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the alice experiment at the lhc is primarily dedicated to study heavy ion collisions .
such collisions produce a high multiplicity environment ( up to 8000 tracks per unit rapidity ) which requires excellent tracking capabilities , in particular for secondary vertex reconstruction for the study of beauty and charm physics .
the detector system dedicated to this task is the its @xcite .
the spd ( see figure [ fig : barrel ] ) constitutes the two innermost layers ( situated at radii 3.9 and 7.2 cm , respectively ) of the its , which also includes two layers of silicon drift and two layers of silicon strip detectors at larger radii .
each spd module ( `` ladder '' ) consists of five readout chips bump bonded to a p - in - n sensor of @xmath0 thickness .
each chip contains 8192 read - out cells corresponding to pixels of dimension @xmath1 ( @xmath2 ) @xmath3 @xmath4 ( z ) .
two ladders together with on - detector read out electronics ( multi - chip module ) form a half - stave .
two half - staves are combined to cover the whole length of the spd barrel ( 28.6 cm ) . the total number of staves in the spd is 60 ( 20 in the inner layer , 40 in the outer ) , with in total 1,200 read - out chips and 9.83 million read - out channels .
the acceptance of the spd is @xmath5 and @xmath6 for the inner and outer layer , respectively .
prototypes of the spd modules in the form of single assemblies in a stand - alone configuration , have been tested in high energy particle beams at the cern sps .
the primary objective was the validation of the pixel asics and of the read - out electronics .
the setup was not optimized for precision measurements of the position resolution , but an accurate determination of the intrinsic spatial precision could anyway be performed .
data runs were taken in 2002 and 2003 with proton / piolsn beams of 350 gev / c and 120 gev / c momentum , respectively .
the set - up consisted of a similar basic configuration : 4 reference planes were used to define tracks which were then projected into a test plane which was situated in between the reference planes . in the 2003 run ,
a prototype full read - out chain with two ladders was installed as part of the configuration and read out separately .
many parameters of the pixel asic , in particular the threshold , can be remotely adjusted via on - chip digital to analog converters ( dac ) .
the prototype in the test plane was studied under different conditions ( threshold scan , different inclination angles w.r.t . the beam and bias voltage scan ) .
clusters of hit pixels correlated with a track were then used to estimate the combined detector / reconstruction efficiency , which was found to be @xmath7 99 % in a wide range of threshold values including the normal working point , see figure [ fig : efficiency ] .
[ htb ]     thick sensors .
the threshold shown is the setting of a 8-bit dac such that a lower dac setting corresponds a to higher threshold .
a setting of 214 is equivalent to approximately 2000 e@xmath8 .
the normal working point is around dac = 200 . ]
the intrinsic precision as a function of various parameters has been calculated using an iterative method @xcite@xcite .
the intrinsic precision at the normal working point is found to be ( 11.1 @xmath9 0.2 ) @xmath10 in the @xmath2 direction .
the dependence of intrinsic precision on threshold and angle of incidence is shown in figure [ fig : resolution ] .
a higher multiplicity region was also investigated using a 158 a gev / c indium ion beam on a pb target .
the data analysis is under way .
discussions of the 2002 and 2003 spd beam tests can also be found in @xcite @xcite and @xcite .
+    [ htb ]     sensor ) as a function of threshold for tracks normal incidence angle ( upper figure ) and as a function of angle for two different thresholds ( lower figure),title="fig : " ]   sensor ) as a function of threshold for tracks normal incidence angle ( upper figure ) and as a function of angle for two different thresholds ( lower figure),title="fig : " ]    in 2004 a combined its beam test was carried out . the setup consisted of two planes of spd (
each equipped with one half stave ) , two planes of sdd and two planes of ssd . for the spd
, the full read - out chain was tested using the final components . as part of the integration test ,
the alice daq and trigger system were for the first time used for more than one sub - detector system .
furthermore , beam test data is used for code validation of the alice software framework aliroot @xcite .
particle tracking in alice is currently based on geant3 @xcite . the energy deposited by geant
is transformed into the number of electron - hole pairs ( 3.6 ev/(e , h ) ) and then compared with a threshold .
threshold fluctuations and noise are taken into account pixel by pixel .
charge sharing is simulated assuming gaussian diffusion of e / h - pairs . in each geant step , the diffusion variance is evaluated : @xmath11 , where @xmath12 is the drift path and @xmath13 .
@xmath14 is taken to be the hole diffusion coefficient : @xmath14 = 11 cm @xmath15s @xcite and @xmath16 , where the hole mobility is taken to be @xmath17 = 450 cm@xmath15vs @xcite and @xmath19 is the electric field per unit length ( which depends on the bias voltage ) .
this model gives a qualitatively good description of the beam test data ( see figure [ fig : clustertype ] ) and tuning of the model parameters is currently under way .
+ geant3 is not supported anymore since 1996 . therefore , alice implements the possibility to use geant4 @xcite and the more broadly physics validated fluka @xcite in its detector simulation .
the code which interfaces the alice software package aliroot @xcite with these packages has been validated using the beam test of 2004 .
for example , it is confirmed that differences between fluka and geant3 do not affect the simulation of the spd .
data analysis of the proton beam tests in 2002 and 2003 shows excellent performance of the alice spd with @xmath7 99 % efficiencies and intrinsic precision @xmath20 11 @xmath21 in the @xmath2 coordinate .
given the successful exploitation of the previous beam tests , the 2004 beam test integrating all three its silicon detector technologies did not need to entirely focus on detector and read out electronics performance .
the detectors were rather used to test alice daq , trigger and data analysis which is used to validate the alice software framework .
+ simulation of the spd using geant3 including gaussian diffusion of charges gives a good description of the beam test data of 2002 and 2003 already with simple assumptions on the model parameters .
further improvement is expected from currently performed tuning of its parameters .
newer particle tracking packages , like geant4 or the more extensively physics validated fluka are implemented within the alice detector simulation and the code is validated using the 2004 beam test data .
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prepared for international conference on advanced monte carlo ( mc2000 ) , lisbon , portugal , 23 - 26 oct 2000 _ see http://aliceinfo.cern.ch/offline
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 .    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,    ,