Search for Exotic Mesons in γp → π+π+π−n with CLAS at Jefferson Lab

In addition to ordinary q-qbar pairs, quantum chromodynamics (QCD) permits many other possibilities in meson spectra, such as gluonic hybrids, glueballs, and tetraquarks. Experimental discovery and study of these exotic states provides insight on the nonperturbative regime of QCD. Over the past twenty years, some searches for exotic mesons have met with controversial results, especially those obtained in the three-pion system. Prior theoretical work suggests that in photoproduction, gluonic hybrids might be found at enhanced levels relative to that found in pion production. To that end, the E004-05 experiment at Jefferson Lab's CEBAF Large Acceptance Spectrometer (CLAS) has recently acquired a high statistics photoproduction dataset, using a liquid hydrogen target and tagged photons from a 5.71 GeV electron beam. The CLAS experimental apparatus was modified to maximize forward acceptance for peripheral production of mesons. The resulting data contain the world's largest 3π photoproduction dataset. A sample of 510K γp → π+π+π-n events have been subjected to a partial-wave analysis in search of the spin-parity exotic π1(1600) meson. Results show strong evidence for the production of the well-known nonexotic a2(1320) and π2(1670), as well as some evidence for the a1(1260). However, we find no evidence of π1(1600) production in γp → π+π+π-n. Possible explanations include misinterpretation of previous analyses claiming the existence of the π1(1600), or a preference for production mechanisms forbidden to charge-exchange photoproduction.