MPI@LHC 2022 is the thirteenth conference of a series of successful joint theory/experiment workshops that bring together the world's leading experts from theory and LHC experiments to discuss the latest progress on the physics relevant to the Multiple Partonic Interactions.
This year it will take place in a hybrid mode (online and in-person) in Madrid, hosted by the IFT, UAM/CSIC.
The conference will cover the following topics, divided in working groups:
WG1: Minimum Bias, Underlying Event and Monte Carlo generators
WG2: Double Parton Scattering
WG3: High multiplicities and small systems
WG4: Diffraction and small-x
WG5: Heavy Ion collisions
Previous editions of the workshop
MPI@LHC'21 Lisbon, Portugal: https://indico.lip.pt/event/688/
MPI@LHC'19 Prague, Czech Republic: https://indico.cern.ch/event/816226/
MPI@LHC'18 Perugia, Italy: https://indico.cern.ch/event/736470/
MPI@LHC'17 Shimla, India: https://indico.cern.ch/event/625304/
MPI@LHC'16 San Cristóbal de las Casas, Mexico: http://www.nucleares.unam.mx/MPI2016/
MPI@LHC’15 Trieste, Italy: http://indico.ictp.it/event/a14280/
MPI@LHC'14 Krakow, Poland: https://indico.cern.ch/event/305160/
MPI@LHC'13 Antwerp, Belgium: http://indico.cern.ch/event/231843/
MPI@LHC'12 CERN Geneva, Switzerland: https://indico.cern.ch/event/184925/
MPI@LHC'11 DESY Hamburg, Germany: http://mpi11.desy.de/
MPI@LHC'10 Glasgow, Scotland: http://www.mpi2010.physics.gla.ac.uk
MPI@LHC'08 Perugia, Italy: http://www.pg.infn.it/mpi08/
This talk presents ATLAS recent measurements of distributions sensitive to Underlying event, the hadronic activity observed in relationship with the hard scattering in the event using the full ATLAS dataset at center-of-mass energy of 13 TeV. Measurement of charged-particle distributions as a function of Upsilon momentum and different Upsilon states will be discussed. The measurement benefits from the heavy-ion style approach to remove combinatorial and pileup backgrounds leading to increased sensitivity. In addition, charged-particle distributions measured in top-antitop events decaying leptonically will be shown. The measured distributions can constrain models of color reconnection mechanism in Monte-Carlo generators.
Multiple measurements of high-multiplicity pp and p-A collisions at LHC energies have revealed that these small collision systems exhibit some of the quark-gluon plasma features, e.g. collective behaviour and strangeness enhancement, formerly thought to be achievable only in heavy-ion collisions. The dependence on multiplicity suggests significant final-state interactions. A proposed method to narrow down the origin of the phenomena is to study the effect of Multi-Parton Interactions (MPIs). Although the MPIs cannot be measured directly, the event observable $R_{\mathrm{T}}$, quantifying the magnitude of the underlying event, has been suggested as an experimental proxy.
Measurements of the charged particle production as a function of $R_{\mathrm{T}}$ in pp, p-Pb and Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}}=5.02~\mathrm{TeV}$ with the ALICE experiment will be presented in the toward, away and transverse regions. The UE contributions measured in the transverse region are subtracted from the toward and the away regions to search for jet-like modifications in small systems. The jet-like signals are studied both as a function of $R_{\mathrm{T}}$ and leading particle $p_{\mathrm{T}}$.
Finally, the results on the production of $\pi$, K, p, $\phi$ and $\Xi$ as a function of $R_{\mathrm{T}}$ in pp collisions at $\sqrt{s}=13~\mathrm{TeV}$ are presented to explore the particle species dependence. In particular, the focus will be on the very low (high) $R_{\mathrm{T}}$ to isolate the “jet” (UE) contributions. All the above results are compared with predictions from QCD-inspired Monte Carlo event generators such as PYTHIA and EPOS-LHC.
With the 13 TeV proton-proton collisions measured by the CMS detector at the LHC, a detailed measurement of the Z+jet production provides ideal conditions for the hadronic jet production study, which provides a stringent test for perturbative QCD. It also helps to understand the non-perturbative QCD models by reflecting the soft-gluon emission kinematics in the low Z pt region. The differential cross sections of the Z(l+l-)+jets are measured as a function of jet multiplicity in various Z pT regions. The azimuthal correlation between the Z boson and the leading jet as well as that between the two leading jets are also measured for different Z pT. The measurements are compared to various theoretical predictions to evaluate the matrix
+element caluculation, the parton density models and the multi-parton interaction (MPI) effects.
On the MC tuning side, two color reconnection models, QCD-inspired and gluon-move are implemented to the PYTHIA8 generator. New sets of parameters are obtained by tuning the parameters of the color reconnection models and multi-parton interactions simultaneously to the 1.96, 7 and 13 TeV measurements that are sensitive to underlying event (UE) contributions. The predictions of the new tunings are compared for observables in various processes, including the minimum bias observables, the strange baryon particle multiplicity, the jet substructure, the Drell-Yan kinematics and UE activity and jet structures in top pair productions.
Hadronization is a complex quantum process whereby quarks and gluons become hadrons. The widely-used models of hadronization in event generators are based on physically-inspired phenomenological models with many free parameters. We propose an alternative approach whereby neural networks are used instead. Deep generative models are highly flexible, differentiable, and compatible with Graphical Processing Unit (GPUs). We make the first step towards a data-driven machine learning-based hadronization model by replacing a component of the hadronization model within the Herwig event generator (cluster model) with a Generative Adversarial Network (GAN). We show that a GAN can reproduce the kinematic properties of cluster decays. Furthermore, we integrate this model into Herwig to generate entire events that can be compared with the output of the public Herwig simulator and with e+e- data.
We present first soft physics and tuning results from the upcoming Sherpa 3.0 release. The new Sherpa release contains updated soft physics modules and is currently re-tuned and tested. Tuning results, in particular for hadronisation and underlying event observables will be discussed.
Monte Carlo Event Generators contain several free parameters that cannot be inferred from first principles and need to be tuned to better model the data. With increasing precision of perturbative theoretical calculations to higher orders and hence decreasing theoretical uncertainties, it becomes crucial to study the systematics of non-perturbative phenomenological models. A recent attempt was made at tuning the combination of the angular-ordered parton shower (AOPS) in Herwig7 and Lund string hadronization model (AOPS+string) to LEP data with a new approach called Autotunes [1]. However, the resultant tune shows worse performance to important observables like LEP event shapes when compared to the previous tunes of the AOPS+cluster model. Since the AOPS and the string hadronization model perform well in other tunes of Herwig7 and Pythia8, we would naively expect them to perform better together and thus investigate further by tuning with the Professor approach adopted in [2]. We present the results of our tune and compare it with the Herwig7 default (AOPS+cluster model) and Autotunes tunes.
[1] High dimensional parameter tuning for event generators, J.Belm, L.Gellersen, Eur.Phys.J.C 80 (2020) 1, 54
[2] Systematic event generator tuning for the LHC, A.Buckley et al, Eur.Phys.J.C 65 (2010) 331-357
In a factorization approach to double parton scattering, the initial state is described by double parton distributions (DPDs). These functions are currently poorly constrained by experiment, but provide a view on interesting correlations between partons in the hadron. We show that DPDs can be calculated from first principles using lattice QCD via the quasi-PDF approach, opening up a new way to constrain these distributions. Specifically, we argue that there exists a factorization formula between the physical lightcone-DPDs and the lattice calculable quasi-DPDs that is governed by a perturbative matching kernel. We verify the perturbative nature of the matching kernel at one-loop order by showing that the lightcone- and quasi-DPDs share the same infrared behaviour and that the matching kernel is free of logarithms involving infrared energy scales.
For the description of double hard interactions, double parton distributions (DPDs) are of major importance. We relate their Mellin Moments to the correlation functions of two local currents which we calculate on the lattice. We calculate all the Wick contractions for the first moment for the nucleon, considering both polarisation and flavor interference. We furthermore compare our results to the SU(6) quark model. Finally, we test whether DPDs can be factorized into a convolution of generalized parton distributions (GPDs).
With the advent of high luminosity accelerators, multi parton scattering (MPS)
physics at high-energy hadron colliders like the Tevatron at Fermilab and the
Large Hadron Collider (LHC) at CERN, in particular those observing double
parton scattering (DPS), has become one of the hottest topics of the modern
experimental particle physics. DPS results are usually interpreted in terms of
the so-called ``pocket formula''. This formula defines that the value of DPS
effective cross section should be universal, independent of the final states or
the phase space. However, there are some models that predict correlations
between partons inside a hadron. This leads us to a new class parton
distribution function, double PDF, that are not tantamount to the simple
multiplication of the standard single PDF.
Standard wisdom tells us that DPS should be more preferable at high energies.
This is why most of the phenomenological analysis of a possible impact of the
double PDF effect are focused on LHC energies. However, effects are mainly
expected at higher values of the Bjorken-$x$ or with a significant gap between
the $x$ values. Even for double $W$ boson production the typical value for the
Bjorken-$x$ is $\langle x \rangle \sim 0.01$.
In contrast to that, preferable conditions can be easily achieved at lower
energy experiments like the Spin Physics Detector at the NICA collider, where
the DPS contribution could be relatively small, but is still expected to be
far from zero.
In this talk we discuss the possible impact of the evolution of the double PDF
on the double $J/\psi$ production and the DPS effective cross sections at NICA
energies within the Gaunt--Stirling model.
The production of J/ψ pairs in high energy hadron collisions has been studied by several experiments. Despite the experimental and theoretical efforts, the origin of the process and the relative weight of different production mechanisms still remain unknown. At different center of mass energies the process can be described by single- and double-parton scattering sub-processes and can also be related to the hypothesis of the intrinsic charm of hadrons and the existence of exotic all-charm tetraquark states. Such states were predicted by theory for the first time in 1975 and have recently been experimentally observed.
The COMPASS experiment at CERN uses a 190 GeV/c negative pion beam scattering off different nuclear targets to study dimuon pair production. In this talk, the COMPASS results on J/psi pair production will be presented and compared to theoretical models.
CMS results on Double Parton Scattering
The GS09 and Pythia double PDFs (dPDFs) have both been constructed to satisfy (in some sense) momentum and valence number constraints. I compare the two sets of dPDFs to each other and to the "naive" dPDFs constructed out of a product of single PDFs, in the context of their predictions for double Drell-Yan production. I explain the differences between these predictions in terms of the procedures used to construct the dPDF sets. Based on arXiv:2208.08197
Double parton scattering (DPS) provides access to information about nucleon structure not accessible in single parton scattering: spatial, spin, and colour correlations between the partons inside the nucleon. For short distances between the two probed partons the double parton distributions (DPDs) containing this information can be calculated in perturbation theory. In this talk I will discuss how a consistent treatment of quark mass effects can be achieved in the evaluation of these short distance $1 \to 2$ splitting DPDs. In addition I will briefly illustrate how - even without a full calculation from Feynman diagrams - the massive splitting can be approximated at next-to-leading order in the strong coupling.
In this contribution, we present possible new advantages of observing double parton
scattering (DPS) processes initiated by photon-proton interactions. The observation of
DPS processes could lead to access fundamental information on double parton distribution
functions (dPDFs) of the protons [3]. These new quantities, appearing in the DPS cross
section, represent a novel and promising tool to access the 3D partonic structure of the
proton. In fact, dPDFs encode double parton correlations in hadrons which cannot be
accessed through, e.g., GPDs. Up to date, however, dPDFs are almost unknown and, in
particular, their dependence on the transverse distance of partons. However, as proved
in Refs. [1, 2] the information on the partonic proton structure cannot be extracted from
proton-proton collisions data. Therefore, in Ref. [3] we proposed the possibility to observe
DPS in processes initiated by quasireal photons. At such low virtualities, the photon will
fluctuate hadronically and/or electromagnetically in a q − q̄ pair which then initiates a
double parton scattering with the proton. The key idea is that the photon transverse size
could be almost controlled by measuring the virtuality and, in turn, the interaction rate in
the which encodes information on the transverse proton structure. We estimated the DPS
cross section for the four jets production in the HERA kinematics, and concluded that
DPS processes in photoproduction gives a significant fraction of the four jet production
cross sections. Recently, we started the study of the different mechanisms contributing to
photoproduction of quarkonium pairs in NRQCD at the EIC, namely via unresolved and
resolved photons. In the latter case, we study the relevance of double parton scatterings
along the lines of Refs. [3, 4, 5, 6, 7, 8, 9]. Like for the hadroproduction case, quarkonium-
pair photoproduction probes, in different kinematical domains, very different interesting
phenomena which will be made accessible at the future US EIC.
References
[1] M. Rinaldi and F. A. Ceccopieri, JHEP 1909, 097 (2019)
[2] M. Rinaldi and F. A. Ceccopieri, Phys. Rev. D 97, no. 7, 071501 (2018)
[3] F. A. Ceccopieri and M. Rinaldi, Phys. Rev. D 105 (2022) no.1, L011501
[4] J. P. Lansberg, H. S. Shao, N. Yamanaka, Y. J. Zhang and C. Noûs, Phys. Lett. B
807 (2020), 135559
[5] J. P. Lansberg, Phys. Rept. 889 (2020), 1-106 [arXiv:1903.09185 [hep-ph]].
[6] J. P. Lansberg, H. S. Shao, N. Yamanaka and Y. J. Zhang, Eur. Phys. J. C 79 (2019)
no.12, 1006 [arXiv:1906.10049 [hep-ph]].
[7] H. S. Shao and Y. J. Zhang, Phys. Rev. Lett. 117 (2016) no.6, 062001
[8] J. P. Lansberg and H. S. Shao, Nucl. Phys. B 900 (2015), 273-294
[9] J. P. Lansberg and H. S. Shao,
doi:10.1016/j.physletb.2015.10.083
In this work, we compute the double-inclusive gluon production cross-section within the framework of the dense-dilute Color Glass Condensate (CGC) approach. As a novelty, we provide the contribution accounting for the quantum correction in which the two gluons come from a single projectile source, thus giving renewed insights about a breaking of the accidental symmetry of the CGC, which straightforwardly reveals the presence of odd harmonics at leading order in the eikonal expansion.
Many new particles, mostly hadrons, are produced in high-energy collisions between atomic nuclei. The most popular models describing the hadron production process are based on the creation, evolution, and decay of resonances, strings, or quark-gluon plasma. The validity of these models is under vivid discussion, and it seems that a common framework for this discussion is missing. Here we introduce the diagram of high-energy nuclear collisions, where domains of the dominance of different hadron-production processes in the laboratory-controlled parameters, the collision energy, and the nuclear-mass number of colliding nuclei are indicated.
We argue the recent experimental results, in particular by NA61/SHINE, locate boundaries between the domains, allowing for the first time to sketch an example diagram. Finally, we discuss the immediate implications for experimental measurements and model development following the sketch.
The experimental observations of anisotropic flows in proton-proton and proton-nucleus collisions at RHIC and LHC energies has stimulated a big interest in these small systems due to the possible formation of short-lived droplets of quark-gluon plasma. We investigate the effects of nonequilibrium dynamics in relativistic proton-nucleus collisions by comparing a microscopic nonequilibrium transport approach, the Parton-Hadron-String-Dynamics (PHSD), with a macroscopic 2D+1 viscous hydrodynamical model, VISHNew. The initial conditions for the hydrodynamic evolution are extracted from PHSD at different times in order to study its impact on the subsequent medium evolution. We investigate the nonequilibrium dynamics by analysing quantities such as the energy density and the viscous corrections. We also discuss the role of the collective flows. The origin of the elliptic flow in high-multiplicity proton-nucleus collisions is still under debate; we address this issue by applying the event-shape engineering through the transverse spherocity observable, capable of classifying different final-state event topologies. Moreover, we discuss the development of the directed flow and in particular of its splitting, focusing on the connection with the baryon transport to midrapidity and the electromagnetic fields. The investigation of such anisotropic flow coefficients may allow to gain further insights into the mechanisms responsible for the QGP-like effects in small systems as well as to the transport properties of the produced medium, such as the electric conductivity which drives the evolution of the electromagnetic fields.
[1] L. Oliva, W. Fan, P. Moreau, S.A. Bass and E. Bratkovskaya, Phys. Rev. C 106, 044910 (2022)
[2] L. Oliva, P. Moreau, V. Voronyuk and E. Bratkovskaya, Phys. Rev. C 101, 014917 (2020)
We will give an overview of the progress in correlations and collectivity from the initial state point of view within the Color Glass Condensate effective theory.
The latest results on the high multiplicity and small system produced within the CMS Collaboration will be discussed.
This talk presents recent ATLAS measurements from small collision systems. A particular focus will be on studies of jet production in p+Pb collisions to search for the jet quenching phenomenon in small systems. Further, the measurement of longitudinal flow decorrelation in pp collisions will be discussed. This study provides information on the initial longitudinal dynamics in small collision systems. These measurements provide stringent tests of the theoretical understanding of the initial state in heavy-ion collisions.
In this talk, we present our ongoing work on the inclusion of string interactions, namely rope hadronization in heavy ion collisions, as an alternative to QGP. Rope hadronization, where strings in close proximity form higher-order colour multiplets, has been earlier observed to explain QGP-like effects in p-p collisions such as strangeness enhancement.
The main challenge involved in implementing such mechanisms in dense environments is that the majority of strings in dense systems lie in random spatial orientation, which makes calculating the force among all strings during an event a complex problem.
In our approach, we use a special Lorentz frame, where a pair of string pieces are in parallel planes with respect to each other. Hence, for a whole Pythia/Angantyr event, every possible string pair is boosted to this frame to calculate the rope effects.
This novel approach of rope hadronization can produce the cumulative result of generating QGP-like effects in dense environments, for both small and large systems. In this talk, we present our new implementation (https://arxiv.org/abs/2205.11170) - the Gleipnir framework - in Pythia8/Angantyr and results from strangeness yield analyses performed in high-multiplicity p-p and heavy-ion collisions with comparison to data for minimum bias events.
The studies as a function of the charged-particle multiplicity allowed the discovery of heavy-ion-like effects in pp collisions. However, to some degree, the multiplicity classifiers used so far bias the sample towards hard processes like multi-jet final states. This limitation of the existing event classifiers makes it difficult to perform, for example, jet-quenching searches in high-multiplicity pp collisions. In this work I will discuss different approaches that have been developed over the last few years. The studies range from machine learning to a new event classifier called flattenicity. The resulting event classifiers have shown a strong sensitivity to multi-parton interactions.
We study the proposal by Kharzeev-Levin to determine entanglement entropy in Deep Inelastic Scattering (DIS) from parton distribution functions (PDFs) and relates the former to the entropy of final state hadrons. We find several uncertainties in the current comparison to data, in particular uncertainties related to the overall normalization, the relation between charged versus total hadron multiplicity in the comparison to experimental results as well as different methods to determine the number of partons in Deep Inelastic Scattering. We further provide a comparison to data based on leading order HERA PDF as well as PDFs obtained from an unintegrated gluon distribution subject to next-to-leading order Balitsky-Fadin-Kuraev-Lipatov and Baltisky-Kovchegov evolution. Within uncertainties we find good agreement with H1 data. We provide also predictions for entropy at lower photon virtualities, where non-linear QCD dynamics is expected to become relevant.
In this talk, we review some recent advances in understanding the interplay between transverse momentum dependent gluon distributions on the one hand, and the saturation phenomenon at low $x$ on the other hand. Using the example of forward heavy-quark pair hadro- or leptoproduction, we argue that saturation corrections are needed to properly take the gauge-dependence and polarisation properties of the gluon distribution into account. Moreover, we give an outline of how nonlinear JIMWLK evolution can be combined in a consistent way with the resummation of Sudakov logarithms.
The two-scale evolution of Collins-Soper-Sterman-type for the unintegrated PDF in the regime qT << mu is derived from the BFKL evolution in physical rapidity with longitudinal momentum conservation constraint in coordinate space form. Comparisons of analytic results with Monte-Carlo implementation of the evolution are also done. This work is a continuation of the calculation done in Phys. Rev. D 104, 054039 (2021).
Strong electromagnetic fields of accelerated heavy ions at the LHC can be used to probe photon-photon and photon-hadron interactions in a new kinematic regime. ALICE has extensively studied these processes in ultra-peripheral collisions (UPC) characterized by impact parameters larger than the sum of nuclear radii. In recent years, photon-induced processes have been also observed in heavy ion collisions with nuclear overlap.
In this contribution, we review recent ALICE results on the coherent $J/\psi$ photoproduction in Pb-Pb UPCs probing nuclear gluon distributions at low Bjorken-x, and the first measurement at the LHC of the dissociative $J/\psi$ photoproduction off protons sensitive to quantum fluctuations of the proton structure. Latest ALICE measurements on the coherent $J/\psi$ photoproduction in collisions with nuclear overlap will be also presented. Finally, the prospects for future measurements of photon-induced processes in the LHC Run 3 and 4 will be discussed.
In this contribution we analyze the double vector meson production in photon–hadron interactions at the LHC and present predictions for the $\rho \rho$, $\phi \phi$, $J/\Psi J/\Psi$, $\rho J/\Psi$ and $\phi J/\Psi$ production considering the double scattering mechanism. We estimate the total cross sections and rapidity distributions at LHC energies and compare our results with the predictions for the double vector meson production in $\gamma \gamma$ interactions at hadronic colliders. We present predictions for the different rapidity ranges probed by the ALICE and LHCb detectors. Our results indicate that the analysis of the double vector meson production at the LHC can be useful to constrain the double-scattering mechanism and improve our understanding of the QCD dynamics.
It is explained that the Good – Walker model of diffraction is viable only for small t which are parametrically smaller than the inverse square of the nucleon radius. Example of J/\psi production in inelastic rapidity gap kinematics is considered and it is argues that the knockout mechanism becomes important/ dominates already at -t ~ 0.5 GeV^2. Restrictions on the distribution of gluons in nucleons from the MPI phenomenology are also considered.
On behalf of the ZEUS Collaboration.
The exclusive photoproduction reactions γp → J/ψ(1S)p and γp → ψ(2S)p have
been measured at an ep centre-of-mass energy of 318 GeV with the ZEUS detector
at HERA using an integrated luminosity of 373 pb−1. The measurement was made
in the kinematic range 30 < W < 180 GeV, Q2 < 1 GeV2 and |t| < 1 GeV2, where
W is the photon–proton centre-of-mass energy, Q2 is the photon virtuality and t
is the squared four-momentum transfer at the proton vertex. The decay channels
used were J/ψ(1S) → μ+μ−, ψ(2S) → μ+μ− and ψ(2S) → J/ψ(1S)π+π− with
subsequent decay J/ψ(1S) → μ+μ−. The ratio of the production cross sections,
R = σψ(2S)/σJ/ψ(1S), has been measured as a function of W and |t| and compared
to previous data in photoproduction and deep inelastic scattering and with predic-
tions of QCD-inspired models of exclusive vector-meson production, which are in
reasonable agreement with the data
The back-to-back di-$\pi^{0}$ correlation functions in forward directions have been observed to be suppressed in $p(d)+$A collisions relative to $p$$+$$p$ collisions. The nonlinear gluon dynamics in nuclei is one of the possible mechanisms responsible for the suppression. In addition, theorists pointed out that the double-parton scattering (DPS) can be an alternative explanation of the suppression in $d+$A collisions. During the 2015 and 2016 RHIC runs, STAR collected data with the Forward Meson Spectrometer (FMS, 2.6 < $\eta$ < 4.0) in $p$$+$$p$, $p+$Al, $p$+Au, and $d$+Au collisions at $\sqrt{s_{\rm{_{NN}}}}=200$ GeV. The recorded datasets enable the measurements of di-$\pi^{0}$ azimuthal correlations in $p$$+$$p$ and $p+$A collisions with different mass numbers. Meanwhile, a comparison of the correlations in $d+$Au and $p+$Au collisions provides an opportunity to study the impact of DPS. In this talk, we will present the results on di-$\pi^{0}$ correlations at forward rapidities in $p$$+$$p$ and $p+$A collisions, together with the preliminary results in $d$+Au collisions.
Relativistic heavy-ion collisions are complex systems that involve physics with multiple length scales. Despite its tiny size and short lifetime, heavy-ion collisions emit various species of particles, carrying different information about the underlying dynamics of the Quark-Gluon Plasma (QGP). The soft hadrons exhibit collective behaviors. QCD jets can probe the QGP at multiple length scales. Electromagnetic radiations and W and Z bosons have negligible interactions with the QGP; hence, they carry information at their production points. Similar to modern astronomy, such a variety of particle measurements form multi-messenger heavy-ion physics. The JETSCAPE framework is an open-source theoretical tool to study all these observables in a unified setting. By drawing constraints from multiple aspects of heavy-ion measurements, the JETSCAPE framework can comprehensively describe heavy-ion collisions. In this talk, I will discuss the recent progress of the JETSCAPE framework in the quantitative descriptions of jet observables and deriving Baysian global constraints on QGP transport properties. I will also highlight the future extension of the JETSCAPE framework to study small systems and heavy-ion collisions at large net baryon densities.
The Angantyr model is used in Pythia8 to generate complete final states for collisions involving heavy ions. This talk will describe the main features of Angantyr, but the main focus will be on recent developments of models for final-state string interactions, and the resulting collective effects in pp, pA and AA collisions in Pythia8/Angantyr.
The three point light-cone correlation function of color charge densities is evaluated in light-cone gauge. The proton is approximated as a non-perturbative 3-quark state plus perturbative corrections due to the emission of a gluon which is not required to be soft. The resulting expressions provide the C-odd contribution to the initial conditions for high-energy evolution of the dipole scattering amplitude to small x.
We also present numerical estimates for the perturbative 3-gluon exchange amplitude (hard Odderon) as a function of dipole size, impact parameter, their relative azimuthal angle, and light-cone momentum cutoff x. We find that this amplitude increases as x decreases from 0.1 to 0.01. At yet lower x, the reversal of this energy dependence would reflect the onset of universal small-x renormalization group evolution.
We study, to all orders in perturbative QCD, the universal behavior of the saturation momentum $Q_s(L)$ controlling the transverse momentum distribution of a fast parton propagating through a dense QCD medium with large size $L$. Due to the double logarithmic nature of the quantum evolution of the saturation momentum, its large $L$ asymptotics is obtained by slightly departing from the double logarithmic limit of either next-to-leading log (NLL) BFKL or leading order DGLAP evolution equations. At fixed coupling, or in conformal N = 4 SYM theory, we derive the large $L$ expansion of $Q_s(L)$ up to order $\alpha_s^{3/2}$. In QCD with massless quarks, where conformal symmetry is broken by the running of the strong coupling constant, the one-loop QCD $\beta$-function fully accounts for the universal terms in the $Q_s(L)$ expansion. Therefore, the universal coefficients of this series are known exactly to all orders in $\alpha_s$.
ALICE is a detector dedicated to study heavy-ion collisions and the physics of strongly interacting matter at extreme energy densities, where a phase of matter called quark-gluon plasma is formed. In this talk, a selection of recent results of Pb-Pb collisions obtained with ALICE is presented, including results from soft to hard probes. Small systems results obtained in pp and p-Pb collisions with high multiplicity are also discussed, since they can exhibit some features as seen in heavy-ion collisions. Lastly, the future perspectives after the detector upgrades are presented.
Heavy ion collisions allow access to novel QCD and QED studies in a laboratory. This opportunity is actively pursued by the CMS experiment at the LHC and is an integral part of its experimental program. Precision measurements of the properties of quark-gluon plasma (QGP) and the strong electromagnetic fields produced in heavy ion collisions at high energies are among recent research pursuits. This talk will present highlights from CMS Collaboration on various QGP and QED probes, such as jets, electroweak bosons, heavy flavor hadrons, quarkonia, and leptons.
This talk presents an overview of recent ATLAS measurements in heavy ion collision systems. These include multiple measurements of jet production and structure, which probe the dynamics of the hot, dense Quark-Gluon Plasma formed in relativistic nucleus-nucleus collisions, and measurements of quarkonia and heavy flavor production to probe the QGP medium properties. Furthermore, the photo-nuclear events can provide a clean probe of the partonic structure of the nucleus analogous to deep inelastic scattering. This talk will also discuss a new measurement of dijet production in ultra-peripheral Pb+Pb collisions.
Over the last decades, the theoretical picture of how hadronic jets interact with nuclear matter has been extended to account for the medium’s finite longitudinal length and expansion. However, only recently a first-principle approach has been developed that allows to couple the jet evolution to the medium flow and anisotropic structure in the dilute limit. In this talk, we will show how to extend this approach to the dense regime, where the resummation of multiple in-medium scatterings is necessary. Particularly, we will consider the modifications of the single particle momentum broadening distribution and single gluon production rate in evolving matter. The resummation is performed by either computing the opacity series or starting from the all order BDMPS-Z formalism. We will also discuss the (novel) resulting modifications to jets' substructure.
Transport based models are widely used to describe the evolution of hard probes in the quark gluon plasma. However, so far such descriptions are strictly only valid in the limit where the medium is assumed to be homogeneous in the transverse direction to the propagation. In this talk, I will discuss how to generalize parton transport to include medium anisotropies. I will show that the resulting evolution laws include quantum corrections to the usual Boltzmann transport. Finally, I will show that such terms lead to power corrections to the jet quenching parameter.
We report about the very recent release (4.0.0) of the EPOS event generator, with many new developments.
The basic principle of treating (nucleonic or partonic) scatterings in parallel (the main element of the EPOS approach), based on S-matrix theory, has been used for two decades. But there is a major problem. For inclusive cross sections (but only for those!), important cancellations occur, leading to factorization (in pp) or binary scaling (in AA). In a parallel scattering scenario, these cancellations must come out (they cannot be imposed), which requires very high precision and good strategies -- and this is provided with EPOS4. We discuss these theoretical issues, and applications from 4GeV to 13TeV.
Beauty and charm quarks are ideal probes of pertubative Quantum Chromodymanics in proton-proton collisions, owing to their large masses. In this talk the role of multi-parton interactions in the production of doubly-heavy hadrons is discussed. New methods of speeding up simulations with Pythia, a Monte Carlo event generator, when generating events containing heavy quarks have been developed, enabling the production of large samples with multiple heavy-quark pairs. Significantly higher production rates of doubly-heavy hadrons are predicted in models that allow heavy quarks originating from different parton-parton interactions (within the same hadron-hadron collision) to combine to form such hadrons. A set of experimental measurements capable of differentiating these additional contributions are suggested. Quantitative predictions are sensitive to the modelling of colour reconnections.
A generic expression to compute triple parton scattering (TPS) cross sections in high-energy proton-proton (p-p) and proton-nucleus (p-A) collisions will be presented as a function of the corresponding single-parton-scattering (SPS) cross sections and the transverse parton distribution in the proton encoded in the σeff,TPS effective parameter. The value of σeff,TPS is found to be closely related to the effective cross section that characterizes double-parton scatterings (DPS). Estimates for triple charm (ccbar) and bottom (bbbar) production (including quarkonia bound states) in pp and p-A collisions at LHC, FCC, and ultrahigh cosmic-ray energies will be presented based on next-to-next-to-leading order perturbative calculations for the corresponding SPS cross sections.
We study the possible contribution of color correlations to the observable final states in Double Parton Scattering (DPS). We show that there is a specific class of Feynman diagrams related to the so-called 1->2 processes when the Sudakov suppression of color correlations is partially relaxed.
Therefore, the contributions of color correlations do not decrease with the transverse scales. We explain that while the contribution of color correlations in GPD may be significant, there is an additional source of smallness in the cross sections since the created particles and complex conjugated ones in both hard processes must be close to each other in space-time. Consequently, the color correlations contribute up to 2-5% in the four jet final states in Tevatron and LHC kinematics. However, the effective relaxation of Sudakov suppression gives hope that although they are small relative to color singlet correlations, they eventually can be observed.
This talk is based on the preprint: arXiv:2210.13282 by B. Blok and J. Mehl.
Collinear double parton distributions (DPDFs) possess a non-trivial color structure that can be classified by a projection onto irreducible representations. The DGLAP evolution equations then depend on the color representation of the respective DPDF. In the color singlet case, the DGLAP splitting kernels are identical to the ones for ordinary single parton distributions. For the color non-singlet kernels, this is not the case. At leading order in pertubation theory, they can be deduced with little effort from the color singlet ones. At next-to-leading order, the procedure becomes more involved and the kernels were unknown until now.
In this talk, I will present for the first time the full splitting kernels for unpolarized, longitudinal and quark transversity DPDFs at NLO. They were obtained using two different and independent methods. The first one is based on graph-by-graph results for PDF splitting kernels, the second one utilizes the matching formula for transverse momentum dependent matrix elements. As a by-product, the NLO anomalous dimension for rapidity evolution of DPDFs in the gluon decuplet and 27-multiplet was also obtained for the first time.
Various mechanisms potentially capable to tame the energy rise of interaction cross sections and of particle production in hadronic collisions are analysed. In particular, a phenomenological implementation of higher twist corrections to parton scattering processes, as realized in the QGSJET-III Monte Carlo generator, is described in some detail. The relation of the mechanism to ad hoc parametrizations of the energy-dependent transverse momentum cutoff for minijet production, commonly employed by general purpose Monte Carlo generators is discussed.
We calculate the total cross section and transverse momentum distributions
for the production of the enigmatic $\chi_{c1}(3872)$ (or X(3872))
assuming different scenarios:
$c \bar c$ state and $D^{0*} {\bar D}^0 + D^0 {\bar D}^{0*}$ molecule.
The derivative of the $c \bar c$ wave function needed in the first
scenario is taken from a potential model calculations.
Compared to earlier calculations of molecular state we include not only
single parton scattering (SPS) but also double parton scattering (DPS)
contributions.
The latter one seems to give smaller contribution than the SPS one.
The upper limit for the DPS production of
$\chi_{c1}(3872)$ is much below the CMS data.
We compare results of our calculations with existing experimental data
of CMS, ATLAS and LHCb collaborations.
Reasonable cross sections can be obtained in either $c \bar c$
or molecular $D {\bar D}^*$ scenarios for $X(3872)$, provided one takes
into account both directly produced $D^0, \bar D^0$, as well as
$D^0, \bar D^0$ from the decay of $D^*$. However arguments related
to the lifetime of $D^*$ suggest that the latter component may be not active.
With these reservations, also a hybrid scenario is not excluded.
The presentation will be based on:\
A. Cisek, W. Schafer and A. Szczurek, arXiv:2203.07827.
Measurements of charm and beauty production in proton-proton (pp) collisions are an important test of perturbative QCD calculations, based on the factorisation approach. Recent results of charm baryon-to-meson ratios in pp collisions at midrapidity have highlighted a significant enhancement of these ratios compared to measurements in e+e– collisions, suggesting that the fragmentation of charm quarks into mesons is not universal across different collision systems.
The study of open and hidden heavy-flavour hadron production in pp collisions as a function of the charged-particle multiplicity is a powerful tool to study the interplay between hard and soft QCD processes responsible for particle production in hadronic collisions and to retrieve information on the role of multi-parton interactions in heavy-flavour production. Furthermore, recent LHC results highlighted the presence of collective-like effects among the particles produced in high-multiplicity pp and proton-lead (p–Pb) collisions, similarly to what observed in heavy-ion collisions, where a quark-gluon plasma state is produced. Extending these studies to the heavy-flavour sector is crucial for a better understanding of the source of these features, which is currently debated.
The excellent capabilities of the ALICE experiment for particle identification, track and decay-vertex reconstruction allow us to precisely reconstruct a large set of heavy-flavour species down to very low transverse momentum.
In this contribution, the latest ALICE results of open heavy-flavour and quarkonium production will be discussed and compared to predictions from different models. A particular focus will be given to measurements of baryon-to-meson production yields in the charm sector in pp collisions, self-normalised yields of open and hidden heavy-flavour hadrons as a function of the event multiplicity in pp collisions, and elliptic-flow coefficient in high-multiplicity pp and p–Pb collisions.
I give an overview of recent theoretical work devoted to understanding the observations of collective behavior in small systems (p-p and p-A collisions, or peripheral A-A collisions). I discuss the progress made and some future prospects in relation to the two main facets of this issue, represented, respectively, by the initial state of the collisions and their subsequent dynamics. I highlight new interdisciplinary connections triggered by the investigation of collective phenomena with light ions and finite particle numbers.
Two-particle correlations are emerging as powerful tools for studying the medium produced in heavy-ion collisions. In particular, two-particle transverse momentum correlations enable measurements of the collision dynamics sensitive to momentum currents. Their evolution with collision multiplicity, as proxy of the system lifetime, gives information about the specific shear viscosity, $\eta/s$, and the system relaxation time, $\tau_{\pi}$.
Particle production measured at low momenta in small collision systems, such as pp and p--Pb, at LHC energies points to manifestations of collective behavior, longitudinal long-range correlations, and large azimuthal anisotropy, that suggests those small systems may produce short-lived QGP matter similar to that observed in Pb--Pb collisions.
In this talk, measurements of two-particle transverse momentum correlators in pp, p--Pb, and Pb--Pb collisions are presented. The strength and shape of the correlators are studied as functions of the produced particle multiplicity to identify evidence for longitudinal broadening that might reveal the presence of viscous effects expected to arise in fluid-like systems produced in those collisions. The measured correlators, their strengths and shapes, are additionally compared to model predictions and their evolution, along the pp, p--Pb, and Pb--Pb systems, is discussed.
The very forward energy, measured at large rapidities, is a powerful tool for characterising proton fragmentation in pp and p–Pb collisions. The correlation of forward energy with particle production at midrapidity gives direct insights into the initial stages of high-energy hadronic collisions.
The forward energy can also be studied in relation with the large transverse momentum particles produced at midrapidity. Such studies complement the measurements of the underlying event, usually interpreted in the framework of models implementing centrality-dependent multiple parton interaction.
Results about the correlation between the very forward energy and particle production at midrapidity in pp collisions at $\sqrt{s}$ = 13 TeV and in p-Pb collisions at $\sqrt{s_{\rm NN}}$ = 8.16 TeV, measured with the ALICE detector, will be discussed.
Finally, these results will be compared with expectations of hadronic interaction event generators, such as PYTHIA and EPOS, to test the models capability in describing both the multi-parton-interaction-dependent UE and the forward fragmentation, observables mainly driven by non-perturbative QCD physics.
Correlations between charged particles provide important insight about the hadronization process. The analysis of the momentum difference between charged hadrons in pp, p-lead, and lead-lead collisions of various energies is performed in order to study the dynamics of hadron formation. The spectra of correlated hadron chains are explored and compared to the predictions based on the quantized fragmentation of a three dimensional QCD helix string. This provides an alternative view of the two-particle correlation phenomenon typically attributed to the Bose-Einstein correlation. It provides insight into the mismodelling of low transverse momentum production of charged particles observed in ee, pp and heavy ion collisions.
The collective behaviour, associated with the quark-gluon plasma formation in ultrarelativistic heavy-ion collisions, has been observed also in high-multiplicity collisions of small systems such as pp and p--Pb at the LHC. Using the data recorded with the ALICE experiment, thanks to its unique particle identification capability and the forward multiplicity detectors, it is possible to study ultra-long-range correlations of identified particles. Such analyses allow to extract the flow coefficients of many different particle species free of non-flow contamination employing the template fit method. While the mass ordering has been observed before, a baryon-meson splitting in the intermediate $p_{\rm T}$ region, with a statistical significance larger than 3$\sigma$, is observed for the first time. Such a behaviour cannot be explained by the jet fragmentation alone, thus advocating the contribution from the quark coalescence that shows the evidence of partonic collectivity.
On behalf of the ZEUS Collaboration.
Collective behaviour of final-state hadrons, and multiparton interactions are studied in high-multiplicity ep scattering at a centre-of-mass energy 𝑠√ = 318 GeV with the ZEUS detector at HERA. Two- and four-particle azimuthal correlations, as well as multiplicity, transverse momentum, and pseudorapidity distributions for charged- particle multiplicities 𝑁ch ≥ 20 are measured. The dependence of two-particle correlations on the virtuality of the exchanged photon shows a clear transition from photoproduction to neutral current deep inelastic scattering. For the multiplicities studied, neither the measurements in photoproduction processes nor those in neutral current deep inelastic scattering indicate significant collective behaviour of the kind observed in high-multiplicity hadronic collisions at RHIC and the LHC. Comparisons of PYTHIA predictions with the measurements in photoproduction strongly indicate the presence of multiparton interactions from hadronic fluctuations of the exchanged photon.
In this talk, we show that the same QCD formalism that accounts for the suppression of high-pT hadron spectra in heavy-ion collisions predicts a medium-enhanced pair production in high-pT jets. We also present an experimental strategy to quantify such an enhancement by measuring the production of ccbar-tagged jets at high-pT in ultrarelativistic heavy-ion collisions at the LHC.
Measurements of quarkonia production in ultra-peripheral heavy-ion collisions are of important value to studying photon-photon and photon-nucleus interactions, the partonic structure of nuclei, and mechanisms of the vector-meson output. LHCb is a single-arm forward detector with a pseudorapidity coverage from 2 to 5. We will present a new measurement of the coherent $J/\psi$ and $\psi(2S)$ production in ultra-peripheral collisions using PbPb data collected by the LHCb detector. This gives the highest precision currently accessible in the forward rapidity. Comparisons to theoretical calculations and other experimental results will also be discussed.
In this talk, we will present the latest measurements of heavy flavor flow from the CMS collaboration.
I will present recent development of hadronization and colour reconnection in the Herwig 7 event generator.
The elastic scattering of protons at 13 TeV is measured in the range of protons??? transverse momenta allowing the access to the Coulomb-Nuclear-Interference region. The data were collected thanks to dedicated special LHC beta* = 2.5km optics. The total cross section as well as rho-parameter, the ratio of the real to imaginary part of the forward elastic scattering amplitude, are measured and compared to various models and to results from other experiments. The measurement of exclusive production of pion pairs at the LHC using 7 TeV data is also presented. This represents the first use of proton tagging to measure an exclusive hadronic final state at the LHC.
CMS results on diffraction and low x
We present a phenomenological analysis of events with two high transverse momentum (pT_{T}T) jets separated by a large (pseudo-)rapidity interval void of particle activity, also known as jet-gap-jet events. In the limit where the collision energy $\sqrt{s}$ is much larger than any other momentum scale, the jet-gap-jet process is described in terms of perturbative pomeron exchange between partons within the Balitsky-Fadin-Kuraev-Lipatov (BFKL) limit of perturbative quantum chromodynamics (QCD). The BFKL pomeron exchange amplitudes, with resummation at the next-to-leading logarithmic approximation, have been embedded in the PYTHIA8 Monte Carlo event generator. Standard QCD dijet events are simulated at next-to-leading order in αs_{s}s matched to parton showers with POWHEG+PYTHIA8. We compare our calculations to measurements by the CDF, D0, and CMS experiments at center-of-mass energies of 1.8, 7 and 13 TeV. The impact of the theoretical scales, the parton densities, final- and initial-state radiation effects, multiple parton interactions, and $p_T$ thresholds and multiplicities of the particles in the rapidity gap on the jet-gap-jet signature is studied in detail. With a strict gap definition (no particle allowed in the gap), the shapes of most distributions are well described except for the CMS azimuthal-angle distribution at 13 TeV. The survival probability is surprisingly well modelled by multiparton interactions in PYTHIA8. Without multiparton interactions, theoretical predictions based on two-channel eikonal models agree qualitatively with fits to the experimental data.
We study inclusive dijet production in deep inelastic scattering at NLO within the Color Glass Condensate effective field theory. We begin by considering this process in general small-$x$ kinematics. We first show that the differential cross-section is infrared and collinear safe. We demonstrate the factorization of large rapidity logarithms that can then be resummed via JIMWLK renormalization, and we extract the NLO impact factor for which we provide explicit expressions.
We then specialize in the transverse back-to-back kinematics where this process is sensitive to unpolarized and linearly polarized parts of the Weizsäcker-Williams (WW) gluon distribution. We isolate in the impact factor the large Sudakov double and single logarithms at finite $N_c$. We show that small-$x$ and Sudakov resummation can be performed simultaneously provided that the small-$x$ evolution of the WW distribution, formulated in terms of the projectile rapidity, is amended by a kinematic constraint that imposes lifetime ordering of successive gluon emissions. We also comment on non-logarithmically enhanced terms in the impact factor that can break TMD factorization at NLO in the saturation regime. We conclude by discussing on-going efforts towards the numerical evaluation of the NLO dijet cross-section at the EIC.
Refs:
[1] P. Caucal, F. Salazar and R. Venugopalan, JHEP 2021 (11), 1-108 18,2021
[2] P. Caucal, F. Salazar, B. Schenke and R. Venugopalan, arXiv:2208.13872
We study dijet production within the small-x improved TMD factorization, which is a formalism based on the Color Class Condensate suitable to study relatively hard jet production processes and to take into account gluon saturation. The formalism also allows to perform the Sudakov resummation as well as implements the power corrections that are necessary to access the dijet decorrelation region. We study both the full b-space Sudakov resummation as well as the simplified momentum space approach, which is more suitable for Monte Carlo simulations. We compute observables for the updated ATLAS kinematics as well as for the future FoCal kinematics and discuss modifications due to final state shower and hadronization.