Observation of a New Narrow Axial-Vector Mesona1(1420)

Abstract

The COMPASS Collaboration at CERN has measured diffractive dissociation of 190 GeV/c pions into the π(-)π(-)π(+) final state using a stationary hydrogen target. A partial-wave analysis (PWA) was performed in bins of 3π mass and four-momentum transfer using the isobar model and the so far largest PWA model consisting of 88 waves. A narrow peak is observed in the f0(980)π channel with spin, parity and C-parity quantum numbers J(PC)=1(++). We present a resonance-model study of a subset of the spin-density matrix selecting 3π states with J(PC)=2(++) and 4(++) decaying into ρ(770)π and with J(PC)=1(++) decaying into f0(980)π. We identify a new a1 meson with mass (1414(-13)(+15)) MeV/c2 and width (153(-23)(+8)) MeV/c2. Within the final states investigated in our analysis, we observe the new a1(1420) decaying only into f0(980)π, suggesting its exotic nature.

New a1(1420) state: Structure, mass, and width

“…The prediction for the mass of the J PC ¼ 1 þþ tetraquark state obtained there agrees with data of Ref. [3].…”

“…It is interesting that by classifying a 1 ð1420Þ as an axial-vector meson the COMPASS Collaboration did not exclude its interpretation as an exotic state [3]. One of reasons pointed out there is observation of only a 1 ð1420Þ → f 0 ð980Þπ decay mode of the new meson a 1 ð1420Þ.…”

“…The conventional theory of mesons that considers them as particles composed of a quark and antiquark qq meets with abundance of observed light states that should be included into this scheme. In fact, the family of axial-vector mesons contained till now five states with the spin-parities J PC ¼ 1 þþ [2], recently enlarged due to discovery by COMPASS Collaboration of a new resonance a 1 ð1420Þ with the same quantum numbers [3]. Some of these states including a 1 ð1420Þ meson were listed in Ref.…”

Threshold and Kinematic Effects for the Exotic Resonance-Like Structures

“…Thus, the ATS mechanism implies a peak structure with I G , J PC = 1 − , 1 ++ in the 3π channel. The recent observation of an isovector a 1 (1420) by the COMPASS Collaboration in π − p → a 1 (1420) ± π ∓ n → π + π − π 0 n [15] can be regarded as a direct evidence for the ATS mechanism [16].…”

Production of $$a_1$$ a 1 in heavy meson decays

“…Recently the COMPASS collaboration [26,27] has reported the observation of a light resonance-like state with quantum numbers I G (J PC ) = 1 − (1 ++ ) in the P-wave f 0 (980)π final state with f 0 (980) → π + π − . The signal was also confirmed by the VES experiment [28] in the π − π 0 π 0 final state.…”