Part of the energy created in deuterium-tritium fusion reactors is carried away from plasma by a high-intensity neutron flux, which is then absorbed by the reactor’s inner walls. The neutron flux can be used to sustain the reaction by the following mechanism: the walls are coated with lithium-rich $\textit{breeding blankets}$, in which a fraction of neutrons interacts with lithium, creating tritium, which can be, in turn, used a fuel for the main reaction. The interactions of neutrons with the materials within the breeding blanket can also result in the production of dark sector particles, feebly interacting light scalars or pseudoscalars, via nuclear transitions. We estimate the potential size of such dark sector flux outside the reactor and consider possible detection methods at current and future thermonuclear fusion reactors. In our analysis, we take into account all other current bounds, recasting also the SNO axion bound for a CP even scalar. We find that year-long searches at current and future reactors can set leading constraints on dark scalar- and dark pseudoscalar-nucleon couplings.

Israel

Chaja Baruch

Mexico

Patrick Fitzpatrick

United States

Tony Menzo

Yotam Soreq

Sokratis Trifinopoulos

Jure Zupan

Searching for exotic scalars at fusion reactors

<p>The Cabibbo-Kobayashi-Maskawa (CKM) matrix, which controls flavor mixing between the three generations of quark fermions, is a key input to the standard model of particle physics. In this paper, we identify a surprising connection between quantum entanglement and the degree of quark mixing. Focusing on a specific limit of <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mn>2</mn><mo stretchy="false">→</mo><mn>2</mn></math> quark scattering mediated by electroweak bosons, we find that the quantum entanglement generated by scattering is minimized when the CKM matrix is almost (but not exactly) diagonal, in qualitative agreement with observation. With the discovery of neutrino masses and mixings, additional angles are needed to parametrize the Pontecorvo-Maki-Nakagawa-Sakata (PMNS) matrix in the lepton sector. Applying the same logic, we find that quantum entanglement is minimized when the PMNS matrix features two large angles and a smaller one, again in qualitative agreement with observation, plus a hint for suppressed <math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><mi>C</mi><mi>P</mi></math> violation. We speculate on the (unlikely but tantalizing) possibility that minimization of quantum entanglement might be a fundamental principle that determines particle physics input parameters.</p>

Jesse Thaler

Flavor patterns of fundamental particles from quantum entanglement?

A Muon (Synchrotron) Ion Collider (MuSIC) can be the successor to the Electron-Ion Collider at Brookhaven National Laboratory, as well as the ideal demonstrator facility for a future multi-TeV Muon Collider. Besides its rich nuclear physics and Standard Model particle physics programs, in this work we show that the MuSIC with a TeV-scale muon beam offers also a unique opportunity to probe New Physics. In particular, the relevant searches have the potential to surpass current experimental limits and explore new regimes of the parameter space for a variety of Beyond the Standard Model scenarios including: lepton-flavor violating leptoquarks, muonphilic vector boson interactions, axion-like particles coupling to photons, and heavy sterile neutrinos. Depending on the particular case, the sensitivity of the searches in the MuSIC may span a wide range of energy scales, namely from sub-GeV particles to the few TeV New Physics mediators. Our analysis demonstrates that the MuSIC can strike a powerful chord in the search for New Physics, thanks to unique combination of features that amplify its capabilities.

Hooman Davoudiasl

Hongkai Liu

Roman Marcarelli

New physics at the Muon (Synchrotron) Ion Collider: MuSIC for several scales

The so-called metastability bound asserts that an unnaturally small Higgs mass is a necessary condition for electroweak vacuum metastability, offering a new approach toward solving the hierarchy problem. So far, this result relies on the assumption of a negative Higgs mass parameter, or equivalently, on electroweak spontaneous symmetry breaking. We derive a new, corresponding bound for the case of a positive mass parameter. When the new bound is significantly more restrictive than or comparable to its established counterpart, it may offer an explanation for the sign of the Higgs mass parameter, and thus, spontaneous symmetry breaking itself. New physics at scales <math><mrow><mi>O</mi><mo>(</mo><mn>1</mn><mi>–</mi><mn>10</mn><mo>)</mo><mtext> </mtext><mtext> </mtext><mi>TeV</mi></mrow></math> can lower these bounds as far as the TeV scale. As an illustration, we consider vacuum stability in the presence of additional TeV-scale fermions with Yukawa couplings to the Higgs, as well as a dimension-six term parametrizing new physics in the UV. This scenario requires new physics that couples strongly to the Higgs, and can potentially be probed at future colliders. Finally, to allow for comparison with concrete mechanisms predicting metastability, we provide the mass-dependent lifetime of the electroweak vacuum for this model.

Sean Benevedes

Thomas Steingasser

Spontaneous symmetry breaking, gauge hierarchy, and electroweak vacuum metastability

CERN

Carlotta Accettura

United Kingdom

Dean Adams

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

The emission of collinear radiation off an elementary lepton can be factorised from the hard scattering process by introducing Parton Distribution Functions of a Lepton (LePDF), which, contrary to protons, can be derived from first principles. In case of multi-TeV lepton colliders, such as the muon colliders currently being proposed, the complete structure of Standard Model interactions must be taken into account. In this work we solve numerically the corresponding DGLAP equations at the double-log order and provide public files with LePDFs for both muons and electrons, including polarisation effects. We discuss several interesting aspects of the resulting PDFs and compare them with the Effective Vector Approximation, showing that the latter fails to describe well the vector bosons PDFs at small momentum fractions, unless it is extended to higher orders.

Francesco Garosi

David Marzocca

LePDF: Standard Model PDFs for high-energy lepton colliders

The combination of precise determinations of <math><msub><mi>V</mi><mrow><mi>u</mi><mi>s</mi></mrow></msub></math> and <math><msub><mi>V</mi><mrow><mi>u</mi><mi>d</mi></mrow></msub></math> hints toward a violation of the Cabibbo-Kobayashi-Maskawa matrix first row unitarity at about <math><mn>3</mn><mi>σ</mi></math> level. Conversely, the recent measurement of the <math><mi>W</mi></math>-boson mass by the CDF Collaboration exhibits significant tension with the Standard Model prediction, intensifying the conflict that may arise in models addressing the unitarity violation. We demonstrate that one vectorlike <math><mi>S</mi><mi>U</mi><mo>(</mo><mn>2</mn><msub><mo>)</mo><mi>L</mi></msub></math> doublet with mass of a few TeV mixing with light quarks and the top quark can simultaneously account for the two anomalies, without conflicting with flavor-changing phenomena and electroweak observables. Moreover, another tension in the value of the Cabibbo angle is also reported at the <math><mn>3</mn><mi>σ</mi></math> level, between two determinations of <math><msub><mi>V</mi><mrow><mi>u</mi><mi>s</mi></mrow></msub></math> obtained from semileptonic <math><mi>K</mi><mo>ℓ</mo><mn>3</mn></math> and leptonic <math><mi>K</mi><mi>μ</mi><mn>2</mn></math> kaon decays. We show that the vectorlike doublet can be at the origin of this discrepancy and the substantial positive shift in the <math><mi>W</mi></math>-boson mass. This unique feature of the vectorlike quark doublet may render it a crucial puzzle piece in new physics scenarios addressing the Cabibbo-Kobayashi-Maskawa matrix unitarity problem. The model can be potentially probed in future colliders.

Benedetta Belfatto

Cabibbo angle anomalies and oblique corrections: The remarkable role of the vectorlike quark doublet

We propose a new mechanism to dynamically select the electroweak scale during inflation. An axionlike field <math><mi>ϕ</mi></math> that couples quadratically to the Higgs moves with a large initial velocity towards a critical point <math><msub><mi>ϕ</mi><mi>c</mi></msub></math> where the Higgs becomes massless. When <math><mi>ϕ</mi></math> crosses this point, it enters a region where the Higgs mass is tachyonic and this results into an explosive production of Higgs particles. Consequently, a backreaction potential is generated and the field <math><mi>ϕ</mi></math> is attracted back to <math><msub><mi>ϕ</mi><mi>c</mi></msub></math>. After a series of oscillations around this point it is eventually trapped in its vicinity due to the periodic term of the potential. The model avoids trans-Planckian field excursions, requires very few <math><mi>e</mi></math>-folds of inflation and it is compatible with inflation scales up to <math><msup><mn>10</mn><mn>5</mn></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>. The mass of <math><mi>ϕ</mi></math> lies in the range of hundreds of GeV to a few TeV and it can be potentially probed in future colliders.

Belgium

Miguel Vanvlasselaer

Attracting the electroweak scale to a tachyonic trap

Rare flavour-changing neutral-current transitions b → sμ + μ  −  probe higher energy scales than what is directly accessible at the LHC. Therefore, the presence of new physics in such transitions, as suggested by the present-day LHCb anomalies, would have a major impact on the motivation and planning of future high-energy colliders. The two most prominent options currently debated are a proton-proton collider at 100 TeV (FCC-hh) and a multi-TeV muon collider (MuC). In this work, we compare the discovery prospects at these colliders on benchmark new physics models indirectly detectable in b → sμ + μ  −  decays but beyond the reach of the high-p  T  searches at the HL-LHC. We consider a comprehensive set of scenarios: semileptonic contact interactions, Z′ from a gauged   <math> <mi>U</mi> <msub> <mfenced> <mn>1</mn> </mfenced> <mrow> <msub> <mi>B</mi> <mn>3</mn> </msub> <mo>−</mo> <msub> <mi>L</mi> <mi>μ</mi> </msub> </mrow> </msub> <mspace width="0.25em"></mspace> </math>  $$ \textrm{U}{(1)}_{B_3-{L}_{\mu }} $$  and   <math> <mi>U</mi> <msub> <mfenced> <mn>1</mn> </mfenced> <mrow> <msub> <mi>L</mi> <mi>μ</mi> </msub> <mo>−</mo> <msub> <mi>L</mi> <mi>τ</mi> </msub> </mrow> </msub> </math>  $$ \textrm{U}{(1)}_{L_{\mu }-{L}_{\tau }} $$ , the scalar leptoquark S 3, and the vector leptoquark U 1. We find that a 3 TeV MuC has a sensitivity reach comparable to the one of the FCC-hh. However, for a heavy enough mediator, the new physics effects at a 3 TeV MuC are only observed indirectly via deviations in the highest energy bin, while the FCC-hh has a greater potential for the discovery of a resonance. Finally, to completely cover the parameter space suggested by the bsμμ anomalies, among the proposed future colliders, only a MuC of 10 TeV (or higher) can meet the challenge.

Admir Greljo

Jakub Salko

New physics in b → sμμ: FCC-hh or a muon collider?