Current Work
as
a probe of nucleon substructure.
A photon can convert into a particle-antiparticle pair, in this case an
electron-positron pair. This photon is said to have a mass.
Since it has mass, this photon can be at rest! These photons allow us to look inside
neutrons and protons (nucleons), at the charges (quarks) within.
Charged currents give rise to magnetic fields - thus the charge motion is
probed as well. The higher the photon mass the "deeper" we can probe the nucleons.
Shown below is a simulated
event as imaged by the modified RMC spectrometer.
This event shows an electron-positron pair moving outward from the liquid-hydrogen
target at the center. Since they are charged they bend in the strong magnetic field. The
illuminated segments show that charged sensitive panels (scintillators) have fired.
The pattern of these segments will be used to identify the events. Here we see that
the charge definitely came from the target. The crosses in the drift chamber cells are
calculated from the amount of time it takes for ionization from the charged tracks
to reach sense wires.
The photon mass is calculated from the measured electron and positron momenta.
The number of counts in small ranges of photon mass then defines a spectrum that can be
compared to a theoretical prediction. Parameters that enter the theory can be fine-tuned
so as to reproduce the data. This then is equivalent to a measurement of that parameter,
of course assuming that the detector is very well understood.
