Cherenkov Radiation from Jets in Heavy-ion Collisions Page: 1 of 4
This article is part of the collection entitled: Office of Scientific & Technical Information Technical Reports and was provided to Digital Library by the UNT Libraries Government Documents Department.
The following text was automatically extracted from the image on this page using optical character recognition software:
Cherenkov Radiation from Jets in Heavy-ion Collisions
V. Koch, A. Majumder, and Xin-Nian Wang
Nuclear Science Division, Lawrence Berkeley National Laboratory, 1 Cyclotron road, Berkeley, CA 94720
(Dated: July 26, 2005)
The possibility of Cherenkov-like gluon bremsstrahlung in dense matter is studied. We point out
that the occurrence of Cherenkov radiation in dense matter is sensitive to the presence of partonic
bound states. This is illustrated by a calculation of the dispersion relation of a massless particle in
a simple model in which it couples to two different massive resonance states. We further argue that
detailed spectroscopy of jet correlations can directly probe the index of refraction of this matter,
which in turn will provide information about the mass scale of these partonic bound states.
PACS numbers: 12.38.Mh, 11.10.Wx, 25.75.Dw
mal gluon bremsstrahlung induced by multiple parton
scattering, one can identify these associated soft hadrons
with those from the hadronization of radiated gluons.
Because of the Landau-Pomeranchuck-Midgal (LPM) in-
terference, the angular distribution of the induced gluon
bremsstrahlung does peak at an angle 0 2/wgL away
from the initial jet direction [15, 16]. However, this angle
decreases with the length of the jet propagation or with
the centrality of the nuclear collisions. This is currently
not supported by the experimental data [13, 14].
Two other known phenomena can, however, result
in such an emission pattern: Mach cones generated by
the hydrodynamical propagation of energy deposited by
a quenched jet along its path [17, 18] and Cherenkov
gluon radiation. The angle of particle emission from the
generated Mach cone is determined by the velocity of
sound which can be calculated in lattice QCD. In the
case of Cherenkov gluon radiation, the situation is less
clear. While the general phenomenon has been discussed
[19, 20], the essential question, whether the index of re-
fraction, which determines the cone angle, is larger than
unity in deconfined QCD matter has not been addressed.
Indeed, calculations in the Hard Thermal Loop (HTL)
approximation of QCD  do not allow for Cherenkov
gluon radiation . Quenched lattice QCD calculations
also indicate a time-like dispersion relation for large mo-
menta (p >> T) at T > T, . The situation, however,
is unclear for soft modes p < T and at around T,.
In this Letter, we start with the realization that
a large index of refraction and therefore Cherenkov-
like gluon bremsstrahlung can only result from coherent
gluon scattering off partonic bound states in the QGP.
The situation is analogous to photons in a gas, where
the coherent scattering off atoms in the gas allows for
Cherenkov radiation, but not in a gas of single elementary
charged particles. Thus the observation of Cherenkov-
like bremsstrahlung in heavy-ion collisions would serve
as an signal for the presence of bound states in the QGP.
Obviously, a large index of refraction, corresponding
to a space-like dispersion relation, requires attractive in-
teraction. This is where the bound states enter the pic-
ture: It is natural to assume that these bound states
The goal of high-energy heavy-ion collisions is to cre-
ate and explore a novel state of matter in which quarks
and gluons are deconfined over distances considerably
larger than that of a hadron. Lattice QCD calculations
 have predicted such a transition with a rapid rise in
the entropy density at a critical temperature of about
T, ~ 170 MeV. However, the entropy density is seen
to level off somewhat below the ideal gas limit. Calcu-
lations with a more sophisticated resummation of quasi-
particle modes  within the hard-thermal-loop approx-
imation  improve upon the ideal gas picture but are
still above the lattice QCD results near and just above
T,, suggesting that the plasma may possess a somewhat
more complex structure in this regime. Indeed, recent
lattice QCD calculations of spectral functions [4, 5] find
the presence of charmonium states above T,. This has
led to the suggestion that at moderate temperatures,
T ~ 1 - 2 T, there exist many bound states  both
in the color singlet and other colored representations,
though lattice QCD results on baryon-strangeness cor-
relations can rule out the presence of many light bound
states involving only quarks and anti-quarks . Further-
more, strong collective flow observed in experiments at
the Relativistic Heavy-Ion Collider (RHIC)  also sug-
gest a strongly interacting plasma. Therefore, the nature
of the relevant degrees of freedom in the matter created
at RHIC needs to be further explored.
It is the purpose of this Letter to argue and demon-
strate that one can probe the resonance structure of the
dense matter via the production of Cherenkov-like soft
hadrons along the path of quenched jets. Jet quenching
or medium modification of the jet structure has emerged
as a new diagnostic tool for the study of partonic prop-
erties of the dense matter . The modification goes
beyond a mere suppression of inclusive spectra of lead-
ing hadrons  and has been extended to include the
modification of two-hadron correlations [11, 12]. Of par-
ticular interest for the present work is the experimental
observation that soft hadrons correlated with a quenched
jet have an angular distribution that is peaked at a fi-
nite angle away from the jet [13, 14], whereas they peak
along the jet direction in vacuum. In the picture of nor-
Here’s what’s next.
This article can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Article.
Koch, Volker; Majumder, Abhijit & Wang, Xin-Nian. Cherenkov Radiation from Jets in Heavy-ion Collisions, article, July 26, 2005; Berkeley, California. (https://digital.library.unt.edu/ark:/67531/metadc884824/m1/1/: accessed May 25, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.