Flagged LLT polynomials, nonsymmetric plethysm, and nonsymmetric Macdonald polynomials

The plethystic transformation $f[X] \mapsto f[X/(1-t)]$ and LLT polynomials are central to the theory of symmetric Macdonald polynomials. In this work, we introduce and study nonsymmetric flagged LLT polynomials. We show that these admit both an algebraic and a combinatorial description, that they Weyl symmetrize to the usual symmetric LLT polynomials, and we conjecture that they expand positively in terms of Demazure atoms. Additionally, we construct a nonsymmetric plethysm operator $\Pi_{t,x}$ on $\mathfrak{K}[x_1,\ldots,x_n]$, which serves as an analogue of $f[X] \mapsto f[X/(1-t)]$. We prove that $\Pi_{t,x}$ remarkably maps flagged LLT polynomials defined over a signed alphabet to ones over an unsigned alphabet. Our main application of this theory is to formulate a nonsymmetric version of Macdonald positivity, similar in spirit to conjectures of Knop and Lapointe, but with several new features. To do this, we recast the Haglund-Haiman-Loehr formula for nonsymmetric Macdonald polynomials $\mathcal{E}_{\mu }(x;q,t)$ as a positive sum of signed flagged LLT polynomials. Then, after applying a suitable stable limit of $\Pi_{t,x}$ to a stable version of $\mathcal{E}_{\mu }(x;q,t)$, we obtain modified nonsymmetric Macdonald polynomials which are positive sums of flagged LLT polynomials and thus are conjecturally atom positive, strengthening the Macdonald positivity conjecture.