Physics Archives – Page 115 of 145

Pomerons return to Blois

Twenty years ago the first “Blois Workshop” was organized by Basarab Nicolescu of the University Paris VI and Jean Tran Thanh Van of the University Paris Sud in the historic Château de Blois. Now, the 11th conference in this international biannual series focusing on elastic scattering and diffraction returned to Blois on 15-21 May 2005. Organized by the original team plus Maurice Haguenauer of the Ecole Polytechnique, it set the scene for future high-energy studies in this field.

CCEblo1_10-05 — The spiral staircase of the Château de Blois was an attractive motif for the meeting. (Drawing by T van Vlijmen; reproduced with permission.)

Blois Workshops have taken place all over the world, and the meeting is now a scientific forum for researchers trying to unravel the foundations of elastic scattering and diffraction from first principles in quantum chromodynamics (QCD). Originally a rather specialized field, this has moved towards the centre of high-energy QCD studies. This is particularly because measurements at HERA and Fermilab show that diffractive events, where a scattered proton remains intact in a high-energy inelastic collision, constitute a surprisingly high proportion of the entire rate. Recent measurements from the ZEUS and H1 detectors at HERA show that approximately 10% of the deep inelastic lepton-proton scattering cross-section is diffractive.

High-energy elastic and diffractive scattering is traditionally explained in Regge theory as a result of pomeron exchange. Here the system exchanged between projectile and target carries the quantum numbers of the vacuum. Events with large gaps in the rapidity distribution occur even in hard collisions involving very high momentum transfers. Such “hard diffraction” has now become firmly established, initially at the Intersecting Storage Rings (ISR) and Super Proton Synchrotron (SPS) at CERN in proton and antiproton collisions, and now most clearly at HERA in positron-proton collisions, and in proton-proton collisions at the Relativistic Heavy Ion Collider (RHIC) at Brookhaven and the Tevatron at Fermilab.

With the advent of QCD, hard diffraction became attributed to the exchange of two or more gluons with net zero colour, and these processes are now an important observable for understanding fundamental aspects of the strong interactions. At the conference, Peter Landshoff from Cambridge and Sandy Donnachie from Manchester reviewed the apparent dichotomy between the soft and hard aspects of pomeron exchange and the phenomenological manifestations, such as the remarkable growth with energy of the cross-section for hard diffractive electroproduction of vector mesons. Theorists are beginning to develop formalisms that encompass the transition between hadron and quark-gluon degrees of freedom and the duality between the two descriptions.

QCD also predicts the existence of a C-odd three-gluon exchange, which through interference with the pomeron exchange leads to remarkable charge asymmetries in diffractive reactions.

CCEblo2_10-05 — Participants at the 11th meeting included many of those present at the first Blois Workshop held 20 years ago.

One interesting nuclear diffractive phenomenon is the demonstration of QCD “colour transparency” by the E791 fixed-target experiment at Fermilab, which measured the diffractive dissociation of a high-energy 500 GeV/c pion into two high-transverse-momentum jets while leaving the target nucleus intact. The experiment confirmed the remarkable prediction, based on the gauge interactions of QCD, that the small quark-antiquark Fock component of the pion projectile interacts coherently on every nucleon in the nucleus without absorption or energy loss, in dramatic contrast with traditional Glauber theory. The diffractive dijet experiment also provides crucial information on the quark-antiquark wavefunction of the pion. Other diffractive experiments that explore the structure of the photon are now in progress at HERA.

Contrary to parton-model expectations, the rescattering of the quarks with the spectator constituents shortly after the nucleon has been struck by the lepton critically affects the final state in deep inelastic scattering (DIS). The rescattering of the struck quark from gluon exchange generates dominantly imaginary diffractive amplitudes. This gives rise to a dijet effective “hard pomeron” exchange – a rapidity gap between the target and the diffractive system – while leaving the target intact. The diffractive cross-section measured in diffractive deep inelastic scattering can be interpreted in terms of the quark and gluon constituents of an effective pomeron as in the model by Gunnar Ingelman and Peter Schlein. Since the gluon exchange occurs after the interaction of the lepton current, the pomeron cannot be considered a pre-existing constituent of the target proton. The rescattering contributions to the DIS structure functions are not included in the target proton’s wavefunctions computed in isolation, and cannot be interpreted as parton probabilities. The resulting gluon exchange matches closely the phenomenology of the soft colour interaction model. Gluon exchange in the final state also leads to Bjorken-scaling the Sivers single-spin asymmetry, a T-odd correlation between the spin of the target proton and the production plane of a produced hadron or quark jet.

The connections between diffraction and coherent effects in nuclei such as shadowing and anti-shadowing are also now being understood. Diffractive deep inelastic scattering on a nucleon leads to nuclear shadowing at leading twist as a result of the destructive interference of multistep processes within the nucleus. In addition, multistep processes involving Reggeon exchange lead to anti-shadowing. In fact, since Reggeon couplings are flavour-specific, anti-shadowing is predicted to be non-universal, depending on the type of current and even the polarization of the probes in nuclear DIS.

Saturation under focus

A central focus of the 2005 Blois conference was the physics of “saturation”, a QCD phenomenon that limits particle production when the underlying gluonic scattering subprocesses significantly overlap in space and time. At very high energies, the gluon density is so high that two scatterings have the same probability as one. The theory of saturation is based on the Balitsky-Kovchegov equation and its extensions, and has analogues with stochastic methods used in other areas of statistical physics. The effects of saturation can be observed in the small-x, high-energy domain of deep inelastic lepton scattering at HERA, thus providing a window into nonlinear aspects of QCD.

The theory of saturation predicts a parameterization of the HERA data (“geometrical scaling”) that gives a remarkably good description of the deep inelastic structure functions at small-x in terms of a single scaling variable. The high occupation number of gluons can even lead to the formation of a “colour glass condensate”, which may be causing a decrease of particle creation at forward rapidities in heavy-ion collisions at RHIC.

The nonlinear gluon interactions of QCD also underlie the physics of the hard Balitsky-Fadin-Lipatov-Kuraev pomeron, which is postulated to control the energy dependence of hard reactions, as well as the distribution of particle production at very small values of x and extreme values of rapidity.

Remarkably, Juan Maldacena’s “anti-de-Sitter-/conformal-field-theory (AdS/CFT) duality” between conformal gauge field theory and string theory in 10 dimensions has begun to make an impact on QCD studies. The mapping of quark and gluon physics onto the fifth dimension of anti-de Sitter space is providing insight into the gluonium spectrum that controls the pomeron trajectory and light-quark hadron spectroscopy. Moreover, it explains the success of QCD counting rules for hard elastic scattering reactions using the methods of Joseph Pochinsky and Matt Strassler.

The AdS/CFT correspondence also explains the dominance of the quark interchange mechanism in hard exclusive reactions, and gives a model for the basic light-front wavefunctions of hadrons that incorporates conformal scaling at short distance and colour confinement at large distances. Lattice gauge theory is also making an impact on the QCD physics of high-energy collisions.

Much of the phenomenological work in pomeron physics was pioneered by loyal participants in the Blois conference series. Many of the original participants of the first Blois Workshop attended the 20th anniversary conference and presented their current work. While the discussions at the first Blois Workshop centred on results from the ISR at CERN and the first data from the SPS as a proton-antiproton collider (with predictions for the Tevatron and Superconducting Super Collider projects), the XIth International Conference had speakers reporting the latest experimental results from the Tevatron, from polarized proton-proton (and proton-carbon) experiments using fixed and jet targets at RHIC, and from HERA at DESY. HERA II running has started and a significant and welcome statistical increase in diffraction data is expected before the machine closes down in 2007.

The experimental efforts under way regarding forward physics at the Large Hadron Collider (LHC) at CERN were extensively discussed; the large increase in reach in x and Q² in proton-proton and nucleus-nucleus collisions will provide significant tests and advances for QCD-inspired diffraction phenomenology and calculations. There was also speculation that the Froissart bound for the total proton-proton cross-section will be saturated at the LHC, with its value controlled not by pion exchange but by the exchange of the lightest glueball, as originally predicted by Nicolescu. An exciting possibility for the future is observing the Higgs boson in doubly tagged diffractive collisions pp → p + H + p at the LHC; the Higgs would be found as a peak in the missing mass spectrum, rather than in a specific decay channel.

A number of presentations addressed existing and new cosmic-ray detector arrays, where the discrepancy above the Greisen-Zatsepin-Kuzmin cut-off between the excess in the Akena Giant Air Shower Array (AGASA) and the fall-off in the High Resolution Fly’s Eye (HiRes) experiment continues to generate interest. Some proposals for very forward instrumentation at the LHC are dedicated to providing benchmarks for simulations of cosmic-ray shower initiation: the Centauro and Strange Object Research (CASTOR) detector in TOTEM/CMS for forward electromagnetic showers, and in the proposed LHCf for the measurement of forward π⁰s. Zero-degree neutron calorimeters, which are operating successfully in the experiments at RHIC, will have counterparts in the heavy-ion programme at the LHC, but would also be very useful and complementary tools in the measurement of diffraction in proton-proton collisions.

The diffractive production of vector mesons and single hard photons at HERA provides a way to select and determine different generalized parton distributions (GPDs), the quantum-mechanical parton wavefunctions, of the proton. Early results from HERMES and from the H1 and ZEUS experiments seem to be in close agreement with current GPD models. A number of talks discussed the physics of other exclusive diffractive reactions such as two-photon collisions, which are sensitive to the vector meson distribution amplitudes as well as the exchange mechanism. Double-charm production, an indication of an intrinsic heavy-charm component in the proton wavefunction, was demonstrated by the SELEX experiment at Fermilab. The production of pentaquarks and other exotic quark bound states was reviewed with the conclusion that the situation is still very confused, with seemingly contradictory results.

At this anniversary, a number of overviews were presented surveying the progress in diffractive physics, both experimentally and theoretically, made over the past 20 years. Alan Krisch of Michigan told the story of his pioneering measurements of hadronic spin effects, and the extraordinarily large spin correlations that were discovered in large-angle elastic proton-proton scattering and that are still only partially understood. Konstantin Goulianos of Rockefeller University gave an overview of diffraction, both soft and hard, as measured at the Tevatron and its connection to results from HERA. Gunnar Ingelman of Uppsala presented the enormous evolution in understanding of hard diffraction since it was first observed by UA8 at CERN’s proton-antiproton collider.

The overall atmosphere of the conference was one of expectation: for the data from the new runs that have started at the Tevatron and at HERA II, and from the LHC which will start running around the time of the next Blois conference. Indeed, the next two meetings may well see significant progress in the understanding of both soft and hard diffraction.

KamLAND detects geoneutrinos

The Kamioka liquid-scintillator antineutrino detector (KamLAND) has made the first observation of “geoneutrinos”. This comes just over 50 years since George Gamow, in a letter to Fred Reines in 1953, pointed out the possibility of detecting antineutrinos of terrestrial origin. KamLAND, which has already confirmed neutrino oscillations by detecting antineutrinos emitted from nuclear reactors, has opened up a new window of research, exploring the deep interior of the Earth by detecting geoneutrinos.

CCEnew2_09-05 — A view of the top of the KamLAND detector. Image: Berkeley KamLAND group.

Geoneutrinos are created in the beta decays of radioactive isotopes in the Earth. The current geochemical and geophysical models suggest that the radiogenic power from the ²³⁸U and ²³²Th decay chains is 16 TW, approximately half the total measured heat-dissipation rate from the Earth. This heat helps to drive convective flows in the mantle and the outer core, resulting in plate tectonics, volcanism and terrestrial magnetism. Thus radiogenic heat is a key factor in understanding the Earth’s dynamics, formation and evolution. However, since geophysicists have never had a direct way to determine how uranium and thorium are distributed in the Earth’s interior, measuring their concentration inside the Earth sheds new light on geophysics.

CCEnew3_09-05 — Left: Electron antineutrino energy spectrum of 152 candidate events with energies less than 3.4 MeV. The lower solid line shows the total expected background spectrum including the reactor (dashed blue line), ¹³C(α, n)¹²C reactions (thin dotted brown line) and accidental coincidence (dot-dashed purple line) events. Combining the expected geoneutrino signals of ²³⁸U (dot-dashed red line) and ²³²Th (dotted green line), gives the total expected spectrum shown by the upper solid line. Right: Δχ² as a function of the total number of ²³⁸U and ²³²Th geoneutrino candidates, assuming a Th:U mass ratio of 3.9. The band gives the value predicted by the current geological model of the Earth.

KamLAND consists of about 1 kt of liquid scintillator, located in the Kamioka mine in Japan. It can detect geoneutrinos from the ²³⁸U and ²³²Th decay chains through an inverse beta-decay process with a threshold energy of 1.8 MeV. Using data collected between 9 March 2002 and 30 October 2004 with a detector live-time of 749 days, 25 geoneutrino events were obtained after subtracting the number of expected background events, mostly from reactor antineutrinos. Combining the event rate and energy spectrum of candidates yields between 4.5 and 54.2 geoneutrinos, with a central value of 28 at the 90% confidence interval (see figures). This assumes a Th:U mass ratio of 3.9, the value derived from chondritic meteorites and commonly understood to be the same for all materials in the solar system.

The result is consistent with the central value of 19 predicted by a geological model, and constrains the flux of geoneutrinos from uranium and thorium to less than 1.62 × 107 cm⁻² s^−-1 at 99% confidence limits. Although the present data have limited statistical power, they nevertheless directly provide an upper limit of 60 TW for the radiogenic power of uranium and thorium in the Earth.

These investigations should pave the way to more accurate measurements, which may develop into a new field of neutrino geophysics. There is a programme currently under way to reduce the radioactive content of the KamLAND detector, but further background reduction will require a new detector location, far away from nuclear reactors. In the future, a worldwide network of geoneutrino detectors would allow the production of a tomographic image of the radiogenic heat distribution.

Belle observes b → d

The Belle collaboration, with a detector operating at the KEKB facility, has recently reported that they have observed the rare b → d transitions. After analyzing 386 million B meson pairs, they have identified 35 events in which the B meson decays into either a ρ or an ω meson with an accompanying photon, implying a branching fraction

Br(B → (ρ, ω)γ) = 1.34 + 0.34 − 0.31 (stat) + 0.14 − 0.10 (syst) × 10⁻⁶ with a significance of 5.5σ. From this they derive the ratio of CKM matrix elements |V_td⁄V_ts| = 0.200 + 0.026 − 0.025 (exp) + 0.038 − 0.029 (theo). This is the first time that such B decays have been observed, due to the low branching fraction.

Belle has also reported evidence for signals in B⁺ → K⁰bar K⁺ and B⁰ → ⁰K⁰bar with significances of 3.0σ and 3.5σ respectively, albeit from a sample of 275 million BBbar pairs. These decay modes are examples of hadronic b → d transitions. The corresponding branching fractions are measured to be Br(B → K⁰bar K⁺) = 1.0 ± 0.4 ± 0.1 × 10⁻⁶ and Br(B⁰ → K⁰K⁰bar) = 0.8 ± 0.3 ± 0.1 × 10⁻⁶.

Slow antiprotons galore

In the 50 years since Owen Chamberlain, Emilio Segrè, Clyde Wiegand and Tom Ypsilantis discovered the antiproton in October 1955, an extremely diverse range of research topics has developed that involve antiproton beams with kinetic energies of order kilo-electron-volts or less. This was the subject of the Workshop on Physics with Ultra Slow Antiproton Beams, held 14-16 March 2005 at RIKEN, Japan.

CCEslo1_09-05 — Participants enjoy an excellent buffet during the workshop.

The workshop was motivated by the recent progress in manipulating large numbers of ultra-slow antiprotons that has been made by the antihydrogen and antiprotonic-helium collaborations working at CERN’s Antiproton Decelerator (AD). The latest of these developments was in summer 2004. That was when the Monoenergetic Ultra-Slow Antiproton Source for High-Precision Investigations (MUSASHI) group of the ASACUSA collaboration first slowed the 5.3 MeV pulsed AD beam in a radio-frequency quadrupole decelerator (RFQD) to some tens of kilo-electron-volts, then confined and cooled more than 1 million antiprotons in a large multi-ring Penning trap. The trapping efficiency of about 4% is approximately 100 times higher than any previously achieved. The group also succeeded in extracting antiprotons from the trap as an ultra-slow DC beam of 10-500 eV. The fact that this unique beam can, in principle, be transported for some distance without serious loss makes beam sharing for a variety of experiments a real possibility.

Although the workshop was announced only two months beforehand, it attracted some 70 participants from all the related fields, and covered subjects ranging from fundamental questions about charge-parity-time-reversal (CPT) symmetry and gravitation, to the structure of exotic nuclei, atomic collisions and atomic physics. This report relates just a few of these topics; a full account will soon be published in the Proceedings series of the American Institute of Physics.

The early days of antiproton research were reviewed by John Eades of the University of Tokyo. Eades turned back the pages of scientific history in a talk entitled “The Antiproton and How It Was Discovered”, quoting the thoughts and opinions of some of the main participants, made both at the time and in retrospect. He underlined the initial doubts and inconsistencies that surrounded Paul Dirac’s relativistic-wave equation of 1930, and its final triumph as the positron, antiproton and other antiparticles were discovered.

Klaus Jungmann of the Kernfysisch Versneller Instituut (KVI), Groningen, gave a comprehensive overview of the current status of low-energy antiprotons and other exotic particles, and the experimental opportunities they offer as windows on fundamental forces and symmetries in nature. On the theoretical side, Ralf Lehnert of Vanderbilt University pointed to the large gap that will remain in our understanding of nature at the smallest scales until a consistent quantum theory is developed that underlies both the Standard Model and general relativity. He discussed the so-called Standard Model Extension (SME) as a theoretical framework that may bridge this gap, and which incorporates all Lorentz- and CPT-violating corrections compatible with key principles of physics . The SME predicts diurnal variations in spectroscopic measurements of matter and antimatter atoms, and could therefore be a guiding principle in designing future antihydrogen experiments.

Antihydrogen atoms and antihydrogen ions

The past three years have seen important progress by both the ATHENA and the Antihydrogen Trap (ATRAP) collaborations in synthesizing and experimenting with antihydrogen atoms at the AD. Some of the main results concern the accumulation of large numbers of positrons and antiprotons in “nested” Penning traps of various geometrical designs, leading to the observation of high formation rates for antihydrogen atoms. An unexpected consequence is that these antihydrogen atoms seem to be created before their constituent antiprotons have been fully cooled, with the result that they are themselves too hot to be easily stored and manipulated with existing techniques. Moreover, they are primarily formed in highly excited Rydberg states, while it is the ground and first-excited states that are of most interest for testing CPT invariance.

These obstacles to preparing usable antihydrogen atoms for physics experiments demand new ideas in trap design, going beyond the configuration of the nested electrostatic potential well used so far. Thus, Jeff Hangst of Aarhus described the present status of the high-field-gradient magnetic multipole trap proposed by the newly formed Antihydrogen Laser Physics Apparatus (ALPHA) collaboration; and Dieter Grzonka of Jülich reported on tests made on long-term electron storage in the ATRAP collaboration’s quadrupole magnet, which has a more moderate field gradient.

The storage of neutral atoms of antihydrogen requires the presence of magnetic field gradients to drive the so-called low-field-seeking atomic-spin states towards field minima, and will be essential to carry out high-precision antihydrogen spectroscopy. Since it appears that the atoms are produced in highly excited Rydberg states, they must be stored for long enough to allow them to relax to the ground state. Discussions at the workshop centred on various multipole and quadrupole trap designs that are likely to be useful in preparing such ground-state antihydrogen atoms.

Further new designs involve the so-called “cusp trap”, consisting of a potential well formed by two oppositely directed Helmholtz-coil fields, and a high-Q RF trap resonating at two frequencies, which can store positively and negatively charged particles simultaneously.

Ryugo Hayano of the University of Tokyo summarized both the present status of precision spectroscopy of antiprotonic helium and the development of the two-frequency RF trap for antihydrogen synthesis. In the latter, positrons and antiprotons may recombine within a volume of around 1 mm³, and thus form a source for an antihydrogen atomic beam. Sextupole magnets installed in such a beam could select and analyse specific antihydrogen spin alignments to measure the hyperfine structure of the antihydrogen ground state, much as was done with ordinary hydrogen atoms several decades ago.

Because of their larger mass, muons probe CPT-violation effects at a distance 200 time closer to the antiproton nucleus than positrons and electrons do.

Akihiro Mohri of RIKEN, Japan, showed that stable long-term storage of an electron plasma has been achieved at finite temperature in a cusp trap and that this can also trap synthesized antihydrogen atoms in low-field-seeking states. When the temperature of antihydrogen atoms and the magnetic field of the cusp trap are properly set, antihydrogen atoms in the ground state are selectively guided and focused along the magnetic axis, enabling an intensity-enhanced spin-polarized antihydrogen beam to be prepared.

A new path towards gravitational experiments with antihydrogen was proposed by Patrice Perez of CEA/Saclay, who discussed synthesis of antihydrogen ions (Hbar⁺). These could be formed via two-step reactions (pbar →Hbar →Hbar ⁺) when a 13 keV antiproton beam passes through a dense cloud of positronium atoms. The resulting Hbar⁺ ions would then be trapped, sympathetically cooled with laser-cooled alkali-earth ions, and finally ionized to the neutral state by a laser-detachment process to create the ultra-cold Hbar atoms necessary for detecting the extremely weak gravitational interaction.

Kanetada Nagamine of KEK proposed studying muonic antihydrogen (μ⁺pbar), the antimatter equivalent of muonic hydrogen (μ^–p), as an alternative to antihydrogen. The advantage of comparisons between μ^–p and μ⁺pbar is that because of their larger mass, muons probe CPT-violation effects at a distance 200 times closer to the antiproton nucleus than positrons and electrons do.

Further studies

Collision dynamics with antiprotons is also a potentially important subject, in which the antiproton behaves like a heavy electron. Although the Coulomb force is understood, its collision dynamics are not well known when more than three particles are involved. A familiar, puzzling example is the double ionization of helium by fast antiprotons, the cross-section for which is about twice as large as that for protons having the same velocity. Almost 20 years have passed since this observation, but it is not yet fully understood theoretically. This contrasts with the case of bound systems such as antiprotonic helium (pbarHe⁺⁺), where the observed transition levels have been theoretically accounted for at the level of one part per billion.

Joachim Ullrich of the Max Planck Institute, Heidelberg, discussed the importance of studying collision dynamics with antiproton energies in the range of 100 keV for which the time required to traverse atoms or molecules is of the order of 100 attoseconds (as). Since this is comparable to the orbital period of outer-shell electrons in atoms or molecules, crucial information on collision dynamics involving electron-electron correlation can be extracted.

Antiprotonic atoms have long been used to probe neutron density distributions in stable nuclei through studies of antiprotonic X-ray spectra, radiochemistry of the residual nuclei, and the charged pions emitted when the antiprotons annihilate. An antiproton captured in an electronic orbit de-excites to successively lower atomic levels until its overlap with the nucleus becomes appreciable. At this point annihilation takes place with a proton or a neutron near the “surface” of the nucleus (atomic number A), the actual charge state being identifiable from the charge balance of emerging pions; a nucleus of atomic number A-1 results.

Michiharu Wada of RIKEN proposed extending the pion-detection method by storing antiprotons and unstable nuclei in a nested trap. The charge-balance method can be applied to various nuclei including those for which the A-1 nuclei have no bound states. Slawomir Wycech of the Soltan Institute, Warsaw, emphasized that all these measurements test neutron density distributions in different regions of nuclei and yield complementary information on the rms and higher moments of density profiles as low as 0.001 of the central neutron density.

Looking to future antiproton facilities Paul Kienle of the Technischen Universität München discussed the possibility of an antiproton-ion collider at GSI’s Facility for Antiproton and Ion Research (FAIR), with energies of 30 MeV and 740 AMeV for protons and ions respectively. Cross-sections for antiproton absorption on protons and neutrons would be measured by detecting residual nuclei with A-1, using Schottky and recoil detectors respectively. This would permit rms radii for protons and neutrons to be determined separately in stable and short-lived nuclei by means of antiproton absorption at medium energies. A general discussion around the subject of ultra-slow antiproton physics ended this extremely fruitful workshop.

Antiproton physics takes another big LEAP forward

The biennial International Conference on Low Energy Antiproton Physics, LEAP-05, took place in May at the Gustav-Stresemann-Institute in Bonn. Organized by the Jülich Research Centre, it brought together about 150 physicists, including experienced and active users of the former Low Energy Antiproton Ring (LEAR) at CERN and the existing Antiproton Decelerator (AD), as well as potential users of the future Facility of Antiproton and Ion Research (FAIR) at the Gesellschaft für Schwerionenforschung (GSI). The meeting enabled researchers who are interested in using the exciting tool of antiprotons to exchange knowledge about the physics and techniques. The programme covered the whole field of research with antiprotons, from atomic physics at low energies to hadronic reactions at high energies. The conference showed that the field is evolving, with new physics being studied at existing and planned facilities.

CCElea1_09-05 — The Wolfgang Paul Lecture Hall, packed long before the public lectures began. Image: Dieter Grzonka and Tomek Rozek.

Antiprotons at work

The AD began operating in 2000 and its experiments have reported spectacular production rates of antihydrogen atoms as well as topical observations of antiprotonic helium atoms. Though limited to low-energy antiproton research with only a pulsed extracted beam, the AD is regarded as the successor of LEAR after it closed down at the end of 1996, together with the Antiproton Accumulator (AA) and Antiproton Collector (AC). The AC machine was modified to become the AD – a decelerator to slow down the antiproton beam from a momentum of 3.57 GeV/c to 100 MeV/c. During deceleration, the beam undergoes stochastic and electron cooling. The extracted beam intensity is about 3 × 10⁷ antiprotons in a pulse of 90 ns, repeated every 86 s.

CCElea2_09-05 — Emiko Hiyama, from Nara Women University, Japan, talking about three- and four-body structures of S = −2 hypernuclei.Image: Dieter Grzonka and Tomek Rozek.

The AD delivers antiprotons only at the lowest energy that was available at LEAR, i.e. 5 MeV. For one experiment – ASACUSA – the antiproton beam is further slowed down to about 60 keV with a radio-frequency quadrupole decelerator (RFQD). A possible additional decelerator ring, ELENA, to serve all experiments, would have a cooled beam and would bring a major improvement if installed.

CCElea3_09-05 — Karting at the Michael Schumacher Centre made a change from antiproton deliberations for participants at LEAP-05. Image: Dieter Grzonka and Tomek Rozek.

Plans are being realized for a new antiproton facility at GSI, where antiprotons with energy high enough for physics with strange and, especially, charm mesons will be available, in addition to very-low-energy antiprotons. An accelerator complex for research with both ion and antiproton beams is planned. This would provide an outstanding new experimental facility for studying matter at the level of atoms, protons and neutrons and their sub-nuclear constituents: quarks and gluons.

Research on fundamental symmetries was a very important part of the scientific programme at LEAP-05. Over the past 50 years, experimental tests have made physicists discard certain assumptions about symmetry: first, that physics is invariant under parity (P); and second, that it is invariant under the charge-parity (CP) transformation. Direct CP violation has been established in the decays of K-mesons and, recently, B-mesons. On the other hand, CP plus time-reversal (T) invariance, CPT, is believed to hold – and is partly experimentally verified – to a high degree of accuracy.

The symmetries under T, CP and CPT transformations are connected, and the CPT theorem demands that for each particle or element the equivalent antiparticle has the same mass, lifetime, spin and isospin – but an opposite value for all of the additive quantum numbers. The proof (or disproof) of the validity of this basic symmetry may be the key to such fundamental aspects as the universe’s matter-antimatter asymmetry. Physics is still in a phase where it is important to accumulate highly precise experimental data from different leptonic and/or hadronic systems. In this respect, the role of matter-antimatter asymmetry – especially baryonic proton-antiproton physics – is significant.

Probing how antiprotons interact with matter at very low energies is still a topical field for precise studies of the electromagnetic and strong forces and their interplay. High-precision spectroscopy of meta-stable antiprotonic atoms has produced very interesting and unique results. With the accuracy achieved in investigations of antiprotonic helium atoms, the CPT theorem can be tested to a level comparable to the existing bounds from other systems.

An alternative approach is the production and comparison of hydrogen and antihydrogen. A reasonable requirement for a new and unique CPT test of this kind is that it is eventually more stringent than existing tests with leptons and baryons. To make the required high-precision spectroscopic measurements, the hydrogen and antihydrogen atoms have to be at such low temperatures that laser cooling of trapped atoms, which is possible owing to the development of a continuous Lyman-alpha laser, appears to be necessary. Once all the basic technical requirements to produce antihydrogen atoms have been explored and optimized, tests of the gravitational force on antimatter will also be possible, free from the problems associated with charged particles.

When describing the nucleon-antinucleon (N-Nbar) interaction, it is implicitly assumed that the six-quark N-Nbar system can be regarded as a product of quark-antiquark nucleon wave functions with a complex potential that is dominated by the distance between the nucleons. Such a potential predicts a spectrum of many states, if the annihilation part is ignored. There is a rich dynamics of resonances or bound states around thresholds, where the annihilation effects are less dominant, since the phase space for the decay into meson resonances is more restricted. However, the transition from an N-Nbar system to a multiquark state, where quarks and antiquarks interact directly by gluon exchange, must be fully understood before invoking exotic mechanisms based on details of the interaction. New dedicated experiments could determine the energy and the quantum numbers of an N-Nbar system, clarifying the long-range interaction.

Probing the strong interaction

Antiproton beams are an excellent tool for addressing the regime of strong coupling. In antiproton-proton annihilations, particles with gluonic degrees of freedom as well as particle-antiparticle pairs are copiously produced, allowing spectroscopic studies with unprecedented statistics and precision. Phenomena arise that represent open problems in quantum chromodynamics (QCD) as they have their origin in the specific properties of the strong interaction and represent a major intellectual challenge. Quarks are confined within hadrons; the hadron mass does not balance with the summed mass of the quarks contained; and the characteristic self-interaction among gluons should allow for the existence of glue-balls and hybrids, consisting mainly of gluons and/or glue plus a quark-antiquark pair, respectively.

The charmonium system has turned out to be a powerful tool for understanding the strong interaction. The spectroscopy of the charm-anticharm system helped in tuning potential models of mesons in which the gluon condensate is determined. The gluon condensate is closely related to the charmonium masses since it is the gluon and quark-antiquark condensates that represent the energy density of the QCD vacuum. The QCD spectrum is much richer than in the simple quark model, as the gluons, which mediate the strong force between quarks, can also act as the principle components of entirely new types of hadronic matter: glueballs and hybrids.

The additional degrees of freedom carried by gluons allow glueballs and hybrids to have exotic quantum numbers that are forbidden for normal mesons and other fermion-antifermion systems. Such exotic systems can be identified by observing an overpopulation in the experimental meson spectrum, and by comparing their properties with predictions from models for lattice QCD considerations. Antiproton annihilation experiments have produced very promising results for gluonic hadrons.
A special session spiritedly discussed applications of antimatter, radiation and particle detection, covering well established medical treatments, diagnostic routines, plans for future developments and using nuclear physics to locate land-mines to reduce injuries, especially to children.

One highlight of the conference was a public presentation in the overcrowded Wolfgang Paul Lecture Hall at the University of Bonn, where more than 600 people listened to presentations on modern, high-quality physics and its excitements. At least some listeners were disappointed when these lectures stopped after four hours! The entire LEAP-05 was a brilliant preview of the physics to come from using antiprotons as a special and very effective tool.

• The conference was sponsored by Forschungszentrum Jülich; Deutsche Forschungsgemeinschaft; HiEnergy Technologies, Inc; Deutsche Telekom Stiftung; iseg Spezialelektronik GmbH; Bicron; W-IE-NER, Plein & Baus GmbH; and Pfeiffer Vacuum.

DEAR pins down kaonic hydrogen

The DEAR (DAFNE Exotic Atoms Research) experiment at the DAFNE φ factory at Frascati has performed the most accurate determination of the effect of the strong interaction on the binding energy of kaonic hydrogen.

Kaonic hydrogen is an exotic atom where the electron is replaced by a K^–, and it turns out to be an excellent laboratory for studies of quantum chromodynamics. Especially interesting is the determination of the strangeness content of the nucleon, which has traditionally been determined from low-energy kaon-nucleon scattering amplitudes. A significantly more accurate approach has now been discovered, which involves measuring the ground-state X-ray transitions in kaonic hydrogen atoms.

The DEAR collaboration took advantage of the low-energy monoenergetic kaons from the decay of φ mesons resonantly produced by e⁺e^– collisions at one of the two interaction points at DAFNE. The kaons travelled through the thin beam pipe of DEAR and stopped in a gaseous hydrogen target. CCD detectors with a pixel size of 22.5 x 22.5 μm² cooled to 165 K detected the X-rays emitted.

The DEAR experiment follows the steps of the KpX experiment at KEK in Japan, which first measured the ground-state X-ray peak of kaonic hydrogen. DEAR’s values are about a factor of two more accurate, and roughly 40% lower than those of the Japanese collaboration. The ground-state shift, ε_1s, was measured to be -193 ± 37 (stat.) ± 6 (syst.) eV, with a 1s strong interaction width of Γ_1s = 249 ± 111 (stat.) ± 30 (syst.) eV.

DEAR has also become the first experiment to observe transitions from different excited states, clearly identifying K_α, K_β and K_γ lines.

Variation in weak force found by E158

The E158 experiment at the Stanford Linear Accelerator Center has made a landmark observation: the strength of the weak force acting on two electrons lessens when the electrons are far apart. The results will be published in Physical Review Letters.

CCEnew3_07-05 — Predicted variation of sin²θ_w^eff as a function of momentum transfer Q (solid line) and its estimated theoretical uncertainty (shaded). Results of previous low-energy experiments are shown with the Particle Data Group value (PDG2004) at the scale of the Z mass.

Because there is an asymmetry in how the weak force acts, there is a difference between how often left- and right-handed electrons scatter via a Z particle (the neutral carrier of the weak force). Two years ago, the team made the first observation of this parity-violation effect in electron-electron interactions.

For the new results, E185 used its improved precision asymmetry measurement to calculate the long-distance (low momentum transfer, Q) weak charge of the electron, which determines the strength of the weak force between two electrons. The result is the world’s best determination of the weak mixing angle at low energy: sin²θ_w^eff = 0.2397 ± 0.0010 (stat.) ± 0.0008 (syst.), evaluated at Q² = 0.026 GeV².

Previous experiments at SLAC and CERN measured the electron’s weak charge at high momentum transfer (short distances). E158’s long-distance measurement observes this weak charge to be half the size of the charge at short distances. Comparing the short-distance measurements with the long-distance results demonstrates (with 6σ significance) the variation of the strength of the weak force with distance, and confirms an important aspect of Standard Model theory. Using the result for sin²θ_w^eff, E158 finds the electron’s weak charge to be -0.041 ± 0.006 – half the value expected if there were no variation.

E158 was also sensitive to indirect signals from hypothetical Z’ particles, suggesting they are at least 10 times the mass of the Z.

Lattice QCD and CLEO-c tackle the CP challenge

Problems in theories such as quantum chromodynamics (QCD) that involve strong coupling are among the most intractable in physics. The difficulties of accurate calculations are particularly vexing when it comes to studying charge-parity (CP) violation – a necessary ingredient for explaining the absence of the antimatter produced in the Big Bang, and a vital topic in particle physics. Progress in making accurate QCD calculations in this sector of the Standard Model could have far-reaching consequences, because the larger theory in which the Standard Model is embedded, even if not strongly coupled ab initio, will almost certainly have strongly coupled sectors.

CCEqcd1_07-05 — Candidates for the decay D⁺ → μ⁺ν are found by computing the mass of the missing neutrinos in tagged events. The plot illustrates the distribution of the squares of missing neutrino masses for the full CLEO-c data sample. The event candidates between the arrows in the insert (blue) meet all selection criteria.

The amount of CP violation that is consistent with our current understanding of the Standard Model is not enough to account for the disappearance of the antimatter produced along with the matter. The decay of B mesons is the most promising arena in which to search for other sources of CP violation, and the B-factory experiments, BaBar and Belle, have been spectacularly successful in observing and studying CP violation in those decays.

However, the search for CP violation beyond the Standard Model involves comparison of the angles of the “unitarity triangle” measured in CP violation experiments, with the lengths of the sides determined from more conventional measurements. These lengths are determined from elements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix, which describes the relative strengths of the weak decays of quarks. The problem is that quarks do not appear alone, but in strongly interacting combinations such as mesons and baryons. So, there is always a strong-interaction parameter that relates decay measurements to the underlying CKM matrix element. So far, the uncertainties in these parameters severely limit the precision of measurements of the CKM matrix elements.

One example of such a strong-interaction parameter, called f_B, is required to extract the CKM matrix element called V_td from measurements of B⁰B⁰bar mixing. This parameter is a measure of the separation of the b quark and the anti-d quark in a B meson, and could, in principle, be measured in B⁺ → μ⁺ν decay. However, this decay is so slow that an accurate measurement is impossible, even with the enormous data samples that the BaBar and Belle collaborations have accumulated. This is one of the parameters that lattice QCD (LQCD) theorists can calculate, but accuracy has been limited. Recent progress in LQCD holds the promise of precise calculations of the parameters, including f_B, that are required to determine CKM matrix elements, but until now there has been no effective direct experimental test of the precision of these calculations.

CCEqcd2_07-05 — The CLEO detector contains 900 t of iron and 250,000 detection elements. Image: Courtesy Cornell – Wilson Laboratory.

Five years ago, some LQCD theorists and members of the CLEO collaboration at the Cornell Laboratory for Elementary-Particle Physics realized that comparing measurements of D meson decays with LQCD calculations could test the LQCD calculations needed for extracting CKM matrix elements. In particular, in the decay D⁺ → μ⁺ν there is a parameter called f_D⁺ that is analogous to f_B. The rate for D⁺ → μ⁺ν decay is larger than the corresponding B meson decay rate, so it can be measured accurately. If good agreement is found between the experimental and LQCD values for f_D⁺, that would inspire confidence in LQCD calculations of f_B. It is also generally believed that the ratio f_B:f_D⁺ can be calculated more accurately than either one, so f_B could be determined from an experimental measurement of f_D⁺ and a LQCD calculation of the ratio. This caused the CLEO collaboration and a group of LQCD theorists to embark on D meson decay experiments and the corresponding LQCD calculations, with goals of accuracies of a few per cent. However, they had technical difficulties to overcome.

Although there have been LQCD calculations since the 1970s, their accuracy has been limited to 20% because of a simplification – the “quenched approximation”. Predictions from “unquenched” LQCD calculations that match experimental results to a few per cent are needed to demonstrate that the calculations can be done at the desired level of accuracy. The LQCD theorists were the first to reach the goal of a few per cent in their calculations of important parameters (not f_D⁺ or f_B) in the b and c quark systems. However, their results were “postdictions” not predictions, because the corresponding experimental results had already been published. Still, this success motivated the group of LQCD theorists to embark on their calculations of the strong-interaction parameters needed to determine CKM matrix elements.

Measurement of f_D⁺ is one of the main goals of the CLEO-c physics programme at the Cornell Electron Storage Ring (CESR). However, obtaining high luminosity was a major challenge for the CESR accelerator group. Although CESR had made progress in luminosity since its first operation in 1979, much of those gains would be lost in reducing the energy from the b quark threshold region, near 5 GeV per beam, to the c quark threshold region near 2 GeV per beam, where the measurement could be made most readily. As the energy of the beams is decreased, the synchrotron radiation “damping” required for high luminosity is substantially reduced. The solution was the installation of 12 “wiggler” magnets to increase the damping at the lower energies. These magnets were installed in 2003 and 2004 in CESR-c, and the CLEO-c programme then began in earnest, funded by a five-year grant from the National Science Foundation.

The first engineering run of CESR-c with six wigglers yielded an integrated luminosity of 60 pb^-1 in e⁺e^– collisions at a total energy of 3.77 GeV, the peak of the ψ (3770) resonance. This is substantially more than the luminosities available to either the MARK III or the BES II collaborations, at SLAC and the Institute of High Energy Physics in Beijing respectively, which previously took data at the same energy. Subsequent runs with 12 wigglers brought the total integrated luminosity to 281 pb^-1. This is the most desirable energy for measuring D meson decays because the ψ (3770) decays only to D⁺D^– or D⁰D⁰bar pairs, making very clean “tagged” measurements possible. In tagged measurements pioneered by the MARK III collaboration, if one D meson, D^– for example, is reconstructed in an event, then the rest of the particles in that event must be from the decay of a D⁺ meson. Coupled with the excellent resolution and large acceptance of the CLEO-c detector, tagging provides a very clean sample of D⁺ meson decays, which is an ideal arena for searching for rare decays such as D⁺ → μ⁺ν.

The CLEO collaboration found eight D⁺ → μ⁺ν events (with an estimated background of one event) in the first 60 pb^-1 of CLEO-c data. This provided a rough measurement of f_D⁺ to an accuracy of 20%, which has now been published (Bonvicini 2004). The larger data sample and improvements in selection criteria produced a yield of 50 candidates for D⁺ → μ⁺ν decay with an estimated background of 2.9 ± 0.5, enough candidates to yield an error in f_D⁺ below 10%.

While the CLEO collaboration, with the help of their CESR colleagues, was accumulating and analysing these data, a group of LQCD theorists was also hard at work calculating f_D⁺. It became clear that both groups could have substantial results just in time for the Lepton-Photon Symposium in Uppsala at the end of June. Since both communities felt that it was very important for the LQCD result to be a real prediction, they agreed to embargo both of their results until the conference. On the second day of the symposium, Marina Artuso of Syracuse University reported the preliminary CLEO-c result f_D⁺ = 223 ± 16_-9⁺⁷ MeV (CLEO-c 2005), and Iain Stewart of MIT reported the LQCD result from the Fermilab, MIMD Lattice Calculation (MILC) and High Precision QCD (HPQCD) collaborations, f_D⁺ = 201 ± 3 ± 17 MeV (Aubin et al. 2005). For both results the errors are statistical and systematic, respectively. The two results agree well within the errors of about 8% for each.

The agreement between the results motivates both communities to continue comparing LQCD calculations with experiments. On the LQCD side, important next steps include improvements in algorithms that can reduce systematic errors and precision calculations of the “form factors” involved in semileptonic decays of D and B mesons. The CLEO collaboration plans to utilize its data sample to measure form factors in semileptonic D decay and take more data to reduce errors. The LQCD theorists and the CLEO collaboration both aim to reduce errors to below 5%. The CLEO collaboration is also planning to explore the threshold region for D_sD_sbar production to search for an energy at which the tagging techniques can be applied to make the first accurate measurements of D_s meson decay, including f_{D_s} and D_s semileptonic decay form factors. The Fermilab, MILC and HPQCD group has already predicted the value of f_{D_s}.

Analysis method measures angle γ

Up to a few years ago, no significant measurement of the angle γ in the unitarity triangle of B-meson physics was expected to come out of the current B-factories. However, a recent proposal to measure γ in B → DK decays using a Dalitz plot analysis has revolutionized the field. Results are emerging from both the B-factories at KEK and at SLAC.

Determinations of the Cabibbo-Kobayashi-Maskawa (CKM) matrix elements provide important checks on the consistency of the Standard Model and ways to search for new physics. The CKM matrix parameterizes the mixing of different quarks as seen by the weak interaction and provides the Standard Model interpretation for charge-parity (CP) violation.

The nine complex elements of the matrix are related by unitarity constraints to a series of equations. One such relationship of specific interest to B-physics can be represented by a triangle, referred to as the “unitarity triangle”. The angles of the unitarity triangle are referred to as α, β and γ (ϕ₂, ϕ₁ and ϕ₃ respectively in Japan). Although β has already been measured with an accuracy of a few degrees, it is more difficult to measure α and γ accurately.

.The new analysis uses three-body decay of the neutral D, D⁰ → K_sπ⁺π^– from the channels B^± → D⁰K^±, B^± → D*K^± and B^± → DK*^±. In the Dalitz plot analysis on the three-body decay of the D the invariant mass of the K_sπ⁺ system is plotted versus the K_sπ^– system in two dimensions, helping a measurement of an asymmetry when looking at B⁺ compared with B^– decays. The method also utilizes more event information and is thus more sensitive compared with a 1D approach.

Using a data sample of 253 fb^-1, the Belle collaboration at KEK obtains 276 signal candidates for B^± → D⁰K^±, 69 candidates for B^± → D*K^± and 56 candidates for B^± → DK*^± (Abe et al. 2004 and 2005). Combining the first two channels yields the result γ = 68° ± 14° ± 13° ± 11°. The first error is statistical, the second is experimental systematics and the third is model uncertainty. The statistical significance of CP violation is 98%. This is not quite enough to claim observation of direct CP violation just yet, but it is getting close.

The BaBar collaboration at SLAC is also working on a similar analysis and their preliminary result stands at γ = 70° ± 26° ± 10° ± 10° (Aubert et al. 2004). Such values of γ agree with what is expected by the Standard Model and global fits of other information; moreover, the Dalitz plot method is fast becoming an established tool for measuring γ.

Mystery deepens as pentaquarks refuse to make an appearance

Preliminary data on the hot topic of the search for pentaquarks were presented at the April Meeting of the American Physical Society by the Jefferson Laboratory’s CEBAF Large Acceptance Spectrometer (CLAS) collaboration. Quantum chromodynamics (QCD) does not forbid exotic, pentaquark states comprising four quarks and an antiquark, but the jury is still out as to whether such a state has been observed. Several experiments have published positive results while an equal number of different experiments have found nothing. The new result adds to the negative evidence.

CCEnew3_06-05 — New results from the g11 experiment at Jefferson Laboratory’s CLAS detector, seen here with the “clam-shell” open, are adding to the pentaquark mystery. Image: JLab.

The g11 experiment at the CLAS detector is a fixed-target photoproduction experiment in which a tagged photon beam, with photon energies individually measured, at an energy of 1.6-3.8 GeV hits a proton target. Data-taking was completed in 2004 with 70 pb^-1 of integrated luminosity. The collaboration searched for the Θ⁺(1540) produced together with a neutral kaon in the reaction γp → Θ⁺Kbar⁰, where the Kbar⁰ is detected via its K⁰_s component decaying into π⁺π^–.

The Θ⁺ is expected to decay into a neutron and a K⁺, and the neutron is reconstructed from the missing mass in the reaction. No signal is seen in the nK⁺ mass spectrum, putting a limit on the production cross-section for γp → Θ⁺K⁰bar of less than 4 nb at a 95% confidence level.

This result is at odds with a published analysis of CLAS, where a Θ⁺ signal was seen with a 7.8 σ significance in the reaction γp → Θ⁺π⁺K^–. The earlier study was performed on 5 pb^-1 of data, where a couple of severe geometry cuts had to be applied to the original nK⁺ distribution to reveal the Θ⁺ signal. An experiment at higher energy to verify this result is planned.

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