[Paper Review] Fitting CoGeNT Modulation with an Inelastic, Isospin-Violating $Z'$ Model

Haipeng An, F. Gao|arXiv (Cornell University)|Aug 19, 2011

Dark Matter and Cosmic Phenomena3 citations

TL;DR

This paper proposes an inelastic, isospin-violating $Z'$ model with a 13.3 GeV dark matter particle and a 33 keV mass gap to explain the annual modulation signal observed by the CoGeNT experiment. The model fits the data while evading constraints from XENON100 by leveraging isospin-violating couplings and inelastic scattering, with $M_{Z'} \lesssim 2m_\chi$ required by collider limits.

ABSTRACT

We reanalyze the annual modulation data observed by CoGeNT experiment and show that if the annually modulated anomaly detected by CoGeNT is induced by collision between dark matter particle and nucleus, it can be fitted by a $Z'$ model with inelastic dark matter and isospin-violating interaction, and the constraint from XENON100 can be avoided. This $Z'$ model is strongly constrained by collider physics that the upper bound of the mass of $Z'$ is around twice of the mass of dark matter.

Motivation & Objective

• To explain the annual modulation signal in CoGeNT's low-energy recoil data, which remains unexplained by standard elastic WIMP models.
• To resolve the tension between the unmodulated event rate and the observed modulation in CoGeNT.
• To reconcile the CoGeNT signal with the stringent XENON100 limits on light WIMP-nucleon scattering.
• To construct a viable $Z'$ model that avoids collider constraints while fitting the data.

Proposed method

• Uses a leptophobic $Z'$ portal with isospin-violating couplings to quarks in the SM to mediate inelastic dark matter scattering.
• Introduces a 33 keV mass gap between the dark matter ground state and its excited state, enabling inelastic scattering with keV-scale nuclear recoils.
• Applies a model-independent translation of differential scattering rates between detectors using low-energy nuclear recoil spectra.
• Imposes constraints from monojet + missing transverse energy at Tevatron and LHC to limit $Z'$ mass and couplings.
• Performs Fourier analysis on monthly binned CoGeNT data to extract modulation amplitude and significance.
• Uses the phase space suppression of three-body final states to reduce collider cross sections and satisfy constraints.

Experimental results

Research questions

• RQ1 Can an inelastic, isospin-violating $Z'$ model explain the annual modulation observed in CoGeNT's low-energy recoil data?
• RQ2 How can such a model evade the stringent XENON100 limits on light WIMP scattering?
• RQ3 What are the collider constraints on the $Z'$ boson in this model, and what mass range is allowed?
• RQ4 Why is the 'down-scattering' process less effective at generating observable annual modulation?
• RQ5 What is the required dark matter mass and mass gap to fit the CoGeNT modulation data?

Key findings

• A 13.3 GeV inelastic dark matter particle with a 33 keV mass gap fits the CoGeNT modulation data with a good statistical significance.
• The model evades XENON100 constraints due to isospin-violating couplings, which suppress the scattering cross section in xenon relative to germanium by a factor of ~20.
• The $Z'$ boson mass is constrained to be less than twice the dark matter mass, $M_{Z'} \lesssim 2m_\chi$, due to collider limits from monojet + MET searches.
• The 'down-scattering' process (excited state decaying to ground state) produces less velocity-dependent modulation than 'up-scattering', making it less effective for fitting the data.
• The statistical uncertainty of the modulation amplitude is estimated as $\delta\mathcal{M} = \sqrt{2/(N\bar{F})}$, with $N$ months of data and $\bar{F}$ average event rate per month.
• The model requires a small DM velocity dispersion ($v_0 \sim 200$ km/s) to ensure sufficient population of dark matter particles with enough kinetic energy for inelastic scattering.