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The Search for Heavy Majorana Neutrinos

Anupama Atre, Tao Han, Silvia Pascoli, Bin Zhang

TL;DR

The paper investigates how heavy Majorana neutrinos can induce lepton-number-violating processes in both low-energy decays and high-energy colliders, focusing on a single accessible heavy state $N_4$ with mass $m_4$ and mixing $V_{\ell 4}$. It derives decay widths and LV transition rates in two mass regimes ($m_4 \ll m_W$ and $m_4 \gg m_W$), and uses these to constrain $|V_{\ell 4}V_{\ell' 4}|$ from tau decays and rare meson decays, as well as to project collider sensitivities for like-sign dilepton signatures at Tevatron and LHC. The work shows that resonant enhancement can overcome suppression factors, yielding strong current bounds (e.g., from $K^+ \rightarrow \ell^+ \ell^+ \pi^-$) that can reach or exceed $|V_{\ell 4}V_{\ell' 4}| \sim 10^{-9}$ in some mass windows, and that collider searches can probe $m_4$ up to several hundred GeV with mixing down to $10^{-6}$–$10^{-7}$ depending on channel and luminosity. It highlights the complementary roles of $0\nu\beta\beta$ bounds, EW precision tests, and LV searches in mapping the heavy-neutrino parameter space, and emphasizes that an observed LV signal would signal a Majorana neutrino and physics beyond the SM.

Abstract

The Majorana nature of neutrinos can be experimentally verified only via {\it lepton-number} violating processes involving charged leptons. We study 36 lepton-number violating ($\lv$) processes from the decays of tau leptons and pseudoscalar mesons. These decays are absent in the Standard Model but, in presence of Majorana neutrinos in the mass range $\sim 100 \mev$ to $5 \gev$, the rates for these processes would be enhanced due to their resonant contribution. We calculate the transition rates and branching fractions and compare them to the current bounds from direct experimental searches for $\dl=2$ tau and rare meson decays. The experimental non-observation of such $\lv$ processes places stringent bounds on the Majorana neutrino mass and mixing and we summarize the existing limits. We also extend the search to hadron collider experiments. We find that, at the Tevatron with $8 {fb}^{-1}$ integrated luminosity, there could be $2σ$ ($5σ$) sensitivity for resonant production of a Majorana neutrino in the $μ^\pm μ^\pm$ modes in the mass range of $\sim 10 - 180 {\gev} (10 - 120 {\gev})$. This reach can be extended to $\sim 10 - 375 {\gev} (10 - 250 mbox{\gev})$ at the LHC of 14 TeV with $100 {fb}^{-1}$. The production cross section at the LHC of 10 TeV is also presented for comparison. We study the $μ^\pm e^\pm$ modes as well and find that the signal could be large enough even taking into account the current bound from neutrinoless double-beta decay. The signal from the gauge boson fusion channel $W^+ W^+\to \ell^+_1 \ell^+_2$ at the LHC is found to be very weak given the rather small mixing parameters. We comment on the search strategy when a $τ$ lepton is involved in the final state.

The Search for Heavy Majorana Neutrinos

TL;DR

The paper investigates how heavy Majorana neutrinos can induce lepton-number-violating processes in both low-energy decays and high-energy colliders, focusing on a single accessible heavy state with mass and mixing . It derives decay widths and LV transition rates in two mass regimes ( and ), and uses these to constrain from tau decays and rare meson decays, as well as to project collider sensitivities for like-sign dilepton signatures at Tevatron and LHC. The work shows that resonant enhancement can overcome suppression factors, yielding strong current bounds (e.g., from ) that can reach or exceed in some mass windows, and that collider searches can probe up to several hundred GeV with mixing down to – depending on channel and luminosity. It highlights the complementary roles of bounds, EW precision tests, and LV searches in mapping the heavy-neutrino parameter space, and emphasizes that an observed LV signal would signal a Majorana neutrino and physics beyond the SM.

Abstract

The Majorana nature of neutrinos can be experimentally verified only via {\it lepton-number} violating processes involving charged leptons. We study 36 lepton-number violating () processes from the decays of tau leptons and pseudoscalar mesons. These decays are absent in the Standard Model but, in presence of Majorana neutrinos in the mass range to , the rates for these processes would be enhanced due to their resonant contribution. We calculate the transition rates and branching fractions and compare them to the current bounds from direct experimental searches for tau and rare meson decays. The experimental non-observation of such processes places stringent bounds on the Majorana neutrino mass and mixing and we summarize the existing limits. We also extend the search to hadron collider experiments. We find that, at the Tevatron with integrated luminosity, there could be () sensitivity for resonant production of a Majorana neutrino in the modes in the mass range of . This reach can be extended to at the LHC of 14 TeV with . The production cross section at the LHC of 10 TeV is also presented for comparison. We study the modes as well and find that the signal could be large enough even taking into account the current bound from neutrinoless double-beta decay. The signal from the gauge boson fusion channel at the LHC is found to be very weak given the rather small mixing parameters. We comment on the search strategy when a lepton is involved in the final state.

Paper Structure

This paper contains 25 sections, 132 equations, 27 figures, 7 tables.

Table of Contents

  1. Introduction
  2. Majorana neutrinos in extension of the standard model
  3. Formalism for Heavy Neutrino Mixing
  4. Current Constraints on $\hbox{$N_4$}$ Masses And Mixing
  5. Mixing with $\nu_e$
  6. Mixing with $\nu_\mu$
  7. Mixing with $\nu_\tau$
  8. Electroweak Precision Tests
  9. Neutrinoless Double Beta Decay ($0 \nu \beta \beta$)
  10. Lepton-Number Violating Decays
  11. Decay Modes of Heavy Majorana Neutrino
  12. Decay Modes of Heavy Majorana Neutrino with mass $m_4 \ll m_W$
  13. Decay Modes of Heavy Majorana Neutrino with mass $m_4 > m_W$
  14. Lepton-Number Violating Tau Decays
  15. Lepton-Number Violating Rare Meson Decays
  16. ...and 10 more sections

Figures (27)

  • Figure 1: A generic diagram for $\Delta L=2$ processes via Majorana neutrino exchange.
  • Figure 2: Bounds on $|V_{e4}|^2$ versus $m_4$ in the mass range 10 eV--10 MeV. The excluded regions with contours labeled $^{187}$Re Galeazzi:2001py, $^3$H Hiddemann:1995ce , $^{63}$Ni Holzschuh:1999vy , $^{35}$S Holzschuh:2000nj , $^{20}$F and Fermi$_2$Deutsch:1990ut refer to the bounds from kink searches. All the limits are given at 95% C.L. except for the ones from Ref. Deutsch:1990ut which are at 90% C.L.. The areas delimited by short dashed (blue) contour labeled Borexino and solid (cyan) contour labeled Bugey are excluded at 90% C.L. by searches of $\hbox{$N_4$}$ decays from the Borexino Counting Test facility Back:2003ae and Ref. Hagner:1995bn respectively. The region with long-dash-dotted (grey) contour, labelled $\pi \rightarrow e \nu$, is excluded by peak searches Britton:1992pg. The dotted (maroon) line labeled $0\nu\beta\beta$ indicates the bound from searches of neutrinoless double beta-decay Benes:2005hn.
  • Figure 3: Bounds on $|V_{e4}|^2$ versus $m_4$ in the mass range 10 MeV--100 GeV. The areas with solid (black) contour labeled $\pi \rightarrow e \nu$ and double dash dotted (purple) contour labeled $K \rightarrow e \nu$ are excluded by peak searches Britton:1992pgknupeak. Limits at 90% C.L. from beam-dump experiments are taken from Ref. Bernardi:1987ek (PS191), Ref. Badier:1986xz (NA3) and Ref. Bergsma:1985is (CHARM). The limits from contours labeled DELPHI and L3 are at 95% C.L. and are taken from Refs. Abreu:1996pa and Adriani:1992pq respectively. The excluded region with dotted (maroon) contour is derived from a reanalysis of neutrinoless double beta decay experimental data Benes:2005hn.
  • Figure 4: Limits on $|V_{\mu 4}|^2$ versus $m_4$ in the mass range 100 MeV--100 GeV come from peak searches and from $\hbox{$N_4$}$ decays. The area with solid (black) contour labeled $K \rightarrow \mu \nu$Kusenko:2004qc is excluded by peak searches. The bounds indicated by contours labeled by PS191 Bernardi:1987ek, NA3 Badier:1986xz, BEBC BEBC, FMMF Gallas:1994xp, NuTeV Vaitaitis:1999wq and CHARMII Vilain:1994vg are at 90% C.L., while DELPHI Abreu:1996pa and L3 Adriani:1992pq are at 95% C.L. and are deduced from searches of visible products in $\hbox{$N_4$}$ decays. For the beam dump experiments, NA3, PS191, BEBC, FMMF and NuTeV we give an estimate of the upper limit for the excluded values of the mixing angle.
  • Figure 5: Bounds on $|V_{\tau 4}|^2$ versus $m_4$ from searches of decays of heavy neutrinos, given in Ref. Orloff:2002de (CHARM) and in Ref. Astier:2001ck (NOMAD) at 90% C.L., and in Ref. Abreu:1996pa (DELPHI) at 95% C.L.
  • ...and 22 more figures