Probing the existence of a new charged vector boson decaying into heavy neutral leptons using ultra-peripheral heavy ion collisions at ATLAS

TL;DR

The paper investigates the potential of Ultra-peripheral Collisions (UPCs) at the LHC to reveal new physics in the Vector Scotogenic Model, focusing on a charged vector boson $V^{\pm}$ decaying to heavy neutral leptons $N_L$ via a dimuon+MET final state. Using ATLAS-level lead-ION UPC data and simulations with a UFO/MadGraph workflow, the authors perform a random-cut optimization across angular and kinematic observables to maximize signal significance, revealing 95% CL exclusions for several low-mass scenarios and a 5$\sigma$ discovery only for $(M_V^{\pm}, M_{N_L})=(20,10)$ GeV. They further extend the search to HL-LHC with ultra-peripheral proton-proton collisions, where higher luminosity enables exclusions of the whole studied region $100<M_V^{\pm}<200$ GeV and $5<M_{N_L}<200$ GeV at 95% CL and 5$\sigma$ discovery in most cases. The work demonstrates UPCs as a clean environment to probe BSM particles that are challenging to detect in conventional proton-proton collisions and highlights the HL-LHC's capacity to broaden the accessible parameter space for $V^{\pm}\to N_L$ decays.

Abstract

In this paper we explore the potential of Ultra-peripheral Collisions in the LHC to investigate new physics, focusing specifically on the production of new charged vector bosons ${V}^{\pm}$ that decay into heavy neutral leptons $N_{L}$ in the context of the Vector Scotogenic Model. We show that the ATLAS experiment searching for dilepton+met final states through UPCs of lead ions, can prove the existence of new charged vector bosons in the mass range 5-100 GeV, a region not explored by the LEP-II experiment. Our analysis identifies regions in the parameter space where the signal can be distinguished from the background with high statistical significance. Within the mass range 5 GeV $<M_{V^{\pm}}, M_{N_L}<$ 100 GeV, ATLAS can exclude scenarios with 95\% of C.L. for specific mass scenarios such as (30 GeV, 20 GeV), (30 GeV, 10 GeV) and (20 GeV, 10 GeV). In a discovery context, ATLAS could reach a significance of 5$σ$ only for (20 GeV, 10 GeV). Furthermore, we show that in the absence of new physics signals at ATLAS, the High Luminosity LHC with UPCs of protons could explore further into larger mass ranges, specifically 100 GeV $< M_{V^{\pm}}<$ 200 GeV and 5 GeV $<M_{N_L}<$ 200 GeV. We find that the HL-LHC can exclude this mass range with 95\% of C.L., and most scenarios can achieve a discovery significance of 5$σ$

Figures (13)

• Figure 1: Typical ultra-peripheral collision process of lead ions, where $P_{a}$ and $P_{b}$ represent possible final states.
• Figure 2: Main Feynman diagram contributions for the process ${}^{208}_{82}Pb+{}^{208}_{82}Pb \rightarrow {\mu}^{-} + {\mu}^{+} + MET + {}^{208}_{82}Pb^{*} + {}^{208}_{82}Pb^{*}$ in the (a) background and (b) signal cases.
• Figure 3: Cross-section as a function of (${M}_{{V}^{\pm}}$,${M}_{{N}_{L}}$), where (a) shows the cross-section being compared with the exclusion region suggested by LEP-II, while (b) shows the cross-section points where ${N}_{S} \geq 1$.
• Figure 4: Angular observables for a pair of muons in three different scenarios where (a) is the acoplanarity, (b) is the separation in the $\eta \times \phi$ plane, and (c) is the cosine of the separation angle. The mass values are given in GeV.
• Figure 5: Kinematic observables for muons in three different scenarios where (a) is the muon transverse momentum, (b) is the anti-muon transverse momentum, (c) is the invariant mass of a pair of muons, and (d) is the missing transverse energy. The mass values are given in GeV.