Physics Archives – Page 143 of 144

Electron–positron pioneer

On 16 November 1998 the INFN Laboratori Nazionali di Frascati celebrated the memory of Bruno Touschek, the spiritual father of electron-positron physics, on the 20th anniversary of his premature death. On the occasion, Frascati announced the institution of the Bruno Touschek grant for high-school students who excel in science.

After escaping from a concentration camp during the Second World War, the Austrian-born Touschek began work in Göttingen and Glasgow, and eventually reached Rome in 1952. On 7 March 1960 he gave a historic seminar at Frascati that would change the face of physics. Pointing out the importance of carrying out a systematic study of electron-positron collisions, he suggested that this could be achieved by constructing a single magnetic ring in which electrons and positrons circulate at the same energy but in opposite directions. Soon afterwards, the first electron-positron accumulation ring, AdA, was built under his leadership in Frascati.

The memorial meeting, held in the main auditorium of the laboratory, named after Touschek, was the occasion for distinguished physicists to review the past, present and future of this still dynamic field of research. After Touschek’s close friends and collaborators, Giorgio Salvini and Carlo Bernardini, recalled his brilliant personality and the “glorious days” of AdA, Jacques Haissinski described the second step of this pioneering work – the move of AdA to Orsay, and the subsequent construction of the ACO ring in that laboratory.

bruno2_2-99 — Carlo Bernardini, a colleague of Bruno Touschek, addresses the memorial meeting.

Revolutionary results then flowed from electron-positron rings all over the world. In particular, in the early 1970s electron-positron collisions helped change our view of hadrons (particles that, like the proton and the pion, and unlike the electron, carry the “strong” force) from elementary to composite particles. This was covered by Massimo Testa, who underlined the major role of Adone, Frascati’s second-generation electron-positron machine, in these developments.

Emilio Picasso’s comprehensive review of electron- positron machines, from AdA to the world’s largest ring, LEP at CERN, introduced the present role of electron- positron physics. Guido Altarelli stressed the prominent role of LEP and its US counterpart, Stanford’s SLC, in the detailed confirmation of the Standard Model, our best description of elementary particle interactions so far.

LEP is in the forefront of the search for physics beyond the Standard Model. One hint of such new physics might be the recent results from the SuperKamiokande Collaboration in Japan, looking at neutrinos produced by cosmic rays in the Earth’s atmosphere and which suggest neutrino oscillations. CERN Director-General Luciano Maiani showed why this is so exciting and underlined the importance of conducting accelerator experiments under controlled conditions to explore neutrino masses and mixings. Projects for such experiments are already under way, both in Japan and the USA, but there is still room for Europe to participate in such a venture. The question “Is a meaningful international scientific programme possible?” begs an answer.

For the immediate future, Frascati will again be in the forefront of electron-positron physics with the new DAFNE phi-factory, which obtained its first collisions last March. Miro Preger explained how the machine has already exceeded a luminosity of 1030 per sq.cm per s operating with only one bunch of electrons and one of positrons. In the near future the machine is expected to run with more than 100 bunches per beam to reach the design record luminosity of 5 x 10³².

The reason behind such a high luminosity was explained by Paolo Franzini: it will allow about 5000 phis per second to be produced and will detect their subsequent decays into charged or neutral kaon pairs. Their study should shed light on the long-standing puzzle of the violation of CP symmetry. A new big detector, KLOE, built for this purpose, is a common effort by more than 120 physicists from Italy, Germany, the USA, Russia, China and Israel, and initial results are eagerly awaited.

For the longer term future, the electron-positron community is focusing on projects for a new very-high-energy linear collider, as presented by the final speaker, Marcello Piccolo. Three independent study groups have already addressed this issue in Europe, Japan and the USA. A costed design is expected for 2001; after a selection and approval procedure, construction could start in 2003.

In the new century, Touschek’s ideas will continue to guide our exploration of the fundamental interactions.

The mysteries of cosmic rays

Cosmic rays, the extraterrestrial particles which rain down on the Earth, extend to energies greater than those available via the biggest laboratory machines. This ultra-high-energy frontier is the traditional focus of the International Symposium on Very High Energy Cosmic Ray Interactions, and the most recent event at the Italian Gran Sasso laboratory highlighted the continual enigma of the universe’s highest particle energies.

High-altitude emulsion chamber experiments record the tracks left by these particles. The Pamir experiment, at an altitude of 4400 metres in Central Asia, confirmed earlier observations of coplanar sheets of hadrons from primary particles with energies above 8000 TeV.

cosmic1_2-99 — One of the mysterious “Centauro” events seen by the BrazilJapan collaboration operating X-ray emulsion chambers at an altitude of 5200 m on Mt Chacaltaya in the Bolivian Andes. Given the number of hadrons seen in the lower chamber (left) physicists are intrigued by the relative lack of corresponding electromagnetic effects in the upper chamber (right).

cosmic2_2-99 — One of the mysterious “Centauro” events seen by the BrazilJapan collaboration operating X-ray emulsion chambers at an altitude of 5200 m on Mt Chacaltaya in the Bolivian Andes. Given the number of hadrons seen in the lower chamber (left) physicists are intrigued by the relative lack of corresponding electromagnetic effects in the upper chamber (right).

This phenomenon is seen in multiple “halo” events with total visible electromagnetic energy above 700 TeV recorded in X-ray emulsion chambers. (Haloes are large black spots on the film, up to several square centimetres.) The events have several separate haloes whose centres lie in a straight line even after having passed through the atmosphere. A number of phenomenological models, some invoking unusual heavy penetrating hadrons, attempt to explain this, but the process remains a mystery.

The long-standing BrazilJapan collaboration operating X-ray emulsion chambers at 5200 m on Mt Chacaltaya in the Bolivian Andes, described in detail a recent clean example of a “Centauro” event.

Centauros were first reported in 1980 by the BrazilJapan team and confirmed in 1984 by the Pamir collaboration. These events contain relatively few particles, but which are almost entirely hadrons, with very few photons. They show that at these energies, hadrons can be generated without neutral pions or eta mesons (which decay into photons).

Man and horse

In Greek mythology, a Centaur was highly asymmetric, with the top half of a man and the legs of a horse. The latest physics Centauro is totally free of photons and with a similar appearance to the original Centauro I. Centauro events have always been a puzzle and remain the subject of speculation.

Other mysterious phenomena seen by these experiments include anomalous cascades penetrating very large thicknesses of densely absorbing material.

It is certainly difficult to explain the exotic phenomena seen by such high-altitude emulsion chamber experiments using conventional physics. This was underlined at the meeting by simulations described by M Tamada of Kinki, Osaka.

Presenting the status of today’s Standard Model, Guido Altarelli of CERN laid special emphasis on the riddles posed by the observation of ultra-high-energy cosmic rays, above 10²⁰ eV.

Increased understanding strengthens the links between cosmic ray and accelerator experiments. An important part of the conference was devoted to this topic, with status reports from major laboratories.

Of particular interest to cosmic-ray physics is the search for the quark-gluon plasma, the precursor of nuclear matter, in heavy ion collisions. The subject was reviewed by Jürgen Schukraft of CERN with special emphasis on the recent data from lead ion experiments at CERN.

Another major focus of the conference was the extreme cosmic-ray energy spectrum. The energies of primary particles extend from around 10⁹ eV (1 GeV) to above 10²⁰ eV, the latter being more than 100 000 times the energies at which it has been possible to observe the primaries directly with balloon- or satellite-borne experiments. Information on this very-high-energy region comes instead from indirect methods via the investigation of extensive atmospheric air showers (EAS) using detectors on the ground.

Quarks in hadrons and nuclei

35 years ago, in the autumn of 1963, the idea of quarks as the elementary constituents of all nuclear and hadronic matter was conceived, both at Caltech in Pasadena by Murray Gell-Mann and at CERN by George Zweig. However, it took about another 10 years before it was realized that with the help of the quarks a relativistic field theory of all strong interaction phenomena could be formulated: quantum chromodynamics (QCD).

Today QCD is the basic and comprehensive theory of strong interactions, covering both nuclear and particle physics. However, solving QCD at low and at high energies requires different approaches, and attempting to describe strong interaction phenomena via QCD in both nuclear and in high-energy physics remains a great challenge.

quark1_2-99 — Reconciling the quark/gluon picture of high-energy particle physics with the properties of nuclear matter is not straightforward. The narrow bridge was crossed in September when particle and nuclear physicists met in the unusual setting of the Hall of Armed Knights of the thousand-year-old Rothenfels castle above the small town of Oberwölz in Styria, Austria.

This problem was highlighted recently when about 60 particle and nuclear physicists working in hadron physics met for a symposium on “Quarks in Hadrons and Nuclei” in the unusual setting of the Hall of Armed Knights of the thousand-year-old Rothenfels castle above the small town of Oberwölz in Styria, Austria.

Topics

Physics topics included constituent quark models, structure functions of hadrons, spin structure of the nucleon, meson-baryon physics, chiral perturbation theory, diffractive processes, lattice gauge theory, quark masses etc.

The meeting began with a general historical account by Harald Fritzsch (Munich) of the development of strong interaction theory. This history dates back to a 1932 paper on the need for a new strong force inside the atomic nuclei by Werner Heisenberg, and culminates in the formulation of the fundamentals of QCD in 1972-3. This property of “confinement” shows that quarks exist as quasi-free particles when close together, but feel increasingly strong binding forces when they try to move apart.

Among the many facets of QCD that have emerged over the past 25 years is the problem of quarkgluon dynamics at low energies, responsible for the properties of hadrons and nuclei as building blocks of matter. Effective models of QCD (such as the constituent quark model) work remarkably well at low energies though a fundamental derivation from fundamental QCD is still lacking.

At low energies the nucleon appears to be a composite structure of three valence (constituent) quarks, while at high energy it can appear as a complicated mixture of current quarks and antiquarks as well as gluons. These dual pictures of the nucleon and the link between them are still not fully understood.

Progress and new attempts in the description of hadrons in terms of constituent quarks were reported by M Beyer (Rostock) and W Lucha (Vienna). The delicate problem of the composition of the spin of the nucleon from low to high energies was addressed from the experimental and the theoretical sides. While the constituent (valence) quark model suggests that (most of) the spin of a nucleon should arise from the quark degrees of freedom, experiments described by K Rith (Erlangen) indicate that the quark contribution to the nucleon spin is only about 30%. Theoretical attempts to solve this problem were reviewed by M Karliner (Tel Aviv). A Vogt (Leiden) reported insights from deep-inelastic structure functions. Whatever the final solution to the nucleon spin problem will be, it is clear that gluonic degrees of freedom plays a role and that gluons are either directly or indirectly contributing to the nucleon spin. (This is currently being addressed by the HERMES experiment at DESY’s HERA electron ring.)

Since the early days of QCD physicists have predicted the existence of hadronic objects “glueballs” formed primarily of gluons rather than quarks, or “hybrids” with both quarks and gluons as constituents. These aspects of QCD and their connections with elastic-like diffractive processes were covered by F Close (CERN), P Landshoff (Cambridge), and P Minkowski (Bern). While the existence of glueballs and hybrids is now generally accepted on theoretical grounds, it is only recently that coherent evidence has begun to emerge supporting predictions that the simplest gluonic mesons exist between 1.4 and 1.8 GeV.

Some basic properties of hadrons and nuclei, such as their masses, are directly related to the structure of the hadronic vacuum, which influences the otherwise difficult-to-calculate (nonperturbative) aspects of QCD. The influence of the hadronic vacuum can be described via QCD sum rules. M Shifman (Minneapolis) gave an account of recent developments and also reviewed 20 years of the sum-rule technique. R Rückl (Würzburg) described specific applications for exclusive decays of heavy mesons. The question of quark masses was covered by M Jamin (Heidelberg).

Gauge theory using an underlying lattice rather than a continuum, pioneered by Kenneth Wilson to describe the otherwise difficult-to-handle aspects of QCD, has undergone a tremendous development. A number of variants have been developed and it became clear from the talks of F Jegerlehner (DESY, Zeuthen) and A Schäfer (Regensburg) that lattice QCD will continue to be a powerful tool for QCD problems at low energies.

Another salient feature of low-energy QCD is the role played by topological properties of gluonic field configurations like colour magnetic monopoles and instantons. They might be responsible for quark confinement and also play a decisive role in the formation and dynamics of the lightest mesons, as was discussed by F Lenz (Erlangen) and H Reinhardt (T¸bingen).

In the same context, with respect to mesonbaryon physics and more generally any hadron properties, consideration of chiral (left handed and right-handed) symmetry and chiral-symmetry breaking turns out to be important. G Ecker (Vienna) gave an instructive summary of chiral perturbation theory, and P Kroll (Wuppertal) addressed exclusive charmonium decays.

While QCD relies on quarks and gluons, quark and gluonic degrees of freedom are relatively inconspicuous for the dynamics of nuclei. However, as emphasized by A Thomas (Adelaide), nuclear matter cannot be described solely by nucleons moving in a nuclear potential; quark and gluonic aspects also need to be taken into account.

The symposium was co-organized by the Institutes for Theoretical Physics of the Ludwig-Maximilians-Universität Munich and the Karl-Franzens-Universität Graz, with H Fritzsch and W Plessas chairing the organizing committee. It was funded by the Province of Styria, the Austrian Federal Ministry for Science and Transportation, and the German W.E. Heraeus Foundation. It was also supported by sponsors from commerce and industry as well as the town of Oberwölz.

Neutrinos stop going West

Research using neutrino beams began at CERN in 1963 using particles from the PS proton synchrotron. The highlight of the PS neutrino act was the discovery of the weak neutral current in 1973, and in 1977 the new SPS synchrotron took over the neutrino role.

Twenty-one years later, with the completion of the CHORUS and NOMAD experiments in September, an era of neutrino physics at least in its traditional setting of the West Area of the SPS has now drawn to a close.

Plans for this facility were laid in 1971 during plans for CERN’s “300 GeV Project” which became the 450 GeV SPS. The BEBC bubble chamber, then under construction, was a fixed point through which the beams had to pass, but two possibilities were considered for the primary proton target, where the pion and kaon parents of the neutrinos would be produced. The alternatives were a surface beam, or a beam beginning at the SPS, 40 m below ground and pointing upwards at an inclination of 4.25°. The “underground” solution was chosen because it allowed operation at any energy possible from the SPS.

cern1_12-98 — The neutrino cavern of CERN’s West Area beam. Protons from the SPS strike a beryllium target (hidden by blocks in the foreground). Immediately after the target is a beam collimator and then can be seen the first of the “magnetic horns” that focus the parent particles before they decay to produce neutrinos.

In addition to BEBC, an electronic detector, that of the WA1 collaboration, was approved and both were available when the first SPS neutrino beams appeared early in 1977.

Over the years four types of beam have been available: the basic diet of wide-band horn-focused beams; the added spice of narrow-band beams using momentum-selected parent particles; “beam dump” beams obtained by dumping the proton beam into a massive target; and low-energy beams from the smaller PS accelerator.

This versatility allowed a wide variety of physics to be covered. A key factor also was the provision of detectors to monitor the flux of the accompanying muons, which allowed absolute determinations of the neutrino flux, always a problem in neutrino beams.

In 1977 BEBC was joined by the Gargamelle bubble chamber, moved from the South West Area at the PS, scene of the neutral current discovery. A year later the 100 t CHARM electronic detector became operational.

First studies

Initial physics centred on studies of neutral currents, on the quark structure of matter, and on quantum chromodynamics (QCD), the field theory of quarks and gluons. The results obtained were, and still are, still important input for tests of the Standard Model.

The targets for the neutrinos were “heavy” nuclei (iron in WA1 and marble in CHARM, freon or propane in Gargamelle, or neon in BEBC) or “free” nucleons in hydrogen and deuterium in BEBC. The latter enabled the distributions of up and down quarks in the nucleon to be measured. As well as BEBC, there was also a 1.5 t liquid hydrogen target upstream of the WA1 detector.

In December 1997 a novel type of experiment was carried out – the “beam dump”. The primary SPS proton beam was pointed toward the detectors and dumped into an intervening massive copper target. In such a target the pions and kaons, the usual parents of neutrinos, do not have time to decay before being absorbed, so that the neutrino supply from these sources is reduced by a factor of more than a thousand. However, any neutrinos produced in the decays of very short-lived parents will be unaffected. In fact such “prompt” neutrinos were observed and are now known to be due to the decays of charmed particles, although at the time measurements had claimed that charm production by protons was much too small. These initial results were confirmed by further dump experiments in 1979 and 1981, which also set limits on the production of tau neutrinos.

Gargamelle took part in the first beam dump experiment and in wide-band and narrow-band running in 1978. However, in October of that year leaks detected in the chamber were found to be due to deep cracks in the steel vessel, which unfortunately it was not possible to repair. However, in its short career at the SPS, Gargamelle nevertheless succeeded in observing for the first time a touchstone weak interaction the purely leptonic process in which a muon neutrino hits an electron, producing an electron neutrino and a muon. After a few years gathering rust in the West Area, Gargamelle is now proudly displayed in CERN’s Microcosm Park.

cern2_12-98 — A “textbook” picture from the BEBC bubble chamber. A neutrino interacts with a proton in the liquid hydrogen to produce a negative muon, a proton and an excited charmed meson (D^*). The D^* decays to a charmed D⁰ meson plus a positive pion and the D⁰ itself decays to a negative kaon and another positive pion. After stopping in the liquid the kaon interacts with another proton to produce a hyperon.

During its lifetime BEBC recorded many thousands of neutrino interactions. This was much less than in the electronic detectors, but the enormous amount of detail available in bubble chamber pictures allowed the study of particular final state particles, notably charmed particles, and the measurement of production cross-sections, masses and decay modes.

During 1984, WA1 and CHARM made very accurate determinations of basic electroweak parameters in high statistics studies of neutral-current interactions using the narrow-band beams. These measurements still serve as an “anchor point” to which the precision high-energy measurements made at CERN’s LEP electronpositron collider can be related, giving an insight into electroweak radiative corrections, and hence to the mass of the Higgs boson.

Throughout the lifetime of the West Area facility, the possibility of neutrino oscillations electron, muon and tau neutrino types transforming among themselves has been the object of continual conjecture and study. The well understood neutrino beams and the variety of detectors available allowed very stringent limits to be placed on such oscillations, within the kinematic limits imposed by the beam energies and the locations of the detectors.

Narrow band, wide band and beam-dump beams have all been exploited to search for neutrino oscillations. The existence of oscillations runs counter to neutrino orthodoxy, requiring that neutrinos have mass.

In order to explore very small neutrino masses, a special neutrino beam was derived from the CERN PS with a new “near station” 130 m from the target. WA1, CHARM and BEBC took part in this search in 1983-84. However, no oscillation evidence was found.

Muon neutrinos

Of particular interest is the possibility of muon neutrino/tau neutrino oscillations. With the West Area well served with muon neutrinos, specialized detectors (NOMAD using kinematic techniques, and CHORUS using nuclear emulsion to observe the final state tau leptons) began operation in 1993.

From 1983-91 the WA79 (CHARM II) experiment detected several thousand events in which a muon (anti)neutrino scatters off an electron, remaining a a muon (anti)neutrino. (One event of this type, observed in Gargamelle in 1972, was the precursor of the discovery of neutral currents.) These purely leptonic interactions provided a valuable measurement of electroweak mixing, free of uncertainties due to the hadronic structure of nuclear matter.

However, the end of neutrino beams in the West Area does not mean the end of neutrino physics at CERN. The evidence for neutrino oscillations in neutrinos from the Sun, from cosmic rays in the atmosphere and from accelerators, indicates that much longer “base lines” are required.

One possibility, not pursued at CERN, was to take advantage of the upward inclination of the West Area beam and install a detector in the Jura mountains 17 kilometres from the target. Instead, interest has turned to the Neutrino Beam for Gran Sasso project in which a beam generated at CERN could be pointed south-east towards the Italian Gran Sasso Laboratory, 730 kilometres away.

More light on the Cherenkov effect

Cherenkov radiation is one of the main techniques for particle identification, but details of the underlying theory are still under debate. A group of researchers from Comenius University, Bratislava, CERN and JINR recently carried out measurements using CERN’s high-energy beam of lead ions. With the assistance of the NA49 experiment, they used air, helium and various crystals as media.

news5_12-98 — Ring the changes – first photograph of the Cherenkov radiation from a high-energy lead-ion beam at *CERN.*

The photo shows the result of passing the beam through a biaxial crystal of triglycine sulphate. The circle is the image of the Cherenkov radiation via the focusing lens, and the two elliptical bands are the Cherenkov radiation (the crystal has two different refractive indices). The light is also not uniform due to the alignment of the optical axis of the crystal.

Cherenkov radiation derives its name from Pavel Cherenkov, who as a young PhD student at Moscow’s Lebedev Institute in the early 1930s, was assigned by Sergei Vavilov the task of investigating what happens to the radiation from a piece of radium when it is immersed in a fluid. Such radioactive materials give off an eerie blue light, such as that seen in a “swimming pool” nuclear reactor.

Initially, this was thought to be fluorescence, similar to that seen when X-rays strike a screen, but Vavilov and Cherenkov were not convinced. After heroic investigations, where Cherenkov would typically prepare for a working day by staying in a totally dark room for one hour, he found that the radiation was produced by electrons and was essentially independent of the liquid used, thereby ruling out fluorescence.

The explanation for the effect came in 1937 from Ilya Frank and Igor Tamm, who explained that the radiation is a shock wave resulting from a charged particle moving through a material faster than the velocity of light in the material the optical equivalent of the sonic boom produced by an aircraft as it accelerates beyond the speed of sound.

The “Cherenkov” radiation propagates as a cone whose opening angle depends on the particle velocity. When this cone hits a flat surface, a characteristic ring is seen.

Further elucidation came in the 1950s when Cherenkov rings were photographed by Valentin Zrelov using proton beams at the Joint Institute for Nuclear Research (JINR), Dubna, near Moscow. In 1958, Cherenkov, Frank and Tamm shared the Nobel prize for their work. Vavilov had died earlier.

Cosmic mystery

A recent paper by Glennys Farrar of Rutgers and Peter Biermann of the Max Planck Institute for Radio Astronomy in Bonn suggests interesting possibilities for the orbits of cosmic particles.

Cosmic rays, particles arriving from outer space, are generally believed to be the result of cosmic fireworks like supernovae. But this is not the whole story.

The energy spectrum of cosmic rays extends above 10²⁰ eV, more than a million times the energy of CERN’s future LHC proton collider. Quite apart from the difficulty of imagining a mechanism which can produce such astronomical energies, the fact that such energies are seen at all is an enigma.

The universe is filled with cosmic background radiation, the faint echo of the Big Bang, discovered by Penzias and Wilson in 1965. Shortly after this discovery, it was pointed out that cosmic particles gradually lose energy by scattering off these photons. Theorists calculate that because of this continual attrition, no cosmic particle should be able to maintain an energy above about 5×10¹⁹ eV.

But some of them do. A handful of ultra-high-energy particles have been recorded which have somehow negotiated this brick wall. Perhaps some new kind of ultra-high-energy cosmic particle “uhecron” is able to shake off the interaction with the cosmic background radiation. Another possibility, put forward by Sidney Coleman and Sheldon Glashow, is that these extreme energies encounter relativistic effects which under ordinary conditions are too small to be noticeable.

The paper by Farrar and Biermann points out other interesting features of these extreme cosmic energies. Normally, galactic and intergalactic magnetic fields make charged particles loop around in tangled orbits so that it is impossible to tell from which direction they have come. However, the higher the energy, the “stiffer” these orbits become, so that the very high energy ones continue to move more or less in their original direction.

Farrar and Biermann point out that the handful of events above 10²⁰ eV appear to come from the direction of quasars, stellar beacons from the dawn of time. If the particles are indeed quasar generated, some additional explanation is still needed for why these signals from the early universe are not eroded by the cosmic background radiation.

Particles for export

gran1_11-98 — Scientists meet in November to discuss how they could use neutrinos sent 730 kilometres from CERN to Italy.

This summer’s neutrino news underlined how little we really know about neutrinos. New data, notably from the SuperKamiokande detector, 1000 metres underground in a Japanese mine, show that the behaviour of neutrinos, the most inactive of known particles, is probably much more complex than originally supposed and poses many new physics questions.

Neutrinos come in three varieties electron, muon, or tau according to which kind of weakly interacting particle (lepton) they escort. The new results suggest that this lepton allegiance can be temporary, changing as the neutrinos fly through matter. Neutrinos could be Nature’s floating voters a neutrino which sets out preferring the company of muons could arrive favouring taus instead.

Some of the neutrinos picked up by SuperKamiokande and other underground detectors derive from cosmic rays crashing into the Earth’s atmosphere. These collisions produce unstable particles such as pions and kaons, which subsequently decay, and whose decay products can in turn decay, producing neutrinos. At face value, there should be twice as many muon-type neutrinos as electron-type. However, for some time, such underground detectors have been seeing fewer muonic neutrinos than expected.

Neutrino oscillations NOW

Neutrino oscillations continuous changes of neutrino type or “flavour” might not only explain the observed lack of neutrinos from the Sun and the atmosphere, but may also provide a very sensitive probe for tiny mass differences between neutrinos of different flavour. These possibilities were on the agenda on 6 September when 130 physicists from all over the world gathered in an Amsterdam brewery for the Neutrino Oscillation Workshop NOW’98.

Recently the SuperKamiokande underground experiment in Japan presented evidence for neutrino oscillations and thereby neutrino mass. Although these masses are believed to be much smaller than that of any other particle, neutrinos are so abundant that they would even with such a tiny mass contribute significantly to the mass of the universe and to its gravitational fate.

now1_11-98 — Discussions oscillate at NOW’98 in Amsterdam. Left to right: Y Declais (hidden), V Palladino, E Bellotti, S Bilenky, P Zucchelli, P J Litchfield, H T Wong.

At NOW’98, recent results presented in morning sessions provided ground for discussions on the future directions of neutrino oscillation physics during the afternoons.

The workshop opened with a talk by M Nakagawa (Meijo), a godfather of neutrino oscillations, who covered the underlying theory from a historical perspective. Currently available data and their phenomenological implications were reviewed by G L Fogli (Bari), while P G Langacker (Pennsylvania) focused on fundamental aspects of neutrino masses and mixing.

Atmospheric neutrino data were presented by W Gajewski (Irvine), of course paying much attention to the recent SuperKamiokande result. K Nishikawa (KEK) explained how from next year, neutrinos produced at the KEK laboratory will be detected by SuperKamiokande, 250 kilometres away. This will be the first of probably several “long baseline” experiments; in the US plans are being drawn up to direct a neutrino beam over 730 kilometres from Fermilab to the Soudan mine, while in Europe a neutrino beam could be directed from CERN to the Italian Gran Sasso underground laboratory at a similar distance.

The afternoon sessions concentrated on the theory of neutrino oscillations, on solar and atmospheric neutrino experiments, short, medium and long-baseline experiments, reactor experiments and on neutrino beams.

In one morning session, S Sarkar (Oxford) brought cosmology down to earth, showing how measurements of the cosmological microwave background radiation and scenarios for the development of large-scale structures in the universe can constrain the mass of neutrinos and may even require neutrinos to be massive.

Twenty-five years of neutral currents

A quarter of a century ago, after raging controversy and nailbiting doubt, an experiment at CERN discovered “neutral currents” in neutrino interactions. For the first time, the weak force had been seen to act without shuffling electric charges

The saga has been covered many times, notably by Don Perkins of the Gargamelle collaboration speaking at the 1992 “Rise of the Standard Model” historical seminar held at the Stanford Linear Accelerator Center (SLAC). Five years ago, for the 20th anniversary of the discovery, several CERN Courier articles recalled those momentous times, when two complementary experiments Gargamelle at CERN and the E1A electronic experiment at Fermilab had to resolve difficult problems chasing a will-o’-the-wisp physics effect, while in the wings many physicists steadfastly refused to believe in neutral currents.

garga1_11-98 — Neutral current in 1973 the Gargamelle bubble chamber at CERN captured this historic photo showing how an invisible neutrino has jogged an electron.

In 1973 CERN had yet to reach full scientific maturity. European physicists were not used to making major discoveries at their accelerators and were sometimes hesitant to swim against powerful currents of opinion. The discovery enabled CERN to attain research maturity.

It was not the first time in modern physics history that dogma had to be revised virtually overnight. For a quarter of a century, physicists had believed that quantum mechanisms are unaffected by space reflection (parity) and particleantiparticle charge conjugation. The dramatic overthrow of these principles in 195657, following a bold hypothesis by T D Lee and C N Yang, underlined how much weak interaction physics could be ignored by so many for so long.

The weak interaction scene had been set in the early 1930s when Enrico Fermi formulated his classic theory of beta decay, in which a neutron spontaneously decays into a proton, an electron and an antineutrino four particles meeting at a single space-time point. In this process, electric charge gets shuffled around, an initial neutral particle producing two oppositely charged particles.

Rearranging the four legs of the Fermi interaction gave other reactions, such as a neutrino interacting with a neutron to produce a proton and an electron. But electric charge was always rearranged the interaction was always a “charged current”.

In principle, other reactions could be imagined for example a neutrino or an electron bouncing off a proton in which electric charge was not rearranged: a “neutral current”. But no such reactions with neutrinos had been seen, while the corresponding effect with electrons was in any case blanketed by electromagnetic scattering.

garga2_11-98 — This 1967 aerial view of CERN shows (top left) the mound over the 28 GeV PS proton synchrotron with, pointing downwards and to the right, the beamline feeding the Gargamelle heavy-liquid bubble chamber, where neutral currents were discovered in 1973. In 1976 Gargamelle was laboriously reinstalled further away to work with the neutrino beams from CERN’s new 450 GeV SPS proton synchrotron.

In the 1960s, with no experiment ever having seen a weak neutral current effect, some physicists spoke of a “no neutral current selection rule”. Occasionally a new search for such effects was mounted, but nothing was found and the disbelief in weak neutral currents grew.

With the emergence of modern quantum electrodynamics in the 1940s following the work of Feynman, Schwinger and Tomonaga, and of Dyson, physicists realized the importance of “renormalization”, the process of carefully constructing a theory so that it did not throw up nonsensical infinite probabilities for things that were clearly finite. In quantum electrodynamics, for example, the mass and electric charge of the participating particles (which have to be put in by hand anyway) can be redefined to sidestep these infinities. Quantum electrodynamics is “renormalizable”.

However, the Fermi theory of weak interactions as it stood could not be made renormalizable. While some theorists took this warning lightly, others declared “there is no theory of weak interactions”.

25 years of asymptotic freedom

Like members of a close-knit family, quarks and gluons behave like free particles when they are very close together, but feel much stronger forces if they are separated. This result, paradoxical at first sight and embodied in the term “asymptotic freedom”, is basically why quarks and gluons cannot be isolated as free particles. They can only be studied in their native habitat, using high-energy particle beams to spy on them deep inside protons and neutrons and other strongly interacting particles.

The idea was pointed out in two landmark papers by David Gross and Frank Wilczek at Princeton and by David Politzer at Harvard, published in the June 1973 issue of Physical Review Letters. In his presentation at the 1992 history seminar at Stanford, now published in The Rise of the Standard Model (edited by Lillian Hoddeson, Laurie Brown, Michael Riordan and Max Dresden, Cambridge University Press, 1997), David Gross described this breakthrough: “For me, the discovery of asymptotic freedom was totally unexpected. Like an atheist who has just received a message from a burning bush, I became an immediate true believer.”

qcd1_11-98 — The at-first curious idea of asymptotic freedom, first published in 1973, provided the key to quantum chromodynamics (QCD), the field theory of quarks and gluons. Asymptotic freedom pioneers 't Hooft (left Utrecht) and David Gross (right now at Santa Barbara), attended the 6th international QCD conference (QCD 98) held in Montpellier from 28 July.

But the idea had been noticed elsewhere. In his book In Search of the Ultimate Building Blocks (Cambridge University Press, 1997), Gerard ‘t Hooft relates: “In 1972 a small conference took place in Marseille. On arriving at Marseille airport, I discovered that (prominent field theorist) Kurt Symanzik and I had shared the same plane… He had been trying to understand Bjorken scaling [the behaviour seen in high-energy scattering when the incoming projectile particle transfers a lot of momentum to the target Ed.] in a quantum field theory, but had limited himself to what he considered to be the prototype of all field theories, a simple spin zero model. Unfortunately it had the wrong scaling behaviour.

” ‘If only I could turn this scaling behaviour round, ‘ Symanzik said,’then you would get a theory where particles at close distance behave almost as free particles, but when they separate to larger distances they would feel much stronger forces.’

” ‘Well, ‘ I [‘t Hooft] cried,’that is exactly what you get in a Yang-Mills (spin one) gauge theory!’

“Symanzik replied: ‘You should publish this quickly, because this would be very important.’ ”

“Much to my later regret, I did not follow this sensible advice, ” says ‘t Hooft, whose 1971 work on the renormalizability of Yang-Mills theories had underlined the importance of gauge fields for understanding particle behaviour. However,’t Hooft did air his idea for spin one fields following Symanzik’s talk at the Marseille meeting.

This idea was the key to quantum chromodynamics (QCD), the field theory of quarks and gluons. A host of theoreticians contributed to the subsequent development of the theory, many of whom, including Gross (now at Santa Barbara) and ‘t Hooft (Utrecht), attended the 6th international QCD conference (QCD 98) held in Montpellier from 28 July. The QCD series, now an established event in the particle physics calendar, is run by Stephan Narison of Montpellier.

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