Initial State Effects Research Articles

An ab initio study of the O1s and Mg1s, Mg2s, Mg2p core electron binding energies in bulk MgO

Embedded cluster models combined with wavefunction based quantum chemical ab initio methods were used to calculate the O1s and Mg1s, Mg2s, Mg2p core electron binding energies (CEBEs) of bulk MgO. The calculations were performed at three levels of theory: Koopmans' theorem (KT, initial state effects), ΔSCF (relaxation of the electronic wavefunction after core ionization, final state effects, with inclusion of scalar relativistic effects), and approximate coupled cluster (CC) approach to account for electron correlation. Several basis sets as well as MgO clusters of various design and size were employed to study their influence on the CEBEs. The contribution of electron correlation being in the order of 1 eV, but very much different for the different core levels, was analyzed in detail. Our final results are 529.5 eV for the O1s CEBE and 1303.5, 87.7 and 48.8 eV for Mg1s, Mg2s, Mg2p, respectively (all relative to the Fermi level). They agree with recent experimental XPS data within about 0.5 eV.

Off-shell initial state effects, gauge invariance and angular distributions in the Drell–Yan process

We discuss production of Drell–Yan lepton pairs at hadron colliders in the framework of the parton Reggeization approach, which includes off-shell initial state effects in a gauge-invariant way. Other possible prescriptions to restore gauge-invariance of hard-scattering coefficient with off-shell initial-state partons are also investigated and significant differences for the resulting structure functions are found, especially for the FUU(cos⁡2ϕ). We compare our numerical results for qT-spectra of the lepton pair with experimental data, obtained by E-288 collaboration (S=19.4 and 23.8 GeV) and find a good agreement. Also we perform predictions for the Drell–Yan structure functions at NICA pp-collider (S=24GeV).

Importance of initial and final state effects for azimuthal correlations in p+Pb collisions

We study the influence and interplay of initial state and final state effects in the dynamics of small systems, focusing on azimuthal correlations at different multiplicities. To this end we introduce a new model, matching the classical Yang-Mills dynamics of pre-equilibrium gluon fields (IP-GLASMA) to a perturbative QCD based parton cascade for the final state evolution (BAMPS) on an event-by-event basis. Depending on multiplicity of the event, we see transverse momentum dependent signatures of the initial, but also the final state in azimuthal correlation observables, such as v2 {2PC}(pT). In low-multiplicity events, initial state correlations dominate for transverse momenta pT > 2 GeV, whereas in high-multiplicity events and at low momenta final state interactions dominate and initial state correlations strongly affect v2 {2PC}(pT) for pT > 2 GeV as well as the pT integrated v2 {2PC}. Nearly half of the final pT integrated v2 {2PC} is contributed by the initial state in low-multiplicity events, whereas in high-multiplicity the share is much less. Based on Ref. [M. Greif, C. Greiner, B. Schenke, S. Schlichting, Z. Xu, Phys. Rev. D96 (9) (2017) 091504], we are now able to carry out a systematic multiplicity scan, probing the dynamics on the border of initial state dominated to final state dominated – but not yet hydrodynamic regime.

Recent Quarkonium Studies from the PHENIX Experiment

Quarkonium suppression in nucleus-nucleus collisions is a powerful tool to probe the density and temperature of the medium created in heavy ion collisions. Forward rapidity measurements in p(d)+Au collisions are essential to understand how quarkonium states are affected by initial state effects, formation time, and local particle multiplicity. Earlier measurements in Au+Au collisions showed a stronger suppression of forward J/ψ compared to mid-rapidity results, indicating the possibility of a smaller contribution of regenerated quarkonium states at forward rapidity. These proceedings report on the latest quarkonium studies performed by the PHENIX collaboration in the rapidity range 1.2<|y|<2.2.

Elucidating the multiplicity dependence of mathrm {J}/psi production in proton\u2013proton collisions with PYTHIA8

A study of prompt and non-prompt mathrm {J}/psi production as a function of charged-particle multiplicity in inelastic proton–proton (pp) collisions at a centre-of-mass energy of sqrt{s} = 13 TeV based on calculations using the PYTHIA8 Monte Carlo is reported. Recent experimental data shows an intriguing stronger-than-linear increase of the self-normalized mathrm {J}/psi yield with multiplicity; several models, based on initial or final state effects, have been able to describe the observed behaviour. In this paper, the microscopic reasons for this behaviour, like the role of multiple parton interactions, colour reconnections and auto-correlations are investigated. It is observed that the stronger-than-linear increase and the transverse momentum (p_mathrm {T}) dependence, contrary to what is predicted by the other available models, can be attributed to auto-correlation effects only. In absence of auto-correlation effects, the increase of the yield of mathrm {J}/psi with multiplicity – and in general for all hard processes – is weaker than linear for multiplicities exceeding about three times the mean multiplicity. The possibility of disentangling auto-correlation effects from other physical phenomena by measuring the charged-particle multiplicity in different pseudo-rapidity and azimuthal regions relative to the mathrm {J}/psi direction is investigated. In this regard, it is suggested to extend the experimental measurements of mathrm {J}/psi production as a function of the charged-particle multiplicity by determining the multiplicity in several azimuthal regions and in particular in the Transverse region with respect to the direction of the mathrm {J}/psi meson.

Off-shell initial state effects in Drell-Yan process at hadron colliders

We discuss production of Drell-Yan lepton pairs at hadron colliders in the framework of the parton Reggeization approach, which includes off-shell initial state effects in a gauge-invariant way. We have compared our numerical results forqT-spectra of the lepton pair with the experimental data, obtained by E-288 collaboration ($\rad{}{S} = 19.4$ and 23.8 GeV) and found a good agreement. We have also made predictions for the Drell-Yan structure functions at NICApp-collider ($\rad{}{S} = 24\,{\rm{GeV}}$).

Double-core-hole states in CH3CN: Pre-edge structures and chemical-shift contributions.

Spectra reflecting the formation of single-site double-core-hole pre-edge states involving the N 1s and C 1s core levels of acetonitrile have been recorded by means of high-resolution single-channel photoelectron spectroscopy using hard X-ray excitation. The data are interpreted with the aid of ab initio quantum chemical calculations, which take into account the direct or conjugate nature of this type of electronic states. Furthermore, the photoelectron spectra of N 1s and C 1s singly core-ionized states have been measured. From these spectra, the chemical shift between the two C 1s-1 states is estimated. Finally, by utilizing C 1s single and double core-ionization potentials, initial and final state effects for the two inequivalent carbon atoms have been investigated.

Measurements of low-pT electrons from semileptonic heavy-flavour hadron decays at mid-rapidity in pp and Pb-Pb collisions at sqrt{s_{mathrm{NN}}}=2.76 TeV

Transverse-momentum (pT) differential yields of electrons from semileptonic heavy-flavour hadron decays have been measured in the most central (0–10%) and in semi-central (20–40%) Pb–Pb collisions at sqrt{s_{mathrm{NN}}}=2.76 TeV. The corresponding production cross section in pp collisions has been measured at the same energy with substantially reduced systematic uncertainties with respect to previously published results. The modification of the yield in Pb–Pb collisions with respect to the expectation from an incoherent superposition of nucleon-nucleon collisions is quantified at mid-rapidity (|y| < 0.8) in the pT interval 0.5–3 GeV/c via the nuclear modification factor, RAA. This paper extends the pT reach of the RAA measurement towards significantly lower values with respect to a previous publication. In Pb–Pb collisions the pT-differential measurements of yields at low pT are essential to investigate the scaling of heavy-flavour production with the number of binary nucleon-nucleon collisions. Heavy-quark hadronization, a collective expansion and even initial-state effects, such as the nuclear modification of the Parton Distribution Function, are also expected to have a significant effect on the measured distribution.

Recent results on hard processes in p+Pb, Pb+Pb, and pp collisions from the ATLAS Experiment at the LHC

In relativistic heavy-ion collisions, a hot medium with a high density of unscreened colour charges is produced. Hard processes from parton-parton scattering in the early stages of the collision can be used as probes of the medium since the initial state is mostly understood such that differences from pp collisions in the final state can be attributed to interactions with the medium. The initial and final state effects are separated by comparing p+Pb and pp collisions. Recent results from the ATLAS experiment at the LHC on the production of electroweak bosons, jets, and quarkonia are discussed in p+Pb and Pb+Pb collisions, compared to pp. Recent results on ultra-peripheral collisions are also shown.

PHENIX results on collectivity in small systems

Recent results from small collision systems at RHIC and LHC indicate that many of the signatures of collective behavior observed in AA collisions are also present in small systems in high-multiplicity events. Using the extraordinary versatility of RHIC in selecting different colliding species, the PHENIX experiment has collected data in p+Al, p+Au, d+Au, and 3He+Au at a nucleon-nucleon center-of-mass energy of 200 GeV and conducted a comprehensive set of anisotropic flow measurements. These geometry-controlled experiments provide a unique testing ground for theoretical models that produce azimuthal particle correlations based on initial and/or final state effects. In this paper, we present measurments of elliptic and triangular flow for inclusive charged particles, and v2(pT) measurements for identified pions, and protons. Detailed model comparisons and the implications for the origin of collectivity in p/d/3He+Au collisions are discussed.

Nuclear modification factors of strange and multi-strange particles in pPb collisions with the CMS experiment

Recent observation of collective effects in high-multiplicity pp and pA collisions raise the question of whether QGP can also be formed in the smaller systems. Systematic studies of the strange particle abundance and nuclear modification factors can shed light on this issue. The CMS experiment has excellent strange-particle reconstruction capabilities over a broad kinematic range in pp and pPb collisions. The spectra of and hadrons have been measured in various rapidity regions as a function of pT in pp and pPb collisions at 5.02 TeV. Based on the measurements of these spectra, nuclear modification factors of , and in mid-rapidity are measured. Since pPb is an asymmetric system, the nuclear modification factor of , and in Pb-going direction are compared to those in p-going direction. These final results for nuclear modification factors measured out to high-pT can be helpful in discussing the implications of collective behavior and energy loss. In addition, the measurement of the forward-backward rapidity yield asymmetries of and Λ as a function of pT provide sensitivity to initial state effects, such as shadowing in the nuclear parton distributions.

Kinetic evolution and correlation of fluctuations in an expanding quark gluon plasma

Evolution of spatially anisotropic perturbation created in the system formed after Relativistic Heavy Ion Collisions has been studied. The microscopic evolution of the fluctuations has been examined within the ambit of Boltzmann Transport Equation (BTE) in a hydrodynamically expanding background. The expansion of the background composed of quark gluon plasma (QGP) is treated within the framework of relativistic hydrodynamics. Spatial anisotropic fluctuations with different geometries have been evolved through Boltzmann equation. It is observed that the trace of such fluctuation survives the evolution. Within the relaxation time approximation, analytical results have been obtained for the evolution of these anisotropies. Explicit relations between fluctuations and transport coefficients have been derived. The mixing of various Fourier (or k) modes of the perturbations during the evolution of the system has been explicitly demonstrated. This study is very useful in understanding the presumption that the measured anisotropies in the data from heavy ion collisions at relativistic energies imitate the initial state effects. The evolution of correlation function for the perturbation in pressure has been studied and shows that the initial correlation between two neighbouring points in real space evolves to a constant value at later time which gives rise to Dirac delta function for the correlation function in Fourier space. The power spectrum of the fluctuation in thermodynamic quantities (like temperature estimated in this work) can be connected to the fluctuation in transverse momentum of the thermal hadrons measured experimentally. The bulk viscous coefficient of the QGP has been estimated by using correlations of pressure fluctuation with the help of Green–Kubo relation. Angular power spectrum of the anisotropies has been estimated in the appendix.

Collectivity in small systems - Initial state vs. final state effects

Observations of long rang azimuthal correlations in small collision systems (p+p/A) have triggered an enormous excitement in the heavy-ion community. However, it is presently unclear to what extent the experimentally observed correlations should be attributed to initial state momentum correlations and/or the final state response to the initial state geometry. We discuss how a consistent theoretical description of the nonequilibrium dynamics is important to address both effects within a unified framework and present first results from weakly coupled non-equilibrium simulations in [1] to quantify the relative importance of initial state and final state effects based on theoretical calculations.

Recent results from the ALICE Experiment at LHC

ALICE (A Large Ion Collider Experiment) at the LHC performed high statistics measurements in Pb-Pb collisions at the top LHC energy, complemented with large recent reference datasets in elementary proton-proton collisions at the same energy. Elementary pp collisions are serving as baseline for testing QCD properties and allow the study of the changes induced by the hot and dense medium produced in heavy ion collisions. Key observables like nuclear modification factors, jet production, flow phenomena and spectra for identified particles, related to the different stages of collision evolution, are presented and compared with the most recent results from p-Pb and Xe-Xe collisions, thus allowing to probe both initial cold nuclear matter and final state effects, combined with the system size dependence of the measurements.

Transverse momentum spectra and nuclear modification factors of identified charged hadrons in p-Pb and Pb-Pb collisions at √sNN = 5.02 TeV with ALICE

The ALICE experiment has measured the production of identified light-flavour hadrons in p-Pb and Pb-Pb collisions at 5.02 TeV in a wide range of transverse momentum. The newest ALICE results on pion, kaon and proton transverse momentum spectra, yield ratios and nuclear modification factors will be presented and discussed in comparison to lower energy results. In particular, the production of identified hadrons in most central Pb-Pb collisions relative to pp collisions is found to be strongly suppressed at high transverse momenta (pT &gt; 8 GeV/c) whereas in p-Pb collisions the nuclear modification factors are consistent with unity. This indicates that the strong suppression of high-pT hadrons measured in central Pb-Pb collisions is not due to an initial state effect but instead to the energy loss of partons traversing a hot and dense QCD medium.

Polarization observables in few nucleon systems with CLAS

The CEBAF Large Acceptance Spectrometer (CLAS), housed in Hall-B at the Thomas Jefferson National Accelerator Facility provides us with the experimental tools to study strongly-interacting matter and its dynamics in the transition from hadronic to partonic degrees of freedom in nuclear interactions. In this paper we discuss the progress made in understanding the relevant degrees of freedom using polarisation observables of deuteron photodisintegration in the few-GeV photon-energy region. We also address progress made in studying the interaction between Hyperons and Nucleons via polarisation observables, utilising high-statistics experiments that provided us with the large data samples needed to study final-state interactions, as well as perform detailed studies on initial-state effects. The polarisation observables presented here provide us with unique experimental tools to study the underlying dynamics of both initial and final-state interactions, as well as the information needed to disentangle signal from background contributions.

Importance of initial and final state effects for azimuthal correlations in p+Pb collisions

We investigate the relative importance of initial and final state effects on azimuthal correlations of gluons in low and high multiplicity p+Pb collisions. To achieve this, we couple Yang-Mills dynamics of pre-equilibrium gluon fields (IP-GLASMA) to a perturbative QCD based parton cascade for the final state evolution (BAMPS) on an event-by-event basis. We find that signatures of both the initial state correlations and final state interactions are seen in azimuthal correlation observables, such as $v_2\left\lbrace2PC\right\rbrace(p_T)$, their strength depending on the event multiplicity and transverse momentum. Initial state correlations dominate $v_2\left\lbrace2PC\right\rbrace(p_T)$ in low multiplicity events for transverse momenta $p_T>2~{\rm GeV}$. While final state interactions are dominant in high multiplicity events, initial state correlations affect $v_2\left\lbrace2PC\right\rbrace(p_T)$ for $p_T>2~{\rm GeV}$ as well as the pT integrated $v_2\left\lbrace2PC\right\rbrace$.

Nuclear modification of forward Drell-Yan production at the LHC

Forward Drell-Yan production at high energy can provide important constraints on gluon densities at small $x$, in the saturation regime. In this work we focus on the nuclear modification of this process, which could be measured at the LHC in the near future. For this we employ the color dipole approach, using the optical Glauber model to relate the dipole cross section of a nucleus to the one of a proton. Combining these results with our earlier results for forward $J/\psi$ production, we compute the ratio of the nuclear modification factors of these two processes. This observable was recently suggested as a way to distinguish between initial and final state effects in forward particle production.

Towards tomography of quark\u2013gluon plasma using double inclusive forward-central jets in Pb\u2013Pb collision

We propose a new framework, merging High Energy Factorization with final-state jet quenching effects due to interactions in a quark–gluon plasma, to compute di-jet rates at mid-rapidity and forward rapidity. It allows one to consistently study the interplay of initial-state effects with medium interactions, opening the possibility for understanding the dynamics of hard probes in heavy-ion collisions and the QGP evolution in rapidity.

Study of heavy meson production in p–Pb collisions at [formula omitted] TeV in the general-mass variable-flavour-number scheme

We study inclusive charm and bottom production, for both D and B mesons, in p–Pb collisions at the LHC. Numerical results for pT-differential production cross sections are obtained at next-to-leading-order in the general-mass variable-flavour-number scheme. We compare our results with recent data from ALICE, LHCb and CMS at a centre-of-mass energy of 5 TeV and find good agreement. A comparison with p–p cross sections does not reveal the presence of nuclear initial-state interaction effects that could be expected to become visible as deviations of the ratio of p–Pb and p–p cross sections from one.