Non-divergence operators structured on homogeneous Hörmander vector fields: heat kernels and global Gaussian bounds

Let $X_{1},...,X_{m}$ be a family of real smooth vector fields defined in $\mathbb{R}^{n}$, $1$-homogeneous with respect to nonisotropic dilations and satisfying Hörmander's rank condition at $0$ (and therefore every point $\mathbb{R}^{n}$). The are not assumed translation invariant any Lie group structure. us consider the nonvariational evolution operator \[ \mathcal{H}:=\sum_{i,j=1}^{m}a_{i,j}(t,x)X_{i}X_{j}-\partial_{t} , \] where $(a_{i,j}(t,x))_{i,j=1}^{m}$ is symmetric uniformly positive $m\times m$ matrix entries $a_{ij}$ bounded Hölder continuous functions on $\mathbb{R}^{1+n}$, ``parabolic'' distance induced by fields. We prove existence global heat kernel $\Gamma(\cdot;s,y)\in C_{X,\mathrm{loc}}^{2,\alpha}(\mathbb{R}^{1+n} \setminus\{(s,y)\})$ for $\mathcal{H}$, such that $\Gamma$ satisfies two-sided Gaussian bounds $\partial_{t}\Gamma, X_{i}\Gamma,X_{i}X_{j}\Gamma$ satisfy upper strip $[0,T]\times\mathbb{R}^{n}$. also scale-invariant parabolic Harnack inequality $\mathcal{H} $, standard corresponding stationary \mathcal{L}:=\sum_{i,j=1}^{m}a_{i,j}(x)X_{i}X_{j} coefficients.