Number Of Light Neutrino Species Research Articles

Improved Bhabha cross section at LEP and the number of light neutrino species

In e+e− collisions, the integrated luminosity is generally measured from the rate of low-angle Bhabha interactions e+e−→e+e−. In the published LEP results, the inferred theoretical uncertainty of ±0.061% on the predicted rate is significantly larger than the reported experimental uncertainties. We present an updated and more accurate prediction of the Bhabha cross section in this letter, which is found to reduce the Bhabha cross section by about 0.048%, and its uncertainty to ±0.037%. When accounted for, these changes modify the number of light neutrino species (and its accuracy), as determined from the LEP measurement of the hadronic cross section at the Z peak, to Nν=2.9963±0.0074. The 20-years-old 2σ tension with the Standard Model is gone.

Beam-beam effects on the luminosity measurement at LEP and the number of light neutrino species

In e+e− collisions, electromagnetic effects caused by large charge density bunches modify the effective acceptance of the luminometer system of the experiments. These effects consequently bias the luminosity measurement from the rate of low-angle Bhabha interactions e+e−→e+e−. Surprisingly enough, the magnitude of this bias is found to yield an underestimation of the integrated luminosity measured by the LEP experiments by about 0.1%, significantly larger than the reported experimental uncertainties. When accounted for, this effect modifies the number of light neutrino species determined at LEP from the measurement of the hadronic cross section at the Z peak.

Probing nonstandard neutrino cosmology with terrestrial neutrino experiments

Neutrino masses and the number of light neutrino species can be tested in a variety of laboratory experiments and also can be constrained by particle astrophysics and precision cosmology. A conflict between these various results could be an indication of new physics in the neutrino sector. In this paper we explore the possibility for reconciliation of otherwise discrepant results in a simple model containing a light scalar field which produces Mass Varying Neutrinos (MaVaNs). We extend previous work on MaVaNs to consider issues of neutrino clumping, the effects of additional contributions to neutrino mass, and reconciliation of eV mass sterile neutrinos with cosmology.

Dark photons as fractional cosmic neutrino masquerader

Recently, Weinberg proposed a Higgs portal model with a spontaneously broken global U(1) symmetry in which Goldstone bosons may be masquerading as fractional cosmic neutrinos. We extend the model by gauging the U(1) symmetry. This gives rise to the so-called dark photon and dark Higgs. The dark photons can constitute about 0.912 (0.167) to the effective number of light neutrino species if they decouple from the thermal bath before the pions become non-relativistic and after (before) the QCD transition. Restriction on the parameter space of the portal coupling and the dark Higgs mass is obtained from the freeze-out condition of the dark photons. Combining with the collider data constraints on the invisible width of the standard model Higgs requires the dark Higgs mass to be less than a few GeV.

CONSTRAINTS ON LEFT–RIGHT SYMMETRIC AND E6 SUPERSTRING MODELS FROM THE NUMBER OF LIGHT NEUTRINO SPECIES

Using the experimental value for the rate [Formula: see text], we derive constraints on the number of light neutrino species with the invisible width method in the framework of a left–right symmetric model (LRSM) and a E6 superstring model as a function of their respective mixing angles for the neutral vector gauge bosons. Using the above LEP result we get the constraints -1.6×10-3≤ ϕ ≤ 1.1 × 10-3 and -1.3 × 10-2 ≤ ϕE6 ≤ 0.4 × 10-2, which are stronger than those obtained in previous studies of these models.

Single- and Multi-Photon Events with Missing Energy in e+e- Collisions at LEP

A brief review of searches for physics beyond the Standard Model using single- and multi-photon events with missing energy at LEP is given here. These include searches for supersymmetry, large extra dimensions, and anomalous neutrino production. Recent results from all four LEP experiments are presented, including improved limits on the new scale of gravity for models with large extra dimensions and the most precise direct measurement of the number of light neutrino species. Status and prospects of the LEP combined searches are also discussed.

Precision primordial4Hemeasurement from the CMB

Big bang nucleosynthesis (BBN) and the cosmic microwave background (CMB) are two major pillars of cosmology. Standard BBN accurately predicts the primordial light element abundances ${(}^{4}\mathrm{He},$ D, ${}^{3}\mathrm{He}$ and ${}^{7}\mathrm{Li}),$ depending on one parameter, the baryon density. Light element observations are used as a baryometer. The CMB anisotropies also contain information about the content of the Universe which allows an important consistency check on the big bang model. In addition CMB observations now have sufficient accuracy to not only determine the total baryon density, but also resolve its principal constituents H and ${}^{4}\mathrm{He}.$ We present a global analysis of all recent CMB data, with special emphasis on the concordance with BBN theory and light element observations. We find ${\ensuremath{\Omega}}_{B}{h}^{2}{=0.0250}_{\ensuremath{-}0.0026}^{+0.0019}$ and ${Y}_{p}{=0.250}_{\ensuremath{-}0.014}^{+0.010}$ (fraction of baryon mass as ${}^{4}\mathrm{He})$ using CMB data alone, in agreement with ${}^{4}\mathrm{He}$ abundance observations. The determination of ${Y}_{p}$ allows us to constrain the relativistic degrees of freedom during BBN, measured through the effective number of light neutrino species, ${N}_{\ensuremath{\nu},eff}{=3.02}_{\ensuremath{-}0.79}^{+0.85},$ in accord with the standard model of particle physics. With this concordance established we show that the inclusion of standard, ${N}_{\ensuremath{\nu},eff}\ensuremath{\equiv}3,$ BBN theory priors significantly reduces the volume of parameter space. In this case, we find ${\ensuremath{\Omega}}_{B}{h}^{2}{=0.0245}_{\ensuremath{-}0.0028}^{+0.0015}$ and ${Y}_{p}{=0.2493}_{\ensuremath{-}0.0010}^{+0.0007}.$ We also find that the inclusion of deuterium abundance observations reduces the ${Y}_{p}$ and ${\ensuremath{\Omega}}_{B}{h}^{2}$ ranges by a factor of $\ensuremath{\sim}2.$ Further light element observations and CMB anisotropy experiments will refine this concordance and sharpen BBN and the CMB as tools for precision cosmology.

Signatures of relativistic neutrinos in CMB anisotropy and matter clustering

We present a detailed analytical study of ultrarelativistic neutrinos in cosmological perturbation theory and of the observable signatures of inhomogeneities in the cosmic neutrino background. We note that modification of perturbation variables that removes all the time derivatives of scalar gravitational potentials from the dynamical equations simplifies their solution notably. The used perturbations of particle number per coordinate, not proper, volume are generally constant on superhorizon scales. In real space the analytical approach can be extended beyond fluid models to neutrinos. The faster cosmological expansion due to the neutrino background affects the acoustic and damping angular scales of the cosmic microwave background (CMB). But we find that equivalent changes can be produced by varying other standard parameters, including the primordial helium abundance. The low-l integrated Sachs-Wolfe effect is also not sensitive to neutrinos. However, the gravity of neutrino perturbations suppresses the CMB acoustic peaks for the multipoles with $l\ensuremath{\gtrsim}200$ while it enhances the amplitude of matter fluctuations on these scales. In addition, the perturbations of relativistic neutrinos generate a unique phase shift of the CMB acoustic oscillations that, for adiabatic modes, cannot be caused by any other standard physics. The origin of the shift is traced to neutrino free-streaming velocity exceeding the sound speed of the photon-baryon plasma. We find that from a high-resolution, low-noise instrument such as CMBPOL the effective number of light neutrino species can be determined with an accuracy of $\ensuremath{\sigma}{(N}_{\ensuremath{\nu}})\ensuremath{\simeq}0.05--0.09,$ depending on the constraints on the helium abundance.

Single- and multi-photon events with missing energy in e+e− collisions at LEP

Single- and multi-photon events with missing energy are selected in 619 pb−1 of data collected by the L3 detector at LEP at centre-of-mass energies between 189 and 209 GeV. The cross sections of the process e+e−→νν̄γ(γ) are found to be in agreement with the Standard Model expectations, and the number of light neutrino species is determined, including lower energy data, to be Nν=2.98±0.05±0.04. Selection results are given in the form of tables which can be used to test future models involving single- and multi-photon signatures at LEP. These final states are also predicted by models with large extra dimensions and by several supersymmetric models. No evidence for such models is found. Among others, lower limits between 1.5 and 0.65 TeV are set, at 95% confidence level, on the new scale of gravity for the number of extra dimensions between 2 and 6.

Searches for new physics in photonic final states at LEP

A brief review of searches for physics beyond the Standard Model in photonic final states at LEP is given here. These include searches for supersymmetry, large extra dimensions and contact interactions. Recent results from all four LEP experiments are presented, including improved limits on the new scale of gravity for models with large extra dimensions and the most precise direct measurement of the number of light neutrino species. Status and prospects of the LEP combined searches are also discussed.

Primordial nucleosynthesis with CMB inputs: probing the early universe and light element astrophysics

Cosmic microwave background (CMB) determinations of the baryon-to-photon ratio η∝Ω baryon h 2 will remove the last free parameter from (standard) big bang nucleosynthesis (BBN) calculations. This will make BBN a much sharper probe of early universe physics, for example, greatly refining the BBN measurement of the effective number of light neutrino species, N ν,eff . We show how the CMB can improve this limit, given current light element data. Moreover, it will become possible to constrain N ν,eff independent of 4 He, by using other elements, notably deuterium; this will allow for sharper limits and tests of systematics. For example, a 3% measurement of η, together with a 10% (3%) measurement of primordial D/H, can measure N ν,eff to a 95% confidence level of σ 95%( N ν,eff )=1.8 (1.0) if η∼6.0×10 −10. If instead, one adopts the standard model value N ν,eff =3, then one can use η (and its uncertainty) from the CMB to make accurate predictions for the primordial abundances. These determinations can in turn become key inputs in the nucleosynthesis history (chemical evolution) of galaxies thereby placing constraints on such models.

Precise determination of the Z resonance parameters at LEP: “Zedometry”

This final analysis of hadronic and leptonic cross-sections and of leptonic forward-backward asymmetries in ${\mathrme}^+{\mathrme}^-$ collisions with the OPAL detector makes use of the full LEP 1 data sample comprising $161 \mathrm{pb}^{-1}$ of integrated luminosity and $4.5\times10^6$ selected Z decays. An interpretation of the data in terms of contributions from pure Z exchange and from $\gamma/$ Z interference allows the parameters of the Z resonance to be determined in a model-independent way. Our results are in good agreement with lepton universality and consistent with the vector and axial-vector couplings predicted in the Standard Model. A fit to the complete dataset yields the fundamental Z resonance parameters: $m_{\mathrm{Z}} = (91.1852 \pm 0.0030)$ GeV, $\Gamma_{\mathrm{Z}} = (2.4948 \pm 0.0041)$ GeV, $\sigma^0_{\mathrm{h}} = (41.501 \pm 0.055)$ nb, $R_{\ell} = 20.823 \pm 0.044$ , and $A_{\mathrm{FB}}^{0,\ell} = 0.0145 \pm 0.0017$ . Transforming these parameters gives a measurement of the ratio between the decay width into invisible particles and the width to a single species of charged lepton, $\Gamma_{\mathrm{inv}}/\Gamma_{\ell\ell} = 5.942 \pm 0.027$ . Attributing the entire invisible width to neutrino decays and assuming the Standard Model couplings for neutrinos, this translates into a measurement of the effective number of light neutrino species, $N_{\nu} = 2.984 \pm 0.013$ . Interpreting the data within the context of the Standard Model allows the mass of the top quark, $m_{\mathrm{t}} = (162 ^{+29}_{-16})$ GeV, to be determined through its influence on radiative corrections. Alternatively, utilising the direct external measurement of $m_{\mathrm{t}}$ as an additional constraint leads to a measurement of the strong coupling constant and the mass of the Higgs boson: $\alpha_{\mathrm{s}}(m_{\mathrm{Z}}) = 0.127 \pm 0.005$ and $m_{\mathrm{H}} = (390^{+750}_{-280})$ GeV.

Photonic events with missing energy in $\mathrme^+\mathrme^-$ collisions at $\sqrt{s}$ = 189 GeV

Photonic events with large missing energy have been observed in $\rm e^+e^-$ collisions at a centre-of-mass energy of 189 GeV using the OPAL detector at LEP. Results are presented for event topologies consistent with a single photon or with an acoplanar photon pair. Cross-section measurements are performed within the kinematic acceptance of each selection, and the number of light neutrino species is measured. Cross-section results are compared with the expectations from the Standard Model process $\mathrme^+\mathrme^-\to \nu\overline{\nu}$ + photon(s). No evidence is observed for new physics contributions to these final states. Upper limits on $\sigma(\mathrme^+\mathrme^-\to\mathrm{X}\mathrm{Y})\cdot\mathrm{BR}(\mathrm{X}\to\mathrm{Y}\gamma)$ and $\sigma(\mathrme^+\mathrme^-\to\mathrm{XX})\cdot\mathrm{BR}^2(\mathrm{X}\to\mathrm{Y}\gamma)$ are derived for the case of stable and invisible $\mathrm{Y}$ . These limits apply to single and pair production of excited neutrinos ( $\mathrm{X} = \nu^*, \mathrm{Y} = \nu$ ), to neutralino production ( $\mathrm{X}={{{\tilde{\chi}}^{0}}_{2}}, \mathrm{Y}={{{\tilde{\chi}}^{0}}_{1}}$ ) and to supersymmetric models in which $\mathrm{X} ={{{\tilde{\chi}}^{0}}_{1}}$ and $\mathrm{Y}={\tilde{\mathrm{G}}}$ is a light gravitino. The case of macroscopic decay lengths of particle X is considered for $\mathrme^+\mathrme^- \to \mathrm{XX}$ , $\rm X \to Y \gamma$ , when $M_{\mathrm Y}\approx 0$ . The single-photon results are also used to place upper limits on superlight gravitino pair production as well as graviton-photon production in the context of theories with additional space dimensions.

CONSTRAINING MODELS WITH SUPERLIGHT GRAVITINO, SCALAR AND PSEUDOSCALAR PARTICLES

In some supergravity models there are light weakly coupled scalar (S) and pseudoscalar (P) particles. These particles arise following a superlight gravitino. In these models the decay S/P→γγ exists. We examine constraints on this process considering these photons as responsible by the extragalactic background light. We also consider the amount of S/P particles produced through the fusion of the cosmic background photons and contributing to the effective number of light neutrino species during primordial nucleosynthesis. We obtain bounds on the gravitino mass complementary to the existing ones.

Measurements of cross sections and forward-backward asymmetries at the Z resonance and determination of electroweak parameters

We report on measurements of hadronic and leptonic cross sections and leptonic forward-backward asymmetries performed with the L3 detector in the years 1993-95. A total luminosity of 103 pb^-1 was collected at centre-of-mass energies \sqrt{s} ~ m_Z and \sqrt{s} ~ m_Z +/- 1.8 GeV which corresponds to 2.5 million hadronic and 245 thousand leptonic events selected. These data lead to a significantly improved determination of Z parameters. From the total cross sections, combined with our measurements in 1990-92, we obtain the final results: m_Z = 91189.8 +/- 3.1 MeV, Gamma_Z = 2502.4 +/- 4.2 MeV, Gamma_had = 1741.1 +/- 3.8 MeV, Gamma_l = 84.14 +/- 0.17 MeV. An invisible width of Gamma_inv = 499.1 +/- 2.9 MeV is derived which in the Standard Model yields for the number of light neutrino species N_nu = 2.978 +/- 0.014. Adding our results on the leptonic forward-backward asymmetries and the tau polarisation, the effective vector and axial-vector coupling constants of the neutral weak current to charged leptons are determined to be \bar{g}_V^l = -0.0397 +/- 0.0017 and \bar{g}_A^l = -0.50153 +/- 0.00053.Including our measurements of the Z -> b \bar{b} forward-backward and quark charge asymmetries a value for the effective electroweak mixing angle of sin^2\bar{\theta}_W = 0.23093 +/- 0.00066 is derived. All these measurements are in good agreement with the Standard Model of electroweak interactions. Using all our measurements of electroweak observables an upper limit on the mass of the Standard Model Higgs boson of m_H < 133 GeV is set at 95% confidence level.

Helium Abundance in the Most Metal‐deficient Blue Compact Galaxies: I Zw 18 and SBS 0335−052

We present high-quality spectroscopic observations of the two most-metal deficient blue compact galaxies known, I Zw 18 and SBS 0335-052 to determine the helium abundance. The underlying stellar absorption strongly influences the observed intensities of He I emission lines in the brightest NW component of I Zw 18, and hence this component should not be used for primordial He abundance determination. The effect of underlying stellar absorption, though present, is much smaller in the SE component. Assuming all systematic uncertainties are negligible, the He mass fraction derived in this component is Y = 0.243+/-0.007. The high signal-to-noise ratio spectrum (> 100 in the continuum) of SBS 0335-052 allows us to measure the helium mass fraction with a precision better than 2% -- 5% in nine different regions along the slit. Assuming all systematic uncertainties are negligible, the weighted mean He mass fraction in SBS 0335-052 is Y = 0.2437+/-0.0014 when the three He I 4471, 5876 and 6678 emission lines are used, and is 0.2463+/-0.0015 when the He I 4471 emission line is excluded. The weighted mean helium mass fraction in the two most metal-deficient BCGs I Zw 18 and SBS 0335-052, Y=0.2462+/-0.0015, after correction for the stellar He production results in a primordial He mass fraction Yp = 0.2452+/-0.0015. The derived Yp leads to a baryon-to-photon ratio of (4.7+/-1.0) 10^{-10}, consistent with the values derived from the primordial D and 7Li abundances, and supporting the standard big bang nucleosynthesis theory. For the most consistent set of primordial D, 4He, and 7Li abundances we derive an equivalent number of light neutrino species 3.0+/-0.3 (95% C.L.).

Measurement of the Z resonance parameters at LEP

The properties of the Z resonance are measured from the analysis of 4.5 million Z decays into fermion pairs collected with the \Aleph detector at L EP. The data are consistent with lepton universality. The resonance parameters are measured to be $\MZ=(91.1885 \pm 0.0031)~\Gevcc$, $\GZ= (2.4951 \pm 0.0043)~\GeV$, $\spol=(41.559 \pm 0.058)$~nb and, combining the three lepton flavours $\Rl= 20.725\pm 0.039$. The corresponding number of light neutrino species is $N_{\nu}=2.983\pm0.013$ and the strong coupling constant is $\alpha_s(\MZ) = 0.114\pm {0.004} \pm 0.002_{\rm{QCD}} +\,0.005\,\log_{10}\left[\frac{\MH}{150\,{\rm GeV}/c^2 }\right]\,$. The lepton pair forward-backward asymmetry is measured to be $\Afbpol=0.0173 \pm 0.0016$ from which the effective weak mixing angle is derived: $\swsqeffl = 0.23089\pm 0.00089\,$. The measurement of the leptonic width $\Gll=84.02\pm0.15$~MeV leads to a determination of the effective $\rho$ parameter $\rho_{\rm eff}^{\rm lept} = 1.0064 \pm 0.0018$. The data support the Standard Model and favour a light Higgs.

Determination of the number of light neutrino species from single photon production at LEP

A determination of the number of light neutrino families performed by measuring the cross section of single photon production in e +e − collision near the Z resonance is reported. From an integrated luminosity of 100 pb −1 , collected during the years 1991–94, we have observed 2091 single photon candidates with an energy above 1 GeV in the polar angular region 45°< θ γ <135°. From a maximum likelihood fit to the single photon cross section, the Z decay width into invisible particles is measured to be Γ inv =498±12 (stat)±12 (sys) MeV . Using the Standard Model couplings of neutrinos to the Z, the number of light neutrino species is determined to be N ν =2.98±0.07(stat)±0.07(sys).

Primordial nucleosynthesis, majorons and heavy tau neutrinos

We determine the restrictions imposed by primordial nucleosynthesis upon a heavy tau neutrino, in the presence of v τ annihilations into majorons, as expected in a wide class of particle physics models of neutrino mass. We determine the equivalent number of light neutrino species N eq as a function of m v τ , and the v τ − v τ -majoron coupling g. We show that for theoretically plausible g values ≳ 10 -4 present nucleosynthesis observations can not rule out v τ masses in the MeV range. Moreover, these models give N eq ⩽ 3 in the v τ mass region 1–10 MeV, for very reasonable values of g ⩾ 3 × 10 -4. The evasion of the cosmological limits brings new interest to the improvement of the present laboratory limit on the v τ mass which can be achieved at a tau-charm factory.

Big Bang nucleosynthesis in crisis?

A new evaluation of the constraint on the number of light neutrino species (${N}_{\ensuremath{\nu}}$) from big bang nucleosynthesis suggests a discrepancy between the predicted light element abundances and those inferred from observations, unless the inferred primordial $^{4}\mathrm{He}$ abundance has been underestimated by 0.014\ifmmode\pm\else\textpm\fi{}0.004 ($1\ensuremath{\sigma}$) or less than 10% (95% C.L.) of $^{3}\mathrm{He}$ survives stellar processing. With the quoted systematic errors in the observed abundances and a conservative chemical evolution parametrization, the best fit to the combined data is ${N}_{\ensuremath{\nu}}=2.1\ifmmode\pm\else\textpm\fi{}0.3$ ($1\ensuremath{\sigma}$) and the upper limit is ${N}_{\ensuremath{\nu}}&lt;2.6$ (95% C.L.). The data are inconsistent with the standard model (${N}_{\ensuremath{\nu}}=3$) at the 98.6% C.L.