Collision Systems Research Articles

High-pT suppression in small systems

We present first results for leading hadron suppression in small collision systems, from a convolved radiative and collisional pQCD energy loss model which receives a short path length correction to the radiative energy loss. We find that the short path length correction is exceptionally large for light flavor final states at high-pT , for all system sizes. We numerically investigate the self-consistency of various assumptions underlying the radiative energy loss model, in an effort to understand the size of the short path length correction. This calculation shows that the large formation time assumption, which is utilized by most contemporary energy loss models, is invalid for a large portion of the phenomenologically relevant parameter space.

3D multi-system Bayesian calibration with energy conservation to study rapidity-dependent dynamics of nuclear collisions

Considerable information about the early-stage dynamics of heavyion collisions is encoded in the rapidity dependence of measurements. To leverage the large amount of experimental data, we perform a systematic analysis using three-dimensional hydrodynamic simulations of multiple collision systems — large and small, symmetric and asymmetric. Specifically, we perform fully 3D multi-stage hydrodynamic simulations initialized by a parameterized model for rapidity-dependent energy deposition, which we calibrate on the hadron multiplicity and anisotropic flow coefficients. We utilize Bayesian inference to constrain properties of the early- and late-time dynamics of the system, and highlight the impact of enforcing global energy conservation in our 3D model.

Insights on strange quark hadronization in small collision system with ALICE: Multiple strange hadrons and Σ± baryons

Among the most iconic results of Run-1 and Run-2 of the LHC is the observation of enhanced production of (multi-)strange to non-strange particles, gradually rising from low-multiplicity to high-multiplicity pp or p–Pb collisions and reaching values close to those measured in peripheral Pb–Pb collisions [1]. The observed behaviour cannot be quantitatively reproduced by any of the available QCD-inspired MC generators. In this contribution two extensions of this study are presented: the measurement of Σ baryons and the first measurement of the full Probability Density Function (PDF) for KS0, Λ, Ξ− and Ω−, therefore extending the study of strangeness production beyond the average of the distribution. This novel method represents a unique opportunity to test the connection between charged and strange particle multiplicity production.

Insight into the light-flavour particle production mechanism from studies of the transverse spherocity dependence in pp collisions at √s =13 TeV with ALICE at the LHC

The Large Hadron Collider (LHC) Run 1 and Run 2 data revealed heavy-ion-like features such as enhanced strangeness production and long-range azimuthal correlation in high-multiplicity pp collisions paving the way to rethink particle production in small collision systems. Event shape observables like transverse spherocity are sensitive to isotropic and jet-like topologies, which are useful tools to distinguish the pp collisions dominated by soft or hard physics. The interplay between multiplicity and transverse spherocity on light-flavour particle production can be understood by comparing the results obtained by selecting multiplicity and/or transverse spherocity. This contribution presents recent results on light-flavour particle production (π, K, p, ф, K*0, K0s, Λ, ξ) at midrapidity obtained by the ALICE experiment in pp collisions at √s = 13 TeV as a function of event multiplicities and transverse spherocity. The results are even obtained by going to the most extreme selections such as the highest 0–1% in multiplicity and the highest 0–10% in transverse sphe rocity. The results include the transverse momentum spectra, yields, 〈pT〉 and their ratios. These measurements will be compared with the Monte Carlo (MC) predictions obtained from models such as PYTHIA8, EPOS and Herwig7.

The role of longitudinal decorrelations for measurements of anisotropic flow in small collision systems

Within a (3+1)D viscous hydrodynamic model we compute anisotropic flow in small system collisions as performed at the Relativistic Heavy Ion Collider and measured by the STAR and PHENIX Collaborations. We emphasize the importance of the rapidity dependence of the geometry for interpreting the differences encountered in measurements by the two collaborations.

An experimental overview: Collective dynamics

One of the most studied phenomena in heavy-ion physics is the collective emission of particles. Historically, the observation of collectivity in nucleus-nucleus collisions has been viewed as proof of the creation of a strongly interacting medium, as neither independent particle emission nor hadronic interactions alone could account for measurements. Intriguingly, collective particle emission was recently also measured in small collision systems, ranging from proton-proton to even high-multiplicity e+e− collisions. In these proceedings, I will elaborate on the current status of the experimental findings regarding collectivity and discuss their implications for the future.

Measurement of two-particle correlations and flow coefficients in high multiplicity e+e− collisions using archived ALEPH data at 91-209 GeV

We present measurements of two-particle angular correlations of charged particles emitted in high-energy collisions using data collected by the ALEPH detector at LEP between 1992 to 2000. The correlation functions are measured over a wide range of pseudorapidity and azimuthal angle as a function of charged particle multiplicity. Previous measurement with LEP1 data at √S = 91 GeV shows no significant long-range correlations in either lab coordinate or thrust coordinate analyses, with associated yield distributions in agreement with predictions from the PYTHIA v6.1 event generator. The use of higher collision energy LEP2 data allows access to not only higher event multiplicity but also additional production channels beyond the e+e- → y* / Z → q¯q process. Notably, the highest multiplicity bin (Ntrk ≥ 50) suggests a tantalizing disagreement with MC and implies the potential to search for collective phenomena in small systems. This measurement is pushing the studies of long-range correlation to the smallest collision system limit and includes the first flow coefficient (vn) measurement in e+e− collisions, which uses a Fourier decomposition analysis to quantify the anisotropy in the azimuthal two-particle correlation as a function of charged particles’ transverse momentum. This work supplements our understanding of small-system references to long-range correlations observed in proton-proton, proton-nucleus, and nucleus-nucleus collisions.

Overview of recent CMS results

Recent results from the CMS Collaboration are presented. These measurements include a full physics program using ultraperipheral collisions such as photon-photon and photon-ion interactions, small collision systems including proton-proton and proton-lead collisions, and many measurements of hadronic ion-ion collisions. The properties of the quark-gluon plasma produced in ion-ion collisions are studied in detail. The measurements examine the number of degrees of freedom of the medium, the strength of jet quenching effects in the medium, the role of heavy flavor in hadronization processes, and more.

The inhibited response of accessory minerals during high‐temperature reworking

AbstractU–Pb zircon and monazite geochronology are considered to be among the most efficient and reliable methods for constraining the timing of high‐temperature (HT) metamorphic events. However, the reliability of these chronometers is coupled to their ability to participate in reactions. A case study examining the responsiveness of zircon and monazite has been conducted using granulite facies metapelitic and metamafic lithologies in the Warumpi Province, central Australia. In some instances, metapelitic granulites from this locality are polymetamorphic, with an early M1 assemblage containing orthopyroxene, cordierite, biotite, quartz, ilmenite and magnetite, and an M2 assemblage represented by garnet, sillimanite, orthopyroxene, cordierite, biotite, sapphirine, ilmenite and magnetite. M2 metamorphism is linked to HT peak conditions of 8–10 kbar and 850–915°C. Detrital and metamorphic zircon and monazite from these rocks dominantly record U–Pb dates of 1670–1610 Ma and have trace element compositions suggesting they grew prior to peak M2 garnet in the rock. Lu–Hf geochronology from M2 garnet gives ages of c. 1150 Ma. Zircon and monazite are therefore suggested to have remained largely inert during HT metamorphism. We attribute the relatively minor response of zircon and monazite during high‐temperature Mesoproterozoic metamorphism to the localized development of refractory bulk compositions at c. 1630 Ma during M1 metamorphism. This created refractory Mg–Al‐rich bulk compositions that were unable to undergo significant partial melting, despite experiencing subsequent temperatures of ~900°C at c. 1150 Ma. In contrast, metapelitic and metamafic rocks in the area that did not develop refractory bulk compositions during M1 metamorphism were able to partially melt and record c. 1150 Ma accessory mineral U–Pb ages. These results contribute to a small, but growing number of case studies investigating the systematics of the U–Pb system in zircon and monazite in polymetamorphic HT terranes and their apparent resistance to isotopic resetting. Where disequilibrium is apparent, garnet Lu–Hf geochronology can form an important tool to interrogate the significance of accessory U–Pb ages. In the Warumpi Province in central Australia, c. 1640 Ma zircon U–Pb ages had previously been interpreted to reflect the formation of HT garnet‐bearing granulites during a collisional event. Instead, the garnet‐bearing assemblages formed at c. 1150 Ma during the Mesoproterozoic, calling into question the existence of a late Palaeoproterozoic collisional system in central Australia.

Parameterizing smooth viscous fluid dynamics with a viscous blast wave

Blast wave fits are widely used in high energy nuclear collisions to capture essential features of global properties of systems near kinetic equilibrium. They usually provide temperature fields and collective velocity fields on a given hypersurface. We systematically compare blast wave fits of fluid dynamic simulations for Au+Au collisions at and Pb+Pb collisions at TeV with the original simulations. In particular, we investigate how faithful the viscous blast wave introduced in Yang and Fries (2022 Phys. Rev. C 105 014910) can reproduce the given temperature and specific shear viscosity fixed at freeze-out of a viscous fluid dynamic calculation, if the final spectrum and elliptic flow of several particle species are fitted. We find that viscous blast wave fits describe fluid dynamic pseudodata rather well and reproduce the specific shear viscosities to good accuracy. However, extracted temperatures tend to be underpredicted, especially for peripheral collisions. We investigate possible reasons for these deviations. We establish maps from true to fitted values. These maps can be used to improve raw fit results from viscous blast wave fits. Although our work is limited to two specific, albeit important, parameters and two collision systems, the same procedure can be easily generalized to other parameters and collision systems.

Geomorphological and geo/thermo-chronological responses of Indian plate’s deformation during neogene- quaternary time along the Eastern Himalayan Syntaxis: Formation of Manabhum anticline

India-Eurasia and India-Burma collision systems encircle Eastern Himalayan Syntaxis (EHS) in the north and the southeast. The main active structure responsible for the uplift of the Quaternary and Neogene rocks in the easternmost part of the EHS is the NW-SE trending Mishmi Thrust (MT). The spectacular antiformal Manabhum Hill developed during the Quaternary as a ramp antiform over a splay of Mishmi Thrust in its foreland region. The current work presents the deformation scenario, uplift rate, and landform development along the Mishmi Thrust from the Neogene to the Recent for the first time. The surface deformation and the rock and sediment deformation histories have been interpreted from the geomorphology, structure and petrology of the rocks and sediments. We use new (U + Th + Sm)/He low-temperature thermochronology (LTT) of zircon and apatite and Optically Stimulated Luminescence (OSL) dates to calculate the uplift rate for the Neogene to early Quaternary and late Quaternary time intervals, respectively. The Neogene uplift rate varies between 0.91 and 2.3 mm/yr, whereas the early Quaternary uplift rate is ∼3.1 mm/yr. The increased uplift rate during the Quaternary period with significant deformation of sediments indicates the non-rigid character of the Indian plate in this sector, which experienced compression and clockwise rotation.

Vertical tearing of subducting plates controlled by geometry and rheology of oceanic plates

Lateral non-uniform subduction is impacted by continuous plate segmentation owing to vertical tearing of the subducting plate. However, the dynamics and physical controls of vertical tearing remain controversial. Here, we employed 3D numerical models to investigate the effects of trench geometry (offset by a transform boundary) and plate rheology (plate age and the magnitude of brittle/plastic strain weakening) on the evolution of shear stress-controlled vertical tearing within a homogenous subducting oceanic plate. Numerical results suggest that the trench offset geometry could result in self-sustained vertical tearing as a narrow shear zone within the intact subducting oceanic plate, and that this process of tearing could operate throughout the entire subduction process. Further, the critical trench offset length for the maturation of vertical tearing is impacted by plate rheology. Comparison between numerical modelling results and natural observations suggests that vertical tearing attributed to trench offset geometry is broadly developed in modern subduction and collision systems worldwide.

Excitation function of thermal freeze-out parameters and their correlations from strange hadrons spectra in Au–Au collision at 54.4 GeV

We present a systematic study of Gold–Gold (Au–Au) collision system at 54.4 GeV conducted by the STAR Collaboration at RHIC. The transverse momentum spectra of the strange hadrons, namely KS0, Λ, Λ̄, Ξ̄+ and Ξ− are studied at mid-rapidity |y|<0.5 for seven centrality classes. We applied the blast wave model with Tsallis statistics (TBW) with two flow profiles (n0 = 1 and n0 = 2) in order to compare the results and to check the sensitivity of n0 to the freeze-out parameters. It is found that the TBW model with both the flow profiles can describe the particle spectra. In addition, the bulk properties in terms of kinetic freeze-out temperature (T0), transverse flow velocity (βT), the non-extensive parameter (q), the kinetic freeze-out volume (V), the mean transverse momentum (〈pT〉), and the multiplicity parameter (N0) are also extracted. All the parameters decrease with decreasing the event centrality, however, the non-extensive parameter q has the opposite behavior. There is an abrupt decrease seen in 0–5% to 30–40% centrality, while a slight decrease appears in 40–60% and 60–80% centrality in T0, βT, V, and 〈pT〉. On the other hand, the parameter q follows the same scenario with an opposite trend with centrality. T0 and 〈pT〉 are larger for massive particles while βT, q, V and N0 are larger for lighter particles. The larger T0 and 〈pT〉, and smaller βT, V, and q for the massive particles affirm that the heavy particles freeze-out early. Moreover, the multiple kinetic freeze-out and volume differential freeze-out scenarios are reported, as the former increase while the latter decrease with particle mass. In addition, T0, q, V, and N0 obtained from TBW model with n0 = 1 are compatible to that of n0 = 2. On the other hand, βT is slightly larger in n0 = 2, while 〈pT〉 is slightly larger in n0 = 1. Furthermore, we also reported some important correlations among the extracted parameters. Among these correlations, the positive correlation exist between T0 and βT, T0 and V, T0 and 〈pT〉, T0 and N0, βT and 〈pT〉, βT and V, βT and N0, V and N0, and 〈pT〉 and N0, while the negative correlation of T0 and q, and 〈pT〉 and q.

Centrality and System Size Dependence among Freezeout Parameters and the Implications for EOS and QGP in High-Energy Collisions.

Utilizing the Modified Hagedorn function with embedded flow, we analyze the transverse momenta (pT) and transverse mass (mT) spectra of π+ in Au-Au, Cu-Cu, and d-Au collisions at sNN = 200 GeV across various centrality bins. Our study reveals the centrality and system size dependence of key freezeout parameters, including kinetic freezeout temperature (T0), transverse flow velocity (βT), entropy-related parameter (n), and kinetic freezeout volume (V). Specifically, T0 and n increase from central to peripheral collisions, while βT and V show the opposite trend. These parameters also exhibit system size dependence; T0 and βT are smaller in larger collision systems, whereas V is larger. Importantly, central collisions correspond to a stiffer Equation of State (EOS), characterized by larger βT and smaller T0, while peripheral collisions indicate a softer EOS. These insights are crucial for understanding the properties of Quark-Gluon Plasma (QGP) and offer valuable constraints for Quantum Chromodynamics (QCD) models at high temperatures and densities.

Measurements of H2O-O2 line shape parameters and the determination of the intermolecular potential for modified complex Robert-Bonamy calculations

A series of spectra, one pure H2O and four H2O-O2 mixtures, were measured for the ν2 band in the 1200–1950 cm−1 region at room temperature using a single-pass copper cell housed in the Bruker 125HR high-resolution Fourier transform spectrometer (FTS) at Jet Propulsion Laboratory (JPL). All the five spectra were fit simultaneously using a multispectrum non-linear least-squares fitting software, Labfit programme suite, which adopts a speed-dependent Voigt line shape profile using full line mixing taken into account through a relaxation matrix operation. O2-broadened half-width and O2-induced frequency shift coefficients were determined for more than 350 transitions of H2 16O. These data were then checked using the smooth variation and paring rules [Brown et al. J Mol Spectrosc. 2007;246:1-21, Ma et al. Mol Phys. 2011;109:1925-41] and 186 transitions that obey the rules with less than 3% difference were chosen. Using these selected data, Modified Complex Robert-Bonamy calculations were made to determine the intermolecular potential for the H2O-O2 collision system. Some 93 potential were investigated resulting in a final potential (potential 74) that agrees with the selected half-width measurements having an average percent difference of 0.052 and a standard deviation of 5.108% between measurement and calculations.

Hyperon Polarization along the Beam Direction Relative to the Second and Third Harmonic Event Planes in Isobar Collisions at sqrt[s_{NN}]=200 GeV.

The polarization of Λ and Λ[over ¯] hyperons along the beam direction has been measured relative to the second and third harmonic event planes in isobar Ru+Ru and Zr+Zr collisions at sqrt[s_{NN}]=200 GeV. This is the first experimental evidence of the hyperon polarization by the triangular flow originating from the initial density fluctuations. The amplitudes of the sine modulation for the second and third harmonic results are comparable in magnitude, increase from central to peripheral collisions, and show a mild p_{T} dependence. The azimuthal angle dependence of the polarization follows the vorticity pattern expected due to elliptic and triangular anisotropic flow, and qualitatively disagrees with most hydrodynamic model calculations based on thermal vorticity and shear induced contributions. The model results based on one of existing implementations of the shear contribution lead to a correct azimuthal angle dependence, but predict centrality and p_{T} dependence that still disagree with experimental measurements. Thus, our results provide stringent constraints on the thermal vorticity and shear-induced contributions to hyperon polarization. Comparison to previous measurements at RHIC and the LHC for the second-order harmonic results shows little dependence on the collision system size and collision energy.

Multiplicity Dependence of the Freeze-Out Parameters in Symmetric and Asymmetric Nuclear Collisions at Large Hadron Collider Energies

Strange hadron transverse momentum spectra are analyzed in symmetric pp and PbPb and asymmetric pPb collision systems for their dependence on rapidity and event charged-particle multiplicity. The thermodynamically consistent Tsallis models with and without flow velocity are used to reproduce the experimental data, extracting the freeze-out parameters to gain insights into the underlying physics of the collision processes by looking into the parameters change with different multiplicities, particle types, and collision geometries. We found that with an increase in the event multiplicity, the average transverse flow velocity, effective, and kinetic freezeout temperatures increase, with heavier strange particle species exhibiting a more significant increase. The value of the non-extensivity parameter decreases with an increase in the multiplicity of the particles. For heavier particles, larger Teff and T0 and smaller q have been observed, confirming the quick thermalization and equilibrium for massive particles. Furthermore, the differences in parameter values for particle species are more significant in pp and pPb collisions than in PbPb collisions. In addition, in symmetric pp and PbPb collisions, parameter values (q,T0,βT) show more significant shifts for heavier particles compared to the lighter ones. In contrast, in asymmetric pPb collisions, both heavier and lighter particles display uniform linear progression.

State-selective charge exchange cross sections in collisions of highly-charged sulfur ions with helium and molecular hydrogen

The state-selective cross section data are useful for understanding and modeling the x-ray emission in celestial observations. In the present work, using the cold target recoil ion momentum spectroscopy, for the first time we investigated the state-selective single electron capture processes for S q+–He and H2 (q = 11–15) collision systems at an impact energy of q × 20 keV and obtained the relative state-selective cross sections. The results indicate that only a few principal quantum states of the projectile energy level are populated in a single electron capture process. In particular, the increase of the projectile charge state leads to the population of the states with higher principal quantum numbers. It is also shown that the experimental averaged n-shell populations are reproduced well by the over-barrier model. The database is openly available in Science Data Bank at 10.57760/sciencedb.j00113.00091.

Jet modification in absence of QGP-medium: the role of multiparton interactions and color reconnection* *D. Banerjee acknowledges the Inspire Fellowship research grant (DST/INSPIRE Fellowship/2018/IF180285)

Recent studies on high-multiplicity events in small collision systems (proton-proton and proton-lead) have drawn considerable research interest toward the possibility of the formation of partonic medium in such systems. One of the important consequences of the formation of dense partonic medium is the quenching of high-momentum final-state particles, resulting in several experimental observations such as suppression in nuclear modification factor , modification of jet shape observable and jet fragmentation ( ) distributions, etc. In this work, we study and for inclusive charged-particle jets in proton-proton (pp) collisions at = 13 TeV using the PYTHIA 8 Monash 2013 Monte Carlo simulation. We show that the color reconnection (CR) and multiparton interaction (MPI) mechanisms in PYTHIA 8 can lead to an increased rate of jet production. We also find that the mechanisms of MPI and CR and change in the gluonic contribution in high-multiplicity events result in significant modification of and compared to those in minimum bias events for 10 20 GeV/c. We notice a direct connection of and gluonic contribution with the amount of modification in : the larger the number of MPIs and/or gluonic contribution, the larger the amount of modification of .

Pseudo-rapidity distributions of charged particles in asymmetric collisions using Tsallis thermodynamics

The pseudo-rapidity distributions of the charged particles produced in the asymmetric collision systems p+Al, p+Au and 3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}He+Au at sNN=200\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{s_\ ext {NN}} = 200$$\\end{document} GeV are evaluated in the framework of a fireball model with Tsallis thermodynamics. The fireball model assumes that the experimentally measured particles are produced by fireballs following the Tsallis distribution and it can effectively describe the experimental data. Our results as well as previous results for d+Au collisions at sNN=200\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{s_\ ext {NN}} = 200$$\\end{document} GeV and p+Pb collisions at sNN=5.02\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sqrt{s_\ ext {NN}} = 5.02$$\\end{document} TeV validate that the fireball model based on Tsallis thermodynamics can provide a universal framework for pseudo-rapidity distribution of the charged particles produced in asymmetric collision systems. We predict the centrality dependence of the total charged particle multiplicity in the p+Al, p+Au and 3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^3$$\\end{document}He+Au collisions. Additionally, the dependences of the fireball model parameters (y0a\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$y_{0a}$$\\end{document}, y0A\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$y_{0A}$$\\end{document}, σa\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _{a}$$\\end{document} and σA\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sigma _{A}$$\\end{document}) on the centrality and system size are studied.