Jobs – Page 15 of 505

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.”

Beams back for a bumper year

As winter bids farewell, the recommissioning of the CERN accelerator complex is gathering pace, with diverse communities eagerly awaiting particle beams in their experiments. Following the year-end technical stop, beam entered Linac4 on 5 February, two days ahead of schedule. It was then sent to the PS Booster and reached the PS on 21 February. Following SPS beam commissioning beginning in March, the first particle beams are scheduled to enter the LHC on 11 March, initially with one to a few bunches at most.

The expectations for 2024 are high. For the LHC, the focus is on proton–proton luminosity production, aiming at an unprecedented accumulation of up to 90 fb^–1. This, together with the luminosity forecast for the 2025 run, should provide a sizeable analysis dataset to keep physicists busy during Long Shutdown 3. The 2024 LHC run will conclude with lead–lead collisions, scheduled from 6 to 28 October.

The injector chain also has an ambitious year ahead, serving a busy fixed-target programme. Physics is set to start in the PS East Area on 22 March, followed by the PS n_TOF facility on 25 March. Physics in ISOLDE, downstream of the PS Booster, will start on 8 April, followed by the SPS North Area on 10 April. The antimatter factory is set to start delivering antiprotons to its experiments on 22 April, while the AWAKE facility will run for 10 weeks and the SPS HiRadMat facility for four one-week runs.

Beyond this busy physics programme, many machine development studies and tests are planned in all the machines. One of these tests will take place between mid-March and early June to configure the Linac3 source to produce magnesium ions, which will be accelerated in Linac3, injected into LEIR, and possibly even into the PS. This test will help assess the feasibility and performance of magnesium beams in the accelerator complex, for potential future applications in the LHC and the SPS North Area.

As the countdown to 11 March continues, the operations and expert teams are working diligently to prepare the machines and the beams for another successful physics run.

Potent accelerators in microquasar jets

Supernova remnants (SNRs) are excellent candidates for the production of galactic cosmic rays. Still, as we approach the “knee” region in the cosmic-ray spectrum (in the few-PeV regime), other astrophysical sources may contribute. A recent study by the High Energy Stereoscopic System (H.E.S.S.) observatory in Namibia sheds light on one such source, called SS 433, a microquasar located nearly 18,000 light-years away. It is a binary system formed by a compact object, such as a neutron star or a stellar-mass black hole, and a companion star, where the former is continuously accreting matter from the latter and emitting relativistic jets perpendicular to the accretion plane.

The jets of SS 433 are oriented perpendicular to our line of sight and constantly distort the SNR shell (called W50, or the Manatee Nebula) that was created during the black-hole formation. Radio observations reveal the precessing motion of the jets up to 0.3 light-years from the black hole, disappearing thereafter. At approximately 81 light-years from the black hole, they reappear as collimated large-scale structures in the X- and gamma-ray bands, termed “outer jets”. These jets are a fascinating probe into particle-acceleration sites, as interactions between jets and their environments can lead to the acceleration of particles that produce gamma rays.

Excellent resolution

The H.E.S.S. collaboration collected and analysed more than 200 hours of data from SS 433 to investigate the acceleration and propagation of electrons in its outer jets. Being an imaging air–shower Cherenkov telescope, H.E.S.S. offers excellent energy and angular resolutions. The gamma-ray image showed two emission regions along the outer jets, which overlap with previously observed X-ray sources. To study the energy dependence of the emission, the full energy range was split into three parts, indicating that the highest energy emission is concentrated closer to the central source, i.e. at the base of the outer jets. A proposed explanation for the observations is that electrons are accelerated to TeV energies, generate high-energy gamma rays via inverse Compton scattering, and subsequently lose energy as they propagate outwards to generate the observed X-rays.

Monte Carlo simulations modelled the morphology of the gamma-ray emission and revealed a significant deceleration in the velocity of the outer jets at their bases, indicating a possible shock region. With a lower limit on the cut-off energy for electron injection into this region, the acceleration energies were found to be > 200 TeV at 68% confidence level. Additionally, protons and heavier nuclei can also be accelerated in these regions and reach much higher energies as they are affected by weaker energy losses and carry higher total energy than electrons.

These jets are a fascinating probe into particle-acceleration sites

SS 433 is, unfortunately, ruled out as a contributor to the observed cosmic-ray flux on Earth. Considering the age of the system to be 30,000 years and proton energies of 1 PeV, the distance traversed by a cosmic-ray particle is much smaller than even the lowest estimates for the distance to SS 433. Even with a significantly larger galactic diffusion coefficient or an age 40 times older, it remains incompatible with other measurements and the highest estimate on the age of the nebula. While proton acceleration does occur in the outer jets of SS 433, these particles don’t play a part in the cosmic-ray flux measured on Earth.

This study, by revealing the energy-dependent morphology of a galactic microquasar and constraining jet velocities at large distances, firmly establishes shocks in microquasar jets as potent particle-acceleration sites and offers valuable insights for future modelling of these astrophysical structures. It opens up exciting possibilities in the search for galactic cosmic-ray sources at PeV energies and extragalactic ones at EeV energies.

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Acceleration and decay

Evolving landscape

Excellent resolution