The Measurement of the Number of Light Neutrino Species at LEP

Abstract

Within weeks of the start of the data taking at the LEP accelerator, the ALEPH, DELPHI, L3 and OPAL experiments were able to confirm the existence of just three light neutrino species. This measurement relies on the Standard Model relation between the 'invisible' width of the Z-boson and the cross-sections for Z-boson production and subsequent decay into hadrons.The full data sample collected by the experiments at and around the Z-boson resonance allows a high-precision measurement of the number of light neutrino species as 2.9840 ± 0.0082. The uncertainty is mostly due to the understanding of the low-angle Bhabha scattering process used to determine the experimental luminosity.This result is independently confirmed by the elegant direct observation of the e − e + → ννγ process, through the detection of an initial-state-radiation photon in otherwise empty detectors.This result confirms expectations from the existence of three charged leptons species, and contributes to the fields of astrophysics and cosmology. Alongside other LEP achievements, the precision of this result is a testament to the global cooperation underpinning CERN's fourth decade. LEP saw the onset of large-scale collaboration across experiments totaling over 2000 scientists, together with a strong partnership within the wider high-energy physics community: from accelerator operations to the understanding of theoretical processes.

Cosmological Neutrinos

“…On the other hand, neutrino oscillation experiments have placed a lower limit of 0.06 eV on the sum of the neutrino masses. 8,9 This mass range shows that Ω ν h 2 1, as follows from equation (13). If we take < m ν >∼ 1 eV and f ν = 3 in equation ( 13), we obtain a value for Ω ν of ∼ 0.03.…”

“…11,12 A more precise determination of the number of neutrino flavors comes from a determination of the mass width of the Z boson using the LEP accelerator at CERN. 13 The following discussion therefore implicitly assumes f = 3 in equation ( 4):…”

Neutrino non-standard interactions: A status report

“…Here we have chosen m H + = 700 GeV, so its contribution to NSI is sub-dominant. Also shown here are the additional LEP constraints from monophoton searches [157] and from neutrino scattering experiment BOREX-INO [158], as well as the global fit constraints on ττ [28] from neutrino oscillation plus COHERENT data and the future DUNE sensitivity in the 300 and 850 kt.MW.yr configurations. The most stringent limit comes from IceCube atmospheric neutrino data which requires | ττ − µµ | < 9.3% [159,160], but is applicable to ττ , since both ττ and µµ cannot be large simultaneously in the Zee model due to LFV constraints on µτ [151].…”

Report on scipost_201909_00005v1

“…Here we have chosen m H + = 700 GeV, so its contribution to NSI is sub-dominant. Also shown here are the additional LEP constraints from monophoton searches [153] and from neutrino scattering experiment BOREXINO [154], as well as the global fit constraints on ε τ τ [24] from neutrino oscillation plus COHERENT data. We find that ε τ τ as large as 43% is still allowed, with the upper limit coming from BOREXINO.…”