Global healthcare company Novartis has announced plans to acquire Advanced Accelerator Applications (AAA), a spin-off radiopharmaceutical firm established by former CERN physicist Stefano Buono in 2002. With an expected price of $3.9B, said the firm in a statement, the acquisition will strengthen Novartis’ oncology portfolio by introducing a new therapy platform for tackling neuroendocrine tumours. Trademarked Lutathera, and based on the isotope lutetium-177, the technology was approved in Europe in September 2017 for the treatment of certain neuroendocrine tumours and is under review in the US.
With its roots in nuclear-physics expertise acquired at CERN, AAA started its commercial activity with the production of radiotracers for medical imaging. The successful model made it possible for AAA to invest in nuclear research to produce innovative radiopharmaceuticals. “We believe that the combination of our expertise in radiopharmaceuticals and theragnostic strategy together with the global oncology experience and infrastructure of Novartis, provide the best prospects for our patients, physicians and employees, as well as the broader nuclear medicine community,” said Buono, who is CEO of AAA.
In November, Fermilab became a research member of CERN openlab – a public-private partnership between CERN and major ICT companies established in 2001 to meet the demands of particle-physics research. Fermilab researchers will now collaborate with members of the LHC’s CMS experiment and the CERN IT department to improve technologies related to physics data reduction, which is vital for gaining insights from the vast amounts of data produced by high-energy physics experiments.
The work will take place within an existing CERN openlab project with Intel on big-data analytics. The goal is to use industry-standard big-data tools to create a new tool for filtering many petabytes of heterogeneous collision data to create manageable, but still rich, datasets of a few terabytes for analysis. Using current systems, this kind of targeted data reduction can often take weeks, but the Intel-CERN project aims to reduce it to a matter of hours.
The team plans to first create a prototype capable of processing 1 PB of data with about 1000 computer cores. Based on current projections, this is about one twentieth of the scale of the final system that would be needed to handle the data produced when the High-Luminosity LHC comes online in 2026. “This kind of work, investigating big-data analytics techniques is vital for high-energy physics — both in terms of physics data and data from industrial control systems on the LHC,” says Maria Girone, CERN openlab CTO
At 10.50 a.m. on 22 November 2017, the third-generation light source SESAME in Jordan produced its first X-ray photons, signalling the start of the regional laboratory’s experimental program. Researchers sent a beam of monochromatic light through the XAFS/XRF (X-ray absorption fine structure/X-ray fluorescence) spectroscopy beamline, the first to come on stream at SESAME and targeted at research ranging from solid state physics to environmental science and archaeology.
Obtaining first light is an important step in the commissioning of a new synchrotron light source, and the milestone comes 10 months after SESAME circulated its first electrons (CERN Courier March 2017 p8). Nevertheless, it is just one step on the way to full operation. The SESAME synchrotron is currently operating with a beam current of just over 80 milliamps while the design value is 400 milliamps. Over the coming weeks and months as experiments get underway, the current will be gradually increased.
SESAME’s initial research program will be carried out at two beamlines, the XAFS/XRF beamline and an infrared spectro-microscopy beamline that is scheduled to join the XAFS/XRF beamline this year. A third beamline devoted to materials science will come on stream in 2018. “After years of preparation, it’s great to see light on target,” said XAFS/XRF beamline scientist Messaoud Harfouche. “We have a fantastic experimental programme ahead of us, starting with an experiment to investigate heavy metals contaminating soils in the region.”
The free-electron X-ray laser SwissFEL at the Paul Scherrer Institute (PSI) in Switzerland has hosted its inaugural experiment, marking the facility’s first science result and demonstrating that its many complex components are working as expected. Construction of 740m-long SwissFEL began in April 2013, with the aim of producing extremely short X-ray laser pulses for the study of ultrafast reactions and processes.
Between 27 November and 4 December 2017, PSI researchers and a research group from the University of Rennes in France conducted the first in a series of pilot experiments.
The high-energy X-ray light pulses enabled the team to investigate the electrical and magnetic properties of titanium pentoxide nanocrystals, which have potential applications in high-density data storage. This and further pilot experiments will help hone SwissFEL operations before regular user operations begin in January 2019.
The possibility that dark-matter particles may interact via an unknown force, felt only feebly by Standard Model (SM) particles, has motivated an effort to search for so-called dark forces.
The force-carrying particle for such hypothesised interactions is referred to as a dark photon, A’, in analogy with the ordinary photon that mediates the electromagnetic interaction. While the dark photon does not couple directly to SM particles, quantum-mechanical mixing between the photon and dark-photon fields can generate a small interaction. This provides a portal through which dark photons may be produced and through which they might decay into visible final states.
The minimal A’ model has two unknown parameters: the dark photon mass, m(A’), and the strength of its quantum-mechanical mixing with the photon field. Constraints have been placed on visible A’ decays by previous beam-dump, fixed-target, collider, and rare-meson-decay experiments.
However, much of the A’ parameter space that is of greatest interest (based on quantum field theory arguments) is currently unexplored. Using data collected in 2016, LHCb recently performed a search for the decay A’→μ+μ– in a mass range from the dimuon threshold up to 70 GeV. While no evidence for a signal was found, strong limits were placed on the A’–photon mixing strength. These constraints are the most stringent to date for the mass range 10.6 < m(A’) < 70 GeV and are comparable to the best existing limits on this parameter.
Furthermore, the search was the first to achieve sensitivity to long-lived dark photons using a displaced-vertex signature, providing the first constraints in an otherwise unexplored region of A’ parameter space. These results demonstrate the unique sensitivity of the LHCb experiment to dark photons, even using a data sample collected with a trigger that is inefficient for low-mass A’ decays. Looking forward to Run 3, the number of expected A’→μ+μ− decays in the low-mass region should increase by a factor of 100 to 1000 compared to the 2016 data sample. LHCb is now developing searches for A’→e+e− decays which are sensitive to lower-mass dark photons, both in LHC Run 2 and in particular Run 3 when the luminosity will be higher. This will further expand LHCb’s dark-photon programme.
Despite many negative searches during the last decade and more, supersymmetry (SUSY) remains a popular extension of the Standard Model (SM). Not only can SUSY accommodate dark matter and gauge–force unification at high energy, it offers a natural explanation for why the Higgs boson is so light compared to the Planck scale. In the SM, the Higgs boson mass can be decomposed into a “bare” mass and a modification due to quantum corrections. Without SUSY, but in the presence of a high-energy new physics scale, these two numbers are extremely large and thus must almost exactly oppose one another – a peculiar coincidence called the hierarchy problem. SUSY introduces a set of new particles that each balances the mass correction of its SM partner, providing a “natural” explanation for the Higgs boson mass.
Thanks to searches at the LHC and previous colliders, we know that SUSY particles must be heavier than their SM counterparts. But if this difference in mass becomes too large, particularly for the particles that produce the largest corrections to the Higgs boson mass, SUSY would not provide a natural solution of the hierarchy problem.
New SUSY searches from ATLAS using data recorded at an energy of 13 TeV in 2015 and 2016 (some of which were shown for the first time at SUSY 2017 in Mumbai from 11–15 December) have extended existing bounds on the masses of the top squark and higgsinos, the SUSY partners of the top quark and Higgs bosons, respectively, that are critical for natural SUSY. For SUSY to remain natural, the mass of the top squark should be below around 1 TeV and that of the higgsinos below a few hundred GeV.
ATLAS has now completed a set of searches for the top squark that push the mass limits up to 1 TeV. With no sign of SUSY yet, these searches have begun to focus on more difficult to detect scenarios in which SUSY could hide amongst the SM background. Sophisticated techniques including machine learning are employed to ensure no signal is missed.
First ATLAS results have also been released for higgsino searches. If the lightest SUSY particles are higgsino-like, their masses will often be close together and such “compressed” scenarios lead
to the production of low-momentum particles. One new search at ATLAS targets scenarios with leptons reconstructed at the lowest momenta still detectable. If the SUSY mass spectrum is extremely compressed, the lightest charged SUSY particle will have an extended lifetime, decay invisibly, and leave an unusual detector signature known as a “disappearing track”.
Such a scenario is targeted by another new ATLAS analysis. These searches extend for the first time the limits on the lightest higgsino set by the Large Electron Positron (LEP) collider 15 years ago. The search for higgsinos remains among the most challenging and important for natural SUSY. With more data and new ideas, it may well be possible to discover, or exclude, natural SUSY in the coming years.
Now that all the particles predicted in the Standard Model (SM) have been discovered, most recently the Higgs boson in 2012, experiments at the LHC are active on two fronts: a deeper scrutiny of the SM and the search for new particles produced by beyond-SM (BSM) physics. Recent studies of rare processes involving the top quark serve both purposes. On one hand, they probe SM predictions and parameters in regions not accessed so far. On the other hand, if BSM couplings to the massive particles of the third generation of the SM are substantial, rare processes involving the top quark are golden candidates to reveal signs of BSM physics.
Based on data taken during 2016, the CMS Collaboration has recently published two such studies of rare top quark processes: the production of a single top quark associated with a Z boson and one or more jets (tZq) and the production of four top quarks (tttt). Detecting these processes is very difficult due to their tiny cross sections (about 0.8 pb for tZq and 0.01 pb for tttt in proton–proton collisions at 13 TeV), which means that no more than a few hundred tZq events and a dozen tttt events were expected after selection. If this was not challenging enough, these events have to be separated from an overwhelming amount of background from several other SM processes. To achieve a sufficient control of the background, the analyses are restricted to final states containing multiple electrons and muons. Furthermore, the tZq analysis uses multivariate techniques to classify event candidates according to their topologies.
In both analyses, the signal is extracted with maximum-likelihood fits performed simultaneously in the control regions with different selections. As a result, CMS was able to report evidence of the tZq process with a significance of 3.1 standard deviations (3.7 expected) against the background-only hypothesis, and a cross section of 0.123+0.033–0.031 (stat) +0.029–0.023 (syst) pb, in agreement with the SM. CMS also reported a small excess of tttt events over the background-only hypothesis, with a significance of 1.6 standard deviations (1.0 expected), and derived an upper limit of 0.0208+0.0112–0.0069 pb on the tttt production cross section. The high energy and the large integrated luminosities provided by the LHC have opened a new window on precision physics, in which measurements of rare processes involving top quarks play a central role.
As more LHC data become available, these studies will provide more stringent tests of the SM while increasing the chances of revealing BSM processes.
In a heavy-ion collision, a longitudinal asymmetry arises due to unequal numbers of participating nucleons from the two colliding nuclei, causing a shift in the centre-of-mass (CM) of the overlapping “participant zone” with respect to the nucleon–nucleon CM.
The asymmetry may be expressed as α = (A-B)/(A+B), where A and B are the number of nucleons participating from the two colliding nuclei. This shifts the rapidity (y0) of the participant zone with respect to the nucleon-nucleon CM rapidity, where y0≅ ½ ln (A/B).
First results on the asymmetry in the longitudinal direction and its effect on the pseudorapidity distributions in lead-lead collisions at a nucleon-nucleon CM energy of 2.76 TeV have been obtained with the ALICE detector, allowing investigations of the effect of variations in the initial conditions on other measurable quantities.
Since the number of participants cannot be measured directly, the asymmetry in an event was estimated by measuring the energy in the forward neutron-zero-degree-calorimeters (ZNs) in the ALICE detector. The observed distribution of asymmetry in ZNs, αzn, is used to classify events into symmetric and asymmetric by a choice of αzn. A Monte Carlo simulation using a Glauber model for the colliding nuclei is tuned to reproduce the spectrum in the ZNs and provides a relation between the measurable longitudinal asymmetry and the shift in the rapidity of the participant zone formed by the unequal number of participating nucleons.
The effect of the longitudinal asymmetry was measured on the pseudorapidity distributions of charged particles in the mid and forward regions by taking the ratio of the pseudorapidity distributions from events corresponding to different regions of asymmetry (see figure). The coefficients of a polynomial fit to the ratio characterise the effect of the asymmetry, with the coefficient of the linear term in the polynomial expansion, c1, showing a linear dependence on the mean value of y0.
This analysis confirms that the longitudinal distributions are affected by the rapidity-shift of the participant zone with respect to the nucleon-nucleon CM frame, highlighting the relevance of nucleon numbers in the production of charged particles, even at high energies. The method is potentially a new event classifier for the study of initial state fluctuations and different particle production mechanisms.
Since 2008, astronomers have been puzzled by a mysterious feature in the cosmic-ray energy spectrum. Data from the PAMELA satellite showed a significant increase in the ratio of positrons to electrons at energies above 10 GeV. This unexpected positron excess was subsequently confirmed by both the Fermi-LAT satellite and the AMS-02 experiment onboard the ISS (CERN Courier December 2016 p26–30), sparking many explanations, ranging from dark-matter annihilation to positron emission by nearby pulsars. New measurements by the High-Altitude Water Cherenkov (HAWC) experiment now seem to rule out the second explanation, hinting at a more exotic origin of the positron excess.
Although standard cosmic-ray propagation models predict the production of positrons from interactions of high-energy protons travelling through the galaxy, the positron fraction is expected to decrease as a function of energy. One explanation for the excess is the annihilation of dark-matter particles with masses of several TeV, which would result in a bump in the electron–positron fraction, with the measured increase perhaps being the rising part of such a bump. According to other models, however, the excess is the result of positron production by astrophysical sources such as pulsars (rapidly spinning neutron stars). Since these charged particles lose energy due to interactions with interstellar magnetic and radiation fields they must be produced relatively close to Earth, making nearby pulsars a prime suspect.
HAWC, situated near the city of Puebla in Mexico, detects charged particles created in the Earth’s atmosphere from collisions between high-energy photons and atmospheric nuclei. The charged particles produced in the resulting shower produce Cherenkov radiation in HAWC’s 300 water tanks, their high altitude location making HAWC the most sensitive survey instrument to measure astrophysical photons in the TeV range. This allows the study of TeV-scale photon emission from nearby pulsars, such as Geminga and PSR B0656+14, to investigate if these objects could be responsible for the positron excess.
Pulsars are thought to emit electrons and positrons with energies up to several hundred TeV, which diffuse into the interstellar medium, but the details of the emission, acceleration and propagation of these leptons are not well understood. The TeV photons measured by HAWC are produced as the electrons and positrons emitted by the pulsars interact with low energy photons in the interstellar medium. One can, therefore, use the intensity of the TeV photon emission and the size of the emitting region to indirectly measure the high-energy positrons. The HAWC data show the large emitting regions of both the pulsars Geminga and PSR B0656+14 (see figure). The spectral and spatial features of the TeV emission were then inserted in a diffusion model for the positrons, allowing the team to calculate the positron flux from these sources reaching Earth. The results, published in Science, indicate that the positron flux from these sources reaching Earth is significantly smaller than that measured by PAMELA and AMS-02.
These indirect measurements of the positron emission appear to rule out a significant contribution of the local positron flux by these two pulsars, making it unlikely that pulsars are the origin of the positron excess. More exotic explanations such as dark matter, or other astrophysical sources such as micro-quasars and supernovae remnants, are not ruled out, however. Results from gamma-ray observations of such sources, along with more detailed measurements of the lepton flux at even higher energies by AMS-02, DAMPE or CALET, are therefore highly anticipated to fully solve the mystery of the cosmic positron excess.
For the past 40 years, CERN’s accelerator complex has been served by a little-known linear accelerator called Linac2. Commissioned in 1978, the 50 MeV linac was constructed to provide a higher beam intensity to the newly built Proton Synchrotron Booster (PSB).
It superseded Linac1, which accelerated its first beam in 1958 and was the only supplier of protons to the CERN Proton Synchrotron (PS) for the following 20 years. Linac1 was sent into retirement in 1992, having spent 33 years accelerating protons as well as deuterons, alpha particles and oxygen and sulphur ions, and is now an exhibit in the CERN Microcosm. Linac3 took over CERN’s ion production in 1994, but today Linac2 is still injecting protons into the PS and SPS from where they end up in the Large Hadron Collider (LHC).
Although the construction of this workhorse of the CERN accelerator chain was an important step forward for CERN, and contributed to major physics discoveries, including the W, Z and Higgs bosons, Linac2’s relatively low energy and intensity are not compatible with the demanding requirements of the LHC luminosity upgrade (HL-LHC). Persistent vacuum problems in the accelerating vessels over the past years also raise major concerns for the performance of the LHC. For this reason, in 2007, it was decided to replace Linac2 with a more suitable injector for the LHC’s future.
A decade later, in spring 2017, the 160 MeV Linac4 was fully commissioned and entered a stand-alone operation run to assess and improve its reliability, prior to being connected to the CERN accelerator complex. The machine’s overall availability during this initial run reached 91 per cent – an amazing value for an accelerator whose beam commissioning was completed only a few months earlier. The Linac4 reliability run will continue well into 2018, sending the beam round-the-clock to a dump located at the end of the accelerating section under the supervision of the CERN Control Centre (CCC) operation team. After a consolidation phase to address any teething troubles identified during the reliability run, Linac4 will be connected to the next accelerator in the chain, the PSB, in 2019 at the beginning of the LHC Long Shutdown 2. Test beams will be made available to the PSB as soon as 2020, and from 2021 all protons at CERN will come from the new Linac4, marking the end of a 20 year-long journey of design and construction that has raised many challenges and inspired innovative solutions.
Linac4 has the privilege of being the only new accelerator built at CERN since the LHC. With an accelerating length of 86 m, plus 76 m of new transfer line, Linac4 is definitely the smallest accelerator in the LHC injection chain. Yet it plays a fundamental role in the preparation of the beam. The linac is where the beam density is generated under the influence of the strong defocusing forces coming from Coulomb repulsion (space charge), and where negative ions initially at rest (containing protons emerging from a bottle of hydrogen gas) are progressively brought close to the relativistic velocities required for acceleration in a synchrotron. This rapid increase in beam velocity requires the use of complex and differentiated mechanical designs to accelerate and focus the beam. Combined with the need for high accelerating gradients (the beam passes only once through the linac), particular demands were placed on Linac4 to achieve the high values of availability required by the first element of the acceleration chain.
The main improvements provided by Linac4 stem from the use of negative hydrogen ions instead of protons and from a higher injection energy into the PSB. Negative hydrogen ions – a proton with two electrons – are converted into protons by passing them through a thin carbon foil, after their injection into the PSB to strip them of electrons. This charge-exchange technique involves progressively injecting the negatively charged ions over the circulating proton beam to achieve a higher particle density. After injection, both beams pass through the stripping foil leaving only protons in the beam. This provides an extremely flexible way to load particles into a synchrotron, making the accumulation of many turns possible with a tight control of the beam density.
Extensive modifications
However, the use of hydrogen ions does not come without complications. It requires extensive modifications to the injection area of the synchrotron and a complex ion source in front of the linac. The other key element for generating the high-brightness beams required by the LHC upgrade is the increase of the injection energy in the PSB by more than a factor three with respect to the present Linac2, which reduces space-charge effects at the PSB injection and allows the accumulation of more intense beams.
On top of these crucial advantages for the HL-LHC, Linac4 is designed to be more flexible and more environmentally clean than Linac2. Modulation at low energy of the beam-pulse structure, the option of varying beam energy during injection and a useful margin in the peak beam current will help prepare the large variety of beams required by the injector complex, at the same time reducing beam loss and activation in the PSB. Linac4 is also designed for the long term. Having originated from studies at the end of the 1990s, the goal was to progressively replace the PS complex (Linac2, PSB and PS) with more modern accelerators capable of higher intensities for the future needs of the LHC and other non-LHC programmes. Alas, this ambitious staged approach was later discarded to give priority to the consolidation of CERN’s older synchrotrons, but Linac4 retains features related to the old staged programme that could be exploited to adapt the CERN injector complex to future physics programmes. Examples are the orientation of the Linac4 tunnel, which leaves space for future extensions to higher energies, and its pulse-repetition frequency. The latter is currently limited by the rise time of the PSB magnets to about 1 Hz, but this could be upgraded up to 50 Hz were the PSB to be replaced one day by another accelerator.
Last but not least, Linac4 is a model for the successful reuse of old equipment. All its accelerating structures operate at a frequency of 352 MHz, which is precisely that of the old Large Electron Positron (LEP) collider. Linac4 reuses a large quantity of LEP’s RF components, such as klystrons and waveguides, which were carefully stored and maintained following LEP’s closure in 2000. However, the LEP klystrons installed in Linac4 will gradually be replaced in pairs by modern klystrons with twice the power.
Reaching Linac4’s required performance and reliability posed several problems in the design and construction of the new linac. The first challenge was to build a reliable source of negative hydrogen ions, starting from a new design developed at CERN that profited from the experience of other laboratories such as DESY and Brookhaven National Laboratory. The ion source is a complex device that starts from a bottle of hydrogen similar to the one used in Linac2 and generates ions in a plasma heated by a high-frequency wave of several dozen kilowatts. Following some initial difficulties, the new ion source is now steadily providing the minimum beam intensity required by the LHC, while improvements are still ongoing.
Accelerating elements
After the ion source, the challenge for the main Linac4 accelerating section has been to integrate focusing and accelerating elements in the small linac cells, achieving a good power efficiency at the same time. These requirements motivated the use of four different types of accelerating structure: an RF quadrupole (RFQ)to take the energy to 3 MeV; a drift-tube linac (DTL) of the Alvarez type to 50 MeV; a cell-coupled drift-tube linac (CCDTL) to 102 MeV; and finally a Pi-mode structure (PIMS) to the final energy of 160 MeV. Most of these accelerating sections include important innovations. The CCDTL and PIMS structures are a world-first developed specifically for Linac4 and used for the first time to accelerate a beam. The DTL includes a novel patented mechanism to support and adjust the drift tubes and makes use for the first time at CERN of a long focusing section made of 108 permanent magnet quadrupoles. To these innovations we had to add a novel scheme to “chop” the beam pulse at low energy, a simplified RFQ mechanical design, and finally the flexible and upgradeable beam optics design.
In spite of a general trend towards superconducting accelerators, Linac4 is entirely normal-conducting. This is a logical choice for a low-energy linear accelerator injecting into a synchrotron and operating at low duty cycle. Linac4 is pulsed, and the short particle beam is in the linac only for a tiny fraction of time. Although as much as 24 MW of RF power are needed for acceleration during the beam pulse, the average power to the accelerating structures will be only 8 kW, out of which only about 6 kW are dissipated in the copper, the rest going to the beam. The power required to cool Linac4 to cryogenic temperatures would be much higher than the power lost into the copper structures.
The construction of Linac4 is a great example of international collaboration, expanding well beyond the boundaries of CERN. Already in the R&D phase between 2004–2008, Linac4 collected support from the European Commission and a group of Russian institutes supported by the ISTC international organisation. The construction of the accelerator received important contributions from a large number of collaborating institutes. These included CEA and CNRS in France, BINP and VNIITF in Russia, NCBJ in Poland, ESS Bilbao in Spain, INFN in Italy and RRCAT in India. Organising this wide network of collaborations was a great challenge, but the results were excellent both in terms of technical quality of the components and in terms of developing a common working culture.
Linac4 brought proton-linac technology back to Europe. Since the construction of Linac2 in 1978 and of the HERA injector at DESY a few years later, all new proton linac developments took place in the US and in Japan. The development effort coordinated by CERN for the construction of Linac4 allowed bringing back to Europe the latest developments in linac technology described above, with a strong involvement of European companies. A measure of the success of this endeavour is the fact that many technical solutions developed for Linac4 will be now adopted by the normal-conducting section of the new European Spallation Source linac under construction at Lund, Sweden.
The inauguration of Linac4 on 9 May 2017 marked the coronation of a long project. The ground-breaking on so-called “Mount Citron” (made in the 1950s with the spoil from the construction of the PS ring) took place in October 2008 and the new linac building started to take shape. Construction extended over the mandate of three CERN Directors General. It’s expected that Linac4 will have a long life – at least as long as Linac2 – and play a vital role at the high-luminosity LHC and beyond.
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