1. 1 March 11 th, 2008 S. Chevalier-Thery, Moriond QCD – Top Mass from Cross Sections Top Mass from Cross Sections Solene Chevalier-Thery LPTHE, CEA-IRFU/SPP D0 collaboration Work in collaboration with : - U.Bassler (SPP CEA Saclay) - M.Cacciari (LPTHE Paris) - F.Deliot (SPP CEA Saclay) - U.Heintz (Boston University)

2. 2 March 11 th, 2008 S. Chevalier-Thery, Moriond QCD – Top Mass from Cross Sections Why extracting top mass from cross sections ?  Direct measurement already exists and is more precise.  However : which mass do we really extract from Pythia : LO, NLO, which renormalization scheme…?  Alternative : extract mass from NLO+NLL cross section measurement this gives a mass in a well-defined renormalization scheme

3. 3 March 11 th, 2008 S. Chevalier-Thery, Moriond QCD – Top Mass from Cross Sections First approach  Both experimental and theoretical cross sections depend on top mass : their intersection gives the top mass.  Both cross sections have uncertainties : the intersection of the uncertainty bands gives the uncertainty on the top mass.  Extracted top mass (D0 note 5459) :  Lepton+jet channel  Dilepton channel  In agreement with the central value from direct measurement : mtop = 172.6 ± 1.4 GeV. (see U.Heintz talk) Dilepton channel Lepton+jet channel

4. 4 March 11 th, 2008 S. Chevalier-Thery, Moriond QCD – Top Mass from Cross Sections Second approach : use of probabilities  Theoretical t-tbar cross section depends on :  the PDFs  the factorization scale μ F, the renormalization scale μ R  According to the factorization theorem, the total cross section of t-tbar pair production at the Tevatron is : PDF Partonic cross section  The theoretical and experimental cross sections suffer from uncertainties.  If you know the shape of these uncertainties, you can combine them according to probability rules to obtain the probability of having a given top mass. Theoretical dependencies Experimental dependencies  The experimental cross section is measured as : Evaluated by MC Slight dependance with top mass  Experimental t-tbar cross section depends on :  systematics  statistics

5. 5 March 11 th, 2008 S. Chevalier-Thery, Moriond QCD – Top Mass from Cross Sections Combining uncertainties  We want to determine the probability density function (p.d.f) for the top mass f(m t ). So we have to know the different p.d.f.s for all the sources of uncertainties :  Experimental uncertainty : taken gaussian f exp (σ|m t )  Theoretical uncertainties :  PDFs : taken gaussian and calculated from CTEQ or MRST sets f th,PDF (σ|m t )  renormalization and factorization scales f th,μ (σ|m t ) Experimental gaussian : centered on experimental curve cross section p.d.f. cross section Gaussian probability due to the PDFs uncertainty f(m t ) = f exp (σ|m t ) x ( f th,PDF (σ|m t ) f th,μ (σ|m t ) ) σ σ_min σ_max 100% cross section p.d.f. σ σ_min σ_max 90% cross section p.d.f. 10% δσ1δσ1 δσ1δσ1 δσ2δσ2 δσ2δσ2 σ σ_min σ_max 60% cross section p.d.f. 10% δσ1δσ1 δσ1δσ1 δσ2δσ2 δσ2δσ2 30% x Different step function for the probability due to the scale uncertainty 1 2 3

6. 6 March 11 th, 2008 S. Chevalier-Thery, Moriond QCD – Top Mass from Cross Sections Results  Different choices for theoretical uncertainty have limited impact on mass extraction (+/- 1 GeV for the central value and the error). Uncertainty +/- 7 GeV Uncertainty +/- 10 GeV

7. 7 March 11 th, 2008 S. Chevalier-Thery, Moriond QCD – Top Mass from Cross Sections Conclusions  We can extract a value for top quark mass, in good agreement with the direct measurement but with greater uncertainty.  The uncertainties could be reduced using :  New data sets (more precise)  New PDFs sets with smaller uncertainties  Better higher order calculation (NNLO?)  This extraction depends on the choice for the shape of the p.d.f. due to the scales uncertainties. Work in progress.