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Spin identity

Researchers grapple with facinating phenomenon of spinning quarks in the nucleon



Spin is an essential and fascinating phenomenon in the physics of elementary particles. Ever since spin was first defined by Goudsmit and Uhlenbech in 1925, it has played a dramatic role in elementary parŽticle physics, sometimes refuting theories and at other times supporting them. During Experiment E99-117 at Jefferson Lab, an international collaboration consisting of more than 80 physicists from 22 institutions and led by Jian-Ping Chen (JLab), Zein-Eddine Meziani (Temple Univ.) and Xiaochao Zheng (then a Ph.D. student at M.I.T.), collected precision data on the spin of the neutron. Results from this experiment provide evidence that our current understanding of spin is not totally valid.

In high-school physics lessons, nucleons simply consist of three quarks. A more complete picture includes these three so-called valence quarks, plus a sea of quark-antiquark pairs that pop in and out of the vacŽuum, and gluons exchanged between quarks. Experiments where both the electron beams and the target spins are polarized can provide information about how quarks' spins are oriented inside the target proton or neutron, helping us to understand the fundaŽmental structure of matter and the strong forces holding it together. In the early 1980's people thought that the quarks' spin should contribute a majority of the proton spin. But in 1987, the pioneering spin experiment by the European Muon Collaborations at CERN showed that the sum of the spins carried by the quarks in a proton add up to only about one-eighth of the proton's spin. This "proton spin crisis" triggered tremendous efforts to further study the source of spin of protons and neutrons. Now after 20 years of study, we know that the nucleon spin comes not only from the spin of quarks, but also from the quarks' orbital angular momentum, and from the angular momentum of gluons, the particles that hold the quarks together.

Therefore, one attractive place to study the nucleon spin structure is the valence quark region, where the nucleon can be viewed as being made of only three valence quarks, while other components -- gluon, strangeŽness (s quarks) and other sea quarks -- are scarce and the nucleon is relaŽtively easier to study. In particular, it is expected when a valence quark carries a majority of the nucleon energy, it should have negligible orbital angular momentum (OAM) and its spin should align to the nucleon's spin. Since the nucleon is primarily made of two flavors of valence quarks, the up (u) and the down (d) quarks, it is necessary to combine informaŽtion from both protons and neutrons to decompose the nucleon spin into different valence quark flavors. Due to the lack of precision neutron data, the valence quarks' spin orientations have not been explored until recently. Experiment E99-117 collected, for the first time, precision data on valence quarks' spin distribution in the neutron. The experiment ran in Hall A from June 1 to Sept. 29, 2001. In the experiŽment, a polarized beam of electrons was sent into a polarized helium-3 (He-3) target. This target was used because the He-3 nuclei is made of one neutron and two protons with their spins anti-aligned with each other, hence most of the helium-3 spin comes from the neutron. Electrons scattered from the target were detected in Hall A's two High Resolution Spectrometers (HRS). The nucleon spin asymmetry was formed by comparing the counts of scattered electrons for opposite electron beam helicity states and then corrected for the helium-3 nuclear effects.

When we combined the experiment's neutron results with previous data on the proton, we found that while the spin of the valence up quark is aligned parallel to the proton spin, this is not true for the valence down quark. This new result disagrees with our previous expectations and indicates that valence quarks' OAM is not negligible for the kinematic region explored at JLab. On the other hand, predictions from relativistic constituent quark models, which takes into account the quark OAM through relativistic effects, agrees well with the new data. Extensions of this measurement are being planned as one of the "flagship" experiments for the upgraded JLab. The doubled beam energy (12 GeV) will allow a test of our understanding of the nucleon spin at a cleaner valence and more energetic region.

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