Subatomic Physics

11.14. The Weak Current of Hadrons at High Energies 369
Welcome to
NuTeV
Toroid
(Muon Spectrometer)
Target-Calorimeter
(Iron-Scintillator-Drift Chamber)
Figure 11.13: The NuTev detector in Fermilab. The steel target region is instrumented with
counters and spark chambers to detect the interaction point and to track muons downstream.
The toroids permit measurement of final state muon momenta. The large size is apparent by
comparison with the individual shown.
on the laboratory energy is shown in Fig. 11.14.
Figure 11.14: Total charged current neutrino
and antineutrino cross sections plotted against
energy. [From F. Eisele, Rep. Prog. Phys. 49,
233 (1986).]
The factor of 3 difference between σν
and σ¯ν in Fig. 11.14 can be understood
from angular momentum conservation.
Neutrinos are purely left-handed, antineutrinos
right-handed. For massless
quarks and leptons, only the left-handed
components of these particles participate
in charged current weak interactions,
as we will detail in Chapter 13.
Then, as shown in Fig. 11.15, angular
momentum can be conserved for backward
scattering of neutrinos, but not of
antineutrinos. The consequence is that
the angular distribution of neutrinos is
isotropic, but that of antineutrinos is
[(1 + cos θ)/2] 2 . The resulting decrease
in the integral of the differential cross
section accounts for the smaller antineutrino
total cross section.
For elastic scattering, form factors are important and the effective size of the
target particle provides a scale. Consequently, the cross section as a function of the
laboratory energy flattens out after an initial rise. Lee and Yang used the conserved
vector current hypothesis of Gell-Mann and Feynman to compute the expected cross
sections; their result is shown in Fig. 11.16.
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