Precision Calculations for Collider Physics

Transcription

1 SFB Arbeitstreffen März 2005 Precision Calculations for Collider Physics Michael Krämer (RWTH Aachen) Radiative corrections to Higgs and gauge boson production Combining NLO calculations with parton showers Michael Krämer Page 1 SFB Arbeitstreffen März 2005

2 Introduction Exploring BSM physics at the LHC may be challenging Example: MSSM Higgs sector two Higgs doublets to give mass to up- and down-quarks 5 physical states: h, H, A, H ± MSSM Higgs sector determined by tan β = v 2 /v 1 and M A Consider σ MSSM (gg h γγ) σ SM (gg h γγ) [Dedes, Heinemeyer, Su, Weiglein] < >1.5 msugra gg >h >γγ differences < 10% Needs precision measurements & calculations for production cross sections and branching ratios tanβ (GeV) M A Michael Krämer Page 2 SFB Arbeitstreffen März 2005

3 Introduction Precision physics at hadron colliders requires the calculation of QCD corrections at (N)NLO; the calculation of electroweak corrections at NLO; the resummation of large logarithmic corrections; matching of fixed order calculations with parton showers & hadronization; the precision determination of input pdfs. Michael Krämer Page 3 SFB Arbeitstreffen März 2005

4 Higgs Production at the LHC Recent progress for SM Higgs production includes NNLO QCD calculations for pp H [Harlander, Kilgore; Anastasiou, Melnikov; Ravindran, Smith, van Neerven; Anastasiou, Melnikov, Petriello; Blümlein, Ravindran;... (02-05)] NNLO QCD calculations for pp HZ, HW [Brein, Djouadi, Harlander (04)] (N)NLO QCD calculations for pp Q QH [Beenakker, Dittmaier, MK, Plümper, Spira, Zerwas; Dawson, Jackson, Orr, Reina, Wackeroth; Harlander, Kilgore; Campbell, Ellis, Maltoni, Willenbrock;... (01-05)] NLO QCD calculations for pp qqh [Figy, Oleari, Zeppenfeld; Berger, Campbell (03-04)] NLO EWK calculations for pp HZ, HW [Ciccolini, Dittmaier, MK (03)] (N)NLL resummation for pp H [Kulesza, Sterman, Vogelsang; Berger, Qiu; Catani, de Florian, Grazzini;... (03-04)] matching of NLO calculation with parton shower MC Herwig for pp H [Frixione, Webber (04)]; NNLO splitting functions and PDF fits, error estimates for PDF fits [Moch, Vermaseren, Vogt; MRST; CTEQ (02-05)]. Michael Krämer Page 4 SFB Arbeitstreffen März 2005

5 Associated b bh production: two calculational schemes 4-flavour scheme 5-flavour scheme + exact g b b splitting & mass effects no summation of ln(m H /M b ) terms + summation of ln(m H /M b ) terms LL approximation to g b b splitting Comparison at LO Comparison at (N)NLO [Harlander, Kilgore; Dittmaier, MK, Spira (04)] 10 3 σ(pp bb _ h + X) [fb] s = 14 TeV µ = (2m b + M h )/ bb _ h (NNLO) 10 gg bb _ h (NLO) M h [GeV] Michael Krämer Page 5 SFB Arbeitstreffen März 2005

6 W H/ZH production: NNLO QCD and NLO EWK corrections at LO through q q V V +H q q V V H Tevatron: important Higgs discovery channel LHC: measurement of Higgs couplings? QCD uncertainties small: σ NNLO < 3% (cf Drell-Yan) [Brein, Djouadi, Harlander (03)] K WH (LHC) QCD EW corrections significant: K EW(NLO) 0.9 K QCD(NNLO) [Ciccolini, Dittmaier, MK (03)] QCD+EW M H [GeV] Michael Krämer Page 6 SFB Arbeitstreffen März 2005

7 W W production: a background to Higgs searches pp H W W l ν l ν is an important Higgs search channel W W pair production is the dominant background: σ BR 5 times larger than signal a Higgs mass peak cannot be reconstructed from leptonic W decays theoretical control of the background is important experimental selection cuts may enhance the importance of higher-order corrections for background processes example: gg W W l ν l ν [Binoth, Ciccolini, Kauer, MK (05)] g d W e d ν µ g d W + u ν e µ + formally NNLO but enhanced by Higgs search cuts gg q q (NLO) corr. σ tot 54 fb 1373 fb 4% σ bkg 1.39 fb 4.8 fb 30% Michael Krämer Page 7 SFB Arbeitstreffen März 2005

8 Higgs and gauge boson production: (potential) projects the calculation of NLO QCD/EWK corrections to SUSY-Higgs production: pp Q Qh/H/A, pp Q Q H ± the calculation of EWK effects in gauge boson production pp Z/W : multiple photon emission, Sudakov logarithms the calculation of higher-order corrections to Higgs and SUSY searches: gluon-gluon induced backgrounds Michael Krämer Page 8 SFB Arbeitstreffen März 2005

9 NLO calculations with parton showers (work with Soper and Mrenna) N k LO calculations + allow precision test of QFTs break down for certain kinematic configurations do not provide realistic final states are limited to IR safe observables Parton shower Monte Carlo programs + include certain summations (soft/collinear emissions) + provide realistic final states do not include hard emissions are based on LO perturbation theory Aim: Perform NLO calculations with (LL) summation of soft/collinear logarithms and realistic hadronic final states match NLO calculations with parton shower Monte Carlo programs Problem of double counting: parton showers include part of the short-distance physics already included in NLO calculations See also work by Frixione, Nason, Webber Michael Krämer Page 9 SFB Arbeitstreffen März 2005

10 NLO calculations with parton showers Consider event shape variables in e + e 3 jets Examine df 3 /dm where f 3 is the fraction of events that have three jets and M is the mass of a jet (Durham algorithm, y cut = 0.05) NLO calculation NLO Pythia 0.4 e + e 3 jets: jet-mass distribution df 3 /dm [GeV -1 ] ( s = M Z ; µ = s/6, k T algorithm, y cut = 0.05) 1 e + e 3 jets: jet-mass distribution df 3 /dm [GeV -1 ] ( s = M Z ; µ = s/6, k T algorithm, y cut = 0.05) 0.2 NLO NLO + PS + Had NLO M [GeV] wrong jet structure for M M [GeV] realistic jet structure for M 0 accurate (NLO) prediction for rate Michael Krämer Page 10 SFB Arbeitstreffen März 2005

11 NLO calculations and parton showers: (potential) projects comprehensive phenomenological analysis of e + e 3 jets: dependence on matching scheme, systematic comparison with Pythia, Herwig, experimental data,... extension to hadron collider processes Michael Krämer Page 11 SFB Arbeitstreffen März 2005