2,692 research outputs found
Impact of the Nuclear Modification of the Gluon Densities on J/Psi production in pPb collisions at sqrt(s_NN) = 5 TeV
We update our previous studies of nuclear-matter effects on J/Psi production
in proton-nucleus for the recent LHC pPb runs at sqrt(s_NN)=5 TeV. We have
analysed the effects of the modification of the gluon PDFs in nucleus, using an
exact kinematics for a 2->2 process, namely g+g->J/Psi+g as expected from LO
pQCD. This allows to constrain the transverse-momentum while computing the
nuclear modification factor for different rapidities, unlike with the usual
simplified kinematics. Owing to the absence of measurement in pp collisions at
the same sqrt(s_NN) and owing to the expected significant uncertainties in
yield interpolations which would hinder definite interpretations of nuclear
modification factor --R_pPb--, we have derived forward-to-backward and
central-to-peripheral yield ratios in which the unknown proton-proton yield
cancel. These have been computed without and with a transverse-momentum cut,
e.g. to comply with the ATLAS and CMS constraints in the central-rapidity
region.Comment: 5 pages, 16 figures, LaTeX. v2: predictions on R_CP and 3 references
added; introduction slightly extende
J/\psi\ and \psi' production in proton(deuteron)-nucleus collisions: lessons from RHIC for the proton-lead LHC run
We study the impact of different cold nuclear matter effects both on J/\psi\
and \psi' production, among them the modification of the gluon distribution in
bound nucleons, commonly known as gluon shadowing, and the survival probability
for a bound state to escape the nucleus --the nuclear absorption. Less
conventional effects such as saturation and fractional energy loss are also
discussed. We pay a particular attention to the recent PHENIX preliminary data
on \psi' production in dAu collisions at sqrt{s}=200 GeV, which show a strong
suppression for central collisions, 5 times larger than the one obtained for
J/\psi\ production at the same energy. We conclude that none of the
abovementioned mechanisms can explain this experimental result.Comment: 4 pages, 2 tables, 2 figures, contribution to Rencontres du Vietnam,
'Heavy Ion Collisions in the LHC Era', 15-21 July 2012, Quy Nhon, Vietna
On the theoretical and experimental uncertainties in the extraction of the J/psi absorption cross section in cold nuclear matter
We investigate the cold nuclear matter effects on production, whose
understanding is fundamental to study the quark-gluon plasma. Two of these
effects are of particular relevance: the shadowing of the parton distributions
and the nuclear absorption of the pair. If 's are not
produced {\it via} a process as suggested by recent theoretical
works, one has to modify accordingly the way to compute the nuclear shadowing.
This naturally induces differences in the absorption cross-section fit to the
data. A careful analysis of these differences however requires taking into
account the experimental uncertainties and their correlations, as done in this
work for Au collisions at \sqrtsNN=200\mathrm{GeV}, using several
shadowing parametrisations.Comment: 6 pages, 1 table, 3 figures, Submitted to J. Phys. G, talk given at
the International Conference on Strangeness in Quark Matter (SQM2009),
Buzios, Brasil, Sep. 27 - Oct. 2, 200
Conserved current for the Cotton tensor, black hole entropy and equivariant Pontryagin forms
The Chern-Simons lagrangian density in the space of metrics of a
3-dimensional manifold M is not invariant under the action of diffeomorphisms
on M. However, its Euler-Lagrange operator can be identified with the Cotton
tensor, which is invariant under diffeomorphims. As the lagrangian is not
invariant, Noether Theorem cannot be applied to obtain conserved currents. We
show that it is possible to obtain an equivariant conserved current for the
Cotton tensor by using the first equivariant Pontryagin form on the bundle of
metrics. Finally we define a hamiltonian current which gives the contribution
of the Chern-Simons term to the black hole entropy, energy and angular
momentum.Comment: 13 page
Detecting Repetitions and Periodicities in Proteins by Tiling the Structural Space
The notion of energy landscapes provides conceptual tools for understanding
the complexities of protein folding and function. Energy Landscape Theory
indicates that it is much easier to find sequences that satisfy the "Principle
of Minimal Frustration" when the folded structure is symmetric (Wolynes, P. G.
Symmetry and the Energy Landscapes of Biomolecules. Proc. Natl. Acad. Sci.
U.S.A. 1996, 93, 14249-14255). Similarly, repeats and structural mosaics may be
fundamentally related to landscapes with multiple embedded funnels. Here we
present analytical tools to detect and compare structural repetitions in
protein molecules. By an exhaustive analysis of the distribution of structural
repeats using a robust metric we define those portions of a protein molecule
that best describe the overall structure as a tessellation of basic units. The
patterns produced by such tessellations provide intuitive representations of
the repeating regions and their association towards higher order arrangements.
We find that some protein architectures can be described as nearly periodic,
while in others clear separations between repetitions exist. Since the method
is independent of amino acid sequence information we can identify structural
units that can be encoded by a variety of distinct amino acid sequences
Open-beauty production in Pb collisions at =5 TeV: effect of the gluon nuclear densities
We present our results on open beauty production in proton-nucleus collisions
for the recent LHC Pb run at =5 TeV. We have analysed the
effect of the modification of the gluon PDFs in nucleus at the level of the
nuclear modification factor. Because of the absence of measurement in
collisions at the same energy, we also propose the study of the
forward-to-backward yield ratio in which the unknown proton-proton yield
cancel. Our results are compared with the data obtained by LHCb collaboration
and show a good agreement.Comment: 6 pages, 3 figures, Proceedings IS2013 submitted to Nuclear Physics
CauchyâRiemann equations and J-symplectic forms
AbstractLet (ÎŁ,j) be a Riemann surface. The almost complex manifolds (M,J) for which the J-holomorphic curves Ï:ÎŁâM are of variational type, are characterized. This problem is related to the existence of a vertically non-degenerate closed complex 3-form on ÎŁĂM (see Theorem 4.3 below), which determines a family of J-symplectic structures on (M,J) parametrized by ÎŁ
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