[Paper Review] Probing for chiral $Z^\prime$ gauge boson through scattering measurement experiments

Kento Asai, Arindam Das+3 more|arXiv (Cornell University)|Jul 19, 2023

Particle physics theoretical and experimental studiesPhysics and Astronomy3 citations

TL;DR

This paper investigates chiral $Z^\prime$ gauge bosons in a $U(1)_X$-extended Standard Model, where left- and right-handed fermions couple differently to $Z^\prime$, leading to distinctive $Z^\prime$-mediated scattering processes. It derives stringent constraints on $g_X$ and $M_{Z^\prime}$ using recast data from FASER$\nu$(2), SND@$\text{LHC}$, COHERENT, NA64, MUonE, BaBaR, LHCb, CMS, and LEP, showing scattering and beam-dump experiments probe $Z^\prime$ masses from 0.02 to 150 GeV.

ABSTRACT

Motivated by the observation of tiny neutrino mass can not be explained within the framework of Standard Model (SM), we consider extra gauge extended scenarios in which tiny neutrino masses are generated through seesaw mechanism. These scenarios are equipped with beyond the standard model (BSM) neutral gauge boson called $Z^\prime$ in the general $U(1)_X$ symmetry which is a linear combination of $U(1)_Y$ and $U(1)_{B-L}$. In this case, left and right handed fermions interact differently with the $Z^\prime$. The $Z^\prime$ gives rise to different processes involving neutrino-nucleon, neutrino-electron, electron-nucleus and electron-muon scattering processes. By comparing with proton, electron beam-dump experiments data, recast data from searches for the long-lived and dark photon at BaBaR, LHCb and CMS experiments, the electron and muon $g-2$ data, and the data of the dilepton and dijet searches at the LEP experiment, we derive bounds on the gauge coupling and the corresponding gauge boson mass for different $U(1)_X$ charges and evaluate the prospective limits from the future beam-dump scenarios at DUNE, FASER(2) and ILC. We conclude that large parameter regions could be probed by scattering, beam-dump and collider experiments in future.

Motivation & Objective

• To explore the phenomenology of chiral $Z^\prime$ gauge bosons in a $U(1)_X$-extended SM where left- and right-handed fermions have different $U(1)_X$ charges.
• To derive constraints on the $U(1)_X$ gauge coupling $g_X$ and $Z^\prime$ mass $M_{Z^\prime}$ using $Z^\prime$-mediated scattering processes.
• To compare existing and prospective experimental bounds from scattering, beam-dump, and collider experiments to assess sensitivity to $Z^\prime$ in the 0.02–150 GeV range.
• To evaluate the complementarity of different experiments—such as FASER$\nu$(2), NA64, LHCb, and future ILC-BD—across varying $Z^\prime$ mass regimes.
• To assess the viability of $Z^\prime$ detection via $g-2$ measurements, showing they are less sensitive due to universal fermionic couplings.

Proposed method

• Recasting existing experimental data from FASER$\nu$(2), SND@$\text{LHC}$, COHERENT, NA64, MUonE, and others to derive limits on $g_X$ and $M_{Z^\prime}$ for $Z^\prime$-mediated neutrino-nucleon, electron-nucleus, and electron-muon scattering.
• Using $Z^\prime$-mediated $t$-channel processes in $U(1)_X$ models with chiral couplings, where fermion charges under $U(1)_X$ break left-right symmetry.
• Applying anomaly cancellation conditions to ensure gauge and mixed gauge-gravity anomaly freedom, requiring three generations of right-handed neutrinos with non-universal $U(1)_X$ charges.
• Recasting dark photon search data from BaBaR, LHCb, and CMS to constrain $g_X$ and $M_{Z^\prime}$, especially in the 1–150 GeV range.
• Analyzing $g-2$ data from electron and muon experiments to constrain $Z^\prime$ contributions, assuming universal couplings across fermion generations.
• Comparing results with dilepton and dijet searches from LEP to assess complementarity and robustness of bounds across different mass and coupling regimes.

Figure 1: Number of muon neutrino pass through the FASER $\nu$ (left panel) and SND@LHC (right panel) detectors. Thirty energy bins are defined uniformly on the logarithmic scale in [10, $10^{4}$ ] GeV.

Experimental results

Research questions

• RQ1 How do chiral $U(1)_X$ charges affect $Z^\prime$ couplings to left- and right-handed fermions in $\mathcal{G}_{\rm SM} \otimes U(1)_X$ models?
• RQ2 What are the strongest experimental constraints on $g_X$ and $M_{Z^\prime}$ for $Z^\prime$-mediated scattering processes in the 0.02–150 GeV range?
• RQ3 How do beam-dump experiments like NA64, E141, and DUNE compare in sensitivity to $Z^\prime$ compared to scattering experiments like FASER$\nu$(2) and JSNS2?
• RQ4 What role do $g-2$ measurements play in constraining $Z^\prime$ couplings when fermion couplings are generation-independent?
• RQ5 Which future experiments—FASER$\nu$(2), ILC-BD, or DUNE—offer the most promising sensitivity to light $Z^\prime$ bosons?

Key findings

• Scattering experiments such as FASER$\nu$(2), SND@$\text{LHC}$, and JSNS2 can probe $Z^\prime$ masses in the 0.02–0.2 GeV range, with sensitivities reaching $g_X \sim 10^{-6}$ for $M_{Z^\prime} \leq 0.04$ GeV.
• Dark photon searches at LHCb provide stringent limits for $M_{Z^\prime} \geq 0.21$ GeV, with $g_X \in [3.5 \times 10^{-6}, 10^{-4}]$ in the 0.21–70 GeV range, and tighter bounds near $M_{Z^\prime} \approx 10$ GeV.
• Limits from BaBaR and CMS dark photon searches are strongest around $M_{Z^\prime} \approx 1$ GeV and $3$ GeV, with $g_X \lesssim 10^{-5}$ in narrow windows.
• The $g-2$ data from muon and electron experiments yield weaker constraints than scattering and beam-dump experiments due to universal fermionic couplings in the model.
• Future beam-dump experiments at FASER$\nu$(2), ILC-BD, and DUNE are expected to probe $g_X \sim 10^{-6}$ for $M_{Z^\prime} \leq 0.04$ GeV, with JSNS2 bounds crossing future sensitivities depending on $x_H$.
• Existing beam-dump experiments (e.g., $\nu$-cal, E137, NA64) rule out $g_X \in [10^{-6}, 0.01]$ for $M_{Z^\prime} \leq 0.08$ GeV, with DUNE providing weaker bounds in this mass range.

Figure 2: The neutrino-proton scattering cross section $\sigma_{\nu p}$ divided by the incoming neutrino energy $E_{\nu}$ . The gauge coupling $g_{X}$ is set to the unity.