139 research outputs found
A Photon Peak due to Strong Coupling
We show that if a flavour-less vector meson remains bound after
deconfinement, and if its limiting velocity in the quark-gluon plasma is
subluminal, then this meson produces a distinct peak in the spectrum of thermal
photons emitted by the plasma. We also demonstrate that this effect is a
universal property of all strongly coupled, large-\nc plasmas with a gravity
dual. For the the corresponding peak lies between 3 and 5 GeV and
could be observed at the LHC.Comment: 4 pages, 2 figures - To appear in the conference proceedings for
Quark Matter 2009, March 30 - April 4, Knoxville, Tennessee. V2: minor
correction
A New Mechanism of Quark Energy Loss
We show that a heavy quark moving sufficiently fast through a quark-gluon
plasma may lose energy by Cherenkov-radiating mesons. We demonstrate that this
takes place in all strongly coupled, large-Nc plasmas with a gravity dual. The
energy loss is exactly calculable in these models despite being an
O(1/Nc)-effect. We discuss phenomenological implications for heavy-ion
collision experiments.Comment: 4 pages, 4 figures; v2: plot modified, conclusions unchange
Interference effects in medium-induced gluon radiation
As a step towards understanding the in-medium evolution of a hard jet, we
consider the interference pattern for the medium induced gluon radiation
produced by a color singlet quark-antiquark antenna embedded in a QCD medium
with size L. We focus on the typical kinematics for medium-induced gluon
radiation in the BDMPS-Z regime, that is, short formation times \tau_f << L and
relatively large emission angles \theta >>\theta_c = 2/\sqrt{\hat q L^3}, with
\hat q the `jet quenching' parameter. We demonstrate that, for a dipole opening
angle \theta_{q\bar q} larger than \theta_c, the interference between the
medium-induced gluon emissions by the quark and the antiquark is parametrically
suppressed with respect to the corresponding direct emissions. Physically, this
is so since the direct emissions can be delocalized anywhere throughout the
medium and thus yield contributions proportional to L. On the contrary, the
interference occurs only between gluons emitted at very early times, within the
characteristic time scales for quantum and color coherence between the two
emitters, which in this regime are much smaller than L. This implies that, for
\theta_{q\bar q} >> \theta_c, the medium-induced radiation by the dipole is
simply the sum of the two BDMPS-Z spectra individually produced by the quark
and the antiquark, without coherence effects like angular ordering. For
\theta_{q\bar q} << \theta_c, the BDMPS-Z radiation by the dipole vanishes.Comment: 47 pages, 7 figures. Improved physical discussion in Sect. 2. New
section added (Sect. 6) on possible implications for in-medium jet evolution.
More references added. Conclusions unchange
Introductory lectures on jet quenching in heavy ion collisions
Jet quenching has become an essential signal for the characterization of the
medium formed in experiments of heavy-ion collisions. After a brief
introduction to the field, we present the full derivation of the medium-induced
gluon radiation spectrum, starting from the diagrammatical origin of the Wilson
lines and the medium averages and including all intermediate steps. The
application of this spectrum to actual phenomenological calculations is then
presented, making comparisons with experimental data and indicating some
improvements of the formalism to the future LHC program. The last part of the
lectures reviews calculations based on the AdS/CFT correspondence on the medium
parameters controlling the jet quenching phenomenon.Comment: 63 pages, 17 figures, Presented at the XLVII Cracow School of
Theoretical Physics, Zakopane, Poland, June 14-22, 200
Eccentricity fluctuations make flow measurable in high multiplicity p-p collisions
Elliptic flow is a hallmark of collectivity in hadronic collisions. Its
measurement relies on analysis techniques which require high event multiplicity
and could be applied so far to heavy ion collisions only. Here, we delineate
the conditions under which elliptic flow becomes measurable in the samples of
high-multiplicity () p-p collisions, which will soon be
collected at the LHC. We observe that fluctuations in the p-p interaction
region can result in a sizable spatial eccentricity even for the most central
p-p collisions. Under relatively mild assumptions on the nature of such
fluctuations and on the eccentricity scaling of elliptic flow, we find that the
resulting elliptic flow signal in high-multiplicity p-p collisions at the LHC
becomes measurable with standard techniques.Comment: 4 pages, 3 figures. v2 accepted by PR
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