Theoretical Uncertainties in Vector Theoretical Uncertainties in Vector Boson Production at the LHC Scott Yost The Citadel Charleston South Carolina Charleston, South Carolina with N.E. Adam, V. Halyo, W.-H. Zhu y (Princeton) PHENO 2009 – Madison, Wisconsin – May 12, 2009
W and Z Production at the LHC Vector Boson Production will be an important process at p p the LHC: � Standard candle for the precision luminosity measurement (1%). � Precision EW parameter measurements � Precision EW parameter measurements � Constraints on PDFs via Z/W rapidity. � Important for detector calibration. p � New physics searches: Z’ predicted by various SM extensions – few TeV range accessible. 2 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
Precision Event Generator � An event generator is desired at the 1% precision level. � The present best event generators incorporate NLO QCD QCD with a parton shower: MC@NLO or POWHEG. ith a parton sho er MC@NLO or POWHEG � NNLO QCD is available but not interfaced to a shower: � NNLO QCD is available, but not interfaced to a shower: Vrap (Anastasiou, Dixon, Melnikov, Petriello) and FEWZ (Melnikov, Petriello). � Electroweak corrections cannot be neglected. HORACE (Carloni Calame) HORACE (Carloni-Calame) 3 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
Theoretical Uncertainty Studies y � We decided to study the state of the art programs to � We decided to study the state of the art programs to determine how well Z production could really be calculated at this time. [Adams, Halyo, Yost: JHEP 05 (2008) 062] � The results can be useful in selecting experimental cuts to minimize systematic errors, as well as in cuts to minimize systematic errors as well as in identifying the most fruitful course for improving the precision. � The analysis was extended to W production. [Adams, Halyo, Yost, Zhu: JHEP 09 (2008) 133] 4 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
Theoretical Uncertainty Studies y � These studies focused on three areas: � Electroweak Corrections � NNLO QCD � Parton Distribution Functions � The basic generators used were HERWIG 6.5 and G MC@NLO. � Electroweak corrections were evaluated using � Electroweak corrections were evaluated using PHOTOS and HORACE 3.1. � NNLO QCD was calculated using FEWZ. 5 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
Electroweak Effects 2 at LHC energies, NLO QED naively � Since α � α s g , Q y s should enter at a comparable level to NNLO QCD. � But EW corrections are enhanced by big logs (generically log n � s/m 2 � ) which increase at high energy: NLO QED and QCD can be comparable. energy: NLO QED and QCD can be comparable. � This especially affects new physics searches (Z’, …) in the TeV range, where the W and Z begin to look increasingly “massless”. 6 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
Electroweak Effects � We calculated the EW effects in HORACE3.1 � Event Generator with LO QCD + Shower and O ( α ) EW + FSR photon shower. � We also compared PHOTOS (W ą s) � Add-on program that generates photon radiation from charged final-state particles from charged final-state particles. � What is the best way to incorporate mixed QCD/EW effects? � PHOTOS can be run with NLO QCD. � HORACE cannot, but has more complete EW. 7 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
Cut Selections � The comparisons were made for sets of experimental cuts typical of those that might be used in a Luminosity measurement, or precision W/Z parameter measurements. � We are interested not just in the total cross section, but also in the detector acceptances for these cuts. Th The error estimates for acceptances and total cross ti t f t d t t l sections can differ greatly. Acceptance = σ (cut)/ σ (total) σ (cu )/ σ ( o a ) ccep a ce σ (total) already includes basic “generator-level” cuts, such as a lower bound on M ll in Z production to remove the photon-dominated region. th h t d i t d i 8 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
Cut Selections: EW Calculation We used the following cuts: g θ ⎛ ⎞ η = ⎜ ⎟ log cot → γ → q q Z / l l ⎝ 2 ⎠ | η l | M ll p Tl CUT Loose > 40 GeV > 5 GeV < 50 40 < M ll < 140 GeV 40 < M < 140 GeV Tight Tight > 20 GeV > 20 GeV < 2 < 2 ± → → ± ν q ' q W l | η l | p Tl p T ν CUT Basic > 25 GeV > 20 GeV < 1 Larger η 1 < | η l | < 2.2 > 25 GeV > 20 GeV Higher p T ν Higher > 25 GeV > 25 GeV > 30 GeV > 30 GeV < 1 < 1 9 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
Electroweak Effects: Z Production Compare HORACE and PHOTOS in Z Production: � EW correction in σ : 1 – 3%. Difference: 0.2 – 0.4% � Recommendation: Use MC@NLO + PHOTOS. Fractional EW Contribution in Z Production Fractional EW Contribution in Z Production Cross Section Acceptance Comparison 0.0 0.0 0.0 (%) %) %) Contribution (% Contribution (% OS - HORACE ( Cross Section ‐ 1.0 ‐ 0.4 ‐ 1.0 PHOTOS ‐ 2.0 ‐ 0.8 ‐ 2.0 HORACE EW C EW C PHOTO Acceptance ‐ 3.0 ‐ 3.0 ‐ 1.2 ‐ 4.0 ‐ 4.0 ‐ 1.6 Cut: Cut: Cut: se ght se ght se ght loos loos loos tig tig tig 10 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
Electroweak Effects: W Production Compare HORACE and PHOTOS for W Production: p � Using PHOTOS for W production is not as well motivated: the W can radiate also. Use HORACE if EW is important. Fractional EW Contribution in W Production Fractional EW Contribution in W Production Cross Section Acceptance 0.0 5.0 tion (%) n (%) 4.0 -0.5 HORACE W+ 3.0 3 0 W Contribution EW Contribut -1.0 2.0 PHOTOS W+ -1.5 1.0 -2.0 HORACE W- 0.0 -2.5 EW -1.0 1 0 PHOTOS W- -3.0 -2.0 -3.5 -3.0 Note sign basic gher η er p T ν none basic her η er p T ν Cut: Cut: difference difference. highe hig hig high 11 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
Number of Photons Generated Comparison of the numbers of photons gnerated by PHOTOS (black) or HORACE (red). [Born level (no photon generation) = blue] Z Production W + Production W + Production Z Production 12 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
Total Photon P T T Comparison of photon P T in PHOTOS and HORACE: � HORACE gives slightly more photon P T . Z Production W + Production Larger scale Larger scale 13 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
Cut Selections: NNLO Calculation We used the following cuts: (generator cut: M ll > 40GeV) g (g ) ll 11.5 o – 25.2 o → γ → q q Z / l l | η l | CUT M ll p Tl Basic > 40 GeV > 20 GeV < 2 1.5 � | η l | < 2.3 Angle Slice > 40 GeV > 20 GeV 79 < M ll < 104 GeV M ll Z Peak Z Peak 79 104 GeV > 20 GeV 20 GeV < 2 2 ± → → ± ν q ' q W l | η l | p Tl p T ν CUT Basic > 25 GeV > 20 GeV < 1 Larger η 1 < | η l | < 2.2 > 25 GeV > 20 GeV Higher p T ν Higher > 25 GeV > 25 GeV > 30 GeV > 30 GeV < 1 < 1 14 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
Z: NNLO QCD Contribution The size of the NNLO correction calculated with two PDFs: Fractional NNLO Contribution in Z Production Cross Section Acceptance 5 5 4 4 4 4 tribution (%) 3 3 ribution (%) 2 2 MRST 1 1 NNLO Cont NNLO Contr 0 0 CTEQ -1 -1 -2 -2 -3 -3 -3 -3 -4 -4 none basic ngle slice Z peak basic gle slice Z peak ang an 15 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
W: NNLO QCD Contribution The same calculation for W production: p Fractional NNLO Contribution in W Production Cross Section Acceptance 1 2 0 0 1 W+ MRST tribution (%) -1 ribution (%) 0 -2 W+ CTEQ -1 -3 W- MRST W- MRST -2 2 NNLO Cont NNLO Contr -4 -3 W- CTEQ -5 -4 -6 -5 -7 -7 -8 -6 none basic higher η gher p T ν basic higher η gher p T ν hig h hi 16 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
NNLO Dependence on P T Cut: Z p T NNLO contribution dependence on lepton P T cuts for Z production section ance cross s accept 17 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
NNLO Dependence on P T Cut: W + p T NNLO contribution dependence on lepton P T cuts for W + production section ance cross s accept 18 S. Yost, The Citadel S. Yost, The Citadel PHENO 2009 – Madison, Wisconsin – May 12, 2009
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