Precision study of the $η\toμ^+μ^-γ$ and $ω\toμ^+μ^-π^0$ electromagnetic transition form-factors and of the $ρ\toμ^+μ^-$ line shape in NA60

Abstract

The NA60 experiment studied low-mass muon pair production in proton-nucleus (p-A) collisions using a 400 GeV proton beam at the CERN SPS. The low-mass dimuon spectrum is well described by the superposition of the twobody and Dalitz decays of the light neutral mesons η, ρ, ω, η and φ, and no evidence of in-medium effects is found. A new high-precision measurement of the electromagnetic transition form factors of the η and ω was performed, profiting from a 10 times larger data sample than the peripheral In-In sample previously collected by NA60. Using the pole-parameterisation |F(M)| 2 = (1 − M 2 /Λ 2 ) −2 we find Λ −2 η = 1.934 ± 0.067 (stat.) ± 0.050 (syst.) (GeV/c 2 ) −2 and Λ −2 ω = 2.223 ± 0.026 (stat.) ± 0.037 (syst.) (GeV/c 2 ) −2 . An improved value of the branching ratio of the Dalitz decay ω → µ + µ − π 0 is also obtained, with BR(ω → µ + µ − π 0 ) = [1.41 ± 0.09 (stat.) ± 0.15 (syst.)] × 10 −4 . Further results refer to the ρ line shape and a new limit on ρ/ω interference in hadron interactions.

What can we learn about light-meson interactions at electron–positron colliders?

“…(b) Form factor |F ωπ | 2 in the two experimentally accessible kinematic regions 4m 2 e < s < (m ω − m π ) 2 and s > (m ω + m π ) 2 . Form factor data in the decay region is taken from NA60 [345] (stars) and A2 [346] (crosses). The line is the VMD parameterization from Eq.…”

“…The data in the 2m < √ s < m ω − m π region for the form factor is obtained from ω → π 0 + − decays. The two most precise results from the NA60 [345] and A2 [346] experiments are included. In NA60, the ω mesons are produced in pp or p-nucleus collisions and the ω → π 0 µ + µ − decay is extracted from inclusive dimuon invariant-mass distributions.…”

Analysis of Dalitz decays with intrinsic parity-violating interactions in resonance chiral perturbation theory

“…Ratio Model PDG Model in the isospin limit In the following, TFFs for Dalitz decay of vector mesons are analyzed. In particular, we fit |F V I P i | 2 for (i, I) = (1, 2), (1,3) and (2,3), in each bin for di-lepton invariant mass. In order to minimize the statistic,…”

“…(192) the experimental data extracted from Refs. [1,[33][34][35][36][37][38] are adopted for parameter estimation. In the fitting procedure, two cases: c + 34 < 0 and c + 34 > 0 are considered.…”

“…Using these experimental data, we can test the validity of QCD effective theories which include resonances of vector meson. As a recent result, high-precision data of transition form factors (TFFs) for ω → π 0 µ + µ − and η → γµ + µ − are measured by the NA60 collaboration [1] in proton-nucleus (p-A) collisions. Moreover, the measurement of the branching ratio and the TFF of η → γe + e − has been carried out by the BES III collaboration [2].…”