High Energy Pp Collisions Research Articles

Disentangling the development of collective flow in high energy proton proton collisions with a multiphase transport model

AbstractIn this work, we investigate the collective flow development in high energy proton proton (pp) collisions with a multiphase transport model (AMPT) based on PYTHIA8 initial conditions with a sub-nucleon structure. It is found that the PYTHIA8 based AMPT model can reasonably describe both the charged hadron productions and elliptic flow experimental data measured in pp collisions at $$\sqrt{s}=13$$ s = 13 TeV. By turning on the parton and hadron rescatterings in AMPT separately, we find that the observed collective flow in pp collisions is largely developed during the parton evolution, while no significant flow effect can be generated with the pure hadronic rescatterings. It is also shown that the parton escape mechanism is important for describing both the magnitude of the two-particle cumulant and the sign of the four-particle cumulants. We emphasize that the strong mass ordering of the elliptic flow results from the coalescence process in the transport model and can thus be regarded as unique evidence related to the creation of deconfined parton matter in high energy pp collisions.

Diquarks and the production of charmed baryons

Utilizing a quark model characterized by parameters that effectively replicate the masses of ground state hadrons, we illustrate that (us) or (ds) diquarks exhibit greater compactness in comparison to (ud) diquarks. Concretely, the binding energy of the (us) diquark - defined as the diquark's mass minus the combined masses of its individual quarks - is found to be stronger than that of the (ud) diquark. This heightened attraction present in (us) diquarks could lead to enhanced production of Ξc/D particles in high-energy pp or ultrarelativistic heavy-ion collisions.

Heavy-flavor transport and hadronization in a small fireball

We study heavy-flavor hadron production in high-energy pp collisions, assuming the formation of a small, deconfined and expanding fireball where charm quarks can undergo rescattering and hadronization. We adopt the same in-medium hadronization mechanism developed for heavy-ion collisions, which involves Local Color-Neutralization (LCN) through recombination of charm quarks with nearby opposite color charges from the background fireball. Diquark excitations in the hot medium favor the formation of charmed baryons. The recombination process, involving closely aligned partons from the same fluid cell, effectively transfers the collective flow of the system to the final charmed hadrons. This framework can qualitatively reproduce the observed experimental findings in heavy-flavor particle-yield ratios, pT -spectra and ellipticflow coefficients. Our results provide new, complementary support to the idea that the collective phenomena observed in small systems have the same origin as those observed in heavy-ion collisions.

Bottom-hadron production in high-energy pp and heavy-ion collisions

The hadro-chemistry of bottom quarks produced in hadronic collisions encodes valuable information on the mechanism of color-neutralization in these reactions. We first compute the chemistry of bottom-hadrons in high-energy pp collisions employing statistical hadronization with a largely augmented set of states beyond the currently measured spectrum. This enables a comprehensive prediction of fragmentation fractions of weakly decaying bottom hadrons for the first time and a satisfactory explanation of the existing measurements in pp collisions at the LHC. Utilizing the bottom hadro-chemistry thus obtained as the baseline, we then perform transport simulations of bottom quarks in the hot QCD matter created in PbPb collisions at the LHC energy and calculate the pertinent bottom-hadron observables. The transverse momentum (pT) dependent modifications of the bottom baryon-to-meson ratio (Λb0/B-) relative to their pp counterparts are highlighted as a result of bottom quark diffusion and hadronization in the Quark-Gluon Plasma (QGP). We finally summarize the heavy quark (charm vs bottom) diffusion coefficients as extracted from transport simulations and compare them to result from recent full lattice QCD computations.

Measurement of the Sensitivity of Two-Particle Correlations in pp Collisions to the Presence of Hard Scatterings.

A key open question in the study of multiparticle production in high-energy pp collisions is the relationship between the "ridge"-i.e., the observed azimuthal correlations between particles in the underlying event that extend over all rapidities-and hard or semihard scattering processes. In particular, it is not known whether jets or their soft fragments are correlated with particles in the underlying event. To address this question, two-particle correlations are measured in pp collisions at sqrt[s]=13 TeV using data collected by the ATLAS experiment at the LHC, with an integrated luminosity of 15.8 pb^{-1}, in two different configurations. In the first case, charged particles associated with jets are excluded from the correlation analysis, while in the second case, correlations are measured between particles within jets and charged particles from the underlying event. Second-order flow coefficients, v_{2}, are presented as a function of event multiplicity and transverse momentum. These measurements show that excluding particles associated with jets does not affect the measured correlations. Moreover, particles associated with jets do not exhibit any significant azimuthal correlations with the underlying event, ruling out hard processes contributing to the ridge.

Quantifying the underlying event: investigating angular dependence of multiplicity classes and transverse-momentum spectra in high-energy pp collisions at LHC energies

We present a study of the transverse momentum spectra of charged hadrons and their evolution as a function of their azimuthal position with respect to the leading particle in proton–proton collisions at = 13 TeV using the PYTHIA8 Monte Carlo event generator with the Monash 2013 tune. The spherocity distribution in the same azimuthal bins is reported. The Multiplicity and midrapidity transverse momentum spectra of charged hadrons have been analysed in the non-extensive statistical framework. The results corresponding to the Underlying Event are reported. This work provides a possible understanding of the interplay between hard scatterings and the Underlying Event in proton–proton collisions.

Charmed meson production based on dipole transverse momentum representation in high energy hadron-hadron collisions available at the LHC

The production of D-mesons in high-energy pp collisions at the LHC kinematic regime is analyzed with the color dipole approach in the momentum representation. We present predictions for the D-meson differential cross section in terms of the transverse momentum and rapidity distributions taking into account the nonlinear behavior of the QCD dynamics. Comparison between our results and the corresponding experimental measurements reported by the ALICE and LHCb Collaborations in different rapidity bins is performed. We show that the D-meson production in the high energy limit can be properly addressed by using the QCD dipole transverse momentum distributions.

Femtoscopic study on the ΛΛ-NΞ interaction

The correlation functions of pΞ − and ΛΛ pairs from high-energy pp collisions are investigated in the coupled-channel formalism. The NΞΛΛ coupled-channel potentials obtained in the lattice QCD calculation at almost physical quark masses are employed. The pΞ − correlation function shows the large enhancement from the pure Coulomb case, while the ΛΛ correlation function shows the moderate enhancement from the pure quantum statistics case. This agreement indicates that both the NΞ and ΛΛ interactions are moderately attractive without having quasibound or bound state.

The role of the underlying event in the enhancement in high-energy pp collisions

We study the enhanced production of charmed baryons relative to charmed mesons in proton–proton collisions at LHC energies. Utilizing PYTHIA 8 simulations with enhanced color-reconnection we propose methods based on the comparative use of several event-activity classifiers to identify the source of the charmed-baryon enhancement. We conclude that in this enhanced color-reconnection scenario the excess production is primarily linked to the underlying event activity and not to the production of high-momentum jets.

Femtoscopic study of coupled-channels NΞ and ΛΛ interactions

The momentum correlation functions of S = -2 baryon pairs (p Xi^- and Lambda Lambda) produced in high-energy pp and pA collisions are investigated on the basis of the coupled-channel formalism. The strong interaction is described by the coupled-channel HAL QCD potential obtained by the lattice QCD simulations near physical quark masses, while the hadronic source function is taken to be a static Gaussian form. The coupled-channel effect, the threshold difference, the realistic strong interaction, and the Coulomb interaction are fully taken into account for the first time in the femtoscopic analysis of baryon-baryon correlations. The characteristic features of the experimental data for the p Xi^- and Lambda Lambda pairs at LHC are reproduced quantitatively with a suitable choice of non-femtoscopic parameters and the source size. The agreement between theory and experiment indicates that the N Xi (Lambda Lambda) interaction is moderately (weakly) attractive without having a quasi-bound (bound) state.

Investigating high energy proton proton collisions with a multi-phase transport model approach based on PYTHIA8 initial conditions

The striking resemblance of high multiplicity proton-proton (pp) collisions at the LHC to heavy ion collisions challenges our conventional wisdom on the formation of the quark-gluon plasma (QGP). A consistent explanation of the collectivity phenomena in pp will help us to understand the mechanism that leads to the QGP-like signals in small systems. In this study, we introduce a transport model approach connecting the initial conditions provided by PYTHIA8 with subsequent AMPT rescatterings to study the collective behavior in high energy pp collisions. The multiplicity dependence of light hadron productions from this model is in reasonable agreement with the pp sqrt{s}=13 TeV experimental data. It is found in the comparisons that both the partonic and hadronic final state interactions are important for the generation of the radial flow feature of the pp transverse momentum spectra. The study also shows that the long range two particle azimuthal correlation in high multiplicity pp events is sensitive to the proton sub-nucleon spatial fluctuations.

Charged-particle multiplicity dependence of charm-baryon-to-meson ratio in high-energy proton-proton collisions

We propose that the charged-particle multiplicity dependence of the charm-baryon-to-meson ratio observed in high-energy pp collisions can be explained by canonical treatment of quantum charges in the statistical hadronization model (SHM). Taking the full particle listings of PDG complemented by additional charm-baryon states from relativistic quark model predictions, we evaluate the canonical partition function and the charm-hadron chemical factors that measure the canonical suppression arising from the requirement of strict conservation of quantum charges. We demonstrate that, while charm number conservation induces common suppression on the production of both charm-baryons and -mesons, baryon (strangeness) number conservation causes further suppression on charm-baryons (charm-strange mesons) relative to nonstrange charm-mesons, thereby resulting in a decreasing Λc/D0 (Ds/D0) ratio toward smaller multiplicity events. The charm-hadron thermal densities thus computed are then used as pertinent weights to perform charm-quark fragmentation simulations yielding pT-dependent Λc/D0 and Ds/D0 ratios at varying multiplicities in fair agreement with ALICE measurements.

Hadroproduction of scalar P -wave quarkonia in the light-front kT -factorization approach

In this work, we present a thorough analysis of scalar P -wave χQ0, Q = c, b quarkonia electromagnetic form factors for the γ∗γ∗→ χQ0 couplings, as well as their hadroproduction observables in k⊥-factorisation using the light-front (LF) potential approach for the quarkonium wave function. The electromagnetic form factors are presented as functions of photon virtualities. We discuss the role of the Melosh spin-rotation and relativistic corrections estimated by comparing our results with those in the standard nonrelativistic QCD (NRQCD) approach. Theoretical uncertainties of our approach are found by performing the analysis with two different unintegrated gluon densities and with five distinct models of the Qoverline{Q} interaction potentials consistent with the meson spectra. The results for the rapidity and transverse momentum distributions of χQ0 produced in high-energy pp collisions at sqrt{s} = 13 TeV are shown.

Monte-Carlo study of long-range correlations of average transverse momentum and multiplicity for strange particles in pp-collisions at the LHC energies

This study is motivated by the puzzling enhanced production of multi-strange hadrons observed for the first time by the ALICE Collaboration in high-multiplicity pp-collisions at at the LHC. We investigate this effect of strangeness enhancement in pp-collisions in the framework of the MC event generator PYTHIA 8 where the collectivity processes are taken into account by the formation of a so-called flavour rope. The latter is hadronized with a larger effective string tension providing the increase of strangeness yield. This concept of the flavour rope, as a source of multi-strange hadrons, extended in rapidity, is being tested by the study of so called long-range correlations (LRC) between the average transverse momentum 〈pT〉 and multiplicity n for charged particles containing strange quarks produced in high energy pp-collisions.The dependencies of correlation coefficients bn−n, bPT−n and bPT−PT on the gap between forward and backward pseudorapidity windows, and on the width of the forward pseudorapidity windows, are studied and the results are discussed.

Chemical potentials of light hadrons and quarks from yield ratios of negative to positive particles in high energy pp collisions

We describe the transverse momentum spectra of π±, K±, p, and produced in proton–proton (pp) collisions at different collision energies by using a Tsallis–Pareto-type function, and obtain the yield ratios, kπ, kK, and kp, of negative to positive particles from the fitting results of transverse momentum spectra and the extracted normalization constants. The transverse momentum dependent and energy dependent chemical potentials of light hadrons (π, K, and p) and quarks (u, d, and s) in pp collisions are then extracted successively from the yield ratios. The six types of transverse momentum dependent chemical potentials show the trend of being close to zero in low-pT region and away from zero in high-pT region. Meanwhile, the energy dependent chemical potentials seem to decrease slightly with the increase of the collision energy, and the limiting values of the six types of chemical potentials are zero at very high energy in pp collisions, which confirms that the partonic interactions possibly play an important role at RHIC and LHC, especially at LHC.

From Tsallis Scheme of high-energy pp collisions to Generalized Multiplicity Distribution with Modified Combinants

An analysis of the generalized multiplicity distribution, (GMD), has been made in high energy pp collisions. An attempt is made to reconcile the generating function of GMD with Tsallis statistics in order to find an interpretation of Tsallis parameter,(q). The modified combinants, (Cj’s), for both GMD and NBD are obtained using the generating functions of these distributions. Our results show that single NBD does not lead to the oscillatory behaviour of the observed Cj. However, for GMD the corresponding Cj not only oscillate but also show the fading-down feature of the experimentally observed Cj. This could help in future to analyse the physical process which is responsible for these oscillations.

The Angantyr model for heavy-ion collisions in Pythia8

We present a new model for building up complete exclusive hadronic final states in high energy nucleus collisions. It is a direct extrapolation of high energy pp collisions (as described by Pythia), and thus bridges a large part of the existing gap between heavy ion and high energy physics phenomenology. The model is inspired by the old Fritiof model and the notion of wounded nucleons. Two essential features are the treatment of multi-parton interactions and diffractive excitation in each NN sub-collision. Diffractive excitation is related to fluctuations in the nucleon partonic sub-structure, and fluctuations in both projectile and target are here included for the first time. The model is able to give a good description of general final-state properties such as multiplicity and transverse momentum distributions, both in pA and AA collisions. The model can therefore serve as a baseline for understanding the non-collective background to observables sensitive to collective behaviour. As Pythia does not include a mechanism to reproduce the collective effects seen in pp collisions, such effects are also not reproduced by the present version of Angantyr. Effects of high string density, shown to be able to reproduce e.g. higher strangeness ratios and the ridge in pp, will be added in future studies.

Transverse momentum spectra of hadrons in high energy pp and heavy ion collisions

We present a study of transverse momentum (pT) spectra of unidentified charged particles in pp collisions at RHIC and LHC energies from to 13 TeV using Tsallis/Hagedorn function. The power law of Tsallis/Hagedorn form gives very good description of the hadron spectra in pT range from 0.2 to 300 GeV/c. The power index n of the pT distributions is found to follow a function of the type with asymptotic value a = 6.8. The parameter T governing the soft bulk contribution to the spectra remains almost same over wide range of collision energies. We also provide a Tsallis/Hagedorn fit to the pT spectra of hadrons in pPb and different centralities of PbPb collisions at . The data/fit shows deviations from the Tsallis distribution which become more pronounced as the system size increases. We suggest simple modifications in the Tsallis/Hagedorn power law function and show that the above deviations can be attributed to the transverse flow in low pT region and to the in-medium energy loss in high pT region.

Elliptic flow in small systems due to elliptic gluon distributions?

We investigate the contributions from the so-called elliptic gluon Wigner distributions to the rapidity and azimuthal correlations of particles produced in high energy pp and pA collisions by applying the double parton scattering mechanism. We compute the ‘elliptic flow’ parameter v2 as a function of the transverse momentum and rapidity, and find qualitative agreement with experimental observations. This shall encourage further developments with more rigorous studies of the elliptic gluon distributions and their applications in hard scattering processes in pp and pA collisions.

Reconstruction of semileptonically decaying beauty hadrons produced in high energy pp collisions

It is well known that in b-hadron decays with a single unreconstructible final state particle, the decay kinematics can be solved up to a quadratic ambiguity, without any knowledge of the b-hadron momentum. We present a method to infer the momenta of b-hadrons produced in hadron collider experiments using information from their reconstructed flight vectors. Our method is strictly agnostic to the decay itself, which implies that it can be validated with control samples of topologically similar decays to fully reconstructible final states. A multivariate regression algorithm based on the flight information provides a b-hadron momentum estimate with a resolution of around 60% which is sufficient to select the correct solution to the quadratic equation in around 70% of cases. This will improve the ability of hadron collider experiments to make differential decay rate measurements with semileptonic b-hadron decays.