[Jlab-seminars] Theory Center Seminar

[Mary Fox]

Theory Center Seminar
Monday, Oct. 24, 2016
1:00 p.m. (coffee at 12:45 p.m.)
CEBAF Center, Room L102

*Challenges of the Three-Pion System *

Mikhail Mikhasenko
Bonn University

High-energy peripheral reactions provide an excellent opportunity to 
study the excitation
spectrum of hadrons. The COMPASS experiment at CERN has measured the 
diffractive scattering
of pions to the 3-pion final state with unprecedented statistical 
precision. Partial wave analysis
techniques have been employed to obtain an expansion of the reaction 
cross section in terms of
partial waves with quantum numbers J^PC M^epsilon.

Since around a hundred of observed resonances in the light sector are 
coupled to pionic systems,
the spin-density matrices from COMPASS PWA are extremely valuable data 
to identify ordinary and,
possibly, exotic mesons. One very interesting candidate to the latter is 
a resonance-like behaviour in
the 1++ 0+ f_0 pi P-wave which has been listed in the latest PDG as 
a_1(1420).

The aim of our analysis is an extraction of three pion scattering 
amplitudes from mass-dependence of
the COMPASS spin-density matrices using analyticity and unitarity 
constraints.

First I am considering the isobar model and the quasi-two-body 
unitarity. Using K-matrix approach, we
built the amplitude for scattering of a quasi-two-body final state (pi 
pi-subchannnel resonance + pion),
and include a unitarization procedure to incorporate non-resonant 
long-range production processes via
pion exchange, i.e. "Deck"-like processes. An application of the model 
to 2-+ sector will be shown.

The second part will be dedicated to a_1(1420) phenomenon. We show that 
the peculiar kinematics of
pi pi pi - Kbar K pi rescattering for the invariant mass around 1.4 GeV 
produces the resonance-like
behaviour. A mechanism called Triangle Singularity is likely responsible 
for the a_1(1420) appearance.
I will discuss a systematic way to deal with the rescattering.

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