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Right handed neutrino production from $Z^\prime$ interactions in forward search experiments

ShivaSankar K. A, Souvik Das, Arindam Das, Sanjoy Mandal

Abstract

We study two general $U(1)$ extensions of the Standard Model (SM) those generate tiny neutrino masses via the seesaw mechanism after general $U(1)$ breaking. These models predict a new neutral gauge boson ($Z'$) and right-handed neutrinos (RHNs), the latter introduced for anomaly cancellation and neutrino mass generation. In both scenarios, left- and right-handed fermions couple differently to the $Z'$, and RHNs mix with light neutrinos, enabling variety of decay modes. Focusing on the high-luminosity LHC (HL-LHC) and the future FASER2 experiment, we explore RHN pair production from $Z'$ decays in two cases: (i) long-lived $Z'$ decays to visible modes and long-lived RHNs, and (ii) short-lived $Z'$ decays to long-lived RHNs, which further decay visibly inside FASER2. We estimate projected limits on the general $U(1)$ gauge coupling, $Z'$ mass, RHN mass, and light-heavy neutrino mixing for various $U(1)$ charge assignments, and compare them with current experimental bounds.

Right handed neutrino production from $Z^\prime$ interactions in forward search experiments

Abstract

We study two general extensions of the Standard Model (SM) those generate tiny neutrino masses via the seesaw mechanism after general breaking. These models predict a new neutral gauge boson () and right-handed neutrinos (RHNs), the latter introduced for anomaly cancellation and neutrino mass generation. In both scenarios, left- and right-handed fermions couple differently to the , and RHNs mix with light neutrinos, enabling variety of decay modes. Focusing on the high-luminosity LHC (HL-LHC) and the future FASER2 experiment, we explore RHN pair production from decays in two cases: (i) long-lived decays to visible modes and long-lived RHNs, and (ii) short-lived decays to long-lived RHNs, which further decay visibly inside FASER2. We estimate projected limits on the general gauge coupling, mass, RHN mass, and light-heavy neutrino mixing for various charge assignments, and compare them with current experimental bounds.

Paper Structure

This paper contains 12 sections, 47 equations, 11 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Model
  3. Case-I
  4. Case-II
  5. Neutrino mass
  6. $Z^\prime$ interactions with the fermions
  7. RHN interactions and partial decay modes
  8. Long-lived particle search experiments
  9. Results and discussion
  10. Long-lived $Z'$+long-lived RHNs
  11. Short-lived $Z'$+long-lived RHNs
  12. Conclusions

Figures (11)

  • Figure 1: Branching ratios of $Z^\prime$ into different modes considering $x_\Phi=1$ for U$(1)_X$ case. In the top row branching ratios are shown as a function of $x_H$ for $M_{Z^\prime}=0.1$ (1) GeV in the left (right) panel considering $M_{N_{1,2}} = M_{Z^\prime}/3$ and $M_{N_{3}} > M_{Z^\prime}/2$. In the middle and bottom rows, branching ratios are shown as a function of $M_{Z^\prime}$ for fixed $x_H$.
  • Figure 2: Branching ratios of $Z^\prime$ into different modes for the 'alternative' case. In the top row branching ratios are shown as a function of $x_H$ for $M_{Z^\prime}=0.1$ (1) GeV in the left (right) panel considering $M_{N_{1,2}} = M_{Z^\prime}/3$ and $M_{N_{3}} > M_{Z^\prime}/2$. In the middle and bottom rows, branching ratios are shown as a function of $M_{Z^\prime}$ for fixed $x_H$.
  • Figure 3: Decay length of $Z'$ as a function of gauge coupling $(g_X)$, $M_{Z'}$ and $x_H$ for $U(1)_X$ scenario considering the decay of $Z^\prime$ at rest frame. The black contour stands for the decay length $L_{Z'}=620$ m relevant for FASER2 detector.
  • Figure 4: Decay length of $Z'$ as a function of gauge coupling $(g_X)$, $M_{Z'}$ and $x_H$ for the 'alternative' scenario considering the decay of $Z^\prime$ at rest frame. The black contour stands for the decay length $L_{Z'}=620$ m relevant for FASER2 detector.
  • Figure 5: Branching ratio of RHNs in different modes (top panel) and its decay length (bottom panel) considering decays of RHN at the rest frame. The dashed black line in each of the upper panel stands for the branching ratio of the RHN to the visible mode. The black contour in the bottom panel stands for the decay length $L_{N}=620$ m relevant for FASER2 detector.
  • ...and 6 more figures