Muon neutrinos definitely disappear en route from Fermilab in Illinois to Soudan in Minnesota. This is the conclusion from the first results of the Main Injector Neutrino Oscillation Search (MINOS), presented at a seminar at Fermilab on 30 March, which showed that MINOS has observed the disappearance of a significant fraction of these neutrinos. The observation is consistent with the phenomenon of neutrino oscillation, in which neutrinos change from one kind to another, and corroborates earlier observations of muon-neutrino disappearance, made by the Super-Kamiokande and KEK-to-Kamioka (K2K) experiments in Japan.
The Fermilab side of the MINOS experiment comprises a beam-line in a 1220 m long tunnel pointing towards Soudan. The tunnel holds the carbon target and beam-focusing elements that generate neutrinos from protons accelerated by Fermilab’s Main Injector accelerator. A neutrino detector, the MINOS “near detector” located 100 m underground on the Fermilab site, measures the composition and intensity of the neutrino beam as it leaves the laboratory. The Soudan side of the experiment features the 6000 tonne “far” detector about 700 m underground, which measures the properties of the neutrinos after their 725 km trip to northern Minnesota.
If neutrinos did not change as they travel away from Fermilab, the MINOS detector in Soudan should have recorded 177±11 muon neutrinos. Instead, the collaboration found only 92 muon-neutrino events – a clear observation of muon-neutrino disappearance. The deficit as a function of energy is consistent with the hypothesis of neutrino oscillations, which can occur only if different neutrino types have different masses. The MINOS observations yield a value of Δm2, the square of the mass difference between two types of neutrinos, equal to 0.0031 ±0.0006 (statistical uncertainty) ±0.0001 (systematic uncertainty) eV2.
In the oscillation scenario, muon neutrinos can transform into electron neutrinos or tau neutrinos, but alternative models – such as neutrino decay and extra dimensions – are not yet excluded. The MINOS collaboration will need to record much more data to test more precisely the exact nature of the disappearance process. Over the next few years, the experiment should collect about fifteen times more data, yielding more results with higher precision.
The MINOS neutrino experiment follows on from the K2K long-baseline neutrino experiment in Japan. From 1999-2001 and 2003-2004, K2K sent neutrinos created at an accelerator at the KEK laboratory to a detector in Kamioka, a distance of about 240 km. Compared with K2K, the distance in the MINOS experiment is three times longer, and both the intensity and the energy of the MINOS neutrino beam are higher. These advantages have enabled the MINOS experiment to observe in less than a year about three times as many neutrinos as K2K did in around four years. Later this year the CERN Neutrinos to Gran Sasso project will start delivering muon neutrinos to the Gran Sasso National Laboratory in Italy.
• The MINOS experiment includes about 150 scientists, engineers, technical specialists and students from 32 institutions in six countries, including Brazil, France, Greece, Russia, the UK and the US. The US Department of Energy provides the major share of the funding, with additional funding from the US National Science Foundation and the UK’s Particle Physics and Astronomy Research Council. For more information on the experiment see www-numi.fnal.gov/.
The Belle experiment has recently revealed evidence for a rare and long-sought missing-energy decay of the B meson, B– → τ–ν. This has allowed the Belle Collaboration to measure the B-meson decay constant, fB, for the first time. The results were announced at the Flavor Physics and CP Violation conference in Vancouver, and have been submitted to Physical Review Letters (Ikado et al. 2006).
The Belle experiment is a collaborative effort of scientists from universities and laboratories in America, Asia, Australia and Europe. It operates at the KEK High Energy Physics Laboratory in Japan – home to KEKB, the world’s highest-luminosity particle accelerator, which recently achieved a peak luminosity of 1.6 × 1034 cm-2s-1.
In the decay mode B– → τ–ν, the B meson (a strongly interacting bound state of a b quark and anti-u quark) transforms into a final state containing only leptons. Previously, because this decay process had not been seen, researchers had to rely entirely on either calculations in lattice quantum chromodynamics (QCD) or models to obtain the parameter fB, which is needed to interpret many other measurements in particle physics, including the Cabibbo-Kobayashi-Maskawa unitarity triangle constraints from Bd-Bbard mixing.
The decay mode B– → τ–ν is especially hard to find. Not only is it rare – about 1 in 10,000 charged B decays contains such an event – but tau leptons often decay to an electron or muon together with two neutrinos, which escape the detector unseen. This means that the experimental signature is simply a single charged track accompanied by missing energy, and is frequently mimicked by less-interesting background processes.
The Belle experiment operates at the U(4S) resonance where each B meson is produced accompanied by an anti-B meson partner and nothing else. The experimental breakthrough that allowed the discovery of the missing-energy decay mode involved detecting all the decay products of the B meson accompanying the sought-after decay, thereby constraining the energy and momentum of the missing or undetected particles. This technique has a very low efficiency and is only possible because of the unprecedented luminosity of the KEKB accelerator, which provided the Belle experimenters with 457 million charged B mesons to study. Even so, this was barely sufficient to discover this rare and unusual process.
Based on the events that they have found, the Belle team reports a preliminary value of fB = 176+28-23 (stat.) +20-19 (syst.) MeV, which is compatible with the most recent calculations in unquenched lattice QCD. Conversely, if fB is taken from lattice QCD, the Belle measurement of B → τν gives a tight constraint on charged Higgs masses at high tanβ in extensions of the Standard Model, where tanβ is the ratio of vacuum expectation values.
This breakthrough in the detection of a rare missing-energy decay is the first step towards the observation of exotic decays such as B → Kννbar, B → dark matter, and other possible types of unusual and new physics processes. Although the experimental technique for observing a rare missing-energy mode has now been established, two orders of magnitude more BBbar pairs are probably needed to find B → Kννbar and exploit all of the possibilities this technique has to offer. This will be possible at the proposed KEK Super B-Factory facility.
Two more rounds of data taken by the Hall A Proton Parity Experiment (HAPPEx) at the US Department of Energy’s Jefferson Lab have provided the most precise constraint yet on nucleon strangeness. The result, presented at the American Physical Society April meeting in Dallas, reveals that the strange-quark contribution to the proton’s overall charge distribution and magnetic moment is small. It amounts to no more than 1% of the proton’s charge radius and no more than 4% of its magnetic moment – and in both cases, the final value could be consistent with zero.
It may seem unusual that strange quarks should be important in determining the properties of the proton as, unlike up and down quarks, they are not thought of as permanent residents of the proton. However, the strange quark may appear as part of the proton’s quark-gluon sea, the seething mass of particles that constantly blink in and out of existence due to strong force energy.
A useful method of accessing strange quarks is through parity-violating electron scattering, in which the interference of the electromagnetic force and neutral weak force is measured by scattering a beam of polarized electrons off target particles. Since the electromagnetic force is parity-symmetric, while the weak force is not, a longitudinally polarized electron beam allows experimenters to separate the electromagnetic and weak components, and by comparing their strengths they can disentangle the contributions of the up, down and strange quarks.
The HAPPEx Collaboration measured a combination of strange-quark contributions to the charge distribution and magnetization of the proton, which are represented via GsE and GsM, the strange electric and magnetic form factors, respectively. To disentangle the two form factors, the collaboration took data on two different targets: hydrogen and helium (4He). 4He has no net spin and hence no magnetic moment, and so allowed the researchers to isolate GsE.
HAPPEx took data on both targets during 2005, using a longitudinally polarized 3 GeV electron beam from Jefferson Lab’s Continuous Electron Beam Accelerator Facility. A gallium arsenide superlattice photocathode provided an average beam polarization of 86% with rapidly flipping helicity. The beam was sent into a 20-cm long cryogenic aluminum target vessel containing either hydrogen or 4He in Jefferson Lab’s Hall A. Septum magnets then deflected elastically scattered electrons, which were at a forward angle of 6°, to the Hall A High Resolution Spectrometers (HRS), located at 12.5°.
The HRS allowed a very clean separation of elastic events, with an average value of momentum-transfer squared, Q2 = 0.1 (GeV/c)2. A Cherenkov electromagnetic shower calorimeter covered the distribution of elastic events in the spectrometer focal plane. The signal was integrated over each period of constant helicity. A blinding factor was placed on the data and removed only a week before the result was presented in Dallas.
The HAPPEx results indicate small values for the strange form factorss GsM = 0.12±0.24 and GsE = -0.002±0.017. While these results are consistent with previous results from HAPPEx (Aniol et al. 2006) and world data, they reveal that the large values and possible radical Q2 dependence of the strange form factors suggested by previous data in this kinematic region, are highly unlikely. Also, while these new data are accurate enough to eliminate many models of strangeness content, they do not rule out sizable contributions at higher Q2. They are also compatible with a new analysis of world data, the result of which is in excellent agreement with modern calculations based on non-perturbative quantum chromodynamics using lattice methods and chiral extrapolation (Young et al. 2006).
During their long lifetimes stars generate their energies by nuclear fusion in their interiors, which are generally accepted to be the breeding grounds for carbon and heavier elements. The heaviest elements made this way are iron and nickel; heavier elements are thought to be built by slow and/or rapid neutron-capture reactions, the s- and r-processes. Although these mechanisms for nucleosynthesis have been known for some time, the abundances of some heavy elements have remained a mystery. Now Carla Fröhlich of the Universität Basel and Gabriel Martínez-Pinedo of the Gesellschaft für Schwerionenforschung, Darmstadt, and colleagues have proposed a novel nucleosynthesis that might solve these puzzles.
When a massive star forms a supernova, part of the matter in the stellar interior forms a neutron star, and the liberated energy, mainly in the form of neutrinos, contributes to the ejection of the stellar mantle into the interstellar medium. The temperature of the deepest ejected layers is so hot that nuclei are decomposed into free protons and neutrons. The tremendous flux of neutrinos and antineutrinos, which accompanies the birth of the neutron star, can be absorbed by the nucleons and so determines the relative abundance of protons and neutrons and hence the composition of the nuclei that form when the ejected matter reaches cooler regions.
During the later stages of the explosion the matter is expected to become rich in neutrons, so supernovae are believed to be the site of heavy-element production by the r-process. However, it has been realized very recently that during the first second of the explosion the ejected material is rich in protons.
When Fröhlich and colleagues studied the nucleosynthesis in this proton-rich environment they discovered possible solutions to two long-standing problems. First, they could reproduce the abundances of elements such as scandium, copper and zinc, for which calculations had previously fallen notoriously short. More surprisingly, they also noticed the appearance of heavier elements such as strontium, molybdenum, ruthenium and beyond (C Fröhlich et al. 2006).
This heavy-element production can be attributed to a novel nucleosynthesis process, which Fröhlich and colleagues named the νp process after the two main contributors: proton capture, which transports matter sequentially to higher charges, and (anti)neutrinos, which are captured by free protons and so change the protons to neutrons. This presence of neutrons allows the flow in element creation to circumvent long-lived nuclei such as 56Ni and 64Ge, so enabling the synthesis of heavier elements.
The νp process is a primary process, that is, it should occur in all core-collapse supernovae. As a consequence there should already be fingerprints of νp nucleosynthesis in the earliest and most primitive stars. Indeed, finding strontium in the most metal-poor, and hence oldest, star observed so far came as a big surprise last year. This might now be explained as debris from the νp process that had operated in an earlier supernova. Further observations of elemental abundances in metal-poor stars combined with progress in supernova modelling and improved knowledge of the nuclei involved – as expected from future facilities such as the Facility for Antiproton and Ion Research – will help to disentangle the importance of the νp process for the abundances of the elements in the universe.
The CDF collaboration at Fermilab has announced the precision measurement of the matter-antimatter transitions for the B0s meson, which consists of a bottom quark bound to a strange anti-quark. The announcement came less than a month after the news that the D0 collaboration had measured the first upper and lower bounds on the oscillation frequency.
In a seminar at Fermilab on 10 April, the CDF Collaboration reported on their analysis of 1 fb-1 of proton-antiproton collision data acquired by the CDF detector between February 2002 and January 2006, during Tevatron Run II. Within the 700-member CDF Collaboration – from 61 institutions and 13 countries – a team of 80 researchers from 27 institutions performed the data analysis leading to the precision measurement just one month after the data-taking was completed.
The team used semileptonic and hadronic decays of the B0s and found a signature consistent with B0s-Bbar0s oscillations, with a probability that the data could randomly fluctuate to mimic such a signature of 0.5%. Analysis yielded a preliminary result for the B0s-Bbar0s oscillation frequency, Δms, of 17.33+0.42-0.21(stat.)±0.07(sys.) ps-1 – in agreement with D0’s result of 17 < Δms Δ 21 ps-1. The CDF Collaboration also derived a value for the ratio of the related parameters of the Cabibbo-Kobayashi-Maskawa matrix, |Vtd|/|Vts| = 0.208+0.008-0.007 (stat.+sys.).
This precision measurement from CDF will immediately be interpreted within different theoretical models, in particular in the context of supersymmetry. General versions of supersymmetry predict an even faster transition rate than has been measured, so some of those theories can be ruled out based upon this result. More information will come from combining precise measurements of B0s-Bbar0s oscillations and searching for the rare decay of B0s mesons into muon pairs. Both the D0 and CDF experiments expect to achieve improved results in these areas in the near future.
Researchers at the Lawrence Livermore National Laboratory (LLNL) in California have made the most precise test so far of quantum electrodynamics (QED). In studies of highly ionized, lithium-like uranium, they have measured the two-loop Lamb shift for the first time (Beiersdorfer et al. 2005).
QED is a well-established theory that describes at the quantum level all phenomena involving the electromagnetic force. Its extremely accurate predictions have been tested by various experiments, including measurements of the tiny shift in the energy levels in hydrogen discovered by Willis Lamb in 1951, owing to the self-interaction of the electron. Tests of so-called one-loop QED (self-energy and vacuum polarization) confirmed the theoretical predictions with high precision, and theorists and experimentalists are now looking to evaluate higher-order QED processes.
Highly charged ions offer an opportunity for high-accuracy calculations of atomic properties within QED, in that they provide a strong-field environment and relatively simple spectra. These conditions allow high-precision measurements of some transitions. Moreover, measurements of lithium-like systems such as U89+ are more sensitive to higher-order QED terms than those of hydrogen-like systems.
Using the SuperEBIT high-energy electron-beam ion trap at LLNL, the researchers measured the 2s(1/2)-2p(1/2) transition in U89+ with an accuracy that is nearly an order of magnitude better than previously available. The team monitored X-ray emission from the ions with a high-purity germanium detector, and used a spectrometer specifically developed for this experiment for spectroscopy at extreme ultraviolet wavelengths.
The results allow the researchers to infer a two-loop Lamb shift in lithium-like U89+ of 0.20 eV. This is also in excellent agreement with the recent calculation of the two-loop Lamb shift for the 1s level in hydrogen-like U91+.
It is slightly more than 30 years since the discovery of the J/ψ, the first bound state of a charmed quark, c, and its antiquark cbar, near a mass of 3.1 GeV/c2. This discovery ushered in the era of heavy-flavour physics, which now includes studies of the tau lepton and its neutrino, and the b and t quarks. As the mass of the charmed quark is quite large, the velocities of the c and cbar in a bound state are small enough that many important features of these states can be described using non-relativistic potential models. Also, at typical separations of the quark and antiquark, the shape of the ccbar potential is somewhat like that of the Coulomb potential. Hence, many features of ccbar states – collectively called charmonium – are familiar from the physics of the hydrogen atom, or more precisely, from the spectroscopy and dynamics of positronium, a bound state of an electron and a positron.
After its discovery, the J/ψ was soon identified as a 3S1 ccbar bound state, that is, a spin-triplet (S = 1) S-wave (L = 0) level with total spin J = 1. Several other ccbar levels were observed soon after, including the ψ(2S), or ψ’, an excited version of the J/ψ; several orbitally excited triplet P-wave (3PJ = 0, 1, 2) levels χcJ; a D-wave level at 3.77 GeV/c2; and a spin-singlet 1S0 level known as the ηc(1S). Figure 1 illustrates the low-mass charmonium spectrum and the principal transitions between charmonium states expected from the analogy of ccbar states with positronium states. Among the low-mass states expected, only the ηc(2S), an excited version of the ηc(1S), and the hc, a spin-singlet P-wave 1P1 level, steadfastly refused to make significant appearances, despite reported sightings that were not confirmed.
A few years ago, with the conclusion of its 20-year programme of studies of the decays and spectroscopy of the bottom quark, the CLEO Collaboration at the Cornell Laboratory for Elementary-Particle Physics turned its attention to the study of charm and charmonium. The National Science Foundation supported converting the Cornell Electron Storage Ring (CESR) to CESR-c, including installing wiggler magnets to enhance luminosity in the charm threshold region (see CERN Courier May 2003 p7). The new programme benefits enormously from the versatility of the CLEO detector, upgraded to CLEO-c (figure 2), which is unrivalled by other detectors that have operated in this energy region. This latest version of the CLEO detector features excellent charged-particle tracking, neutral-shower energy resolution, and particle identification.
The ease of studying the lower-mass charmonium states is due in part to their narrow decay widths (long lifetimes), which are much smaller than the mass differences among the states. Above the charm threshold, where production of a pair of charmed mesons, D, becomes possible – that is, above 2MD ≈ 3.73 GeV/c2 – charmonium states are much broader and they may overlap, so the spectroscopy becomes more complicated.
The charmonium spectrum provides fundamental information about the nature of the strong force holding quarks together. If current ideas about the nature of the interquark force are correct, the mass of the hc, M(hc), is expected to be near the spin-weighted average of the masses of the χcJ levels, 〈M(3PJ)〉 ≈ 3525 MeV/c2. This prediction for M(hc) is based on the expectation that the dominant spin-dependent interquark force is Coulomb-like, as predicted by quantum chromodynamics (QCD), the theory of the strong force. It is borne out by calculations in lattice gauge theory, which predict a difference of at most a few mega-electron-volts/c2 between the masses of the spin-singlet and spin-triplet P-wave states.
Charm and charmonium data taken at CLEO so far include a sample of slightly more than 3 million ψ(2S) decays, as well as continuum data below the ψ(2S), charm data just above DDbar threshold, and data at higher energies for a nascent programme of Ds investigations. The ψ(2S) data were used to search for the isospin-violating transition ψ(2S) → π0hc. A similar transition in the bbbar system (from the Υ(3S) level) was proposed some time ago as a way to search for the 1P1 state hb (Voloshin 1986). The transition was expected to occur with a branching fraction of only about 10-3, and so substantial suppression of background was required. The hc was expected to decay to the ηc and a photon of energy around 500 MeV with a branching fraction of roughly 40%, so this photon was sought in coincidence with the slow π0 (energy around 160 MeV) from the first transition.
The search for the hc using just the 160 MeV π0 and 500 MeV photon at CLEO produced good results (Rosner et al. 2005 and Rubin et al. 2005). Analyses of this inclusive signature yielded M(hc) = 3524.9±0.7±0.4 MeV/c2 and a product of branching fractions Bψ’Bh ≡ B(ψ(2S) → π0hc)B(hc → γηc) = (3.5±1.0±0.7) × 10-4, both in good agreement with expectations.
The inclusive hc signal sits on a considerable background. Further reduction of this background is possible if one reconstructs the decay of ηc into specific final states. The hc peak stands out quite distinctly under such circumstances (figure 3). This exclusive analysis yielded values of the hc mass and product branching fraction consistent with those of the inclusive measurement, but with slightly larger errors. However, as a result of the low background, the statistical significance of the exclusive measurement is higher than that of the inclusive measurement, providing a more conclusive observation of the existence of the hc. The combined inclusive and exclusive analyses yield M(hc) = 3524.4±0.6±0.4 MeV/c2 and Bψ’Bh (4.0±0.8±0.7) × 10-4, very close to theoretical expectations.
The hc thus lies at 1.0±0.6±0.4 MeV/c2 below the average 3PJ mass, supporting the QCD prediction and indicating little contribution from a long-range spin-dependent quark-confining force or coupled-channel effects, which could cause a displacement from this value. It is barely consistent with an interesting (but non-relativistic) bound that predicted the hc should lie no lower than 〈M(3PJ)〉 (Stubbe and Martin 1991).
An independent experiment at Fermilab, E835, has produced additional evidence that the hc is nearly degenerate with 〈M(3PJ)〉 (Andreotti et al. 2005). By forming hc candidates using collisions of antiprotons in the Accumulator Ring with protons in a gas-jet target, the E835 Collaboration found 13 candidates for the process pbarp →hc → γgηc → γ(γγ). Utilizing the carefully controlled energy of the antiproton, the team found M(hc) = 3525.8±0.2 0.2 MeV/c2 and a decay width Γ <1 MeV.
The CLEO Collaboration plans to collect more ψ(2S) data, enabling a better measurement of the hc mass and production rate. It is hoped that the predictions of lattice gauge theories will keep pace with these improvements.
Further discoveries
The hc is not the only new charmonium state below charmed threshold to which CLEO has contributed substantially. Several years ago, the Belle Collaboration observed a candidate for ηc(2S) in B→ K(KSKπ) (Choi et al. 2002) and e+e– → J/ψ + X (Abe et al. 2002), the mass of which was incompatible with that of the previously claimed observation. By studying its production in photon-photon collisions, the CLEO collaboration has confirmed the presence of this state (Asner et al. 2004), as has the BaBar Collaboration. The mass of the ηc(2S) is found to be only 48±5 MeV/c2 below the corresponding spin-triplet ψ(2S) state, a hyperfine splitting that is considerably less than the difference of 117 MeV/c2 seen in the 1S charmonium states, that is, between the J/ψ and the ηc(1S). This difference may well be due to the proximity of the charmed meson-pair threshold, which can lower the mass of the ψ(2S) by tens of MeV/c2.
Researchers at the CLEO Collaboration found that the product Γ(ηc(2S) → γγ)B(ηc(2S) → KSKπ) is only 0.18±0.05±0.02 times the corresponding product for ηc(1S). This could pose a problem for descriptions of charmonium if the branching ratios to KSKπ are equal. More likely, the heavier ηc(2S) has more decay modes available to it, so its branching ratio to KSKπ is likely to be less than that of the hc(1S).
Altogether it is remarkable that more than 30 years after the first discovery, charmonium continues to yield new information and new challenges to elementary-particle physics, thanks to improvements in collider luminosities and detector capabilities. Recent advancements include surprises from charmonium spectroscopy above charm threshold to which CLEO is also contributing.
With the recent discoveries of the hc and the ηc(2S), all of the expected bound states below charm threshold have now been observed. With the exception of the mass of the ηc(2S), the observed masses and branching fractions are in quantitative agreement with theoretical expectations, while the lower-than-expected ψ(2S) – ηc(2S) mass splitting stresses the importance of the nearby DDbar- threshold. The quantitative agreement between theory and experiment for the states below charm threshold provide a firm foundation for developing an understanding of the new states found above the threshold.
The D0 collaboration at Fermilab has published the first direct two-sided bound on the oscillation frequency of the B0s, the meson comprising a strange quark (s) and a bottom antiquark (Bbar). The result is consistent with what is expected from the Standard Model, within a 90% confidence level. The phenomenon in which a B0d meson (with a down quark, d, instead of the strange quark) converts into its antiparticle Bbar0d is well established, and its oscillation frequency Δmd has been measured precisely (Heavy Flavour Averaging Group 2006). The value of the corresponding measure of the oscillation frequency of a B0s meson into its antiparticle Bbar0s – Δms – was until now much more poorly known.
The D0 collaboration is an international team of 700 physicists from 90 institutions and 20 countries working at Fermilab’s Tevatron, which provides high-energy proton-antiproton collisions for two experiments, D0 and CDF. The data for the D0 result were taken from 1 fb-1 of total collision data, yielding more than a billion events (Abazov et al. 2006). The 90% confidence level means that the result does not qualify as a discovery, although it does provide a very strong indication. However, according to Rob Roser, co-spokesperson of CDF, within the next month or so the CDF collaboration should provide a result with greater precision.
The D0 result already provides some interesting constraints on supersymmetry. “The D0 value of 17 < Δms < 21 ps-1 limits the contributions to the oscillation process that could be made by supersymmetric particles,” explains D0 co-spokesperson Terry Wyatt, from the University of Manchester. “The basic idea is that supersymmetric particles may be exchanged in the box diagrams that are responsible for B0s mixing.” Several theoretical models of supersymmetry predict a much faster oscillation of B0s, and the D0 result now disfavours these models.
Almost every current theoretical model of neutrino masses introduces sterile (“right-handed”) fields, which mix with the ordinary (“left-handed”) neutrinos. Ordinary neutrinos have no electrical charge and interact through the weak force, but there may also exist rogue sterile neutrinos that feel only gravity. Most models make these new particles very heavy, while also trying to explain the small masses of ordinary neutrinos. Now Peter Biermann of the Max Planck Institut for Radioastronomy, Bonn, and Alexander Kusenko of University of California, Los Angeles, have suggested that if some of the sterile neutrinos are relatively light, they could resolve several astrophysical puzzles. In particular, sterile neutrinos with kilo-electron-volt (keV) masses could account for dark matter, the origin of the rapid motion of observed pulsars and re-ionization of the universe (Biermann and Kusenko 2006).
These relatively light sterile neutrinos were the topic of a recent workshop, Sterile Neutrinos in Astrophysics and Cosmology, held in Crans Montana in March. The meeting looked not only at how keV sterile neutrinos can solve a variety of problems in astrophysics, but also at how their existence might be detected.
Dark-matter sterile neutrinos could decay into a lighter neutrino and an X-ray photon, and this seems to be the most promising path to discovery. The workshop brought together particle physicists and X-ray observers, who presented the current limits and discussed ways to search for dark-matter neutrino decays. One important feature of dark matter in the form of the sterile neutrinos is the smoothing of structures on small scales. This “warm” dark matter – in contrast with the “cold” and “hot” alternatives – would be indistinguishable from cold dark matter on large scales, but it would yield stellar structures with the smallest size relative to the dark-matter particle mass. Recent studies of dwarf spheroid satellite galaxies have reported seeing the minimal halo size, indicative of warm dark matter.
The same decays into X-ray photons happening in the early universe could have produced enough ionization to catalyse a rapid production of molecular hydrogen, which is the most important cooling agent for primordial gas. Enriched with molecular hydrogen, haloes of gas would cool and collapse, forming the first stars. These stars could have re-ionized the universe, in agreement with observations of the Wilkinson Microwave Anisotropy Probe.
The role of sterile neutrinos in pulsars originates in supernova explosions, where sterile neutrinos with a mass of several keVs from the cooling nascent neutron star would be emitted preferentially in one direction, set by the star’s magnetic field. Although the neutrinos would not interact with the magnetic field, they would scatter off fermions polarized along the magnetic field in the neutron star. The anisotropy of sterile-neutrino emission would be sufficient to give the neutron star a recoil velocity of hundreds of kilometres a second. This agrees with observations of pulsars – magnetized rotating neutron stars – all of which have very large velocities. The origin of these velocities is a long-standing puzzle, which would have a simple explanation if sterile neutrinos exist.
On 15 October 1991 the highest-energy cosmic-ray particle ever measured struck Earth’s atmosphere tens of kilometres above the Utah Desert. Colliding with a nucleus, it lit up the night for an instant and then was gone. The Fly’s Eye detector at the Dugway Proving Grounds in Utah captured the trail of light emitted as the cascade of secondary particles created in the collision made the atmosphere fluoresce. The Fly’s Eye researchers measured the energy of the unusual ultra-high-energy cosmic-ray event – dubbed the “Oh-My-God (OMG) event” – at 320 exa-electron-volts (EeV), or 320 × 1018 eV. In SI units, the particle, probably a proton, hit the atmosphere with a total kinetic energy of about 5 J. For a microscopic particle this is a truly macroscopic energy – enough to lift a mass of 1 kg half a metre against gravity. On 3 December 1993, on the opposite side of the world, the Akeno Giant Air Shower Array (AGASA) in Japan recorded another OMG event with an energy of 200 EeV. In this case the cosmic ray was recorded using a large array of detectors on the ground to measure the extended air shower (EAS) resulting from the primary cosmic ray interacting with the atmosphere.
Since these first observations at least a dozen OMG events have been recorded, confirming the phenomenon and mystifying cosmic-ray physicists. It seemed that particles with energies more than about 50 EeV should not reach Earth from any plausible source in the universe more than around 100 million parsecs distant, as they should rapidly lose their energy in collisions with the 2.7 K cosmic-microwave background radiation from the Big Bang – the Greisen-Zatsepin-
Kuzmin limit. While many explanations have been proposed, experiments have so far failed to decipher a clear message from these highly energetic messengers, and the existence of the OMG events has become a profound puzzle. Now a new eye on these ultra-high-energy events has come into focus, based on the great plain of the Pampa Amarilla in western Argentina. The Pierre Auger Observatory (PAO), with its unprecedented collecting power, has begun to study cosmic rays at the highest energies.
However signs of the extreme-energy universe may also come in a different guise – not as a single OMG event but rather as bursts of events of more-modest energy. On 20 January 1981, near Winnipeg, a cluster of 32 EASs – with an estimated mean energy of 3000 tera-electron-volts – was observed within 5 min (Smith et al. 1983). Only one such event would have been expected. This observation was the only one of its kind during an experiment that recorded 150,000 showers in 18 months. In the same year an Irish group reported an unusual simultaneous increase in the cosmic-ray shower rate at two recording stations 250 km apart (Fegan et al. 1983). The event, recorded in 1975, lasted 20 s and was the only one of its kind detected in three years of observation.
There have since been a few hints of such “correlated” cosmic-ray phenomena seen by some small cosmic-ray experiments dotted around the world, such as a Swiss experiment that deployed four detector systems in Basel, Bern, Geneva and Le Locle, with a total enclosed area of around 5000 km2. In addition, the Baksan air-shower-array group has presented evidence from data from 1992 to 1996 for short bursts of super-high-energy gamma rays from the direction of the active galactic nucleus Markarian 501. The AGASA collaboration has also reported small-scale clustering in arrival directions, and possibly in the arrival times of these clustering events.
One mechanism that could generate correlated showers over hundreds of kilometres is the photodisintegration of high-energy cosmic-ray nuclei passing through the vicinity of the Sun, first proposed by N M Gerasimova and Georgy Zatsepin back in the 1950s. Other more recent and more exotic examples of phenomena that could give rise to large-area non-random cosmic-ray correlations include relativistic dust grains, antineutrino bursts from collapsing stellar systems, primordial black-hole evaporation and even mechanisms arising from the presence of extra dimensions.
Whichever way the high-energy universe is incarnated on Earth, the signs should be exceedingly rare, requiring large numbers of detectors deployed over vast areas to provide a reasonable signal. The detection of a single OMG particle requires dense EAS arrays and/or atmospheric fluorescence detectors, with detector spacings of the order of a kilometre, as in the PAO. Detection of cosmic-ray phenomena correlated over very large areas requires even bigger detection areas, which at present are economically feasible only with more sparse EAS arrays (on average much fewer than one detector per km2). In fact, global positioning system (GPS) technology makes it possible to perform precision timing over ultra-large areas, enabling a number of detector networks to be deployed as essentially one huge array. An example is the Large Area Air Shower array, which started taking data in the mid-1990s. It comprises around 10 compact EAS arrays spread across Japan, forming a sparse detector network with an unprecedented enclosed area of the order of 30,000 km2.
Now, however a new dimension to cosmic-ray research has opened up. In 1998 in Alberta, building on a proposal first presented in 1995, the first node of a new kind of sparse very-large-area network of cosmic-ray detectors began to take data. The innovative aspect of the Alberta Large-area Time-coincidence Array (ALTA) is that it is deployed in high schools. By the end of 1999 three high-school sites were operating, each communicating with the central site at the University of Alberta. In 2000 the Cosmic Ray Observatory Project (CROP), centred at the University of Nebraska, set up five schools with detectors from the decommissioned Chicago Air Shower Array. Around the same time the Washington Large-area Time-coincidence Array (WALTA) installed its first detectors.
The ALTA, CROP and WALTA projects have a distinct purpose – to forge a connection between two seemingly unrelated but equally important aims. The first is to study the extreme-energy universe by searching for large-area cosmic-ray coincidences and their sources; the second is to involve high-school students and teachers in the excitement of fundamental research. These “educational arrays”, with their serious research purpose, provide a unique educational experience, and the paradigm has spread to many other sites in North America. The detector systems are simple but effective. Following the ALTA/CROP model they use a small local array of plastic scintillators, which are read by custom-made electronics and which use GPS for precise coincidence timing with other nodes in a network of local arrays over a large area. Most of the local systems forming an array use three or more detectors, which, with a separation of the order of 10 m and a hard-wired coincidence, allow accurate pointing at each local site. Today the ALTA/CROP/WALTA arrays involve more than 60 high schools and there are three further North American educational arrays in operation: the California High School Cosmic Ray Observatory (CHICOS) and the Snowmass Area Large-scale Time-coincidence Array (SALTA) in the US, and the Victoria Time-coincidence Array (VICTA) in Canada. At least seven more North American projects are planned.
The CHICOS array is the largest ground-based array in the Northern Hemisphere. Its detectors, donated by the CYGNUS collaboration, are deployed on more than 70 high-school rooftops across 400 km2 in the Los Angeles area. Each site has two 1 m2 plastic scintillator detectors separated by a few metres. Local pointing at each site is not possible, nor is it required as CHICOS uses GPS pointing across multiple sites to concentrate on the search for single ultra-high-energy cosmic-ray air showers. Recently the collaboration reported their results at the 29th International Cosmic Ray Conference in Pune, India (McKeown et al. 2005).
Innovative detection techniques have also been employed in this burgeoning collaboration between researchers and high-school students and teachers in North America. A prime example is the project for Mixed Apparatus for Radar Investigation of Cosmic Rays of High Ionization (MARIACHI), based at Brookhaven National Laboratory, New York. The plan is for the experiment to detect ultra-high-energy cosmic rays using the passive bistatic radar technique, where stations continuously listen to a radio frequency that illuminates the sky above it. The ionization trails of ultra-high-energy cosmic-ray showers – as well as meteors, micro-meteors and even aeroplanes – in the field of the radio beam will reflect radio waves into the high-school-based detectors. These schools will also be equipped with conventional cosmic-ray air-shower detectors. The technique, if successful, will speed the construction of ultra-large-area cosmic-ray detectors.
The European endeavour
Across the Atlantic, schools in many European countries are also getting involved in studying the extreme-energy universe (see figure 1). In 2001 physicists from the University of Wuppertal proposed SkyView – the first European project to suggest using high-school-based cosmic-ray detectors. This ambitious project proposed an immense 5000 km2 array, the size of the PAO, using thousands of universities, colleges, schools and other public buildings in the North Rhine-Westphalia area. Roughly a year later CERN entered the field with a collaborative effort to distribute cosmic-ray detectors from the terminated High Energy Gamma Ray Astronomy project in schools around Dusseldorf. A test array of 20 counters was set up at Point 4 on the tunnel for the Large Electron-Positron (LEP) collider, with the aim of studying coincidences with counters installed about 5 km away at Point 3 as part of cosmic-ray studies by the L3 experiment on the LEP.
Also in 2002 the High School Project on Astrophysics Research with Cosmics (HiSPARC), initiated by physicists from the University of Nijmegen in the Netherlands, joined the European effort. HiSPARC now has five regional clusters of detectors being developed in the areas of Amsterdam, Groningen, Leiden, Nijmegan and Utrecht. Around 40 high schools are participating so far and more are joining. In March 2005 the HiSPARC array registered an event of energy 8 × 1019 eV, in the ultra-high-energy “ankle” region of the cosmic-ray energy spectrum, which was also reported at the international conference in Pune (Timmermans 2005).
The HiSPARC collaboration is also planning to use a recent and exciting development of the Low Frequency Array (LOFAR) Prototype Station (LOPES) experiment in Karlsruhe. Using a relatively simple radio antenna, LOPES detects the coherent low-frequency radio signal that accompanies the showers of secondary particles from ultra-high-energy cosmic rays. A large array of these low-frequency radio antennas, the LOFAR observatory, is already being constructed in the Netherlands. Such technology can also be exploited by high-school-based observatories around the world to expand their capability rapidly to become effective partners in the search for point sources of ultra-high-energy particles.
Elsewhere, the School Physics Project was initiated in Finland and is now under development. Also in 2002 the Stockholm Educational Air Shower Array (SEASA) was proposed to the Royal Institute of Technology in Stockholm. SEASA has two stations of cosmic-ray detectors running at the AlbaNova University Centre and the first cluster of stations for schools in the Stockholm area is now in the production stage. Meanwhile, in the Czech Republic the Technical University in Prague and the University of Opava in the province of Silesia – working closely with the ALTA collaboration – each have a detector system taking data, with a third to be deployed this summer.
A number of other European efforts are gearing up, including two that have links to the discovery of cosmic-ray air showers in 1938 by Pierre Auger, Roland Maze and Thérèse Grivet-Meyer working at the Paris Observatory. The Reseau de Lycées pour Cosmiques (RELYC) project, centred on the College de France/Laboratoire Astroparticule et Cosmologie in Paris, is preparing to install detectors in high schools close to where Auger and colleagues performed their ground-breaking experiments. The Roland Maze project is centred on the Cosmic Ray Laboratory of the Andrzej Soltan Institute for Nuclear Studies in Lodz, Poland, where it continues a long tradition in studies of cosmic-ray air showers initiated in partnership with Maze some 50 years ago. The plans are to deploy detectors in more than 30 local high schools. In the UK, physicists from King’s College London in collaboration with the Canadian ALTA group will place detector systems in the London area during 2006. In northern England, Preston College is continuing to work on a pilot project, initiated in 2001, to develop an affordable cosmic-ray detection system as part of the Cosmic Schools Group Proposal, involving the University of Liverpool and John Moores University in Liverpool. Finally, a project to set up cosmic-ray telescopes with GPS in 10 Portuguese high schools is underway, spearheaded by the Laboratório de Instrumentação e Física Experimental de Partículas and the engineering faculty of the Technical University in Lisbon.
While the majority of the European projects are based on plastic scintillators, the Italian Extreme Energy Events (EEE) project has opted instead for multigap resistive plate chambers (MRPCs) as their basic detector element. These allow a precise measurement of the direction and time of arrival of a cosmic ray. The aim of this project, the roots of which date back to 1996, is to have a system of MRPC telescopes distributed over a surface of 106 km2, for precise detection of extreme-energy events (Zichichi 1996). These chambers are similar to those that will be used in the time-of-flight detector for the ALICE experiment at CERN’s Large Hadron Collider. Three MRPC chambers form a detector “telescope” that can reconstruct the trajectories of cosmic muons in a shower. At present 23 schools from across Italy are involved in the pilot project, with around 100 others on a waiting list from the length and breadth of the Italian peninsula. More than 60 MRPCs have been built at CERN by teams of high-school students and teachers under the guidance of experts from Italian universities and the INFN.
Most of the major groups in Canada and the US have formed a loose collaboration – the North American Large-area Time Coincidence Arrays (NALTA) – with more than 100 detector stations spread across North America (figure 2). The aim is to share educational resources and information. However, it is also planned to have one central access point where students and researchers can use data from all of the NALTA sites, creating in effect a single giant array. Such a combined network across North America could eventually consist of thousands of cosmic-ray detectors, with the primary research aim of studying ultra-high-energy cosmic-ray showers and correlated cosmic-ray phenomena over a very large area. Until the PAO collaboration constructs its second array in Colorado, US, the NALTA arrays, along with their European counterparts, will dominate the ground-based investigation of the extreme-energy universe in the Northern Hemisphere.
The European groups are also developing a similar collaboration, called Eurocosmics. It is clear that a natural next step is to combine the North American and European networks into a worldwide network that could contribute significantly to elucidating the extreme-energy universe. Such a network could aid and encompass other efforts throughout the world, including in developing countries where it could provide a natural bridgehead into the global scientific culture.
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