Abstract
The DAMA collaboration reported an annually modulated rate with a phase compatible with a Dark Matter induced signal. We point out that a slowly varying rate can bias or even simulate an annual modulation if data are analyzed in terms of residuals computed by subtracting approximately yearly averages starting from a fixed date, rather than a background continuous in time. In the most extreme case, the amplitude and phase of the annual modulation reported by DAMA could be alternatively interpreted as a decennial growth of the rate. This possibility appears mildly disfavoured by a detailed study of the available data, but cannot be safely excluded. In general, a decreasing or increasing rate could partially reduce or enhance a true annual modulation, respectively. The issue could be clarified by looking at the full time-dependence of the DAMA total rate, not explicitly published so far.
Highlights
JHEP04(2020)137 backgrounds with a period of roughly one year
Typical contaminants that could be present at the beginning of data-taking are the cosmogenic 3H with T1/2 = 12.3 yr, and 210Pb with T1/2 = 22.3 yr, which have about 40% and 16% of the energy spectrum falling in the Region of Interest (RoI), respectively
The DAMA collaboration reported an annual modulation in the single-hit scintillation events that peaks around June, 2nd, as predicted by a DM signal
Summary
We discuss, from a general point of view, how an apparently periodic signal can be mimicked by a time-dependent total rate without any modulation. R0(t) ≡ C, one can consider the average of the rate over any desired time interval ∆, and subtract this quantity from R(t). That becomes a sawtooth wave with period ∆ and amplitude B∆/2 if the procedure is repeated over several data-taking intervals of equal length ∆. — the length ∆ of the time interval over which the background subtraction is periodically applied — and a nonzero modulation with period ∆ has been generated from a non-periodic rate. In the presence of a modulated signal as in eq (2.1), its amplitude A and phase φ will be modified by the fitting procedure. In the simple case where the duration of the cycles is taken equal to the period of the signal one has
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