1. 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 Theoretical uncertainties on W/Z cross-sections

2. 5 September 2008 Marc Goulette 2/16 Outline 1. Introduction 2. W and Z cross sections, pdfs and selection cuts 3. Electron acceptances for W and Z 4. Study of acceptance differences 1. Pythia documentation 2. Main sources of difference (ISR, k T ) (except pdfs) 3. Comment about the intrinsic k T of the incoming partons 4. Problem in interface between Herwig and Photos 5. Impact of QED corrections and recombination 6. pdfs 5. Overall systematic uncertainties 6. NNLO corrections 7. Ratio of W to Z cross sections 8. Conclusion and outlook

3. 5 September 2008 Marc Goulette 3/16 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, Herwig 6.510+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: p T leptons > 25 GeV and |  | < 2.5 Introduction

4. 5 September 2008 Marc Goulette 4/16 Cross sections, pdfs and filter cuts Cross sectionsPythiaHerwigMC@NLONNLO*  W  e (pb)17210 (  132)**17315 (  132)19885 (  138)22097 (  692)  Z  ee (pb)1731 (  42)1676 (  16)1985 (  155)2018 (  64) * Ref: hep-ph/0307219 (S. Alekhin), 2003 ** only statistical errors LEPTON FILTER: p T _cut = 0.0  _cut = 5.0 STANDARD CUTS: p T e > 25 GeV for W p T e > 20 GeV for Z p T > 25 GeV |  | < 2.5 excluding the crack (1.37  1.52) Electron and photon merged if  R =  (  2 +  2 ) < 0.04 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) pdfs

5. 5 September 2008 Marc Goulette 5/16 Acceptances as a function of  max with p T leptons > 25 GeV (W) and p T e > 20 GeV (Z) and |  e | < 2.5 (crack excluded) Acc. for W  e and Z  ee (STD) W  e Z  ee HW/PY: 0.7 % for W and 2 % for Z HW/NLO: 7 % for W and 11 % for Z  max 2.5 MC@NLO 51.02  0.10 HERWIG 45.27  0.14 PYTHIA 46.20  0.07 HERWIG/PYTHIA 97.98  0.34 HERWIG/MC@NLO 88.72  0.33 PYTHIA/MC@NLO 90.55  0.23  max 2.5 MC@NLO 36.84  0.10 HERWIG 34.27  0.15 PYTHIA 34.51  0.07 HERWIG/PYTHIA 99.29  0.48 HERWIG/MC@NLO 93.02  0.47 PYTHIA/MC@NLO 93.94  0.30 Acceptance with all effects switched OFF (no UE, ISR, k T nor ME) give: HERWIG/PYTHIA 98.42  0.38 HERWIG/PYTHIA 98.98  0.32

6. 5 September 2008 Marc Goulette 6/16 Study of acceptance differences - check main distributions (see note or previous talks…) - check printouts, variables, comparison with standalone mode - study individual sources of differences: - ISR - k T - 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 p T W and p T Z - NNLO corrections Slide 9 Slides 10 Slide 11 Slide 13 slide 8 Slide 10 slide 7

7. 5 September 2008 Marc Goulette 7/16 Problem in Pythia documentation 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 ?) (MeV) Px Py Pz Should be a perfect delta function

8. 5 September 2008 Marc Goulette 8/16 Impact of ISR on acceptances ISR xu xd xc xs _ _ Fluctuations for MC@NLO due to sligthly less statistics (100 k compared to 500 k) and most probably due to different pdfs model !

9. 5 September 2008 Marc Goulette 9/16 Intrinsic k T of the incoming partons Tail for Pythia (doc ? Heavy quarks ? Other ?) Mean 1.753 GeV Ptrms=1.753 GeV  Mean 2.37 GeV RMS=935 MeV (mstp 91) RMS=1187 MeV Pythia Herwig (needed to reproduce Tevatron data on p T Z ) Slight difference in parameters between Pythia and Herwig: kTkT (Note: k T not switched on by default in Herwig) Wmass

10. 5 September 2008 Marc Goulette 10/16 Impact of QED corrections Problem in interface between Herwig and Photos for Z: 7% !!  So no numbers discussed for Herwig here Note: - no electron/photon(s) recombination  - 2% on acceptance - with recombination  effect vanishes - to be checked for systematics with more realistic simulation 1.1 % effect 2.3 % 2.1 %

11. 5 September 2008 Marc Goulette 11/16 Pdf impact on acceptances W - NLO 6.1 W + NLO 6.1 5 W + MC@NLO 6.1 W - MC@NLO 6.1 Thanks to Joey Huston. Calculations for the W acceptance using ResBos W + MC@NLO 3.2 W - MC@NLO 3.2  syst ~ + 0.72 % - 0.69 % + 0.89 % - 0.90 % + 0.66 % - 0.69 % + 0.68 % - 0.84 % + 0.89 % - 0.90 %  syst =  [  (acc(i)-mean) 2 ]) (stat is not significant) 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. 5 September 2008 Marc Goulette 12/16 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. 5 September 2008 Marc Goulette 13/16 NNLO corrections Ref: hep-ph/0603182 (Melnikov & Petrielo), 2006 W - production: pp  W - X  e - X  From 1.8 to 2.8 % if p T e min > 20, 30 GeV, so take ~ 2.3 % for p T e min > 25 GeV _ 1) Comparison with NNLO predictions: 2) NNLO corrections: p T e min (GeV) A (NLO)A (NNLO)A (MC@NLO) My data, A (MC@NLO) A (my data)/ A (NNLO) in % 200.4880.4920.485 0.4835  0.0007 1.8 300.3780.376- 0.3654  0.0007 2.8 - 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. 5 September 2008 Marc Goulette 14/16 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 N w sig - N w bck A Z  Z F N Z sig - N Z bck A W  W R =

15. 5 September 2008 Marc Goulette 15/16 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: _ Ratio (A Z /A W )PythiaHerwig ALL off 1.0653  0.011.0714  0.01 STD photos on 1.1330  0.01 n.a (*) STD photos off 1.1321  0.01 n.a ISR on 1.3375  0.021.3291  0.02 kT on 1.1756  0.011.1587  0.01 ME on 1.1469  0.011.1536  0.01 UE on 1.1466  0.011.1502  0.01 Don’t know yet what to deduce from that… Photos seems ok, but the other sources ? (*) due to problem in interface

16. 5 September 2008 Marc Goulette 16/16 Conclusion and outlook I)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 % II)Systematic uncertainties - use 1.23 % for W and 1.19 % for Z with current knowledge - Point out that dominant sources (ISR, k T, pdfs) will be further constrained with data: p T W, p T Z spectrum Z  ee found with one electron with |  e |>>2.5 III)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 p T W, different p T Z, anything else ?) - Probably do this only at 14 TeV ?