arXiv subjects

Ivan Vitev

Publications and source records attributed to Ivan Vitev.

At least 109 records · Page 6Linked to original sources

arXiv2012

${\cal O}(α_s^3)$ analysis of inclusive jet and di-jet production in heavy ion reactions at the Large Hadron Collider

Jets physics in heavy ion reactions is an important new area of active research at the Relativistic Heavy Ion Collider (RHIC) and at the Large Hadron Collider (LHC) that paves the way for novel tests of QCD multi-parton dynamics in dense nuclear matter. At present, perturbative QCD calculations of hard probes in elementary nucleon-nucleon reactions can be consistently combined with the effects of the nuclear medium up to $ {\cal O}(α_s^3) $. While such accuracy is desirable but not necessary for leading particle tomography, it is absolutely essential for the new jet observables. With this motivation, we present first results and predictions to $ {\cal O}(α_s^3) $ for the recent LHC lead-lead (Pb+Pb) run at a center-of-mass energy of 2.76 TeV per nucleon-nucleon pair. Specifically, we focus on the suppression of the single and double inclusive jet cross sections. Our analysis includes not only final-state inelastic parton interactions in the QGP, but also initial-state cold nuclear matter effects and an estimate of the non-perturbative hadronization corrections. We demonstrate how an enhanced di-jet asymmetry in central Pb+Pb reactions at the LHC, recently measured by the ATLAS and CMS experiments, can be derived from these results. We show quantitatively that a fraction of this enhancement may be related to the ambiguity in the separation between the jet and the soft background medium and/or the diffusion of the parton shower energy away from the jet axis through collisional processes. We point to a suite of measurements that can help build a consistent picture of parton shower modification in heavy ion collisions at the LHC.

hep-ph↗

arXiv2011

Dihadron momentum imbalance and correlations in d+Au collisions

We calculate in perturbative QCD the transverse momentum imbalance of dijet and dihadron production in high energy p+A (d+A) collisions. We evaluate the effect of both initial- and final-state multiple scattering, which determines the strength of this transverse momentum imbalance. Combining this new result with the suppression of the cross section in d+Au collisions, which arises from cold nuclear matter energy loss and coherent power corrections, we are able to describe the dihadron correlations measured by both PHENIX and STAR collaborations at RHIC, including mid-mid, mid-forward, and forward-forward rapidity hadron pairs.

hep-ph↗

arXiv2011

An effective theory for jet propagation in dense QCD matter: jet broadening and medium-induced bremsstrahlung

Two effects, jet broadening and gluon bremsstrahlung induced by the propagation of a highly energetic quark in dense QCD matter, are reconsidered from effective theory point of view. We modify the standard Soft Collinear Effective Theory (SCET) Lagrangian to include Glauber modes, which are needed to implement the interactions between the medium and the collinear fields. We derive the Feynman rules for this Lagrangian and show that it is invariant under soft and collinear gauge transformations. We find that the newly constructed theory SCET$_{\rm G}$ recovers exactly the general result for the transverse momentum broadening of jets. In the limit where the radiated gluons are significantly less energetic than the parent quark, we obtain a jet energy-loss kernel identical to the one discussed in the reaction operator approach to parton propagation in matter. In the framework of SCET$_{\rm G}$ we present results for the fully-differential bremsstrahlung spectrum for both the incoherent and the Landau-Pomeranchunk-Migdal suppressed regimes beyond the soft-gluon approximation. Gauge invariance of the physics results is demonstrated explicitly by performing the calculations in both the light-cone and covariant $R_ξ$ gauges. We also show how the process-dependent medium-induced radiative corrections factorize from the jet production cross section on the example of the quark jets considered here.

hep-ph↗

arXiv2011

Medium-induced parton splitting kernels from Soft Collinear Effective Theory with Glauber gluons

We derive the splitting kernels for partons produced in large $Q^2$ scattering processes that subsequently traverse a region of strongly-interacting matter using a recently-developed effective theory \SCETG. We include all corrections beyond the small-$x$ approximation, consistent with the power counting of \SCETG. We demonstrate how medium recoil, geometry and expansion scenarios, and phase space cuts can be implemented numerically for phenomenological applications. For the simplified case of infinite transverse momentum kinematics and a uniform medium, we provide closed-form analytic results that can be used to validate the numerical simulations.

hep-ph↗

arXiv2011

NLO analysis of inclusive jet, tagged jet and di-jet production in Pb+Pb collisions at the LHC

We present results and predictions at next-to-leading order for the recent LHC lead-lead run at a center-of-mass energy of 2.76 TeV per nucleon-nucleon pair. Specifically, we focus on the suppression the single and double inclusive jet cross sections and demonstrate how the di-jet asymmetry, recently measured by ATLAS and CMS, can be extracted from this general result. The case of jets tagged by an electroweak boson is exemplified by the $Z^0$+jet channel. We predict a signature transition from enhancement to suppression of the tagged jet related to the medium-induced modification of the parton shower. Finally, we clarify the relation between the suppression of inclusive jets, tagged jets and di-jets and the quenching of inclusive particles on the example of the recent ALICE charged hadron attenuation data.

hep-ph↗

arXiv2011

Photon-tagged heavy meson production in high energy nuclear collisions

We study the photon-triggered light and heavy meson production in both p+p and A+A collisions. We find that a parton energy loss approach that successfully describes inclusive hadron attenuation in nucleus-nucleus reactions at RHIC can simultaneously describe well the experimentally determined photon-triggered light hadron fragmentation functions. Using the same framework, we generalize our formalism to study photon-triggered heavy meson production. We find that the nuclear modification of photon-tagged heavy meson fragmentation functions in A+A collision is very different from that of the photon-tagged light hadron case. While photon-triggered light hadron fragmentation functions in A+A collisions are suppressed relative to p+p, photon-triggered heavy meson fragmentation functions can be either enhanced or suppressed, depending on the specific kinematic region. The anticipated smaller energy loss for $b$-quarks manifests itself as a flatter photon-triggered $B$-meson fragmentation function compared to that for the $D$-meson case. We make detailed predictions for both RHIC and LHC energies. We conclude that a comprehensive comparative study of both photon-tagged light and heavy meson production can provide new insights in the details of the jet quenching mechanism.

hep-ph↗

arXiv2011

Parton showers as sources of energy-momentum deposition in the QGP and their implication for shockwave formation at RHIC and at the LHC

We derive the distribution of energy and momentum transmitted from a primary fast parton and its medium-induced bremsstrahlung gluons to a thermalized quark-gluon plasma. Our calculation takes into account the important and thus far neglected effects of quantum interference between the resulting color currents. We use our result to obtain the rate at which energy is absorbed by the medium as a function of time and find that the rate is modified by the quantum interference between the primary parton and secondary gluons. This Landau-Pomeranchuk-Migdal type interference persists for time scales relevant to heavy ion phenomenology. We further couple the newly derived source of energy and momentum deposition to linearized hydrodynamics to obtain the bulk medium response to realistic parton propagation and splitting in the quark-gluon plasma. We find that because of the characteristic large angle in-medium gluon emission and the multiple sources of energy deposition in a parton shower, formation of well defined Mach cones by energetic jets in heavy ion reactions is not likely.

hep-ph↗

arXiv2010

Jet tomography of high-energy nucleus-nucleus collisions at next-to-leading order

We demonstrate that jet observables are highly sensitive to the characteristics of the vacuum and the in-medium QCD parton showers and propose techniques that exploit this sensitivity to constrain the mechanism of quark and gluon energy loss in strongly-interacting plasmas. As a first example, we calculate the inclusive jet cross section in high-energy nucleus-nucleus collisions to ${\cal O}(α_s^3)$. Theoretical predictions for the medium-induced jet broadening and the suppression of the jet production rate due to cold and hot nuclear matter effects in Au+Au and Cu+Cu reactions at RHIC are presented.

hep-ph↗

arXiv2009

A light-cone wavefunction approach to open heavy flavor dynamics in QCD matter

We calculate the lowest order charm and beauty parton distribution functions in and fragmentation functions into D and B mesons using the operator definitions of factorized perturbative QCD. In the vacuum, we find the leading corrections that arise from the structure of the final-state hadrons. Quark-antiquark potentials extracted from the lattice are employed to demonstrate the existence of open heavy flavor bound-state solutions in the QGP in the vicinity of the critical temperature. We provide first results for the in-medium modification of the heavy quark distribution and decay probabilities in a co-moving plasma. In an improved perturbative QCD description of heavy flavor dynamics in the thermal medium, we combine D and B meson formation and dissociation with parton-level charm and beauty quark quenching to obtain predictions for the heavy meson and non-photonic electron suppression in Cu+Cu and Pb+Pb collisions at RHIC and the LHC, respectively.

hep-ph↗

arXiv2009

Direct photon production in d+A and A+A collisions at RHIC

Direct photon production in minimum bias d+Cu and d+Au and central Cu+Cu and Au+Au collisions at center of mass energies $\sqrt{s}=62.4$ GeV and 200GeV at RHIC is systematically investigated. We study the jet quenching effect, the medium-induced photon bremsstrahlung and jet-photon conversion in the hot QGP. We account for known cold nuclear matter effects, such as the isospin effect, the Cronin effect, shadowing and cold nuclear matter energy loss. It is shown that at high $p_T$ the nuclear modification factor for direct photons $R_{AA}^γ (p_T) < 1$ is dominated by cold nuclear matter effects and there is no evidence for large cross section amplification due to medium-induced photon bremsstrahlung and jet-photon conversion in the medium. Comparison of numerical simulations to experimental data also rules out large Cronin enhancement and incoherent photon emission in the QGP but the error bars in the current experimental data cannot provide further constraints on the magnitudes of other nuclear matter effects.

nucl-th↗

arXiv2008

A systematic study of direct photon production in heavy ion collisions

A theoretical derivation of photon bremsstrahlung, induced by the interactions of an energetic quark in a hot and dense quark-gluon plasma, is given in the framework of the reaction operator approach. For the physically relevant case of hard jet production, followed by few in-medium interactions, we find that the Landau-Pomeranchuk-Migdal suppression of the bremsstrahlung photon intensity is much stronger than in the previously discussed limit of on-shell quarks and a large number of soft scatterings. We present the first systematic study of direct photon production in minimum bias d+Cu and d+Au and central Cu+Cu and Au+Au heavy ion collisions at the Relativistic Heavy Ion Collider at center of mass energies of 62.4 GeV and 200 GeV. We find that the contribution of the final-state photon production at pT < 5 GeV is limited to 35%, and at high transverse momenta, the modification of the direct photon cross section is dominated by initial-state cold nuclear matter effects.

hep-ph↗

arXiv2008

The theory and phenomenology of jets in nuclear collisions

We review selected results from a recent in-depth study of jet shapes and jet cross sections in ultra-relativistic reactions with heavy nuclei at the LHC arXiv:0810.2807 [hep-ph]. We demonstrate that at the highest collider energies these observables become feasible as a new, differential and accurate test of the underlying QCD theory. Our approach allows for detailed simulations of the experimental acceptance/cuts that help isolate jets emerging from a dense QGP. We show for the first time that the pattern of stimulated gluon emission can be correlated with a variable quenching of the jet rates and provide an approximately model-independent approach to determining the characteristics of the medium-induced bremsstrahlung spectrum. The connection between such cross section attenuation and the in-medium jet shapes is elucidated.

hep-ph↗

arXiv2008

A theory of jet shapes and cross sections: from hadrons to nuclei

For jets, with great power comes great opportunity. The unprecedented center of mass energies available at the LHC open new windows on the QGP: we demonstrate that jet shape and jet cross section measurements become feasible as a new, differential and accurate test of the underlying QCD theory. We present a first step in understanding these shapes and cross sections in heavy ion reactions. Our approach allows for detailed simulations of the experimental acceptance/cuts that help isolate jets in such high-multiplicity environment. It is demonstrated for the first time that the pattern of stimulated gluon emission can be correlated with a variable quenching of the jet rates and provide an approximately model-independent approach to determining the characteristics of the medium-induced bremsstrahlung spectrum. Surprisingly, in realistic simulations of parton propagation through the QGP we find a minimal increase in the mean jet radius even for large jet attenuation. Jet broadening is manifest in the tails of the energy distribution away from the jet axis and its quantification requires high statistics measurements that will be possible at the LHC.

hep-ph↗

arXiv2008

Theoretical developments in heavy and light flavor energy loss

Recent developments in the many-body perturbative QCD theory of inelastic parton interactions in dense nuclear matter and the phenomenology of strongly-interacting hard probes in heavy ion collisions are reviewed. We highlight the progress that has been made toward consistent comparison between radiative and collisional energy loss, the exploration of novel heavy flavor suppression mechanisms in the quark-gluon plasma, and the determination of the stopping power of cold nuclear matter. Future directions and opportunities for jet physics in nuclear collisions, enabled by the unprecedentedly high center of mass energies at the LHC, are also discussed. We propose that the physics of jet shapes and a generalizations of the well-understood inclusive particle suppression in the QGP will provide a new differential, and accurate test of the underlying QCD theory and a new precision tool for jet tomography at the LHC.

hep-ph↗

arXiv2007

Non-Abelian energy loss in cold nuclear matter

We use a formal recurrence relation approach to multiple parton scattering to find the complete solution to the problem of medium-induced gluon emission from partons propagating in cold nuclear matter. The differential bremsstrahlung spectrum, where Landau-Pomeranchuk-Migdal destructive interference effects are fully accounted for, is calculated for three different cases: (1) a generalization of the incoherent Bertsch-Gunion solution for asymptotic on-shell jets, (2) initial-state energy loss of incoming jets that undergo hard scattering and (3) final-state energy loss of jets that emerge out of a hard scatter. Our analytic solutions are given as an infinite opacity series, which represents a cluster expansion of the sequential multiple scattering. These new solutions allow, for the first time, direct comparison between initial- and final-state energy loss in cold nuclei. We demonstrate that, contrary to the naive assumption, energy loss in cold nuclear matter can be large. Numerical results to first order in opacity show that, in the limit of large jet energies, initial- and final-state energy losses exhibit different path length dependences, linear versus quadratic, in contrast to earlier findings. In addition, in this asymptotic limit, initial-state energy loss is considerably larger than final-state energy loss. These new results have significant implications for heavy ion phenomenology in both p+A and A+A reactions.

hep-ph↗

arXiv2007

Collisional dissociation of heavy mesons in dense QCD matter

In the framework of the reaction operator approach we calculate and resum the multiple elastic scattering of a fast $q \bar{q}$ system traversing dense nuclear matter. We derive the collisional broadening of the meson's transverse momentum and the distortion of its intrinsic light cone wave function. The medium-induced dissociation probability of heavy mesons is shown to be sensitive to the opacity of the quark-gluon plasma and the time dependence of its formation and evolution. We solve the system of coupled rate equations that describe the competition between the fragmentation of $c$- and $b$-quarks and the QGP-induced dissociation of the $D$- and $B$-mesons to evaluate the quenching of heavy hadrons in nucleus-nucleus collisions. In contrast to previous results on heavy quark modification, this approach predicts suppression of $B$-mesons comparable to that of $D$-mesons at transverse momenta as low as $p_T \sim 10$ GeV. It allows for an improved description of the large attenuation of non-photonic electrons in central Au+Au reactions at RHIC.

hep-ph↗

arXiv2007

Novel heavy flavor suppression mechanisms in the QGP

We revisit the question of the measured, unexpectedly large, heavy flavor suppression, $R_{AA}(p_T) \ll 1$, in nucleus-nucleus collisions at RHIC and compare two new theoretical approaches to the $D$- and $B$-meson quenching. In the first model, radiative energy loss, collisional energy loss and heavy quark-resonance interactions are combined to evaluate the drag and diffusion coefficients in the quark-gluon plasma and the mixed phase. These are applied in a relativistic Fokker-Planck equation to simulate the heavy $c$- and $b$-quark suppression rate and elliptic flow $v_2(p_T)$. In the second model, the fragmentation probability for heavy quarks and the medium-induced decay probability for heavy hadrons are derived. These are implemented in a set of coupled rate equations that describe the attenuation of the observable spectra from the collisional dissociation of heavy mesons in the QGP. An improved description of the non-photonic electron $R_{AA}(p_T)$ at RHIC can be obtained. In contrast to previous results, the latter approach predicts suppression of $B$-mesons comparable to that of $D$-mesons at transverse momenta as low as $p_T \sim 10$ GeV.

hep-ph↗

arXiv2006

PQCD approach to parton propagation in matter

We review recent theoretical developments in understanding the many-body perturbative QCD dynamics of strongly-interacting hard probes in dense nuclear matter. The relation between initial- and final-state parton scattering in nuclei and the quark-gluon plasma and experimental measurements of the Cronin effect, nuclear shadowing, jet quenching, modified di-hadron correlations and forward rapidity particle suppression in p+A and A+A reactions is clarified. Our approach emphasizes the process dependence of nuclear effects and outlines the techniques for their dynamical calculation and incorporation in simulations that build upon collinear factorization in QCD.

hep-ph↗