The Search for Exotic Mesons in gamma p -> pi+pi+pi-n with CLAS at Jefferson Lab Page: 2 of 5
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XIV International Conference on Hadron Spectroscopy (hadron2011), 13-17 June 2011, Munich, Germany
exotic, sighted first in pion production experiments [4], [5]. CLAS acceptance is optimized
for baryon spectroscopy, so the partial-wave analysis was performed on a smaller set of
83K events where background from A and N* decays had been removed by kinematic cuts.
No evidence for a 1-+ exotic signal was found [6], a direct challenge to claims that gluonic
hybrids could be produced on an equal footing with the a2(1320). However, statistics were
insufficient to rule out a 7r1(1600) produced at the same level as it was observed in pion
production, at a few percent of the a2.
In order to look for nr1(1600) photoproduction at lower levels, the HyCLAS experiment
was proposed in 2004, with data taken in 2008 as a member of the CLAS g12 rungroup.
2 Features of the CLAS g12 Dataset
The g12 run of CLAS was completed in June of 2008, acquiring 26 billion events of various
topologies. From these 26B triggers we have isolated 6 million exclusive -yp C+r+r-n
events by vertex and timing cuts, with the neutron selected via missing mass. These events
have either a mesonic topology, where -y p Xn 7+ 7+-n, or a baryonic topology,
where -y p N*p c+gr+r-n, as can be seen by the light-shaded distributions in Figure
1. Our analysis depends on having a pure sample of mesonic events, so we selected events
with the following criteria:
I t37 - tmin I = t' < 0.105 GeV2/c4; 0ab (T+) < 25 (both ?T+)
An example of the post-cut n n mass distributions, as well as the resulting 37c mass distribu-
tion can also be found in Figure 1.
3 Partial-Wave Analysis of -y p 7+p+p--n in CLAS g12 Data
We then subjected the final sample of 502K events to a partial-wave analysis based on
the helicity formalism. Decay amplitudes are calculated in the reflectivity basis to ensure
parity conservation, and then each is paired with a complex production amplitude. Those
production amplitudes were then varied in a likelihood fit to find the most probable mixture
of states given the set of input events. We then examine the norm of each production
amplitude to look for peaks, as well as examining the phase difference between pairs of
production amplitudes to look for movement corresponding to the interference of two
Breit-Wigner distributions.
In particular, the fit presented contains 19 waves spread among four JPC: 1++, 2++, 1-+,
and 2-+. Since the helicity of the photon is constrained to be 1 and 7c exchange is the
dominant production mechanism, we should not observe any spin-0 states, one of the most
complicated features of pion-production analyses. Thus, most of the intensity is allocated2
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Bookwalter, Craig. The Search for Exotic Mesons in gamma p -> pi+pi+pi-n with CLAS at Jefferson Lab, article, December 1, 2011; Newport News, Virginia. (https://digital.library.unt.edu/ark:/67531/metadc837595/m1/2/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.