Solved: The Case of the Missing “Excited” Nucleons

Discovery could provide a deeper understanding of the dynamics of the three quarks enslaved inside the nucleon.

Image courtesy of Thomas Jefferson National Accelerator Facility

The Science

Scientists conducting experiments at the Thomas Jefferson National Accelerator Facility (TJNAF) have discovered some key, heavier mass (“excited”) versions of the nucleon (i.e. neutron and proton) called “N*s” (pronounced N-stars). These N*s were previously thought to be “missing” – predicted but not in existence.

The Impact

The discovery of some of the famous “missing N* resonances” at TJNAF eliminates a theoretical hypothesis regarding why these N*s might not exist. This “diquark” hypothesis, now shown to be incorrect, suggested that two of the three quarks inside N*s were fused into a “diquark” that acted as a single particle.

Summary

Modern experiments using electron beams at TJNAF and the CEBAF Large Acceptance Spectrometer (CLAS) detector address a precise question: how do the fundamental constituent particles of the Standard Model (quarks, antiquarks and gluons) assemble to form the composite “strongly interacting” particles observed in nature? Experimenters at TJNAF recently discovered five strongly-interacting unstable particles of a type known as “N* baryon resonances.” These composite particles, which are dominantly composed of three quarks, were predicted to exist in supercomputer studies of the theory of quarks and gluons, “Quantum Chromodynamics,” as well as by the original quark model of baryons. Since these predicted “missing resonances” had long eluded discovery, a conjecture arose that the presence of diquarks - a hypothetical strong pairing of two of the three quarks inside a baryon - might actually preclude their existence. The advanced experimental capability provided by the CLAS detector resolved the issue by finding some of these previously missing N* resonances; the crucial breakthrough was to search for the missing N*s in unusual decays that produced strange quarks. Since the previously “missing” N*s evidently do exist, this discovery eliminated the diquark conjecture regarding why these particular strongly-interacting composite particles were absent. The new particles discovered by CLAS have now been included in the Particle Data Group’s definitive 2012 Review of Particle Properties.

Contact

Dr. Volker Burkert
Thomas Jefferson National Accelerator Facility
[email protected]

Funding

Office of Science Nuclear Physics (NP) program

Publications

A.V. Anisovich et al. “Properties of baryon resonances from a multichannel partial wave analysis.” Eur. Phys. J. A 48 15 (2012). [Also available at http://arxiv.org/abs/1112.4937]

CLAS Collaboration (B. Dey et al.). “Differential cross sections and recoil polarizations for the reaction gamma p -> K+ Sigma0.” Phys. Rev. C 82 025202 (2010). [Also available at http://arxiv.org/abs/1006.0374]

CLAS Collaboration (M.E. McCracken et al.). “Differential cross section and recoil polarization measurements for the gamma p to K+ Lambda reaction using CLAS at Jefferson Lab.” Phys. Rev. C 81 025201 (2010). [Also available at http://arxiv.org/abs/0912.4274]

CLAS Collaboration (R.K. Bradford et al.) . “First Measurement of Beam-Recoil Observables Cx and Cz in Hyperon Photoproduction.” Phys. Rev. C 75 035205 (2007). [Also available at http://arxiv.org/abs/nucl-ex/0611034].