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In a simple model of the reaction, a circularly polarized virtual photon strikes an oppositely polarized up quark inside the proton (1). The spin of this quark flips (2) and the quark recoils from its neighbours, stretching a gluonic flux-tube between them (3). When the stored energy is sufficient, the tube breaks and a strange quark-antiquark pair is produced (4, 5).

JLab results put new spin on the vacuum

New measurements from the US Department of Energy's Jefferson Lab (JLab) in Virginia are challenging existing ideas on how quark-antiquark pairs are produced from "nothing" — that is, the vacuum. Members of the CEBAF Large Acceptance Spectrometer (CLAS) collaboration have studied the spin transfer from a polarized electron beam to a produced lambda particle, with surprising results.

The experiment recorded collisions between a 2.567 GeV longitudinally polarized electron beam and a proton target in which the electron emerges together with a polarized lambda (Λ0) and a kaon (K+). The large acceptance of CLAS enabled the team to detect the outgoing electron and the kaon, as well as the proton from the decay of the lambda, over a wide range of scattering angles — in effect, a wide range of momentum transfer from the electron to the quark system. The team was thus able to measure the angular dependence of the lambda polarization.

At the quark level, the reaction studied corresponds to the creation, from the available kinetic energy, of a strange quark-antiquark pair, in addition to the original quarks in the proton. In a simple model of the reaction dynamics, a circularly polarized virtual photon (emitted by the polarized electron) strikes an oppositely polarized up quark inside the proton. The spin of the struck quark flips in direction and the quark recoils from its neighbours, stretching a flux-tube of gluonic matter between them. When the stored energy in the flux-tube is sufficient, the tube is "broken" by the production of a strange quark-antiquark pair.

Using this simple picture, the CLAS team found that they could explain the measured angular dependence of the lambda polarization if the quark pair is produced with the spins in opposite directions, or anti-aligned. This is unexpected because according to the popular triplet-P-zero (3P0) model, a quark-antiquark pair is produced with vacuum quantum numbers, and that means their spins should be aligned. The new results imply that the 3P0 model may not be as widely applicable as was previously thought.

Winston Roberts, a theorist at JLab and Old Dominion University, finds the CLAS measurement very interesting. "If they are right, it means we have to rethink what we thought we understood about our models for baryon decays," he said. "The CLAS results may also be saying something about what we understand of baryons themselves - our knowledge of how to describe scattering processes such as the one they measure, or even that there may be oddities or peculiarities, dare I say 'strangeness', in the way strange quark-antiquark pairs are produced."

The CLAS team itself expects further reaction from theorists. "Polarized lambda production is obviously sensitive to the spin dynamics of quark-pair creation," said Mac Mestayer of JLab. "We eagerly await confirmation, or refutation, of the conclusions of our simple model by realistic theoretical calculations." Meanwhile, the collaboration is planning further experiments, as Daniel Carman of Ohio University and lead author of the recent paper explains. "Our group is continuing this exciting research by extending our arguments to test our picture of the dynamics in different reactions."

The results certainly show that we still do not fully understand the basic structure of the vacuum. Twentieth-century quantum field theories filled the once-empty space with virtual particles. Now JLab physicists are working to measure the spin of those particles, helping us to understand the vacuum better as well as the matter that populates it.

Further reading

D S Carman et al. 2003 Phys. Rev. Lett. 90 131804.