| Abstract: | The COMPASS collaboration at CERN has measured diffractive dissociation of
190 GeV$/c$ pions into the $pi^-pi^-pi^+$ final state using a stationary
hydrogen target. A partial-wave analysis (PWA) was performed in bins of $3pi$
mass and four-momentum transfer using the isobar model and the so far largest
PWA model consisting of 88~waves. A narrow $J^{PC} = 1^{++}$ signal is observed
in the $f_0(980),pi$ channel. We present a resonance-model study of a subset
of the spin-density matrix selecting $3pi$ states with $J^{PC} = 2^{++}$ and
$4^{++}$ decaying into $
ho(770),pi$ and with $J^{PC} = 1^{++}$ decaying
into $f_0(980),pi$. We identify a new $a_1$ meson with mass
$(1414^{+15}_{-13})$ MeV$/c^2$ and width $(153^{+8}_{-23})$ MeV$/c^2$. Within
the final states investigated in our analysis, we observe the new $a_1(1420)$
decaying only into $f_0(980),pi$, suggesting its exotic nature. To our
knowledge, such a state has never been predicted. |
REGISTER
Did you like this article? 
