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Register a new userObservation of a new narrow axial-vector meson $a_1(1420)$
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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 $rho(770),pi$ 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.
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By analyzing a 2.93~$rm fb^{-1}$ data sample of $e^+e^-$ collisions, recorded at a center-of-mass energy of 3.773 $rm ,GeV$ with the BESIII detector operated at the BEPCII collider, we have searched for the semileptonic $D^+$ transition into the axial-vector meson ${bar{K}}_{1}(1270)^{0}$. The $D^{+} rightarrow {bar{K}}_{1}(1270)^{0}e^{+} u_{e}$ decay is observed for the first time with a statistical significance greater than $10sigma$. Its decay branching fraction is determined to be ${mathcal B}[D^+to bar K_1(1270)^0 e^+ u_e]=(2.30pm0.26pm 0.18 pm 0.25)times10^{-3}$, where the first and second uncertainties are statistical and systematic, respectively, and the third originates from the input branching fraction of ${bar{K}}_{1}(1270)^{0} rightarrow K^{-} pi^{+} pi^{0}$.
The COMPASS experiment recently discovered a new isovector resonance-like signal with axial-vector quantum numbers, the $a_1(1420)$, decaying to $f_0(980)pi$. With a mass too close to and a width smaller than the axial-vector ground state $a_1(1260)$, it was immediately interpreted as a new light exotic meson, similar to the $X$, $Y$, $Z$ states in the hidden-charm sector. We show that a resonance-like signal fully matching the experimental data is produced by the decay of the $a_1(1260)$ ground state into $K^ast(Kpi)bar{K}$ and subsequent rescattering through a triangle singularity into the coupled $f_0(980)pi$ channel. The amplitude for this process is calculated using a novel method based on partial-wave projections. For the first time, the triangle singularity model is fitted to the partial-wave data of the COMPASS experiment. Despite having less parameters, this fit shows a slightly better quality than the one using a resonance hypothesis and thus eliminates the need for an additional resonance in order to describe the data. We thereby demonstrate for the first time that a resonance-like structure in the experimental data can be described by rescattering through a triangle singularity, providing evidence for a genuine three-body effect.
We report the observation of a new $D_{sJ}$ meson produced in $B^{+} to bar{D}^{0} D_{sJ} to bar{D}^{0} D^{0} K^{+}$. This state has a mass of $M=2708 pm 9 ^{+11}_{-10} rm{MeV}/{it c}^{2}$, a width $Gamma = 108 pm 23 ^{+36}_{-31} ~rm{MeV}/ {it c}^{2}$ and a $1^{-}$ spin-parity. The results are based on an analysis of 449 million $Bbar{B}$ events collected at the $Upsilon(4S)$ resonance with the Belle detector at the KEKB asymmetric-energy $e^{+} e^{-}$ collider.
We analyze the low-$Q^2$ behavior of the axial form factor $G_A(Q^2)$, the induced pseudoscalar form factor $G_P(Q^2)$, and the axial nucleon-to-$Delta$ transition form factors $C^A_5(Q^2)$ and $C^A_6(Q^2)$. Building on the results of chiral perturbation theory, we first discuss $G_A(Q^2)$ in a chiral effective-Lagrangian model including the $a_1$ meson and determine the relevant coupling parameters from a fit to experimental data. With this information, the form factor $G_P(Q^2)$ can be predicted. For the determination of the transition form factor $C^A_5(Q^2)$ we make use of an SU(6) spin-flavor quark-model relation to fix two coupling constants such that only one free parameter is left. Finally, the transition form factor $C^A_6(Q^2)$ can be predicted in terms of $G_P(Q^2)$, the mean-square axial radius $langle r^2_Arangle$, and the mean-square axial nucleon-to-$Delta$ transition radius $langle r^2_{ANDelta}rangle$.
We study mixing between conventional and hybrid mesons in vector and axial vector charmonium using QCD Laplace sum-rules. We compute meson-hybrid cross correlators within the operator product expansion, taking into account condensate contributions up to and including those of dimension-six as well as composite operator renormalization-induced diagrams. Using measured masses of charmonium-like states as input, we probe known resonances for nonzero coupling to both conventional and hybrid meson currents, a signal for meson-hybrid mixing.
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