KM3NeT Constraint on Lorentz-Violating Superluminal Neutrino Velocity

Abstract: Lorentz invariance is a fundamental symmetry of spacetime and foundational to modern physics. One of its most important consequences is the constancy of the speed of light. This invariance, together with the geometry of spacetime, implies that no particle can move faster than the speed of light. In this article, we present the most stringent neutrino-based test of this prediction, using the highest energy neutrino ever detected to date, KM3-230213A. The arrival of this event, with an energy of $220^{+570}_{-110}\,\text{PeV}$, sets a constraint on $\delta \equiv c_\nu^2-1 < 4\times10^{-22}$.

• The paper presents a test of Lorentz invariance by analyzing the high-energy neutrino event KM3-230213A for superluminal propagation evidence.
• It employs decay width calculations and conservative decay length estimates to constrain the LIV parameter δ with high precision.
• The findings, with δ limits as low as ~4.2×10^-22, challenge previous estimates and underscore the potential of advanced neutrino observatories.

KM3NeT Constraint on Lorentz-Violating Superluminal Neutrino Velocity

The paper "KM3NeT Constraint on Lorentz-Violating Superluminal Neutrino Velocity" presents an analysis that utilizes high-energy neutrino events detected by the KM3NeT observatory to test the limits of Lorentz invariance, a cornerstone of modern physics. In particular, the study focuses on the superluminal propagation of neutrinos, which, if observed, would imply a violation of Lorentz symmetry, providing significant evidence in favor of new physics beyond the standard model.

Methodology and Key Findings

The experiment is set against the backdrop of seeking Lorentz invariance violations (LIV), which can manifest as deviations in the constancy of the speed of light or erroneous interactions at high energies. LIV can be quantified through a parameter $\delta$, defined as $\delta = c_\nu^2 - 1$, where $c_\nu$ is the speed of the neutrino. A non-zero $\delta$ would imply superluminal neutrinos.

The analysis uses the ultra-high-energy neutrino event KM3-230213A, detected by KM3NeT, with an energy estimation of $220^{+570}_{-110} \text{ PeV}$—the highest recorded to date. The study targets this neutrino due to its high energy and probable extragalactic origin, which suggests a significant traveled distance, thereby allowing for competitive LIV constraints.

The decay width $\Gamma$ of a superluminal neutrino due to processes like electron-positron pair production ($\nu \rightarrow \nu + e^+ + e^-$) is utilized to infer the value of $\delta$. The methodology involves calculating the decay length $L = 10 c_\nu / \Gamma$, a customary approach that takes into account ten decay lengths, which are conventionally set for conservative assumptions.

Results

Calculating over a spectrum of probable propagation distances (ranging from the galactic scale to intergalactic distances), the paper sets an upper limit on the LIV parameter $\delta$. For a conservative galactic distance, $L\approx 4\times10^{20}\,\text{m}$, the constraint reads $\delta < 1.8^{+3.9}_{-1.7} \times 10^{-21}$. For more plausible intergalactic distances, presumed to be around 1 Mpc, the limit tightens to $\delta < 4.2^{+9.2}_{-3.7} \times 10^{-22}$.

These results present one of the most stringent limits on $\delta$ to date, challenging previous estimates obtained using other neutrino sources and lower-energy neutrinos. The competitive edge of these results is attributed to both the unprecedented energy of KM3-230213A and the extended baseline presumed for its origin.

Discussion and Future Directions

Establishing such a low value of $\delta$ indicates no observable LIV at the precision level achieved by KM3NeT. This suggests that if LIV exists, its effects are exceedingly minute or occur beyond the current observational capabilities.

However, future progress in detecting even higher energy neutrinos or advancements in theoretical modeling of astrophysical sources could further tighten these constraints. Additionally, improving spectral modeling and adopting enhanced detector arrays could provide new insights into fundamental symmetries.

Furthermore, while superluminal propagation primarily explored in this study may seem limited in its capacity to explore deeper aspects of LIV, combining these methods with phenomena such as cosmogenic neutrino spectral studies or leveraging other neutrino observatories could yield even more stringent conditions.

The work reflects the potential of using neutrino observations not only to constrain possible extensions to the standard model but also to explore the boundaries of our understanding of spacetime symmetries as posited by quantum gravity theories. As neutrino telescope accuracy and range advance, they hold promise as unique tools in probing fundamental physics beyond current paradigms.