A direct constraint on dimension-eight operators from

Abstract

We study the radiative decay Z → ννγ within an effective Lagrangian approach. Using the search for energetic single-photon events in the data collected by the L3 Collaboration, we get direct bounds on dimension-six and dimension-eight operators associated with the τ -neutrino magnetic moment and the anomalous electromagnetic properties of the Z boson. As a by-product of our calculation, we reproduce the L3 result for the bound on µ ντ .The effective Lagrangian approach concerning the local SU(2) L × U(1) Y symmetry linearly realized [1] has been used recently to explore the consequences of physics beyond the Standard Model (SM) at lepton [2], hadron [3] and γγ colliders [4]. Also, this approach has been used to constrain the anomalous electromagnetic couplings of the W boson, the t quark [5] and the neutrinos [6] from the known experimental bounds on the rare decays b → sγ [7] and µ → eγ [8]. In the present letter we point out that the recent measurement of energetic single-photons at LEP arising from the radiative decay Z → ννγ [9] leads to direct constraints on dimension-eight and dimensionsix operators associated with the anomalous electromagnetic properties of the Z vector boson and the τ -neutrino magnetic moment, respectively.The radiative decay Z → ννγ can not be induced at the tree level in the SM. In the effective Lagrangian approach this decay could proceed through

Bounds on the dipole moments of the tau-neutrino via the process $e^{+}e^{-}\rightarrow \nu \bar{\nu}\gamma$ in a 331 model

“…is obtained at q 2 = 0 from a beam-dump experiment with assumptions on the D s production cross section and its branching ratio into τ ν τ [39], thus severely restricting the cosmological annihilation scenario [40]. Our results in Tables 1 and 2 Collaboration [18], and with others limits reported in the literature [15,16,22,36].…”

“…from measurements of the Z 1 invisible width at LEP [22]; µ ντ ≤ 2.62 × 10 −6 in the effective Lagrangian approach at the Z 1 pole [36];…”

New Physics Effects in Rare Z Decays

“…The ZZ * γ effective vertex can be parametrized in terms of four form factors h Z i . Two of them are CP-conserving and two are CP-violating [21,22] …”

“…The bound obtained for the coefficients of these operators is given by ǫ 8 < 0.165 [22], with the following definition for this dimensionless coupling…”

Search for electromagnetic properties of the neutrino inγeandγγcollisions at CLIC

“…The magnetic moment couple neutrinos to photon through the dimension-6 effective Lagrangian term [19,[22][23][24]…”