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Parton-level study of Z  l + l - for luminosity measurement

Parton-level study of Z  l + l - for luminosity measurement. Motivation PDF uncertainties Parton-level study & rate estimation Relaxed cuts & Conclusions. Ricardo Gon ç alo. Z l + l - for luminosity measurement. Proposed in: Dittmar, Pauss & Zurcher Phys.Rev.D56 7284 (97)

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Parton-level study of Z  l + l - for luminosity measurement

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  1. Parton-level study of Z  l+l- for luminosity measurement • Motivation • PDF uncertainties • Parton-level study & rate estimation • Relaxed cuts & Conclusions Ricardo Gonçalo

  2. Zl+l- for luminosity measurement • Proposed in: Dittmar, Pauss & Zurcher Phys.Rev.D56 7284 (97) • Basically just count nr of Z  ee and Z   • Shown in Rome meeting by J.Pinfold and Bryan L.Caron (see also next slide): • tlumi = 8.6 – 28 hours • First Year Operations • L = 2 x1033cm-2 s-1 • W : (12.0 – 2.4) Hz • Z : (2.0 – 0.4) Hz • Significant problem seems to be PDF uncertainties (several % uncertainty) ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  3. NNLO Calculations & PDFs (J.Pinfold’s talk) Anastasiou et al., Phys.Rev. D69:094008, 2004 • Bands indicate the uncertainty from varying the renormalization (mR) and factorization (mF) scales in the range: MZ/2 < (mR = mF) < 2MZ • At LO: ~ 25 - 30 % x-s error • At NLO: ~ 6 % x-s error • At NNLO: < 1 % x-s error Similar improvement in calculation for W at NLO and NNLO VRAP code at http://www.slac.stanford.edu/~lance/Vrap/ • PDFs - See talk from A. Tricoli from the Standard Model session for some estimated PDF uncertainties upon measured W→ lnh distributions. Currently PDFs contribute several percent to the errors. ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  4. What do the PDF uncertainties look like at Q2~MZ? MRST2001 - 30 PDF “eigenvector” sets ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  5. P1 X1P1 (x1+x2).P=q Z0 X2P2 P2 Can we improve the situation? • The idea is to try to select Z bosons produced almost at rest in the lab frame to try to reduce the range of Bjorken x in the hard scatter. • To do this, select events with Z decaying to two leptons back-to-back in the R-Z plane using pseudorapidity (1= -20.1) • Should get for free lepton energies close to 45GeV and back-to-back leptons in phi => can also cut on these variables • Hopefully this would limit x to a region where PDF uncertainties are small. • At LO we should have log10x~-2.2: ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  6. Naïve case • Pythia 6.321; CTEQ5L PDF set; 200k events: • No initial-state parton showers • No multiple interactions • Only ppZ (ISUB=1) • Simple cuts only: • Preselection: 2 electrons or muons of opposite charge from a Z decay (truth only, no mis-ID) • Selection: • |l|<1 to have a clean sample • 2 = l0.1 to select Z “at rest” in beam direction ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  7. Naïve case ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  8. Naïve case • Lepton pT above 25GeV for selected sample  good trigger efficiency • Hadronic pT low (note this also includes very forward particles which are missed by detector) • Expect hadronic pT to increase in more realistic case ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  9. Naïve case • mZ calculated from stable lepton momentum only, no correction for FSR (and plot slightly distorted by generator-level cut) • All seems to work for naïve case but resolutions will be smeared when initial PS included • Most critical is the effect on x ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  10. P1 X1P1 X2P2 Z0 P2 With initial-state parton showers • Initial-state parton showers • Approximates higher-order processes • Range of x now very wide but still limited to x>6.5x10-3 • This may still be good to minimise PDF errors due to DGLAP evolution integrals ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  11. With initial-state parton showers ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  12. With initial-state parton showers • Lepton pT> ~20 GeV  trigger efficiency should still be ok ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  13. With initial-state parton showers Z0’s momentum still pretty much ok ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  14. Apart from the low statistics, some (naïve?) estimates can be made: Assume preselection is all that can be seen in detector with 100% acceptance in |eta|<2.5 Production rate is: L~1033cm-2s-1 ~ 1nb-1s-1; x-section ~ 43nb (Pythia) => R=43Hz Other factors (ex.BR to e and mu ~2x3.36% =>2.89Hz; etc…) Preselection (|eta|<2.5; Elep>9) gives ~2.9%-3.0% (~6000sel./200k gen.) Reduction factor from the eta cuts is: 1.9%+-0.2% (no initial PS) and 2.1%+-0.2% (with PS) This means: Preselection: ~1.3Hz Additional “back-to-back” cuts: ~0.026Hz And so (at low luminosity): 3.8x103s ~ 1 hour to get 10% statistical accuracy (~100 ev) 15x103s ~ 4.2 hours to get 5% 384x103s ~106 hours to get 1% Other acceptance factors on top of these would be trigger and reconstruction/tagging efficiencies for e/mu which could be ~0.8-0.5 or less… Rates and efficiencies Must relax cuts to get higher rates! ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  15. Relaxed angular cuts • Same sample as the previous one (parton showers ON) • Relaxed cuts on back-to-back-ness of leptons: • |l|<2.5 instead of |l|<1 • 2 = l0.3 instead of 2 = l0.1 • Sharp edge on Bjorken x still in the same place • But minimum lepton pT now very low (next 2 slides) • Also wide leptonrange means that crack regions become relevant etc • This means other sources of systematic uncertainty become important ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  16. Relaxed angular cuts ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  17. Relaxed angular cuts • Minimum lepton pT is now very low even if energy still ~45 GeV ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  18. Relaxed angular cuts • With these cuts: • Preselection: 6080 sel./200k gen. • After angular cuts: 534 ev. (factor of ~4 over previous cuts) • But events clearly not as clean as before ATLAS-UK Higgs meeting - Glasgow, 24 June 05

  19. Production rate is: L~1033cm-2s-1 ~ 1nb-1s-1; x-section~43nb (Pythia)=> R=43Hz Preselection (|eta|<2.5; Elep>9) gives ~2.9%-3.0% (~6000sel./200k gen.) Selection (eta cuts): 8.8%+-0.4% So: Preselection: ~1.3 Hz Selection: ~0.11 Hz 10% stat. accuracy in ~15mins. 5% stat accuracy in ~1 hour 1% stat accuracy in ~24 hours Conclusions: The numbers in these page start to look encouraging But: This is a parton level study Detector and trigger acceptances need to be taken into account Systematic uncertainties may make us loose what we may gain from limiting the region in x that we're sensitive to. Relaxed angular cuts: rates and conclusion ATLAS-UK Higgs meeting - Glasgow, 24 June 05

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