Elementary-Particle Physics (1986) / Chapter Skim
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3 What We Have Learned in the Past Two Decades
3 What We Have Learned in the Past Two Decades
Pages 48-80
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From page 48... ...
For example, the A+ + (pronounced delta plus plus) shown in Figure 3.1(A)
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From page 49... ...
WHAT WE HAVE f EARNED IN THE PAST ~0 DECADES 49 is- (ddd)
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From page 50... ...
Thus although the hadron classification scheme based on quarks was widely accepted, the actual physical existence of quarks was questioned. The Discovery of the Charmed Quark During the years from 1964 through 1973, considerable progress was made, both experimentally and theoretically, in support of the idea of physical quarks.
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From page 51... ...
An analogous two-body system is the hydrogen atom, composed of a single electron orbiting around a single proton. The different energy levels of the excited states of hydrogen account for the discrete lines in the spectrum of light emitted by hydrogen; the spectral lines are produced by photons emitted in a transition from an excited level to a less-excited level, and their energy is equal to the difference in energy levels of the initial and final states.
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From page 52... ...
and its antiparticle, they should show a spectrum of energy levels similar to those of positronium. However, since the charmed quark is about 3000 times more massive than the electron, and since the force holding the quarks together in charmonium is the nuclear force (about 100 times stronger than the electric force)
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From page 53... ...
Experimenters at the SPEAR electron-positron collider at the Stanford Linear Accelerator Center (SLAC) began to look at the particles being produced in this machine to see if there might be charged leptons other than the electron or muon being created in the collisions.
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From page 54... ...
Subsequent studies of the tau lepton at SPEAR and other electronpositron colliders showed that it behaved the same way as the electron and muon with respect to the weak and electromagnetic force and that it did not respond to the strong force. Further studies of the decay of the tau lepton demonstrated that it had its own unique neutrino associated with it.
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From page 55... ...
Extensive studies of upsilon particles and B mesons have been and are being made, particularly at electronpositron colliders. For example, Figure 3.5 shows the spectrum of the upsilon family of particles, obtained at the Cornell Electron Storage Ring (CESR)
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From page 57... ...
Each of these states consists of a b quark bound to a ~ quark. For comparison the two 35~ states of the ~ family are shown on the left.
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From page 58... ...
58 ELEMENTARY-PARTICLE PHYSICS Generation Particle Charge Mass ~ electron (e) I ~ electron neutrino (~e)
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From page 59... ...
with neither hadron being changed, and inelastic scattering, where more hadrons are created in the collision. The basic quantitative understanding of this vast body of data has so far been limited to some areas where the new theory of quantum chromodynamics can be applied, as discussed in Chapter 3 in the section on Strong interactions among Quarks.
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From page 60... ...
The wiggly lines are gluons that carry the strong force between the quarks. When the struck quark is knocked away from the two spectator quarks, hadrons are produced by the strong interaction between the scattered quark and the spectator quarks.
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From page 61... ...
IU_U) quarks u scattered qua r k vp = muon neutrino ~~ = negative muon W = W intermediate boson p = proton u = up quark d = down quark FIGURE 3.8 The inelastic scattering of a neutrino on a proton is analogous to the inelastic scattering of an electron on a proton (Figure 3.7)
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From page 62... ...
All these measurements agree with the values expected from the quark model. Evidence of the underlying quark structure of hadrons can be found in many different kinds of experiments.
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/ FIGURE 3.9 In (a) the central black dot represents the point where the electron and positron annihilated.
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From page 64... ...
This gives a total of four jets, two along the direction of the original colliding particles and two more in the directions of the scattered quarks. The details of the process through which the scattered quarks form hadrons, called hadronization, cannot be exactly calculated yet because of its great complexity.
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From page 65... ...
Having profited from the idea that the weak and electromagnetic interactions are at least analogous, one is prompted to ask whether they might actually be related. In relativistic quantum theories, interactions are mediated or carried by force particles.
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From page 66... ...
The idea that the weak and electromagnetic interactions so different in apparent strength—have a common origin provides an estimate of the W's mass of approximately 100 times the proton mass. To advance from these general notions of analogy and synthesis to a viable theory of the weak and electromagnetic interactions has required a half century of experimental discoveries and precision measurements and of theoretical insights and inventions.
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From page 67... ...
A self-consistent theory could- not be based on the known force particles (the photon and the conjectured W) alone but required in addition an electrically neutral weak force particle Z° and an auxiliary object known as the Higgs particle.
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From page 68... ...
68 1 o~ Us rat ao ~ TIC I J 1 '' - - 1 rr)
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From page 69... ...
To extract the intermediate bosons from the background of ordinary events requires an elaborate detector that can recognize and measure the characteristic decay products amid the debris of a violent collision. The most characteristic signal for W decay is an energetic electron emitted transverse to the direction of the colliding beams and an undetected neutrino with equal and opposite transverse momentum.
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From page 70... ...
These seemed at first to indicate that the quark model was nothing more than a convenient mnemonic recipe. In pursuing and resolving these puzzles' physicists have found a dynamical basis for the quark model that promises to give a complete description of the strong interactions.
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From page 71... ...
We still do not understand completely why quarks apparently interact only weakly when they are close together and yet cannot be pulled apart. To see why this is surprising, and to learn how it might come about, it is helpful to consider the interactions of electrically charged objects.
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From page 72... ...
When the color symmetry among red, blue, and green quarks is taken as the basis for a gauge theory, the resulting interactions among quarks are mediated by force particles called gluons. There are eight gluons corresponding to the distinct coloranticolor combinations.
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From page 73... ...
The steady weakening of the charge at short distances is known as asymptotic freedom because quarks become effectively free at very small separations. In the regime of short distances probed in violent high-energy collisions, the strong interactions are sufficiently feeble that reaction rates may be calculated using the diagrammatic methods developed for QED.
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From page 74... ...
The resulting threejet events are commonplace in the electron-positron annihilations studied at the PEP and PETRA storage rings. The highest energies yet attained in collisions of the fundamental constituents are those reached in proton-antiproton interactions at the CERN SppS machine.
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From page 75... ...
This program makes extremely heavy demands on computer time and has spurred the development and implementation of new computer architectures. Already calculations of this sort have yielded suggestive evidence that quarks and gluons are indeed permanently confined in QCD.
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From page 76... ...
This would raise the possibility of a common origin for the strong, weak, and electromagnetic interactions. A unification is also suggested by the fact that both QCD and the electroweak theory are gauge theories, with similar mathematical structure.
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From page 77... ...
In this way the long-standing mystery of the electric neutrality of stable matter is explained, because the proton and electron must have equal and opposite charges if quarks are combined with leptons in extended families. One branch of the first extended family is the set (dreg Doreen dare ever.
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From page 78... ...
Some unified theories also successfully relate the masses of some quarks and leptons in the same generation, but the meaning of these partial successes is less clear. The prediction of proton instability is a key consequence of unified theories, and dedicated experiments have been mounted to search for proton decay.
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From page 79... ...
However, the discovery of the cosmic microwave background radiation (together with many supporting pieces of evidence) makes it likely that the universe began in a hot big bang of extremely high-energy density.
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From page 80... ...
. - ~ FIGURE 3.19 The lrvine-Michigan-Brookhaven experiment searching for proton decay uses 2000 photomultiplier tubes arrayed in an 8000-cubic-meter tank of very pure water.
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