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Theory Center Seminar<br>
Monday, Oct. 24, 2016<br>
1:00 p.m. (coffee at 12:45 p.m.)<br>
CEBAF Center, Room L102<br>
<br>
<b>Challenges of the Three-Pion System </b><br>
<br>
Mikhail Mikhasenko<br>
Bonn University<br>
<br>
High-energy peripheral reactions provide an excellent opportunity to
study the excitation <br>
spectrum of hadrons. The COMPASS experiment at CERN has measured the
diffractive scattering<br>
of pions to the 3-pion final state with unprecedented statistical
precision. Partial wave analysis<br>
techniques have been employed to obtain an expansion of the reaction
cross section in terms of <br>
partial waves with quantum numbers J^PC M^epsilon. <br>
<br>
Since around a hundred of observed resonances in the light sector
are coupled to pionic systems,<br>
the spin-density matrices from COMPASS PWA are extremely valuable
data to identify ordinary and, <br>
possibly, exotic mesons. One very interesting candidate to the
latter is a resonance-like behaviour in <br>
the 1++ 0+ f_0 pi P-wave which has been listed in the latest PDG as
a_1(1420). <br>
<br>
The aim of our analysis is an extraction of three pion scattering
amplitudes from mass-dependence of<br>
the COMPASS spin-density matrices using analyticity and unitarity
constraints. <br>
<br>
First I am considering the isobar model and the quasi-two-body
unitarity. Using K-matrix approach, we <br>
built the amplitude for scattering of a quasi-two-body final state
(pi pi-subchannnel resonance + pion), <br>
and include a unitarization procedure to incorporate non-resonant
long-range production processes via <br>
pion exchange, i.e. "Deck"-like processes. An application of the
model to 2-+ sector will be shown. <br>
<br>
The second part will be dedicated to a_1(1420) phenomenon. We show
that the peculiar kinematics of<br>
pi pi pi - Kbar K pi rescattering for the invariant mass around 1.4
GeV produces the resonance-like <br>
behaviour. A mechanism called Triangle Singularity is likely
responsible for the a_1(1420) appearance.<br>
I will discuss a systematic way to deal with the rescattering. <br>
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