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Yacine Mehtar-Tani

Publications and source records attributed to Yacine Mehtar-Tani.

At least 55 records · Page 3Linked to original sources
arXiv2020

Dynamical grooming of QCD jets

We propose a new class of infrared-collinear (IRC) and Sudakov safe observables with an associated jet grooming technique that removes dynamically soft and large angle branches. It is based on identifying the hardest branch in the Cambridge/Aachen re-clustering sequence and discarding prior splittings that occur at larger angles. This leads to a dynamically generated cut-off on the phase space of the tagged splitting that is encoded in a Sudakov form factor. In this exploratory study we focus on the mass and momentum sharing distributions of the tagged splitting which we analyze analytically to modified leading logarithmic accuracy and compare to Monte-Carlo simulations.

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arXiv2020

On gauge invariance of transverse momentum dependent distributions at small x

The interplay between the small x limit of QCD amplitudes and QCD factorization at moderate x has been studied extensively in recent years. It was finally shown that semiclassical formulations of small x physics can have the form of an infinite twist framework involving Transverse Momentum Dependent (TMD) distributions in the eikonal limit. In this work, we demonstrate that small x distributions can be formulated in terms of transverse gauge links. This allows in particular for direct and efficient decompositions of observables into subamplitudes involving gauge invariant sub-operators which span parton distributions.

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arXiv2019

A background estimator for jet studies in p+p and A+A collisions

Experimentally, jet physics studies face an unavoidable task: distinguishing, at the detector level, the particles produced in the hard partonic scattering from the ones created in unrelated soft processes such as pileup interactions in high-luminosity proton-proton scattering or the underlying event in heavy-ion collisions. The fluctuating nature of the background constitutes the main source of uncertainty for any subtraction algorithm. Aiming at mitigating the effect of such fluctuations, we present a new method to estimate the background contribution to the transverse momentum on a jet-by-jet basis. Our approach is based on estimating the median background momentum density stored above a $p_{\rm T}$-cut applied at the constituent level and an experimentally accessible correction term related to the signal contribution below the cut. This allows to trade part of the uncertainty due to background contamination for that of the signal below the cut, similarly to SoftKiller method. We propose to reduce the fluctuations of the latter by exploiting intrinsic correlations among the soft and hard sectors of QCD jets. Our data-driven approach is tested against PYTHIA8 and JEWEL di-jet events embedded in a thermal background and compared to the area-median and SoftKiller methods. The main result of this study is a $\sim 5\!-\!40\%$ improvement on the resolution of the reconstructed jet $p_T$ compared to previous methods in a high-luminosity proton-proton scenario. Its applicability in a heavy-ion context is also discussed.

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arXiv2019

Improved opacity expansion for medium-induced parton splitting

We present a new expansion scheme to compute the rate for parton splittings in dense and finite QCD media. In contrast to the standard opacity expansion, our expansion is performed around the harmonic oscillator whose characteristic frequency depends on the typical transverse momentum scale generated in the splitting. The first two orders account for the high frequency regime that is dominated by single hard scatterings together with the regime of multiple soft scatterings at low frequency. This work generalizes the findings of Ref. \cite{Mehtar-Tani:2019tvy} beyond the leading logarithmic approximation allowing to account also for the Bethe-Heitler regime and compare to the full numerical results from Ref. \cite{CaronHuot:2010bp}. We investigate the sensitivity of our results to varying the separation scale that defines the leading order. Finally, the application to Monte Carlo event generators is discussed.

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arXiv2019

Gluon bremsstrahlung in finite media beyond multiple soft scattering approximation

We revisit the calculation of the medium-induced gluon spectrum in a finite QCD and develop a new approach that goes beyond multiple soft scattering approximation. We show by expanding around the harmonic oscillator that the first two orders encompass the two known analytic limits: single hard and multiple soft scattering regimes, valid at high and low frequencies, respectively. Finally, we investigate the sensitivity of our results to the infrared and observe that for large media the spectrum is weakly dependent on the infrared medium scale.

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arXiv2019

Higher-order corrections to jet quenching

We compute the inclusive jet spectrum in the presence of a dense QCD medium by going beyond the single parton energy loss approximation. We show that higher-order corrections are important yielding large logarithmic contributions that must be resummed to all orders. This reflects the fact that jet quenching is sensitive to fluctuations of the jet substructure.

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arXiv2018

Quantifying jet modifications with substructure

The striking suppression and modification patterns that are observed in jet observables measured in heavy-ion collisions with respect to the proton-proton baseline have the potential to constrain the spatio-temporal branching process of energetic partons in a dense QCD medium. The mechanism of jet energy loss is intricately associated with medium resolution of jet substructure fluctuations. This naturally affects the behavior of the suppression of jets at high-pT, inducing an explicit dependence on jet scales. In this contribution, we review recent work on using the insight from multi-parton quenching to calculate leading-logarithmic corrections to the single-inclusive jet spectrum, and discuss its impact on a wide range of observables, including jet substructure.

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arXiv2018

Radiative energy loss of neighboring subjets

We compute the in-medium energy loss probability distribution of two neighboring subjets at leading order, in the large-$N_c$ approximation. Our result exhibits a gradual onset of color decoherence of the system and accounts for two expected limiting cases. When the angular separation is smaller than the characteristic angle for medium-induced radiation, the two-pronged substructure lose energy coherently as a single color charge, namely that of the parent parton. At large angular separation the two subjets lose energy independently. Our result is a first step towards quantifying effects of energy loss as a result of the fluctuation of the multi-parton jet substructure and therefore goes beyond the standard approach to jet quenching based on single parton energy loss. We briefly discuss applications to jet observables in heavy-ion collisions.

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arXiv2018

Universal quark to gluon ratio in medium-induced parton cascade

We investigate the radiative break-up of a highly energetic quark or gluon in a high-temperature QCD plasma. Within an inertial range of momenta $T \ll ω\ll E$, where $E$ denotes the energy of the original hard parton (jet) and $T$ the temperature of of the medium, we find that, as a result of the turbulent nature of the underlying parton cascade, the quark to gluon ratio of the soft fragments tends to a universal constant value that is independent of the initial conditions. We discuss implications of this result to jet quenching physics and the problem of thermalization of the quark-gluon plasma in heavy ion collisions.

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arXiv2018

Sudakov suppression of jets in QCD media

We compute modifications to the jet spectrum in the presence of a dense medium. We show that in the large-$N_c$ approximation and at leading logarithmic accuracy the jet nuclear modification factor factorizes into a quenching factor associated to the total jet color charge and a Sudakov suppression factor which accounts for the energy loss of jet substructure fluctuations. This factor, called the jet collimator, implements the fact that subjets, that are not resolved by the medium, lose energy coherently as a single color charge, whereas resolved large angle fluctuations suffer more quenching. For comparison, we show that neglecting color coherence results in a stronger suppression of the jet nuclear modification factor.

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arXiv2016

Nonlocal wave turbulence in non-Abelian plasmas

We investigate driven wave turbulence in non-Abelian plasmas, in the framework of kinetic theory where both elastic and inelastic processes are considered in the small angle approximation. The gluon spectrum, that forms in the presence of a steady source, is shown to be controlled by nonlocal interactions in momentum space, in contrast to the universal Kolmogorov-Zakharov spectra. Assuming strongly nonlocal interactions, we show that inelastic processes are dominant in the IR and cause a thermal bath to form below the forcing scale, as a result of a detailed balance between radiation and absorption of soft gluons by the hard ones. Above the forcing scale, the inelastic collision term reduces to an inhomogeneous diffusion-like equation yielding a spectrum that spreads to the UV as $t^{1/2}$, similarly to elastic processes. Due to nonlocal interactions the non-universal turbulent spectrum is not steady and flattens when time goes on toward the thermal distribution. This analysis is complemented by numerical simulations, where we observe that in the explored time interval the spectral exponent of the nonlocal turbulent cascade is close to $-2$ in agreement with simulations of classical Yang-Mills equations.

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arXiv2016

Groomed jets in heavy-ion collisions: sensitivity to medium-induced bremsstrahlung

We argue that contemporary jet substructure techniques might facilitate a more direct measurement of hard medium-induced gluon bremsstrahlung in heavy-ion collisions, and focus specifically on the "soft drop declustering" procedure that singles out the two leading jet substructures. Assuming coherent jet energy loss, we find an enhancement of the distribution of the energy fractions shared by the two substructures at small subjet energy caused by hard medium-induced gluon radiation. Departures from this approximation are discussed, in particular, the effects of colour decoherence and the contamination of the grooming procedure by soft background. Finally, we propose a complementary observable, that is the ratio of the two-pronged probability in Pb-Pb to proton-proton collisions and discuss its sensitivity to various energy loss mechanisms.

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arXiv2016

The thermalization of soft modes in non-expanding isotropic quark gluon plasmas

We discuss the role of elastic and inelastic collisions and their interplay in the thermalization of the quark-gluon plasma. We consider a simplified situation of a static plasma, spatially uniform and isotropic in momentum space. We focus on the small momentum region, which equilibrates first, and on a short time scale. We obtain a simple kinetic equation that allows for an analytic description of the most important regimes. The present analysis suggests that the formation of a Bose condensate, expected when only elastic collisions are present, is strongly hindered by the inelastic, radiative, processes.

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arXiv2016

Theoretical Developments in QCD Jet Energy Loss

We review the recent developments in the theory of jet-quenching. First, we analyze the coherent vacuum cascade and incoherent medium-induced cascade separately. We then discuss the interplay between the two kinds of cascade and the resulting partial decoherence of the inner jet structure. Finally, we report on recent calculations of higher-order corrections. In particular, the dominant radiative corrections to jet observables that yield the renormalization of the jet-quenching parameter are addressed.

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arXiv2015

Jet Structure in Heavy Ion Collisions

We review recent theoretical developments in the study of the structure of jets that are produced in ultra relativistic heavy ion collisions. The core of the review focusses on the dynamics of the parton cascade that is induced by the interactions of a fast parton crossing a quark-gluon plasma. We recall the basic mechanisms responsible for medium induced radiation, underline the rapid disappearance of coherence effects, and the ensuing probabilistic nature of the medium induced cascade. We discuss how large radiative corrections modify the classical picture of the gluon cascade, and how these can be absorbed in a renormalization of the jet quenching parameter $\hat q $. Then, we analyze the (wave)-turbulent transport of energy along the medium induced cascade, and point out the main characteristics of the angular structure of such a cascade. Finally, color decoherence of the in-cone jet structure is discussed. Modest contact with phenomenology is presented towards the end of the review.

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arXiv2015

Energy flow along the medium-induced parton cascade

We discuss the dynamics of parton cascades that develop in dense QCD matter, and contrast their properties with those of similar cascades of gluon radiation in vacuum. We argue that such cascades belong to two distinct classes that are characterized respectively by an increasing or a constant (or decreasing) branching rate along the cascade. In the former class, of which the BDMPS, medium-induced, cascade constitutes a typical example, it takes a finite time to transport a finite amount of energy to very soft quanta, while this time is essentially infinite in the latter case, to which the DGLAP cascade belongs. The medium induced cascade is accompanied by a constant flow of energy towards arbitrary soft modes, leading eventually to the accumulation of the initial energy of the leading particle at zero energy. It also exhibits scaling properties akin to wave turbulence. These properties do not show up in the cascade that develops in vacuum. There, the energy accumulates in the spectrum at smaller and smaller energy as the cascade develops, but the energy never flows all the way down to zero energy. Our analysis suggests that the way the energy is shared among the offsprings of a splitting gluon has little impact on the qualitative properties of the cascades, provided the kernel that governs the splittings is not too singular.

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arXiv2014

Angular distribution of medium-induced QCD cascades

We provide a complete description of the angular distribution of gluons in a medium-induced QCD cascade. We identify two components in the distribution, a soft component dominated by soft multiple scatterings, and a hard component dominated by a few hard scatterings. The typical angle that marks the boundary between these two components is determined analytically as a function of the energy of the observed gluon and the size of the medium. We construct the complete solution (beyond the diffusion approximation) in the regime where multiple branchings dominate the dynamics of the cascade in the form of a power series in the number of collisions with the medium particles. The coefficients of this expansions are related to the moments of the distribution in the diffusion approximation and are determined analytically. The angular distribution may be useful in phenomenological studies of jet shapes in heavy-ion collisions.

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