Ⅱ-5. Hadron Physics (Theory)

RIKEN Accel. Prog. Rep. 48 (2015)

Strong enhancement of jet-medium coupling in the quark-gluon
plasma near transition temperature†
J. Liao∗1,∗2†∗1∗2Condensed from the articles in Phys. Rev. D. 89, 126006(2014) and in arXiv:1411.3673.
Physics Department and Center for Exploration of Energyand Matter, Indiana University
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A new, deconﬁned form of QCD matter known as
the quark-gluon plasma is created in relativistic heavyion collisions. In such collisions, highly energetic jets
provide unique probe of the quark-gluon plasma properties. The observable quantifying jet energy loss isthe nuclear modiﬁcation factor, RAA as well as its
azimuthal anisotropy characterized by its second harmonic coeﬃcient v2 . Both RHIC and LHC measurements have shown a sizable v2 in the high pt region.
Recently the temperature dependence of jet-medium
coupling, which may be quantiﬁed by the so-called
jet transport coeﬃcient q̂(T ), has attracted signiﬁcant interest. As was ﬁrst found in1) , the geometricanisotropy v2 at high pt is particularly sensitive to suchtemperature dependence, and a simultaneous description of high pT RAA and v2 at RHIC requires a strong
enhancement of jet-medium coupling in the near-Tc region. Furthermore, the near-Tc enhancement predicts
a visible reduction of average opaqueness of the ﬁreballfrom RHIC to LHC which was conﬁrmed by analyzing
how the RAA evolves with beam energy2,3) . Therefore
phenomenologically there are robust evidences for suchnear-Tc enhancement, while a precise theoretical determination of such nontrivial T-dependence has not been
known owing to the highly nonperturbative dynamics
in the near-Tc regime.
One important approach for strongly coupled quarkgluon plasma is to use holographic QCD models. It
is particularly important to introduce non-conformal
physics that is most prominent around Tc . By constructing such a model4) , we have found that: 1) there
are strong non-conformal, non-perturbative dynamics
going on in the near-Tc region; 2) such dynamics leads
to non-monotonic behavior in QGP thermodynamics
as shown by the strong near-Tc peak of trace anomaly(which is well modeled by holography); 3) the same
dynamics leads to non-monotonic behavior in QGP
transport properties and in particular strong near-Tc
enhancement of jet-medium coupling q̂/T 3 (see Fig.1).
More recently we have developed a new jet quenching framework, CUJET3.05) , that is shown to accountwell for both high pT single inclusive hadron suppression RAA and its azimuthal anisotropy v2 at both
RHIC and the LHC energies. CUJET3.0 model includes two new nonperturbative eﬀects in the QCDtransition temperature range T ∼ 140 − 250 MeV: (1)the Polyakov loop suppression of color-electric scatter-0.2

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0.6Fig. 2. Temperature dependence of jet-medium couplingfrom the CUJET3.0 model5) .

ing and (2) the enhancement of scattering due to emergent magnetic monopoles near Tc . We ﬁnd that theCUJET3.0 jet transport parameter q̂(E, T )/T 3 peaks
near Tc and has very strong nonconformal E and T
dependence up to T ∼ 400 MeV. Extrapolating downto E = 2 GeV, we ﬁnd a striking new connection between bulk perfect ﬂuidity with η/s ∼ 0.1 near Tc andhigh pT high T perturbative jet quenching.
References
1) J. Liao and E. Shuryak, Phys. Rev. Lett. 102, 202302(2009).
2) X. Zhang and J. Liao, Phys. Rev. C 89, no. 1, 014907(2014).
3) X. Zhang and J. Liao, Phys. Rev. C 87, 044910 (2013).
4) D. Li, J. Liao and M. Huang, Phys. Rev. D 89, no. 12,126006 (2014).
5) J. Xu, J. Liao and M. Gyulassy, arXiv:1411.3673 [hepph].

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