arXiv subjects

Martin Spousta

Publications and source records attributed to Martin Spousta.

At least 19 recordsLinked to original sources

arXiv2024

Flavor and path-length dependence of jet quenching from inclusive jet and {\gamma}-jet suppression

We employ the parametric approach to analyze jet suppression measured using the nuclear modification factor of inclusive jets, $b$-jets, and jets from $\gamma$-jet events. With minimum model assumptions, we quantify the magnitude of the average energy loss and its transverse momentum dependence. Then, we quantify the impact of fluctuations in the energy loss and nuclear PDFs on the measured jet suppression. The Glauber and Trajectum models are used to estimate the average path length that a jet is expected to traverse in the medium. With this information, we quantify the path-length dependence of the average energy loss, which is found to support the physics picture of the radiative nature of parton energy loss. Using the obtained parameterizations, we evaluate jet $v_2$, quantifying its sensitivity to details of path-length modeling. We also provide model-independent predictions for the magnitude of energy loss expected in upcoming oxygen-oxygen collisions and briefly discuss the path-length dependence of energy loss of $b$-jets. Finally, we analyze intriguing features seen in the suppression of jets in the $\gamma$-jet system.

hep-ph↗

arXiv2024

QCD challenges from pp to AA collisions -- 4th edition

This paper is a write-up of the ideas that were presented, developed and discussed at the fourth International Workshop on QCD Challenges from pp to AA, which took place in February 2023 in Padua, Italy. The goal of the workshop was to focus on some of the open questions in the field of high-energy heavy-ion physics and to stimulate the formulation of concrete suggestions for making progresses on both the experimental and theoretical sides. The paper gives a brief introduction to each topic and then summarizes the primary results.

hep-ex↗

arXiv2023

Isolating perturbative QCD splittings in heavy-ion collisions

We define a new strategy to scan jet substructure in heavy-ion collisions. The scope is multifold: (i) test the dominance of vacuum jet dynamics at early times, (ii) capture the transition from coherent to incoherent jet energy loss, and (iii) study elastic scatterings in the medium, which are either hard and perturbative or soft and responsible for jet thermalisation. To achieve that, we analyse the angular distribution of the hardest splitting, $\theta_{\rm hard}$, above a transverse momentum scale, $k_t^{\rm min}$, in high-$p_t$ jets. Sufficiently high values of $k_t^{\rm min}$ target the regime in which the observable is uniquely determined by vacuum-like splittings and energy loss, leaving the jet substructure unmodified compared to proton-proton collisions. Decreasing $k_t^{\rm min}$ enhances the sensitivity to the relation between energy loss and the intra-jet structure and, in particular, to observe signatures of colour decoherence at small angles. At wider angles it also becomes sensitive to hard elastic scatterings with the medium and, therefore, the perturbative regime of medium response. Choosing $k_t^{\rm min}\approx 0$ leads to order one effects of non-perturbative origin such as hadronisation and, potentially, soft scatterings responsible for jet thermalisation. We perform a comprehensive analysis of this observable with three state-of-the-art jet-quenching Monte Carlo event generators. Our study paves the way for defining jet observables in heavy-ion collisions dominated by perturbative QCD and thus calculable from first principles.

hep-ph↗

arXiv2023

Pileup density estimate independent on jet multiplicity

The hard-scatter processes in hadronic collisions are often largely contaminated with soft background coming from pileup in proton-proton collisions, or underlying event in heavy-ion collisions. There are multiple methods to remove the effect of pileup for jets. Two such methods, Area Subtraction and Constituent Subtraction, use the pileup density as the main ingredient to estimate the magnitude of pileup contribution on an event-by-event basis. The state-of-the-art approaches to estimating pileup density are sensitive to the number of hard-scatter jets in the event. This paper presents a new pileup-density estimation method that minimizes the sensitivity on the presence of hard-scatter jets in the event. Using a detector-level simulation, we provide a comparison of the new method with the state-of-the-art estimation methods. We observe a significantly lower bias for the estimated pileup density when using the new method. We conclude that the new method has the potential to significantly improve pileup mitigation in proton-proton collisions or the underlying event subtraction in heavy-ion collisions.

hep-ph↗

arXiv2022

On macroscopic residual QCD force of electrodynamics

We explore a connection between virtual particles of quantum electrodynamics and quantum chromodynamics (QCD) which is predicted to give rise to a residual attractive interaction measurable as a macroscopic force. We calculate the asymptotic behavior of relevant scattering amplitudes, perform their resummation, and analyze the sign of the resulting interaction. Then, we calculate the primary experimentally observable consequences of this Standard Model force. We discuss the impact of this force at terrestrial scales and at astrophysical scales. In particular, we quantify the impact of this force on the warm ionized medium present in galaxies and the intracluster medium present in cluster of galaxies.

hep-ph↗

arXiv2022

Multipartonic cascades in expanding media

In this work, we introduce both gluon and quark degrees of freedom for describing the partonic cascades inside the medium. We present numerical solutions for the set of coupled evolution equations with splitting kernels calculated for the static, exponential and Bjorken expanding media to arrive at medium-modified parton spectra for quark and gluon initiated jets respectively. We discuss novel scaling features of the partonic spectra between different types of media. Next, we study the inclusive jet $R_{AA}$ by including phenomenologically driven combinations of quark and gluon fractions inside a jet. In addition, we have also studied the effect of the nPDF as well as vacuum like emissions on the jet $R_{AA}$. Differences among the estimated values of quenching parameter for different types of medium expansions are noted. Next, the impact of the expansion of the medium on the rapidity dependence of the jet $R_{AA}$ as well as jet $v_2$ are studied in detail. Finally, we present qualitative results comparing the sensitivity of the time for the onset of the quenching for the Bjorken profile on these observables. All the quantities calculated are compared with the recent ATLAS data.

hep-ph↗

arXiv2021

Multi-partonic medium induced cascades in expanding media

Going beyond the simplified gluonic cascades, we introduce both gluon and quark degrees of freedom for partonic cascades inside the medium. We then solve the set of coupled evolution equations numerically with splitting kernels calculated for static, exponential, and Bjorken expanding media to arrive at medium-modified parton spectra for quark and gluon initiated jets. Using these, we calculate the inclusive jet $R_{AA}$ where the phenomenologically driven combinations of quark and gluon jet fractions are included. Then, the rapidity dependence of the jet $R_{AA}$ is examined. We also study the path-length dependence of jet quenching for different types of expanding media by calculating the jet $v_2$. Additionally, we study the sensitivity of observables on effects from nuclear modification of parton distribution functions, vacuum-like emissions in the plasma, and the time of the onset of the quenching. All calculations are compared with recently measured data.

hep-ph↗

arXiv2021

Jet quenching in expanding medium

Comprehensive understanding of medium-induced radiative energy loss is of a paramount importance in describing observed jet quenching in heavy-ion collisions. In this work, we have calculated the medium-modified gluon splitting rates for different profiles of the expanding partonic medium, namely profiles for static, exponential, and Bjorken expanding medium. Here in this study, we have used the Baier-Dokshitzer-Mueller-Peigne-Schiff-Zakharov (BDMPSZ) formalism for multiple soft scatterings with a time-dependent transport coefficient for characterizing the expanding medium. Using the kinetic rate equation, we have numerically evaluated the distribution of the medium evolved gluon spectra for different medium profiles. We have presented a calculation of the jet $Q_{AA}$ which quantifies a sensitivity of the inclusive jet suppression on the way how the medium expands. Comparisons of predicted jet $Q_{AA}$ with experimental data from the LHC are also presented.

hep-ph↗

arXiv2020

Jet quenching and scaling properties of medium-evolved gluon cascade in expanding media

We present a study of the impact of the expansion of deconfined medium on single-gluon emission spectra and the jet suppression factor ($Q_{AA}$) within the BDMPS-Z formalism. These quantities are calculated for three types of media (static medium, exponentially decaying medium and Bjorken expanding medium). The distribution of medium-induced gluons and the jet $Q_{AA}$ are calculated using the evaluation of in-medium evolution with splitting kernels derived from the gluon emission spectra. Scaling behavior of splitting kernels is derived for low-x and high-x regimes in the asymptote of large times and its impact on the resulting jet $Q_{AA}$ is discussed. For the full phase space of the radiation, the scaling of jet $Q_{AA}$ with an effective quenching parameter is presented.

hep-ph↗

arXiv2020

Parametric analysis of COVID-19 expansion in European countries in the period of February to June 2020

The data on number of registered cases of COVID-19 disease in twenty European countries is analyzed by the least-squares fitting procedure with generic analytic functions. Three regimes of the expansion of the disease are identified and quantified -- early exponential expansion, damped exponential, and linear expansion. Differences among countries in the early expansion period are quantified. The velocity of the expansion in the exponential regime lies within one standard deviation from the average value for 11 countries. The number of infected individuals at the initial time is excessively high for Italy, 7 standard deviations from the average value. Method for predicting the expansion based on extrapolation in the parametric space is presented. One-week predictions based on extrapolations have average precision of 18% and 29% during the later period of the damped exponential expansion for the case of Italy and Czechia, respectively. The method based on extrapolations in the parametric space may provide an elementary method to quantify the impact of restrictive measures on the spreading of the disease.

q-bio.PE↗

arXiv2019

Universality in Hadronic and Nuclear Collisions at High Energy

Recent experimental results in proton-proton and in proton-nucleus collisions at Large Hadron Collider energies show a strong similarity to those observed in nucleus-nucleus collisions, where the formation of a quark-gluon plasma is expected. We discuss the comparison between small colliding systems and nucleus-nucleus collisions, for: a)~the strangeness suppression factor $\gamma_s$ and yields of multi-strange hadrons; b)~the average transverse momentum, $p_t$, with particular attention to the low $p_t$ region where soft, non-perturbative effects are important; c)~the elliptic flow scaled by the participant eccentricity. The universal behavior in hadronic and nuclear high energy collisions emerges for all these observables in terms of a specific dynamical variable which corresponds to the entropy density of initial system in the collision and which takes into account the transverse size of the initial configuration and its fluctuations.

hep-ph↗

arXiv2019

Medium-induced cascade in expanding media

Detailed insight into the interplay between parton energy loss and the way deconfined medium created in heavy-ion collisions expands is of great importance for improving the understanding of the jet quenching phenomenon. In this paper we study the impact of the expansion of deconfined medium on the single-gluon emission spectrum, its resummation and the jet suppression factor ($Q_{AA}$) within the BDMPS-Z formalism. We calculate these quantities for three types of expansion scenarios, namely static, exponentially decaying and Bjorken expanding media. The distribution of medium-induced gluons is calculated using an evolution equation with splitting kernels derived from the gluon emission spectra. A universal behavior of splitting kernels is derived in the regime of soft gluon emissions when evaluated at a common effective evolution time $\tau_{eff}$. Novel scaling features of the resulting gluon distribution and jet $Q_{AA}$ are discussed. For realistic spectra valid beyond the soft-gluon emission limit, where the results are obtained by a numerical solution of the evolution equation, these features are partially replaced by a scaling expected from considering an averaged jet quenching parameter along the trajectory of propagation. Further we show that differences arising from different types of the medium expansion can be to a large extent scaled out by appropriate choice of the quenching parameter. Sizable differences among the values of the quenching parameter for different types of medium expansion point to the importance of the medium expansion for precise modeling of the jet quenching phenomenon.

hep-ph↗

arXiv2019

Pileup and Underlying Event Mitigation with Iterative Constituent Subtraction

The hard-scatter processes in hadronic collisions are often largely contaminated with soft background coming from pileup in proton-proton collisions, or underlying event in heavy-ion collisions. This paper presents a new background subtraction method for jets and event observables (such as missing transverse energy) which is based on the previously published Constituent Subtraction algorithm. The new subtraction method, called Iterative Constituent Subtraction, applies event-wide implementation of Constituent Subtraction iteratively in order to fully equilibrate the background subtraction across the entire event. Besides documenting the new method, we provide guidelines for setting the free parameters of the subtraction algorithm. Using particle-level simulation, we provide a comparison of Iterative Constituent Subtraction with several existing methods from which we conclude that the new method has a significant potential to improve the background mitigation in both proton-proton and heavy-ion collisions.

hep-ph↗

arXiv2018

Novel tools and observables for jet physics in heavy-ion collisions

Studies of fully-reconstructed jets in heavy-ion collisions aim at extracting thermodynamical and transport properties of hot and dense QCD matter. Recently, a plethora of new jet substructure observables have been theoretically and experimentally developed that provide novel precise insights on the modifications of the parton radiation pattern induced by a QCD medium. This report, summarizing the main lines of discussion at the 5th Heavy Ion Jet Workshop and CERN TH institute "Novel tools and observables for jet physics in heavy-ion collisions" in 2017, presents a first attempt at outlining a strategy for isolating and identifying the relevant physical processes that are responsible for the observed medium-induced jet modifications. These studies combine theory insights, based on the Lund parton splitting map, with sophisticated jet reconstruction techniques, including grooming and background subtraction algorithms.

hep-ph↗

arXiv2017

Flavor aspects of parton energy loss

Understanding flavor dependence of the parton energy loss is one of key tasks of the jet quenching physics. In these proceedings we provide a summary of recent works on a quantification of the flavor dependence of parton energy loss along with a summary and discussion of a subset of contributions presented at the Hard Probes 2016 conference which are related to the flavor aspects of parton energy loss.

hep-ph↗

arXiv2016

On similarity of jet quenching and charmonia suppression

We quantify the magnitude of the parton energy loss in jets in lead-lead collisions at the LHC. We extract the effective color factor characterizing the difference between the in-medium radiation of quark-initiated jets and gluon-initiated jets from the data. Then, we examine the energy loss of prompt charmonia and we point to a remarkable similarity between the quenching of light-quark-initiated jets and the prompt charmonia suppression. Finally, we discuss possible sources of this similarity.

hep-ph↗

arXiv2015

Overview of new results from ATLAS heavy ion physics program

In this short paper we provide an overview of new results from the ATLAS physics program at the LHC as of spring 2015. We separately summarize the results from p+Pb collisions and Pb+Pb collisions along with some of their interpretations.

hep-ex↗

arXiv2015

Interpreting Single Jet Measurements in Pb+Pb Collisions at the LHC

Results are presented from a phenomenological analysis of recent measurements of jet suppression and modifications of jet fragmentation functions in Pb+Pb collisions at the LHC. Particular emphasis is placed on the impact of the differences between quark and gluon jet quenching on the transverse momentum ($p_{T}^{jet}$) dependence of the jet $R_{AA}$ and on the fragmentation functions, $D(z)$. Primordial quark and gluon parton distributions were obtained from PYTHIA8 and were parameterized using simple power-law functions and extensions to the power-law function which were found to better describe the PYTHIA8 parton spectra. A simple model for the quark energy loss based on the shift formalism is used to model $R_{AA}$ and $D(z)$ using both analytic results and using direct Monte-Carlo sampling of the PYTHIA parton spectra. The model is capable of describing the full $p_{T}^{jet}$ , rapidity, and centrality dependence of the measured jet $R_{AA}$ using three effective parameters. A key result from the analysis is that the $D(z)$ modifications observed in the data, excluding the enhancement at low-$z$, may result primarily from the different quenching of the quarks and gluons. The model is also capable of reproducing the charged hadron $R_{AA}$ at high transverse momentum. Predictions are made for the jet $R_{AA}$ at large rapidities where it has not yet been measured and for the rapidity dependence of $D(z)$.

hep-ph↗