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Title: How do quarks and gluons lose energy in the QGP?

Abstract

RHIC introduced the method of hard scattering of partons as an in-situ probe of the the medium produced in A+A collisions. A suppression, R AA ≈ 0.2 relative to binary-scaling, was discovered for π⁰ production in the range 5 < ρ T < 20 GeV/c in central Au+Au collisions at √s NN = 200 GeV, and surprisingly also for single-electrons from the decay of heavy quarks. Both these results have been confirmed in Pb+Pb collisions at the LHC at √s NN = 2.76 TeV. Interestingly, in this ρ T range the LHC results for pions nearly overlap the RHIC results. Thus, due to the flatter spectrum, the energy loss in the medium at LHC in this ρ T range must be ~ 40% larger than at RHIC. Unique at the LHC are the beautiful measurements of the fractional transverse momentum imbalance 1 – (ρ-carot T2/ρ-carot T1) of di-jets in Pb+Pb collisions. At the Utrecht meeting in 2011, I corrected for the fractional imbalance of di-jets with the same cuts in p-p collisions and showed that the relative fractional jet imbalance in Pb+Pb/p-p is ≈ 15% for jets with 120 < ρ-carot T1 < 360 GeV/c. CMS later confirmed this muchmore » smaller imbalance compared to the same quantity derived from two-particle correlations of di-jet fragments at RHIC corresponding to ρ-carot T jet ≈ 10 – 20 GeV/c, which appear to show a much larger fractional jet imbalance ≈ 45% in this lower ρ-carot T range. The variation of apparent energy loss in the medium as a function of both ρ T and √s NN is striking and presents a challenge to both theory and experiment for improved understanding. There are many other such unresolved issues, for instance, the absence of evidence for a q-carot effect, due to momentum transferred to the medium by outgoing partons, which would widen the away-side di-jet and di-hadron correlations in a similar fashion as the k T-effect. Another issue well known from experiments at the CERN ISR, SpS and SpS collider is that parton-parton hard-collisions make negligible contribution to multiplicity or transverse energy production in p-p collisions–soft particles, with ρ T < 2 GeV/c, predominate. Thus an apparent hard scattering component for A+A multiplicity distributions based on a popular formula, dN AA ch/dη = [(1 - x) (N part)dN pp ch/dη2 + x (N colldN pp ch/dη], seems to be an unphysical way to understand the deviation from N part scaling. Based on recent p-p and d+A measurements, a more physical way is presented along with several other stimulating results and ideas from recent d+Au (p+Pb) measurements.« less

Authors:
 [1]
  1. Brookhaven National Lab. (BNL), Upton, NY (United States)
Publication Date:
Research Org.:
Brookhaven National Lab. (BNL), Upton, NY (United States)
Sponsoring Org.:
USDOE Office of Science (SC)
OSTI Identifier:
1193197
Report Number(s):
BNL-107861-2015-JA
Journal ID: ISSN 1742-6588; KB0201021
Grant/Contract Number:  
SC00112704
Resource Type:
Journal Article: Accepted Manuscript
Journal Name:
Journal of Physics. Conference Series
Additional Journal Information:
Journal Volume: 589; Conference: 9. International Workshop on High-ρT Physics at LHC, Grenoble (France), 24-28 Sep 2013; Journal ID: ISSN 1742-6588
Publisher:
IOP Publishing
Country of Publication:
United States
Language:
English
Subject:
73 NUCLEAR PHYSICS AND RADIATION PHYSICS; nuclear; physics; radiation

Citation Formats

Tannenbaum, M. J. How do quarks and gluons lose energy in the QGP?. United States: N. p., 2015. Web. doi:10.1088/1742-6596/589/1/012019.
Tannenbaum, M. J. How do quarks and gluons lose energy in the QGP?. United States. https://doi.org/10.1088/1742-6596/589/1/012019
Tannenbaum, M. J. Tue . "How do quarks and gluons lose energy in the QGP?". United States. https://doi.org/10.1088/1742-6596/589/1/012019. https://www.osti.gov/servlets/purl/1193197.
@article{osti_1193197,
title = {How do quarks and gluons lose energy in the QGP?},
author = {Tannenbaum, M. J.},
abstractNote = {RHIC introduced the method of hard scattering of partons as an in-situ probe of the the medium produced in A+A collisions. A suppression, RAA ≈ 0.2 relative to binary-scaling, was discovered for π⁰ production in the range 5 < ρT < 20 GeV/c in central Au+Au collisions at √sNN = 200 GeV, and surprisingly also for single-electrons from the decay of heavy quarks. Both these results have been confirmed in Pb+Pb collisions at the LHC at √sNN = 2.76 TeV. Interestingly, in this ρT range the LHC results for pions nearly overlap the RHIC results. Thus, due to the flatter spectrum, the energy loss in the medium at LHC in this ρT range must be ~ 40% larger than at RHIC. Unique at the LHC are the beautiful measurements of the fractional transverse momentum imbalance 1 – (ρ-carotT2/ρ-carotT1) of di-jets in Pb+Pb collisions. At the Utrecht meeting in 2011, I corrected for the fractional imbalance of di-jets with the same cuts in p-p collisions and showed that the relative fractional jet imbalance in Pb+Pb/p-p is ≈ 15% for jets with 120 < ρ-carotT1 < 360 GeV/c. CMS later confirmed this much smaller imbalance compared to the same quantity derived from two-particle correlations of di-jet fragments at RHIC corresponding to ρ-carotT jet ≈ 10 – 20 GeV/c, which appear to show a much larger fractional jet imbalance ≈ 45% in this lower ρ-carotT range. The variation of apparent energy loss in the medium as a function of both ρT and √sNN is striking and presents a challenge to both theory and experiment for improved understanding. There are many other such unresolved issues, for instance, the absence of evidence for a q-carot effect, due to momentum transferred to the medium by outgoing partons, which would widen the away-side di-jet and di-hadron correlations in a similar fashion as the kT-effect. Another issue well known from experiments at the CERN ISR, SpS and SpS collider is that parton-parton hard-collisions make negligible contribution to multiplicity or transverse energy production in p-p collisions–soft particles, with ρT < 2 GeV/c, predominate. Thus an apparent hard scattering component for A+A multiplicity distributions based on a popular formula, dNAAch/dη = [(1 - x) (Npart)dNppch/dη2 + x (NcolldNppch/dη], seems to be an unphysical way to understand the deviation from Npart scaling. Based on recent p-p and d+A measurements, a more physical way is presented along with several other stimulating results and ideas from recent d+Au (p+Pb) measurements.},
doi = {10.1088/1742-6596/589/1/012019},
url = {https://www.osti.gov/biblio/1193197}, journal = {Journal of Physics. Conference Series},
issn = {1742-6588},
number = ,
volume = 589,
place = {United States},
year = {2015},
month = {3}
}

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