Dark Matter implications of DAMA/LIBRA-phase2 results (1804.01231v3)

Abstract: Recently, the DAMA/LIBRA collaboration released updated results from their search for the annual modulation signal from Dark Matter (DM) scattering in the detector. Besides approximately doubling the exposure of the DAMA/LIBRA data set, the updated photomultiplier tubes of the experiment allow a lower recoil energy threshold of 1\,keV electron equivalent compared to the previous threshold of 2 keV electron equivalent. We study the compatibility of the observed modulation signal with DM scattering. Due to a conspiracy of multiple effects, the new data at low recoil energies is very powerful for testing the DM hypothesis. We find that canonical (isospin conserving) spin-independent DM-nucleon interactions are no longer a good fit to the observed modulation signal in the standard halo model. The canonical spin-independent case is disfavored by the new data, with best fit points of a DM mass of $\sim 8\,$GeV, disfavored by $5.2\,\sigma$, or a mass of $\sim 54\,$GeV, disfavored by $2.5\,\sigma$. Allowing for isospin violating spin independent interactions, we find a region with a good fit to the data with suppressed effective couplings to iodine for DM masses of $\sim 10\,$GeV. We also consider spin-dependent DM-nucleon interactions, which yield good fits for similar DM masses of $\sim 10\,$GeV or $\sim 45\,$GeV

Dark Matter Implications of DAMA/LIBRA-Phase2 Results

The extensive search for interactions between dark matter (DM) and ordinary matter, extending beyond merely gravitational effects, remains a defining challenge in modern physics. The DAMA/LIBRA collaboration stands apart from the general consensus of null results by reporting a statistically significant detection of an annual modulation signal, which they claim is consistent with dark matter interactions. The recent results from their DAMA/LIBRA-phase2 upgrades have heightened the ability to probe dark matter hypotheses by reducing the detection threshold to 1 keV electron equivalent, thus expanding sensitivity at lower recoil energies.

The paper approaches these new results by critically evaluating the compatibility of the observed modulation signal with three types of dark matter interactions: canonical spin-independent (SI) interactions, isospin-violating (IV) spin-independent interactions, and spin-dependent (SD) interactions, all framed within the standard halo model. The nature of these interactions describes how DM may scatter off nucleons in the detector. Notably, the work reveals that the canonical isospin-conserving SI framework no longer provides a satisfactory fit to the data, disfavored at the 5.2σ level for a low mass (∼8 GeV) hypothesis, and at the 2.5σ level for a high mass (∼54 GeV) hypothesis.

A key insight from the paper is the prominence of isospin-violating interactions, where the effective coupling to iodine is suppressed. This coupling configuration allows an acceptable fit to DAMA's data for DM masses around 10 GeV. Here, the best fit points require significant isospin violation, providing a coupling ratio $f_n/f_p \approx -0.666$, closely approximating the scenario where iodine scattering effectively vanishes ($f_n/f_p \approx -0.716$). This interpretation highlights nuanced interaction methodologies, meriting further experimental validation particularly as the DAMA/LIBRA phosphorylation exceeds previous exposure limits.

In exploring spin-dependent interactions, the paper delineates different scenarios: proton-only, neutron-only, and mixed SD interactions. Crucially, each type yields viable fits to the new modulation data with low and high mass regions identified at approximately 10 GeV and 45 GeV respectively. This comprehensive approach allows the paper to articulate a multifaceted parameter space where competing models of DM behavior might converge with experimental results.

For the SD neutron-only scenario, the most notable result was the affinity for high-mass DM candidates at approximately 50 GeV, in contrast to the proton-only candidate preference towards lower-mass spectrums (approximately 10 GeV). In examining the mixed SD interactions, negative values of the ratio $a_n/a_p$ yielded an effective DM-proton coupling fit, reaffirming that acceptable fits can emerge under varied experimental conditions and theoretical suppositions.

The handling of uncertainties within experimental parameters such as quenching factors indicates robustness in the theoretical predictions despite variations. It is noteworthy that energy-dependent quenching factors do significantly alter the predicted best-fit points, notably exacerbating the difficulty for canonical SI interactions. This emphasizes the necessity of precise calibration in experimental parameters.

Broader implications of these findings persist. If additional direct detection experiments confirm DAMA's observed modulation signals, particularly through independent means with NaI crystal targets, our understanding of dark matter interactions will necessitate a profound revision, potentially favoring IV or SD interaction scenarios. Additionally, these results provide insights into the nuanced behavior of dark matter-nucleon couplings—a critical characteristic that might distinguish among competing particle physics models of DM.

In conclusion, the DAMA/LIBRA-phase2 results present a fertile testing ground for evaluating the extent to which dark matter might engage with ordinary matter through mechanisms not aligned with canonical DM models. The rigorous analysis demonstrates that alternatives such as isospin-violating spin-independent interactions and spin-dependent interactions remain plausible contenders for elucidating this long-standing cosmic mystery. Future experimental work, marrying increased exposure with model variety, will remain pivotal in reconciling these signatures with the broader tapestry of dark matter physics.