Doubly charged scalars frequently emerge in many well-motivated extensions of the Standard Model, particularly in frameworks that aim to explain the origin of neutrino masses. Their distinct electric charge and clean leptonic signatures make them especially compelling from the standpoint of experimental searches. In this work, we explore the sensitivity of the LHC full run II, including photon-photon fusion, and Future Circular Collider in its hadron-hadron configuration (FCC-hh) to such states, assuming they decay promptly and exclusively into charged leptons either conserving or violating lepton flavor. We find that the FCC-hh, operating at 100 TeV, is uniquely positioned to probe doubly charged scalars with masses up to 7 TeV and possibly establish the mechanism behind neutrino masses.

Brazil

Letícia Guedes

Gabriela Hoff

Chile

Farinaldo Queiroz

Y. Oviedo-Torres

France

Y. Villamizar

Search for a doubly charged scalar at the LHC and FCC-hh

The Left-Right model is a popular extension of the Standard Model that features the new gauge bosons, $$W^{\pm }_R$$ and $$Z_R$$ . Collider searches for a Left-Right symmetry are often concentrated on the charged right-handed current. However, in this work, we take advantage of the dilepton data at the LHC with center-of-mass energy of 13 TeV and 139 $$\hbox {fb}^{-1}$$ of integrated luminosity to place lower mass bounds in the $$Z_R$$ mass based on the $$p\,p \rightarrow Z_{R} \rightarrow \ell ^{+} \, \ell ^{-}+X$$ process. We vary the $$SU(2)_R$$ gauge coupling from $$g_{R}=0.4$$ to $$g_{R}=1.0$$ , and find that the LHC data impose $$M_{Z_R}>5.4$$ TeV and $$M_{Z_{R}}>6.1$$ TeV, respectively. Lastly, we put our findings into perspective with $$W_R$$ searches at the LHC and show that our limits cover an unexplored region of parameter space, where the right-handed neutrino is heavier than the $$W_R$$ boson.

Gabriela Lichtenstein

M. Neves

Ricardo Silva

Updated bounds on the minimal left-right symmetric model from LHC dilepton resonance searches

Light scalars that couple to leptons are common figures in beyond the Standard Model endeavors. Considering a scalar that has universal and couplings to leptons only, we compute this leptophilic scalar contribution to the $$e^{-}e^{+} \rightarrow \tau ^{+}\tau ^{-} S $$ production cross section with $$S \rightarrow e^{+}e^{-}(\mu ^{+}\mu ^{-})$$ . We later compare the expected signal with recent data from the BELLE collaboration collected near the resonance $$\Upsilon (4S)$$ with $$\mathscr {L}=626 fb^{-1}$$ of integrated luminosity to place limits on the couplings-mass plane for the 4 MeV-6.5 GeV mass range. We then extended this analysis to a more general one $$e^{-}e^{+} \rightarrow {\ell }^{+}{\ell }^{-} S$$ production cross section where $$\ell =e,\mu ,\tau $$ with $$S \rightarrow e^{+}e^{-}(\mu ^{+}\mu ^{-})$$ , showing that BELLE constitutes an excellent laboratory for light scalars, where can be derived constraints stronger than those derived stemming from the g-2 of the electron or muon.

D. Cogollo

Yoxara Villamizar

J. Zamora-Saa

Search for sub-GeV scalars in $$e^+e^-$$ collisions

In this study, we implement the type-II seesaw mechanism for Dirac neutrino masses within the framework of a 3-3-1 model. To this end, we introduce a scalar sextet and impose both lepton number conservation and invariance under a discrete $\textit{Z}$ symmetry in the Lagrangian. This mechanism naturally generates small Dirac neutrino masses by allowing the soft breaking of the $\textit{Z}$ symmetry through a unique term in the scalar potential, while preserving lepton number. Specifically, we explore the realization of this model at low-energy scales. Regarding flavor implications, we analyze its contributions to the rare decay processes $\textit{μ}$ → $\textit{eγ}$ and $$ \mu \to \overline{e} ee $$ . In the cosmological context, we analyze the influence of right-handed neutrinos on the effective number of neutrino species, $\textit{N}$ , through interactions mediated by the $\textit{Z}$ boson. Our findings establish a lower bound of $$ {m}_{Z^{\prime }} $$ > 4.4 TeV, which slightly exceeds the current lower limit set by the Large Hadron Collider (LHC).

Sweden

Portugal

Vinícius Oliveira

Patricio Escalona

Lucia Angel

C. Pires

Type-II seesaw mechanism for Dirac neutrinos and its implications on $\textit{N}$ eff and lepton flavor violation in a 3-3-1 model

We give a fresh look at the WIMP paradigm by considering updated limits and prospects for direct and indirect dark matter detection and covering realistic dark matter models, beyond the so-called simplified models, which have been the target of experimental scrutiny. In particular, we investigate dark matter scenarios featuring dwindled direct detection signals, due to loop or momentum suppression. Therefore, this review extends previous reviews in different aspects and motivates the search for WIMP dark matter in light of the present and near-future detectors.

Italy

Giorgio Arcadi

Spain

David Cabo-Almeida

United States

Maíra Dutra

India

Pradipta Ghosh

Germany

Manfred Lindner

Yann Mambrini

Jacinto Neto

Mathias Pierre

Stefano Profumo

The waning of the WIMP: endgame?

CERN

Carlotta Accettura

United Kingdom

Dean Adams

Rohit Agarwal

Claudia Ahdida

Chiara Aimè

Nicola Amapane

David Amorim

Paolo Andreetto

Fabio Anulli

Robert Appleby

Artur Apresyan

Aram Apyan

Sergey Arsenyev

Pouya Asadi

Egypt

Mohammed Mahmoud

Aleksandr Azatov

John Back

Lorenzo Balconi

Laura Bandiera

Roger Barlow

Nazar Bartosik

Emanuela Barzi

Fabian Batsch

Matteo Bauce

J. Berg

Andrea Bersani

Alessandro Bertarelli

Alessandro Bertolin

Kevin Black

Fulvio Boattini

Alex Bogacz

Maurizio Bonesini

Bernardo Bordini

Salvatore Bottaro

Luca Bottura

Alessandro Braghieri

Marco Breschi

Natalie Bruhwiler

Xavier Buffat

Laura Buonincontri

Philip Burrows

Graeme Burt

Dario Buttazzo

Barbara Caiffi

Marco Calviani

Simone Calzaferri

Daniele Calzolari

Rodolfo Capdevilla

Christian Carli

Fausto Casaburo

Massimo Casarsa

Luca Castelli

Maria Catanesi

Lorenzo Cavallucci

Gianluca Cavoto

Francesco Celiberto

Luigi Celona

Alessandro Cerri

Gianmario Cesarini

Cari Cesarotti

Grigorios Chachamis

Antoine Chance

Switzerland

Siyu Chen

Yang-Ting Chien

Mauro Chiesa

Anna Colaleo

Francesco Collamati

Gianmaria Collazuol

Marco Costa

Nathaniel Craig

Camilla Curatolo

Canada

David Curtin

Giacomo Molin

Magnus Dam

Heiko Damerau

Sridhara Dasu

Jorge Blas

Stefania Curtis

Ernesto Matteis

Stefania Rosa

Jean-Pierre Delahaye

Dmitri Denisov

Türkiye

Haluk Denizli

Christopher Densham

Radovan Dermisek

Luca Luzio

Elisa Meco

Biagio Micco

Keith Dienes

Eleonora Diociaiuti

Tommaso Dorigo

Alexey Dudarev

Robert Edgecock

Filippo Errico

Marco Fabbrichesi

Stefania Farinon

Anna Ferrari

Jose Somoza

Netherlands

Frank Filthaut

Davide Fiorina

Erratum: Towards a muon collider

A muon collider would enable the big jump ahead in energy reach that is needed for a fruitful exploration of fundamental interactions. The challenges of producing muon collisions at high luminosity and 10 TeV centre of mass energy are being investigated by the recently-formed International Muon Collider Collaboration. This Review summarises the status and the recent advances on muon colliders design, physics and detector studies. The aim is to provide a global perspective of the field and to outline directions for future work.

Towards a muon collider

The Hubble constant inferred from the 6-parameter fit to the CMB power spectrum conflicts with the value obtained from direct measurements via type Ia supernova and Cepheids observations. We write down effective operators involving spin-0, spin-1/2, and spin-1 dark matter that lead to the relativistic production of dark matter particles at early times, and consequently lead to an increase in the number of relativistic degrees of freedom. This mechanism which is amenable to CMB, BBN, and structure formation observables can sufficiently raise the value of the Hubble constant derived from CMB and reconcile local and CMB probes of the Hubble constant. This mechanism alone increases  $$H_0$$  <math> <msub> <mi>H</mi> <mn>0</mn> </msub> </math>   up to  $$70\, \textrm{km}\, \textrm{s}^{-1} \, \textrm{Mpc}^{-1}$$  <math> <mrow> <mn>70</mn> <mspace width="0.166667em"></mspace> <mtext>km</mtext> <mspace width="0.166667em"></mspace> <msup> <mtext>s</mtext> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> <mspace width="0.166667em"></mspace> <msup> <mtext>Mpc</mtext> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </mrow> </math>  , and with the help of a Phantom-like cosmology, reach  $$H_0 \simeq $$  <math> <mrow> <msub> <mi>H</mi> <mn>0</mn> </msub> <mo>≃</mo> </mrow> </math>   71–73  $$\textrm{km} \,\textrm{s}^{-1} \textrm{Mpc}^{-1}$$  <math> <mrow> <mtext>km</mtext> <mspace width="0.166667em"></mspace> <msup> <mtext>s</mtext> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> <msup> <mtext>Mpc</mtext> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </mrow> </math>  . Lastly, we outline the region of parameter space which reproduces  $$H_0 \simeq $$  <math> <mrow> <msub> <mi>H</mi> <mn>0</mn> </msub> <mo>≃</mo> </mrow> </math>   71–73  $$\textrm{km} \,\textrm{s}^{-1} \, \textrm{Mpc}^{-1}$$  <math> <mrow> <mtext>km</mtext> <mspace width="0.166667em"></mspace> <msup> <mtext>s</mtext> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> <mspace width="0.166667em"></mspace> <msup> <mtext>Mpc</mtext> <mrow> <mo>-</mo> <mn>1</mn> </mrow> </msup> </mrow> </math>   while obeying all relevant constraints.

Alvaro Jesus

Nelson Pinto-Neto

Joseph Silk

Dêivid Silva

The hubble rate trouble: an effective field theory of dark matter

We discuss a model where a mixed warm and hot keV neutrino dark matter rises naturally. We arrange active and sterile neutrinos in the same SU(3) L  multiplet, with the lightest sterile neutrino being dark matter. The other two heavy sterile neutrinos, through their out-of-equilibrium decay, contribute both to the dilution of dark matter density and its population, after freeze-out. We show that this model features all ingredients to overcome the overproduction of keV neutrino dark matter, and explore the phenomenological implications for Big Bang Nucleosynthesis and the number of relativistic degrees of freedom.

C. S. Pires

A model for mixed warm and hot right-handed neutrino dark matter

Neutrinos are massless in the Standard Model. The most popular mechanism to generate neutrino masses are the type I and type II seesaw, where right-handed neutrinos and a scalar triplet are augmented to the Standard Model, respectively. In this work, we discuss a model where a type I + II seesaw mechanism naturally arises via spontaneous symmetry breaking of an enlarged gauge group. Lepton flavor violation is a common feature in such setup and for this reason, we compute the model contribution to the $$\mu \rightarrow e\gamma $$ <math><mrow><mi>μ</mi><mo>→</mo><mi>e</mi><mi>γ</mi></mrow></math>  and $$\mu \rightarrow 3e$$ <math><mrow><mi>μ</mi><mo>→</mo><mn>3</mn><mi>e</mi></mrow></math>  decays. Moreover, we explore the connection between the neutrino mass ordering and lepton flavor violation in perspective with the LHC, HL-LHC and HE-LHC sensitivities to the doubly charged scalar stemming from the Higgs triplet. Our results explicitly show the importance of searching for signs of lepton flavor violation in collider and muon decays. The conclusion about which probe yields stronger bounds depends strongly on the mass ordering adopted, the absolute neutrino masses and which much decay one considers. In the 1–5 TeV mass region of the doubly charged scalar, lepton flavor violation experiments and colliders offer orthogonal and complementary probes. Thus if a signal is observed in one of the two new physics searches, the other will be able to assess whether it stems from a seesaw framework.