Search for Relativistic Magnetic Monopoles with Eight Years of IceCube Data

We present an all-sky 90% confidence level upper limit on the cosmic flux of relativistic magnetic monopoles using 2886 days of IceCube data. The analysis was optimized for monopole speeds between $0.750c$ and $0.995c$, without any explicit restriction on the monopole mass. We constrain the flux of relativistic cosmic magnetic monopoles to a level below $2.0\times {10}^{-19}{\mathrm{cm}}^{-2}{\mathrm{s}}^{-1}{\mathrm{sr}}^{-1}$ over the majority of the targeted speed range. This result constitutes the most strict upper limit to date for magnetic monopoles with $\beta \gtrsim 0.8$ and up to $\beta \sim 0.995$ and fills the gap between existing limits on the cosmic flux of nonrelativistic and ultrarelativistic magnetic monopoles.

Figure 1

(Left) Expected event rate of monopoles as a function of number of photoelectrons ($n_{\mathrm{PE}}$) and zenith angle of the track reconstruction (${\theta }_{\mathrm{zen}}$). The color scale is normalized to the expected monopole event rate assuming a flux equal to the previous IceCube limit [14]. (Right) Same for astrophysical neutrinos, with the color scale representing the event rate normalized to the measured flux. $\cos ({\theta }_{\mathrm{zen}})=-1$ corresponds to vertically up-going tracks, while $\cos ({\theta }_{\mathrm{zen}})=1$ to vertically down-going tracks. Events with less than ${10}^{4.6}$ $n_{\mathrm{PE}}$ have been rejected by an early cut in step I, while the black line shows the effect of the final, angular-dependent, step I selection (see the text for details). Events below the black line are discarded by this selection.

Figure 2

(Left) BDT score for the signal (monopoles, normalized to the previous IceCube limit [14]) and the different background components (astrophysical electron, muon, and tau neutrinos, as well as their sum, normalized according to the measured flux [25]). (Right) The model rejection potential as a function of BDT score, showing a clear minimum at 0.047 that defines the cut value that provides the best sensitivity of the analysis.

Figure 3

Zenith averaged effective area for monopoles as a function of true particle $\beta$ for the two levels of the analysis. The shaded area in the step II curve represents the systematic uncertainty.

Figure 4

90% confidence level upper limit on the cosmic flux of relativistic monopoles as a function of true particle $\beta$ obtained in the present analysis assuming zero background (dark green curve). Also included are previous results of IceCube [14, 15], ANTARES [16], and Baikal [17]. The limits are valid for monopoles with the given $\beta$ at the detector. The Parker bound [37, 38] is shown as reference.