A. Pulvirenti

The ALICE Silicon Pixel Detector: readiness for the first proton beam

The Silicon Pixel Detector (SPD) is the innermost element of the ALICE Inner Tracking System (ITS). The SPD consists of two barrel layers of hybrid silicon pixels surrounding the beam pipe with a total of ≈ 107 pixel cells. The SPD features a very low material budget, a 99.9% efficient bidimensional digital response, a 12 μm spatial precision in the bending plane (r) and a prompt signal as input to the L0 trigger. The SPD commissioning in the ALICE experimental area is well advanced and it includes calibration runs with internal pulse and cosmic ray runs. In this contribution the commissioning of the SPD is reviewed and the first results from runs with cosmic rays and circulating proton beams are presented.

Performance of ALICE silicon pixel detector prototypes in high energy beams

The two innermost layers of the ALICE inner tracking system are instrumented with silicon pixel detectors. Single chip assembly prototypes of the ALICE pixels have been tested in high energy particle beams at the CERN SPS. Detection efficiency and spatial precision have been studied as a function of the threshold and the track incidence angle. The experimental method, data analysis and main results are presented.

The Assembly of the first Sector of the ALICE Silicon Pixel Detector

The Silicon Pixel Detector (SPD) is the innermost part of the Inner Tracking System (ITS) of the ALICE experiment at LHC. 240 detector ladders containing in total about 10 million pixel cells with dimension 50 × 425 µm2, have to be assembled on a carbon fibre support. The mounting procedure of the basic SPD modules (Half-Staves) and the assembly of the barrel sectors are presented. Results on the assembly of the first sector are reported.

Design, production and first operation of the ALICE silicon pixel detector system

The ALICE Silicon Pixel Detector (SPD) constitutes the two innermost barrel layers of the ALICE experiment. The SPD is the detector closest to the interaction point, mounted around the beam pipe with the two layers at r=3.9 cm and 7.6 cm distance from beam axis. In order to reduce multiple scattering the material budget per layer in the active region has been limited to ≈1% X0. The SPD consists of 120 hybrid silicon pixel detectors modules with a total of ~107 cells. The on-detector read-out is based on a multi-chip-module containing 4 ASICs and an optical transceiver module. The readout electronics, located in the control room, is housed in 20 VME boards; it is the interface to the ALICE trigger, data acquisition, control system and detector electronics. In this contribution the SPD detector components design and production are reviewed. First operation results are reported. SPD detector overview I. The SPD [1] consists of 120 detector modules, the halfstaves, which are arranged in two cylindrical layers at 3.9 and 7.6 cm from the beam axis. Each detector module comprises two ladders; a ladder consists of 5 pixel chips [2] with 8192 pixel cells each, bump bonded to a sensor using Sn-Pb bumps of 20 μm diameter [3]. In order to achieve the lowest material budget, the pixel chips are thinned to 150 μm and the sensor thickness is 200 μm. In total the SPD contains 9.83 x 106 pixels. At the end of each half-stave a multi chip module (MCM) [4] reads out the 10 pixel chips. The MCM contains 4 ASICs, the rx40 [5] to receive an LHC synchronous clock and serial data on optical fibers, the digital pilot chip [6] to configure and read-out the pixel chips, the 800 Mbit/s serializer chip GOL [7] to send the data on one optical fiber from the detector to the control room and the analog pilot chip [8] to provide bias voltages to the pixel chip. The electrical connection between the pixel chip and the MCM is done via a aluminum based multi-layer flat cable, the pixel bus [9]. An aluminium-kapton foil, the grounding foil, is electrically separating the half-stave from the carbon fiber support structure. Cooling pipes are directly integrated into the carbon fiber structure [10]. Copper and kapton flat cables deliver electrical power to the half staves. SPD system components II.

Beam Test Performance and Simulation of Prototypes for the ALICE Silicon Pixel Detector

The silicon pixel detector (SPD) of the ALICE experiment in preparation at the Large Hadron Collider (LHC) at CERN is designed to provide the precise vertex reconstruction needed for measuring heavy flavor production in heavy ion collisions at very high energies and high multiplicity. The SPD forms the innermost part of the Inner Tracking System (ITS) which also includes silicon drift and silicon strip detectors. Single assembly prototypes of the ALICE SPD have been tested at the CERN SPS using high energy proton/pion beams in 2002 and 2003. We report on the experimental determination of the spatial precision. We also report on the first combined beam test with prototypes of the other ITS silicon detector technologies at the CERN SPS in November 2004. The issue of SPD simulation is briefly discussed.

Study of hadronic resonances in ALICE in a first physics scenario

ALICE (ALICE Collaboration 2008 JINST 3 S08002) is the LHC experiment mainly devoted to heavy-ion physics and to the quark-gluon plasma study. However, it has also excellent properties for pp physics at the unexplored energy regime available at LHC ( 14 TeV). Hadronic resonances play a fundamental role in the characterization of the strong interacting medium created in the collisions and can be sensitive to some expected QGP signatures like the chiral symmetry restoration and the strangeness enhancement. In this respect, a particularly important role is played by the (1020) resonance. The ability of ALICE to measure (1020) resonances by their K+K− decay has been investigated using simulated minimum bias PYTHIA events reconstructed with an implementation of the detector configuration (alignment, calibration, etc) which will be available for the first data. Its transverse momentum spectrum and its yield as a function of the charged event multiplicity have been evaluated with a statistics of some million of events. This shows how physics with resonances can be done right after the first high energy collisions at the LHC.

Study of short-lived resonances in ALICE

The study of short-lived resonances is a powerful way to investigate the collision dynamics and to understand the properties of the hot and dense matter which is produced in ultrarelativistic heavy ion collisions. In this respect the reconstruction of such resonances is also important in pp collisions, since they provide a baseline for the study of AA collisions. The ability of the ALICE detector to identify the K*(892)0, Λ*(1520) and (1020) by their hadronic decay in pp collisions at LHC energies has been investigated. Minimum bias PYTHIA events at two different colliding energies (900 GeV and 14 TeV) have been generated and fully reconstructed. An accurate study has been done on the background evaluation and on the influence of the particle identification accuracy. Moreover, global detection efficiency as a function of rapidity and transverse momentum has been evaluated.

The Silicon Pixel Detector for ALICE Experiment

The Inner Tracking System (ITS) of the ALICE experiment is made of position sensitive detectors which have to operate in a region where the track density may be as high as 50 tracks/cm2. To handle such densities detectors with high precision and granularity are mandatory. The Silicon Pixel Detector (SPD), the innermost part of the ITS, has been designed to provide tracking information close to primary interaction point. The assembly of the entire SPD has been completed.

Short lived resonances in ALICE

The ability of the ALICE detector to reconstruct the K*(892)0 resonance in p–p and Pb–Pb collisions at LHC energies has been investigated, by means of a detailed study of simulated events. PYTHIA events for p–p collisions and HIJING events for Pb–Pb collisions have been generated and fully reconstructed. The K*(892)0 was identified by its hadronic decay into Kπ through invariant mass analysis. The combinatorial background has been estimated using an event-mixing technique for p–p events and a like-sign technique for the Pb–Pb ones. The role of the particle identification of the decay product is also discussed.