The Sherpa approach of calculating multijet backgrounds. [Theory seminar @ FNAL] - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Jan Winter a Aim: improved description of multijet final states Sherpa at a glance CKKW method ... merging tree-level MEs and PSs Survey of application examples ALPGEN vs. Sherpa studies (see EPJC53 (2008) 473) Current developments a Sherpa authors: T. Gleisberg, S. H¨ oche, F . Krauss, M. Sch¨ onherr, F . Siegert, S. Schumann, J. W. http://www.sherpa-mc.de/ Jan Winter TH seminar, February 14, 2008 – p.1
Challenge – Physics at ... ... future (and present) hadron linear collider experiments. and C AMPBELL , H USTON , S TIRLING LHC physics: reveal EWSB mechanism, proton - (anti)proton cross sections large rates of (B)SM final states 9 9 10 10 LHC: is a QCD machine Multijets 8 8 10 10 σ tot huge production phase space 7 7 10 10 Tevatron LHC 6 6 10 10 Prior to new physics: need to under- 5 5 10 10 σ b stand SM physics/backgrounds 4 4 10 10 -1 -2 s V + jets, V V + jets, Q ¯ 3 3 Q + jets, single t ’s 10 10 33 cm jet > √ s/20) σ jet (E T events/sec for L = 10 2 2 10 10 VBF and g – g fusion, Higgs production σ (nb) σ W 1 1 10 10 σ Z 0 0 10 10 SUSY particles and decay chains jet > 100 GeV) σ jet (E T -1 -1 10 10 -2 -2 10 10 -3 -3 σ t 10 10 jet > √ s/4) -4 σ jet (E T -4 10 10 σ Higgs (M H = 150 GeV) -5 -5 10 10 Today’s signals will be tomorrow’s backgrounds. -6 -6 10 10 σ Higgs (M H = 500 GeV) -7 -7 10 10 0.1 1 10 √ s (TeV) Jan Winter TH seminar, February 14, 2008 – p.2
Challenge – Physics at ... ... future (and present) hadron linear collider experiments. and LHC physics: reveal EWSB mechanism, large rates of (B)SM final states LHC: is a QCD machine Multijets huge production phase space Prior to new physics: need to under- stand SM physics/backgrounds V + jets, V V + jets, Q ¯ Q + jets, single t ’s VBF and g – g fusion, Higgs production SUSY particles and decay chains The signal-to-background puzzle. E.g. Higgs in weak boson fusion: Nice rapidity gap. Signal/background ratio depends on central jet veto. Loss of gap structure for higher orders in QCD? Central jet veto to be modified? Backgrounds well modelled? Signal spoiled by UE? Jan Winter TH seminar, February 14, 2008 – p.2
Challenge – Physics at ... ... future (and present) hadron linear collider experiments. and LHC physics: reveal EWSB mechanism, large rates of (B)SM final states LHC: is a QCD machine Multijets huge production phase space Prior to new physics: need to under- stand SM physics/backgrounds V + jets, V V + jets, Q ¯ Q + jets, single t ’s VBF and g – g fusion, Higgs production SUSY particles and decay chains The signal-to-background puzzle New-physics discovery signalled by enhanced rate of hard events? Signal: leptons + jets + E / T . Is SM backround precisely known? Is it sufficient using PSs only? Jet properties depend on nature of new physics. Jan Winter TH seminar, February 14, 2008 – p.2
Challenge – Physics at ... ... future (and present) hadron linear collider experiments. and LHC physics: reveal EWSB mechanism, large rates of (B)SM final states LHC: is a QCD machine Multijets huge production phase space Prior to new physics: need to under- stand SM physics/backgrounds V + jets, V V + jets, Q ¯ Q + jets, single t ’s VBF and g – g fusion, Higgs production SUSY particles and decay chains Need for tools that model ...!!! Jet production Jet evolution Hadronization Jan Winter TH seminar, February 14, 2008 – p.2
Example: jet mass B AUR , O RR arXiv:0707.2066 � m jet � ∝ √ α s p jet T , ... At higher orders light quark and gluon jets acquire a mass which depends on jet algorithm and ∆ R separation. However, non-perturbative QCD effects may significantly contribute. Before we search for new physics, we want to make sure Jets are always defined according to some algorithm. that detector and reconstruction And different algorithms will give different results. algorithms operate properly. Jan Winter TH seminar, February 14, 2008 – p.3
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