Tagged jets and jet reconstruction as a probe of QGP induced partonic energy loss
aa r X i v : . [ h e p - ph ] N ov September 14, 2018 14:27 WSPC/INSTRUCTION FILE neufeld˙prague
International Journal of Modern Physics Ec (cid:13)
World Scientific Publishing Company
TAGGED JETS AND JET RECONSTRUCTION AS A PROBE OFQGP INDUCED PARTONIC ENERGY LOSS
R.B. NEUFELD
Theoretical Division, Los Alamos National LabLos Alamos, New Mexico, USA, [email protected]
Received (received date)Revised (revised date)Recent experimental advances at the Relativistic Heavy Ion Collider (RHIC) and thelarge center-of-mass energies available to the heavy-ion program at the Large HadronCollider (LHC) will enable strongly interacting matter at high temperatures and densi-ties, that is, the quark-gluon plasma (QGP), to be probed in unprecedented ways. Amongthese exciting new probes are fully-reconstructed inclusive jets and the away-side hadronshowers associated with a weakly or electromagnetically interacting boson, or, taggedjets. Full jet reconstruction provides an experimental window into the mechanisms ofquark and gluon dynamics in the QGP which is not accessible via leading particles andleading particle correlations. Theoretical advances in this growing field can help resolvesome of the most controversial points in heavy ion physics today. I here discuss thepower of jets to reveal the spectrum of induced radiation, thereby shedding light on theapplicability of the commonly used energy loss formalisms and present results on theproduction and subsequent suppression of high energy jets tagged with Z bosons in rel-ativistic heavy-ion collisions at RHIC and LHC energies using the Gyulassy-Levai-Vitev(GLV) parton energy loss approach.
The suppression of energetic partons in the QGP, or jet quenching , is one ofthe most important results from the heavy-ion program at RHIC 1 ,
2. The lim-ited center-of-mass energies available at RHIC have mostly restricted experimentalmeasurements thus far to leading particle suppression relative to p + p collisions 3.Unfortunately, leading particle suppression alone cannot discriminate between par-tonic energy loss formalisms or extract quantitatively the jet quenching propertiesof the QGP 4.In order to constrain the underlying quantum chromodynamic (QCD) the-ory of jet quenching, new and more differential observables are needed. Par-ticularly promising are jet shapes 5 , , eptember 14, 2018 14:27 WSPC/INSTRUCTION FILE neufeld˙prague R.B. NEUFELD ! min (1) ! min ( R (1) R ( !" Fig. 1. A schematic illustration of the jet reconstruction cone radius, R = p (∆ φ ) + (∆ y ) , andthe particle tower energy selection, ω min . As these parameters are varied, the amount of energyrecovered in jet reconstruction (and hence the jet cross section) also varies.
10 20 30 40 50 E T [GeV] σ ( R ) / σ ( R ) R=0.6/R=0.8R=0.4/R=0.8R=0.2/R=0.8
10 20 30 40 50 E T [GeV] R AA j e t ( R , p T m i n = ) PHENIX π R=0.2 R=0.4 R=0.6 R=0.8 R=1.0 Central Au+Au
Fig. 2. The figure gives predictions for jet R AA as a function of jet reconstruction cone size R = p (∆ φ ) + (∆ y ) using the GLV partonic energy loss formalism. The plot is for impactparameter b = 3 fm in Au+Au collisions at √ s = 200 GeV per nucleon. The sensitivity of thesuppression on cone size R is reflective of the angular distribution of medium-induced radiation.The insert shows the ratios of jet cross sections for selected values of R . Ref. 5, which shows a schematic illustration of the jet reconstruction cone radius, R = p (∆ φ ) + (∆ y ) , and the particle tower energy selection, ω min . As one variesthese parameters, the amount of energy recovered in jet reconstruction (and hencethe jet cross section) likewise varies. This variation can be exploited to reveal thestructure of the underlying medium induced bremsstrahlung spectrum, as is demon-strated in Fig. 2, which is taken from Ref. 6. The Figure shows predictions for theeptember 14, 2018 14:27 WSPC/INSTRUCTION FILE neufeld˙prague TAGGED JETS AND JET RECONSTRUCTION AS A PROBE OF QGP INDUCED PARTONIC ENERGY LOSS sensitivity of jet R AA on jet reconstruction cone size R using the Guylassy-Levai-Vitev 10 (GLV) partonic energy loss formalism. The plot was done for impactparameter b = 3 fm in Au+Au collisions at √ s = 200 GeV per nucleon. Thedependence of the suppression on cone size R directly reflects the angular distri-bution of medium-induced radiation. The heavy-ion program at the LHC and thefuture RHIC upgrades will enable a high statistics experimental measurement ofthis feature for the first time. Tagged jets also provide an exciting opportunity to quantify the jet quenchingproperties of the QGP. Tagged jets are high energy partons produced in associationwith the tagging particle. Electroweak bosons are ideal tags to study partonic energyloss because, once produced, they have negligible medium induced modifications.In the limit of leading order (LO) kinematics, the tagging particle serves as a signalfor the initial associated jet energy. In this way, one hopes to obtain the amount anddistribution of partonic energy loss by reconstructing the tagged jet. The leptonicfinal states of the Z boson are an especially attractive jet tag because the largeinvariant mass of the Z boson makes it easy to distinguish from the backgroundgenerated in a heavy-ion collision. The heavy-ion program at the LHC will enablethe experimental measurement of this feature for the first time.As mentioned above, at LO the kinematics ensure that the tagging Z bosonserves as an exact measure of the associated jet p T (magnitude of momentum trans-verse to the beam line). This is demonstrated in Fig. 3 where I present results 7 p T jet [GeV] d σ / dp T j e t [ pb / G e V ] LONLO R = 0.8NLO R = 0.4NLO R = 0.2 s =4 TeV|y jet |<2.5 |y µ |<2.5M z -3 Γ z 5. However, at NLO thiscorrespondence is largely washed away due to the possibility of an extra jet in thefinal state. This relatively large smearing effect at NLO suggests great care must betaken in experimentally tagging the initial associated jet energy in AA collisions.I next consider results for jets associated with Z / γ ∗ → µ + µ − in Pb+Pb col-lisions at √ s = 4 TeV/nucleon pair. In this proceedings I will not consider coldnuclear matter effects 12, but will leave that for a future study. In Pb+Pb collisionsit is essential to include jet energy loss through medium-induced bremsstrahlung.The partonic energy loss in medium for the results that follow was obtained fromthe GLV energy loss formalism 10. The NLO results for tagging Z / γ ∗ requiredto have 92 . < p T < . 50 100 150 200 250 300 p T jet [GeV] d σ / dp T j e t [ pb / G e V ] Pb+Pb α s = 0.3Pb+Pb α s = 0.5p+p p T jet [GeV] I AA |y jet |<2.5 |y µ |<2.5M z -3 Γ z 5. The results areshown for two different values of jet cone radius R = 0 . R = 0 . p T . As the jet cone radius isincreased, the medium modified curves approach the p + p result, as more and more of the mediuminduced bremsstrahlung is recovered in the jet. two different values of jet cone reconstruction radius, R = 0 . , . p T differential cross section per nucleon pair for Pb+Pbis superimposed with the result from p+p which was shown in Fig. 3. As is clearfrom the figure, the effect of medium induced energy loss is to shift the curve tolower values of p T . It is also clear from the figure that as the jet cone radius is in-eptember 14, 2018 14:27 WSPC/INSTRUCTION FILE neufeld˙prague TAGGED JETS AND JET RECONSTRUCTION AS A PROBE OF QGP INDUCED PARTONIC ENERGY LOSS creased, the medium modified curves approach the p + p result, as more and more ofthe medium induced bremsstrahlung is recovered in the jet. This occurs regardlessof the strength of the medium coupling, α s . As the jet cone radius is decreased,the sensitivity to α s becomes more important, as less of the medium induced ra-diation is recovered. This sensitivity of the result in Pb+Pb to jet cone radius isdirectly related to the angular distribution of the medium-induced bremsstrahlungspectrum.In summary, recent experimental advances at RHIC and the upcoming heavy-ion program at the LHC will usher in a new era of experimental capabilities to probethe quark-gluon plasma. Among these new probes, jet shapes and jets tagged withweakly interacting probes are especially promising, particularly in light of the factthat leading particle suppression alone is not sufficient to discriminate between par-tonic energy loss formalisms or to extract quantitatively the jet quenching propertiesof the QGP.I have here discussed the power of jets to reveal the spectrum of induced radia-tion, thereby shedding light on the applicability of the commonly used energy lossformalisms and presented results on the production and subsequent suppression ofhigh energy jets tagged with Z bosons in relativistic heavy-ion collisions using theGLV parton energy loss approach. The results suggest that higher order produc-tion processes will make experimentally tagging the initial associated jet energywith electroweak bosons difficult. It is also observed that the dependence of thesuppression in Pb+Pb relative to p+p on jet reconstruction radius R is a sensitiveprobe of the spectrum of medium induced radiation. It is worthwhile to here men-tion that tagged jets may provide an exciting opportunity to probe the mediumresponse to hard partons 13, especially if the NLO smearing effects are broughtunder sufficient theoretical control. Acknowledgements I wish to thank my collaborators Ivan Vitev and Ben-Wei Zhang, and also JaroslavBielcik and Jana Bielcikova for hosting an excellent workshop. This work was sup-ported in part by the US Department of Energy, Office of Science, under ContractNo. DE-AC52-06NA25396. References 1. M. Gyulassy, I. Vitev, X. N. Wang and B. W. Zhang, arXiv:nucl-th/0302077.2. P. Jacobs and X. N. Wang, Prog. Part. Nucl. Phys. , 443 (2005).3. S. S. Adler et al. [PHENIX Collaboration], Phys. Rev. Lett. , 202301 (2006).4. S. A. Bass et al. , Phys. Rev. C , 024901 (2009).5. I. Vitev, S. Wicks and B. W. 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