Physics monitor

the results continuing to emerge from them with the declared objectives of charges (magnetic monopoles) do the HERA electron-proton collider at the world's major Laboratories. T.D. not exist. Particle theorists are not so DESY, Hamburg, where the Zeus Lee drew attention to the need for sure, and for a long time magnetic and H1 experiments look deep inside future studies to understand CP monopoles have been tentatively the proton, showing that its gluon violation - the delicate asymmetry included on the theoretical menu. content increases as the momentum between matter and antimatter. The role of these monopoles has fraction decreases. With a large now become crucial. kinematical range now covered, Information from Klaus Monig and Also playing a central role is the these results correlate well with data Clara Matteuzzi idea of supersymmetry. In a quantum from fixed target experiments. theory, basic particles, like quarks The two HERA experiments con­ and leptons (fermions), interact tinue to see 'rapidity gaps', where through force-carrying particles events pile up in kinematical bands. Spotlight on quantum (bosons) like the photon of electro- Many people have understood this in magnetism, the W and Z of the weak terms of the incoming electron black holes nuclear force and the gluon of the bouncing off a proton constituent in a strong inter-quark force. In super- 'colourless' way ('colour' is for the article theorists are getting symmetry, each fermion has addi­ quark-gluon force what electric P unusually excited these days as tional boson partners, and vice versa. charge is for the electromagnetic new ideas and different approaches So far, no evidence for super- force). Recently on the market is a converge in the search for a picture symmetry has been found, but the new idea explaining rapidity gaps which describes all the underlying underlying ideas are so convincing with a full-colour quark-gluon mecha­ mechanisms of Nature. that its existence is almost taken for nism. Although the final picture has yet to granted among theorists. Super- Also for proton structure, the enig­ emerge, the outline is becoming symmetry would have governed the matic spin content of proton constitu­ clearer. While the intellectual moun­ mechanics of the Big Bang, but as ents is underlined in additional data tain range to be crossed was once the temperature fell, supersymmetry from experiments at SLAC and at intimidating, these new develop­ 'froze' out and became almost CERN (July 1994, page 19). ments are beginning to point to a way invisible. The neutrino sector is difficult to over. In the late 1970s, Klaus Montonen summarize, as invariably somewhere A series of recent topical workshops and David Olive pointed out that if a result refuses to conform. The have highlighted these developments magnetic monopoles are included in muon-neutrino content from cosmic and leap-frogged ahead - including a supersymmetric quantum picture, ray interactions in the atmosphere is String 95 at the University of South­ the electric and magnetic sectors are still significantly less than expected. ern California, Los Angeles, this in some respects mutually comple­ This year has also seen much spring and a conference on Mirror mentary. Magnetic charges provide controversy in the continuing search Symmetry and S-duality held in June additional calculational leverage, for neutrino oscillations (June, page at Trieste's International Centre for sidestepping the traditional problem 13). Klaus Winter attempted to paint Theoretical Physics. Closing the of having to solve the equations of a tidy picture at Beijing and looked Trieste meeting, prominent theorist the theory through sometimes unsat­ forward to new results. Ed Witten said "this is one of the isfactory approximations. Heavy quark effective theory, where most exciting conferences in which I Subsequently, the ideas were heavy quarks are assumed to be have ever participated". enlarged to include 'dyons' - particles infinitely heavier than the lighter With profound problems to be having both electric and magnetic ones, is now a useful calculation tool, overcome, the new ideas now on the charges, providing a much richer getting answers right to within ten per market at first look very unconven­ scenario. cent. tional. Classical electromagnetism Theories with sufficient numbers of Summarizing the physics scene at exploits the parallels between elec­ supersymmetric particles can provide Beijing, Sam Ting enumerated the tricity and magnetism but accepts the an appealing correspondence ('dual­ results coming in and compared everyday wisdom that free magnetic ity') between different sectors of the

4 CERN Courier, October 1995 Physics monitor

theory. These theories are naturally 'superstrings' in multi-dimensional but often indicate that deeper down finite, with no troublesome infinities spaces (usually ten dimensions). in the theory, something significant is thrown up by the calculations. While For the first time, superstrings offer happening. this looks superficially attractive, real the possibility of a full quantum With gravity an essential part of the physics unfortunately does not theory of gravity, a goal which has superstring picture, quantum black behave so nicely, and 'renormal- long eluded theorists. holes can occur. Classically, black ization' constraints have to be im­ In physics, writing down a set of holes are concentrations of matter so posed to remove unwanted infinities. basic equations is not the bottom dense that nothing can escape from Last year, Nathan Seiberg and Ed line. Solving them is quite another them. On the scale of Big Bang Witten wrote a milestone paper problem. Many physicists are con­ dynamics, even these black holes, (December 1994, page 3) in which vinced that the compelling features of classically at least several times massless monopoles in a field theory these superstring theories could larger than the Sun, can happily exist with less supersymmetry (and there­ provide a natural framework for the as massless objects. These quantum fore requiring renormalization) equations of a Theory of Everything. black hole ideas had been seeded ensured that quarks were automati­ The revolution in quantum electrody­ early in the 1980s, but the implica­ cally 'confined' in larger particles. namics resulting from Richard tions of this leap in imagination are While quarks are the natural constitu­ Feynman's introduction of simple only now being appreciated. ents of all nuclear particles, they are diagrams in the late 1940s is a good The new work has shown that never encountered as free particles, example of how an intuitive ingredi­ presence of massless black holes and one of the big puzzles was ent can make a difficult theory more can sidestep some of the problems of always how to ensure that these assimilable. the internal six-dimensional spaces. quarks are automatically locked The ten dimensions of superstrings These holes help iron out trouble­ inside nucleons. Seiberg and include the four of conventional some topological twists when the six- Witten's toy model of quark confine­ space-time, complemented by a six- dimensional internal spaces curl up. ment made theorists sit up and take dimensional internal (Calabi-Yau) Building on these ideas, S. Kachru notice. As well as providing physics space, which at some stage has to and C. Vafa from Harvard have been insight, the new picture also sug­ 'compactify' - curling up on itself to able to show that the Seiberg-Witten gested a simpler way of handling the become invisible. The invisibility of results extend to a class of string underlying four-dimensional geom­ these additional dimensions was theories with the same amount of etry. originally a handicap, as they could inbuilt supersymmetry as the theories Including electromagnetism and the curl up in all sorts of ways and their considered by the latter in their strong and weak nuclear forces is not role in the underlying unified theory seminal work, demonstrating how the end of the story. To get right back was not clear. these valuable results can be ex­ to Big Bang conditions, theorists The big question was how to apply tended to obtain exact answers in have to bring in gravity too. Gravity - the Seiberg-Witten ideas to theories including the long-sought operating over very large distances superstrings. Seminal ideas by A. goal of quantum gravity and with and always attractive - is very differ­ Sen of Bombay's Tata Institute were complicated dynamics. ent to the other forces of Nature. followed by recent advances by C. With these ideas in place and their With such widely different behaviour, Hull and P. Townsend in the UK. At implications beginning to be appreci­ from the long-range gravitational pull the Trieste meeting, A. Strominger of ated, theorists can glimpse the first between galaxies to the intricate Santa Barbara explained how certain outlines of a Theory of Everything. inter-quark mechanisms at work kinds of multidimensional superstring inside nuclear particles, putting all can 'compactify' to four-dimensional these forces together is an intimidat­ space-time scenarios. ing prospect. Previous efforts to reconcile invis­ In trying to do this, many theorists ible internal geometries had come have come to believe that the basic across singularities - places where elements are not pointlike particles at the mathematics breaks down. all, but even smaller two-dimensional Singularities are awkward to handle,

CERN Courier, October 1995 5