Medium induced QCD cascades: broadening and rescattering during branching

In a series of previous papers, we have presented a new approach, based on perturbative QCD, for the evolution of a jet in a dense quark-gluon plasma. In the original formulation, the plasma was assumed to be homogeneous and static. In this work, we extend our description and its Monte Carlo implementation to a plasma obeying Bjorken longitudinal expansion. Our key observation is that the factorisation between vacuum-like and medium-induced emissions, derived in the static case, still holds for an expanding medium, albeit with modified rates for medium-induced emissions and transverse momentum broadening, and with a modified phase-space for vacuum-like emissions. We highlight a scaling relation valid for the energy spectrum of medium-induced emissions, through which the case of an expanding medium is mapped onto an effective static medium. We find that scaling violations due to vacuum-like emissions and transverse momentum broadening are numerically small. Our new predictions for the nuclear modification factor for jets RAA, the in-medium fragmentation functions, and substructure distributions are very similar to our previous estimates for a static medium, maintaining the overall good qualitative agreement with existing LHC measurements. In the case of RAA, we find that the agreement with the data is significantly improved at large transverse momenta pT ≳ 500 GeV after including the effects of the nuclear parton distribution functions.

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“…√ ωq/4ᾱ s for ω ω br (see also the related studies in [42][43][44][45][46]). In appendix A, we generalise the calculation of ref.…”

Section: Jhep04(2021)209mentioning

confidence: 99%

Jet radiation in a longitudinally expanding medium

Caucal

Iancu

Soyez

2021

J. High Energ. Phys.

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“…That is, in the soft and collinear limit, the spectrum is given by the product of the time integral of the broadening distribution and the medium induced energy rate. This result is not tied with the IOE approach and has been previously obtained in the literature in the context of BDMPS-Z calculations [19] and exploited in Monte-Carlo simulations [54,56]. We proceed to compute eq.…”

Section: Jhep09(2021)153mentioning

confidence: 54%

“…In this case, we recover the well-known factorization formula given by eq. (3.89), often used in jet quenching phenomenology [19,[54][55][56]. In particular, this result implies that in the soft limit the differential spectrum can be written as the product between the LO term and powers ofq 0 q that correspond to higher order contributions.…”

Section: Discussionmentioning

confidence: 95%

“…In parallel, we would like to implement the formulas that we have derived in this paper in a suitable Monte Carlo framework such as refs. [54,56]. which implies that the roots are in general not unique: either there is no real solution, there is a single solution or at most two real solutions.…”

Section: Jhep09(2021)153mentioning

confidence: 99%

“…[49] in the soft regime. This new scheme, dubbed the Improved Opacity Expansion (IOE), has been shown to successfully meet this goal when applied to the description of the single particle broadening probability [50] and to the medium-induced gluon energy spectrum [51][52][53], which constitute the major tools used in a multitude of well established phenomenological models of jet quenching [43,[54][55][56][57][58][59]. In a nutshell, this framework is built as a series expansion of the in-medium scattering cross section where the zeroth order term encodes the multiple scattering solution and higher N n LO orders 1 in the series account for n hard scatterings with the medium.…”

Section: Introductionmentioning

confidence: 99%

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Medium-induced radiative kernel with the Improved Opacity Expansion

Barata

Mehtar-Tani

Soto-Ontoso

et al. 2021

J. High Energ. Phys.

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We calculate the fully differential medium-induced radiative spectrum at next-to-leading order (NLO) accuracy within the Improved Opacity Expansion (IOE) framework. This scheme allows us to gain analytical control of the radiative spectrum at low and high gluon frequencies simultaneously. The high frequency regime can be obtained in the standard opacity expansion framework in which the resulting power series diverges at the characteristic frequency ωc ∼ $$ \hat{q} $$ q ̂ L2. In the IOE, all orders in opacity are resumed systematically below ωc yielding an asymptotic series controlled by logarithmically suppressed remainders down to the thermal scale T « ωc, while matching the opacity expansion at high frequency. Furthermore, we demonstrate that the IOE at NLO accuracy reproduces the characteristic Coulomb tail of the single hard scattering contribution as well as the Gaussian distribution resulting from multiple soft momentum exchanges. Finally, we compare our analytic scheme with a recent numerical solution, that includes a full resummation of multiple scatterings, for LHC-inspired medium parameters. We find a very good agreement both at low and high frequencies showcasing the performance of the IOE which provides for the first time accurate analytic formulas for radiative energy loss in the relevant perturbative kinematic regimes for dense media.

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A unified picture of medium-induced radiation

Isaksen

Takács

Tywoniuk

2023

J. High Energ. Phys.

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We revisit the picture of jets propagating in the quark-gluon plasma. In addition to vacuum radiation, partons scatter on the medium constituents resulting in induced emissions. Analytical approaches to including these interactions have traditionally dealt separately with multiple, soft, or rare, hard scatterings. A full description has so far only been available using numerical methods. We achieve full analytical control of the relevant scales and map out the dominant physical processes in the full phase space. To this aim, we extend existing expansion schemes for the medium-induced spectrum to the Bethe-Heitler regime. This covers the whole phase space from early to late times, and from hard splittings to emissions below the thermal scale. Based on the separation of scales, a space-time picture naturally emerges: at early times, induced emissions start to build from rare scatterings with the medium. At a later stage, induced emissions due to multiple soft scatterings result in a turbulent cascade that rapidly degrades energy down to, and including, the Bethe-Heitler regime. We quantify the impact of such an improved picture, compared to the current state-of-the-art factorization that includes only soft scatterings, by both analytical and numerical methods for the medium-induced energy distribution function. Our work serves to improve our understanding of jet quenching from small to large systems and for future upgrades of Monte Carlo generators.

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Medium induced QCD cascades: broadening and rescattering during branching

Cited by 17 publications

References 41 publications

Jet radiation in a longitudinally expanding medium

Jet radiation in a longitudinally expanding medium

Medium-induced radiative kernel with the Improved Opacity Expansion

A unified picture of medium-induced radiation

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