1 / 19

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)

ova
Télécharger la présentation

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

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related