Comparison of Zγγ Cross Sections: Bozzi vs SHERPA

1. Diphoton+MET: Z() Cross Section Max Baugh, Ryan Reece, Bruce Schumm SCIPP 15 May 2017

2. ISSUE • Z background contributions are small; we have always estimated them directly from MC • Z simulated with SHERPA MC  Leading order + parton-shower corrections • Nominal Cross section taken from metadata • There exists an NLO calculation (Bozzi et al., arXiv:1107.3149v1 [hep-ph]) with theoretically controlled uncertainties • Project: Compare total cross sections from Bozzi and Sherpa and come to some conclusion or other

3. FURTHER ISSUE • SHERPA sample generated at 13 TeV with certain cuts • Bozzi calculation was for 14 TeV with different cuts • Biggest differences are photon pT cut (30 GeV for Bozzi and 50 GeV for SHERPA), then beam energy • Must figure out a way to compare apples to apples • Approach: Use MadGraph to reproduce Bozzi, then change MadGraph cuts to compare to SHERPA and hope for agreement. • We don’t see agreement (and we didn’t in Run I either; too small to worry about for our strong production 2015 analysis)

4. MadGraph vs. Bozzi Thanks to UCSC grad Max Baugh for doing the work (will include him on the Note) Cuts used in 14 TeVBozzi calculation: • pT,> 30 GeV; || < 2.5 • R > 0.4 • Various parton-photon separation cuts that shouldn’t make any difference at leading order (and seem not to) Results for total Leading Order Z(ll) cross section, per species l of neutrino: ~5% agreement

5. Now apply SHERPA cuts to MadGraph Cuts used in 13 TeV SHERPA generation: • pT,> 50 GeV; || < 9999. • R > 0.2 • M > 10 GeV • “Photon isolation cut” Apply cuts in black to 13 TeV Z() MadGraph calculation (couldn’t figure out how to easily apply green cuts, but note that those would lower the MadGraph result further. (Again, per neutrino species)

6. Compare Bozzi to Sherpa via MadGraph Bozzi paper is NLO calculation, with ~20% scale/factorization uncertainties; paper provides “K factor” (ratio of NLO to LO) as a function of several kinematic quantities K ~ 1.6, and is fairly stable with respect to these quantities (photon pT cut, R, Etmiss) Apply this NLO/LO correction to the LO MadGraph calculation with the SHERPA cuts; this is our “Bozzi” NLO calculation carried over to the Z MC sample. Compare to the SHERPA cross section from the metadata (again, per neutrino species): Translated Bozzi calculation about 45% lower than SHERPA, which is consistent with our 8 TeV calculation

7. Another Note Compare W(e) to Z() at next-to-leading order (Bozzi vs. Bozzi), for 30 GeV pT cut • Again, per lepton species • Direct from paper; no need for MadGraph • Full NLO calculation • For the W background, we have a confirming control region  Naively, we expect Z to be significantly smaller than W

8. Conclusions • Using MadGraph, we translated the Bozzi NLO calculation for the Z cross section onto the SHERPA MC sample • The Bozzi calculation produces has an estimated uncertainty of less that 20%, but the cross section is about 45% lower than that determined by SHERPA • We also observed this for the 8 TeV analysis. • Some cuts made during the SHERPA generation were not applied to the translation of the Bozzi result to the SHERPA phase space, but these would only increase the discrepancy (probably slightly) • I might proposed that we lower the Z cross section in our analysis by (45  45)%, as we did for 8 TeV.