download report - Sapienza

Scientific Report 2007-2009
Particle physics
P8. Interactions between nuclei at LHC: ALICE experiment
High-energy heavy-ion physics aims to study how collective
phenomena and macroscopic properties, involving
many degrees of freedom, emerge from the microscopic
laws of elementary-particle physics. The most interesting
case of collective phenomena is the occurrence
of phase transitions in quantum fields at characteristic
energy densities; this would affect the current understanding
of both the structure of the Standard Model
at low energy and of the evolution of the early Universe.
In ultra-relativistic nuclei collisions it is expected to attain
energy densities which reach and exceed the critical
energy density 1 GeV fm 3 , predicted by lattice calculations
of Quantum Chromo Dynamics (QCD) for a
phase transition of nuclear matter to a deconfined state
of quarks and gluons, thus making the QCD phase transition
the only one predicted by the Standard Model that
is presently within reach of laboratory experiments.
ALICE is a general-purpose heavy-ion experiment primarily
designed to study the physics of strongly interacting
matter and the quark-gluon plasma (QGP) formed
in nucleus-nucleus collisions at the LHC [1]. Its detectors
measure and identify mid-rapidity hadrons, electrons
and photons produced in the collision, and reconstruct
particle tracks, including short lived ones, in an
environment with large multiplicity of charged particles.
A forward muon arm detects and identifies muons covering
a large rapidity domain. Hadrons, electrons and
photons are detected and identified in the central rapidity
region by a complex system of detectors immersed in
a moderate (0.5 T) magnetic field. Tracking relies on a
set of high granularity detectors: an Inner Tracking System
(ITS) consisting of six layers of Silicon Detectors,
a large-volume Time-Projection Chamber (TPC) and a
high-granularity Transition-Radiation Detector (TRD).
Particle identification in this central region is performed
by measuring energy loss in the ITS and TPC, transition
radiation in the TRD, Time Of Flight (TOF) with a
high-resolution array of multigap Resistive Plate Chambers,
Cherenkov radiation with a High-Momentum Particle
Identification Detector (HMPID), photons with a
high granularity crystal photon Spectrometer (PHOS)
and a low granularity electromagnetic calorimeter (EM-
CAL). Additional detectors located at large rapidities
complete the central detection system to characterize the
event and to provide the interaction triggers.
The ALICE Rome group is involved in the ultrarelativistic
nuclei collisions study since 1994, participating
to the WA97 and NA57 experiments at the CERN
SPS. The aim was to obtain clear evidence of an
enhancement of the (multi)strange baryons/antibaryons
ratio production in the Pb-Pb collisions compared to
p-Be collisions; this fact could represent a strong signal
for a phase transition from ordinary matter to a Quark
Gluon Plasma state. The results obtained in the NA57
experiment seem to confirm this important signature
[2]. Since 2004 the ALICE Rome group participates
Figure 1: A Silicon Drift Detector on the assembly jig.
to the realization of the Silicon Drift Detector (SDD)
that constitutes the intermediate layers of the ITS [3].
The ITS consists of six coaxial cylinders: two innermost
ones form the Silicon Pixel Detectors, two intermediate
ones the Silicon Drift Detectors, two outermost ones
the Silicon Strip Detectors. The number, position and
segmentation of the layers are optimized for efficient
track finding and high impact parameter resolution.
The SDD front-end electronics is based on three types of
ASICs, two of them, PASCAL and AMBRA, assembled
on an hybrid circuit (see fig. 1) which is directly bonded
to the sensor, and one, CARLOS, located at each end
of a ladder. The Alice Rome group was responsible for
the production and certification of the PASCAL and
AMBRA chips since 2005. To this purpose a validation
system was built up in the Rome laboratory using a
semiautomatic Probe Station in a class 100 Clean Room
equipped with hardware and software tools projected by
the Rome group. In 2007, The Rome group collaborated
to the assembly of the SDD modules and ladders in
Torino, and finally to the installation inside the ITS in
the ALICE site at CERN. During 2008 and 2009 the
ALICE Rome group participated to the data taking in
the cosmic rays runs used for the intercalibration of
the whole ALICE detector. In december 2009, LHC
machine started with p-p collisions at 900 and 2360
GeV and ALICE gave the first physics paper on the
charged particles multiplicity in the detector [4].
References
1. K.Aamodt et al., JINST 3, S08002 (2008).
2. F. Antinori et al., J.Phys. G: Nucl.Part.Phys 34, 403
(2007).
3. S. Beole et al., Nucl.Instr.Meth. in Phys. Res. A582,
733 (2007).
4. K. Aamodt et al., Eur.Phys.J. C, S10052 (2009).
Authors
F. Meddi, S. Di Liberto 1 , M.A. Mazzoni 1 , G.M. Urciuoli 1
http://www.roma1.infn.it/exp/alice
Sapienza Università di Roma 115 Dipartimento di Fisica