ew corrections for cms z and drell-yan measurements dimitri bourilkov university of florida for the...

EW Corrections forCMS Z and Drell-Yan

Measurements

Dimitri BourilkovUniversity of Florida

For the CMS Collaboration

Working Group on Electroweak precision measurements at the LHC, and PDF4LHC,

CERN, October 9, 2012

Electroweak (EW) corrections around the Z peak and for high mass Drell-Yan

Review of CMS Z (W) and Drell-Yan results Reached/expected experimental precision

control QCD and EW corrections in precision calculations

Calculations with FEWZ3.1: a tool to calculate simultaneously QCD corrections up to NNLO and EW corrections at NLO: O(αs

2) + O(αEW) for Z/γ∗ → ℓℓ (arXiv:1208.5967)

MRST2004QED PDF used (includes QED corrections to DGLAP evolution of PDFs)

Introduction

D.Bourilkov, Florida, CMS EW Corrections in Z and DY 2

Non-resonant contributions from s-channel * exchange

Gauge invariance of photonic O() corrections splitting in QED (not discussed

here) and pure weak O() corrections: Near Z peak:

QED corrections described by radiator functions Weak corrections absorbed in line shape parameters

Invariant mass Ml+

l- directly related to off-

shellness of Z boson

EW corrections depend on the scales probed; important for mass determinations (W), for high invariant masses of Z/*

Electroweak Corrections for Z/*


At the TeV scale, large and significant impact of pure weak corrections; factors like Sudakov logs (from vertex and box corrections) modify the inclusive cross sections: ~ W ln2(2/MW

2)

At = 1 TeV this is ~ 25 * W

Effects ~ 10 % in the TeV region Data in 2012 @ TeV will start probing at

this level of precision

Electroweak Corrections for Z/*


Early Studies

• LHC SM workshop “SM Physics (and

more) at the LHC” CERN Yellow Report 2000-004 http://arxiv.org/abs/hep-ph/0003275

• Studies for 14 TeV• Highlighted the

importance of pure EW corrections


Z Cross Sections @ 8 TeV

https://cdsweb.cern.ch/record/1460098

• Dedicated low pile-up run at 8 TeV• Pair mass 60-120 GeV• Lepton PT > 25 GeV, || < 2.1 for muons, || < 2.5 for

electrons (excluding barrel-endcap transition)

• Lineshape described well by POWHEG+PYTHIA


Inclusive Z Cross Sections @ 8 TeV

10710 Z events, very low background

Experimental uncertainties 2.8-1.9% for ee, Systematics dominated by lumi (4.4 %) and theory uncertainty, mainly acceptance (2.6-1.9%)

EW corrections smaller


W/Z Ratios and @ 8 TeV

• Satisfactory agreement with NNLO predictions: FEWZ + MSTW2008


W/Z Ratios and @ 8 TeV

Only PDF uncertainties are included in the theory uncertainties


Drell-Yan Mass Spectrum


• Lepton pairs with PT > 14,9 GeV , > 20 GeV

ee• FSR effects unfolded (“Born” leptons, before QED FSR)

• NNLO predictions with FEWZ+MSTW08• NNLO effects at low mass important


Drell-Yan Double Diff. Cross Section

• Double differential cross section in M & Y

• In CMS acceptance, for , normalized to Z cross section

• 6 bins in M, 24 or 12 bins in |Y|• Low mass needs more work,

improve theory predictions• 45-60 and 60-120 GeV: excellent

agreement with POWHEG and FEWZ

• Z peak bin systematics limited• 120-200 and 200-1500 GeV:

good agreement with POWHEG and FEWZ

• High mass bin statistics limited; will improve with 8 TeV data


Forward-backward Asymmetry

http://cdsweb.cern.ch/record/1430637

• POWHEG+PYTHIA+CT10• Forward-backward asymmetry will become sensitive

to EW corrections


Z PT

• Z PT spectrum at

7 TeV http://arxiv.org/abs/1110.4973

• Theory predictions: QCD at NNLO

• New physics can show up in the tail


Z’ Search @ 7 TeV

http://lanl.arxiv.org/abs/1206.1849

• Accumulating events > 1 TeV• Entering the Sudakov zone


Z’ Search @ 8 TeV


• Expanding the reach above 1 TeV• By the end of the 2012 run the statistical

precision will start to probe EW corrections


Calculations with FEWZ_3.1.b2 (good backward

compatibility with FEWZ2): a tool to calculate simultaneously QCD corrections up to NNLO and EW corrections up to NLO for Z and DY production

Results for QCD NLO EW NLO shown MRST2004QED PDF used

G scheme to “minimize” higher order EW

corrections “Standard” input file for muons Roughly in CMS acceptance:

PT > 25 GeV, || < 2.4 for both leptons

EW Corrections with FEWZ3.1


EW Corrections with FEWZ3.1: Mass


EW Corrections with FEWZ3.1: Afb


EW Corrections with FEWZ3.1: Y


EW Corrections with FEWZ3.1: PT


EW corrections will grow in importance for the complete 2012 data set

New tools e.g. FEWZ3.1 (beta) available to study these effects

First round of calculations looks promising

Refine the studies: QCD NNLO EW NLO Realistic experimental cuts for electrons

and muons

Outlook


EW Corrections in Z and DY 22D.Bourilkov, Florida, CMS

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