Light Collision Research Articles

Small-x structure of oxygen and neon isotopes as seen by the Large Hadron Collider

Results on collisions of 16O nuclei performed at the Relativistic Heavy Ion Collider (RHIC) have been presented for the first time at Quark Matter 2023 by the STAR collaboration. 16O+16O collisions are also expected to take place in the near future at the Large Hadron Collider (LHC) at much higher beam energies. We explore the potential of beam-energy-dependent studies for this system to probe small-x dynamics and QCD evolution. We perform 3 + 1D IP-Glasma simulations to predict the rapidity dependence of the initial geometry of light-ion collisions, focusing on 16O+16O and 20Ne+20Ne collisions at √SNN = 70 GeV and 7 TeV. The choice of 20Ne is motivated by its strongly elongated geometry, which may respond differently to the effect of the highenergy evolution compared to the more spherical 16O. We find that smearing induced by soft gluon production at high energy causes mild variations in the initial-state eccentricities as a function of the collision energy. These effects could be resolved in future experiments and deserve further investigation.

Jet quenching from heavy to light ion collisions

We perform an analysis of jet quenching in heavy and light ion collisions for scenarios without and with quark-gluon plasma formation in pp collisions. We find that the results for these scenarios are very similar, and both of them are in reasonable agreement with data for heavy ion collisions. However, their results become differ significantly for light nuclei. Using the parameters fitted to heavy ion data on the nuclear modification factor RAA, we make predictions for 0.2 and 7 TeV O+O collisions that can be verified by future experiments at RHIC and the LHC.

Correlations of High-pT Hadrons and Jets in ALICE

There are two prominent experimental signatures of quark–gluon plasma creation in ultra-relativistic heavy-ion collisions: the jet quenching phenomenon and the azimuthal-momentum space-anisotropy of final-state particle emission. Recently, the latter signature was also observed in lighter collision systems such as p–Pb or pp. This raises a natural question of whether in these systems, the observed collectivity is also accompanied by jet quenching. In this paper, we overview ALICE measurements of the jet quenching phenomenon studied using semi-inclusive distributions of track-based jets recoiling from a high-transverse momentum ( p T ) hadron trigger in Pb–Pb and p–Pb collisions at LHC energies. The constructed coincidence observable, the per trigger normalized yield of associated recoil jets, is corrected for the complex uncorrelated jet background, including multi-partonic interactions, using a data-driven statistical subtraction method. In the p–Pb data, the observable was measured in events with different underlying event activity and was utilized to set an upper limit on the average medium-induced out-of-cone energy transport for jets with resolution parameter R = 0.4 . The associated jet momentum shift was found to be less than 0.4 GeV/c at 90% confidence.

The onsets of deconfinement and fireball of NA61/SHINE

The NA61/SHINE experiment at the CERN SPS is pursuing a rich programme on strong interactions, covering the study of onset of deconfinement and aims to discover the critical point of strongly interacting matter by performing an energy and system-size scan at the full CERN SPS momentum range. These scans of p+p, p+Pb, Be+Be, Ar+Sc and Pb+Pb have been mostly completed with Xe+La last year (more Pb+Pb to to taken this year). Results from the different reactions are now emerging. As a surprise, some measurements did not scale smoothly. In particular, for the K+/π+ ratio, Be+Be collisions behaved similarly to p+p (as superposition of nucleon collisions), while Ar+Sc was closer to Pb+Pb collisions. This step can not be explained by onset of deconfinement, and may indicate that there is also a onset of fireball in relativistic heavy ion collisions. A review of the results as well as possible interpretation will be presented. The theoretical models (SMES, PHSD) describe onset of deconfinement at the heaviest system relatively well. However, no model describes the behaviour of data at previously unmeasured collisions of light and intermediate size ions. The onset of fireball is not described by any model.

Anisotropic (v1 and v2) Flow in Relativistic Heavy-Ion Collisions at Energies between 4 GeV and 200 GeV

Basic features of directed and elliptic flows of identified hadrons in heavy-ion collisions at intermediate and high energies are considered within two transport string models, UrQMD and QGSM. Both models indicate changing of the sign of proton directed flow at midrapidity from antiflow to normal flow with decreasing energy of collisions. The origin of this effect is traced to hadron rescattering in baryon-rich remnants of the colliding nuclei. To distinguish the effect of rescattering from the flow softening caused by creation of quark-gluon plasma one has to compare heavy-ion and light-ion collisions at the same energy. Both directed and elliptic flows at midrapidity are formed within t = 10-12 fm/c. The differences in the development of elliptic flows of mesons and baryons are found at high energies. These differences can be explained by dissimilar freeze-out conditions, thus suggesting simultaneous study of particle collective flow and freeze-out.

Results of the LHCf experiment and the forward measurements at the LHC

The Large Hadron Collider is intensively used to test hadronic interaction models used in cosmic-ray physics because its maximum (designed) collision energy √s = 14 TeV corresponds to the interaction of a 1017 eV cosmic-ray proton hitting the atmosphere. In this paper various types of forward experiments at the LHC, where the particles relevant to the air shower development are observed, are reviewed. Recent results of a dedicated experiment for precise forward measurement, the LHC forward, are discussed in detail. A future possibility at the LHC, light ion collisions, is also discussed.

Complete Glauber calculations of reaction and interaction cross sections for light-ion collisions

The differences of the interaction and reaction cross sections for collisions between relativistic light ions of mass number in the range of A < 40 are calculated on the basis of Glauber theory. In the optical approximation of Glauber theory, these differences are about 1% of the interaction cross sections. Upon taking into account all diagrams in Glauber theory, these differences reach 3 to 4% of the interaction cross section.

Two-gluon correlations in heavy–light ion collisions

Abstract We derive the cross-section for two-gluon production in heavy–light ion collisions in the saturation/Color Glass Condensate framework. This calculation includes saturation effects to all orders in one of the nuclei (heavy ion) along with a single saturation correction in the projectile (light ion). The calculation of the correlation function predicts (qualitatively) two identical ridge-like correlations, near- and away-side. This prediction was later supported by experimental findings in p + A collisions at the LHC. Concentrating on the energy and geometry dependence of the correlation functions we find that the correlation function is nearly center-of-mass energy independent. The geometry dependence of the correlation function leads to an enhancement of near- and away-side correlations for the tip-on-tip U + U collisions when compared with side-on-side U + U collisions, an exactly opposite behavior from the correlations generated by the elliptic flow of the quark–gluon plasma.

Projectile electron loss in collisions of light charged ions with helium

We investigate the single-electron loss processes of light charged ions (Li1+,2+, C2+,3+,5+, and O2+,3+) in collisions with helium. To better understand the experimental results, we propose a theoretical model to calculate the cross section of projectile electron loss. In this model, an ionization radius of the incident ion was defined under the classical over-barrier model, and we developed “strings" to explain the processes of projectile electron loss, which is similar with the molecular over-barrier model. Theoretical calculations are in good agreement with the experimental results for the cross section of single-electron loss and the ratio of double-to-single ionization of helium associated with one-electron loss.

Multiplicity distributions inp+p,p+A, andA+Acollisions from Yang-Mills dynamics

We compute transverse momentum and momentum integrated multiplicity distributions consistently in the IP-Glasma model for proton-proton and proton-lead collisions at the LHC, in deuteron-gold collisions at RHIC, and in heavy ion collisions at both RHIC and LHC energies. Several sources of sub-nucleon scale contributions to the multiplicity distributions are identified. Our results, which are constrained by inclusive and diffractive deeply inelastic scattering data from HERA, are compared to measured distributions for a range of collision energies. These results are an essential first step in quantifying the relative role of initial and final state effects on multiparticle correlations in light and heavy ion collisions.

Dence Cold Matter

Possible way to create dense cold baryonic matter in the laboratory is discussed. The density of this matter is comparable or even larger than the density of neutron star core. The properties of this matter can be controlled by trigger conditions. Experimental program for the study of properties of dense cold matter for light and heavy ion collisions at initial energy range √sNN~2-3GeV is proposed..

Long-range rapidity correlations in heavy–light ion collisions

We study two-particle long-range rapidity correlations arising in the early stages of heavy ion collisions in the saturation/Color Glass Condensate framework, assuming for simplicity that one colliding nucleus is much larger than the other. We calculate the two-gluon production cross section while including all-order saturation effects in the heavy nucleus with the lowest-order rescattering in the lighter nucleus. We find four types of correlations in the two-gluon production cross section: (i) geometric correlations, (ii) HBT correlations accompanied by a back-to-back maximum, (iii) away-side correlations, and (iv) near-side azimuthal correlations which are long-range in rapidity. The geometric correlations (i) are due to the fact that nucleons are correlated by simply being confined within the same nucleus and may lead to long-range rapidity correlations for the produced particles without strong azimuthal angle dependence. Somewhat surprisingly, long-range rapidity correlations (iii) and (iv) have exactly the same amplitudes along with azimuthal and rapidity shapes: one centered around Δ ϕ = π with the other one centered around Δ ϕ = 0 (here Δ ϕ is the azimuthal angle between the two produced gluons). We thus observe that the early-time CGC dynamics in nucleus–nucleus collisions generates azimuthal non-flow correlations which are qualitatively different from jet correlations by being long-range in rapidity. If strong enough, they have the potential of mimicking the elliptic (and higher-order even-harmonic) flow in the di-hadron correlators: one may need to take them into account in the experimental determination of the flow observables.

Some Aspects of Transfer Reactions in Light and Heavy Ion Collisions

Transfer reactions played a key role in unraveling many properties of the nucleus, in particular they have been essential to define the relevance of the independent particle motion and to define the properties of two-particle correlations. In view of the now available radioactive beams, it is important to summarize some aspects of the transfer processes that will be relevant to define the properties of nuclei close to the neutron and proton drip lines.

Production of hypernuclei in peripheral collisions of relativistic ions

Abstract Formation of hypernuclei in peripheral collisions of relativistic light and heavy ions is studied theoretically within the transport and statistical approaches. New mechanisms for the formation of strange nuclear systems via capture of hyperons by slightly excited spectator matter and their subsequent disintegration are investigated. These processes lead to production of specific and exotic hypernuclei, which may not be accessible in other reactions. Similar mechanisms processing via absorption of strange particles by nuclei can take place in reactions initiated by electrons, antiprotons and other hadrons. It is demonstrated that our approach is consistent with experimental data.

Influence of the isotope effect on the charge exchange in slow collisions of Li, Be, and C ions with H, D, and T

The influence of the isotope effect (mass dependence) on the charge-exchange process in low-energy collisions of light ions with hydrogen isotopes (H, D, and T) is studied using the adiabatic theory of transitions in slow collisions developed by E. Solov'ev [Sov. Phys. Usp. 32, 228 (1989)]. Results of the numerical calculations are presented for the charge-exchange probabilities and cross sections of Li, Be, and C ions colliding with hydrogen isotopes and for the inverse reactions.

Exclusive measurement of two-pion production in the [formula omitted] reaction

The results from the first kinematically complete measurement of the d d → He 4 π π reaction are reported. The aim was to investigate a long standing puzzle regarding the origin of the peculiar ππ -invariant mass distributions appearing in double pion production in light ion collisions, the so-called ABC effect. The measurements were performed at the incident deuteron energies of 712 MeV and 1029 MeV, with the WASA detector assembly at CELSIUS in Uppsala, Sweden. We report the observation of a characteristic enhancement at low ππ -invariant mass at 712 MeV, the lowest energy yet. At the higher energy, in addition to confirming previous experimental observations, our results reveal a strong angular dependence of the pions in the overall centre of mass system. The results are qualitatively reproduced by a theoretical model, according to which the ABC effect is described as resulting from a kinematical enhancement in the production of the pion pairs from two parallel and independent N N → d π sub-processes.

The WASA detector facility at CELSIUS

The WASA 4 pi multidetector system, aimed at investigating light meson production in light ion collisions and eta meson rare decays at the CELSIUS storage ring in Uppsala is presented. A unique fea ...

Open Heavy Flavor Production from Light Ion Collisions at RHIC

PHENIX measures leptons at mid and forward rapidities and extracts leptons resulting from semi-leptonic decays of heavy quarks. We present PHENIX results related to heavy quark production, specifically the invariant cross section of the non-photonic single electrons produced at midrapidity in p+p and d+Au collisions at sqrt(sNN) = 200 GeV. The measured cross section for {\it p+p} collisions is compared with the NLO prediction, and a possible excess over the prediction is noted. The measured cross section for {\it d+Au} collisions scales with the number of colliding nucleon pairs from the {\it p+p} spectra.

System size dependence of hadron production in A+A collisions at 40 A GeV beam energy

Rapidity and transverse mass spectra of charged kaons and negative pions measured in C+C, Si+Si and minimum bias Pb+Pb collisions at 40 A GeV are reported. A monotonous increase of the inverse slope parameter with system size is observed. The ⟨K+⟩/⟨K−⟩ ratio rises with ⟨NW⟩ from the light ion collisions to peripheral Pb+Pb and stays constant from there on. A steep rise of the ⟨K+⟩/⟨π+⟩ ratio is observed for the small systems with the ratio in Si+Si collisions being slightly higher than in peripheral Pb+Pb collisions. The measurement of the ⟨K−⟩/⟨π−⟩ ratio in Si+Si collisions is significantly higher than the ratio in peripheral Pb+Pb collisions; it is rather close to the observed ⟨K−⟩/⟨π−⟩ ratio in central Pb+Pb collisions.

CELSIUS PHYSICS: PAST, PRESENT AND FUTURE

The experimental program on meson production in light-ion collisions at the CELSIUS cooler/storage ring of the The Svedberg Laboratory in Uppsala, is reviewed. Production of pions and eta-mesons have been studied in pp and pd collisions using the PROMICE/WASA detector. This program is now continued at CELSIUS with the WASA4π detector together with studies of meson decays.