WIMP Annual Modulation with Opposite Phase in Late-Infall Halo Models

TL;DR

The paper analyzes how a non-virialized late-infall halo, as proposed by Sikivie, alters WIMP direct-detection signatures. By contrasting the standard Gaussian, truncated halo with Sikivie's stream-dominated velocity distribution, it derives the angular flux, Earth-frame speed modulation, and recoil spectra, revealing a phase reversal of the annual modulation in the late-infall model. It also predicts a characteristic pattern of WIMP arrival directions and a step-like, non-sinusoidal recoil spectrum modulated by the annual cycle, with end-points that depend on time. The work implies that experimental searches should not assume a fixed modulation phase and that directional detection could provide crucial insights into the halo's structure.

Abstract

We show that in the late-infall model of our galactic halo by P. Sikivie the expected phase of the annual modulation of a WIMP halo signal in direct detection experiments is opposite to the one usually expected. If a non-virialized halo component due to the infall of (collisionless) dark matter particles cannot be rejected, an annual modulation in a dark matter signal should be looked for by experimenters without fixing the phase a-priori. Moreover, WIMP streams coming to Earth from directions above and below the galactic plane should be expected, with a characteristic pattern of arrival directions.

Figures (5)

• Figure 1: Sky map in galactic coordinates of the WIMP flux as seen by an observer moving with the Sun for the standard halo model with $\overline{v} = 220$ km/s and $v_{\rm esc}=650$ km/s. Lighter colors indicate larger flux intensities. The WIMP wind comes mostly from the direction of the Sun motion (white cross).
• Figure 2: Sky map in galactic coordinates of the WIMP flux as seen by an observer moving with the Sun for Sikivie's late-infall halo model, smoothing each stream with a gaussian with velocity dispersion $\overline{v}_i = 30$ km/s. Lighter colors indicate larger flux intensities. WIMPs come from the direction of the streams (the bright spots). The most intense streams lie around the direction opposite to the Sun motion (white circle). This reverses the phase of the annual modulation of a halo WIMP signal due to the motion of the Earth around the Sun.
• Figure 3: Annual modulation in the mean WIMP speed on Earth as a function of time. The time axis starts January 1 and covers a year. Solid curves in SLI halo model (we also include possible velocity dispersions); dashed curve in the standard halo model. The phase of the modulation is opposite in the two models.
• Figure 4: Recoil-energy spectrum at the maximum and minimum of the annual modulation in the mean WIMP speed on Earth. Step-like curves in the Sikivie's late-infall halo model; smooth curves in the standard halo model. The phase of the modulation in the two models is opposite at low energies, the same at intermediate energies, opposite again at high energies. Notice that the end-point energy of each step in the SLI model is annually modulated.
• Figure 5: Annual modulation of the recoil-energy spectrum at several fixed energies. Solid curves in the Sikivie's late-infall halo model; dashed curves in the standard halo model. Beyond the end-point energy of the lowest step, the modulation of the recoil-energy spectrum in the SLI halo model may be poorly approximated by a sinusoidal (cfr. the 25keV curve, e.g.).