SPD consecutive triggers

1. SPD consecutive triggers Míriam 21 April 2010

2. Introduction • Afterpulse-like activity at the SPD noticed in different contexts like calibration triggers or LHC collisions. What are the sources? Are they common? • SPD>0 (cosmic) run • Collisions Calibration BXIds: 876 y 2655 LED system N SPD40 High rate SPD>0 in BXIds: 882910… 26612690… BXId

3. Cosmics + noise • SPDMult>0 trigger run • No LHC beam ~470Hz (Threshold 0.5 MIP) 637134 events

4. Consecutive triggers Fraction of events ---- no cells or PMT in common At least one cell in common PMT in common, but no cells Prob consecutive triggers 1.2·10-5 Fraction of triggers which could be afterpulse: 2.4·10-4 (integrated tail from >Next2) Compatible with BCN measurements ‘Next2’ could include spill-over From BX>40 2.5·10-5vs 5.4·10-6 of same VFE random coincidence 9.5·10-6vs 9·10-9 of same cell coincidence

5. Afterpulse candidates (same hit or VFE also at T0) SPD Multiplicity of afterpulse candidates afterpulse candidates Number of consecutive triggers from the same event Same VFE Same cell

6. Collisions • SPDMult>0 (downscaled by factor 40) and SPDMult>2 trigger channels. SPD>0 triggers At least one PMT in common At least one cell in common SPD>2 triggers At least one PMT in common At least one cell in common +67% SPD40 triggers wrt collision BXID +1.7% SPD triggers wrt collision BXID Relative 54.7% of tail (>33%) 15.1% of tail (>1.3%) 82.5% of tail 45.7% of tail 45% of the tail unrelated with collision signal

7. SPDMult>0 trigger Threshold 0.8 MIP No cells or PMT in common At least one cell in common PMT in common, but not same cell Integrated tail (blue + red) 37% aditional triggers (10.2%) Not compatible with ~2% expected from afterpulsing (from cosmic + noise run) BUT related with regions with signal in the collisions SPDMult>2 trigger Integrated tail (blue + red) 1.4% (0.8%)

8. SPD>0 SPD Multiplicity of events at BXId 1786 (‘T0’) Only one consecutive trigger from the same event (rejection factor 40) <>=34 VFEs After-triggers mainly 1 VFE SPD Multiplicity of events with after-trigger <>=53 VFEs SPD Multiplicity of after-triggers <>=1.04 VFEs

9. SPD>2 SPD Multiplicity of events at BXId 1786 (‘T0’) Number of consecutive triggers from the same event Mainly once <>=33 VFEs SPD Multiplicity of events with after-triggers After-triggers mainly 1 or 2 VFE <>=52 VFEs SPD Multiplicity of after-triggers <>=1.5 VFEs

10. SPD>2 SPD PS T0 Next 32

11. SPD PS Next 37 ~half of after-trigger events >1 VFE But distance between hits from different VFE is below 50 cm for 70% of these Physics bkg?

12. Increasing the SPD Multiplicity cut Current level of after triggers is not a problem. (If at some point it become a problem, the cure can be to increase the threshold). • Eff = efficiency on SPD>0 events at BXID 1786 • After-triggers = fraction of events with a consecutive trigger (up to 200 BX) SPDMult>4

13. Summary • Trying to find if SPD trigger tails is consistent with possible afterpulsing • 0.025 % is too low to explain ~10-40% tail in the SPD>0 trigger • Barcelone measurements estimate 0.01% • Tail duration does not fit with AP duration • For beam-beam interactions is about 200 cycles (5 s) • Barcelone measurements, AP duration is < 1 s • Physics background? • For the long-lived component of the after-triggers (SPD>2), a clear tendency is seen for the hits to be clustered in the same detector region. The fact that this happens also in contiguous VFEs points towards physics origins.

14. Backup

15. Cosmics + noise SPD rates in Hz (LHC off) SPD40: 11.75 Hz SPD: 1.6 Hz

16. Cosmics + noise AP Initially one cell (629631 evts) Prob consecutive triggers 1.2·10-5 same VFE 1.2·10-7 same cell 3.2·10-9 Integrated tail (red+ blue >=Next3) 2.3·10-4

17. Next2 Cosmics + noise Next4 Next6 Next8

18. Cosmics + noise SPD Multiplicity of events with afterpulsing (*) Any event (*) >Next2 triggers with at least one PMT in common No afterpulses ‘T0’ with afterpulses