements to be made in a relatively compact space. CMS concentrates on accurate measurement of muon momentum together with high precision electromagnetic calorimetry to reconstruct the energy and direction of both electrons and photons.
In Bristol we are involved with two types of search for exotic new particle, which might show up in the early months of data taking. We have devised a novel technique to search for massive particles with a long lifetime which might come to rest in the detector material before decaying. We are also looking for new high-mass particles that decay to a pair of electrons. Both of these studies are motivated by recent theoretical developments. We also have a programme of work measuring the production and properties of the top quark, to enhance our understanding of the most massive of the known Standard Model particles.
We have been involved with the design and construction of parts of the electromagnetic calorimeter and the first level trigger for CMS, as well as the Grid software that is essential for managing, distributing and processing the vast quantity of data produced by the detector. We are actively studying and planning for future upgrades of the detector to cope with the increases in the LHC performance over a ten-year timescale.
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Future Detector R&DAlmost all modern particle physics experiments make use of silicon sensors to detect and track charged particles. In such solid-state devices, energy deposited by a high-energy particle interacting with the sensor material is detected as a small electric charge. Silicon allows the fabrication of very small sensitive elements (less than 100μm in length and/or width) with correspondingly precise position information.
The Bristol group are actively researching and developing new solid-state particle detector technologies, for use both in future particle physics experiments and in other fields. Potential applications range from the innermost detectors at the proposed International Linear Collider (ILC), through upgrades to the LHC experiments to medical physics and homeland security. Some of our current projects are described below.
The Bristol group is a member of the SPiDeR collaboration, a UK project developing silicon pixel sensors with advanced technological features. Finely-segmented detectors with in-built intelligence could be used in future high energy physics experiments as tracking layers or in digital calorimeters.
Tracking detectors need a minimum of material, and Bristol is leading the Low-Mass project which is using novel materials such as foams to make the supporting elements for thin silicon sensors.
Diamond is superior to silicon as a detector material in terms of speed and radiation tolerance. We are working with academic and industrial partners to develop artifically-grown diamond detectors for particle physics and dosimetry applications.
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