arXiv subjects

Jamal Jalilian-Marian

Publications and source records attributed to Jamal Jalilian-Marian.

At least 37 records · Page 2Linked to original sources

arXiv2016

Relating $\hat{q}$, $η/s$ and $ΔE$ in an expanding Quark-Gluon Plasma

We use linear viscous hydrodynamics to describe the energy and momentum deposited by a fast moving parton in a quark gluon plasma. This energy-momentum is in turn used to compute the probability density for the production of soft partons by means of the Cooper-Frye formula. We use this probability density to render manifest a relation between the average transverse momentum given to the fast moving parton from the medium $\hat{q}$, the entropy density to shear viscosity ratio $η/s$ and the energy lost by the fast moving parton $ΔE$ in an expanding medium under similar conditions to those generated in nucleus-nucleus collisions at the LHC. We find that $\hat{q}$ increases linearly with $ΔE$ for both trigger and away side partons that have been produced throughout the medium. On the other hand, $η/s$ is more stable with $ΔE$. We also study how these transport coefficients vary with the geometrical location of the hard scattering that produces the fast moving partons. The behavior of $\hat{q}$, with $ΔE$ is understood as arising from the length of medium the parton traverses from the point where it is produced. However, since $η/s$ is proportional to the ratio of the length of medium traversed by the fast parton and the average number of scatterings it experiences, it has a milder dependence on the energy it loses. This study represents a tool to obtain a direct connection between transport coefficients and the description of in-medium energy loss within a linear viscous hydrodynamical evolution of the bulk.

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arXiv2016

The RHIC Cold QCD Plan for 2017 to 2023: A Portal to the EIC

The exploration of the fundamental structure of strongly interacting matter has always thrived on the complementarity of lepton scattering and purely hadronic probes. As the community eagerly anticipates a future electron ion collider (EIC) in the U.S., an outstanding scientific opportunity remains to complete "must-do" measurements in p+p and p+A physics in the years preceding the EIC. As we describe in this document, these measurements will be essential to fully realize the scientific promise of the EIC by provid-ing a comprehensive set of measurements in hadronic collisions that, when combined with data from the EIC, will establish the validity and limits of factorization and universality. The outlined program will on the one hand lay the groundwork for the EIC, both scientifically and in terms of refining the experimental requirements for the physics program at the EIC, and thus be the natural next step on the path towards an electron-ion collider. On the other hand, while much of the physics in this program is unique to proton-proton and proton-nucleus collisions and offers discovery potential on its own, when combined with data from the EIC it will provide a broad foundation to a deeper understanding of fundamental QCD.

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arXiv2016

Jet asymmetry and momentum imbalance from $2 \to 2$ and $2 \to 3$ partonic processes in relativistic heavy-ion collisions

We study momentum imbalance as a function of jet asymmetry in high-energy heavy-ion collisions. To implement parton production during the collision, we include all Leading Order (LO) $2\to 2$ and $2\to 3$ parton processes in pQCD. The produced partons lose energy within the quark gluon plasma and hadronize collinearly when they leave it. The energy and momentum deposited into the plasma is described using linear viscous hydrodynamics with a constant energy loss per unit length and a total energy loss given by a Gaussian probability centered around a mean value $\bar{\mathcal{E}}$ and a half-width $Δ{\mathcal{E}}$. We argue that the shape of the asymmetry observed by the CERN-CMS Collaboration can indeed be attributed to parton energy loss in the medium and that a good description of data is achieved when one includes a slight enhancement coming from the contribution of $2\to 3$ parton processes that modifies the asymmetry distribution of the dijet events. We compare our results to CMS data for the most central collisions and study different values for $\bar{\mathcal{E}}$ and $Δ{\mathcal{E}}$.

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arXiv2015

Small-$x$ QCD evolution of $2 n$ Wilson line correlator: the weak field limit

We write down explicit expressions for the $x$-evolution (equivalent to energy or rapidity evolution) of $2 n$ ($n = 1, 2, ...$) Wilson lines using the JIMWLK equation and the Color Glass Condensate formalism. We investigate the equation in the weak gluon field limit (linear regime) by expanding the Wilson lines in powers of the gluon field and show that it reduces to the BJKP equation describing the evolution of a state of $2 n$ Reggeized gluons with energy. We also make available for download a {\it Mathematica} program which provides this expression for any value of $n$.

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arXiv2012

Monte Carlo approach for hadron azimuthal correlations in high energy proton and nuclear collisions

We use a Monte Carlo approach to study hadron azimuthal angular correlations in high energy proton-proton and central nucleus-nucleus collisions at the BNL Relativistic Heavy Ion Collider (RHIC) energies at mid-rapidity. We build a hadron event generator that incorporates the production of $2\to 2$ and $2\to 3$ parton processes and their evolution into hadron states. For nucleus-nucleus collisions we include the effect of parton energy loss in the Quark-Gluon Plasma using a modified fragmentation function approach. In the presence of the medium, for the case when three partons are produced in the hard scattering, we analyze the Monte Carlo sample in parton and hadron momentum bins to reconstruct the angular correlations. We characterize this sample by the number of partons that are able to hadronize by fragmentation within the selected bins. In the nuclear environment the model allows hadronization by fragmentation only for partons with momentum above a threshold $p_T^{\tiny{thresh}}=2.4$ GeV. We argue that one should treat properly the effect of those partons with momentum below the threshold, since their interaction with the medium may lead to showers of low momentum hadrons along the direction of motion of the original partons as the medium becomes diluted.

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arXiv2012

Prompt photon production and photon-hadron correlations at RHIC and the LHC from the Color Glass Condensate

We investigate inclusive prompt photon and semi-inclusive prompt photon-hadron production in high energy proton-nucleus collisions using the Color Glass Condensate (CGC) formalism which incorporates non-linear dynamics of gluon saturation at small x via Balitsky-Kovchegov equation with running coupling. For inclusive prompt photon production, we rewrite the cross-section in terms of direct and fragmentation contributions and show that the direct photon (and isolated prompt photon) production is more sensitive to gluon saturation effects. We then analyze azimuthal correlations in photon-hadron production in high energy proton-nucleus collisions and obtain a strong suppression of the away-side peak in photon-hadron correlations at forward rapidities, similar to the observed mono-jet production in deuteron-gold collisions at forward rapidity at RHIC. We make predictions for the nuclear modification factor R_{p(d)A} and photon-hadron azimuthal correlations in proton(deuteron)-nucleus collisions at RHIC and the LHC at various rapidities.

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arXiv2011

Hadron production in pA collisions at the LHC from the Color Glass Condensate

We investigate the contribution of inelastic and elastic processes to single inclusive hadron production in proton-proton and proton (deuteron)-nucleus collisions at RHIC and the LHC. Using the hybrid formulation which includes both elastic and inelastic contributions, supplemented with the running-coupling Balitsky-Kovchegov equation, we get a good description of RHIC data. It is shown that inclusion of the inelastic terms makes the transverse momentum dependence of the production cross section steeper in the mid-rapidity region but does not affect the cross section in the very forward region. The inelastic processes also lead to a sharper increase of the nuclear modification factor R_{pA} with increasing p_T. We also make predictions for the nuclear modification factor in proton-nucleus collisions at the LHC (\sqrt{s}=4.4 and 8.8 TeV) at various rapidities using the Color Glass Condensate framework.

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arXiv2011

On perturbative limits of quadrupole evolution in QCD at high energy

We consider the perturbative (weak field) limit of the small $x$ QCD evolution equation for quadrupole, the normalized trace of four Wilson lines in the fundamental representation, which appears in di-hadron angular correlation in high energy collisions. We linearize the quadrupole evolution equation and then expand the Wilson lines in powers of $g\, A_μ$ where $A_μ$ is the gauge field. The quadratic terms in the expansion ($\sim g^2\, A^2$) satisfy the BFKL equation as has been recently shown. We then consider the quartic terms ($\sim g^4\, A^4$) in the expansion and show that the linearized quadrupole evolution equation, written in terms of color charge density $ρ$, reduces to the well-known BJKP equation for the imaginary part of four-reggeized gluon exchange amplitude. We comment on the possibility that the BJKP equation for the evolution of a $n$-reggeized gluon state can be obtained from the JIMWLK evolution equation for the normalized trace of $n$ fundamental Wilson lines when non-linear (recombination) terms are neglected.

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arXiv2011

Renormalization group evolution of multi-gluon correlators in high energy QCD

Many-body QCD in leading high energy Regge asymptotics is described by the Balitsky-JIMWLK hierarchy of renormalization group equations for the x evolution of multi-point Wilson line correlators. These correlators are universal and ubiquitous in final states in deeply inelastic scattering and hadronic collisions. For instance, recently measured di-hadron correlations at forward rapidity in deuteron-gold collisions at the Relativistic Heavy Ion Collider (RHIC) are sensitive to four and six point correlators of Wilson lines in the small x color fields of the dense nuclear target. We evaluate these correlators numerically by solving the functional Langevin equation that describes the Balitsky-JIMWLK hierarchy. We compare the results to mean-field Gaussian and large N_c approximations used in previous phenomenological studies. We comment on the implications of our results for quantitative studies of multi-gluon final states in high energy QCD.

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arXiv2011

Three-hadron angular correlations from pQCD at RHIC and LHC

We study three-hadron azimuthal angular correlations in high energy proton-proton and central nucleus-nucleus collisions at RHIC and LHC at mid-rapidity. We use the LO parton matrix elements for 2 \rightarrow 3 processes and include the effect of parton energy loss in the Quark-Gluon Plasma using the modified fragmentation function approach. For the case when the produced hadrons have either same or not too different momenta, we observe two away side peaks at 2pi/3 and 4pi/3. We consider the dependence of the angular correlations on energy loss parameters that have been used in studies of single inclusive hadron production at RHIC. Our results on the angular dependence of the cross section agree well with preliminary data by the PHENIX collaboration. We comment on the possible contribution of 2 \rightarrow 3 processes to di-hadron angular correlations and how a comparison of the two processes may help characterize the plasma further.

hep-ph↗

arXiv2011

Two-gluon correlations and initial conditions for small-x evolution

We derive the effective action of the hard large-x valence charges up to fourth order in their density. Such non-Gaussian weight functionals contribute at leading order in N_c to the connected two-gluon production diagrams which determine di-hadron correlations. The corresponding diagrams are not necessarily (highly) suppressed by the density of valence charges since their infrared divergences differ from those obtained in a Gaussian theory. Therefore, it appears prudent to include such higher dimensional operators when determining initial ensembles for JIMWLK evolution of higher n-point functions of Wilson lines.

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arXiv2011

The broad away side of azimuthal correlations: 3 vs 2 final state particles in high energy nuclear collisions

In high energy heavy ion collisions at RHIC there are important aspects of the medium induced dynamics, that are still not well understood. In particular, there is a broadening and even a double hump structure of the away-side peak appearing in azimuthal correlation studies in Au+Au collisions which is absent in p+p collisions at the same energies. These features are already present but suppressed in p+p collisions: 2 to 3 parton processes produce such structures but are suppressed with respect to 2 to 2 processes. We argue that in A+A collisions the different geometry for the trajectories of 3 as opposed to 2 particles in the final state, together with the medium induced energy loss effects on the different cross sections, create a scenario that enhances processes with 3 particles in the final state, which gives on average this double hump structure.

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arXiv2010

Two-hadron correlations in the Color Glass Condensate formalism

Two-hadron correlations are a sensitive probe of the dynamics of gluon saturation and the Color Glass Condensate formalism where the degrees of freedom are Wilson lines of the gluon field. It is shown that unlike structure functions in DIS and single hadron production in proton-nucleus collisions, higher point functions of Wilson lines appear in two-hadron production cross section. We derive equations for the energy ($x$) evolution of these higher point functions using the JIMWLK Hamiltonian and show that dipole approximation, employed in the literature to fit the di-hadron data measured by the STAR collaboration in the forward rapidity region, breaks down. This necessitates an investigation of the full hierarchy of the JIMWLK evolution equations for these higher point functions and their solutions. This can then be used to make a quantitative analysis of the di-hadron correlations in the forward rapidity region in deuteron-gold collisions at RHIC and in the long range rapidity correlations observed in proton-proton collisions at the LHC.

hep-ph↗

arXiv2010

The ridge in proton-proton collisions at the LHC

We show that the key features of the CMS result on the ridge correlation seen for high multiplicity events in sqrt(s)=7TeV proton-proton collisions at the LHC can be understood in the Color Glass Condensate framework of high energy QCD. The same formalism underlies the explanation of the ridge events seen in A+A collisions at RHIC, albeit it is likely that flow effects may enhance the magnitude of the signal in the latter.

hep-ph↗

arXiv2010

Forward dijets in high-energy collisions: evolution of QCD n-point functions beyond the dipole approximation

Present knowledge of QCD n-point functions of Wilson lines at high energies is rather limited. In practical applications, it is therefore customary to factorize higher n-point functions into products of two-point functions (dipoles) which satisfy the BK evolution equation. We employ the JIMWLK formalism to derive explicit evolution equations for the 4- and 6-point functions of fundamental Wilson lines and show that if the Gaussian approximation is carried out before the rapidity evolution step is taken, then many leading order N_c contributions are missed. Our evolution equations could specifically be used to improve calculations of forward dijet angular correlations, recently measured by the STAR collaboration in deuteron-gold collisions at the RHIC collider. Forward dijets in proton-proton collisions at the LHC probe QCD evolution at even smaller light-cone momentum fractions. Such correlations may provide insight into genuine differences between the JIMWLK and BK approaches.

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arXiv2009

Three and two-hadron correlations in \sqrt{s_{NN}}=200 GeV proton-proton and nucleus-nucleus collisions

We compare the azimuthal correlations arising from three and two hadron production in high energy proton-proton and nucleus-nucleus collisions at \sqrt{s_{NN}}=200 GeV, using the leading order matrix elements for two-to-three and two-to-two parton-processes in perturbative QCD. We first compute the two and three hadron production cross sections in mid-rapidity proton-proton collisions. Then we consider Au + Au collisions including parton energy loss using the modified fragmentation function approach. By examining the geometrical paths the hard partons follow through the medium, we show that the two away-side partons produced in two-to-three processes have in average a smaller and a greater path length than the average path length of the away-side parton in two-to-two processes. Therefore there is a large probability that in the former processes one of the particles escapes while the other gets absorbed. This effect leads to an enhancement in the azimuthal correlations of the two-to-three with respect to the two-to-two parton-processes when comparing to the same processes in proton-proton collisions since in average the particle with the shortest path length looses less energy with respect to the away side particle in two-to-two processes. We argue that this phenomenon may be responsible for the shape of the away-side in azimuthal correlations observed in mid-rapidity Au + Au collisions at RHIC.

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arXiv2009

From RHIC to EIC: Nuclear Structure Functions

We study the nuclear structure function $F_2^A$ and its logarithmic derivative in the high energy limit (small $x$ region) using the Color Glass Condensate formalism. In this limit the structure function $F_2$ depends on the quark anti-quark dipole-target scattering cross section $N_F (x_{bj}, r_t, b_t)$. The same dipole cross section appears in single hadron and hadron-photon production cross sections in the forward rapidity region in deuteron (proton)-nucleus collisions at high energy, i.e. at RHIC and LHC. We use a parameterization of the dipole cross section, which has successfully been used to describe the deuteron-gold data at RHIC, to compute the nuclear structure function $F_2^A$ and its log $Q^2$ derivative (which is related to gluon distribution function in the double log limit). We provide a quantitative estimate of the nuclear shadowing of $F_2^A$ and the gluon distribution function in the kinematic region relevant to a future Electron-Ion Collider.

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arXiv2008

Saturation physics and angular correlations at RHIC and LHC

We investigate the angular correlation between pions and photons produced in deuteron-gold collisions at RHIC and proton-lead collisions at LHC using the Color Glass Condensate formalism and make predictions for the dependence of the production cross section on the angle between the pion and photon at different rapidities and transverse momenta. Measuring this dependence would shed further light on the role of high gluon density and saturation dynamics at RHIC and LHC.

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