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Testing the unitarity of the light neutrino mixing matrix

E. Gabrielli, A. Lind, L. Marzola, K. Müürsepp, E. Nardi

Abstract

We propose a novel test of the unitarity of the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing matrix at collider experiments. Our approach exploits the incomplete cancellation between $t$-channel neutrino exchange and $s$-channel gauge-boson contributions that arises in the presence of violation of the flavor-diagonal PMNS unitarity conditions, leading to an anomalous growth of the cross section with energy. Such effects are generic in extensions of the Standard Model in which light neutrinos mix with heavier states, and can manifest at colliders as long as the characteristic energy of the process remains below the mass threshold of the new degrees of freedom. After briefly reviewing these scenarios, we employ our strategy to derive model-independent bounds on flavor diagonal unitarity-violating effects using LEP-II data. We then present sensitivity projections for future lepton and hadron colliders, demonstrating that they are well suited to probe the unitarity of the neutrino mixing matrix with this method.

Testing the unitarity of the light neutrino mixing matrix

Abstract

We propose a novel test of the unitarity of the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) mixing matrix at collider experiments. Our approach exploits the incomplete cancellation between -channel neutrino exchange and -channel gauge-boson contributions that arises in the presence of violation of the flavor-diagonal PMNS unitarity conditions, leading to an anomalous growth of the cross section with energy. Such effects are generic in extensions of the Standard Model in which light neutrinos mix with heavier states, and can manifest at colliders as long as the characteristic energy of the process remains below the mass threshold of the new degrees of freedom. After briefly reviewing these scenarios, we employ our strategy to derive model-independent bounds on flavor diagonal unitarity-violating effects using LEP-II data. We then present sensitivity projections for future lepton and hadron colliders, demonstrating that they are well suited to probe the unitarity of the neutrino mixing matrix with this method.
Paper Structure (6 sections, 48 equations, 4 figures, 2 tables)

This paper contains 6 sections, 48 equations, 4 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Neutrino mass models and PMNS non-unitarity
  3. Unitarity violation at lepton colliders
  4. Lepton collider bounds on $\delta_{\alpha}$
  5. Unitarity tests at hadron colliders
  6. Conclusions

Figures (4)

  • Figure 1: Tree level Feynman diagrams for the processes $\ell_\alpha^+\ell_\alpha^-\to W^+W^-$, $\alpha=e$, $\mu$, $\tau$. As explained in the text, the Higgs boson contribution is neglected. The arrows on the fermion lines indicate the momentum flow.
  • Figure 2: The ratio of the total cross section including the NP contributions (Eq. (\ref{['eq:diff_xs']})) for $\delta_\alpha = 0.01$ (blue solid line) and $\delta_\alpha = 0.1$ (green solid line), relative to the SM prediction ($\delta_\alpha = 0$) as a function of the center of mass energy.
  • Figure 3: The angular distribution of the differential cross section at two fixed energies: $\sqrt{s} = 350 \text{ GeV}$ (blue solid line: including the NP contribution with $\delta_\alpha = 0.1$, blue dashed line: SM result) and $\sqrt{s} = 3 \text{ TeV}$, corresponding to the center of mass energies of FCC-ee and CLIC (green solid line: including the NP contribution with $\delta_\alpha = 0.1$, green dashed line: SM result).
  • Figure 4: Tree level Feynman diagrams for the processes $p p\to jj\, \ell_\alpha^+ \ell_\alpha^-$, $(\alpha=e,\,\mu,\,\tau)$. The arrows on the fermion lines indicate the momentum flow.