Benjamin FUKS Michael KLASEN Fabienne LEDROIT Qiang LI Julien MOREL ATLAS Exotics group

1. Precisionprediction for Z’-productionat LHC-Comparison between Resummation, MC@NLO and Pythia Benjamin FUKS Michael KLASEN Fabienne LEDROIT Qiang LI Julien MOREL ATLAS Exotics group IN2P3 – CNRS - LPSC - Grenoble 15/ 01 / 2008

2. Introduction • 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) LO mass and pT Spectrum Without any showering or ME correction : the 3 mass spectra and cross sections coincide perfectly

3. Z’ Studies in ATLAS We mainly study γ/Z/Z’ simulations coming from Pythia generator : • Pythia 6.403 is based on : • LO matrix element calculations • Showering • Lund string hadronization model. • 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 ) • The mass spectra are compatible • The pTSpectra : • 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 … Normalized to LO predictions

4. NLO calculation Using 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. • 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 Normalized to LO predictions MC@NLO gives very accurate results (very similar to the theoretical ones) and offers an easy implementation in ATHENA full chain.

5. Theoritical 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.

6. PDF uncertainties on theoriticalK-Factor and pT spectrum • Theoretical 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 peak Scale uncertainties (9%) are larger than the PDF ones

7. Present status in CSC vs joint resummation results (σNLO – σLO) / σLO σNLO / σLO MRST2004 Results obtained by F. Heinemann LO calculation  K-factor (from Stirling private communication) PDF Reweighting on NLO calculation With MRST : K-factor ≈ 1.34 @ 1 TeV Results obtained with joint resummation With MRST : K-factor ≈ 1.29 @ 1 TeV 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).

8. Conclusion • Pythia+ K-factor does a good job for the mass spectrum but : • We need to know the K-factor • The pythiapT 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-factorsand 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