New Physics Effects in Rare Z Decays

Abstract

Virtual effects induced by new physics in rare Z decays are reviewed. Since the expected sensitivity of the giga-Z linear collider is of the order of 10 −8 , we emphasize the importance of any new physics effect that gives a prediction above this limit. It is also pointed out that an improvement on the known experimental constraints on rare Z decays will provide us with a critical test of the validity of the standard model at the loop level.

Indirect searches for sterile neutrinos at a high-luminosity Z-factory

“…(Although there is still a tension between the most recent Planck results on extra light neutrinos (relics) and reactor anomalies, in this work we focus on the rôle of (heavier) sterile fermions, which are not expected to contribute as light relativistic degrees of freedom [24].) Rare charged lepton flavour violating (cLFV) Z decays have been extensively discussed in the context of SM extensions involving massive (Majorana and/or Dirac) neutrinos [3,5,[25][26][27][28]; similar studies were carried using an effective theory approach [29][30][31][32], some also exploring a possible complementarity with low-energy cLFV searches.…”

“…While the recent MEG bound on µ → eγ decays excludes important regions of the parameter space, 5 the contribution of the Z penguin diagrams to cLFV 3-body decays and µ − e conversion in nuclei severely constrains the flavour violating Z ∓ 1 ± 2 vertex (see also [29][30][31]). This is especially manifest in the case of Z → eµ decays, since the severe constraints from BR(µ → 3e) and CR(µ − e, Au) typically preclude BR(Z → eµ) 10 −13 ; however, and for a regime of very heavy sterile states (m 4 10 4 GeV), the "3 + 1 model" can nevertheless account for BR(Z → eµ) within FCC-ee reach.…”

Search for Higgs andZBoson Decays toJ/ψγandϒ(nAad1, Abbott2, Abdallah3 et al. 2015Phys. Rev. Lett.845980View full textAdd to dashboardCite

“…Rare decay modes of the Z boson have attracted attention focused on establishing their sensitivity to new physics [11]. Several estimates of the SM branching fraction for the decay Z → J=ψγ are available [12][13][14] with the most recent being ð9.96 AE 1.86Þ × 10 −8 [14].…”

Indirect searches for sterile neutrinos at a high-luminosity Z-factory

“…Despite the potential of this simple "toy-model" to account for significant LFV Z decay branching fractions (which could be as large as O(10 −6 )), these cannot be reconciled with current bounds on low-energy cLFV processes (see Table 1), to which the sterile states also contribute. While the recent MEG bound on µ → eγ decays excludes important regions of the parameter space 5 [18][19][20]). This is especially manifest in the case of Z → eµ decays, since the severe constraints from BR(µ → 3e) and CR(µ−e, Au) typically preclude BR(Z → eµ) 10 −13 ; however, and for a regime of very heavy sterile states (m 4 10 4 GeV), the "3+1 model" can nevertheless account for BR(Z → eµ) within FCC-ee reach.…”

“…(Although there is still a tension between the most recent Planck results on extra light neutrinos (relics) and reactor anomalies, in this work we focus on the rôle of (heavier) sterile fermions, which are not expected to contribute as light relativistic degrees of freedom [14].) Rare charged lepton flavour violating (cLFV) Z decays have been extensively discussed in the context of SM extensions involving massive (Majorana and/or Dirac) neutrinos [5,[15][16][17]; similar studies were carried using an effective theory approach [18][19][20][21], some also exploring a possible complementarity with low-energy cLFV searches.…”