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Observation of single-diffractive W production with CMS: a feasibility study

Observation of single-diffractive W production with CMS: a feasibility study. Antonio Vilela Pereira ( Universit à degli Studi di Torino & INFN Torino) On behalf of the CMS Collaboration. DIS 2008 - London. Outline. W   selection Observation of single-diffractive (SD) W signal

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Observation of single-diffractive W production with CMS: a feasibility study

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  1. Observation of single-diffractive W production with CMS: a feasibility study Antonio Vilela Pereira (Università degli Studi di Torino & INFN Torino) On behalf of the CMS Collaboration DIS 2008 - London

  2. Outline • W   selection • Observation of single-diffractive (SD) W signal • Background & Systematics (Large Rapidity Gap) NB: No pile-up Effective luminosity for single-interactions – 100 pb-1 DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  3. Motivation • Rediscover hard-diffraction at LHC • Sensitive to quark component of diffractive PDF’s (dPDF’s) • Probe Rapidity Gap Survival Probability (S2) – connection to multiple partonic interactions and soft rescattering effects … dPDF (Large Rapidity Gap) Ratio of diffractive to inclusive W’s measured at Tevatron (~1%) (cf. K. Goulianos’s talk) DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  4. W selection W   selection: (same as inclusive analysis) 1 isolated  pT > 25 GeV 50 < MT < 200 GeV || < 2.0  =  - (,ETmiss) < 1 rad n jets(ET > 40 GeV)  3 n muons(pT > 20 GeV) < 2 Overall efficiency: Pomwig SD W: 34% (2.4k evts/100pb-1) Pythia W: 28% (600k evts/100pb-1) NB: Diffractive W: Pomwig v2.0beta (dPDF: NLO H1 2006 Fit B) DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  5. Forward detectors CMS Hadronic Forward (HF) Hadronic Forward (HF) (3.0 < || < 5.0) (3.0 < || < 5.0) CASTOR CASTOR ZDC ZDC TOTEM T2 TOTEM T2 (|| > 8.1) (|| > 8.1) (5.2 < || < 6.6) (5.2 < || < 6.6) Not present in start-up TOTEM T1 TOTEM T1 TOTEM: Total Cross Section, Elastic Scattering and Diffraction Dissociation at the LHC DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  6. Selecting diffractive events • Diffractive events historically selected through Large Rapidity Gaps (LRG’s) in (forward) hadronic final state • Select diffractive events based on multiplicities in CMS Forward Hadron Calorimeter (HF) and/or CASTOR • In addition, look at activity in central tracker Generated particles – energy weighted Diffractive sample generated with gap in –plus side For selection: exploit different multiplicity distributions in forward and central regions (cf. Tevatron, HERA) DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  7. Observation of diffractive signal • Selection of Rapidity Gap side: • Observed gap side: side with lowest energy sum in HF (HF-plus vs HF-minus) • Pre-selection on central Track Multiplicity • Maximum number of reconstructed tracks in central tracker (|| < 2.0, pT > 0.9 GeV) • HF tower multiplicity in “low ” vs. HF tower multiplicity in “ high ” region • HF tower multiplicity vs. CASTOR sector multiplicity in gap side CMS (3.0 < || < 5.0) HF HF ZDC CASTOR ZDC CASTOR (|| > 8.1) (5.2 < || < 6.6) DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  8. HF multiplicity at “low ” vs HF multiplicity at “high” “low ”: 3.0 < || < 4.0 “high ”: 4.0 < || < 5.0 Excess in low multiplicity visible Non-diffractive background clusters in the low multiplicity (signal) region DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  9. HF tower multiplicity vs CASTOR -sector multiplicity in gap side • CASTOR: (5.2 < || < 6.6) • no  segmentation, 16 -sectors • Generator level information: number of -sectors with energy above threshold (E > 10 GeV) • For start-up (2008), installed only in one side (z-minus) • Reject events in which observed gap side is not the z-minus (CASTOR) side Potential signal increase by factor ~2 with CASTOR in both sides Clear peak at low multiplicity DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  10. Effect of central track pre-selection Increased non-diffractive background when loosening the track pre-selection Diffractive signal less visible when no track pre-selection is present – but still visible ~ 100 – 200 events in zero multiplicity bin (S/B ~ 6 - 20) DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  11. Backgrounds • Non-diffractive W   • Inclusive QCD events • Applied full selection to sample of “-enriched” QCD sample (Pythia) • Inclusive W selection: ~ 50k events/100pb-1 (< 10% of inclusive W rate) • Negligible contribution at low HF/CASTOR multiplicity (< 1% level wrt diffractive W yields) • Proton-dissociative events: ppXN (with X containing a W boson and N a low mass state into which the proton diffractively dissociates) • Cross section expected to be of the same order as signal • If N escapes detection (limited forward coverage), it’s an irreducible background (but it’s also diffraction) • Around 50% can be rejected by vetoing on ZDC • Expect enhancement in signal region by ~ 30% (cf. S. Ovyn’s talk) http://cms-physics.web.cern.ch/cms-physics/public/DIF-07-001-pas-v4.pdf DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  12. Systematics • Rapidity Gap Survival Probability (S2): • Assumed 5% (as indicated by theory) • Ndiff S2 • Extraction of diffractive yields provides S2 measurement • W selection and reconstruction: from inclusive analysis (cf. S. Bolognesi’s talk) • Same for diffractive and non-diffractive • Negligible effect on acceptance • UE description: • Generator level study: Pythia Tune DWT (default) vs Tune A • Selection applied on full simulated Alpgen W events • Potential background enhancement by factor 3 – 5 in signal region http://cms-physics.web.cern.ch/cms-physics/public/EWK-07-002-pas-v8.pdf DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  13. Summary • Procedure discussed to observe single-diffractive W   production with an effective 100pb-1 for single interactions • Diffractive selection based on forward hadron multiplicities in HF and CASTOR calorimeters, as well as multiplicity in central tracker • For S2 = 0.05, expected O(100) reconstructed signal events per 100pb-1 • S/B of up to 20 with CASTOR • This study emphasizes the need of CASTOR (ideally on both sides) • TOTEM T1/T2 information (similar acceptance as HF/CASTOR) should add to the background rejection DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  14. BACKUP DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  15. MC samples • Diffractive W samples: • Pomwig v2.0beta • Proton PDF: CTEQ6M • dPDF: NLO H1 2006 Fit B (IP(0) = 1.111, ' = 0.06 GeV-2, BIP = 5.5 GeV-2) • Non-diffractive samples: • Pythia W • Pythia Inclusive QCD (-enriched) • Alpgen W • Cross sections: • - Diffractive W:  ~ 69 pb (NLO PDFs) (Assumes S2 = 5%) • - Non-difractive W:  ~ 21 nb (NLO cross section) This gives a ratio of diffractive to inclusive cross sections of ~0.3% DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  16.  distribution MX=(s)1/2  = p/p DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  17. Choosing the gap side • Gap side: side with lowest energy sum in HF (HF-plus vs. HF-minus) • Energy thresholds for counting HF towers minimizing the probability of choosing the gap side incorrectly • HF alone has limited • coverage for gap side • selection Fraction of times the gap side is chosen incorrectly DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  18. HF tower multiplicity in gap side • HF multiplicities in gap side for diffractive (Pomwig) vs non-diffractive (Pythia) W events • Gap side: side with lowest energy sum in HF (HF-plus vs HF-minus) • Pre-selection on number of reconstructed tracks in central tracker (|| < 2.0, pT > 0.9 GeV) Diffractive signal cannot be clearly distinguished in “data” sample DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  19. Effect on track pre-selection – HF “low ” vs HF “high ” S/B for zero multiplicity bin: O(1) or less Diffractive signal most visible when track pre-selection is tighter > 200 events in zero multiplicity bin even for strict track pre-selection DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  20. CMS Forward detectors (D. d’Enterria hep-ex/0708.0551) (3.0 < || < 5.0) CMS (3.0 < || < 5.0) Not present in start-up CASTOR CASTOR ZDC ZDC HF HF T2 (|| > 8.1) (|| > 8.1) T2 (5.2 < || < 6.6) (5.2 < || < 6.6) DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

  21. CASTOR & ZDC • CASTOR: Centauro And Strange Object Research • Quartz tungsten sampling calorimeter • 5.2 < || < 6.6 – charged & neutrals • EM + HAD sections • CMS Zero Degree Calorimeter • Quartz tungsten sampling calorimeter • || > 8.1 - neutrals • EM + HAD sections DIS08 Observation of single-diffractive W production with CMS A. Vilela Pereira

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