[Paper Review] Viable models for large non-standard neutrino interactions

Yasaman Farzan|arXiv (Cornell University)|Dec 15, 2016

Neutrino Physics Research28 references3 citations

TL;DR

This paper proposes two viable models based on a light $U(1)'$ gauge symmetry with a $Z'$ boson at $O(10\,$MeV) that generate large neutral current non-standard neutrino interactions (NSI) with couplings of order $5\times10^{-5}$--$10^{-4}$. These NSI can produce discernible effects in long-baseline neutrino oscillation experiments like DUNE and NO\nu A, while remaining consistent with existing experimental bounds through careful flavor and chirality structure in couplings to quarks and neutrinos.

ABSTRACT

Thanks to high precision long baseline neutrino experiments such as NO$ν$A and DUNE, possible effects of Non-Standard neutrino Interactions (NSI) on neutrino oscillation data have received renewed interest in the last two years. It is however challenging to build models that can give rise to NSI with sizeable couplings discernible at neutrino oscillation experiments without violating the various existing experimental bounds. We introduce two viable models that can lead to neutral current NSI with sizable couplings. Both models are based on a new $U(1)^\prime$ gauge symmetry with $Z^\prime$ gauge boson of mass $O(10~{ m MeV})$. We will highlight the common phenomenological features of these models and suggest ways to test them.

Motivation & Objective

• To construct viable models that generate large non-standard neutrino interactions (NSI) with couplings sizable enough to be detectable in long-baseline oscillation experiments like DUNE and NO\nu A.
• To ensure these models remain consistent with existing experimental constraints from neutrino scattering, solar neutrino experiments, and cosmology.
• To identify observable signatures that can test the models in astrophysical and terrestrial experiments.
• To explore the phenomenological implications of flavor-diagonal and flavor-off-diagonal NSI structures arising from a common $U(1)'$ gauge symmetry.

Proposed method

• Introduce a new $U(1)'$ gauge symmetry with a $Z'$ gauge boson of mass $O(10\,$MeV), coupling to neutrinos and first-generation quarks.
• Construct models where the effective four-fermion NSI Lagrangian arises via $Z'$ exchange, parameterized as $\mathcal{L}_{\rm NSI} = -2\sqrt{2}G_F \epsilon_{\alpha\beta}^{fX} (\bar{\nu}_\alpha \gamma^\mu P_L \nu_\beta)(\bar{f}\gamma_\mu P_X f)$.
• Impose non-chiral couplings to quarks ($\epsilon^{fL}_{\alpha\beta} = \epsilon^{fR}_{\alpha\beta}$) and set electron couplings to zero to preserve solar neutrino flux constraints.
• Ensure the $Z'$ decays dominantly to neutrinos for $m_{Z'} < m_\pi$, enabling resonant interactions with relic neutrinos.
• Use the $Z'$ exchange to generate large NSI couplings $\epsilon_{\alpha\beta} \sim O(1)$ in matter, sufficient to affect oscillation patterns.
• Derive constraints from existing experiments (e.g., CCFR, CHARM, NuTeV) and cosmological bounds (e.g., big bang nucleosynthesis).

Experimental results

Research questions

• RQ1 Can viable models be constructed that yield large neutral current NSI couplings ($\epsilon \sim O(1)$) without violating existing experimental bounds?
• RQ2 How can a light $Z'$ boson ($m_{Z'} \sim 10\,$MeV) mediate sizable NSI while remaining consistent with neutrino scattering and solar neutrino data?
• RQ3 What are the key astrophysical and cosmological signatures of such a $Z'$ that could test the model?
• RQ4 Can the model explain the LMA-Dark solution to the solar neutrino problem via large off-diagonal NSI?
• RQ5 What are the observable effects of the $Z'$ on supernova neutrino emission and high-energy cosmic neutrino spectra?

Key findings

• The models generate NSI couplings of order $5\times10^{-5}$--$10^{-4}$, large enough to produce measurable effects in long-baseline neutrino oscillation experiments like DUNE and NO\nu A.
• The $Z'$ boson with mass $O(10\,$MeV) can be thermally produced in supernova cores and decay back to neutrinos with a decay length of $\sim10^{-9}$ km, prolonging the duration of neutrino emission.
• Resonant production of the $Z'$ by high-energy cosmic neutrinos interacting with relic neutrinos can produce a dip in the energy spectrum at $E_\nu \sim 500\,$TeV--1 PeV for $m_{Z'} \sim 10\,$MeV.
• The $Z'$ coupling to neutrinos and quarks can be probed by direct dark matter detection experiments: current bounds from CDMLite ($\sqrt{g_B g_\nu} \lesssim 5\times10^{-5}$) already constrain part of the parameter space, and future experiments like LUX-ZEPLIN and SuperCDMS will fully probe it.
• The models predict a measurable dip in the cosmic neutrino spectrum due to $Z'$ resonance, with optical depth $\gtrsim 1$ for the relevant coupling range, making it potentially detectable in IceCube data.
• The models are consistent with big bang nucleosynthesis, requiring $m_{Z'} > 5\,$MeV, and avoid constraints from solar neutrino experiments by setting $\epsilon^{eL,R}_{\alpha\beta} = 0$ and equal left/right couplings for $u$ and $d$ quarks.