arXiv subjects

Magdalena Djordjevic

Publications and source records attributed to Magdalena Djordjevic.

At least 37 records · Page 2Linked to original sources

arXiv2021

Effects of demographic and weather parameters on COVID-19 basic reproduction number

Timely prediction of the COVID-19 progression is not possible without a comprehensive understanding of environmental factors that may affect the infection transmissibility. Studies addressing parameters that may influence COVID-19 progression relied on either the total numbers of detected cases and similar proxies and/or a small number of analyzed factors, including analysis of regions that display a narrow range of these parameters. We here apply a novel approach, exploiting widespread growth regimes in COVID-19 detected case counts. By applying nonlinear dynamics methods to the exponential regime, we extract basic reproductive number R0 (i.e., the measure of COVID-19 inherent biological transmissibility), applying to the completely naive population in the absence of social distancing, for 118 different countries. We then use bioinformatics methods to systematically collect data on a large number of demographics and weather parameters from these countries, and seek their correlations with the rate of COVID-19 spread. In addition to some of the already reported tendencies, we show a number of both novel results and those that help settle existing disputes: the absence of dependence on wind speed and air pressure, negative correlation with precipitation; significant positive correlation with society development level (human development index) irrespective of testing policies, and percent of the urban population, but an absence of correlation with population density per se. We find a strong positive correlation of transmissibility on alcohol consumption, and the absence of correlation on refugee numbers, contrary to some widespread beliefs. Significant tendencies with health-related factors are reported, including a detailed analysis of the blood type group showing consistent tendencies on Rh factor, and a strong positive correlation of transmissibility with cholesterol levels.

q-bio.PE↗

arXiv2021

Understanding infection progression under strong control measures through universal COVID-19 growth signatures

Widespread growth signatures in COVID-19 confirmed case counts are reported, with sharp transitions between three distinct dynamical regimes (exponential, superlinear and sublinear). Through analytical and numerical analysis, a novel framework is developed that exploits information in these signatures. An approach well known to physics is applied, where one looks for common dynamical features, independently from differences in other factors. These features and associated scaling laws are used as a powerful tool to pinpoint regions where analytical derivations are effective, get an insight into qualitative changes of the disease progression, and infer the key infection parameters. The developed framework for joint analytical and numerical analysis of empirically observed COVID-19 growth patterns can lead to a fundamental understanding of infection progression under strong control measures, applicable to outbursts of both COVID-19 and other infectious diseases.

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arXiv2020

From high $p_\perp$ theory and data to inferring anisotropy of Quark-Gluon Plasma

High $p_\perp$ theory and data are commonly used to study high $p_\perp$ parton interactions with QGP, while low $p_\perp$ data and corresponding models are employed to infer QGP bulk properties. On the other hand, with a proper description of high $p_\perp$ parton-medium interactions, high $p_\perp$ probes become also powerful tomography tools, since they are sensitive to global QGP features, such as different temperature profiles or initial conditions. This tomographic role of high $p_\perp$ probes can be utilized to assess the spatial anisotropy of the QCD matter. With our dynamical energy loss formalism, we show that a (modified) ratio of $R_{AA}$ and $v_2$ presents a reliable and robust observable for straightforward extraction of initial state anisotropy. We analytically estimated the proportionality between the $v_2/(1-R_{AA})$ and anisotropy coefficient $ε_{2L}$, and found surprisingly good agreement with full-fledged numerical calculations. Within the current error bars, the extraction of the anisotropy from the existing data using this approach is still inaccessible. However, with the expected accuracy improvement in the upcoming LHC runs, the anisotropy of the QGP formed in heavy ion collisions can be straightforwardly derived from the data. Such a data-based anisotropy parameter would present an important test to models describing the initial stages of heavy-ion collision and formation of QGP, and demonstrate the usefulness of high $p_\perp$ theory and data in obtaining QGP properties.

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arXiv2020

Dynamical energy loss formalism: from describing suppression patterns to implications for future experiments

Understanding properties of Quark-Gluon Plasma requires an unbiased comparison of experimental data with theoretical predictions. To that end, we developed the dynamical energy loss formalism which, in distinction to most other methods, takes into account a realistic medium composed of dynamical scattering centers. The formalism also allows making numerical predictions for a wide number of observables with the same parameter set fixed to standard literature values. In this proceedings, we overview our recently developed DREENA-C and DREENA-B frameworks, where DREENA is a computational implementation of the dynamical energy loss formalism, and where C stands for constant temperature QCD medium, while B stands for the medium modeled by 1+1D Bjorken expansion. At constant temperature our predictions overestimate $v_2$, in contrast to other models, but consistent with simple analytical estimates. With Bjorken expansion, we have a good agreement of the predictions with both $R_{AA}$ and $v_2$ measurements. We find that introducing medium evolution has a larger effect on $v_2$ predictions, but for precision predictions it has to be taken into account in $R_{AA}$ predictions as well. Based on numerical calculations and simple analytical derivations, we also propose a new observable, which we call path length sensitive suppression ratio, for which we argue that the path length dependence can be assessed in a straightforward manner. We also argue that $Pb+Pb$ vs. $Xe+Xe$ measurements make a good system to assess the path length dependence. As an outlook, we expect that introduction of more complex medium evolution (beyond Bjorken expansion) in the dynamical energy loss formalism can provide a basis for a state of the art QGP tomography tool - e.g. to jointly constrain the medium properties from the point of both high pt and low pt data.

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arXiv2020

Extracting the temperature dependence in high-$p_\perp$ particle energy loss

The suppression of high-$p_\perp$ particles is one of the main signatures of parton energy loss during its passing through the QGP medium, and is reasonably reproduced by different theoretical models. However, a decisive test of the reliability of a certain energy loss mechanism, apart from its path-length, is its temperature dependence. Despite its importance and comprehensive dedicated studies, this issue is still awaiting for more stringent constraints. To this end, we here propose a novel observable to extract temperature dependence exponent of high-$p_{\perp}$ particle's energy loss, based on $R_{AA}$. More importantly, by combining analytical arguments, full-fledged numerical calculations and comparison with experimental data, we argue that this observable is highly suited for testing (and rejecting) the long-standing $ΔE/E \propto L^2 T^3$ paradigm. The anticipated significant reduction of experimental errors will allow direct extraction of temperature dependence, by considering different centrality pair in $A + A$ collisions (irrespective of the nucleus size) in high-$p_\perp$ region. Overall, our results imply that this observable, which reflects the underlying energy loss mechanism, is very important to distinguish between different theoretical models.

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arXiv2019

Shape of the quark gluon plasma droplet reflected in the high-p_T data

We show, through analytic arguments, numerical calculations, and comparison with experimental data, that the ratio of the high-p_T observables v_2/(1-R_AA) reaches a well-defined saturation value at high p_T, and that this ratio depends only on the spatial anisotropy of the quark gluon plasma (QGP) formed in ultrarelativistic heavy-ion collisions. With expected future reduction of experimental errors, the anisotropy extracted from experimental data will further constrain the calculations of initial particle production in heavy-ion collisions and thus test our understanding of QGP physics.

hep-ph↗

arXiv2019

Exploring the initial stages in heavy-ion collisions with high-pT RAA and v2 theory and data

Traditionally, low-pT sector is used to infer the features of initial stages before QGP thermalization. On the other hand, recently acquired wealth of high-pT experimental data paves the way to utilize the high-pT particles energy loss in exploring the initial stages. We here study how four different commonly considered initial-stage scenarios which have the same temperature profile after, but differ in the 'temperature' profile before thermalization affect predictions of high-pT RAA and v2 observables. Contrary to common expectations, we obtain that high-pT v2 is insensitive to the initial stages of medium evolution, being unable to discriminate between different conditions. On the other hand, RAA is sensitive to these conditions, however, within the current errorbars, the sensitivity is not sufficient to distinguish between different initial stages. Moreover, we also reconsider the validity of widely-used procedure of fitting the energy loss parameters, individually for different initial-stage cases, to reproduce the experimentally observed RAA. We here find that previously reported sensitivity of v2 to different initial states is mainly a consequence of the RAA fitting procedure, which may lead to incorrect conclusions. On the other hand, if a global property, in particular the same average temperature, is imposed to tested temperature profiles, high sensitivity of high-pT v2 is again obtained. We however show that this sensitivity would not be a consequence of differences in initial, but rather final, stages. Consequently, the simultaneous study of high-pT RAA and v2, with consistent energy loss parametrization and stringently controlled temperature profiles, is necessary to assess sensitivity of different variables to differences in initial and final stages.

hep-ph↗

arXiv2019

DREENA-B framework: first predictions of $R_{AA}$ and $v_2$ within dynamical energy loss formalism in evolving QCD medium

Dynamical energy loss formalism allows generating state-of-the-art suppression predictions in finite size QCD medium, employing a sophisticated model of high-$p_\perp$ parton interactions with QGP. We here report a major step of introducing medium evolution in the formalism though $1+1D$ Bjorken (``B'') expansion, while preserving all complex features of the original dynamical energy loss framework. We use this framework to provide joint $R_{AA}$ and $v_2$ predictions, for the first time within the dynamical energy loss formalism in evolving QCD medium. The predictions are generated for a wide range of observables, i.e. for all types of probes (both light and heavy) and for all centrality regions in both $Pb+Pb$ and $Xe+Xe$ collisions at the LHC. Where experimental data are available, DREENA-B framework leads to a good joint agreement with $v_2$ and $R_{AA}$ data. Such agreement is encouraging, i.e. may lead us closer to resolving $v_2$ puzzle (difficulty of previous models to jointly explain $R_{AA}$ and $v_2$ data), though this still remains to be thoroughly tested by including state-of-the-art medium evolution within DREENA framework. While introducing medium evolution significantly changes $v_2$ predictions, $R_{AA}$ predictions remain robust and moreover in a good agreement with the experimental data; $R_{AA}$ observable is therefore suitable for calibrating parton-medium interaction model, independently from the medium evolution. Finally, for heavy flavor, we observe a strikingly similar signature of the dead-cone effect on both $R_{AA}$ and $v_2$ - we also provide a simple analytical understanding behind this result. Overall, the results presented here indicate that DREENA framework is a reliable tool for QGP tomography.

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arXiv2019

Calculating hard probe radiative energy loss beyond soft-gluon approximation: Examining the approximation validity

The soft-gluon approximation, which implies that radiated gluon carries away a small fraction of initial parton's energy, is a commonly used assumption in calculating radiative energy loss of high momentum partons traversing QGP created at RHIC and LHC. While soft-gluon approximation is convenient, different theoretical approaches reported significant radiative energy loss of high $p_{\perp}$ partons, thereby questioning its validity. To address this issue, we relaxed the soft-gluon approximation within DGLV formalism. The obtained analytical expressions are quite distinct compared to the soft-gluon case. However, numerical results for the first order in opacity fractional energy loss lead to small differences in predictions for the two cases. The difference in the predicted number of radiated gluons is also small. Moreover, the effect on these two variables has an opposite sign, which when combined results in almost overlapping suppression predictions. Therefore, our results imply that, contrary to the commonly held doubts, the soft-gluon approximation in practice works surprisingly well in DGLV formalism. Finally, we also discuss generalizing this relaxation in the dynamical QCD medium, which suggests a more general applicability of the conclusions obtained here.

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arXiv2018

How to test path-length dependence in energy loss mechanisms: analysis leading to a new observable

When traversing QCD medium, high $p_\perp$ partons lose energy, which is typically measured by suppression, and also predicted by various energy loss models. A crucial test of different energy loss mechanisms is how the energy loss depends on the length of traversed medium (so-called path-length dependence). The upcoming experimental results will allow to, at least in principle, for the first time, clearly observe how the energy loss changes with the size of the medium, in particular, by comparing already available $Pb+Pb$ measurements with now upcoming $Xe+Xe$ data at the LHC. However, in practice, to actually perform such test, it becomes crucial to chose an optimal observable. With respect to this, a ratio of observed suppression for the two systems may seem a natural (and frequently mentioned) choice. We, however, show that extracting the path-length dependence from this observable would not be possible. We here provide an analytical derivation based on simple scaling arguments, as well as detailed numerical calculations based on our advanced energy loss framework, showing that a different observable is suitable for this purpose. We call this observable path-length sensitive suppression ratio ($R_L^{AB}$) and provide our predictions before experimental data become available. This predictions also clearly show that this observable will allow a simple comparison of the related theoretical models with the experimental data, and consequently to distinguish between different (underlying) energy loss mechanisms, which is in turn crucial for understanding properties of created QCD medium.

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arXiv2018

Joint $R_{AA}$ and $v_2$ predictions for $Pb+Pb$ collisions at the LHC within DREENA-C framework

In this paper, we presented our recently developed DREENA-C framework, which is a fully optimized computational suppression procedure based on our state-of-the-art dynamical energy loss formalism in constant temperature finite size QCD medium. With this framework, we for the first time, generated joint $R_{AA}$ and $v_2$ predictions within our dynamical energy loss formalism. The predictions are generated for both light and heavy flavor probes, and different centrality regions in $Pb+Pb$ collisions at the LHC, and compared with the available experimental data. Despite the fact that DREENA-C does not contain medium evolution (to which $v_2$ is largely sensitive) and the fact that other approaches faced difficulties in explaining $v_2$ data, we find that DREENA-C leads to qualitatively good agreement with this data, though quantitatively, the predictions are visibly above the experimental data. Intuitive explanation behind such results is presented, supporting the validity of our model, and it is expected that introduction of evolution in the ongoing improvement of DREENA framework, will lead to better joint agreement with $R_{AA}$ and $v_2$ data, and allow better understanding of the underlying QCD medium.

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arXiv2017

Complex suppression patterns distinguish between major energy loss effects in Quark-Gluon Plasma

Interactions of high momentum partons with Quark-Gluon Plasma created in relativistic heavy-ion collisions provide an excellent tomography tool for this new form of matter. Recent measurements for charged hadrons and unidentified jets at the LHC show an unexpected flattening of the suppression curves at high momentum, exhibited when either momentum or the collision centrality is changed. Furthermore, a limited data available for B probes indicate a qualitatively different pattern, as nearly the same flattening is exhibited for the curves corresponding to two opposite momentum ranges. We here show that the experimentally measured suppression curves are well reproduced by our theoretical predictions, and that the complex suppression patterns are due to an interplay of collisional, radiative energy loss and the dead-cone effect. Furthermore, for B mesons, we predict that the uniform flattening of the suppression indicated by the limited dataset is in fact valid across the entire span of the momentum ranges, which will be tested by the upcoming experiments. Overall, the study presented here, provides a rare opportunity for pQCD theory to qualitatively distinguish between the major energy loss mechanisms at the same (nonintuitive) dataset.

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arXiv2016

Mass tomography at different momentum ranges in quark-gluon plasma

We here show that at lower momentum (i.e., $p_\perp \sim 10$GeV) single particle suppression for different types of probes exhibit a clear mass hierarchy, which is a direct consequence of the differences in the energy loss, rather than the differences in the initial distributions. On the other hand, we predict that the mass hierarchy is not expected at high momentum (i.e., $p_\perp \sim 100$GeV); i.e., while we surprisingly predict that suppression for charged hadrons will be somewhat smaller than the suppression for heavy mesons, we find that this difference will be a consequence of fragmentation functions, not the finite mass effects. That is, apart from the fragmentation functions, the probes of different masses exhibit nearly the same suppression in the high momentum region. We also argue that the same insensitivity on the probe types also appears for jets. In particular, the experimental data in the momentum regions where they exist for both types of probes, show similar suppressions of charged hadrons and inclusive jet data. Interestingly, we also find that our state-of-the-art suppression predictions for high momentum single particles are also in agreement with the jet suppression data, where the reasons behind this agreement yet remain to be understood. Finally, the available jet data also show (though with large error bars) an overlap between b jets (heavy) and inclusive jets (light) probes. Consequently, our results suggest that single particles in the momentum region below 50 GeV present an excellent tool for mass tomography, while high momentum single particles and (possibly) jets are somewhat insensitive to the details of the interaction with quark-gluon plasma.

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arXiv2015

Heavy flavor suppression predictions at 5.1 TeV Pb+Pb collisions at LHC

High momentum hadron suppression is considered to be an excellent probe of jet-medium interactions in QCD matter created in ultra-relativistic heavy ion collisions. We previously showed that our dynamical energy loss formalism can accurately explain suppression measurements at 200 GeV Au+Au collisions at RHIC and 2.76 TeV Pb+Pb collisions at the LHC. With the upcoming LHC measurements at notably higher collision energies, there is a question of what differences, with respect to the current (2.76 TeV) measurements, can be expected. In this paper we concentrate on heavy flavor suppression at the upcoming 5.1 TeV Pb+Pb collisions energy at the LHC. Naively, one would expect a notably ($\sim 30\%$) larger suppression at 5.1 TeV collision energy, due to estimated (significant) energy loss increase when transitioning from 2.76 to 5.1 TeV. Surprisingly, more detailed calculations predict nearly the same suppression results at these two energies. We show that this unexpected result is due to an interplay of the following two effects, which essentially cancel each other: i) flattening of the initial distributions with increasing collision energies, and ii) significantly slower than naively expected increase in the energy loss. Therefore, the obtained nearly the same suppression provides a clear (qualitative and quantitative) test of our energy loss formalism.

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arXiv2015

Importance of different energy loss effects in jet suppression at RHIC and LHC

Jet suppression is considered to be an excellent probe of QCD matter created in ultra-relativistic heavy ion collisions. Our theoretical predictions of jet suppression, which are based on our recently developed dynamical energy loss formalism, show a robust agreement with various experimental data, which spans across different probes, experiments (RHIC and LHC) and experimental conditions (i.e. all available centrality regions). This formalism includes several key ingredients, such as inclusion of dynamical scattering centers, finite size QCD medium, collisional energy loss, finite magnetic mass and running coupling. While these effects have to be included based on theoretical grounds, it is currently unclear what is their individual importance in accurately interpreting the experimental data, in particular because other approaches to suppression predictions commonly neglect some - or all - of these effects. To address this question, we here study the relative importance of these effects in obtaining accurate suppression predictions for D mesons (clear energy loss probe) at top RHIC and LHC energies. We obtain that several different ingredients are responsible for the accurate predictions, i.e. the robust agreement with the data is a cumulative effect of all the ingredients, though inclusion of the dynamical scattering centers has the largest relative importance.

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arXiv2014

Heavy flavor puzzle at RHIC: analysis of the underlying effects

Suppressions of light and heavy flavor observables are considered to be excellent probes of QCD matter created in ultra-relativistic heavy ion collisions. Suppression predictions of quark and gluon jets appear to suggest a clear hierarchy according to which neutral pions should be more suppressed than D mesons, which in turn should be more suppressed than single electrons. However, joint comparison of neutral pion (light probe) and non-photonic single electron (heavy probe) suppression data at RHIC unexpectedly showed similar jet suppression for these two probes, which presents the well-known heavy flavor puzzle at RHIC. We here analyze which effects are responsible for this unexpected result, by using the dynamical energy loss formalism. We find that the main effect is a surprising reversal in the suppression hierarchy between neutral pions and D mesons, which is due to the deformation of the suppression patterns of light partons by fragmentation functions. Furthermore, we find that, due to the decay functions, the single electron suppression approaches the D meson suppression. Consequently, we propose that these two effects, taken together, provide a clear intuitive explanation of this longstanding puzzle.

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arXiv2014

RHIC and LHC jet suppression in non-central collisions

Understanding properties of QCD matter created in ultra-relativistic heavy-ion collisions is a major goal of RHIC and LHC experiments. An excellent tool to study these properties is jet suppression of light and heavy flavor observables. Utilizing this tool requires accurate suppression predictions for different experiments, probes and experimental conditions, and their unbiased comparison with experimental data. With this goal, we here extend our dynamical energy loss formalism towards generating predictions for non-central collisions; the formalism takes into account both radiative and collisional energy loss, dynamical (as opposed to static) scattering centers, finite magnetic mass, running coupling and uses no free parameters in comparison with experimental data. Specifically, we here generate predictions for all available centrality ranges, for both LHC and RHIC experiments, and for four different probes (charged hadrons, neutral pions, D mesons and non-prompt $J/ψ$). We obtain a very good agreement with all available non-central data, and also generate predictions for suppression measurements that will soon become available. Finally, we discuss implications of the obtained good agreement with experimental data with different medium models that are currently considered.

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arXiv2013

LHC jet suppression of light and heavy flavor observables

Jet suppression of light and heavy flavor observables is considered to be an excellent tool to study the properties of QCD matter created in ultra-relativistic heavy ion collisions. We calculate the suppression patterns of light hadrons, D mesons, non-photonic single electrons and non-prompt $J/ψ$ in Pb+Pb collisions at LHC. We use a theoretical formalism that takes into account finite size {\it dynamical} QCD medium with finite magnetic mass effects and running coupling, which is integrated into a numerical procedure that uses no free parameters in model testing. We obtain a good agreement with the experimental results across different experiments/particle species. Our results show that the developed theoretical formalism can robustly explain suppression data in ultra relativistic heavy ion collisions, which strongly suggests that pQCD in Quark-Gluon Plasma is able to provide a reasonable description of the underlying jet physics at LHC.

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