Monte Carlo Event Generator Research Articles

Investigating correlated fluctuations of conserved charges with net-Λ fluctuations in Pb–Pb collisions at ALICE

Event-by-event fluctuations of conserved charges – such as electric charge, strangeness, and baryon number – in ultrarelativistic heavy-ion collisions provide insight into the properties of the quark-gluon plasma and the QCD phase diagram. They can be related to the higher moments of the multiplicity distributions of identified particles, such as the Λ baryon which carries both strangeness and baryon number and is thus of particular interest. We present the first measurement of net-Λ fluctuations in Pb–Pb collisions at sNN=5.02TeV as a function of centrality and the pseudorapidity acceptance of the measurement. The results are compared to expectations of the effects of global baryon number conservation as well as to predictions from the HIJING Monte Carlo event generator. In this analysis the Identity Method is applied in a novel way to account for the combinatoric background in the invariant mass distribution.

Exploring jet profiles in Pb-Pb collisions at 5.02 TeV with the ALICE detector

In this contribution, we present measurements of inclusive jet production with the ALICE detector in Pb-Pb collisions at sNN=5.02TeV. Production cross sections of charged particle jets and fully reconstructed jets are measured with jet resolution parameters R = 0.2 and 0.3. To quantify the medium induced parton energy loss, jet nuclear modification factors (RAA) are measured by utilizing POWHEG+Pythia8 predictions as reference cross section in pp collisions. We also calculate the ratio of jet production cross sections for jets reconstructed with different cone radii to explore the jet radial profile in Pb-Pb collisions. The ratios of jet cross sections are compared to a pp reference predicted by POWHEG+Pythia8 and in-medium energy loss model prediction by the JEWEL Monte Carlo event generator.

Learning new physics from a machine

We propose using neural networks to detect data departures from a given reference model, with no prior bias on the nature of the new physics responsible for the discrepancy. The virtues of neural networks as unbiased function approximants make them particularly suited for this task. An algorithm that implements this idea is constructed, as a straightforward application of the likelihood-ratio hypothesis test. The algorithm compares observations with an auxiliary set of reference-distributed events, possibly obtained with a Monte Carlo event generator. It returns a p-value, which measures the compatibility of the reference model with the data. It also identifies the most discrepant phase-space region of the data set, to be selected for further investigation. The most interesting potential applications are model-independent new physics searches, although our approach could also be used to compare the theoretical predictions of different Monte Carlo event generators, or for data validation algorithms. In this work we study the performance of our algorithm on a few simple examples. The results confirm the model-independence of the approach, namely that it displays good sensitivity to a variety of putative signals. Furthermore, we show that the reach does not depend much on whether a favorable signal region is selected based on prior expectations. We identify directions for improvement towards applications to real experimental data sets.

Ultrahigh-energy cosmic rays: Anomalies, QCD, and LHC data

Measurements of proton and nuclear collisions at the Large Hadron Collider at nucleon-nucleon c.m. energies up to $ \sqrt {S_{NN} } = 13\,{\rm{TeV}} $ have improved our understanding of hadronic interactions at the highest energies reached in collisions of cosmic rays with nuclei in the earth atmosphere, up to $ \sqrt {S_{NN} } $ ≈ 450 TeV. The Monte Carlo event generators (epos, qgsjet, and sibyll) commonly used to describe the air showers generated by ultrahigh-energy cosmic rays (UHECR, with ECR ≈ 1017-1020 eV) feature now, after parameter retuning based on LHC Run-I data, more consistent predictions on the nature of the cosmic rays at the tail of the measured spectrum. However, anomalies persist in the data that cannot be accommodated by the models. Among others, the total number of muons (as well as their maximum production depth) remains significantly underestimated (overestimated) by all models. Comparisons of epos, qgsjet, and sibyll predictions to the latest LHC data, and to collider MC generators such as pythia, indicate that improved description of hard multiple minijet production and nuclear effects may help reduce part of the data-model discrepancies, shed light on the UHECR composition approaching the observed ECR ≈ 1020 eV cutoff, and uncover any potential new physics responsible for the observed anomalies.

Multiplicity Dependence of Strangeness and Charged Particle Production in Proton--Proton Collisions

In this contribution, the production rates and the transverse momentum distributions of strange hadrons are reported as a function of charged particle multiplicity. In this analysis, the data collected in proton-proton collisions at $\sqrt{s}$ = 13 TeV with the ALICE detector at the LHC are used. It is found that the production rate of $K_{S}^{0}$, $\Lambda$, $\Xi^{\pm}$, and $\Omega$ increases with multiplicity faster than that for charged particles. The higher the strangeness content of the hadron, the more pronounced is the increase. Moreover, the energy and multiplicity dependence of charged particle production in pp collisions are presented and the results are compared to predictions from Monte Carlo (MC) event generators. It turns out that the average multiplicity density increases steeply with center-of-mass energy for high multiplicity classes.

Production of Σ0 Hyperons at LHC with ALICE

The first measurements of the transverse momentum (pT) spectra and integrated yields of Σ0 and Σ−0 baryons in proton-proton (pp) collisions at √s = 7 TeV are reported. The Σ0(Σ−0) signal is reconstructed via the Λ (Λ−) + γ decay channel by invariant mass analysis. The Λ (Λ−) baryon is reconstructed by its weak decay into p + π−(p− + π+), while the photon is detected exploiting the unique capability of the ALICE detector to measure low energy photons via conversion into e+e− pairs. A significant disagreement between the data and the predictions from the QCD-inspired Monte Carlo event generator PYTHIA is observed for the pT spectrum. The yield ratio of Σ0(Σ−0) to Λ is shown as a function of pT . Finally, the new data point for the integrated yield ratio of Σ0(Σ−0) to Λ from pp collisions at 7 TeV is found to be similar to the results obtained in different elementary collison systems (e+e−, pp) measured at collision energies below 100 GeV.

Muon-electron scattering at next-to-leading order accuracy

The next-to-leading order electro-weak radiative corrections to the µ±e- → µ±e- process are reviewed and their relevance is discussed for the MUonE experiment, proposed at CERN. The aim of MUonE is the high precision measurement of the QED running coupling constant in the space-like region, from which the full hadronic contribution can be extracted and used to provide a new and independent determination of the leading-order hadronic correction to the muon g − 2. In this context, the required accuracy demands that radiative corrections are accounted for at the highest level of precision and implemented into a Monte Carlo event generator for data analysis. The first step towards the final goal of theoretical precision, which will require the full set of NNLO corrections and resummation of higher orders, is the inclusion of NLO electro-weak corrections.

Exclusive LHC physics with heavy ions: SuperChic 3

We present results of the updated SuperChic 3 Monte Carlo event generator for central exclusive production. This extends the previous treatment of proton–proton collisions to include heavy ion (pA and AA) beams, for both photon and QCD-initiated production, the first time such a unified treatment of exclusive processes has been presented in a single generator. To achieve this we have developed a theory of the gap survival factor in heavy ion collisions, which allows us to derive some straightforward results about the A scaling of the corresponding cross sections. We compare against the recent ATLAS and CMS measurements of light-by-light scattering at the LHC, in lead-lead collisions. We find that the background from QCD-initiated production is expected to be very small, in contrast to some earlier estimates. We also present results from new photon-initiated processes that can now be generated, namely the production of axion-like particles, monopole pairs and monopolium, top quark pair production, and the inclusion of W loops in light-by-light scattering.

Status of the BabaYaga event generator

The Monte Carlo event generator BabaYaga has been developed for high precision simulation of QED processes (e+e−→ e+ e− , e+e−→ µ+µ− and e+e−→ γγ) at flavour factories, chiefly for luminometry purposes, with an estimated theoretical accuracy at the 0.1% level or better. QED radiative corrections are included by means of a Parton Shower in QED matched with exact next-to-leading order corrections to reach the required accuracy. The latter is assessed by means of consistent comparisons to independent calculations and an estimate of the size of missing higher-order corrections. The main theoretical framework is overviewed and the status of the generator is summarized.

Kinematic strangeness production in cluster hadronization

We present a modification to the non-perturbative strangeness production mechanisms in the Monte-Carlo event generator Herwig in order to make the processes more dynamic and collective. We compare the model to a series of observables for soft physics at both LEP and LHC.

Study of Pseudorapidity and Transverse-Momentum Distributions of Charged Particles in pp Interactions at √s = 13 TeV Using Hadron Production Models

Pseudorapidity (η) and Transverse momentum (pT) distributions of charged particles produced in proton-proton (pp) collisions at √s = 13 TeV are studied. Transverse momentum distributions of charged particles are considered in the Pseudorapidity region of |η| < 0.8 and in the transverse momentum range of 0 < pT < 20 GeV/c. EPOS1.99, EPOS-LHC and QGSJETII-04 models are used to perform simulations. In order to check the validity of simulations performed using these Monte-Carlo event generators, the results are compared to the measurements of ALICE experiment at the LHC CERN. For the pT distribution, the EPOS1.99 and QGSJET underpredict the experimental data while the EPOS-LHC model reproduced the distribution very well. EPOS-LHC model describes the experimental data very well as compared to the other two models for the ratio of pT spectra of charged particles at √s = 13 TeV to 7 TeV. EPOS-LHC also describes the pseudorapidity distribution very well while the EPOS 1.99 and QGSJETII-04 underpredict and overpredict the distribution respectively.

Measurement of photon–jet transverse momentum correlations in 5.02 TeV Pb + Pb and pp collisions with ATLAS

Jets created in association with a photon can be used as a calibrated probe to study energy loss in the medium created in nuclear collisions. Measurements of the transverse momentum balance between isolated photons and inclusive jets are presented using integrated luminosities of 0.49 nb−1 of Pb + Pb collision data at sNN=5.02 TeV and 25 pb−1 of pp collision data at s=5.02 TeV recorded with the ATLAS detector at the LHC. Photons with transverse momentum 63.1<pTγ<200 GeV and |ηγ|<2.37 are paired with all jets in the event that have pTjet>31.6 GeV and pseudorapidity |ηjet|<2.8. The transverse momentum balance given by the jet-to-photon pT ratio, xJγ, is measured for pairs with azimuthal opening angle Δϕ>7π/8. Distributions of the per-photon jet yield as a function of xJγ, (1/Nγ)(dN/dxJγ), are corrected for detector effects via a two-dimensional unfolding procedure and reported at the particle level. In pp collisions, the distributions are well described by Monte Carlo event generators. In Pb + Pb collisions, the xJγ distribution is modified from that observed in pp collisions with increasing centrality, consistent with the picture of parton energy loss in the hot nuclear medium. The data are compared with a suite of energy-loss models and calculations.

Towards a fully massive five-flavor scheme

In this work we explore first necessary steps to contruct a fully massive version of a variable flavour number scheme. In particular we focus, as an example, on an extension of the five-flavour scheme, where instead of neglecting explicit initial state quark mass effects, we retain all massive dependence, while keeping the resummation properties of the massless five-flavour scheme. We name this scheme five-flavour-massive (5FMS) scheme. Apart from consistently modified parton distribution functions, we provide all the ingredients that are needed to implement this scheme at MC@NLO accuracy, in a Monte Carlo event generator. As proof of concept we implement this scheme in SHERPA, and perfom a comparison of the new scheme with traditional ones for the simple process of scalar particle production in bottom quark fusion.

Measurements of the differential jet cross section as a function of the jet mass in dijet events from proton-proton collisions at sqrt{s}=13 TeV

Measurements of the differential jet cross section are presented as a function of the jet mass in dijet events, in bins of jet transverse momentum, with and without a jet grooming algorithm. The data have been recorded by the CMS Collaboration in proton-proton collisions at the LHC at a center-of-mass energy of 13 TeV and correspond to an integrated luminosity of 2.3 fb−1. The absolute cross sections show slightly different jet transverse momentum spectra in data and Monte Carlo event generators for the settings used. Removing this transverse momentum dependence, the normalized cross section for ungroomed jets is consistent with the prediction from Monte Carlo event generators for masses below 30% of the transverse momentum. The normalized cross section for groomed jets is measured with higher precision than the ungroomed cross section. Semi-analytical calculations of the jet mass beyond leading logarithmic accuracy are compared to data, as well as predictions at leading order and next-to-leading order, which include parton showering and hadronization. Overall, in the normalized cross section, the theoretical predictions agree with the measured cross sections within the uncertainties for masses from 10 to 30% of the jet transverse momentum.

Angular distributions in electroweak pion production off nucleons: Odd parity hadron terms, strong relative phases, and model dependence

The study of pion production in nuclei is important for signal and background determinations in current and future neutrino oscillation experiments. The first step, however, is to understand the pion production reactions at the free nucleon level. We present an exhaustive study of the charged-current and neutral-current neutrino and antineutrino pion production off nucleons, paying a special attention to the angular distributions of the outgoing pion. We show, using general arguments, that parity violation and time-reversal odd correlations in the weak differential cross sections are generated from the interference between different contributions to the hadronic current that are not relatively real. Next, we present a detailed comparison of three, state of the art, microscopic models for electroweak pion production off nucleons, and we also confront their predictions with polarized electron data, as a test of the vector content of these models. We also illustrate the importance of carrying out a comprehensive test at the level of outgoing pion angular distributions, going beyond comparisons done for partially integrated cross sections, where model differences cancel to a certain extent. Finally, we observe that all charged and neutral current distributions show sizable anisotropies, and identify channels for which parity violating effects are clearly visible. Based on the above results, we conclude that the use of isotropic distributions for the pions in the center of mass of the final pion-nucleon system, as assumed by some of the Monte Carlo event generators, needs to be improved by incorporating the findings of microscopic calculations.

Measurement of charged particle spectra in minimum-bias events from proton\u2013proton collisions at sqrt{s}=13,text {TeV}

Pseudorapidity, transverse momentum, and multiplicity distributions are measured in the pseudorapidity range |eta | < 2.4 for charged particles with transverse momenta satisfying p_{mathrm {T}} > 0.5,text {GeV} in proton–proton collisions at a center-of-mass energy of sqrt{s} = 13,text {TeV} . Measurements are presented in three different event categories. The most inclusive of the categories corresponds to an inelastic mathrm {p}mathrm {p} data set, while the other two categories are exclusive subsets of the inelastic sample that are either enhanced or depleted in single diffractive dissociation events. The measurements are compared to predictions from Monte Carlo event generators used to describe high-energy hadronic interactions in collider and cosmic-ray physics.

EKHARA 3.0: An update of the EKHARA Monte Carlo event generator

The Monte Carlo event generator EKHARA was upgraded during last years. The upgrades presented here contain: (a) the inclusion of new final states e+e−→e+e−η, e+e−→e+e−η′, e+e−→e+e−χci and e+e−→e+e−χci(→J∕ψ(→μ+μ−)γ); b) new γ∗−γ∗−P transition form factors and c) the radiative corrections to the reactions e+e−→e+e−P. For the upgrades (a) and (b), we present here only the pieces missing in other publications, mostly algorithms used in the phase space generation. The radiative corrections are presented here for the first time. A new algorithm of the phase space generation for the reaction e+e−→e+e−Pγ being its main part. A comparison with GGRESRC generator is presented. Big differences between the radiative corrections calculated by the EKHARA and the GGRESRC generators are observed. New version program summaryProgram Title: EKHARA 3.0Program Files doi:http://dx.doi.org/10.17632/g5nmv83g5z.1Licensing provisions: GPLv3Programming language: FORTRAN 77Supplementary material: The following publications are relevant for the upgrades presented in Section 2: [3,4,5,6]Journal reference of previous version: H. Czyż, S. Ivashyn, Comput. Phys. Commun. 182 (2011) 1338.Does the new version supersede the previous version?: YesReasons for the new version: Major upgrades.Summary of revisions: The upgrades contain the inclusion of the new final states e+e−→e+e−η, e+e−→e+e−η′, e+e−→e+e−χci and e+e−→e+e−χci(→J∕ψ(→μ+μ−)γ), new γ∗−γ∗−P transition form factors and the radiative corrections to the reactions e+e−→e+e−P.Nature of problem: The program is constructed to help in measurements of transition form factors at meson factories. To do this one needs a good description of the already existing data and calculation of the radiative corrections.Solution method: The models of various form factors were developed in [3,4,5,6] and are implemented in the program. For the radiative corrections a new algorithm of generation of the phase space for the reactions e+e−→e+e−Pγ was developed and is presented here.Additional comments including Restrictions and Unusual features: The program needs a quadruple precision version of a FORTRAN compiler.

Measurement of jet fragmentation in 5.02 TeV proton–lead and proton–proton collisions with the ATLAS detector

A measurement of the fragmentation functions of jets into charged particles in p+Pb collisions and pp collisions is presented. The analysis utilizes 28 nb−1 of p+Pb data and 26 pb−1 of pp data, both at sNN= 5.02 TeV, collected in 2013 and 2015, respectively, with the ATLAS detector at the LHC. The measurement is reported in the centre-of-mass frame of the nucleon–nucleon system for jets in the rapidity range |y⁎|<1.6 and with transverse momentum 45 <pT< 260 GeV. Results are presented both as a function of the charged-particle transverse momentum and as a function of the longitudinal momentum fraction of the particle with respect to the jet. The pp fragmentation functions are compared with results from Monte Carlo event generators and two theoretical models. The ratios of the p+Pb to pp fragmentation functions are found to be consistent with unity.

Measurement of the underlying event activity in inclusive Z boson production in proton-proton collisions at sqrt{s}=13 TeV

This paper presents a measurement of the underlying event activity in proton-proton collisions at a center-of-mass energy of 13 TeV, performed using inclusive Z boson production events collected with the CMS experiment at the LHC. The analyzed data correspond to an integrated luminosity of 2.1 fb−1. The underlying event activity is quantified in terms of the charged particle multiplicity, as well as of the scalar sum of the charged particles’ transverse momenta in different topological regions defined with respect to the Z boson direction. The distributions are unfolded to the stable particle level and compared with predictions from various Monte Carlo event generators, as well as with similar CDF and CMS measurements at center-of-mass energies of 1.96 and 7 TeV respectively.

Precise determination of αS(MZ) from global fits of e+e− data to NNLO+NNLL predictions

We present a comparison of the computation of energy-energy correlations and Durham algorithm jet rates in e+e− collisions at next-to-next-to-leading logarithmic accuracy matched with the next-to-next-to-leading order perturbative prediction to LEP, PEP, PETRA, SLC and TRISTAN data. With these predictions we perform global extractions of the strong coupling constant taking into account non-perturbative effects modelled with modern Monte Carlo event generators that simulate NLO QCD corrections.