Physics Archives – Page 141 of 144

What’s the quark matter?

cernnews6_5-99 — J/psi production yield per nucleonnucleon collision, measured in leadlead collisions by the NA50 experiment at CERN, relative to the normal nuclear absorption pattern inferred from other collision processes, as a function of L, the total length of nuclear matter traversed by the produced state. The abrupt fall in yield at higher L values shows that, in the violent collisions when the lead nuclei hit each other head-on, the quarks and antiquarks find it more difficult to stick together.

Careful analysis of data collected by the NA50 experiment studying high-energy heavy-ion collisions at CERN shows clear signs of new behaviour, suggesting that under these conditions the colliding nuclear particles briefly fuse together to form a new kind of matter.

In ordinary matter, quarks and gluons are confined inside nucleons, the component particles of nuclei. However, this has not always been the case. In the first split second after the Big Bang, when the temperature exceeded 10¹³°, quarks and gluons roamed around in a uniform “soup”. When the temperature dipped, the free-ranging quarks and gluons suddenly “froze” into strongly interacting particles (hadrons), where they have remained ever since. The only known way for them to leave this confinement is via high-energy nuclear collisions “Little Bangs” when small pockets of hot and dense nuclear matter simulate post-Big Bang conditions.

Over the past 20 years, lab physics experiments have gradually increased the energy of their nuclear beams in the search for this “quarkgluon plasma”. As well as providing sufficient input energy to create Little Bangs, another challenge is to recognize clearly the deconfined state once it has been recreated.

One suggestion, which was made in 1986 by Tetsuo Matsui and Helmut Satz, was to look among the emerging particles for states like the J/psi – a meson composed of a charmed quark and antiquark bound together.

Approaching plasma conditions, the attractive force between the quark and the antiquark will be screened by gluons and lighter quarks, and less charmed quark antiquark pairs will bind into J/psi states.

However, an absorption effect also results from interactions of the produced J/psis with the nucleons while traversing the surrounding nuclear matter. Fortunately, this conventional absorption mechanism can be understood from the study of lighter collision systems, as has been done at CERN’s SPS synchrotron with proton, oxygen and sulphur beams.

A sudden drop in the rate of J/psi formation, after accounting for the normal nuclear absorption, is considered to be a clear signature of quarkgluon plasma formation.

In 1996, colliding 158 GeV/nucleon lead beams on a solid lead target and using an improved experimental set-up, NA50 saw 190000 J/psis via their decay into muon pairs, four times the data collected in 1995. For peripheral lead-lead collisions, where the density of nuclear matter is least, NA50 sees the expected nuclear absorption effects, extrapolated from studies with lighter nuclei.

However, for more violent lead-lead collisions, more energy is transferred and there is a maximum density of hot nuclear matter. Under these conditions, quarks and antiquarks find it more difficult to stick together and the J/psi production rate dramatically decreases.

Under these conditions the quarks and gluons in the colliding lead nuclei briefly “forget” about their 15 billion year nuclear heritage and revert to their primeval state.

As well as the clear signs of J/psi suppression seen by NA50, other encouraging signs that collective quark-gluon behaviour is not far away come from other experiments at CERN using heavy-ion beams, notably NA45; seeing an excess of light electron-positron pairs; the increased yield of multiply strange particles by WA97/NA57; and several intriguing observations from the big NA49 study.

This bodes well for the experiments that are preparing to take their first data at the end of the year at the higher energies of Brookhaven’s RHIC heavy ion collider. Their measurements should confirm beyond reasonable doubt the current indications that high-energy nuclear collisions lead to a transition from confined to deconfined matter, where quarks and gluons are no longer bound inside hadrons.

Later this year, Brookhaven’s RHIC collider will start exploring a higher-energy frontier for heavy-ion physics, with gold nuclei at 200 GeV per nucleonnucleon collision.

Meanwhile, CERN’s SPS experiments NA45, NA49 and NA57 convinced by the results found at 158 GeV per nucleon, will devote their 1999 beam time to a low-energy run with lead ions at 40 GeV per nucleon. The aim is to study the onset of the anomalous phenomena seen at the full SPS energy and to fill in the energy gap between existing SPS results and lower-energy data from the CERN and Brookhaven synchrotrons.

The return of antimatter

anti1_5-99 — CERN Antiproton Decelerator project leader Stephan Maury at the AD during a live antimatter “Webcast” from CERN.

This year should see the start of physics with CERN’s new Antiproton Decelerator ring, marking the return of antiparticle physics to the CERN research stage three years after the closure of the LEAR low-energy antiproton ring in 1996.

The Antiproton Decelerator (AD) was built from CERN’s former Antiproton Collector ring, which was commissioned in 1987 to supplement the original Antiproton Accumulator (AA; meanwhile, elements of the AA have been sent to the Japanese KEK laboratory).

The task of the AD will be to take the antiprotons, which are produced by 26 GeV/c momentum protons hitting a target and selected at the optimum 3.57 GeV/c momentum level, and, as its name implies, decelerate them to much lower energies, using electron and stochastic cooling to control the beams.

Late last year the AD had a foretaste of particles the much more readily available protons, in this case. The antiproton debut is scheduled to take place soon after the restart of the CERN machines this spring, with the physics programme following in September.

On the menu are the ATHENA and ATRAP experiments, which will use magnetic trapping to manufacture atoms of antihydrogen. Following the first synthesis of chemical antimatter at LEAR in 1995, physicists have been eagerly awaiting a chance to revisit atomic antimatter country to see whether there is any difference between the behaviour of matter and antimatter.

Also on the menu is the ASACUSA experiment by a Japanese-European collaboration, which aims to continue the exploration of antiprotonic atoms atoms in which an orbital electron has been replaced by an antiproton.

Telegrams from the antiworld

anti2_5-99 — Some “atoms” with orbital antiprotons live long enough to be examined by laser spectroscopy. (This “trick” image is a superposition of two images: one with the people and one with the laser.)

Antiprotonic atoms, in which an antiparticle is bound to an ordinary nucleus, carry important messages about the antiworld and are much easier to make than anti-atoms. Among antiprotonic atoms, protonium (a “nuclear” proton and an “orbital” antiproton) is particularly interesting because it is the simplest two-body system consisting of a strongly interacting particleantiparticle pair.

An isolated protonium atom will not be destroyed by collisions with atoms of the medium in which it was produced and can only de-excite by giving off radiation. The lifetime can then easily exceed microseconds. The difficulty will be to produce the atoms in isolation.

Antiprotonic helium is a special case. An experiment at CERN discovered that this exotic atom can survive a very large number of collisions and survive long enough to be studied by laser spectroscopy.

Isolated antiprotonic lithium would also be of great interest because its antiproton orbit should be far outside the residual pair of electrons. It should then be able to descend a ladder of these slow electromagnetic transitions, which ends only when it approaches the electrons.

In studying the interactions of antiprotons with matter, it is important to understand their ionization effects how antiprotons strip electrons from ordinary atoms.

An experiment at LEAR by a collaboration involving Aarhus, PSI Villigen, University College London and St Patrick’s Maynooth measured the ionization of hydrogen by antiprotons within the 30-1000 keV energy range, where the antiprotons can be considered to be “fast and heavy” (see next article). The experimentally observed effects concur with theoretical calculations.

However, at lower energies, where there are as yet no data, theoretical analysis becomes more difficult and different calculations disagree, although they suggest an at the most weak energy dependence.

The study of the ionization of helium by antiprotons, with removal of one or both electrons, was pioneered at LEAR and is also ripe for further investigation.

These additional physics objectives form an integral part of the ASACUSA experimental programme, which involves some 50 researchers from 19 research institutes and in which Japanese physicists play a prominent role.

Antiprotons cannot do this, but when their energy drops still further (below a few tens of electron volts) they will readily be captured by the nucleus (see previous article) and form antiprotonic atoms.

These effects showed up clearly in the very-low-energy domain of antiproton physics opened up at CERN’s LEAR low-energy antiproton ring, and groups from Aarhus and Tokyo carried out many atomic interaction experiments as a guide to a better theoretical understanding of these many-body collisions (see previous article).

In the LEAR era, such experiments injected high-energy antiprotons into metallic foils or high-density gases, which degraded the antiprotons to electron volt energies and (in some experiments) provided the target atoms in which they were finally captured.

If the target density or thickness could be made so small that only one collision occured, much more precise and better-controlled experiments on the atomic interactions of antiprotons would be possible, and the dynamics of antiprotonic atom formation could be studied in detail. At such low target densities the absence of collisions after the capture process should also ensure that all antiprotonic atoms are stable enough to be brought under the penetrating eye of laser spectroscopy (see previous article).

The thin-target condition, where a beam particle enters a target and makes a single interaction, is, in a sense, “business as usual” for high-energy particle experiments, yet it constitutes one of ASACUSA’s more difficult longer-term goals. The solution is to separate the deceleration of the antiprotons from the atomic interaction (or antiprotonic atom formation) to be studied.

However, the electron volt antiprotons required for these experiments have a millionth of the energy that even the AD can provide. This energy gap will be crossed in two stages. First, the AD will be supplemented by a decelerating Radio Frequency Quadrupole (under construction in CERN PS division) to reduce the energy to tens of kilo electron volts. The antiprotons will then be confined in a Penning trap that is being constructed at Tokyo University, cooled to cryogenic temperatures, and reaccelerated to a given electron volt-scale energy.

Finally, the reaccelerated antiprotons will be introduced into low pressure gas targets or jets or ultrathin foils. These experiments should start in 2000, after the first round of experiments (on antiprotonic helium) is complete.

Per Ardua ad ASACUSA

At CERN’s AD Antiproton Decelerator, the ASACUSA collaboration is already preparing to greet the first AD antiprotons with a barrage of laser and microwave beams. ASACUSA stands for Atomic Spectroscopy And Collisions Using Slow Antiprotons, and, as this name implies, the experimenters’ joblist will include studies of the interaction of antiprotons with atoms at super-low energies, both as a means of understanding the formation of antiprotonic atoms, and as a subject in its own right.

Most physicists learn early in their career that it is impossible to find exact solutions for problems with more than two interacting bodies. Unfortunately, nature’s arrangements do not include making life easy for physicists most of the phenomena that they find interesting (including those mentioned above) turn out to involve three bodies or more. Often physicists can avoid this handicap, sometimes by taking advantage of the fact that the masses and/or energies of some bodies may be much larger or smaller than those of other bodies; sometimes by using approximation methods; and sometimes by employing both approaches.

The many-body problem of the interaction of charged particle projectiles, such as protons and antiprotons, with atoms has repeatedly engaged many of the most agile minds of 20th-century physics. If, in such collisions, the incident particle is much heavier than the electrons in the target atom and its encounter with the atom is short- lived enough to be treated as a small perturbation, it will follow a straight, charge-independent, constant-velocity path through the atom and will not be deflected by electric fields.

This approximation, together with a few additional assumptions (for example, that the nucleus is too small a target to play a significant role), leads to the familiar BetheBloch formula for the cumulative energy loss from multiple atomic encounters of charged particles passing through matter of everyday importance in every particle physics experiment.

The “fast and heavy” approximation can at best hold down to projectile velocities about equal to that of the target atom’s electrons: about 25 keV for nucleons approaching hydrogen atoms. At lower energies the charge independence assumption will also be lost, because the projectile stays in the atom long enough to feel the nucleus. Among the more dramatic ultralow-energy effects is that of projectile protons repeatedly capturing and losing electrons.

Is spacetime symmetric?

anti3_5-99 — Ace particle trapper Gerald Gabrielse of Harvard (left) with Alan Kostelecky, organizer of the recent meeting on spacetime symmetries at Indiana, Bloomington.

The synthesis of antihydrogen (a lone positron orbiting a nuclear antiproton) at CERN in 1995 showed that antimatter is not merely a theoretical dream. Later this year, experiments at CERN’s new Antiproton Decelerator (AD) will begin investigating the properties of antihydrogen, their objective being to search for tiny differences in behaviour between matter and antimatter. Any such disparity would have deep implications for our understanding of space and time, as was highlighted at a recent meeting on spacetime symmetries held at Indiana University, Bloomington.

At the microscopic level the universe seems invariant both under CPT (the combination of charge conjugation, C, parity inversion, P and time reversal, T) and relativistic Lorentz transformations (rotations and boosts). However, these symmetries could be violated by effects at the Planck scale, at distances so small (10³³ cm) and energies so high (10¹⁹ GeV) that the gravitational force between two particles becomes comparable to the other forces of physics. Although such effects would be very small, they might be detected in sensitive experiments.

If nature is CPT invariant, the masses of a particle and its antiparticle should be exactly equal. Recent experiments at Fermilab and CERN have established mass equality for the neutral kaon and antikaon to about one part in 1019. This astonishing precision can be compared to measuring the distance between the Earth and the nearest stars (a few light years) to an accuracy of about 1 cm.

Opening the meeting, Bruce Winstein, spokesman for Fermilab’s KTeV experiment, summarized the status of these experiments and the KLOE experiment at Frascati’s DAPHNE collider. An ambitious proposal to improve the current bound by more than an order of magnitude in a dedicated CPT kaon experiment was presented by Gordon Thomson of Rutgers. Measurements constraining CPT violation in the B-meson system to about one part in 1016, recently performed by the OPAL and DELPHI collaborations at CERN using data from LEP, were reviewed by Martin Jimack of CERN.

A general extension of the standard model and quantum electrodynamics that includes CPT and Lorentz violation was presented by meeting organizer Alan Kostelecky of Indiana. This can be employed to identify promising observable signals that arise from a broad class of theories with CPT and Lorentz violation, including those in which Lorentz symmetry is spontaneously broken in an underlying unified theory at the Planck scale. Malcolm Perry of the University of Cambridge reviewed the status of string and M (membrane) theory and described a new mechanism for CPT violation that involves the dilaton field.

One crucial test of spacetime symmetries is to compare the properties of stable particles with those of their antiparticles. This is possible with high-precision measurements made in electromagnetic traps. New results were presented by experimentalist Richard Mittleman from Hans Dehmelt’s group at Washington. An analysis of several months of data from an experiment with single trapped electrons placed a bound of six parts in 10²¹ on a combination of Lorentz- and CPT-violating quantities. Another new bound was reported by Gerald Gabrielse of Harvard, who constrained certain Lorentz-violating quantities to four parts in 10²⁶ by comparing the cyclotron frequencies of an antiproton and a hydrogen ion in an electromagnetic trap. A bold plan for testing spacetime symmetries is to perform spectroscopic measurements on antihydrogen and compare them with those of hydrogen. This requires the production of trapped antihydrogen, soon to begin, employing CERN’s Antiproton Decelerator (AD). Talks at the meeting outlined the goals of the AD’s two key trapped antihydrogen collaborations, ATRAP and ATHENA.

Comparisons between specialized atomic clocks can provide sharp tests of spacetime symmetries. These experiments are, in principle, capable of discerning Lorentz violation at the remarkable level of about one part in 10³¹. Astrophysical observations are interesting too, because small effects could be amplified as light travels over astronomical distances. One possibility is to look for radiowave birefringence on cosmological scales. Roman Jackiw of MIT presented a theoretical study of such effects, while other talks described possible experiments along these lines.

Organized by particle theorist Alan Kostelecky and attended by about 70 physicists from about half a dozen countries, the meeting was the first conference specifically focusing on this topic.

Don’t be afraid of the dark

dark1_5-99 — Dark room – a session of the Second International Conference on Dark Matter in Astro- and Particle Physics in Heidelberg.

The invisible dark matter of the universe weighs heavily on cosmology. However, whatever and wherever this invisible material is, it must be made of something, and the most plausible candidates are relic particles from the early phase of the universe. The search for dark matter, mostly using non-accelerator experiments, has become an established part of particle physics.

These questions were examined when physicists from all over the world met in Heidelberg for the Second International Conference on Dark Matter in Astro- and Particle Physics (DARK98). The goal was to shed light on theoretical backgrounds from particle physics and cosmology, to discuss the results of dark matter detection experiments and to examine future projects.

The most compelling evidence for both baryonic (nuclear) and non-baryonic dark matter comes from observations of the rotation curves of galaxies. In particular, the rotation curves of dwarf spirals are completely dark matter dominated, pointed out Andreas Burkert (Heidelberg). The rotation curve of one of the best measured dwarf spirals can only be fitted to theoretical predictions if both an outer cold dark matter halo and an inner spherical distribution of massive compact baryonic objects (MACHOs) is assumed.

The search for MACHOs in the halo of our own galaxy in the form of planets, white and brown dwarfs or primordial black holes exploits the gravitational microlensing effect the temporary brightening of a background star as an unseen object passes close to the line of sight. For several years a number of groups have been monitoring the brightness of millions of stars in the Magellanic clouds, as Kim Griest (San Diego) and Marc Moniez (Orsay) explained.

MACHOs or WIMPs?

Several candidates have already been detected and if interpreted as dark matter would make up half of the amount needed in the galactic halo. However, no stellar candidate seems to be able to explain the observations. MACHOs could be an exotic form of baryonic matter, like primordial black holes, or they could be located outside the halo of our galaxy.

The leading non-baryonic dark matter candidates are the so-called weakly interacting massive particles (WIMPs). If WIMPs populate the halo of our galaxy, they could be detected directly in laboratory experiments, or indirectly through their annihilation products in the halo the centre of the Sun or Earth.

Blas Cabrera (Stanford) gave an overview of the direct detection experiments. The goal is to look for the elastic scattering of WIMPs off nuclei in a low-background target detector. The Stanford Cold Dark Matter Search (CDMS) experiment, he explained, uses detectors of ultrapure germanium and silicon operated at a temperature of 20 mK. The simultaneous measurement of both ionization and phonon signals allows nuclear recoil events to be differentiated from electron interactions a very effective background suppression method. For the moment, the experiment is located at the Stanford Underground Facility, 10.6 m below ground, but the goal is to operate the detector in the deep Soudan mine in Minnesota.

The DAMA experiment, presented by Rita Bernabei (Rome), is running 115.5 kg of sodium iodide detectors in the Gran Sasso underground laboratory near Rome. Its high statistics open the possibility of looking for WIMPs via a variation in the event rate owing to the movement of the Sun in the galactic halo and the Earth’s rotation around the Sun. The analysis of about 13 kg/yr reveals a positive WIMP annual modulation signal, which meanwhile has been confirmed with higher statistics from 54 kg/yr. However, a further confirmation by DAMA and by other experiments must be awaited.

The Heidelberg group reported on the two most sensitive germanium experiments the HeidelbergMoscow experiment and Heidelberg Dark Matter Search (HDMS) both of which are located in the Gran Sasso Laboratory. The HeidelbergMoscow experiment, which also searches for neutrinoless double beta decay in enriched germanium-76, currently gives the most stringent limits on WIMPnucleon scattering for raw data.

HDMS, a dedicated dark matter experiment, aims to improve this limit by one order of magnitude. Like the HeidelbergMoscow experiment, it looks for a small ionization signal inside a high-purity germanium crystal.

dark2_5-99 — Simulation of the GErmanium in NItrogen Underground Setup (GENIUS) double beta decay and dark matter search using 1 ton more of enriched germanium-76 in liquid nitrogen.

With the expected sensitivity, HDMS will be able to test, like CDMS, the complete DAMA evidence region. The new project of the Heidelberg group, GENIUS, presented by Laura Baudis, aims for a sensitivity that is a thousand times as good as that of present experiments. GENIUS will operate in its dark matter version 40 “naked” germanium crystals (100 kg) in a 12 x 12 m tank of liquid nitrogen. Reaching the target sensitivity, it could test almost the complete parameter space predicted for certain supersymmetric particles, thus deciding whether WIMPs make up the dominant part of our galactic halo.

Terrestrial indirect detection experiments search for high-energetic neutrinos as annihilation products of WIMPs in the centre of the Earth or the Sun. The MACRO experiment in Gran Sasso looks for an excess of neutrino induced upward-going muons, explained Teresa Montaruli (Bari). No WIMP annihilation signal has been found, but the sensitivity of the experiment sets stringent upper limits on the flux of upward-going muons and thus excludes significant portions of the parameter space predicted for the supersymmetric particles.

An alternative indirect signature for dark matter particles would be a distorted spectrum of secondary antiprotons owing to the pair annihilation of neutralinos in the halo. Pierre Salati (Annecy) compared the measured low-energetic antiproton flux by the BESS balloon experiment with theoretical predicted fluxes. While there is some room left for a possible signal of exotic origin, this cannot be seen as evidence for a supersymmetry induced signal, he claimed. To disentangle such a signal from the secondary antiproton flux much more sensitive detectors, like the Alpha Magnetic Spectrometer (AMS), are needed.

Superheavy dark matter

Recently a new class of dark matter candidates superheavy dark matter have emerged. If one gives up the assumption that the particle was in thermal equilibrium in the early universe, explained Edward Kolb (Chicago), then its present abundance is no longer determined by annihilation and much heavier particles the formidable sounding WIMPZILLAs are allowed. There are two necessary conditions for WIMPZILLAs: they must be stable, or at least have a lifetime much greater than the age of the universe; and their interaction rate must be sufficiently weak that thermal equilibrium with the primordial plasma was never obtained. Kolb presented a number of ways in which such a particle could have been created, like gravitational production during the transition between an inflationary and a matter- or radiation-dominated universe, and during the defrosting phase after inflation.

Like the new millennium, dark matter could be just around the corner. The next meeting DARK2000 will take place in Heidelberg. DARK98 was organized by H V Klapdor-Kleingrothaus (with Laura Baudis as scientific secretary) from the Max Planck Institut für Kernphysik, Heidelberg.

CP violation gets clearer

news2_4-99 — Fermilab’s KTeV experiment – contribution to CP violation.

New data from the KTeV experiment at Fermilab blow away some of the fog around the mystery of CP violation and underline the effects suggested by earlier results from CERN. It is now clearly established that CP is violated in the way the six known quarks decay and transform into each other.

In 1956, physicists were shocked to discover that the weak force is sensitive to direction and can differentiate left from right. With theoretical foundations crumbling, physicists proposed a new girder to support their theories: this time the combined CP symmetry mirror that changes particles to antiparticles as it reflects from left to right. In the CP mirror, a right-handed particle reflects as a left-handed antiparticle, and vice versa.

If CP symmetry is good, the neutral kaon should exist in two forms: a longlived one decaying into three pions, and a shortlived one decaying into two pions. In 1964, physicists received another shock when they found that, in the decays of the neutral kaon, CP too is violated. Longlived kaons can decay into two pions.

There are two possible explanations for this CP violation. The longlived kaon could be a mixture of two states that are even and odd under CP reflection. This has long been known to be the case: the even-CP state can decay into two pions and introduce a CP-violating component for the longlived kaon.

The other possibility, called direct CP violation, is that the CP-odd state decays directly into the “forbidden” two pion mode.

What causes the kaon states to mix CP? Using the six known quarks, this can be accommodated in the quark transformations, which subtly rearrange the incoming and outgoing quark configurations, switching a neutral kaon to its antiparticle. However, CP mixing could also be due to kaons transforming into each other via some other mechanism. In this case, direct CP violation CP violation in the decay process would not be possible.

To unravel these two alternatives demands the careful measurement of direct CP violation via the “ratio of ratios”: the ratio of longlived kaons decaying into two neutral pions to those going into two charged pions,
divided by the same ratio for shortlived kaons. If this ratio of ratios turns out to be different from one, this demonstrates that quark transformations are responsible for CP violation.

For several years the two main experiments NA31 at CERN and E731 at Fermilab begged to differ, the former giving the difference of the ratio from unity (divided by a numerical factor) of 2.3 ±0.65 x 10^-3, and the latter giving a much smaller figure, compatible with zero. Physicists held their breath.

Now the KTeV experiment, using 20% of its data collected in 1996 and 1997, comes in at 2.8 ±0.41 x 10^-3, in tune with the earlier CERN figure, but slightly higher. CP would appear to be violated directly in the decay process in such a way that quark mechanisms contribute.

Meanwhile, the big NA48 next-generation CERN study has been collecting data and will be the next to report. Some 35 years after its discovery, CP violation remains a mystery, but at least the mystery is gradually becoming clearer. The new result is good news for new experiments setting out to measure CP violation using B particles, containing the fifth, “beauty”, or “b”, quark, where the levels of CP violation are now expected to be much higher than those using neutral kaons.

Superstrings, black holes and gauge theories

Quantum field theories have had great success in describing elementary particles and their interactions, and a continual objective has been to apply these successful methods to gravity as well.

The natural length scale for quantum gravity to be important is the Planck length, l_p: 1.6 x 10^-33 cm. The corresponding energy scale is the Planck mass, M_p: 1.2 x 10¹⁹ GeV. At this scale the effect of gravity is comparable to that of other forces and is the natural energy for the unification of gravity with other interactions. Evidently this energy is far beyond the reach of present accelerators. Thus, at least in the near future, experimental tests of a unified theory of gravity with the other interactions are bound to be indirect.

When we try to quantize the classical theory of gravity, we encounter short-distance (high-energy) divergences (infinities) that cannot be controlled by the standard renormalization schemes of quantum field theory. These have a physical meaning: they signal that the theory is only valid up to a certain energy scale. Beyond that there is new physics that requires a different description.

Quantum gravity

Such a phenomenon is not unfamiliar. The short-distance divergences of Fermi’s original theory of the weak interactions (with four particles meeting at one space-time point) signal that the description is only valid for energies less than the masses of the W and Z carrier particles. The divergences are resolved by introducing these particles in the Glashow-Salam-Weinberg theory.

We expect that the divergences of quantum gravity would similarly be resolved by introducing the correct short distance description that captures the new physics. Although years of effort have been devoted to finding such a description, only one candidate has emerged to describe the new short-distance physics: superstrings.

quantum1_4-99 — Fig. 1: Particles are the different vibrational modes of a string: either open or closed.

This theory requires radically new thinking. In superstring theory, the graviton (the carrier of the force of gravity) and all other elementary particles are vibrational modes of a string (figure 1). The typical string size is the Planck length, which means that, at the length scales probed by current experiments, the string appears point-like.

The jump from conventional field theories of point-like objects to a theory of one-dimensional objects has striking implications. The vibration spectrum of the string contains a massless spin-2 particle: the graviton. Its long wavelength interactions are described by Einstein’s theory of General Relativity. Thus General Relativity may be viewed as a prediction of string theory!

The quantum theory of strings, using an extended region of space-time, sidesteps short-distance divergences and provides a finite theory of quantum gravity. Besides the graviton, the vibration spectrum of the string contains other excited oscillators that have the properties of other gauge particles, the carriers of the various forces. This makes the theory a promising (and so far the only) candidate for a unification of all of the particle interactions with gravity.

In the absence of direct experimental data to confront string theory, research in this field is largely guided by the requirement for the internal consistency of the theory. This turns out to dictate very stringent constraints. Again, this is not unfamiliar to particle physicists. When resolving the short-distance divergences of Fermi weak interaction theory, the space-time and internal symmetries provide stringent constraints and guide us to the solution.

Superstring theories

Two important features of string theory are implied by the consistency requirement. First, superstring theory is consistent only in 9+1 space-time dimensions. This seems to contradict the fact that the world that we see has 3+1 space-time dimensions. Second, there are five consistent superstring theories: Type IIA, Type IIB, Type I, E₈x E₈ Heterotic and S0(32) Heterotic. Type I is a theory of unoriented open and closed strings; the others are theories of oriented closed strings. Which should be preferred?

All five possess supersymmetry, hence the name superstrings. According to supersymmetry theory, any boson (integer spin particle) has a fermionic (half-integer) superpartner and vice versa. One way to view supersymmetric field theories is as field theories in superspace a space with extra fermionic quantum dimensions. Many physicists expect that supersymmetry exists at the tera electron volt scale, so that the new ‘superpartner’ particles could be seen by CERN’s LHC proton collider.

Compactification and stringy geometry

The time and the three spatial dimensions that we see are approximately flat and infinite. They are also expanding. Just after the Big Bang they were highly curved and small. It is possible that while these four dimensions expanded, other dimensions did not expand, remaining small and highly curved.

Superstring theory says that we live in 9+1 space-time dimensions, six of which are small and compact, while the time and three spatial dimensions have expanded and are infinite.

How can we see these extra six dimensions? As long as our experiments cannot reach the energies needed to probe such small distances, the world will look to us 3+1-dimensional and the extra dimensions can be probed only indirectly via their effect on 3+1-dimensional physics. It is not known at what energies the hidden compact dimensions will open up. The possibility that new dimensions or strings will be seen by the LHC is not ruled out by the current experimental data.

Superstring theory employs the KaluzaKlein mechanism to unify gravitation and gauge interactions using higher dimensions. In such theories the higher dimensional graviton field appears as a graviton, photon or scalar in the 3+1-dimensional world, depending on whether its spin is aligned along the infinite or compact dimensions.

The number of consistent compactifications of the extra six co-ordinates is large. Which compactification to choose is the prize question. The answer is hidden in the dynamics of superstring theory. Using a limited (perturbative) framework, one can attempt a qualitative study of the 3+1-dimensional phenomenology obtained from different compactifications. It is encouraging that some of these compactifications result in 3+1 dimensional models that have qualitative features such as gauge groups and matter representations of plausible grand unification models. Interestingly the low-mass fermions appear in families, the number of which is determined by the topology of the compact space.

T-duality

Not all of the compactifications are distinguishable. To illustrate this, take one spatial co-ordinate to be a circle of radius R. There are two types of excitations. The first, which is familiar from theories of point-like objects, results from the quantization of momentum along the circle. These are called KaluzaKlein excitations. The second type arises from the closed string winding around the circle. These are called winding mode excitations. This is a new feature that does not exist in point-like theories. When we map the size of the radius, R, of the circle to its inverse, 1/R, with the string scale set to one, the two types of excitations are exchanged and the theory remains invariant. There is no way to distinguish the compactification on a circle of radius R from a compactification on a circle of radius 1/R. This means that the classical geometrical concepts break down at short distances and the classical geometry is replaced by stringy geometry.

In physical terms, this implies a modification of the well known uncertainty principle. The spatial resolution, D_x , has a lower bound dictated not just by the inverse of the momentum spread, D_p, but also by the string size. The mapping of the radius of the compactification to its inverse, exchanging KaluzaKlein excitations with winding modes, is called T-duality.

Another example of stringy geometry is “mirror symmetry”. In the previous example, both circles had the same topology and different sizes. In contrast, mirror symmetry is an example of stringy geometry where two six dimensional spaces, called Calabi-Yau manifolds, with different topology are not distinguishable by the string probe.

Probing the pomeron

Particle physics experiments ultimately depend on the way pairs of particles scatter off each other. A simple example of scattering is a game of billiards, in which the cue ball scatters off the other balls in carefully chosen directions. In physics language, billiards is an “elastic scattering” the balls simply bounce off each other. While in billiards the balls do not shatter, in elementary particle scattering the objective is usually to create havoc, the balls knocking bits off each other or even disintegrating totally, creating new balls in a complicated game of snooker that is played under the rules of relativity and quantum mechanics.

scatter1_4-99 — Fig. 1: The exchange of an invisible light quantum (photon) controls the electromagnetic interaction between two electrons.

The interaction of ordinary billiard balls can be understood by their elasticity. After an initial compression as they collide, two balls subsequently spring back into shape and recoil away. For the more complex process of particle interactions, physicists understand the interaction via invisible particles transferring momentum from one visible particle to another as they pass each other. Such an “exchange process” is illustrated in figure 1, which shows the electromagnetic interaction between two electrons mediated by the exchange of an invisible light quantum or “photon”.

For the high-energy scattering of particles that feel the strong force, such as protons, the same physical concept applies, but now the exchange is made by whole families of related particles. One such is the rho family, named after its lightest member, the spin-1 rho meson, which comprises the rho and its cousin spin-3, spin-5, …. recurrences at higher mass.

scatter2_4-99 — Fig. 2: The Regge trajectory simulates the collective effect of exchanging all members of a family of particles. When t is negative it is the square of the momentum transferred in the exchange. Positive t is the squared mass of the physical particles of spin 1, 3, 5,…

Regge theory (after the Italian physicist Tullio Regge) gives the collective effect of the exchange of all of the members of such a family in terms of a Regge trajectory a(t), which is described mathematically as a function of the invariant momentum transfer, t. The rho trajectory is shown in figure 2 for the “time-like” region of positive t corresponding to the real physical particles of spin-1, spin-3, spin-5, etc, and in the “space-like” region of negative t relevant for the exchange in a scattering process. The trajectory is well described by a straight line: a(t) = a0 + a‘t = 0.55 + 0.86t. This is an experimental result: it cannot be predicted by Regge theory. It turns out that the trajectories of all of the dominant meson-exchange families lie close to this line they are almost degenerate.

Regge theory

Regge theory tells us that the energy dependence of the sum of everything that happens the “total cross-section” is dependent on the value of the trajectory at t = 0. Specifically, it is given by the square of the collision energy, s, raised to the power a₀-1. As all of the meson trajectories are nearly degenerate and have a₀ = 0.55, the energy dependence is s^-0.45, roughly as the inverse of the square root of s, so the prediction of Regge theory for meson exchange is that the cross-section should decrease with increasing energy and do so at a well defined rate.

scatter3_4-99 — Fig. 3: Energy behaviour of protonproton and protonantiproton (upper curve) scattering. If the increase is presumed to be due to the exchange of a “pomeron”, then physicists have to discover what the pomeron is.

While such behaviour is indeed seen at collision energies below about 15 GeV, at higher energies the total cross-section at first flattens out and then begins to rise slowly, an effect first hinted at in cosmic-ray data, demonstrated unambiguously for the first time at the CERN ISR Intersecting Storage Rings and the rising trend confirmed at the CERN’s protonantiproton collider and at Fermilab’s Tevatron. The cross-section for protonproton and protonantiproton scattering is shown in figure 3. The simplest assumption to make is that this rising cross-section is due to the exchange of another Regge trajectory and so also gives a simple power of s. To produce a rising cross-section it must be such that a₀ = 1 + e with e positive. This is the phenomenological or “soft” pomeron, named after the Russian physicist Igor Pomeranchuk.

The parameter e is universal, independent of the particles being scattered. Fitting the total cross-sections for protonproton, protonantiproton, positive and negative pionproton, positive and negative kaonproton and photonproton scattering over the centre of-mass energy range 10 GeV to 1.8 TeV gives e = 0.095. Extrapolation to cosmic-ray energies of 30 TeV shows no deviation from the fit. However, the data errors are much greater than those at current accelerator energies and we will need to await the arrival of the LHC to provide the precise data necessary to test the theory fully at these very high energies.

Physical particles have quarks as their constituents: three quarks for the proton and antiproton, plus a quark and an antiquark for the mesons. One view of high-energy particle scattering is that the pomeron interacts with these valence quarks. In the fits to the total cross-sections the ratios of the strengths of pomeron exchange in pionproton and protonproton or protonantiproton scattering is almost exactly 2/3. This can be taken as evidence that the pomeron does indeed couple to single valence quarks in a hadron the “additive quark rule”.

An alternative viewpoint is that the pomeron interacts both with the valence quarks and with the gluons that bind the quarks together to form the physical particles, and the cross-section ratios simply reflect the different sizes of these particles. This issue is still unresolved.

Symmetry violation in a new setting

The CDF experiment at Fermilab’s Tevatron protonantiproton collider has produced heroic new evidence for the violation of “CP” symmetry, a hypothetical operation which takes a particle into a mirror image of its antiparticle.

Although a tiny effect in particle physics, CP violation could be the cause of the matterantimatter asymmetry of the universe a Big Bang which presumably produced equal amounts of matter and antimatter resulted in a universe populated entirely by matter.

So far, the only quantitative evidence for this subtle violation has been in the decays of neutral kaons, at a few per mil. CDF now sees tentative evidence for CP violation in the decays of neutral B mesons, in which the strange quark of the neutral kaon is replaced by a heavy b-quark. CDF looks at B decays producing a J/psi and a short lived neutral kaon.

The vital parameter is measured to be 0.79±0.44, a non-zero value indicating CP violation. While CP violation is not completely understood and therefore cannot be predicted from scratch, the measured value is in line with expectations based on the interconsistency of many Standard Model measurements. Several months ago, CDF published an analysis based on a subset of its B decay sample. The new result uses its full available sample.

With a new generation of electronpositron colliders hoping to open up this area of B physics, we plan to publish more on these initial BCP violation pointers soon.

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