Physics Archives – Page 131 of 145

Superbends expand the scope of Berkeley’s ALS

A superconducting separator dipole — US contribution to the LHC – superconducting separator dipole made by the US Brookhaven Laboratory. Credit: CERN

At first it was a perfect match. The physical constraints of its site at the Lawrence Berkeley National Laboratory on a hillside above the University of California’s Berkeley campus, the research interests of its initial proponents and the fiscal realities of the times all pointed to the same conclusion in the early 1980s: the Advanced Light Source (ALS) should be a third-generation, but low-energy, synchrotron radiation source designed for highest brightness in the soft-X-ray and vacuum-ultraviolet spectral regions.

While the ALS has turned out to be a world leader in providing beams of soft X-rays – indeed, furnishing these beams remains its core mission – there has nonetheless been a steadily growing demand from synchrotron radiation users for harder X-rays with higher photon energies. The clamour has been strongest from protein crystallographers whose seemingly insatiable appetite for solving structures of biological macromolecules could not be satisfied by the number of crystallography beamlines available worldwide.

The question was how to provide these X-rays in a cost-effective way without disrupting the thriving research programmes of existing ALS users. Superconducting bend magnets (superbends) provided the answer for the ALS and a proposal was adopted (a proposal that was originally made in 1993 by Alan Jackson of Berkeley and Werner Joho of Switzerland’s Paul Scherrer Institute) to replace some of the normal combined-function (gradient) magnets in the curved arcs of the storage ring with superconducting dipoles that could generate higher magnetic fields and, thus, synchrotron light with a higher critical energy.

A team headed by David Robin, the leader of the ALS Accelerator Physics Group, took on the pioneering task of retrofitting superconducting bend magnets into the magnet lattice of an operating synchrotron light source. In particular, three 5 Tesla superbends were to replace the 1.3 Tesla centre gradient magnets in Sectors 4, 8, and 12 of the 12-fold symmetric ALS triple-bend achromat storage-ring lattice. The long project culminated early last October when, after a six-week shutdown to install and commission the superbends, the ALS reopened for users with a new set of capabilities.

The superbends have extended the spectral range of the ALS to 40 keV for hard-X-ray experiments. They do not degrade the high brightness of the ALS in the soft-X-ray region, for which the ALS was originally designed, nor do they degrade other performance specifications, such as beam stability, lifetime and reliability. They do not require that any straight sections normally occupied by high-brightness undulators be sacrificed to obtain high photon energies by filling them with high-field, multipole wigglers. Superbend magnets are already serving the first of a new set of protein crystallography beamlines. Ultimately, 12 new beamlines for crystallography and other applications, such as microtomography and diamond-anvil-cell high-pressure experiments, will be constructed.

Superbend history

The ALS was originally based on an electron storage ring with a 198 m circumference and a maximum beam energy of 1.9 GeV to provide peak performance in the vacuum-ultraviolet and soft-X-ray spectral regions. One way for the ALS to respond to the demand that arose in later years for higher photon energies would have been to use some of its scarce straight sections for high-field, multipole wigglers. Later, in 1997 the ALS did install one such wiggler – a device that provides the hard X-rays for an extremely productive protein crystallography beamline (Beamline 5.0.2) operated by the Berkeley Center for Structural Biology.

However, the drawback of the wiggler route was immediately obvious: many wigglers would limit the number of high-brightness undulators that give the ALS its state-of-the-art, soft-X-ray performance and that justified its construction in the first place. Moreover, a wiggler cannot readily service more than one beamline capable of the demanding multiwavelength anomalous diffraction experiments that many crystallographers want to perform, whereas a bend magnet can. In the end, the ALS adopted the superbend alternative proposed by Jackson and Joho – a choice that brought along some imposing challenges.

Superconductivity is no stranger to synchrotron light sources, where superconducting bend magnets have been used in small (mini) synchrotrons dedicated to X-ray lithography. In addition superconducting insertion devices in straight sections are, if not common, a venerable technology. Unlike wigglers and undulators in straight sections, however, superbends would be an integral part of the storage-ring lattice in a large multi-user facility and could not simply be turned off in case of failure or malfunction. So, the stakes were very high – the pay-off would be an expanded spectrum of photons to offer users; the risks included the possibility of ruining a perfectly good light source or, at the very least, causing unacceptable downtime.

Diagram showing flux and brightness — The flux and brightness of the ALS superbends (red) show a dramatic increase at higher photon energies relative to those of the normal bend magnets (blue). Credit: CERN

In 1993, newly hired accelerator physicist Robin was set to work on preliminary modelling studies to see how superbends could fit into the storage ring’s magnetic lattice and to determine whether the lattice symmetry would be broken as a result. He concluded that three superbends with fields of 5 Tesla, deflecting the electron beam through 10° each, could be successfully incorporated into the storage ring. Later, beginning in 1995, Clyde Taylor of Berkeley’s Accelerator and Fusion Research Division (AFRD) led a laboratory-directed R&D project to design and build a superbend prototype.

By 1998 the collaboration (which included the ALS Accelerator Physics Group, the AFRD Superconducting Magnet Program and Wang NMR Inc) had produced a robust magnet that reached the design current and field without quenching. The basic design has remained unchanged through the production phase. It includes a C-shaped iron yoke with two oval poles protruding into the gap. A mile-long length of superconducting wire made of niobium-titanium alloy in a copper matrix winds more than 2000 times round each pole. The operating temperature is about 4 K.

With the strong support of ALS advisory committees and Berkeley laboratory director Charles Shank, Brian Kincaid – at that time the ALS director – made the decision to proceed with the superbend upgrade, and his successor, Daniel Chemla, made the commitment to follow through. The superbend project team, now including members of Berkeley’s engineering division, held a kick-off meeting in September 1998 with Robin as project leader, Jim Krupnick as project manager and Ross Schlueter as lead engineer. Christoph Steier then joined the team a year later as lead physicist.

Subsequently, the success of wiggler Beamline 5.0.2, combined with some pioneering work on normal bend-magnet beamlines by Howard Padmore and members of his ALS Experimental Systems Group, led to the formation of user groups from the University of California, the Howard Hughes Medical Institute and elsewhere that were willing to help finance superbend beamlines, further adding to the momentum of the project.

Superbend team work pays off

For the next three years, the superbend team worked towards making the ALS storage ring the best understood such ring in the world. In every dimension of the project, from beam dynamics to the cryosystem, from the physical layout inside the ring to the timing of the shutdowns, there was very little margin for error. To study the beam dynamics, the accelerator physicists adapted an analytical technique used in astronomy called frequency mapping (CERN Courier January 2001 p15). This provided a way to “experiment” with the superbends’ effect on beam dynamics both theoretically and experimentally before the superbends were installed.

Another technical challenge was to design a reliable, efficient and economical cryosystem capable of maintaining a 1.5 ton cold mass at 4 K with a heat leakage of less than 1 watt. Wang NMR was contracted to construct the superbend systems (three plus one spare). Wang designed a self-sustaining cryogenic system based on a commercial cryocooler, leads made of high-temperature superconductors and a back-up cryogenic reservoir.

By 1998 the collaboration had produced a robust magnet that reached the design current and field without quenching

Following some preparatory work during previous shutdowns, the installation of the superbends began in August 2001. The initial installation plan was very tight. In one 11-day period, the superbend team removed three normal gradient magnets and a portion of the electron-beam injection line in straight section 1 just upstream of Sector 12; installed the superbends; modified cryogenic systems; and completed extensive control system upgrades. They also installed many other storage-ring items and prepared for start-up with a beam.

After the installation phase, the goal was to commission the ALS with superbends and return the beam to users by 4 October. This schedule allowed the month of September to commission the ring (with the exception of a four-day break for the installation of the front ends for two superbend beamlines) and a three-day period for beamline realignment. However, commissioning proceeded much faster than had been expected and it was less than two weeks after the start of the installation when the machine was ramped up to full strength, and the effects of the superbends on the performance of the storage ring were fully evaluated.

Because so much was at stake, the storage ring had been studied and modelled down to the level of individual bolts and screws to ensure a smooth, problem-free installation into the very confined space within the storage ring. This attention to detail also paid off in the rapid commissioning. To take one example, the superbends were very well aligned, as demonstrated by a stored beam with little orbit distortion and small corrector-magnet strengths.

At the end of the first day, a current of 100 mA and an energy of 1.9 GeV were attained. At the end of the first weekend, the injection rate and beam stability were near normal. By the end of the first week, the full 400 mA beam current was ramped to 1.9 GeV and studies of a new, low-emittance lattice with a non-zero dispersion in the straight sections (designed to retain the high brightness that the storage ring had without superbends) were begun. By the end of the second week, test spectra taken in some beamlines showed no change in quality due to the presence of superbends.

Since reopening for business in October, the ALS has not experienced any significant glitches that might be associated with such a major change. Overall the ALS has made good on its promises to users of installing and commissioning the superbends without disrupting or delaying their research programmes and operating them with no adverse effects on performance in the bread-and-butter soft-X-ray spectral region, as demonstrated by the values of the storage-ring parameters.

Superbend beamlines are already taking data and more are under construction or planned. Three superbend protein-crystallography beamlines are now taking data, and researchers at the first of these to come on line have already solved 15 structures. Three more crystallography beamlines are on the way. Non-crystallography beamlines currently in the works include one for tomography and one for high-pressure research with diamond-anvil cells, two areas for which superbends are even more advantageous than they are for protein crystallography, because they more fully exploit the higher photon energies that superbends can generate. Many other areas, including microfocus diffraction and spectroscopy, would also benefit enormously through the use of the superbend sources.

In summary, a new era at Berkeley’s ALS is under way.

Rare kaon decay tests Standard Model

An experiment at Long Island’s Brookhaven laboratory has just reported a long-awaited result: the observation of only the second decay of a positive kaon into a positive pion accompanied by a neutrino and an antineutrino.

The E787 experiment at the laboratory’s Alternating Gradient Synchrotron began 12 years ago with the aim of detecting this extremely rare kaon decay. The first sighting was reported four years ago, and some 6 trillion events have since been analysed in the quest for confirmation.

cernnews2_3-02 — Searching for rare decays: Brookhaven’s E787 experiment. (Brookhaven National Laboratory.)

The Standard Model of particle physics forbids the direct decay of positive kaons into positive pions. Instead the decay must proceed via a two-step process involving massive gauge bosons. That’s what makes it so rare.

Theory predicts that such two-step decays should occur just a few times for every hundred billion kaon decays. The theoretical uncertainty on rare kaon decays is very small, so measuring them is an important test of the Standard Model, as well as being a sensitive indicator for new physics.

A new collaboration, known as KOPIO, has formed round the core of E787 to study another rare kaon decay – that of the long-lived neutral kaon to a neutral pion accompanied by a pair of neutrinos. KOPIO has support from Canada and Japan and is awaiting congressional approval in the US for National Science Foundation funding.

Quantum gravity comes in from the cold

Gravity and quantum mechanics rarely mix in laboratory circles. The weakness of the gravitational interaction makes measuring its effects difficult at the quantum level. However, researchers at the Institut Laue-Langevin (ILL) in Grenoble, France, have now observed quantum effects of gravity on ultracold neutrons trapped in the Earth’s gravitational field. Their technique relies on bouncing neutrons off a reflective surface and observing quantization in the height of the bounce.

cernnews3_3-02 — Quantum states: a team including (left to right) Valery Nesvizhevsky, Hans Boerner and Alexander Petukhov, seen here with their apparatus, has observed energy quantization in a gravitational field.

The key to the experiment was using ultracold neutrons from the ILL reactor. Neutrons do not bounce, except when they strike a surface at very grazing incidence. Instead, they are absorbed or transmitted. However, by firing neutrons with a velocity of less than 8 cm/s over a horizontal mirror, the ILL researchers were able to reduce the vertical component of the neutrons’ velocity as they fell under gravity to just 1.7 cm/s. These neutrons bounced along the mirror like flat pebbles across a pond until they were captured by a detector at the far end of the mirror. A neutron absorber could be positioned at varying heights above the mirror, allowing the researchers to identify the lowest-energy neutrons passing through the apparatus.

The ILL apparatus behaved as a neutron trap, bound from below by the mirror and from above by gravity. According to quantum mechanics, neutrons in such a trap should occupy discrete gravitational energy levels just as electrons trapped by nuclei occupy discrete electromagnetic energy levels.

This result shows that the lowest level occupied by neutrons in the trap is 1.41 x 10^-12 eV. Comparison with the minimum energy for an electron in a hydrogen atom, 13.6 eV, shows why the quantization of energy in a gravitational potential has not been seen before.

The next step is to use a more intense beam and a trap mirrored on all sides to prolong the period of entrapment and thus improve the resolution of the apparatus. This will allow a precision test of the equivalence between gravitational and inertial mass.

Team reports neutrinoless double beta decay

Scientists at the Max Planck Institute for Nuclear Physics in Heidelberg, Germany, have reported evidence of neutrinoless double beta decay. This result comes from members of the long-running Heidelberg-Moscow double beta decay experiment at Italy’s Gran Sasso underground laboratory. Neutrinoless double beta decay violates lepton number conservation, a fundamental tenet of particle physics. If confirmed, the consequences for particle physics will be profound.

cernnews4_3-02 — The Heidelberg-Moscow double beta decay experiment at the Gran Sasso laboratory.

In the conventional picture of particle physics, neutrinos and antineutrinos are distinct. Beta decay proceeds via the transformation of a neutron into a proton with the emission of a neutrino and an electron. However, if the neutrino is its own antiparticle, a rather more exotic process becomes possible. In this case, two successive neutron decays could occur, with the neutrino emitted by the first being absorbed by the second. Two electrons would emerge from the nucleus with no neutrinos, leaving a nucleus that contains two more protons than the original.

In the latest paper from the Heidelberg-Moscow collaboration (2001 Modern Physics Letters A 16 (37) 2409-2420), the authors claim evidence for neutrinoless double beta decay with a half-life of around 1.5 x 10²⁵ years from 10 years of running with an enriched sample of germanium-76. This would be compatible with a neutrino mass of around 0.4 eV, which is difficult to reconcile with neutrino mass results from other experiments and would require a special neutrino mass scenario. A previous limit from this experiment was reported in an earlier issue of CERN Courier.

ESA mission will study ultimate energies

Joining the Pierre Auger Observatory in the long term will be ESA’s Extreme Universe Space Observatory (EUSO), which is scheduled to fly on the International Space Station from 2008. Comprising a wide-angle telescope that is sensitive to the ultraviolet region, the EUSO array will look down on Earth, some 400 km below, to pick up the fluorescence that is generated by cosmic rays above the Greisen, Zatsepin, Kuzmin (GKZ) cut-off.

EUSO will take in much larger areas than the Auger Observatory, with its 60° field of view sweeping over 150,000 sq. km of the Earth’s atmosphere at every pass. As cosmic-ray showers develop in the atmosphere, light is also generated through the Cherenkov effect, with photons being emitted in a narrow cone that follows the particle trajectories. On reaching the surface this light is reflected, resulting in a terminal flash in the EUSO telescope that will be used to reconstruct the cosmic-ray track.

cernnews8_3-02 — ESA's Extreme Universe Space Observatory, EUSO, will pick up the fluorescence generated by cosmic rays.

Conceptually simple, the telescope’s 2.5 m diameter optics consist of Fresnel lenses that focus light onto a plane surface made up of multinode photomultipliers with 250,000 pixels. Each pixel covers approximately 1 sq. km of the Earth’s surface. This resolution is sufficient because at such high energies a cosmic shower can cover dozens of square kilometres when it reaches the Earth.

The total number of excited pixels is governed by the random background noise from residual light on a moonless night. The experiment therefore only takes data when the telescope passes through the Earth’s shadow; for the EUSO telescope, night lasts only 45 min and is repeated 16 times every 24 h. In the three years scheduled for the EUSO mission, some 3000 events above the GKZ cut-off are expected.

Milky Way reveals new information about the density of baryonic matter in the universe

Ordinary matter composed of baryons – essentially protons and neutrons – is responsible for only a tenth of the gravitation observed in the universe. This result has just been independently confirmed by measuring the abundance of helium-3 ions in our galaxy.

cernastrow1_3-02 — The US National Radio Astronomy Observatory’s 140 ft radiotelescope at Green Bank, West Virginia. (NRAO/AUI.)

The abundance of helium-3 is observed by measurements of its 8.665 GHz spin-flip transition. Observations made using the US National Radio Astronomy Observatory’s radiotelescope at Green Bank, West Virginia, show that the concentration of helium-3 is almost the same in star-forming regions as in the rest of the Milky Way. This suggests that stars enrich the interstellar medium with helium-3 very little and therefore that the quantity of helium-3 in our galaxy has hardly increased since the Big Bang.

cernastrow2_3-02 — The Milky Way is yielding information about baryonic matter density in the universe. (John Gleason, Celestial Images.)

The abundance of baryons with respect to photons in the universe is linked directly to the concentration of helium-3 relative to hydrogen. Measurements of helium-3 in our galaxy indicate that ordinary matter – in the form of baryons – represents only about 4% of the total matter and energy content of the universe. The rest of the universe appears to consist of non-baryonic dark matter of an as yet unknown nature and “dark energy”, the repulsive force that is considered to be responsible for accelerating the rate of expansion of the universe.

Cornell’s laboratory is at the crossroads

cernlab1_1-02 — Football field physics: the Cornell Electron Storage Ring runs beneath the university’s sports fields.

The Cornell Laboratory for Nuclear Studies was established in 1946 by scientists returning from Los Alamos after the Second World War. Its given mission was “to investigate the particles of which atomic nuclei are composed and to discover more about the nature of the forces which hold these particles together”. Under Robert R Wilson’s guidance, a succession of electron accelerators ensued, culminating in a $10 million proposal for a 10 GeV machine to be built under the university’s sports grounds. By the time this was completed, in 1968, Wilson had moved on to become the founding director of Fermilab, but during his time at Cornell he had instilled in the laboratory a “can-do” spirit that remains to this day. Wilson believed in value for money and in getting things done by being clever, no matter how tight the budget.

By the 1970s, following pioneering work in the early 1960s at Frascati and the Stanford Linear Accelerator Center (SLAC), colliding beam machines were gaining ground. Cornell’s new director, Boyce McDaniel, decided to build an electron-positron collider at Cornell using the 10 GeV synchrotron as an injector. His first task was to convince the National Science Foundation, Cornell’s main funding agency, that a synchrotron could provide the necessary positron currents for such a machine. Happily for Cornell, accelerator expertise was on hand in the form of Maury Tigner, who had built a table-top electron storage ring in 1959 while a student of Wilson’s. Tigner had also been the first scientist to bring the idea of building an electron-positron collider to Cornell following conversations with Bjørn Wiik at Hamburg’s DESY laboratory. It was natural for McDaniel to appoint Tigner as project leader.

Building the storage ring

Tigner’s immediate challenge was to find a way of getting an intense beam of positrons into the proposed electron-positron storage ring. With the linac that Cornell used to feed the synchrotron, the process of building up a single intense bunch of positrons by accelerating individual bunches through the accelerator chain would be a long and difficult one. Tigner’s solution, which was later described as “fiendishly clever” by Karl Berkelman (the laboratory’s director through much of the Cornell Electron Storage Ring era), overcame the problem by accelerating multiple bunches and then coalescing them through a sequence of particle gymnastics in the synchrotron and the storage ring.

Under Tigner’s scheme the storage ring would be 61/60 of the circumference of the synchrotron and it would be filled with 60 bunches. In this way each bunch – starting with bunch number 2 – could be diverted into the synchrotron in turn. After n – 1 turns in the synchrotron, bunch n would be aligned with bunch 1 and could be reinjected into the storage ring, where it would coalesce.

The physics motivation for building a storage ring received a boost in November 1974 when the J/psi particle was discovered by Burton Richter at SLAC and by Sam Ting at Brookhaven. Six months later, Cornell submitted its proposal. It was then that the facility acquired its grand title. “For a while it was open season on creative names,” recalled Berkelman. “One of the wackiest I remember was suggested by Hywel White: CORNell COlliding Beams, or CORNCOB.” Eventually McDaniel ended the debate with Cornell Electron Storage Ring (CESR).

In 1979, CESR collided its first beams using Tigner’s novel coalescing scheme and the facility ran this way for several years. It was the first step in a proud tradition of Cornell particle gymnastics that would see CESR hold the world luminosity record for many years and that would be copied by other labs around the world. In the pursuit of ever-higher luminosity for CESR, Cornell physicist Rafael Littauer came up with the idea of putting more bunches in the storage ring by making the beams follow eccentric, pretzel-shaped orbits. Later on, Robert Meller’s idea of colliding the beams at a small angle, thus permitting yet more bunches, allowed the luminosity to be pushed still higher. Both of these ideas have been adopted by other labs and allow CESR to run today with a total of 45 on 45 colliding bunches in the ring.

A question of serendipity

The discovery of b anti-b quark bound states – upsilon particles – by Leon Lederman’s group at Fermilab in 1977 was “a fabulously serendipitous gift of nature that would guarantee the productivity of CESR for decades”, said Berkelman. The resonance found by Lederman’s group had a mass in the 9.4-10.4 GeV range, precisely where CESR would be looking. This was an extremely happy coincidence for Cornell, since the size of the ring, and hence its energy range, was determined by no more fundamental a parameter than the size of Cornell’s sports ground.

cernlab4_1-02 — Cornell’s 1979 announcement to the world about its electron storage ring.

CESR’s detectors, CLEO and CUSB, soon resolved Lederman’s resonance into three separate peaks and McDaniel chose to announce the lab’s new facility to the world in the form of a greetings card showing these peaks. The more orthodox announcement came in the form of CLEO’s first paper, which was submitted to Physical Review Letters in February 1980. Cornell went on to play a leading role in the world of b-quark physics until 2000, when the dedicated B-factories at SLAC and KEK came into operation.

Serendipity is not the only thing that has kept Cornell in the world particle physics spotlight. “We take accelerator physics and technology very seriously as a branch of physics,” said Berkelman. It is this approach that has turned Cornell into a recognized centre of excellence in the field, with influence well beyond its Ithaca campus. Many innovations in accelerator physics, such as the concept of a linear collider and the innovation of an energy-recovery linac, have found fertile ground at Cornell over the years.

Superconducting radiofrequency is Cornell’s forte, with a tradition going back to the 1960s. Cornell was first to apply superconducting RF technology to cyclic accelerators for particle physics, installing a superconducting cavity in the 10 GeV synchrotron as early as 1975. This presented quite a challenge, since heat-load due to synchrotron radiation could easily warm the cavities to above their critical temperature. The technology developed for the synchrotron was adopted for the Continuous Electron Beam Accelerator Facility (CEBAF) – now known as the Jefferson Laboratory – in Virginia, the construction of which began in 1987 and occupied a large part of the Cornell group.

By then, Hasan Padamsee, who chose to remain at Cornell, was promoting the idea of superconducting RF for linear colliders, and in 1990 Cornell hosted the first TESLA workshop. This time it was Bjørn Wiik’s turn to take home an idea, and DESY soon became the standard bearer for the TESLA project.

Today, Cornell remains at the forefront of accelerator R&D, and the tradition that began when CEBAF adopted Cornell technology continues. As well as being used at CEBAF and TESLA, superconducting RF technology pioneered at Cornell is now finding applications in light sources, free-electron lasers, spallation neutron sources and radioactive ion-beam facilities.

Polarized colliders may prove to be the key in mapping out proton spin structure

cernbas1_1-02 — Brookhaven’s RHIC will soon be handling polarized protons. Electrons could also be on the menu.

The problem with proton spin is that it does not add up. A recent workshop, which was held at the European Centre for Theoretical Studies (ECT) in Trento, Italy, brought together 34 leading theorists and experimentalists from 12 countries to discuss the theoretical and experimental status of the “proton-spin problem”, to identify the key quantities and to formulate a strategy for measuring them.

Particular emphasis was given to comparing the contributions expected from new polarized (spin-oriented) scattering experiments at CERN, DESY (Hamburg) and SLAC (Stanford), polarized proton-proton collisions at Brookhaven’s new Relativistic Heavy-Ion Collider (RHIC), an electron-ion collider (EIC), also at Brookhaven and a possible new polarized HERA electron-proton collider at DESY.

For polarized HERA, the spin of the existing proton beam would be made directional. Looking further ahead, the THERA scheme plans to use electrons from the TESLA superconducting electron-positron linacs and polarized protons in HERA.<textbreak=Missing spin>Polarized scattering experiments at CERN, DESY and SLAC in which the incident beams probe deep inside the target particles suggest that only about 20-30% of the proton’s spin is carried by the intrinsic spin of its quark and antiquark constituents – less than half the prediction of quark models. Where is the spin coming from? This mystery has inspired vast theoretical and experimental activity to analyse and understand the spin structure of the proton.

The present status of experimental data from fixed-target experiments was reviewed by Alessandra Fantoni (Frascati and HERMES) and Fabienne Kunne (Saclay and SMC/COMPASS). Detailed presentations also covered polarized deep-inelastic measurements at small parton momentum x (x is the fractional momentum of the quark; Barbara Badelek, Uppsala), hard exclusive processes (Moskov Amarian, NIKHEF) and nuclear effects (Valeria Muccifora, Frascati). Albert De Roeck (CERN) and Naohito Saito (RIKEN-Brookhaven) reviewed the new physics possibilities at polarized electron-proton colliders (HERA, THERA and EIC) and at polarized proton-proton collisions at RHIC.

The spin decomposition of the proton was discussed in the opening talk by Robert Jaffe (MIT), who underlined the theoretical subtleties in the definition and measurability of quark orbital angular momentum and the physics prospects for measuring spin from transversely polarized targets.

cernbas3_1-02 — Albert de Roeck (right) of CERN and Moskov Amarian (NIKHEF, Amsterdam) at the recent spin workshop in Trento.

The prospects for measuring the spin-flavour structure of the proton were discussed by Jechiel Lichenstadt (Tel Aviv), Giovanni Ridolfi (Genova) and Gaby Rädel (Palaiseau), who emphasized that tightly confined sprays (“jets”) of particles from quark-antiquark pairs deep inside high-energy, polarized electron-proton collisions provide a “gold-plated” measurement of gluon polarization in the proton. This mechanism is not dependent on theoretical interpretation.

Steven Bass (Trento) stressed the possible connection between the spin structure of the proton and the long range gluon dynamics that are responsible for the unaccountably large mass of the h´ meson (the famous “U(1) problem” of quantum chromodynamics). These ideas could be tested through elastic neutrino-proton scattering, which provides complementary information about the spin structure of the proton. Rex Tayloe (Indiana) reported on an exciting possibility for a definitive neutrino-proton elastic experiment using the miniBooNE set up at Fermilab.

Polarized electron-proton colliders could map out the spin-dependent structure function, g1, down to x of about 10^-4, providing powerful new constraints on small x physics in addition to decisive measurements of gluon polarization in the proton. Pointing out complementary studies, Anthony Thomas (Adelaide) emphasized that polarized deep-inelastic scattering at large x (almost unity) is a sensitive probe of the valence quark structure of the nucleon. Precision measurements of the large-x region are planned at Jefferson Laboratory.

Sum rule status

In polarized photoproduction, the present status of tests of the Drell-Hearn-Gerasimov (DHG) sum rule was reported by Klaus Helbing (Erlangen) and Zein-Eddine Meziani (Temple). The DHG sum rule relates spin dependent total cross-sections to the anomalous magnetic moment of the proton target and is derived from fundamental principles. Any violation of this sum rule would challenge our present understanding of spin in QCD.

The theory and status of spin transfer reactions, in which the spin of both the target and the outgoing particle is measured, was discussed by Jacques Soffer (Marseille). The potential of polarized colliders to probe new physics was reviewed by Jean-Marc Virey (Provence). Contributions were made on developments in QCD parton phenomenology, transverse polarization observables, single-spin asymmetries and exclusive channels, and this motivated a great deal of discussion about the physics potential of future experiments to unravel the spin structure of the proton.

cernbas2_1-02 — Time for spin? The HERA electron-proton collider at DESY, Hamburg.

The workshop ended with a presentation and comparison of the physics prospects of polarized electron-proton collider projects – polarized HERA (Albert De Roeck, CERN) and the electron-ion collider (EIC; Abahy Deshpande, RIKEN-Brookhaven and Witek Krasny, Paris), which are currently under discussion respectively at DESY and, particularly, at Brookhaven. A workshop on the EIC project is scheduled for March 2002 at Brookhaven. Possible future fixed-target programmes were also discussed by Wolf-Dieter Nowak (DESY Zeuthen).

Polarized proton-proton and electron-proton colliders are potentially a very useful tool for the investigation of the spin and chiral structure of any new physics beyond the minimal electroweak Standard Model that might be revealed with the corresponding unpolarized colliders. Building on the programme of polarized proton-proton collisions currently under way at RHIC in the US, it is worthwhile to investigate the physics potential of future polarized proton-proton collisions in CERN’s LHC. The workshop revealed considerable enthusiasm within the spin community for possible future developments with the LHC.

Radioactive beam research gets new facility

cernnews2_1-02 — CERN’s new REX-ISOLDE boosts radioactive ions to energies of up to 2.2 MeV per nucleon.

CERN’s new nuclear physics facility, REX-ISOLDE, was commissioned at the end of October, opening up new horizons for the laboratory’s nuclear physics community. REX-ISOLDE builds on CERN’s existing radioactive beam facility, taking radioactive ions and boosting them to energies of up to 2.2 MeV per nucleon.

ISOLDE, the isotope separator on line, has a history stretching back over 30 years supporting experiments from basic nuclear physics to the life sciences. To date, these have focused on radioactive nuclei at energies of less than 60 keV – an upper limit that was starting to constrain the facility’s potential. REX, the newly commissioned radioactive beam experiment, remedies this by opening up the 0.8-2.2 MeV range for exploration.

REX-ISOLDE has been built round a new linear accelerator, which was funded and constructed by a broad European collaboration. ISOLDE’s 60 keV ions are accumulated in a Penning trap, charge bred in an Electron Beam Ion Source and then finally accelerated in the linac. The first beams to be accelerated consisted of neutron rich sodium isotopes, which reached 2 MeV per nucleon on 30 October.

Researchers at the facility’s main detector system – a gamma detector array known as Miniball – are now starting early experiments probing whether the magic numbers N = 20 and N = 28 of the nuclear shell model are still valid for very neutron-rich nuclei. Future experiments will address topics that include the structure of nuclei with equal numbers of protons and neutrons, proton radioactivity and nuclear astrophysics.

Experiment reveals neutrino surprise

NuTev Detector — View of the NuTeV neutrino detector at the Fermilab Tevatron, which used a 700 tonne sandwich of steel target plates interspersed with drift chambers and scintillators.

Just as physicists were getting used to the idea of all particle physics measurements agreeing with each other and with the all-embracing Standard Model, a major experiment at Fermilab has announced a surprising neutrino measurement.

The NuTeV collaboration at Fermilab’s Tevatron compares the different types of neutrino interaction and finds a vital parameter to be 0.2277, not 0.2227. At the level of precision being explored in particle physics, this is a major upset and needs confirmation. However, the NuTeV experiment has now terminated, and the Fermilab Tevatron has ceased operations for fixed target studies, such as those using neutrino beams.

Over a period of 15 months in 1996 and 1997, NuTeV shone beams of neutrinos and their antiparticles at a 700 tonne target. The energy of the neutrinos was 125 GeV, and that of the antineutrinos was 115 GeV.

Neutrinos do not interact readily with matter – at NuTeV, only one in a billion neutrinos registered a hit inside the target. Together, some 2 million neutrino and antineutrino hits were patiently collected. Neutrinos, which are electrically neutral, almost massless, particles, can interact with other matter through the weak nuclear interaction in one of two ways. In the classic form, related to nuclear beta decay, the neutrino changes a nuclear proton into a neutron (or vice versa) and an electrically charged muon (or electron) is emitted. This type of reaction shuffles round the charges of the participating particles and is therefore known as a “charged current”.

In 1973, neutrinos were also discovered to be capable of interacting with matter without permuting electric charge – a “neutral current”. This discovery was vital evidence in favour of the then new “electroweak” theory, which unifies weak interactions and electromagnetism. By looking to see whether neutrino interactions were accompanied by a muon, NuTeV could distinguish between charged and neutral current interactions – only the former produced an outgoing muon.

By comparing the ratio of neutral and charged current production by neutrinos and antineutrinos, the experiment finds a value for the vital mixing parameter (Weinberg angle), which dictates the weights of the electromagnetic and weak effects in the combined electroweak theory and also relates the masses of the Z boson (the particle that mediates the neutral current) and the W boson (the charged-current carrier).

The history of the neutrino has been full of surprises

Beginning with high-energy neutrino studies at CERN in the late 1970s, and continuing with precision measurements of Z and W properties in proton-antiproton colliders at CERN and Fermilab and at electron-positron colliders at CERN and SLAC in the 1990s, physicists had built up a precision picture of weak interaction parameters. However, the latest NuTeV result does not fit in with this.

The history of the neutrino has been full of surprises. The prediction of such a bizarre particle was itself a surprise, and the fact that neutrinos could be observed was another. More came when physicists discovered that neutrinos come in different forms, which could even transform into each other. Is the latest NuTeV result a blip or another neutrino surprise? Only time will tell.

Muon magnetism OK

A heroic reappraisal of complicated calculations means that an intriguing physics anomaly has gone away, but in an unexpected way. Last year a precision measurement of the muon’s magnetism at Brookhaven reported a slight disagreement with the expected value. In some quarters, this was heralded as possible evidence for new physics. After carefully re-examining the underlying calculations, experts found that there was, in fact, a mathematical error in the predicted value. The Brookhaven team’s measured value has not changed, but is no longer a puzzle. The muon’s magnetism looks to be in order.

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