HLT MET Noise Filters in Run2011B Alex Mott Caltech Review of - - PowerPoint PPT Presentation

Alex Mott

Caltech

HLT MET Noise Filters in Run2011B

Review of Noise Filters

• HBHE noise filters were deployed online earlier this year
• Based on the offline Hcal HBHE Noise filters used in the

PromptReco

• For HLT, add an additional safety factor to avoid rejecting

possible good physics events

• If there are two or more additional RBXs with energy > 40

GeV, do not reject the event

• This filter was shown to overclean at HLT less then 1 ppm
• We intended to deploy this filter as part of the default HLT

MET this fall

• Move to beta*=1m delayed this
• Performance of the filters degraded in Run 2011B –

needed time to study

Scaling with Luminosity

• The online HBHE Noise Filter is becoming considerably less

efficient in rejecting noise as pileup increases

• The offline filter seems to be much less sensitive to the increase

in pileup

MET PD Run2011A Run2011B High PU Fill – ZeroBiasHPF0 PD Offline Filter Online Filter Fraction of Events Rejected

Online Filter Inefficiency

• One issue is with the online safety cut
• At higher pileup, even Zero Bias events pass the cuts a

substantial fraction of the time

• Raising the cut is dangerous
• The pileup falls off substantially over the course of the run
• Even at peak PU, there is a fairly large range of pileup

possible event-by-event

High PU Fill ZeroBiasHPF0 Fraction of events With 3rd RBX E>40 GeV

40 GeV 50 GeV 60 GeV

CaloMET > 60 GeV

Frac of Events Passing 3rd RBX Cut

Modifying the Online Filter

• The 40 GeV threshold is too low for the expected peak 2012

pileup conditions

• Simple raising the threshold is suboptimal for long fills with

large pileup decay

• Two better options for modifying the limit:
• Inst. Lumi dependent: set the energy threshold as a function
• f the luminosity (need to check if this is technically possible)
• Event-by-Event: do something like L1FastJet to subtract the

pileup energy on an event by event basis – keep 40 GeV threshold

• Perhaps we should consider adding a second, higher mass cut

and accepting events passing this regardless of the noise filter

• i.e. for the HLT_MET200 path, only check the online noise

filter for MET between 200 and 300 GeV

• This would suppress the trigger rate while still keeping full

efficiency for very high MET events

Reducing the PU Effect

• One Idea: Apply an energy correction

analogous to L1FastJet

• Order the RBXs by energy
• Compute the median energy of all RBXs >

5GeV except the 3 highest energy

• Subtract this from the energy of the

third highest energy RBX, treat this as the energy for the purpose of the safety cut

High PU Fill ZeroBiasHPF0 Run177201 ZeroBias Run177201 ZeroBias Energy of 3rd RBX Energy Subtracted By this algorithm Frac of events passing online safety cut

Performance on High MET Events

• Its possible that these events are more empty than normal zero

bias

• Would need to tune the subtraction algorithm
• Would accomplish a 50% reduction on the

HLT_MET200_HBHENoiseFiltered path at the current pileup

• This is just a first test
• Suggestions for subtraction algorithm welcome

Run 177201 MET PD HLT_MET200 skim

• On very high MET events, the

reduction isn't as dramatic as on ZeroBias

• This is not fully understood
• Would expect most of these events to

be RBX noise overlaid on Zero Bias

• If this effect is real, need to

understand why

• Perhaps there is other pathology

in high MET events

Removing the Noise from MET

• Try to estimate the MET the event would have

had if not for the HBHE Noise

• Correct the MET in two possible ways
• Set the energy in all hits in the noisy RBX to 0
• Treat the noisy energy as having energy equal to

the second highest energy RBX

• Choose the larger of these two values as the

“corrected MET”

• Try to define this in the safest way possible to

always over estimate the corrected MET

• Apply a looser threshold to this corrected MET

Corrected MET Trigger

• Accept the event is cMET is greater than half the HLT MET threshold
• Half is just an initial test –> can be tuned
• Could conceivably try to recover events with MET lost by opposing HBHE

Noise

• This is a very rare case, and may not be worth the effort
• This method could, in principle, reject a hard interaction coincident with

HBHE Noise

• This event either had little real MET, so shouldn't have passed a MET

trigger anyway

• Or had unbalanced energy coincident with the noise → probably

unrecoverable offline anyway

MET RECO MET > 200 GeV? Reject Event NO HBHE Noise? YES Accept Event NO MET RECO Remove Noise Recompute MET MET > 100 GeV? YES NO

Effect of the MET Cleaning

Run 177201 MET PD HLT_MET200 Skim Events rejected by offline noise filter 80% reduction in MET200 Trigger Rate

Conclusion

• The noise cleaning efficiency of the online filters has decreased

in Run2011B

• This is an inevitable consequence of the increased pileup
• Simply raising the threshold of the RBX safety cut is one

possibility

• This doesn't take into account the change in PU conditions

during the fill

• First attempt at introducing some pileup scaling in the cut

shows modest gains in noise rejection efficiency

– Retune the scaling to maximize rejection – Safety for signal still needs to be evaluated

• An alternative to doing the scaling is to try to estimate the MET

without the spike and cut on that

• This shows the potential to greatly reduce rates
• Needs careful study for signal safety!!