120 research outputs found
Has saturation physics been observed in deuteron-gold collisions at RHIC?
We have addressed the question of whether saturation (CGC) has been observed
in deuteron-gold collisions at RHIC. We have made a detailed analysis of the
Cronin peak characteristic of the nuclear modification factor measured for d-Au
collisions at mid-rapidity. The Cronin peak which is obtained around GeV may be reproduced at the proper height only by boosting the saturation
momentum by a huge non-perturbative additional component. At forward rapidity,
we get a quantitative agreement with data, reproducing hadron production
spectra and the ratio using a recently developed description of the
small-x physics.Comment: 4 pages, 2 figures. Talk given at 41st Rencontres de Moriond: QCD and
Hadronic Interactions, La Thuile, Italy, 18-25 Mar 200
Jet (de)coherence in Pb-Pb collisions at the LHC
We study the modifications of jets created in heavy-ion collisions at LHC
energies. The inherent hierarchy of scales governing the jet evolution allows
to distinguish a leading jet structure, which interacts coherently with the
medium as a single color charge, from softer sub-structures that will be
sensitive to effects of color decoherence. We argue how this separation comes
about and show that this picture is consistent with experimental data on
reconstructed jets at the LHC, providing a quantitative description
simultaneously of the jet nuclear modification factor, the missing energy in
di-jet events and the modification of the fragmentation functions. In
particular, we demonstrate that effects due to color decoherence are manifest
in the excess of soft particles measured in fragmentation functions in Pb-Pb
compared to proton-proton collisions.Comment: 5 pages, 2 figure
Sudakov suppression of jets in QCD media
We compute modifications to the jet spectrum in the presence of a dense
medium. We show that in the large- approximation and at leading
logarithmic accuracy the jet nuclear modification factor factorizes into a
quenching factor associated to the total jet color charge and a Sudakov
suppression factor which accounts for the energy loss of jet substructure
fluctuations. This factor, called the jet collimator, implements the fact that
subjets, that are not resolved by the medium, lose energy coherently as a
single color charge, whereas resolved large angle fluctuations suffer more
quenching. For comparison, we show that neglecting color coherence results in a
stronger suppression of the jet nuclear modification factor.Comment: 5 pages, 1 figur
Radiative energy loss of neighboring subjets
We compute the in-medium energy loss probability distribution of two
neighboring subjets at leading order, in the large- approximation. Our
result exhibits a gradual onset of color decoherence of the system and accounts
for two expected limiting cases. When the angular separation is smaller than
the characteristic angle for medium-induced radiation, the two-pronged
substructure lose energy coherently as a single color charge, namely that of
the parent parton. At large angular separation the two subjets lose energy
independently. Our result is a first step towards quantifying effects of energy
loss as a result of the fluctuation of the multi-parton jet substructure and
therefore goes beyond the standard approach to jet quenching based on single
parton energy loss. We briefly discuss applications to jet observables in
heavy-ion collisions.Comment: 34 pages, 15 figure
Groomed jets in heavy-ion collisions: sensitivity to medium-induced bremsstrahlung
We argue that contemporary jet substructure techniques might facilitate a
more direct measurement of hard medium-induced gluon bremsstrahlung in
heavy-ion collisions, and focus specifically on the "soft drop declustering"
procedure that singles out the two leading jet substructures. Assuming coherent
jet energy loss, we find an enhancement of the distribution of the energy
fractions shared by the two substructures at small subjet energy caused by hard
medium-induced gluon radiation. Departures from this approximation are
discussed, in particular, the effects of colour decoherence and the
contamination of the grooming procedure by soft background. Finally, we propose
a complementary observable, that is the ratio of the two-pronged probability in
Pb-Pb to proton-proton collisions and discuss its sensitivity to various energy
loss mechanisms.Comment: 20 pages, 5 figure
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