Introduction to Event Generators Bryan Webber Cavendish Laboratory University of Cambridge Introduction to Event Generators 1 Bryan Webber, MCnet School, 2014 Introduction to Event Generators • Lecture 1: Monte Carlo event generation ✤ theoretical foundations and limitations • Lecture 2: Parton showering ✤ parton splitting, dipoles, coherence • Lecture 3: Non-perturbative issues ✤ hadronization and underlying event • Lecture 4: Overview of results ✤ jets, W, Z, top, Higgs, BSM Introduction to Event Generators 2 Bryan Webber, MCnet School, 2014 MC Event Generators HERWIG http://projects.hepforge.org/herwig/ Angular-ordered parton shower, cluster hadronization v6 Fortran; Herwig++ PYTHIA http://www.thep.lu.se/∼torbjorn/Pythia.html kt-ordered parton shower, string hadronization v6 Fortran; v8 C++ SHERPA http://projects.hepforge.org/sherpa/ Dipole-type parton shower, cluster hadronization C++ “General-purpose event generators for LHC physics”, A Buckley et al., arXiv:1101.2599, Phys. Rept. 504(2011)145 Introduction to Event Generators 3 Bryan Webber, MCnet School, 2014 OtherOther Relevant Software relevant software Some examples(with (with apologies apologies for many for omissions): omissions) Other event/shower generators: PhoJet, Ariadne, Dipsy, Cascade, Vincia Matrix-element generators: MadGraph/MadEvent, CompHep, CalcHep, Helac, Whizard, Sherpa, GoSam, aMC@NLO Matrix element libraries: AlpGen, POWHEG BOX, MCFM, NLOjet++, VBFNLO, BlackHat, Rocket Special BSM scenarios: Prospino, Charybdis, TrueNoir Mass spectra and decays: SOFTSUSY, SPHENO, HDecay, SDecay Feynman rule generators: FeynRules PDF libraries: LHAPDF Resummed (p ) spectra: ResBos ? Approximate loops: LoopSim Jet finders: anti-k and FastJet ? Analysis packages: Rivet, Professor, MCPLOTS Detector simulation: GEANT, Delphes Constraints (from cosmology etc): DarkSUSY, MicrOmegas Standards: PDF identity codes, LHA, LHEF, SLHA, Binoth LHA, HepMC Can be meaningfully combined andSjöstrand, used for LHC Nobel physics! Symposium, May 2013 IntroductionTorbj¨orn Sj¨ostrandto Event Generators Challenges for4 QCD TheoryBryan Webber, slide MCnet 21/24 School, 2014 Overview of Results http://mcplots.cern.ch/ Introduction to Event Generators 5 Bryan Webber, MCnet School, 2014 Jets Introduction to Event Generators 6 Bryan Webber, MCnet School, 2014 Jet pT 7000 GeV pp Jets 7000 GeV pp Jets 107 6 Jet Transverse Momentum (anti-k (0.4)) Jet Transverse Momentum (anti-k (0.4)) 10 T 6 T 10 ATLAS 3M events ATLAS 3M events [pb/GeV] ≥ [pb/GeV] ≥ T 5 Herwig++ (Def) T Herwig++ (Def) 10 105 /dp Pythia 8 (Def) /dp Pythia 8 (Def) σ Sherpa (Def) σ Sherpa (Def) d d 104 104 Rivet 1.8.3, Rivet 1.8.3, 3 103 10 2 102 10 10 10 1 1 -1 ATLAS_2011_S9128077 10 ATLAS_2011_S9128077 10-1 Herwig++ 2.6.3, Pythia 8.176, Sherpa 1.4.3 Herwig++ 2.6.3, Pythia 8.176, Sherpa 1.4.3 10-2 mcplots.cern.ch mcplots.cern.ch 200 400 600 800 0 200 400 600 800 p (leading jet) [GeV] p (2nd jet) [GeV] T T Ratio to ATLAS Ratio to ATLAS 1.5 1.5 1 1 0.5 0.5 200 400 600 800 0 200 400 600 800 Leading jet Second jet Introduction to Event Generators 7 Bryan Webber, MCnet School, 2014 Jet pT 7000 GeV pp Jets 7000 GeV pp Jets 104 6 10 Jet Transverse Momentum (anti-k (0.4)) Jet Transverse Momentum (anti-k (0.4)) T T 5 10 ATLAS 3M events ATLAS 3M events [pb/GeV] ≥ [pb/GeV] 3 ≥ T Herwig++ (Def) T 10 Herwig++ (Def) 4 /dp 10 Pythia 8 (Def) /dp Pythia 8 (Def) σ Sherpa (Def) σ Sherpa (Def) d d 103 Rivet 1.8.3, 102 Rivet 1.8.3, 102 10 10 1 10-1 1 10-2 -3 10 ATLAS_2011_S9128077 ATLAS_2011_S9128077 10-1 Herwig++ 2.6.3, Pythia 8.176, Sherpa 1.4.3 Herwig++ 2.6.3, Pythia 8.176, Sherpa 1.4.3 10-4 mcplots.cern.ch mcplots.cern.ch 100 200 300 400 500 100 150 200 p (3rd jet) [GeV] p (4th jet) [GeV] T T Ratio to ATLAS Ratio to ATLAS 1.5 1.5 1 1 0.5 0.5 100 200 300 400 500 100 150 200 Extra jets from parton showers Introduction to Event Generators 8 Bryan Webber, MCnet School, 2014 1 Event shapes provide information about the properties of hadronic final states from particle collisions. Suitably defined event-shape variables were among the first observables proposed to test the theory of quantum chromodynamics (QCD) [1, 2] and have been important in en- + abling progress in the theory. At e e− and ep colliders, event shapes have played a crucial role in the extraction of the strong coupling constant as. They have been essential in tuning the parton shower and non-perturbative components of Monte Carlo (MC) event generators and have provided a laboratory for developing and testing analytical probes of the hadronization process. More recently, a large set of event-shape variables suitable for pp colliders has been proposed [3]. An important aspect of these variables is their normalization to the measured sum of transverse momentum or energy of all the objects in the event. It is thus expected that energy-scale uncertainties should cancel to a large extent. Event-shape variables represent a valuable tool for early measurements of the properties of QCD multijet events at the Large Hadron Collider (LHC) and the tuning of MC models [4]. This Letter presents the first measurement of hadronic event shapes with a data sample of 7 TeV proton-proton collisions collected with the Compact Muon Solenoid (CMS) detector at 1 the LHC. The data sample corresponds to an integrated luminosity of 3.2 pb− . A detailed description of the CMS experiment can be found elsewhere [5]. CMS uses a right- handed coordinate system, with the origin located at the nominal collision point, the x-axis pointing towards the center of the LHC ring, the y-axis pointing up (perpendicular to the LHC plane), and the z-axis along the anticlockwise beam direction. The polar angle q is measured from the positive z-axis, the azimuthal angle f is measured in the xy plane, and the pseudora- pidity is defined as h = ln[tan(q/2)]. The central feature of the CMS apparatus is a super- − conducting solenoid, of 6 m internal diameter, providing an axial field of 3.8 T. Within the field volume are the silicon pixel and strip tracker,Jet the crystalevent electromagnetic shapes calorimeter (ECAL), and the brass/scintillator hadron calorimeter7000 GeV pp (HCAL). MuonsJets are measured7000 inGeV gas-ionization pp Jets ) ) C m,C detectors embedded in the steel return yoke.Central Transverse In Thrust the (125 < p < region 200) h < 1.74, the HCALCentral Transverse cells Minor have (90 < p < 125) 1 0.2 3M events CMS | | CMS ≥ 4.1M events widths of 0.087 in pseudorapidity andHerwig++ 0.087 (Def) rad in azimuth (f≥ ). In the (h, f) Herwig++plane, (Def) and for Pythia 8 (Def) Pythia 8 (Def) h < 1.48, the HCAL cells map on to 5Sherpa (Def)5 ECAL crystal arrays to form calorimeterSherpa (Def) towers 0.15 1/N dN/d ln(T 1/N dN/d ln(1-T | | ⇥ Rivet 1.8.3, projecting radially outwards from close to the nominal interactionRivet 1.8.3, point. At larger values of h , the size of the towers increases and the matching ECAL arrays contain fewer crystals. A | | 0.1 10-1 preshower detector consisting of two planes of silicon sensors interleaved with lead is located in front of the ECAL at h > 1.479. In addition to the barrel and endcap detectors, CMS has | | 0.05 extensive forward calorimetry covering the region 3.0 < h < 5.0. CMS_2011_S8957746 | | CMS_2011_S8957746 0 Herwig++ 2.6.3, Pythia 8.176, Sherpa 1.4.3 Herwig++ 2.6.3, Pythia 8.176, Sherpa 1.4.3 10-2 mcplots.cern.ch mcplots.cern.ch Two event-shape variables have been studied:-10 the central-5 transverse thrust t-6 , and the-4 central-2 ln(1-T ) ? C ln(T ) thrust minor Tm, . The two variables probe different QCD radiativeC processes and are mostly m,C C Ratio to CMS Ratio to CMS sensitive to the modeling of two-1.5 and three-jet topologies. The term central1.5 ( ) indicates that C the input to the calculation of these quantities are jets in the central region of the detector 2 ( h < 1.3), where sub-leading contributions1 in the calculation of the event-shape1 variables are | | less significant, and systematic uncertainties on the jet reconstruction are smaller. 0.5 0.5 The central transverse thrust is definedmeasure as-10 [3] of the-5 momentum out of this-6 plane and-4 is defined-2 as Âi ~p ,i nˆ T Âi ~p ,i nˆ T, t , Tc1 max | ? · |,Tm, | ? ⇥ (1) C |. (2) ? C ⌘ ⌘− nˆ T Âi p ,i C ⌘ Âi p ,i ? ? where p ,i is the transverseIntroduction momentum to Event GeneratorsTwo-jet of selected events jet thati. The are axis wellnˆ T9 which balanced maximizes have low the values sum,Bryan Webber, of these MCnet twoSchool, variables, 2014 while isotropic ? and thus minimizes t , , is called the thrustmultijet axis eventsnˆ T, . The have central high transversevalues. thrust is a measure ? C C of the momentum in the plane defined by nˆ T, and the beam axis. The central thrust minor is a The transverseC momenta of jets are used as input to the event-shape calculation. Jets are recon- structed using individual particles that have been identified, and whose energies have been measured, using a particle flow technique [6], which combines information from all subde- tectors: charged tracks in the tracker and energy deposits in the electromagnetic and hadronic calorimeters, as well as signals in the preshower detector and the muon system.