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Oxford LHCb and
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Denys Wilkinson Building Keble Road Oxford OX1 3RH |
LHCb Detector
LHCb RICH
LHCb Physics
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LHCb Physics and Oxford Activities
At present, the specific physics foci of the Oxford group are
- Measurement of the unitarity triangle γ,
- Charm physics.
The general physics programme at LHCb compasses a large and varied set of
precise measurements of B and D meson decays that will
provide information on physics beyond the Standard Model in a manner which
is complementary to the direct particle searches carried out at ATLAS and
CMS. Rare processes that are dominated by suppressed Feynman diagrams
are driven by heavy virtual particles in so-called `loops'. For example,
in the B mixing processes shown to the left, the dominant Standard Model
contributions come from the diagrams with top quarks in the box loops.
In general, however, new physics particles must also contribute to these
loops, giving experimental signatures inconsistent with the Standard Model
expectations.
The primary interest of B decays lies in the multitude of modes which can be studied to learn more about CP violation - the difference in behaviour between matter and anti-matter. In the Standard Model CP violation in introduced as a complex phase in the 3x3 unitarity `Cabibbo-Kobayashi-Maskawa (CKM) matrix', which parameterises the couplings in weak charged current quark interactions. A geometrical way to express this understanding is through the unitarity triangle, which is representation of the relationship between certain CKM elements in the (ρ-bar, η-bar) plane, where ρ-bar and η-bar are two of the four parameters of the matrix, η-bar being the imaginary component. If quark coupling and CP violation is fully described by the Standard Model, all experimental measurements will give results consistent with the uniarity triangle picture. As there is no known reason why new physics will have the same quark coupling and CP violation structure as the Standard Model, contributions from this source will give rise to discrepancies between measurements.
Shown at right is the present status of the unitarity triangle as presented
by the
CKMfitter group.
Each coloured band represents a different measurement, most
of which come from the B-meson sector, performed at
BaBar and
Belle.
All measurements have associated contours which overlap in a yellow oval in
the region
LHCb will significantly improve the sensitivity of many of the measurements contributing to the unitarity triangle, in particular the very poor existing constraints on the angle γ as indicated by the beige shading. Making a precise measurement of γ is one of the most important goals of flavour physics in the coming decade. Taking on this challenge is a priority of the Oxford LHCb group.
There exist many other extremely important measurements which are best considered independently of the unitarity triangle context. These include:
- The search for the very small CP violating phase
associated with Bs mixing, accessed through decays such as
Bs → J/ψ φ . New physics contributions will in general lead to larger effects. - The search for extremely rare B decays, most notably
Bs → μμ which is predicted to occur at a rate of4 x 10-9 in the Standard Model, but can be enhanced greatly by new physics models such as large tan(β) SUSY. - The study of kinematical distributions associated with particular B
decay modes, such as the forward-backward asymmetry in
B → K* μμ . - The search for CP violation in charm decays. In general, any observation of this pheomenon would reveal the presence of new physics processes at work. Charm physics at LHCb is an active research area of the Oxford group.
More information on the LHCb physics programme can be found in LHCb's web pages.