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Title: Precision study of the η → μ +μ -γ and ω → μ +μ -π 0 electromagnetic transition form-factors and of the ρ → μ +μ - line shape in NA60

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 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 22) -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. Finally, further results refer to the ρ line shape and a new limit on ρ/ω interference in hadron interactions.
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Publication Date:
Report Number(s):
BNL-113815-2017-JA
Journal ID: ISSN 0370-2693; R&D Project: PO 024; KA2501032
Grant/Contract Number:
SC0012704
Type:
Accepted Manuscript
Journal Name:
Physics Letters. Section B
Additional Journal Information:
Journal Volume: 757; Journal ID: ISSN 0370-2693
Publisher:
Elsevier
Research Org:
Brookhaven National Lab. (BNL), Upton, NY (United States)
Sponsoring Org:
USDOE Office of Science (SC), High Energy Physics (HEP) (SC-25)
Contributing Orgs:
NA60 Collaboration
Country of Publication:
United States
Language:
English
Subject:
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSICS; Lepton pairs; Transition form factor; Conversion decays; Rho meson
OSTI Identifier:
1354708