Technology Archives – Page 111 of 117

Weaving a better tomorrow: the future of the Web

WWW8 volunteers — Technical support at the World Wide Web conferences is traditionally supplied by volunteers. The WWW8 crew is seen here in front of the Web history wall, assembled during the conference to mark the Web’s 10th anniversary. Volunteers pay their own way to get to the conference for a week’s hard work, just for the pleasure of being there. Credit: CERN

“It was a weird conference,” said Ethernet inventor and self-styled technology pundit, Bob Metcalfe, summing up the eighth World Wide Web conference (WWW8). “Imagine,” he continued, “sitting there listening to a senior executive of IBM wearing a tee-shirt and a beard.” Appearances were not deceptive as Big Blue’s vice president for Internet Technology, John Patrick, captured the spirit of the conference. “Power to the people,” he said, would be the driving force behind the computing industry as we enter the new millennium. For if one thing is abundantly clear, it’s that the political geography of information technology has been turned on its head by personal computing and the World Wide Web. “Stand aside, besuited corporate executives”, came the message. Make way for the altruistic geeks: the future belongs to them.

It’s rare to find such an optimistic bunch of people. The pony-tail count may have been way above average and the word “cool” still cool, but WWW8 delegates have their hearts in the right place. They are the ones who have made the Web, motivated only by the fun of playing with computers and the belief that the Web can make the world a better place. Some were concerned at Microsoft’s conference sponsorship. There was grumbling that the delegates’ pack included complementary Microsoft CD-ROMs (for Windows only). “Next year,” one delegate was overheard to say, while tucking into a spring roll and sipping Chardonnay at the evening reception (courtesy of Bill Gates), “Microsoft will have bought the World Wide Web.” However, his fears were not universally held. There is just too much grass-roots stuff going on out there for one company, however powerful, to take over completely.

Information revolution

It may seem from the outside that the information revolution has arrived, but in John Patrick’s view, “we’re right at the beginning”. The Web’s inventor, Tim Berners-Lee, doesn’t even go that far. The Web we’re going to see emerging over the coming decade, he believes, is none other than the one he had running almost 10 years ago on an obscure computer called a Next cube at CERN. “Ask him about control-shift-N,” said one delegate, referring to the combination of key strokes that instructed that early browser/editor to create a new document linked to the one you were already in. That simple manoeuvre encapsulates Berners-Lee’s vision of what the Web should be, “a common space in which we could all interact”, a medium in which we’d all be creators, not just consumers. Expediency prevented that reality from coming sooner as Berners-Lee and his team at CERN concentrated on providing Web services to the particle physics community leaving the stage free for the entrance of Mosaic, a browser with no editing capacity, in 1994.

Even when the passive Web took off, Berners-Lee did not abandon his dream. To most users of the Web the choice of browsers comes down to two: Netscape Navigator and Internet Explorer. However, there’s actually a lot more choice available. Many of the early browsers can still be found, and there are new companies turning out more. The Web consortium (W3C) has produced a browser/editor, called Amaya, that allows the kind of interactive Web use that Berners-Lee envisaged from the start. If you want to see what the Web was meant to be, open Navigator or Explorer for the last time, go to “http://www.w3.org” and click on “Amaya browser/editor”.

Improving the Web

Content that the Web is finally catching up with his original vision, Berners-Lee is now devoting his energies to improving it. The Web’s biggest problem is caused by its success. There’s so much information out there that it’s often hard to find what you want. The answer, according to Berners-Lee, is what he calls the semantic Web. The kind of information on the Web today is understandable to humans but not to computers. If, for example, Berners-Lee wanted to buy a yellow car in Massachusetts and his neighbour wanted to sell a primrose automobile in Boston, how would his search engine know that what he wanted was right on his doorstep? If a current W3C project is successful, some kind of logical schema will tell the search engine that primrose is just a kind of yellow and that automobiles and cars are in fact the same thing.

Reminding delegates that there’s nothing new under the Sun was IBM’s John Patrick who spelled out his vision of how the Internet is poised to change our lives. Top of his list of next big things was instant messaging, which is just around the corner. Curiously familiar to anyone who used BITNET or DECNET in the 1980s, instant messaging is a sort of halfway house between e-mail and the telephone. Patrick demonstrated IBM’s version by typing in “How is the weather in Heidelberg” to a colleague in Germany. Out boomed the mechanical words “Wie ist das Wetter in Heidelberg”, followed, presumably after the Heidelberger had typed “Es ist kalt und regnerisch”, by “It is cold and rainy”. That’s fine if all you want to do is discuss the weather, but IBM’s translation software might have problems with more complex topics. Nevertheless, it served to show what’s coming.

This article was adapted from text in CERN Courier vol. 39, September 1999, pp26–28

Muon ring could act as a neutrino factory

neutrinos1_7-99 — Traditionally, neutrino physics has been a large-scale operation, needing big detectors to intercept the dispersed particles. The CHARM II detector at CERN extended over a distance of 40 m. Neutrino detectors at muon storage rings would be more compact.

With CERN’s LHC collider now committed to beginning its experimental programme in the year 2005, and with research and development work well advanced for a new generation of linear electronpositron colliders, a totally new avenue of particle physics machines that is currently being enthusiastically explored is that involving muon colliders.

Muons, which are heavy relatives of the electron, come in postively and negatively charged versions with properties that are broadly similar to those of electrons. These oppositely charged beams could be made to collide with each other to produce a wealth of particles.

As a result of their being more than 200 times as heavy as electrons, muons that are bent by a magnetic guide field lose much less energy than electrons and could be housed in far smaller rings. Several generations of muon colliders could be accommodated on existing laboratory sites. Initial thinking towards muon colliders was outlined by Fermilab’s Steve Geer in the December 1997 issue (“Muon colliders move nearer”).

Neutrinos from muons

One interesting possibility en route to a full muon collider scenario with two muon beams is to employ a single circulating muon beam. As they decay, the stored muons would provide an intense source of neutrinos with properties that are very different those of conventional synthetic neutrino beams.

The first step would be an intense 2 GeV drive beam of protons from a linear accelerator to produce pions at a secondary target. These pions decay into muons, captured in an intermediate storage ring. The muons would then be accelerated in a booster synchrotron up to 20 GeV or higher.

The faster muons travel, the longer they appear to live.

At rest, a muon lives on average for approximately 2.2 µs. Without the intervention of relativity, even a high-energy muon would travel only around 600 m before it decayed. Under these conditions, few cosmic-ray muons would reach the Earth’s surface and muon storage would not be possible. However, the lifetimes of fast-moving muons are stretched by relativity, thus, the faster muons travel, the longer they appear to live. High-energy cosmic-ray muons easily reach the Earth, and high-energy muons could be stored.

However, for this to occur it is essential for the muons to be accelerated quickly, therby endowing them with an increased lifetime as rapidly as possible. The muon storage “ring” would have 300 m straight sections in which most of the stored muons would decay into electrons, neutrinos and antineutrinos.

Negatively charged muons decay into electrons and equal numbers of muon-type neutrinos and electron-type antineutrinos, while positively charged muons decay into positrons and equal numbers of muon-type antineutrinos and electron-type neutrinos.

Energy spectra

The energy spectrum of the produced neutrinos depends only on the muon energy and their spin orientation. That of the muonneutrinos peaks towards the upper end of the spectrum, towards the energy of the muons, while the electronneutrinos are softer. Decay electrons would be swept away by the storage ring bending magnets.In addition to having a clear energy spectrum, the flux of neutrinos produced in this way can be calculated from the muon population of the storage ring. The intense neutrino fluxes available via this route warrants the label “neutrino factories”.

Neutrinos at accelerator laboratories are traditionally produced via the decay of kaons or pions. The neutrino detector has to be some distance from the production target to allow the pions and kaons sufficient time to decay, and the accompanying muons have to be screened off. A pure neutrino beam is only established some distance away, so that neutrino physics has always been a large-scale operation, requiring large detectors to intercept the dispersed particles.

However, with muon storage rings, the electrons from muon decay could easily be screened off, providing a pure neutrino beam very close to the intense source. New-look compact neutrino detectors could be mounted using state-of-the-art detector technologies to pinpoint the neutrino interactions and thereby open up a new era of neutrino physics.

Oscillating neutrinos

Classically, neutrinos were supposed to be massless and came in three immutable kinds electron, muon and tau according to their particle allegiance. However, experiments studying neutrinos coming the Sun and from collisions of cosmic rays in the atmosphere have led physicists to believe that neutrinos are not immutable or massless. Other hints for neutrino oscillations come from an experiment at Los Alamos.

Endowed with tiny masses, neutrinos can transform into each other in flight, so that what leaves the Sun as an electron-neutrino could become a muon-neutrino as it flies through space, or what leaves an accelerator facility as a muon-neutrino could transform into a tau-neutrino.

Such effects are subtle, and measurements of the neutrino masses and mixing parameters are difficult. However, the intense fluxes provided by neutrino factories would provide optimal conditions, particularly compared with existing (and planned) sources of accelerator neutrinos.

The Graal of particle physics

“One very powerful way of experimentally investigating the strongly interacting particles (hadrons) is to look at them, to probe them with a known particle; in particular the photon (no other is known as well). This permits a much finer control of viariables, and probably decreases the theoretical complexity of the interactions,” wrote Richard Feynman (1992 PhotonHadron Interactions, Addison Wesley).

Synchrotron radiation, which is an example of one type of photon, is produced when charged particles are bent in a magnetic field. This radiation is used over a range of science as a microstructure probe. The European Synchrotron Radiation Facility (ESRF) in Grenoble, France, is devoted to producing synchrotron radiation for the study of atomic, molecular and more complex organic and inorganic systems. The radiation is produced by a beam of 6.04 GeV electrons circulating in a storage ring. The electrons radiate synchrotron radiation in the bending magnets of the ring lattice and in magnetic insertion devices wigglers and undulators that shake the beam.

However, the new Graal experiment at the ESRF generates radiation in another way to probe nuclear and nucleon structure. Graal has been realized by a French-Italian-Russian collaboration with primary financial support from INFN in Italy and IN2P3 in France through several Italian laboratories and universities (Laboratori Nazionali di Frascati, Laboratori Nazionali del Sud, and the universities of Catania, Genova, Roma II and Torino), two French Institutes (Institut des Sciences Nucleaires and Institut de Physique Nucleaire) and the Institute for Nuclear Research of the Russian Academy of Science.

In this way, Graal extends the ESRF resolving power to nuclear and that of nucleon structure down to a spatial resolution of 0.2 fm

Undulators are normally composed of physical magnets, but Graal uses a micro-undulator in the ESRF straight section D7,which is a beam of ultraviolet laser light moving against the electron beam. The electrons scatter the laser photons, transferring energy to them and producing narrowly collimated gamma rays.

At the ESRF, a laser beam in the near-ultraviolet produces gamma rays with a maximum energy of 1.47 GeV, a maximum linear polarization of 98% and an intensity of a few millions of photons per second. In this way, Graal extends the ESRF resolving power to nuclear and that of nucleon structure down to a spatial resolution of 0.2 fm (0.2 x 10^-13 cm).

The first advantage of this technique over normal electron bremsstrahlung synchrotron radiation is the almost flat energy spectrum. Polarization is the second advantage: photons scattered in the electron direction maintain their polarization. Therefore, at the higher end of the spectrum, the polarization is very close to that of the laser light. Rotating or changing the polarization of the gamma rays is easily accomplished by rotating or changing the polarization of the laser light.

A gamma beam of a useful intensity for studying photonuclear reactions was first produced at Frascati using the Adone storage ring. After this success, several more such beams were produced (table I). A beam with a maximum energy of 2.4 GeV is now under construction at Spring8 in Japan.

Graal’s main goal is the study of photonuclear reactions in the intermediate energy region, where the nucleons cannot be treated as elementary particles and their internal degrees of freedom cannot be ignored, but away from the asymptotic freedom of quarks and gluons. In this region, many excited baryon states are clearly visible and many others await careful exploration

As highlighted by Feynman’s quotation, photons are an interesting probe of hadronic structure because the interaction is given by the product of the electromagnetic vector potential and the hadronic current. The former is well known from quantum electrodynamics, and the relative weakness of electromagnetic coupling makes second-order effects small, thus models of photonuclear reactions are possible. Moreover, the possibility of using linearly and circularly polarized gamma rays makes several single- and double-polarization observables experimentally accessible, providing strong constraints on theoretical models. A linearly polarized photon beam introduces a fixed direction for the electric field, so that the reaction yield is no longer cylindrically symmetric with respect to the beam direction.

In a circularly polarized beam, the photons have well defined helicity and their spins are aligned parallel or antiparallel to their momentum. Parity conservation in photoreactions dictates the overall form of the interaction. The asymmetry in the weak decay of the (strange) lambda provides information on lambda polarization, so it is possible to measure the correlation between the gamma and lambda polarizations in the photoproduction of strange particles.

Ultraviolet photon beam

In Graal, an argon-ion laser provides a beam of ultraviolet photons. A three-lens zoom focuses them at the centre of the laserelectron interaction region. Two precisely adjustable mirrors align the laser light with the electron beam 35 m away to within 3 µrad. A retardation plate rotates the plane of linear polarization of the beam. The ultraviolet enters the storage ring vacuum system through a quartz window and is subsequently reflected through 90° by a beryllium mirror coated with aluminium. This mirror lines up the laser beam with the electron beam. (Beryllium minimizes the absorption of backscattered gamma rays that travel in the opposite direction to the ultraviolet.)

Electrons that have transferred part of their energy to a photon, move with the electron bunch along the straight section but, owing to their lower energy, veer away in the next dipole and become separated from the unscattered electrons by a few centimetres. Measuring the distance between a scattered electron and the electron beam is a measure of its energy loss and therefore of the gamma energy.

The detection of the scattered electron and the measurement of its precise position are done with the tagging detector, which comprises plastic scintillators and silicon microstrips. The microstrips give the position of the electron and the scintillators give its precise timing. The electron timing correlates an event in the hadronic detector with the corresponding electron, thus providing the energy of the gamma ray that produced it. It also provides a precise starting signal for measuring the time of flight (TOF) of photoproduced particles. The jitter of the TOF start pulse, provided by the scintillators, is reduced to 120 ps, effectively synchronizing this pulse with the phase of the accelerating radiofrequency of the ring. This is possible because the electrons travel in short bunches separated by 2.8 ns.

The Graal hadronic detector covers the entire solid angle except for two small entry and exit holes along the beam axis. The detector is made of three parts. In the central part, between 25° and 155°, the emerging particles pass through two cylindrical wire chambers and a barrel of 32 thin plastic scintillators, then enter a calorimeter made of 480 BGO crystals, each 24 cm long, and arranged, like an orange, in 32 sectors of 15 crystals each.

The main features of the Graal detector are high efficiency, good energy resolution for gamma-ray detection and complete angular coverage.

Particles emitted at angles of less than 25° go through two plane wire chambers and three plastic scintillator walls. The first two thin walls are used to measure the specific ionization of the particles. Then a thick wall, with alternating layers of plastic scintillator and lead, measures the total charged particle energy and detects neutrons and gamma rays. All three walls provide a measurement of the position and time of flight of the particles.

Particles emitted backwards encounter two plastic scintillator discs separated by lead. Each disc has a small central hole for the passage of the beam and is viewed by 12 photomultipliers to reconstruct the position and timing of a particle. The responses of the two discs allow charged particles and gamma rays to be differentiated.

The main features of the Graal detector are high efficiency, good energy resolution for gamma-ray detection and complete angular coverage. The detector is well suited to events producing several photons, like the photoproduction of neutral pions and etas, and the identification of the various eta decay channels. The first results to emerge are extensive measurements of the beam polarization asymmetries for the photoproduction of positive and neutral pions and etas. The two-photon and three-neutral-pion (giving six photons) decay channels of the eta have been detected simultaneously.

Polarization asymmetries, derived experimentally from the ratio of successive measurements with the same apparatus, are immune to otherwise common systematic experimental errors, such as the knowledge of the solid angle, the efficiency of the apparatus, the measurement of the dose and the size of the target. From a theoretical point of view, polarization asymmetries are given by the interference of different amplitudes and are therefore more sensitive to small, hitherto unobserved, contributions if b is much less than a, then ab is more sensitive to b than is a² + b².

Event discrimination

Another advantage of full solid-angle apparatus, with a high overall efficiency for the detection of gamma rays, is its ability to discriminate rare events, where only one or a few photons are produced, from the more frequent events containing many gammas.

One example is Compton scattering, which is about 50 times as rare as neutral pion photoproduction and can be difficult to single out using only kinematics. However, Compton scattering has only one photon in the final state, while neutral pion photoproduction has two. Another example is the rare decay of the eta into a neutral pion, which has four photons in the final state compared with the frequent decay into three neutral pions, which has six.

Graal is now in full operation. It can collect data for more than six months per year a large fraction of the time that the ESRF ring is available to the experimenters. A 10 mK dilution refrigerator and a 16 T magnet are now being delivered for the construction of a polarized target. With polarized targets, double-polarization experiments will be possible in all channels.

Electron clouds with copper linings

electrons1_7-99-2 — Prototype beam pipes for CERN’s LHC collider, showing the inner beam screen to shield the surrounding superconducting magnets from synchrotron radiation emitted by the beams.

As a beam of charged particles is bent in a magnetic field, the particles lose energy by synchrotron radiation. The amount of energy lost is far greater for light particles than for heavy ones. Protons, the particles to be accelerated in CERN’s forthcoming Large Hadron Collider (LHC), are some 2000 times as heavy as the electrons and positrons of the laboratory’s current LEP accelerator, thus they lose far less of their energy to synchrotron radiation.

However, although the proportion of energy lost at the LHC will be much less than at LEP, the number of photons emitted by beam particles will, surprisingly, be much greater. Moreover, these photons will give rise to a number of undesirable effects that must be carefully controlled in a cryogenic vacuum environment. They will stimulate the release of gas molecules trapped in the walls of the LHC’s vacuum chamber, they will cause the emission of electrons (photo-emission), that in turn may liberate secondary electrons, and these will cause heating.

Beam screen solution

A partial solution will come in the form of beam screens that are designed to prevent the LHC’s bending magnets from overheating and losing their superconductivity. These metallic tubes lining the LHC vacuum chamber will be held in the 5-20 K range, whereas the magnets will operate at just 1.9 K. The beam screens will carry away heat generated by so-called image currents, induced by the charge of the beams, as well as by the synchrotron radiation.

However, the story doesn’t end there. Within the LHC’s bending magnets, synchrotron radiation will tend to strike the outside wall of the beam screen along a very tight band. Electrons liberated by this radiation will immediately be turned back by the magnetic field and reabsorbed. The only problem this will cause is heating of the beam screen, which is easily dealt with.

However, if a photon is reflected and strikes the roof or floor of the beam screen, liberating electrons there, these electrons will be accelerated in a spiralling path by the positive charge of the passing proton bunch towards the opposite wall (bottom or top) of the beam screen, where they may release more electrons. These in turn will be attracted by the following proton bunch and may be accelerated back to the top or bottom, releasing more electrons in a potential runaway situation. This is called multipacting, a phenomenon that is familiar to designers of accelerating cavities.

With the LHC, however, multipacting will not be quite the problem it may at first seem. The process is self-damping to a certain extent because, when an electron cloud has formed close to the walls of the beam screen, it inhibits further electron emission. Keeping this cloud manageable, however, is a fine art. Calculations suggest that, as long as the maximum number of secondary electrons produced per primary electron is less than 1.3, then runaway will not occur.

electrons2_7-99 — Screening the beam. Conrad Grunhagel makes adjustments to the COLDEX experiment.

Unfortunately, the current beam screens give a coefficient of around 2.4, so work is under way to finetune the design. Some improvement will occur naturally as the beam screens become conditioned by exposure to synchrotron radiation. But to reach the levels required at the LHC demands an in-depth understanding of factors such as the reflectivity of the emitted photons, the primary and secondary electron yields and the amount of gas release stimulated by electrons and photons.

In 1996 a group of researchers in CERN’s LHC division began a study of these quantities using both synchrotron radiation from the Electron Positron Accumulator (normally used to collect particles for LEP) and a laboratory electron source. The group’s goal is to optimize the design of the beam screens by seeing how different manufacturing techniques influence their behaviour on exposure to synchrotron radiation. The current design is a 50 µm layer of copper, chosen for its low electrical resistivity, covering a rigid structure of high manganese steel, chosen for its low magnetic susceptibility. Low resistivity is important in allowing the image currents induced by the LHC’s protons to move as freely as possible to keep resistive heating to a minimum.

electrons3_7-99 — One possible consequence of synchrotron radiation being emitted in an LHC bending arc. A photon (black) emitted from a proton bunch (1, black crosses) liberates an electron (yellow) from the beam screen. This strikes the opposite wall, where it causes heating and liberates low-energy secondary electrons (red), which drift back across the vacuum. Secondary electrons above a certain energy reach the beam screen before the arrival of the following proton bunch (2). Some are absorbed, and some are reflected and accelerated by this following bunch across the vacuum until they strike the beam screen. Meanwhile, lower-energy secondary electrons are also accelerated by this following bunch so that they too strike the beam screen. The following bunch emits synchrotron radiation, which may give rise to a similar effect or cause gas molecules trapped on the beam screen to be released.

Three beam-screen models have been tested at room temperature: a mirror-like copper laminate, electroplated copper and a sawtooth form. The results from the tests suggest that there may be some advantages with the sawtooth design, but further investigations are necessary to be sure that there are no unforeseen drawbacks with it.

The next step is to study beam screen behaviour at cryogenic temperatures. Last year the group began COLDEX (cold experiment). This was designed to measure synchrotron radiation-induced gas release, but it is ideal for all the other measurements as well. When LEP shuts down in November, the COLDEX apparatus will be inserted into the EPA ring to investigate how LHC beam screens operate with positively charged beams passing through them. The results should help to ensure that the LHC’s electron clouds have, if not a silver lining, at least a copper one.

New Fermilab machine dedicated

cernnews1_7-99 — Fermilab’s new 2 mile circumference, 150 GeV main injector (foregound) injects particles into the larger Tevatron.

The world’s newest particle accelerator Fermilab’s 150 GeV main injector officially began its career on 1 June. The US Energy Secretary Bill Richardson, the Speaker of the House of Representatives Dennis Hastert and Illinois Governor George Ryan joined Fermilab staff and visiting scientists in celebrating the on-time, under-budget completion of the $260 million project.

“It has taken seven years to reach this dedication day a long time,” said Fermilab director John Peoples, whose 10 year term in office has spanned the entire project.

The new main injector will, literally, be a major boost for Fermilab’s centrepiece machine the superconducting Tevatron proton synchrotron and protonantiproton collider.

In 1991 a $2.2 million challenge grant from the state of Illinois enabled Fermilab to take the first steps towards building the new main injectors. Federal funding was approved in October 1991, and construction got under way in 1993.

The main injector team worked together so well that a new storage ring the antiproton recycler was added to the accelerator complex without increasing the total project budget or delaying its scheduled completion.

The recycler, which shares the new, 2 mile, circular tunnel with the main injector, uses permanent magnets to retrieve, store and literally recycle antiprotons that would previously have been discarded.

The Tevatron, which began operations in 1983, was previously fed by Fermilab’s original main ring, closed in 1997 after 25 years of service. The Tevatron and the main ring shared the same 4 mile circumference tunnel. As the Tevatron injector, the main ring was a bottleneck in the antiproton supply.

LEP hits 100

cernnews2_7-99 — A “family album” photograph of the final module of superconducting accelerating cavities for CERN's LEP electronpositron collider. These cavities power LEP beams to 100 GeV.

CERN’s LEP electronpositron collider walked onto the stage for its 1999 season and, after its customary greeting at the Z resonance (45 GeV per beam), impressed the waiting audience by quickly taking a shot of electrons to a record 100 GeV. This showed how smoothly its complement of 288 superconducting accelerating cavities can pull together, supplying 3.15 GV (3.3 GV without beam).

Soon after, colliding beams were established with 98 GeV electrons and positrons for radiofrequency tests. After these spectacular opening fireworks, high-energy physics got under way with 96 GeV electron and positron beams. Collision rates were high, with luminosities well above 10³¹ cm^-2s^-1, with sizable beam currents and good integrated luminosities (12 inverse picobarns per day).

Just 10 years ago, LEP began operations equipped with room temperature copper radiofrequency accelerating cavities, supplying 45 GeV per beam. From 1995, equipped with superconducting cavities, LEP’s beam energy was increased to 65 GeV, then to 80.5 GeV in 1996 with more superconducting accelerating power. Last year, LEP ran routinely at 94.5 GeV per beam.

At these new high energies, the LEP experiments are treading on potentially very fertile physics ground and could soon reveal what makes the electroweak theory tick.

Magnetic detector sees cosmic-ray anomalies

cernnews5_7-99 — Fig. 3: Antiparticle puzzle. In the equatorial region, AMS sees about four times as many low-energy positrons as it does electrons.

cernnews4_7-99 — Fig. 2: Cosmic-ray proton distribution measured by the AMS experiment aboard the Space Shuttle at various latitudes.

As reported briefly in the June issue, the Alpha Magnetic Spectrometer (AMS) presents several intriguing effects, which include unexpected distributions of cosmic-ray particles, from its 1998 trial Space Shuttle flight.

From 2 to 12 June 1998, AMS was the primary payload of NASA’s Space Shuttle Discovery in orbit 400 km above the Earth. This was a shakedown mission prior to deploying AMS on the International Space Station in a few years time.

AMS is a sophisticated magnetic detector of the type normally used in high-energy physics laboratories. During the 10 day voyage, AMS recorded the tracks of millions of cosmic-ray particles. It was the first time that such a sophisticated physics detector had been deployed in space and the first time so much information on cosmic particles had been recorded. The first results from this mission have been eagerly awaited.

The advertised goal of AMS was to search for signs of cosmic antimatter. In a universe created from a Big Bang that must have generated matter and antimatter in equal initial amounts, there should be signs of this primordial antimatter, with antinuclei built of antiprotons and antineutrons. However, our universe appears to be built up entirely of matter and no experiment has ever detected any primordial antimatter. AMS set out to look for antinuclei above the screen of the atmosphere, but a sample of almost 3 million cosmic helium nuclei arriving from outer space did not reveal one helium antinucleus. AMS sees no primordial antimatter, but 3 million nuclei is not many and the search continues. However, AMS did see several other unexpected effects, which show that the behaviour of cosmic rays is much more complicated than had been thought.

In orbit, AMS was able to intercept cosmic rays arriving at different latitudes as the Earth turned. Cosmic-ray protons have a range of energies and the Earth’s magnetic field should repel less energetic particles. This terrestrial magnetic repulsion becomes weaker at higher latitudes, and more particles of low energy should be seen nearer the poles, with a magnetic cut-off at each latitude. However, AMS finds that, below a certain proton energy for each latitude, there is no magnetic cut-off and the distribution increases strongly instead (figure 2).

When the Space Shuttle flips over, AMS can also collect cosmic particles moving upwards, away from the Earth. Few high-energy, upwards-moving protons are seen, but the spectrum fills up rapidly for lower-energy particles. In a band extending over 4000 km at the Space Shuttle orbit altitude of 400 km, below about 6 GeV AMS saw as many protons moving upwards as downwards. It is as though these particles are confined in a magnetic toroid around the equator.

A similar effect is found with electrons, but here it is interesting to compare the levels of electrons and their antiparticles positrons. If cosmic-ray electrons and positrons are created in pairs by high-energy gamma rays, there should be as many electrons as there are positrons. However, in the equatorial region AMS sees about four times as many low-energy positrons as it does electrons.

AMS also looked at the distribution of helium nuclei. In a final conundrum, around the equator and at low energies, AMS only sees the helium-3 isotope. Helium-3 is very rare on Earth but was one of the protonuclei formed during the one minute, or so, of the universe’s primordial nucleosynthesis.

AMS is a major international collaboration covering Europe, the US, China and Taiwan and is led by Sam Ting of MIT longtime spokesman of the L3 experiment at CERN’s LEP electronpositron collider.

Accelerating power for LHC

cernnews6_7-99 — A protoype cryomodule containing superconducting accelerating cavities for CERN’s LHC proton collider.

For CERN’s new LHC proton collider, superconducting magnets will not be the only super- conducting technology in the 27 km ring.

When the collider was commissioned in 1989, the energy of CERN’s LEP electronpositron collider was 50 GeV per beam. After a dedicated period of running around the Z particle resonance, LEP’s energy has been increased to 100 GeV per beam.

Behind the success story is the conversion of the machine from conventional radiofrequency to superconducting cavities to feed accelerating power to the circulating beams. Early on, research and development work for LEP showed that cavities made of niobium-coated copper were more effective than those of the more expensive solid niobium. The LHC is set to use this technology from the outset.

The LHC’s radiofrequency must be a multiple of 200 MHz, the operating frequency of the upstream SPS synchrotron, to allow rapid transfer of many SPS proton bunches, but not so high as to make for operational incompatibility. Thus it is set at twice that of the SPS.

The radiofrequency scenarios for LEP’s electrons and positrons, and the LHC’s protons, are very different, even though they both use a 27 km ring. Electrons, being very light particles, lose a lot of energy per turn by synchrotron radiation, which has to be replaced continually by the “accelerating” cavities. Most of LEP’s radiofrequency power is transferred to the beam and then dissipated by synchrotron radiation.

Proton beams lose little energy in this way. The main role of the LHC cavities is to keep the many proton bunches tightly bunched to ensure high luminosity at the collision points and to deliver power to the beam during energy ramping. Matching these radiofrequency conditions using conventional copper cavities would lead to unacceptable displacement of the beam crossing points.

Superconducting cavities with small losses and large stored energy are the best solution. This leads to a design using single-cell accelerating cavities with large beam tubes, similar to those considered for the new generation of electronpositron colliders.

The LHC will use eight cavities per beam, each capable of delivering 2 MV (an accelerating field of 5 MV/m) at 400 MHz. The cavities will operate at 4.5 K (the LHC magnets will use superfluid helium at 1.8 K). For the LHC they will be grouped in fours in cryomodules, with two cryomodules per beam, and installed in a long, straight section of the machine where the interbeam distance will be increased from the normal 195 to 420 mm. The cavities are being made by spinning and electron-beam welding, with the surface niobium being added by magnetron sputtering.

For LHC cavities, an ingenious mechanical tuner has been designed and successfully tested, to cope with the larger detuning range of the LHC cavities and their increased stiffness (compared with LEP cavities).

The experience gained with LEP couplers, which were once a very critical element of the LEP2 project, has led to the design of state-of-the-art LHC couplers, which link the cavity to the RF power system.

B-factory sees first B events

cernnews7_7-99 — An early event recorded by the BaBar detector at the asymmetric B factory at SLAC.

On the morning of 26 May 1999, physicists recorded the first events in the large BaBar detector at the asymmetric B-factory at the Stanford Linear Accelerator Center (SLAC). The factory began operating last year and was formally inaugurated on 23 October 1998 (CERN Courier December 1998).

The collider was constructed by SLAC, Lawrence Berkeley and Lawrence Livermore National Laboratories with $177 million of US government funds. The BaBar detector was built by scientists and engineers from 73 institutions in the US, Canada, China, France, Germany, the UK, Italy, Norway and Russia. It has cost about $110 million in all, with 40% of the total coming from foreign sources.

The goal of the detector is to study CP violation in reactions involving B particles, containing the fifth (“beauty”, “bottom” or simply “b”) quark.

The first published results should be available by next year.

Precision physics progress

cernnews8_7-99 — The main magnet (covered with foil to control temperature) of the spectrometer for precision measurements of the beam energy of CERN’s LEP electronpositron collider ring.

CERN’s LEP electronpositron collider has underlined the importance of precision measurements in particle physics. For this work it is vitally important to know the beam energy of this huge machine as accurately as possible.

In 1983, the UA1 and UA2 teams at CERN’s protonantiproton collider saw the Z particle the electrically neutral carrier of the weak interaction via a handful of events. By the mid-1990s the four experimental teams at CERN’s LEP electronpositron collider had accumulated millions of Zs.

The object of this massive data sample was to establish the consistency of the underlying Standard Model of particle physics. Before the sixth “top” quark was found at Fermilab’s Tevatron protonantiproton collider in 1994, this LEP data provided a valuable indication of where the top quark then the missing element of the Standard Model should lie.

cernnews9_7-99 — Schematic of the new spectrometer, showing the surrounding pick-ups to track the path of the beam and the absorbers to soak up synchrotron radiation.

With the discovery of the top quark, and the underlying Standard Model of physics in such good shape, the central requirement now is to pin down the parameters of the long-sought Higgs particle, which is responsible for the symmetry breaking at the heart of the Standard Model. Precision measurements could also aid the search for discrepancies signalling new physics effects beyond the Standard Model.

This “precision physics” continued as LEP moved away from its initial target at the Z resonance and moved to higher energies to explore the production of the W, the electrically charged partner of the Z, where a precision fix of the W mass could serve as a valuable benchmark. Before LEP went to higher energies, W mass information came via protonantiproton collision physics.

The first LEP W mass measurements in 1996 relied on determining the production threshold, but, as LEP energies were increased further, the W mass was determined by the kinematics of the production processes.

This need for precision has driven a continual need to measure the energy of the particle in LEP as accurately as possible, hopefully to within 10-20 MeV, or about 1 part in 10 000.

Thus began a programme to identify and compensate for external factors that could influence the beam energy measurement. Tiny tidal effects in LEP, amplified by the acceleration process, can contribute up to 40 MeV to the beam energy. Gravitational effects, owing to the neighbouring Jura mountains and the level of water in Lake Geneva, have to be allowed for. LEP energy calibration has even become an expensive way of monitoring the passage of French TGV high-speed trains.

The main technique for accurately measuring LEP beam energy has been resonant depolarization, in which the spin alignment of the stored electrons is destroyed. However, this is only precise at lower energies, around 60 GeV per beam. With LEP now approaching 100 GeV per beam, the low energy measurements have to be extrapolated over a long distance, limiting the attainable precision.

To sidestep this, LEP embarked on a project to mount a magnetic spectrometer inside the ring that will measure the beam energy directly via magnetic deviation.

The spectrometer is a steel dipole magnet, mounted in the LEP ring between the standard dipole magnets, with steel laminations in concrete, which are used to guide LEP’s particles. Beam position monitors either side of the spectrometer dipole track LEP particles as they enter and leave, the objective being to measure variations in the bending of the particles to an accuracy of 10^-4. The adjacent monitors measuring to within 1 µm.

As LEP’s particles are bent round the ring, they continually emit a “screech” of synchrotron radiation that can reach levels of almost 1 kW/m. This emission is not dangerous in an uninhabited ring and disappears as soon as the machine is switched off. During operation, however, this radiation can heat up mechanical supports, producing expansions and contractions of the order of microns, which would mask the spectrometer measurements. To avoid this, the monitors are shielded by copper absorbers to soak up the synchrotron radiation. A stretched wire positioning system checks the relative position of the beam position monitors to within a 1 µm.

The system is now installed and recording preliminary data prior to regular operation later this year. The new spectrometer provides a vivid example of precision microengineering and electronic read-out for physics.

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