Hc 5: Outline Introduc:on Produc:on and iden:fica:on of par:cles Basic concepts: par:cle- an:par:cle, leptons and hadrons Fundamental interac:ons and force carriers Conserva:on laws and symmetries The Standard model: Quark model Book by Tipler, chapter 12.2 41
Ordering scheme for elementary par:cles (repe::on) Baryons have half- integer spin proton, an:- proton, neutron Δ(1232) resonances, Λ(1116), Mesons have integer spin π 0, π ± (140) η(547), σ(600), ρ(770), ω(782), φ(1019), K ± (493), K 0 s (498), Leptons do not interact strongly electron, muon, tau and neutrinos Note: Leptons are elementary par:cles, hadrons not Baryons and mesons are hadrons 42
Structure of ma5er Building blocks: Elementary par:cles Interac:ons between par:cles This lecture 43
Concept of interac:ons Force between two objects Descrip:on of an effect of unknown cause In general a central force: F(r) = 1/r 2 Informa:on exchange with finite velocity (v c) Describe interac:on by exchange of par:cle(s) Four fundamental interac:ons 44
Fundamental interac:ons (1) are coupled to a certain par:cle property Interac(on Gravita:onal Electromagne:c Strong Weak Par(cle property mass charge colour charge flavour (or weak charge) 45
Fundamental interac:ons (2) Interac:on is mediate by exchange par:cle - boson with integer spin Interac(on Gravita:onal Electromagne:c Strong Weak Force carrier or exchange par(cle graviton photon gluon W ± and Z 0 boson Graviton is postulated and not discovered yet. 46
Fundamental interac:ons (3) Underlying field theory (more in dedicated quantum field theory courses) Interac(on Gravita:onal Electromagne:c Strong Weak Field theory (Quantum- ) Gravity Quantum Electrodynamics Quantum Chromo- dynamics Electro- weak theory 47
Feynman diagrams Rules Line for par:cle; ver:ces for interac:on Par:cle considered in :me - 1- dimensional space frame Par:cle (right arrow) and an:- par:cle (lel arrow) Feynman diagrams give computa:onal rules (more in quantum- field theory courses) t x Compton effect Annihila:on process Book Tipler, page 584 48
Strong interac:on: Quarks Hadrons have substructure Evident from deep- inelas:c electron sca5ering experiments Hadrons consist of quarks q (more in the lecture Quark model ) Baryons (q, q, q) Mesons (q, an:- q) Strong interac:on between quarks, not between leptons No evidence for free (frac:onally charged) quarks Why? Deep- inelas6c electron sca7ering 49
Quark proper:es Quark flavour Quarks have different flavours: up, down, charm, As for the leptons, three genera:on of quarks exist 50
Strong interac:on: Gluons Quarks have an addi:onal property: colour (or strong charge ) Why needed? Colour charge states: red, blue and green and their an:- colour Hadrons are colour neutral states Gluons also carry colour (special!) à They interact with each other 51
Gluon colour charge Possible combina:ons of red, blue and green (an:- ) colour states An:- symmetric colour wave func:on required à 8 Independent colour states Colour singlet (rr +bb +gg)/ 3 (rb +br )/ 2 (rg +gr )/ 2 (bg +gb )/ 2 (rr bb )/ 2 Colour octet i(rb br )/ 2 i(rg gr )/ 2 i(bg gb )/ 2 (rr +bb 2gg )/ 6 52
Feynman diagram of interac:on between quarks and gluons Gluons are represented as spirals carry (colour, an:- colour) charge 53
Poten:al of the strong interac:on Strong force described by Quantum Chromo- dynamics Poten:al is not pure 1/r (gluons massless) since gluons interact with each other linear increase of the field at long distances r Field energy of the string is k = 1 GeV/fm Electromagne:c Field lines Strong interac:on 54
(Running) coupling constant α s Asympto:c freedom at short distance the coupling is weak at large distances it is strong quarks only in bound states α s 0.2 at E = 30 GeV Confinement Free quarks not observed in nature Remember in QED: α = e 2 4πε 0!c 1 137 = 0.007 Q: momentum transfer 55
Quark confinement At short distance, e.g., inside a proton, α s is small and the quarks are free Pull one quark out, α s increases and the force tries to keep it in 56
Hadronisa:on Primary produced quarkanti-quark pair (QCD string) forms more quark- anti-quark pairs, which fragments into hadrons This process is called hadronisation and leads to a reduction of the field energy The produced hadrons are seen as two back-to-back orientated jets 57
Jets A jet is a collimated spray of par:cles JADE experiment, DESY-HERA ALICE experiment, CERN-LHC 58
Weak interac:on New interac:on necessary to describe β and strange par:cle decays Weak interac:on happens for all quarks (having flavour) and leptons changes only quark flavour but not lepton flavour The process is therefore not possible. Exchange par:cles µ e +γ charge current: W ± boson (mass = 80 GeV/c 2 ) neutral current: Z 0 boson (mass = 91 GeV/c 2 ) Electro- weak field theory (more in quantum field theory course) 59
Weak interac:on (cont d) Charged vector bosons change flavour of quarks and leptons A à B + W + 60
Dominant weak decays of quarks quark Each downward arrow emits e + + ν e Each upwards arrow emits e - + ν e Two less possible path are shown with dashed arrows 61
Weak interac:on (cont d) Charged vector bosons change flavour of quarks and leptons A à B + W + Neutral vector boson causes change of energy A à A + Z 0 A = quark or lepton 62
Feynman diagrams for weak interac:ons β decay 63
Feynman diagrams for weak interac:ons β - decay n p+e +ν e Decay of Λ 0 par-cle Λ 0 p+e +ν e 64
Discovery of W ± and Z 0 bosons Produced in proton- an6- proton collisions: Z 0 à e + e - decay Predicted by S. Glashow, S. Weinberg and A. Salam (1973) Discovered by C. Rubbia (and S. van der Meer), UA1 experiment at CERN (1983) W ± (80) and Z 0 (91) are one of the heaviest elementary par:cles 65
Three genera:ons of par:cles Shape and height of the signal of the Z 0 boson are theore:cal related to the number of quark and lepton genera:ons But why? 66
How many elementary par:cle states exist? Answer: Quarks (incl. colour) 36 Leptons 12 Exchange par:cles 13 - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - 61 ======================== 67
Summary Hc 5 Strong interac:on colour charge gluons (carry colour!) colour field and strings Weak interac:on flavour charge Outlook: Why are certain interac:ons possible and other not? à Addi:onal quantum numbers à Conserva:on laws and symmetries W ± and Z 0 bosons Feynman diagrams 68
Ques:ons? 69