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Theoretical uncertainties in W/Z cross-sections

This paper discusses the evaluation of theoretical uncertainties in the W and Z cross-sections at the LHC. Topics covered include electron acceptances, acceptance differences, Pythia documentation, main sources of difference, QED corrections, pdfs, overall systematic uncertainties, NNLO corrections, and the ratio of W to Z cross-sections.

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Theoretical uncertainties in W/Z cross-sections

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  1. Theoretical uncertainties on W/Z cross-sections Marc Goulette ATLAS CAT physics Meeting, CERN, 5 September 2008 Thanks a lot to: M. Boonekamp, J. Butterworth, A. Cooper-Sarkar, E.Dobson, N. Ellis, D. Froidevaux, L. Gilbert, I. Hinchliffe, J. Huston, B. Kersevan, E. Richter Was, M. Seymour, T. Sjostrand, G. Stavropoulos and T. Weidberg. Recent paper: Evaluation of the theoretical uncertainties in the Wl Cross Sections at the LHC (N. E. Adam, V. Halyo, S. A. Yost, W. Zhu), Aug. 08

  2. Outline • Introduction • W and Z cross sections, pdfs and selection cuts • Electron acceptances for W and Z • Study of acceptance differences • Pythia documentation • Main sources of difference (ISR, kT) (except pdfs) • Comment about the intrinsic kT of the incoming partons • Problem in interface between Herwig and Photos • Impact of QED corrections and recombination • pdfs • Overall systematic uncertainties • NNLO corrections • Ratio of W to Z cross sections • Conclusion and outlook

  3. Introduction • We and Zee events at the generator level. • Aim: Determine the systematic uncertainty on W/Z cross sections. • Systematic uncertainty on cross-section is dominated by uncertainties determined on acceptances. • More details: ATLAS note submitted (ATL-COM-2008-010) • Last talk: 13 December 07, ATLAS SM group • http://indico.cern.ch/conferenceDisplay.py?confId=12059 • Update with release 12.0.6, standard DC3-jobOptions • Monte Carlo Generators: • Pythia 6.4, Herwig6.510+Jimmy 4.0, MC@NLO 3.2 • Between 100 K and 500 K events for the 3 generators • Some comments about the generation: • Same Mass and same Width of the bosons for all generators • Herwig & Pythia cross sections: agreement better than 1% (see next slide) • Same parton density function (pdfs) for all (CTEQ6L and CTEQ6M) • Standard cuts: pTleptons > 25 GeV and || < 2.5

  4. Cross sections, pdfs and filter cuts • PYTHIA: • LHAPDF version 4.0 • CTEQ6L1-LO with LO alpha_s • HERWIG: • HWLHAPDF modpdf 10042 • LHAPDF version 4.0 • CTEQ6L1-LO with LO alpha_s • MCATNLO: • HWLHAPDF modpdf 10000 • LHAPDF version 4.0 • CTEQ6m (NLO version of CTEQ6L1-LO) * Ref: hep-ph/0307219 (S. Alekhin), 2003 ** only statistical errors pdfs • LEPTON FILTER: • pT_cut = 0.0 • _cut = 5.0 • STANDARD CUTS: • pTe > 25 GeV for W • pTe > 20 GeV for Z • pT > 25 GeV • || < 2.5 excluding the crack (1.371.52) • Electron and photon merged if R = (2+2) < 0.04

  5. Acc. for We and Zee (STD) Acceptances as a function of max with pTleptons > 25 GeV (W) and pTe > 20 GeV (Z) and |e| < 2.5 (crack excluded) Zee We HW/PY: 0.7 % for W and 2 % for Z HW/NLO: 7 % for W and 11 % for Z Acceptance with all effects switched OFF (no UE, ISR, kT nor ME) give:

  6. Study of acceptance differences slide 7 - check main distributions (see note or previous talks…) - check printouts, variables, comparison with standalone mode - study individual sources of differences: - ISR - kT - ME - UE - impact of the electroweak (EWK) corrections (Photos) - impact of the electron/photon recombination - pdfs - Impact of boson mass and width (difference between W and Z) - Impact of the individual cuts - Re-weighting method on pTW and pTZ - NNLO corrections slide 8 Slide 9 Slides 10 Slide 10 Slide 11 Slide 13

  7. Problem in Pythia documentation Py Px (MeV) (MeV) Pz (MeV) Should be a perfect delta function Problem here … • - We should have : Px_s~+Px_c = Px_W and the same for Py and Pz … • Torbjorn S. checked on 10 K events (Thanks !): nothing in standalone mode • Info from common blocs not directly accessible in Athena • Should this be fixed (so that same problem is not rediscovered later ?)

  8. Impact of ISR on acceptances ISR Fluctuations for MC@NLO due to sligthly less statistics (100 k compared to 500 k) and most probably due to different pdfs model ! _ xu xd _ xc xs

  9. Intrinsic kT of the incoming partons Wmass Tail for Pythia (doc ? Heavy quarks ? Other ?) Slight difference in parameters between Pythia and Herwig: kT Pythia Mean 1.753 GeV (needed to reproduce Tevatron data on pTZ) RMS=935 MeV (mstp 91) Herwig Ptrms=1.753 GeV  Mean 2.37 GeV RMS=1187 MeV (Note: kT not switched on by default in Herwig)

  10. Impact of QED corrections Problem in interface between Herwig and Photos for Z: 7% !! 2.3 % 2.1 %  So no numbers discussed for Herwig here 1.1 % effect Note: - no electron/photon(s) recombination  - 2% on acceptance - with recombination  effect vanishes - to be checked for systematics with more realistic simulation

  11. Pdf impact on acceptances + 0.89 % - 0.90 % syst ~ Calculations for the W acceptance using ResBos + 0.72 % - 0.69 % W- NLO 6.1 W-MC@NLO 3.2 + 0.89 % - 0.90 % W- MC@NLO 6.1 W+ NLO 6.1 5 W+ MC@NLO 6.1 + 0.66 % - 0.69 % W+MC@NLO 3.2 + 0.68 % - 0.84 % syst = [(acc(i)-mean)2]) (stat is not significant) Thanks to Joey Huston. Please note that MC experts said that this recipe is not good enough because moving to another pdfs model could introduce an uncertainty on the acceptances (shift of the central value of the set) that could be higher than the fluctuations …

  12. Overall systematic uncertainty • From the Pythia / Herwig comparison (i.e at LO), we get the following uncertainties (using in the worst case 20 % of the impact on the acceptances from offon, and 10 % for QED): • For W: 0.11 % (ISR), 0.38 % (kT), 0.0 % (ME), 0.19 % (UE) and 0.11 % (Photos). • For Z: 0.34 % (ISR), 0.10 % (kT), 0.02 % (ME), 0.04 % (UE) and 0.0 % (Photos). • In addition we have an estimate of the pdfs uncertainty from CTEQ (at NLO):  0.90 % (for W), same applied for Z (resbos data. not available). • Overall syst. excluding crack is: ~1.01 % for W and ~0.97 % for Z for effects studied here. • Is there anything missing ? Yes, impact of higher orders.

  13. NNLO corrections 1) Comparison with NNLO predictions: Ref: hep-ph/0603182 (Melnikov & Petrielo), 2006 _ W- production: pp  W- X  e- X  From 1.8 to 2.8 % if pTe min > 20, 30 GeV, so take ~ 2.3 % for pTe min > 25 GeV 2) NNLO corrections: • - Remember that LO to NLO gave a 7 % difference for We • Comparison here between NLO and NNLO gives ~ 2.3 % • Reasonable assumption ? Assume systematic of ~ 30 % of acceptance difference between NLO and NNLO (0.76 %) and add it (quadr.) to previous overall systematic: 1.23 % for W and 1.19 % for Z

  14. Ratio of We to Zee • - Precision measurements of the ratio of We to Zee production cross sections • are used to test the Standard Model. • The ratio of cross sections can be expressed in terms of measured quantities: • where F is the correction factor for converting the measured */Zee cross section • into the cross section for pure Z boson exchange. • Uncertainties from Background dominant until we have a lot of data (QCD corrections) • Efficiencies depends on Trigger and Reconstruction. • Uncertainty on the ratio of the efficiencies should also be less big than the individual • uncertainties. 1 Nwsig - Nwbck AZZ . . . R = F NZsig - NZbck AWW

  15. Which uncertainties cancel in the ratio ? • The integrated luminosity terms cancel completely in the ratio calculation. • Uncertainties on the acceptances arising from the PDF model are significantly • smaller for the ratio than for either individual acceptance. See for instance a nice • paper produced in 2007 by CDF: • “Measurements of inclusive W and Z cross sections in pp collisions at s = 1.96 TeV”. • Can’t be shown here because pdfs data not available for Z. • In my study: _ Don’t know yet what to deduce from that… (*) due to problem in interface Photos seems ok, but the other sources ?

  16. Conclusion and outlook • Final numbers for acceptances with high statistics - use MC@NLO as central value and extract acceptances - W:  = 36.84  0.10 % (with final definition of crack) - Z:  = 51.02  0.10 % • Systematic uncertainties - use 1.23 % for W and 1.19 % for Z with current knowledge - Point out that dominant sources (ISR, kT, pdfs) will be further constrained with data: • pTW, pTZ spectrum • Zee found with one electron with |e|>>2.5 • Outlook - Need to recompute acceptance with large statistics using MC@NLO and full simulation/reconstruction - Requires different evgen files beyond standard configuration (no QED, different pTW, different pTZ, anything else ?) - Probably do this only at 14 TeV ?

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