Number of light neutrino types from colliders 1

Number of Light Neutrino Typesfrom Collider Experiments
Revised June 2020 by C.-J. Lin (LBNL). Written by D. Karlen (University of Victoriaand TRIUMF).
The most precise measurements of the number of light neutrino types, Nν , come fromstudies of Z production in e+ e− collisions. The invisible partial width, Γinv , is determinedby subtracting the measured visible partial widths, corresponding to Z decays into quarks
and charged leptons, from the total Z width. The invisible width is assumed to be due toNν light neutrino species each contributing the neutrino partial width Γν as given by theStandard Model. In order to reduce the model dependence, the Standard Model value forthe ratio of the neutrino to charged leptonic partial widths, (Γν /Γℓ )SM = 1.991 ± 0.001,
is used instead of (Γν )SM to determine the number of light neutrino types:
ΓNν = inv
Γℓ



Γℓ
Γν



.

(1)

SM

The combined result from the four LEP experiments is Nν = 2.984 ± 0.008 [1]. Recentanalyses applied corrections to the LEP result [1] by including the effect of correlated
luminosity systematics and also using an improved Bhabha cross section calculation [2,3]to obtain Nν = 2.9963 ± 0.0074.
In the past, when only small samples of Z decays had been recorded by the LEP
experiments and by the Mark II at SLC, the uncertainty in Nν was reduced by using
Standard Model fits to the measured hadronic cross sections at several center-of-massenergies near the Z resonance. Since this method is much more dependent on theStandard Model, the approach described above is favored.
Before SLC and LEP, limits on the number of neutrino generations were placed byexperiments at lower-energy e+ e− colliders by measuring the cross section of the processe+ e− → ννγ. The ASP, CELLO, MAC, MARK J, and VENUS experiments observed atotal of 3.9 events above background [4], leading to a 95% CL limit of Nν < 4.8. This
process has a much larger cross section at center-of-mass energies near the Z mass andhas been measured at LEP by the ALEPH, DELPHI, L3, and OPAL experiments [5].
These experiments have observed several thousand such events, and the combined resultis Nν = 3.00 ± 0.08. The same process has also been measured by the LEP experiments
at much higher center-of-mass energies, between 130 and 208 GeV, in searches for newphysics [6]. Combined with the lower energy data, the result is Nν = 2.92 ± 0.05.
Experiments at pp colliders also placed limits on Nν by determining the total Z
width from the observed ratio of W ± → ℓ± ν to Z → ℓ+ ℓ− events [7]. This involved acalculation that assumed Standard Model values for the total W width and the ratio ofW and Z leptonic partial widths, and used an estimate of the ratio of Z to W production
cross sections. Now that the Z width is very precisely known from the LEP experiments,
the approach is now one of those used to determine the W width.

S. Navas et al. (Particle Data Group), Phys. Rev. D 110, 030001 (2024)
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Number of light neutrino types from colliders

References:
1. ALEPH, DELPHI, L3, OPAL, and SLD Collaborations, and LEP Electroweak
Working Group, and SLD Electroweak Group, and SLD Heavy Flavour Group, Phys.
Reports 427, 257 (2006).
2. P. Janot and S. Jadach, Phys. Lett. B803, 135319 (2020).
3. G. Voutsinas et al., Phys. Lett. B800, 135068 (2020).
4. VENUS: K. Abe et al., Phys. Lett. B232, 431 (1989);
ASP: C. Hearty et al., Phys. Rev. D39, 3207 (1989);
CELLO: H.J. Behrend et al., Phys. Lett. B215, 186 (1988);
MAC: W.T. Ford et al., Phys. Rev. D33, 3472 (1986);
MARK J: H. Wu, Ph.D. Thesis, Univ. Hamburg (1986).
5. L3: M. Acciarri et al., Phys. Lett. B431, 199 (1998);
DELPHI: P. Abreu et al., Z. Phys. C74, 577 (1997);
OPAL: R. Akers et al., Z. Phys. C65, 47 (1995);
ALEPH: D. Buskulic et al., Phys. Lett. B313, 520 (1993).
6. DELPHI: J. Abdallah et al., Eur. Phys. J. C38, 395 (2005);
L3: P. Achard et al., Phys. Lett. B587, 16 (2004);
ALEPH: A. Heister et al., Eur. Phys. J. C28, 1 (2003);
OPAL: G. Abbiendi et al., Eur. Phys. J. C18, 253 (2000).
7. UA1: C. Albajar et al., Phys. Lett. B198, 271 (1987);
UA2: R. Ansari et al., Phys. Lett. B186, 440 (1987).

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