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Test beam data analysis at Padova/LNL F. Antinori1, D. Fabris1, M. Lunardon1,2, S. Moretto1,2, M. Cinausero3 1 INFN Sezione di Padova 2 Dep.of Physics of the University of Padova 3 INFN Laboratori Nazionali di Legnaro D.Fabris - SPD Meeting May 2004 Contents • Introduction – Ideal precision of binary detectors – Track incidence angles • • • • • Analysis program Study of cluster size variations at 0º A first look at the 20º data Conclusions Outlook D.Fabris - SPD Meeting May 2004 Ideal precision of binary detectors • In the simplest model of a position detector with a channel size d and binary read-out, assuming that: – each channel outputs is 1 if a particle has passed within the channel boundaries, 0 otherwise – the hit distribution inside each channel is uniform the expected precision is given by the rms of a flat distribution: d 12 • The precision can be improved if the charge is shared between neighbouring pixels (clusters) • For instance, in an ideal situation where the cluster size is only determined by geometry and diffusion, neglecting dE/dx fluctuations, for normal incidence, the minimum (best) precision, obtained when the numbers of clusters of size 2 and 1 are the same, is as much as a factor 2 better: d 2 12 D.Fabris - SPD Meeting May 2004 • Numerically, for the ALICE SPD, with a pitch of 50 µm, we have: R = N(cl. size 2) / N(cl. size 1) – for R = 0 ~ 14.4 µm – for R = 1 ~ 7.2 µm – simulation ~ 12 µm Eventually, we should be able to extract a measurement of the detector precision from the track residuals on the test plane (plane 2) once: – the alignment of the reference planes is optimized – the effect of the tracking precision on the track residuals has been evaluated As a preliminary step, we are studying the variation of the cluster size distributions with the threshold, in order to see if it is worthwhile to attempt an optimization D.Fabris - SPD Meeting May 2004 Track incidence angles In the following, we evaluate incidence angles expected due to the SPD geometry and the magnetic field • from the geometry, with field off: <j> = 0º (layer 1) <j> = 18º (layer 2, due to the turbo layout) – with a maximum deviation from the mean due to the finite rj width of the ladder of about: • 9º (layer 1) • 5º (layer 2) • the angles due to the deviation in the magnetic field are much smaller: j ≈ y’ ≈ x/r, where r(m) = p(GeV/c)/0.3/B(T) is the curvature; for p = 1 GeV/c, B = 0.5 T, the deviation is about: • 0.34º @ 4 cm (~ layer 1) • 0.6º @ 7 cm (~ layer 2) D.Fabris - SPD Meeting May 2004 Summing up, we should consider incidence angles with a spread of a few degrees around: j = 0º (layer 1) j = 18º (layer 2) For layer 1, at incidence around normal, the precision, as we saw, is expected to be maximum when N(cl. size 1) ~ N(cl. size 2) For layer 2, given: – the values of the incidence angles (tan j ~ 0.3) – the geometry of the detector (pitch = ¼ thickness) we should also have a look at N(cl. size 3) vs N(cl. size 2) D.Fabris - SPD Meeting May 2004 Analysis program The aim is to study the variations of the cluster size with the threshold For this study, we have implemented: – masking of noisy pixels – a simple, rough tracking program, in order to: • reject events if no beam is reconstructed in the reference planes • control the detector efficiency on plane 2 D.Fabris - SPD Meeting May 2004 Proton/pion beam at 120 GeV Setup # 3 D.Fabris - SPD Meeting May 2004 Definition of a mask for noisy pixels Plane 1 Plane 2 With mask D.Fabris - SPD Meeting May 2004 Plane 4 Analysis program ( I ) Approximate alignment of the planes using the centroids of the beam distributions Calibration in mm 0 1 1 Ch mm 1 3 2 4 mm Ch D.Fabris - SPD Meeting May 2004 Analysis program ( II ) Considering the planes 0 and 3 as reference for the tracking Events with only 1 cluster on the planes 0,1 and 3,4 and with 0 or 1 clusters on plane 2 Relative distances of the tracks in planes 1, 3, 4 w.r.t. plane 0 within 3σ of the distributions Distance planes 0-3 mm Relative distances of the tracks in the plane 0 and 2 within 5σ The beam must hit plane 2 away from the plane borders (at least 10 rows, 2 cols) D.Fabris - SPD Meeting May 2004 mm Distance planes 0-2 Cluster Size variation with the Threshold at 0o Vth 80 Vth 200 Vth 120 Vth 210 D.Fabris - SPD Meeting May 2004 Vth 170 Vth 214 N(cl. size 2) / N(cl. size 1) vs Threshold 2 1.5 1 Best resolution expected around ~ Vth 210 0.5 0 0 50 100 150 200 250 Threshold (VTH) 1.2 Efficiency on plane 2 vs Threshold Max. efficiency for Vth ≥ 130 efficiency ratio cluster size 2/1 Cluster size variation at 0o 1 0.8 0.6 0.4 0.2 0 0 50 D.Fabris - SPD Meeting May 2004 100 150 Threshold (VTH) 200 250 First look at the 20o data • Preliminary look at cluster size distributions (without tracking conditions) D.Fabris - SPD Meeting May 2004 Cluster Size variation with the Threshold at 20o Vth 100 Vth 160 Vth 120 Vth 180 D.Fabris - SPD Meeting May 2004 Vth 140 Vth 210 N(cl. Size 2)/N(cl.Size 1) vs Threshold ratio clustre size 2/1 Cluster size variation at 20o 2 1.5 1 0.5 0 0 50 100 150 200 250 ratio cluster size 3/2 Threshold (VTH) 6 5 4 N(cl. Size 3)/N(cl. Size 2) vs Threshold 3 2 1 0 0 50 100 150 200 250 Threshold (VTH) D.Fabris - SPD Meeting May 2004 Conclusions • The data show substantial variations of the cluster size with the threshold, in both the 0º and the 20º settings. Therefore, there may be room for optimization... • This should be studied further, since it may be possible to improve the precision to better than the simulation values • We may eventually want run at different threshold settings on layer 1 and layer 2 • During the next beam test, we may want to collect some more data around 20º for various Vth settings around 180 mV D.Fabris - SPD Meeting May 2004 Outlook • In the sharing of the upcoming work, we propose to take care of the analysis of the 20º data D.Fabris - SPD Meeting May 2004