InvisibleZ-boson decays ate+e−colliders

Abstract

�-coupling, 0.45 < gL < 0.5, while the right-handed one is only mildly bounded, |gR| � 0.2. We show that measurements at a future e + e linear collider at different center-of-mass energies, p s = mZ and p s � 170 GeV, would translate into a markedly more precise measurement of the Z�¯ �-couplings. A statistically significant deviation from Standard Model predictions will point toward different new physics mechanisms, depending on whether the discrepancy appears in the direct or the indirect measurement of the invisible Zwidth. We discuss some scenarios which illustrate the ability of different invisible Z-boson decay measurements to constrain new physics beyond the Standard Model. The four LEP collider experiments have performed several precise measurements of the properties of the Z-boson [1, 2, 3, 4], the heavy, neutral partner of the W-boson and the photon. These measurements are part of the evidence that the Standard Model of the Electroweak Interactions (SM) works extremely well, up to energies of several hundred GeV. One of these measurements is associated with the “invisible Z-boson width” (invisible Z-width). Assuming that the SM is correct, this measurement can be translated into a count of the number of neutrino species. The current value of the invisible Z-width agrees quite well with the SM expectation that there are three very light (mν ≪ 1 GeV) neutrino species. This is often interpreted as evidence that the SM contains three and only three families of fermionic fields, meaning that there is no fourth sequential generation. It is remarkable that this result is in agreement with cosmological constraints on the number of relativistic species around the time of Big Bang nucleosynthesis, which seem to indicate the existence of three very light neutrino species [5]. It is interesting to note that the LEP result is precise enough to probe whether the “number of neutrinos,” Nν, deviates slightly from three. Indeed, it is often quoted that the most precise LEP numbers can be translated into Nν = 2.9841 ±0.0083 [2], about two sigma away from the SM expectation, Nν = 3. While not statistically significant, this result has invited theoretical speculations, some of which involve suppressing the Zν¯-couplings with respect to the SM value.