Jobs – Page 21 of 511

Giuseppe Fidecaro 1926–2024

Experimental physicist Giuseppe Fidecaro, who joined CERN in 1956 and continued there long into his retirement, passed away on 28 March.

Born in Messina, Italy in 1926, Giuseppe studied physics at the University of Rome, graduating in 1947 under the supervision of Edoardo Amaldi. Amaldi had become interested in cosmic rays and asked young “Pippo” to help him build a large detector to study the scattering of mesons on an iron target to explore the nuclear force. Between 1952 and 1954, Giuseppe continued to work on cosmic rays at the Tête Grise laboratory, 3500 m above Cervinia, where Maria Cervasi, whom he had met during his studies at Rome, also worked.

In 1953 Amaldi, who had become secretary general of the provisional CERN, suggested that Giuseppe spend time at the University of Liverpool to learn from the new synchrocyclotron being built there. He went to CERN with Maria, by then his wife, in 1956 and began preparing experiments for the 600 MeV Synchrocyclotron (SC), which came into operation in August 1957.

In January 1958, during a conference in New York, Giuseppe attended a presentation by Feynman describing the universal “V–A” theory of weak interactions. He heard that the theory lacked a key experimental ingredient: the decay of a pion into an electron and neutrino, predicted to occur 10 000 times less frequently than to a muon and a neutrino, which had not been observed in two experiments performed by well-known physicists. Upon his return to CERN, Pippo decided with the other members of the SC group that this would be the target of the next experiment. A device was immediately designed and built, and 40 events in perfect agreement with the V–A prediction were presented by Pippo in September 1958. The news put the newly born CERN on the map of the world of particle physics and laid the groundwork for the future discoveries of neutral currents, the W and Z bosons, and the Higgs field.

In 1960, with the start-up of the PS, Giuseppe led his group to measure – using a system of precision scintillators – the antiproton–proton cross section. The following year, he became professor at the University of Trieste and established a group that carried out a series of important scattering measurements at the PS and the SPS, in particular using polarised targets, during the 1970s. Following the proposal and execution of an experiment at the ILL in Grenoble searching for possible neutron–antineutron oscillations, in 1990 he presented an article “Fixed target B-physics at the Large Hadron Collider” at the LHC workshop in Aachen, which proposed, among other things, the use of a very intense proton beam extracted from the accelerator with a crystal, similar to what had been envisaged for the Superconducting Super Collider. This, and discussions with Giovanni Carboni and Walter Scandale, were at the origin of the RD22 collaboration, which for the first time proved the possibility of high-efficiency proton extraction from an accelerator using a bent crystal – a technique that is now used in LHC beam collimation.

Outside physics, Giuseppe made numerous contributions to CERN. In the early 1960s he was a member of the founding committee of the International Center for Theoretical Physics. In 1975 he was appointed as co-chair of a joint scientific committee set up under a collaboration agreement between CERN and the former USSR concerning the use of atomic energy, a responsibility he held until 1986. He was also tasked with coordinating cooperation with JINR in Dubna.

Giuseppe officially retired in 1991 but, together with Maria, continued his work at CERN as an honorary member of the personnel until as recently as 2020, during which time he devoted himself to research in the history of physics. He produced reports of rare beauty and precision, notably three well-documented articles on the contributions of Bruno Pontecorvo, whose friend he became in Dubna in 1989. Giuseppe was also known to CERN visitors, featuring prominently in the film shown in the Synchrocyclotron exhibition. Maria Fidecaro, with whom his rich human and scientific journey was deeply entwined, passed away in September 2023.

Education and outreach in particle physics

The imposing structure of CERN Science Gateway has been likened to a space station. In fact, it was CERN’s technical buildings and underground tunnels that were the inspiration for chief architect Renzo Piano. Its three pavilions and two tubes house exhibitions, hands-on laboratories, artworks, a 900-seat auditorium, a shop and a restaurant – all connected by a 220 m-long bridge and nestled amongst 400 trees and 13,000 shrubs. It has a net-zero carbon footprint, with 2000 m² of solar panels on the pavilion roofs providing all the energy needed, while feeding 40% back into the CERN grid. The Gateway is free to enter and open all year, every day except Mondays, offering the capacity to welcome up to 500,000 visitors of all ages per year.

We look at the importance of reaching out as far and wide as possible

The following articles of expert exposition and opinion lift the lid on CERN Science Gateway. In addition to hearing from the teams behind its content, we explore the broader issues surrounding the theory and practice of education, communication and outreach in particle physics – beginning with what these three terms mean today (From the cosmos to the classroom). Exploring the Gateway’s exhibition spaces, authors reflect on four stunning art installations (Beautiful minds collide), the secrets of success for an interactive exhibit (Interactive exhibits: theory and practice) and the simple power of objects (The power of objects). Following a deep-dive into the new educational labs (Hands on, minds on, goggles on!), learn about CERN’s physics-education research (Why research education?), the impact of its hugely popular teacher programmes (Inspiring the inspirers), and how particle physics is or is not integrated in school curricula (Particle physics in school curricula). From empowering children to aspire to science (Empowering children to aspire to science) to taking physics to festivals (Going where the crowd is), and transcending physical and neurological boundaries (Expanding the senses), three articles emphasise the importance of reaching out as far and wide as possible. Last but certainly not least, we consider the invaluable role played by physicists (Physicists go direct and Time for an upgrade) and weave the rich experiences of CERN guides throughout these articles. Feel inspired? Your nifty red Science Gateway vest awaits!

Pushing accelerator frontiers in Bern

Novel accelerator concepts will play an important role in future accelerators for high-energy physics. Two relevant scenarios being explored in the framework of the European Union I.FAST project are the generation of relativistic single electrons with gigahertz repetition rate for dark-matter searches, and the rapid acceleration of muons with GV/m accelerating fields for experiments at the energy frontier. The topical workshop “Gigahertz Rate and Rapid Muon Acceleration”, held in Bern from 10 to 13 December 2023, addressed the latest developments in these and related topics.

The first part of the workshop was devoted to dark-matter searches and dielectric laser acceleration (DLA). For dark-matter searches, multiple experiments are proposed across different classes (muons vs electrons and positrons, appearance vs disappearance experiments, etc), and an adequate background rejection is important. Promising advanced accelerator technologies are DLA for single electrons – perhaps also muons – and plasma-wakefield accelerators for muons and pions.

Some dark matter-experiments look for an appearance that requires a high flux of incoming particles. For electrons, the standard is set by BDX at JLab, for protons by the proposed SHiP experiment at CERN, and for photons by the proposed Gamma Factory at CERN. In addition, appearances could be seen at existing collider experiments such as the LHC. Other dark-matter experiments search for disappearance. They rely on DC-like electron beams, with prominent examples being LDMX at SLAC and the newly proposed DLA–DMX at PSI. A DC-like muon beam could be explored by the M3 experiment at Fermilab.

Paolo Crivelli (ETH Zürich) described the NA64 experiment as one of the most prominent examples of ongoing accelerator-based dark-matter searches, and presented the first results using a high-energy muon beam. The proposed LDMX experiment at SLAC, presented by Silke Möbius (University of Bern), may set a new standard for indirect dark-matter searches, while advanced concepts employing dielectric laser acceleration, in particular when integrating the accelerating structure with laser oscillator, could achieve many orders of magnitude higher rates of single high-energy electrons entering into an LDMX-type detector.

Uwe Niedermayer (TU Darmstadt), Stefanie Kraus (University Erlangen-Nürnberg) and Raziyeh Dadashi (PSI/EPFL) reviewed the state of the art in DLA plus future plans. Yves Bellouard (EPFL) discussed advances in high-repetition-rate lasers and micro/nano-structures, which suggests that the proposed combined laser-accelerator structures are within reach. Of course, the detector time resolution would also need to be improved tremendously to keep pace with the higher rate of the accelerator.

Acceleration and decay

The second part of the workshop was devoted to the plasma acceleration of non-ultra relativistic and rapidly decaying particles, such as muons and pions. Vladimir Shiltsev (Fermilab) and Daniel Schulte (CERN) presented tentative parameters and ongoing R&D efforts towards a muon collider. Shiltsev also discussed the intriguing possibility of low-emittance muon sources based on plasma-wakefield accelerators, while Alexander Pukhov (Heinrich Heine University Düsseldorf) and Chiara Badiali (IST Lisbon) discussed how plasma acceleration could bring slow particles, such as muons, to relativistic velocities.

The workshop fostered numerous heated discussions and uncovered unresolved issues, which included the “Bern controversy” regarding the ultimate limits of luminosity for PeV energies. Muons are considered particles of choice for future accelerators at the energy frontier. Both low- and high-energy muons have useful applications. Is there an Angstrom limit to the beam diameter? Are tiny beta functions possible? Can plasmas help to overcome such limitations? Understanding and modelling non-point-like particle luminosity is another important topic, also relevant for the Gamma Factory.

The workshop showed how advanced accelerator concepts can jump-start dark-sector searches

The final part of the workshop assembled a roadmap and perspective. DLA studies are to be maintained and, if possible, accelerated. A reasonable target is achieving a gradient of 500 MeV/m and an energy gain of 0.05 GeV in five years on a single wafer, while an integrated DLA laser oscillator could be foreseen five to seven years from now. Plasma-wakefield acceleration of muons could conceivably be tested either at CERN–AWAKE or PSI. It was proposed, as a first step, to put a solid target or tape into the AWAKE set up.

The gamma factory, presented by Witek Krasny (LPNHE), was recognised as an intense source of polarised muons and positrons. For muon-acceleration studies, the dephasing issue, linked to the muons’ non-ultrarelativistic energy, seems to be resolved. A demonstrator experiment for muon plasma acceleration is called for. Open questions include when and where?

Overall, the Bern workshop showed how advanced accelerator concepts can jump start dark-sector searches and muon/pion acceleration. High-repetition-rate acceleration of single electrons for dark-matter searches, using dielectric laser accelerators, and applying high-gradient plasma acceleration to muon and/or pion beams, are intriguing and far-forward looking topics.

A global forum for high-energy physics

The International Committee for Future Accelerators (ICFA) was formally founded in 1977 as a working group in IUPAP’s commission 11 (C11, Particles and Fields). Today it remains the place for discussions on all aspects of particle physics, in particular on the large accelerators that are at the heart of the field, and on the strategic deliberations in the various regions of the world. Although ICFA has no means of ensuring that any of its resolutions are carried out, it can act as the “conscience” of the field, and its recommendations can also influence national or regional activities. Among the currently 16 members, which include directors of CERN, Fermilab, IHEP, KEK and DESY, three are from Europe, three from the US, two from Russia, two from Japan, and one each from China and Canada. Three further members collectively represent smaller countries and regions, and the functions of chair and secretary rotate through the Americas, Europe and Asia, usually every three years.

A significant fraction of ICFA’s work is carried out within a set of seven panels, which meet regularly and assemble expertise on more technical or detailed aspects of particular importance to the field. One is devoted to the International Linear Collider (ILC). For more than two decades, ICFA has promoted the realisation of the ILC, for which a global design effort was put in place in 2005. In parallel, an international collaboration under CERN’s leadership had been working on the Compact Linear Collider (CLIC). Recognising the synergies between the two concepts, ICFA established a single coordinating structure, the Linear Collider Collaboration (LCC), in 2012. Also that year, the Japanese high-energy physics community proposed to host the ILC in Japan as a global project.

The LCC mandate came to an end in 2020, when ICFA put in place the ILC International Development Team (IDT) and its working groups. In June 2021 the IDT developed a proposal for the “preparatory laboratory” as a first step towards the realisation of the ILC in Japan.

Evolving landscape

While the IDT is continuing its work, the global Higgs-factory landscape has evolved since the early days of the ILC: more – linear and circular – studies and proposals are on the table, not least as demonstrated by the P5 report in the US. ICFA will soon discuss in what way its discussions and structures need to be adapted to better reflect this evolving landscape.

In November 2023 ICFA established a new panel devoted to the “data lifecycle”, which involves everything from data acquisition, processing, distribution, storage, access, analysis, simulation and preservation, to management, software, workflows, computing and networking. The panel, which replaces two previous ones on related topics, was created in response to the growing importance of data management and open science in recent years. Its membership is currently being put together with the aim to develop ideas and strategies for workforce development and professional recognition mechanisms.

For more than two decades, ICFA has promoted the realisation of the ILC

ICFA’s farthest-reaching and most visible activity is the ICFA Seminar. The 13th ICFA seminar on “Future Perspectives in High-Energy Physics” took place at DESY from 28 November to 1 December 2023. For the first time in six years (the prior ICFA seminar had taken place in 2017 in Ottawa, Canada), this select crowd of scientists, lab directors and funding agency representatives could come together in person for updates and discussions. One highlight was the panel discussion between the directors of KEK, CERN, Fermilab and IHEP, in which views on a future global strategy were discussed. The seminar concluded on a festive note with the formal passing of the ICFA chair baton from Stuart Henderson (JLAB) to Pierluigi Campana (INFN), who will lead ICFA for the next three years.

ICFA is the only global representation of the particle-physics community, and the ideal discussion forum for global strategic developments, especially large international collider projects. In view of the current situation with numerous opportunities for future facilities – not least a future Higgs factory, but also smaller and more diverse projects – the committee and its panels look forward to serving the field of particle physics through continued advocacy, exploration, discussion and facilitation.

Physics community pays tribute to Peter Higgs

Peter Higgs has passed away at the age of 94. An iconic figure in modern science, Higgs in 1964 postulated the existence of the eponymous Higgs boson. Its discovery at CERN in 2012 was the crowning achievement of the Standard Model (SM) of particle physics – a remarkable theory that explains the visible universe at the most fundamental level.

Alongside Robert Brout and François Englert, and building on the work of a generation of physicists, Higgs postulated the existence of the Brout–Englert–Higgs (BEH) field. Alone among known fundamental fields, the BEH field is “turned on” throughout the universe, rather than flickering in and out of existence and remaining localized. Its existence allowed matter to form in the early universe some 10^–11 s after the Big Bang, thanks to the interactions between elementary particles (such as electrons and quarks) and the ever-present BEH field. Higgs and Englert were awarded the Nobel Prize for Physics in 2013 in recognition of these achievements.

An immensely inspiring figure for physicists around the world
Fabiola Gianotti

“Besides his outstanding contributions to particle physics, Peter was a very special person, an immensely inspiring figure for physicists around the world, a man of rare modesty, a great teacher and someone who explained physics in a very simple yet profound way,” said CERN’s Director-General Fabiola Gianotti, expressing the emotion felt by the physics community upon his loss. “An important piece of CERN’s history and accomplishments is linked to him. I am very saddened, and I will miss him sorely.”

Peter Higgs’ scientific legacy will extend far beyond the scope of current discoveries. The Higgs boson – the observable “excitation” of the BEH field which he was the first to identify – is linked to some of most intriguing and crucial outstanding questions in fundamental physics. This still quite mysterious particle therefore represents a uniquely promising portal to physics beyond the SM. Since discovering it in 2012, the ATLAS and CMS collaborations have already made impressive progress in constraining its properties – a painstaking scientific study that will form a central plank of research at the LHC, high-luminosity LHC and future colliders for decades to come, promising insights into the many unanswered questions in fundamental science.

Dieter Proch 1943–2024

Dieter Proch, who made significant contributions to accelerator science, passed away unexpectedly on 27 February 2024 at the age of 80.

Dieter studied physics at the University of Bonn, where he joined the group of Helmut Piel, which had just started working on superconducting accelerator resonators. He then followed Piel, who had accepted an appointment as professor at the newly founded University of Wuppertal, and completed his doctorate on measurements of superconducting accelerator resonators. Soon after, he analysed the serious problem of so-called one-point multipacting in superconducting resonators prevalent at the time. Together with Wuppertal colleagues, he proposed changing the shape of resonators to have a spherical profile, which solved the multipacting problem. Subsequently, Dieter completed research stays at Cornell and CERN, where in 1981 he contributed to the development of spherical superconducting resonators for LEP II to double the energy of LEP. He then took up a permanent position at DESY, where he remained for almost 27 years until June 2009.

During his first years at DESY, Dieter’s focus was on the development of superconducting accelerator structures for the HERA accelerator that was being planned. He was head of the “Superconducting acceleration sections” experimental programme, where he demonstrated organisational talent as well as scientific and technical skills. Within a few years he pushed superconducting resonators from theoretical considerations to preliminary technological studies, and the operation of experimental resonators in the PETRA accelerator.

In the mid-1980s, Dieter took over a group focusing on superconducting accelerator technology. The group was responsible for the design, manufacturing, testing, installation and operation of the superconducting resonators in HERA.

In addition, Dieter was one of the founders of the international TESLA collaboration. Under his leadership, a groundbreaking infrastructure for the treatment, assembly and testing of superconducting accelerator resonators was built at DESY. This development work made it possible to increase the originally targeted field gradients from 25 to 35 MV/m. He organised close collaborations with many laboratories in Germany, Europe, Asia and the US. Particularly noteworthy here are Peking University and Tsinghua University, both of which appointed Dieter as a visiting professor.

As a globally recognised expert and deputy chair of the TESLA technology collaboration, Dieter served on important committees for many years, such as the advisory board for SNS at Oak Ridge. At DESY, the FLASH and European XFEL user systems are based on his fundamental work. The SRF Workshop, which later became a recognised international conference, was always particularly close to his heart. The scientific reputation that DESY enjoys worldwide was significantly influenced by Dieter. He also collaborated on several articles for the Handbook of Accelerator Physics and Engineering.

Dieter’s contributions continue to shape our understanding and advancement of accelerator technology. We thank him very much and will always remember him fondly.

Balancing matter and antimatter in Pb–Pb collisions

ALICE figure 1 — **Fig. 1.** Baryon chemical potential, μ_B, extracted from experimental data as a function of the centre-of-mass energy per nucleon pair. Data points are compared with a phenomenological parametrisation of μ_B. The inset shows μ_B extracted at two LHC energies. Source: arXiv:2311.13332

When lead ions collide head-on at the LHC they deposit most of their kinetic energy in the collision zone, forming new matter at extremely high temperatures and energy densities. The hot and dense zone quickly expands and cools down, leading to the production of approximately equal numbers of particles and antiparticles at mid-rapidity. However, in reality the balance between matter and antimatter can be slightly distorted.

The collision starts with matter only, i.e. protons and neutrons from the incoming beam. During the collision process, incoming lead nuclei interact while penetrating each other, and most of their quantum numbers are carried away by particles travelling close to the beam direction. Due to strong interactions among the quarks and gluons, quantum numbers of the colliding ions are transported to mid-rapidity rather than to the ions themselves. This leads to an imbalance of baryons originating from the initial state, which has more baryons than antibaryons.

This matter–antimatter imbalance can be quantified by determining two global system properties: the chemical potentials associated with the electric charge and baryon number (denoted μ_Q and μ_B, respectively). In a thermodynamic description, the chemical potentials determine the net electric-charge and baryon-number densities of the system. Thus, μ_B measures the imbalance between matter and antimatter, with a vanishing value indicating a perfect balance.

In a new, high-precision measurement, the ALICE collaboration reports the most precise characterisation so far of the imbalance between matter and antimatter in collisions between lead nuclei at a centre-of-mass energy per nucleon pair of 5.02 TeV. The study was carried out by measuring the antiparticle-to-particle yield ratios of light-flavour hadrons, which make up the bulk of particles produced in heavy-ion collisions. The measurement using the ALICE central barrel detectors included identified charged pions, protons and multi- strange Ω^– baryons, in addition to light nuclei, ³He, triton and the hypertriton (a bound state of a proton, a neutron and a Λ-baryon). The larger baryon content of these light nuclei makes them more sensitive to baryon-asymmetry effects.

The medium created in lead–lead collisions at the LHC is nearly electrically neutral and baryon-number-free at mid-rapidity

The analysis reveals that in head-on lead–ion collisions, for every 1000 produced protons, approximately 986 ± 6 antiprotons are produced. The chemical potentials extracted from the experimental data are μ_Q = -0.18 ± 0.90 MeV and μ_B = 0.71 ± 0.45 MeV. These values are compatible with zero, showing that the medium created in lead–lead collisions at the LHC is nearly electrically neutral and baryon-number-free at mid-rapidity. This observation holds for the full centrality range, from collisions where the incoming ions peripherally interact with each other up to the most violent head-on processes, indicating that quantum-number transport at the LHC is independent of the size of the system formed.

The values of μ_B are shown in figure 1 as a function of the centre-of-mass energy of the colliding nuclei, along with lower-energy measurements at other facilities. The recent ALICE result is indicated by the red solid circle, along with a phenomenological parametrisation of μ_B. The decreasing trend of μ_Bobserved as a function of increasing collision energy indicates that different net-baryon-number density conditions can be explored by varying the beam energy, reaching almost vanishing net-baryon content at the LHC. The inset gives the μ_B values extracted at two LHC energies. It shows that the new ALICE result is almost one order of magnitude more precise than the previous estimate (violet), thanks to a more refined study of systematic uncertainties.

The present study with improved precision characterises the vanishing baryon-asymmetry at the LHC, posing stringent limits to models describing baryon-number transport effects. Using the data samples collected in LHC Run 3, these studies will be extended to the strangeness sectors, enabling a full characterisation of quantum-number transport at the LHC.

Belle II back in business

On 20 February the Belle II detector at SuperKEKB in Japan recorded its first e⁺e^– collisions since summer 2022, when the facility entered a scheduled long shutdown. During the shutdown, a new vertex detector incorporating a fully implemented pixel detector, together with an improved beam pipe at the collision point, was installed to better handle the expected increases in luminosity and backgrounds originating from the beams. Furthermore, the radiation shielding around the detector was enhanced, and other measures to improve the data-collection performance were implemented.

Belle II, for which first collisions were recorded in the fully instrumented detector in March 2019, aims to uncover new phenomena through precise analysis of the properties of B mesons and other particles produced by the SuperKEKB accelerator. Its long-term goal is to accumulate a dataset 50 times larger than that of the former Belle experiment.

Iodine vapours impact climate modelling

FLOTUS quartz flow-tube system — **Enhancement** A new quartz flow-tube system called FLOTUS was added to the CLOUD experiment in 2023 to allow pre-ageing of organic vapours before injection into the chamber. Credit: CERN-PHOTO-202303-068-2

Climate models are missing an important source of aerosol particles in polar and marine regions, according to new results from the CLOUD experiment at CERN. Atmospheric aerosol particles exert a strong net cooling effect on the climate by making clouds brighter and more extensive, thereby reflecting more sunlight back out to space. However, how aerosol particles form in the atmosphere remains poorly understood, especially in polar and marine regions.

The CLOUD experiment, located in CERN’s East Area, maintains ultra-low contaminant levels and precisely controls all experimental parameters affecting aerosol formation growth under realistic atmospheric conditions. During the past 15 years, the collaboration has uncovered new processes through which aerosol particles form from mixtures of vapours and grow to sizes where they can seed cloud droplets. A beam from the Proton Synchrotron simulates, in the CLOUD chamber, the ionisation from galactic cosmic rays at any altitude in the troposphere.

Globally, the main vapour driving particle formation is thought to be sulphuric acid, stabilised by ammonia. However, ammonia is frequently lacking in polar and marine regions, and models generally underpredict the observed particle-formation rates. The latest CLOUD study challenges this view, by showing that iodine oxoacids can replace the role of ammonia and act synergistically with sulphuric acid to greatly enhance particle-formation rates.

“Our results show that climate models need to include iodine oxoacids along with sulphuric acid and other vapours,” says CLOUD spokesperson Jasper Kirkby. “This is particularly important in polar regions, which are highly sensitive to small changes in aerosol particles and clouds. Here, increased aerosol and clouds actually have a warming effect by absorbing infrared radiation otherwise lost to space, and then re-radiating it back down to the surface.”

The new findings build on earlier CLOUD studies which showed that iodine oxoacids rapidly form particles even in the complete absence of sulphuric acid. At iodine oxoacid concentrations that are typical of marine and polar regions (between 0.1 and 5 relative to those of sulphuric acid), the CLOUD data show that the formation rates of sulphuric acid particles are between 10 and 10,000 times faster than previous estimates.

“Global marine iodine emissions have tripled in the past 70 years due to thinning sea ice and rising ozone concentrations, and this trend is likely to continue,” adds Kirkby. “The resultant increase of marine aerosol particles and clouds, suggested by our findings, will have created a positive feedback that accelerates the loss of sea ice in polar regions, while simultaneously introducing a cooling effect at lower latitudes. The next generation of climate models will need to take iodine vapours and their synergy with sulphuric acid into account.”

The promise of laser-cooled positronium

Consisting only of an electron and a positron, positronium (Ps) offers unique exploration of a purely leptonic matter–antimatter system. Traditionally, experiments have relied on formation processes that produce clouds of Ps with a large velocity distribution, limiting the precision of spectroscopic studies due to the large Doppler broadening of the Ps transition lines. Now, after almost 10 years of effort, the AEgIS collaboration at CERN’s Antiproton Decelerator has experimentally demonstrated laser-cooling of Ps for the first time, opening new possibilities for antimatter research.

“This is a breakthrough for the antimatter community that has been awaited for almost 30 years, and which has both a broad physics and technological impact,” says AEgIS physics coordinator Benjamin Rienacker of the University of Liverpool. “Precise Ps spectroscopy experiments could reach the sensitivity to probe the gravitational interaction in a two-body system (with 50% on-shell antimatter mass and made of point-like particles) in a cleaner way than with antihydrogen. Cold ensembles of Ps could also enable Bose–Einstein condensation of an antimatter compound system that provides a path to a coherent gamma-ray source, while allowing precise measurements of the positron mass and fine structure constant, among other applications.”

Laser cooling, which was applied to antihydrogen atoms for the first time by the ALPHA experiment in 2021 (CERN Courier May/June 2021 p9), slows atoms gradually during the course of many cycles of photon absorption and emission. This is normally done using a narrowband laser, which emits light with a small frequency range. By contrast, the AEgIS team uses a pulsed alexandrite-based laser with high intensity, large bandwidth and long pulse duration to meet the cooling requirements. The system enabled the AEgIS team to decrease the temperature of the Ps atoms from 380 K to 170 K, corresponding to a decrease in the transversal component of the Ps velocity from 54 to 37 km s^–1.

The feat presents a major technical challenge since, unlike antihydrogen, Ps is unstable and annihilates with a lifetime of only 142 ns. The use of a large bandwidth laser has the advantage of cooling a large fraction of the Ps cloud while increasing the effective lifetime, resulting in a higher amount of Ps after cooling for further experimentation.

“Our results can be further improved, starting from a cryogenic Ps source, which we also know how to build in AEgIS, to reach our dream temperature of 10 K or lower,” says AEgIS spokesperson Ruggero Caravita of INFN-TIFPA. “Other ideas are to add a second cooling stage with a narrower spectral bandwidth set to a detuning level closer to resonance, or by coherent laser cooling.”

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Acceleration and decay

Evolving landscape