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1、Introduction to High Energy Physics,Chapter 5: Lepton and Quark Scattering,5.1 Electron-Positron Collision e+e-m+m-,Understanding of quarks and their substructure come from experiments on deep inelastic lepton-nucleon scattering. Its simply interpreted as elastic scattering of the lepton by a point-
2、like constituent of the nucleon, soon to be identified as a quark. The detailed lepton-nucleon scattering experiments gave the first information on the nature of quark-quark interactions. This evidence was reinforced and enlarged by studies of e+e- annihilation to hadrons at high energy.,2,5.1 Elect
3、ron-Positron Collision e+e-m+m-,3,e+e- annihilation,e-m- scattering,5.1 Electron-Positron Collision e+e-m+m-,Figure 5.1(c) shows the collisions in the CME, with the z-axis defining the incident direction. Clearly the initial state is either the combination RL with Jz=+1 or LR with Jz= 1, so that the
4、 exchanged photon is transverse, where R refers to the positively charged lepton. Let q be the angle of emission of the m+m- pair with respect to the z-axis. The probability amplitude for this pair is given by the d-function.,4,5.1 Electron-Positron Collision e+e-m+m-,5,5.1 Electron-Positron Collisi
5、on e+e-m+m-,If considering the parity-violating weak amplitude arising from Z0 exchange, we can observe the backward forward asymmetry.,6,Di-muon spectrum from ATLAS,7,Di-muon spectrum from CMS,8,5.2 e+e-hadrons,e+e- annihilation to hadrons Charged and neutral mesons are produced in the form of two
6、oppositely directed jets. These are interpreted as a two-stage process involving the production and subsequent fragmentation of a quark-antiquark pair. The cross section for this process as a function of cms energy is shown in Figure 5.3. One observes a general fall-off of the cross section as 1/s.
7、Point-like process,9,5.2 e+e-hadrons,The evidence for the process becomes more apparent upon plotting the cross section ratio: The magnitude of R is easily calculated if the point-like constituents are taken to be Quarks acting independently.,10,5.2 e+e-hadrons,11,5.2 e+e-hadrons,12,5.2 e+e-hadrons,
8、13,5.3 Electron Muon Scattering m+e- m+e-,14,5.3 Electron Muon Scattering m+e- m+e-,Evaluating these quantities in the cms in which the 3-momentum of each particle has magnitude p and the angle between the e- and m+ lines of flight is q.,15,5.3 Electron Muon Scattering m+e- m+e-,16,5.4 Neutrino Elec
9、tron Scattering ne- ne-,17,5.4 Neutrino Electron Scattering ne- ne-,18,5.4 Neutrion Electron Scattering ne- ne-,19,5.5 Elastic lepton-nucleon Scattering,Early experiments with electron (and neutrino) beams from accelerators, incident on nucleon targets, had concentrated on the study of so-called exc
10、lusive processes. In particular, measurements were made of the elastic or quasi-elastic processes, where nucleon recoils elastically from the impact. These reactions are dominated by form factors, which determine how the scattering is reduced from its value for a pointlike nucleon.,20,5.5 Elastic le
11、pton-nucleon Scattering,21,5.5 Elastic lepton-nucleon Scattering,22,5.5 Elastic lepton-nucleon Scattering,At present, there is no detailed theoretical model of form factors, the above formula can be interpreted in terms of an exponential distribution of charge and magnetic moment in the nucleon. In
12、terms of quark model, the form factor just measures the amplitude that under an impact, the three valence quarks, plus the other constituents gluons and quark-antiquark pairs hold together and recoil coherently.,23,5.6 Deep Inelastic Scattering,It is found that the cross sections for deep inelastic
13、scattering, in contrast to the elastic cross sections, are large and only weakly q2-dependent. This is a signal of elastic scattering by pointlike constituents (quarks and gluons) inside the nucleon.,24,5.7 Deep Inelastic Scattering and Quarks,25,5.7 Deep Inelastic Scattering and Quarks,26,5.8 Exper
14、imental Results on Quark Distributions,The total neutrino-nucleon cross section is proportional to energy. The observed ration R0.45 derived from experimental results, indicates that the nucleon contains antiquarks as well as quarks, /0.15,27,5.8 Experimental Results on Quark Distributions,The relat
15、ion between electron-nucleon and neutrino-nucleon scattering (confirmed): x 0.5 means 50% of the nucleon momentum is carried by partons without weak or EM couplings In fact, this fraction is carried by gluons that mediate the Quark interactions.,28,Test,15 + 5 presentation of your research project Outline Motivation and Physics Goals Introduction Methods (experimental or theoretical) Results and Discussions Future Plan Summary and Conclusions,29,
