Beams of protons are back in CERN’s Proton Synchrotron (PS), having circulated around the accelerator on 20 June for the first time in more than 15 months. The PS restart followed on from the restart of beam in Linac 2 and then the PS Booster (PSB) on 2 June.
The beam made it into the PS on schedule, thanks to the efforts of the PSB specialist teams, who were called upon on many occasions for hardware interventions during the preceding weeks of hardware commissioning and “cold” tests. These included fixing vacuum leaks, re-configuring timing links, correcting magnet connections and, in one instance, replacing an entire magnet with a spare, owing to a water leak. Operations, radiofrequency and instrumentation teams then needed to adjust the settings for beam acceleration and extraction from the PSB to the PS. With beam back in the PS, beam commissioning tests could begin. During this final phase, all of the beam diagnostics – from beam current to bunch spacing – need to be checked, first with low-intensity beams (1011 protons) before moving to higher-intensity levels (1012 protons). The PS will then be ready to send beams to the East Area and the neutron facility, nToF, where physics is planned to start in late July. ISOLDE, the only experimental facility connected directly to the PSB, will be the first user to receive its beams, with physics set to restart in mid-July. The physics programme in the Super Proton Synchrotron is set to restart in the autumn.
At the LHC, the teams closed the last of the 1695 outer magnet bellows on 18 June, marking the end of the Superconducting Magnets And Circuits Consolidation (SMACC) project. Leak tests on the entire machine are proceeding well, and by late June they had been completed in sector 5-6 and were under way in sector 7-8. At the same time, sector 6-7 – the first to be cooled – had reached 20 K, and will be maintained at this temperature during continuity testing of the copper stabilizer. Four sectors should be cool by the end of the summer, and all eight sectors of the LHC are scheduled to be cooled to the nominal temperature of 1.9 K in late autumn.
Beam is expected back into the LHC early in 2015, and the restart of the LHC physics programme is planned for spring 2015, with a collision energy of 13 TeV instead of the previous 7–8 TeV.
The ALPHA experiment at CERN’s Antiproton Decelerator (AD) has made a new precision measurement of the electric charge of antihydrogen atoms, finding it to be compatible with zero to eight decimal places. This is the first time that the charge of an antiatom has been measured to high precision. The ALPHA collaboration studied the trajectories of antihydrogen atoms released from the experiment’s system of particle traps in the presence of an electric field. If the antihydrogen atoms had a charge, the field would deflect them. The analysis, based on 386 events, gives the value of the antihydrogen electric charge as (–1.3±1.1±0.4) × 10–8.
Experiments at the Jülich Cooler Synchrotron (COSY) have found compelling evidence for a new state in the two-baryon system, with a mass of 2380 MeV, width of 80 MeV and quantum numbers I(JP) = 0(3+). The structure, containing six valence quarks, constitutes a dibaryon, and could be either an exotic compact particle or a hadronic molecule. The result answers the long-standing question of whether there are more eigenstates in the two-baryon system than just the deuteron ground-state. This fundamental question has been awaiting an answer since at least 1964, when first Freeman Dyson and later Robert Jaffe envisaged the possible existence of non-trivial six-quark configurations.
The new resonance was observed in high-precision measurements carried out by the WASA-at-COSY collaboration. The first signals of the new state had been seen before in neutron–proton collisions, where a deuteron is produced together with a pair of neutral pions. Now this state has also been observed in polarized neutron–proton scattering and extracted using the partial-wave analysis technique – the generally accepted ultimate method to reveal a resonance. In the SAID partial-wave analysis, the inclusion of the new data produces a pole in the 3D3 partial wave at (2380±10 – i 40±5) MeV.
The mass of the new state is amazingly close to that predicted originally by Dyson, based on SU(6) symmetry breaking. Moreover, recent state-of-the-art Faddeev calculations by Avraham Gal and Humberto Garcilazo reproduce the features of this new state very well. The quantum numbers favour this state as a dibaryon resonance – the “inevitable” non-strange dibaryon predicted by Terry Goldman and colleagues in 1989.
On 7 May, the newly upgraded Continuous Electron Beam Accelerator Facility (CEBAF) delivered the first electron beams to its new experimental complex at the US Department of Energy’s (DOE’s) Jefferson Lab. The success capped a string of accelerator commissioning milestones that were needed for approval to restart experimental operations following CEBAF’s first major upgrade.
CEBAF is an electron-accelerator facility that employs superconducting radiofrequency (SRF) technology to investigate the quark structure of the nucleus. The first large-scale application of SRF technology in the US, it was originally built to circulate electrons through 1–5 passes to provide 4 GeV electron beams. As a result of the operators’ experience in running the machine at its peak potential, the original installation eventually achieved operational energies of 6 GeV.
In May 2012, the accelerator was shut down for its 12 GeV upgrade. This $338-million project, which will double CEBAF’s maximum energy, includes the construction of a fourth experimental hall (Hall D), as well as upgrades to equipment in three existing halls (Halls A, B and C).
Accelerator operators began the painstaking task of bringing the accelerator back online last December. By 5 February, they had achieved the full upgrade-energy acceleration of 2.2 GeV in one pass through the machine. Then on 1 April, the operators exceeded CEBAF’s previous maximum energy. The accelerator delivered three-pass, 6.11-GeV electron beams with 2 nA average current onto a target in Hall A, and recorded the first data of the 12-GeV era, holding the pattern for more than an hour.
The operators continued to push the upgraded machine, and early on 7 May the energy was increased to 10.5 GeV through the entire 5.5 passes. In the last minutes of the day, 10.5 GeV beam was delivered into the new Hall D complex. Having met all of the major milestones in the 12-GeV project for the DOE approval step, Critical Decision-4A (Accelerator Project Completion and Start of Operations), staff and users are now looking forward to demonstration of 12-GeV energy and beam delivery to Jefferson Lab’s experimental halls for commissioning and the start of experiments.
Supernova explosions, triggered when the fuel within a star reignites or its core collapses, launch a shock wave that sweeps through a few light-years of space in only a few hundred years. The remnants of these explosions are now recognized widely as one of nature’s major particle accelerators. The theory is that charged particles increase in energy through repeated encounters with magnetic “mirrors” or changing magnetic fields in the shocks. Now, a team of researchers has brought some of these processes down to Earth, in an experiment to investigate the turbulent amplification of magnetic fields in the supernova remnant, Cassiopeia A, which was first seen about 300 years ago in the constellation Cassiopeia.
Radio observations of Cassiopeia A have revealed regions within the expanding remnant that are consistent with synchrotron radiation emission from giga-electron-volt electrons spiralling in a magnetic field of a few milligauss – 100 times higher than expected from the standard shock compression of the interstellar medium. The origin of such high magnetic fields, which help to make Cassiopeia A a particularly effective particle accelerator and bright radio source, appears to lie with regions of turbulence that could amplify the magnetic field and that could be related to puzzling irregular “knots” seen in optical observations. One explanation for these knots is that the shock produces turbulence as it passes through a region of space that already contains dense clumps or clouds of gas.
To investigate these possibilities, an international team led by Gianluca Gregori at Oxford University used the Vulcan laser facility at the UK’s Rutherford Appleton Laboratory to focus three laser beams onto a carbon rod 0.5 mm thick in a chamber filled with low-density gas. The heat generated made the rod explode, creating a blast that expanded through the surrounding gas, mimicking a supernova shock wave. To simulate the clumps that might surround an exploding star, the team introduced a mesh of fine plastic wires 0.4 mm thick with cells 1.1 mm square at a distance of 1 cm from the rod. Using hydrodynamical scaling relations, the team can relate the experimental conditions 0.3 μs after the laser burst to Cassiopeia A as it is now, about 310 years after the supernova explosion. With the same scaling, the wire thickness corresponds to a distance of about one parsec in the remnant.
The researchers used various techniques to monitor the evolution of the shock wave, including an induction coil to measure the magnetic fields produced. The measurements show that the grid produces additional turbulent flow and gives rise to magnetic-field components that are 2–3 times larger than without it. The results are also in good agreement with the output from numerical simulation code, in particular, the magnetohydrodynamic code FLASH, developed by Don Lamb at Chicago University. The simulations reproduce well the position of the shock, the peak electron density and temperature – with and without the grid – and confirm that the magnetic field is indeed enhanced as a result of induced turbulence created as the shock moves through the grid.
These results demonstrate that the amplification of the magnetic field within the Cassiopeia A “particle accelerator” might indeed arise from the interaction of the shock with a clumpy interstellar medium. Importantly, the experiment also gives valuable confirmation of the simulations, providing for the first time an experimental means to validate the simulation codes used for many astrophysical phenomena.
A German/Japanese collaboration working at the University of Mainz has performed the first direct high-precision measurement of the magnetic moment of the proton – which is by far the most accurate to date. The result is consistent with the currently accepted value of the Committee on Data for Science and Technology (CODATA), but is 2.5 times more precise and 760 times more accurate than any previous direct measurement. The techniques used will feature in the Baryon-Antibaryon Symmetry Experiment (BASE) – recently approved to run at CERN’s Antiproton Decelerator (AD) – which aims at the direct high-precision measurement of the magnetic moments of the proton and the antiproton with fractional precisions at the parts-per-billion (ppb) level, or better.
Prior to this work, the record for the most precise measurement of the proton’s magnetic moment had stood for more than 40 years. In 1972, a group at Massachusetts Institute of Technology measured its value indirectly by performing ground-state hyperfine spectroscopy with a hydrogen maser in a magnetic field. This experiment measured the ratio of the magnetic moments of the proton and the electron. The results, combined with theoretical corrections and two additional independent measurements, enabled the calculation of the proton magnetic moment with a precision of about 10 parts in a billion.
In an attempt to surpass the record, the collaboration of scientists from Mainz University, the Max Planck Institute for Nuclear Physics in Heidelberg, GSI Darmstadt and the Japanese RIKEN institute applied the so-called double Penning trap technique to a single proton for the first time (see figure 1). One Penning trap – called the analysis trap – is used for the non-destructive detection of the spin state, through the continuous Stern-Gerlach effect. In this elegant approach, a strong magnetic inhomogeneity is superimposed on the trap, so coupling the particle’s spin-magnetic-moment to its axial oscillation frequency in the trap. By measuring the axial frequency, the spin quantum state of the trapped particle can be determined. And by recording the quantum-jump rate as a function of a spin-flip drive frequency, the spin precession frequency νL is obtained. Together with a measurement of the cyclotron frequency νc of the trapped particle, the magnetic moment of the proton μp is obtained finally in units of the nuclear magneton, μp/μN = νL/νc.
This approach has already been applied with great success in measurements of the magnetic moments of the electron and the positron. However, the magnetic moment of the proton is about 660 times smaller than that of the electron, so the proton measurement requires an apparatus that is orders of magnitude more sensitive. To detect the proton’s spin state, the collaboration used an extremely strong magnetic inhomogeneity of 300,000 T/m2. However, this limits the experimental precision in the frequency measurements to the parts-per-million (ppm) level. Therefore a second trap – the precision trap – was added about 45 mm away from the strong magnetic-field inhomogeneity. In this trap the magnetic field is about 75,000 times more homogeneous than in the analysis trap.
To determine the magnetic moment of the proton, the first step was to identify the spin state of the single particle in the analysis trap. Afterwards the particle was transported to the precision trap, where the cyclotron frequency was measured and a spin flip induced. Subsequently the particle was transported back to the analysis trap and the spin state was analysed again. By repeating this procedure several hundred times, the magnetic moment was measured in the homogeneous magnetic field of the precision trap. The result, extracted from the normalized resonance curve (figure 2), is the value μp = 2.792847350(9)μN, with a relative precision of 3.3 ppb.
In the BASE experiment at the AD the technique will be applied directly to a single trapped antiproton and will potentially improve the currently accepted value of the magnetic moment by at least a factor of 1000. This will constitute a stringent test with baryons of CPT symmetry – the most fundamental symmetry underlying the quantum field theories of the Standard Model of particle physics. CPT invariance implies the exact equality of the properties of matter–antimatter conjugates and any measured difference could contribute to understanding the striking imbalance of matter and antimatter observed on cosmological scales.
Relativistic heavy-ion collisions produce large numbers of particles that do not move individually, but rather as an organized group, with a collective motion known as flow. Flow studies at Brookhaven’s Relativistic Heavy-Ion Collider (RHIC) contributed to the surprising realization that the hot and dense matter created in the collisions behaves like a perfect liquid and not as a hadron gas. Now, the ALICE collaboration at the LHC has looked further into how these effects vary for different particle species.
In relativistic heavy-ion collisions the collective motion, or flow, is governed by the spatial anisotropy of the almond-shaped overlap region of the colliding nuclei and the initial density inhomogeneities of the fireball. These features are transformed, through interactions between the produced particles, into an anisotropy in momentum space. The degree of this transformation depends on the ratio of shear viscosity to entropy, η/s, which quantifies the friction of the created matter. This resulting anisotropy in particle production can be quantified by a Fourier analysis of the azimuthal distribution relative to the system’s symmetry plane, characterized by Fourier coefficients, vn. The second harmonic, v2, is known as the elliptic-flow coefficient.
One of the major outcomes from RHIC was the measurement of the elliptic flow of identified particles. These results led to the conclusion that the matter created acts as a system where the value of η/s is very close to the lower bound of ħ/4πkB conjectured within anti-de Sitter/conformal field theory – i.e. a nearly perfect liquid. At low values of transverse momentum (pT), for pT < 2 GeV/c, the experiments found an interesting mass-ordering of v2(pT), attributed to the interplay between elliptic and radial flow.
Radial flow tends to create a depletion in the particle pT spectrum at low values, which increases with increasing particle mass and transverse velocity. When introduced in a system that exhibits azimuthal anisotropy, this depletion becomes larger along the shorter axis of the anisotropy, thereby reducing v2. The net result is that at a fixed value of pT, heavier particles have a smaller value of v2 than lighter ones. In the intermediate pT region (2 < pT < 5 GeV/c), the v2 of baryons is larger than that of mesons. This phenomenon was conjectured to originate within a picture where flow develops at the partonic level and quarks coalesce into hadrons during hadronization. The proposed mechanism was argued to lead to the observed hierarchy in the flow values – the so-called number of constituent quarks (NCQ) scaling.
The ALICE collaboration, profiting from the unique particle-identification capabilities that the detector set-up provides, had measured v2 in PbPb collisions at √sNN = 2.76 TeV for different centrality intervals and various particles: π, K, p, Λ, Ξ–, Ω– (and their antiparticles), K0s and φ. The figure illustrates how v2 develops for different particle species within the same centrality interval in central (left plot) and peripheral (middle plot) PbPb collisions.
A clear mass ordering is seen for all centralities in the low-pT region (i.e. pT ≤ 2 GeV/c). Comparisons with hydrodynamic calculations in this transverse-momentum range indicate that the produced matter at the LHC seems to favour a value of η/s smaller than twice the quantum mechanical limit. In the intermediate pT region (pT > 2 GeV/c), although the particles tend to group according to their type (i.e. mesons and baryons), the NCQ scaling, if any, is only approximate. In particular, the φ-meson, with a mass close to that of p and Λ, seems to follow the baryon band in central events and shifts progressively to the band of mesons for peripheral collisions. This seems to indicate that the mass, rather than the number of constituent quarks, is the driving force of the v2 evolution with pT also in the intermediate region.
With the discovery of a Higgs boson at the LHC two years ago, the last piece of the Standard Model puzzle fell into place. Yet, several mysteries remain, one of which is the enigma of the origin of dark matter. One of the most popular classes of models predicts that the dark matter is made of weakly interacting neutral and colourless particles, χ, with mass ranging from a few to a few hundred giga-electron-volts. The LHC, with its high-energy collisions, provides an excellent pace to search for such particles, and the CMS collaboration has been taking a new look at ways in which they could be produced.
Until recently, the main experimental method to look for dark-matter particles was to exploit their elastic scattering on nuclei inside sensitive detectors, working typically at low temperatures. These direct-detection experiments aim to observe the scattering by measuring the momentum of the recoiling nucleus. While interesting hints for dark-matter detection in various mass ranges have been reported by some of these experiments, none of these hints have been confirmed by later, more precise measurements.
Several years ago, a new idea appeared: to look for the production of pairs of dark-matter particles in high-energy particle collisions, like those at the LHC, via a process described by the same Feynman diagram as the scattering of the dark-matter particles on quarks inside the nuclei, but “rotated” by 90°. While such direct-detection experiments look for the process qχ → qχ, experiments at the LHC can look for qq → χχ. The challenge is to trigger on these events, because dark-matter particles would leave no trace in the detector. One possibility is to search for a more complicated process, where an additional particle, for example a gluon or a photon, is produced together with the dark matter.
The CMS experiment has performed a number of such searches, which are referred to collectively as mono-X searches, because they look for a single object, X, recoiling against the invisible particles. Recently, these searches have been extended to more complicated signatures, for example the production of dark matter in association with a pair of top quarks, which are produced in abundance at the LHC. The new analysis looks for top-quark pairs that are recoiling against a large amount of “missing” transverse momentum, carried away by dark-matter particles.
As figure 1 shows, a new measurement by CMS of the production of top-quark pairs in association with missing transverse momentum sets stringent limits in the plane of the dark-matter particle mass Mχ vs an effective interaction energy scale, M* (CMS Collaboration 2014a). The interaction of dark matter with the known particles is usually assumed to be carried by new “messenger” particles. If the messengers are heavy – which would be a good reason why they have not yet been seen – the interaction can be approximated via a point-like interaction with an effective energy scale of M*. This is similar to Enrico Fermi’s effective theory of muon decay, where the messenger – a W boson – is much heavier than the muon.
Another interesting way to look for dark matter is based on precision measurements of the properties of the Higgs boson. If the mass of the dark-matter particle is less than roughly half of the Higgs boson mass, for instance, Mχ < 60 GeV, then it is possible to look for a direct decay of the Higgs boson into a χχ pair. This decay is called “invisible” because its products are not detected.
The CMS collaboration recently published a search for such invisible Higgs-boson decays, where the production of the Higgs is tagged either by the presence of a Z boson (associated ZH production), or by the presence of two forward jets, characteristic of vector-boson fusion (CMS Collaboration 2014b). The upper limits set on the invisible branching fraction of the Higgs boson are 51% and 58% at a 90% and 95% confidence level, respectively. The former limit can be translated to limits on the mass of the dark-matter particle vs its interaction cross-section with a nucleon, which allows for a direct comparison with the limits coming from various direct-detection experiments, as figure 2 shows. The limits are set for various types of dark-matter particle: scalar, vector, or a Majorana fermion. They are significantly more stringent than the direct-detection limits for low masses for dark matter, emphasizing the complementarity of the searches by the LHC and the direct-detection experiments.
The Standard Model of particle physics has been extremely successful in predicting a vast variety of phenomena – so successful, that it is easy to forget that some of its predictions have not yet been verified. A very important one, related intimately to electroweak symmetry breaking, is that the gauge bosons (γ, W and Z) can interact with each other through quartic interactions. Four such interactions are allowed in the Standard Model: WWWW, WWZZ, WWZγ and WWγγ. The other boson combinations are forbidden on symmetry grounds. Now the ATLAS collaboration has found evidence for a process involving the first of these three – the WWWW interaction.
The WWWW and WWZZ interactions, in particular, are of great theoretical interest. If there were no Higgs boson, the rate of these processes would become unphysically large. With the discovery of a Higgs boson, they have remained interesting as a way to study electroweak symmetry breaking and even to probe for new heavier Higgs bosons. At the LHC, the WWWW interactions can be studied through the radiation of W bosons from quarks (see figure). This is a rare process, making the interaction of W bosons particularly rare and difficult to detect.
The ATLAS collaboration has presented the first evidence of this rare process involving a quartic WWWW interaction in a paper submitted to Physical Review Letters. The ATLAS analysis selected events with two same-charge W bosons (reconstructed through their leptonic decays to electron or muon and their respective (anti)neutrinos) and two jets. The background from other Standard Model processes is reduced by using the fact that these processes rarely produce two leptons with the same electric charge, together with the knowledge that the quarks that recoil off the radiated W bosons will produce jets that are separated widely and have a particularly large invariant mass.
Data collected by the ATLAS experiment show an excess of these events across the predicted background, with a statistical significance of 3.6σ. The measured fiducial cross-section is 1.3±0.4 fb, in agreement with the Standard Model expectation of 0.95±0.06 fb, and provides the first step into a previously untouched segment of the Standard Model.
At the Quark Matter 2014 conference, held in Darmstadt on 19–25 May, ATLAS presented a variety of new results based on lead–lead (PbPb) and proton–lead (pPb) data collected during Run 1 of the LHC. The PbPb results included new measurements of event-by-event correlations and fluctuations of collective flow, high-statistics measurements of photon and W production, and studies of jet quenching using charged particles, single jets, nearby jet pairs and jet-fragmentation functions. The results from pPb data included precision measurements of long-range pseudorapidity correlation and associated azimuthal structures, and high-pT production of charged particles, Z bosons and jets. Here are some of the highlights.
Following the first observation of highly asymmetric dijets in PbPb collisions, the study of jet quenching has been an essential part of the heavy-ion physics programme at the LHC. Measurements of the production of electroweak bosons provide important control data for the study of jet quenching, as well as for investigating nuclear modifications to parton distribution functions. ATLAS presented new results on the measurement of W-boson production via the electron and muon decay modes in PbPb collisions at √sNN = 2.76 TeV, with yields of W bosons obtained as a function of centrality and pseudorapidity. A good agreement was found between the two decay channels. The yields are in agreement with the predictions based on modified next-to-leading-order calculations, while leading-order calculations underestimate the yield.
Jets produced in heavy-ion collisions can interact with the medium that is produced in the collisions and lose energy through the phenomenon of jet quenching. This energy loss suppresses the rate of jets produced in these collisions, relative to proton–proton collisions, where no such effects are present. Using the high-statistics proton–proton data set from 2013 as the new reference, ATLAS presented the most precise measurement of jet suppression to date. In central collisions, the jet yields are suppressed by more than a factor of two below a jet transverse momentum, pT, of around 150 GeV (see figure 1), but the suppression is found to be reduced at higher pT.
ATLAS presented first results on direct correlations between the elliptic flow coefficient, v2, and higher-order flow harmonics, v3, v4 and v5, in PbPb collisions. This correlation is obtained via an event-shape engineering technique, in which events within the same centrality interval are divided into different classes according to the observed ellipticity in the forward pseudorapidity. The correlation in v2 for two different ranges in pT (figure 2(a)) shows non-trivial centrality dependence but is linear within a narrow centrality interval. This linearity indicates that viscous effects are controlled by the size of the system, and not its overall shape. The v3–v2 correlations, shown in figure 2(b), reveal a surprising anticorrelation between the ellipticity and triangularity of the initial geometry, which is not accessible via the traditional measurements. The v4–v2 and v5–v2 correlations provide the most direct and detailed picture of the interplay between the linear and nonlinear collective dynamics in the final state of the PbPb collisions.
Turning to the pPb runs, the large data sample collected at √sNN = 5.02 TeV in 2013 allowed ATLAS to measure the jet production over the widest kinematic range ever probed in proton–nucleus collisions. This measurement has the potential to reveal how the hard partonic content inside matter is modified deep inside the high-density nucleus, and explores the interplay between hard processes and collision geometry – a major topic at the conference. When considering collisions of all impact parameters, the rate of jets was found to be slightly above what would be expected just from the proton’s collisions with individual nucleons in the lead nucleus. This slight excess is generally consistent with models of the modified parton densities in nuclei. However, when pPb collisions are selected by “centrality”, unexpected effects appear (figure 3). The rate of jets is suppressed strongly in apparently central events (with small impact parameter) and enhanced in those that appear peripheral (large impact parameter). Furthermore, the modifications of the jet rate have a striking pattern as a function of the energy and rapidity, implying that the modifications might originate in the proton, rather than the nuclear, wave function.
Using the same pPb data set, ATLAS also performed a detailed study of the long-range pseudorapidity correlation and its azimuthal structure, as characterized by the first five Fourier harmonics, v1–v5. The study extended the previous measurements at the LHC of v2 and v3 to higher pT and events with higher charged-particle multiplicities. Moreover, the measurements of v1, v4 and v5 were presented in this context for the first time. As figure 4 shows, the pT dependences of vn are found to be similar to PbPb collisions with comparable multiplicities, suggesting that the collective flow – the main attribute of the dense system created in PbPb collisions – might also be present in pPb interactions.
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