When charged high-energy particles crash past noble-gas molecules, they leave a trail of ionisation in their wake. These tiny signals can be amplified using electric fields, and read out by electronics, revealing particle tracks with beautiful precision. This is the time-honoured concept behind the LHC’s gaseous detectors – an indispensable concept, thanks to its ability to instrument large volumes of a detector in an affordable way.
Unfortunately, environmentally harmful chlorofluorocarbons known as freons also play an essential role, dampening runaway effects to make sure that the amplified signals aren’t swallowed up by electronics noise. Physicists at the LHC are working on consolidating strategies for eliminating the current risks, and are studying novel “eco-gases” for the next generation of detectors. These were the topics of the workshop recently hosted online by CERN. To read more, check out the full report in the CERN Courier magazine.
These now-iconic detectors are critical to a new era of exploration for the ATLAS experiment. In the coming years, a major upgrade to the LHC – known as the High-Luminosity LHC – aims to crank up the collider's luminosity by a factor of ten beyond its design value. This will generate even more collisions, allowing ATLAS physicists to probe phenomena that are even rarer in nature.
A massive upgrade of the ATLAS experiment is underway to prepare for this increased luminosity. The first major system to be upgraded is the muon spectrometer, with the New Small Wheels set to be installed on either end of the experiment in summer and autumn 2021. The wheels use novel small-strip Thin Gap Chambers (sTGC) and Micromegas detectors. These new technologies will give ATLAS much more stringent selection criteria for muons, and better handle high background and pile-up rates – the two main requirements for the High-Luminosity LHC.
The New Small Wheels were built in ATLAS institutes around the world and mounted on the wheels at CERN over the course of several years. Following the installation of the last "wedge" of detectors, the first New Small Wheel is now complete, with just final testing and commissioning pending.
The photos below showcase the logistical complexity behind this milestone. Look forward to watching the installation of this “small” behemoth, set to be broadcast live on CERN and ATLAS channels this summer.
To find out more and view more photos, visit the ATLAS website.
After two nerve-wracking months dedicated to the installation of the ALICE detector’s new Inner Tracking System (ITS), Corrado Gargiulo’s mechanical engineering team, in charge of the installation, can relax: the delicate procedure has been successfully completed and ALICE’s innermost subdetector is poised to collect its first data in the coming weeks.
With its 10 m2 of active silicon area and nearly 13 billion pixels, the new ITS is the largest pixel detector ever built. The detector lies sandwiched between the beam pipe and the Time Projection Chamber, which was installed in 2020, deep in the ALICE detector. By reconstructing primary and secondary particle vertices and improving the momentum and angle resolution for particles reconstructed by the Time Projection Chamber, the ITS is instrumental in identifying the particles born out of the powerful lead–lead collisions in the core of the ALICE detector.
The upgrade of the ITS will significantly increase the resolution of the vertex reconstruction, making the subdetector fit for future runs with higher luminosity, as part of a comprehensive overhaul of ALICE’s subdetectors striving for this very objective. The current upgrade relies on new pixel sensors called ALPIDE, which also make up the new Muon Forward Tracker (MFT), installed a few months ago. Each of those chips contains more than half a million pixels in an area of 15 × 30 mm2 and features an impressive resolution of about 5 μm in both directions – the secret to the subdetector’s improved performances. They are organised in seven layers, the innermost three forming the inner barrel, while the outermost four make up the outer barrel. The collected data is then transmitted with a bit rate of up to 1.2 Gb/s to a system of about 200 readout boards located 7 m away from the detector. The data is then aggregated and eventually sent to ALICE’s computing farm, where it is sequenced and processed.
The insertion of the heart of the ALICE detector around its beam pipe required surgical-like precision. The installation unfolded in two stages, as the two barrels making up the ITS had to be lowered separately, two months apart. The outer barrel got the ball rolling: it was loaded onto a truck in March and transported from Meyrin to Point 2, where the mating of its two halves and its insertion in the detector were carried out smoothly.Installation of the Outer Barrel of the new silicon Inner Tracking System of ALICE inside the solenoidal magnet. (Image: CERN)
But the outer barrel was the easy part, at least compared to its inner counterpart whose insertion was complicated by its position right by the beam pipe. Luckily, weeks of rehearsals and careful alignment studies using metrological surveys proved their worth, and after an intense week of insertion and mating of the component’s two halves, which involved a few late-evening sessions for the experts, the delicate manoeuvre was completed in the late evening of 12 May. Preliminary tests showed no damage occurred during the installation, proving that the teams’ hard work paid off.
The ITS is now fully ready for stand-alone tests with cosmic rays, in view of joining the MFT for a common commissioning phase. The final steps before taking data at the LHC are the installation scheduled for next month of the Fast Interaction Trigger, the last of the ALICE subdetectors that has yet to join this formidable machine, and an overarching commissioning phase starting in July. With the milestone of the ITS installation now behind them, the ALICE collaboration is looking ahead to Run 3 with growing confidence and excitement.
It’s been a decade in space for the Alpha Magnetic Spectrometer (AMS) – and a decade of amazing cosmic discoveries. On its final flight on 16 May 2011, space shuttle Endeavour delivered the AMS detector, which was assembled at CERN, to the International Space Station. And by 19 May 2011 the detector was installed and sending data back to Earth – to NASA in Houston and then from NASA to CERN for analysis. Ten years and more than 175 billion cosmic rays later, AMS has delivered scientific results that have changed and confounded our understanding of the origin of these particles and how they journey through space at almost the speed of light.
Cosmic rays come in many species. They are mainly the atomic nuclei of hydrogen, that is, protons, but also include the nuclei of heavier elements as well as electrons and the antimatter counterparts of protons and electrons. And they fall into two main types: primary and secondary. Primary cosmic rays are mostly produced in supernovae explosions in the Milky Way and beyond, and they can travel for millions of years before reaching AMS. Secondary cosmic rays are created in interactions between the primary cosmic rays and the interstellar medium.
AMS measures the properties of the cosmic rays that reach it to try and shed light on the origin of dark matter, antimatter and cosmic rays as well as to explore new phenomena. Highlights from the many AMS results obtained in its first ten years include a result showing that the numbers, or more precisely the “fluxes”, of several types of secondary cosmic rays are all surprisingly identical to one another and very different from those of primary cosmic rays. AMS also reported an analysis of the flux of cosmic-ray positrons, the antimatter particles of electrons, indicating that at high energies these cosmic rays predominantly originate either from the annihilation of dark matter particles in space or from other cosmic sources such as fast-spinning stars called pulsars.
Other highlights include a result showing that, contrary to expectations, primary cosmic rays have at least two distinct classes, one made of light nuclei and the other made of heavy nuclei. Intriguingly, however, a more recent study revealed that iron nuclei – the most abundant primary cosmic rays after silicon nuclei and the heaviest cosmic rays measured by AMS until now – belong unexpectedly not to the same class as the other heavy nuclei but instead to the class of light nuclei.
“It’s impossible to do justice to all of the AMS results, but one thing is clear,” says AMS spokesperson Samuel Ting. “Over the past ten years, AMS has challenged time and again conventional theory of cosmic-ray origin and propagation, transforming our understanding of these cosmic particles.”
AMS continues to collect data, following the successful completion of a series of spacewalks – unparalleled in complexity for a space intervention – that have extended its remaining lifetime to match that of the International Space Station. And if the results obtained in the past decade are anything to go by, more cosmic discoveries will no doubt be in store.
After several years of complex design, manufacture and planning, the CMS collaboration, in close cooperation with experts from CERN’s Vacuum, Surfaces and Coating group (TE department), have in recent months been installing the new heart of the detector: the beam pipe. This fragile, 36-m-long component, in which the LHC beams collide at the Interaction Point, will be one of the last elements of the experiment to be installed before closing the detector.
The design of the new beam pipe has to comply with the numerous demands of physics, vacuum and integration requirements. From either side of the Interaction Point, the cylindrical section of the central chamber, with a diameter of 43.4 mm, has been extended from 1.6 m to 3.1 m to be compatible with the Phase 2 Tracker sub-detector that will be installed during LS3.
An important change with respect to the previous layout of the beam pipe consists in a new vacuum pumping group, moved away from the detector at 16 m from the Interaction Point, to facilitate maintenance.
Another key motivation behind the beam pipe layout change is the reduction of the radiation dose received by personnel during interventions. The new aluminium alloy used for the beam pipe reduces the induced radioactivity by a factor of 5 compared to the stainless steel used for the old beam pipe. This alloy has been chosen as the main material of the experimental vacuum chambers for Run 3 and the HL-LHC era.
After a series of acceptance tests, the vacuum chamber segments were equipped with a set of temperature sensors and then wrapped with heating foils, the so-called “bake-out jackets”, that will be used to heat up the beam pipe from ambient temperature to 230 °C after the installation. The bake-out will activate the non-evaporable getter (NEG) material already coating the inner surface of the vacuum chambers, which will act as a distributed vacuum pump, constantly absorbing any residual gas. This will clean up stray particles and help to achieve the ultra-high vacuum that is essential inside the beamline of any particle collider to prevent collisions between the circulating beam particles and residual gas molecules. Such collisions would scatter the beam, creating a noisy background for the detector and degrading the beam life.Installation of the optical fibres for monitoring the central segment of the new CMS vacuum chamber. (Image: CERN)
Following a detailed installation sequence of the vacuum pipe segments, in parallel, at both ends of the experiment, the mechanical installation of the chambers and all their operational and temporary supports was completed at the end of April.Insertion of the central vacuum chamber across the CMS Tracker. (Image: CERN)
With the mechanical installation completed, a global leak test will be performed on the chambers, with the aim of reaching an ultimate pressure of 10−11 millibars. Then, the detector endcaps will be positioned in the bake-out configuration for a duration of 168 hours. A final step of ultra-pure neon injection will complete the activity, readying the new beam pipe for Run 3.
This is an extract of an article published on the CMS website. You can read the full version here.
The ATLAS PhD Grant is a flagship programme of the CERN & Society Foundation. It was established in 2013 by former ATLAS spokespersons Fabiola Gianotti and Peter Jenni, using their award money from the Breakthrough Prize in Fundamental Physics. In 2014, the first batch of students began their grant periods. Now in its eighth year, the ATLAS PhD Grant relies on private contributions through the CERN & Society Foundation to continue its legacy.
Due to the ongoing global pandemic, this year’s award ceremony was broadcast live on Facebook and LinkedIn, with CERN & Society donors and members of the CERN Management joining in remotely. The recipients of the 2021 ATLAS PhD Grant were announced as Ana Luisa Carvalho (LIP, Portugal) and Humphry Tlou (University of the Witwatersrand, South Africa). These talented and motivated students will receive 1.5 years of funding for their studies at CERN, under the supervision and training of ATLAS collaboration experts.
“When Fabiola Gianotti and I received the Fundamental Physics Prize, it was clear to us that we wanted to give something back to ATLAS,” said Peter Jenni, speaking at the event. “We remembered our own time as students at CERN and wanted to give others the same opportunity. CERN is a great learning environment – not just for physics, but to experience working closely with people from different countries and cultures.”
Ana Luisa Carvalho and Humphry Tlou each gave a short speech of thanks, extracts of which can be read in the full version of the article here. You can also watch a recording of the full ceremony or visit the CERN & Society website to learn more about the ATLAS PhD Grant.
Its name may suggest it is the stuff of science fiction, but it’s not. SciFi – the new scintillating-fibre particle-tracking detector of the LHCb experiment – is very real, and its first pieces have just journeyed 100 metres down to be installed in the underground cavern that houses the experiment. The construction of the detector and its installation in the LHCb cavern are part of the ongoing upgrade work that is transforming LHCb so it can sustain a fivefold increase in the rate of proton–proton collisions when the Large Hadron Collider starts up again in 2022.
The scintillating-fibre detector is no ordinary particle detector. As the name indicates, the detector is made of scintillating fibres – optical fibres that emit light when a particle interacts with them. The fibres also contain additional scintillator dyes that shift the light’s wavelength from ultraviolet to blue-green, such that it can travel the length of the fibre and be recorded by devices called silicon photomultipliers, which convert the light to electrical signals.
Such detector technology is not new, but it has had to be refined to achieve the scale and precision of the SciFi detector. Scientists had to painstakingly examine and wind more than 10 000 kilometres of fibre to produce the multi-layer ribbons needed for the detector modules – no mean feat.
“It took more than a dozen partner institutes in nine different countries working together since 2014 to make SciFi a reality,” says Blake Leverington, who is coordinating the assembly of the 12 separate pieces that will make up the complete detector. “The lowering of the first four SciFi pieces into the LHCb cavern is an exciting and satisfying moment for us.”
The remaining eight pieces are being assembled and will be installed before the LHC proton beams return in the spring of 2022. Watch this space for more milestones in the transformation of LHCb in time for the next LHC run.
Find out more about the SciFi detector in this story.
The Crystal Clear collaboration (experiment RD-18) began in 1991 as part of the R&D programme run by the Detector Research and Development Committee (DRDC) to address the formidable challenges posed by the future LHC. The objective was clear: identify the most suitable scintillating crystals to pave the way for the discovery of the Higgs boson. Now, 30 years on, it is clear that the collaboration has exceeded all expectations. Not only did it contribute to one of the greatest physics discoveries of the twenty-first century, but it also went on to help drive innovation in the medical technology sector.
Amidst the large-scale R&D efforts to develop the detectors for the future LHC, the collaboration quickly set to work in studying scintillating crystals whose scintillation mechanisms were still a mystery. In 1994, that research led to the recommendation to use lead tungstate (PbWO4 or PWO), a material combining the advantages of high density, fast scintillation and good resistance to radiation with relatively low manufacturing costs, for the construction of the CMS electromagnetic calorimeter and the ALICE PHOS detector. That recommendation was followed, as both detectors are made from PWO crystals.
The purpose of an electromagnetic calorimeter is to measure the energy of photons, electrons and positrons. The particles’ energy is transformed into light as they pass through the crystals and is then detected by a photodetector whose signal is analysed to identify the original particle. Notably, it was in the heart of the CMS electromagnetic calorimeter that the Higgs boson was identified by its decay into two photons.
Starting in 1995, in parallel with its R&D work on scintillators for high-energy physics, the Crystal Clear collaboration branched out into medical applications with the development of several positron emission tomography (PET) devices for imaging in nuclear medicine. PET uses scintillating crystals for the coincidence detection of pairs of photons resulting from electron–positron annihilation. The collaboration started by developing ClearPET prototypes, PET cameras for small animals(1), then moved on to ClearPEM prototypes for detecting breast cancer(1) and, more recently, the EndoTOFPET-US prototype for detecting pancreatic and prostate cancer.
Today, the collaboration’s efforts to improve the coincidence time resolution (CTR) of these tomography machines continue, the target being a CTR of 10 picoseconds (as against more than 200 picoseconds for commercial PET cameras), which would improve image quality while reducing the time spent in the scanner and the dose administered to the patient(2). To this end, the collaboration is exploring new detection concepts, including the development of scintillating nanomaterials.
The Crystal Clear collaboration is also currently pursuing its initial R&D work on future detectors. “Detectors for future accelerators will have to deal with unprecedented constraints on their components. Developing fast, radiation-resistant crystals and coming up with new ways to use them will be vital to designing detectors based on the scintillators of tomorrow,” says collaboration spokesperson Étiennette Auffray, laying out a vision for the future of Crystal Clear.
The technologies developed for high-CTR PET cameras, known as time-of-flight PET cameras, inspired the insertion of a layer of LYSO (lutetium-yttrium oxyorthosilicate) crystals, called the “barrel timing layer”, in the CMS central barrel between the tracker and the electromagnetic calorimeter, which will measure the time of flight of each particle. A “Spaghetti Calorimeter”, or “SpaCal”, made up of an absorber and scintillating crystal fibres, is also being studied as part of the EP department’s R&D programme. It could replace the central part of the current LHCb electromagnetic calorimeter.
Unfortunately, the current health situation prevents the members of the Crystal Clear collaboration from celebrating its 30th anniversary, at least for now. But despite that, Crystal Clear, which has always moved with the times, is looking resolutely forward to a bright future for scintillating crystals.
Target-based experiments are plentiful at CERN, be it at the Antiproton Decelerator, the ISOLDE facility or the North Area. They provide the Laboratory with a variety of secondary particles through the interaction of the target’s components with high-energy proton beams from the accelerator complex. One example is the n_TOF (Neutron Time-Of-Flight) facility, where a spallation target is used to produce a neutron beam. After ten years of service, the old n_TOF neutron spallation target was removed and a third-generation target successfully installed in the facility this month. This achievement marks the culmination of four years of development led by the Sources, Targets and Interactions (STI) group in the Systems (SY) department, which is responsible for the operation of the n_TOF facility.
The n_TOF collaboration (numbering more than 120 physicists) hopes to find answers to the questions posed by the processes of nucleosynthesis (how are chemical elements produced outside of nuclear fusion during Big Bang nucleosynthesis and within stars, and what role do neutrons play in this phenomenon?), as well as to much more pragmatic issues, such as nuclear waste disposal. To achieve this, n_TOF scientists work with a high-quality neutron beam produced by the collision of high-energy protons (20 GeV/c, 7 ns wide) from the Proton Synchrotron (PS) with the lead nuclei of the spallation target. The neutrons “knocked” from the target assembly by the proton beam fly towards and collide with experimental samples, after being moderated by water, doped with enriched boron. Their time of flight and the number of decay products allows the calculation of the probability of interaction (cross section). This makes it possible to take unprecedented measurements of isotopes of elements such as osmium, thulium and beryllium, to name just a few, which help to shed light on nucleosynthesis processes.
The old spallation target, a 1.2-tonne water-cooled monolithic lead cylinder, had to retire after ten years of receiving high-energy protons. Its replacement is made up of six separate U-shaped lead blocks – weighing a total of 1.5 tonnes – a new design that offers several logistical advantages. First of all, it allows the beam-heated lead to be cooled with gaseous nitrogen at ambient pressure instead of with water, which will significantly reduce the pollution of the circuit by removing the erosion and corrosion mechanisms induced by the water in direct contact with lead. Secondly, the new target was designed to house an additional demineralised water moderator tank on its top, across one of the two neutron beam tracks. This new moderator tank will improve the resolution of the measurements of a neutron’s time of flight in the vertical flight path, a crucial aspect of n_TOF’s research. Thirdly, it further improves the physics performance of the facility.
Finally, new modified target shielding was installed in order to provide access to the target area for inspection and operational purposes, as well as to irradiate materials in a field representative of CERN's accelerator systems and evaluate their long-term behaviour within the framework of the Radiation to Materials component of the R2E (Radiation to Electronics) project at CERN. In addition, it would make it possible – if required – to develop an experimental test station much closer to the spallation target than the two existing ones, significantly increasing the measurable number of neutrons per proton pulse. While the construction of this additional experimental station is still under review, the new spallation target leaves n_TOF scientists poised for at least ten more years of world-class neutron research at CERN.Diagram of the n_TOF facility. EAR1 and EAR2 are the two experimental areas situated at the end of the neutron beam lines. (Image: CERN)
For more pictures of the installation, go to: https://cds.cern.ch/record/2759329?ln=en.
The world's largest and most powerful particle accelerator is getting a new experiment. In March 2021, the CERN Research Board approved the ninth experiment at the Large Hadron Collider: SND@LHC, or Scattering and Neutrino Detector at the LHC. Designed to detect and study neutrinos, particles similar to the electron but with no electric charge and very low mass, the experiment will complement and extend the physics reach of the other LHC experiments.
SND@LHC is especially complementary to FASERν, a neutrino subdetector of the FASER experiment, which has just recently been installed in the LHC tunnel. Neutrinos have been detected from many sources, but they remain the most enigmatic fundamental particles in the universe. FASERν and SND@LHC will make measurements of neutrinos produced at a particle collider for the first time, and could thus open a new frontier in neutrino physics.
SND@LHC is a compact apparatus consisting of a neutrino target followed downstream by a device to detect muons, the heavier cousins of electrons, produced when the neutrinos interact with the target. The target is made from tungsten plates interleaved with emulsion films and electronic tracking devices. The emulsion films reveal the tracks of the particles produced in the neutrino interactions, while the electronic tracking devices provide time stamps for these tracks. Together with the muon detector, the tracking devices also measure the energy of the neutrinos.
Like FASERν, SND@LHC will be able to detect neutrinos of all types – electron neutrinos, muon neutrinos and tau neutrinos. Unlike FASERν, which is located on one side of the ATLAS detector and along the LHC’s beamline (the line travelled by particle beams in the collider), SND@LHC will be positioned slightly off the beamline, on the opposite side of ATLAS. This location will allow SND@LHC to detect neutrinos produced at small angles with respect to the beamline, but larger than those covered by FASERν.
“The angular range that SND@LHC will cover is currently unexplored,” says SND@LHC spokesperson Giovanni De Lellis. “And because a large fraction of the neutrinos produced in this range come from the decays of particles made of heavy quarks, these neutrinos can be used to study heavy-quark particle production in an angular range that the other LHC experiments can’t access.”
SND@LHC will be installed in an unused tunnel that links the LHC to the Super Proton Synchrotron over the course of 2021, and it is expected to begin taking data when the LHC starts up again in 2022.The SND@LHC experiment consists of an emulsion/tungsten target for neutrinos (yellow) interleaved with electronic tracking devices (grey), followed downstream by a detector (brown) to identify muons and measure the energy of the neutrinos. (Image: Antonio Crupano/SND@LHC)
Find out more about SND@LHC in this Experimental Physics newsletter article.
Geneva, 31 March 2021. The ALPHA collaboration at CERN has succeeded in cooling down antihydrogen atoms – the simplest form of atomic antimatter – using laser light. The technique, known as laser cooling, was first demonstrated 40 years ago on normal matter and is a mainstay of many research fields. Its first application to antihydrogen by ALPHA, described in a paper published today in Nature, opens the door to considerably more precise measurements of the internal structure of antihydrogen and of how it behaves under the influence of gravity. Comparing such measurements with those of the well-studied hydrogen atom could reveal differences between matter and antimatter atoms. Such differences, if present, could shed light on why the universe is made up of matter only, an imbalance known as matter–antimatter asymmetry.
“The ability to laser-cool antihydrogen atoms is a game-changer for spectroscopic and gravitational measurements, and it could lead to new perspectives in antimatter research, such as the creation of antimatter molecules and the development of anti-atom interferometry,” says ALPHA spokesperson, Jeffrey Hangst. “We’re over the moon. About a decade ago, laser cooling of antimatter was in the realm of science fiction.”
The ALPHA team makes antihydrogen atoms by taking antiprotons from CERN’s Antiproton Decelerator and binding them with positrons originating from a sodium-22 source. It then confines the resulting antihydrogen atoms in a magnetic trap, which prevents them from coming into contact with matter and annihilating. Next, the team typically performs spectroscopic studies, that is, it measures the anti-atoms’ response to electromagnetic radiation – laser light or microwaves. These studies have allowed the team to, for example, measure the 1S–2S electronic transition in antihydrogen with unprecedented precision. However, the precision of such spectroscopic measurements and of planned future measurements of the behaviour of antihydrogen in the Earth’s gravitational field in ongoing experiments is limited by the kinetic energy or, equivalently, the temperature, of the antiatoms.
This is where laser cooling comes in. In this technique, laser photons are absorbed by the atoms, causing them to reach a higher-energy state. The anti-atoms then emit the photons and spontaneously decay back to their initial state. Because the interaction depends on the atoms’ velocity and as the photons impart momentum, repeating this absorption–emission cycle many times leads to cooling of the atoms to a low temperature.
In their new study, the ALPHA researchers were able to laser-cool a sample of magnetically trapped antihydrogen atoms by repeatedly driving the anti-atoms from the atoms’ lowest-energy state (the 1S state) to a higher-energy state (2P) using pulsed laser light with a frequency slightly below that of the transition between the two states. After illuminating the trapped atoms for several hours, the researchers observed a more than tenfold decrease in the atoms’ median kinetic energy, with many of the anti-atoms attaining energies below a microeletronvolt (about 0.012 degrees above absolute zero in temperature equivalent).
Having successfully laser-cooled the anti-atoms, the researchers investigated how the laser cooling affected a spectroscopic measurement of the 1S–2S transition and found that the cooling resulted in a narrower spectral line for the transition – about four times narrower than that observed without laser cooling.
“Our demonstration of laser cooling of antihydrogen atoms and its application to 1S–2S spectroscopy represents the culmination of many years of antimatter research and developments at CERN’s Antiproton Decelerator. This is by far the most difficult experiment we have ever done,” says Hangst.
“Historically, researchers have struggled to laser-cool normal hydrogen, so this has been a bit of a crazy dream for us for many years,” says Makoto Fujiwara, the first proponent of the idea of using a pulsed laser to cool trapped antihydrogen in ALPHA. “Now, we can dream of even crazier things with antimatter.”
FASER* (Forward Search Experiment), CERN’s newest experiment, is now in place in the LHC tunnel, only two years after its approval by CERN’s Research Board in March 2019. FASER is designed to study the interactions of high-energy neutrinos and search for new, as-yet-undiscovered light and weakly interacting particles. Such particles are dominantly produced along the beam collision axis and may be long-lived particles, travelling hundreds of metres before decaying. The existence of such new particles is predicted by many models beyond the Standard Model that attempt to solve some of the biggest puzzles in physics, such as the nature of dark matter and the origin of neutrino masses.
FASER is located along the beam collision axis, 480 m from the ATLAS interaction point, in an unused service tunnel that formerly connected the SPS to the LEP collider – an optimal position for detecting the particles into which light and weakly interacting particles will decay.
The first civil engineering works started in May 2020. “Because of the sloped geometry of the tunnel, the beam collision axis was actually passing under the ground,” says Jamie Boyd, FASER spokesperson. “Measurements from the CERN survey team showed that, by excavating a 50-cm-deep trench, sufficient space would be created to house the 5-m-long FASER detector.” In the summer, the first services and power systems were installed, and in November, FASER’s three magnets were put in place in the trench.The installation of FASER’s three magnets took place in November, in the narrow trench excavated by CERN’s SCE team. (Image: CERN)
A pretty simple experiment
At the entrance to the detector, two scintillator stations are used to veto charged particles coming through the cavern wall from the ATLAS interaction point; these are primarily high-energy muons. The veto stations are followed by a 1.5-m-long dipole magnet. This is the decay volume for long-lived particles decaying into a pair of oppositely charged particles. After the decay volume is a spectrometer consisting of two 1-m-long dipole magnets with three tracking stations, which are located at either end and in between the magnets. Each tracking station is composed of layers of precision silicon strip detectors. Scintillator stations for triggering and precision time measurements are located at the entrance and exit of the spectrometer.
The final component is the electromagnetic calorimeter. This will identify high-energy electrons and photons and measure the total electromagnetic energy. The whole detector is cooled down to 15 °C by an independent cooling station.
“FASER uses spare pieces from the ATLAS (for the tracker) and LHCb (for the calorimeter) experiments, which made possible its installation during Long Shutdown 2, so quickly after its approval,” highlights Jamie Boyd.
FASER will also have a subdetector called FASERν, which is specifically designed to detect neutrinos. No neutrino produced at a particle collider has ever been detected, despite colliders producing them in huge numbers and at high energies. FASERν is made up of emulsion films and tungsten plates to act as both the target and the detector to see the neutrino interactions. FASERν should be ready for installation by the end of the year. The whole experiment will start taking data during Run 3 of the LHC, starting in 2022.
“We are extremely excited to see this project come to life so quickly and smoothly,” says Jamie Boyd. “Of course, this would not have been possible without the expert help of the many CERN teams involved!”
The muon spectrometer is made up of several thousand chambers and is the outermost layer of the ATLAS detector. It identifies and measures the momentum of muons that fly out of the collision point. Key to this is a precise understanding of the muon spectrometer’s geometry.
At small scales, the geometry of the muon spectrometer is almost constantly changing, albeit slowly. Small temperature variations make the chambers and their support structures contract, expand and deform. Further, some of the chambers are mounted on the ATLAS toroid magnets, which themselves can occasionally move and deform.
The muon spectrometer is therefore equipped with an optical alignment system that monitors in real time the positions of chambers relative to each other and to calibrated reference objects in the detector, as well as their deformations. This information can be combined with data from muon tracks in order to fully understand the muon spectrometer’s position.
But when new chambers are added or existing ones repaired, the spatial relationship between the alignment sensors and active detector elements is altered. Such changes require the entire muon spectrometer to be realigned.
ATLAS physicists implemented a new alignment procedure for the data-taking periods of 2017 and 2018. The resulting alignment is almost – but not quite – perfect. Judging from observed deviations, the alignment is accurate to around 50 μm in a large part of the spectrometer volume, with some slightly poorer regions being closer to 100 μm.
In other words: the entire muon spectrometer has been kept aligned to better than the diameter of a human hair. Such incredible precision is key to an experiment’s success, as evidenced by the excellent ATLAS results from Run 2 data.
Read the full ATLAS Experiment Briefing to learn more.
The 2020 ATLAS Outstanding Achievement Awards ceremony was held online on 11 February 2021. Established in 2014, the awards recognise outstanding contributions in support of the ATLAS experiment, covering all areas except physics analysis.
The Collaboration Board Chair Advisory Group, which selected the winners, received a total of 61 nominations – of individuals or teams – for 32 ATLAS activities.
“The ATLAS Outstanding Achievement Awards provide a way of acknowledging the diverse efforts that keep the experiment producing high-quality data,” says Al Goshaw from Duke University (USA) and the Awards Committee Chair. This year’s awards highlighted eight activities involving a total of 21 people, recognising technical work on the detector operation, upgrade, software, computing and combined performance. Meet the winners on the collaboration website!The Thesis Award winners take a Zoom photo with ATLAS Spokesperson Karl Jakobs and Collaboration Board Chair Aleandro Nisati. (Image: ATLAS Collaboration/CERN)
Also on 11 February, ATLAS celebrated its PhD students, a key cohort of the collaboration who make unique and crucial contributions to the experiment while working on their degree. Every year, their work is acknowledged through the ATLAS Thesis Awards. The theses that receive awards can cover any area of ATLAS physics, including detector development, operations, software and performance studies, and physics analysis.
“We received 41 nominations this year, which is more than in any previous year,” says Jessica Leveque from the Laboratoire d'Annecy de Physique des Particules (France) and the ATLAS 2020 Thesis Awards Committee Chair.
This year’s winners are: Christina Agapopoulou (University of Paris-Saclay), Milene Calvetti (University of Pisa), Jennet Dickinson (University of California, Berkeley), Kurt Hill (University of Colorado, Boulder), Luigi Marchese (University of Oxford), Cristiano David Sebastiani (University of Rome “La Sapienza”), Cecilia Tosciri (University of Oxford) and Marco Valente (University of Geneva).
Read the full articles on the ATLAS collaboration website:
Andreas Hoecker is just the fifth person to serve as ATLAS Spokesperson since the collaboration was established in 1992. Since joining ATLAS in 2005, he has taken on several coordination roles, including Data Preparation Coordinator and Physics Coordinator. Andreas enters his newest role with great familiarity, having served for the past four years as Deputy to Spokesperson Karl Jakobs (Freiburg University).
Indeed, the new management team has several familiar faces. Continuing on in their roles are Deputy Spokesperson Manuella Vincter (Carleton University), Technical Coordinator Ludovico Pontecorvo (CERN), Resource Coordinator David Francis (CERN) and Upgrade Coordinator Francesco Lanni (Brookhaven National Laboratory). Joining them is Deputy Spokesperson Marumi Kado (University of Rome I and INFN Rome), who previously served as ATLAS Physics Coordinator.
“Karl has been a fantastic Spokesperson for the collaboration,” says Andreas. “He also recruited an extremely competent and dedicated team; I’m pleased they’ll all be continuing on in their roles and I welcome Marumi’s fresh perspective. We’re embarking on an exciting but challenging period for the collaboration, as we continue to prioritise critical developments of the experiment alongside data analysis. Success will require us to fully commit and focus our forces.”
The new management team will guide the ATLAS collaboration over a two-year term, overseeing the final push before Run 3 of the LHC begins in 2022 as well as preparations for the High-Luminosity LHC (HL-LHC). “We have a lot of tasks to complete before we can return to data-taking,” says Ludovico Pontecorvo. “Foremost among them are the final installation and commissioning of the Phase I upgrades of the detector, which will improve, among other things, the trigger capabilities of ATLAS.”
The coming years will then see work for the HL-LHC kick into full gear. The upgrades will require a lot more than clicking ‘install and restart’. “Every layer and system of the ATLAS experiment is impacted, with work ranging from complex new electronics installations to the complete replacement of the inner detector,” says Francesco Lanni. “Just like the original construction of the experiment, this work has been distributed throughout ATLAS institutes around the world. This is a colossal endeavour, and ensuring its success is one of our highest priorities.”
Alongside these extensive detector priorities, ATLAS management remains committed to maintaining the pace of high-quality physics analysis. “The data we collected during Run 2 of the LHC (2015-2018) has proven a veritable treasure trove,” says Andreas. “We will continue exploring it, probing rare and new processes, and conducting ever-more detailed studies.” These findings, combined with the upcoming Run 3 data, are a source of great anticipation for the physics results yet to come.
That the management team starts their new term during a global pandemic – with many colleagues still working from home – is an unfortunate reality. Nevertheless, they remain optimistic. “I continue to be impressed by the dedication and resilience that ATLAS members have shown during these difficult times,” adds Andreas. “As Spokesperson, I look forward to open engagement with members, as they continue to bring new ideas to the field and accomplish wonders for this experiment.”