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Physics selection and pileup
rejection for pp multiplicity
Evgeny Kryshen
Some details on LHC12h
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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:
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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
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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
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Pileup rejection crucial for high-multiplicity studies
Available pileup rejection tools: AliAnalysisUtils
SPD (min contrib = 5, min zdist = 0.8)
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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
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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)
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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)
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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
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CSHM8 gives factor 100 gain wrt CINT7 in CL1
and TRK high-mult. events
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Only factor 10 gain in V0M high-mult. events
due to weak forward-central correlation
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CSHM8 can be used for the extention of
multiplicity quantiles towards very highmultiplicity events
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Backup
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Motivation
CMS:
• 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%
6304 muons
rel.error/bin ~1.5% -> 50% error on V2
37580 muons
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CMS all bins
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Multiplicity distributions in LHC12h
V0M
0.1% corresponds to nTracklets >76
Tail not yet understood
Probably residual background
n tracklets
V0A
V0C
0.1% corresponds to V0A>160
*Equalized V0 multiplicities are shown
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High multiplicity trigger rates (LHC12h)
no TPC and/or V0 in these runs
Interaction rate
CSHM8-B rate
CSHM8-B rate/Interaction rate
CSHM8:
• 0TVX
• >=120 outer FO chips
CSHM8-B rate ~ 200-250 Hz
@ 200kHz interaction rate
Ratio strongly depends
on background conditions
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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 classes
Trigger 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)
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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
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