Experiment Research
E06-010, Measurement of Single-Spin Asymmetry in a Semi-Inclusive Reaction on a Transversely Polarized Helium-3 Target
and
E07-013, Target Normal Single-Spin Asymmetry in Inclusive Deep-Inelastic Scattering with a Polarized Target
Since the late 1970s, an interesting phenomenon called "target single-spin asymmetry" has been observed. In this phenomenon, pions or kaons produced from a proton or neutron polarized perpendicular (transverse) to the incident beam demonstrate a clear left-right preference with respect to the direction of the incident beam. For example, a recent experiment at DESY showed that positively charged pions favor the left side in the reaction, while negatively charged pions favor the right side.
How does this happen? How do the quarks in a nucleon contribute to this left-right difference? Could it be that within the transversely polarized nucleon, the quarks have their spins aligned in the transverse direction already (i.e. a quark transversity distribution)? Do different types of quarks behave the same way in contributing to the left-right difference?
In E06-010, researchers will scatter high-energy electrons from a vertically polarized neutron target (helium-3) and observe the charged hadrons produced in the reaction (pions and kaons). Researchers will observe if a different number of particles is produced when the target neutron's spin direction is flipped from up to down.
E07-013 will also take data at the same time to address a slightly different left-right asymmetry that is two orders of magnitudes smaller. The experiment concerns inclusive deep-inelastic scattering, where a high-energy electron probe exchanges either one or two virtual photons with a quark in a nucleon. Only the scattered electron probe is observed.
In the case of one virtual photon exchange, the process is time-reversal invariant. In other words, the process is exactly the same when we exchange past with future: the initial state and final state look alike. Therefore, the number of electrons scattered to the left side should be exactly the same as those scattered to the right side (no left-right asymmetry or single-spin asymmetry is allowed).
However, when two virtual photons are exchanged, the situation becomes different; the final state and the initial state are not the same anymore, and the process is asymmetrical. Therefore, more electrons will scatter to one preferred side, and the interference between the two processes (one- vs. two-photon exchange) could generate a target single-spin asymmetry at the order of 0.01% An experiment conducted in 1969 at Stanford University set an upper limit on this type of asymmetry to less than 3%. After 40 years, E07-013 hopes to improve upon this upper limit by two orders of magnitude, or perhaps, for the first time, clearly demonstrate a non-zero asymmetry.