Partonic Medium Research Articles

Jet charge in heavy-ion collisions

Jet quenching effects have been widely used to study the properties of strongly-interacting matter, quark-gluon plasma, in heavy-ion collisions. Flavor tagging in heavy-ion collisions plays an important role to reveal the medium parton showers for quark and gluon evolution. Combining with kinematic information, the average jet charge can be used to separate the contribution of different jet flavors, which is defined as the momentum- weighted sum of the charges of hadrons inside a given jet. Using soft-collinear effective theory with medium interactions, we investigate the factorization of the jet charge in QCD medium. We provide predictions for jet charge distributions and their modifications compared to the ones in proton-proton collisions.

Study of jet-medium interactions using jet shape observables in heavy ion collisions at LHC energies with JEWEL

Based on a perturbative quantum chromodynamics inspired dynamical model of jet-medium interactions, Jewel, we have studied possible modifications to inclusive jet yields and a set of jet shape observables, namely, the fragmentation functions and radial momentum distributions when jets propagate through a deconfined partonic medium created in collisions of heavy nuclei at Large Hadron Collider (LHC) energies. Jets are reconstructed with anti-kT algorithm in the pseudorapidity range for resolution parameter R = 0.2, 0.3 and 0.4. For background subtraction, a Jewel-compatible 4-Momenta subtraction technique (4MomSub) has been used. The modification of inclusive jet-yields in Pb–Pb collisions relative to proton–proton interactions, quantified by , are seen to be in reasonable agreement with ALICE, ATLAS and CMS data over a broad transverse momentum range. Jewel is able to capture the qualitative features of the modifications to the fragmentation functions and radial momentum distributions in data but not always quantitatively. This quantitative discrepancy may be related to the simplified treatment of recoil partons in the background model and partly due to background subtraction procedure itself. Nevertheless, observed modifications to jet shape observables in Jewel corroborates the fact that in-medium fragmentation is harder and more collimated than the fragmentation in vacuum. We further observe that these modifications depend on the transverse momentum of jets and it seems that medium resolves the core structure of low momentum jets below 100 GeV c–1 at LHC energies.

Interplaying mechanisms behind single inclusive jet suppression in heavy-ion collisions

The suppression factor for single inclusive jets in Pb+Pb collisions at the Large Hadron Collider (LHC) has a weak dependence on the transverse momentum $p_T$ and remains almost the same at two colliding energies, $\sqrt{s}=2.76$ and 5.02 TeV, though the central rapidity density of bulk hadrons increases by about 20\%. This phenomenon is investigated within the Linear Boltzmann Transport (LBT) model, which includes elastic and inelastic processes based on perturbative QCD for both jet shower and recoil medium partons as they propagate through a quark-gluon plasma (QGP). With the dynamic evolution of the QGP given by the 3+1D CLVisc hydrodynamic model with event-by-event fully fluctuating initial conditions, single inclusive jet suppression in Pb+Pb collisions from LBT agrees well with experimental data. The weak $\sqrt{s}$ and $p_T$-dependence of the jet suppression factor at LHC are found to result directly from the $\sqrt{s}$-dependence of the initial jet $p_T$ spectra and slow $p_T$-dependence of the jet energy loss. Contributions from jet-induced medium response, influence of radial expansion, both of which depend on jet-cone size, and jet flavor composition all conjoin to give a slow $p_T$-dependence of jet energy loss and the single jet suppression factor $R_{\rm AA}$, their dependence on $\sqrt{s}$ and jet-cone size. Single inclusive jet suppression at $\sqrt{s}=200$ GeV is also predicted that actually decreases slightly with $p_T$ in the $p_T<50$ GeV/$c$ range because of the steeper initial jet spectra though the $p_T$-dependence of the jet energy loss is weaker than that at LHC.

Jet Shapes of Isolated Photon-Tagged Jets in Pb-Pb and pp Collisions at sqrt[s_{NN}]=5.02 TeV.

The modification of jet shapes in Pb-Pb collisions, relative to those in pp collisions, is studied for jets associated with an isolated photon. The data were collected with the CMS detector at the LHC at a nucleon-nucleon center-of-mass energy of 5.02TeV. Jet shapes are constructed from charged particles with track transverse momenta (p_{T}) above 1 GeV/c in annuli around the axes of jets with p_{T}^{jet}>30 GeV/c associated with an isolated photon with p_{T}^{γ}>60 GeV/c. The jet shape distributions are consistent between peripheral Pb-Pb and pp collisions, but are modified for more central Pb-Pb collisions. In these central Pb-Pb events, a larger fraction of the jet momentum is observed at larger distances from the jet axis compared to pp, reflecting the interaction between the partonic medium created in heavy ion collisions and the traversing partons.

Linearized Boltzmann-Langevin model for heavy quark transport in hot and dense QCD matter

In relativistic heavy-ion collisions, the production of heavy quarks at large transverse momenta is strongly suppressed compared to proton-proton collisions. In addition an unexpectedly large azimuthal anisotropy was observed for the emission of charmed hadrons in non-central collisions. Both observations pose challenges to the theoretical understanding of the coupling between heavy quarks and the quark-gluon plasma produced in these collisions. Transport models for the evolution of heavy quarks in a QCD medium offer the opportunity to study these effects - two of the most successful approaches are based on the linearized Boltzmann transport equation and the Langevin equation. In this work, we develop a hybrid transport model that combines the strengths of both of these approaches: heavy quarks scatter with medium partons using matrix-elements calculated in perturbative QCD, while between these discrete hard scatterings they evolve using a Langevin equation with empirical transport coefficients to capture the non-perturbative soft part of the interaction. With the hybrid transport model coupled to a state-of-the-art event-by-event bulk evolution model based on 2+1D relativistic viscous fluid dynamics, we study the azimuthal anisotropy and nuclear modification factor of heavy quarks in Pb+Pb collisions at $\sqrt{s} = 5.02$ TeV. The parameters of our model are calibrated using a Bayesian analysis comparing to available $D$-meson and $B$-meson data at the LHC. Using the calibrated model, we study the implications on heavy-flavor transport properties and predict novel observables.

Discrepancy in low transverse momentum dileptons from relativistic heavy-ion collisions

The dilepton transverse momentum spectra and invariant mass spectra for low $p_T <0.15$~GeV/c in Au+Au collisions of different centralities at $\sqrt{s_{NN}}$ = 200 GeV are studied within the parton-hadron-string dynamics (PHSD) transport approach. The PHSD describes the whole evolution of the system on a microscopic basis, incorporates hadronic and partonic degrees-of-freedom, the dynamical hadronization of partons and hadronic rescattering. For dilepton production in p+p, p+A and A+A reactions the PHSD incorporates the leading hadronic and partonic channels (also for heavy flavors) and includes in-medium effects such as a broadening of the vector meson spectral functions in hadronic matter and a modification of initial heavy-flavor correlations by interactions with the partonic and hadronic medium. The transport calculations reproduce well the momentum integrated invariant mass spectra from the STAR Collaboration for minimum bias Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV, while the description of the STAR data - when gating on low $p_T < 0.15$ GeV/c - is getting worse when going from central to peripheral collisions. An analysis of the transverse momentum spectra shows that the data for peripheral (60-80\%) collisions are well reproduced for $p_T>0.2$ GeV/c while the strong peak at low $p_T < 0.15$ GeV/c, that shows up in the experimental data for the mass bins ($0.4 < M < 0.7$ GeV and $1.2 < M < 2.6$ GeV), is fully missed by the PHSD and cannot be explained by the standard in-medium effects. This provides a new puzzle for microscopic descriptions of low $p_T$ dilepton data from the STAR Collaboration.

Multiple jets and γ-jet correlation in high-energy heavy-ion collisions

γ-Jet production is considered one of the best probes of the hot quark–gluon plasma in high-energy heavy-ion collisions since the direct γ can be used to gauge the initial energy and momentum of the associated jet. This is investigated within the Linear Boltzmann Transport (LBT) model for jet propagation and jet-induced medium excitation. With both parton energy loss and medium response from jet-medium interaction included, LBT can describe experimental data well on γ-jet correlation in Pb+Pb collisions at the Large Hadron Collider. Multiple jets associated with direct γ production are found to contribute significantly to γ-jet correlation at small pTjet<pTγ and large azimuthal angle relative to the opposite direction of γ. Jet medium interaction not only suppresses the leading jet at large pTjet but also sub-leading jets at large azimuthal angle. This effectively leads to the narrowing of γ-jet correlation in azimuthal angle instead of broadening due to jet-medium interaction. The γ-jet profile on the other hand will be broadened due to jet-medium interaction and jet-induced medium response. Energy flow measurements relative to the direct photon is illustrated to reflect well the broadening and jet-induced medium response.

Novel subjet observables for jet quenching in heavy-ion collisions

Using a novel observable that relies on the momentum difference of the two most energetic subjets within a jet varDelta S_{12} we study the internal structure of high-energy jets simulated by several Monte Carlo event generators that implement the partonic energy-loss in a dense partonic medium. Based on inclusive jet and dijet production we demonstrate that varDelta S_{12} is an effective tool to discriminate between different models of jet modifications over a broad kinematic range. The new quantity, while preserving the collinear and infrared safety of modern jet algorithms, it is experimentally attractive because of its inherent resilience against backgrounds of heavy-ion collisions.

Open Charm Measurements at CERN SPS Energies with the New Vertex Detector of the NA61/SHINE Experiment

The heavy-ion programme of the NA61/SHINE experiment at the CERN SPS has been expanded to allow precise measurements of exotic particles with short lifetime. The study of open charm meson production is a sensitive tool for new detailed investigations of the properties of hot and dense matter formed in nucleus-nucleus collisions. In particular, it offers new possibilities for studies of such phenomena as in- medium parton energy loss and quarkonium dissociation and possible regeneration, thus providing new information to probe deconfinement. A new high resolution Vertex Detector was designed for the NA61/SHINE experiment for measurements of the very rare processes of open charm production in nucleus- nucleus collisions at the SPS. It will meet the challenges of track registration and of very precise spatial resolution in primary and secondary vertex reconstruction required for the identification of

Measurement of the Splitting Function in pp and Pb-Pb Collisions at sqrt[s_{NN}]=5.02 TeV.

Data from heavy ion collisions suggest that the evolution of a parton shower is modified by interactions with the color charges in the dense partonic medium created in these collisions, but it is not known where in the shower evolution the modifications occur. The momentum ratio of the two leading partons, resolved as subjets, provides information about the parton shower evolution. This substructure observable, known as the splitting function, reflects the process of a parton splitting into two other partons and has been measured for jets with transverse momentum between 140 and 500GeV, in pp and PbPb collisions at a center-of-mass energy of 5.02TeV per nucleon pair. In central PbPb collisions, the splitting function indicates a more unbalanced momentum ratio, compared to peripheral PbPb and pp collisions.. The measurements are compared to various predictions from event generators and analytical calculations.

Signatures of chiral symmetry restoration and its survival throughout the hadronic phase interactions

The effect of the chiral symmetry restoration (CSR) on observables from heavy-ion collisions is studied in the energy range [see formula in PDF]=3–20 GeV within the Parton-Hadron-String Dynamics (PHSD) transport approach. The PHSD includes the deconfinement phase transition as well as essential aspects of CSR in the dense and hot hadronic medium, which are incorporated in the Schwinger mechanism for the hadronic particle production. We adopt different parametrizations of the nuclear equation of state from the non-linear σ - ω model, which enter in the computation of the quark scalar density for the CSR mechanism, in order to estimate the uncertainty in our calculations. For the pion-nucleon ∑-term we adopt ∑π ≈ 45 MeV which corresponds to a ’world average’. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at [see formula in PDF]=3–20 GeV, realizing an increase of the hadronic particle production in the strangeness sector with respect to the non-strange one. We identify particle abundances and rapidity spectra to be suitable probes in order to extract information about CSR, while transverse mass spectra are less sensitive ones. Our results provide a microscopic explanation for the "horn" structure in the excitation function of the K+/π+ ratio: the CSR in the hadronic phase produces the steep increase of this particle ratio up to [see formula in PDF] ≈ 7 GeV, while the drop at higher energies is associated to the appearance of a deconfined partonic medium.

Application of the momentum kick model to pbpb collisions at $$\sqrt {{s_{NN}}} $$ s N N = 2.76 TeV at the LHC

The “ridge structure” in the Δφ-Δη correlation has been observed in high-energy heavy-ion collisions in AuAu collisions at the Relativistic Heavy Ion Collider (RHIC), and in pp, pPb, and PbPb collisions at the Large Hadron Collider (LHC). It is known that hydrodynamic models are the most successful in explaining the phenomenon. However, there has been some doubt as to whether hydrodynamic flows could be produced in small systems, such as pp collisions, enough to generate the ridge structure. This question leads us to introduce a dynamical process between particles involved in a collision event. In this process, a near-side jet, arising close to the surface, collides with medium partons and the collided particles obtain a momentum transfer along the jet direction to be the ridge particles. Now that there exist several analysis results, based on this approach, in regard to AuAu collisions at the STAR and PHENIX and pp collisions at the LHC, we extend the application to high-energy PbPb collisions at the LHC. We conclude that the kinematic description can explain the ridge behavior formed in high-energy collisions at the LHC, independent of a scale of a collision system.

Evidence for chiral symmetry restoration in heavy-ion collisions

We study the effect of the chiral symmetry restoration (CSR) on heavy-ion collisions observables in the energy range sNN=3–20GeV within the Parton-Hadron-String Dynamics (PHSD) transport approach. The PHSD includes the deconfinement phase transition as well as essential aspects of CSR in the dense and hot hadronic medium, which are incorporated in the Schwinger mechanism for particle production. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at sNN=3–20GeV, realizing an increase of the hadronic particle production in the strangeness sector with respect to the non-strange one. Our results provide a microscopic explanation for the horn structure in the excitation function of the K+/π+ ratio: the CSR in the hadronic phase produces the steep increase of this particle ratio up to sNN≈7GeV, while the drop at higher energies is associated to the appearance of a deconfined partonic medium. Furthermore, the appearance/disappearance of the horn structure is investigated as a function of the system size. We additionally present an analysis of strangeness production in the (T,μB)-plane (as extracted from the PHSD for central Au+Au collisions) and discuss the perspectives to identify a possible critical point in the phase diagram.

Effect of medium recoil and pT broadening on single inclusive jet suppression in high-energy heavy-ion collisions in the high-twist approach

Jet energy loss and single inclusive jet suppression in high-energy heavy-ion collisions are studied within a pQCD parton model that includes both elastic and radiative interactions between jet shower and medium partons as they propagate through the quark-gluon plasma. The collisional energy loss of jets with a given cone-size is found to be relatively small comparing with the radiative energy loss. However the effect of transverse momentum broadening due to elastic scattering is significant in the calculation of radiative energy loss within the higher-twist formalism. The nuclear modification factors for single inclusive jets with different cone-sizes are calculated and compared to experimental data as measured by ALICE and ATLAS experiments in Pb+Pb collisions at $\sqrt{s_{\rm NN}}=2.76$ TeV. Results on jet suppression in Pb+Pb collisions at $\sqrt{s_{\rm NN}} = 5.02$ TeV are also presented.

Neutral meson production in pp, p–Pb and Pb–Pb collisions with ALICE at the LHC

The ALICE experiment is mainly devoted to the study of QGP expected to be created in heavy-ion collisions at the LHC. In the early stage of such collisions, partons can be scattered and photons can be produced in energetic hard processes. High energy partons will interact with the QGP, and from the measurement of the particles originated from the parton fragmentation in vacuum, one can infer transport properties of the QGP. Therefore, study of high-pT hadrons production is important to constrain the properties of the hot and dense medium. The strong suppression of the high-pT hadrons in heavy-ion collisions has been explained by the energy loss of the partons in medium and it can be investigated experimentally via the nuclear modification factor of hadrons.ALICE measured neutral mesons (π0 and η) at mid-rapidity in a wide pT range in pp, p-Pb and Pb-Pb collisions. Neutral mesons are reconstructed via complementary methods, using the electromagnetic calorimeters, PHOS and EMCal, and by the central tracking system, identifying photons converted to e+e− pairs in the material of the inner barrel detectors (ITS and TPC). Measurement of neutral meson spectra in pp collisions at s=0.9,2.76,7 and 8 TeV provides valuable comparison data for pQCD calculations and allows to study scaling properties of hadron production at the LHC energies. The study of neutral mesons production in p-Pb collisions helps to disentangle initial and final state effects on the suppression itself. The latest results of neutral meson measurement in pp, p-Pb and Pb-Pb collisions will be shown in this paper.

Effect of collisional energy loss on particle correlations in AMPT

Jet quenching is a powerful tool to study medium properties of relativistic heavy ion collisions via jet-medium interactions. Jet quenching studies have so far focused on high transverse momentum (pT) particle suppression. Jet shapes at low to intermediate pT, containing rich information on jet-medium interactions, have been less explored. In this talk, I will present a recent study, using a multiphase transport (AMPT) model, of effects on particle correlations from collisional energy loss of partons traversing the heavy ion medium. We follow the parton cascading history so that medium partons (associated particles) which have interacted with a high-pT probe parton (hard probe trigger particle) can be uniquely identified and hence no subtraction of combinatorial background is needed. Results on particle correlation shapes will be presented as a function of pT, the number of parton-parton collisions suffered by the probe parton, and the azimuthal angle of the probe parton relative to the reaction plane. These results reveal pathlength dependence of collisional energy loss.

Chiral symmetry restoration versus deconfinement in heavy-ion collisions at high baryon density

The effect of the chiral symmetry restoration (CSR) on observables from heavy-ion collisions is studied in the energy range within the Parton-Hadron-String Dynamics (PHSD) transport approach. The PHSD includes the deconfinement phase transition as well as essential aspects of CSR in the dense and hot hadronic medium, which are incorporated in the Schwinger mechanism for the hadronic particle production. We adopt different parametrizations of the nuclear equation of state from the non-linear σ − ω model, which enter in the computation of the quark scalar density for the CSR mechanism, in order to estimate the uncertainty in our calculations. For the pion-nucleon Σ-term we adopt Σπ ≈ 45 MeV which corresponds to some ‘world average’. Our systematic studies show that chiral symmetry restoration plays a crucial role in the description of heavy-ion collisions at , realizing an increase of the hadronic particle production in the strangeness sector with respect to the non-strange one. We identify particle abundances and rapidity spectra to be suitable probes in order to extract information about CSR, while transverse mass spectra are less sensitive. Our results provide a microscopic explanation for the “horn” structure in the excitation function of the K+/π+ ratio: the CSR in the hadronic phase produces the steep increase of this particle ratio up to , while the drop at higher energies is associated to the appearance of a deconfined partonic medium.

Medium response in JEWEL and its impact on jet shape observables in heavy ion collisions

Realistic modeling of medium-jet interactions in heavy ion collisions is becoming increasingly important to successfully predict jet structure and shape observables. In Jewel, all partons belonging to the parton showers initiated by hard scattered partons undergo collisions with thermal partons from the medium, leading to both elastic and radiative energy loss. The recoiling medium partons carry away energy and momentum from the jet. Since the thermal component of these recoils’ momenta is part of the soft background activity, comparison with data requires the implementation of a subtraction procedure. We present two independent procedures through which background subtraction can be performed and discuss the impact of the medium recoil on jet shape observables. Keeping track of the medium response significantly improves the Jewel description of jet shape measurements.

Energy loss of heavy quarks and B and D meson spectra in PbPb collisions at LHC energies

We study the production and evolution of charm and bottom quarks in hot partonic medium produced in heavy ion collisions. The heavy quarks loose energy in the medium which is reflected in the transverse momentum spectra of heavy mesons. The collisional energy loss of heavy quarks has been calculated using QCD calculations. The radiative energy loss is obtained using two models namely reaction operator formalism and generalized dead cone approach. The nuclear modification factors, RAA as a function of transverse momentum by including shadowing and energy loss are calculated for D0 and B+ mesons in PbPb collisions at sNN=5.02 TeV and for D0 mesons at sNN=2.76 TeV and are compared with the recent measurements. The radiative energy loss from generalized dead cone approach alone is sufficient to produce measured D0 meson RAA at both the LHC energies. The radiative energy loss from reaction operator formalism plus collisional energy loss gives good description of D0 meson RAA. For the case of B+ meson, the radiative energy loss from generalized dead cone approach plus collisional energy loss gives good description of the CMS data. The radiative process is dominant for charm quarks while for the bottom, both the radiative process and the elastic collisions are important.

Two-particle correlation via Bremsstrahlung

Ridge is the well-known structure in two-particle angular correlations at highenergy heavy-ion collisions. This structure is physically understood through elliptic and higher-order flows at nucleus-nucleus collisions. This behavior is also found in small systems, such as proton-proton collisions, recently. However, Ridge structure in small system is hard to be understood using hydrodynamics, since small systems are not dense enough to produce the Quark-Gluon plasma. Thus, we try to describe this phenomena through kinematic interaction between jets and medium partons. In high-energy heavy-ion collision, the energetic particles called jets go out in specific direction and lose their energy while passing through the medium. During such process, photons/gluons are emitted from interaction between jets and medium partons. We concentrate on energy loss via photon radiations, known as Bremsstrahlung. Recently, two symmetric double scattering processes between jet particle and medium parton are reported to be able to produce certain constructive interference, which gives collective motion and medium partons are aligned along incoming jet particles. We conjecture that similar behavior might happen in Bremsstrahlung processes, and therefore we consider the two symmetric diagrams of photon emission and medium parton scattering. We expect these two amplitudes to give constructive interference leading to the collective motion of medium. We check the correlation between emitted photon and final jet, and those between medium parton and final jet for high-energy jet. To describe parton momentum distribution in medium, we use the Maxwell-Boltzmann distribution. We discover collective motion in both angular correlations. We also check the tendency of the angular correlation for two particles according to the incident angle of jet particle, energy of emitted photon and temperature of systems, respectively. We can conclude that collective motion is able to be understood through kinematic description.