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We discuss the upper limit, $k_{\text{max}}$, of the transverse-momentum integration performed in the $k_t$-factorization formula. Based on explicit calculations in the Yukawa theory and the study of seminal papers, we argue that $k_{\text{max}}$ is equal to the factorization scale $\mu_F$ used to factorize the cross section into an off-shell hard coefficient and a universal factor. There is consequently a relation between $k_{\text{max}}$ and the definition of unintegrated parton densities (UPDFs). The use of an inconsistent relation leads potentially to the overestimation of the cross section, which has been observed, e.g., in D-meson production. One of our conclusions is that UPDFs related to collinear PDFs by an integration up to $\mu\sim Q$, where $Q$ is the hard scale and $\mu$ the scale in the collinear PDFs, imply that $k_{\text{max}}^2 \sim Q^2$. Integrating the transverse-momentum significantly above may result in the overestimation of the cross section. On the opposite, for UPDFs related to the collinear ones by an integration of the transverse momentum up to infinity, any $k_{\text{max}}^2 > Q^2$ is fine., Comment: 21 pages

Nearly ten years ago, Kang, Ma, Qiu, and Sterman derived an evolution equation for a $Q\bar{Q}$ pair fragmenting into a quarkonium. In this study we explore the consequence of this evolution for the color-evaporation model, focusing on $J/\psi$ transverse-momentum ($p_t$) distributions in proton-proton collisions. We show that, as expected, it softens the spectrum obtained by fixed-order calculations. While next-to-leading-order calculations strongly overestimate data at large $p_t$, ours, including the (approximate) $Q\bar{Q}$ evolution and next-to-leading-order cross sections computed with Madgraph, are in good agreement with experiments. Since our study with the color-evaporation model shows a significant effect of the $Q\bar{Q}$ evolution at large $p_t$, a determination of scale-dependent long-distance-matrix elements of non-relativistic QCD could be necessary. To describe data at small and intermediate $p_t$, we use the $k_t$-factorization approach, and we argue that quarkonia data could help constrain unintegrated parton densities., Comment: 24 pages, 16 figures

We recently introduced new concepts, implemented in EPOS4, which allow to consistently accommodate factorization and saturation in high energy proton-proton and nucleus-nucleus collisions, in a rigorous parallel scattering framework. EPOS4 has a modular structure and in this paper, we present in detail how the "single scattering module" (the main EPOS4 building block) is related to perturbative QCD, and how these calculations are performed, with particular care being devoted to heavy flavor contributions. We discuss similarities and differences compared to the usual pQCD approach based on factorization., Comment: 36 pages, 39 figures

Within the framework of $k_t$-factorization, we compute the differential cross section for the production of $B$ and $D$ mesons, using a general-mass variable-flavor-number scheme. Our calculations include all relevant $2\to 2$ processes. We explain how to include the $2\to 1$ process in our calculations, but argue this is not (numerically) relevant at moderate transverse momentum due to its cancellation with the subtraction term. We apply this formalism to $pp$ collisions and compare our results with ALICE and LHCb data at central and forward rapidity., Comment: 21 pages, 18 figures

We investigate the possible existence of graviballs, a system of bound gravitons, and show that two gravitons can be bound together by their gravitational interaction. This idea connects to black hole formation by a high-energy $2\to N$ scattering and to the gravitational geon studied by Brill and Hartle. Our calculations rely on the formalism and techniques of quantum field theory, specifically on low-energy quantum gravity. By solving numerically the relativistic equations of motion, we have access to the space-time dynamics of the (2-gravitons) graviball formation. We argue that the graviball is a viable dark matter candidate and we compute the associated gravitational lensing., Comment: 25 pages, 6 figures, 2 videos as ancillary files. Includes two new sections

We present a perturbative QCD based model for vacuum and in-medium hadronization. The effects of induced energy loss and nuclear absorption have been included. The main objective is the determination of the relative contribution of these mechanisms to the multiplicity ratio observable, measured in semi-inclusive deep-inelastic scattering off deuterium and nuclear targets. This is directly related to the determination of the production length, $Lp$, necessary for a quark to turn into a prehadron. We compare our results with HERMES data for multiplicity ratio and $p_t$-broadening, and show that the description of the whole data set, keeping the model parameters fixed, puts strong constrains on $Lp$. Contrary to induced-energy-loss based models, we find an important contribution from nuclear absorption at HERMES energies. Finally, we discuss some consequences of our study for the LHC physics, and we present the model predictions for the future EIC experiment., Comment: 25 pages, 16 figures

The aim of this paper is to understand resonance production (and more generally particle production) for different collision systems, namely proton-proton (pp), proton-nucleus (pA), and nucleus-nucleus (AA) scattering at the LHC. We will investigate in particular particle yields and ratios versus multiplicity, using the same multiplicity definition for the three different systems, in order to analyse in a compact way the evolution of particle production with the system size and the origin of a very different system size dependence of the different particles., Comment: Proceedings of the 17th International Conference on Strangeness in Quark Matter (SQM 2017), https://www.epj-conferences.org/articles/epjconf/abs/2018/06/epjconf_sqm2018_09002/epjconf_sqm2018_09002.html

The influence of final-state radiation (FSR) of heavy quarks on observables in high-energy proton-proton collisions is studied. The transverse momentum correlation of D and Dbar mesons, which have been emitted with an azimuthal difference angle close to 180 degrees, is identified as an observable which is sensitive to the FSR process. We demonstrate this by performing calculations with the EPOS3+HQ model and with the event generator Pythia 6. The initial symmetric pT = pT' correlation for back-to-back pairs does not completely vanish in EPOS3+HQ, neither for the final DDbar pairs nor for the ccbar pairs before hadronisation. Also a significant difference in the shape of the correlation distribution for EPOS3+HQ and Pythia 6 is observed. Therefore, particle correlations in pp data offer the possibility to study several aspects of energy loss in heavy-ion collisions., Comment: 8 pages, 6 figures, submitted to Eur. Phys. J. C, references added

Recent experimental studies of the multiplicity dependence of heavy quark (HQ) production in proton-proton collisions at 7 TeV showed a strong non-linear increase of the HQ multiplicity as a function of the charged particle multiplicity. We try to understand this behavior using the EPOS3 approach. Two issues play an important role: multiple scattering, in particular its impact on multiplicity fluctuations, and the collective hydrodynamic expansion. The results are very robust with respect to many details of the modeling, which means that these data contain valuable information about very basic features of the reaction mechanism in proton-proton collisions., Comment: 11 pages, 20 figures

There is little doubt that in heavy ion collisions at the LHC and RHIC, we observe a hydrodynamically expanding system, providing strong evidence for the formation of a Quark Gluon Plasma (QGP) in the early stage of such collisions. These observations are mainly based on results on azimuthal anisotropies, but also on particle spectra of identified particles, perfectly compatible with a hydrodynamic evolution. Surprisingly, in p-Pb collisions one observes a very similar behavior, and to some extent even in p-p. We take these experimental observations as a strong support for a unified approach to describe proton-proton (p-p), proton-nucleus (p-A), and nucleus-nucleus (A-A) collisions, with a plasma formation even in tiny systems as in p-p scatterings., Comment: Invited plenary talk Quark Matter 2014

There is little doubt that hydrodynamic flow has been observed in heavy ion collisions at the LHC and RHIC, mainly based on results on azimuthal anisotropies, but also on particle spectra of identified particles, perfectly compatible with hydrodynamic expansions. Surprisingly, in p-Pb collisions one observes a very similar behavior. So do we see flow even in p-Pb? We will try to answer this question., Comment: Talk given at the "Rencontres de Moriond" 2014

Experimental transverse momentum spectra of identified particles in p-Pb collisions at 5.02 TeV show many similarities to the corresponding Pb-Pb results, the latter ones usually being interpreted in term of hydrodynamic flow. We analyse these data using EPOS3, an event generator based on a 3D+1 viscous hydrodynamical evolution starting from flux tube initial conditions, which are generated in the Gribov-Regge multiple scattering framework. An individual scattering is referred to as Pomeron, identified with a parton ladder, eventually showing up as flux tubes (or strings). Each parton ladder is composed of a pQCD hard process, plus initial and final state linear parton emission. Nonlinear effects are considered by using saturation scales $Q_{s}$, depending on the energy and the number of participants connected to the Pomeron in question. We compute transverse momentum ($p_{t}$) spectra of pions, kaons, protons, lambdas, and $\Xi$ baryons in p-Pb and p-p scattering, compared to experimental data and many other models. In this way we show in a quantitative fashion that p-Pb data (and even p-p ones) show the typical ``flow effect'' of enhanced particle production at intermediate $p_{t}$ values, more and more visible with increasing hadron mass.

We show that a fluid dynamical scenario describes quantitatively the observed mass splitting of the elliptical flow coefficients v2 for pions, kaons, and protons. This provides a strong argument in favor of the existence of a fluid dynamical expansion in pPb collisions at 5TeV.

With short resumes and highlights the discussions in the different working groups of the workshop MPI@LHC 2012 is documented.

Multinucleon transfer reactions in 40Ca+96Zr and 90Zr+208Pb have been measured at energies close to the Coulomb barrier in a high resolution gamma-particle coincidence experiment. The large solid angle magnetic spectrometer PRISMA coupled to the CLARA gamma-array has been employed. Trajectory reconstruction has been applied for the complete identification of transfer products. Mass and charge yields, total kinetic energy losses, gamma transitions of the binary reaction partners, and comparison of data with semiclassical calculations are reported. Specific transitions in 95Zr populated in one particle transfer channels are discussed in terms of particle-phonon couplings. The gamma decays from states in 42Ca in the excitation energy region expected from pairing vibrations are also observed., Comment: 10 pages, 9 figures

The dynamics and thermodynamics of phase transition in hot nuclei are studied through experimental results on multifragmentation of heavy systems (A$geq$200) formed in central heavy ion collisions. Different signals indicative of a phase transition studied in the INDRA collaboration are presented and their consistency is stressed.

Thanks to the high detection quality of the INDRA array, signatures related to the dynamics (spinodal decomposition) and thermodynamics (negative microcanonical heat capacity) of a liquid-gas phase transition have been simultaneously studied in multifragmentation events in the Fermi energy domain. The correlation between both types of signals strongly supports the existence of a first order phase transition for hot nuclei., Comment: 9 pages, 2 figures, Invited talk to Nucleus-nucleus 2003 Moscow June 2003

The fragmentation of the quasi-projectile is studied with the INDRA multidetector for different colliding systems and incident energies in the Fermi energy range. Different experimental observations show that a large part of the fragmentation is not compatible with the statistical fragmentation of a fully equilibrated nucleus. The study of internal correlations is a powerful tool, especially to evidence entrance channel effects. These effects have to be included in the theoretical descriptions of nuclear multifragmentation., Comment: 13 pages, 26 figures, submitted to Physical Review C

The dynamics and thermodynamics of phase transition in hot nuclei are studied through experimental results on multifragmentation of heavy systems (A>200) formed in central heavy ion collisions. Different signals indicative of a phase transition studied in the INDRA collaboration are presented and their consistency is stressed., Comment: 10 pages, 10 figures. Invited talk to IWM2001 (International Workshop on Multifragmentation and related topics), Catania, Italy - 28 nov-1 dec 2001