The search for particles that could constitute dark matter in the universe relies on detecting their interplay with the Standard Model particles through a three-pronged approach: via direct-detection experiments, via indirect-detection experiments, and with hermetic detectors at colliders, covering the full 4π-phase space. Because dark matter behaves as a weakly interacting neutral particle, it escapes the detectors without interacting, so in collider experiments its production is inferred by measuring the imbalance in transverse momentum left in the detector. At the LHC, a search for the pair production of dark-matter particles can be performed by looking for events with a large momentum imbalance in association with initial-state radiation of either a jet or a photon – the “monojet” or “monophoton” searches.
The CMS collaboration now has results based on proton–proton collision data collected at a centre-of-mass energy of 8 TeV, amounting to 20 fb–1 of integrated luminosity. In the analysis, both monojet and monophoton searches employ a “cut-and-count” approach. A set of cuts is applied to select potential dark-matter events and, at the same time, to reduce the contamination from Standard Model processes.
One of the dominant and irreducible backgrounds for both searches is the decay of the Z boson into neutrinos, which accounts for roughly 60–70% of the total monojet/monophoton events. The searches look for an excess of events above those expected from the Standard Model processes. In the absence of an excess, limits can be placed on the pair production of dark-matter particles. The results are presented within the framework of an effective field theory where a contact interaction is assumed between the dark-matter and Standard Model particles. Because the effective field theory is not valid for the full parameter space probed at the LHC, the searches are also interpreted in the context of a simplified model with an s-channel mediator. Both assumptions are depicted in the Feynman diagrams in figure 1.
The results (see figure 2) show that CMS extends the sensitivity to spin-independent dark-matter–Standard Model interactions including a vector operator to dark-matter masses that are lower (below 5 GeV) than is currently accessible to the direct-detection experiments. For spin-dependent interactions that include an axial-vector operator, the sensitivity of CMS (not shown here) extends down to dark-matter–nucleon cross-sections of 10–41 cm2. If the particle mediating the dark-matter–Standard Model interaction is accessible at LHC energies, CMS has the opportunity to search for the mediator itself. Figure 3 shows the constraints placed on the mass and coupling strengths of vector-mediator interactions in the monophoton analysis.
The LHC plays a significant role in the search for dark matter and complements well the searches by the direct-detection experiments. The CMS collaboration is now looking forward to intensifying the search with data at 13 TeV and opening up a completely new energy regime to spot hints of dark-matter particles.
EuroCirCol, the EC-funded part of the Future Circular Collider (FCC) study that will develop the conceptual design of an energy-frontier hadron collider, officially started on 1 June. The “kick-off” event at CERN on 2–4 June brought together 62 participants to constitute governance bodies, commit to the project plan and align the organization, structures and processes of 16 institutions from 10 countries. The goal of the project is to conceive a post-LHC research infrastructure around a 100-km circular hadron collider capable of reaching 100-TeV collisions. The project will run for four years, with a total estimated budget of €11.2 million, which includes a €2.99 million contribution from the European Commission’s Horizon 2020 programme on developing new world-class research infrastructures.
EuroCirCol will deliver a design for a hadron collider as part of the broader FCC study (CERN Courier April 2014 p16). It will provide input to an accelerator-infrastructure road map, taking into account European and global interests by the time of the next update of the European Strategy for Particle Physics in 2018. It was the only one of 39 submissions to receive the maximum points from reviewers, a clear sign that high-energy physics remains a top priority for the European Commission.
EuroCirCol is organized around four technical work packages. The first two are to develop the collider’s lattice and beam optics, including the experimental regions. A third develops prototypes and tests a novel cryogenics beam-vacuum system that can respond to the challenges of the high levels of synchrotron radiation expected at such a collider. This work also pioneers collaboration between the particle-physics and light-source communities, with opportunities to improve existing synchrotron-radiation facilities and to reduce cost and performance of fourth- or fifth-generation light sources. The fourth work package will study a viable design for a 16-T accelerator magnet, as part of a worldwide study of conductor R&D for the High-Luminosity LHC project and the FCC.
The EuroCirCol project is set to create opportunities for doctoral and postdoctoral assignments in the areas of beam optics and accelerator technologies, in the participating institutes. It will also provide excellent training opportunities for the next generation of accelerator physicists, under the guidance of world-renowned experts in the field.
As a building block in the globally co-ordinated strategy of the FCC study to produce a global design for a global machine, EuroCirCol’s main outcome will be to lay the foundations for subsequent research-infrastructure development that will strengthen Europe as a leader in global research co-operation over the coming decades.
Astronomers using ESO’s Very Large Telescope (VLT) have discovered a very bright galaxy in the early universe, and found strong evidence that it contains first-generation stars. These massive luminous stars – previously purely theoretical – are made of primordial material from the Big Bang, and produced the first heavy elements. The newly found galaxy is three times brighter than the brightest distant galaxy known up to now.
Astronomers have long theorized the existence of a first generation of stars – known as Population III stars – born out of hydrogen, helium and trace amounts of lithium, the only elements produced by Big Bang nucleosynthesis. All of the heavier chemical elements, such as oxygen, nitrogen, carbon and iron, were forged by nuclear fusion in the cores of stars. The Population III stars would have been enormous – several hundred or even a thousand times more massive than the Sun. They would have exploded as supernovae after only about 2 million years, which is less than a thousandth of the Sun’s lifetime.
A team led by David Sobral, of the University of Lisbon and Leiden Observatory, has used the VLT to peer back into the ancient universe, to a period known as re-ionization, approximately 800 million years after the Big Bang. Instead of conducting a narrow and deep study of a small area of the sky, they broadened their scope to produce the widest survey of very distant galaxies ever attempted. Their expansive study was made using the VLT, with help from the W M Keck Observatory and the Subaru Telescope, as well as the NASA/ESA Hubble Space Telescope. The team discovered and confirmed a number of surprisingly bright, very young galaxies at a redshift, z, of around seven. One of these, labelled CR7 – for COSMOS Redshift 7, but also as an allusion to the footballer Cristiano Ronaldo, who is known as CR7 – is by far the brightest galaxy ever observed so early in the history of the universe.
The X-shooter and SINFONI instruments on the VLT found strong ionized-helium emission in CR7 but, crucially and surprisingly, no sign of any heavier elements in a bright area of the galaxy. This suggests that the team has discovered the first evidence for clusters of Population III stars that had ionized gas within a galaxy in the early universe. Bluer and somewhat redder clusters of stars were found within CR7, indicating that the formation of Population III stars had occurred in waves, as had been predicted. What the team directly observed was the last wave of Population III stars, suggesting that such stars should be easier to find than previously thought: they reside among regular stars, in brighter galaxies, not just in the earliest, smallest and dimmest galaxies, which are so faint as to be extremely difficult to study.
The team considered two alternative theories: that the source of the light was either an active galactic nuclei, or Wolf–Rayet stars. The lack of heavy elements, together with other evidence, strongly refutes both these theories. The team also considered that the source may be a direct-collapse black hole, which would itself be an exotic, so far purely theoretical, object. The lack of a broad emission line, and the fact that the hydrogen and helium luminosities are much greater than what has been predicted for such a black hole, indicates that this scenario is also rather unlikely.
Synchrotron-light sources have become an essential tool in many branches of medicine, biology, physics, chemistry, materials science, environmental studies and even archaeology. There are some 50 storage-ring-based synchrotron-light sources in the world, including a few in developing countries, but none in the Middle East. SESAME is a 2.5-GeV, third-generation light source under construction near Amman. When it is commissioned in 2016, it will not only be the first light source in the Middle East, but arguably also the region’s first true international centre of excellence.
The members of SESAME are currently Bahrain, Cyprus, Egypt, Iran, Israel, Jordan, Pakistan, the Palestinian Authority and Turkey (others are being sought). Brazil, China, the European Union, France, Germany, Greece, Italy, Japan, Kuwait, Portugal, the Russian Federation, Spain, Sweden, Switzerland, the UK and the US are observers.
SESAME will: foster scientific and technological capacities and excellence in the Middle East and neighbouring regions, and help prevent or reverse the brain drain; build scientific links and foster better understanding and a culture of peace through collaboration between peoples with different creeds and political systems.
The origins of SESAME
The need for an international synchrotron light-source in the Middle East was recognized by the Pakistani Nobel laureate Abdus Salam, one of the fathers of the Standard Model of particle physics, more than 30 years ago. This need was also felt by the CERN-and-Middle-East-based Middle East Scientific Co-operation group (MESC), headed by Sergio Fubini. MESC’s efforts to promote regional co-operation in science, and also solidarity and peace, started in 1995 with the organization in Dahab, Egypt, of a meeting at which the Egyptian minister of higher education, Venice Gouda, and Eliezer Rabinovici of MESC and the Hebrew University in Israel – and now a delegate to the CERN and SESAME councils – took an official stand in support of Arab–Israeli co-operation.
At the request of Fubini and Herwig Schopper, the German government agreed to donate the components of BESSY I to SESAME
In 1997, Herman Winick of SLAC and the late Gustav-Adolf Voss of DESY suggested building a light source in the Middle East using components of the soon-to-be decommissioned BESSY I facility in Berlin. This brilliant proposal fell on fertile ground when it was presented and pursued during workshops organized in Italy (1997) and Sweden (1998) by MESC and Tord Ekelof, of MESC and Uppsala University. At the request of Fubini and Herwig Schopper, a former director-general of CERN, the German government agreed to donate the components of BESSY I to SESAME, provided that the dismantling and transport – eventually funded by UNESCO – were taken care of by SESAME.
The plan was brought to the attention of Federico Mayor, then director-general of UNESCO, who called a meeting of delegates from the Middle East and neighbouring regions at the organization’s headquarters in Paris in June 1999. The meeting launched the project by setting up an International Interim Council with Schopper as chair. Jordan was selected to host SESAME, in a competition with five other countries from the region. It has provided the land and funded the construction of the building.
In May 2002, the Executive Board of UNESCO unanimously approved the establishment of the new centre under UNESCO’s auspices. SESAME formally came into existence in April 2004, when the permanent council was established, and ratified the appointments of Schopper as president and of the first vice-presidents, Dincer Ülkü of Turkey and Khaled Toukan of Jordan. A year later, Toukan stepped down as vice-president and became director of SESAME.
Meanwhile, the ground-breaking ceremony was held in January 2003, and construction work began the following August. Since February 2008, SESAME has been working from its own premises, which were formally opened in November 2008 in a ceremony held under the auspices of King Abdullah II of Jordan, and with the participation of Prince Ghazi Ben Mohammed of Jordan and Koïchiro Matsuura, then director-general of UNESCO. In November 2008, Schopper stepped down as president of the Council and was replaced by Chris Llewellyn Smith, who is also a former director-general of CERN. In 2014, Rabinovici and Kamal Araj of Jordan became vice-presidents, replacing Tarek Hussein of Egypt and Seyed Aghamiri of Iran.
SESAME users
As at CERN, the users of SESAME will be based in universities and research institutes in the region. They will visit the laboratory periodically to carry out experiments, generally in collaboration. The potential user-community, which is growing rapidly, already numbers some 300, and is expected eventually to grow to between 1000 and 1500. It is being fostered by a series of Users’ Meetings – the 12th, in late 2014, attracted more than 240 applications, of which only 100 could be accepted. The training programme, which is supported by the International Atomic Energy Agency, various governments and many of the world’s synchrotron laboratories, and which includes working visits to operational light sources, is already bringing significant benefits to the region.
In 2002, the decision was taken to build a completely new main storage ring, with an energy of 2.5 GeV – compared with the 1 GeV that would have been provided by upgrading the main BESSY 1 ring – while retaining refurbished elements of the BESSY I microtron to provide the first stage of acceleration and the booster synchrotron. As a result, SESAME will not only be able to probe shorter distances, but will also be a third-generation light source, i.e. one that can accommodate insertion devices – wigglers and undulators – to produce enhanced synchrotron radiation. There are light sources with higher energy and greater brightness, but SESAME’s performance (see table) will be good enough to allow users – with the right ideas – to win Nobel prizes.
Progress has not been as rapid as had been hoped, owing mainly to lack of funding, as discussed below. The collapse of the roof under an unprecedented snowfall in December 2013, when it even snowed in Cairo, has not helped. Nevertheless, despite working under the open sky throughout 2014, the SESAME team successfully commissioned the booster synchrotron in September 2014. The beam was brought to the full energy of 800 MeV, essentially without loss, and the booster is now the highest-energy accelerator in the Middle East (CERN Courier November 2014 p5).
The final design of the magnets for the main ring and for the powering scheme was carried out by CERN in collaboration with SESAME. Construction of the magnets is being managed by CERN using funds provided by the European Commission. The first of 16 cells was assembled and successfully tested at CERN at the end of March, and installation will begin later this year (CERN Courier May 2015 p6). If all goes well, commissioning of the whole facility – initially with only two of the four accelerating cavities – should begin in June next year.
The scientific programme
SESAME will nominally have four “day-one” beamlines in Phase 1a, although to speed things up and save money, it will actually start with just two. Three more beamlines will be added in Phase 1b.
One of the beamlines that will be available next year will produce photons with energies of 0.01–1 eV for infrared spectromicroscopy, which is a powerful tool for non-invasive studies of chemical components in cells, tissues and inorganic materials. A Fourier transform infrared microscope, which will be adapted to this beamline, has already been purchased. Meanwhile, 11 proposals from the region to use it with a conventional thermal infrared source have been approved. The microscope has been in use since last year, and the first results include a study of breast cancer by Fatemeh Elmi of the University of Mazandaran, Iran, with Randa Mansour and Nisreen Dahshan, who are PhD students in the Faculty of Pharmacy, University of Jordan. When SESAME is in operation, the infrared beamline will be used in biological applications, environmental studies, materials and archaeological sciences.
An X-ray absorption fine-structure and X-ray fluorescence beamline, with photon energies of 3–30 keV, will also be in operation next year. It will have potential applications in materials and environmental sciences, providing information on chemical states and local atomic structure that can be used for designing new materials and improving catalysts (e.g. for the petrochemical industries). Other applications include the non-invasive identification of the chemical composition of fossils and of valuable paintings.
It is hoped that macro-molecular crystallography and material-science beamlines, with photon energies of 4–14 keV and 3–25 keV, respectively, will be added in the next two years, once the necessary funding is available. The former will be used for structural molecular biology, aimed at elucidating the structures of proteins and other types of biological macromolecules at the atomic level, to gain insight into mechanisms of diseases to guide drug design (as used by pharmaceutical and biotech companies). The latter will use powder diffraction for studies of disordered/amorphous material on the atomic scale. The use of powder diffraction to study the evolution of nanoscale structures and materials in extreme conditions of pressure and temperature has become a core technique for developing and characterizing new smart materials.
In Phase 1b, soft X-ray (0.05–2 keV), small and wide-angle X-ray scattering (8–12 keV) and extreme-ultraviolet (10–200 eV) beamlines will be added. They will be used, respectively, for atomic, molecular and condensed-matter physics; structural molecular biology and materials sciences; and atomic and molecular physics, in a spectral range that provides a window on the behaviour of atmospheric gases, and enables characterization of the electrical and mechanical properties of materials, surfaces and interfaces.
The main challenges
The main challenge has been – and continues to be – obtaining funding. Most of the SESAME members have tiny science budgets, many are in financial difficulties, and some have faced additional problems, such as floods in Pakistan and the huge influx of refugees in Jordan. Not surprisingly, they do not find it easy to pay their contributions to the operational costs, which are rising rapidly as more staff are recruited, and will increase even faster when SESAME comes into operation and is faced with paying large electricity bills at $0.36/kWh and rising. Nevertheless, increasing budgets have been approved by the SESAME Council. As soon as the funding can be found, a solar-power plant, which would soon pay for itself and ease the burden of paying the electricity bill, will be constructed. And SESAME has always been open to new members, who are being sought primarily to share the benefits but also to share the costs.
So far, $65 million has been invested, including the value to SESAME of in-kind contributions of equipment (from Jordan, Germany, the UK, France, Italy, the US and Switzerland), cash contributions to the capital budget (from the EU, Jordan, Israel, Turkey and Italy), and manpower and other operational costs that are paid by the members (but not including important in-kind contributions of manpower, especially from CERN and the French light source, SOLEIL).
SESAME is a working example of Arab–Israeli–Iranian–Turkish–Cypriot–Pakistani collaboration.
Thanks to the contributions already made and additional funding to come from Iran, Israel, Jordan and Turkey, which have each pledged voluntary contributions totalling $5 million, most of the funds that are required simply to bring SESAME into operation next year are now available. At the SESAME Council meeting in May, Egypt announced that it will also make a voluntary contribution, which will narrow the immediate funding gap. More will, however, be needed, to provide additional beamlines and a properly equipped laboratory, and additional funds are being sought from a variety of governments and philanthropic organizations.
The ongoing turbulence in the Middle East has only had two direct effects on SESAME. First, sanctions are making it impossible for Iran to pay its capital and operational contributions, which are much needed. Second, discussions of Egypt joining other members in making voluntary contributions were interrupted several times by changes in the government.
Outlook
SESAME is a working example of Arab–Israeli–Iranian–Turkish–Cypriot–Pakistani collaboration. Senior scientists and administrators from the region are working together to govern SESAME through the Council, with input from scientists from around the world through its advisory committees. Young and senior scientists from the region are collaborating in preparing the scientific programme at Users’ Meetings and workshops. And the extensive training programme of fellowships, visits and schools is already building scientific and technical capacity in the region.
According to the Italian political theorist Antonio Gramsci, there is a perpetual battle between the optimism of the will and the pessimism of the brain. Several times during its history, SESAME has faced seemingly impossible odds, and pessimists might have given up. Luckily, however, the will prevailed, and SESAME is now close to coming into operation. There are still huge challenges, but we are confident that thanks to the enthusiasm of all those involved they will be met and SESAME will fulfil its founders’ ambitious aims.
In early April, members of the Baikal collaboration deployed and started operation of the first cluster of the Gigaton Volume Detector (Baikal-GVD). Named “Dubna”, the cluster comprises 192 optical modules arranged at depths down to 1300 m. The modules are glass spheres that house photomultiplier tubes to detect Cherenkov light from the charged particles emerging from neutrino interactions in the water of the lake. By 2020, GVD is set to consist of 10–12 clusters covering a total volume of about 0.4 km3 (GVD phase-1). This is about half the size of the present world leader – the IceCube Neutrino Observatory at the South Pole (CERN Courier December 2014 p30). A planned further extension should then lead towards a second stage containing 27 clusters in a telescope with a total volume of about 1.5 km3.
Neutrino detection in Lake Baikal will be an important part of the effort to understand better the high-energy processes that occur in far-distant astrophysical sources, to determine the origin of cosmic particles of the highest energies ever registered, to search for dark matter, to study properties of elementary particles, and to learn a great deal of new information about the structure and evolution of the universe as a whole. Together with KM3NeT in the Mediterranean Sea, the other future Northern-hemisphere neutrino telescope (CERN Courier July/August 2012 p31), GVD will allow an optimal view to the central parts of the Galaxy.
The start of the Baikal neutrino experiment dates back to 1 October 1980, when a laboratory of high-energy neutrino astrophysics was established at the Institute for Nuclear Research of the former Academy of Sciences of the USSR in Moscow – now the Institute for Nuclear Research of the Russian Academy of Sciences (INR RAS). This laboratory later became the core of the Baikal collaboration, including at various times the Joint Institute for Nuclear Research (JINR) in Dubna, Irkutsk State University, Moscow State University, DESY-Zeuthen, the Nizhni Novgorod State Technical University, the Saint Petersburg State Marine Technical University, and other scientific research organizations in Russia, Hungary and Germany. At present, the participation of institutes from the Czech Republic, Slovakia and Poland is under discussion.
The idea to register neutrinos in large-scale Cherenkov detectors in natural water was expressed for the first time by Moisey Markov, then at Dubna, at the 10th International Conference on High-Energy Physics, in 1960. Two decades later, Alexander Chudakov, of INR, proposed using Lake Baikal as a site both for tests and for future large-scale neutrino telescopes. The choice of this lake – the largest and deepest freshwater reservoir in the world – was determined by the high transparency of its water, its depth, and the ice cover that allows the installation of deep-water equipment during two months in winter.
The predecessor of GVD was constructed during 1993–1998. Named NT200, it comprised 192 photodetectors placed on eight vertical strings at a depth of 1100–1200 m. NT200 covered some 100,000 m3 of fresh water (an order of magnitude less than the present Dubna cluster). Already in 1994, data taken with only 36 of the final 192 photodetectors showed two neutrino events. These two events were the first of several-hundred-thousand atmospheric neutrinos since recorded by deep-underwater and under-ice experiments. Scientific research with NT200 covered a wide programme, most notably the search for a cosmic diffuse neutrino flux leading to tight limits on that flux (CERN Courier July/August 2005 p24). Moreover, limits were derived on the flux of magnetic monopoles and on muons from dark-matter annihilation in the centre of the Earth and the Sun. Last but not least, the NT200 infrastructure was used for innovative environmental studies.
A notable breakthrough in the field came in 2012, when IceCube detected the first high-energy “astrophysical” neutrinos, i.e. high-energy neutrinos generated beyond the solar system (CERN Courier July/August 2013 p35). That marked the birth of high-energy neutrino astronomy, and underlined the need to develop neutrino telescopes of similar capacity in the Northern hemisphere, to be able to study high-energy neutrino sources across the whole celestial sphere. JINR, with many years of experience as a participant in the Baikal neutrino project, recognized this opportunity and decided to treat activities related to Baikal-GVD as a scientific priority.
Baikal-GVD will have a modular structure formed from functionally independent clusters of vertical strings of optical modules. This modular structure will allow data acquisition at early stages in the construction of the facility. The choice of the telescope structure will also allow adjustment of its configuration in response to changes in scientific priorities at different times.
Prototypes of all of the basic elements of the GVD telescope system were designed, manufactured and tested during 2006–2010. The final stage of complex in-situ testing started in 2011 and finished in 2015 with the development of the Dubna cluster. Its 192 optical modules are arranged down to depths of 1300 m on eight vertical strings, each 345-m long. Different from NT200, the optical modules are not grouped in pairs, resulting in 192 space points per cluster (instead of only 96 for NT200). Moreover, the former custom-made, hybrid QUASAR phototube has been replaced by a conventional 10-inch photomultiplier with a high-sensitivity photocathode. The mechanical structure has been simplified compared with NT200, and a totally new system for front-end and trigger electronics and for data acquisition has been designed and implemented.
Deployment of the Dubna cluster is an exciting step towards a next-generation neutrino telescope in Lake Baikal. Such a telescope will be a cornerstone of a future worldwide neutrino observatory, with detectors at the South Pole, in the Mediterranean Sea and in Lake Baikal. The Baikal collaboration pioneered this technology in the 1980s and 1990s, and measured neutrinos generated in the Earth’s atmosphere. Two decades later, the long-awaited discovery by IceCube of the first high-energy neutrinos from far beyond the Earth and the solar system has given increased motivation to projects for similar large detectors in the Northern hemisphere. IceCube has lifted the curtain that hides the high-energy neutrino universe, but just by a little. In the future, Baikal-GVD will help to chart this new cosmic territory fully.
At 10.40 a.m. on 3 June, the LHC operators declared “stable beams” for the first time at a beam energy of 6.5 TeV. It was the signal for the LHC experiments to start taking physics data for Run 2, this time at a collision energy of 13 TeV – nearly double the 7 TeV with which Run 1 began in March 2010. After a shutdown of almost two years and several months re-commissioning without and with beam, the world’s largest particle accelerator was back in business. Under the gaze of the world via a live webcast and blog, the LHC’s two counter-circulating beams, each with three bunches of nominal intensity (about 1011 protons per bunch), were taken through the full cycle from injection to collisions. This was followed by the declaration of stable beams and the start of Run 2 data taking.
The occasion marked the nominal end of an intense eight weeks of beam commissioning (CERN CourierMay 2015 p5 and June 2015 p5) and came just two weeks after the first test collisions at the new record-breaking energy. On 20 May at around 10.30 p.m., protons collided in the LHC at 13 TeV for the first time. These test collisions were to set up various systems, in particular the collimators, and were established with beams that were “de-squeezed” to make them larger at the interaction points than during standard operation. This set-up was in preparation for a special run for the LHCf experiment (“LHCf makes the most of a special run”), and for luminosity calibration measurements by the experiments where the beams are scanned across each other – the so-called “van der Meer scans”.
Progress was also made on the beam-intensity front, with up to 50 nominal bunches per beam brought into stable beams by mid-June. There were some concerns that an unidentified obstacle in the beam pipe of a dipole in sector 8-1 could be affected by the higher beam currents. This proved not to be the case – at least so far. No unusual beam losses were observed at the location of the obstacle, and the steps towards the first sustained physics run continued.
The final stages of preparation for collisions involved setting up the tertiary collimators (CERN Courier September 2013 p37). These are situated on the incoming beam about 120–140 m from the interaction points, where the beams are still in separate beam pipes. The local orbit changes in this region both during the “squeeze” to decrease the beam size at the interaction points and after the removal of the “separation bumps” (produced by corrector magnets to keep the beams separated at the interaction points during the ramp and squeeze). This means that the tertiary collimators must be set up with respect to the beam, both at the end of the squeeze and with colliding beams. In contrast, the orbit and optics at the main collimator groupings in the beam-cleaning sections at points 7 and 3 are kept constant during the squeeze and during collisions, so their set-up remains valid throughout all of the high-energy phases.
By the morning of 3 June, all was ready for the planned attempt for the first “stable beams” of Run 2, with three bunches of protons at nominal intensity per beam. At 8.25 a.m, the injection of beams of protons from the Super Proton Synchrotron to the LHC was complete, and the ramp to increase the energy of each beam to 6.5 TeV began. However, the beams were soon dumped in the ramp by the software interlock system. The interlock was related to a technical issue with the interlocked beam-position monitor system, but this was rapidly resolved. About an hour later, at 9.46 a.m, three nominal bunches were once more circulating in each beam and the ramp to 6.5 TeV had begun again.
At 10.06 a.m., the beams had reached their top energy of 6.5 TeV and the “flat top” at the end of the ramp. The next step was the “squeeze”, using quadrupole magnets on both sides of each experiment to decrease the size of the beams at the interaction point. With this successfully completed by 10.29 a.m., it was time to adjust the beam orbits to ensure an optimal interaction at the collision points. Then at 10.34 a.m., monitors showed that the two beams were colliding at a total energy of 13 TeV inside the ATLAS and CMS detectors; collisions in LHCb and ALICE followed a few minutes later.
At 10.42 a.m., the moment everyone had been waiting for arrived – the declaration of stable beams – accompanied by applause and smiles all round in the CERN Control Centre. “Congratulations to everybody, here and outside,” CERN’s director-general, Rolf Heuer, said as he spoke with evident emotion following the announcement. “We should remember this was two years of teamwork. A fantastic achievement. I am touched. I hope you are also touched. Thanks to everybody. And now time for new physics. Great work!”
The eight weeks of beam commissioning had seen a sustained effort by many teams working nights, weekends and holidays to push the programme through. Their work involved optics measurements and corrections, injection and beam-dump set-up, collimation set-up, wrestling with various types of beam instrumentation, optimization of the magnetic model, magnet aperture measurements, etc. The operations team had also tackled the intricacies of manipulating the beams through the various steps, from injection through ramp and squeeze to collision. All of this was backed up by the full validation of the various components of the machine-protection system by the groups concerned. The execution of the programme was also made possible by good machine availability and the support of other teams working on the injector complex, cryogenics, survey, technical infrastructure, access, and radiation protection.
Over the two-year shutdown, the four large experiments ALICE, ATLAS, CMS and LHCb also went through an important programme of maintenance and improvements in preparation for the new energy frontier.
Among the consolidation and improvements to 19 subdetectors, the ALICE collaboration installed a new dijet calorimeter to extend the range covered by the electromagnetic calorimeter, allowing measurement of the energy of the photons and electrons over a larger angle (CERN Courier May 2015 p35). The transition-radiation detector that detects particle tracks and identifies electrons has also been completed with the addition of five more modules.
A major step during the long shutdown for the ATLAS collaboration was the insertion of a fourth and innermost layer in the pixel detector, to provide the experiment with better precision in vertex identification (CERN Courier June 2015 p21). The collaboration also used the shutdown to improve the general ATLAS infrastructure, including electrical power, cryogenic and cooling systems. The gas system of the transition-radiation tracker, which contributes to the identification of electrons as well as to track reconstruction, was modified significantly to minimize losses. In addition, new chambers were added to the muon spectrometer, the calorimeter read-out was consolidated, the forward detectors were upgraded to provide a better measurement of the LHC luminosity, and a new aluminium beam pipe was installed to reduce the background.
To deal with the increased collision rate that will occur in Run 2 – which presents a challenge for all of the experiments – ATLAS improved the whole read-out system to be able to run at 100 kHz and re-engineered all of the data acquisition software and monitoring applications. The trigger system was redesigned, going from three levels to two, while implementing smarter and faster selection-algorithms. It was also necessary to reduce the time needed to reconstruct ATLAS events, despite the additional activity in the detector. In addition, an ambitious upgrade of simulation, reconstruction and analysis software was completed, and a new generation of data-management tools on the Grid was implemented.
The biggest priority for CMS was to mitigate the effects of radiation on the performance of the tracker, by equipping it to operate at low temperatures (down to –20 °C). This required changes to the cooling plant and extensive work on the environment control of the detector and cooling distribution to prevent condensation or icing (CERN Courier May 2015 p28). The central beam pipe was replaced by a narrower one, in preparation for the installation in 2016–2017 of a new pixel tracker that will allow better measurements of the momenta and points of origin of charged particles. Also during the shutdown, CMS added a fourth measuring station to each muon endcap, to maintain discrimination between low-momentum muons and background as the LHC beam intensity increases. Complementary to this was the installation at each end of the detector of a 125 tonne composite shielding wall to reduce neutron backgrounds. A luminosity-measuring device, the pixel luminosity telescope, was installed on either side of the collision point around the beam pipe.
Other major activities for CMS included replacing photodetectors in the hadron calorimeter with better-performing designs, moving the muon read-out to more accessible locations for maintenance, installation of the first stage of a new hardware triggering system, and consolidation of the solenoid magnet’s cryogenic system and of the power distribution. The software and computing systems underwent a significant overhaul during the shutdown to reduce the time needed to produce analysis data sets.
To make the most of the 13 TeV collisions, the LHCb collaboration installed the new HeRSCheL detector – High Rapidity Shower Counters for LHCb. This consists of a system of scintillators installed along the beamline up to 114 m from the interaction point, to define forward rapidity gaps. In addition, one section of the beryllium beam pipe was replaced and the new beam pipe support-structure is now much lighter.
The CERN Data Centre has also been preparing for the torrent of data expected from collisions at 13 TeV. The Information Technology department purchased and installed almost 60,000 new cores and more than 100 PB of additional disk storage to cope with the increased amount of data that is expected from the experiments during Run 2. Significant upgrades have also been made to the networking infrastructure, including the installation of new uninterruptible power supplies.
First stable beams was an important step for LHC Run 2, but there is still a long way to go before this year’s target of around 2500 bunches per beam is reached and the LHC starts delivering some serious integrated luminosity to the experiments. The LHC and the experiments will now run around the clock for the next three years, opening up a new frontier in high-energy particle physics.
• Complied from articles in CERN’s Bulletin and other material on CERN’s website. To keep up to date with progress with the LHC and the experiments, follow the news at bulletin.cern.ch or visit www.cern.ch.
Over the past decade and more, cosmology on one side and particle physics on the other have approached what looks like a critical turning point. The theoretical models that for many years have been the backbone of research carried out in both fields – the Standard Model for particle physics and the Lambda cold dark matter (ΛCDM) model for cosmology – are proving insufficient to describe more recent observations, including those of dark matter and dark energy. Moreover, the most important “experiment” that ever happened, the Big Bang, remains unexplained. Physicists working at both extremes of the scale – the infinitesimally small and the infinitely large – face the same problem: they know that there is much to search for, but their arms seem too short to reach still further distances. So, while researchers in the two fields maintain their specific interests and continue to build on their respective areas of expertise, they are also looking increasingly at each other’s findings to reconstitute the common mosaic.
Studies on the nature of dark matter are the most natural common ground between cosmology and particle physics. Run 2 of the LHC, which has just begun, is expected to shed some light on this area. Indeed, while the main outcome of Run 1 was undoubtedly the widely anticipated discovery of a Higgs boson, Run 2 is opening the door to uncharted territory. In practical and experimental terms, exploring the properties and the behaviour of nature at high energy consists in understanding possible signals that include “missing energy”. In the Standard Model, this energy discrepancy is associated with neutrinos, but in physics beyond the Standard Model, the missing energy could also be the signature of many undiscovered particles, including the weakly interacting massive particles (WIMPs) that are among the leading candidates for dark matter. If WIMPs exist, the LHC’s collisions at 13 TeV may reveal them, and this will be another huge breakthrough. Because supersymmetry has not yet been ruled out, the high-energy collisions might also eventually unveil the supersymmetric partners of the known particles, at least the lighter ones. Missing energy could also account for the escape of a graviton into extra dimensions, or a variety of other possibilities. Thanks to the LHC’s Run 1 and other recent studies, the Standard Model is so well known that future observation of an unknown source of missing energy could be confidently linked to new physics.
Besides the search for dark matter, another area where cosmology and particle physics meet is in neutrino physics. The most recent result that collider experiments have published for the number of standard (light) neutrino types is Nν = 2.984±0.008 (ALEPH et al. 2006). While the search for a fourth right-handed neutrino is continuing with ground-based experiments, satellite experiments have shown that they can also have their say. Indeed, recent results from ESA’s Planck mission yield Neff = 3.04±0.18 for the effective number of relativistic degrees of freedom, and the sum of neutrino masses is constrained to Σmν < 0.17 eV. These values, derived from Planck’s data of temperature and polarization CMB anisotropies in combination with data from baryonic acoustic oscillation experiments, are consistent with standard cosmological and particle-physics predictions in the neutrino sector (Planck Collaboration 2015a). Although these values do not completely rule out a sterile neutrino, especially if thermalized at a different background temperature, its existence is disfavoured by the Planck data (figure 1).
Ground-based experiments have observed the direct oscillation of neutrinos, which proves that these elusive particles have a nonzero mass.
Working out absolute neutrino masses is no easy task. Ground-based experiments have observed the direct oscillation of neutrinos, which proves that these elusive particles have a nonzero mass. However, no measurement of absolute masses has been performed yet, and the strongest upper limit (about one order of magnitude more accurate than direct-detection measurements) on their sum comes from cosmology. Because neutrinos are the most abundant particles with mass in the universe, the influence of their absolute mass on the formation of structure is as big as their role in many physics processes observed at small scales. The picture in the present Standard Model might suggest (perhaps naively) that the mass distribution among the neutrinos could be similar to the mass distribution among the other particles and their families, but only experiments such as KATRIN – the Karslruhe Tritium Neutrino experiment – are expected to shed some light on this topic.
In recent years, cosmologists and particle physicists have shown a common interest in testing Lorentz and CPT invariances. The topic seems to be particularly relevant for theorists working on string theories, which sometimes involve mechanisms that lead to a spontaneous breaking of these symmetries. To find possible clues, satellite experiments are probing the cosmic microwave background (CMB) to investigate the universe’s birefringence, which would be a clear signature of Lorentz invariance and, therefore, CPT violation. So far, the CMB experiments WMAP, QUAD and BICEP1 have found a value of α – the rotation angle of the photon-polarization plane – consistent with zero. Results from Planck on the full set of observations are expected later this year.
Since its discovery in 2012, the Higgs boson found at the LHC has been in the spotlight for physicists studying both extremes of the scale. Indeed, in addition to its confirmed role in the mass mechanism, recent papers have discussed its possible role in the inflation of the universe. Could a single particle be the Holy Grail for cosmologists and particle physicists alike? It is a fascinating question, and many studies have been published about the particle’s possible role in shaping the early history of the universe, but the theoretical situation is far from clear. On one side, the Higgs boson and the inflaton share some basic features, but on the other side, the Standard Model interactions do not seem sufficient to generate inflation unless there is an anomalously strong coupling between the Higgs boson and gravity. Such strong coupling is a highly debated point among theoreticians. Also in this case, the CMB data could help to rule out or disentangle models. Recent full mission data from Planck clearly disfavour natural inflation compared with models that predict a smaller tensor-to-scalar ratio, such as the Higgs inflationary model (Planck Collaboration 2015b). However, the question remains open, and subject to new information coming from the LHC’s future runs and from new cosmological missions.
AMS now has results based on more than 6 × 1010 cosmic-ray events.
In the meantime, astroparticle physics is positioning itself as the area where both cosmology and particle physics could find answers to the open questions. An event at CERN in April provided a showcase for experiments on cosmic rays and dark matter, in particular the latest results from the Alpha Magnetic Spectrometer (AMS) collaboration on the antiproton-to-proton ratio in cosmic rays and on the proton and helium fluxes. Following earlier measurements by PAMELA – the Payload for Antimatter Matter Exploration and Light nuclei Astrophysics – which took data in 2006–2011, AMS now has results based on more than 6 × 1010 cosmic-ray events (electrons, positrons, protons and antiprotons, as well as nuclei of helium, lithium, boron, carbon, oxygen…) collected during the first four years of AMS-02 on board the International Space Station. With events at energies up to many tera-electron-volts, and with unprecedented accuracy, the AMS data provide systematic information on the deepest nature of cosmic rays. The antiproton-to-proton ratio measured by AMS in the energy range 0–500 GeV shows a clear discrepancy with existing models (figure 2). Anomalies are also visible in the behaviour of the fluxes of electrons, positrons, protons, helium and other nuclei. However, although a large part of the scientific community tends to interpret these observations as a new signature of dark matter, the origin of such unexpected behaviour cannot be easily identified, and discussions are still ongoing within the community.
It may seem that the universe is playing hide-and-seek with cosmologists and particle physicists alike as they probe both ends of the distance scale. However, the two research communities have a new smart move up their sleeves to unveil its secrets – collaboration. Bringing together the two ends of the scales probed by the LHC and by Planck will soon bear its fruits. Watch this space!
The last week of May saw a gathering of 390 physicists from 27 countries and four continents on the island of Elba. The 13th edition of the Pisa Meeting on Advanced Detectors for Frontier Physics took place in the secluded Biodola area. The conference, which takes place every three years, is based on a consolidated format, aiming at an interdisciplinary exchange of ideas: all sessions are plenary, with a round table on a topic of interest (CERN Courier July/August 2006 p31). The programme for this year was built on a record number of contributions (more than 400), out of which 327 were selected for either oral (66) or poster presentations. Eight industries were present throughout the meeting, with stands to display their products and to discuss ongoing and future R&D projects.
The opening session saw an introductory talk by Toni Pich of Valencia that described the situation in frontier physics today. The discovery of a particle associated with the Brout–Englert–Higgs mechanism has opened a whole new field of investigation to explore, in addition to the “known unknowns”. Among these, revealing the nature of dark matter and of neutrino masses is the main priority. In the following talk, CERN’s Michelangelo Mangano discussed the search for supersymmetry, as well as different possibilities for signals of new physics that will be explored with high priority from the start of Run 2 at the LHC.
A key event was the round table organized on the second day of the meeting, with 13 people representing nine laboratories (CERN, the Institute of High Energy Physics (IHEP) in Beijing, Fermilab, PSI, TRIUMF, the European Spallation Source, KEK and the Japan Proton Accelerator Research Complex) and four funding agencies (the US Department of Energy, the Institut national de physique nucléaire et de physique des particules (IN2P3), the Istituto Nazionale di Fisica Nucleare (INFN) and the UK’s Science and Technology Facilities Council). The topic for discussion was “Synergies and complementarity among laboratories”, in view of the challenges of the coming decades and of the growing role of CERN as the place where the energy frontier will be explored. The presentation about the future of high-energy physics in China by Gang Chen of IHEP was particularly enlightening, giving a perspective and an impressive plan spanning the middle of this century. Representatives of the funding agencies discussed the nearer future, where – besides the High Luminosity LHC project – a strong neutrino programme is foreseen. The lively exchange among the scientists at the table and participants on the floor left everyone with a vivid perception that what Sergio Bertolucci, CERN’s director for research and computing, defined as “co-opetition” among different institutions in high-energy physics, must move forward and become part of the texture of daily work. Several participants stressed that although CERN is central, regional laboratories have an important role because they relate directly to the host nations. Demonstrating the societal impact of research in high-energy physics to politicians and to the public at large is a key point in obtaining support for the whole field.
Each Pisa meeting has a number of standard sessions on gas and solid-state detectors, particle and photon identification, calorimetry and advanced electronics, astroparticle physics, and the application of high-energy-physics techniques in other fields. The presentations, both oral and with posters, demonstrated that significant improvements in existing detectors and current techniques are still possible. The topics presented covered dedicated R&D as well as novel ideas, some developed in a beneficial crossover with other areas, ranging from material science to nanotechnology and chemistry. In a dedicated session, speakers from the LHC experiments noted that the detectors are now performing well and are ready to help harvest the physics at 13 TeV that will come from the LHC’s Run 2.
As the field keeps changing, so does the conference. This year, a new session was introduced to offer adequate space to applied superconductivity. The technique is now fundamental, not just to provide stronger magnetic fields for accelerators and spectrometers, but also in specialized detectors. The review talk by Akira Yamamoto of KEK and CERN outlined the new frontier of superconducting magnets, both in terms of achievable field and of stored energy/mass ratio. Emanuela Barzi and Alexander Zoblin presented the R&D programme for high-field superconducting magnets at Fermilab. The laboratory that pioneered the use of superconducting magnets in accelerators now aims to be able to build magnets suitable for the Future Circular Collider design study (CERN Courier April 2014 p16). The use of superconducting materials to detect photons was discussed in two talks, by Martino Calvo of CNRS Grenoble and Roberto Leoni of IFN-CNR, Rome. The use of cryogenic detectors – bolometers, kinetic-inductance detectors, transition-edge sensors, to name but a few – was discussed by Flavio Gatti of INFN Genova, in a review of the large number of posters on the subject presented at the conference.
The meeting saw the awarding of the first Aldo Menzione Prize. Among his many activities, Aldo was one of the founders of the Pisa meeting and recipient of the W K H Panokfsky Prize in 2009. He passed away in December 2012 (CERN Courier April 2013 p37), and to honour his memory, the Frontier Detectors for Frontier Physics (FDFP) association that organizes the conference series, established an award to be assigned at each meeting to “a distinguished scientist who has contributed to the development of detector techniques”. The recipients of the prize on this first occasion were David Nygren, now of the University of Texas at Arlington, for the invention of the time-projection chamber, and Fabio Sauli, now of the TERA Foundation, for the invention of the gas electron-multiplier, or GEM. The prizes were presented by Donata Foà, Aldo’s widow, and Angelo Scribano, the president of the FDFP.
At the end of the conference dinner, several awards were also assigned by an international jury. Elsevier established two Elsevier Young Scientist Awards to honour the late Glenn Knoll, who was an editor of Nuclear Instruments and Methods (NIM). These were presented by Fabio Sauli, on behalf of NIM, to Filippo Resnati of CERN and Joana Wirth of the Technische Universität München, respectively, for his talk on the “Charge transfer properties through graphene for applications in gaseous detectors”, and for her poster on “CERBEROS: a tracking system for secondary pion beams at the HADES spectrometer”. Three FDFP awards to “talented young scientists active in the development of detection techniques and contributing, by talk or poster, to the scientific programme” were conferred by Angelo Scribano to Lars Graber of the University of Göttingen for his talk on “A 3D diamond detector for particle tracking”, Roberto Acciarri of Fermilab for a poster on “Experimental study of breakdown electric fields in liquid argon” and Raffaella Donghia of INFN-LNF for her poster on “Time performances and irradiation tests of CsI crystals read-out by MPPC”.
Concluding the conference, the chair, Marco Grassi of INFN-Pisa, provided a few statistics. He remarked that 36% of the participants were below 35 years old and nearly all of them – 96% – contributed to the conference programme with oral presentations or posters. This demonstrates that the field of detector development is attractive and has a strong basis on which it can grow, as long as, at a national level, institutes can continue to recruit these young scientists. This, as Catherine Clerc from IN2P3 reminded everybody during the round table, is the most pressing challenge in many European countries.
Following the restart of CERN’s flagship accelerator in early April, commissioning the LHC with beam is progressing well. In the early hours of 10 April, the operations team successfully circulated a beam at 6.5 TeV for the first time – a new world record – but this was only one of many steps to be taken before the accelerator delivers collisions at this beam energy.
The operators reached another important milestone on 21 April, when they succeeded in circulating a nominal-intensity bunch. The first commissioning steps in particular take place with low-intensity (probe) beams – single bunches of 5 × 109 protons. The nominal intensity, in contrast, is a little over 1 x 1011 protons per bunch, and when the LHC is in full operation later this year, some 2800 bunches will circulate in each beam.
To handle the higher number of protons per bunch and the higher number of bunches safely, a number of key systems have to be fully operational and set up with beam. These include the beam-dump system, the beam-interlock system and the collimation system. The latter involves around 100 individual pairs of jaws, each of which has to be positioned with respect to the beam during all of the phases of the machine cycle. Confirmation that everything is as it should be is made by deliberately provoking beam losses and checking that the collimators catch the losses as they are supposed to.
On 2 May, this set-up procedure allowed a nominal-intensity bunch in each beam to be taken to 6.5 TeV. Four days later, collisions were produced at the injection energy of 450 GeV, enabling the experiment teams to record events and check alignment and synchronization of the detectors. One of the important steps in reaching this stage is to commission the “squeeze” – the final phase in the LHC cycle of injection, ramp and squeeze. During this phase, the strengths of the magnetic fields either side of a given experiment are adjusted to reduce the beam size at the corresponding interaction point.
• To find out more, see the LHC reports in CERN Bulletin: bulletin.cern.ch.
The Republic of Turkey became an associate member state of CERN on 6 May, following notification that Turkey has ratified an agreement signed last year, granting this status to the country. Turkey’s new status will strengthen the long-term partnership between CERN and the Turkish scientific community. Associate membership will allow Turkey to attend meetings of the CERN Council. Moreover, it will allow Turkish scientists to become members of the CERN staff, and to participate in CERN’s training and career-development programmes. Finally, it will allow Turkish industry to bid for CERN contracts, thus opening up opportunities for industrial collaboration in areas of advanced technology.
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