Physics selection and pileup rejection for pp multiplicity

Evgeny Kryshen

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Some details on LHC12h• Typical filling scheme: 50ns_1374_1368_0_1262_144bpi12inj• 2500 main-satellite collisions per orbit (25ns separation between main-satellite bunches). • Minimum bias (CINT7) cross section: ~56 mb• Runining strategy:

– VBAND>200kHz – switch off V0 and run with CINT8 suite– VBAND<200kHz – swith on V0 and run with CINT7 suite

• Average number of interactions per bunch crossing µ ~ rate/(2500*11223) µ = 0.007 @ VBAND=200 kHz

• Conditions similar to those expected in high-multiplicity runs in RUN2 (25ns filling scheme with ~2500 colliding bunches per orbit)

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Tracklet-vs-cluster cut• Tracklet-vs-cluster cut not included in default physics selection• Standard cut: nClusters< 64+4*nTracklets• Huge remaining background after applying standard V0-based physics selection• Up to 90% of high-multiplicity triggers are rejected with tracklet-vs-cluster cut

CINT7 CSHM8

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Pile-up rejection• Pileup rejection crucial for high-multiplicity studies• Available pileup rejection tools: AliAnalysisUtils

SPD (min contrib = 5, min zdist = 0.8) Multi-vertexer (min contrib = 5)

• In general pileup is not a big issue in LHC12h.• Pileup rejection probability grows with multiplicity as expected (todo: check slope).• TODO: Why CSHM8 is less sensitive to pileup than CINT7 (requirement of T0Pileup rejection in SHM8

on the level of PS)?• TODO: Do we introduce any bias on observables with pileup rejection?

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More puzzles on pileup rejection

• No T0Pileup is required for CSHM8 in this case

• Puzzling behaviour of pileup probability vs TRK/V0M

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Physics selection and pileup rejection (CINT7)

• Inclusion of PS + TRK-CLS very important for proper determination of multiplicity quantiles especially for high-multiplicity events

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Physics selection and pileup rejection (CSHM8)

• Inclusion of PS + TRK-CLS very important for proper determination of multiplicity quantiles especially for high-multiplicity events

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LHC12h statistics: CINT7 vs CSHM8

• CSHM8 gives factor 100 gain wrt CINT7 in CL1 and TRK high-mult. events

• Only factor 10 gain in V0M high-mult. events due to weak forward-central correlation

• CSHM8 can be used for the extention of multiplicity quantiles towards very high-multiplicity events

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Backup

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MotivationCMS:• Dedicated high-multiplicity trigger based on track multiplicity• Statistics: 980 nb-1 at 7 TeV (easily achievable with ALICE, just

need efficient high-multiplicity trigger)• Near-side ridge most pronounced in ~0. 1% most “central”

events at 1<pt<2 GeV/c.Possible goals for ALICE:• Search for near-side ridge structures (with and without

subtraction)• Extend CMS measurement to forward rapidity• Search for unforeseen

Goal of this analysis: • study background and pile-up rejection performance as

function of multiplicity• study various multiplicity estimators • study high-multiplicity trigger rates • study two-particle correlations in LHC12h period, select

~0.1% most “central” events in bin 1<pt <2 GeV/c to estimate required statistics for run 2

• write analysis note on these feasibility studies

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muon-ITStrack correlations with dimuon like-sign trigger

rel.error/bin ~3% rel.error/bin ~1.5% -> 50% error on V26304 muons 37580 muons

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CMS all bins

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Multiplicity distributions in LHC12h

Tail not yet understoodProbably residual background

0.1% corresponds to nTracklets >76

0.1% corresponds to V0A>160

n tracklets

*Equalized V0 multiplicities are shown

V0M

V0A V0C

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High multiplicity trigger rates (LHC12h)Interaction rate

CSHM8-B rate

CSHM8-B rate/Interaction rate

CSHM8-B rate ~ 200-250 Hz @ 200kHz interaction rate

Ratio strongly depends on background conditions

CSHM8:• 0TVX • >=120 outer FO chips

no TPC and/or V0 in these runs

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High multiplicity trigger lumi• Too high L0b rates observed -> high multiplicity trigger was downscaled after run 190050

• Simple luminosity estimate: L = L0b(C0TVX)/σ(C0TVX)*Lifetime(CSHM8)

• Large fraction of events is rejected due to background/pile-up (see next slides).

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Clusters and trigger classesTrigger classes:• HM: a high-multiplicity trigger based on SPD or V0 or T0 multiplicity – further studies needed to take

decision which is preferred– In LHC12h+i, HM = CSHM8 = 0TVX + >=120 outer FO chips– If we want to use V0-like centrality estimator, better to trigger on high V0 mult. (see slide 10)

• HMMSL: HM in coincidence with single muon trigger (MSL): in case HM is downscaled or taken with central barrel only

Clusters:• “rare” mode:

– HM in cluster ALL– If HM in cluster CENT (or FAST), request in addition HMMSL in cluster ALL

• “min-bias” mode: add high-multiplicity classes in fast clusters to collect high-mult sample in a shadow of min-bias data taking– Barrel:

• VFAST: V0+T0+SPD+SSD - ITStrack-ITStrack correlations (no SDD)• UFAST: V0+T0+SPD – tracklet-tracklet correlations (no momentum info)

– Muon-barrel:• MUON: V0+T0+SPD+SSD+MCH+MTR –ITStrack-muon correlations (no SDD)• MFAST: V0+T0+SPD+MCH+MTR – tracklet-muon correlations

• Possibility to study tracklet-muon and tracklet-tracklet correlations was demonstrated on p-Pb data but momentum information is still desirable (so VFAST and MFAST is preferred)

High multiplicity trigger:

Let’s analyse LHC12h period and select ~0.1% most “central” events in bin 1<pt <2 GeV/c to estimate required statistics

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Remark on HM + MSL rate

CSHM8 in coincidence with single muon trigger (0MSL trigger input):

• (CSHM8 & 0MSL)/CSHM8 ~ 0.0075

• CSHM8 rate ~ 200 Hz @ interaction rate ~200kHz

• => (CSHM8 & 0MSL) ~ 1.5 Hz @ interaction rate ~200kHz

• => HM + MSL rate is negligibly small

• This is a pessimistic estimate since • background conditions will hopefully improve• Background rejection can be improved with different HM trigger logic