[Paper Review] Isospin-Violating Dark Matter Benchmarks for Snowmass 2013

Jonathan L. Feng, Jason Kumar+2 more|arXiv (Cornell University)|Jul 6, 2013

Dark Matter and Cosmic Phenomena29 references3 citations

TL;DR

This paper introduces isospin-violating dark matter (IVDM) as a framework that generalizes direct detection searches by introducing the neutron-to-proton coupling ratio $f_n/f_p$ as a free parameter, enabling reinterpretation of conflicting low-mass dark matter signals. It demonstrates that IVDM can reconcile apparent tensions between experiments like CoGeNT, CDMS-Si, and XENON100 by altering nuclear response functions, especially when $f_n/f_p \approx -0.70$, and emphasizes the need for multi-target experiments to resolve the $f_n/f_p$ ambiguity.

ABSTRACT

Isospin-violating dark matter (IVDM) generalizes the standard spin-independent scattering parameter space by introducing one additional parameter, the neutron-to-proton coupling ratio f_n/f_p. In IVDM the implications of direct detection experiments can be altered significantly. We review the motivations for considering IVDM and present benchmark models that illustrate some of the qualitatively different possibilities. IVDM strongly motivates the use of a variety of target nuclei in direct detection experiments.

Motivation & Objective

• To address the theoretical and phenomenological limitations of assuming isospin invariance in direct dark matter detection experiments.
• To motivate the use of isospin-violating dark matter (IVDM) as a more general framework that allows independent proton and neutron couplings.
• To demonstrate how IVDM can resolve apparent inconsistencies among low-mass dark matter signals from experiments like CoGeNT, CDMS-Si, and DAMA.
• To show that the neutron-to-proton coupling ratio $f_n/f_p$ can be constrained using cross-section measurements from at least three different target nuclei.
• To explore complementary probes via collider and indirect detection experiments for IVDM models with enhanced couplings or light mediators.

Proposed method

• Introduces the isospin-violating dark matter (IVDM) framework by generalizing the standard spin-independent scattering parameter space with the addition of $f_n/f_p$, the ratio of dark matter couplings to neutrons and protons.
• Derives the degradation factor $D_p^Z = \sigma_N^Z / \sigma_p$ in Eq. (1), which relates the normalized-to-nucleon cross section $\sigma_N^Z$ to the proton cross section $\sigma_p$ and depends quadratically on $f_n/f_p$.
• Uses the ratio of normalized cross sections from two experiments with different target elements to determine $f_n/f_p$ up to a two-fold ambiguity.
• Employs a third target material to break the degeneracy and fully constrain $f_n/f_p$.
• Applies the formalism to benchmark models such as $Z$-mediated ($f_n/f_p = -13.3$) and argophobic ($f_n/f_p = -0.82$) interactions.
• Evaluates the impact of IVDM on experimental sensitivity by comparing exclusion contours and signal regions in the $(m_X, \sigma_p)$ plane for different $f_n/f_p$ values.

Experimental results

Research questions

• RQ1 How does relaxing isospin invariance affect the interpretation of direct detection data from low-mass dark matter experiments?
• RQ2 Can isospin-violating dark matter explain apparent inconsistencies between signal regions from CoGeNT, CDMS-Si, and DAMA while remaining consistent with null results from XENON100?
• RQ3 What is the minimal number of direct detection experiments with different target nuclei required to uniquely determine $f_n/f_p$?
• RQ4 How do collider and indirect detection probes complement direct detection in constraining IVDM models?
• RQ5 What are the implications of destructive interference in IVDM for dark matter annihilation and production cross sections?

Key findings

• The $f_n/f_p = -0.70$ benchmark allows the CDMS-Si signal region to remain consistent with XENON100 null results, while the CoGeNT and CDMS-Ge ROIs become marginally consistent.
• For $f_n/f_p = -0.70$, the DAMA and CRESST signal regions remain in tension with XENON100 bounds, indicating that not all low-mass signals can be reconciled without further assumptions.
• The degradation factor $D_p^Z$ depends quadratically on $f_n/f_p$, enabling determination of the coupling ratio from two experiments with different targets, though a third is needed to resolve the two-fold ambiguity.
• IVDM enhances dark matter annihilation and collider production cross sections when destructive interference requires larger individual couplings to quarks, increasing sensitivity to indirect and collider probes.
• Neutrino searches from dark matter capture in the Sun provide a complementary probe, especially for targets with low neutron content, offering a distinct signature from xenon or germanium detectors.
• The $Z$-mediated benchmark ($f_n/f_p = -13.3$) and argophobic model ($f_n/f_p = -0.82$) are shown to be physically motivated in various UV-complete scenarios, supporting the broader relevance of IVDM.