[Paper Review] Neutrino magnetic and electric dipole moments: From measurements to parameter space

D. Aristizábal Sierra, O. G. Miranda+2 more|arXiv (Cornell University)|Dec 23, 2021

Neutrino Physics Research51 references31 citations

TL;DR

This paper develops a parameter space vector formalism to map experimental limits on neutrino magnetic and electric dipole moments into fundamental Hamiltonian parameters, revealing that CP phases in the Majorana case can create 'blind spots'—regions where signals vanish despite large couplings. In contrast, Dirac case limits tightly constrain couplings, and the framework enables reconciliation of conflicting bounds from LSND, GEMMA, and XENON1T experiments.

ABSTRACT

Searches for neutrino magnetic moments/transitions in low energy neutrino scattering experiments are sensitive to effective couplings which are an intricate function of the Hamiltonian parameters. We study the parameter space dependence of these couplings in the Majorana (transitions) and Dirac (moments) cases, as well as the impact of the current most stringent experimental upper limits on the fundamental parameters. In the Majorana case we find that for reactor, short-baseline and solar neutrinos, CP violation can be understood as a measurement of parameter space vectors misalignments. The presence of nonvanishing CP phases opens a blind spot region where -- regardless of how large the parameters are -- no signal can be observed in either reactor or short-baseline experiments. Identification of these regions requires a combination of different data sets and allows for the determination of those CP phases. We point out that stringent bounds not necessarily imply suppressed Hamiltonian couplings, thus allowing for regions where disparate upper limits can be simultaneously satisfied. In contrast, in the Dirac case stringent experimental upper limits necessarily translate into tight bounds on the fundamental couplings. In terms of parameter space vectors, we provide a straightforward mapping of experimental information into parameter space.

Motivation & Objective

• To map experimental upper limits on neutrino magnetic/electric dipole moments to fundamental Hamiltonian parameters.
• To identify how CP phases in the Majorana case can create blind spots where signals vanish despite large couplings.
• To reconcile conflicting experimental bounds from LSND, GEMMA, and XENON1T using a unified parameter space framework.
• To provide a basis-independent, vector-based formalism for effective couplings in reactor, short-baseline, and solar neutrino experiments.
• To distinguish between the implications of stringent bounds in the Majorana vs. Dirac neutrino cases.

Proposed method

• Introduces a parameter space vector notation to represent effective couplings in mass and flavor eigenstate bases.
• Derives general matricial expressions for effective couplings in short-baseline, reactor, and solar neutrino experiments.
• Uses unitarity of the lepton mixing matrix to analyze CP phase-dependent alignments and blind spots in the Majorana case.
• Applies phase-averaged calculations for solar neutrinos to derive observable couplings.
• Maps experimental data from LSND, GEMMA, and XENON1T into parameter space to test consistency.
• Distinguishes between Dirac and Majorana cases in terms of how experimental bounds constrain fundamental couplings.

Experimental results

Research questions

• RQ1 Can CP phases in the Majorana case lead to regions where no signal is observed despite large couplings, known as blind spots?
• RQ2 How do stringent experimental bounds on effective couplings constrain the underlying Hamiltonian parameters in the Dirac and Majorana cases?
• RQ3 Can the conflicting bounds from LSND, GEMMA, and XENON1T be simultaneously satisfied within a single parameter space framework?
• RQ4 What is the role of CP phases in determining the magnitude and sign of effective neutrino magnetic moments?
• RQ5 How can experimental data be systematically mapped into parameter space using a vector formalism?

Key findings

• CP violation in the Majorana case can be interpreted as misalignment of parameter space vectors, leading to blind spots where no signal is detectable.
• Blind spots arise when CP phases cause destructive interference, rendering effective couplings zero even for large fundamental parameters.
• In the Majorana case, stringent experimental bounds do not necessarily imply suppressed Hamiltonian couplings, allowing for parameter regions where multiple bounds are simultaneously satisfied.
• In the Dirac case, experimental upper limits directly translate into tight constraints on fundamental couplings due to the absence of destructive interference.
• The parameter space vector formalism enables a direct, basis-independent mapping of experimental data into Hamiltonian parameter space.
• The framework demonstrates the viability of reconciling LSND’s bound with the more stringent GEMMA and XENON1T limits through CP phase-dependent alignments.