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doi:10.1016/j.physletb.2016.07.046

Cited by 53 publications

(37 citation statements)

References 41 publications

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“…This Letter presents such a measurement of the FF in high-p T jets azimuthally balanced by a prompt, isolated photon in pp and Pb+Pb collisions at a center-of-mass energy of 5.02 TeV per nucleon pair, using data samples with integrated luminosities of 25 pb −1 and 0.49 nb −1 , respectively. Photon-hadron p T correlations in gold-gold collisions were measured at the Relativistic Heavy Ion Collider [19,20]. A measurement of the γ-tagged jet FF at the LHC compared the FF at detector level with theoretical calculations that parameterize the detector smearing effects [21].Following previous measurements in ATLAS [5,6], the FF for a jet to contain a charged particle with a given p T , η and φ [22] is expressed as D(p T ) = (1/N jet )(dN ch (p T )/dp T ) or D(z) = (1/N jet )(dN ch (z)/dz) where N jet is the total number of jets, N ch is the number of charged particles associated with a jet, and the longitudinal momentum fraction, z, is defined as p T cos (∆R) /p jet T , ∆R = ((η jet − η part ) 2 + (φ jet − φ part ) 2 ) 1/2 .…”

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Comparison of Fragmentation Functions for Jets Dominated by Light Quarks and Gluons from pp and Pb+Pb Collisions in ATLAS

Aaboud¹,

Aad²,

Abbott³

et al. 2019

Phys. Rev. Lett.

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The ATLAS Collaboration Charged-particle fragmentation functions for jets azimuthally balanced by a high-transversemomentum, prompt, isolated photon are measured in 25 pb −1 of pp and 0.49 nb −1 of Pb+Pb collision data at 5.02 TeV per nucleon pair recorded with the ATLAS detector at the Large Hadron Collider. The measurements are compared to predictions of Monte Carlo generators and to measurements of inclusively selected jets. In pp collisions, a different jet fragmentation function in photon-tagged events from that in inclusive jet events arises from the difference in fragmentation between light quarks and gluons. The ratios of the fragmentation functions in Pb+Pb events to that in pp events are used to explore the parton color-charge dependence of jet quenching in the hot medium. In relatively peripheral collisions, fragmentation functions exhibit a similar modification pattern for photon-tagged and inclusive jets. However, photontagged jets are observed to have larger modifications than inclusive jets in central Pb+Pb events.Ultrarelativistic nucleus-nucleus collisions create a quark-gluon plasma, a hot, dense, and long-lived system of deconfined quarks and gluons. The high density of unscreened color charges causes hard-scattered partons with large transverse momentum (p T ) to lose energy as they traverse the medium, a phenomenon referred to as jet quenching. In lead-lead (Pb+Pb) collisions at the Large Hadron Collider (LHC), jet production rates at fixed p T are suppressed relative to proton-proton (pp) collisions [1][2][3][4]. Since the parton shower develops inside the quark-gluon plasma, the momentum distributions of hadrons in the quenched jet are also modified. Measurements of the jet fragmentation function (FF) for inclusively produced jets in Pb+Pb collisions [5-7] exhibit differences from pp collisions. In these measurements, jets are selected by their final-state p T , i.e. after the effects of quenching, which may result in a bias towards jets that have suffered only modest modifications and complicates interpretation of the data [8, 9]. Alternatively, the initial parton p T can be tagged with a particle unaffected by the medium, such as a photon (γ) [10][11][12].The photon approximately balances the parton p T before quenching and thus selects populations of jets in pp and Pb+Pb collisions with identical initial conditions. A jet recoiling against a prompt photon is more likely to be initiated by the showering of a light quark, whereas inclusive jets are mostly initiated by gluons. Thus γ-tagged jets can provide information about how energy loss depends on the color charge of the initiating parton. Finally, the photon selection equally samples all geometric production points, whereas the inclusive selection may be biased towards jets which have lost less energy or were produced near the surface of the medium [13][14][15].Many theoretical models of jet quenching have highlighted the value of γ-tagged jet measurements [16][17][18], inviting systematic comparisons of these with inclusive jet measur...

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confidence: 99%

Comparison of Fragmentation Functions for Jets Dominated by Light Quarks and Gluons from pp and Pb+Pb Collisions in ATLAS

Aaboud¹,

Aad²,

Abbott³

et al. 2019

Phys. Rev. Lett.

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“…PHENIX estimated the average parton transverse momentum k T at RHIC energies in pp [106], and measured the ratio z T = p h T /p γ T , a proxy for the fragmentation function [170]. A similar measurement has been published by STAR in [171]. Various proton PDF sets have been tested for instance by back-to-back isolated photon-jet measurements in pp at √ s =7 TeV by ATLAS [172].…”

Section: Photon-jet Photon-hadron and Photon-photon Correlationsmentioning

confidence: 99%

“…The power of the method is illustrated in Fig. 16, right plot, from [171], where (associated) away-side charged hadron yields are measured as a function of z T = p T assoc /p T trig in pp and Au+Au with high p T direct photon and π 0 triggers. The measured D(z T ) fragmentation functions for photon and π 0 triggers are very similar in pp, but quite different in Au+Au, where the trigger π 0 now comes from a parton which already lost energy in the medium.…”

Section: Photon -Hadron/jet Correlations: Energy Loss In the Mediummentioning

confidence: 99%

“…and, in particular, comparing the high-and low-end of the z T distributions in principle one can discriminate between a picture where the medium substantially modifies the parton shower (modified FF) and one where mostly a single parton carries the energy through the medium and the lost energy shows up only at extremely low energies and angles [171]. 45 It is hard to over-emphasize the importance of treating π 0 (hadrons) and photons simultaneously.…”

Section: Photon -Hadron/jet Correlations: Energy Loss In the Mediummentioning

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Direct real photons in relativistic heavy ion collisions

David

2020

Rep. Prog. Phys.

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Direct real photons are arguably the most versatile tools to study relativistic heavy ion collisions. They are produced, by various mechanisms, during the entire space-time history of the strongly interacting system. Also, being colorless, most the time they escape without further interaction, i.e. they are penetrating probes. This makes them rich in information, but hard to decypher and interpret. This review presents the experimental and theoretical developments related to direct real photons since the 1970s, with a special emphasis on the recently emerged "direct photon puzzle", the simultaneous presence of large yields and strong azimuthal asymmetries of photons in heavy ion collisions, an observation that so far eluded full and coherent explanation. CONTENTS arXiv:1907.08893v1 [nucl-ex] 21 Jul 2019 21. Hydrodynamical models 48 2. Initial state, (fast) thermalization 50 3. Transport calculations 50 4. Other ideas 52 VII. Concluding remarks 54 VIII. Acknowledgements 55 References 56 3 I. INTRODUCTIONThe centuries old quest to reveal the "ultimate" constituents of matter and the laws of Nature governing them, and the realization from quantum mechanics that mapping smaller and smaller objects requires ever larger energy probes, made high energy physics one of the dominant scientific disciplines of the XXth century. At first we took advantage of the Universe as an "accelerator" providing high energy probes (cloud chamber and emulsion experiments), but soon we started to build our own accelerators, constantly increasing their energy and luminosity. The most spectacular and best known results came in elementary particle physics, but astrophysics and nuclear physics were a close second, benefiting enormously from the progress in experimental and theoretical tools. The central question in particle physics was the substructure of hadrons and the nature of confinement, while high energy nuclear physics' foremost concern was the behavior of high density nuclear matter, including its collectivity and possible phase transition into partonic matter, which in turn evoked the early stages of the Universe. Albeit particle and nuclear physics came with different perspectives, both were concerned with the strong interaction and its underlying theory, quantum chromodynamics (QCD), so the birth of a new discipline at their intersection, relativistic heavy ion physics, was almost inevitable. From Princeton and Berkeley to Dubna the race for larger and larger ions, energies and luminosities was on. Hadrons and nuclear fragments were studied extensively, and the applicability of thermo-and hydrodynamics gradually recognized. An excellent review of these first facilities and early developments -both experimental and theoretical -can be found in [1] by Goldhaber, while a (literally) insightful account of the genesis and achievements of RHIC and LHC was given by Baym in [2].Although lepton and photon production in hadronic and nuclear collisions was studied almost since the birth of quantum mechanics and quantum electrodynamics [3][4][...

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