1 Behaviour of the Silicon Strip Detector modules for the Alice experiment: simulation and test with minimum ionizing particles Federica Benedosso Utrecht, February 2004

2 Outlook ● The Alice experiment ● The simulation program. Results ● Beam test and data analysis ● Simulated data and experimental data ● Calibration of the simulation parameters

3 Physics: Phase Diagram ● QGP is present just after Big Bang – Quarks and gluons deconfinement – Chiral symmetry restoration ● Disappear by cooling ● Is it actually present in neutron stars? (low temperature, high density) ● Experiments reproduce high temperature conditions

4 Physics: QGP Evidences ● QGP produced by Pb-Pb collisions ● 5 TeV per nucleon in the centre of mass ● 2x10 4 particles per event ● QGP signatures – J/  suppression – Increasing of electronic couples – Strangeness enhancement BNL  Au-Au  200 GeV per nucleon CERN  Pb-Pb fixed target  150 GeV per nucleon Data acquisition start: 2007 p-p collisions p-Pb collisions Pb-Pb collisions

5 The Alice LHC Two principal components: – Central part for studying of hadronic and electronic signals ● Magnet ● TPC ● ITS ● Other detectors – Muonic spectrometer for studying quarks in the dense matter

6 Inner Tracking System ITS ● Determination of primary and secondary vertices ● Particle identification and tracking of low momentum particles ● Silicon technology ● 6 layers – 2 SPD (inner) – 2 SDD (intermediate) – 2 SSD (outer)

7 Silicon Strip Detector ● In the outer layers of ITS, r=40cm (748 modules) and r=45cm (950 modules) ● Area=73x40mm², W=300  m  768 strip per side, pitch 95  m, stereo angle 35 mrad  Resolution: 15  m (x) and 700  m (y)  Readout: 12 chip, each connected to 128 strip. The ADCs convert charges in numbers x y module Readout chip and hybrid Microcables

8 Reconstruction Efficiency Small stereo angle ● Very different resolutions (15  m and 700  m) ● Decreasing of ghost number (useful because of multiplicity) ● Readout only on two side true ghost

9 The Simulation Program AliRoot is the framework for simulation, reconstruction and analysis of the Alice data. It completes step of different programs ● Production of the physical event ● Transport of the particles through the detectors (Geant) and production of hit (information of position and energy loss) GEOMETRY ● Response simulation (digitisation) GEOMETRY ● Points and tracks reconstruction GEOMETRY The geometry is basilar in every step

10 Beam Test Geometry ● Goal: calibration of the SSD module response using the beam test data ● Conditions for simulation as realistic as possible  Only one SSD module (or a telescope)  Beam of 7 GeV/c  ● Two choice to introduce the right geometry: x Writing a stand-alone program (Geant) Using AliRoot ● Because of the object-oriented structure, algorithms of simulation, digitisation and reconstruction don't need changes

11 Response Simulation 1/2 Knowledge of the hits allows identification of the activated strips and their signal ● Energy loss “almost” continuous and electron/hole production ● The pairs migrate to the strips whit Gaussian distribution ● A Gaussian noise is added to the signal

12 Response Simulation 2/2 ● Capacitive coupling: to each strip is added a ratio of the signal of the previous strip and of the subsequent strip ● Conversion in ADC channels ● Written on disk only the values over a fixed threshold Each particle actives one or more strips (charge distribution, noise, coupling). The set of strips activated by the same particle is called cluster

13 Simulation Results Strip signal Cluster size Cluster signal Noise 2 activated strip Main peak 1 strip clusters P N P P N N

14 Beam Test ● Measure performed at CERN using a beam of 7 GeV/c  ( minimum ionizing ) from PS ● Study of modules response: noise, power supply, surface scan, tracking  Tests with single modules and telescope  Development of a specific data analysis program particles/minute pulsed beam (3 burst/min) collected 250 events/minute from 5000 to ev/run 163 run with single module 231 run with telescope Trigger configuration

15 The Analysis Program: the Structure ● Kernel development: from raw data to signals – C-programs linked with ROOT libraries – Shell scripts for looping over runs ● BeTeA (Beam Test Analyzer): from signals to point reconstruction – Wrapping of the C-programs – AliRoot classes – Graphic interface

16 The Analysis Program: the Graphic Interface

17 Noise Run: Pedestal We need noise run to know the background noise and analyze the signal in a second step ● For each strip, the pedestal is the charge average on the events ● To avoid high signal (caused by particles) we use arbitrary thresholds ( pedestal=0,  =50 ) ● If a strip produces systematically a signal over thresholds, it is noticed as not working ● We iterate the calculus using as threshold the pedestal and the standard deviation found at previous step ● The pedestal is subtracted strip by strip and event by event P side N side Bad strips Before After

18 Noise Run: Common Mode ● Common mode is an offset change due to the read-out chip ● It is the average, event by event, of the signal of the 128 strips read by the chip itself Before After: now it is the NOISE Bad strips ???

19 Noise Run: Power Supply ● The noise changes as a function of the bias of the power supply ● We have the best bias when the noise is the lowest M1 M2 M3

20 Signal Run ● Calculus and subtraction of pedestal and common mode performed as for the noise runs Iterative method doesn't need preliminary analysis of a noise run: improvement of analysis speed ● Cluster shape is the charge distribution among the strips; it allows to find the capacitive couplings

21 Signal Run: Cluster Finding ● Two methods implemented – Used a first threshold to identify events due to passing of particles and a second one (lower if necessary) for the other strips forming the cluster – Geometric method: each strip crosses some strip of the opposite side: the cluster is reconstructed only if there is a geometrical correspondence between the two sides ● The implemented methods give similar results

22 Signal to Noise Ratio P N S/N=43 S/N=17

23 Telescope Tests ● Tracking – Calculus of spatial resolution – Preliminary results: x=15  m y=650  m  Different modules: the noise of P-side is similar to N-side  Work in progress...

24 Calibration of the Parameters ● Increasing of the conversion factor from electron/hole pairs to ADC channels: from 40 to 65 channels per pairs ● Noise changing: used the measured values. On N-side the value is twice the P-side one ● Increasing of capacitive coupling (from 2% until to 7%), using the ratios obtained from cluster shape

25 Comparison After the Calibration ??? Strip signal Cluster size Cluster signal Simulation Experimental data

26 Conclusions ● Calibrated the conversion factor, the noise, the capacitive coupling – Signal distribution reproduced for clusters – Cluster size reproduced as shape – Charge distribution for the strip is still quite different ● The calibration is satisfactory, but the model can be improved (capacitive coupling...) ● Modules redrawn because of the noise problem ● Analysis of telescope run still in progress...