SPD spill-over and the subtractor Míriam Calvo 23 June 2010

25 ns Subtractor set at 20% of signal at previous BX The same fraction for any size of the pad Fraction of (mip) signal at each integration window (12x12 pad) Subtractor setting 2 E. Aguiló, et al. LHCb Notes and Next1/T0 = 17% spill-over

Time distribution for non-empty bunch-crossing (14 TeV MC) 3 Next1/T0 = 32% The fraction of signal observed after the current BX is explained by:  The time response of the detector.  Very slow secondaries produced mainly in interactions of initial particles with the detector material.  Decay of long lived particles (neutrons, muons, pions and kaons).  Backsplash particles produced in the material of ECAL and HCAL. Average fraction of Next1/T0 SPD occupancies (14 TeV MC) With subtractorWithout subtractor Inner3%11% Middle5%14% Outer10%21% Subtractor corrects for this one After integrating and applying the threshold…

Next1/T0 occupancies in 2010 data 4 A factor ~2 lower than in MC  lower beam energies and luminosity  effect of 1 ns delay on data (TAE events mid of April)  slightly less spill-over Inner, 1.3%Middle, 3% Outer, 6% VFE 69 Subtractor is working as expected

‘Pile-up’ on consecutive crossings Occupancy Next1+Next2 in 2010 data TeV beams (TAE events, Calo trigger) Problematic channels of VFE 69 removed from the plot Inner, 0.04%Middle, 0.04% Outer, 0.06% Max. cell following BXs is 0.4%

6 In the case of consecutive collisions… Max. pile-up due to spill-over of the order of 4·10 -3 (average 5·10 -4 ) Photon mis-ID due to spill-over of previous collisions is lower than the 1.5% mis-ID due to the interaction of the photon with the scintillator material + 0.9% due to backsplash in the same BX But pile-up with other interaction would depend on the fraction of non-empty bunch crossings with at least one interaction e.g. 15 MHz/40 MHz ~ 0.37 in nominal conditions If crossings every 50 ns; max. pile-up 2.4·10 -3 (from >=Next2) occupancy BX>=Next2 (up to Next200) occupancy BX>=Next3 (up to Next200)

Summary The subtractor is supposed to correct the spill-over of the SPD pulseshape (~17%). – In the electronics set at 20% of previous BX. – No plans to modify this setting. – Fix problem with VFE 69 in a long technical stop. But still there is the spill-over due to particles that actually arrive later. – Higher at the outer region, as expected. – No correction for this. Max ‘pile-up’ due to spill-over of the order of 4·10 -3 (if consecutive collisions). – Smaller than other effects as backsplash, photon interaction… of the order of 2% in photon misidentification. 7

BACKUP 8

9 Shape for different scintillator sizes innermiddle outer

10 Next1/T0 fraction with subtractor Next1/T0 fraction without subtractor innermiddle outer 7 TeV per beam

T0 occupancies (3.5 TeV collisions) SPD Mult <>=105.5 Calo triggered events Max occupancy ~15.4% Min occup ~0.1% 11

Occupancies Next1, Next2 76.7% of T0 events 31% of T0 events SPD Mult <>=3.2 <>=1.4 Max occupancy Next1+Next2 ~ 0.5% 12