# UCL HEP Seminars 1996

27th November 1996 : Dr C. Damerell (RAL)

The international e+e- linear collider programme: physics prospects and recent design developements

The future e+e- linear collider was one of the options studied in detail at the recent Snowmass Workshop 'New Directions for High Energy Physics'. This talk (which includes material presented in the closing parallel session) summarises recent progress on the accelerator and detector design, and contrasts the physics prospects with other options discussed during the workshop.

20th November 1996 : Prof. I Percival (QMW)

Quantum mechanics, cat-fleas and gravit

By analogy with the use of Brownian motion to detect flucuations on the atomic scale, it is shown that modern matter interferometry experiments might detect fluctuations of space-time on a Planck scale, despite the small values of the Planck length and time.

13th November 1996 : Dr B. Webber (Cavendish Laboratory)

"Hadronisation" corrections to QCD observables

Predictions in perturbative QCD refer to final states consisting of quarks and gluons, rather than the hadrons actually observed in experiments. It has therefore been customary to apply `hadronization'' corrections, based on Monte Carlo models, to the theory before comparing with experiment. Although this procedure seems to fit the data quite well, the corrections applied have not been well justified theoretically. Empirically, they behave like inverse powers of a large momentum scale of the process, for example the centre-of-mass energy in e+e- annihilation or the quark mass in heavy quark processes. Recent theoretical work has suggested that such power-suppressed corrections can arise from divergences of the perturbation series at high orders, which are called `renormalons''. This talk will explore the insights into hadronization corrections that can be obtained from the renormalon approach.

6th November 1996 : Dr C. Sutton (Oxford)

Communicating science

30th October 1996 : Dr O Teryaev (Dubna and Birkbeck)

QCD single spin asymmetries in the RHIC and HERA-N programmes

23rd October 1996: Dr J. McGovern (Manchester)

Chiral symmetry in nuclei

Partial restoration in nuclear matter of the chiral symmetry of QCD is discussed together with some of its possible signals. Estimates of corrections to the leading, linear dependence of the quark condensate are found to be small, implying a significant reduction of that condensate in matter. The importance of the pion cloud for the scalar quark density of a single nucleon indicates a close connection between chiral symmetry restoration and the attractive two-pion exchange interaction between nucleons. This force is sufficiently long-ranged that nucleons in nuclear matter will feel a significant degree of symmetry restoration despite the strong correlations between them. Expected consequences of this include reductions in hadron masses and decay constants. Various signals of these effects are discussed, in particular the enhancement of the axial charge of a nucleon in matter.

16 October 1996 : Prof E. Leader (Birkbeck)

Report on the 12th Int. Symp. on High Energy Spin Physics, Amsterdam

9th October 1996 : Prof D. Miller (UCL) and Dr J. Lauber (UCL)

Report on the INternational Conference on High Energy Physics, Warsaw

Wednesday 5th June 1996 : Dr Dhiman Chakraborty (State University of New York at Stony Brook)

The current status of top physics at the Tevatron

The Tevatron accelerator at Fermilab has recently concluded a run in the collider mode before switching to the fixed-target mode of operation for the next 3 years. Each of the two collider experiments, CDF and D0, has collected over 100 pb**-1 worth of data which is about twice what the last batch of publications were based on. The first results from this full data sample have just started to come out. In the arena of top physics, both experiments have revised their calculations of production cross-section and the mass of top quarks with reduced statistical and systematic errors. These as well as some new results from more difficult final states and a breif outline of future prospects will be presented.

Wednesday 29th May 1996 : Dr Mike Charlton (University College)

AntiHydrogen

Antihydrogen was recently observed at high energies at CERN. We briefly review this experiment, but suggest that the primary motivations for studying this object, which include CPT and WEP tests, can only be addressed by its controlled production at very low energies. Techniques which will allow this are described, including the capture and cooling of antiprotons to temperatures below 20K and the accumulation of dense positron plasmas.

Wednesday 22nd May 1996 : Nick Allen (Brunel University)

Electroweak Measurements at SLD

Wednesday 15th May 1996 :Dr George Lafferty (Manchester)

Inclusive particle production in hadronic Z decay

An overview will be given of the physics results from the study of inclusive particle production in multihadronic Z decays at LEP, the primary aim of which is to reach an understanding of the non-perturbative hadronisation process in QCD. Topics covered will include: aspects of quark and gluon fragmentation; local parton-hadron duality; QCD-based models of parton hadronisation; two-particle correlations; and spin effects in fragmetation.

Wednesday 8th May 1996 : Barry Macevoy (Imperial College)

Defect kinetics in silicon detector material

A numerical kinetics model has been used to investigate the evolution of complex defects in high resistivity silicon detector material during fast neutron irradiation to levels expected at the CERN LHC. The complex V_2O is identified as a candidate for a deep-level acceptor state which gives rise to experimentally observed changes in the effective doping concentration. The importance of the initial oxygen impurity concentration in determining the radiation tolerance of the detectors is demonstrated. The characteristics of devices heavily irradiated with Colbalt 60 photons are modelled satisfactorily by using a semiconductor simulation in conjunction with the kinetics model. It is postulated that inter-defect transitions between divacancy states in the terminal damage clusters are responsible for apparent discrepancies in the modelling of data from neutron-irradiated devices. This mechanism (if correct) may have important consequences for te prospects of "defect-engineering" a radiation hard device.