From $B_c$ mesons to the baryon asymmetry: a unified $B$ Mesogenesis Framework

TL;DR

This work presents a unified B mesogenesis framework that translates SM CP violation into the observed baryon asymmetry via three B-meson channels: neutral B_q^0-b_q^0 mixing, charged B_c^+  B^+ decays, and a novel hybrid B_c^+  B_q^0 mechanism with subsequent oscillations. It refines neutral-meson contributions using time-integrated decay rates, finding a ~0.4 suppression for the B_d^0 term while the B_s^0 term remains essentially unchanged, thereby alleviating tension from its negative sign. The paper introduces a systematic study of B_c^+  BM decays, providing branching-ratio predictions via leading-order factorization and a data-driven D  M_1M_2 approach, and identifies two B_c^+ sources of mesogenesis with charm CP violation accessible at percent-level BRs. By combining neutral, charged, and hybrid mechanisms and scanning over direct CP asymmetries and fragmentation fractions, the authors show that successful baryogenesis is achievable in broad parameter space, underscoring the viability of unified B mesogenesis and motivating future measurements of B_c^+ modes and charm CP violation to test the framework.

Abstract

$B$ mesogenesis offers an interesting mechanism to generate the baryon asymmetry of the universe by converting the CP violation of the Standard Model into a net baryon number asymmetry. In this work we refine and extend the $B$ mesogenesis framework by incorporating all relevant $B$ meson channels active after low-temperature reheating. We first update the known neutral-meson contribution using time-integrated decay rates. While the $B_s^0$ contribution remains essentially unchanged, we find a suppression of the $B_d^0$ term by a factor $\sim 0.4$ with respect to previous analyses, alleviating the tension associated with its expected negative sign. We then perform a systematic study of $B_c^+$ decays, which are basically unexplored. We provide branching-ratio predictions using both leading-order factorization and a data-driven approach inspired by $D\to M_1M_2$ decays. These estimates allow us to quantify two $B_c^+$ sources of mesogenesis: the previously discussed $B_c^+ \to B^+ M^0$ channel and a new mechanism introduced in this work, $B_c^+ \to B_q^0 M$, in which the asymmetry is generated by combining direct CP violation with neutral-meson oscillations. Interestingly, in these channels the charm quark decays. Therefore, these decays give access to {\it charm CP violation} in modes with percent level branching ratios. With moderate assumptions, we find that $B_c^+$ mesogenesis can match or exceed the neutral contribution. Finally, we combine all three mechanisms and explore their viability in terms of the direct CP asymmetry, neutral-meson mixing parameters and early-universe fragmentation fractions. We find that successful baryogenesis can be achieved in a broad parameter space, showing the viability of $B$ mesogenesis. Future measurements of $B_c^+$ modes are thus highly anticipated to further probe the viability of unified $B$ mesogenesis.

Figures (5)

• Figure 1: Illustration of the decay of a $B$ meson into a visible SM baryon $\mathfrak{B}$ and dark matter states $\xi$ and $\phi$, mediated by the heavy field $Y$.
• Figure 2: Schematic representation of the combined $B$ mesogenesis model. $B$ mesons in the purple-shaded area arise from hadronization of the $b$ quarks produced in $\Phi$ decays. Direct CP violation in decays is denoted by black arrows, while CP violation arrising through $B_q^0$--$\bar{B}_q^0$ mixing is shown in orange. Green arrows indicate the decay paths responsible for generating the baryon asymmetry, whereas red dashed arrows denote channels that, although allowed, do not contribute to it.
• Figure 3: Leading topological diagrams for $B_c^+ \to B_d^0 \pi^+$ and $B_c^+ \to B^+ \pi^0$.
• Figure 4: Estimates of the individual contributions to the baryon asymmetry from each $B_c^+ \to BM$ decay channel using $a_q^{\rm theo}$ in \ref{['eq:atheo']}. We fix $\mathcal{A}_\mathrm{CP}^\mathrm{dir}(B_c^+ \to BM) = 10^{-3}$, $\mathcal{B}(B \to \mathfrak{B}\xi\phi)=10^{-3}$ and use our leading-order approach for the calculation of the $B_c^+\to BM$ branching ratios. Orange bars correspond to $B_c^+ \to B^+ M^0$ modes, green bars to $B_c^+ \to B_d^0 M^+$ and blue bars to $B_c^+ \to B_s^0M^+$.
• Figure 5: Viability of the unified $B$ mesogenesis framework as a function of the fragmentation fraction $f_c$ and $\mathcal{A}_\mathrm{CP}^\mathrm{dir}$ for $\Delta_B=10^{-8}$ (solid) and $10^{-7}$ (dashed). We fix $\mathcal{B}(B\to f)=10^{-3}$ and use leading-order factorization to estimate the $B_c^+$ branching ratios. Orange (pink) regions correspond to experimental (theoretical) determinations of the semileptonic asymmetries $a_q$ at $1\sigma$.