Physics selection and pileup rejection for pp multiplicity

1. Physics selection and pileup rejection for pp multiplicity EvgenyKryshen

2. 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)

3. 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

4. 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?

5. More puzzles on pileup rejection • No T0Pileup is required for CSHM8 in this case • Puzzling behaviour of pileup probability vs TRK/V0M

6. Physics selection and pileup rejection (CINT7) • Inclusion of PS + TRK-CLS very important for proper determination of multiplicity quantiles especially for high-multiplicity events

7. Physics selection and pileup rejection (CSHM8) • Inclusion of PS + TRK-CLS very important for proper determination of multiplicity quantiles especially for high-multiplicity events

8. 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

9. Backup

10. 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

11. muon-ITStrack correlations with dimuon like-sign trigger rel.error/bin ~3% rel.error/bin ~1.5% -> 50% error on V2 6304 muons 37580 muons

12. CMS all bins

13. 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

14. High multiplicity trigger rates (LHC12h) no TPC and/or V0 in these runs Interaction rate • CSHM8: • 0TVX • >=120 outer FO chips CSHM8-B rate ~ 200-250 Hz @ 200kHz interaction rate CSHM8-B rate CSHM8-B rate/Interaction rate Ratio strongly depends on background conditions

15. 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).

16. 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-ITStrackcorrelations (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-Pbdata 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

17. 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