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A proton is known for its longevity, but what is its lifetime? While manyGrand Unified Theories predict the proton decay with a finite lifetime, we showthat the Standard Model (SM) and some versions of Ultra Unification (whichreplace sterile neutrinos with new exotic gapped/gapless sectors, e.g.,topological or conformal field theory under global anomaly cancellationconstraints) with a discrete baryon plus lepton symmetry permit a stableproton. For the 4d SM with $N_f$ families of 15 or 16 Weyl fermions, inaddition to the continuous baryon minus lepton U(1)$_{\bf B - L}$ symmetry,there is also a compatible discrete baryon plus lepton $\mathbb{Z}_{2N_f, \bf B+ L}$ symmetry. The $\mathbb{Z}_{2N_f, \bf B + L}$ is discrete due to the ABJanomaly under the BPST SU(2) instanton. Although both U(1)$_{\bf B - L}$ and$\mathbb{Z}_{2N_f, \bf B + L}$ symmetries are anomaly-free under the dynamicalSM gauge field, it is important to check whether they have mixed anomalies withthe gravitational background field and higher symmetries (whose charged objectsare Wilson electric or 't Hooft magnetic line operators) of SM. We can alsoreplace the U(1)$_{\bf B - L}$ with a discrete variant $\mathbb{Z}_{4,X}$ for$X \equiv 5({\bf B - L})-\frac{2}{3} {\tilde Y}$ of electroweak hypercharge${\tilde Y}$. We explore a systematic classification of candidate perturbativelocal and nonperturbative global anomalies of the 4d SM, including all thesegauge and gravitational backgrounds, via a cobordism theory, which controls theSM's deformation class. We discuss the proton stability of the SM and UltraUnification in the presence of discrete ${\bf B + L}$ symmetry protection, inparticular (U(1)$_{\bf B - L} \times \mathbb{Z}_{2N_f,\bf B +L})/{\mathbb{Z}_2^{\rm F}}$ or $(\mathbb{Z}_{4,X} \times \mathbb{Z}_{2N_f, \bfB + L})/{\mathbb{Z}_2^{\rm F}}$ symmetry with the fermion parity$\mathbb{Z}_2^{\rm F}$.

Proton Stability: From the Standard Model to Beyond Grand Unification