LPSC - Grenoble Julien MOREL 1Precision prediction for Z’-production

PrecisionPrecision predictionprediction for Z’- for Z’-productionproduction atat LHC LHC--

Comparison between Resummation, Comparison between Resummation, MC@NLO and PythiaMC@NLO and Pythia

Benjamin FUKSMichael KLASEN

Fabienne LEDROITQiang LI

Julien MOREL

ATLAS Exotics groupIN2P3 – CNRS - LPSC - Grenoble

15/ 01 / 2008

LPSC - Grenoble Julien MOREL 2Precision prediction for Z’-production

This work has been performed on a typical Z’, reachable at the LHC :•Z’χ from E6 gauge group (GUT) at 1 TeV

•Mass range 900-1200 GeV•arXiv:0711.0749 (accepted for publication in NPB)

IntroductionIntroduction

, ,u d s

, , 'Z Z

, ,u d s

e

e

LO mass and pT Spectrum

Without any showering or ME

correction :

the 3 mass spectra and cross sections coincide perfectly

LPSC - Grenoble Julien MOREL 3Precision prediction for Z’-production

Z’ Studies in ATLASZ’ Studies in ATLAS

We mainly study γ/Z/Z’ simulations coming from Pythia generator : To improve the patron level

predictions, Pythia includes ”corrections” :•Parton Shower (soft and collinear region) (≈LL)•Power Shower (full space phase)•Matrix Elements (normalization to ME of additional hard parton )

Pythia 6.403 is based on :•LO matrix element calculations•Showering•Lund string hadronization model.

Normalized to LO

predictions

The mass spectra are compatible

The pT Spectra :•Peak at 5 GeV •Tail up to 700 GeV•No divergence at low pT

ME improve the pT shape but it’s still LO calculations … not complete NLO at all …

LPSC - Grenoble Julien MOREL 4Precision prediction for Z’-production

NLO calculationNLO calculationUsing the NLO calculations for Z’:We have implemented the γ/Z/Z’ process with full interference in MC@NLO and in the joint resummation formalism.

MC@NLO gives very accurate results (very similar to the theoretical ones) and offers an easy implementation in

ATHENA full chain.

Normalized to LO

predictions

Mass spectrum :

•MC@NLO and resummation K-factors agrees perfectly.

•Pythia gives good shape but not normalization.

pT spectrum :

•Impressive agreement between MC@NLO and theoretical prediction (M/2) for the pT spectrum.

•Pythia prediction rises and falls more steeply

LPSC - Grenoble Julien MOREL 5Precision prediction for Z’-production

Theoritical uncertainties on the Cross sectionTheoritical uncertainties on the Cross section

Cross section integrated on pT and mass (900-1200 GeV)

LO and Pythia : agree→ Small dependence due to μF .

NLO : Very large dependence due to μR

MC@NLO : agree with NLO central value→ Small dependence due to LL resummation (in the PS).

Joint resummation :→Smaller than NLO due to resummation→Larger MC@NLO dependence but more realistic.

Scale uncertainties around 9% with joint resummation.

LPSC - Grenoble Julien MOREL 6Precision prediction for Z’-production

PDF PDF uncertaintiesuncertainties on on theoriticaltheoritical K-Factor and pT K-Factor and pT spectrumspectrumTheoretical prediction using joint resummation

•Used PDF = CTEQ6M •Error obtained with the 40 error PDF included in the set

(Lower band =min ; Upper band = max)

PDF uncertainties on K-factor vs Mass ≈3%

PDF uncertainties on pT spectrum

≈7% on the 10 GeV peakScale uncertainties (9%) are larger than the PDF ones

LPSC - Grenoble Julien MOREL 7Precision prediction for Z’-production

MRST 2004

Present status in CSC vs joint resummation resultsPresent status in CSC vs joint resummation results

Results obtained by F. HeinemannLO calculation K-factor (from Stirling

private communication)

PDF Reweighting on NLO calculationWith MRST : K-factor ≈ 1.34 @

1 TeV

(σNLO – σLO) / σLO

Results obtained with joint resummation

With MRST : K-factor ≈ 1.29 @ 1 TeV

σNLO / σLO

K-Factor from Stirling code seem to be larger, but we don’t know exactly how they are computed.

Joint resummation gives similar results than full NNLO calculation at the Z’ peak (as expected).

LPSC - Grenoble Julien MOREL 8Precision prediction for Z’-production

ConclusionConclusion

Pythia + K-factor does a good job for the mass spectrum but :•We need to know the K-factor•The pythia pT spectrum is not compatible with NLO one.

We have implemented γ/Z/Z’ process with full interference in MC@NLO and resummation.We can use it to compute K-factors and generate events with the correct pT spectrum.We have computed the uncertainties on theoretical K-factors:•PDF around 3%•Scales around 9%

Our modified program has been endorsed by the original authors and can be found on the MC@NLO web page : http://www.hep.phy.cam.ac.uk/theory/webber/MCatNLO/

Or at http://lpsc.in2p3.fr/klasen/software