Physics monitor

Together at the Tau 94 Workshop in Montreux (left to right) tau lepton pioneer Martin Perl, Montreux organizing committee chairman Gigi Rolandi, and workshop series pioneer Michel Davier.

beyond the Standard Model. From the experimental viewpoint, Martin Perl pointed to future perspectives at LEP, CESR and BEPC (Beijing) where tau research continues. He stressed the advantages of the planned B-factories and the physics possibilities of a tau-charm factory, indicating the detector specifications needed for this research. Michael Turner of Chicago high­ lighted the interesting cosmological and astrophysical consequences of a tau neutrino mass between 1 MeV and 30 MeV, with the additional cosmological mischief that could be wrought by unstable tau neutrinos. The summary talk by Richard Stroynowski of Dallas emphasized the precision that has been achieved In the future the strange tau sector Charged currents in this very active field, and under­ is expected to provide more quantita­ lined the usefulness of the tau lepton tive and valuable information about as a laboratory to study quark field One full day was devoted to SU(3) symmetry breaking. theory (quantum chromodynamics - charged current tau decays, with Non-strange hadronic decays have QCD) and resonances, etc. With data from the ARGUS (DESY), been measured by ALEPH with a most of the results still limited by CLEO and LEP collaborations. global analysis yielding 'startlingly' statistics, more stringent tests will Michel Davier of Orsay in his review small errors, reaching better than one come from future tau data. said that tau data is in a 'healthy' percent precision in dominant chan­ state. The structure of tau charged nels. The Tau 94 workshop, chaired by currents is in excellent agreement Gigi Rolandi, was the third in a series with the conventional vector/axial- pioneered by Michel Davier, and was vector picture. Neutral currents sponsored by CERN and the Swiss Despite measurements still being National Science Foundation. statistics limited, LEP experiments LEP results with tau pair events give very precise leptonic branching probing neutral current interactions From Ricard Alemany ratios, emphasizing LEP's excellent were shown by Andrei Kounine of tau physics capabilities. One notable Massachusetts. Frequent beam technique is the impact parameter energy calibrations with the resonant approach used by OPAL. With depolarization method improve the Colourless different methods used to measure precision of the Z mass and width. the tau lifetime mutually consistent, There was increased precision also confinement for quarks the electron-muon and tau-muon on the electron and tau Z couplings universalities can be tested at the from the LEP and SLC (Stanford) The enigma of quarks is that they are 0.3% level. collaborations, however still limited there, hidden deep inside nucleons Hadronic decays, reviewed by Brian by statistics. and other strongly interacting parti­ Heltsley of Cornell, have also made cles, but refuse to come out. The progress, with an 'explosion' of The final day looked at the tau as a tighter the quark bonds are stretched, results in the tau strange sector, and probe for new physics, where the more difficult they are to break. with charged and neutral kaons well Charles Nelson of Binghampton This dogma has been accepted for identified. surveyed possible tau territory some thirty years but has never been

CERN Courier, December 1994 3 rnysics monitor

Small-x (momentum fraction carried by struck quark) data on nucleon structure from the HERA electron-proton collider at DESY, Hamburg, from H1 and ZEUS experiments. The curves show different theoretical predic­ tions, bracketed by curves assuming very different gluon distributions at low x. The momentum density of gluons inside the proton clearly increases in this range. mathematically proved. result is a gratifying combination of The latest excitement with theorists years of theoretical endeavour. is a proof that, under certain assump­ The papers are numbers 9408099 tions and in model environments far and 9407087 on the high energy from reality, quarks can only exist in physics theory preprints bulletin colourless combinations. This is the board on World Wide Web. first theoretical proof that quarks cannot exist alone and explains why hadrons - clusters of quarks, each carrying fractional electric charge - only have familiar integral electric charges. As well as their electric charge, HADRONS-94 quarks have another charge-like Soft interactions at property called 'colour', denoted as red, green and blue. Although quarks large distances are not really coloured, the descrip­ sections - was associated with tion is useful because, according to Ten years ago the Institute for hadronic reactions only. Now, the the hypothesis of colour confinement, Theoretical Physics (known since small-x data on nucleon structure quarks always combine to make 1992 as the Bogolubov Institute after from the HERA electron-proton colourless hadrons in the same way its founder) of the Academy of collider at DESY, Hamburg, provide that different coloured light mixes to Science of the Ukraine initiated what new information on the nature of the make white. Colour confinement also has become a very successful series mysterious pomeron, establishing a forbids hadrons with fractional of annual meetings on strong interac­ link between "soft" (small momentum electric charges, but the hypothesis tions at large distances. transfer) hadron-hadron and the that they are forbidden has never Although sometimes overshadowed violent interactions of "hard" deep been proven theoretically until now. by the successes of the Standard inelastic lepton-hadron scattering. In two recent compelling papers, Model and the theoretical entice­ Latest data on the small-x behav­ Nathan Seiberg of Rutgers and Ed ments of supertheories; the Hadrons iour of the proton structure and Witten of Princeton show that quarks series has overcome political barriers diffractive behaviour were presented are confined in colourless hadrons. and financial chaos to bring together at the workshop by both HERA Confinement is shown in the wonder­ physicists from diverse backgrounds experiments - H1 (Christophe Royon) land of N=2 supersymmetry (where to discuss central physics issues. and ZEUS (Hong Joo Kim). The data there are two supersymmetric part­ The latest workshop in the series in the x range 103 -104 are compat­ ners for each ordinary particle); by was held from September 7-11 in ible with a power 0.2-0.5 increase in 'soft-breaking' the proof is extended Uzhgorod (Ungvar), a small univer­ the richness of the underlying struc­ to the one-to-one N=1 sector. How­ sity town in the westernmost reaches ture. The Gribov-Lipatov-Altarelli- ever much remains to be done to of the Ukraine, bordering on Hun­ Parisi QCD fit for these data shows reach the real world. gary, Poland, Romania and Slovakia. that the momentum density of gluons The new development confirms The main topics were: elastic and inside the proton could increase superconductor analogies which diffractive scattering; probing the tenfold in this range. describe how, if two magnetic nucleon at small x (where x is the Hard scattering in both photo- monopoles were placed inside a momentum fraction carried by the production and deep inelastic scatter­ superconductor, the lines of magnetic struck quark); spin physics; hadron ing has been observed with concen­ flux would just go directly from one to spectroscopy; and collective proper­ trations in distinct kinematical the other, forming a confining poten­ ties of strongly interacting matter. regions, an effect called 'rapidity gap' tial. Seiberg and Witten use ingen­ Until recently, the pomeron - a in the trade. For the diffractive ious constructions and many different hypothetical vacuum quantum photoproduction events, the hadronic past results to show that colour is number exchange governing the final states show a predominantly similarly confined. This theoretical asymptotic behaviour of total cross two-jet structure, each jet having

4 CERN Courier, December 1994