arXiv subjects

Antoni Szczurek

Publications and source records attributed to Antoni Szczurek.

At least 163 records · Page 9Linked to original sources

arXiv2017

On the Implications of Energy and Momentum Conservation for Particle Emission in A+A Collisions at SPS Energies

We construct a simple model of heavy ion collisions, local in the impact parameter plane, and appropriate for the SPS energy range. This model can be regarded as a new realization of the "fire-streak" approach, originally applied to studies of lower energy nucleus-nucleus reactions. Starting from local energy and momentum conservation, we nicely describe the broadening of the pion rapidity distribution when going from central to peripheral Pb+Pb collisions at $\sqrt{s_{NN}}$=17.3 GeV. The results of our calculations are compared with SPS experimental data. We discuss the resulting implications on the role of energy and momentum conservation for the dynamics of particle production in heavy ion collisions. A specific space-time picture emerges, where the longitudinal evolution of the system strongly depends on the position in the impact parameter (${b_x}$, ${b_y}$) plane. This picture is consistent with our earlier findings on the longitudinal evolution of the system as deduced from electromagnetic effects on charged pion directed flow, and can provide an explanation for specific low-$p_T$ phenomena seen in the fragmentation region of Pb+Pb collisions.

nucl-th↗

arXiv2016

Central production of $ρ^{0}$ in $pp$ collisions with single proton diffractive dissociation at the LHC

We consider the $pp \to pp ρ^{0} π^{0}$ and $pp \to pn ρ^{0} π^{+}$ processes at LHC energies. Our description is based on the nonperturbative framework of tensor pomeron and tensor reggeon exchanges. We discuss the Drell-Hiida-Deck type mechanism with centrally produced $ρ^{0}$ meson associated with a very forward/backward $πN$ system. The considered processes constitute an inelastic (non-exclusive) background to the $p p \to p p ρ^0$ reaction in the case when only the centrally produced $ρ^{0}$ meson decaying into $π^{+} π^{-}$ is measured, the final state protons are not observed, and only rapidity-gap conditions are checked experimentally. We compare our results for the $γπ^{+} \to ρ^{0} π^{+}$ reaction with the experimental data obtained by the H1 collaboration at HERA. We present several differential distributions for the $pp \to pn ρ^{0} π^{+}$ reaction and estimate the size of the proton dissociative background to the exclusive $p p \to p p ρ^0$ process. The ratio of integrated cross sections for the inelastic $p p \to p N ρ^0 π$ processes, where $p N ρ^0 π$ stands for $p n ρ^0 π^{+}$ plus $p p ρ^0 π^{0}$, to the reference reaction $pp \to \ pp ρ^0$ is of order of (7--10)\%. We present also the ratios of the $ρ^0$ rapidity and transverse momentum distributions for the inelastic $p p \to p N ρ^0 π$ versus the elastic $pp \to \ pp ρ^0$ reaction. Our results may be used to investigate the $γπ\to ρ^{0} π$ process at LHC energies.

hep-ph↗

arXiv2016

Semiexclusive production of $J/ψ$ mesons in proton-proton collisions with electromagnetic and diffractive dissociation of one of the protons

We calculate the cross sections for both electromagnetic and diffractive dissociation of protons for semiexclusive production of $J/ψ$ mesons in proton-proton collisions at the LHC. Several differential distributions in missing mass ($M_X$), or single-particle variables related exclusively to the $J/ψ$ meson are calculated for $\sqrt{s}$ = 7 TeV as an example. The cross sections and distributions are compared to the cross section of the purely exclusive reaction $p p \to p p J/ψ$. We show the corresponding ratio as a function of $J/ψ$ meson rapidity. We compare the distributions for purely electromagnetic and purely diffractive proton excitations/dissociation. We predict cross sections for electromagnetic and diffractive excitations of similar order of magnitude.

hep-ph↗

arXiv2016

New Results for Ultraperipheral Heavy Ion Collisions

We discuss diphoton semi(exclusive) production in ultraperipheral $PbPb$ collisions at energy of $\sqrt{s_{NN}}=$ 5.5 TeV (LHC). The nuclear calculations are based on equivalent photon approximation in the impact parameter space. The cross sections for elementary $γγ\to γγ$ subprocess are calculated including three different mechanisms: box diagrams with leptons and quarks in the loops, a VDM-Regge contribution with virtual intermediate hadronic excitations of the photons and the two-gluon exchange contribution (formally three-loops). We got relatively high cross sections in $PbPb$ collisions. This opens a possibility to study the $γγ\to γγ$ (quasi)elastic scattering at the LHC. We find that the cross section for elastic $γγ$ scattering could be measured in the lead-lead collisions for the diphoton invariant mass up to $W_{γγ} \approx 15-20$ GeV. We identify region(s) of phase space where the two-gluon exchange contribution becomes important ingredient compared to box and nonperturbative VDM-Regge mechanisms. We discuss also first results concerning production of two $e^+ e^-$ pairs in UPCs of heavy ions. We considered only double scattering mechanism.

nucl-th↗

arXiv2016

Diphoton production in lead-lead and proton-proton UPC

We discuss diphoton semi(exclusive) production in ultraperipheral $PbPb$ collisions at energy of $\sqrt{s_{NN}}=$ 5.5 TeV (LHC). The nuclear calculations are based on equivalent photon approximation in the impact parameter space. The cross sections for elementary $γγ\to γγ$ subprocess are calculated including three different mechanisms: box diagrams with leptons and quarks in the loops, a VDM-Regge contribution with virtual intermediate hadronic excitations of the photons and the two-gluon exchange contribution (formally three-loops) to elastic photon-photon scattering in the high-energy approximation. We got relatively high cross sections in $PbPb$ collisions ($306$ nb). This opens a possibility to study the $γγ\to γγ$ (quasi)elastic scattering at the LHC. We find that the cross section for elastic $γγ$ scattering could be measured in the lead-lead collisions for the diphoton invariant mass up to $W_{γγ} \approx 15-20$ GeV. We identify region(s) of phase space where the two-gluon exchange contribution becomes important ingredient compared to box and nonperturbative VDM-Regge mechanisms. We perform a similar analysis for the $pp \to ppγγ$ reaction at energy of $\sqrt{s_{NN}}=7$ and $100$ TeV.

hep-ph↗

arXiv2016

Diffractive processes at the LHC within $k_t$ -factorization approach

We discuss the single diffractive production of $c \bar c$ pairs and charmed mesons at the LHC. In addition to standard collinear approach, for a first time we propose a $k_t$-factorization approach to the diffractive processes. The transverse momentum dependent (the unintegrated diffractive parton distributions) in proton are obtained with the help of the Kimber-Martin-Ryskin prescription where collinear diffractive PDFs are used as input. In this calculation the transverse momentum of the pomeron is neglected with respect to transverse momentum of partons entering the hard process. The results of the new approach are compared with those of the standard collinear one. Significantly larger cross sections are obtained in the $k_t$-factorization approach where some part of higher-order effects is effectively included. Some correlation observables, like azimuthal angle correlation between $c$ and $\bar c$, and $c \bar c$ pair transverse momentum distribution were obtained for the first time.

hep-ph↗

arXiv2016

Double-parton scattering effects in double charm production within gluon fragmentation scenario

We discuss charm $D^0 D^0$ meson-meson pair production in the forward rapidity region related to the LHCb experimental studies at $\sqrt{s}$ = 7 TeV. We consider double-parton scattering mechanisms of double $c \bar c$ production and subsequent standard $cc \to D^{0}D^{0}$ scale-independent hadronization as well as new double $g$ and mixed $g c\bar c $ production mechanisms with $gg \to D^{0}D^{0}$ and $gc \to D^{0}D^{0}$ scale-dependent hadronization. The new scenario with gluon fragmentation components results also in a new single-parton scattering mechanism of $gg$ production which is also taken here into account. Results of the numerical calculations are compared with the LHCb data for several correlation observables. The new mechanisms lead to a larger cross sections and to slightly different shapes of the calculated correlation observables.

hep-ph↗

arXiv2016

Four-jet production in the $k_t$-factorisation

We discuss the single-parton and double-parton scattering (SPS or DPS) effects in four-jet production at the LHC. The calculations of both single-parton and double-parton scattering components are done in the high-energy (or $k_{T}$)-factorization approach. Here we follow our recent developments of relevant methods and tools. The calculations are performed for kinematical situations relevant for two experimental measurements (ATLAS and CMS) at the LHC. We compare our results to those reported by the ATLAS and CMS collaborations for different sets of kinematical cuts. A special attention is given to the optimization of kinematical conditions in order to enhance the relative contribution of DPS in four-jet sample. Several differential distributions are calculated and carefully discussed in the context of recent and future searches for DPS effects at the LHC. The dependences of the relative DPS amount is studied as a function of rapidity of jets, rapidity distance, and various azimuthal correlations between jets. The regions with an enhanced DPS contribution are identified.

hep-ph↗

arXiv2016

Drell-Yan production at forward rapidities: a hybrid factorization approach

We discuss the Drell-Yan production of dileptons at high energies in the forward rapidity region of proton-proton collisions in a hybrid high-energy approach. This approach uses unintegrated gluon distributions in one proton and collinear quark/antiquark distributions in the second proton. We compute various distributions for the case of low-mass dilepton production and compare to the LHCb and ATLAS experimental data on dilepton mass distributions. In distinction to dipole approaches, we include four Drell-Yan structure functions as well as cuts at the level of lepton kinematics. The impact of the interference structure functions is rather small for typical experimental cuts. We find that both side contributions ($g q/\bar q$ and $q/\bar q g$) have to be included even for the LHCb rapidity coverage which is in contradiction with what is usually done in the dipole approach. We present results for different unintegrated gluon distributions from the literature. Some of them include saturation effects, but we see no clear hints of saturation even at small $M_{ll}$.

hep-ph↗

arXiv2016

Exclusive production of two and four pions in proton-proton scattering

We consider exclusive $pp \to pp π^{+} π^{-}$ and $pp \to pp π^{+} π^{-}π^{+} π^{-}$ reactions at high energies. The calculation is based on a tensor pomeron model and the amplitudes for the processes are formulated in an effective field-theoretic approach. In the case of $pp \to pp π^{+} π^{-}$ process we consider both diffractive and photoproduction mechanisms and we include the non-resonant $π^{+}π^{-}$ continuum and the resonance $f_{0}(500)$, $f_{0}(980)$, $f_{2}(1270)$, $ρ(770)$ contributions. We discuss how two pomerons couple to tensor meson $f_{2}(1270)$ and the interference effects of resonance and dipion continuum. We find that the relative contribution of resonances $ρ(770)$, $f_2(1270)$ and dipion continuum strongly depends on the cut on proton transverse momenta. In the case of exclusive central $4 π$ production we include the contribution via the intermediate $σσ$ and $ρρ$ states. For both processes the theoretical results have been compared with the experimental data and predictions for planned or being carried out experiments (e.g. STAR, ATLAS-ALFA) are presented.

hep-ph↗

arXiv2016

Charm quark and meson production in association with single-jet at the LHC

We discuss charm quark/antiquark and charmed meson production in association with one extra jet (gluon, quark, antiquark) at the LHC. The calculations are performed both in collinear and $k_T$-factorization approaches. Different unintegrated gluon distribution functions are used in the $k_T$-factorization approach. Several predictions for the LHC are presented. We show distributions in rapidity and transverse momenta of $c$/$\bar c$ (or charmed mesons) and the associated jet as well as some two-dimensional observables. Interesting correlation effects are predicted, \textit{e.g.} in azimuthal angles $φ_{c\bar{c}}$ and $φ_{c\mathrm{\textit{-jet}}}$. We have also discussed a relation of the $2 \to 2$ and $2 \to 3$ partonic calculations in the region of large transverse momenta of charm quarks/antiquarks as well as the similarity of the next-to-leading order collinear approach and the $k_T$-factorization approach with the KMR unintegrated parton distribution functions. Integrated cross sections for $D^{0}$ + jet production for ATLAS detector acceptance and for different cuts on jet transverse momenta are also presented.

hep-ph↗

arXiv2016

Inclusive production of J/$ψ$ and $ψ'$ mesons at the LHC

We discuss the prompt production of $J/ψ$ mesons in proton-proton collisions at the LHC within a NRQCD $k_{t}$-factorization approach using Kimber-Martin-Ryskin (KMR) unintegrated gluon distributions (UGDF). We include both direct color-singlet production ($gg \to J/ψ$g) as well as a feed-down from $χ_{c} \to J/ψγ$ and $ψ' \to J/ψX$. The production of the decaying mesons ($χ_{c}$ or $ψ'$)is also calculated within NRQCD $k_{t}$-factorization. The corresponding matrix elements for $gg \to J/ψ$, $gg \to ψ'$ and $gg \to χ_{c}$ include parameters of the nonrelativistic spatial wave functions of quarkonia at $r=0$, which are taken from potential models from the literature. We get the ratio of the corresponding of the cross sections for $χ_{c}(2)$-to-$χ_{c}(1)$ much closer to experimental data than obtained in recent analyses. Differential distributions in rapidity of $J/ψ$ and $ψ'$ are calculated and compared to experimental data of the ALICE and LHCb collaborations. We discuss possible onset of gluon saturation effects at forward/backward rapidities. One can describe the experimental data for $J/ψ$ production within model uncertainties with color-singlet component only. Therefore our theoretical results leave only a relatively small room for the color-octet contributions.

hep-ph↗

arXiv2016

Double scattering production of two positron-electron pairs in ultraperipheral heavy-ion collisions

We present first measurable predictions for electromagnetic (two-photon) double scattering production of two positron-electron pairs in ultraperipheral heavy-ion collisions at LHC. Measureable cross sections are obtained with realistic cuts on electron/positron (pseudo)rapidities and transverse momenta for the ALICE and ATLAS or CMS experiments. The predictions for total and differential cross sections are presented. We show also two-dimensional distributions in rapidities of the opposite-sign (from the same or different subcollisions) and of the same-sign ($e^+ e^+$ or $e^- e^-$) electrons and in rapidity distance between them. Expected number of events are presented and discussed. Our calculations strongly suggest that relevant measurements with the help of ATLAS, CMS and ALICE detectors are possible in a near future.

nucl-th↗

arXiv2016

Search for walking technipion in four-jet sample

We discuss how to search for a possibe signal of the recently observed 750 GeV enhancement in the diphoton channel in the four-jet production. In the present studies we assume that the produced state is pseudoscalar. This fact, when combined with specificity of the corresponding amplitude, allows to improve the signal-to-background (S/B) ratio. We discuss in detail how to impose cuts on jets in rapidity and transverse momenta in order to find optimal S/B ratio and not to loose too much statistics. Our study suggest a measurement of two soft (low cut on $p_t$) large-rapidity jets and two hard (high cut on $p_t$) mid-rapidity jets. Azimuthal correlation between the soft external jets may be useful to further improve the situation. Several differential distributions in rapidities and transverse momenta of jets as well as dijet invariant mass are shown. The integrated cross sections corresponding to different cuts are collected in a table and number of events are presented.

hep-ph↗

arXiv2016

Electromagnetic effects on meson production: a new tool for studying the space-time evolution of heavy ion collisions

We review our studies of spectator-induced electromagnetic (EM) effects on the emission of charged mesons in the final state of ultrarelativistic heavy ion collisions. We argue that these effects offer sensitivity to the distance $d_E$ between the charged meson formation zone at freeze-out and the spectator system. As such, they can serve as an independent, new tool to probe the space-time and longitudinal evolution of the system created in the collision. As a phenomenological application for this tool in the context of resonance production and decay, we obtain a first estimate of the time of pion emission from EM effects. This we compare to existing HBT data.

nucl-th↗

arXiv2016

Photoproduction of $J/ψ$ and $Υ$ states in exclusive and proton-dissociative diffractive events

The amplitude for $γp \to V p$, where $V$ is a $J/ψ$ or $Υ$ ground state or excited vector meson, is calculated in a pQCD $k_{T}$-factorization approach. We use this amplitude to predict the cross section for exclusive photoproduction of $J/ψ, ψ', Υ$ mesons in proton-proton collisions. Calculations are performed for a variety of unintegrated gluon distributions, and we compare our results to LHCb data. Here we especially focus on the possibilty of constraining saturation effects. Compared to earlier calculations we include both Dirac and Pauli electromagnetic form factors. We discuss the role of the $Q\bar Q$ light-cone wave functions for differential distributions for ratios such as $σ(ψ')/σ(J/ψ)$. Absorption effects are always taken into account. We also discuss the related diffractive production in proton dissociative events. Here we concentrate on electromagnetic dissociation , which is calculable without additional free parameters. Besides being of interest in their own right, dissociative events constitute an important experimental background to exclusive production.

hep-ph↗

arXiv2016

Forward production of Drell-Yan dileptons at high energies and low dilepton invariant masses in a $k_t$-factorization approach: Do we see onset of saturation?

We discuss Drell-Yan production of dileptons at high energies in forward rapidity region in a hybrid high-energy approach. This approach uses unintegrated gluon distributions in one proton and collinear quark/antiquark distributions in the second proton. Corresponding momentum-space formula for the differential cross sections in high-energy approximation has been derived. The relation to the commonly used dipole approach is discussed. We conclude and illustrate that some results of the dipole approaches are too approximate, as far as kinemtics is considered, and in fact cannot be used for real experiments. We find that the dipole formula is valid only in very forward/backward rapidity regions ($|y| >$ 5). Some differential cross sections for low-mass dilepton production are shown and compared to the LHCb and ATLAS experimental data. In distinction to dipole approaches, we include four Drell-Yan structure functions (the impact of interference structure functions is rather small for typical experimental cuts). We find that both side contributions ($g q/\bar q$ and $q/\bar q g$) have to be included even for the LHCb rapidity coverage which is in contradiction with what is usually done in the dipole approach. We present results for different unintegrated gluon distributions from the literature (some of them include saturation effects). We see no clear hints of saturation even at small $M_{ll}$.

hep-ph↗

arXiv2016

Interpreting the 750 GeV diphoton signal as technipion

We discuss whether the enhancement in the diphoton final state at $M_{γγ}$ = 750 GeV, observed recently by the ATLAS and CMS Collaborations could be a neutral pseudoscalar technipion $\tildeπ^0$. We considered two distinct minimal models for the dynamical electroweak symmetry breaking. Here we concentrate only on two-flavor vector-like technicolor model and we assume that the two-photon fusion is a dominant production mechanism. We include contributions of $2 \to 1$, $2 \to 2$ and $2 \to 3$ partonic processes. All the mechanisms give similar contributions to the cross section. With the strong Yukawa (technipion-techniquark) coupling $g_{TC} \simeq 20$ we roughly obtain the measured cross section of the "signal". With such value of $g_{TC}$ we get a relatively small total decay width $Γ_{\rm tot}$. We discuss also the size of the signal at lower energies (LHC, Tevatron) for $γγ$ final states, where the enhancement was not observed. We predict a measurable cross section for neutral technipion production associated with one or two soft jets. The technipion signal is compared with the Standard Model diphoton background contributions. We observe the dominance of inelastic-inelastic $γγ$ processes. We predict the signal cross section for purely exclusive $p p \to p p γγ$ procesess at $\sqrt{s}$ = 13 TeV to be about 0.2 fb. Such a cross section would be, however, difficult to measure with the planned integrated luminosity. We conclude that in all considered cases the signal is below the background or/and below the threshold set by statistics although some tension can be seen.

hep-ph↗