Regular Series

Vol. 51 (2020), No. 6, pp. 1191 - 1597

Cracow Epiphany Conference on LHC Physics: Standard Model and Beyond



My Life with Kacper Z.

Acta Phys. Pol. B 51, 1199 (2020)

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This is a written version of a talk given by the author on the occasion of Professor Kacper Zalewski \(85^\mathrm {th}\) birthday.

Baryon Number in High-energy Collisions

Acta Phys. Pol. B 51, 1207 (2020)

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Distributions of baryons in high-energy collisions of hadrons and nuclei contain important information on the mechanism underlying multiparticle production processes. In this work, we present a phenomenological study of new, high precision experimental data on non-strange baryon spectra in \(pp\) and \(p\)C reactions at \(\sqrt {s_{NN}}=17.3\) GeV, made in the framework of the Dual Parton Model. These new experimental data are not subject to limitations imposed to earlier data sets used in such studies. We find that the classical mechanism proposed by Capella and Tran Thanh Van cannot describe the full distribution of final-state baryons in \(p\)C reactions in which the proton projectile undergoes multiple collisions with carbon nucleons. This reveals a new class of baryonic final states, characterized by long-distance transfers of baryon number in rapidity space. If confirmed, our observation brings important implications for antiproton reactions with nuclear targets, suggesting the need for a new experimental programme aimed at studies of such processes.

High Precision Lepton Pair Bremsstrahlung with PHOTOS

Acta Phys. Pol. B 51, 1221 (2020)

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I present results for the final-state emissions of lepton pairs in decays of heavy intermediate states such as \(Z\) boson. Numerical distributions of relevance for the LHC are shown. A testing framework allowing comparison of spectra by PHOTOS with an exact one solution is developed. Exact matrix element of \(2f\to Z/\gamma ^*\to 4f\) process has been installed into  PHOTOS and has been tested. Approximation of exact matrix element is proposed and future research is suggested in order to factorize approximated matrix element.

Nested Soft–Collinear Subtractions in NNLO QCD Computations

Acta Phys. Pol. B 51, 1231 (2020)

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Thanks to a remarkable progress in understanding of how to perform NNLO QCD computations, many processes at hadron collisions have been recently computed to that precision. Despite these developments, the search for the optimal subtraction scheme that allows us to handle infrared and collinear singularities in an efficient and general way is still ongoing. In the following, I will review the nested soft–collinear subtraction scheme proposed in F. Caola, K. Melnikov, R. Röntsch, Eur. Phys. J. C 77, 248 (2017). This scheme seems to possess many features of the possible optional scheme; for example, it is analytic, fully local and highly modular. I will describe an application of this scheme to the description of deep inelastic scattering of a proton with an electron (DIS) that, together with results on colour singlet production and decay, completes the set of building blocks that are required for the application of this scheme to arbitrary processes at hadron colliders.

Fully Bayesian Unfolding in High-energy Physics

Acta Phys. Pol. B 51, 1241 (2020)

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The process of unfolding is a crucial part of many particle physics analyses, representing the correction of measured spectra in data for the finite detector efficiency, acceptance and resolution from the detector to particle or from the particle to parton levels. Compared to other commonly used methods, the Fully Bayesian Unfolding (FBU) returns not only an unfolded value and its uncertainty, but provides the full binned posterior probability density. This study focuses on the dependence of unfolding results on the regularization parameter strength \(\tau \) applied to different high-energy physics spectra.

The Cosmological Constant and Higgs Mass with Emergent Gauge Symmetries

Acta Phys. Pol. B 51, 1251 (2020)

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We discuss the Higgs mass and cosmological constant in the context of an emergent Standard Model, where the gauge symmetries “dissolve” in the extreme ultraviolet. In this scenario, the cosmological constant scale is suppressed by power of the large scale of emergence and expected to be of similar size to neutrino masses. Cosmology constraints then give an anthropic upper bound on the Higgs mass.

Effects of Top-quark Decay Modeling in \(t\bar {t}\gamma \) Production at the LHC

Acta Phys. Pol. B 51, 1267 (2020)

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We present a systematic comparison of different approaches for the modeling of \(t\bar {t}\gamma \) final states with leptonic decays at the LHC. On the one hand, we consider a complete calculation at NLO QCD accuracy which includes all resonant and non-resonant diagrams. On the other hand, we consider predictions in the narrow-width approximation with top-quark decays modeled at various accuracies. In this way, we quantify the impact of the off-shell effects in \(t\bar {t}\gamma \) production. We also discuss the relative importance of double-, single- and non-resonant contributions in the complete calculation. Finally, we investigate the fraction of isolated photons from decays of top quarks, which represent a background for measurements of anomalous \(t\gamma \) couplings.

Searches for a Singly Charged Higgs Boson at CMS

Acta Phys. Pol. B 51, 1279 (2020)

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We present recent results on searches for a singly charged Higgs boson from the CMS experiment. The searches are based on proton–proton collision data at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 35.9 fb\(^{-1}\). Various production and decay modes of a singly charged Higgs boson in two-Higgs-doublet models and the Georgi–Machacek model were investigated. No statistically significant evidence of a singly charged Higgs boson was found, and upper limits were set on the production and decay rate of a singly charged Higgs boson in each search channel.

QCD Corrections to Higgs Boson Decaying into Bottom Quarks

Acta Phys. Pol. B 51, 1287 (2020)

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The largest decay channel of the Higgs boson is to bottom quarks. In these proceedings, we describe tools that facilitate high-precision simulation of the \(H \to \bb \) decay. We present an event generator, constructed using the MiNLO method, which allows us to consistently match the next-to-next-to-leading order (NNLO) QCD corrections to Higgs boson decaying into massless \(b\) quarks with a parton shower (PS). Furthermore, we present an NNLO QCD calculation of the Higgs boson decay into \(b\) quarks with full treatment of the \(b\)-quark mass. These calculations provide a state-of-the-art description of the \(H \to \bb \) decay and, therefore, they will be important for studying the Higgs boson itself.

\(b \rightarrow s\ell ^+ \ell ^-\) Transitions at LHCb

Acta Phys. Pol. B 51, 1299 (2020)

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These proceedings show the most recent results of lepton universality test in \(B^+ \rightarrow K^+ \ell ^+ \ell ^-\) decays and searches for the flavour-violating \(B^+ \rightarrow K^+ \mu ^\pm e^\mp \) decays using data samples from the LHCb experiment. No signal was observed and lepton universality measured shows 2.5 \(\sigma \) deviation from the Standard Model prediction.

Production of Vector Mesons in \(pp \to pVp\) Reactions with Electromagnetic Dissociation of Protons

Acta Phys. Pol. B 51, 1305 (2020)

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We discuss the semi-exclusive production of vector mesons in proton–proton collisions with electromagnetic dissociation of one of the protons. Several differential distribution in missing mass (\(M_{X}\)) or single-particle variables related exclusively to the produced vector meson are calculated for the \(pp\) center-of-mass energies of 7 and 13 TeV. The cross sections and some differential distributions are compared to their counterparts for purely exclusive reaction \(pp \to pVp\). For electromagnetic dissociation, the important property is that the \(p\gamma ^{\star } \to Xp\) transitions are given by the electromagnetic structure function of proton. In our calculations, we use different parametrizations of the structure function and discuss how it is constrained by the data on virtual photoabsorption on a proton.

Electroweak Measurements with the CMS Detector

Acta Phys. Pol. B 51, 1315 (2020)

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Electroweak measurements are at the core of the Large Hadron Collider physics program. The most recent results obtained by the CMS experiment with Drell–Yan, \(W\) and multi-boson events will be reviewed. These include the measurement of the electroweak mixing angle, the differential distributions in Drell–Yan events, electroweak production of one and two vector bosons in association with two jets. No deviations from the Standard Model predictions are found and stringent bounds are set on anomalous triple and quartic gauge couplings.

Search for a New Heavy Resonance in Final States with Di-jet or Di-\(b\)-jets with the ATLAS Detector

Acta Phys. Pol. B 51, 1327 (2020)

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A search of new heavy resonances decaying into a pair of jets in \(pp\) collisions with the ATLAS detector at LHC is reported. The dataset corresponds to an integrated luminosity of up to 139 fb\(^{-1}\) collected between 2015 and 2018 at \(\sqrt {s}=13\) TeV. No evidence of significant excess of events above the smooth background shape is observed. Upper cross-section limits for several types of signal hypotheses are provided at 95% C.L.

Experimental Results in the Extended Higgs Sector

Acta Phys. Pol. B 51, 1333 (2020)

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An Extended Higgs sector could solve many problems of the Standard Model and is, therefore, investigated by several analyses of the ATLAS experiment at the LHC. Selected searches for additional Higgs bosons and other predicted effects of an extended Higgs sector based on \(139\) fb\(^{-1}\) of data taken between 2015 and 2018 at a center-of-mass energy of 13 TeV are summarized.

QZE and IZE in a Simple Approach and the Neutron Decay

Acta Phys. Pol. B 51, 1345 (2020)

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We discuss a simple and analytically solvable measurement model which describes the famous Quantum Zeno Effect (QZE) and Inverse Zeno Effect (IZE), that correspond to the slow down and to the increase of the decay rate caused by measurements (or, more general, by the interaction of an unstable state with the detector and the environment). Within this model, one can understand quite universal features of the QZE and IZE: by considering an unstable quantum state, such as an unstable particle, whose decay width as a function of energy is \({\mit \Gamma }(\omega )=g^{2}\omega ^{\alpha },\) then — under quite general assumptions — the QZE occurs for \(\alpha \in (0,1)\), while the IZE for \(\alpha \in (-\infty ,0)\cup (1,\infty ).\) This result is also valid for more realistic measurement models than the one described in this work. We then apply these considerations to the decay of the neutron, for which \(\alpha =5.\) Hence, the realization of the IZE for the neutron decay (and for the majority of weak decays) is in principle possible. Indeed, trap experiments find a lifetime that is \(8.7\pm 2.1\) s shorter than beam experiments, suggesting that the IZE could have taken place.

Exotic Searches at LHCb

Acta Phys. Pol. B 51, 1357 (2020)

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Jet physics offers a wonderful environment to study both the Standard and beyond the Standard Model physics. The most recent LHCb cross-section measurements of the top-quark pair production and the production of \(W\) boson with quark pairs at 8 TeV are presented. Results show a very good agreement with NLO predictions. Additionally, updated constraints on the Hidden Valley pion signal strength are shown, alongside with a new ongoing search for \(b'\) and \(Z'\) in the process of \(Z\)-boson production associated with two \(b\) jets at 13 TeV.

On the Universality of the KRK Factorization Scheme

Acta Phys. Pol. B 51, 1363 (2020)

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The factorization scheme (FS) abbreviated as KRK FS including a new definition of the PDFs for initial hadrons was formulated while developing KrkNLO scheme of matching QCD NLO corrections for the hard process with the parton shower heavy-boson production in hadron–hadron collision and for deep inelastic lepton–hadron scattering. KRK FS (originally called Monte Carlo FS) can be regarded as a variant of the \(\overline {\mathrm {MS}}\) system. It is, therefore, trivially universal, that is process-independent. The question of its universality is formulated differently: As the basic role of KRK FS is to drastically simplify NLO corrections, the question is now whether the same single variant of PDFs in the KRK FS is able to achieve the same maximal simplification of the NLO corrections for all processes with one or two initial hadrons and any number of the final hadrons? Our answer is positive and the proof is elaborated in the present note within the Catani–Seymour subtraction methodology. KRK FS is mandatory in the KrkNLO method of matching NLO calculation and parton shower — a much simpler alternative of POWHEG and/or MC@NLO. However, the use of KRK FS and the corresponding PDFs simplifies NLO calculations for any other method of calculating NLO corrections and for arbitrary processes as well.

Higgs Cross-section Measurements in the Four-lepton Final State at CMS

Acta Phys. Pol. B 51, 1379 (2020)

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Cross sections of the Higgs (\(H\)) boson are measured in the \({H}\rightarrow {Z}{Z}\rightarrow 4\ell \) (\(\ell ={e},\mu \)) decay channel. The full data sample of proton–proton collisions at a center-of-mass energy of \(13\) TeV is used, corresponding to an integrated luminosity of \(137.1~{\mathrm {fb}}^{-1}\) recorded in 2016, 2017, and 2018 by the CMS detector at the LHC. The results include signal-strength modifier \(\mu \), simplified template cross-sections and fiducial cross-sections (inclusive and differential) measurements. All results are found to be compatible with the SM predictions, within the measurements precision.

Alignment Studies at Belle II Vertex Detector

Acta Phys. Pol. B 51, 1385 (2020)

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The Belle II is the next generation of famous \(B\)-physics experiments and they would like to continue in success of predecessors. To provide better measurements and extend limits, the upgrades were necessary. One of the main upgrades was expanding a strip silicon detector using a pixel detector. Higher precision requires more sophisticated alignment procedure, parametrization, validation and monitoring techniques. Since the first commissioning of vertex detector, a lot of experiences have been acquired and many of features have been observed.

Exclusive Four-pion Photoproduction in Ultra-peripheral Heavy-ion Collisions at RHIC and LHC Energies

Acta Phys. Pol. B 51, 1393 (2020)

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We study the photoproduction of exclusive \(2\pi ^+2\pi ^-\) mesons in ultra-peripheral heavy-ion collisions at the RHIC and LHC energies. Predictions in photon–nucleus interactions are calculated for various resonances at central and forward rapidities. The recent H1 preliminary data are utilized to improve the description of the poorly known \(\gamma p \to 4\pi ^\pm p\) process. We present the comparisons of our results to the available STAR data at RHIC, and made predictions for the LHC energies.

Searching for BSM Higgs and Gauge Bosons Decaying to Two Tau Leptons Using 36 fb\(^{-1}\) of Data Collected at \(\sqrt {s}=13\) TeV with the ATLAS Detector

Acta Phys. Pol. B 51, 1405 (2020)

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A search for additional heavy neutral Higgs bosons and \(Z'\) bosons decaying to two tau leptons is performed using data corresponding to an integrated luminosity of 36.1 fb\(^{-1}\) from proton–proton collisions at \(\sqrt {s} = 13\) TeV recorded by the ATLAS detector at the LHC during 2015 and 2016. The data are in good agreement with the Standard Model and results are interpreted in several benchmark scenarios.

Soft QCD at ATLAS and CMS

Acta Phys. Pol. B 51, 1411 (2020)

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A short overview of the recent results of soft QCD measurements in proton–proton collisions from the ATLAS and CMS collaborations at the Large Hadron Collider is presented. Measurements of double parton scattering and the underlying event in inclusive production, and differential cross sections for single diffractive dissociation at centre-of-mass energies \(\sqrt {s} = 8\) and 13 TeV are discussed.

Top Precision for Associated Top-pair Production Processes at the LHC

Acta Phys. Pol. B 51, 1425 (2020)

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The studies of the associated production processes of a top-quark pair with a colour-singlet boson, e.g. Higgs, \(W\) or \(Z\), are among the highest priorities of the LHC programme. Correspondingly, improvements in precision of theoretical predictions for these processes are of central importance. In this paper, we review our latest results on resummation of soft gluon corrections. The resummation is carried out using the direct QCD Mellin space technique in three-particle invariant mass kinematics. We discuss the impact of the soft-gluon corrections on predictions for total cross sections and differential distributions.

Searching for Odderon in Central Exclusive Processes at the LHC

Acta Phys. Pol. B 51, 1437 (2020)

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There seems to be recently an evidence for \(C = -1\) odderon exchange in proton–proton elastic scattering at high energies. Here, we discuss three different central-exclusive-production processes \(pp \to pp (\phi \to K^{+}K^{-})\), \(pp \to pp (\phi \to \mu ^{+}\mu ^{-})\), and \(pp \to pp \phi \phi \to pp K^{+}K^{-}K^{+}K^{-}\), as a possible source of information for soft odderon exchange. The theoretical results are calculated within the tensor-Pomeron and vector–odderon model for soft reactions. We include absorptive corrections at the amplitude level. To describe the low-energy data measured in the past by the WA102 Collaboration, we consider also subleading processes with reggeized vector–meson exchanges and reggeons. We discuss possible evidences for odderon exchange at the low energies and try to make predictions at the LHC.

Production of Heavy Particle Pairs via Photon–Photon Processes at the LHC

Acta Phys. Pol. B 51, 1449 (2020)

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We discuss production of \(W^+ W^-\) pairs and \(t \bar t\) quark–antiquark pairs in proton–proton collisions induced by two-photon fusion including transverse momenta of incoming photons. The unintegrated inelastic fluxes (related to proton dissociation) of photons are calculated based on modern parametrizations of deep inelastic structure functions in a broad range of \(x\) and \(Q^2\). We focus on processes with single and double proton dissociation. Highly excited remnant systems hadronise producing particles that can be vetoed in the calorimeter. We calculate associated gap survival factors. The gap survival factors depend on the process, mass of the remnant system and collision energy. The rapidity gap survival factor due to remnant fragmentation for double dissociative (DD) collisions is smaller than that for single dissociative (SD) process. We observe approximate factorisation: \(S_{\mathrm {R,DD}} \approx S_{\mathrm {R,SD}}^2\) when imposing rapidity veto. For the \(W^+W^-\) final state, the remnant fragmentation leads to a taming of the cross section when the rapidity gap requirement is imposed. Moreover, for \(t \bar t\) quark–antiquark pairs such a condition reverses the hierarchy observed for the case when such a condition is taken into account. Our results imply that for the production of such heavy objects as \(t\) quark and \(\bar t\) antiquark, the virtuality of the photons attached to the dissociative system is very large (\(Q^2 \lt \) 10\(^{4}\) GeV\(^2\)). A similar effect is observed for the \(W^+ W^-\) system.

Production of \(\nu _{\tau }\) and \(\bar {\nu }_{\tau }\) in Fixed Target SHiP Experiment

Acta Phys. Pol. B 51, 1461 (2020)

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We discuss production cross sections of \(\nu _{\tau }\) and \({\bar \nu }_{\tau }\) coming from the direct \(D_s^{\pm } \to \tau ^{\pm }\nu _{\tau }/{\bar \nu }_{\tau }\) and chain \(D_s^{\pm } \to \tau ^+/\tau ^- \to {\bar \nu }_{\tau }/\nu _{\tau }\) decays in \(p+^{96}\)Mo scattering with proton beam \(E_{\mathrm {lab}}=400\) GeV i.e. at \(\sqrt {s}_{NN}=27.4\) GeV. We include two different \(D_s^{\pm }\) meson production mechanisms: via charm fragmentation \(c \to D_s^{+}\) and \(\bar c \to D_s^-\) as well as via subleading fragmentation of strange quarks/antiquarks \(s \to D_s^-\) and \(\bar s \to D_s^+\). Estimates of a number of observed \(\nu _{\tau } / \bar {\nu }_{\tau }\) in the \(\nu _{\tau } / \bar {\nu }_{\tau } +^{208}\!\mathrm {Pb}\) reaction, with 2 m long target are given.

Diamond Detectors for Radiation Monitoring and Beam Abort at Belle II

Acta Phys. Pol. B 51, 1473 (2020)

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The Belle II experiment will be at the forefront of indirect searches for non-Standard-Model physics using billions of heavy quarks and \(\tau \) leptons produced in high-intensity 10 GeV electron–positron collisions from the SuperKEKB collider. The intense beams needed to achieve the required precisions are associated with high beam-background radiation that may damage the inner detectors. A dedicated radiation-monitoring and beam-abort system, based on artificial diamond sensors, ensures protection and safe data-taking conditions. I briefly outline the system and illustrate the operational experience and performance during 2019 physics operations.

Tests of Lepton Flavour Universality in \(b\to c\ell \nu \) Decays at the LHCb Experiment

Acta Phys. Pol. B 51, 1479 (2020)

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Several measurements, aimed at testing the lepton flavour universality hypothesis, have been performed by the \(B\)-physics experiments which exploit both neutral current (\(b\to s \ell \ell \)) and charged current (\(b\to c\ell \nu \)) decays. The combination of the measurements carried out in the charged current sector shows possible hints of deviations with respect to the Standard Model predictions. This document reports the analyses, performed by the LHCb experiment, of semi-tauonic decays with both leptonic and the hadronic decays of the \(\tau \) lepton.

Spacetime Information as a Guiding Principle for Color Reconnection in Herwig 7

Acta Phys. Pol. B 51, 1485 (2020)

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A new model of color reconnection in the Monte Carlo generator Herwig 7 is presented. It is based on the minimization of a boost-invariant distance of the parton system, where all partons have momentum as well as spacetime position assigned. We test the influence of both types of variables, namely the rapidity span and transverse distance, on the actual need to reconnect the system to better describe soft physics measurements. We find reasonable agreement with the data and conclude that spacetime topology of the event can be useful for hadron collision modeling.

Investigation of the CP Properties of VBF Higgs Production Using the Decay to a Pair of tau Leptons with the ATLAS Detector

Acta Phys. Pol. B 51, 1497 (2020)

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Recent studies at the LHC have led to the observation of the Higgs boson decay to tau leptons, with a rate compatible with the Standard Model expectation. The observation opens the way to a more in-depth investigation of the properties of the production and decay of a Higgs boson into tau leptons, including whether or not the Higgs boson couplings involved in this process violate CP conservation. The analysis presented in these proceedings focuses on events collected by the ATLAS experiment where Higgs bosons are produced via vector-boson fusion in order to investigate the tensor structure of their coupling to electroweak gauge bosons. For this, a profile likelihood fit using a matrix-element observable is employed in the decay to tau leptons to test whether a CP-odd component is present in the coupling. A measurement of a non-zero value would be an indication of CP violation in the Higgs sector. No evidence of CP violation was found.

NNLO QCD Calculations with the Sector-improved Residue Subtraction Scheme

Acta Phys. Pol. B 51, 1503 (2020)

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Precise predictions for total and differential cross sections at hadron colliders are at the heart of LHC physics. The lack of new ‘smoking-gun’ physics signals requires precise comparisons between measurements and Standard Model predictions to get a handle on New Physics effects. Tremendous efforts have been made to push perturbative calculations to higher orders such that NNLO QCD calculations are now state-of-the-art for most \(2\to 1\) and \(2\to 2\) hard scattering processes. Upcoming five-point two-loop amplitudes and refined subtractions schemes for real radiation contributions allow now first steps in the direction of \(2\to 3\) scattering processes.

Effects of In-medium \(k_{\mathrm {T}}\) Broadening on Di-jet Observables

Acta Phys. Pol. B 51, 1515 (2020)

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We study the influence of in-medium transverse momentum broadening on observables of jet pairs produced in nuclear collisions. These di-jets were obtained numerically via the Monte Carlo simulations of nuclear collisions, where the partons within the colliding nuclei were described via unintegrated parton densities, and their subsequent propagation within the medium of a quark–gluon plasma (QGP). For jet-evolution in the QGP both, kicks transverse to the momentum of the incoming particle as well as medium-induced radiation were considered. After favorable comparison of our results with experimental LHC-data on jet-quenching, we make predictions for the decorrelation of di-jets. In particular, we study deviations from a transverse momentum broadening that follows a Gaussian distribution.

Comments on the Space-time Evolution of the System Created in the Nucleus–Nucleus Collision at High Energy

Acta Phys. Pol. B 51, 1525 (2020)

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The space-time evolution of the system created in the high-energy nucleus–nucleus collision is a complex, non-perturbative phenomenon which consequently largely escapes model-independent theoretical predictions. In this paper, we summarize the main findings gathered on this phenomenon from studies of spectator-induced electromagnetic effects as well as rapidity spectra in light- and heavy-ion systems at the CERN SPS. Main emphasis is put upon the evolution of the system in the longitudinal direction, where the CERN SPS energy regime is probably the highest where reasonably complete experimental coverage can be provided. Implications of these studies for other energy regimes, in particular the LHC, are shortly addressed.

Relativistic Hydrodynamics for Spin-polarized Media

Acta Phys. Pol. B 51, 1537 (2020)

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We summarize the key ingredients of the recently proposed formalism of relativistic perfect-fluid hydrodynamics with spin. Based on the underlying kinetic theory definitions for the equilibrium distribution functions, we obtain the evolution equations governing the system’s expansion. Employing the Bjorken symmetry, we study the spin polarization dynamics of the system.

Introducing the Chirality-flow Formalism

Acta Phys. Pol. B 51, 1547 (2020)

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In QCD, we are used to describing the SU(3) color space in terms of a flow of color. At the algebra level, the Lorentz group consists of two copies of the (complexified) \(\mathfrak {su}(2)\) algebra, so one may anticipate that a similar way of thinking about the spacetime structure of scattering amplitudes should exist. In this article, we argue that this is indeed the case, and introduce the chirality-flow formalism for massless tree-level QED and QCD. Within the chirality-flow formalism, scattering amplitudes can directly be written down in terms of Lorentz-invariant spinor inner products, similar to how the color structure can be described in terms of a color flow.

From Transition Electromagnetic Form Factors \(\gamma ^* \gamma ^* \eta _c(1S,2S)\) to the Production of \(\eta _c(1S,2S)\) at the LHC

Acta Phys. Pol. B 51, 1559 (2020)

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We review our recent results for production of \(\eta _c(1S)\) and \(\eta _c(2S)\) in the \(\gamma ^* \gamma ^* \to \eta _c(1S,2S)\) fusion and in proton–proton collisions via gluon–gluon fusion. The quarkonium wave functions are calculated by solving Schrödinger equation for different \(c \bar c\) potentials. Using the Terentev prescription, the light-cone wave functions are obtained. The light-cone wave functions are used then to calculate \(\gamma ^* \gamma ^* \to \eta _c\) transition form factors. The theoretical results are compared to the Belle experimental data for \(\eta _c(1S)\). In addition, we discuss our results for two-photon decay width. We present also results of our calculations for proton–proton collisions obtained within \(k_{\mathrm {T}}\)‑factorization approach for different unintegrated gluon distributions. The results for hadroproduction of \(\eta _c(1S)\) are compared to the LHCb experimental data.

On the Possibility of Having Roman Pots Around IP8 in Run 4 and Beyond

Acta Phys. Pol. B 51, 1577 (2020)

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Proton trajectories along LHC Beam 1 (clockwise direction) in the vicinity of the LHCb Interaction Point (IP8) for the most recent LHC Run 4 optics (HLLHC1.5) were presented. On this basis, three possible locations of forward proton detectors were identified: 150, 180 and 430 m from IP8. For these locations, geometric acceptances were estimated. For the proton relative energy loss, \(\xi \), these limits are \(0.05 \lt \xi \lesssim 0.1\), \(0.025 \lt \xi \lesssim 0.1\) and \(0.003 \lt \xi \lt 0.013\), respectively. The influence of boost of the central system due to the acceptance of LHCb detector on \(\xi \) was discussed. Finally, the impact of pile-up was presented.

Ultra-peripheral Vector Meson Photoproduction in Heavy-ion Collisions in CMS

Acta Phys. Pol. B 51, 1585 (2020)

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In this document, an introduction to ultra-peripheral collisions (UPCs) is given. The recent results on UPCs obtained by the Compact Muon Solenoid Collaboration are presented. They are also shown in conjunction with theoretical predictions. Finally, the analysis of ultra-peripheral \({\mit \Upsilon }\) photoproduction in the new PbPb data collected at the end of 2018 by the CMS is discussed. This analysis is still in progress.

Particle Correlations at LHCb

Acta Phys. Pol. B 51, 1593 (2020)

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Studies of particle correlations provide an insight into hadronization, the process that after many years of research is still not fully described. Past and ongoing analyses at the LHCb investigate this phenomenon in the unique forward kinematic region of the detector (\(2 \lt \eta \lt 5\)), expanding the knowledge gathered by other experiments. Studies of the Bose–Einstein Correlations for pairs and triplets of identical pions unveil temporal and spatial properties of the hadronic source as well as characteristics of the particle emission.

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