Physics Archives – Page 99 of 144

Spin and snakes come to the land of Jefferson

CCspi1_05_09 — Anatoly Kondratenko, one of the inventors of the Siberian snake, outside the symposium’s building on the beautiful Virginia campus, which was originally designed by Thomas Jefferson.
Image credit: M Leonova.

The International Spin Physics Symposia series started at the Argonne National Laboratory in 1974, just after its 12 GeV Zero Gradient Synchrotron (ZGS) had accelerated the world’s first polarized proton beam. Paul Dirac gave the keynote lecture in which he reviewed the history of spin, starting with the first ideas in the 1920s. The 18th Symposium, SPIN 2008, was held in October 2008 at the University of Virginia, which was founded by Thomas Jefferson in 1821. Jefferson became third president of the US and is best known as the main author of the US Declaration of Independence. He had a keen interest in science and was president of the American Philosophical Society. As a fitting tribute, an appropriately-dressed person – who claimed to be Jefferson – gave the after-dinner talk at SPIN 2008. Speaking with a polite 1800s Virginian accent, he gave wise scientific advice that is as relevant now as it was in the early 19th century.

Symposia highlights

SPIN 2008 was attended by 282 high-energy and nuclear spin physicists from around the world. There were 195 parallel talks and 37 plenary talks, so this report mentions only a few of the exciting highlights. After the welcoming talks, the ever-enthusiastic Elliot Leader of Imperial College, London, opened the symposium with a rousing lecture on “The power of spin: a scalpel-like probe of theoretical ideas”. He was followed by Klaus Rith of Erlangen, who gave a detailed experimental overview in his talk that addressed selected highlights of spin experiments and their technological challenges.

CCspi2_05_09 — Maria Leonova of Michigan beside a statue of Thomas Jefferson. She discussed recent data from SPIN@COSY and the proper equations for the deuteron spin-depolarizing resonance strengths from RF dipole and RF solenoid magnets.
Image credit: Vasily Morozov.

The main highlight of the symposium was the success of Brookhaven’s RHIC in its operations as a polarized-proton collider. The machine has produced a great deal of high-quality data in 100 GeV-on-100 GeV collisions and had a brief but successful test of stored 250 GeV polarized protons. For this impressive achievement, Thomas Roser, Mei Bai and their team of polarized-beam experts used two Siberian snakes in each RHIC ring together with two partial Siberian snakes in the venerable Alternating Gradient Synchrotron, which serves as the injector for RHIC. These operations were possible thanks to some vital external contributions. James Simons, mathematics professor at Stony Brook, a Brookhaven trustee and now a “renaissance-technologies” billionaire, provided $13 million to allow a 6 month polarized run of RHIC. Moreover, the long-term support of Akito Arima – a nuclear theorist who became a member of the Japanese Diet and science minister – resulted in more than $20 million for RHIC’s four superconducting Siberian snakes and other essential hardware. The funds were transferred from Japan to Brookhaven via the RIKEN research institute.

One interesting result was the measurement by the BRAHMS experiment at RHIC of the left–right spin asymmetry, A_n, in the inclusive production of π⁺ and π^– mesons, which was presented by Christine Aidala of the University of Massachusetts at Amherst. The data show that, despite the prediction of perturbative QCD (PQCD) that spin would be unimportant at high energy, the inclusive A_n at large Feynman-x reached the same value of about 40% at 3900 GeV² (P_Lab ≈ 2 TeV/c) as at Argonne’s ZGS, Brookhaven’s AGS and Fermilab at momentum values of 12, 22 and 200 GeV/c, respectively (figure 1). This result might encourage PQCD theorists to define more clearly what is meant by “high” energy.

CCspi3_05_09 — Fig. 1. Measurements at RHIC by BRAHMS of the left–right spin asymmetry, A_n, in the inclusive production of π⁺ and π^– mesons, show no change compared with results at lower energies – contrary to PQCD predictions.

In an overview of the transverse spin structure of the nucleon, Mario Anselmino of Turin reported on the very large observed transverse spin effects, which are still not fully understood. Karl Slifer of the University of Virginia gave a talk on what polarized electron scattering has revealed about the spin content of the nucleon. He discussed the theoretical implications of recent polarized-electron experiments, many of which were done at the 6 GeV polarized-electron ring at the Thomas Jefferson National Accelerator Facility (Jefferson Lab). The use of polarized radioactive beams was the subject of an interesting talk by Koichiro Asahi of Tokyo Institute of Technology.

Speakers also covered the more experimental aspects of spin studies, namely the production of polarized beams and polarized targets. Richard Milner of the Massachusetts Institute of Technology gave an excellent review of the progress towards a future polarized-electron ring, which would allow collisions with either polarized protons or polarized nuclear ions stored in a much larger ring – possibly one of the rings at RHIC. Erhard Steffens of Erlangen, the new chair-elect of the International Spin Physics Committee, summarized the discussions at the second workshop on ‘How to Polarize Antiprotons’, held in August 2007 at the Cockcroft Institute at the Daresbury Laboratory in the UK. There has been significant progress on this challenging topic during the 22 years since Owen Chamberlain and Alan Krisch organized the first Polarized Antiproton Workshop in 1985 at Bodega Bay near Berkeley, but there is still no clearly defined solution to this difficult problem.

CCspi4_05_09 — Mei Bai talks about the Siberian snakes at RHIC.
Image credit: M Leonova.

Werner Meyer of Bochum reviewed the continuing progress, since SPIN 2006 in Kyoto, on cryogenic polarized proton and deuteron targets. Brookhaven’s Anatoly Zelenski then described the recent progress on polarized-ion sources – the subject of a joint paper with Alexander Belov of the Institute for Nuclear Research, Troitsk. This progress is important because these polarized sources feed RHIC. Indeed, Brookhaven and the Spin Physics Committee sponsored a recent workshop on this topic at Brookhaven. Matt Poelker of Jefferson Lab reviewed progress on polarized-electron sources and polarimeters, which was the subject of another recent spin workshop at the laboratory. These sources and polarimeters are vital to progress in polarized-electron experiments.

Anatoly Kondratenko of Novosibirsk, who along with Yaroslav Derbenev invented Siberian snakes in the 1970s, gave an interesting talk on his more recent idea, now named Kondratenko Crossing (KC). This uses a symmetric spin-resonance crossing pattern that forces the resonance’s depolarizing effects to cancel themselves. Richard Raymond of Michigan reported on new data from the SPIN@COSY team at the Cooler Synchrotron (COSY) at the Forschungszentrum in Jülich. The results show that KC works, at least for RF-solenoid-induced resonances with deuterons. In the same parallel session accelerator pioneer Ernest Courant, of Brookhaven, still going strong at 89, discussed his new theoretical work on the behaviour of stored polarized beams.

There was also a special evening plenary session where the director (or proxy) of each of the major laboratories involved in spin-physics studies reported on their plans at GSI-Darmstadt, Brookhaven, Jefferson Lab, IHEP-Protvino, JINR-Dubna, J-PARC, and COSY-Jülich. On the last day, Thomas Roser, the new past-chair of the Spin Physics Committee, gave an excellent lecture on the future of high-energy polarized beams. The symposium ended with closing remarks from committee chair, Kenichi Imai of Kyoto. He announced that SPIN 2010 would be hosted by Forschungszentrum Jülich, while a high priority would be given to SPIN 2012 being hosted somewhere in Russia.

• For more information about SPIN 2008, see www.faculty.virginia.edu/spin2008/.

Laser-pulse blasts set antiparticle production record

CCnew2_04_09 — Hui Chen sets up targets for the antimatter experiment at the Jupiter laser facility.
Image credit: LLNL.

The latest record for antiparticle density created in the laboratory has not come from an accelerator facility but from the Lawrence Livermore National Laboratory’s Jupiter laser facility. Hui Chen and colleagues blasted picosecond laser pulses carrying 10²⁰ Wcm^–2 from the Titan laser onto gold targets some 1 mm thick. Part of each laser pulse created a plasma and part drove the plasma’s electrons into the gold. The gold nuclei then slowed down the electrons, producing photons that converted into electron–positron pairs. The result was an estimated 10¹⁶ positrons/cubic centimetre.

In addition to being intrinsically interesting this work could aid better understanding of astrophysical phenomena such as gamma-ray bursts. It could also lead to new ways to produce positron sources, which at present are limited to positron-emitting radioisotopes and pair-creation from high-energy photons at accelerators.

NSCL researchers constrain nuclear symmetry energy at low density

By analysing collisions between several combinations of tin nuclei, researchers at the Michigan State University National Superconducting Cyclotron Laboratory (NSCL) have refined the understanding of nuclear symmetry energy. Their work marks the first successful theoretical explanation of common observables that are related to symmetry energy in heavy-ion experiments. The results should help in discerning the properties of neutron stars, particularly in the crust region.

CCnew6_04_09 — Betty Tsang adjusts a detector that is used to make precise measurements of particles produced in high-speed collisions of nuclei.
Image credit: NSCL.

The nuclear attraction between a neutron and a proton is, on average, stronger than that between two protons or two neutrons. The nuclear contribution to the difference between the binding energy of a system of all neutrons and another with equal numbers of protons and neutrons is known as the symmetry energy. To allow for this difference, formulae to calculate nuclear masses include a symmetry-energy term. This term often takes a form that assumes the symmetry energy to be independent of density, even though its value inside the nucleus, at normal density, should exceed its value at the surface, where the density is lower and the ratio of proton to neutron densities differs from that for the nuclear interior.

The symmetry energy of a stable nucleus reflects typical nuclear densities of about 2–3 × 10¹⁴ g/cm³; it contributes modestly to the binding energy but influences significantly the stability of nuclei against beta decay. Despite the sensitivity of nuclear masses to its average value, the precise understanding of the dependence of symmetry energy on density has proved elusive, leading to large uncertainties in theoretical predictions for properties of nuclei that are very rich in neutrons. The effects of symmetry energy loom even larger in environments that have unusual ratios of protons to neutrons and much larger ranges of density, such as in neutron stars. There, the dependence of the symmetry energy upon density is one of the most uncertain parts of the mathematical palette describing the forces at play.

Now, Betty Tsang, Bill Lynch, Pawel Danielewicz and colleagues have helped to constrain understanding of the density dependency of symmetry energy by studying how it affects heavy-ion reactions at NSCL’s Coupled Cyclotron Facility (Tsang et al. 2009). In two experiments, the team directed various beams of tin nuclei at stationary targets of tin. The four combinations included a beam of ¹²⁴Sn (50 protons and 74 neutrons) on a target of ¹²⁴Sn, ¹¹²Sn (62 neutrons) on ¹¹²Sn, ¹²⁴Sn on ¹¹²Sn, and ¹¹²Sn on ¹²⁴Sn. This allowed the researchers to create and study nuclear matter with different neutron-to-proton ratios over a range of density, which could be varied by adjusting the energy of the beam and the centrality of the collisions.

The team collected data on several observables, including isospin diffusion, which probes the neutron-to-proton ratio of neutron-rich projectile nuclei after collisions with neutron-deficient target nuclei. During grazing collisions at relative velocities of 0.3 c, a neck region with reduced density can form between projectile and target nuclei through which neutrons and protons can diffuse. The stronger the symmetry energy is in this neck region, the more likely the neutron-to-proton ratios in the projectile and target nuclei will equilibrate and become equal. A second observable involves comparisons of the energy spectra of neutrons and protons in central head-on collisions. In this case the symmetry energy expels neutrons from the central overlap region of the projectile and target nuclei; the ratio of neutron-to-proton emission then provides a probe of the variation in symmetry energy as the system compresses and expands during the collision.

By comparing the experimental data to results obtained with theoretical models developed by their Chinese colleagues, YingXun Zhang and Zhuxia Li at the China Institute of Atomic Energy, the team obtained constraints on the density dependence of symmetry energy at densities ranging from normal down to around one third nuclear matter density. The results will help to describe the inner crust of neutron stars, where the density of nuclear matter is in the 1–2 × 10¹⁴ g/cm³ range. The role of symmetry energy at the cores of such stars, where the density of nuclear matter reaches 8 × 10¹⁴ g/cm³, is currently associated with the largest uncertainty in descriptions of neutron stars.

New Zealand meeting looks at dark matter

The 7th Heidelberg International Conference on Dark Matter in Astrophysics and Particle Physics – Dark 2009 – was held at Canterbury University in Christchurch on 18–24 January. The event saw 56 invited talks and contributions, which provided an exciting and up-to-date view of the development of research in the field. The participants represented well the distribution of dark-matter activities around the world: 25 from Europe, 11 from the US, 5 from Japan and Korea, 14 from Australia and New Zealand, and 1 from Iran. The programme covered the traditionally wide range of topics, so this report looks at the main highlights.

CCdar1_04_09 — Participants of DARK 2009 at the monument for Robert Falcon Scott, who started his Antarctic expedition from Christchurch in 1910.

The conference started with an overview of searches for supersymmetry at the LHC and dark matter by Elisabetta Barberio of the University of Melbourne. To date, the only evidence for cold dark matter from underground detectors is from the DAMA/LIBRA experiment in the Gran Sasso National Laboratory, as Pierluigi Belli from the collaboration explained. This experiment, which looks for an expected seasonal modulation of the signal for weakly interacting massive particles (WIMPs), now has a significance of 8.4 σ. Unfortunately, all other direct searches for dark matter do not currently have the statistics to look for this signal. Nevertheless, Jason Kumar from Hawaii described how testing the DAMA/LIBRA result at the Super-Kamiokande detector might prove interesting.

Later sessions covered other searches for dark matter. Tarek Saab from Florida gave an overview of ongoing direct searches in underground laboratories, including recent results from the Cryogenic Dark Matter Search experiment in the Soudan mine, and Nigel Smith of the UK’s Rutherford Appleton Laboratory presented results from the ZEPLIN III experiment in the Boulby mine. Irina Krivosheina of Heidelberg and Nishnij Novgorod discussed the potential offered by using bare germanium detectors in liquid nitrogen or argon for dark-matter searches, on the basis of the results from the GENIUS-Test-Facility in the Gran Sasso National Laboratory. Chung-Lin Shan of Seoul National University reported on how precisely WIMPs can be identified in experimental searches in a model-independent way.

Searching for signals from dark-matter annihilation in X-rays and weighing supermassive black holes with X-ray emitting gas were subjects for Tesla Jeltema of the University of California Observatories/Lick Observatory and David Buote of the University of California, Irvine. Stefano Profumo of the University of California, Santa Cruz, provided an overview of fundamental physics with giga-electron- volt gamma rays. Iris Gebauer of Karlsruhe addressed the excess of cosmic positrons indicated by the Energetic Gamma Ray Experiment Telescope, which are still under discussion, as well as the new anomalies observed by the Payload for Antimatter Matter Exploration and Light-Nuclei Astrophysics (PAMELA, PAMELA finds an anomalous cosmic positron abundance ) satellite experiment and the Advanced Thin Ionization Calorimeter (ATIC) balloon experiment. These results and the limits that they set on some annihilating dark matter (neutralino or gravitino) models were also discussed by Kazunori Nakayama of Tokyo and Koji Ishiwata of Tohoku.

Other presentations outlined results and prospects for the AMANDA, IceCube and ANTARES experiments, which study cosmic neutrinos – though there is still a long way to go before they have conclusive results. Emmanuel Moulin of the Commissariat à l’énergie Atomique/Saclay presented results from imaging atmospheric Cherenkov telescopes, in particular the recent measurements from HESS, which exploited the fact that dwarf spheroidal galaxies, such as Canis Major, are highly enriched in dark matter and are therefore good candidates for its detection. Unfortunately, the results do not yet have the sensitivity of the Wilkinson Microwave Anisotropy Probe in restricting either the minimal supersymmetric Standard Model or Kaluza–Klein scenarios.

Leszek Roszkowski of Sheffield gave an overview of supersymmetric particles (neutralinos) as cold dark matter, while scenarios of gravitino dark matter and their cosmological and particle-physics implications were presented by Gilbert Moultaka of the University of Montpellier and Yudi Santoso of the Institute for Particle Physics Phenomenology, Durham. Dharam Vir Ahluwalia of the University of Canterbury put the case for the existence of a local fermionic dark-matter candidate with mass-dimension one, on the basis of non-standard Wigner classes. However, as the proposed fields, as outlined in detail by Ben Martin of Canterbury, do not fit into Steven Weinberg’s formalism of quantum-field theory, this suggestion led to dispute between other experts. An interesting candidate for dark matter was presented by Norma Susanna Mankoc-Borstnik of the University of Ljubljana, who proposed a fifth family as candidates for forming dark matter.

Dark energy and the cosmos

Dark energy was a major topic at the conference. Chris Blake of Swinburn University of Technology in Melbourne presented the prospects for the WiggleZ survey at the Anglo-Australian Telescope, the most sensitive experiment of this kind, and Matt Visser of Victoria University in Wellington gave a cosmographic analysis of dark energy. On the theoretical side there are diverging approaches to dark energy, including attempts to explain it in a “radically conservative way without dark energy”, as David Wiltshire of Canterbury University, Christchurch, explained.

A particular highlight was the presentation by Terry Goldman of Los Alamos, which discussed a possible connection between sterile fermion mass and dark energy. His conclusion was that a neutrino with mass of 0.3 eV could solve the problem of dark energy. This possibility was qualitatively supported by results of non-extensive statistics in astroparticle physics that Manfred Leubner of the University of Innsbruck presented, in the sense that dark energy is expected to behave like an ordinary gas. Goldman’s suggestion is also of interest with respect to the final result of the Heidelberg–Moscow double-beta-decay experiment, reported by Hans Klapdor-Kleingrothaus, which predicts a Majorana neutrino mass of 0.2–0.3 eV.

Danny Marfatia of the University of Kansas discussed mass-varying neutrinos in his presentation about phase transition in the fine structure constant. He proposed that the coupling of neutrinos to a light scalar field might explain why Ω_{dark energy} is of the same order as Ω_matter. Possible connections between dark matter and dark energy with models of warped extra dimensions and the hierarchy problem were outlined by Ishwaree Neupane of the University of Canterbury and Yong Min Cho of Seoul National University.

Dark mass and the centre of the galaxy was the topic of a special session in which Andreas Eckart of the University of Cologne presented recent results on the luminous accretion onto the dark mass at the centre of the Milky Way. Patrick Scott of Stockholm University discussed dark stars at the galactic centre, while Benoit Famaey of the Université Libre de Bruxelles and Felix Stoehr of the Space Telescope European Coordinating Facility/ESO in Garching discussed the distribution of dark and baryonic matter in galaxies. Primordial molecules and the first structures in the universe were the topics addressed by Denis Puy of the Univesité Montpellier II. Youssef Sobouti of the Institute of Advanced Studies on Basic Science in Zanjan, Iran, presented a theorem on a “natural” connection between baryonic dark matter and its dark companion, while Matthias Buckley of the California Institute of Technology put forward ideas about dark matter and “dark radiation”.

Gravity also came under scrutiny. David Rapetti of SLAC explored the potential of constraining gravity with the growth of structure in X-ray galaxy clusters, while Agnieszka Jacholkowska of IN2P3/Centre National de la Recherche Scientifique gave an experimental view of probing quantum-gravity effects with astrophysical sources. In a special session on general relativity, Roy Patrick Kerr of Canterbury University gave an interesting historical lecture entitled “Cracking the Einstein Code”.

To conclude, the lively and highly stimulating atmosphere of Dark 2009 reflected a splendid future for research in the field of dark matter in the universe and for particle physics beyond the Standard Model. The proceedings will be published by World Scientific.

Nara workshop looks at heavy quarkonia

The 6th International Workshop on Heavy Quarkonia took place in Nara in December 2008, attracting some 100 participants. It was the latest in a series organized by the Quarkonium Working Group (QWG), a collaboration of theorists and experimentalists particularly interested in research related to the physics of quarkonia – bound states of heavy quark–antiquark pairs (Brambilla et al. 2004). The talks and discussions in three round tables emphasized the latest advances in the understanding of quarkonium production, the discovery of the η_b, the properties of the X, Y and Z narrow resonances, as well as the use of quarkonium states as probes of the QCD matter formed in high-energy nuclear collisions. The meeting ended with a series of talks about how the Antiproton Annihilations at Darmstadt (PANDA) experiment and the LHC experiments should improve and complement present knowledge.

New states

CCqua1_04_09 — Fig. 1. CDF’s J/ψπ⁺π^– invariant mass distribution, showing the very prominent X(3872) resonance.

The nature and properties of the X, Y and Z narrow resonances, recently discovered in B-factories (and thought to be quarkonium states), were extensively discussed at the workshop. Presentations from the Belle, BaBar and CDF collaborations provided new information on the masses, branching ratios, quantum numbers and production properties of these particles. Using approximately 6000 signal events in J/ψ → π⁺π^– decays, CDF obtained the most precise determination of the X(3872) mass: 3871.61±0.16(stat.) ±0.19(syst.) MeV/c², a value extremely close to the D⁰D^*0 mass threshold, 3871.8±0.36 MeV/c² (figure 1). Given present uncertainties, the interpretation of the X(3872) as a “molecular” D⁰D^*0bound state remains possible but not compulsory. In addition the CDF collaboration reported a very accurate mass measurement for the B_c^± of 6275.6±2.9(stat.) ±2.5(syst.) MeV/c², obtained by studying the mass spectrum of B_c⁺ → J/ψπ⁺ decays (and the charge conjugates). The CDF and D0 experiments also measured the B_c lifetime, through the study of semileptonic decays. The measurements are of comparable precision, leading to a world average lifetime of 0.459±0.037 ps for the only observed charged quarkonium.

CCqua2_04_09 — Fig. 2. BaBar’s inclusive photon spectrum from U(2S) decays, showing evidence of η_b production in U(2S) → γη_b decays.

Another hot topic was the BaBar experiment’s discovery of the long-sought-after bottomonium ground state, the η_b. On the basis of a record amount of event samples collected early in 2008 (more than two hundred million U(2S) and U(3S) events), the BaBar collaboration announced in July the observation of the η_b in the rare magnetic-dipole transition U(3S) → γη_b. At the Nara workshop, BaBar showed preliminary evidence for the U(2S) → γη_b decay, which confirms the earlier observation (figure 2). The measured mass for the η_b is 71.4 (stat.) ±2.7(syst.) MeV smaller than the U(1S) mass. This mass difference is almost twice the value calculated in perturbative QCD, 39±11(theor.) (δα_s) MeV, hence challenging the expectation that non-perturbative corrections should be only a few million electron-volts.

The Belle collaboration reported an improved measurement of the inclusive cross-section for the production of a J/ψ meson plus additional charmed particles. The new result is around 15% lower than their previous value and, in combination with a new calculation of next-to-leading-order (NLO) corrections, brings theory and experiment into reasonable agreement – albeit with large uncertainties – potentially solving a long-standing quarkonium-production puzzle.

The workshop also heard about new calculations of NLO corrections to the colour-singlet quarkonium-production mechanism, which confirm that the ratio between the colour-singlet and colour-octet production rates is larger than previously thought. The same calculations predict that J/ψs produced via the colour-singlet mechanism should exhibit a stronger longitudinal polarization in the helicity frame than is observed in the data from CDF. In the case of J/ψ photoproduction, new NLO calculations of the colour-singlet contribution fail to reproduce the polarization measurements made at HERA. If it turns out that feed-down effects do not modify the observed polarizations significantly then these discrepancies might indicate that a colour-octet contribution is required to bring the polarization predictions and experiment into agreement. There was also a discussion on the consistency of the measurements of the J/ψ polarization by the E866, HERA-B and CDF experiments. The seemingly contradictory data sets are surprisingly well reproduced if one models the polarization along the direction of relative motion of the colliding partons by assuming that, for directly produced J/ψs, it changes continuously from fully longitudinal at low total momentum to fully transverse at asymptotically high total momentum.

Heavy ions

Another interesting line of research in the QWG’s activities has to do with the use of heavy-quarkonium states as particularly informative probes of the properties of the high-density QCD matter produced in high-energy heavy-ion collisions. Contrary to early expectations, however, currently available J/ψ suppression measurements cannot be seen as “smoking-gun signatures” that would show beyond reasonable doubt the creation of a deconfined state of quarks and gluons. Indeed, the present experimental picture is blurred by several “cold nuclear matter” effects, including shadowing of the parton densities, final-state nuclear absorption of fully formed charmonium states (or of pre-resonances) and initial-state parton-energy loss. Furthermore, there needs to be a careful evaluation of feed-down contributions to the production yields of the J/ψs (and their own “melting” patterns). Presentations in Nara showed recent progress in the understanding of these topics and there were detailed discussions concerning the quarkonium properties in finite-temperature QCD. Future measurements of the U family at the LHC should open a better window into this interesting landscape.

CCqua3_04_09 — QWG participants gather in the sunshine to pose for the traditional group photo during the meeting in Nara.
Image credit: Kenkichi Miyabayashi.

The next International Workshop on Heavy Quarkonia will take place at Fermilab in May 2010. Meanwhile, quarkonium aficionados are eagerly awaiting the first results from the LHC. More than 30 years after the serving of the charmonium and bottomonium families as revolutionary entrées, quarkonium physics remains high in the menus of many physicists, providing a table d’hôte where to test the properties of perturbative and non-perturbative QCD and validate the continually improving computational tools. Sprinkled with enough puzzles to spice up the meal, quarkonium physics will continue to please the most discerning appetites for years to come.

PAMELA pins down cosmic antiproton flux

CCnew3_03_09 — The antiproton-to-proton flux ratio measured by PAMELA compared with theoretical calculations for pure secondary production of antiprotons.

The satellite experiment Payload for Antimatter Matter Exploration and Light-nuclei Astrophysics (PAMELA) has made a new measurement of the antiproton-to-proton flux ratio in cosmic rays with energies up to 100 GeV. The results, which represent a great improvement in statistics compared with data published previously, provide significant constraints on exotic sources of cosmic antimatter.

The PAMELA experiment has been in low Earth-orbit on the Resurs-DK1 satellite since its launch in June 2006. During 500 days of data collection it has identified 1000 antiprotons with energies in the range 1–100 GeV, including 100 antiprotons with an energy above 20 GeV. This is a larger data sample at higher energies than any other experiment has obtained.

Cosmic antiprotons can be made in particle (mainly proton) collisions with interstellar gas but they could also have more exotic origins, for example, in the annihilation of dark-matter particles. Finding out more about the actual production mechanisms requires detailed studies of the antiproton energy spectrum over a wide energy range, which in turn depend on data with good statistics, as PAMELA now provides.

Analysis of the data from PAMELA show that the antiproton-to-proton flux ratio rises smoothly to about 10 GeV, before tending to level off. The results match well with theoretical calculations that assume only secondary production of antiprotons by cosmic rays propagating through the galaxy. This places limits on contributions from other, more exotic sources.

ISOLTRAP weighs in with new noble results

CCnew4_03_09 — Fig. 1. Cross-sectional view of the VADIS source showing the ionization chamber, electric potential lines (green) and ion trajectories (red).

Georg Christoph Lichtenberg , the 18th-century philosopher scientist, said: “To see something new, you must build something new.” This adage certainly applies on the nuclear scale at CERN’s On-Line Isotope Mass Separator, ISOLDE, the pioneering rare-isotope factory. Measurements with the Penning-trap mass spectrometer ISOLTRAP, have determined new masses for several isotopes of the noble gases, xenon and radon, while discovering a new isotope of radon along the way.

ISOLDE is CERN’s longest-running facility and has always been at the forefront of development. Now the facility is the key player in the European sixth framework design study for EURISOL, a next-generation facility for isotope separation online (ISOL). At ISOLDE the short-lived nuclides are created using 1.4 GeV protons from CERN’s PS Booster. Once produced in the target, these rare species must be ionized efficiently to form secondary beams that can be accelerated and mass-separated for use in experiments. Thus, all ISOLDE targets have a built-in chemically selective ion source.

CCnew5_03_09 — Fig. 2. Observed decay curve of the newly discovered ²²⁹Rn isotope from which a half-life of 12 s was determined. The inset shows a time-of-flight cyclotron resonance curve from ISOLTRAP, from which the ²²⁹Rn mass was determined.

One of the tasks of EURISOL (in conjunction with the HighInt Marie-Curie Training programme) is the development of an efficient ion source that can accommodate the 50-fold increase in proton beam intensity that will become available at CERN through the upcoming Linac 4 and Superconducting Proton Linac upgrades. This has led to a prototype, the Versatile Arc Discharge Ion Source (VADIS). Its principle rests on the optimization of both the discharge-current densities within the ion-source geometry and the extracted ion-beam intensities (figure 1). The version designed for the selective ionization of noble gases increases ionization efficiency by over an order of magnitude.

VADIS was employed at ISOLDE in 2008 with spectacular results. The experiment in question involved another pioneering facility, ISOLTRAP. ISOLTRAP in effect weighs radioactive nuclides created by ISOLDE using the elegant technique of exciting the cyclotron motion of a single ion in a magnetic field. Knowledge of the mass gives access to the nuclear binding energy, which is not only a rich source of information for nuclear structure, size and shape, but also determines the amount of energy available for radioactive decay and for reactions of major importance for modelling nucleosynthesis, the cooking of elements in stars.

ISOLTRAP first weighed isotopes of xenon ionized by VADIS, determining masses for four more of them. The team then focused its efforts on the neutron-rich isotopes of radon, with impressive results. The experiment determined seven new masses, one for an isotope, ²²⁹Rn, that had never previously been observed in the laboratory. As there was no information to confirm this isotope’s identity, the experimenters needed to take particular care to make sure it was indeed what they thought it to be. As a result, they also determined the half live of this nuclide (figure 2), marking the first discovery of a nuclide by Penning-trap mass spectrometry (Neidherr et al.). To make things even more interesting, the new radon masses show a unique pattern that provides a link to a special type of nuclear octupole deformation, predicted to occur in this region of the nuclear chart.

Finding the way to polarized antiprotons

CCnew6_03_09 — Fig. 1. The COSY ring with electron cooler, cluster target, and proton–deuteron polarization analyser (polarimeter).

The QCD physics potential of experiments with high-energy polarized antiprotons is enormous, but until now high-luminosity experiments have been impossible. This situation would change dramatically with the production of a stored beam of polarized antiprotons, and the realization of a double-polarized high-luminosity antiproton–proton collider. Recent measurements at the Cooler Synchrotron (COSY) at Jülich have for the first time studied the influence of unpolarized electrons on polarized protons, settling a puzzle over the magnitude of such effects.

The collaboration for Polarized Antiproton Experiments (PAX) has proposed a physics programme that would be possible with a double-polarized proton–antiproton collider at the new Facility for Antiproton and Ion Research (FAIR), which is to be built at GSI in Darmstadt (PAX Collaboration 2006). The original idea was to use polarized electrons to produce a polarized beam of antiprotons (Rathmann et al. 2005). This triggered further theoretical work on the subject and a group from Mainz proposed using co-moving electrons or positrons (e) at slightly different velocities from the orbiting protons or antiprotons (p) as a means to polarize the stored beam (Walcher et al. 2007). When the relative velocities, v, between the e and p are adjusted so that v/c is about 0.002, a numerical calculation by the Mainz group predicts the cross-section for the ep spin-flip to be as large as about 2 × 10¹³ b. Analytical predictions for the same quantity by a group from Novosibirsk, however, yield a range well below one millibarn (Milstein et al. 2008).

CCnew7_03_09 — Fig. 2. Ratio of the beam polarization with or without electron beam as a function of the average relative velocity. The horizontal bars indicate the range of velocities that contribute to the measurement. The vertical bars are statistical uncertainties.

To provide an experimental answer to the puzzle, the collaborations for PAX and for the Apparatus for Studies of Nucleon and Kaon Ejectiles experiment joined forces at COSY, where they mounted an experiment that used the electrons in the electron cooler as a target and measured the effect of the electrons on the polarization of a 49.3 MeV proton beam orbiting in COSY. Instead of studying the build-up of polarization in an unpolarized beam, the teams studied the inverse by observing the depolarization of an initially (vertically) polarized beam; they measured the proton-beam polarization using the analysing power of proton–deuteron elastic scattering on a deuterium cluster jet target (figure 1).

Figure 2 shows the results, with the ratio of the measured beam polarizations, P_E and P₀ (Oellers et al. 2009). P_E represents the measured polarization corresponding to well defined changes of the electron velocity with respect to the protons. This was achieved by detuning the accelerating voltage in the electron cooler by a specific amount compared to the nominal voltage. P₀ is the polarization measured when the electron beam was off (i.e. no electron target was present). No depolarization effect on the proton beam could be detected within the statistical precision of the measurement. This translates into an upper limit for the ep transverse and longitudinal spin-flip cross-sections of 1.5 × 10⁷ b at a relative velocity of v = 0.002, six orders of magnitude below the numerical predictions. After the completion of the experiment, the Mainz group uncovered a numerical overestimation in their original estimates (Walcher et al. 2009).

The result rules out the practical use of polarized leptons to polarize a beam of antiprotons with present-day technologies. This leaves spin-filtering as the only proven method to polarize a stored beam in situ, a technique that exploits the spin- dependence of the strong interaction using a polarized internal target (Rathmann et al. 1993). At present, a complete quantitative understanding of all underlying processes is lacking, so the PAX collaboration aims to use stored protons in COSY for high-precision polarization build-up studies with transverse and longitudinal polarization. Under these circumstances, the build-up process itself can be studied in detail because the spin-dependence of the proton–proton interaction around 50 MeV is completely known. The internal polarized target and the target polarimeter required for these investigations are currently set up to be installed together with a large-acceptance detector system for the determination of the beam polarization in a dedicated low-β section at COSY.

In contrast to the proton–proton system, the experimental basis for predicting the polarization build-up by spin filtering in a stored antiproton beam is practically non-existent. Therefore, it is of high priority to perform a series of dedicated spin- filtering experiments using stored antiprotons. The Antiproton Decelerator at CERN is a unique facility at which stored antiprotons in the appropriate energy range are available with characteristics that meet the requirements for the first-ever antiproton polarization build-up studies.

CDF and D0 report single top quark events

CCnew8_03_09 — Examples of single top quark candidates in D0 (this image) and CDF. In both events the top quark decays and produces a b quark jet, a muon and a neutrino. In the CDF event (see other image), the arrow indicates the direction of the escaping neutrino.
Image credit: D0 and CDF.

Almost 14 years to the day after the announcement of the discovery of the top quark in 1995, the CDF and D0 collaborations at Fermilab have announced the observation of top quarks produced singly in proton–antiproton collisions, rather than in top antitop pairs. On 4 March, the two teams submitted their independent results to Physical Review Letters. Unlike pair-production of top quarks, which occurs through the strong interaction, the production of single top quarks occurs through the weak interaction and has important implications for possible new physics beyond the Standard Model.

Only one in every 20,000 million proton–antiproton collisions produces a single top quark, and to make matters worse, the signal of these rare occurrences is easily mimicked by other “background” processes that occur at much higher rates. Both teams have previously published evidence for single top production at Fermilab’s Tevatron, CDF last year and D0 in 2007. These earlier papers reported significance levels of 3.7 σ and 3.6 σ for CDF and D0, respectively. Now both teams report the first observation of the process with a significance of 5.0 σ, based on 3.2 fb^–1 of proton–antiproton collision data in CDF and 2.3 fb^–1 in D0.

CCnew9_03_09 — Examples of single top quark candidates in D0 (see other image) and CDF. In both events the top quark decays and produces a b quark jet, a muon and a neutrino. In the CDF event (this image), the arrow indicates the direction of the escaping neutrino.
Image credit: D0 and CDF.

Examples of single top quark candidates in D0 (see other image) and CDF. In both events the top quark decays and produces a b quark jet, a muon and a neutrino. In the CDF event (this image), the arrow indicates the direction of the escaping neutrino.
Image credit: D0 and CDF.

The analyses also constrain the magnitude of |V_tb|, an important parameter of the Standard Model’s Cabibbo-Kobayashi Maskawa (CKM) matrix, which describes how quarks can change from one type to another. If the CKM matrix describes the intermixing of only three generations of quarks – with top and bottom forming the third generation – the value of |V_tb| should be close to one. In the new analysis CDF finds |V_tb| = 0.91 ± 0.11(stat.+syst.) ± 0.07(theor.), while D0 reports |V_tbf^L| = 1.07±0.12 where f^Lis the strength of the left-handed coupling between the W boson and the top and bottom quarks.

In addition to its inherent success, discovering single top quark production has presented the collaborations with challenges similar to the search for the Higgs boson, in terms extracting an extremely small signal from a large background. Advanced analysis techniques pioneered for the single top discovery are now in use in both collaborations for the Higgs boson search.

Fermi sees most powerful gamma-ray burst

CCast1_03_09 — The X-ray afterglow of GRB 080916C appears orange and yellow in this view, which merges images from Swift’s ultraviolet/optical and X-ray telescopes.
Image credit: NASA/Swift/Stefan Immler.

The Fermi Gamma-ray Space Telescope has observed the evolution of a gamma-ray burst over six orders of magnitude in photon energy. The combination of its brightness and its remote distance makes it by far the most energetic gamma-ray blast ever seen. Furthermore, the observed delay of the highest-energy emission gives a lower limit on the strength of quantum-gravity effects.

Since the launch of the Swift satellite in November 2004, up to a few gamma-ray bursts (GRBs) are routinely detected every day. The phenomenon now seems commonplace and only the record-breaking bursts attract public attention.

After the “Rosetta stone” GRB 030329 and the “naked-eye” GRB 080319B, here comes the “extreme” GRB 080916C. This giant burst was observed by Fermi, which was launched into space last year. It is one of the rare bursts detected up to giga-electron-volt energies by the Large Area Telescope (LAT), the main instrument aboard Fermi. In five months the LAT has detected only 3 GRBs out of 58 that were in its field of view, according to the positions provided by the secondary instrument, the Gamma-ray Burst Monitor (GBM).

The burst of 16 September 2008, GRB 080916C, was the brightest observed so far and the only one with a distance determined by an observed redshift. The redshift of z = 4.35 ± 0.15, measured by the Gamma-Ray Burst Optical/Near-Infrared Detector (GROUND) on the 2.2 m Max Planck Telescope at La Silla, in Chile, locates the collapsing-star event at a distance of 12.2 thousand million light-years. This cosmological distance means that GRB 080916C was intrinsically extremely luminous – at least twice as much as the previous record-holder, GRB 990123, which was observed by the Energetic Gamma-Ray Experiment Telescope aboard the Compton Gamma-Ray Observatory.

The Fermi LAT and Fermi GBM collaborations have jointly published a detailed analysis of the emission of this extreme burst. The combined GBM and LAT spectra – covering the range from 8 keV to 300 GeV – are consistent with a very simple spectral shape. Spectra were extracted for five distinct epochs during the evolution of the burst and all have the simple form of a Band function, which smoothly joins low- and high-energy power laws. A simple physical interpretation for such spectra is synchrotron radiation of charged particles in a magnetic field, but this cannot be confirmed, because the synchrotron self-Compton emission expected in this case could not be detected.

The most interesting result is probably the evidence of a consistently increasing delay of higher-energy radiation during the second peak of the GRB emission. This time lag can be intrinsic to the source or induced by quantum-gravity effects along the path from the remote source to the telescope. The delay by about 16 s of the most energetic photon – 13 GeV – with respect to the on-set of the burst allows the researchers to derive a lower limit on the quantum-gravity mass of only about one order of magnitude below the Planck mass. The question of whether the observed delay is intrinsic to the source or results from its long journey through the quantum foam of space–time will eventually be solved with the detection of several other bursts with known redshift and measurable time delays.

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