We consider a simple Higgs portal dark matter (DM) model, where the Standard Model is extended with a complex singlet scalar. The imaginary part of the scalar becomes a massive and stable pseudo-Nambu-Goldstone boson, serving as the DM candidate, while the real part gives rise to a second (dark) Higgs boson. We focus on the freeze-in production of the DM, paying particular attention to low-reheating temperature scenarios, where the dark Higgs can be a long-lived particle (LLP). We also explore the phenomenology of this dark Higgs at the LHC and the Future Circular Collider in hadron-hadron mode, discussing its discovery prospects in regions of parameter space consistent with current DM constraints. Our results emphasize the impact of the cosmic reheating dynamics on the DM freeze-in production, and their critical role in interpreting collider signatures. Furthermore, our findings suggest that LLP searches may provide insights into the fundamental dynamics of reheating.

United Arab Emirates

Nicolás Bernal

Chile

Giovanna Cottin

Bastián Díaz Sáez

Manuel López

Testing frozen-in pNGB dark matter with a long-lived dark Higgs

We study light neutralinos $$ \left({\overset{\sim }{\chi}}_1^0\right) $$ with masses ranging from 10 GeV to several hundred GeV within the framework of R-parity-violating (RPV) supersymmetry. These light neutralinos can be long-lived, decaying with a macroscopic displacement (order cm) inside the LHC main detectors. Complementing previous works on the subject, here we focus on their production through the electroweak pair production of left-chiral sleptons $$ \left({\tilde{e}}_L\right) $$ , with the signal process $\textit{pp}$ → $$ {\tilde{e}}_L^{+}{\tilde{e}}_L^{-} $$ → $$ {e}^{+}{\overset{\sim }{\chi}}_1^0{e}^{-}{\overset{\sim }{\chi}}_1^0 $$ . In contrast to the previous study with a singly produced slepton, where the RPV coupling $$ {\lambda}_{111}^{\prime } $$ induces $\textit{both}$ the production and decay of the light neutralino, in our scenario the production proceeds through Drell-Yan-like processes that are essentially independent of RPV couplings. Correspondingly, we implement a displaced-vertex search strategy for which our numerical analysis shows that the high-luminosity LHC can probe $$ {\lambda}_{111}^{\prime } $$ values up to three orders of magnitude smaller, and neutralino masses up to about four times larger than those accessible in the previously studied single-slepton production scenario.

Juan Helo

Fabián Hernández-Pinto

Nicolás Neill

China

Zeren Wang

Probing light neutralinos from pair-produced sleptons with displaced vertices at the high-luminosity LHC

A correction to this paper has been published: https://doi.org/10.1007/JHEP05(2025)238

Spain

Rebeca Beltrán

Germany

Julian Günther

Martin Hirsch

Italy

Arsenii Titov

Erratum to: Heavy neutral leptons and top quarks in effective field theory

We study the phenomenology of heavy neutral leptons (HNLs) at the LHC in effective field theory, concentrating on $\textit{d}$ = 6 operators with top quarks. Depending on the operator choice and HNL mass, the HNLs will be produced either from proton-proton collisions in association with a single top, or via non-standard decays of top quarks. For long-lived HNLs we estimate the sensitivity reach of different detectors to various operators with top quarks and the HNLs for the high-luminosity phase of the LHC. For certain operators, ATLAS and some far detectors (MATHUSLA and ANUBIS) will be able to probe the associated new-physics scale as large as 12 TeV and 4.5 TeV, respectively, covering complementary HNL-mass ranges.

Heavy neutral leptons and top quarks in effective field theory

Neutrino masses and dark matter (DM) might have a common origin. The $\textit{scotogenic}$ model can be considered the proto-type model realizing this idea, but many other variants exist. In this paper we explore the phenomemology of a particular DM neutrino mass model, containing a triplet scalar. We calculate the relic density and check for constraints from direct detection experiments. The parameter space of the model, allowed by these constraints, contains typically a long-lived or quasi-stable doubly charged scalar, that can be searched for at the LHC. We reinterpret existing searches to derive limits on the masses of the scalars of the model and estimate future sensitivities in the high-luminosity phase of the LHC. The searches we discuss can serve to constrain also many other 1-loop neutrino mass models.

Carolina Arbeláez

Téssio Melo

Long-lived particle phenomenology in one-loop neutrino mass models with dark matter

We study axion-like particles (ALPs) with quark-flavor-violating couplings at the LHC. Specifically, we focus on the theoretical scenario with ALP-top-up and ALP-top-charm interactions, in addition to the more common quark-flavor-diagonal couplings. The ALPs can thus originate from decays of top quarks which are pair produced in large numbers at the LHC, and then decay to jets. If these couplings to the quarks are tiny and the ALPs have   <math> <mi>O</mi> <mfenced> <mn>10</mn> </mfenced> </math>  $$ \mathcal{O}(10) $$  GeV masses, they are long-lived, leading to signatures of displaced vertex plus multiple jets, which have the advantage of suppression of background events at the LHC. We recast a recent ATLAS search for the same signature and reinterpret the results in terms of bounds on the long-lived ALP in our theoretical scenario. We find that the LHC with the full Run 2 dataset can place stringent limits, while at the future high-luminosity LHC with 3 ab−1 integrated luminosity stronger sensitivities are expected.

Taiwan

South Korea

Kingman Cheung

Fei-Tung Chung

Quark flavor violation and axion-like particles from top-quark decays at the LHC

Erratum to: Searching for light neutralinos with a displaced vertex at the LHC

Many models beyond the Standard Model predict light and feebly interacting particles that are often long-lived. These long-lived particles (LLPs) in many cases can be produced from meson decays. In this work, we propose a simple and quick reinterpretation method for models predicting LLPs produced from meson decays. With the method, we are not required to run Monte-Carlo simulation, implement detector geometries and efficiencies, or apply experimental cuts in an event analysis, as typically done in recasting and reinterpretation works. The main ingredients our method requires are only the theoretical input, allowing for computation of the production and decay rates of the LLPs. There are two conditions for the method to work: firstly, the LLPs in the models considered should be produced from a set of mesons with similar mass and lifetime (or the same meson) and second, the LLPs should, in general, have a lab-frame decay length much larger than the distance between the interaction point and the detector. As an example, we use this method to reinterpret exclusion bounds on heavy neutral leptons (HNLs) in the minimal “3+1” scenario, into those for HNLs in the general effective-field-theory framework as well as for axion-like particles. We are able to reproduce existing results, and obtain new bounds via reinterpretation of past experimental results, in particular, from CHARM and Belle.

Reinterpretation of searches for long-lived particles from meson decays

We study the LHC discovery potential in the search for heavy neutral leptons (HNL) with a new signature: a displaced shower in the CMS muon detector, giving rise to a large cluster of hits forming a displaced shower. A new Delphes module is used to model the CMS detector response for such displaced decays. We reinterpret a dedicated CMS search for neutral long-lived particles decaying in the CMS muon endcap detectors for the minimal HNL scenario. We demonstrate that this new strategy is particularly sensitive to active-sterile mixings with τ leptons, due to hadronic τ decays. HNL masses between ~ 1–6 GeV can be accessed for mixings as low as |V  τN |2~10−7, probing unique regions of parameter space in the τ sector.

United States

Cristián Peña

Christina Wang

Si Xie

Long-lived heavy neutral leptons with a displaced shower signature at CMS

In the framework of the low-energy effective field theory of the Standard Model extended with heavy neutral leptons (HNLs), we calculate the production rates of HNLs from meson decays triggered by dimension-six operators. We consider both lepton-number-conserving and lepton-number-violating four-fermion operators involving either a pair of HNLs or a single HNL. Assuming that HNLs are long-lived, we perform simulations and investigate the reach of the proposed far detectors at the high-luminosity LHC to (i) active-heavy neutrino mixing and (ii) the Wilson coefficients associated with the effective operators, for HNL masses below the mass of the B-meson. We further convert the latter to the associated new-physics scales. Our results show that scales in excess of hundreds of TeV and the active-heavy mixing squared as small as 10 −15 can be probed by these experiments.

Long-lived heavy neutral leptons from mesons in effective field theory