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Research Highlights: Nuclear Physics

Precision

The Proton's Electromagnetic Structure


The Proton's Electromagnetic Structure (top ^)

For more than 20 years it has been assumed, based on the available data, that the charge and magnetization distributions in the proton were proportional to one another (corresponding to µGe/Gm=1). New data from Hall A shows that this is not true, and is leading to a re-examination of the dynamics governing the proton's quark wavefunctions.

For more information, see: M. K. Jones, et al., Phys. Rev. Lett. 84, 1398 (2000), O. Gayou, et. al., Phys. Rev. Lett. 88, 092301 (2002) and http://hallaweb.jlab.org/


Final HAPPEX

results of the HAPPEX experiment

Strange Quarks in the Proton (top ^)

Strange quark-antiquark pairs are constantly bubbling up in the proton. In hadronic language, the proton can dissociate into a virtual hyperon-strange meson system (e.g., lambdaK). In quark language, S Bar 2 pairs are constantly being made virtually by gluon-mediated interactions inside the proton. In either case, one expects a substantial probability of strange quarks and a corresponding contribution to the charge and magnetization distributions of the proton. (A predicted strange quark contribution of 10% of the proton magnetic moment is typical). To disentangle this contribution from the dominant effects of the u and d quarks requires the use of the weak interaction (Z exchange) as a probe. The HAPPEX experiment shows that the actual contribution is considerably smaller than expected in most models. For more information see: K.A. Aniol et al., Phys. Rev. Lett. 82, 1096 (1999), and http://hallaweb.jlab.org/


Quark Model

Spectrum of 3 Confined Spin 1/2 Quarks

Missing Excited States of the Nucleon (top ^)

If the proton is made of three spin-1/2 quarks, then its "atomic" excitation spectrum must contain a certain number of levels with certain quantum numbers. At this time more than half of the low-lying states needed to test this basic picture are missing.

Recent data from CLAS promises to rectify this situation. The first figure below shows CLAS's kinematic coverage in Q2 and total recoil mass W (left panel) and decay channel identification capabilities in W and the missing mass of the mesonic system recoiling against the proton (right panel) of the CLAS. One clearly sees, for example, the dominant pion decay of the delta resonance [delta(1232)] and the eta decay of the N*(1520) and N*(1680) states. The region from 1.9 to 2.1 GeV in excitation is where many of the missing states are expected to be observed. A large number of events are seen in this region in the omega channel, consistent with theoretical predictions and with the fact that they haven't been seen in earlier experiments (most of which involved pion excitation and therefore wouldn't couple well to these states).

Nucleon

Left: CLAS's kinematic coverage in Q2 and total recoil mass W
Right: decay channel identification capabilities in W and the missing mass of the mesonic system recoiling against the proton of the CLAS

The second figure shows the overall quality of CLAS data compared to that available from earlier experiments (even though only a few percent of the CLAS data set is presented!). The data are clearly of a quality sufficient to perform the phase shift analysis that will be essential to identify the missing states. For more information see: http://www.jlab.org/Hall-B/

CLAS Data

overall quality of CLAS data compared to that available from earlier experiments


Quasi-Elastic

e12C->epX, when plotted as a function of the missing momentum (Pm) and missing energy (Em) carried by X

Testing the Limits of Nuclear Many Body Theory (top ^)

New data on the reaction e12C->epX, when plotted as a function of the missing momentum (Pm) and missing energy (Em) carried by X (see Figure) shows the expected pion production channel (events that are Fermi-smeared around the diagonal line), and the expected quasifree events where X is a recoiling A=11 system (the events clustered around Em=O). However, it also clearly shows a large number of events (the green "peninsula") consistent with two body correlations.

In addition to epX events, e2HX and e3HX have been seen:

Events 1

e2HX and e3HX

as well as events in which the nucleus is seen to shatter into all possible numbers of protons:

Events 2

the nucleus is seen to shatter into all possible numbers of protons

For more information, see: http://www.jlab.org/Hall-B/


Deuteron A

The "t20 experiment

Probing the Limits of Standard Nuclear Physics (top ^)

The deuteron is not a spherical nucleus. In the standard proton-neutron picture of this simplest nucleus, it's shape is largely determined by pion exchange which leads to strong noncentral (namely "tensor") interactions. While clearly correct at large distances, many expected this description to break down as the proton and neutron quark wavefunctions began to overlap one another. The "t20 experiment" has measured this shape at high momentum transfers (Q2) corresponding to distances of order the proton radius and seen no departure from standard nuclear physics.

When combined with earlier data from CEBAF's Hall A:

BCD Shapes 1

When combined with earlier data from CEBAF's Hall A

For more information see: L. C. Alexa et al., Phys. Rev. Lett. 82, 1374 (2000).

One can deduce the intrinsic shape of the deuteron:

BCD Shapes 2

the intrinsic shape of the deuteron

For more information see: D. Abbott et al., Phys. Rev. Lett. 82, 1379 (1999); D. Abbott, et al., Eur. Phys. J. A7, 421 (2000); D. Abbott, et al., Phys. Rev Lett. 84, 5053 (2000) and http://www.jlab.org/Hall-C/


Exotic Meson

the Exotic Meson experiment

Gluonic Excitation Cross Sections Are Large (top ^)

Glue plays a central role in strongly interacting matter: The mass of the proton is about half gluonic, and the most novel feature of QCD, quark confinement, is due to gluonic forces. The most basic experimental signature for the presence and dynamics of glue is the spectrum of gluonic excitations of strongly interacting matter, but no states of this type have yet been established. It had long been believed that searches for gluonic excitations of Q Bar 2 mesons with "exotic" quantum numbers would be the most fruitful method for discovering these states and extracting information from them on the dynamics underlying quark confinement.

However, planned searches (e.g., at the 12 GeV CEBAF Upgrade) were based on the speculation that the production cross section for such states were sizeable. In Phys. Rev. D 60, 114016 (1999) it is shown, using an exact duality of the heavy quark limit, that the cross sections for such states is large in the flux tube model, and that the physics underlying these large cross sections can be seen in operation in the model-independent region where perturbative QCD can be applied. Once identified, this mechanism for gluonic excitation can be tracked into ordinary hadrons, with the important conclusion that gluonic excitation cross sections are comparable to ordinary excited state cross sections once the threshold for their production is crossed.