Measurement of the groomed jet radius and momentum splitting fraction in pp and Pb-Pb collisions at 5.02 TeV

This article presents groomed jet substructure measurements in pp and Pb–Pb collisions at √sNN = 5.02 TeV with the ALICE detector. The Soft Drop grooming algorithm provides access to the hard parton splittings inside a jet by removing soft wide-angle radiation. We report the groomed jet mo- mentum splitting fraction, zg, and the (scaled) groomed jet radius, θg. Charged-particle jets are reconstructed at midrapidity using the anti-kT algorithm with resolution parameters R = 0.2 and R = 0.4. In heavy-ion collisions, the large underlying event poses a challenge for the reconstruction of groomed jet observables, since fluctuations in the background can cause groomed parton splittings to be misidentified. By using strong grooming conditions to reduce this background, we report these observables fully corrected for detector effects and background fluctuations for the first time. A nar- rowing of the θg distribution in Pb–Pb collisions compared to pp collisions is seen, which provides direct evidence of the modification of the angular structure of jets in the quark–gluon plasma. No significant modification of the zg distribution in Pb–Pb collisions compared to pp collisions is ob- served. These results are compared with a variety of theoretical models of jet quenching, and provide constraints on jet energy-loss mechanisms and coherence effects in the quark–gluon plasma.

More reading: https://arxiv.org/abs/2107.12984 

RNC Program.

MEASUREMENT OF THE ANGLE BETWEEN JET AXES in PP AND PBPB COLLISIONS - Qualitatively new NEW STYDY of jet quenching

This letter presents the first measurement of the angle between different jet axes (denoted as ∆Raxis) in Pb–Pb collisions. The measurement is carried out in the 0–10% most-central events at √sNN = 5.02 TeV. Jets are assembled by clustering charged particles at midrapidity using the anti-kT algorithm with resolution parameters R = 0.2 and 0.4 and transverse momenta in the intervals 40 < pch jet < 140 GeV/c and 80 < pch jet < 140 GeV/c, respectively. Measurements at these low transverse momenta enhance the sensitivity to quark–gluon plasma (QGP) effects. A comparison to models im- plementing various mechanisms of jet energy loss in the QGP shows that the observed narrowing of the Pb–Pb distribution relative to pp can be explained if quark-initiated jets are more likely to emerge from the medium than gluon-initiated jets. These new measurements discard intra-jet pT broadening as described in a model calculation with the BDMPS formalism as the main mechanism of energy loss in the QGP. The data are sensitive to the angular scale at which the QGP can resolve two independent splittings, favoring mechanisms that incorporate incoherent energy loss.

More reading: https://arxiv.org/abs/2303.13347 and https://arxiv.org/abs/2211.08928 

RNC Program.

Measurement of inclusive and leading subjet fragmentation in pp and PbPb collisions

This article presents new measurements of the fragmentation properties of jets in both proton–proton (pp) and heavy-ion collisions with the ALICE experiment at the Large Hadron Collider (LHC). We report distributions of the fraction zr of transverse momentum carried by subjets of radius r within jets of radius R. Charged-particle jets are reconstructed at midrapidity using the anti-kT algorithm with jet radius R = 0.4, and subjets are reconstructed by reclustering the jet constituents using the anti-kT algorithm with radii r = 0.1 and r = 0.2. In proton–proton collisions, we measure both the inclusive and leading subjet distributions. We compare these measurements to perturbative calcula- tions at next-to-leading logarithmic accuracy, which suggest a large impact of threshold resummation and hadronization effects on the zr distribution. In heavy-ion collisions, we measure the leading sub- jet distributions, which allow access to a region of harder jet fragmentation than has been probed by previous measurements of jet quenching via hadron fragmentation distributions. The zr distributions enable extraction of the parton-to-subjet fragmentation function and allow for tests of the universality of jet fragmentation functions in the quark–gluon plasma (QGP). We find no significant modification of zr distributions in Pb–Pb compared to pp collisions. However, the distributions are also consistent with a hardening trend for zr < 0.95, as predicted by several jet quenching models. As zr → 1 our results indicate that any such hardening effects cease, exposing qualitatively new possibilities to dis- entangle competing jet quenching mechanisms. By comparing our results to theoretical calculations based on an independent extraction of the parton-to-jet fragmentation function, we find consistency with the universality of jet fragmentation and no indication of factorization breaking in the QGP.

More reading: https://arxiv.org/abs/2204.10270 

RNC Program.

Measurements of the groomed jet radius and momentum splitting fraction with the soft drop and dynamical grooming algorithms in pp collisions at 5 TEV

This article presents measurements of the groomed jet radius and momentum splitting fraction in pp collisions at s = 5.02 TeV with the ALICE detector at the Large Hadron Collider. Inclusive charged-particle jets are reconstructed at midrapidity using the anti-kT algorithm for transverse momentum 60 < pch jet < 80 GeV/c. We report results using two different grooming algorithms: soft drop and, for the first time, dynamical grooming. For each grooming algorithm, a variety of grooming settings are used in order to explore the impact of collinear radiation on these jet substructure ob- servables. These results are compared to perturbative calculations that include resummation of large logarithms at all orders in the strong coupling constant. We find good agreement of the theoretical predictions with the data for all grooming settings considered.

More reading: https://arxiv.org/abs/2204.10270 

RNC Program.

Measurements of strange baryons and mesons within jets scheds light on hadronization and collective effects

The production of Λ baryons and K0 mesons (V0 particles) was measured in p–Pb collisions at 5.02 TeV and pp collisions at s = 7 TeV with ALICE at the LHC. The production of these strange particles is studied separately for particles associated with hard scatterings and the underlying event to shed light on the baryon-to-meson ratio enhancement observed at intermediate transverse momentum (pT) in high multiplicity pp and p–Pb collisions. Hard scatterings are selected on an event-by-event basis with jets reconstructed with the anti-kT algorithm using charged particles. The production of strange particles associated with jets pch > 10 and pch > 20 GeV/c in p–Pb T, jet T, jet collisions, and with jet pch > 10 GeV/c in pp collisions is reported as a function of pT. Its de- T, jet pendence on angular distance from the jet axis, R(V0, jet), for jets with pch > 10 GeV/c in p–Pb T, jet collisions is reported as well. The pT-differential production spectra of strange particles associated with jets are found to be harder compared to that in the underlying event and both differ from the inclusive measurements. In events containing a jet, the density of the V0 particles in the underlying event is found to be larger than the density in the minimum bias events. The Λ/K0S ratio associated with jets in p–Pb collisions is consistent with the ratio in pp collisions and follows the expectation of jets fragmenting in vacuum. On the other hand, this ratio within jets is consistently lower than the one obtained in the underlying event and it does not show the characteristic enhancement of baryons at intermediate pT often referred to as “baryon anomaly” in the inclusive measurements.

More reading: https://arxiv.org/abs/2105.04890 and https://arxiv.org/abs/2211.08936  

RNC Program.

Photoproduction of KAON pairs in ultra-peripheral collisions

K+K− pairs may be produced in photonuclear collisions, either from the decays of photoproduced φ(1020) mesons, or directly as non-resonant K+K− pairs. Measurements of K+K− photoproduc- tion probe the couplings between the φ(1020) and charged kaons with photons and nuclear targets. We present the first measurement of coherent photoproduction of K+K− pairs on lead ions in ultra- peripheral collisions using the ALICE detector, including the first invesstigation of direct K+K− production. There is significant K+K− production at low transverse momentum, consistent with coherent photoproduction on lead targets. In the mass range 1.1 < MKK < 1.4 GeV/c2 above the φ(1020) resonance, for rapidity |yKK| < 0.8 and pT,KK < 0.1 GeV/c, the measured coherent pho- toproduction cross section is dσ/dy = 3.37 ± 0.61 (stat.) ± 0.15 (syst.) mb. The centre-of-mass energy per nucleon of the photon–nucleus (Pb) system WγPb,n ranges from 33 to 188 GeV, far higher than previous measurements on heavy-nucleus targets. The cross section is larger than expected for φ (1020) photoproduction alone. The mass spectrum is fit to a cocktail consisting of φ (1020) decays, direct K+K− photoproduction, and interference between the two. The confidence regions for the amplitude and relative phase angle for direct K+K− photoproduction are presented.

More reading: https://arxiv.org/abs/2311.11792 

RNC Program.

Identifying groomed jet splittings in heavy-ion collisions

Measurements of jet substructure in heavy-ion collisions may provide key insight to the nature of jet quenching in the quark-gluon plasma. Jet grooming techniques from high-energy physics have been applied to heavy-ion collisions in order to isolate theoretically controlled jet observables and explore possible modification to the hard substructure of jets. However, the grooming algorithms used have not been tailored to the unique considerations of heavy-ion collisions, in particular to the experimental challenge of reconstructing jets in the presence of a large underlying event. We report a set of simple studies illustrating the impact of the underlying event on identifying groomed jet splittings in heavy-ion collisions, and on associated groomed jet observables. We illustrate the importance of the selection of grooming algorithm, as certain groomers are more robust to these effects, while others, including those commonly used in heavy-ion collisions, are susceptible to large background effects -- which, when uncontrolled, can mimic a jet quenching signal. These experimental considerations, along with appropriate theoretical motivation, provide input to the choice of grooming algorithms employed in heavy-ion collisions.

More reading: 

https://arxiv.org/abs/2006.01812 

RNC Program.

Searching for the dead cone effects with iterative declustering of heavy-flavor jets

We present a new method to expose the dead cone effect at colliders using iterative declustering techniques. Iterative declustering allows to unwind the jet clustering and to access the subjets or branches at different depths of the jet tree. Our method consists of declustering the heavy flavour- tagged jet using Cambridge-Achen algorithm following the branch containing the heavy flavour at each step and registering the kinematics of the complementary untagged prong. The kinematics of the complementary untagged prong fill a Lund map representing the gluon radiation off the heavy flavour quark at each step of the vacuum shower.

Using Pythia8 MC, we show that a simple cut on the Lund plane introduced by ln(kT) > 0, suppresses hadronisation effects and the angular separation between the jet prongs becomes very sensitive to flavour effects. A clear suppression for heavy flavour jets relative to inclusive jets in the region of splitting angles delimited by the relation θ < mQ/E is observed, where mQ is the mass of the heavy quark and E is the energy of the radiator or splitting prong.

More reading: 

https://arxiv.org/abs/1812.00102 

First measurement by ALICE - Nature 

RNC Program.

Explainable machine learning of the underlying physics of high-energy particle collisions

ML Generative Adversarial Network learns internal workings of QCD - the Altarelli-Parisi splitting function - using information encoded in final state particle distributions. We present an implementation of an explainable and physics-aware machine learning model capable of inferring the underlying physics of high-energy particle collisions using the information encoded in the energy-momentum four-vectors of the final state particles. We demonstrate the proof-of-concept of our White Box AI approach using a Generative Adversarial Network (GAN) which learns from a DGLAP-based parton shower Monte Carlo event generator. We show, for the first time, that our approach leads to a network that is able to learn not only the final distribution of particles, but also the underlying parton branching mechanism, i.e. the Altarelli-Parisi splitting function, the ordering variable of the shower, and the scaling behavior. 

More reading: https://arxiv.org/abs/2012.06582 

RNC Program. Work supported by LBNL's LDRD program.

The information content of jet quenching and machine learning assisted observable design

ML guides future directions of the experimental program focused on the understanding microscopic structure of the quark-gluon plasma created in heavy-ion collisions at collider experiments (RHIC and LHC).  Jets produced in high-energy heavy-ion collisions are modified compared to those in proton-proton collisions due to their interaction with the deconfined, strongly-coupled quark-gluon plasma (QGP). In this work, we employ machine learning techniques to identify important features that distinguish jets produced in heavy-ion collisions from jets produced in proton-proton collisions. We formulate the problem using binary classification and focus on leveraging machine learning in ways that inform theoretical calculations of jet modification.

More reading: https://arxiv.org/abs/2111.14589 

RNC Program. Work supported by LBNL's LDRD program.

Machine learning-based jet and event classification at the Electron-Ion Collider with applications to hadron structure and spin physics

ML guides future directions of the experimental program accessible with the new Electron-Ion Collider at Brookhaven National Laboratory.  We explore machine learning-based jet and event identification at the future Electron-Ion Collider (EIC). We study the effectiveness of machine learning-based classifiers at relatively low EIC energies, focusing on (i) identifying the flavor of the jet and (ii) identifying the underlying hard process of the event. We propose applications of our machine learning-based jet identification in the key research areas at the future EIC and current Relativistic Heavy Ion Collider program, including enhancing constraints on (transverse momentum dependent) parton distribution functions, improving experimental access to transverse spin asymmetries, studying photon structure, and quantifying the modification of hadrons and jets in the cold nuclear matter environment in electron-nucleus collisions.

More reading: https://arxiv.org/abs/2210.06450 

RNC & NSD-Theory programs within NSD.

Is infrared-collinear safe information all you need for jet classification?

Next level of exploration of effects of hot and dense quark-gluon plasma on jets propagating within its volume - new insight for experimental program at RHIC and the LHC. Machine learning-based jet classifiers are able to achieve impressive tagging performance in a variety of applications in high energy and nuclear physics. However, it remains unclear in many cases which aspects of jets give rise to this discriminating power, and whether jet observables that are tractable in perturbative QCD such as those obeying infrared-collinear (IRC) safety serve as sufficient inputs. In this article, we introduce a new classifier, Jet Flow Networks (JFNs), in an effort to address the question of whether IRC unsafe information provides additional discriminating power in jet classification.

More reading: https://arxiv.org/abs/2305.08979 

RNC Program. Work supported by LBNL's LDRD program.

Quantum simulation of non-equilibrium dynamics and thermalization in the Schwinger model

We present simulations of non-equilibrium dynamics of quantum field theories on digital quantum computers. As a representative example, we consider the Schwinger model, a 1+1 dimensional U(1) gauge theory, coupled through a Yukawa-type interaction to a thermal environment described by a scalar field theory. We use the Hamiltonian formulation of the Schwinger model discretized on a spatial lattice. With the thermal scalar fields traced out, the Schwinger model can be treated as an open quantum system and its real-time dynamics are governed by a Lindblad equation in the Markovian limit. The interaction with the environment ultimately drives the system to thermal equilibrium. In the quantum Brownian motion limit, the Lindblad equation is related to a field theoretical Caldeira-Leggett equation. By using the Stinespring dilation theorem with ancillary qubits, we perform studies of both the non-equilibrium dynamics and the preparation of a thermal state in the Schwinger model using IBM's simulator and quantum devices. The real-time dynamics of field theories as open quantum systems and the thermal state preparation studied here are relevant for a variety of applications in nuclear and particle physics, quantum information and cosmology.

More reading: https://arxiv.org/abs/2106.08394 

RNC Program.

Quantum simulation of open quantum systems in heavy-ion collisions

We present a framework to simulate the dynamics of hard probes such as heavy quarks or jets in a hot, strongly-coupled quark-gluon plasma (QGP) on a quantum computer. Hard probes in the QGP can be treated as open quantum systems governed in the Markovian limit by the Lindblad equation. However, due to large computational costs, most current phenomenological calculations of hard probes evolving in the QGP use semiclassical approximations of the quantum evolution. Quantum computation can mitigate these costs, and offers the potential for a fully quantum treatment with exponential speedup over classical techniques. We report a simplified demonstration of our framework on IBM Q quantum devices, and apply the Random Identity Insertion Method (RIIM) to account for CNOT depolarization noise, in addition to measurement error mitigation. Our work demonstrates the feasibility of simulating open quantum systems on current and near-term quantum devices, which is of broad relevance to applications in nuclear physics, quantum information, and other fields.

More reading: https://arxiv.org/abs/2010.03571 

RNC Program.