Hunting long-lived doubly charged scalars at the HL-LHC

Abstract

This work studies the collider phenomenology of long-lived doubly charged scalars. After reviewing the existing searches for the doubly charged scalar in both the prompt and the long-lived regimes, we identify an intermediate range of proper decay length, namely, $\mathcal{O}(0.1\mathrm{mm})\lesssim c\tau \lesssim \mathcal{O}(100\mathrm{mm})$, where conventional searches lose sensitivity. We investigate the prospects for probing the doubly charged scalar both in the presence and in the absence of the $\Delta L=2$ Yukawa coupling of the $SU(2{)}_L$ complex triplet. In the presence of this coupling, $H^{\pm \pm }$ can be long-lived only for masses in the range $100$-$150\mathrm{GeV}$, whereas in the fermiophobic scenario (i.e., in the absence of the $\Delta L=2$ coupling), the range extends to $\mathrm{TeV}$ scale. We propose a displaced-vertex search at the HL-LHC for doubly charged scalars with masses up to $\approx 1\mathrm{TeV}$ and $c\tau =10$-$100\mathrm{mm}$ in the fermiophobic scenario, while a benchmark point with a doubly charged scalar of mass $120\mathrm{GeV}$ and $c\tau =5\mathrm{mm}$ is considered when the complex triplet scalar couples to leptons. We show that a cut on the invariant mass of the displaced vertex as reconstructed from the associated tracks can strongly suppress Standard Model backgrounds. We present projected limits on the Drell-Yan pair-production cross section for the doubly charged scalar at $\sqrt{s}=14\mathrm{TeV}$ with an integrated luminosity of $3000{\mathrm{fb}}^{-1}$, considering two illustrative assumptions for the residual background. Displaced-vertex searches thus probe a region complementary to those covered by prompt and heavy stable charged-particle searches.

Comments: 39 pages, 11 figures, 6 tables