The Gap Trigger Problem

1
The Gap Trigger Problem

• Tamsin Edwards
• QCD Meeting 10 March 2003

2
Hard Diffractive Events

• Characterized by
• high pT jets
• gap(s) - proton and/or antiproton do not break up
• How to define in a trigger?
• - at the moment veto on hits in the Luminosity
  Monitors
• - in the future also cut on energy in forward
  calorimeter

3
Luminosity Monitors
Two arrays of 24 scintillators mounted on
calorimeter end caps
2.7 lt ? lt 4.4
LM South
LM North

• At the moment
• no read-out of individual scintillators
• only HIT or NO HIT info for each side
• determined by voltage thresholds

4
Fastz halo events
min bias (fast z)
coincidence
antiproton halo
S then N
N then S
proton halo
5
Diffractive events
gap north
S only
gap south
N only
gap south north
neither
6
Triggers motivation

• Triggers
• JT_25TT_NG 25GeV jet
• JT_25TT_GAPS 25GeV jet no hit LM South
• JT_25TT_GAPN 25GeV jet no hit LM North
• JT_25TT_GAPSN 25GeV jet no hit LM South no
  hit LM North
• min_bias
• zero_bias
• Contamination in gap samples, particularly GAPS
• Mike Strang, Pavel Demine have looked at this
• Appears to be part diffractive, part
  non-diffractive
• GAPS currently has higher prescale than GAPN to
  compensate for the difference in rates we are
  losing genuine diffractive events
• Is there a timing problem in the L1 and/or
  terms...?

7
L1 and/or terms
Trigger decisions are made with 256 L1 a/o terms.
Five of these are

• fastz
• timing coincidence
• ahalo
• timing S then N
• phalo
• timing N then S

- only one of these three can be issued at a time

• lm south
• hit S
• lm north
• hit N

• both should be fired if fastz
• or ahalo or phalo fired
• - gap triggers veto these

8
L1 and/or terms

• In an event
• a/o terms are fired or not fired
• sent in groups of 16 by sub-detectors to the
  Trigger Framework
• TFWK waits for 25 ticks (some groups are very
  slow) then makes the L1 decision
• So the and/or terms could miss the boat somewhere
  and not be included in the L1 decision.
• ...a not-fired lm a/o term looks to the trigger
  like a gap...

9
Data Cuts

• Data
• trigger list CMT-9.20
• reconstructed with p13.05.00
• Cuts
• zero bias sample
• no primary vertex (to get rid of beam)
• all others
• at least 1 pvtx
• at least 5 tracks matched to pvtx
• zpvtx lt 50cm
• metc lt 0.7 pT of leading jet
• 2 jets with pt gt 25 GeV (except min bias sample)

10
GAPS GAPN events

• None should have fastz/ahalo/phalo fired
• GAPS very contaminated (89)
• GAPN also contaminated (24)

11
NG GAPSN events

• NG if fastz fired
• 5.9 lm south did not fire
• 0.1 lm north did not fire

12
Min bias zero bias events

• min bias
• 3 lm south does not fire
• 0.2 lm north does not fire

13
Conclusion

• LM N S terms are sent to the Trigger Framework
  with fastz, which we know gets there on time, so
  that isnt the problem
• The likely culprit the N S read-out
• depends on the cables from the LM, pre-amps etc
• we need to have this checked
• Rich Partridge is going to re-examine it...

in fact, the TFWK timing is set to fastz!