Precision study of the $\eta \to {\mu }^{+}{\mu }^{-}\gamma$ and $\omega \to {\mu }^{+}{\mu }^{-}{\pi }^0$ electromagnetic transition form-factors and of the $\rho \to {\mu }^{+}{\mu }^{-}$ 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 two-body and Dalitz decays of the light neutral mesons $\eta$, $\rho$, $\omega$, ${\eta }^{\prime }$ and $\varphi$, and no evidence of in-medium effects is found. A new high-precision measurement of the electromagnetic transition form factors of the $\eta$ and $\omega$ was performed, profiting from a 10~times larger data sample than the peripheral In-In sample previously collected by NA60. Using the pole-parameterisation $|F(M){|}^2=(1-M^2/{\mathrm{\Lambda }}^2{)}^{-2}$ we find ${\mathrm{\Lambda }}_{\eta }^{-2}=1.934\pm 0.067$~(stat.) $\pm 0.050$~(syst.)~(GeV/$c^2$)${}^{-2}$ and ${\mathrm{\Lambda }}_{\omega }^{-2}=2.223\pm 0.026$~(stat.) $\pm 0.037$~(syst.)~(GeV/$c^2$)${}^{-2}$. An improved value of the branching ratio of the Dalitz decay $\omega \to {\mu }^{+}{\mu }^{-}{\pi }^0$ is also obtained, with $BR(\omega \to {\mu }^{+}{\mu }^{-}{\pi }^0)=[1.41\pm 0.09(stat.)$ $\pm 0.15(syst.)]\times 10^{-4}$. Further results refer to the $\rho$ line shape and a new limit on $\rho /\omega$ interference in hadron interactions.

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• The hadronic light-by-light contribution to the muon's anomalous magnetic moment
• Towards an improved understanding of $\eta \to {\gamma }^{\ast }{\gamma }^{\ast }$
• Dilepton Radiation from Strongly Interacting Systems
• What can we learn about light-meson interactions at electron-positron colliders?
• Measurement of the $\omega \to {\pi }^{+}{\pi }^{-}{\pi }^0$ Dalitz plot distribution
• Dielectron production in proton-proton collisions at $\sqrt{s}$ = 7 TeV
• Measurements of Branching Ratios for $\eta$ Decays into Charged Particles
• Light-front holographic radiative transition form factors for light mesons
• Radiative corrections to the ${\eta }^{(\prime )}$ Dalitz decays
• Double-Hit Signature of Millicharged Particles in 3D segmented neutrino detector
• Two-photon decays and transition form factors of ${\pi }^0$, $\eta$, and ${\eta }^{\prime }$ in large-$N_c$ chiral perturbation theory
• Event generators for $\eta /{\eta }^{\prime }$ decays at BESIII
• Observation of the hindered electromagnetic Dalitz decay $\psi (3686)\to e^{+}{e}^{-}{\eta }_c$
• Non-Abelian axial anomaly, axial-vector duality, and the pseudoscalar glueball
• Analysis of Dalitz decays with intrinsic parity violating interactions in resonance chiral perturbation theory
• Improved measurements of the Dalitz decays $\eta /{\eta }^{\prime }\to \gamma e^{+}{e}^{-}$
• Nuclear dependence of light neutral meson production in p-A collisions at 400 GeV with NA60
• Novel approach to investigate $\eta$ decays via ${\eta }^{\prime }\to \pi \pi \eta$
• Possible manifestation of the 2p pionium in particle physics processes
• Measurement of the $\eta$ transition form factor through ${\eta }^{\prime }\to {\pi }^{+}{\pi }^{-}\eta$ decay
• New high-statistics measurement of the ${\pi }^0\to e^{+}{e}^{-}\gamma$ Dalitz decay at the Mainz Microtron
• Data-Driven Predictions for Dark Photon and Millicharged Particle Production
• A revision of radiative corrections to double-Dalitz decays