Hard Parton Research Articles

HYDJET++ model as a hot mixture of jets and hydrodynamics at ultra-relativistic heavy-ion collisions

HYDJET++ model combines the description of soft processes with the treatment of hard partons propagating hot and dense nuclear medium. The model is employed for the analysis of PbPb collisions at LHC energies. The cross-talk of elliptic v2 and triangular v3 flow in the model generates both even and odd harmonics of higher order, as well as long-range azimuthal dihadron correlations (“ridge” structure). The simple modification of the model via introducing the distribution over spatial anisotropy parameters permits HYDJET++ to reproduce both v2 and v3 flow fluctuations and related to it eccentricity fluctuations of the initial state. The data on momentum spectra and elliptic flow of charmed mesons are reproduced by HYDJET++ including thermal and non-thermal charm production mechanisms. A tolerable agreement of HYDJET++ predictions with the data for hard probes (including such observables as high-pT hadron and jet nuclear modification factors, and specific modification of longitudinal and radial jet profiles) testifies in favour of successful treatment of the hard component based on PYQUEN event generator.

RAA vs. v2 of heavy and light hadrons within a linear Boltzmann transport model

We establish a linear Boltzmann transport (LBT) model coupled to hydrodynamical background to study hard parton evolution in heavy-ion collisions. Both elastic and inelastic scatterings are included in our calculations; and heavy and light flavor partons are treated on the same footing. Within this LBT model, we provide good descriptions of heavy and light hadron suppression from RHIC to the LHC energies. Possible solutions to the RAA vs. v2 puzzle are discussed.

Direct virtual photon production in Au+Au collisions at [formula omitted

We report the direct virtual photon invariant yields in the transverse momentum ranges 1<pT<3 GeV/c and 5<pT<10 GeV/c at mid-rapidity derived from the dielectron invariant mass continuum region 0.10<Mee<0.28 GeV/c2 for 0–80% minimum-bias Au+Au collisions at sNN=200 GeV. A clear excess in the invariant yield compared to the nuclear overlap function TAA scaled p+p reference is observed in the pT range 1<pT<3 GeV/c. For pT>6 GeV/c the production follows TAA scaling. Model calculations with contributions from thermal radiation and initial hard parton scattering are consistent within uncertainties with the direct virtual photon invariant yield.

Jets in heavy ion collisions

High energy collisions of heavy nuclei permit the study of nuclear matter at temperatures and energy densities so high that the fundamental theory for strong interactions, QCD, predicts a phase transition to a plasma of quarks and gluons. This matter, called a Quark Gluon Plasma (QGP), has been studied experimentally for the last decade and has been observed to be a strongly interacting liquid with a low viscosity. High energy partons created early in the collision interact with the QGP and provide unique probes of its properties. Hard partons fragment into collimated sprays of particles called jets and have been studied through measurements of single particles, correlations between particles, and measurements of fully reconstructed jets. These measurements demonstrate partonic energy loss in the QGP and constrain the QGP’s properties. Measurements of the jet structure give insight into the mechanism of this energy loss. The information we have learned from studies of jets and challenges for the field will be reviewed.

Medium-induced gluon radiation in hard forward parton scattering in the saturation formalism

We derive the medium-induced, coherent gluon radiation spectrum associated with the hard forward scattering of an energetic parton off a nucleus, in the saturation formalism and within the Gaussian approximation for the relevant correlators of Wilson lines. The calculation reproduces the simple expression for the spectrum previously obtained in the opacity expansion formalism, and rigorously specifies its validity range. The connection between the calculations in the opacity expansion and saturation formalisms is made apparent. This study may serve as a first step in order to implement consistently induced coherent energy loss and gluon shadowing in `saturation-based models' of hadron nuclear suppression in proton-nucleus collisions.

New detector for studies of cumulative processes in hadron collisions in NA61(SHINE) at the CERN SPS

The increase of luminosity of the SPS beams expected after 2020 allows considering the investigation of rather rare processes. In particular, so-called cumulative particle production can be studied in hadron collisions by measurements of secondary particle yields in regions kinematically forbidden for reactions with free nucleons. Such processes could be either a result of hard parton collisions with some large-density multi-quark configuration or of the formation of heavy baryonic resonances. Measurements in the backward hemisphere in fixed target experiment should provide the event-by-event data that could be used, along with those from the forward region, for a correlation analysis, thus resulting in new constraints on models. In this report the preliminary design, ideas, technology and the first GEANT simulations of a proposed new detector are presented and discussed.

The effect of momentum deposition during fireball evolution on flow anisotropy

The highest LHC energies give rise to the production of many pairs of hard partons which deposit four-momentum into the expanding fireball matter. We argue that it is necessary to include momentum deposition during fireball evolution into 3 + 1 dimensional hydrodynamic simulations of the collision. This influence cannot be accounted for simply by modifying the initial conditions of the simulation. The resulting contribution to flow anisotropy is correlated with the fireball geometry and causes an increase of the elliptic flow in non-central collisions. The results are presented for various scenarios with energy and momentum deposition to clearly demonstrate this effect.

Photon bremsstrahlung from quark jet via transverse and longitudinal scattering: Single versus multiple scattering

We study the production of jet-bremsstrahlung photons through the scattering with the constituents of a dense nuclear matter within the framework of deep-inelastic scattering off a large nucleus. Applying a gradient expansion up to the second order for the exchanged three-dimensional momentum between jet and medium, we derive the single photon bremsstrahlung spectrum with the inclusion of the contributions from the transverse broadening as well as the longitudinal drag and diffusion of the hard parton's momentum. We also compare the medium-induced photon radiation spectra for single scattering and from the resummation of multiple scatterings. It is found that the coupling between different scatterings can give additional contribution to medium-induced photon radiation, while for small momentum exchange, the leading contribution from the drag and diffusions to the photon emission spectra remain the same for single and multiple scatterings.

Measurements of D-meson production with ALICE

Heavy quarks (charm and beauty) are produced in hard parton scattering processes in the initial stages of hadronic collisions. The study of their production can give us different information in each type of collisions. In pp collisions at the LHC it provides a test of perturbative QCD calculations at the highest collision energy. In p-Pb collisions it allows one to characterise the nuclear modification of the parton distribution functions and the kT-broadening effect due to soft scatterings of the partons in the incoming nuclei. Moreover, it provides insight into possible final state effects that could occur in p-Pb collisions, like energy loss of heavy quarks in the cold nuclear matter and their collective behaviour. In Pb-Pb collisions, heavy-flavour production can be used to study the properties of the high energy-density and strongly-interacting medium created in heavy-ion collisions. Passing through all the phases of the system evolution, heavy quarks lose energy via gluon radiation and elastic collisions in the medium. Therefore, the study of their production can be used to test parton energy-loss models.In these proceedings we will focus on the ALICE results of prompt D mesons measured via the full reconstruction of their hadronic decays. We will present results on open-charm production in pp collisions at s=7 TeV, p-Pb collisions at sNN=5.02 TeV and Pb-Pb collisions at sNN=2.76 TeV.

Differential measurements of dielectron and direct virtual photon productions at STAR

Dileptons and photons have been serving as penetrating probes in heavy-ion collisions for quite a long time since the strongly interacting matter predicted by QCD theory. We present the mid-rapidity dielectron results in 0-80% Au+Au collisions at sNN=19.6,39,62.4, and 200 GeV, and the direct virtual photon results in 0-80% Au+Au collisions at sNN=200 GeV at STAR. The excess yields of dielectron with respect to hadronic sources at low mass range are presented, which are consistently described by a broadened ρ spectral function plus thermal radiations. Furthermore, the direct virtual photon yield has been measured and found it is consistent with a model that includes thermal radiation and initial hard parton scattering.

Semi-holography for heavy ion collisions: self-consistency and first numerical tests

We present an extended version of a recently proposed semi-holographic model for heavy-ion collisions, which includes self-consistent couplings between the Yang-Mills fields of the Color Glass Condensate framework and an infrared AdS/CFT sector, such as to guarantee the existence of a conserved energy-momentum tensor for the combined system that is local in space and time, which we also construct explicitly. Moreover, we include a coupling of the topological charge density in the glasma to the same of the holographic infrared CFT. The semi-holographic approach makes it possible to combine CGC initial conditions and weak-coupling glasma field equations with a simultaneous evolution of a strongly coupled infrared sector describing the soft gluons radiated by hard partons. As a first numerical test of the semi-holographic model we study the dynamics of fluctuating homogeneous color-spin-locked Yang-Mills fields when coupled to a homogeneous and isotropic energy-momentum tensor of the holographic IR-CFT, and we find rapid convergence of the iterative numerical procedure suggested earlier.

Background subtraction methods for precision measurements of di-hadron and jet-hadron correlations in heavy ion collisions

Di-hadron correlations and jet-hadron correlations are frequently used to study interactions of the Quark Gluon Plasma with the hard partons that form jets. The existing background subtraction methods for these studies depend on several assumptions and independent measurements of the Fourier coefficients of the combinatorial background. In this paper, we present two background subtraction methods which use the observation that the near-side correlation signal is negligible for large separations between the associated particle and the trigger. The combinatorial background is fit in this background-dominated region. Including the reaction plane dependence of the raw correlation improves these results. To test the accuracy of these methods, a simple model is used to simulate di-hadron and jet-hadron correlations with a combinatorial background similar to that observed in the data. The true signal is compared to the extracted signal. The results are compared to results from two variants of the Zero-Yield-At-Minimum (ZYAM) method. We test these methods for mid-peripheral and central collisions and for di-hadron and jet-hadron correlations. These methods are more precise than the ZYAM method with fewer assumptions about the shape and level of the combinatorial background, even in central collisions where the experimental resolution on the measurement of the reaction plane dependence is poor. These methods will allow more accurate studies of modifications of the away-side jet and will be particularly useful for studies of jet-hadron correlations, where the combinatorial background is poorly constrained from previous studies.

Herwig 7.0/Herwig++ 3.0 release note

A major new release of the Monte Carlo event generator Herwig++ (version 3.0) is now available. This release marks the end of distinguishing Herwig++ and HERWIG development and therefore constitutes the first major release of version 7 of the Herwig event generator family. The new version features a number of significant improvements to the event simulation, including: built-in NLO hard process calculation for all Standard Model processes, with matching to both angular-ordered and dipole shower modules via both subtractive (MC@NLO-type) and multiplicative (Powheg-type) algorithms; QED radiation and spin correlations in the angular ordered shower; a consistent treatment of perturbative uncertainties within the hard process and parton showering. Several of the new features will be covered in detail in accompanying publications, and an update of the manual will follow in due course.

Late time behaviour for Hard Parton Evolution using the Schwinger-Keldysh Formalism

AdS/CFT computations have been used to describe the energy loss of QCD-like particles moving through a strongly coupled plasma, but little is understood regarding the initial conditions of these jets. We are investigating an analytic description of jet initial conditions through the time evolution of hard partons by means of the Schwinger-Keldysh Formalism.We have derived a general formula for the late time expectation value of the total 4- momentum in terms of the differential cross-section and the initial 4-momenta of the system. As a consistancy check, we have found the same expression for limxo→∞ 〈∫d3xTμν(x)〉 for an interacting scalar field theory to leading order.

Quantifying jet transport properties via large $$p_T$$ p T hadron production

Nuclear modification factor $R_{AA}$ for large $p_T$ single hadron is studied in a next-to-leading order (NLO) perturbative QCD (pQCD) parton model with medium-modified fragmentation functions (mFFs) due to jet quenching in high-energy heavy-ion collisions. The energy loss of the hard partons in the QGP is incorporated in the mFFs which utilize two most important parameters to characterize the transport properties of the hard parton jets: the jet transport parameter $\hat q_{0}$ and the mean free path $\lambda_{0}$, both at the initial time $\tau_0$. A phenomenological study of the experimental data for $R_{AA}(p_{T})$ is performed to constrain the two parameters with simultaneous $\chi^2/{\rm d.o.f}$ fits to RHIC as well as LHC data. We obtain for energetic quarks $\hat q_{0}\approx 1.1 \pm 0.2$ GeV$^2$/fm and $\lambda_{0}\approx 0.4 \pm 0.03$ fm in central $Au+Au$ collisions at $\sqrt{s_{NN}}=200$ GeV, while $\hat q_{0}\approx 1.7 \pm 0.3$ GeV$^2$/fm, and $\lambda_{0}\approx 0.5 \pm 0.05$ fm in central $Pb+Pb$ collisions at $\sqrt{s_{NN}}=2.76$ TeV. Numerical analysis shows that the best fit favors a multiple scattering picture for the energetic jets propagating through the bulk medium, with a moderate averaged number of gluon emissions. Based on the best constraints for $\lambda_{0}$ and $\tau_0$, the estimated value for the mean-squared transverse momentum broadening is moderate which implies that the hard jets go through the medium with small reflection.

Finite time calculations for hard parton production relevant to the quark-gluon plasma

AdS/CFT computations have been used to describe the energy loss of QCD-like particles moving through a strongly coupled plasma, but little is understood regarding the initial conditions of these jets. We use the Schwinger-Keldysh finite-time formalism applied to an interacting scalar field theory to derive perturbative expressions detailing the system which exists during the initial stages of a high energy collision.In this paper we calculate 〈ϕ〉(x) for a scalar Yukawa model, demonstrate the finiteness of the energy momentum tensor for λϕ4 to leading order, and derive an expression for the conditional expectation value of operators to aid in the description jet-like behaviour in interacting theories.

Centrality and rapidity dependence of inclusive jet production in [formula omitted] proton–lead collisions with the ATLAS detector

Measurements of the centrality and rapidity dependence of inclusive jet production in sNN=5.02 TeV proton–lead (p+Pb) collisions and the jet cross-section in s=2.76 TeV proton–proton collisions are presented. These quantities are measured in datasets corresponding to an integrated luminosity of 27.8 nb−1 and 4.0 pb−1, respectively, recorded with the ATLAS detector at the Large Hadron Collider in 2013. The p+Pb collision centrality was characterised using the total transverse energy measured in the pseudorapidity interval −4.9<η<−3.2 in the direction of the lead beam. Results are presented for the double-differential per-collision yields as a function of jet rapidity and transverse momentum (pT) for minimum-bias and centrality-selected p+Pb collisions, and are compared to the jet rate from the geometric expectation. The total jet yield in minimum-bias events is slightly enhanced above the expectation in a pT-dependent manner but is consistent with the expectation within uncertainties. The ratios of jet spectra from different centrality selections show a strong modification of jet production at all pT at forward rapidities and for large pT at mid-rapidity, which manifests as a suppression of the jet yield in central events and an enhancement in peripheral events. These effects imply that the factorisation between hard and soft processes is violated at an unexpected level in proton–nucleus collisions. Furthermore, the modifications at forward rapidities are found to be a function of the total jet energy only, implying that the violations may have a simple dependence on the hard parton–parton kinematics.

A semi-holographic model for heavy-ion collisions

We develop a semi-holographic model for the out-of-equilibrium dynamics during the partonic stages of an ultrarelativistic heavy-ion collision. The model combines a weakly-coupled hard sector, involving gluon modes with energy and momenta of the order of the saturation momentum and relatively large occupation numbers, with a strongly-coupled soft sector, which physically represents the soft gluons radiated by the hard partons. The hard sector is described by perturbative QCD, more precisely, by its semi-classical approximation (the classical Yang-Mills equations) which becomes appropriate when the occupation numbers are large. The soft sector is described by a marginally deformed conformal field theory, which in turn admits a holographic description in terms of classical Einstein's equations in $AdS_5$ with a minimally coupled massless `dilaton'. The model involve two free parameters which characterize the gauge-invariant couplings between the hard and soft sectors. Via these couplings, the hard modes provide dynamical sources for the gravitational equations at the boundary of $AdS_5$ and feel the feedback of the latter as additional soft sources in the classical Yang-Mills equations. Importantly, the initial conditions for this coupled dynamics are fully determined by the hard sector alone, i.e. by perturbative QCD, and are conveniently given by the color glass condensate (CGC) effective theory. We also develop a new semi-holographic picture of jets in the QGP by attaching a non-Abelian charge to the endpoint of the trailing string in $AdS_5$ representing a heavy quark. This leads to modified Nambu-Goto equations for the string which govern the (collisional and radiative) energy loss by the heavy quark towards both hard and soft modes.

Jet transport and photon bremsstrahlung via longitudinal and transverse scattering

We study the effect of multiple scatterings on the propagation of hard partons and the production of jet-bremsstrahlung photons inside a dense medium in the framework of deep-inelastic scattering off a large nucleus. We include the momentum exchanges in both longitudinal and transverse directions between the hard partons and the constituents of the medium. Keeping up to the second order in a momentum gradient expansion, we derive the spectrum for the photon emission from a hard quark jet when traversing dense nuclear matter. Our calculation demonstrates that the photon bremsstrahlung process is influenced not only by the transverse momentum diffusion of the propagating hard parton, but also by the longitudinal drag and diffusion of the parton momentum. A notable outcome is that the longitudinal drag tends to reduce the amount of stimulated emission from the hard parton.

Dissecting soft radiation with factorization.

An essential part of high-energy hadronic collisions is the soft hadronic activity that underlies the primary hard interaction. It includes soft radiation from the primary hard partons, secondary multiple parton interactions (MPI), and factorization-violating effects. The invariant mass spectrum of the leading jet in Z+jet and H+jet events is directly sensitive to these effects, and we use a QCD factorization theorem to predict its dependence on the jet radius R, jet p_{T}, jet rapidity, and partonic process for both the perturbative and nonperturbative components of primary soft radiation. We prove that the nonperturbative contributions involve only odd powers of R, and the linear R term is universal for quark and gluon jets. The hadronization model in Pythia8 agrees well with these properties. The perturbative soft initial state radiation (ISR) has a contribution that depends on the jet area in the same way as the underlying event, but this degeneracy is broken by dependence on the jet p_{T}. The size of this soft ISR contribution is proportional to the color state of the initial partons, yielding the same positive contribution for gg→Hg and gq→Zq, but a negative interference contribution for qq[over ¯]→Zg. Hence, measuring these dependencies allows one to separate hadronization, soft ISR, and MPI contributions in the data.