The performance of the ALICE Silicon Pixel Detector has been

studied during the commissioning phase with cosmic tracks in 2008 and with the first proton-proton collisions in 2009 delivered by LHC since the early phase of its commissioning

Terrevoli, C.

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DOI 10.1393/ncc/i2011-10779-yColloquia: IFAE 2010IL NUOVO CIMENTO Vol. 33 C, N. 6 Novembre-Dicembre 2010Performance of the ALICE Silicon Pixel DetectorC. Terrevoli on behalf of the SPD project in the ALICE experimentDipartimento Interateneo di Fisica “M. Merlin” and INFN, Sezione di Bari - Bari, Italy(ricevuto l’ 8 Ottobre 2010; pubblicato online l’8 Febbraio 2011)Summary. — The performance of the ALICE Silicon Pixel Detector has beenstudied during the commissioning phase with cosmic tracks in 2008 and with thefirst proton-proton collisions in 2009 delivered by LHC since the early phase of itscommissioningPACS 29.40.Gx – Tracking and position-sensitive detectors.1. – The ALICE experiment and the Silicon Pixel DetectorALICE (A Large Ion Collider Experiment) is the general-purpose heavy-ion detec-tor at the CERN LHC. Its Inner Tracking System (ITS) consists of six layer barrel ofsilicon detectors located at radii between 4 and 43 cm from the beam axis and cover-ing the pseudo-rapidity range |η| < 0.9 for all vertices located within the length of theinteraction diamond. It contributes to the tracking of particles with momentum below200MeV/c, improves the momentum resolution for particles reconstructed by the largeTime Projection Chamber (TPC), the main tracking system in ALICE, and reconstructsparticles traversing dead region of the TPC. The two innermost layers of the ITS aremade of hybrid Silicon Pixel Detectors (SPD). The SPD is also called vertex detectorbecause it plays a fundamental role in reconstructing the primary vertex and identifyingthe secondary vertices from the decays of heavy flavor and strange particle. The SPDconsists of 9.8M of pixel cells arranged in 1200 read-out chips, each one containing 8192pixel cells. Each chip provides a fast trigger signal if at least one hit is detected. Thetrigger signals from 1200 chips are then combined in a programmable logic unit whichprovides a level-0 trigger signal contribution for the ALICE data acquisition system [1].2. – Cosmic data (2008)2.1. Method to evaluate the efficiency of the innermost SPD layer with cosmic tracks.– The 2008 Cosmic Data acquisition has been taken using the SPD trigger: this triggerselects event having at least a cluster in the top outer half barrel in AND with a clusterin the bottom outer one. To evaluate the inner layer efficiency, we defined a probingtrack using 3 points: 2 points in the outer layer and the 3rd one in the inner layer. Thenc© Societa` Italiana di Fisica 289290 C. TERREVOLI on behalf of the SPD PROJECT in the ALICE EXPERIMENTa compatible cluster is searched for in the inner layer on the hemisphere opposite tothat of the 3rd point. So, if the 4th point is found, a success is added to the efficiencyof the chip containing this cluster; if not, a failure is assigned to the chip intersectedby the probing track. The efficiency calculated by this method is equal to 99.4 ± 0.2%,meanwhile in Monte Carlo the efficiency is calculated to be 99.91 ± 0.02%. This smalldifference between Data and Monte Carlo may be due to the geometry misalignmentwhich in real data is greater than the one presently estimated in simulations.3. – First proton-proton collisions (2009)The first proton-proton collisions in ALICE occurred on 23rd November 2009. Atthat date, the Silicon Pixel Detector was set with 100 Half-Staves switched on and theremaining 20 were off (17%), due to not good thermal conditions. In 2010 few morehalf-staves have been reconfigured and recovered.3.1. Strategy for searching of dead pixels, chip by chip. – The number of hits cumulatedover the available statistics was considered to evaluate the real amount of the dead pixelsin SPD. The dead pixels can be less than the never fired ones because of not enoughstatistics. The method to evaluate the dead pixel number is based on two hypotheses:the track occupancy within the chip is uniform and the detection efficiency is either 0or 1 (dead or alive). The average number of tracks per alive pixel in a chip is Nave =NhitsNpixel−Ndead where Npixel = 8192 and Nhits is the total number of hits. The probabilityto have no hit in a pixel is estimated based on Poissonian probability: Ppoisson(0 | Nave).So the expected number Noff of pixels without any hit can be calculated as the sumbetween the pixels with no signal because lack of statistics and dead pixels: Noff =Npixel · Ppoisson(0 | Nave) + Ndead · (1− Ppoisson(0 | Nave)). This equation can be solvednumerically to give the unknown Ndead in each chip. A procedure to localize whichpixels should be considered as alive among those without hit was developed knowingfrom construction the dead pixels which are mainly located either in the corner of thechip or along columns. We call Nover-dead = Noff − Ndead the number of alive pixelswith no hit and the method developed assigns the dead pixels among those that arenever fired in the pixel hit-map. We call NA the isolated never-fired pixels which aresurrounded only by alive pixels; NB those which have only one adjacent never-firedpixel. If Noff − Ndead < NA: we choose Nover-dead alive pixels randomly from NA. IfNoff −Ndead > NA: all the NA are considered as alive and some of the NB are randomlyconsidered as alive. After this procedure we have estimated the fraction of isolated deadpixels being ∼ 1%, and the total amount of pixels recovered is Nover-dead = 0.24%.3.2. SPD: data and Monte Carlo. – A good agreement is found in the cluster yieldper chip between data and MC, but for Sector 9 because of a problematic configurationof its half-staves, due to cooling problem. This causes a lower efficiency in this sectorwith respect to the MC estimation. The fit of the ratio between data and MC (excludingsector 9) is equal to 1, for both inner and outer layers. Hence we can conclude that theSPD performance in real data is in good agreement with the MC simulation.REFERENCES[1] Aamodt K. et al., JINST, 3 (2008) S08002.

Performance of the ALICE Silicon Pixel Detector

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Performance of the ALICE Silicon Pixel Detector

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