Probing baryon number with missing energy

Abstract

Quark portal interactions $qqqN$ with a light singlet fermion $N$ make baryon number testable through missing transverse energy (MET). We find that present LHC data constrain scales up to 10 TeV (MET plus jet) and 15 TeV (MET plus top). With increasing mass, or larger portal couplings, $N$ becomes less long-lived, and gives clean displaced vertex signatures, which encourage dedicated searches. We also narrow down viable mesogenesis models with color triplet scalars to a mass range $\sim y \, (2-3) \, \text{TeV}$ and couplings $y$ to $b$- and second generation quarks, a window that can be scrutinized by HL-LHC. Interactions also induce rare decays of type baryon (meson) to meson (baryon) plus invisible, which complement high-$p_T$ searches and can prove baryon number violation. We explore charm decays $Λ_c \to (π,K) + \mathrm{invisible}$. Their branching ratios are subject to sizable hadronic uncertainties and require high luminosity flavor facilities such as a Tera-Z facility (FCC-ee, CEPC). Branching ratios of top quarks into one or two $b$-jets plus $N$ can reach few$\times 10^{-6}$.

Figures (16)

• Figure 1: Top panel: vertices for interactions between $\Psi\sim(3,1,2/3)$ and fermions. Bottom panel (left to right): topology relevant for meson mixing; four-quark operators generated at tree level by the model that do not induce meson mixing; and four-quark operators generated by the model that induce meson mixing at loop level, avoided if couplings are allowed only between two quark generations.
• Figure 2: Branching ratios of exclusive $B$ meson (upper row) and hyperon (lower row) decays into the singlet $N (\bar{N})$ in units of $C^2/\Lambda^4 \text{ TeV}^4$. Predictions are based on Refs. Elor:2022jxyAlonso-Alvarez:2021oaj.
• Figure 3: Chiral amplitudes for the process $u_i d_j \to \bar{N} \bar{d}_k$ highlighting the different contractions of the indices. In the high-energy limit interference is only possible between diagrams with identical chiralities. This allows for interference between $\mathcal{O}^{uddN}_{ijk}$ and $\mathcal{O}^{uddN}_{ikj}$, while for $\mathcal{O}^{qqdN}$ all interference terms vanish. Antiquark channels are related by charge conjugation and down-quark fusion channels are related by crossing. The indicated factors of 2 for $C^{qqdN}_{\{ij\}k}$ arise from expanding the doublets in $\mathcal{O}^{qqdN}_{ijk}$ into components, see Eqn. \ref{['eqn:BNV_4F']}.
• Figure 4: The PLFs (\ref{['eq:PLF']}) that allow for BNV initial quark-combinations. The results are given as summed over quark and antiquark combinations and the lower plot shows the relative uncertainties. We use the PDFset NNPDF40_lo_as_01180NNPDF:2021njg with the factorization scale $\mu_F = \sqrt{\hat{s}}$. Solid lines correspond to the central values and the envelope shows the $1 \sigma$-ranges for the PDF uncertainties.
• Figure 5: Number of events in the $(M_N,\Lambda \, / \, \sqrt{C})$-plane with signature of MET (left), a DV (middle) and a prompt decay (right) of the singlet $N$, relative to the total number of produced particles $n_0$ for flavors $C_{111}^{uddN}$, $C_{123}^{uddN}$, $C_{211}^{uddN}$, and, with a top, $C_{311}^{uddN}$ (from top to bottom). The ranges of $M_N$ are shown close to the respective kinematic cutoffs given by the decay products, see Eqn. \ref{['eqn:BNV_total_width']}.