LEMC Workshop, Fermilab, February 2007 Peter Skands Monte Carlo Event Generators for HEP Studies

Peter SkandsEvent Generator Status 2 ►Philosophy of this talk: Mail-order two (2) ACME muon guns  Physics Studies at the Interaction Point ►This talk is meant to give an update on HEP event generators, with special focus on μ + μ -  HEP μ-μ- μ+μ+ Physics at the IP = Primary Vertex

Peter SkandsEvent Generator Status 3 The Event Generator Position Collider Detector Reconstruction Framework Detailed Simulations prerequisite for: Serious physics studies Detector design studies Design reports …

Peter SkandsEvent Generator Status 4 The Monte Carlo Method Hard Part Hundreds of GeV  E cm Parton Showers Multi-GeV Hadron Decays Hadronizatio n ~ 1 GeV ~ m σ hard process, P res P ISR, P FSR P hadronization P decays

Peter SkandsEvent Generator Status 5 ►Starting observation: photon forward brems singularity is process- independent. Fundamental property of gauge theory  also applies to gluon brems. Leading contributions to both QED (photon radiation) & QCD (quark, gluon radiation) can be worked out to all orders once and for all  exponentiated (Altarelli-Parisi) integration kernels ►Iterative (Markov chain) formulation = parton shower can be used to generate the forward singular (“collinear”) parts of QED and QCD corrections to any process to infinite order in the coupling ordered in a measure of resolution  a series of successive factorizations the lower end of which can be matched to a hadronization description at some fixed low hadronization scale ►Limitations misses interference terms relevant in the deep non-singular region kinematic ambiguities and double counting between fixed order part and resummed part Bremsstrahlung: Parton Showers

Peter SkandsEvent Generator Status 6 Pre-LEP Event Generators ► “Seated on a tripod above a crack in the earth, she went into a trance by the stupefying vapors rising from the earth and by chewing laurel leaves. From the incoherent babbling which the priestess spoke in her ecstasy, the temple priests formulated the oracle.” ►There was a certain amount of voodoo involved ►Still, an impressive amount of data could be accounted for ►We now have much better voodoo The Pythia ►ARIADNE, HERWIG, and PYTHIA T. Sjöstrand, S. Mrenna, PS, JHEP 05 (2006) 026

Peter SkandsEvent Generator Status 7 Developments since LEP ►Hard Bremsstrahlung (additional jets, e.g. Z  3,4,5 jets) Parton shower approximation breaks down  large uncertainties Lots of progress in last ~ 5 years: “matching” to matrix elements ►Jet Broadening (soft/collinear parton bremsstrahlung) New generation of theoretical models Improved parton showers implemented in both general-purpose generators (HERWIG(++) and PYTHIA) + work here at Fermilab: VINCIA – a next-generation “antenna” shower ►The Final Femtometer – Hadronization With short-distance parts better under control  non-perturbative side has less wiggle room  also here improved possibility for studies Current studies at hadron colliders  feed back to lepton collider physics

Peter SkandsEvent Generator Status 8 VINCIA ►VINCIA Dipole shower C++ code for gluon showers – running Can evolve in either of 2 different shower evolution variables: pT-ordering (~ ARIADNE) Virtuality-ordering (~ Pythia 6.2 & SHERPA) For each evolution type, an infinite family of radiation functions implemented, all with correct collinear and soft behaviour (= “antenna functions”) ►First parton shower with systematic possibility of variation of shower variable and shower functon  control uncertainties ►To any fixed order, these variations can be absorbed by a new type of matching to matrix elements Dipoles – a dual description of QCD virtual numerical collider with integrated antennae Giele, Kosower, PS : in progress

Peter SkandsEvent Generator Status 9 VINCIA Example: H  gg  ggg VINCIA Unmatched “soft” |A| 2 VINCIA Unmatched “hard” |A| 2 VINCIA Matched “soft” |A| 2 VINCIA Matched “hard” |A| 2 Expect public code and long writeup before summer Giele, Kosower, PS : in progress y 12 y 23 Later: plug-in for Pythia 8 ? Next: Quarks and leptons  lepton colliders y 12 ►First Branching ~ first order in perturbation theory ►Unmatched shower varied from “soft” to “hard” : soft shower has “radiation hole”. Filled in by matching. radiation hole in high-p T region

Peter SkandsEvent Generator Status 10 The Move to C++ (from F77) ►HERWIG++: complete reimplementation Improved parton shower and decay algorithms Eventually to include “CKKW”-style matching ? B.R. Webber; S. Gieseke, D. Grellscheid, A. Ribon, P. Richardson, M. Seymour, P. Stephens,... ►SHERPA: complete implementation, has CKKW matching ME generator + wrappers to / adaptations of PYTHIA, HERWIG F. Krauss; T. Fischer, T. Gleisberg, S. Hoeche, T. Laubrich, A. Schaelicke, S. Schumann, C. Semmling, J. Winter ►PYTHIA 8: selective reimplementation Improved PS and UE, limited number of hard subprocesses Many obsolete features not carried over  simpler, less parameters T. Sjöstrand, S. Mrenna, P. Skands

PYTHIA for the Muon Collider Instructive Examples

Peter SkandsEvent Generator Status 12 Event listing (summary) I particle/jet KS KF orig p_x p_y p_z E m 1 !mu+! !mu-! ======================================================================== ====== 3 !mu+! !mu-! !mu+! !mu-! !Z'0! !nu_e! !nu_ebar! ======================================================================== ====== 10 (Z'0) gamma gamma gamma nu_e nu_ebar gamma gamma ======================================================================== ====== sum: Z’ Radiative Return ►Z’ with mass 1 TeV, with invisible decays (here neutrinos) ►MC running at 1.5 TeV. Radiative return via photon bremsstrahlung Beam Initial State (= beam + initial-state radiation) Final State (at Leading Order) Final State (after bremsstrahlung) Examples using PYTHIA T. Sjöstrand, S. Mrenna, PS, JHEP 05 (2006) Note: also handles full γ/Z 0 /Z’ 0 interference for SM channels Standard PC  ~ 1M evts / hr.

Peter SkandsEvent Generator Status 13 Event listing (summary) I particle/jet KS KF orig p_x p_y p_z E m 1 !mu+! !mu-! ======================================================================== ====== 3 !mu+! !mu-! !mu+! !mu-! !h0! !b! !bbar! ======================================================================== ====== 10 (h0) gamma gamma (ubar) A (g) I (g) I (b) V (u) A (g) I (g) I (g) I (g) I (g) I (g) I (bbar) V ======================================================================== ====== 25 (string) (omega) (pi0) (pi0) (rho-) (B*bar0) (string) (rho0) (K*+) K p (eta') (Deltabar-) pi (rho-) pi (B0) SM Higgs Factory Beam Initial State (= beam) Final State (at Leading Order) Final State (after bremsstrahlung) Legend: (g): gluon (u): quark A I I : colour chain I V Final State (after hadronization) Legend: (pi0): decayed hadron K- : undecayed hadron (string): hadronizing string Examples using PYTHIA T. Sjöstrand, S. Mrenna, PS, JHEP 05 (2006) Standard PC  ~ 1M evts / hr.

Peter SkandsEvent Generator Status 14 A 0 (from 2HDM) Event listing (summary) I particle/jet KS KF orig p_x p_y p_z 1 !mu+! !mu-! ======================================================== ==== 3 !mu+! !mu-! !mu+! !mu-! !A0! !Z0! !h0! !d! !dbar! !tau-! !tau+! ======================================================== ==== 14 (A0) gamma (Z0) (h0) (tau+) (tau-) gamma (d) A (g) I (g) I (g) I (g) I (g) I (g) I (g) I (g) I (g) I (g) I (g) I (g) I (g) I (dbar) V ======================================================== ==== 38 nu_taubar (rho+) nu_tau (rho-) (string) (omega) H ++ (from LR Symmetry) Event listing (summary) I particle/jet KS KF orig p_x p_y p_z 1 !mu+! !mu-! ============================================================ 3 !mu+! !mu-! !mu+! !mu-! !H_L++! !H_L--! !tau+! !tau+! !tau-! !tau-! ============================================================ 13 (H_L++) (H_L--) gamma gamma (tau-) (tau-) (tau+) (tau+) gamma gamma gamma ============================================================ 24 nu_tau pi nu_tau pi (rho0) nu_taubar pi nu_e e nu_taubar pi pi ============================================================ sum: Examples using PYTHIA T. Sjöstrand, S. Mrenna, PS, JHEP 05 (2006) 026

Peter SkandsEvent Generator Status 15Technicolor Event listing (summary) I particle/jet KS KF orig p_x p_y p_z E m 1 !mu+! !mu-! ======================================================================== ====== 3 !mu+! !mu-! !mu+! !mu-! !pi_tc+! !pi_tc-! !u! !bbar! !ubar! !b! ======================================================================== ====== 13 (pi_tc+) (pi_tc-) gamma gamma (u) A (g) I (g) I (g) I (g) I (g) I (g) I (g) I (g) I (g) I (bbar) V (ubar) A (g) I (g) I (g) I (g) I (g) I (g) I (g) I (b) V ======================================================================== ====== 37 (string) p (rho+) pbar (pi0) (pi0) (rho0) Technicolor Strawman model Lowest-lying bound states of lightest Techni-fermion doublet Can be used for studies of a wide range of non-perturbative New Physics models Topcolor, walking Technicolor, extended Technicolor, … Mrenna, Lane Phys.Rev.D67:115011,2003 Examples using PYTHIA T. Sjöstrand, S. Mrenna, PS, JHEP 05 (2006) Standard PC  ~ 250k evts / hr.

Peter SkandsEvent Generator Status 16 The Generator Outlook ►MC Generators in state of continuous development Not much done (yet) specifically for muon colliders, but what’s there is already pretty useful As physics studies pick up, interactions likely to foster more developments  more studies  more developments … ►Get yours today – it’s free  more precise, more reliable theoretical predictions μ-μ- μ+μ+