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1. the SM of Particle Physics in a Nutshell the Electroweak And

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1. the SM of Particle Physics in a Nutshell the Electroweak And 1. The SM of Particle Physics in a Nutshell The electroweak and strong interactions of quarks and leptons are described by renormalizable quantum gauge theories. The principle of invariance of the theory under the transformation of a local gauge symmetry group fixes the dynamics. The particles that transmit the forces are thus called gauge bosons. They have spin 1 and have to be quantized according to Bose-Einstein spin statistics (bosons). The matter particles, the quarks and leptons, have spin and have to be quantized ¡ according to Fermi-Dirac spin statistics (fermions). Force acts on transmitted by electromagnetic all electrically charged particles photon (massless, spin 1) £ ¢ ¢¤£ ¥§¦ weak quarks, leptons, ¥§¦ (massive, spin 1) strong all colored particles 8 gluons (quarks and gluons) (massless, spin 1) PHY521 Elementary Particle Physics Symmetries are mathematically formulated using group theoretical methods: ¡ The transformations of local gauge symmetries are described by unitary § £ ¡ ¤¦¥ matrices, ¢ ( H: hermitian, quadratic matrix), with real, space-time dependent elements. The matrices ¢ form a group called U(n), SU(n) (det(U)=1). ¡ ¡ ¨ © U(n) has and SU(n) parameters, , and generators, , and can be £ written in terms of infinitesimal transformations as follows ( ): £ © ¢ U(n): £ © ¢ SU(n): Example: £ © ¢ Gauge group of the electromagnetic interaction: U(1) with ! . ! is the electric charge. Requiring the Dirac equation, which describes free electrons, to be invariant un- der these transformations leads to electron-photon interaction and the existence of massless photons. PHY521 Elementary Particle Physics Interaction symmetry group gauge theory electromagnetic unbroken local U(1): QED invariance under space-time dep. phase transitions generated by the electric charge strong unbroken local SU(3): QCD invariance under space-time dep. rotations in the 8-dimensional color space electroweak (spontaneously broken) SU(2) U(1): SM of electroweak invariance under space-time dep. interactions rotations in the 3-dim. (weak) isospin space and under phase transitions generated by the (weak) hypercharge, £ ¡ ¤ ¡ ¦ ¥ ( ¢ ) §©¨ PHY521 Elementary Particle Physics PHY521 blets particles The Right-handed Leptons and ¢ ¤ ¥ ¥ : : electric third ' ( antiparticles of £ component ) the and b char ) ¦ ut : quarks symmetry Lepton § ge § quarks the in ¦ ¦ of quantum § units and number of the £ Leptons Quarks are leptons the of ¨ © . weak group arranged , is : quarks Baryon § separately numbers isospin ( § ¦ £ " ¦ of # $ £ the number %& and in © ¨ ¦ conserv & ! weak ! three ( leptons * is is § § the observ ed + f ¦ isospin, amilies for ) Sommerfeld form are ¦ ¦ each ed ha ¤ e v ¥ re singlets xperimentally e f (generations) SU(2) v amily the ersed fine ¦ (with, under same structure ¢ ( in © ¥ ¥ ¡ sign. e.g., SU(2) to £ mass be ¢ of ¦ ¦ constant). conserv ( © ¤ © © left-handed £¥¤ ¥ £ and £ ): ed. for spin ). ¦ ¦ Elementary ¥ ¥ ¥ as dou- ¥ the P article ¥ ) Physics ¡ £ However, recently, In the SM the neutrinos ( ) are considered to be massless. ¢ strong experimental evidence has been found that this might not be the case. This could be the first signal of physics beyond the SM (term paper). Leptons do not carry color charge and thus do not feel the strong force. Quarks carry color charge and each quark flavor comes in three colors. Colored particles are permanently bound in colorless hadrons (“confinement”) (mesons: ¤¦¥ ¤ bound states, baryons: ¤ ¤ ¤ bound states). As a consequence of QCD, quarks are asymptotically free, i.e. the strength of the coupling decreases with increasing momentum transfer of the interaction discussed (term paper). As a consequence of the mechanism which generates mass for the electroweak §©¨ gauge bosons, , (Higgs-Kibble mechanism), the SM predicts the existence of a massive, neutral, spin 0 particle, the Higgs boson (term paper). The Higgs boson is the only SM particle that has not been experimentally observed (yet) (term paper). PHY521 Elementary Particle Physics SM prediction versus experiment An example: ¤¡ ¤ ¢ ¢ £ ¤ ¤ £ 10 PEP PETRA TRISTAN LEP HRS CELLO AMY L3 MAC JADE TOPAZ MARK II MARK J VENUS ] PLUTO nb TASSO [ 1 σ + − + − -1 e e → µ µ 10 -2 10 0 20 40 60 80 100 120 140 160 180 200 √s [GeV] § © © § ¦ ¨ ¦ ¨ ¦ £ £ ¥ (total) cross section © © § ¨ ¦ £ center-of-mass energy from http://l3.web.cern.ch/l3/analysis/lineshape/wwwls.html PHY521 Elementary Particle Physics SM prediction versus experiment An example: ¥ ¥ ¤ ¤ ¢ ¢ ¢ § § ¡ ¡ ¡ ¡ £ £ 30 16/07/2002 LEP (pb) PRELIMINARY WW σ 20 10 YFSWW/RacoonWW no ZWW vertex (Gentle) ν only e exchange (Gentle) 0 160 180 200 from http://lepewwg.web.cern.ch/LEPEWWG/ √s (GeV) PHY521 Elementary Particle Physics SM prediction versus experiment: A possible Higgs signal An example: £ ¢ ¢ ¤ ¤¦¥ £ £ at the LHC ¢¡ 1500 20000 1000 Events / 2 GeV 17500 500 15000 12500 0 Signal-background, events / 2 GeV 10000 105 120 135 105 120 135 mγγ (GeV) mγγ (GeV) from the ATLAS TDR, CERN-LHCC 99-14 PHY521 Elementary Particle Physics.
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