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