arXiv subjects

Alice Ohlson

Publications and source records attributed to Alice Ohlson.

15 recordsLinked to original sources

arXiv2026

SMARTHEP: training PhD students in real-time analysis at the LHC and in industry

In this invited Editorial for Software and Computing for Big Science, we describe the SMARTHEP Innovative Training Network funded via the Marie Sk{\l}odowska-Curie Actions between 2021 and 2025. SMARTHEP trained 12 PhD students to advance machine learning and real-time analysis in high-energy physics experiments and industrial applications. We present the perspective of students, supervisors, and external observers of the network, concerning the work done within the network, the added value compared to ``typical'' PhD positions, and the emerging themes and directions from our experiences in the past four years.

physics.ed-ph↗

arXiv2025

Few is different: deciphering many-body dynamics in mesoscopic quantum gases

Emergent macroscopic descriptions of matter, such as hydrodynamics, are central to our description of complex physical systems across a wide spectrum of energy scales. The conventional understanding of these many-body phenomena has recently been shaken by a number of experimental findings. Collective behavior of matter has been observed in \emph{mesoscopic} systems, such as high-energy hadron-hadron collisions, or ultra-cold gases with only few strongly interacting fermions. In such systems, the separation of scales between macroscopic and microscopic dynamics (at the heart of any effective theory) is inapplicable. To address the conceptual challenges that arise from these observations and explore the universality of emergent descriptions of matter, the EMMI Rapid Reaction Task Force was assembled. This document summarizes the RRTF discussions on recent theoretical and experimental advances in this rapidly developing field. Leveraging technological breakthroughs in the control of quantum systems, we can now quantitatively explore what it means for a system to exhibit behavior beyond the sum of its individual parts. In particular, the report highlights how the (in)applicability of hydrodynamics and other effective theories can be probed across three principal frontiers: the size frontier, the equilibrium frontier, and the interaction frontier.

cond-mat.quant-gas↗

arXiv2021

Probing the Gluon Plasma with Charm Balance Functions

Recent theoretical explanations for how hydrodynamic-like flow can build up quickly in small collision systems (hydrodynamization) has led to a microscopic picture of flow building up in a gluon-dominated phase before chemical equilibrium between quarks and gluons has been attained. The goal of this contribution to Offshell-2021 is to explore consequence of assuming a long-lived gluon-dominated phase, which we shall denote a gluon plasma (GP). As these consequences are naturally enhanced in a large systems, we assume and explore the extreme scenario in which a GP would be created in AA collisions and exist for significant time before the formation of a chemically-equilibrated quark-gluon plasma (QGP). The GP and its formation would be impossible to probe with light-quark hadrons, which are first produced later in this scenario. As charm quarks are produced early in the collision, they can circumvent the limitations of light quarks and we propose charm balance functions as an effective tool to test this idea and constrain the dynamics of the GP.

hep-ph↗

arXiv2019

Measurements of the fluctuations of identified particles in ALICE at the LHC

The event-by-event fluctuations of identified particles in ultrarelativistic nucleus-nucleus collisions give information about the state of matter created in these collisions as well as the phase diagram of nuclear matter. In this proceedings, we present the latest results from ALICE on the centrality and pseudorapidity dependence of net-proton fluctuations, which are closely related to net-baryon fluctuations, as well as net-kaon and net-pion fluctuations. The effects of volume fluctuations and global baryon conservation on these observables are discussed. Furthermore, the correlated fluctuations between different particle species, quantified by the observable $ν_{dyn}$, are also shown as functions of multiplicity and collision energy and are compared with Monte Carlo models. These measurements are performed in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 2.76$ TeV using the novel Identity Method and take advantage of the excellent particle identification capabilities of ALICE.

nucl-ex↗

arXiv2019

Ridges in p-A (and pp) collisions

Correlations between particles separated by several units of pseudorapidity were discovered in high-multiplicity pp and p-Pb collisions at the LHC. These long-range structures observed in two-particle correlation functions are reminiscent of features seen in Pb-Pb collisions, where they are often viewed as a signature of collective behavior and the formation of a quark-gluon plasma (QGP). Therefore, the discovery of these `ridges' in small systems has implications for the study of collectivity in small systems as well as in heavy-ion collisions. The ridges in pp and p-Pb collisions have been studied in the ALICE, ATLAS, CMS, and LHCb experiments to characterize the $p_{\rm{T}}$-, $η$-, and multiplicity-dependences of the ridge yield, as well as its particle composition.

nucl-ex↗

arXiv2019

Collective behavior in small systems

The presence of correlations between particles significantly separated in pseudorapidity in proton-proton and proton-nucleus collisions has raised questions about whether collective effects are observed in small collision systems as well as in heavy-ion collisions. The quantification of these long-range correlations by $v_n$ coefficients is of particular interest. A selection of the latest $v_n$ measurements is presented, including results from the recent $d$+Au beam energy scan at RHIC where a significant non-zero $v_2$ is measured down to low center-of-mass energies ($\sqrt{s_{\rm{NN}}}$ = 39 GeV). Results from a collision system scan - comprising $p$+Au, $d$+Au, and $^3$He+Au collisions - are also shown to address the role of the initial nuclear geometry in the final state anisotropy. Finally, the challenge of measuring multi-particle cumulants, particularly $c_2\{4\}$, in $p$+$p$ collisions is discussed, and new methods for reducing the effects of non-flow are shown to produce a more robust measurement of $v_2\{4\}$ in $p$+$p$ collisions.

nucl-ex↗

arXiv2019

Investigating correlated fluctuations of conserved charges with net-$Λ$ fluctuations in Pb-Pb collisions at ALICE

Event-by-event fluctuations of conserved charges - such as electric charge, strangeness, and baryon number - in ultrarelativistic heavy-ion collisions provide insight into the properties of the quark-gluon plasma and the QCD phase diagram. They can be related to the higher moments of the multiplicity distributions of identified particles, such as the $Λ$ baryon which carries both strangeness and baryon number and is thus of particular interest. We present the first measurement of net-$Λ$ fluctuations in Pb-Pb collisions at $\sqrt{s_{\mathrm{NN}}} = 5.02$ TeV as a function of centrality and the pseudorapidity acceptance of the measurement. The results are compared to expectations of the effects of global baryon number conservation as well as to predictions from the HIJING Monte Carlo event generator. In this analysis the Identity Method is applied in a novel way to account for the combinatoric background in the invariant mass distribution.

nucl-ex↗

arXiv2019

Correlations, multiplicity distributions, and the ridge in pp and p-Pb collisions

Measurements made by the ALICE Collaboration of single- and two-particle distributions in high-energy pp and p-Pb collisions are used to characterize the interactions in small collision systems, tune models of particle production in QCD, and serve as a baseline for heavy-ion observables. The measurements of charged-particle multiplicity density, $\langle dN_{ch}/dη\rangle$, and multiplicity distributions are shown in pp and p-Pb collisions, including data from the top center-of-mass energy achieved at the Large Hadron Collider (LHC), $\sqrt{s}$ = 13 TeV. Two-particle angular correlations in p-Pb collisions are studied in detail to investigate long-range correlations in pseudorapidity which are reminiscent of structures previously thought unique to heavy-ion collisions.

nucl-ex↗

arXiv2018

Differential Correlation Measurements with the Identity Method

We present an extension of the identity method initially introduced for particle yield fluctuation studies towards measurements of differential correlations. The extension is developed and illustrated in the context of measurements of the normalized two-particle cumulant $R_2$ but is adaptable to any correlation measurements, including differential flow measurements. The identity method is also extended to account for an arbitrary number of particle identification devices and signals.

physics.data-an↗

arXiv2015

Recent results on two-particle correlations in ALICE

The discovery of correlations between particles separated by several units of pseudorapidity in high-multiplicity pp and p-Pb collisions, reminiscent of structures observed in Pb-Pb collisions, was a challenge to traditional ideas about collectivity in heavy ion collisions. In order to further explore long-range correlations and provide information to theoretical models, correlations between forward trigger muons and mid-rapidity associated hadrons were measured in p-Pb collisions at $\sqrt{s_{\mbox{NN}}} = 5.02~\mbox{TeV}$. The results demonstrate that the nearside and awayside ridges extend to $Δη\sim \pm 5$ and that the $v_2$ of muons, obtained from subtracting the correlation functions in high- and low-multiplicity events, is $(16\pm6)\%$ higher in the Pb-going than in the p-going direction. The results are compared with AMPT simulations.

nucl-ex↗

arXiv2013

Calculating Jet $v_n$ and the Event Plane in the Presence of a Jet

Advances in measurements of jets and collective phenomena in ultrarelativistic heavy ion collisions have led to further understanding of the properties of the medium created in such collisions. Measurements of the correlations between the axes of reconstructed jets and the reaction plane or second-order participant plane of the bulk medium (defined as jet $v_2$), as well as the higher-order participant planes (jet $v_n$), provide information on medium-induced parton energy loss. Additionally, knowledge of jet $v_n$ as well as the ability to reconstruct the event plane in the presence of a jet are necessary in analyses of jet-triggered particle correlations, which are used to study medium-induced jet shape modification. However, the presence of a jet can bias the event plane calculation, leading to an overestimation of jet $v_2$. This paper proposes a method for calculating jet $v_2$ (and by extension, the higher jet $v_n$ harmonics) and the event plane in an unbiased way, using knowledge of the azimuthal angle of the jet axis from full jet reconstruction.

nucl-ex↗

arXiv2012

Measurements of the Correlation between Reconstructed Jets and the Reaction Plane in STAR at RHIC

The relationship between jet properties and the underlying geometry of the medium produced in heavy ion collisions can be explored through a measurement of the correlation between the axes of reconstructed jets and the reaction plane, defined as "jet v2." Such a measurement provides information on the pathlength dependence of medium-induced parton energy loss and may also be used to assess biases in jet-finding methods. We present first measurements of jet v2 in sqrt(sNN) = 200 GeV Au+Au collisions in the STAR experiment at RHIC. In order to reduce the artificial jet - event plane bias, which results from jet fragments being included in the event plane calculation, detectors at forward pseudorapidity are used to determine the event plane when measuring the v2 of reconstructed jets at mid-rapidity. These measurements demonstrate a non-zero jet v2, which is indicative of pathlength-dependent parton energy loss.

nucl-ex↗

arXiv2012

Jets and Jet-like correlations in STAR

The propagation and modification of hard-scattered partons in the QGP can be studied using various types of jet and jet-like correlation measurements. The STAR detector with its full azimuthal and large pseudorapidity acceptance, as well as its wide transverse momentum (pT) coverage, is well-suited for these measurements. At mid-rapidity, azimuthal correlations of charged hadrons with the axis of a reconstructed trigger jet are used to study the modification of jet shapes and associated hadron yields from p+p to Au+Au. Dihadron correlations with back-to-back high-pT hadron pairs are used to investigate dijets and fragmentation biases. STAR's increased particle identification capabilities due to the Time-Of-Flight detector are utilized to investigate the differences between jet-related and bulk-related particle production. Dihadron correlations with identified trigger particles provide experimental tests of simple recombination theories. The comprehensive set of STAR jet-quenching measurements can be used to further constrain theories of parton energy loss at RHIC.

nucl-ex↗

arXiv2011

Jet-hadron correlations in STAR

Advancements in full jet reconstruction have made it possible to use jets as triggers in azimuthal angular correlations to study the modification of hard-scattered partons in the medium created in ultrarelativistic heavy-ion collisions. This increases the range of parton energies accessible in these analyses and improves the signal-to-background ratio compared to dihadron correlations. Results of a systematic study of jet-hadron correlations in central Au-Au collisions at sqrt(s_NN) = 200 GeV are indicative of a broadening and softening of jets which interact with the medium. Furthermore, jet-hadron correlations suggest that the suppression of the associated hadron yield at high-pT is balanced in large part by low-pT enhancement.

nucl-ex↗

arXiv2011

Jet-hadron correlations in STAR

In recent years, the study of dihadron correlations has been one of the primary methods used to investigate the propagation and modification of hard-scattered partons through the QGP. Due to recent advances in jet-finding algorithms, it is now possible to use reconstructed jets in these correlation studies, extending the kinematic reach compared to dihadron analyses. The results of the jet-hadron correlation analysis indicate a broadening and softening of jets that interact with the medium. Jet-hadron correlations can also be used to assess the systematics of other jet-like correlation analyses, such as 2+1 correlations. It is shown that the jets selected in 2+1 correlations are relatively unmodified. Future work will include an analysis of jet-hadron correlations with respect to the event plane to measure the pathlength dependence of parton energy loss. The first steps in this analysis indicate that complications arise when calculating the event plane in the presence of a jet as well as in calculating jet v2. The data analyzed were collected by the STAR detector in sqrt(s_NN) = 200 GeV Au-Au collisions at the Relativistic Heavy Ion Collider (RHIC).

nucl-ex↗