Physics Archives – Page 17 of 144

Slim, charming protons on the menu in Mainz

The triennial international conference on meson–nucleon physics and the structure of the nucleon (MENU) attracted more than 140 participants to the historic centre of Mainz from 16 to 20 October 2023.

Among MENU 2023’s highlights on nucleon structure, a preliminary analysis by the NNPDF collaboration suggests that the proton contains more charm than anticharm, with Niccolò Laurenti (Università degli Studi di Milano) showing evidence of a non-vanishing intrinsic valence charm contribution to the proton’s wavefunction. Meanwhile, Michael Kohl (Hampton University) concluded that the proton–radius puzzle is still not resolved. To make progress, form-factor measurements in electron scattering must be scrutinised, and the use of atomic spectroscopy data clarified, he said.

Hadron physics

A large part of this year’s conference was dedicated to hadron spectroscopy, with updates from Belle II, BESIII, GlueX, Jefferson Lab, JPAC, KLOE/KLOE-2 and LHCb, as well as theoretical overviews covering everything from lattice quantum chromodynamics to effective-field theories. Special emphasis was also given to future directions in hadron physics at future facilities such as FAIR, the Electron-Ion Collider and the local Mainz Energy-Recovering Superconducting Accelerator (MESA) facility – a future low-energy but high-intensity electron accelerator that will make it possible to carry out experiments in nuclear astrophysics, dark-sector searches and tests of the SM. Among upgrade plans at Jefferson Lab, Eric Voutier (Paris-Saclay) presented a future experimental programme with positron beams at CEBAF, the institute’s Continuous Electron Beam Accelerator Facility. The upgrade will allow for a rich physics programme covering two-photon exchange, generalised polarisabilities, generalised parton distribution functions and direct dark-matter searches.

Highlights on nucleon structure include a preliminary analysis suggesting that the proton contains more charm than anticharm

Hadron physics is also closely related to searches for new physics, as precision observables of the Standard Model are in many cases limited by the non-perturbative regime of quantum chromodynamics. A prime example is the physics of the anomalous magnetic moment of the muon, for which a puzzling discrepancy between data-driven dispersive and lattice–quantum chromodynamics calculations of hadronic contributions to the Standard Model prediction persists (CERN Courier May/June 2021 p25). The upcoming collaboration meeting of the Muon g-2 Theory Initiative in September 2024 at KEK will provide important new insights from lattice QCD and e⁺e^– experiments. It remains to be seen whether the eventual theoretical consensus will confirm a significant deviation from the experimental value, which is currently being updated by Fermilab’s Muon g-2 experiment using their last three years of data.

BESIII passes milestone at the charm threshold

The BESIII collaboration has marked a significant milestone: the completion of its 15-year campaign to collect 20 fb^–1 of e⁺e^– collision data at the ψ(3770) resonance. The sample, collected in two main running periods, 2010–2011 and 2022–2024, is more than 20 times larger than the world’s previous charm-threshold data set collected by the CLEO-c experiment in the US.

BESIII is an experiment situated on the BEPCII storage ring at IHEP in Beijing. It involves more than 600 physicists drawn not only from China but also other nations, including Germany, Italy, Poland, the Netherlands, Sweden and the UK from the CERN member states. The detector has collected data at a range of running points with centre-of-mass energies from 1.8 to 4.95 GeV, most of which are inaccessible to other operating colliders. This energy regime allows researchers to make largely unique studies of physics above and below the charm threshold, and has led to important discoveries and measurements in light-meson spectroscopy, non-perturbative QCD, and charm and tau physics.

The ψ(3770), discovered at SLAC in 1977, is the lightest charmonium state above the open-charm threshold. Charmonium consists of a bound charm quark and anti-charm quark, whereas open-charm states such as D⁰ and D⁺ mesons are systems in which the charm quark co-exists with a different anti-quark. The ψ(3770) can decay into D and anti-D mesons, whereas charmonium states below threshold, such as the J/ψ, are too light to do so, and must instead decay through annihilation of the charm and anticharm quarks.

The sample is more than 20 times larger than the world’s previous charm-threshold data set

Open-charm mesons are also produced in copious quantities at the LHC and at Belle II. However, in ψ(3770) decays at BESIII they are produced in pairs, with no accompanying particles. This makes the BESIII sample a uniquely clean laboratory in which to study the properties of D mesons. If one meson is reconstructed, or tagged, in a known charm decay, the other meson in the event can be analysed in an unbiased manner. When reconstructed in a decay of interest, the unbiased sample of mesons can be used to measure absolute branching fractions and the relative phases between any intermediate resonances in the D decay.

“Both sets of information are not only interesting in themselves, but also vital for studies with charm and beauty mesons at LHCb and Belle II,” explains Guy Wilkinson of the University of Oxford. “For example, measurements of phase information performed by BESIII with the first tranche of ψ(3770) data have been essential input in the world-leading determination of the CP-violating angle γ of the unitarity triangle by LHCb in events where a beauty meson decays into a D meson and an accompanying kaon.” Exploitation of the full 20 fb^–1 sample will be essential in helping LHCb and Belle II realise their full potential in CP-violation measurements with larger data sets in the future, he adds. “Hence BESIII is very complementary to the higher energy experiments, demonstrating the strong synergies that exist between particle-physics facilities worldwide.”

This summer, BEPCII will undergo an upgrade that will increase its luminosity. Over the rest of the decade more data will be taken above and below the charm threshold. In the longer term, there are plans, elsewhere in China, for a Super Tau Charm Facility – an accelerator that would build on the BEPCII and BESIII programme with datasets that are two orders of magnitude larger.

First DESI results shine a light on Hubble tension

The expansion of the universe has been a well-established fact of physics for almost a century. By the turn of the millennium the rate of this expansion, referred to as the Hubble constant (H₀), had converged to a value of around 70 km s^–1 Mpc^–1. However, more recent measurements have given rise to a tension: whereas those derived from the cosmic microwave background (CMB) cluster around a value of 67 km s^–1 Mpc^–1, direct measurements using a local distance-ladder (such as those based on Cepheids) mostly prefer larger values around 73 km s^–1 Mpc^–1. This disagreement between early- and late-universe measurements, respectively, stands at the 4–5σ level, thereby calling for novel measurements.

One such source of new information are large galaxy surveys, such as the one currently being performed by the Dark Energy Spectroscopic Instrument (DESI). This Arizona-based instrument uses 5000 individual robots that optimise the focal plane of the detector to allow it to measure 5000 galaxies at the same time. The goal of the survey is to provide a detailed 3D map, which can be used to study the evolution of the universe by focussing on the distance between galaxies. During its first year of observation, the results of which have now been released, DESI has provided a catalogue of millions of objects.

Primordial imprints

Small fluctuations in the density of the early universe resulted not only in signatures in the CMB, as measured for example by the Planck probe, but also left imprints in the distribution of baryonic matter. Each over-dense region is thought to contain dark matter, baryonic matter and photons. The gravitational force from dark matter on the baryons is countered by radiation pressure from the photons. From the small over-densities, baryons are dragged along by photon pressure until these two types of particles decoupled during the recombination era. The original location of the over-density is surrounded by a sphere of baryonic matter, which typically is at a distance referred to as the sound horizon. The sound horizon at the moment of decoupling, denoted r_d, leaves an imprint that has since evolved to produce the density fluctuations in the universe that seeded large-scale structures.

Constraints on the Hubble constant assuming the flat ΛCDM model — **Hubble tension** 68% credible-interval constraints on the Hubble constant assuming the flat ΛCDM model, showing: DESI baryon acoustic oscillation measurements in combination with other data (blue, bold); corresponding results from the Sloan Digital Sky Survey and combinations thereof (blue); CMB anisotropy measurements from Planck and the Atacama Cosmology Telescope (orange); and measurements using Cepheids or tip-of-the-red-giant branch distance ladders (green). Source: arXiv:2404.03002

This imprint, and how it has evolved over the last 13 billion years, depends on a number of parameters in the standard ΛCDM model of cosmology. Measuring the baryon distribution therefore allows many of the ΛCDM parameters to be constrained. Since the DESI data measure the combination of H₀ and r_d, a direct measurement of H₀ is not possible. However, by using additional data for the sound horizon, taken from CMB measurements and Big Bang nucleosynthesis theory, the team finds values of H₀ that cluster around 67.5 km s^–1 Mpc^–1 (see “Hubble tension” figure). This is consistent with early-universe measurements and differs by more than 3σ from late-universe measurements.

Although these new results do not directly resolve the Hubble tension, they do hint at one potential solution: the need to revise the ΛCDM model. The measurements also allow constraints to be placed on the acceleration of the universe, which depends on the dark-energy equation of state, w. While this is naturally assumed to be constant at w = –1, the DESI first-year results better match a time-evolving equation of state. Although highly dependent on the analysis, the DESI data so far provide results that differ from ΛCDM predictions by more than 2.5σ. The data from the remaining four years of the survey are therefore highly anticipated as these will show whether a change to the standard cosmological model is required.

7th Workshop for Energy for Sustainable Science at Research Infrastructures

Event description

In the current decade, after contrasted indications of dwindling and instable energy resources, energy cost rises and severe evidences of climate change, the sustainability of technical infrastructures has been confirmed as an unavoidable demand. New medium and large dimension research infrastructures are forced to face this challenging scenario.

Extreme performance operation and cutting-edge technologies often lead to high power consumption. The development of next generation research and technological infrastructures and the upgrade of existing ones demand new concepts in terms of sustainability, affecting both to new technological concepts and reconsidering the operation of the facilities.

Besides, the clear increase of carbon-free, renewable energy sources, energy-efficient systems, more advanced energy storage integration and smart grids can reduce strain on the energy supply. But this cannot be an excuse for delaying the transition to sustainable infrastructures. In this regard, collaboration among scientists, engineers, environmental experts and scientific policy makers is crucial for devising long-term strategies for sustainable research infrastructure.

Sustainable development and operation of research infrastructures does not limit to energy efficiency considerations; it also encompasses circular economy concepts and a proper life cycle of materials and components, concluding with a responsible management of wastes. Embracing transformative changes towards sustainability means progress and innovation. Research facilities must take a leading position in environmental consciousness and the pursuit of efficient energy solutions, inspiring other sectors.

Renewable energy sources offer hope for a greener future, including solar, wind, and geothermal technologies. Energy-efficient systems and smart grids can mitigate power demand and reduce strain on traditional energy grids. Integrating energy storage and management systems is essential for a stable power supply.

The main goal of this event is to identify the challenges, technical and strategical, to develop and implement sustainable solutions at research infrastructures. This includes sharing experiences on new energy-efficient technologies, energy management at research infrastructures, review how the energy sustainability is faced on the current research projects, analyze life cycle, and discuss about future aims and trends, among other topics. Specific interest will be paid to involve pre-doctoral students into the workshop discussions, to promote the involvement of the young research staff on the event topics.

To stimulate exchanges and foster contacts and cooperation among the participants, the format of the event has been selected to be in-presence, although speakers from non European institutions will be exceptionally allowed to participate on-line.

CIEMAT, in collaboration with CERN (the European Organization for Nuclear Research), the ESRF (European Synchrotron Radiation Facility), DESY (Deutsches Elektronen-Synchrotron), PSI (the Paul Scherrer Institut), ESS (The European Spallation Source) and ERF (The European Association of National Research Facilities) will host the Seventh Workshop on Energy for Sustainable Science at Research Infrastructures Facilities in Madrid on 25-27 November 2024, as a continuation of a series of events hosted by ESS (2011), CERN (2013), DESY (2015), ELI-NP (2017), PSI (2019) and ESRF (2022).

New pentaquark searches in beauty decays

LHCb figure 1 — **Fig. 1.** Invariant-mass distributions of (left) Λ_c⁺ D_s^– and (right) Λ_c⁺ D⁰ K^– candidates with the results of the fit overlaid. The exclusive Λ_b⁰ decays to Λ_c⁺ D_s^– and Λ_c⁺ D⁰ K^– peak around the nominal Λ_b⁰ mass of 5620 MeV. The partially reconstructed decays Λ_b⁰ → Λ_c⁺ D_s^*– and Λ_b⁰ → Λ_c⁺ D^*0 K^– are shown in orange and green, while nuisance components are shown in different colours. Source: LHCb-PAPER-2023-034

Pentaquarks, bound states of five quarks predicted in the first formulation of the quark model in 1964, have had a troubled history. Following disputed claims of the discovery of light-flavour species over 20 years ago, pentaquarks with hidden charm are now well-established members of the hadronic spectrum. The breakthrough was achieved by the LHCb experiment in 2015 with the observation of P_c⁺ states in the J/ψ p system.

The P_c⁺ quark content (uudcc) implies that decays to two open-charm hadrons, such as Λ_c⁺ D⁰ or Λ_c⁺ D*⁰, are possible. The rates of such decays are important for understanding more about the nature of the P_c⁺ states, as different models predict rates that differ by orders of magnitude. Distinguishing between the proposed mechanisms by which pentaquarks, and excited hadrons in general, are produced and bound allows a better understanding of the dynamics of the strong interaction in the non-perturbative regime.

A new analysis by LHCb of the open-charm hadrons in Λ_b decays was presented at the International Conference on Meson-Nucleon Physics and the Structure of the Nucleon, held in Mainz in October. It concerns the first observation and measurement of the branching fractions of Λ_b⁰ → Λ_c⁺ D(*)⁰ K^– and Λ_b⁰ → Λ_c⁺ D_s^*^– decays using proton–proton collision data collected during LHC Run 2.

All branching fractions are measured relative to the known Λ_b⁰ → Λ_c⁺ D_s^– decay mode, which is reconstructed with the same set of six final-state hadrons: p K^– π⁺ K⁺ π^– K^–. Many systematic uncertainties in the measured ratios therefore cancel out, making the precision on the relative branching fraction of Λ_b⁰ → Λ_c⁺ D⁰ K^– statistically limited. For Λ_b⁰ → Λ_c⁺ D^0* K^– and Λ_b⁰ → Λ_c⁺ D*^– the resulting branching fractions are systematically limited. This is because either a photon or neutral pion is not reconstructed, so their shape in the invariant mass spectrum of the reconstructed particles is more difficult to describe and more affected by the backgrounds (see figure 1, where the components with a missing photon for which a branching fraction is calculated are shown in orange and those with a missing neutral pion in green).

The partially reconstructed Λ_b⁰ → Λ_c⁺ D_s^*^– decay cannot be used directly to search for pentaquarks, but it is an important input to model calculations. In addition, as a two-body decay, it is a powerful test of factorisation assumptions in heavy-quark effective theory.

In the Λ_b⁰ → Λ_c⁺ D(*)⁰ K^– decay, the production process of the P_c⁺ pentaquarks is the same as in the discovery channel, Λ_b⁰ → J/ψ p K^–. A comparison between the measured branching fractions and observed signal yields can thus be used to estimate the expected sensitivity for observing P_c⁺ signals in the open-charm channels. In particular, the rate of a Λ_b⁰ decay to Λ_c⁺ D⁰ K^– is about six times greater than to J/ψ p K^–; however, more than 60 times as much data would be needed to match the currently available Λ_b⁰ → J/ψ p K^– signal yield.

A factor of about 24 in this calculation comes from the branching fractions ratio of J/ψ and open-charm hadrons, given their reconstructed decay modes. The rest is from reconstruction and selection inefficiencies, which favour the four-prong μ⁺μ^– p K^– over the fully hadronic six-body final state. With the upgraded Run 3 detector and now triggerless detector readout, a large part of the inefficiency for fully hadronic final states is recoverable, making pentaquark searches in double open-charm final states more favourable compared to the situation in Run 2.

Tango for two: LHCb and theory

The 13th annual “Implications of LHCb measurements and future prospects” workshop, held at CERN on 25–27 October 2023, drew substantial interest with 231 participants. This collaborative event between LHCb and the theoretical community showcased the mutual enthusiasm for LHCb’s physics advances. The workshop featured five streams highlighting the latest experimental and theoretical developments in mixing and CP violation, heavy ions and fixed-target results, flavour-changing charged currents, QCD spectroscopy and exotics, and flavour-changing neutral currents.

The opening talk by Monica Pepe Altarelli underscored LHCb’s diverse physics programme, solidifying its role as a highly versatile forward detector. While celebrating successes, her talk candidly addressed setbacks, notably the new results in tests of lepton-flavour universality. LHCb detector and computing upgrades for Run 3 include a fully software-based trigger using graphics processing units. The collaboration is also working towards an Upgrade II programme for Long Shutdown 4 (2033–2034) that would position LHCb as a potentially unique global flavour facility.

On mixing and CP violation, the October workshop unveiled intriguing insights in both the beauty and charm sectors. In the beauty sector, notable highlights encompass measurements of the mixing parameter ΔΓ_s and of CP-violating phases such as ϕ_s,d, ϕ_s^ss^s and γ. CP asymmetries were further scrutinised in B → DD decays, accounting for SU(3) breaking and re-scattering effects. In the charm sector, the estimated CP asymmetries considering final-state interactions were found to be small compared to the experimental values related to D⁰ → π^–π⁺ and D⁰ → K^–K⁺ decays. Novel measurements of CP violation in three-body charm hadron decays were also presented.

Unique capabilities

On the theoretical front, discussions delved into the current status of bottom-baryon lifetimes. Recent lattice predictions on the ε_K parameter were also showcased, offering refined constraints on the unitarity triangle. The LHCb experiment’s unique capabilities were discussed in the heavy ions and fixed-target session. Operating in fixed-target mode, LHCb collected data pertaining to proton–ion and lead–ion interactions during LHC Run 2 using the SMOG system. Key highlights included measurements impacting theoretical models of charm hadronisation, global analyses of nuclear parton density functions, and the identification of helium nuclei and deuterons. The first Run 3 data with the SMOG2 upgrade showed promising results in proton–argon and proton–hydrogen collisions, opening a path to measurements with implications for heavy-ion physics and astrophysics.

The session on flavour-changing charged currents unveiled a recent measurement concerning the longitudinal polarisation of D^*⁻ mesons in B⁰ → D^*⁻τ^–ν_τ decays, aligning with Standard Model (SM) expectations. Discussions delved into lepton-flavour-universality tests that showed a 3.3σ tension with predictions in the combined R(D⁽^*⁾) measurement. Noteworthy were new lattice-QCD predictions for charged current decays, especially R(D⁽^*⁾), showcasing disparities in the SM prediction across different lattice groups. Updates on the CKM matrix elements |V_ub| and |V_cb| lead to a reduced tension between inclusive and exclusive determinations. The session also discussed the impact of high-energy constraints of Wilson coefficients on charged-current decays and Bayesian inference of form-factor parameters, regulated by unitarity and analyticity. The QCD spectroscopy and exotics session also featured important findings, including the discovery of novel baryon states, notably Ξ_b(6087)⁰ and Ξ_b(6095)⁰. Pentaquark exploration involved diverse charm–hadron combinations, alongside precision measurements of the Ω⁰_c mass and first observations of b-hadron decays with potential exotic-state contributions. Charmonia-associated production provided fresh insights for testing QCD predictions, and an approach based on effective field theory (EFT) interpreting pentaquarks as hadronic molecules was presented. A new model-independent Born–Oppenheimer EFT framework for the interpretation of doubly heavy tetraquarks, utilising lattice QCD predictions, was introduced. Scrutinising charm–tetraquark decays and the interpretation of newly discovered hadron states at the LHC were also discussed.

During the flavour-changing neutral-current session a new analysis of B⁰ → K^*⁰μ⁺μ^– decays was presented, showing consistency with SM expectations. Stringent limits on branching fractions of rare charm decays and precise differential branching fraction measurements of b-baryon decays were also highlighted. Challenges in SM predictions for b → sℓℓ and rare charm decays were discussed, underscoring the imperative for a deeper comprehension of underlying hadronic processes, particularly leveraging LHCb data. Global analyses of b → dℓℓ and b → sℓℓ decays were presented, alongside future prospects for these decays in Run 3 and beyond. The session also explored strategies to enhance sensitivity to new physics in B^± → π^±μ⁺μ^– decays.

The keynote talk, delivered by Svjetlana Fajfer, offered a comprehensive summary and highlighted existing anomalies that demand further consideration. Tackling these challenges necessitates precise measurements at both low and high energies, with the collaborative efforts of LHCb, Belle II, CMS and ATLAS. Additionally, advancements in lattice QCD and other novel theoretical approaches are needed for precise theoretical predictions in tandem with experimental efforts.

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.

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.”

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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