High Energy Physics
High Energy Physics PHY 245A
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This 3 page Class Notes was uploaded by Horace McClure on Tuesday September 8, 2015. The Class Notes belongs to PHY 245A at University of California - Davis taught by Staff in Fall. Since its upload, it has received 32 views. For similar materials see /class/191841/phy-245a-university-of-california-davis in Physics 2 at University of California - Davis.
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Date Created: 09/08/15
Accelerators and detectors The goal is to give a quick introduction to particle accelerators and detectors the hardware of the eld The reading includes a discussion of some of the underlying physics of particle detection Unfortunately some of that is rather complicated and messy and not an read Fortunately you can easily nd treatments with more discussion eg Jackson and Peskin Also you should take 252B when it is offered it is a whole quarter devoted to these matters It is good to know the following things The basic ideas of how a linear accelerator and a synchrotron work How secondary beams of particles other than protons and electrons are obtained The advantages of colliding beam machines The event rate luminosity cross section relationship The major machines with their particles and energies are given Be fa miliar with them The detector physics includes the ionization energy loss formula the bremsstrahlung results and the relations for Cherenkov radiation Bremsstrahlung and pair production combine to give electromagnetic showers for electrons and photons Jackson has a much more complete discussion of Cherenkov radiation To describe an event one wants to know the identities and fourmomenta of the nal state particles That s what detectors are for The particles don t materialize with little labels on them like they have in Feynman diagrams The main things one needs to know to identify a particle are its charge and The charge is not usually a problem Charged particles interact with the matter in a detector electromagnetically This leaves a trail of ionization that can be ampli ed and seen Also the charge determines the response of the particle to electric and magnetic elds There is no direct measure of the It is obtained from the particle s fourmomentum Thus the threemomentum and the energy or the velocity must be measured The momentum is available from the the particle s path is bent by an applied B field The velocity can be determined by timeof ight ionization density or Cerenkov radiation Direct measures of the energy are available in calorimetery Other clues to particle identity are available if it is observed to decay or interact with matter in the target The common particles that need to be distinguished and measured are 7r K P 11227 and 1 Charged particles leave ionization It indicates the presence of the particle and something of its velocity The rate of energy loss to ionization is a function of the particle velocity See the formula in the Data Booklet Because 23 s are so light they bremsstrahlung and make cascades The size of the shower gives the energy High energy y s pair produce and then cascade There are a number of devices for detecting particles by their ionization The basic idea is to somehow amplify the ionization along the path of the particle so that it can be recorded and analyzed Such devices bubble cham bers proportional counters multiwire proportional chambers MW PC drift chambers time projection chambers TPC spark chambers streamer cham bers flash tubes scintillation counters emulsions semiconductor counters including microstrip and pixel Particles traveling in a medium at greater than the local velocity of light or crossing a boundary between media with different indices of refraction emit EM radiation in a that depends on the velocity of the particle Ierenkov counters and transition radiation counters take advantage of these effects For measuring the total energy of a particle or jet of particles there are EM shower detectors and hadron calorimeters 23 s and 1 s are known by their EM showers 22 s are known indirectly by the fact that they are not seen at all is by the fact that they leave tracks through large amounts of shielding 7r s K s and P s can be differentiated by TOF Cerenkov or ionization if the energy is not too high They are distinguished from the leptons and 1 s by not having the properties above ou can see a little picure at httplifshitzucdavisedukiskisphy245aU6idpdf Pion electromagnetic form factors The proton has two electromagnetic form factors F1 and F2 or GE and GM A quiz question asked how many electromagnetic form factors the pion has I claimed that because the pion is spin zero it can have no magnetic moment and therefore the answer is that it hasjust one form factor This argument was not universally appreciated Here I offer a first draft of a more detailed argument If you see places where it still needs work let me know One counterargurnent was that if the spins in some sense cancel then since the quarks have different masses and charges the moments will not Let s look at the matrix element of the EM current between pions of momentum p and p pq ie the current carries momentum q ltP j qlp One form factor is easy enough to construct Using p and q and current conservation we P39j Qpgt p p Fq2 0 q FqZ 1 In discussing the proton form factors we have seen that a magnetic form factor has the form 1 q Spy 2 with E antisymmetric A magnetic moment comes from a nonzero limit for E as q goes to zero So for a pion magnetic form factor we need an operator of that form with a non vanishing matrix element ltpql2mlpgt We could try the form PLpu pup y qrpu ppqu but when contracted with q as in 2 it leads back to the form 1 Since we already thought we had exhausted the possibilities in 1 this is no surprise However the idea of the counterargurnent above is that there is some other vector or tensor lurking in the pion state that can be used in addition to p and q to construct an object of the right form The problem with this is that it contradicts the statement that the pion is spinless Our study of the irreducible representations of the Poincare group told us that states are labeled by mass and spin and that s it so far as anything with nonetrivial spacetime properties goes Thus if the pion is spinless there is nothing else to work with
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