Monte Carlo Event Generator Research Articles

Transverse momentum distributions of the identified particles in mini–bias non-single diffracted p＋p collisions at 200 GeV

The transverse momentum (pT) distributions of hadrons stem from the probability distributions of deconfined colored partons known as pT distributions. This provides insights into various stages of nuclear collisions, ranging from the initial partonic phase to the later stages, including the directly measurable hadronization process. The objective is to analyze the pT distributions of identified particles: π+, π−, K+, K−, K0s, p, p̄, Λ, Λ̄, Σ+, Σ−. These analyses are conducted in mini-bias non-single diffracted p+p collisions at an energy of 200GeV. The goal is to accurately determine the kinetic freezeout parameters and the extent of statistical nonextensivity. The measurements from the STAR experiment are merged with simulations using DPMJET, PYTHIA, and Sibyll.3d Monte–Carlo Event Generators. The predictions from these models and the actual measurements are compared using statistical distributions employing the Tsallis-type of non-extensive statistics. While Pythia8 displays a closer alignment with the data in certain instances, it was observed that none of the event generators could accurately replicate the data for all charged particles across the entire range of pT. We examined how the effective temperature correlates with the rest mass of different particles. Our analysis revealed that the freezeout process for heavier particles tends to occur at lower temperatures. This suggests that heavier particles might reach an equilibrium state relatively rapidly during the freezeout stage. Additionally, we observed a pattern where the non-extensive parameter q decreases as the mass of the produced particles increases. This implies that lighter particles tend to reach equilibrium after the heavier ones. Furthermore, the average transverse momentum (<pT>) is greater for heavier particles, leading to an enhanced radial flow compared to lighter particles. This rising trend also indicates the likelihood of mini-jet production, contributing to color string fragmentation that becomes more pronounced with the increasing mass of the particles.

Performance of Monte Carlo Event Generators of pp Collisions at NICA Energies

Performance of Monte Carlo Event Generators of pp Collisions at NICA Energies

Determination of Tsallis parameters for K∗(892)± mesons in inelastic pp, pPb and PbPb collisions

Doubly differential transverse momentum spectra of strange mesons [Formula: see text] and [Formula: see text] within rapidity interval of [Formula: see text] are studied by using Monte Carlo event generators in pp, pPb and PbPb collisions at different center-of-mass energies. The predictions of simulations are compared to the ALICE data. The fitting of doubly differential transverse momentum spectra of ALICE data is performed by using Tsallis distribution function. To check the quality of the fitting, chemical potential is also set to zero value. The extracted values of the non-extensive parameter ([Formula: see text]), a chemical potential [Formula: see text] and effective Tsallis temperature [Formula: see text] for different particle species have been obtained from the thermodynamically consistent Tsallis distribution function. It is observed that the effective Tsallis temperature increases at different center-of-mass energies. The predictions of the MC event generators, i.e. PYTHIA8 CR and HIJING 2.0 are compared to the ALICE data, deviations are observed which may be due to the reshuffling of parton colors and string fragmentation.

A Bayesian tune of the Herwig Monte Carlo event generator

The optimisation (tuning) of the free parameters of Monte Carlo event generators by comparing their predictions with data is important since the simulations are used to calculate experimental efficiency and acceptance corrections, or provide predictions for signatures of hypothetical new processes in experiments. We present a tuning procedure that is based on Bayesian reasoning and that allows for a proper statistical interpretation of the results. The parameter space is fully explored using Markov Chain Monte Carlo. We apply the tuning procedure to the Herwig7 event generator with both the cluster and the string hadronization models and a large set of measurements from hadronic Z-boson decays produced at LEP in e + e - collisions. Furthermore, we introduce a coherent propagation of uncertainties from the realm of parameters to the realm of observables and we show the effects of including experimental correlations of the measurements. To allow comparison with the approaches of other groups, we repeat the tuning considering weights for individual measurements.

F0(980) production in inelastic pp collisions at [formula omitted] = 5.02 TeV

The measurement of the production of f0(980) in inelastic pp collisions at s=5.02 TeV is presented. This is the first reported measurement of inclusive f0(980) yield at LHC energies. The production is measured at midrapidity, |y|<0.5, in a wide transverse momentum range, 0<pT<16 GeV/c, by reconstructing the resonance in the f0(980)→π+π− hadronic decay channel using the ALICE detector. The pT-differential yields are compared to those of pions, protons and ϕ mesons as well as to predictions from the HERWIG 7.2 QCD-inspired Monte Carlo event generator and calculations from a coalescence model that uses the AMPT model as an input. The ratio of the pT-integrated yield of f0(980) relative to pions is compared to measurements in e+e− and pp collisions at lower energies and predictions from statistical hadronisation models and HERWIG 7.2. A mild collision energy dependence of the f0(980) to pion production is observed in pp collisions from SPS to LHC energies. All considered models underpredict the pT-integrated 2f0(980)/(π+ +π−) ratio. The prediction from the canonical statistical hadronisation model assuming a zero total strangeness content of f0(980) is consistent with the data within 1.9σ and is the closest to the data. The results provide an essential reference for future measurements of the particle yield and nuclear modification in p–Pb and Pb–Pb collisions, which have been proposed to be instrumental to probe the elusive nature and quark composition of the f0(980) scalar meson.

Azimuthal correlations of heavy-flavor hadron decay electrons with charged particles in pp and p–Pb collisions at pmb {sqrt{s_{mathrm{{NN}}}}} = 5.02 TeV

The azimuthal (Delta varphi ) correlation distributions between heavy-flavor decay electrons and associated charged particles are measured in pp and p–Pb collisions at sqrt{s_{mathrm{{NN}}}} = 5.02 TeV. Results are reported for electrons with transverse momentum 4<p_{textrm{T}}<16textrm{GeV}/c and pseudorapidity |eta |<0.6. The associated charged particles are selected with transverse momentum 1<p_{textrm{T}}<7textrm{GeV}/c, and relative pseudorapidity separation with the leading electron |Delta eta | < 1. The correlation measurements are performed to study and characterize the fragmentation and hadronization of heavy quarks. The correlation structures are fitted with a constant and two von Mises functions to obtain the baseline and the near- and away-side peaks, respectively. The results from p–Pb collisions are compared with those from pp collisions to study the effects of cold nuclear matter. In the measured trigger electron and associated particle kinematic regions, the two collision systems give consistent results. The Delta varphi distribution and the peak observables in pp and p–Pb collisions are compared with calculations from various Monte Carlo event generators.

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.

Extending StringSpinner to handle vector-meson spin

Quark spin effects in hadronization were recently included in the Pythia 8 Monte Carlo event generator for the simulation of the deep inelastic scattering (DIS) process off a polarized proton or neutron target. The spin effects were activated via the external StringSpinner package, which is based on the string+P03 model of polarized hadronization, and were restricted to the production of pseudoscalar mesons in string fragmentation. The struck quark polarization could either be calculated using parametrizations of the transversity PDFs or be freely specified. In this article we present a major extension of StringSpinner that includes vector meson production in the Pythia 8 string fragmentation. The spin-dependent emission and decay processes of the vector mesons are described using the string+P03 model, which systematically accounts for the quark-spin degree of freedom in the fragmentation chain. The new package works with the version 8.3 of the Pythia event generator and it allows for a more complete and quantitative simulation of the final states in polarized semi-inclusive DIS events. Also, it can be used to study the transverse-spin effects for the production of vector mesons. After describing the new version of StringSpinner, examples of its usage in semi-inclusive DIS events are presented. New version program summaryProgram Title: StringSpinnerCPC Library link to program files:https://doi.org/10.17632/37jrpmyz3t.2Developer's repository link:https://gitlab.com/albikerbizi/stringspinner.gitLicensing provisions: GNU GPL v2 or laterProgramming language:C++, FortranJournal reference of previous version: Comput. Phys. Commun. 272 (2022) 108234.Does the new version supersede the previous version?: YesReasons for the new version: Vector meson emission and decay is required for a more complete simulation of the DIS final states.Summary of revisions: The spin-dependence of the string fragmentation is extended to include the production and the decay of polarized vector mesons.Nature of problem: Vector meson contribute strongly to the final hadrons produced in DIS, and they can produce non-trivial effects on the spin-dependent observables.Solution method: Extend the previous version of StringSpinner to include polarized vector meson production and decay using a more complete model of polarized hadronization.

Multiplicity dependence of charged-particle production in pp, p–Pb, Xe–Xe and Pb–Pb collisions at the LHC

Multiplicity (Nch) distributions and transverse momentum (pT) spectra of inclusive primary charged particles in the kinematic range of |η|<0.8 and 0.15 GeV/c<pT<10 GeV/c are reported for pp, p–Pb, Xe–Xe and Pb–Pb collisions at centre-of-mass energies per nucleon pair ranging from sNN=2.76 TeV up to 13 TeV. A sequential two-dimensional unfolding procedure is used to extract the correlation between the transverse momentum of primary charged particles and the charged-particle multiplicity of the corresponding collision. This correlation sharply characterises important features of the final state of a collision and, therefore, can be used as a stringent test of theoretical models. The multiplicity distributions as well as the mean and standard deviation derived from the pT spectra are compared to state-of-the-art model predictions. Providing these fundamental observables of bulk particle production consistently across a wide range of collision energies and system sizes can serve as an important input for tuning Monte Carlo event generators.

Looking for an axion in a haystack of muons

The search for axion-like particles X in muon decays is an excellent opportunity for the MEG II and Mu3e experiments to extend their horizons beyond μ + → e + γ and μ + → e + e - e +. A suitable process for both experiments is the two-body decay μ + → e + X, whose only signature is a monochromatic peak close to the kinematic endpoint of the positron energy spectrum of the μ + → e + νeν̅μ background. The hunt for such an elusive signal in a vast amount of irreducible background requires extremely accurate theoretical predictions to be implemented in a Monte Carlo event generator. This work presents a new state-of-the-art computation of μ + → e + νeν̅μ for polarised muons, accomplished with the McMule framework. The calculation includes next-to-next-leading order QED corrections and logarithmically enhanced terms at even higher orders. The results are also used to estimate the sensitivity of both experiments on the branching ratio of μ + → e + X, in order to evaluate the impact of the theoretical error.

NLO + parton-shower generator for varvec{Wc} production in the POWHEG BOX RES

We present the implementation of a next-to-leading-order plus parton-shower event generator for the hadronic production of a heavy charm quark accompanied by a leptonically-decaying W boson in the POWHEG BOX RES framework. We consider both signatures, i.e. pprightarrow c, ell ^- {bar{nu }}_{ell } and pprightarrow {bar{c}} ,ell ^+ {nu }_{ell }, and we include exactly off-shell and spin-correlation effects, as well as off-diagonal Cabibbo–Kobayashi–Maskawa contributions. We present particle-level results, obtained interfacing our code with the Herwig7.2 and Pythia8.3 shower Monte Carlo event generators, including hadronization and underlying-event effects, and compare them against the data collected by the CMS Collaboration at sqrt{s}=13 TeV.

Inclusive photon production at forward rapidities in pp and p–Pb collisions at sqrt{{{s}}_{textrm{NN}}}={5.02} TeV

A study of multiplicity and pseudorapidity distributions of inclusive photons measured in pp and p–Pb collisions at a center-of-mass energy per nucleon–nucleon collision of sqrt{s_{textrm{NN}}}~=~5.02 TeV using the ALICE detector in the forward pseudorapidity region 2.3 <~eta _textrm{lab} ~< 3.9 is presented. Measurements in p–Pb collisions are reported for two beam configurations in which the directions of the proton and lead ion beam were reversed. The pseudorapidity distributions in p–Pb collisions are obtained for seven centrality classes which are defined based on different event activity estimators, i.e., the charged-particle multiplicity measured at midrapidity as well as the energy deposited in a calorimeter at beam rapidity. The inclusive photon multiplicity distributions for both pp and p–Pb collisions are described by double negative binomial distributions. The pseudorapidity distributions of inclusive photons are compared to those of charged particles at midrapidity in pp collisions and for different centrality classes in p–Pb collisions. The results are compared to predictions from various Monte Carlo event generators. None of the generators considered in this paper reproduces the inclusive photon multiplicity distributions in the reported multiplicity range. The pseudorapidity distributions are, however, better described by the same generators.

Event-Shape-Dependent Analysis of Charm–Anticharm Azimuthal Correlations in Simulations

In high-energy collisions of small systems, by high-enough final-state multiplicities, a collective behaviour is present that is similar to the flow patterns observed in heavy-ion collisions. Recent studies connect this collectivity to semi-soft vacuum-QCD processes. Here we explore QCD production mechanisms using angular correlations of heavy flavour using simulated proton-proton collisions at s=13 TeV with the PYTHIA8 Monte Carlo event generator. We demonstrate that the event shape is strongly connected to the production mechanisms. Flattenicity, a novel event descriptor, can be used to separate events containing the final-state radiation from the rest of the events.

Measurement of the production of charm jets tagged with D0 mesons in pp collisions at sqrt{s} = 5.02 and 13 TeV

The measurement of the production of charm jets, identified by the presence of a D0 meson in the jet constituents, is presented in proton–proton collisions at centre-of-mass energies of sqrt{s} = 5.02 and 13 TeV with the ALICE detector at the CERN LHC. The D0 mesons were reconstructed from their hadronic decay D0 → K−π+ and the respective charge conjugate. Jets were reconstructed from D0-meson candidates and charged particles using the anti-kT algorithm, in the jet transverse momentum range 5 < pT,chjet< 50 GeV/c, pseudorapidity |ηjet| < 0.9 − R, and with the jet resolution parameters R = 0.2, 0.4, 0.6. The distribution of the jet momentum fraction carried by a D0 meson along the jet axis left({z}_{BigVert}^{textrm{ch}}right) was measured in the range 0.4 < {z}_{BigVert}^{textrm{ch}} < 1.0 in four ranges of the jet transverse momentum. Comparisons of results for different collision energies and jet resolution parameters are also presented. The measurements are compared to predictions from Monte Carlo event generators based on leading-order and next-to-leading-order perturbative quantum chromodynamics calculations. A generally good description of the main features of the data is obtained in spite of a few discrepancies at low pT,chjet. Measurements were also done for R = 0.3 at sqrt{s} = 5.02 and are shown along with their comparisons to theoretical predictions in an appendix to this paper.

Transverse momentum broadening of medium-induced cascades in expanding media

In this work, we explore the features of gluonic cascades in static and Bjorken expanding media by numerically solving the full BDIM evolution equations in longitudinal momentum fraction x and transverse momentum {{varvec{k}}} using the Monte Carlo event generator MINCAS. Confirming the scaling of the energy spectra at low-x, discovered in earlier works, we use this insight to compare the amount of broadening in static and expanding media. We compare angular distributions for the in-cone radiation for different medium profiles with the effective scaling laws and conclude that the out-of-cone energy loss proceeds via the radiative break-up of hard fragments, followed by an angular broadening of soft fragments. While the dilution of the medium due to expansion significantly affects the broadening of the leading fragments, we provide evidence that in the low-x regime, which is responsible for most of the gluon multiplicity in the cascade, the angular distributions are very similar when comparing different medium profiles at an equivalent, effective in-medium path length. This is mainly due to the fact that in this regime, the broadening is dominated by multiple splittings. Finally, we discuss the impact of our results on the phenomenological description of the out-of-cone radiation and jet quenching.

Neural network predictions of inclusive electron-nucleus cross sections

We investigate whether a neural network approach can reproduce and predict the electron-nucleus cross sections in the kinematical domain of present and future accelerator-based neutrino oscillation experiments. For this purpose, we consider the large amount of data available to the community via the University of Virginia's Quasielastic Electron Nucleus Scattering Archive and use a residual, fully connected feedforward neural network. We illustrate the training performances of the neural network by comparing its results with experimental data for the electron double-differential cross section on carbon. The agreement between predictions and data is remarkable from quasielastic to deep-inelastic scattering. To test the predicting power of the neural network we consider the numerous kinematical conditions for which experimental cross sections on calcium are available. Furthermore, we show the predictions of the electron-scattering cross sections on oxygen, argon, and titanium: Nuclei of particular interest in the context of present and future accelerator-based neutrino oscillation programs. The agreement between these predictions and the data is comparable to the one of other theoretical models commonly used to calculate electron and neutrino cross sections, such as SuSAv2 and GiBUU. Results obtained with genie, a Monte Carlo event generator, are also discussed for comparison. The good performances obtained with our neural network suggest that neural networks could be exploited for theoretical and experimental investigations of electron- and neutrino-nucleus scattering.

Underlying-event properties in pp and p–Pb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV

We report about the properties of the underlying event measured with ALICE at the LHC in pp and p–Pb collisions at sqrt{s_{textrm{NN}}} = 5.02 TeV. The event activity, quantified by charged-particle number and summed-pT densities, is measured as a function of the leading-particle transverse momentum left({p}_{textrm{T}}^{textrm{trig}}right) . These quantities are studied in three azimuthal-angle regions relative to the leading particle in the event: toward, away, and transverse. Results are presented for three different pT thresholds (0.15, 0.5 and 1 GeV/c) at mid-pseudorapidity (|η| < 0.8). The event activity in the transverse region, which is the most sensitive to the underlying event, exhibits similar behaviour in both pp and p–Pb collisions, namely, a steep increase with {p}_{textrm{T}}^{textrm{trig}} for low {p}_{textrm{T}}^{textrm{trig}} , followed by a saturation at {p}_{textrm{T}}^{textrm{trig}}approx 5 GeV/c. The results from pp collisions are compared with existing measurements at other centre-of-mass energies. The quantities in the toward and away regions are also analyzed after the subtraction of the contribution measured in the transverse region. The remaining jet-like particle densities are consistent in pp and p–Pb collisions for {p}_{textrm{T}}^{textrm{trig}}>10 GeV/c, whereas for lower {p}_{textrm{T}}^{textrm{trig}} values the event activity is slightly higher in p–Pb than in pp collisions. The measurements are compared with predictions from the PYTHIA 8 and EPOS LHC Monte Carlo event generators.

Event-activity-dependent production of strange and non-strange charmed baryons in the enhanced color-reconnection scheme

We investigated the production of charmed baryons with different isospin and strangeness content, compared to both charmed D0 mesons and to the baryon in proton–proton collisions at LHC energies. We used the PYTHIA 8 Monte Carlo event generator with color reconnection beyond leading-color approximation and proposed methods based on event-activity classifiers to probe the source of the charmed-baryon enhancement. We conclude that in the considered model class, the isospin of the charmed-baryon state has a strong impact on the enhancement pattern. Using the observables we propose, upcoming high-precision experimental data will be able to differentiate between mechanisms of strangeness and charm enhancement.

Jet substructure in neutral current deep inelastic e+p scattering at the upcoming Electron-Ion Collider

Predictions are made for the jet substructure of one-jet events produced in electron-proton neutral current deep inelastic scattering at the future Electron-Ion Collider for exchanged four-momentum squared, $Q^2 > 125$ GeV$^2$. Data are simulated using Monte Carlo event generators PYTHIA 8.304 and RAPGAP 3.308 at the center of mass energies $\sqrt{s}$ = 63.2, 104.9 and 141 GeV. Jets and subjets are produced by longitudinally invariant $k_T$ cluster algorithm. The subjet multiplicity distributions and differential jet shapes are measured for different jet sizes and varying jet-resolution parameter. A comparison is presented between the $k_T$ and anti-$k_T$ cluster algorithms for the study of jets and subjets using the simulated data and the HERA data.

Two-particle transverse momentum correlations in pp and p -Pb collisions at energies available at the CERN Large Hadron Collider

Two-particle transverse momentum differential correlators, recently measured in Pb--Pb collisions at energies available at the CERN Large Hadron Collider (LHC), provide an additional tool to gain insights into particle production mechanisms and infer transport properties, such as the ratio of shear viscosity to entropy density, of the medium created in Pb-Pb collisions. The longitudinal long-range correlations and the large azimuthal anisotropy measured at low transverse momenta in small collision systems, namely pp and p-Pb, at LHC energies resemble manifestations of collective behaviour. This suggests that locally equilibrated matter may be produced in these small collision systems, similar to what is observed in Pb-Pb collisions. In this work, the same two-particle transverse momentum differential correlators are exploited in pp and p-Pb collisions at $\sqrt{s} = 7$ TeV and $\sqrt{s_{\rm NN}} = 5.02$ TeV, respectively, to seek evidence for viscous effects. Specifically, the strength and shape of the correlators are studied as a function of the produced particle multiplicity to identify evidence for longitudinal broadening that might reveal the presence of viscous effects in these smaller systems. The measured correlators and their evolution from pp and p--Pb to Pb--Pb collisions are additionally compared to predictions from Monte Carlo event generators, and the potential presence of viscous effects is discussed.