Hard Probes Research Articles

Multipole scattering amplitudes in the color glass condensate formalism

We evaluate the octupole in the large-$N_c$ limit in the McLerran-Venugopalan model, and derive a general expression of the 2n-point correlator, which can be applied in analytical studies of the multi-particle production in the scatterings between hard probes and dense targets.

Revealing the source of the radial flow patterns in proton–proton collisions using hard probes

In this work, we propose a tool to reveal the origin of the collective-like phenomena observed in proton–proton collisions. We exploit the fundamental difference between the underlying mechanisms, color reconnection and hydrodynamics, which produce radial flow patterns in Pythia 8 and Epos 3, respectively. Specifically, we proceed by examining the strength of the coupling between the soft and hard components which, by construction, is larger in Pythia 8 than in Epos 3. We study the transverse momentum () distributions of charged pions, kaons and (anti) protons in inelastic pp collisions at TeV produced at mid-rapidity. Specific selections are made on an event-by-event basis as a function of the charged particle multiplicity and the transverse momentum of the leading jet () reconstructed using the FastJet algorithm at mid-pseudorapidity (). From our studies, quantitative and qualitative differences between Pythia 8 and Epos 3 are found in the spectra when (for a given multiplicity class) the leading jet is increased. In addition, we show that for low-multiplicity events the presence of jets can produce radial flow-like behavior. Motivated by our findings, we propose to perform a similar analysis using experimental data from RHIC and LHC.

10th International Workshop on High-pT Physics in the RHIC/LHC Era

PrefaceThis volume contains selected papers presented at the 10th International Workshop on High-pT Physics in the RHIC/LHC Era. The workshop was held from 9-12 September 2014 at Subatech Nantes, France.The rst workshop of this series was organized 2006 in Trento, Italy. The purpose of the 10th workshop was to offer an opportunity for experimentalists and theoreticians to get together and discuss experimental results from RHIC and LHC - before the start of its secound run - and theoretical developments. Focus was also given to high-pT physics in pA collisions especially at the LHC energies. Emphasis was given on discussion over a large scope : high-pT physics, jets, photons, correlations, hard scattering and hard probes phenomena.The main topics of the workshop were:• Nuclear modi cations of the parton distribution functions• High pT jet production in pp, pA and AA• High pT parton propagation in matter• Nuclear modi cations of the fragmentation functions• Correlations with leading particles• Direct photon and heavy avor tagging• Heavy avors and Quarkonia• Multi parton interactionsAbout 45 participants coming from 10 countries participated in the workshop. 8 invited talks and 21 oral contributions were presented at the conference.

Recent hard probe measurements from STAR

High momentum transfer processes can be reliably calculated by perturbative quantum chromodynamics (pQCD). Products of those hard processes, high transverse momentum particles, jets, and heavy quarks (hard probes), interact with the quark-gluon plasma (QGP) created in relativistic heavy-ion collisions. Properties of the QGP can be studied by comparing hard probe measurements in heavy-ion collisions to those in p+p collisions or from pQCD calculations. I will present recent results from STAR on hard probe measurements, including a wide range of observables: heavy flavor, dijet transverse momentum imbalance, hadron-jet correlations, γ -jet correlations, and dihadron correlations. I will discuss what we have learned from these measurements.

Compton Scattering Measurement to Detect Momentum Distribution of Electrons in Warm Dense Matter

Detailed study of electrons in warm dense matter is a key to understanding material properties. Up to now there are many methods to detect electron spectral features with optical lasers and x-rays. Here we propose and demonstrate a new measurement method using inelastic Compton scattering of hard x-ray probe radiation. By using a high energy photon beam, above 100 keV, the change of the electron momentum distribution is observed through a change of the scattered x-ray spectrum even in a solid density plasmas.

Recent Heavy-Flavor Results Utilizing the FVTX in PHENIX at RHIC

Heavy-flavor and quarkona production are important hard probes to test Quantum Chromodynamics (QCD) and measure the properties of the Quark Gluon Plasma (QGP) created in high energy heavy ion collisions. The new PHENIX mid- and forward-rapidity silicon vertex detectors provide precise Distance of Closest Approach (DCA) determination, allowing charm and bottom yields identi cation, and improve di-muon mass resolution for charmonia studies. The relative ψ(2S) over J/ψ (1S) yields measured in p+p, p+Al and p+Au data at help us to understand nal state effects on the charmonia production. Direct access to B-meson production has been enabled via measurements of non-prompt J/ψ → µ+ + µ− in p+p collisions at and Cu+Au collisions at . The fraction of B-meson decays in J/ψ yields in the p+p measurement agrees with higher energy results from Tevatron and the LHC. This fraction is enhanced in Cu+Au collisions suggesting the B-meson yield is not modi ed by the medium whereas J/ψ is strongly suppressed.

Warriors of Word and Sword

The Battle of Okinawa was the last major ground battle of World War ii. The Tenth u.s. Army that invaded this small piece of Japan was a unique force composed of units from the u.s. Army and others from the u.s. Marine Corps. Much historical literature has focused on the different approaches to ground combat of the two armed services, but they also employed very different policies towards support of the news media. The u.s. Marines were much more supportive than the u.s. Army. The two different policies and styles of news coverage that reporters employed led to coverage favoring the u.s. Marines. Reporting suggested that u.s. Marine procedures were less costly in lives and created enormous concern in the United States about casualty rates, motivating President Harry S. Truman to hold an Oval Office meeting to re-think strategy in the Pacific theater. It would be wrong, though, to argue that the media altered the course of the war. Truman asked hard probing questions about the direction of the war, but General of the Army George C. Marshall acted to ensure that the United States stayed on its current strategic path.

An overview of experimental results from ultra-relativistic heavy-ion collisions at the CERN LHC: Hard probes

The first collisions of lead nuclei, delivered by the CERN Large Hadron Collider (LHC) at the end of 2010, at a centre-of-mass energy per nucleon pair $\sqrt{s_{NN}}$ = 2.76 TeV, marked the beginning of a new era in ultra-relativistic heavy-ion physics. The study of the properties of the produced hot and dense strongly-interacting matter at these unprecedented energies is currently experimentally pursued by all four big LHC experiments, ALICE, ATLAS, CMS, and LHCb. The more than a factor 10 increase of collision energy at LHC, relative to the previously achieved maximal energy at other collider facilities, results in an increase of production rates of hard probes. This review presents selected experimental results focusing on observables probing hard processes in heavy-ion collisions delivered during the first three years of the LHC operation. It also presents the first results from Run 2 heavy-ion data at the highest energy, as well as from the studies of the reference pp and pPb systems, which are an integral part of the heavy-ion programme.

Nuclear collisions at the Future Circular Collider

The Future Circular Collider is a new proposed collider at CERN with centre-of-mass energies around 100 TeV in the pp mode. Ongoing studies aim at assessing its physics potential and technical feasibility. Here we focus on updates in physics opportunities accessible in pA and AA collisions not covered in previous Quark Matter contributions, including Quark-Gluon Plasma and gluon saturation studies, novel hard probes of QCD matter, and photon-induced collisions.

In-medium jet evolution: interplay between broadening and decoherence effects: The XXVth International Conference on Ultrarelativistic Nucleus-Nucleus Collisions

The description of the modifications of the coherence pattern in a parton shower, in the presence of a QGP, has been actively addressed in recent studies. Among the several achievements, finite energy corrections, transverse momentum broadening due to medium interactions and interference effects between successive emissions have been extensively improved as they seem to be essential features for a correct description of the results obtained in heavy-ion collisions. In this work, based on the insights of our previous work [L. Apolinário, N. Armesto, J. G. Milhano, C. A. Salgado, Medium-induced gluon radiation and colour decoherence beyond the soft approximation, JHEP 1502 (2015) 119. arXiv:1407.0599], we explore the physical interplay between broadening and decoherence, by generalising previous studies of medium-modifications of the antenna spectrum [Y. Mehtar-Tani, C. A. Salgado, K. Tywoniuk, Antiangular Ordering of Gluon Radiation in QCD Media, Phys. Rev. Lett. 106 (2011) 122002. arXiv:1009.2965, J. Casalderrey-Solana, E. Iancu, Interference effects in medium-induced gluon radiation, JHEP 08 (2011) 015. arXiv:1105.1760, Y. Mehtar-Tani, C. A. Salgado, K. Tywoniuk, The Radiation pattern of a QCD antenna in a dense medium, JHEP 10 (2012) 197. arXiv:1205.5739] – so far restricted to the case where transverse motion is neglected. The result allow us to identify two quantities controlling the decoherence of a medium modified shower that can be used as building blocks for a successful future generation of jet quenching Monte Carlo simulators: a generalisation of the Δmed parameter of the works of [Y. Mehtar-Tani, C. A. Salgado, K. Tywoniuk, Antiangular Ordering of Gluon Radiation in QCD Media, Phys. Rev. Lett. 106 (2011) 122002. arXiv:1009.2965, Y. Mehtar-Tani, C. A. Salgado, K. Tywoniuk, The Radiation pattern of a QCD antenna in a dense medium, JHEP 10 (2012) 197. arXiv:1205.5739] – that controls the interplay between the transverse scale of the hard probe and the transverse resolution of the medium – and of the Δcoh in [L. Apolinário, N. Armesto, J. G. Milhano, C. A. Salgado, Medium-induced gluon radiation and colour decoherence beyond the soft approximation, JHEP 1502 (2015) 119. arXiv:1407.0599] – that dictates the interferences between two emitters as a function of the transverse momentum broadening acquired by multiple scatterings with the medium.

W and Z bosons with CMS in pp, pPb and PbPb collisions

Electroweak boson production is an important benchmark process in high-energy heavy-ion collisions at the LHC. W and Z bosons do not participate in the strong interaction and their leptonic decays provide medium-blind probes of the initial state of the collisions. The final results on the W and Z production in pPb collisions at 5.02 TeV, combining both the muon and electron channels, will be presented. When compared to theory calculations that include nuclear modifications to the parton distributions, data show a clear sensitivity to this type of effects. The final results in PbPb collisions at 2.76 TeV, compared to pp collisions at the same centre of mass energy, will also be presented. The centrality dependence confirms the binary scaling of hard probes in heavy-ion collisions, while the differential cross sections points to initial state effects small compared to the statistical precision of the available data.

Nuclear effects in the diffraction scattering of protons by nuclei at high energies

Processes of the diffraction scattering of protons on nuclei in the context of developing the HARDPING (Hard Probe Interaction Generator) Monte Carlo event generator are considered with allowance for nuclear effects in the initial and final states. Effects such as the length of formation, energy losses, and the multiple rescattering of incident and produced hadrons are studied.

Ultrafast dynamics of localized magnetic moments in the unconventional Mott insulator Sr2IrO4

We report a time-resolved study of the ultrafast dynamics of the magnetic moments formed by the states in Sr2IrO4 by directly probing the localized iridium 5d magnetic state through resonant x-ray diffraction. Using optical pump–hard x-ray probe measurements, two relaxation time scales were determined: a fast fluence-independent relaxation is found to take place on a time scale of 1.5 ps, followed by a slower relaxation on a time scale of 500 ps–1.5 ns.

Jet fragmentation in p + Pb and Pb + Pb with ATLAS

The modification of jets in heavy ion collisions is a central tool to study the properties of QCD matter at high temperature and its interaction with hard probes. Jet production in heavy ion collisions is observed to be suppressed relative to expectations from pp collisions and the measurement of the longitudinal fragmentation functions in Pb+Pb can provide insight into mechanism of the modification of the parton showering by the QCD medium. In proton-nucleus collisions, a large amount of hot QCD matter is not expected to be made and no evidence of jet suppression is observed. However, an excess is observed in the production of high momentum charged particles in p+Pb collisions relative to pp collisions. The p+Pb fragmentation functions can provide insight into the high momentum charged particle excess and into a possible influence of Pb+Pb jet fragmentation by effects due to the nuclear initial state. The ATLAS measurements of charged particle fragmentation functions in pp, p+Pb, and Pb+Pb collisions are presented.

Hard probes of small systems

This proceedings discusses the experimental status of high-pT jet, hadron, and electroweak boson production measurements in proton- and deuteron–nucleus (p/d+A) collisions at the Relativistic Heavy Ion Collider (RHIC) and the Large Hadron Collider (LHC), with an emphasis on results presented for the first time at the Hard and Electromagnetic Probes conference at McGill University in June 2015.

Hard Probes: After the dust has settled!

A lot has been learnt in the 15 years since the first data on jet modification at the Relativistic Heavy Ion Collider (RHIC). This proceeding will describe the portion of the theory that is unassailable, and attempt to chart a course for the next period of precision measurements and related calculations. In particular, we will focus on the emerging series of higher order calculations, which may reveal the scale, energy and temperature dependence of jet transport coefficients, as well as develop ingredients for a future NLO calculation of jet modification. Connections with the underlying degrees of freedom in the Quark Gluon Plasma (QGP) will be explored. We will discuss the challenges faced by Monte-Carlo event generators and their comparison with a broad spectrum of jet modification data. First principles calculations of transport coefficients, and the determination of new transport coefficients which encapsulate the transition of energy momentum from the jet scale to the medium scale will also be discussed.

Initial state in heavy ion collisions

We briefly review advances in understanding the initial stages of a heavy ion collision. In particular the focus is on moving from parametrizing the initial state to calculating its properties from QCD, consistently with the description of hard probes and dilute-dense scattering experiments. Modeling the event-by-event fluctuating nuclear geometry in initial state calculations has significantly improved in recent years. We also discuss prospects of directly seeing effects of particle correlations created in the initial state in the experimental observables.

STAR Overview of Hard Probe Observables

Parton energy loss, quarkonium sequential melting and particle production from electromagnetic interactions are tools to study Quark Gluon Plasma properties. The STAR detector, with large acceptance at mid-rapidity, excellent particle identification and wide transverse momentum coverage, is able to study these probes in details. In Hard Probes 2015, the STAR collaboration reported measurements of reconstructed jets, heavy-flavor physics, di-lepton production and the performance of new detectors in seven presentations and one poster. Given the rich results from STAR, this overview report will focus on a few selected results on jets and ϒ measurements in Au+Au collisions at sNN=200 GeV, J/ψ production in p+p collisions at sNN=500 GeV, and the di-electron spectrum in the low mass region from the Beam Energy Scan – Phase I.

Theory @ Hard Probes 2015

Overview of the latest theory developments presented at the Hard Probes 2015 conference, held at McGill University, Montreal, Canada, in July 2015.

Experiment summary

The measurement of the production of particles coming from hard scattering processes covers a fundamental role in the characterization of the system formed in heavy-ion collisions, allowing to probe the microscopic processes underlying the interaction of high energy partons with the medium. An impressive amount of measurements related to jet, quarkonia, open heavy flavor, and electroweak signal production in nucleus-nucleus as well as p(d)-nucleus collisions was delivered by experiments at RHIC and LHC in past years. In these proceedings, the main experimental results presented during the Hard Probes conference are summarized.