MeV-scale reheating temperature and thermalization of oscillating neutrinos by radiative and hadronic decays of massive particles (1908.10189v2)

Abstract: From a theoretical point of view, there is a strong motivation to consider an MeV-scale reheating temperature induced by long-lived massive particles with masses around the weak scale, decaying only through gravitational interaction. In this study, we investigate lower limits on the reheating temperature imposed by big-bang nucleosynthesis assuming both radiative and hadronic decays of such massive particles. For the first time, effects of neutrino self-interactions and oscillations are taken into account in the neutrino thermalization calculations. By requiring consistency between theoretical and observational values of light element abundances, we find that the reheating temperature should conservatively be $T_{\rm RH} \gtrsim 1.8$ MeV in the case of the 100% radiative decay, and $T_{\rm RH} \gtrsim$ 4-5 MeV in the case of the 100% hadronic decays for particle masses in the range of 10 GeV to 100 TeV.

• The paper reveals that decay-induced incomplete neutrino thermalization constrains the reheating temperature to above 1.8 MeV for radiative decays and 4–5 MeV for hadronic decays.
• The study employs Monte Carlo analyses incorporating neutrino oscillations and self-interactions to refine predictions of light element abundances from Big Bang Nucleosynthesis.
• The findings imply significant consequences for early Universe physics, extending insights to beyond Standard Model scenarios such as supersymmetry and string theory.

MeV-Scale Reheating and Neutrino Thermalization: Implications for Big Bang Nucleosynthesis

This paper by Hasegawa et al. investigates the constraints on the reheating temperature of the early Universe, particularly focusing on the effects of decays from long-lived massive particles. These particles, primarily decaying via gravitational interaction, hold masses in the range of 10 GeV to 100 TeV and could potentially dominate the cosmic energy density during an early matter-dominated era. The paper offers a detailed examination of how such scenarios can influence Big Bang Nucleosynthesis (BBN) when the reheating temperature is around the MeV scale.

Neutrino Thermalization and Reheating Constraints

At the onset, the paper emphasizes the critical role of neutrino thermalization processes during the reheating period. Unlike prior assumptions in standard cosmology where neutrinos are perfectly thermalized before the onset of BBN, this work posits that the decay of massive particles can lead to an incomplete thermalization of neutrinos. This is primarily due to the late-time entropy production caused by the decay processes.

The paper is innovative in incorporating neutrino self-interactions and oscillations in evaluating neutrino thermalization. The findings reveal that the reheating temperature, $T_{\rm RH}$, must conservatively be greater than 1.8 MeV for scenarios dominated by radiative decays. However, when considering hadronic decays, the bound is tightened, requiring $T_{\rm RH}$ to be between 4 and 5 MeV for most masses and branching ratios considered.

Big Bang Nucleosynthesis Modifications

The paper further advances our understanding of BBN by exploring how these scenarios impact light element abundances. By considering neutrino oscillation effects, the paper identifies modifications in the neutron-to-proton ratio, which significantly influences helium-4 and deuterium yields. The analysis provides updated theoretical predictions, acknowledging the sensitivity of these abundances to both the thermal history and the decay of massive particles.

A robust Monte Carlo analysis quantifies the compatibility of theoretical predictions with observed abundances. It presents a bound on the reheating temperature enforced by both helium and deuterium data, refining prior constraints with an emphasis on neutrino physics and decay processes. The enhanced methodologies yield a conservative lower limit bound, reinforcing that BBN remains a potent probe for reheating dynamics with implications for fundamental physics beyond the Standard Model.

Implications and Future Prospects

The paper successfully delineates the profound implications of MeV-scale reheating on cosmology, specifically elucidating constraints applicable to scenarios featuring decaying massive particles. The implications extend to theoretical frameworks beyond the conventional model, potentially influencing interpretations within supersymmetry and string theory.

Looking ahead, the authors suggest that their findings can be further validated and refined through Cosmic Microwave Background (CMB) observations and large-scale structure surveys. Such data, sensitive to similar thermal dynamics, could offer complementary bounds and bolster the constraints derived from BBN. Moreover, advancing neutrino physics experiments may provide direct insights into self-interaction and oscillation mechanisms synonymous with the conditions postulated in this paper.

In summary, this work is a valuable contribution to the field, challenging existing cosmological assumptions and pushing the boundaries in the paper of the early universe's thermal history.