The Muon Collider is one of the most promising future collider facilities with the potential to reach multi-TeV center-of-mass energy and high luminosity. Due to the significant Higgs boson production cross section in muon-antimuon collisions at such high energies, the collider offers an excellent opportunity for in-depth exploration of Higgs boson properties. It holds the capability to significantly advance our understanding of the Higgs sector to a very high level of precision. However, the presence of beam-induced background resulting from the decay of the beam muons poses unique challenges for detector development and event reconstruction. In this paper, the prospects for measuring various Higgs boson properties at a center-of-mass energy of 3 TeV are presented, using a detailed detector simulation in a realistic environment. The study demonstrates the feasibility of achieving high precision results with the current state-of-the-art detector design. In addition, the paper discusses the detector requirements necessary to achieve this level of accuracy.

Italy

Paolo Andreetto

Nazar Bartosik

Laura Buonincontri

Switzerland

Daniele Calzolari

Vieri Candelise

Massimo Casarsa

Luca Castelli

Mauro Chiesa

Anna Colaleo

Portugal

Giacomo Da Molin

United States

Matthew Forslund

Luca Giambastiani

Alessio Gianelle

Carlo Giraldin

United Kingdom

Karol Krizka

Sergo Jindariani

Anton Lechner

Donatella Lucchesi

Leo Mareso

Belgium

Paola Mastrapasqua

Patrick Meade

Sweden

Alessandro Montella

Simone Pagan Griso

Leonardo Palombini

Nadia Pastrone

Lorenzo Sestini

Rosamaria Venditti

Angela Zaza

Davide Zuliani

Aspects of Higgs Physics at a $$\sqrt{s}=3$$ TeV Muon Collider with detailed detector simulation

The coupling of the Higgs boson to first and second generation fermions has yet to be measured experimentally. There still could be very large deviations in these couplings, as the origin of flavor is completely unknown. Nevertheless, if Yukawa couplings are modified, especially for light generations, there are generically strong constraints from flavor-changing neutral currents (FCNCs). Therefore, it is imperative to understand whether there exists viable UV physics consistent with current data that motivates future Higgs coupling probes. In particular, the charm-quark Yukawa is the next quark coupling that could be measured at the LHC $\textit{if}$ it is a few times larger than the SM and compatible with flavor data. This is difficult to achieve in the context of standard ansatz such as Minimal Flavor Violation. In this paper we show that within the framework of Spontaneous Flavor Violation (SFV), using a Two Higgs Doublet Model as an example, the Higgs can be sufficiently charming that new LHC probes are relevant. In this charming region, we show that new Higgs states near the EW scale with large couplings to quarks are required, providing complementary observables or new constraints on the SM Yukawa couplings. The down-type SFV mechanism enabling the suppression of FCNCs also allows for independent modifications to the up-quark Yukawa coupling, which we explore in detail as well.

Artemis Giannakopoulou

Mauro Valli

How charming can the Higgs be?

The interpretation of Higgs data is typically based on different assumptions about whether there can be additional decay modes of the Higgs or if any couplings can be bounded by theoretical arguments. Going beyond these assumptions requires either a precision measurement of the Higgs width or an absolute measurement of a coupling to eliminate a flat direction in precision fits that occurs when   <math> <mfenced> <mrow> <msub> <mi>g</mi> <mi>hVV</mi> </msub> <mo>/</mo> <msubsup> <mi>g</mi> <mi>hVV</mi> <mi>SM</mi> </msubsup> </mrow> </mfenced> </math>  $$ \left|{g}_{hVV}/{g}_{hVV}^{SM}\right| $$  &gt; 1, where V = W  ± , Z. In this paper we explore how well a high energy muon collider can test Higgs physics without having to make assumptions on the total width of the Higgs. In particular, we investigate off-shell methods for Higgs production used at the LHC and searches for invisible decays of the Higgs to see how powerful they are at a muon collider. We then investigate the theoretical requirements on a model which can exist in such a flat direction. Combining expected Higgs precision with other constraints, the most dangerous flat direction is described by generalized Georgi-Machacek models. We find that by combining direct searches with Higgs precision, a high energy muon collider can robustly test single Higgs precision down to the   <math> <mi>O</mi> <mfenced> <mrow> <mn>.1</mn> <mo>%</mo> </mrow> </mfenced> </math>  $$ \mathcal{O}\left(.1\%\right) $$  level without having to assume SM Higgs decays. Furthermore, it allows one to bound new contributions to the width at the sub-percent level as well. Finally, we comment on how even in this difficult flat direction for Higgs precision, a muon collider can robustly test or discover new physics in multiple ways. Expanding beyond simple coupling modifiers/EFTs, there is a large region of parameter space that muon colliders can explore for EWSB that is not probed with only standard Higgs precision observables.

Precision Higgs width and couplings with a high energy muon collider

CERN

Carlotta Accettura

Dean Adams

Rohit Agarwal

Claudia Ahdida

Chiara Aimè

Nicola Amapane

David Amorim

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

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

Philip Burrows

Graeme Burt

Dario Buttazzo

Barbara Caiffi

Marco Calviani

Simone Calzaferri

Rodolfo Capdevilla

Christian Carli

Fausto Casaburo

Maria Catanesi

Lorenzo Cavallucci

Gianluca Cavoto

Spain

Francesco Celiberto

Luigi Celona

Alessandro Cerri

Gianmario Cesarini

Cari Cesarotti

Grigorios Chachamis

Antoine Chance

Siyu Chen

Yang-Ting Chien

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

Muon colliders are an exciting possibility for reaching the highest energies possible on the shortest timescale. They potentially combine the greatest strengths of e + e  −  and pp colliders by bridging the energy versus precision dichotomy. In this paper we study the sensitivity of Higgs properties that can be achieved with a future 3 or 10 TeV muon collider from single Higgs production. The results presented here represent the first comprehensive picture for the precision achievable including backgrounds and using fast detector simulation with Delphes. Additionally, we compare the results of fast detector simulation with available full simulation studies that include the muon collider specific Beam Induced Background, and show the results are largely unchanged. We comment on some of the strengths and weaknesses of a high energy muon collider for Higgs physics alone, and demonstrate the complementarity of such a collider with the LHC and e + e  −  Higgs factories. Furthermore, we discuss some of the exciting avenues for improving future results from both theoretical and detector R&amp;D that could be undertaken.

High precision higgs from high energy muon colliders

We demonstrate that multiple-Higgs production at the LHC is the most sensitive probe of first- and second-generation quark flavor in the Higgs sector. In models where new scalars couple to light quarks, gigantic di-Higgs and even sizable tri-Higgs production rates can be obtained, which can be used to either discover or severely constrain such theories. As an example, we show that the most stringent bounds on enhanced interactions of the 125 GeV Higgs to the down quark in extended Higgs sectors are obtained by looking for the extra Higgs bosons that provide for such enhancements using the di-Higgs and <math><mi>Z</mi><mi>h</mi></math> topologies. In this context, we set new limits on the 125 GeV Higgs coupling to the down quark as strong as <math><msub><mi>λ</mi><mrow><mi>h</mi><mi>d</mi><mover><mi>d</mi><mo>¯</mo></mover></mrow></msub><mo>≲</mo><mn>20</mn><msubsup><mi>λ</mi><mrow><mi>h</mi><mi>d</mi><mover><mi>d</mi><mo>¯</mo></mover></mrow><mi>SM</mi></msubsup></math>—a dramatic improvement over previously available bounds. Regarding second-generation quark flavor, we obtain new limits in the coupling to strange as strong as <math><msub><mi>λ</mi><mrow><mi>h</mi><mi>s</mi><mover><mi>s</mi><mo>¯</mo></mover></mrow></msub><mo>≲</mo><mn>10</mn><msubsup><mi>λ</mi><mrow><mi>h</mi><mi>s</mi><mover><mi>s</mi><mo>¯</mo></mover></mrow><mi>SM</mi></msubsup></math>. In addition, we show that the currently unexplored triple-Higgs production topology could be a potential discovery channel of a wide variety of extended Higgs sectors at the LHC, including not only models where extra Higgses couple to light quarks, but also more frequently studied theories where they have preferential couplings to the top.

Daniel Egana-Ugrinovic

Samuel Homiller

Multi-Higgs boson production probes Higgs sector flavor

We show that the recent excess presented by KOTO in their search for <math><msub><mi>K</mi><mi>L</mi></msub><mo>→</mo><msup><mi>π</mi><mn>0</mn></msup><mi>ν</mi><mover><mi>ν</mi><mo>¯</mo></mover></math> may be due to weakly coupled scalars produced from Kaon decays. We study two concrete realizations, the minimal Higgs portal and a hadrophilic scalar model, and demonstrate that they can explain the observed events while satisfying existing limits. The simplicity of these models, and their possible relations to interesting UV constructions, provides strong theoretical motivation for a new physics interpretation of the KOTO data.

Light Scalars and the Koto Anomaly

A common lore has arisen that beyond the Standard Model (BSM) particles, which can be searched for at current and proposed experiments, should have flavorless or mostly third-generation interactions with Standard Model quarks. This theoretical bias severely limits the exploration of BSM phenomenology and is especially constraining for extended Higgs sectors. Such limitations can be avoided in the context of spontaneous flavor violation (SFV), a robust and UV complete framework that allows for significant couplings to any up- or down-type quark, while suppressing flavor-changing neutral currents via flavor alignment. In this work we study the theory and phenomenology of extended SFV Higgs sectors with large couplings to any quark generation. We perform a comprehensive analysis of flavor and collider constraints of extended SFV Higgs sectors and demonstrate that new Higgs bosons with large couplings to the light quarks may be found at the electroweak scale. In particular, we find that new Higgses as light as 100 GeV with order <math><mo>∼</mo><mn>0.1</mn></math> couplings to first- or second-generation quarks, which are copiously produced at the LHC via quark fusion, are allowed by current constraints. Furthermore, the additional SFV Higgses can mix with the SM Higgs, providing strong theory motivation for an experimental program looking for deviations in the light quark–Higgs couplings. Our work demonstrates the importance of exploring BSM physics coupled preferentially to light quarks and the need to further develop dedicated experimental techniques for the LHC and future colliders.

Higgs bosons with large couplings to light quarks

We present a systematic spurion setup called aligned flavor violation (AFV) that allows for new physics couplings to quarks that are aligned with standard model (SM) Yukawa couplings, but do not necessarily share their hierarchies nor are family universal. Additionally, we show that there is an important subset of AFV called spontaneous flavor violation (SFV), which naturally arises from UV completions where the quark family number and <math><mrow><mi>C</mi><mi>P</mi></mrow></math> groups are spontaneously broken. Flavor-changing neutral currents are strongly suppressed in SFV extensions of the SM. We study SFV from an effective field theory perspective and demonstrate that SFV new physics with significant and preferential couplings to first or second generation quarks may be close to the TeV scale.

Aligned and Spontaneous Flavor Violation