Slide 1Slide 2Slide 3Slide 4Slide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15Slide 16Slide 17Slide 18Slide 19Slide 20Slide 21Slide 22Slide 23Slide 24Slide 258.882 LHC PhysicsExperimental Methods and MeasurementsResonances: Production, Decay and Reconstruction[Lecture 13, March 18, 2009]C.Paus, LHC Physics: Resonance Production, Decay and Reconstruction 2Organizational Issues Spring break●next week: March 22-27, no lecture/recitation●make sure project 1 is done by then Project 1●received the combined CERN people note: looks good, on first sight will evaluate for Monday after spring break●will meet with Michael and Erik after lecture today Project 2●due April 9, Upsilon cross sections●exact definitions are on the TWiki‘09The Physics Colloquium SeriesThursday, March 19 at 4:15 pm in room 10-250Jeff KimbleCalifornia Institute of Technology "Quantum Networks" SpringFor a full listing of this semester’s colloquia, please visit our website at web.mit.edu/physicsColloquium SeriesPhysicsC.Paus, LHC Physics: Resonance Production, Decay and Reconstruction 4Lecture Outline Resonances: Production, Decay and Reconstruction●resonances: an overview●the zoo of particles and putting order into it●production and decay●vertex reconstruction●reconstruct and selection of resonancesC.Paus, LHC Physics: Resonance Production, Decay and Reconstruction 5General Resonance Example Take scattering experiment●shoot pion on fixed target●observe scattering rate depending on CM energy●most likely creation: at Δ mass and around itcharacteristics – mass and widthwidth = 1 / lifetimeBreit Wigner curveC.Paus, LHC Physics: Resonance Production, Decay and Reconstruction 6Lifetime of Resonances Resonance curve, Breit-Wigner: Lifetime is defined by resonance width●lifetime τ = 1/Γ → width Γ = 1/τ Check whether this makes sense●particle with super short lifetime has a huge width meaning that it “disappears”●particle with a very long lifetime has a small width (very sharp peak) What defines the lifetime of a particle?●time scale or strength of interaction it decays with●strong interaction means short lifetime and broad peak●weak interaction means long lifetime and narrow peakC.Paus, LHC Physics: Resonance Production, Decay and Reconstruction 7Key Dates in Particle Discoveries 1897 electron: Thomson (cathode ray) 1911 atomic nucleus: Rutherford (gold foil experiment) 1918/9 proton: Rutherford (H nuclei observed in α on N2) 1923 photon: Compton confirms quantum nature of x rays 1930 neutrino suggested by Pauli (α spectrum) 1930 positron suggested by Dirac (mathematical issue) 1931 neutron: Chadwick 1937 muon: at first confused with the pion 1946/47 muon is correctly identified and I.I.Rabi comments: ”who ordered that?” 1947 charged pion in cosmic rays (interacts strongly) 1949 positive kaonC.Paus, LHC Physics: Resonance Production, Decay and Reconstruction 8Key Dates in Particle Discoveries 1951 V like particles discovered: Λ0, K0 1952 Δ particles discovered (Δ++,Δ+,Δ0,Δ–)C.Paus, LHC Physics: Resonance Production, Decay and Reconstruction 9The Stories Neutrino story●beta decay understood as: n → p + e– + ν●neutrino was introduced to fix the energy balance Pion story●electron is held by EM force to nucleus (photon)●force should hold neutrons and proton together (pion)●uncertainty principle was used to explain reach of forces●photon is massless: infinitely far reach●pion has to be massive: force reach very limited●discovered in 1947 in cosmic raysC.Paus, LHC Physics: Resonance Production, Decay and Reconstruction 10The Stories Muon story●muon was found in comic rays shortly after Yukawa's pion prediction, and incorrectly identified as pion, mass too small though●was finally correctly identified as the heavier brother of the electron but has no role in ordinary matter.... Strange particles●in cosmic rays a whole bunch of unnecessary particles started popping up: K+, K–, KL, KS●in strong interactions they appeared in conjunction with other new particles: Λ, Σ, Ξ●they were considered to be strange and strange quantum number followed: strangeness has to be conservedC.Paus, LHC Physics: Resonance Production, Decay and Reconstruction 11Ordering the Zoo By the sixties over 100 particles (resonances) were known, categorizing scheme(s) became necessary●according to mass, did not really work●photons: massless●leptons: lightweight (electron, neutrino, muon)●mesons: middle weights (pion, kaon)●baryons: heavyweights (proton,neutron, Λ, Σ, Ξ)●the eightfold way●1960/1 Murray Gell-Mann and Yuval Ne'eman arranged particles in patterns●number eight is one of the recurring schemes and gives it its name●1964 Gell-Mann and George Zweig propose that only three quarks could construct all particles u (2/3), d (-1/3), s (-1/3) following an SU(3) symmetry group (later more SU(N))C.Paus, LHC Physics: Resonance Production, Decay and Reconstruction 12The Eightfold Way Neutron and proton form part of the octoplet which gave the theory its name (8 particles) Different multiplets put all togetherC.Paus, LHC Physics: Resonance Production, Decay and Reconstruction 13The Eightfold Masterpiece “The esoteric world of theoretical physics went into spasms of enthusiasm last week when Brookhaven National Laboratory announced the identification of a new elementary particle. It is not the biggest particle known or the smallest, and it lives only one ten-billionth of a second. But physicists all over the world were stirred up because it has almost precisely the mass that was predicted for it by long-range theory. It was rather as if Columbus, sailing across the Atlantic, had really found Japan just where he thought it would be...” Time magazine article: Feb 28, 1964C.Paus, LHC Physics: Resonance Production, Decay and Reconstruction 14The Eightfold Masterpiecesingle event discoveryBrookhaven National LaboratoryC.Paus, LHC Physics: Resonance Production, Decay and Reconstruction 15Resonance Decays Table of different decaysDecay type MediatorTypical decay timestrong force gluon 1.00E-023electromagnetic force photon 1.00E-016weak force intermediate vector boson 1.00E-013 Practicalities●strong force gives very fast decay: no measurable lifetime but the particle width can be measured●electromagnetic decays: no measurable lifetime nor width●weak decays: lifetimes measurable