### Regular Series

#### Cracow Epiphany Conference on Advances in Heavy Ion Physics

Gluon TMDs from Forward $pA$ Collisions in the CGC

Acta Phys. Pol. B 50, 969 (2019)

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abstract

We calculate the production of a photon and two jets at forward rapidity in proton–nucleus collisions within the hybrid dilute-dense framework in the Color Glass Condensate (CGC) formalism. After obtaining the cross section for the quark-initiated channel, we consider the correlation limit, in which the vector sum of the transverse momenta of the three outgoing particles is small with respect to the individual transverse momenta. In this limit, the cross section simplifies considerably and can be written in a factorized form, sensitive to various unpolarized and linearly-polarized transverse-momentum-dependent gluon distribution functions (gluon TMDs).

On Resonance Contribution to Balance Functions

Acta Phys. Pol. B 50, 981 (2019)

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abstract

It is known that resonance decays influence the shape of the charge-balance functions measured in hadronic collisions. That is reflected in their rapidity and azimuthal widths and the integral and, therefore, has consequences for different model interpretations. In this paper, we show that the contribution from neutral resonance decays can be removed from the balance function in an analytical way, and test the performance of the removal procedure with PYTHIA events. Prospects for applications of the procedure to real-data analysis of balance functions are also discussed.

Testing Production Scenarios for (Anti-)(Hyper-)nuclei with Multiplicity-dependent Measurements at the LHC

Acta Phys. Pol. B 50, 991 (2019)

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abstract

The production of light anti- and hyper-nuclei provides unique observables to characterise the system created in high-energy proton–proton ($pp$), proton–nucleus ($pA$) and nucleus–nucleus ($AA$) collisions. In particular, nuclei and hyper-nuclei are special objects with respect to non-composite hadrons (such as pions, kaons, protons, etc.), because their size is comparable to a fraction or the whole system created in the collision. Their formation is typically described within the framework of coalescence and thermal-statistical production models. In order to distinguish between the two production scenarios, we propose to measure the coalescence parameter $B_A$ for different anti- and hyper-nuclei (that differ by mass, size and internal wave function) as a function of the size of the particle emitting source. The latter can be controlled by performing systematic measurements of light anti- and hyper-nuclei in different collision systems ($pp$, $pA$, $AA$) and as a function of the multiplicity of particles created in the collision. While it is often argued that the coalescence and the thermal model approach give very similar predictions for the production of light nuclei in heavy-ion collisions, our study shows that large differences can be expected for hyper-nuclei with extended wave functions, as the hyper-triton. We compare the model predictions with data from the ALICE experiment and we discuss perspectives for future measurements with the upgraded detectors during the High-Luminosity LHC phase in the next decade.

Overview of Recent Results from the PHENIX Collaboration

Acta Phys. Pol. B 50, 1003 (2019)

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abstract

We present recent results from the PHENIX Collaboration on a variety of observables over a wide-range in system size and collision energy.

Multiplicity Dependence of Two-particle Correlation in $pp$ and $p$Pb at the LHCb

Acta Phys. Pol. B 50, 1013 (2019)

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abstract

This paper shows that two-particle correlation function depends on multiplicity in proton–proton ($pp$) collisions at $\sqrt {s}=13$ TeV and proton–lead ($p$Pb) collisions at $\sqrt {s_{NN}}=5.02$ TeV using results from the LHCb experiment. Results show azimuthal anisotropies in the event shape, the so-called near-side ridge in long-range $\Delta \eta$. The near-side ridge in a small system has been understood as the collective behaviour of emitted particles in the collisions.

Correlations with Fluctuating Strings

Acta Phys. Pol. B 50, 1019 (2019)

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abstract

We present a semi-analytic approach to the forward–backward multiplicity correlations in ultra-relativistic nuclear collisions, based on particle emission from strings with fluctuating end-points. We show that with the constraints from rapidity spectra, one can obtain bounds for the magnitude of the standard measures of the forward–backward fluctuations. The method is generic under the assumption of independent production from sources. For definiteness, we use the wounded quark model for Au+Au and $d$+Au collisions at the energy of $\sqrt {s_{NN}}=200$ GeV.

Recent Results from Proton Intermittency Analysis in Nucleus–Nucleus Collisions from NA61 at CERN SPS

Acta Phys. Pol. B 50, 1029 (2019)

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abstract

The search for experimental signatures of the critical point (CP) of strongly interacting matter is one of the main objectives of the NA61/SHINE experiment at CERN SPS. In the course of the experiment, an energy (beam momentum 13$A$–150$A$ GeV/$c$) and system size ($p$+$p$, $p$+Pb, Be+Be, Ar+Sc, Xe+La) scan is performed. Local proton density fluctuations in transverse momentum space represent an order parameter of the chiral phase transition and are expected to scale according to a universal power-law in the vicinity of the CP; we probe their behaviour through an intermittency analysis of the proton second scaled factorial moments (SSFMs) in transverse momentum space. Previous such analyses revealed power-law behaviour in NA49 “Si”+Si collisions at 158$A$ GeV/$c$; no intermittency was observed in NA49 “C”+C and Pb+Pb collisions at the same energy, and in NA61/SHINE Be+Be collisions at 150$A$ GeV/$c$. Results suggest a baryochemical potential for the critical point in the vicinity of $\sim 250$ MeV. In the present work, we extend the analysis to the NA61/SHINE Ar+Sc system at 150$A$ GeV/$c$. We employ statistical techniques to subtract non-critical background and estimate statistical and systematic uncertainties. Finally, we use Monte Carlo simulations to estimate the likelihood of a spurious signal.

${\mit \Lambda }^+_c \rightarrow p \mu ^+\mu ^-$ Results from LHCb

Acta Phys. Pol. B 50, 1041 (2019)

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abstract

The search for rare decay ${\mit \Lambda }^{+}_{c} \rightarrow p \mu ^{+} \mu ^{-}$ with LHCb data corresponding to an integrated luminosity of 3.0 fb$^{-1}$ is presented. Such decays are highly suppressed in Standard Model and they are sensitive to contributions from new physics phenomena. No significant signal is observed. Using ${\mit \Lambda }_c \rightarrow p \phi$ decay as normalization channel, an upper limit on branching fraction of $B({\mit \Lambda }^{+}_{c} \rightarrow p \mu ^{+} \mu ^{-}) \lt 7.7~(9.6) \times 10^{-8}$ at 90% (95%) C.L. is set. The first observation of ${\mit \Lambda }^{+}_{c} \rightarrow p \omega$ is also reported.

Hydrodynamics with Spin — Recent Developments

Acta Phys. Pol. B 50, 1047 (2019)

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abstract

Recent theoretical developments in hydrodynamics of particles with spin $\frac {1}{2}$ are briefly reviewed.

Overview of Results from NA61/SHINE: Uncovering Critical Structures

Acta Phys. Pol. B 50, 1057 (2019)

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abstract

NA61/SHINE is a multi-purpose experiment to study hadron–proton, hadron–nucleus and nucleus–nucleus collisions at the CERN Super Proton Synchrotron. The experiment performs unique measurements for physics of strong interactions as well as important reference measurements for neutrino and cosmic-ray physics. The results from the strong interaction programme uncover rapid changes in collision-energy and system-size dependence of basic hadron production properties — the critical structures. They are attributed to the onset of deconfinement, onset of fireball and may indicate the critical point of strongly interacting matter.

Wounded Source Models Versus Experimental Results from RHIC

Acta Phys. Pol. B 50, 1071 (2019)

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abstract

The wounded source emission functions, $F(\eta )$, based on the wounded nucleon, quark, and quark–diquark models, are extracted from PHOBOS data on $d$+Au collisions at $\sqrt {s_{_{NN}}}=200$ GeV. Based on these functions, we calculate ${\rm d}N_{\text {ch}}/{\rm d}\eta$ distributions in $p$+Al, $p$+Au, $^3$He+Au and Au+Au collisions at the same energy and compare them with the experimental results.

Light-by-light Scattering in Ultra-peripheral Collisions in the Future

Acta Phys. Pol. B 50, 1077 (2019)

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abstract

This work will be focussed on a study of photon-pair production that can be created through fermionic boxes, resonance scattering, VDM-Regge mechanism, two-gluon exchange and pionic background. Each of these processes dominates at different ranges of two-photon invariant masses. Presented nuclear cross sections are in good agreement with the recently measured ATLAS and CMS data. Predictions including ALICE and LHCb experimental cuts for the next run at the LHC will be shown.

Software Platform for the Monitoring and Calibration of the LHCb Upgrade I Silicon Detectors

Acta Phys. Pol. B 50, 1087 (2019)

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abstract

Run 2 of the proton–proton collision data taking has finished at the end of 2018 and is now followed by the second long shutdown period (LS 2) that is going to be used for various upgrades and modification of all experiments operating at the LHC machine. The LHCb experiment will undergo a major upgrade in that time. In particular, the whole charge particle tracking system will be exchanged and adapted to read-out data at the LHC machine clock (40 MHz). Silicon planar technologies will be used for the vertex detector and Upstream Tracker. In this paper, we report on current status and plans regarding preparation of the high-level software platform for the emulation and monitoring of the silicon detectors of the upgraded LHCb spectrometer. This software is going to be an essential part of both detectors commissioning and daily operation.

Heavy Vector Meson Photoproduction in Ultra-peripheral Collisions at the LHC

Acta Phys. Pol. B 50, 1095 (2019)

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abstract

Pb nuclei, accelerated at the LHC, are sources of strong electromagnetic fields that can be used to measure photon-induced interactions in a new kinematic regime. These interactions can be studied in ultra-peripheral $p$–Pb and Pb–Pb collisions where impact parameters are larger than the sum of nuclear radii and hadronic interactions are strongly suppressed. Heavy quarkonium photoproduction is of particular interest since it is sensitive to gluon distributions in target hadrons. An overview of recent results on heavy vector meson photoproduction in ultra-peripheral Pb–Pb collisions and $p$–Pb collisions at the LHC will be presented, and implications for the study of gluon density distributions and nuclear gluon shadowing will be discussed. In addition, projections for heavy vector meson photoproduction measurements in LHC Run 3 and 4 will be presented.

Spectral Function for Overoccupied Gluodynamics from Classical Lattice Simulations

Acta Phys. Pol. B 50, 1105 (2019)

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abstract

We study the spectral properties of an overoccupied gluonic system far from equilibrium. Using classical Yang–Mills simulations and linear response theory, we determine the statistical and spectral functions. We measure dispersion relations and damping rates of transversally and longitudinally polarized excitations in the gluonic plasma, and also study further structures in the spectral function.

Probing Photonic Content of the Proton Using Photon-induced Dilepton Production in $p+$Pb Collisions at the LHC

Acta Phys. Pol. B 50, 1117 (2019)

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abstract

We propose a new experimental method to probe the photon–parton distribution function inside the proton (photon PDF) at the LHC energies. The method is based on the measurement of dilepton production from the $\gamma p\rightarrow \ell ^+\ell ^-+X$ reaction in proton–lead collisions. We firstly calculate the cross sections for this process with collinear photon PDFs, where we identify optimal choice of the scale, in analogy to deep inelastic scattering kinematics. We then perform calculations including the transverse-momentum dependence of the probed photon. Finally, we estimate rates of the process for the existing LHC data samples.

Strong and Electromagnetic Collective Effects from NA61/SHINE

Acta Phys. Pol. B 50, 1127 (2019)

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abstract

This contribution presents new NA61/SHINE results on collective effects, including spectator-induced electromagnetic (EM) phenomena in charged pion emission in collisions at beam momentum of $150A$ GeV/$c$ as well as directed flow for protons and charged pions as a function of transverse momentum and centrality in Pb$+$Pb collisions at $30A$ GeV/$c$. EM effects bring new, independent information on the space-time properties of the system. They may also suggest presence of the fast-moving participant charge close to the spectator system, as well as relatively fast spectator decay. The results on the directed flow at CERN SPS energies are complementary to STAR results and provide a bridge to FAIR and NICA energies. A specific picture of the longitudinal evolution of the system at CERN SPS energies emerges, where the centrality dependence of pion rapidity spectra is governed predominantly by the simple energy-momentum conservation and the collision geometry.

Extracting the Shear Relaxation Time of Quark–Gluon Plasma from Rapidity Correlations

Acta Phys. Pol. B 50, 1139 (2019)

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abstract

We propose that rapidity-dependent momentum correlations can be used to extract the shear relaxation time $\tau _\pi$ of the medium formed in high-energy nuclear collisions. Shear viscosity drives the initial fluctuations of the medium toward equilibrium at a rate characterized by the shear relaxation time. Momentum fluctuations, in excess of thermal fluctuations, that survive to freeze out, are remnants of the initial state, and influence the rapidity dependence of momentum correlations. We describe a method for calculating the rapidity dependence of two-particle momentum correlations with a second order, causal, diffusion equation that includes Langevin noise as a source of thermal fluctuations. In comparison to RHIC data, we find that the ratio $\tau _\pi /\nu \approx 5$–6, where $\nu =\eta /sT$ is the kinematic viscosity.

Masses of the Doubly-heavy Tetraquarks in a Constituent Quark Model

Acta Phys. Pol. B 50, 1151 (2019)

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abstract

We analyze the masses of the doubly-heavy tetraquark states $T_{QQ}$ using the variational method. We also find that our full model calculations give, in general, smaller binding energy, compared to those from the simplified quark model that treats quark dynamics inside the tetraquark the same as that inside a baryon. We investigate the main origin of this weaker binding energy. The original work is described in W. Park, S. Noh, S.H. Lee, Nucl. Phys. A 983, 1 (2019).

Acta Phys. Pol. B 50, 1159 (2019)

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abstract

Light-by-light (LbyL) scattering, $\gamma \gamma \rightarrow \gamma \gamma$, is a quantum-mechanical process, forbidden by the classical theory of electrodynamics, but possible in Quantum Electrodynamics via a loop diagram. Despite the small cross section, it is theoretically possible to observe this process in ultra-peripheral high-energy heavy-ion collisions. Based on 0.48 nb$^{-1}$ of 2015 Pb+Pb data, a first direct evidence of LbyL scattering was established by the ATLAS Collaboration in 2017. In total, 13 events were found in the signal region with a background expectation of $2.6\pm 0.7$ events. The excess corresponds to 4.4$\sigma$ significance over the background-only hypothesis. In November 2018, the new dataset of Pb+Pb collisions was collected by the ATLAS experiment with an integrated online luminosity of 1.7 nb$^{-1}$. This recent dataset has been employed to perform a preliminary study using the control sample from $\gamma \gamma \rightarrow e^+e^-$ process.

full authors' list

R.S. Bhalerao, G. Giacalone, P. Guerrero-Rodríguez, M. Luzum, C. Marquet, J.-Y. Ollitrault

Relating Eccentricity Fluctuations to Density Fluctuations in Heavy-ion Collisions

Acta Phys. Pol. B 50, 1165 (2019)

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abstract

The magnitude of anisotropic flow in a nucleus–nucleus collision is determined by the energy-density field, $\rho (x,y,z)$, created right after the collision occurs. Specifically, elliptic flow, $v_2$, and triangular flow, $v_3$, are proportional to the anisotropy coefficients $\varepsilon _2$ and $\varepsilon _3$, which are functionals of $\rho$. We express the mean and the variance of $\varepsilon _2$ and $\varepsilon _3$ as a function of the 1- and 2-point functions of $\rho$. These results generalize results obtained previously that were valid only for central collisions or only for identical point-like sources. We apply them to the color glass condensate effective theory using the recently derived expression of the 2-point function.

The Lepton Flavour Violation Decays at the LHCb Experiment

Acta Phys. Pol. B 50, 1177 (2019)

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abstract

These proceedings present a set of searches for lepton flavour violation, performed using decays of $B$ and $D$ mesons, using data collected by the LHCb experiment. None of the described searches resulted in an observation of the signal and upper limits on the branching fractions are established.

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

Acta Phys. Pol. B 50, 1185 (2019)

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abstract

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 $K_{\mathrm {S}}^{0}$, ${\mit \Lambda }$, ${\mit \Xi }^{\pm }$, and ${\mit \Omega }$ 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.

full authors' list

W. Płaczek, A. Abramov, S.E. Alden, R. Alemany Fernandez, P.S. Antsiferov, A. Apyan, H. Bartosik, E.G. Bessonov, N. Biancacci, J. Bieroń, A. Bogacz, A. Bosco, R. Bruce, D. Budker, K. Cassou, F. Castelli, I. Chaikovska, C. Curatolo, P. Czodrowski, A. Derevianko, K. Dupraz, Y. Dutheil, K. Dzierżęga, V. Fedosseev, N. Fuster Martinez, S.M. Gibson, B. Goddard, A. Gorzawski, S. Hirlander, J. Jowett, R. Kersevan, M. Kowalska, M.W. Krasny, F. Kroeger, M. Lamont, T. Lefevre, D. Manglunki, B. Marsh, A. Martens, J. Molson, D. Nutarelli, L.J. Nevay, A. Petrenko, V. Petrillo, S. Radaelli, S. Pustelny, S. Rochester, M. Sapinski, M. Schaumann, L. Serafini, V.P. Shevelko, T. Stoehlker, A. Surzhikov, I. Tolstikhina, F. Velotti, G. Weber, Y.K. Wu, C. Yin-Vallgren, M. Zanetti, F. Zimmermann, M.S. Zolotorev, F. Zomer

Gamma Factory at CERN — Novel Research Tools Made of Light

Acta Phys. Pol. B 50, 1191 (2019)

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abstract

We discuss the possibility of creating novel research tools by producing and storing highly relativistic beams of highly ionised atoms in the CERN accelerator complex, and by exciting their atomic degrees of freedom with lasers to produce high-energy photon beams. Intensity of such photon beams would be by several orders of magnitude higher than offered by the presently operating light sources, in the particularly interesting $\gamma$-ray energy domain of $0.1$–$400\,$MeV. In this energy range, the high-intensity photon beams can be used to produce secondary beams of polarised electrons, polarised positrons, polarised muons, neutrinos, neutrons and radioactive ions. New research opportunities in a wide domain of fundamental and applied physics can be opened by the Gamma Factory scientific programme based on the above primary and secondary beams.

Studying Baryonic Matter with HADES at GSI/FAIR

Acta Phys. Pol. B 50, 1205 (2019)

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abstract

HADES at GSI/FAIR is a versatile detector operating with a few GeV proton, pion and heavy-ion beams. The highlights of the experimental programme and plans for future measurements within Phase-0 of FAIR are presented.

Overview of Heavy Ions from the ATLAS Experiment

Acta Phys. Pol. B 50, 1217 (2019)

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abstract

The experimental data collected by the ATLAS experiment during the LHC Run 2 provide a broad physics program to probe and characterize the Quark–Gluon Plasma created in these collision systems. The large acceptance and high granularity of the detector is well-suited to perform detailed analyses on bulk phenomena, electroweak probes, quarkonia, charged particles and jets. In these proceedings, the latest results on these observables will be described, including comparisons with those produced in lighter $p+$Pb and $pp$ collisions, as well as with theoretical predictions.

Forward Proton Detectors in Heavy-ion Physics

Acta Phys. Pol. B 50, 1229 (2019)

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abstract

The forward proton detectors, already existing at the LHC, are considered in the context of heavy-ion collisions. It is shown that such detectors have the potential to measure nuclear debris originating from spectator nucleons. The geometric acceptance for different nuclei is studied, and how it is affected by the motion of the nucleons in the nucleus and by the experimental conditions. A possibility of reconstructing the impact parameter of the collision from the measurement of the nuclear fragments is discussed.

The Non-equilibrium Attractor: Beyond Hydrodynamics

Acta Phys. Pol. B 50, 1243 (2019)

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abstract

The quark–gluon plasma created in heavy-ion collisions is not in local thermal equilibrium at early times. Despite this, dissipative hydrodynamics describes the evolution of the energy-momentum tensor quite well after only roughly 0.5–1 fm/$c$. This can be understood using the concept of a non-equilibrium dynamical attractor. The attractor is a uniquely identifiable solution to the dynamical equations to which all solutions are drawn as the system evolves. Once solutions collapse onto the non-equilibrium attractor, they are “pseudo-thermalized” in the sense that they have lost information about the precise initial conditions used, but are not yet in exact local thermal equilibrium. Here, I review recent work which demonstrates that there exists a non-equilibrium attractor in full kinetic theory models which goes beyond the usual low-order momentum moments considered in hydrodynamical treatments.

The Role of Nonhydrodynamic Modes in Bjorken Flow

Acta Phys. Pol. B 50, 1251 (2019)

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abstract

When hydrodynamics is pushed to the limits of its applicability, the influence of transient nonhydrodynamic modes becomes important. As hydrodynamics plays an important role in modelling heavy-ion collisions, understanding the nonhydrodynamic sector of QCD is a worthy, but difficult, goal. In this contribution, I describe the role that nonhydrodynamic modes play in toy models of the expanding quark–gluon plasma, with a focus on my work in kinetic theory.

Photon-induced Processes in Semi-central Nucleus–Nucleus Collisions

Acta Phys. Pol. B 50, 1263 (2019)

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abstract

We calculate total and differential cross sections for $J/\psi$ photoproduction in ultrarelativistic lead–lead collisions at the LHC energy $\sqrt {s_{NN}}=2.76$ TeV. We use a simple model based on vector dominance picture and multiple scattering of the hadronic ($c \bar c$) state in a cold nucleus. In our analysis, we use Glauber formulae for calculating $\sigma _{{\mathrm {tot}},J/\psi {\mathrm {Pb}}}$ which is a building block of our model. For semi-central collisions, a modification of the photon flux is necessary. We discuss how to effectively correct photon fluxes for geometry effects. We try to estimate the cross sections for different centrality bins and for $J/\psi$ mesons emitted in forward rapidity range ($2.5\lt y\lt 4$) corresponding to the ALICE experimental results. We discuss similar analysis for dilepton production in ultrarelativistic heavy-ion collisions at very low pair transverse momenta, $P_{\mathrm {T}}\leq 0.15$ GeV. We investigate the interplay of thermal radiation with photon annihilation processes, $\gamma \gamma \to l^+ l^-$, due to the coherent electromagnetic fields of the colliding nuclei. For the thermal radiation, we employ the emission from the QGP and hadronic phases with in-medium vector spectral functions. We first verify that the combination of photon fusion, thermal radiation and final-state hadron decays gives a fair description of the low-$P_{\mathrm {T}}$ invariant-mass as well as $P_{\mathrm {T}}$ distributions as measured recently by the STAR Collaboration in $\sqrt {s_{NN}}=200$ GeV Au+Au collisions for different centralities. The coherent contribution dominates in peripheral collisions, while thermal radiation shows a significantly stronger increase with centrality. We also provide predictions for the ALICE experiment at the LHC. The resulting excitation function reveals a nontrivial interplay of photoproduction and thermal radiation.

Relativistic Fluid Dynamics and Its Extensions as an Effective Field Theory

Acta Phys. Pol. B 50, 1275 (2019)

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abstract

We examine hydrodynamics from the perspective of an effective field theory. The microscopic scale in this case is the thermalization scale, and the macroscopic scale is the gradient, with thermal fluctuations playing the role of $\hbar$. We argue that this method can be applied both, to consistently include thermal fluctuations in the theory and to extend hydrodynamics to systems whose microscopic structure is non-trivial. For the latter, we discuss the case of spin polarization and gauge theories.

Ultra-peripheral Vector Meson Production in CMS

Acta Phys. Pol. B 50, 1289 (2019)

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abstract

In this document, a brief introduction to the ultra-peripheral collisions (UPCs) and two analyses performed with the use of these events will be shown. First analysis is a measurement of the UPC ${\mit \Upsilon }$ photoproduction in the $p$Pb collisions and the second one is the current status of the analysis of ${\mit \Upsilon }$ photoproduction in the PbPb data, which is still in progress and will be the main part of my Ph.D. Thesis.

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