exotic nuclei structure and reaction noyaux exotiques ... - IPN - IN2P3

Quarkonia physics with ALICE
IPNO Participation: L. Bimbot, B. Boyer, V. Chambert, S. Drouet, B. Espagnon, C. Hadjidakis, I. Hrivnacova,
B.Y. Ky, G. Lalu, V. Lafage, Y. Le Bornec, M. Lopez-Noriega, M. Malek, G. Noël, S. Rousseau,
C. Suire, D. Tapia-Takaki, N. Willis and SEP technicians.
Collaboration : INFN Cagliari, IRFU/SPhN, PNPI Gatchina, SINP Kolkata, SUBATECH Nantes, VECC
Kolkata.
Avec le démarrage du LHC, les premières données de physique enregistrées par l'expérience ALICE seront
accessibles à des énergies encore jamais atteintes. En ce qui concerne le groupe de l’IPN d’Orsay
nous avons poursuivi les études portant sur la production des quarkonia : étude de la production du J/
dans le cadre du modèle du Color Glass Condensate en p-p et p-Pb, préparation à la mesure de la section
efficace de production du J/ en p-p et enfin étude de la production de J/ par désintégration des mésons
B en p-p.
On November 23rd 2009, the LHC successfully
delivered the first p-p collisions to ALICE at SPS
injection energy (corresponding to a center of
mass energy of s = 900 GeV). These data provided
such clear physics results that ALICE was
able to submit the first LHC paper on November
28th. This paper was accepted for publication on
December 1st [1] ! The forthcoming two years of
data with collisions in p-p at √s = 7 TeV and leadlead
at √s = 2.76 TeV will open a window on a new
physics never yet explored [2,3].
Physics simulations and data analysis
The data analysis in ALICE constitutes a real challenge.
In order to store and analyse the huge
amount of data produced by the LHC experiments,
the LHC Computing Grid (LCG) has been developed
to provide the requested computing resources.
As a member of the GRIF (Grille de Recherche
d'Île de France) TIER2, the IPN hosts now
416 CPUs (~1000 kSi2K) and 100 To of disk storage.
A total of 442 000 ALICE jobs were executed
on the IPNO cluster during the period covered by
this report.
Fig 1 : Nuclear modification factor as a function of
p T for muons coming from charm or beauty decays
in p+Pb at √s = 8.8 TeV. The quarks are produced
with the CGC model.
The computing power of the GRIF TIER2 was also
intensively used for our local simulations [4]. Indeed
we have initiated in ALICE the study of
charm and beauty production in the framework of
the Color Glass Condensate (CGC) model. All
these simulations, requiring a large amount of CPU
time, have run on the IPNO machines.
The CGC model describes the initial state of nucleons
at very high energy and had some success in
describing heavy ion collision at RHIC energies.
The ALICE IPNO team, in collaboration with theorists,
has worked on the possibility to observe the
onset of the CGC regime in p+p and p+Pb collisions
with the muon spectrometer [5-7]. In Fig 1
one can see the nuclear modification factor as a
function of p T for muons coming from charm and
beauty at forward rapidity (corresponding to the
muon spectrometer acceptance). This factor is defined
as :
R
Pbp
N
dN
coll
Pbp
dN
dydp
where N Pbp (N pp ) is the yield of particles produced
in Pb-p (pp) collisions and is the average
number of binary collisions. This factor is supposed
to be equal to unity if the Pb-p collision is a
superposition of independent nucleon-nucleon collisions.
The CGC calculations show a strong suppression
for the open heavy flavour in p-Pb collisions
at low p T and for the lowest quark mass, as
expected in this model. This work has led to a second
PhD [8] defended in July 2009. This thesis has
demonstrated that CGC effects might be difficult to
observe in the single muon spectra from heavy
flavours as the effects are visible at low p T where
the heavy flavours measurement is challenging. It
was also shown that multiplicity fluctuations of
charged particles in p-p collisions can be expressed
in terms of a pseudo impact-parameter,
driven by a CGC calculation of the parton density,
between the two protons.
pp
T
dydp
T
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