UCL HEP Seminars 1995
Tuesday 12th December 1995 : Dr Greg Heath (Bristol)
Hunting the Higgs Boson - In a Haystack
Particle physics experiments produce enormous raw data rates. Millions of events per second must be filtered on-line to find a few dozen of interest. Filtering processors will be described and the strategies used on them, for current experiments and for the future Large Hadron Collider at CERN.
Wednesday 29th November 1995 : Dr Jeff Forshaw (Manchester)
Report on the Beijing Lepton-Photon Conference
Wednesday 22nd November 1995 : Dr Roger Phillips (DRAL)
Speculations about the KARMEN Anomaly
The KARMEN experiment, studying neutrinos from the pi+ decay chain, has found an anomaly in the time-dependence that suggests a new unstable particle x with m(x)=33.9 MeV, produced via pi -> mu+x . Could this be a new neutrino or what? Pros, cons, ifs and buts will be reviewed.
Wednesday 15th November 1995 : Dr Subir Sarkar (Oxford)
"No-crisis" for Big Bang Nucleosynthesis
The synthesis of the light elements at the end of the `first three minutes' provides the most detailed probe of physical conditions in the early universe and has proved very useful in constraining new physics, viz. the existence of new particles and forces. Recently it was claimed that the observationally inferred abundances of deuterium, helium and lithium are in fact inconsistent with even the Standard Model, thus undermining this programme. I will review the situation and argue that there is no such crisis.
Wednesday 8th November 1995 : Prof John Edgington (QMW)
Neutrino Oscillations - Weighing the Evidence
Do neutrinos possess mass? The Standard Model says no, without giving a reason. So far there is no direct and sustainable experimental evidence for mass, despite occasional hints to the contrary. To some, non-zero mass is more "natural" and cosmologists seeking missing mass have supported this theological view with enthusiasm. Massive neutrinos will have wave functions which are combinations of separate flavour eigenstates; thus production of neutrinos of one flavour and their detection as another flavour is evidence for non-zero mass. Experimental searches for neutrino oscillations will be reviewed, concentrating on the KARMEN experiment at Rutherford Appleton Laboratory and comparing it with the LSND experiment at Los Alamos National Laboratory. Some problems of drawing conclusions will be discussed, including the relevance of a Bayesian approach to data handling.
Wednesday 1st November 1995 : Dr. D. B. Stamenov (INR, Bulgarian Academy of Sciences)
Constraints on "Fixed Point" QCD from the CCFR data on DIS
Wednesday 25th October 1995 : Dr Robert Thorne (DRAL)
Report on the European Physical Society HEP Meeting, Brussels
I will attempt to summarize some of the most important and interesting developments reported at the Meeting. This is clearly a personal choice, but the topics to be discussed include: developments in perturbative QCD, particularly small x physics, high pT jets, and higher twist corrections; particle-astrophysics and the solar neutrino problem; precision measurements (particularly R_b and R_c) and calculations in the standard model, and implications for new physics; and duality in supersymmetric theories.
Wednesday 11th October 1995 : Prof John Dainton (Liverpool)
The Partonic Structure of Diffraction
Recent new measurements of the deep-inelastic scattering of electrons on protons have revealed a new layer in our understanding of the structure of the proton. At the highest possible interaction energies, which are available only at the HERA ep collider in DESY in Hamburg, new measurements have revealed a contribution which is due to the way protons interact with other hadrons. The first measurements of the short distance structure of this contribution reveal that it is partonic, and furthermore that it may be understood in terms of Quantum Chromodynamics (QCD). For the first time, deep-inelastic lepton scattering experiments are able to probe one of the oldest conundrums in physics, namely the way nucleons interact with other nucleons (often generically referred to as diffraction), and to provide measurements which elucidate an understanding in terms of QCD. The new results which lead to these conclusions are presented and discussed.
Wednesday 14th June 1995 : Jason Ward
Measurement of the Photon Structure Function at OPAL
An introduction to how photon structure is studied at an electron-positron collider is given, with motivation for such a study. The new contributions that LEP can make to the measurement of the photon structure function are discussed and the OPAL measurements are presented. These measurements are compared with theory. The photon structure function measurements that we expect from future LEP-II running are also discussed.
Wednesday 31st May 1995 : Prof. Roger Cashmore (Oxford)
The Top Quark - Discovery and Implications
The evidence for the recent discovery of the top quark at Fermilab will be reviewed and the implications for other areas of particle physics discussed. Further progress in top quark physics will be made with upgrades to the Tevatron and the construction of the LHC while a high energy linear e+/e- collider offers intriguing possibilities. These aspects will be developed and put in perspective.
Wednesday 24th May 1995 : Dr Alexei Yung (St. Petersburg / Swansea)
Why Topological Models are Relevant to Physics
An elementary introduction to what is topological field theory is given. The problem how topological models could serve physics is discussed. In particular, the idea that the physical theory could correspond to the broken phase of the topological theory is explained. The example of such breakdown phenomenon in the two dimensional topological sigma model is presented.
Wednesday 17th May 1995 : Dr Paul Harrison (Queen Mary)
A Solution to the Solar and Atmospheric Neutrino ProblemThe physics of solar and atmospheric neutrinos is reviewed, and the evidence of anomolous detection rates is summarised. Vacuum neutrino oscillations are discussed and an especially simple and elegant form for the lepton mixing matrix is proposed, based on a cyclic permutation symmetry among the generations. Predictions are compared with experiment, and an excellent fit is obtained, the data requiring a hierarchical spectrum of mass-squared differences for the neutrinos. Implications for future experiments are discussed.
Wednesday 10th May 1995 : Dr Brian Foster (Bristol)
The BaBar Experiment
I will motivate the study of CP violation in the B system by the need to understand the baryone asymmetry of the Universe. I will discuss the possible mechanisms of CP violation and various formalisms in which to understand and express them. I will discuss the requirements for an experiment to study CP violation in the B system and then show how these are being realised in the design of the BaBar experiment at PEP-II
Wednesday 3rd May 1995 : Dr Alfred Goldhaber (Stony Brook / Cambridge)
An Open Universe from Inflation
A model is described in which inflation occurs, with many of the features customarily associated with inflation, but the universe today may still exhibit appreciable negative curvature, e.g. Omega = 0.2. This is achieved without extremely fine tuning if the inflation proceeds in two stages. The first, or old inflation, stage, is one with a local minimum in the vacuum energy associated with the inflaton field, and implies a de Sitter universe. At some point a bubble forms through a a quantum transition, with new or slow-roll inflation proceeding inside. This bubble evolves into our visible universe, and the slow roll must involve a change in the magnitude of the inflaton field of the same order as the Planck scale. Tuning in the initial value (delta Phi)/Phi around 0.01 to 0.1 would be sufficient to give agreement with current observations. The model has been presented in a recent paper by M. Bucher, A.S. Goldhaber, and N. Turok, and further developed in work by Bucher and Turok.
Wednesday 15th March 1995 : Dr Apostolos Pilaftsis (DRAL)
How Left-Right Asymmetry, Tau Polarization and Lepton Universality Constrain Unified Theories at the Z Peak
We suggest the use of a universality-breaking observable based on lepton asymmetries at the Z peak, which can efficiently constrain the parameter space of unified theories. The new observable is complementary to the common lepton-universality quantity relying on partial width differences and depends critically on the chirality of a possible non-universal Z-boson coupling to like- flavour leptons. The LEP potential of probing universality violation is discussed in representative low-energy extensions of the Standard Model (SM) that may be motivated by supersymmetric grand unified theories, such as the SM with left-handed and/or right-handed neutral isosinglets, the left-right symmetric model, and the minimal supersymmetric SM.
Wednesday 8th March 1995 : Dr Mike Green (RHBNC)
The Search for the Higgs Boson at LEP I and LEP II
One of the most important outstanding issues for the standard model is the origin of mass. The most popular mechanism for generating mass requires the existence of one or more Higgs bosons with a mass below about 1 TeV. Prior to the operation of LEP there was very little opportunity to search for these and only a very small mass range close to zero was excluded. In the first few years of LEP running the whole mass range below 60 GeV has been excluded for the minimum standard model Higgs. At LEP II the sensitivity will be extended up to at least 80 GeV and hopefully higher. Direct searches above this mass will depend on LHC. However virtual effects in electroweak processes and a direct measurement of the top mass can help pin the Higgs down to a restricted mass region
Wednesday 22nd February 1995 : Dr Werner Vogelsang (DRAL)
Prompt Photon Production at Hera
We first review the status of prompt photon production in hadronic collisions, focussing in particular on the situation at high-energy ppbar colliders, where we show how the longstanding discrepancy between data and theory at small pt/S is removed by using steep small-x parton distributrions and also by taking into account the fragmentation contribution in next-to-leading order. Then we turn to prompt photon production at HERA and study its sensitivity to the parton, in particular the gluon, content of the photon in the framework of a complete next-to-leading order calculation. Special attention is paid to the issue of isolation constraints imposed on the cross section by experiment.
Wednesday 15th February 1995 : Dr Tim Greenshaw (Liverpool)
Recent Results from H1
Wednesday 25th January 1995
The Next Collider(s) after LHC : Prof David Miller
The LHC must come first, but then what? There is important physics which other colliders can do but which LHC will find difficult or impossible. Technical and physics questions will be reviewed.
Wednesday 18th January 1995 : Dr Neville Harnew (Oxford)
Exotic Physics Searches at Hera
During the 1993 running period of the HERA e-p collider, the ZEUS and H1 detectors each recorded approximately 550 inverse picobarns of integrated luminosity. I report on searches from both experiments for particles beyond the Standard Model, in which HERA is able to explore an entirely new kinematic range. Topics considered include the hunt for direct, indirect and flavour-changing leptoquarks, predicted in extensions to the Standard Model. I will also review the status of the search for excited electrons, neutrinos and quarks, all of which are predicted in composite models. Finally, I will discuss the prospects for the discovery of SUSY at HERA, both in the R-parity conserving and R-parity violating production and decay modes.