New measurements from the US Department of Energy’s Jefferson Lab (JLab) in Virginia are challenging existing ideas on how quark-antiquark pairs are produced from “nothing” – that is, the vacuum. Members of the CEBAF Large Acceptance Spectrometer (CLAS) collaboration have studied the spin transfer from a polarized electron beam to a produced lambda particle, with surprising results.
The experiment recorded collisions between a 2.567 GeV longitudinally polarized electron beam and a proton target in which the electron emerges together with a polarized lambda (Λ0) and a kaon (K+). The large acceptance of CLAS enabled the team to detect the outgoing electron and the kaon, as well as the proton from the decay of the lambda, over a wide range of scattering angles – in effect, a wide range of momentum transfer from the electron to the quark system. The team was thus able to measure the angular dependence of the lambda polarization.
At the quark level, the reaction studied corresponds to the creation, from the available kinetic energy, of a strange quark-antiquark pair, in addition to the original quarks in the proton. In a simple model of the reaction dynamics, a circularly polarized virtual photon (emitted by the polarized electron) strikes an oppositely polarized up quark inside the proton. The spin of the struck quark flips in direction and the quark recoils from its neighbours, stretching a flux-tube of gluonic matter between them. When the stored energy in the flux-tube is sufficient, the tube is “broken” by the production of a strange quark-antiquark pair.
Using this simple picture, the CLAS team found that they could explain the measured angular dependence of the lambda polarization if the quark pair is produced with the spins in opposite directions, or anti-aligned. This is unexpected because according to the popular triplet-P-zero (3P0) model, a quark-antiquark pair is produced with vacuum quantum numbers, and that means their spins should be aligned. The new results imply that the 3P0 model may not be as widely applicable as was previously thought.
Winston Roberts, a theorist at JLab and Old Dominion University, finds the CLAS measurement very interesting. “If they are right, it means we have to rethink what we thought we understood about our models for baryon decays,” he said. “The CLAS results may also be saying something about what we understand of baryons themselves – our knowledge of how to describe scattering processes such as the one they measure, or even that there may be oddities or peculiarities, dare I say ‘strangeness’, in the way strange quark-antiquark pairs are produced.”
The CLAS team itself expects further reaction from theorists. “Polarized lambda production is obviously sensitive to the spin dynamics of quark-pair creation,” said Mac Mestayer of JLab. “We eagerly await confirmation, or refutation, of the conclusions of our simple model by realistic theoretical calculations.” Meanwhile, the collaboration is planning further experiments, as Daniel Carman of Ohio University and lead author of the recent paper explains. “Our group is continuing this exciting research by extending our arguments to test our picture of the dynamics in different reactions.”
The results certainly show that we still do not fully understand the basic structure of the vacuum. Twentieth-century quantum field theories filled the once-empty space with virtual particles. Now JLab physicists are working to measure the spin of those particles, helping us to understand the vacuum better as well as the matter that populates it.
Two separate experiments in North America – at TRIUMF in Vancouver and at the Indiana University Cyclotron Facility (IUCF) in Bloomington – have made new observations of charge symmetry breaking (CSB). The results have brought fresh input to theoretical attempts to determine the different contributions to the phenomenon, including effects due to the mass difference between quarks.
If charge symmetry were an exact symmetry, neutrons and protons would be indistinguishable except for their electromagnetic interactions. CSB, which can be attributed to a difference in the masses of u and d quarks and their electromagnetic interactions within the nucleon, is seen in small effects such as the neutron-proton mass difference ΔN. The two new measurements have extended the observation of CSB to pion-production reactions in systems of few nucleons that consist of equal numbers of neutrons and protons.
The group working at TRIUMF, an Alberta-Ohio-TRIUMF-UNBC collaboration led by Allena Opper of Ohio University, measured the angular distribution of deuterons from a reaction in which a neutron and a proton combine to produce a deuteron and a neutral pion (np→dπ0). CSB was expected to appear as a small difference in the numbers of deuterons emitted in the forward and backward hemispheres of the centre-of-mass system. At a neutron beam energy of 279.5 MeV (~ 0.8 MeV FWHM), just 4 MeV above threshold, the deuterons emerged within 30 mrad (lab angle) of the beam direction. This made it possible to detect the complete centre-of-mass angular distribution with one field setting of the SASP magnetic spectrometer. The experiment accumulated more than 6 million good events during 10 cycles of production and calibration runs. The observed events were compared with simulated events to account for energy loss and multiple scattering of deuterons in the liquid-hydrogen target, and the spectrometer’s acceptance.
The results can be expressed in terms of the difference in the numbers of deuterons in the forward and backward hemispheres divided by their sum: Afb = 17±8(stat)±5.5(sys) x 10-4. For comparison, a theoretical calculation of Afb identified contributions from the n-p mass difference, from η-π mixing, and from π0 rescattering effects, which are due to the d-u quark mass difference and electromagnetic interactions between the quarks (J A Niskanen 1999, U van Kolck et al. 2000). The solid blue line of figure 1 indicates the shift in the calculated value of Afb when a “large but still reasonable” value for π0 rescattering is included. To reconcile this prediction with the data for Afb and the neutron-proton mass difference would require a quark mass difference within the chiral effective field theory that is less than 2 MeV. Large uncertainties remain, however, because neither the strength of η-π0 mixing nor the η-N coupling is well known.
The IUCF-Argonne-Hillsdale-Minnesota State-Western Michigan collaboration, led by Edward Stephenson, sought evidence for the reaction in which two deuterons combine to form an alpha particle and a neutral pion (dd→απ0). This reaction is forbidden by charge symmetry, so a non-zero cross section would indicate CSB. The group’s method was to make deuterons circulating in the IUCF Cooler Ring collide with deuterons in a gas jet target, and then detect the resulting alpha particles in coincidence with the two gamma rays from the decay of the neutral pion. The challenge was to see a clean signal for the reaction – estimated to be as small as a few picobarns in cross section – in the presence of various backgrounds. The most troublesome background was expected to come from a reaction producing an alpha particle and two gamma rays, but without the formation of a pion – a reaction that is not forbidden by charge symmetry. The IUCF group was able to display the αγγ events in terms of the missing mass, showing a clear peak at the mass of the pion on top of the double radiative capture continuum (figure 2). The cross section measured was 12.7±2.2 picobarns at a beam energy of 228.5 MeV and 15.1±3.1 pb at 231.8 MeV.
This non-zero result has provided fresh stimulus to a team of theorists whose goal is to relate the dd→απ0 cross section, thought to be dominated by η-π mixing, to quark mass differences. The hope is that this cross section, when combined with the n-p mass difference and Afb in np→dπ0, will help unravel the quark mass difference, electromagnetic and meson-mixing contributions to nuclear CSB.
These results and the related theoretical work were the focus of a special session of the April 2003 meeting of the American Physical Society in Philadelphia.
On 28 April, the UK minister for science and innovation, Lord Sainsbury, opened a new research cavern at the Boulby Underground Laboratory for Dark Matter Research, at Boulby in North Yorkshire, UK. The Boulby lab is situated in a working salt and potash mine and houses experiments to detect weakly interacting massive particles (WIMPs), a prime candidate for dark matter in the universe. The laboratory has recently benefited from a £3.1 million Joint Infrastructure Award (JIF) from the UK Particle Physics and Astronomy Research Council, which has provided new enhanced underground laboratories and complementary surface facilities.
Situated more than 1 km beneath the Earth’s surface within a salt and potash mine, the laboratory is isolated from interference from cosmic rays and benefits from an environment with low natural radioactivity. The laboratory operates on behalf of the UK Dark Matter Consortium – the University of Sheffield, the Rutherford Appleton Laboratory, the Imperial College of Science, Technology and Medicine, and the University of Edinburgh.
The Boulby lab currently houses three experiments to detect dark matter – NAIAD (NaI Advanced Array Detector), ZEPLIN I (from ZonEd Proportional scintillation in LIquid Noble gases, now operating with liquid xenon), and DRIFT (Directional Recoil Identification From Tracks). DRIFT is the first experiment to be installed in the new area of the laboratory and is unique because its aim is not only to detect WIMPS, but also to determine which direction they come from.
More and more astronomers are joining the race for the first optical images from gamma-ray bursts. After the precise localization of a new gamma-ray burst, several institutes try to obtain an image of its optical counterpart as quickly as possible. What makes this “sport” so exciting is that nobody knows when or where the next gamma-ray burst will occur. Luck plays an important role in the quest. Among the telescopes around the world that are ready to respond immediately to an alert, only a few are able to take advantage as the bursts occur during the night, well above the horizon and during good weather conditions.
The story of one of the most exciting of these races has recently been published in Nature (Fox et al. 2003). The starter’s shot was the detection by NASA’s High Energy Transient Explorer (HETE) satellite of a gamma-ray burst, named GRB021004, on 4 October 2002 at 12:06:14 UT. Just 193 seconds later, the Japanese Automated Response Telescope at the Institute for Physical and Chemical Research (RIKEN) pointed on the location sent out by HETE to make the first image of the burst. Sharper images were performed a few minutes later by the Near Earth Asteroid Tracking (NEAT) camera mounted on one of the Mount Palomar telescopes near San Diego, California. The fading afterglow of this burst was then followed, for several weeks, by some 40 optical telescopes around the world, as well as by six radio telescopes.
GRB021004 is the second gamma-ray burst, after GRB990123 in January 1999, to be observed in the optical less than 15 minutes after the burst. Although detected slightly later in the optical than GRB990123, it was the best-observed gamma-ray burst at that time. Now, however, all the attention is focused on GRB030329, which is on the way to take the lead over GRB021004. Detected by HETE on 29 March 2003, GRB030329 is one of the brightest and closest gamma-ray bursts on record. With a cosmological redshift of z = 0.17, it is approximately two billion light-years away, as opposed to other bursts such as GRB021004 (z = 2.32) that are located at more than 10 billion light-years away.
The unprecedented number of observations of these two recent gamma-ray bursts has tended to confirm their association with supernovae. The violent flash of gamma rays is thought to arise from ultra-relativistic particles thrown out by a cataclysmic event such as the collapse of a massive star – for example in the “cannonball” model. Such models apply to gamma-ray bursts lasting several seconds; bursts shorter than about 2 seconds are thought to be due to the coalescence of two neutron stars to form a black hole.
Currently, two satellites are able to provide and distribute accurate burst locations within seconds – HETE and INTEGRAL. In December 2003, NASA will launch the Swift spacecraft, which will have an even greater capability to detect and locate bursts, as well as on-board optical, ultraviolet and X-ray telescopes.
The 10th International Workshop on Neutrino Telescopes, held in Venice on 11-14 March, brought together particle physicists, astrophysicists and cosmologists, all captivated by the fascinating properties of neutrinos. A total of 142 participants attended the meeting, which was organized by Milla Baldo Ceolin of Padova University and co-sponsored by the Istituto Nazionale di Fisica Nucleare (INFN) and the Istituto Veneto delle Scienze, Lettere ed Arti. In her opening address, Baldo Ceolin recalled the recent death of George Marx, a leading figure in Hungarian astroparticle physics. Further reminiscences followed, with warm and affectionate recollections of Bruno Pontecorvo by Luigi Radicati di Brozolo from Pisa, who talked about Pontecorvo’s deep human qualities and his invaluable scientific legacy, in particular regarding neutrinos.
The first two days of the workshop were devoted to neutrino oscillations, neutrino masses and mixing angles. John Bahcall of the Institute of Advanced Study, Princeton, began by reminding us of the tremendous challenge that the detection of solar neutrinos represented when it was first proposed. Like the Sun, which shone in Venice during most of the conference and dissolved the last of the winter fog, the joint effort of all experiments on solar neutrinos and solar physics has finally cast light on the long-standing solar neutrino problem. However, warned Bahcall, now that the solar standard model seems to work perfectly well, we should not stop testing it.
The elegance and completeness of the experiments at the Sudbury Neutrino Observatory (SNO) – with its outstanding feature of being sensitive to both charged and neutral current interactions on deuterium – emerged clearly from the talks by SNO project director Art McDonald of Queen’s University, Ontario, and by Richard Hahn from the Brookhaven National Laboratory. The experiment now has evidence at the 5.3σ level for neutrino flavour oscillation to active neutrinos. Yoichiro Suzuki of Tokyo described the contribution that Kamiokande and Super-Kamiokande have made in this field, including the first directional observation of solar neutrinos, the first measurement of the 8B neutrinos from the Sun, and the most precise detection of high-energy solar neutrinos through neutrino-electron scattering, as well as the detection of neutrinos emitted by the explosion of the supernova SN1987A. Kamiokande and Super-Kamiokande also produced the first clear evidence of neutrino oscillations through the distortion of the zenith distribution of atmospheric muon neutrinos. Support for this result came from the MACRO detector at the Gran Sasso Laboratory and Soudan2 in the US. Takaaki Kajita of Tokyo presented an exciting series of measurements in this field, together with the first confirmation of muon neutrino oscillations with a long baseline neutrino beam, which has been made by the K2K experiment. This experimental programme will continue with the long baseline experiment at the Japan Proton Accelerator Research Complex (J-PARC, formerly the Japan Hadron Facility).
Moving to the low-energy region, Vladimir Gavrin of the Institute for Nuclear Research (INR) of the Russian Academy of Sciences (RAS) reminded us of the contributions made by the Baksan Neutrino Observatory. Its most outstanding result was provided by SAGE, the radio-chemical experiment with metallic gallium that has been taking data for 13 years. Thanks to the low threshold of the neutrino capture reaction in the metal, gallium experiments are the only ones that are sensitive to all the components of the solar neutrino flux, in particular the pp neutrinos. On the same topic, Till Kirsten of the Max Planck Institute, Heidelberg, described the evolution over the years of the Gran Sasso Laboratory’s activities on solar neutrinos. The GALLEX experiment announced the first observation of solar pp neutrinos 10 years ago, and also made the first neutrino source calibration of any solar neutrino detector. Today, Gran Sasso’s involvement in solar neutrino physics continues with the Gallium Neutrino Observatory, which began running in 1998; the BOREXINO real-time, low-threshold solar neutrino detector, which is almost ready; and the LUNA experiment that aims to measure the cross section of the fusion reactions at the energy of the solar Gamow peak (that is, the optimum energy for the reactions).
The final brush-stroke to this picture of solar neutrino oscillations – after a quest that has lasted for 50 years – has come from the KamLAND experiment, which has shown the first strong evidence for the disappearance of reactor antineutrinos. As Atsuto Suzuki from Tohoku pointed out, the present KamLAND result is completely consistent with large mixing angle (LMA) solar neutrino oscillations. In addition, the experiment has such a low background that it can observe the antineutrinos from the decay of uranium and thorium in the Earth, and could in the near future provide a measurement of the beryllium-generated solar neutrinos. In this context, Gianno Fiorentini of Ferrara stressed that it is now time to use such geo-neutrinos to determine the radiogenic contribution to the energetics of the Earth. More generally, Gianluigi Fogli of Bari pointed out that we are entering the era of precision tests for several neutrino parameters. In the solar sector, the large angle solution at present includes two close, but separate regions in parameter space (see plots figure). Fogli showed that there is now statistical evidence for matter effects in the Sun, which were also nicely reviewed by Alexei Smirnov of INR/RAS and ICTP, one of the founding fathers of this field.
Artificial neutrino beams will continue to play a fundamental role in the precise determination of neutrino oscillation parameters. Bill Louis from Los Alamos presented the evidence for neutrino oscillation from the LSND experiment, through the appearance of electron antineutrinos in a muon antineutrino beam. He also described the status of MiniBooNe at Fermilab, which should unambiguously confirm or refute the LSND result, and which is now taking data with the results expected in early 2005. The final and complete mapping of the neutrino oscillation parameters will, however, require new facilities and new detectors. Deborah Harris from Fermilab presented future long baseline neutrino experiments, introducing new concepts in neutrino beams such as off-axis neutrino beams, “super beams” and “beta beams”. Ken Peach from the Rutherford Appleton Laboratory illustrated the road towards what appears to be a final neutrino beam facility – the Neutrino Factory – where neutrino beams of unprecedented intensity and purity will be produced by the decays of muons in flight.
The general implications of the recent neutrino results on physics beyond the Standard Model were discussed by Guido Altarelli from CERN. He remarked that the neutrino properties fit nicely, and actually support the framework of grand unification in its supersymmetric version, where dark matter and baryogenesis can be included naturally. In such a context, the dominant source of neutrino masses could be the “see-saw” mechanism, which was reviewed by Steve King of Southampton, Rabindra Mohapatra of Maryland and Ferruccio Feruglio of Padova. Neutrino masses are also considered in models with extra dimensions, as summarized by Qaisar Shafi from the Bartol Research Institute. The challenging and fundamental direct determination of neutrino mass, both with integral spectrometers and cryogenic detectors, was discussed by Christian Weinheimer of Bonn and Angelo Nucciotti of Milano-Bicocca. The relevance of a positive signal in neutrinoless double beta decay for understanding the neutrino spectrum was emphasized by Serguey Petcov of SISSA/INFN and INRNE, Sofia, while Alessandro Strumia of Pisa reminded us about the existing unconfirmed neutrino “anomalies”.
On the third day of the workshop, the discussion moved to neutrino astrophysics. Petr Vogel of Caltech explained that supernova neutrinos are essential for improving our knowledge about the emission models in gravitational collapses. Neutrinos with energies in the MeV range could also shed light on other types of gravitational collapse, such as the one leading to a strange-quark star starting from a neutron star, as described by Arnon Dar of Technion and CERN.
The detection of high-energy cosmic neutrinos represents one of the most exciting future prospects in astrophysics – indeed, in 1988 the first Neutrino Telescope Workshop held in Venice promoted the birth of this new field. Dar, Daniele Fargion of Rome and Francis Halzen of Wisconsin reviewed the theoretical motivations for studying high-energy cosmic neutrinos. Such studies are expected to play an important role in unravelling the mysteries associated with major cosmic accelerators, such as active galactic nuclei and gamma-ray bursters. There are several existing models, and competition between them is fierce, as noted by Alvaro de Rujula of CERN, so observations will be crucial. Sandip Pakvasa of Hawaii pointed out that neutrino decay might affect the flavour composition of astrophysical neutrinos.
Impressive progress has been made in the construction of neutrino telescopes since the first workshop in 1988. As Stephan Hundertmark of Stockholm reported, AMANDA, the muon and neutrino detector array at the South Pole, is now a successfully operating telescope. It currently has the best limit on a neutrino source above the TeV region, with a sensitivity of about 0.1 event per km2 per year. The future of AMANDA will be IceCube, a kilometre-scale neutrino observatory designed to detect neutrinos of all flavours at energies from 107 eV to 1020 eV. The first of its 80 strings will be deployed in 2004, and the detector will be completed in 2009. Closer to Europe, Jürgen Brunner of Marseille reported on ANTARES, the neutrino telescope under construction off the Toulon coast, which will be ready to take data in less than three years. A strong programme has also already begun on the Neutrino Mediterranean Observatory (NEMO), a km3 deep-sea neutrino telescope. The wish of all the participants is that the first “light” through this new window on the universe might be announced in Venice at a future Neutrino Telescope Workshop.
Neutrinos in cosmology were the subject of the fourth and last day of the workshop. Few things in recent years have had the same impact on our view of particle interactions as the recent impressive experimental achievements in cosmology. The data from WMAP, for example, confirm that we are now performing precision tests of cosmological models. Evidence for dark matter was reviewed in great detail by Marco Roncadelli of INFN Pavia, while Rita Bernabei of Rome reported the status of LIBRA. This is the upgraded version of DAMA, the experiment at Gran Sasso that is reporting a signal from dark-matter particles. We know that neutrinos can be, at most, a sub-dominant component of dark matter, and as Sergio Pastor of Valencia recalled, we can infer from the power spectrum of density fluctuations, an upper bound on the sum of neutrino masses of about 1 eV. The canonical dark-matter candidate remains the lightest particle – possibly a neutralino – in a supersymmetric extension of our world. Neutrinos could, however, provide an efficient way of revealing neutralinos captured by the Sun, after annihilation, as suggested by Antonio Masiero of Padova. On a related topic, the observed baryon density could have been produced by leptogenesis, through the CP-violating, non-equilibrium decay of a right-handed neutrino. Franco Buccella of Napoli showed how leptogenesis can be elegantly incorporated into grand unified theories such as SO(10). It is remarkable that the range of neutrino masses required for successful leptogenesis is essentially the same as the one obtained from neutrino oscillations, as was discussed by Wilfried Buchmüller of DESY.
Dark energy is currently one of the most intriguing mysteries of our universe, and was described for the conference by Masiero and Sabino Matarrese of Padova. Dark matter and dark energy are also expected to affect the time variation of fundamental constants in specific frameworks such as string theory, as Thibault Damour of IHES, Bures-sur-Yvette, explained. Neutrinos are apparently not involved here – unless, as Guido Altarelli observed, the equality between the dark-energy scale and the scale of neutrino masses is not a numerical coincidence but is instead an indication of some deep and yet unidentified relationship.
Scrutinized at any length scale, from the microscopic to the cosmological, our world is fascinating. This workshop illustrated that neutrinos are capable of carrying information about our universe from the smallest scales to the largest distances, and this, in the words of Sheldon Glashow of Boston who closed the meeting, is the kind of unification that we should really be looking for.
Leon Lederman, former director of Fermilab, once described the development of particle physics as frequently a story of false trails, crossed wires, sloppy techniques, misconceptions and misunderstandings, occasionally compensated for by strokes of incredible luck. Certainly, this subject is quite often a tale of the unexpected. Nowhere is this more true than for the subject of neutrino physics, and the elucidation of neutral weak currents.
Neutral currents were discovered in 1973 by a collaboration operating with the large heavy-liquid bubble chamber Gargamelle at the CERN proton synchrotron (PS). It was built by a team of physicists and engineers at Saclay, led by André Lagarrigue. Lagarrigue’s plans for Gargamelle were laid following the Siena Conference in 1963, where results were reported from the first PS neutrino experiments with the CERN 1.2 m heavy-liquid chamber, built by a team led by Colin Ramm.
The results that were obtained from this small CERN chamber (Block et al. 1964) included a limit on the ratio of “elastic” neutral-current (NC) events to charged-current (CC) events: σ(νμ + p → νμ + p)/σ(νμ + n → μ– + p) < = 0.03 for events with proton momentum above 250 MeV/c. The accepted ratio is about 0.12. The CERN result was just wrong, following a book-keeping error (the actual 90% confidence limit was < 0.09). As is often the case in physics, the error was uncovered by a graduate student, Michel Paty from Strasbourg, who found a limit < = 0.20 and put this number in his thesis (Paty, 1965). The CERN group intended to publish an erratum, but (since there seemed to be little interest in our limit) we decided to wait for the results from a forthcoming propane run, since that would measure scattering from free as well as bound protons, and we could exploit the better kinematic constraints. However, the propane run was delayed by more than two years and the corrected limit for the ratio of < 0.12 ± 0.06 was not published until 1970 (Cundy et al. 1970).
The Gargamelle collaboration on neutrino physics (Aachen, Brussels, CERN, Ecole Polytechnique, Milan, Orsay and UCL) had formed by 1968, and met in Milan to list the priorities in the forthcoming neutrino experiments (see “Gargamelle’s priorities”). The search for neutral currents was way down at number eight. And it has to be explained here that at that time there were several different models for beating the divergence problems of the Fermi theory, and neutral currents as postulated by the electroweak model of Glashow (1961), Salam (1968) and Weinberg (1967) was just one option. Other models included the “diagonal” model of Gell-Mann, Goldberger, Kroll and Low (1969). In this model, the divergence in non-diagonal processes involving unlike currents, such as in muon decay, were cancelled out, whereas for like currents such as in νe + e → νe + e–, the expected cross-sections could be arbitrarily large. We knew that there was support for this model from a reactor experiment by Reines and Gurr (1970), who initially found a cross-section for this process of more than 100 times the V-A value (which turned out to be yet one more of the many wrong numbers floating around in the neutrino field, although we did not know that at the time). In the 1970 paper of Cundy et al. giving the revised value for the NC:CC ratio, the limit on diagonal coupling was in fact given pride of place above that for neutral currents.
The situation changed dramatically in 1971, following the work on renormalization by Veltman and ‘t Hooft, showing that for the electroweak model, the cross-sections were finite and clearly specified in terms of just one unknown parameter, sin2θw. But it is necessary to emphasize here that the ‘t Hooft paper was – like the other papers on the electroweak model – about the interactions between leptons. It was not clear then how hadrons might be incorporated into the model, and there could be unknown suppression effects, as in the Cabibbo theory. So, many people in the Gargamelle group felt that emphasis should be first of all on a search for the very rare leptonic processes νμbar + e– → νμbar + e– and νμ + e– → νμ + e– where a single electron is projected at a small forward angle (about 2 degrees) to the beam at giga-electron-volt energies. The antineutrino process was preferred since the principal background from νe + n → e– + p was expected to be less than 1%. In a 1.4 million picture antineutrino run, between 5 and 30 antineutrino events were expected, depending on the weak mixing angle. In fact, it turned out that the actual value of sin2θw = 0.23 meant that the number of events was near the minimum: after a cut of E > 0.3 GeV on electron recoil energy and the effect of finite scanning efficiency, only three events were found. But they were gold-plated, unmistakable events, of which the first was found at Aachen in December 1972.
Here we had some real luck. This first event (Hasert et al. 1973) was initially recorded by a scanner, who put it down as a “muon plus gamma ray”. But a sharp-eyed graduate student, Franz-Josef Hasert, realized that it must be something else, and showed it to Prof. Helmut Faissner, who recognized it immediately for what it was. Faissner showed me this event at New Year 1973, and since at that time the expected background in the 100,000 pictures scanned to date was < 0.01 events, I was convinced that this was a real signal and that neutral currents really existed. I would have been less sanguine had I known that, in scanning 14 times as much film over the next year – and accumulating 14 times as much background – only two further events were to be found. Of course, three events could hardly establish a new physical process, but the important thing was that this first observation of a leptonic neutral-current event gave great encouragement to the collaboration to push ahead further with the search for hadronic neutral currents.
The establishment of a neutral-current signal for events containing only hadron secondaries and no muon was a more complex story. Since the neutrino cross-sections on nucleons were more than two orders of magnitude bigger than those on electrons, a clear understanding of the background, rather than of statistics, was the main problem. The first step for the Gargamelle collaboration was to resolve to look for inclusive events of the type ν + N → ν + hadrons, rather than exclusive processes, which would be more difficult to identify, and to select only the highest-energy events, with total hadronic energy E > 1 GeV.
Events with identified hadrons only were called “NC”, for neutral-current candidates, as compared with charged-current (“CC”) events containing a charged lepton (muon). “AS” events were those in which a CC event had an associated “star” due to a secondary neutron in the same picture (figure 1). Suppose that a number B of the NC events are due to neutrons, then if we take a simple one-dimensional model with all particles travelling along the beam and chamber axis, the expected value of B can be easily calculated from the number of AS events in “equation 1”, where L = 600 g cm-2 is the fiducial length of the chamber, λ = 135 g cm-2 is the neutron interaction length in the liquid and λatt ~ 300 g cm-2 is the attenuation length of neutrons in the shield – that is, the distance over which the number of neutrons of a particular energy is reduced by a factor e. This attenuation length was calculated from the shape of the neutron energy spectrum measured in AS events and reasonable assumptions on the elasticity ε of neutron interactions. In fact for a power-law neutron spectrum of the form dN/dE ~ E-n, the attenuation length is simply λatt =λ/[1-<εsup>n-1>] (and typically n ~ 2 and ε ~ 1/2).
Clearly the B:AS ratio simply states that for L » λ, the neutrons entering the front of the chamber are generated by neutrino interactions from an upstream region in the shield of thickness λatt, while inside the chamber the AS neutron stars can be generated by neutrino interactions anywhere inside a distance L, less an amount λ for neutron detection. By using this equation, one calculated B/AS = 0.7, compared with the observed ratio of NC/AS ~ 6.0. This implied that only about 10% of the NC events were due to neutrons. A full Monte-Carlo simulation by Fry and Haidt (Fry and Haidt 1975) using detailed data on the neutrino beam divergence, the angular and energy distribution of secondary neutrons and the elasticity distribution in the propagation of the neutron cascade in the shield gave essentially the same numbers. It happens that the simple one-dimensional approximation works because of the relatively high energy of the hadronic events, so that most of the neutrons do enter the front face of the chamber.
Confirmation that the NC events were produced by neutrinos came from measurement of the longitudinal and radial dependence of the NC:CC event ratio in the chamber (figure 2), which was found to be constant within the statistical errors, whereas a predominantly neutron origin would have shown strong dependence on longitudinal and/or radial distance.
A counter-experiment at Fermilab by the HPWF (Harvard, Pennsylvania, Wisconsin, Fermilab) group was contemporaneous with the Gargamelle experiment, and used a 300 GeV proton beam, compared with the 26 GeV beam from the CERN PS. The results from both Gargamelle (Hasert et al. 1973a, 1974) and HPWF were presented by Myatt at the Bonn Conference in August 1973, as positive evidence for neutral currents. Shortly afterwards, the HPWF result was withdrawn, following modifications to the experiment. Figure 3 shows a diagram of the HPWF set-up. It consisted of a liquid scintillator plus spark chamber detector acting as a target and as a hadron calorimeter, followed by a muon spectrometer containing magnetized iron toroids instrumented with spark chambers. A CC event would register as a hadron shower in the scintillator section, plus a muon penetrating through to the spectrometer, while a NC event would consist of a hadron shower only. From the angular distribution of the muons in the observed CC events, one could calculate the true muon angular distribution at production and hence the proportion of CC events appearing as NC “background” events because the muon missed the spectrometer. After making this correction, the ratio of rates as given at Bonn was found to be: HPWF (mixed ν/νbar beam) NC:CC = 0.29 ± 0.09…which was compatible with the Gargamelle results, as follows: Gargamelle ν beam NC:CC = 0.21 ± 0.03; Gargamelle νbar beam NC:CC = 0.45 ± 0.09.
Following the Bonn Conference, HPWF undertook a major re-configuration of the experiment to enhance the NC signal by reducing the correction for “lost” muons emitted at wide angles in CC events. The areas of the spark chambers in the muon spectrometer were increased, and a large spark chamber, SC4, originally part of the hadron calorimeter, was made part of the muon-detection system by placing a 30 cm thick steel plate in front of it. Previously the first muon chamber had been behind 1.25 m of steel. The reduced steel thickness in front of SC4 unfortunately led to a quite catastrophic increase in punch-through of energetic hadrons, and because of this genuine NC events would be wrongly classified as CC events. By November 1973 the modified HPWF ratio was quoted as NC:CC = 0.05 ± 0.05 – consistent with no neutral-current events at all. This incorrect result was primarily because the hadronic punch-through probability – and hence the proportion of genuine NC events misclassified as CC events – had been underestimated by more than a factor of two. The considerable uncertainties and errors in this experiment were eventually resolved and new values, with a horn-focused beam, were given in April 1974 as NC:CC (ν) = 0.11 ± 0.05: NC:CC (νbar) = 0.32 ± 0.09 (Benvenuti et al. 1974).
However, the damage had been done, and after this debacle many physicists were quite unconvinced of the reality of neutral currents. It was to take another year from the time of the original Gargamelle claim before independent confirmation of their existence came from a bubble-chamber experiment at Argonne, counter-experiments at Brookhaven and a Caltech group in a narrowband beam at Fermilab, all reported at the London Conference in 1974.
In figure 4, the value of the ratio NC:CC for neutrinos is plotted against the value for antineutrinos for the Gargamelle and HPWF experiments, together with the (valence quark model) prediction from the electroweak theory as a function of the mixing parameter sin2θw. The Gargamelle value of 0.38 ± 0.09 was a long way from the accepted figure of 0.23. Probably the discrepancy was due to the strong minimum hadron energy cut of 1 GeV, and the inadequacy of the simple quark model at these relatively low neutrino energies for assessing such effects. It will also be seen that the HPWF result is inconsistent with the electroweak theory for any value of sin2θw. However, the main point is that both experiments had now found a clear neutral-current signal, regardless of whether it agreed with a particular model.
It is perhaps not generally realized today that the measurement at CERN of sin2θw in neutrino experiments has an interesting, and one might even say noble, history. Some of the main evidence at present for physics beyond the Standard Model comes from the detection of neutrino masses and neutrino oscillations, deduced from analysis of atmospheric neutrino interactions over the last few years. Yet a handful of atmospheric neutrino interactions were first recorded more than 40 years ago in deep mine experiments, and were at that time considered to be of little interest. The detection today of large numbers of such interactions in underground multi-kiloton detectors such as SuperKamiokande was originally quite unintentional. They occurred as an unwanted background to the main goal of finding proton decay. In turn, the proton-decay search had been triggered by the initial success of the minimal SU(5) version of GUTs, which arose largely because of wrong CERN results. The experiments in question were in the BEBC chamber with neon or hydrogen fillings in an SPS wideband neutrino beam. They found some anomalously low values for sin2θw and brought the world average down to 0.21, in keeping with the prediction of non-supersymmetric SU(5) (figure 5). This led to the consequent underground searches in three continents for proton decay as a unique test of grand unification. So even wrong experimental results can sometimes have unexpected but very significant and positive consequences.
Finally let me say a few words about the role of Willi Jentschke in the neutral-current story. When the claim to have found neutral currents in Gargamelle was followed by the report from Fermilab that the NC:CC ratio they found was consistent with zero, many physicists in CERN and Europe – but not in the US – believed that the Gargamelle result must be wrong. Indeed, one senior CERN physicist bet so heavily against Gargamelle that he lost half his wine cellar. But Jentschke himself was always very supportive of the experiment. I myself never discussed it formally with him but I did meet him on one occasion in the CERN lift. He told me he was worried about the Gargamelle result, because some people had told him that it could be wrong. I replied that, of course, any experiment could be wrong. For example, looking back over the last 25 years, I had found that 85% of my previous experiments had been wrong.
This seemed to shake him, and he told me that, if the Gargamelle result was wrong, it would be very bad for CERN. My response was that, coming after the “split A2” affair, it would be an absolute disaster. However, I knew the group had gone through the event analysis many times and for almost a year we had searched intensively for some other explanation for the effects observed, without success. So I thought the result was absolutely solid, and he should just ignore rumours from across the Atlantic. I don’t know if my words reassured him, but he got out of the lift with a smile on his face.
The Sun burns through various nuclear reactions with the same net effect – the fusion of four protons to form a nucleus of helium, with the release of energy. The two main sets of reactions are the p-p chain, which involves the lightest nuclei, and the CNO cycle, in which the heavier nuclei of carbon, nitrogen and oxygen play a role. Calculations based on the standard solar model suggest that most of the Sun’s energy comes from the p-p chain, with only 1.5% from the CNO cycle. This prediction is widely assumed to be correct, but can it be tested experimentally?
This was indeed an early goal of solar neutrino experiments. However, the solar neutrinos that proved the least difficult to detect have been the high-energy neutrinos from the 8B decays in a variant of the p-p chain. The low-energy neutrinos from 13N and 15O decays in the CNO cycle are much more problematic. Moreover, electron-neutrinos emitted by the Sun are now known to escape detection in many experiments through oscillation to another variety. Indeed, before the data from the Sudbury Neutrino Observatory (SNO) and SuperKamiokande became available, calculations in which the CNO cycle contributed as much as 99.95% of the solar energy would fit the data.
Now John Bahcall and Carlos Peña-Garay of the Institute of Advanced Study, Princeton, and M C Gonzalez-Garcia of CERN, SUNY and IFIC Valencia, have revisited the problem using all existing solar neutrino data, including that from SNO and SuperKamiokande, as well as the recent reactor data from KamLAND. Their extensive analysis involves 10 free parameters, which include neutrino oscillation parameters as well as various solar neutrino fluxes – including 13N and 15O decays. Their best fit indicates that the energy from the CNO cycle must be less than 7.3% at the 3ρ level. To improve the limits to the 1.5% level of the solar model predictions will be very challenging, but would, say Bahcall and colleagues, provide a stringent test of the theory of stellar evolution.
MACRO, the Monopole, Astrophysics and Cosmic Ray Observatory detector, ceased operation two years ago. As its name makes clear, one of MACRO’s main aims was to search for magnetic monopoles, and recently the MACRO collaboration published the final results of their direct monopole search. It found none, but set the most stringent upper limits on their existence so far, and at the same time set upper limits on nuclearites.
MACRO was located in Hall B of the Gran Sasso Underground Laboratory, under about 1400 m of rock, which reduced the cosmic ray flux to about 1 muon per square metre per hour, or about one million times less than the flux at the Earth’s surface. The detector had a modular structure, with a total volume of 76.5 x 12 x 9.3 m3 and a total acceptance of about 10,000 m2 sr. It used three types of subdetectors – liquid scintillation counters, limited streamer tubes and nuclear track detectors – and was operational from 1989 until the end of 2000.
The magnetic monopole is a hypothetical particle with a single magnetic charge. Classically, such particles are expected in analogy with electrically charged particles, and in order to symmetrize Maxwell’s equations. However, experiments indicate that in nature magnetic effects are due only to magnetic dipoles, which are created by moving electric charges.
Nevertheless, in 1931, while attempting to find a theoretical motivation for the quantization of electric charge, Paul Dirac found a relationship that quantizes the product of a basic electric charge e, times a basic magnetic charge g – namely, eg/c = nh/4π, with n = 1, 2, 3,…. In this case, the quantization of electric charge follows from the existence of a magnetic charge. Moreover, if the basic electric charge is that of the electron, in the symmetric cgs system of units, then the basic magnetic charge is g = 68.5e. This is a large value, and it introduces a numerical asymmetry between electric and magnetic phenomena; it also leads to a large magnetic coupling constant. Such “classical Dirac magnetic monopoles” have been searched for at every new higher energy accelerator, but without success.
During the 1970s, monopoles appeared on the scene again, when theorists found that electric charge is naturally quantized in gauge theories that unify the strong and the electroweak interactions (GUTs). These theories require magnetic monopoles of very large mass, ~1017 GeV. Such a mass cannot be produced at any of our accelerators, existing or planned, but the heavy monopoles could have been produced in the early universe, and could still exist as relic particles in the penetrating cosmic radiation.
MACRO searched for superheavy magnetic monopoles using its three types of subdetectors, either on a stand-alone basis or in combination. It also used different and redundant electronics, which allowed searches in different velocity ranges, cross checks and background studies. No candidate monopole was found, with an upper limit at the 90% confidence level of a flux level of 1.4 x 10-16 cm-2 s-1 sr-1 for monopoles with velocity between 4 x 10-5c to c and magnetic charge with n >= 1 (Ambrosio et al. 2002a). Some indirect searches may yield stronger limits, but only after making several hypotheses that can’t be checked.
The GUT monopole is a complicated object with a very small core and different surrounding regions. If a proton were to hit a monopole core it would catalyse the decay of the proton (Mp → Me+π0). Because of the smallness of the monopole’s core this should be a very rare phenomenon, but if there are baryon number violating terms in the four-fermion virtual condensate around a GUT monopole of up to 1 fm radius, the cross-section could be large and the phenomenon could be observed. MACRO made a dedicated search for this using the streamer tube system and looking for a fast track originating from a slow (10-4 < ß 10sup>-3) incoming particle – the possible monopole. The search excluded large cross-sections and again obtained the best existing limits (Ambrosio et al. 2002b).
As a by-product of these searches for magnetic monopoles, MACRO has also set stringent limits for other exotica, in particular nuclearites. Nuclearites (strangelets or strange quark matter) should consist of aggregates of u, d and s quarks. They would be colour singlets and could be the ground state of QCD. The overall electrical neutrality of strange quark matter would be insured by an electron cloud. Nuclearites could have been produced shortly after the Big Bang, at around the hadronization time, and may have survived as remnants to form part of the cold dark matter. When hitting the Earth with typical galactic velocities (ß ~10-3) they would not ionize, but excite atoms and molecules along their trajectories and should be easily detected in scintillators and nuclear track detectors. MACRO established upper limits at the level of 1.4 x 10-16 cm-2 s-1 sr-1. Nuclearites, like magnetic monopoles, could exist at lower levels than we can at present detect, if indeed they exist at all.
The old spa area of Herlany in Slovakia welcomed more than 50 physicists from over 30 countries last September for the 2002 Hadron Structure conference, which took place in the Educational Centre of the Technical University. The area is famous for its cold-water geyser, which is unique in Europe, and which did not disappoint as it erupted four times during the conference. Nor did the conference itself disappoint, with its mix of theoretical talks and experimental reviews.
The Hadron Structure conferences, which have become one of the major events in the Slovak high-energy physics community, are based on a tradition of more than 30 years. The origins of the conferences can be traced back to the late 1960s, when informal meetings of theoreticians from Bratislava, Budapest and Vienna – the so-called Triangle Meetings – were organized three to six times a year and moved between the different locations. The meetings held in Slovakia were called the Hadron Structure meetings and they gradually developed into a series of conferences.
Although the Triangle Meetings were predominantly devoted to theoretical topics, at Hadron Structure 2002 the theoretical reports were balanced by impressive experimental review talks. The following is only a brief report of the scientific programme, which involved a wide range of high- and medium-energy particle physics and heavy-ion physics.
The LEP experiments presented reports on W boson physics, Higgs boson mass limits, and on the searches for neutralinos and large extra dimensions, as well as electroweak, heavy flavour and QCD measurements at LEP. The results are in good agreement with the Standard Model expectations. The H1 and ZEUS experiments at HERA reviewed results on proton structure functions, inclusive diffraction measurements, open charm and beauty, as well as vector meson production. The beauty results seem in general to be above perturbative QCD predictions. Recent spin physics results from HERMES, as well as the latest results from the HERA-B experiment, were also presented.
In B physics, the two dedicated spectrometers BaBar and Belle presented their results on CP violation in B0 decays, the B0 lifetime and branching fractions. Their measurements of the unitary triangle angle ß are found to be consistent with the expectations of the Standard Model and can be used to constrain extensions of the model.
Moving on to heavy-ion collisions at RHIC, in Brookhaven, the STAR collaboration reported results on transverse momentum distributions, hadronic yields and correlations. The azimuthal correlations at moderately high transverse momenta demonstrate the existence of hard scattering processes at RHIC, while the disappearance of di-jets and the suppression of single inclusive particle production are consistent with the jet-quenching scenario. PHENIX presented results on high-pt charged-particle azimuthal correlations, which may indicate a novel particle production mechanism.
In relativistic nuclear physics, selected problems studied at the Veksler and Baldin Laboratory of High Energies at JINR, Dubna, were reported. These studies make use of the Synchrofasotron Nuclotron acceleration system. A plan to upgrade the Nuclotron and organize a user centre for relativistic nuclear physics and applied research with ions of a few GeV energy is foreseen.
Two review talks at the conference were presented on behalf of the ATLAS collaboration. One of these concerned the overall detector concept, the status of the subsystems and the magnet. The second talk was an overview of the ATLAS physics potential for searches at the LHC for the Higgs boson(s), supersymmetric particles, quark and lepton compositeness, new gauge bosons and extra dimensions.
The conference was organized by the Nuclear Physics Department in the Faculty of Sciences at P J Safárik University in Kosice, in association with the Department of Subnuclear Physics, Institute of Experimental Physics, Slovak Academy of Sciences, Kosice, the Physics Institute, Slovak Academy of Sciences, Bratislava, the Faculty of Mathematics, Physics and Informatics, Comenius University, Bratislava, and the Physics Department, Faculty of Electrical Engineering and Informatics, Technical University, Kosice.
In 1966/7 Steven Weinberg, Abdus Salam and John Ward proposed a local gauge theory, SU(2) x U(1), for a unified description of electromagnetic and weak interactions, with a Higgs mechanism to give mass to the (weak) field quanta. When I arrived as a student at Johns Hopkins University in 1966, Ward was a professor there. I could understand that something exciting was going on from the discussions at the physics seminars, but could not appreciate the importance it would subsequently acquire.
The most striking feature of the weak interactions is their very short range of less than around 10-15 cm, i.e. less than 1% of the size of a nucleon. This is compared with a range of around 10-13 cm for nuclear (strong) forces, and is in stark contrast to the “infinite” range of the electromagnetic force. The short range of the weak interactions implied very massive mediating particles or quanta, W+ and W–, for the charged current, the only known weak interactions at the time. However, the unified description of Weinberg, Salam and Ward had four field quanta, two charged and two neutral, implying that a new type of “neutral current” weak interaction should exist. This would be mediated by the Z0 – a particle that is closely related to the massless photon, in fact it is almost identical except for being very massive. The renormalizability of the theory, shown in 1971 by Gerard ‘t Hooft and Martin Veltman, and by the discovery of the weak neutral currents at CERN in 1973, made this unified electroweak scheme appear plausible. But what could the mass of the W and Z particles be?
Where and how?
The observed linear increase of the neutrino-nucleus cross-sections with incident energy up to Eν ~350 GeV, which was consistent with the (old) Fermi four-fermion point interaction, could not last forever. At a neutrino-nucleon (or rather, neutrino-quark) centre-of-mass energy of the order of 300 GeV the cross-section would reach the S-wave unitarity limit, so the effects of W-exchange had to come in, in order to modify this unacceptable behaviour. The non-deviation from linearity in the measured cross-section indicated mw > 50 GeV, and was consistent with infinite mw. Meanwhile, the charged current and neutral current data from neutrino interactions, when incorporated into the Weinberg-Salam-Ward scheme, were giving a weak mixing angle sin2Θw ~0.3-0.6, which implied mW,Z ~60-100 GeV. Subsequently, measurements of sin2Θw narrowed its value down to around 0.23, providing by 1982/3 a much better estimate of mw ~80 GeV and mz ~90 GeV to within a few GeV. In the late 1970s and early 1980s the forward-backward angular asymmetry, due to γ-Z interference, in e+e– → µ+µ– at the top PETRA energies (√s ~30-40 GeV) also indicated mz < 100 GeV rather than an infinite mz. So, the question was where could these W and Z intermediate vector bosons be produced and how could they be detected?
In 1976 CERN’s SPS began operating with particle beams of energies up to 350-400 GeV onto a fixed target, i.e. with centre-of-mass energies of √s ~30 GeV, which was insufficient for W and Z production. The same year David Cline, Carlo Rubbia and Peter McIntyre proposed transforming the SPS into a proton-antiproton collider, with proton and antiproton beams counter-rotating in the same beam pipe to collide head-on. This would yield centre-of-mass energies in the 500-700 GeV range. Provided the antiproton intensity was sufficient, the W and Z particles could be produced through their couplings to quarks and antiquarks, and detected through their couplings to leptons as prescribed by the Weinberg-Salam-Ward model. Then, in 1979, Weinberg, Salam and Sheldon Glashow were awarded the Nobel prize for electroweak unification and the prediction of weak neutral interactions, which implied the existence of the Z particle. (Ward was no doubt of the same class, but the Nobel prize can only be awarded to three people at most.) This indicated that the theoretical community was more convinced of the existence of the W and Z than most of the experimentalists at the time.
The proton-antiproton collider
CERN meanwhile went ahead with the proton-antiproton collider, and by the summer of 1981 the heroic endeavour of transforming the SPS into a proton-antiproton collider had been accomplished, despite the many uncertainties, including unknown unpredictable beam-beam effects. There is no doubt that Carlo Rubbia, with his enthusiasm, power of conviction and charisma, played a key role in this phase of the project. The first proton-antiproton collisions occurred on 9 July 1981, almost exactly three years since the project had been officially approved. Within hours, the first events that had been seen, detected and reconstructed in UA1’s central tracker were shown by Rubbia at the Lisbon conference (UA1 collaboration 1981).
The PS proton beam at 26 GeV was used on a fixed target to produce antiprotons at ~3.5 GeV, creating about one antiproton per 106 incident protons. The antiprotons were then stacked and stochastically cooled in the antiproton accumulator at 3.5 GeV, and this is where the expertise of Simon Van der Meer and coworkers played a decisive role. With a few times 1011 antiprotons accumulated per day, the cooled (phase-space compactified) antiprotons were reinjected into the PS, accelerated to 26 GeV and injected into the SPS, counter-rotating in the same beam pipe with a proton beam. Both beams were then accelerated to 270 GeV and brought into collision in two interaction regions at √s = 540 GeV. Sufficient luminosity remained for about half a day. The initial luminosity in November/December 1981 was about 1025 cm-2 s-1, but subsequently increased by a factor of 105 over the following years.
The UA1 (Underground Area 1) detector was conceived and designed in 1978/9, with the proposal submitted in mid-1978. At that time we were in barracks on the parking lot in front of building 168, at the same time and place that CDF was designed, for Alvin Tollestrup was spending a year at CERN. UA1 was approved in 1979, and was constructed and essentially functional – including the reconstruction software – by the summer of 1981 (although part of the tracker electronics was still missing). At the time of approval there was a general incredulity in the particle physics community (although not obviously in UA1) that UA1 could be built – and even less operated – in time when compared with the much more focused design and modest size of the UA2 detector. That this was possible was largely thanks to Rubbia’s enlightened absolutism (or more diplomatically to his unrelenting efforts), and to his unbelievable intellectual and professional capabilities and stamina.
The two detectors
UA1 was a huge (~10 x 6 x 6 m3, ~2000 tonnes) and extremely complex detector for its day, exceeding any other collider detector by far. The design was simple, beautiful, economical and, as it turned out, very successful. In the days of initial construction, the collaboration counted around 130 physicists from Aachen, Annecy, Birmingham, CERN, College de France, Helsinki, London/QMC, UCLA-Riverside, Rome, Rutherford, Saclay and Vienna. There was a large, normally conducting dipole magnet with a field of 7 kG perpendicular to the beamline. The collision region was surrounded by a central tracker – a 5.8 m long, 2.3 m diameter drift chamber with 6176 sensitive wires organized in horizontal and vertical planes. Tracks were sampled about every centimetre and could have up to 180 hits, with a resolution of 100-300 µm in the bending plane. This detector was at the cutting edge of technology; it was the first “electronic bubble chamber” and the reconstruction software was done by ex-bubble chamber track reconstructors. The tracker was surrounded by electromagnetic (27 radiation lengths deep) and hadronic calorimeters (about 4.5 interaction lengths deep) down to 0.2° to the beamline. This almost complete coverage in solid angle became known as “hermeticity”. The central electromagnetic calorimeter – which was to play a key role in the subsequent discoveries – was very effectively and economically designed as a lead-scintillator stack in the form of two cylindrical half-shells each subdivided into 24 elements (gondolas). The entire detector was doubly surrounded by ~800 m2 of muon drift chambers with a spatial resolution of ~300 µm. The overall cost of the detector was about 30 million Swiss Francs, and the central ECAL about 3 million – which was probably the best ever investment in particle physics.
While UA1 was designed as a general-purpose detector, UA2 was optimized for the detection of e± from W and Z decays. The emphasis was on calorimetry with a spherical projective geometry – much simpler than that in UA1. There was full coverage in solid angle, except for 20° cones along the beamlines. There were about 500 calorimeter cells with a granularity of about 10° by 15° in polar and azimuthal angles, with a three-fold segmentation in depth in the central region (40-140°) and two-fold segmentation in the forward regions (20-40° and 140-160°) to allow electron-hadron separation. The central calorimetry was, in total, about 4.5 interaction lengths deep, while the forward one was about 1 interaction length (two sections of 18 and six radiation lengths). There was no central magnetic field, but the two forward regions were equipped with magnetic spectrometers (two sets of 12 toroid coils). In the central part there was a vertex detector made of coaxial drift and proportional chambers to detect charged tracks and the collision vertex. Preshower counters improved electron identification through the spatial matching of tracks and clusters. The collaboration counted about 60 physicists, with groups from Bern, CERN, Copenhagen, Orsay, Pavia and Saclay.
The jet run
The first real physics run was in December 1981. Known as the jet run, it was devoted to the search for jets arising from the hard scattering and fragmentation of partons as expected from QCD. The integrated luminosity was about 20 events per µb. The main initial effort in UA1 was based on the tracker, i.e. the measurement of high-momentum tracks and the correlations in azimuth and rapidity between charged particles. Within the collaboration, not enough attention was paid to the searches based on energy clusters in the calorimeters. The UA2 search, based exclusively on calorimetry, was simpler and gave more telling results. At the Paris conference in the summer of 1982, UA2 had clear back-to-back two-jet events, one of which was particularly spectacular with a total transverse energy (Et) of about 130 GeV. The UA1 result was somewhat less elegant. The subsequent studies by UA1 and UA2 were based on calorimetric jet algorithms and the data were selected by total Et or localized Et depositions. This gave an excellent confirmation of QCD expectations in terms of cross-sections, fragmentation functions, angular distributions, etc. But what about the W and Z particles?
On the trail of the W
In the case of the W particle, both experiments looked for Drell-Yan production – that is ubard → W–, udbar → W+ with the antiquarks, qbar, largely from the valence antiquarks in the incident antiprotons, and the quarks from the incident protons, with a fractional momentum x ~mW,Z/√s ~0.2. This identification of incident partons was to facilitate the unambiguous identification of a possible resonance mass peak with the expected properties of the W+/- – namely the spin of 1 and the V-A nature of weak interactions – which should manifest themselves through characteristic forward-backward asymmetries in the decays of the W to a charged lepton and neutrino (W → lν). For the running period at the end of 1982 we expected a luminosity in excess of 1028 cm-2s-1 and an experimental sensitivity of > ~10 events/nb – an increase of 1000 compared with the previous run. The theoretical predictions for the cross-section for W → lν were ~0.5 nb, so few events were expected.
In the run in November/December 1982 the collider attained a peak luminosity of 5 x 1028 cm-2s-1. UA1 collected 18 nb-1 of data, with the total number of recorded triggers about 106 for 109 interactions in the detector. The electron trigger in UA1 was two adjacent gondolas or bouchon petals with > 10 GeV, with a rate of ~1 s-1. The criteria that in December allowed UA1 to select the first five W → eν candidates unambiguously, required an ECAL cluster of > 15 GeV, a hard isolated track of pt > 7 GeV/c roughly pointing to the cluster, missing Et > 14 GeV, and no jet within 30° back-to-back in the plane transverse to the electron candidate. This became known as the Saclay missing Et method. This selection in fact gave six events, five of which turned out to be fully compatible with e±. In these five events, the electron had an Et of ~25 GeV in one case and between 35 and 40 GeV in the others, closely balanced event-by-event by the missing Et. Thanks to the hermeticity of the UA1 design, the resolution on missing Et in UA1 was 7 GeV in hard/jetty events, so the observed missing Et was highly significant in each event (> 5σ). The sixth event had 1.5 GeV of leakage in the HCAL and, upon detailed inspection, turned out to be a case of W → τν → π±π0ν.
In the first weeks of January 1983 an independent search – not based on a missing Et selection, but on stringent electron selection requirements – was performed at CERN. It found the same events, without the tau event, but with an additional event in the endcaps that was below the Saclay/missing Et selection cuts. These events were announced later the same month at the Rome conference and went in the publication announcing the discovery of the W (UA1 collaboration 1983a). The key to this success was the built-in redundancy of UA1 – which allowed the same events to be found by two largely independent methods, resulting in clean samples with no nearby background events – and the fact that the reconstruction software was ready and working. The already perceptible Jacobian peak behaviour giving mw = 81±5 GeV clinched the day.
In the same run UA2 had four W → eν candidates (UA2 collaboration 1983a). The electron identification was based on a calorimetric cluster of more than 15 GeV, with longitudinal and transverse shower profiles consistent with e+/-, track-preshower-calorimetric cluster spatial matching, and electron isolation within a cone of 10°. In the forward-backward regions, where there was a magnetic field, momentum/energy (p/E) matching was enforced but the electron was not required to be isolated. Moreover, events with significant Et opposite to the electron were rejected. These events also had missing Et, but the 20° forward openings resulted in poorer resolution, and thus the separation of events from the background was not so good. In fact one of the consequences of UA1’s hermeticity and the selective power it provided for W → lν events, was that the D0 detector at Fermilab, which was designed in 1983/4, was made as hermetic as possible.
Catching the Z
In April/May 1983 came the next run with 118 nb-1 of integrated luminosity for UA1. This gave an additional sample of 54 W → eν events, giving mw = 80.3 + 0.4-1.3 GeV – and the angular asymmetry in the W decay due to the V-A coupling was unmistakable. The first W → µν events were also seen, but most importantly the first Z → e+e– events and one Z → µ+µ– were found. An express line selected events with two electromagnetic clusters of Et > 25 GeV with small HCAL deposition, and also muon pair events, thereby allowing very fast analysis. The selection of Z → e+e– was much easier than the W selection. The additional requirement of track isolation in the tracker, track-cluster spatial matching and < 1 GeV in the HCAL cell behind the cluster, selected four Z → e+e– events with no visible experimental background in 55 nb-1 of data. At this stage UA1 decided to publish its evidence for the Z. The first mass determination gave mz = 95.5 ± 2.5 GeV and the cross-section for Z decay to lepton pairs was about one-tenth that of the W, as theoretically expected (UA1 collaboration, 1983b).
UA2 accumulated a comparable integrated luminosity during April/May 1983. In the UA2 selection for Z events, while one electron candidate again had to satisfy the same stringent requirements as in the W → eν search, the requirements on the second electron candidate were much looser, essentially a narrow electromagnetic cluster and a cluster-cluster invariant mass of more than 50 GeV. This procedure selected eight events altogether, all clustering in mass around 90 GeV. For three out of these eight events, the second electron candidate in fact also satisfied all the tight electron requirements (UA2 collaboration 1983b). With results from UA1 and UA2, the Z particle was definitely found.
This period, around the end of 1982 and throughout 1983, was an amazing time from both a professional and personal point of view. It was an unforgettable time of extreme effort, tension, excitement, satisfaction and joy. Subsequent runs allowed us to nail down the properties of the W and Z better and initiate other searches that were not always as successful but still extremely interesting and exciting.
The discovery of the W and Z particles was a definitive vindication of the idea of gauge theories as appropriate descriptions of nature at this level, and the unified electroweak model combined with QCD became known as the Standard Model. In the 10 years of experimentation at LEP, this Standard Model became one of the most thoroughly tested theories in physics, down to the level of a part in a thousand. However, in the SU(2) x U(1) scheme with spontaneous symmetry breaking, one of the four scalars that did not disappear into the W± and Z masses has still to be found – and the discovery of the Standard Model Higgs, in the ATLAS and CMS detectors at CERN should eventually complete this story. The discovery of the W and Z at CERN also signalled that the “old side” of the Atlantic regained its eminence in particle physics. “…L’espoir changea de camp, le combat changea d’âme….” (Victor Hugo, “Waterloo”.)
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The technical storage or access is strictly necessary for the legitimate purpose of enabling the use of a specific service explicitly requested by the subscriber or user, or for the sole purpose of carrying out the transmission of a communication over an electronic communications network.
Preferences
The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user.
Statistics
The technical storage or access that is used exclusively for statistical purposes.The technical storage or access that is used exclusively for anonymous statistical purposes. Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you.
Marketing
The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes.
