EW AND SYMMETRY BREAKING
BEYOND THE SM
HEAVY FLAVOUR PHYSICS
NEUTRINO AND NON ACCEL PHYSICS
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PERT AND NON-PERT QCD EW AND SYMMETRY BREAKING BEYOND THE SM HEAVY FLAVOUR PHYSICS NEUTRINO AND NON ACCEL PHYSICS |
Nigel Glover, Adrian Signer, James Stirling
Jets may be produced in hadron-hadron collions, electron-positron annihilation and electron-proton scattering. It is a short distance and therefore perturbative process as the point-like particles scatter and produce quarks or gluons with significant energy transverse to the beam. As the coloured partons separate they lose energy through gluon radiation and finally through the formation of hadrons. The hadrons tend to follow the direction of the parton giving rise to the jet.
Although jet production is a primarily perturbative process, the strength of the quark-gluon coupling means that leading order estimates of cross-sections are only good to about 50%. This is mainly due to the uncertainty in the choiice of renormalisation or factorisation scales. Ultimately the cross section should be independent of these scales and at next-to-leading order, the uncertainty is significantly reduced to about 20% or so. Many cross sections have been calculated to this order and we have produced a variety of next-to-leading order QCD Monte Carlo programs. In hadron collisions, one and two jet production is described by JETRAD, vector boson with jet by DYRAD, direct photoproduction by DPRAD and vector boson pair production by DKS. In electron-positron annihilation, we have written two Monte Carlo's to describe four jet production, MENLO-PARC, and EERAD2.
Extracting useful results from experimental data requires both plentiful data and accurate theoretical calculations. For example, the single jet inclusive transverse energy distribution observed by the CDF collaboration at the TEVATRON indicated possible new physics at large transverse energy. However, because of both theoretical and experimental uncertainties no reliable conclusion could be drawn.
This figure shows the predicted differential cross section for producing jets with transverse energy of 100 GeV at CDF in Run 1. The renormalisation scale dependence is shown for the leading order, next-to-leading order and next-to-next-to-leading order predictions (this is known up to a renormalisation scale independent constant). We see that for renormalisation scales within a factor of two of the jet energy, the renormalisation scale uncertainty is reduced from 20% to 9% to 1%. Interestingly, the experimental statistical error from CDF for this data point is currently about 2%, while the systematic error is about 10%.
At both the LHC and Run II of the TEVATRON, it is necessary to improve the accuracy of the theoretical calculations by including either more particles or next-to-next-to-leading order accuracy . This is a formidable challenge requiring a major theoretical effort to (a) evaluate the two loop matrix elements (b) isolate the infrared singularities from the double bremsstrahlung processes and (c) develop a numerical implementation that is fast, accurate and adaptable to the details of the experimental measurement. For an overview of these issues see the talk by Nigel Glover at the DIS2000 workshop in Liverpool. A group of us are now working towards the two loop matrix elements.