The cosmic ray shadow of the Moon observed with the ANTARES neutrino telescope

Abstract

One of the main objectives of the ANTARES telescope is the search for point-like neutrino sources. Both the pointing accuracy and the angular resolution of the detector are important in this context and a reliable way to evaluate this performance is needed. In order to measure the pointing accuracy of the detector, one possibility is to study the shadow of the Moon, i.e. the deficit of the atmospheric muon flux from the direction of the Moon induced by the absorption of cosmic rays. Analysing the data taken between 2007 and 2016, the Moon shadow is observed with $3.5\sigma$ statistical significance. The detector angular resolution for downward-going muons is $0.{73}^{\circ }\pm 0.{14}^{\circ }.$ The resulting pointing performance is consistent with the expectations. An independent check of the telescope pointing accuracy is realised with the data collected by a shower array detector onboard of a ship temporarily moving around the ANTARES location.

Fig. 1

The visible and invisible sectors of the position of the Moon for ANTARES with respect to the detector horizontal coordinate system. The occurrences of the Moon position are computed at each hour in the period 2007–2016 with the library SkyField [22]. The map is arranged according to the Mollweide equal-area view obtained with the use of the HEALPIX package [23], setting the parameter $NSIDE=64$ (i.e. 49152 pixels)

Fig. 2

The test statistics $\lambda$ distribution for the “Moon shadow” hypothesis $H_1$ (dotted curve) and the “no Moon shadow” hypothesis $H_0$ (smooth curve). The dashed area corresponds to the 50% of the pseudo-experiments where the Moon shadow hypothesis is correctly identified. The shaded area quantifies the expected median significance (here $3.4\sigma$) to observe the Moon shadow

Fig. 3

Measured muon event density as a function of the angular distance $\delta$ from the Moon. Data histogram is shown with statistical errors; the smooth line is the best fit according to Eq. (4); the shaded area corresponds to the apparent radius of the Moon ($0.{26}^{\circ }$)

Fig. 4

Projection of the measured 2-D event distributions, in absence of the Moon shadow ($H_0$), for the field of view coordinates $x=({\alpha }_{\mu }-{\alpha }_{\mathit{Moon}})\times cos{h}_{\mu }$ and $y=h_{\mu }-h_{\mathit{Moon}}$

Fig. 5

Measured distribution of the test statistic $\lambda$ from Eq. (3) in the field of view around the Moon nominal position $O\equiv (0^{\circ },0^{\circ })$, indicated by a white cross. The white dot refers to the coordinates $(0.5^{\circ },0.1^{\circ })$ where the test statistics reaches the minimum (${\lambda }_{\mathit{min}}=-17.05$)

Fig. 6

Contour plots corresponding to different confidence levels (cyan/dashed: 68.27%; green/dot-dashed: 95.45%; red/dotted: 99.73%), computed with the two methods described in the text. In the zoom, the dot represents the position in the FoV where ${\lambda }_{\mathit{min}}=-17.05$. The cross indicates the nominal position of the Moon

Fig. 7

Distribution of the $\Theta ={\lambda }_O-{\lambda }_{\mathit{min}}$ test statistics, obtained with ${10}^5$ pseudo-experiments assuming the Moon in the nominal position O. The 23% of the pseudo-experiments has a test statistic $\Theta$ larger than the measured value ${\Theta }_{\mathit{meas}}=3.68$, indicated for reference by the red-dashed line

Fig. 8

Ship route in the 2011 (red) and 2012 (blue) campaigns around the center of the ANTARES detector

Fig. 9

Left: the difference between the zenith angles of the shower axis (determined as the direction of the ship with respect to the ANTARES location) and of the reconstructed muon underwater. Right: the same for the azimuth angles. Fit results: $\Delta {\theta }_{\mathit{mean}}=-0.{07}^{\circ }\pm 0.{22}^{\circ }$; $\Delta {\varphi }_{\mathit{mean}}=-0.5^{\circ }\pm 0.8^{\circ }$