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Dark-matter research arrives at the crossroads

CCdes1_03_09 — Gabriele Veneziano presents the 2008 DESY Heinrich-Hertz Lecture on Physics entitled “Space, time and matter”. This is for the general public and is held during the workshop.
Image credit: DESY Hamburg.

There is overwhelming evidence that the universe contains dark matter made from unknown elementary particles. Astronomers discovered more than 75 years ago that spiral galaxies, such as the Milky Way, spin faster than allowed by the gravity of known kinds of matter. Since then there have been many more observations that point to the existence of this dark matter.

Gravitational lensing, for example, provides a unique probe of the distribution of luminous-plus-dark matter in individual galaxies, in clusters of galaxies and in the large-scale structure of the universe. The deflection of gravitational light depends only on the gravitational field between the emitter and the observer, and it is independent of the nature and state of the matter producing the gravitational field, so it yields by far the most precise determinations of mass in extragalactic astronomy. Gravitational lensing has established that, like spiral galaxies, elliptical galaxies are dominated by dark matter.

Strong evidence for the fact that most of the dark matter has a non-baryonic nature comes from the observed heights of the acoustic peaks in the angular power spectrum of the cosmic microwave background measured by the Wilkinson Microwave Anisotropy Probe, because the peaks are sensitive to the fraction of mass in the baryons. It turns out that only about 4% of the mass of the universe is in baryons, whereas about 20% is in non- baryonic dark matter – a finding that is also in line with inferences from primordial nucleosynthesis.

A host of candidates

This leaves some pressing questions. What is the microscopic nature of this non- baryonic dark matter? Why is its mass fraction today about 20%? How dark is it? How cold is it? How stable is it?

CCdes2_03_09 — Worldwide, many experiments are trying to unravel the particle constituents of dark matter by searching for indirect signatures at accelerators and in cosmic rays, as well as for direct signatures in the laboratory. The Cryogenic Rare Event Search with Superconducting Thermometers is aiming at the direct detection of WIMPs. In its second phase it is using detectors based on scintillating crystal (CaWO₄) absorbers, operating in a cold box at around 10 mK.
Image credit: CRESST.

Progress in finding the answers to such questions provided the focus for the 2008 DESY b Theory Workshop, which was held on 29 September – 2 October.

Organized by Manuel Drees of Bonn, it sought to combine results from a range of experiments and confront them with theoretical predictions. It is clear that the investigation of the microscopic nature of dark matter has recently entered a decisive phase. Experiments are being carried out around the globe to try to identify traces of the mysterious dark-matter particles. Since the different theoretical candidates appear to have quite distinctive signatures, there are good reasons to expect that from a combination of all of these efforts a common picture will materialize within the next decade.

Theoretical particle physicists have proposed a whole host of candidates for the constituents of non-baryonic dark matter, with fancy names such as axions, axinos, gravitinos, neutralinos and lightest Kaluza–Klein partners. The best-motivated of these occur in extensions of the Standard Model that have been proposed to solve other problems besides the dark-matter puzzle. The axion, for example, arose in extensions that aim to solve the strong CP problem. It later turned out to be a viable dark- matter candidate if its mass is in the micro-electron-volt range. Gravitinos and neutralinos, on the other hand, are the superpartners of the graviton and the neutral bosons, respectively. They arise in supersymmetric extensions of the Standard Model, which aim at a solution of the hierarchy problem and at a grand unification of the strong and electroweak interactions. In fact, neutralinos are natural candidates for dark matter because they have cross-sections of the order of electroweak interactions and their masses are expected to be of the order of the weak scale (i.e. 100 GeV). This leads to the fact that their relic density resulting from freeze-out in the early universe is just right to account for the observed amount of dark matter.

Neutralinos belong to the class of weakly interacting massive particles (WIMPs). Such particles seem to be more or less generic in extensions of the Standard Model at the tera-electron-volt scale, but their stability (or a long enough lifetime) has to be imposed. This is not necessary for super-weakly interacting massive particles (superWIMPs), such as sterile neutrinos, gravitinos, hidden sector gauge bosons (gauginos) and the axino. For example, unstable but long-lived gravitinos in the 5–300 GeV mass range are viable candidates for dark matter and provide a consistent thermal history of the universe, including successful Big Bang nucleosynthesis.

Detecting dark matter

Owing to their relatively large elastic cross-sections with atomic nuclei, WIMPs such as neutralinos are good candidates for direct detection in the laboratory, yielding up to one event per day, per 100 kg of target material. The expected WIMP signatures are nuclear recoils, which should occur uniformly throughout the detector volume at a rate that shows an annual flux modulation by a few per cent. Intriguingly, the DAMA experiment in the Gran Sasso National Laboratory has seen evidence for such an annual modulation.

However, there is some tension with other direct-detection experiments. Theoretical studies have revealed that interpretation in terms of a low-mass (5–50 GeV) WIMP is marginally compatible with the current limits from other experiments. In contrast to DAMA, which looks just for scintillation light, most of the latter exploit at least two observables out of the set (phonons, charge, light) to reconstruct the nuclear recoil-energy.

CCdes3_03_09 — Experiments in space and on Earth are searching for indirect signals of dark matter in the spectra of cosmic radiation. Left: a rendering of the Fermi Gamma-Ray Space Telescope in orbit; centre: a phototube for the IceCube experiment at the South Pole; right: the PAMELA satellite experiment being prepared prior to launch.
Image credit: General Dynamics C4 Systems, IceCube collaboration, PAMELA Project.

Many different techniques based on cryogenic detectors (e.g. the Cryogenic Dark Matter Search), noble liquids (e.g. the XENON Dark Matter Project) or even bubble chambers, are currently employed to search for WIMPs via direct detection. Detectors with directional sensitivity (e.g. the Directional Recoil Identification From Tracks experiment) may not only have a better signal-to-background discrimination but may also be capable of measuring the local dark-matter, phase-space distribution. In summary, these direct experiments are currently probing some of the theoretically interesting regions for WIMP candidates. The next generation of experiments may enter the era of WIMP (astro) physics.

The axion is another dark-matter candidate for which there are ongoing direct- detection experiments. Both the Axion Dark Matter Experiment (ADMX) in the US and the Cosmic Axion Research with Rydberg Atoms in a Resonant Cavity (CARRACK) experiment in Japan exploit a cooled cavity inside a strong magnetic field to search for the stimulation of a cavity resonance from a dark-matter axion–photon conversion in the microwave frequency region, corresponding to the expected axion mass. While they differ in their detector technology – ADMX uses microwave telescope technology whereas CARRACK employs Rydberg atom technology – both experiments are designed to cover the 1–10 μeV mass range. Indeed, if dark matter consists just of axions then it should soon be found in these experiments. The CERN Axion Solar Telescope, meanwhile, is looking for axions produced in the Sun.

There are also of course possibilities for indirect detection. Dark matter may not be absolutely dark. In fact, in regions where the dark-matter density is high (e.g. in the Earth, in the Sun, near the galactic centre, in external galaxies), neutralinos or other WIMPs may annihilate to visible particle–antiparticle pairs and lead to signatures in gamma-ray, neutrino, positron and antiproton spectra. Moreover, superWIMPs (e.g. gravitinos), may also leave their traces in cosmic-ray spectra if they are not absolutely stable.

CCdes4_03_09 — The LHC at CERN will provide the opportunity for indirect detection of dark matter through the discovery of new particles, such as neutralinos.

Interestingly, the Payload for Antimatter Matter Exploration and Light-Nuclei Astrophysics (PAMELA) satellite experiment recently observed an unexpected rise in the fraction of positrons at energies of 10–100 GeV, thereby confirming earlier observations by the High Energy Antimatter Telescope balloon experiment. In addition, the Advanced Thin Ionization Chamber balloon experiment has reported a further anomaly in the electron-plus positron flux, which can be interpreted as the continuation of the PAMELA excess to about 800 GeV. The quantification of these excesses is still quite uncertain, not least because of relatively large systematic uncertainties. It is well established that they cannot be explained by the standard mechanism, namely the secondary production of positrons arising from collisions between cosmic-ray protons and the interstellar medium within our galaxy. However, a very conventional astrophysical source for them could be nearby pulsars.

On a more speculative level, these observations have inspired theorists to search for pure particle-physics models that accommodate all results. Generically, interpretations in terms of WIMP annihilation seem to be disfavoured, because they require a huge clumpiness of the Milky Way dark-matter halo, which is at variance with recent numerical simulations of the latter. This constraint is relaxed in superWIMP scenarios, where the positrons may be produced in the decay of dark-matter particles (e.g. gravitinos).

It is clear that one of the keys to understanding the origin of the excess in the positron fraction is the accurate, separate measurement of positron and electron fluxes, which can be done with further PAMELA data and with the Alpha Magnetic Spectrometer satellite experiment. Furthermore, distinguishing different interpretations of the observed excesses requires a multimessenger approach (i.e. to search for signatures in the radio range, synchrotron radiation, neutrinos, antiprotons and gamma rays).

Fortunately the Fermi Gamma-Ray Space Telescope is in orbit and taking data. Together with other cosmic-ray experiments it will probe interesting regions of parameter space in WIMP and superWIMP scenarios of dark matter.

Dark matter at colliders

Clearly, at colliders the existence of a dark-matter candidate can be inferred only indirectly from the apparent missing energy, associated with the dark-matter particles, in the final state of the collision. However, such a measurement can be made with precision and under controlled conditions. To extract the properties, such as the mass, of dark-matter particles, these final-state measurements have to be compared with predictions from theoretical models. In a supersymmetric extension of the Standard Model, for example, with the neutralino as the lightest superpartner, experiments at the LHC would search for signatures from the cascade decay of gluinos and squarks into gluons, quarks, leptons and neutralinos. This would show up as large missing transverse-energy in events with some jets and leptons. The endpoints in kinematic distributions could then be used for the determination of the dark-matter candidate’s mass, which could be compared with the mass determined eventually by measurements of recoil energy in direct-detection experiments.

This complementarity between direct, indirect and collider searches for dark matter is essential. Although collider experiments might identify a dark-matter candidate and precisely measure its properties, they will not be able to distinguish a cosmologically stable particle from one that is long-lived but unstable. In turn, direct detection cannot tell definitely what kind of WIMP has been observed. Moreover, in many superWIMP dark matter scenarios a direct detection is impossible, while detection at the LHC may be feasible. For example, if the lightest superpartner is a gravitino (or hidden gaugino) and the next-to-lightest is a charged lepton, experiments at the LHC may search for the striking signature of a displaced vertex plus an ionizing track.

In many cases, however, precision measurements from a future electron–positron collider seem to be necessary to exploit fully the collider–cosmology–astrophysics synergy. In addition “low-energy photon- collider” experiments – such as the Axion-Like Particle Search at DESY, the GammeV experiment at Fermilab and the Optical Search for QED magnetic birefringence, axions and photo regeneration at CERN, where the interactions of intense laser beams with strong electromagnetic fields are probed – may give viable insight into the existence of very lightweight, axion-like, dark-matter candidates.

In summary, there is evidence for non-baryonic dark matter that is not made of any known elementary particle. We are today in the exploratory stage to figure out its microscopic nature. Many ideas are currently being explored in theories and in experiments, and more will come. Nature has given us a few clues that we need to exploit. The data coming soon from accelerators, and from direct and indirect detection experiments, will be the final arbiter.

MINOS maps the deepest secrets of the upper atmosphere

A collaborative study by particle physicists and atmospheric researchers has found the first correlations between daily variations in cosmic-ray muons detected deep below ground and large-scale phenomena in the upper atmosphere. The effect suggests that underground muon-detectors could play a valuable role in identifying certain meteorological events and observing long-term trends.

Scott Osprey and colleagues from the UK’s National Centre for Atmospheric Science and Oxford University have worked with members of the Main Injector Neutrino Oscillation Search (MINOS) collaboration in analysing data collected between 2003 and 2007 by the MINOS Far Detector, located 705 m below ground in a disused iron mine at Soudan, in Minnesota. The MINOS experiment intercepts a neutrino beam that goes 725 km from Fermilab to Soudan and studies long-baseline neutrino oscillations; the penetrating muons appear as background noise.

The teams have found a close relationship between the rate of muons detected in MINOS and upper-air temperatures from the European Centre for Medium Range Weather Forecasts. In particular, they discovered strong correlations between the muon rate and the upper-air temperature during short-term events (of around 10 days) in the upper atmosphere, or stratosphere, in winter.

When primary comic rays strike the Earth’s atmosphere they interact, creating pions and kaons. These mesons in turn decay to produce muons – the most energetic of which penetrate deep below the Earth’s surface. The mesons can also interact before they decay, so the number of muons produced depends on the local density of the atmosphere and varies with temperature. An increase in temperature means a decrease in density and, hence, fewer mesons interact and instead decay, increasing the number of muons. Physicists have known of this effect since the Monopole Astrophysics and Cosmic Ray Observatory first observed a seasonal variation in the rate of muons a decade ago (Ambrosio et al. 1997).

CCnew4_03_09 — The MINOS Far Detector is helping to study the weather.
Image credit: Fermilab Visual Media Services.

Most of the mesons that give rise to the muons detected in MINOS occur at altitudes of around 15 km in the region known as the tropopause, where there is little variation in temperature. However, the mesons also occur in the mid-stratosphere – at altitudes where temperatures fluctuate, particularly in winter. For the analysis, the team defined an “effective” temperature based on an average temperature over the altitudes where mesons occur, weighted by the calculated distribution of meson production.

The results show a striking relationship between this temperature and the number of muons, with correlated changes occurring over periods of only a few days (Osprey et al. 2009). The data for the Northern Hemisphere winter of 2004–2005 are particularly interesting. The meteorological data indicate the occurrence of a major phenomenon, known as a sudden stratospheric warming, during February. This was linked to break-up of the winter polar vortex, a polar cyclone that brings cooler weather and which extended over the MINOS site in early February. Prior to that, the 2004–2005 winter had seen the lowest recorded temperatures in the polar stratosphere, and ozone concentration in the polar vortex was anomalously low.

The results show that underground muon data contain information that could identify important short-term meteorological events, over and above the already known seasonal effect. This is interesting for atmospheric researchers, as it provides an independent technique to measure such phenomena. Moreover, physicists have cosmic-ray data from experiments dating back 50 years or more, covering periods when upper-air observations from weather balloons were less reliable than today.

Conference probes the dark side of the universe

During the past decade a consistent quantitative picture of the universe has emerged from a range of observations that include the microwave background, distant supernovae and the large-scale distribution of galaxies. In this “standard model” of the universe, normal baryonic matter contributes only 4.6% to the overall density; the remainder consists of dark components in the form of dark matter (23%) and dark energy (72%). The existence and dominance of dark energy is particularly unexpected and raises fundamental questions about the foundations of modern physics. Is dark energy merely Albert Einstein’s cosmological constant? Is it a new kind of field that evolves dynamically as the universe expands? Or is a new law of gravity needed?

In the search for answers to these questions, more than 250 participants, ranging from senior experts to young students, attended the 3rd Biennial Leopoldina Conference on Dark Energy held on 7–11 October 2008 at the Ludwig Maximilians University (LMU) in Munich. The meeting was organized jointly by the Bonn-Heidelberg-Munich Transregional Research Centre “The Dark Universe” and the German Academy of Sciences Leopoldina, with support from the Munich-based Excellence Cluster “Origin and Structure of the Universe”. The goal of the international symposium was to gain a better understanding of the nature of dark energy by bringing together observers, modellers and theoreticians from particle physics, astrophysics and cosmology to present and discuss their latest results and to explore possible future routes in the rapidly expanding field of dark-energy research.

CCdar1_03_09 — Fig. 1. An illustration of the time evolution of the universe, starting with the Big Bang.
Image credit: NASA/WMAP Science Team.

Around 60 plenary talks at the conference were held in the central auditorium (Aula) of LMU Munich, with lively discussions following in poster sessions (where almost 100 posters were displayed) and during the breaks in the inner court of the university. There were fruitful exchanges between physicists engaged in a range of observations, from ground-based studies of supernovae to satellite probes of the cosmic microwave background (CMB), and theorists in search of possible explanations for the accelerated expansion of the universe, which was first reported in 1998. This acceleration has occurred in recent cosmic history, corresponding to redshifts of about z ≤ 1.

An accelerating expansion

Brian Schmidt of the Australian National University in Canberra gave the observational keynote speech. He led the High-z Supernova Search Team that presented the first convincing evidence for the existence of dark energy – which works against gravity to boost the expansion of the universe – almost simultaneously with the Supernova Cosmology Project led by Saul Perlmutter of the Lawrence Berkeley National Laboratory and the University of California at Berkeley. Adam Riess, a member of the High-z team, presented constraints on dark energy from the latest supernovae data, including those from the Hubble Space Telescope at redshift z > 1. This is where the acceleration becomes a deceleration, owing to the lessening impact of dark energy at earlier times (figure 1).

Both teams independently discovered the accelerating expansion of the universe by studying distant type Ia supernovae. They found that the light from these events is fainter than expected for a given expansion velocity, indicating that the supernovae are farther away than predicted (figure 2, p18). This implies that the expansion is not slowing under the influence of gravity – as might be expected – but is instead accelerating because of some uniformly distributed, gravitationally repulsive substance accounting for more than 70% of the mass-energy content of the universe – now known as dark energy.

Type Ia supernovae arise from runaway thermonuclear explosions following accretion on a carbon/oxygen white dwarf star and after calibration have an almost uniform brightness. This makes them “standard candles”, suitable as tools for the precise measurement of astronomical distances. Wolfgang Hillebrandt of the Munich Max-Planck Institute for Astrophysics presented 3D simulations of type Ia supernova explosions. It is still a matter of debate how standard these so-called “standard candles” really are. Their colour–luminosity relationship is inconsistent with Milky Way-type dust and, as Robert Kirshner of the Harvard-Smithsonian Center for Astrophysics mentioned, the role of dust is generally underestimated. Future supernova observations in the near infrared hold promise because, at these wavelengths, the extinction by dust is five times lower. Bruno Leibundgut of ESO said that infrared observations using the future James Webb Space Telescope will be crucial in solving the problem of reddening from dust.

As Schmidt pointed out, and others detailed in subsequent talks, measurements of the temperature fluctuations in the CMB provide independent support for the theory of an accelerating universe. These were first observed by the Cosmic Background Explorer in 1991 and subsequently in 2000 by the Boomerang and MAXIMA balloon experiments. Since 2003 the Wilkinson Microwave Anisotropy Probe (WMAP) has observed the full-sky CMB with high resolution. Additional evidence came from the Sloan Digital Sky Survey and 2-degree Field Survey. In 2005 they measured ripples in the distribution of galaxies that were imprinted in acoustic oscillations of the plasma when matter and radiation decoupled as protons and electrons combined to form hydrogen atoms, 380,000 years after the Big Bang. These are the “baryonic acoustic oscillations” (BAOs).

Dark-energy candidates

Eiichiro Komatsu of the Department of Astronomy at the University of Texas in Austin, lead author of WMAP’s paper on the cosmological interpretation of the five-year data, said that anything that can explain the observed luminosity distances of type Ia supernovae, as well as the angular-diameter distances in the CMB and BAO data, is “qualified for being called dark energy” (figure 3). Candidates include energy, modified gravity and an extreme inhomogeneity of space.

CCdar2_03_09 — Fig. 2. Expansion of the universe as indicated by luminosity data from type Ia supernovae as a function of redshift, z (A Riess et .).
Image credit: Michael Turner, conference summary; Turner and Huterer 2007.

Although the latter approach was presented in several talks, the impression prevailed that the effects of dark energy are too large to be accounted for through spatial inhomogeneities and an accordingly adapted averaging procedure in general relativity. Komatsu – and many other speakers – clearly favours the Lambda-cold-dark-matter (ΛCDM) model, with a small cosmological constant Λ to account for the accelerated expansion. The dark-energy equation of state is usually taken to be w = p/ρ= –0.94 ± 0.1(stat.) ± 0.1 (syst.) with a negative pressure, p; a varying w is not currently favoured by the data. Several speakers presented various versions of modified gravity. Roy Maartens of the University of Portsmouth in the UK acknowledged that ΛCDM is currently the best model. As an alternative he presented a braneworld scenario in which the vacuum energy does not gravitate and the acceleration arises from 5D effects. This scenario is, however, challenged by both geometric and structure-formation data.

Theoretical keynote-speaker Christof Wetterich of Heidelberg University emphasized that the physical origin, the smallness and the present-day importance of the cosmological constant are poorly understood. In 1988, almost simultaneously with but independently from Bharat Ratra and James Peebles, he proposed the existence of a time-dependent scalar field, which gives rise to the concept of a dynamical dark energy and time-dependent fundamental “constants”, such as the fine-structure constant. Although observations may eventually decide between dynamical or static dark energy, this is not yet possible from the available data.

CCdar3_03_09 — Fig. 3. Evidence for dark energy from observations of type Ia supernovae (SNe), fluctuations of the cosmic microwave background radiation (CMB) and baryonic acoustic oscillations (BAO).
Image credit: Michael Turner, conference summary; Kowalski et . 2008.

Yet another indication for the accelerated expansion comes from the investigation of the weak-lensing effect, as Matthias Bartelmann of Heidelberg University and others explained. This method of placing constraints on dark energy through its effect on the growth of structure in the universe relies on coherent distortions in the shapes of background galaxies by foreground mass structures, which include dark matter. The NASA-DOE Joint Dark Energy Mission (JDEM) is a space probe that will make use of this effect, in addition to taking BAO observations and distance and redshift measurements of more than 2000 type Ia supernovae a year. The project is now in the conceptual-design phase and has a target launch date of 2016. ESA’s corresponding project – the Dark UNiverse Explorer – is part of the planned Euclid mission, scheduled for launch in 2017. There were presentations on both missions.

The first major scientific results from the 10 m South Pole Telescope (SPT) initial survey were the highlight of the report by John Carlstrom, principal investigator for the project. The telescope is one of the first microwave telescopes that can take large-sky surveys with precision. It will be possible to use the resulting size-distribution pattern together with information from other telescopes to determine the strength of dark energy.

CCdar4_03_09 — The South Pole Telescope will allow independent tests for the existence and strength of dark energy.
Image credit: Bradford Benson.

Carlstrom described the detection of four distant, massive clusters of galaxies in an initial analysis of SPT survey data – a first step towards a catalogue of thousands of galaxy clusters. The number of clusters as a function of time depends on the expansion rate, which leads back to dark energy. Three of the detected galaxy clusters were previously unknown systems. They are the first clusters detected in a Sunyaev–Zel’dovich (SZ) effect survey, and are the most significant SZ detections from a subset of the ongoing SPT survey. This shows that SZ surveys, and the SPT in particular, can be an effective means of finding galaxy clusters. The hope is for a catalogue of several thousand galaxy clusters in the southern sky by the end of 2011 – enough to rival the constraints on dark energy that are expected from the Euclid Mission and NASA’s JDEM.

The conference was lively and social activities enabled discussions outside the conference auditorium, particularly during the lunch breaks in nearby Munich restaurants. The presentations and discussions all demonstrated that the search for definite signatures and possible sources of the accelerated expansion of the universe continues to flourish and has an exciting future ahead. The results on supernovae and the CMB have led the way, but there is still much to learn. In his conference summary, Michael Turner of the University of Chicago emphasized that “cosmology has entered an era with large quantities of high-quality data”, and that the quest to understand dark energy will remain a grand scientific adventure. Future observational facilities – such as the Planck probe of the CMB, which is scheduled for launch around Easter 2009, the all-sky galaxy-cluster X-ray mission eROSITA, ESA’s Euclid and NASA’s JDEM – are all designed to produce unprecedented high-precision cosmology results that will shed new light on dark energy.

CMS does a full cosmic-data run

Fig. 1. Event display of a cosmic muon going through the pixel and silicon strip trackers (green and magenta), the electromagnetic and hadron calorimeters (magenta and blue, respectively), and the barrel muon chambers (green).

On 11 November 2008 the conclusion of a month-long, major data-taking run by the CMS collaboration brought a two-year commissioning phase to a successful close. The aim of the Cosmic Run At Four Tesla (CRAFT) was to run CMS continuously as a complete experiment, 24 hours a day, to gain further operational experience even without LHC beams. Data from 300 million cosmic muons were recorded with the solenoid at its operating point of 3.8 T for detailed detector studies. By the end of the exercise more than 7 million tracks in the strip tracker and around 75,000 tracks in the pixel tracker were available for alignment and other studies. The data volume totalled an impressive 400 TB. Runs were reconstructed at the Tier-0 centre with a typical latency of six hours before shipping to several Tier-1 and Tier-2 centres.

The CMS data flow was stressed during CRAFT in a way similar to what is foreseen for LHC operations, with calibration and/or alignment sequences performed for the electromagnetic calorimeter, the tracker and the muon systems during the run. Random triggers added on top of the cosmic-muon triggers emulated the trigger rates that will be experienced at the LHC. The high-level trigger ran a menu similar to the one used for the LHC start-up, with the installed complement of nearly 4500 filter processors for the CMS filter farm (around 40% of the final number) being deployed for the first time at the end of the run. Along with the main cosmic data, special raw-data streams created for specific calibration and alignment purposes were shipped to the Tier-0 centre. Teams residing at the CMS analysis facility in Meyrin and at remote centres including DESY and Fermilab checked the data quality offline and validated the online quality-assignments of the data-quality monitoring system.

Fig. 2. The reconstructed transverse-momentum distribution for cosmic tracks during the run.

The precision of tracker alignment previously obtained with data recorded without a magnetic field is now improving significantly with the data collected during CRAFT because the momentum measurement enables better control of the uncertainty that arises from multiple scattering. The run also allowed an initial alignment of the modules comprising the barrel pixel detector. The collaboration completed a first reprocessing of the CRAFT data, incorporating these newly determined calibration and alignment constants, in early December 2008. Several analysis teams will use these data to perform some basic physics measurements, including measurements of the charge ratio and momentum distribution of cosmic muons.

Residuals for the inner-barrel 7_01_09 — Fig. 3. Distribution of the means of the residuals for the inner-barrel compartment of the silicon strip tracker. Red and green lines show the improvement of the alignment using tracks in the CRAFT data samples (comparing two alignment algorithms, Millepede (MP) and HIP) with respect to the earlier alignment constants: black shows the result with the initial geometry, blue with data from cosmics collected without a magnetic field.

The success of the continuous operation of CMS in “LHC-like” conditions marks the end of a commissioning phase that started two years ago: in November 2006 both the underground experimental cavern and the adjoining service cavern (which is now buzzing with all of the off-detector readout electronics) were empty. The CMS teams are looking back with understandable pride at what has been achieved since then and are looking forward to the challenges of operation with colliding LHC beams in 2009. The commissioning programme to improve the readiness for LHC physics will resume after the annual cooling maintenance, which is expected to take place in late January 2009.

Proton-rich nuclei shed light on heavy-element synthesis in cosmos

Researchers at the National Superconducting Cyclotron Laboratory (NSCL) at Michigan State University (MSU) have measured the half-lives of ¹⁰⁰Sn and ⁹⁶Cd, two nuclei with equal numbers of protons and neutrons that are close to the proton drip line – the proton-rich limit of stability. The result for ¹⁰⁰Sn narrows the error range of previous half-life measurements, while the half-life of ⁹⁶Cd, measured here for the first time, casts a light on the role of the isotope in the rapid proton-capture (rp) process – a key part of heavy-element synthesis in the cosmos. The result for ⁹⁶Cd also implies a new, as-yet-unknown origin for ⁹⁶Ru in the solar system, where its abundance has long remained unexplained.

Daniel Bazin and colleagues at MSU used the same fast-beam fragmentation scheme to create both species. Using the facility’s coupled cyclotrons, the team generated a primary beam of 120 MeV/nucleon ¹¹²Sn and fragmented it on a beryllium target. The resulting radioactive beam was filtered through the A1900 Fragment Separator and newly commissioned RF Fragment Selector. Finally, the filtered secondary beam implanted itself in NSCL’s Beta Counting System, a series of silicon beta-particle detectors flanked by detectors of the laboratory’s segmented germanium array. To track the beta-decay of implanted nuclei, Bazin’s team monitored decay events at the impact site and neighbouring pixels in the detector for 10 s after implantation.

CCnew12_01_09 — Particle identification spectrum of the heavy nuclei transmitted through the Radio-Frequency Fragment Selector. The most intense contaminants normally present at larger A/Z are rejected. The low-Z contaminants are not shown on this figure. The gates indicated on the spectrum were used to select the N = Z nuclei, to reduce cross contamination. (Bazin *et al.* 2008.)

For ¹⁰⁰Sn, the team observed a half-life of 0.55^+0.70_–0.31s. This result is similar to previous measurements made at GSI and yields an average of 0.86^+0.37_–0.20s when combined. The increased precision may bolster understanding of this isotope, which is one of the few “doubly magic” nuclei close to the proton drip line. Its protons and neutrons both form a closed-shell configuration, which affords extra stability to the nucleus.

The measured half-life of ⁹⁶Cd, which was previously unknown, was 1.03^+0.24_–0.21. This is within the range of several theoretical predictions but it is too short to make ⁹⁶Cd a critical “waiting point” in the rp process. This process, along with slow neutron capture and rapid neutron capture, probably accounts for many of the universe’s heavy elements. It occurs in supernovae, X-ray bursts and perhaps other astrophysical environments where seed nuclei join with free protons to form nuclei of increasing atomic number. Build-up stalls at specific stages when the binding of another proton is energetically unfavourable. Nuclei accumulate at these so-called waiting points, generating a spike in the observed isotope abundance. Such a spike exists at ⁹⁶Ru, the product of beta-decay from ⁹⁶Cd, which suggests a waiting point at ⁹⁶Cd.

With the result for ⁹⁶Cd, the half-lives of all expected waiting points along the proton drip line, up to the rp-process’s predicted endpoint, are now known experimentally. However, the half-life that Bazin and collaborators have measured is approximately a tenth of the value required to account for the observed abundance of ⁹⁶Ru. There must be a different explanation – perhaps an unexplored astrophysical process.

Protons and neutrons cosy up in nuclei and neutron stars

Short range correlation reaction — Fig. 1. An illustration of a short-range correlation reaction. By selecting kinematics beyond the Fermi momentum of nucleons in the nucleus, knocking out a proton causes a high-momentum correlated partner nucleon to be emitted from the nucleus, leaving the rest of the system relatively unaffected.

The structure of nuclei is determined by the nature of the strong force: strong repulsion at short distances and strong attraction at moderate distances. This force, which binds the nucleons together while also keeping the structure from collapsing, makes the nucleus a fairly dilute system. This has allowed for calculations that treat the nucleus as a collection of hard objects in an average or mean field to describe many of the properties of nuclear matter. Of course, this simple picture of the nucleus is inaccurate – the nucleons should really be thought of as waves that can strongly overlap for short periods of time. Indeed, recent experiments have shown that about 20% of all nucleons in carbon are in such a state at any given time.

These states of strongly overlapping wave functions are commonly referred to as nucleon–nucleon short-range correlations (SRC). Calculations indicate that, for short periods, these correlations lead to local densities in the nucleus that are several times as high as the average nuclear density of 0.17 GeV/fm³. Such densities are comparable to those predicted in the core of neutron stars. c– whether extremely small (such as helium nuclei) or extremely large (such as neutron stars).

The distinctive experimental features of two-nucleon SRCs are the large back-to-back relative momentum and small centre-of-mass momentum of the correlated pair, where large and small are relative to the Fermi-sea level of about 250 MeV/c. This is shown in figure 1, where a virtual photon is absorbed by one nucleon in a correlated pair, causing both nucleons to be emitted from the nucleus. The large strength of the nucleon–nucleon interaction at short distances means that the relative motion in the pair should be the same in all nuclei, although the absolute probability of a correlation grows with density – with the probability of a nucleon to be part of a pair reaching 25% for iron and heavier nuclei.

Scaling effects

Isolating the signal of the SRC initial state has been difficult at low and medium energies because other processes (such as final-state interactions and meson-exchange currents) mimic this effect. Nevertheless, there has recently been progress using modern accelerators with high-luminosity and high-momentum transfer – as well as with kinematics, where competing mechanisms are suppressed. For electron scattering, this corresponds to luminosities of 10³⁷ cm^–2s^–1; a four-momentum transfer, Q², greater than 1.4 (GeV/c)²; and focusing on kinematics where Bjorken-x, Q²/2mυ, is greater than 1, where υ is the beam energy minus the energy of the scattered electron. For elastic scattering from a free proton, Bjorken-x is exactly 1. At least two nucleons must be involved to have x > 1; x > 2 requires a system with at least three nucleons.

Cross-section ratio — Fig. 2. The cross section ratio ¹²C to ³He as a function of Bjorken-x for Q² >1.4 (GeV/c)². The horizontal solid and dashed lines indicate scaling regions, which have been interpreted as being due to two- and three-nucleon SRCs, respectively. As the cross-sections fall rapidly with Bjorken-x, the absolute strength of the three-nucleon correlation is an order of magnitude smaller than the two-nucleon correlation.

One of the new results has come from inclusive data at high momentum-transfer, Q² > 1.4 (GeV/c)², and x > 1 from the Hall B CEBAF Large Acceptance Spectrometer at the US Department of Energy’s Jefferson Laboratory (K S Egiyan et al. 2006). The measurement was made to check the predicted universality of SRCs by measuring the ratio of the inclusive cross-sections off heavy nuclei to those of light nuclei at sufficiently large Q² and x, where the scattering off slow nucleons in the nucleus does not contribute. The signal predicted to indicate dominance of such correlations is the scaling of the ratios – a weak dependence on x and Q² for 1 < x <2 – which is clearly observed in the data. Continuing this line of reasoning would suggest that a second scaling region arising from three-nucleon correlations should be observed for x > 2. Indeed, a second scaling region does seem to be present, although the statistics are limited (figure 2). These results reflect the dominance of few-nucleon correlations in the high-momentum component of the nucleus.

While the inclusive data clearly suggest strong local correlations, it has taken exclusive data to confirm that the inclusive scaling arises from SRCs, as well as to measure directly what fraction of nucleon-pair types are involved. In exclusive experiments, using a high-momentum probe to remove one fast nucleon from the nucleus effectively breaks a pair and releases the second nucleon of the correlation. Brookhaven National Laboratory and Jefferson Lab have conducted such tests on the carbon nucleus with a hadronic and electromagnetic probe, respectively. They measured momentum transfers of greater than 1.5 GeV/c and a missing momentum greater than the Fermi momentum of 250 MeV/c.

Fraction of SRC pair combinations — Fig. 3. The fractions of SRC pair combinations in ¹²C as obtained from the Jefferson Lab (e,e’pp) and (e,e’pn) reactions, as well as from Brookhaven (p,2pn) data. The results agree with the interpretation of the inclusive data, and show the dominance of proton–neutron pairs.

Both experiments have shown that recoiling nucleons, with a momentum above the Fermi-sea level in the nucleus, are part of a correlated pair and both observed the same strength of proton–neutron correlations (Piasetzky et al. 2006; Subedi et al. 2008). This confirms that the process is accessing a universal property of nuclei unrelated to the probe. The Jefferson Lab’s experiment also observed the proton–proton pairs and used matched-acceptance detectors to determine the ratio of neutron–proton to proton–proton pairs as nearly 20, as figure 3 shows. Calculations explain the magnitude of this neutron–proton to proton–proton ratio as arising from the short-range tensor part, or nucleon–nucleon spin-dependent part, of the nucleon–nucleon force (Sargsian et al. 2005; Schiavilla et al. 2007; Alvioli et al. 2008).

Isolating the signatures of SRCs opens new avenues for the exploration of nucleon–nucleon interactions at short distances, particularly in addressing the long-standing question of how close nucleons have to approach before the nucleons’ quarks reveal themselves. Nucleon degrees of freedom can no longer be used to describe the system.

These studies can also influence calculations of the extremely massive. Without SRCs, a large object, such as a neutron star, could be well approximated as a Fermi gas predominantly of neutrons with a small fraction of protons acting as a separate Fermi gas. With SRCs the protons and neutrons interact, strongly enhancing the high-momentum component of the proton momentum distribution, leading to changes in the physical properties of the system (figure 4).

Momentum space illustration — Fig. 4. Illustration in momentum space of the momentum distribution, k_Fermi, of protons and neutrons in neutron stars. The left side shows the case where the protons and neutrons weakly interact and can be approximated as separate Fermi spheres, with the neutron momentum much greater than the proton momentum. The figure on the right shows how strong neutron–proton SRCs cause protons to escape their Fermi sphere and have higher momentum then would otherwise be allowed.

In the future, inclusive short-range-correlation experiments will improve the statistics of the x > 2 data to show definitively whether or not there is indeed a second scaling. These will use targets such as ⁴⁰Ca and ⁴⁸Ca to measure the dependence on the initial-state proton–neutron ratio. The future exclusive experiments will focus on ⁴He (a nucleus where both full and mean-field calculations can come together) and push the limits of the recoil momentum to extend our understanding of the repulsive part of the nucleon–nucleon potential.

The Pauli principle faces testing times

The Pauli exclusion principle (PEP), and more generally the spin-statistics connection, plays a pivotal role in our understanding of countless physical and chemical phenomena, ranging from the periodic table of the elements to the dynamics of white dwarfs and neutron stars. It has defied all attempts to produce a simple and intuitive proof, despite being spectacularly confirmed by the number and accuracy of its predictions, because its foundation lies deep in the structure of quantum theory. Wolfgang Pauli remarked in his Nobel Prize lecture (13 December 1946): “Already in my original paper I stressed the circumstance that I was unable to give a logical reason for the exclusion principle or to deduce it from more general assumptions. I had the feeling, and I still have it today, that this is a deficiency. The impression that the shadow of some incompleteness fell here on the bright light of success of the new quantum mechanics seems, to me, unavoidable.” Pauli’s conclusion remains basically true today.

CCspi1_01_09 — Participants of SpinStat 2008 filled the entrance stairs of the Stazione Marittima conference centre in Trieste.
Image credit: Erica Novacco.

The PEP was a major theme of the workshop “Theoretical and experimental aspects of the spin-statistics connection and related symmetries” at SpinStat 2008, held in Trieste on 21–25 October at the Stazione Marittima conference centre. Some 60 theoretical and experimental physicists attended, as well as a number of philosophers of science. The aim was to survey recent work that challenges traditional views and to put forward possible new experimental tests, including new theoretical frameworks.

A single framework for discussion

On the theoretical side, several researchers are currently exploring theories that may allow a tiny violation of PEP, such as quon theory, the existence of hidden dimensions, geometric quantization and a new spin-statistics connection in the framework of quantum gravity. Others have done several experiments over the past few years to search for possible small violations of the spin-statistics connection, for both fermions and photons. Thus scientists have recently obtained new limits for the validity of PEP for nuclei, nucleons and electrons, as well as for the validity of Bose–Einstein statistics for photons. These results were presented during the workshop and discussed for the first time in a single framework together with theoretical implications and future perspectives. The aim was to accomplish a “constructive interference” between theorists and experimentalists that could lead towards possible new ideas for nuclear and particle-physics tests of the PEP’s validity, including the interpretation of existing results.

CCspi2_01_09 — Some of the workshop participants gathered for a group photograph on a terrace overlooking the city centre.
Image credit: Catalina Curceanu.

The workshop benefited from the presence of researchers who have devoted a life’s work to the thorough examination of the structure of the spin-statistics connection in the context of quantum mechanics and field theory. In addition, young scientists put forward suggestions and experimental results that may pave the way to interesting future developments.

Oscar W Greenberg of the University of Maryland opened the workshop with a review talk on theoretical developments, with special emphasis on quon theory – which characterizes particles by a parameter q, where q spans the range from –1 to +1 and thus interpolates between fermion and boson – in an effort to develop more general statistics. Greenberg is the originator of this concept and he continues to be a major contributor to its theoretical development, maintaining a high degree of interest in the field. Robert Hilborn of the University of Texas reviewed the past experimental attempts to find a violation. Other theoretical speakers included distinguished scientists such as Stephen Adler, Michael Berry, Aiyalam P Balachandran, Sergio Doplicher, Giancarlo Ghirardi, Nikolaos Mavromatos and Allan Solomon.

CCspi3_01_09 — The public event – a reading of selected parts of the book by George Gamow and Russell Stannard, The New World of Mr Tompkins – was a great success. The cartoon seen here, illustrating the twin paradox of special relativity, is by Javad Alizadeh.
Image credit: Erica Novacco.

The experimental reports included presentations on spectroscopic tests of Bose–Einstein statistics of photons by Dmitry Budker’s group at the University of California and the Lawrence Berkeley National Laboratory, and studies of spin-statistics effects in nuclear decays by Paul Kienle’s group at the Stefan Mayer Institute for Subatomic Physics in Vienna. Other talks included results from the Borexino neutrino experiment and the DAMA/LIBRA dark-matter detector in the Gran Sasso laboratory, the KLOE experiment at Frascati, the NEMO-2 detector in the Fréjus underground laboratory and the dedicated Violation of the Pauli exclusion principle experiment in the Gran Sasso laboratory. Each talk was followed by lively discussions concerning the interpretation of the results. Michela Massimi of University College London closed the workshop with an excellent talk on historical and philosophical issues.

Another highlight was the event held for the general public: a reading of selected parts of the book by George Gamow and Russell Stannard, The New World of Mr Tompkins, where the professor depicted in Gamow’s book was played by a witty Michael Berry from the University of Bristol. This event was a success, especially among the young students who participated so enthusiastically.

Overall, the workshop showed that the field is full of new and interesting ideas. Although nobody expects gross violations of the spin-statistics connection, there could be subtle effects that may point to new physics in a context quite different from that of the LHC.

The workshop was sponsored jointly by the INFN and the University of Trieste. It received generous contributions from the Consorzio per la Fisica, the Hadron Physics initiative (Sixth Framework Programme of the EU) and Regione Friuli–Venezia Giulia.

Brookhaven finds more rare kaon decays

The E949 collaboration at Brookhaven National Laboratory has observed three new events of the rare kaon decay K⁺ → π⁺νν. This brings the total number observed to seven, four of which were found by E949 and three by its predecessor E787. The branching ratio from all seven candidate events is (1.73 + 1.15/–1.05) × 10^–10, which is consistent with the Standard Model prediction of (0.85 ± 0.07) × 10^–10.

CCnew3_10_08 — Kinetic energy vs. range of all events passing all other cuts. The squares show events selected by the latest analysis by the E949 collaboration. The circles and triangles represent events selected by previous E787 and E949 analyses, while the solid (dashed) lines show the limits of the signal regions for the different E949 (E787) analyses. The points near E = 108 MeV are from K⁺ → π⁺π⁰ decays that survived the photon veto cuts. The light-grey points are simulated K⁺ → π⁺νν̅ events that would be accepted by the trigger. (For more details see Artamonov *et al.* 2008.)

The decay, K⁺ → π⁺νν, which is one of the rarest and most challenging particle decays ever observed, is highly sensitive to physics beyond the Standard Model (SM). The uncertainty of the SM prediction, which involves second-order weak interactions – that is, the exchange of two weak force carrier bosons – is less than 10%. Any deviations uncovered by a precise measurement of this branching ratio could unambiguously signal the presence of new physics effects that are predicted in extensions to the SM.

The experimental signature for K⁺ → π⁺νν decay is the detection of a solitary positively charged pion, since the emitted neutrino and anti-neutrino pair interact too weakly to be detected, but unfortunately the sought-after signal resembles many other kaon decay channels. To identify the pion positively and ensure that no other observable decay particles were present, the collaboration created one of the most efficient particle-detection systems ever built. They also employed unbiased “blind” analysis techniques, which were pioneered by E787 and are now frequently used in modern high-energy physics experiments.

The three new events, which were obtained in a sample of 1.7 × 10¹² kaon decays, were observed in a low-energy pion region (see figure). This presented an even greater experimental challenge relative to the high-energy pion region, owing to additional processes that can mimic the K⁺ → π⁺νν decay signature. The total background expected was 0.93 ± 0.17(Stat.) +0.32/–0.24(syst.) events, primarily from π⁺ scattering in the stopping target.

The result confirms detailed predictions of the SM at higher orders. Given the level of statistical uncertainty associated with the result, only limitations on new physics beyond the SM can be inferred. However, a new generation of K⁺ → π⁺νν measurements from the NA62 experiment at CERN aim for a precision comparable to that of the current SM prediction.

• The E949 and E787 experiments at Brookhaven’s Alternating Gradient Synchrotron included more than 100 collaborators from Canada, China, Japan, Russia and the US.

CERN pulls Strings together

CCstr1_10_08 — Strings 2008 drew a large audience from many countries. Participants at the event are pictured on the steps of CERN’s well known Main Building.

The annual “Strings” conference draws together a large number of active researchers in the field from all over the world. As the largest and most important event on string theory, it aims to review the recent developments for experts, rather than give a comprehensive overview of the field. CERN was an attractive venue for the conference this year, with the imminent start-up of the LHC together with the longer-term Theory Institutes on string phenomenology and black holes taking place just before and after the event. Organized by CERN’s Theory Unit, the universities of Geneva and Neuchâtel, and the ETH Zurich, Strings 2008 attracted more than 400 participants from 36 countries. It opened in the presence of CERN’s management and the rector of the University of Geneva, who also represented the state of Geneva. Appropriately, the first talk was by Gabriele Veneziano, formerly of CERN and one of the initiators of string theory following his famous formula invention 40 years ago. There was a welcome reception at the United Nations in Geneva, and the conference banquet was held in the Unimail building at the university.

A framework for unified particle physics

String theory can be seen as a framework for generalizing conventional particle quantum field theory, with applications stretching across a broad range of areas, such as quantum gravity, grand unification, gauge theories, heavy-ion physics, cosmology and black holes. It allows the systematic investigation of many of the important features of such theories by providing a coherent and consistent way of formulating the problems at hand. As Hirosi Ooguri from the California Institute of Technology so aptly said in his summary talk, string theory can be viewed, depending on the application, as a candidate, a model, a tool and/or a language.

The richness of string theory makes it a candidate for a consistent framework that truly unifies all of particle physics, including gravity. It also provides a stage for analysing complicated problems, such as quantum black holes and strongly coupled systems, as in quark–gluon plasma, through the means of idealized, often supersymmetric, models. Moreover, string theory has been proved to be an invaluable tool for doing computations in particle physics in an extremely efficient manner. It also often provides a novel language, with which it miraculously transforms seemingly hard problems into simple ones by reformulating them in a “dual” way. This also includes certain hard problems in mathematics that become simple when translated into the language of string theory.

CCstr2_10_08 — Jean-Dominique Vassalli (centre), rector of the University of Geneva, at the conference dinner with David Gross (right), who gave the final outlook talk, and Ignatios Antoniadis, one of the conference organizers. The dinner was held in the Unimail building at the university.
Image credit: E Gianolio.

The talks displayed all of these four facets of string theory effectively. Essentially there were five key areas on which the conference focused, roughly reflecting the fields of highest activity and progress during the past year. In addition, there were three talks on the LHC and its physics by the project leader, Lyn Evans; CERN’s chief scientific officer, Jos Engelen; and Oliver Buchmuller from CERN and the CMS experiment. These were intended to educate the string community in down-to-earth physics.

The first area covered was string phenomenology, which uses string theory as model and candidate for the unification of all particles and forces. The various approaches for model building reviewed were mostly of a geometrical nature. That is, many properties of the Standard Model can be translated into geometrical properties of the compactification space that is used to make strings look four-dimensional at low energies. While this translation can be pushed a long way qualitatively, it seems exceedingly difficult technically to go much beyond this stage and obtain predictions that would be testable at the LHC. On the other hand, for the most optimistic case in which the string scale is low (namely of the order of the scale of the weak interactions), concrete predictions of string theory are fully possible, as reported in one of the talks.

Another area, which has become highly visible during the past year, is the computation of certain scattering amplitudes, often in theories with extended supersymmetries and notably in N = 8 supergravity. Extensive computations based on string-inspired methods suggest that this theory may be finite, owing to unexpected cancellations of Feynman diagrams. However, some researchers have suggested that Feynman diagrams might not provide the most efficient way to perform quantum field theory; the results may instead point to the existence of a yet-to-be-discovered dual formulation of the theory that would be much simpler. Other related results concern theories with less supersymmetry, as well as amplitudes of phenomenological relevance, such as multi-gluon scattering amplitudes.

It is well known that string theory is a theory not only of strings but also of membranes and other extended objects. A hot topic of the past year has been the “M-brane mini-revolution”. This deals with a novel description of M-theory membranes and has created some controversy about the meaning of the results. Several talks duly reviewed this subject and it became apparent that the issues had not yet been completely settled.

A key topic of every string conference within the last 10 years has been the gauge theory/gravity duality, which maps ordinary gauge theories to gravitational – i.e., string – theories. This year’s focus was mainly on the connection between systems that are strongly coupled – and in a sense hydrodynamical – and gravity. This leads to a stringy, dual interpretation of certain states in heavy-ion physics, such as the quark–gluon plasma. In particular, a link can be made between the decay of glueball states in QCD and the decay of black holes by Hawking radiation. While these ideas seem to work well on a qualitative level, quantitatively solid results are much harder to obtain because of the strongly coupled nature of the physics involved. The significance of this approach is the subject of ongoing debate and collaboration between heavy-ion physicists and string theorists.

A field of permanent activity and conceptual importance is that of black hole physics, to which string theory has made extremely important contributions during the past few years. As reviews at the conference showed, the identification and counting of microscopic quantum states in stringy toy models has been refined and made more precise, even to the level of quantum corrections. Moreover, fascinating connections between black holes and topological strings have been proposed, and testing those connections has been an important field of activity during the past few years. The results of topological string theory have also had a considerable impact on certain areas of mathematics, and have led to fruitful interactions with mathematicians.

Apart from these five focus areas, other subjects were reviewed at the conference. For example, there was a lecture on loop quantum gravity so that the string community could judge whether there might be connections to this seemingly different approach to quantum gravity.

Both during the conference and afterwards, many participants expressed the view that string theory continues to be a healthy, fascinating and important subject for theoretical work. This is despite the fact that the original main goal, namely to explain the Standard Model of particle physics, appears to be much harder to achieve (if, indeed, achievable at all) than initially hoped. In the final outlook talk, David Gross of the Kavli Institute for Theoretical Physics at the University of California, Santa Barbara, presented a picture of string theory as an umbrella that covers most of theoretical physics, similar to the way in which CERN has emerged as an umbrella for the worldwide community of particle physicists.

Getting heavy on Capri

CCfla1_10_08 — Participants in the garden of the Villa Orlandi in Anacapri, where the heavy flavour physics workshop was held.
Image credit: G Ricciardi.

The Second Workshop on Theory, Phenomenology and Experiments in Heavy Flavour Physics took place on 16–18 June in Anacapri, Capri, Italy – the same location as the first meeting in the series in May 2006. The aim of the series is to bring together theoreticians and experimentalists to develop a dialogue on phenomenological issues. The focus is on discussion and interaction among physicists who are active in the field. The topics this year focused on results, especially in B-physics, as well as exploring the potential for heavy flavour physics in both current and future experiments. With participation by invitation only, owing to space problems at the venue, the 60 or so attendees took part in many fruitful and lively discussions after the seminars and during the free time.

During the past decade flavour physics has witnessed unprece-dented experimental and theoretical progress, opening up the era of precision tests of the Standard Model. The quark and lepton sectors of the Standard Model have been subjected to a series of stringent tests, and it has become customary to look for violations of the Standard Model by using unitarity triangles. Updates of the unitarity triangle analyses were presented by Marco Ciuchini from Rome III/INFN and Jérôme Charles from the Centre de Physique Théorique, Marseille, for the UTfit and CKMfitter collaborations, respectively.

Such updates have been possible, not only because of the large amount of data now available, but also through theoretical progress (e.g. in lattice calculations). As Chris Sachrajda from the University of Southampton and Fermilab’s Paul Mackenzie reported, many approximations in typical lattice calculations have been overcome in recent years. Numerous simulations are now being performed that include quark–antiquark pairs, or a pion mass less than or equal to 300 MeV. Moreover, it is now becoming possible to generate configurations on lattices that are sufficiently fine and large to allow direct simulation of the charm quark.

An impressive amount of data has come from the two asymmetric e⁺e^– B factories, PEP-II and KEKB, and their respective detectors, BaBar and Belle. The BaBar experiment concluded data taking in April 2008, having collected a total of 531 fb^–1, of which 433 fb^–1 was on the Υ(4S) peak, corresponding to about 470 × 10⁶ BB pairs. In 2008, BaBar also collected 30 fb^–1 on the Υ(3S) resonance and 14 fb^–1 on the Υ(2S) resonance, with interesting results, such as the first evidence of the η_B – the long-sought bottomonium ground state. By the time of the workshop, Belle had accumulated about 850 fb^–1, with 730 fb^–1 on the Υ(4S) resonance.

The B factories have led the recent progress in knowledge of the unitarity triangle related to the B system, with angles α, β and γ (or φ2, φ1 and φ3). Christoph Schwanda from the Austrian Academy of Sciences and Giuseppe Finocchiaro from the Frascati National Laboratory/INFN reported on measurements of the angles and sides of the unitarity triangle. Paolo Gambino of Torino and Francesca Di Lodovico from Queen Mary, University of London, reviewed the main theoretical problems on the way to the long-sought precise theoretical inclusive determination of ΙV_ubΙ in the Cabibbo–Kobayashi–Maskawa (CKM) matrix.

Golden modes and penguins

The B factories’ golden modes for the extraction of sin2β are b → cc decays, and the latest measurement from BaBar gives sin2β = 0.714 ± 0.032 ± 0.018, in agreement with the results from Belle. Other interesting decays are the b → sqq “penguin dominated” decays, those study of which is motivated not only by the measure of sin2β, but mostly by the search for new physics.

The angle α can be studied in b → uud modes, and its determination is made complicated by the addition of the b → d penguin amplitude to the b → uud tree one. The first measurements of the B⁰ → ρ⁰ρ⁰ decay confirm the indication that the effect of penguin amplitudes is relatively small in ρρ decays, which in fact yield the most stringent constraints on α.

A precise measurement of the unitarity angle, γ, unthinkable when the B factories started, is now becoming possible with the large statistics accumulated by the B factories. Several new measurements of B^± → D⁰ K^± transitions have appeared recently, and strong evidence for direct CP violation in these decays is building up.

With the first runs of the LHC on the horizon, heavy flavour physics is entering a new phase.

Radiative penguin and leptonic B meson decays are another area of interest at B factories because they constitute a powerful probe of new physics. John Walsh of INFN/Pisa reported on recent experimental results in this field.

On the theory side, Luca Trentadue from Parma/INFN discussed the resummation of large logarithmic terms, which otherwise spoil the convergence of the perturbative series in the threshold region, in semileptonic charmed B decays. Cai-Dian Lü of the Institute of High Energy Physics (IHEP), Beijing, analysed charmless two-body decays of the B mesons to light vector and pseudoscalar mesons in the soft-collinear effective theory.

With the first runs of the LHC on the horizon, heavy flavour physics is entering a new phase. Natalia Panikashvili of Michigan, Andrei Starodumov of PSI and Stefania Vecchi of INFN/Ferrara described the role of flavour physics at the LHC for the ATLAS, CMS and LHCb experiments, respectively. Tobias Hurth of CERN/SLAC stressed that a large increase in statistics at LHCb for B_d → K ^*0μ⁺μ^– will make measurements possible with much greater precision, allowing for an indirect search for new physics.

A main goal at the LHC is to measure CP-violating parameters, such as the B_s⁰ mixing phase, which in the Standard Model is predicted to be small, and could be another way of evincing new physics. Luca Silvestrini of INFN/Roma presented a new analysis that claims evidence of new physics through a B_s⁰ mixing phase, that is much larger than expected in the Standard Model. However, this had not been confirmed by independent analyses at the time of the workshop. Future data from the Tevatron or an extended Υ(5S) run of Belle may be of help in assessing the new results. Joaquim Matias of the Institute for High Energy Physics, Barcelona, discussed poss-ible strategies to measure the weak mixing phase of the B_s⁰ system using B mesons decaying into vectors.

CCfla2_10_08 — A boat trip social event offered the opportunity to see more of the island’s spectacular and stunning surroundings.
Image credit: G Ricciardi.

Charm physics, charm spectroscopy, CP violation in charm decays and searches for new physics were all discussed at length during the workshop. Ikaros Bigi of Notre Dame stated that comprehensive and detailed CP studies of charm decays provide a unique window into flavour dynamics. He emphasized the importance of LHCb in achieving the statistics required to find evidence of new physics in the charm sector. There is a powerful programme at LHCb for charm physics, which includes studying D⁰ mixing, observed for the first time at B factories in spring of last year, and CP violation in some specific decays. Pietro Colangelo of INFN/Bari discussed aspects of new charm spectroscopy and David Miller of Purdue presented the latest results from CLEO-c on W annihilation decays of the D⁺ and D_s⁺ mesons. The HERA electron–proton storage ring at DESY had come to the end of its scheduled operation roughly a year before the workshop, but data are still being analysed. Luis Labarga of the Universidad Autónoma de Madrid presented recent results from the H1 and ZEUS collaborations on charm production and fragmentation, in addition to results on beauty production.

The experiment BESII, at the Beijing Electron Positron Collider has accumulated 5.8 × 10⁷ J/ψ events, 1.4 × 10⁷ ψ(2S) events and 35.5 pb^–1 ψ(3770) data. Xiaoyan Shen of IHEP reported on recent results, which include the observation of the Υ(2175) in the J/ψ → ηφf₀(980) decay. In the past few years a wealth of new experimental results on heavy quarkonia and exotic states has become available. Riccardo Faccini of Rome “La Sapienza”/INFN summarized recent developments in the search for excited states of the scalar nonet among the light mesons, and reviewed the experimental evidence for new states. Ruslan Chistov of the Institute of Theoretical and Experimental Physics, Moscow, discussed the experimental status of X, Y and Z states at B factories. More new states, decays and production mechanisms have been discovered in the past few years than in the previous 30 years. Besides the regular quarkonium states (mostly quark–antiquark states), new exotic states have been found the composition of which (molecule, tetraquark, hybrids&ellip;) is currently the centre of a lively debate. It was the subject of a dedicated round table at the workshop, chaired by Nora Brambilla of Milan, with Antonello Polosa of INFN/Rome, Joan Soto of Barcelona and Antonio Vairo of Milan.

Flavour for all

There were many lively discussions on the role of flavour to evince new physics. Andrzej Buras of the Technical University of Munich reviewed several main results on flavour physics beyond the Standard Model, analysing in particular flavour and CP-violating processes in models with supersymmetry, “littlest Higgs” and extra dimensions. Ulrich Nierste of Karlsruhe summarized the role of the decays B⁰ → μ⁺μ^–, B⁺ → τ⁺ν_τ and B → D τ ν_τ in the hunt for new Higgs effects in the minimal supersymmetric Standard Model. Flavour and precision physics in the Randall–Sundrum model were the topics of discussion for Matthias Neubert of the Johannes Gutenberg University, Mainz, while Jernej Kamenik of INFN/Frascati talked about phenomenology in the context of minimal flavour violation.

While waiting for the LHC, Fermilab’s Tevatron has not only demon-strated the possibility of B-physics at hadron machines but also produced measurements that are highly competitive and complementary to those of the B factories. In particular, this was via the unique access that hadron machines have to the B_s sector. Vaia Papadimitriou of Fermilab and the CDF experiment, and Brad Abbott of Oklahoma and D0, presented the latest measurements on the production, spectroscopy, lifetimes and branching fractions for B mesons, B baryons, and quarkonia. Theoretical issues for the measurement of the top mass using jets, and implications for measurements of the top mass at the Tevatron, were discussed by Iain Stewart of the Massachusetts Institute of Technology. Monte Carlo programs have already proved indispensable for making exclusive theoretical predictions at the Tevatron. Christian Bauer from Lawrence Berkeley National Laboratory presented an improved Monte Carlo framework (GenEvA) mainly based on a new notion of phase space.

Tom Browder of Hawaii and Marcello Giorgi of INFN/Pisa made a strong science case for continued heavy flavour physics measurements at future Super-B machines, underlining their complementarity to the LHC programme. One focus for the Super-B factories would be studying, and possibly discovering, new sources of flavour-changing neutral currents and CP-violation. There are plans for a Super-B factory at KEK in Japan based on the existing KEK accelerator and Belle detector, as well as a proposal for a laboratory in Italy, the SuperB project. In both cases the goal is a luminosity of around 1 × 10³⁶ cm^–2s^–1, approximately 60 times as high as achieved at present B factories.

The conference was extremely lively, and most goals of the workshop, such as promoting co-operation and a fruitful exchange of ideas among theoreticians and experimentalists, were fulfilled. Up to now, the data agrees globally with the CKM picture, but there are also hints of discrepancies, which if confirmed could signify new physics. With the advent of the new machines, it will be feas-ible to investigate possible flavour structure and new sources of CP violation beyond the Standard Model through studies of flavour processes. Heavy flavour, and therefore physics, continues to play a fundamental role in particle physics and has an exciting future.

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