Extremely high energy neutrinos from cosmic strings

Superstring theory and other supersymmetric theories predict the existence of relatively light, weakly interacting scalar particles, called moduli, with a universal form of coupling to matter. Such particles can be emitted from cusps of cosmic strings, where extremely large Lorentz factors are achieved momentarily. Highly boosted modulus bursts emanating from cusps subsequently decay into gluons; they generate parton cascades which in turn produce large numbers of pions and then neutrinos. Because of very large Lorentz factors, extremely high energy neutrinos, up to the Planck scale and above, are produced. For some model parameters, the predicted flux of neutrinos with energies $\gtrsim {10}^{21}\mathrm{eV}$ is observable by JEM-EUSO and by the future large radio detectors LOFAR and SKA.

Figure 1

Calculated neutrino fluxes compared with existing upper limits, the Antarctic Impulse Transient Antenna–II (ANITA-II), Radio Ice Cerenkov Experiment (RICE), ANITA 08, and with sensitivities of projects, Japanese Experiment Module–Extreme Universe Space Observatory (JEM-EUSO), Low Frequency Array (LOFAR), and Square Kilometer Array (SKA). Line “$E^{-2}$ cascade” presents the upper limit for cosmogenic neutrinos [40]. Curve “theor. 3” gives the predicted neutrino flux for the gravity-dominated regime with a normalizing set of parameters $p=1$ (ordinary strings), and ${\alpha }_7=m_5=z_{*}/z_{\nu }=1$. This flux is detectable only by SKA. Curve “theor. 2” corresponds to the superstring-motivated example in the gravity-dominated regime (see Sec. 4f) with $p=1$, $z_{*}/z_{\nu }=1$, ${\alpha }_7=3$, and $m_5=4$. This flux is detectable by SKA and LOFAR. Increasing further the product ${\alpha }_7^2{m}_5^{-1/2}$ the flux can be made detectable by JEM-EUSO, though increasing of ${\alpha }_7$ should be limited, because of increasing of $E_{\min }$ due to this factor. The curve “theor. 1” with parameters $p=z_{*}/z_{\nu }=1$, ${\alpha }_7=2$, and $m_5=0.1$ gives the flux detectable by all three future instruments SKA, LOFAR, and JEM-EUSO. In the case of the moduli-dominated regime with parameters considered in Sec. 4g as ${\alpha }_7=2$, $m_5=0.1$, and $z_{*}=100$, neutrino flux is observable by all three detectors, SKA, LOFAR, and JEM-EUSO.