Ultrarelativistic Energies Research Articles

Multiplicity fluctuations and rapidity correlations in ultracentral proton-nucleus collisions

A collision between a proton and a heavy nucleus at ultrarelativistic energy creates particles whose rapidity distribution is asymmetric, with more particles emitted in the direction of the nucleus than in the direction of the proton. This asymmetry becomes more pronounced as the centrality estimator, defined from the energy deposited in a calorimeter, increases. We argue that for high-multiplicity collisions, the variation of the impact parameter plays a negligible role, and that the fluctuations of the multiplicity and of the centrality estimator are dominated by quantum fluctuations, whose probability distribution can be well approximated by a correlated gamma distribution. We show that this simple model reproduces existing data, and we make quantitative predictions for collisions in the 0−0.1% and 0−0.01% centrality windows. We argue that by repeating the same analysis with a different centrality estimator, one can obtain direct information about the rapidity decorrelation in particle production.

EPN2EOS Data Transfer System

EPN2EOS Data Transfer System

Mechanisms for cluster production in heavy-ion collisions near midrapidity

The formation of weakly bound clusters and hypernuclei in the hot and dense environment at midrapidity is a surprising phenomenon observed experimentally in heavy-ion collisions, spanning from low SIS to ultra-relativistic LHC energies. This occurrence, often referred to as the ’ice in a fire’ puzzle, has prompted the exploration of three distinct approaches to elucidate cluster formation: the potential mechanism, involving cluster formation throughout the entire heavy-ion collision via potential interactions between nucleons; the kinetic mechanism, entailing deuteron production through catalytic hadronic reactions; and coalescence at kinetic freeze-out. In this context, we discuss the observables sensitive to the mechanism of cluster production, utilizing a microscopic transport Parton-Hadron-Quantum Molecular (PHQMD) approach.

RF Electron Source Design Modelling for Optimization of Initial e- Beam Parameters for LCS γ-Ray System

"The paper presents results from computer simulations performed on a model of an RF electron source for the Laser Compton Scattering (LCS) γ-ray system. The new LCS γ-ray system is under implementation at ELI-NP/IFIN-HH. Different configurations of the system modelled with the ASTRA software package and different setings on the system components were analysed to optimize the inital beam parameters before acceleration up to ultra-relativistic energy in linac. The properly designed RF electron source contibutes significantly towards delivery a high brightness beam, which is a crucial specification for the LCS γ-rays systems."

Determination of the Neutron Skin of ^{208}Pb from Ultrarelativistic Nuclear Collisions.

Emergent bulk properties of matter governed by the strong nuclear force give rise to physical phenomena across vastly different scales, ranging from the shape of atomic nuclei to the masses and radii of neutron stars. They can be accessed on Earth by measuring the spatial extent of the outer skin made of neutrons that characterizes the surface of heavy nuclei. The isotope ^{208}Pb, owing to its simple structure and neutron excess, has been in this context the target of many dedicated efforts. Here, we determine the neutron skin from measurements of particle distributions and their collective flow in ^{208}Pb+^{208}Pb collisions at ultrarelativistic energy performed at the Large Hadron Collider, which are mediated by interactions of gluons and thus sensitive to the overall size of the colliding ^{208}Pb ions. By means of state-of-the-art global analysis tools within the hydrodynamic model of heavy-ion collisions, we infer a neutron skin Δr_{np}=0.217±0.058 fm, consistent with nuclear theory predictions, and competitive in accuracy with a recent determination from parity-violating asymmetries in polarized electron scattering. We establish thus a new experimental method to systematically measure neutron distributions in the ground state of atomic nuclei.

Radio Pulsars Resonantly Accelerating Electrons

Based on the recently demonstrated resonant wave–wave process, it is shown that electrons can be accelerated to ultra-relativistic energies in the magnetospheres of radio pulsars. The energization occurs via the resonant interaction of the electron wave (described by the Klein–Gordon (KG) equation) moving in unison with an intense electromagnetic (EM) wave; the KG wave/particle continuously draws energy from EM. In a brief recapitulation of the general theory, the high-energy (resonantly enhanced) electron states are investigated by solving the KG equation, minimally coupled to the EM field. The restricted class of solutions that propagate in phase with EM radiation (functions only of ζ=ωt−kz) are explored to serve as a possible basis for the proposed electron energization in the radio pulsars. We show that the wave–wave resonant energization mechanism could be operative in a broad class of radio pulsars with periods ranging from milliseconds to normal values (∼1 s); this could drive the magnetospheric electrons to acquire energies from 100 s of TeVs (millisecond pulsars) to 10 ZeVs (normal pulsars).

Positron Generation and Acceleration in a Self-Organized Photon Collider Enabled by an Ultraintense Laser Pulse.

We discovered a simple regime where a near-critical plasma irradiated by a laser of experimentally available intensity can self-organize to produce positrons and accelerate them to ultrarelativistic energies. The laser pulse piles up electrons at its leading edge, producing a strong longitudinal plasma electric field. The field creates a moving gamma-ray collider that generates positrons via the linear Breit-Wheeler process-annihilation of two gamma rays into an electron-positron pair. At the same time, the plasma field, rather than the laser, serves as an accelerator for the positrons. The discovery of positron acceleration was enabled by a first-of-its-kind kinetic simulation that generates pairs via photon-photon collisions. Using available laser intensities of 10^{22} W/cm^{2}, the discovered regime can generate a GeV positron beam with a divergence angle of around 10° and a total charge of 0.1pC. The result paves the way to experimental observation of the linear Breit-Wheeler process and to applications requiring positron beams.

Particle motion in ultra-strong electromagnetic fields of neutron stars: The influence of radiation reaction

Context. Neutron stars are known to be efficient accelerators that produce particles with ultra-relativistic energies. As a by-product, they also emit copious amounts of photons from radio wavelengths up to gamma rays. Aims. As a follow-up to our previous work on particle acceleration simulation near neutron stars, in this paper, we discuss the impact of radiation reaction on test particles injected into their magnetosphere. We therefore neglect the interaction between particles through the electromagnetic field as well as gravitation. Methods. We integrate numerically the reduced Landau-Lifshitz equation for electrons and protons in the vacuum field of a rotating magnetic dipole based on analytical solutions in a constant electromagnetic field. These expressions are simple in a frame where the electric and magnetic field are parallel. Lorentz transforms are used to switch back and forth between this frame and the observer frame. Results. We found that, though due solely to the Lorentz force, electrons reach Lorentz factors up to γ = 1014 and protons reach them up to γ = 1010.7. When radiation reaction is enabled, electrons reach energies up to γ = 1010.5 and protons reach energies up to γ = 108.3. The second set of values are more realistic since the radiation reaction feedback is predominant within the magnetosphere. Moreover, as expected, symmetrical behaviours between the north and south hemispheres are highlighted, either with respect to the location around the neutron star or with respect to particles of opposite charge to mass ratio (q/m). Consequently, it is useless to simulate the full set of geometrical parameters in an effort to obtain an overview of all possibilities. Conclusions. The study of the influence of the magnetic dipolar moment inclination shows similar behaviours regardless of whether radiation reaction is enabled. Protons (respectively electrons) impact the surface of the neutron star less as the inclination angle increases (decreases for electrons), while if the rotation and magnetic axes are aligned, all the protons impact the neutron star, and all the electrons impact the surface if the rotation and magnetic axes are anti-aligned. Similarly, we still find that particles are ejected away from the neutron star, in some preferred directions and Lorentz factors.

Commissioning and first performances of the ALICE MID RPCs

ALICE (A Large Ion Collider Experiment) at the CERN Large Hadron Collider (LHC) is designed to study pp and Pb–Pb collisions at ultra-relativistic energies. ALICE is equipped with a Muon Spectrometer (MS) to study the heavy charmonia in pp and heavy ion collisions via their muonic decay. At first, in the LHC Run 1 and 2 the selection of interesting events for muon physics in the MS was performed with a dedicated Muon Trigger system based on Resistive Plate Chambers (RPCs) operated in maxi-avalanche mode. During the LHC Long Shutdown 2 (LS2), ALICE underwent a major upgrade of its apparatus in order to fully profit from the increased luminosity of Pb–Pb collisions (from 20 kHz in Run 2 to 50 kHz in Run 3). Many sub-systems of the ALICE experiment are now running in continuous readout (triggerless). In this context the Muon Trigger system became the Muon IDentifier (MID). In order to reduce the RPC ageing and to increase the rate capability, it was decided to use a new front-end electronics FEERIC with a pre-amplification stage to minimize the charge released per hit inside the gas gap. A description of the MID upgrades, together with the results and performances of the RPCs from the commissioning, is presented in this paper.

Shifting physics of vortex particles to higher energies via quantum entanglement

Physics of structured waves is currently limited to relatively small particle energies as the available generation techniques are only applicable to the soft X-ray twisted photons, to the beams of electron microscopes, to cold neutrons, or non-relativistic atoms. The highly energetic vortex particles with an orbital angular momentum would come in handy for a number of experiments in atomic physics, nuclear, hadronic, and accelerator physics, and to generate them one needs to develop alternative methods, applicable for ultrarelativistic energies and for composite particles. Here, we show that the vortex states of in principle arbitrary particles can be generated during photon emission in helical undulators, via Cherenkov radiation, in collisions of charged particles with intense laser beams, in such scattering or annihilation processes as emu rightarrow emu , ep rightarrow ep, e^-e^+ rightarrow pbar{p}, and so forth. The key element in obtaining them is the postselection protocol due to entanglement between a pair of final particles and it is largely not the process itself. The state of a final particle – be it a gamma -ray, a hadron, a nucleus, or an ion – becomes twisted if the azimuthal angle of the other particle momentum is measured with a large error or is not measured at all. As a result, requirements to the beam transverse coherence can be greatly relaxed, which enables the generation of highly energetic vortex beams at accelerators and synchrotron radiation facilities, thus making them a new tool for hadronic and spin studies.

Hadron gas in the presence of a magnetic field using non-extensive statistics: a transition from diamagnetic to paramagnetic system

Non-central heavy-ion collisions at ultra-relativistic energies are unique in producing magnetic fields of the largest strength in the laboratory. Such fields being produced at the early stages of the collision could affect the properties of Quantum Chromodynamics matter formed in the relativistic heavy-ion collisions. The transient magnetic field leaves its reminiscence, which in principle, can affect the thermodynamic and transport properties of the final state dynamics of the system. In this work, we study the thermodynamic properties of a hadron gas in the presence of an external static magnetic field using a thermodynamically consistent non-extensive Tsallis distribution function. Various thermodynamical observables such as energy density (ϵ), entropy density (s), pressure (P) and speed of sound (c s) are studied. Investigation of magnetization (M) is also performed and this analysis reveals an interplay of diamagnetic and paramagnetic nature of the system in the presence of a magnetic field of varying strength. Further, to understand the system dynamics under equilibrium and non-equilibrium conditions, the effect of the non-extensive parameter (q) on the above observables is also studied.

Chiral symmetry restoration at high matter density observed in pionic atoms

According to quantum chromodynamics, vacuum is not an empty space, because it is filled with quark–antiquark pairs. The pair has the same quantum numbers as the vacuum and forms a condensate because the strong interaction of the quantum chromodynamics is too strong to leave the vacuum empty. This quark–antiquark condensation, the chiral condensate, breaks the chiral symmetry of the vacuum. The expectation value of the chiral condensate is an order parameter of the chiral symmetry, which is expected to decrease at high temperatures or high matter densities where the chiral symmetry is partially restored. Head-on collisions of nuclei at ultra-relativistic energies have explored the high-temperature regime, but experiments at high densities are rare. Here we measure the spectrum of pionic 121Sn atoms and study the interaction between the pion and the nucleus. We find that the expectation value of the chiral condensate is reduced at finite density compared to the value in vacuum. The reduction is linearly extrapolated to the nuclear saturation density and indicates that the chiral symmetry is partially restored due to the extremely high density of the nucleus. In quantum chromodynamics, the condensation of quark–antiquark pairs breaks the chiral symmetry of vacuum. Experiments with pionic tin atoms demonstrate that the symmetry is partially restored at high densities.

Plasma heating and particle acceleration in collisionless shocks through astrophysical observations

Supernova remnants (SNRs), the products of stellar explosions, are powerful astrophysical laboratories, which allow us to study the physics of collisionless shocks, thanks to their bright electromagnetic emission. Blast wave shocks generated by supernovae (SNe) provide us with an observational window to study extreme conditions, characterized by high Mach (and Alfvénic Mach) numbers, together with powerful nonthermal processes. In collisionless shocks, temperature equilibration between different species may not be reached at the shock front. In this framework, different particle species may be heated at different temperatures (depending on their mass) in the post-shock medium of SNRs. SNRs are also characterized by broadband nonthermal emission stemming from the shock front as a result of nonthermal populations of leptons and hadrons. These particles, known as cosmic rays, are accelerated up to ultrarelativistic energies via diffusive shock acceleration. If SNRs lose a significant fraction of their ram energy to accelerate cosmic rays, the shock dynamics should be altered with respect to the adiabatic case. This shock modification should result in an increase in the total shock compression ratio with respect to the Rankine–Hugoniot value of 4. Here, I show that the combination of x-ray high resolution spectroscopy (to measure ion temperatures) and moderate resolution spectroscopy (for a detailed diagnostic of the post-shock density) can be exploited to study both the heating mechanism and the particle acceleration in collisionless shocks. I report on new results on the temperatures measured for different ion species in the remnant of the SN observed in 1987 in the Large Magellanic Cloud (SN 1987A). I also discuss evidence of shock modification recently obtained in the remnant of SN 1006 a. D., where the shock compression ratio increases significantly as the angle between the shock velocity and the ambient magnetic field is reduced.

Bremsstrahlung as a probe of baryon stopping in heavy-ion collisions

In collisions between heavy ions at ultra-relativistic energies the participating protons lose energy, which is converted into new particles. As the protons slow down, they emit bremsstrahlung radiation. The yield and angular distribution of the emitted radiation are sensitive probes of how much energy the incoming protons have lost. In this paper, the spectrum of bremsstrahlung radiation is calculated for different stopping scenarios, and the results are compared with the expected yield of photons from hadronic interactions.

Localized mesospheric ozone destruction corresponding to isolated proton aurora coming from Earth’s radiation belt

Relativistic electron precipitation (REP) from the Earth’s radiation belt plays an important role in mesospheric ozone loss as a connection between space weather and the climate system. However, the rapid (tens of minutes) destruction of mesospheric ozone directly caused by REP has remained poorly understood due to the difficulty of recognizing its location and duration. Here we show a compelling rapid correspondence between localized REP and ozone destruction during a specific auroral phenomenon, the called an isolated proton aurora (IPA). The IPA from the Earth’s radiation belt becomes an important spatial and temporal proxy of REP, distinct from other auroral phenomena, and allowing visualizing micro-ozone holes. We found ozone destruction of as much as 10–60% within 1.5 h of the initiation of IPA. Electromagnetic ion cyclotron waves in the oxygen ion band observed as the driver of REP likely affect through resonance with mainly ultra-relativistic (> 2 mega-electron-volts) energy electrons. The rapid REP impact demonstrates its crucial role and direct effect on regulating the atmospheric chemical balance.

An event of extreme relativistic and ultra-relativistic electron enhancements following the arrival of consecutive corotating interaction regions: Coordinated observations by Van Allen Probes, Arase, THEMIS and Galileo satellites

During July to October of 2019, a sequence of isolated Corotating Interaction Regions (CIRs) impacted the magnetosphere, for four consecutive solar rotations, without any interposed Interplanetary Coronal Mass Ejections. Even though the series of CIRs resulted in relatively weak geomagnetic storms, the net effect of the outer radiation belt during each disturbance was different, depending on the electron energy. During the August-September CIR group, significant multi-MeV electron enhancements occurred, up to ultra-relativistic energies of 9.9 MeV in the heart of the outer Van Allen radiation belt. These characteristics deemed this time period a fine case for studying the different electron acceleration mechanisms. In order to do this, we exploited coordinated data from the Van Allen Probes, the Time History of Events and Macroscale Interactions during Substorms Mission (THEMIS), Arase and Galileo satellites, covering seed, relativistic and ultra-relativistic electron populations, investigating their Phase Space Density (PSD) profile dependence on the values of the second adiabatic invariant K, ranging from near-equatorial to off equatorial mirroring populations. Our results indicate that different acceleration mechanisms took place for different electron energies. The PSD profiles were dependent not only on the μ value, but also on the K value, with higher K values corresponding to more pronounced local acceleration by chorus waves. The 9.9 MeV electrons were enhanced prior to the 7.7 MeV, indicating that different mechanisms took effect on different populations. Finally, all ultra-relativistic enhancements took place below geosynchronous orbit, emphasizing the need for more Medium Earth Orbit (MEO) missions.

Harmonic flow correlations in Au+Au reactions at 1.23 AGeV: a new testing ground for the equation-of-state and expansion geometry

Correlations between the harmonic flow coefficients v_1, v_2, v_3 and v_4 of nucleons in semi-peripheral Au+Au collisions at a beam energy of 1.23 AGeV are investigated within the hadronic transport approach ultra-relativistic quantum molecular dynamics (UrQMD). In contrast to ultra-relativistic collision energies (where the flow coefficients are evaluated with respect to the respective event plane), we predict strong correlations between the flow harmonics with respect to the reaction plane. Based on an event-by-event selection of the midrapidity final state elliptic flow of nucleons we show that as a function of rapidity, (I) the sign of the triangular flow changes, (II) that the shape of v_4 changes from convex to concave, and (III) that v_3propto v_1v_2 and v_4propto v_2^2 for all different event classes, indicating strong correlations between all investigated harmonic flow coefficients.

Unruh Effect and Information Entropy Approach

The Unruh effect can be considered a source of particle production. The idea has been widely employed in order to explain multiparticle production in hadronic and heavy-ion collisions at ultrarelativistic energies. The attractive feature of the application of the Unruh effect as a possible mechanism of the multiparticle production is the thermalized spectra of newly produced particles. In the present paper, the total entropy generated by the Unruh effect is calculated within the framework of information theory. In contrast to previous studies, here the calculations are conducted for the finite time of existence of the non-inertial reference frame. In this case, only a finite number of particles are produced. The dependence on the mass of the emitted particles is taken into account. Analytic expression for the entropy of radiated boson and fermion spectra is derived. We study also its asymptotics corresponding to low- and high-acceleration limiting cases. The obtained results can be further generalized to other intrinsic degrees of freedom of the emitted particles, such as spin and electric charge.

Exploring the effect of hadron cascade time on particle production in Xe+Xe collisions at sNN=5.44 TeV using a multiphase transport model

Heavy-ion collisions at ultrarelativistic energies provide extreme conditions of energy density and temperature to produce a deconfined state of quarks and gluons. Xenon (Xe), being a deformed nucleus, further gives access to the effect of initial geometry on final state particle production. This study focuses on the effect of nuclear deformation and hadron cascade-time on the particle production and elliptic flow using a multiphase transport (AMPT) model in $\mathrm{Xe}+\mathrm{Xe}$ collisions at $\sqrt{{s}_{NN}}=5.44\phantom{\rule{0.16em}{0ex}}\mathrm{TeV}$. We explore the effect of hadronic cascade time on identified particle production through the study of ${p}_{\mathrm{T}}$-differential particle ratios. The effect of hadronic cascade time on the generation of elliptic flow is studied by varying the cascade time between 5 and $25\phantom{\rule{0.16em}{0ex}}\mathrm{fm}/c$. This study shows the final state interactions among particles generate additional anisotropic flow with increasing hadron cascade time, especially at very low and high ${p}_{\mathrm{T}}$.

A Possible Gamma-Ray Enhancement Event in Tycho's Supernova Remnant

We report a possible γ-ray enhancement event detected from Tycho’s supernova remnant (SNR), the outcome of a type Ia supernova explosion that occurred in the year 1572. The event lasted for 1.5 yr and showed a factor of 3.6 flux increase mainly in the energy range of 4–100 GeV, while notably accompanied with two 478 GeV photons. Several young SNRs (including Tycho’s SNR) were previously found to show peculiar X-ray structures with flux variations in one- or several-year timescales, such an event at γ-ray energies is for the first time seen. The year-long timescale of the event suggests a synchrotron radiation process, but the hard γ-ray emission requires extreme conditions of either ultrahigh energies for the electrons up to ∼10 PeV (well above the cosmic-ray knee energy) or high inhomogeneity of the magnetic field in the SNR. This event in Tycho’s SNR is likely analogous to the γ-ray flares observed in the Crab Nebula, the comparably short timescales of them both requiring a synchrotron process, and similar magnetohydrodynamic processes such as magnetic reconnection would be at work as well in the SNR to accelerate particles to ultrarelativistic energies. The event, if confirmed, helps reveal the more complicated side of the physical processes that can occur in young SNRs.