In this work, we consider the possibility that the dynamical dark energy hinted at by recent Dark Energy Spectroscopic Instrument data may be mimicked by the effects of additional components in the universe, potentially arising from topological defects. We find that the data does not show a particular preference for the existence of cosmic strings. However, a domain wall contribution at the percent level can improve the fit, yielding a <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="normal">Δ</mi><msup><mi>χ</mi><mn>2</mn></msup><mo>=</mo><mo>−</mo><mn>1.72</mn></math> and <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="normal">Δ</mi><mi>DIC</mi><mo>=</mo><mo>−</mo><mn>0.94</mn></math> compared to the lambda-cold dark matter model (<math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="normal">Λ</mi><mi>CDM</mi></math>). The improvement indicates that topological defects remain a viable and interesting extension to <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi mathvariant="normal">Λ</mi><mi>CDM</mi></math>, meriting further investigation with future cosmological data.

China

Haipeng An

South Korea

Chengcheng Han

Borui Zhang

Topological defects as effective dynamical dark energy

The tiny neutrino masses are most naturally explained by seesaw mechanism through singlet right-handed neutrinos, which can further explain the matter-antimatter asymmetry in the Universe. In this work, we propose a new approach to study cosmological signatures of neutrino seesaw through the interaction between inflaton and right-handed neutrinos, which respects the shift symmetry. In our framework, after inflation the inflaton predominantly decays into right-handed neutrinos and its decay rate is modulated by the fluctuations of the Higgs field, which act as the source of curvature perturbations. This gives a new realization of Higgs modulated reheating, and it produces primordial non-Gaussian signatures that can be measured by the forthcoming large-scale structure surveys. We demonstrate, for the first time, that the measurements of non-Gaussianity by these surveys can probe the neutrino seesaw scale and have the potential to constrain a large portion of the seesaw parameter space, opening up a new window for directly testing the high-scale seesaw mechanism.

Hong-Jian He

Linghao Song

Jingtao You

Cosmological signatures of neutrino seesaw mechanism

The neutrino mass generation via conventional seesaw mechanism is realized at high scales around <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>O</mi><mo stretchy="false">(</mo><msup><mn>10</mn><mn>14</mn></msup><mo stretchy="false">)</mo><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> with natural Yukawa couplings of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>O</mi><mo stretchy="false">(</mo><mn>1</mn><mo stretchy="false">)</mo></math>, making the test of the neutrino seesaw a great challenge. It is intriguing to note that the neutrino seesaw scale is typically around the upper range of the cosmological inflation scale. In this work, we propose a new framework incorporating inflation and the neutrino seesaw in which the inflaton primarily decays into right-handed neutrinos after inflation. This decay process is governed by the inflaton interaction with the right-handed neutrinos that respects the shift symmetry. Through the neutrino seesaw mechanism, fluctuations of the Higgs field can modulate the inflaton decays and contribute to the curvature perturbation. We investigate the induced non-Gaussian signatures and demonstrate, for the first time, that such signatures provide an important means to directly probe the high scale of natural neutrino seesaw. We further analyze the interplay of the non-Gaussianity signatures with the low-energy neutrino experiments, and their interplay with the Higgs self-coupling measurements at the LHC and future colliders.

Cosmological non-Gaussianity from the neutrino seesaw mechanism

Lepton flavor violation (LFV) offers a powerful probe of physics beyond the Standard Model, particularly in models addressing neutrino masses and the baryon asymmetry of the universe. In this study, we investigate LFV processes within the framework of type II seesaw leptogenesis, where the Standard Model is extended by an $S U(2)_L$ triplet Higgs field. We focus on key LFV processes, including $\mu^+\to e^+\gamma$ , $\mu^+ \to e^+e^-e^+$ , and $\mu \rightarrow e$ conversion in nuclei, deriving stringent constraints on the parameter space from current experimental data. We scan the 3$\textit{σ}$ range of neutrino oscillation parameters and identify the most conservative bounds consistent with existing measurements. Our results reveal that the MEG experiment currently provides the most stringent constraints in the normal ordering (NO) scenario, while the SINDRUM experiment offers comparable sensitivity in the inverted ordering (IO) case. Future experiments, such as MEG II, Mu3e, Mu2e, and COMET, are predicted to significantly improve the sensitivity, testing larger regions of the parameter space.

Yijun Han

Sihui Huang

Zhanhong Lei

Probing type II seesaw leptogenesis through lepton flavor violation * Chengcheng Han acknowledges supports from the National Key R&D Program of China (2023YFA1606100), the Sun Yat-Sen University Science Foundation, the Fundamental Research Funds for the Central Universities at Sun Yat-sen University (24qnpy117) and the Key Laboratory of Particle Astrophysics and Cosmology (MOE) of Shanghai Jiao Tong University

In this work we investigate the muon anomalous magnetic dipole moment <math altimg='si1.svg'><msub><mrow><mi>a</mi></mrow><mrow><mi>&#956;</mi></mrow></msub></math> in a model that extends the Standard Model with a scalar triplet and a lepton triplet. Different from previous studies, we find that there is still viable parameter space in this model to explain the discrepancy <math altimg='si2.svg'><mi mathvariant='normal'>&#916;</mi><msub><mrow><mi>a</mi></mrow><mrow><mi>&#956;</mi></mrow></msub><mo linebreak='goodbreak' linebreakstyle='after'>=</mo><msub><mrow><mi>a</mi></mrow><mrow><mi>&#956;</mi></mrow></msub><mo stretchy='false'>(</mo><mrow><mi mathvariant='normal'>Exp</mi></mrow><mo stretchy='false'>)</mo><mo linebreak='goodbreak' linebreakstyle='after'>&#8722;</mo><msub><mrow><mi>a</mi></mrow><mrow><mi>&#956;</mi></mrow></msub><mo stretchy='false'>(</mo><mrow><mi mathvariant='normal'>SM</mi></mrow><mo stretchy='false'>)</mo></math>. While being consistent with the current data of neutrino mass, electroweak precision measurements and the perturbativity of couplings, our model can provide new physics contribution <math altimg='si128.svg'><msubsup><mrow><mi>a</mi></mrow><mrow><mi>&#956;</mi></mrow><mrow><mtext mathvariant='normal'>NP</mtext></mrow></msubsup></math> to cover the central region of <math altimg='si18.svg'><mi mathvariant='normal'>&#916;</mi><msub><mrow><mi>a</mi></mrow><mrow><mi>&#956;</mi></mrow></msub></math> with new scalar and lepton mass as low as around TeV. This mass scale is allowed by the current collider searches for doubly charged scalars and the lepton triplet, and they can be tested at future high energy and/or high luminosity colliders.

Lei Cai

Shi-Ping He

Peiwen Wu

Hybrid Type-II and Type-III seesaw model for the muon <italic>g</italic>&#8239;&#8722;&#8239;2 anomaly

We investigate the effects of nonthermally produced dark matter on large-scale structure formation, focusing on the gravitino dark matter. Our analysis shows that large-scale structure measurements from the Sloan Digital Sky Survey (SDSS) luminous red galaxy (LRG) data offer a promising approach to probing nonthermally produced gravitinos. Specifically, if gravitinos resulting from neutralino decay constitute a fraction <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>F</mi><mi>dec</mi></msub><mo>=</mo><mn>0.35</mn><mo stretchy="false">(</mo><mn>0.1</mn><mo stretchy="false">)</mo></math> of the dark matter component, the data exclude binolike neutralino with a mass <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><msub><mi>m</mi><msubsup><mi>χ</mi><mn>1</mn><mn>0</mn></msubsup></msub><mo>≲</mo><mn>25</mn><mo stretchy="false">(</mo><mn>11</mn><mo stretchy="false">)</mo><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> at 95% C.L. Furthermore, this constraint appears to be largely independent of the gravitino mass.

Liyuan Guo

Weiyi Deng

Probing nonthermal gravitinos through large-scale structure observations

The self-interacting dark matter (SIDM) paradigm offers a potential solution to the small-scale structure problems faced by collisionless cold dark matter. This framework incorporates self-interactions among dark matter particles, typically mediated by a particle with a MeV-scale mass. Recent evidences of nano-Hertz gravitational waves from pulsar timing arrays (PTAs) such as NANOGrav, CPTA, EPTA, and PPTA suggest the occurrence of a first-order phase transition (FOPT) at a MeV-scale temperature. Considering the close proximity between these two scales, we propose that the mediator mass in the SIDM model originates from the spontaneous breaking of a <math><mi>U</mi><mo>(</mo><mn>1</mn><msup><mo>)</mo><mo>′</mo></msup></math> symmetry, which is driven by the FOPT indicated by PTA data. Consequently, the alignment of these two scales is believed to be deeply connected by the same underlying physics. By extensively exploring the parameter space, remarkably, we find that the parameter space favored by SIDM just provides an explanation for the PTA data.

Ke-Pan Xie

Jin Min Yang

Mengchao Zhang

Self-interacting dark matter implied by nano-Hertz gravitational waves

We explore a scenario of high-scale supersymmetry where all supersymmetric particles except gauginos stay at a high-energy scale <math><msub><mi>M</mi><mi>SUSY</mi></msub></math> that is much larger than the reheating temperature <math><msub><mi>T</mi><mi>RH</mi></msub></math>. The dark matter is dominated by a bino component with mass around the electroweak scale and the observed relic abundance is mainly generated by the freeze-in process during the early Universe. Considering the various constraints, we identify two available scenarios in which the supersymmetric sector at an energy scale below <math><msub><mi>T</mi><mi>RH</mi></msub></math> consists of (a) bino or (b) bino and wino. Typically, for a bino mass around 0.1–1 TeV and a wino mass around 2 TeV, we find that <math><msub><mi>M</mi><mi>SUSY</mi></msub></math> should be around <math><msup><mn>10</mn><mrow><mn>13</mn><mo>−</mo><mn>14</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> with <math><msub><mi>T</mi><mi>RH</mi></msub></math> around <math><msup><mn>10</mn><mrow><mn>4</mn><mo>−</mo><mn>6</mn></mrow></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>.

Freeze-in bino dark matter in high-scale supersymmetry

CERN

Carlotta Accettura

United Kingdom

Dean Adams

United States

Rohit Agarwal

Claudia Ahdida

Italy

Chiara Aimè

Nicola Amapane

David Amorim

Paolo Andreetto

Fabio Anulli

Robert Appleby

Artur Apresyan

Aram Apyan

France

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

Spain

Francesco Celiberto

Luigi Celona

Alessandro Cerri

Gianmario Cesarini

Cari Cesarotti

Portugal

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

Germany

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