Event Generator

This is the webpage and data repository for our Monte Carlo generator of correlated nuclear configurations to be used in pA and AA high energy collisions simulations. Our original work about the generation of configurations and the subsequent works on beam fragmentation, flow asymmetries in AA and recent works on color fluctuations are listed below, along with links for downloading the configurations themselves.

Latest results for (xp, λ) correlations at RHIC and LHC are in the bottom of the page

The FORTRAN format to read the configurations is also provided as a reference. Note that in the applications to the pA, AA collisions the impact parameter has to be recentered on event by event basis to account for shift of the center of mass of the system as compared to nominal.

Massimiliano Alvioli, Mark Strikman


A Monte Carlo generator of nucleon configurations in complex nuclei including Nucleon-Nucleon correlations

M. Alvioli, H.-J. Drescher, M. Strikman

Phys. Lett. B680 (2009) 225

We developed a Monte Carlo event generator for production of nucleon configurations in complex nuclei consistently including effects of Nucleon-Nucleon (NN) correlations. Our approach is based on the Metropolis search for configurations satisfying essential constraints imposed by short- and long-range NN correlations, guided by the findings of realistic calculations of one- and two-body densities for medium-heavy nuclei. The produced event generator can be used for Monte Carlo (MC) studies of pA and AA collisions. We perform several tests of consistency of the code and comparison with previous models, in the case of high energy proton-nucleus scattering on an event-by-event basis, using nucleus configurations produced by our code and Glauber multiple scattering theory both for the uncorrelated and the correlated configurations; fluctuations of the average number of collisions are shown to be affected considerably by the introduction of NN correlations in the target nucleus. We also use the generator to estimate maximal possible gluon nuclear shadowing in a simple geometric model.

The configurations obtained using our code for 16O, 40Ca and 208Pb nuclei are linked below. We provide 100k configurations for each nucleus, organized in 10 files. Data can be read using this FORTRAN format; the first three columns are the x,y,z coordinates, while the fourth column represents the isospin variable (0 or 1) of the particular nucleon. Files are compressed due to using WordPress. The file sizes indicated are after inflation.

16O – 6.5 MB each  40Ca – 17 MB each 208Pb – 84 MB each
o16-01.dat ca40-01.dat pb208-01.dat
o16-02.dat ca40-02.dat pb208-02.dat
o16-03.dat ca40-03.dat pb208-03.dat
o16-04.dat ca40-04.dat pb208-04.dat
o16-05.dat ca40-05.dat pb208-05.dat
o16-06.dat ca40-06.dat pb208-06.dat
o16-07.dat ca40-07.dat pb208-07.dat
o16-08.dat ca40-08.dat pb208-08.dat
o16-09.dat ca40-09.dat pb208-09.dat
o16-10.dat ca40-10.dat pb208-10.dat

Beam Fragmentation in Heavy Ion Collisions with Realistically Correlated Nuclear Configurations

M. Alvioli, M. Strikman

Phys. Rev. C83 (2011) 044905

We develop a new approach to production of the spectator nucleons in the heavy ion collisions. The energy transfer to the spectator system is calculated using the Monte Carlo based on the updated version of our generator of configurations in colliding nuclei which includes a realistic account of short-range correlations in nuclei. The transferred energy distributions are calculated within the framework of the Glauber multiple scattering theory, taking into account all the individual inelastic and elastic collisions using an independent realistic calculation of the potential energy contribution of each of the nucleon-nucleon pairs to the total potential. We show that the dominant mechanism of the energy transfer is tearing apart pairs of nucleons with the major contribution coming from the short-range correlations. We calculate the momentum distribution of the emitted nucleons which is strongly affected by short range correlations including its dependence on the azimuthal angle. In particular, we predict a strong angular asymmetry along the direction of the impact parameter b, providing a unique opportunity to determine the direction of b. Also, we predict a strong dependence of the shape of the nucleon momentum distribution on the centrality of the nucleus-nucleus collision.

pb-pb-b05_inter_corr_side_flat pb-pb-b05_inter_corr_rear_flat

Initial State Anisotropies and their Uncertainties in Ultrarelativistic Heavy-Ion Collisions from the Monte Carlo Glauber Model

M. Alvioli, H. Holopainen, K.J. Eskola, M. Strikman

Phys. Rev. C85 (2012) 034902

In hydrodynamical modeling of heavy-ion collisions the initial state spatial anisotropies are translated into momentum anisotropies of the final state particle distributions. Thus understanding the origin of the initial state anisotropies and their uncertainties is important before extracting specific QCD matter properties, such as viscosity, from the experimental data. In this work we review the wounded nucleon approach based on the Monte Carlo Glauber model, charting in particular the uncertainties arising from modeling of the nucleon-nucleon interactions between the colliding nucleon pairs and nucleon-nucleon correlations inside the colliding nuclei. We discuss the differences between the black disk model and a probabilistic profile function approach for the inelastic nucleon-nucleon interactions, and investigate the influence of initial state correlations using state-of-the-art modeling of these.

197Au – no correlations 197Au – central correlations 197Au – full corr. (2b operator chains) 197Au – full corr. (3b operator chains)
(1000 configurations) (1000 configurations) (940 configurations)

(1820 configurations)

Download Download Download Download

208 Pb -208Pb: participant matter (shown: from one of the two colliding nuclei)


Color fluctuations effects in proton-nucleus collisions

M. Alvioli, M. Strikman

Phys. Lett. B 722, 347 (2013)

Color fluctuations in hadron-hadron collisions are responsible for the presence of inelastic diffraction and lead to distinctive differences between the Gribov picture of high energy scattering and the low energy Glauber picture. We find that color fluctuations give a larger contribution to the fluctuations of the number wounded nucleons at a given impact parameter that the fluctuations of the number of nucleons at a given impact parameter. For the impact parameter averaged fluctuations two contributions are comparable. As a result, standard procedures for selecting peripheral (central) collisions lead to selection of configurations in the projectile which interact with smaller (larger) than average strength. We suggest that studies of pA collisions with a hard trigger may allow to observe effects of color fluctuations.




Revealing “flickering” of the interaction strength in pA collisions at the CERN LHC

M. Alvioli, L. Frankfurt, V. Guzey, M. Strikman

Phys. Rev. C90 (2014) 034914

Using the high-energy color fluctuation formalism to include inelastic diffractive processes and taking into account the collision geometry and short-range nucleon-nucleon correlations in nuclei, we assess various manifestations of “flickering” of the parton wave function of a rapid proton in pA interactions focusing at energies available at the CERN Large Hadron Collider (LHC) in soft QCD processes and in the special soft QCD processes accompanying hard processes. We evaluate the number of wounded nucleons, Ncoll—the number of inelastic collisions of projectiles—in these processes and find a nontrivial relation between the hard collision rate and centrality. We study the distribution over Ncoll for a hard trigger selecting configurations in the nucleon with the strength larger or smaller than the average one and argue that the pattern observed in the LHC pA measurements by CMS and ATLAS for jets carrying a large fraction of the proton momentum, xp, is consistent with the expectation that these configurations interact with the strength which is significantly smaller than the average one, a factor of two smaller for xp∼0.5. We also study the leading twist shadowing and the European Muon Collaboration effects for superdense nuclear matter configurations probed in the events with a larger-than-average number of wounded nucleons. We also argue that taking into account energy-momentum conservation does not change the distribution over Ncoll but suppresses hadron production at central rapidities.

fig1 fig2 fig3

Evidence for x-dependent proton color fluctuations in pA collisions at the CERN large hadron collider

M. Alvioli, B.A. Cole, L. Frankfurt, D.V. Perepelitsa, M. Strikman

Phys. Rev. C93 (2016) 011902(R)

The centrality dependence of forward jet production in pA collisions at the Large Hadron Collider (LHC) has been found to grossly violate the Glauber model prediction in a way that depends on the x in the proton. We argue that this modification pattern provides the first experimental evidence for x-dependent proton color fluctuation effects. On average, parton configurations in the projectile proton containing a parton with large x interact with a nuclear target with a significantly smaller than average cross section and have smaller than average size. We implement the effects of fluctuations of the interaction strength and, using the ATLAS analysis of how hadron production at backward rapidities depends on the number of wounded nucleons, make quantitative predictions for the centrality dependence of the jet production rate as a function of the x-dependent interaction strength σ(x). We find that σ(x)∼0.6<σ> gives a good description of the data at x=0.6. These findings support an explanation of the European Muon Collaboration effect as arising from the suppression of small-size nucleon configurations in the nucleus.

fig1 fig3

Mapping color fluctuations in the photon in ultraperipheral heavy ion collisions at the Large Hadron Collider

M. Alvioli, L. Frankfurt, V. Guzey, M. Strikman, M. Zhalov

Phys. Lett. B767 (2017) 450

Using information on photoproduction of light and heavy vector mesons on the nucleon and nuclei, we explicitly model color fluctuations (CFs) in the photon wave function and for the first time make predictions for the distribution over the number of wounded nucleons ν in the inelastic photon-nucleus scattering. We show that CFs lead to a dramatic enhancement of this distribution at ν=1 and large ν>10 compared to the standard Glauber model calculations. We also study discuss the implications of CFs in the photon on the total transverse energy ΣET and other observables in inelastic γA scattering with different triggers. Our predictions can be tested in proton-nucleus and nucleus-nucleus ultraperipheral collisions at the LHC and will help to map CFs, whose first indications have already been observed at the LHC.


Global analysis of color fluctuation effects in proton- and deuteron-nucleus collisions at RHIC and the LHC

M. Alvioli, L. Frankfurt, D. Perepelitsa, M. Strikman

Physical Review D98 (2018) 071502(R)

We test the hypothesis that configurations of a proton with a large-x parton, xp0.1, have a smaller than average size. The QCD Q2 evolution equations suggest that these small configurations also have a significantly smaller interaction strength, which has observable consequences in collisions with nuclei. We perform a global analysis of jet production data in proton- and deuteron-nucleus collisions at RHIC and the LHC. Using a model which takes a distribution of interaction strengths into account, we quantitatively extract the xp-dependence of the average interaction strength, σ(xp), over a wide kinematic range. By comparing the RHIC and LHC results, our analysis finds that the interaction strength for small configurations, while suppressed, grows faster with collision energy than does that for average configurations. We check that this energy dependence is consistent with the results of a method which, given σ(xp) at one energy, can be used to quantitatively predict that at another. This finding further suggests that at even lower energies, nucleons with a large-xp parton should interact much more weakly than those in an average configuration, a phenomenon in line with explanations of the EMC effect for large-xp quarks in nuclei based on color screening.

pA @ LHC dA @ RHIC


Spin-isospin correlated configurations in complex nuclei and neutron skin effect in W+/ production in high-energy proton-lead collisions

M. Alvioli, M. Strikman

Phys.Rev. C100 (2019) 024912

We extend our Monte Carlo generator of global configurations in nuclei to include different spatial distributions of protons and neutrons in heavy nuclei taking into account the difference of spatial correlations between two protons, two neutrons and proton-neutron pairs. These configurations are used for building an event generator for proton-heavy nucleus collisions at the LHC for final states with a hard interaction in the channels where cross section for p-p and p-n scattering differ. Soft interactions are taken into account in the color fluctuation extension of the Glauber algorithm taking into account the inherently different transverse geometry of soft and hard p-N collisions. We use the new event generator to test an interesting observation of Physics Letters B745 (2015) 73-78, that the ratio of W+/W production rates in p-Pb collisions should significantly deviate from the inclusive value for peripheral collisions due to the presence of a neutron skin. We qualitatively confirm expectation of Physics Letters B745 (2015) 73-78, though for a realistic centrality trigger, we find the effect to be a factor of two smaller than the original estimate.

Download configurations
208Pb No NN correlations Central correlations Realistic correlations
48Ca No NN correlations Central correlations Realistic corelations



Revealing minijet dynamics via centrality dependence of

double parton interactions in proton-nucleus collisions

M. Alvioli, M. Azarkin, B. Blok, M. Strikman

Eur. Phys. J. C, 79:482 (2019)

One of the main challenges hampering an accurate measurement of the double parton scattering (DPS) cross sections is the difficulty in separating the DPS from the leading twist (LT) contributions. We argue that such a separation can be achieved, and cross section of DPS measured, by exploiting the different centrality dependence of DPS and LT processes in proton–nucleus scattering. We developed a Monte Carlo implementation of the DPS processes which includes realistic nucleon–nucleon (NN) correlations in nuclei, an accurate description of transverse geometry of both hard and soft NN collisions as well as fluctuations of the strength of interaction of nucleon with nucleus (color fluctuation effects). Our method allows the calculation of probability distributions of single and double dijet events as a function of centrality, also distinguishing double hard scatterings originating from a single target nucleon and from two different nucleons. We present numerical results for the rate of DPS as a function of centrality, which relates the number of wounded nucleons and the sum of transverse energy of hadrons produced at large negative (along the nucleus direction) rapidities, which is experimentally measurable. We suggest a new quantity which allows to test the geometry of DPS and we argue that it is a universal function of centrality for different DPS processes. This quantity can be tested by analyzing existing LHC data. The method developed in this work can be extended to the search for triple parton interactions.


Influence of neutron-skin effect on nuclear isobar collisions at RHIC

J. Hammelmann, A. Soto-Ontoso, M. Alvioli, H. Elfner, M. Strikman

arXiv:1908:10231 [nucl-th]

The unambiguous observation of a Chiral Magnetic Effect (CME)-driven charge separation is the core aim of the isobar program at RHIC consisting of 9640Zr+9640Zr and 9644Ru+9644Ru collisions at √sNN=200 GeV. We quantify the role of the spatial distributions of the nucleons in the isobars on both eccentricity and magnetic field strength within a relativistic hadronic transport approach (SMASH). In particular, we introduce isospin-dependent nucleon-nucleon spatial correlations in the geometric description of both nuclei, deformation for 9644Ru and the so-called neutron skin effect for the neutron-rich isobar i.e. 9640Zr. The main result of this study is a reduction of the magnetic field strength difference between 9644Ru+9644Ru and 9640Zr+9640Zr by a factor of 2, from 10% to 5% in peripheral collisions when the neutron-skin effect is included. Further, we find an increase of eccentricity by up to 10% when deformation is taken into account while neither the neutron skin effect nor the nucleon-nucleon correlations result into a significant modification of this observable with respect to the traditional Woods-Saxon modeling. Our results suggest a significantly smaller CME signal to background ratio for the experimental charge separation measurement in peripheral collisions with the isobar systems than previously expected.


feedback: alvioli(NoSPaM)pg.infn.it – M.Alvioli@uni_perugia – M.Alvioli@CNR_IRPI – M.Alvioli@PennState – ResearchGate – INSPIRE – ArXiV

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