We discuss the landscape of flavor physics at the Circular Electron-Positron Collider (CEPC), based on the nominal luminosity outlined in its Technical Design Report. The CEPC is designed to operate in multiple modes to address a variety of tasks. At the $\textit{Z}$ pole, the expected production of 4 Tera $\textit{Z}$ bosons will provide unique and highly precise measurements of $\textit{Z}$ boson couplings, while the substantial number of boosted heavy-flavored quarks and leptons produced in clean $\textit{Z}$ decays will facilitate investigations into their flavor physics with unprecedented precision. We investigate the prospects of measuring various physics benchmarks and discuss their implications for particle theories and phenomenological models. Our studies indicate that, with its highlighted advantages and anticipated excellent detector performance, the CEPC can explore beauty and $\textit{τ}$ physics in ways that are superior to or complementary with the Belle II and Large-Hadron-Collider-beauty experiments, potentially enabling the detection of new physics at energy scales of 10 TeV and above. This potential also extends to the observation of yet-to-be-discovered rare and exotic processes, as well as testing fundamental principles such as lepton flavor universality, lepton and baryon number conservation, etc., making the CEPC a vibrant platform for flavor physics research. The $\textit{WW}$ threshold scan, Higgs-factory operation and top-pair productions of the CEPC further enhance its merits in this regard, especially for measuring the Cabibbo-Kobayashi-Maskawa matrix elements, and Flavor-Changing-Neutral-Current physics of Higgs boson and top quarks. We outline the requirements for detector performance and considerations for future development to achieve the anticipated scientific goals. The role of machine learning for innovative detector design and advanced reconstruction algorithms is also stressed. The CEPC flavor physics program not only develops new capabilities for exploring flavor physics beyond existing projects but also enriches the physics opportunities of this machine. It should be remarked that, given the richness of the CEPC flavor physics, this manuscript is not meant to be a comprehensive survey, but rather an investigation of representative cases. Uncovering the full potential of flavor physics at the CEPC will require further dedicated explorations in the future.

China

Xiaocong Ai

United States

Wolfgang Altmannshofer

Peter Athron

Xiaozhi Bai

Lorenzo Calibbi

Germany

Lu Cao

Yuzhi Che

Chunhui Chen

Ji-Yuan Chen

Long Chen

Mingshui Chen

Shanzhen Chen

Xuan Chen

Shan Cheng

Cheng-Wei Chiang

Switzerland

Andreas Crivellin

Hanhua Cui

France

Olivier Deschamps

Sébastien Descotes-Genon

Xiaokang Du

Shuangshi Fang

Yu Gao

Yuanning Gao

Li-Sheng Geng

Pablo Goldenzweig

Jiayin Gu

Feng-Kun Guo

Yuchen Guo

Zhi-Hui Guo

Tao Han

Hong-Jian He

Jibo He

Miao He

Xiaogang He

Yanping Huang

Gino Isidori

Quan Ji

Jianfeng Jiang

Xu-Hui Jiang

Slovenia

Jernej F. Kamenik

Tsz Hong Kwok

Gang Li

Geng Li

Haibo Li

Haitao Li

Hengne Li

Honglei Li

Liang Li

Lingfeng Li

Qiang Li

Shu Li

Xiaomei Li

Xin-Qiang Li

Yiming Li

Yubo Li

Yuji Li

Zhao Li

Hao Liang

Zhijun Liang

Libo Liao

Zoltan Ligeti

Jia Liu

Jianbei Liu

Tao Liu

Yi Liu

Yong Liu

Zhen Liu

Xinchou Lou

Peng-Cheng Lu

Italy

Alberto Lusiani

Hong-Hao Ma

Kai Ma

Farvah Mahmoudi

Yajun Mao

Yaxian Mao

David Marzocca

Juan-Juan Niu

Soeren Prell

Huirong Qi

Sen Qian

Zhuoni Qian

Qin Qin

Ariel Rock

Jonathan L. Rosner

Manqi Ruan

Dingyu Shao

Chengping Shen

Xiaoyan Shen

Haoyu Shi

United Kingdom

Liaoshan Shi

Zong-Guo Si

Cristian Sierra

Huayang Song

Shufang Su

Wei Su

Zhijia Sun

Michele Tammaro

Dayong Wang

En Wang

Flavor physics at the CEPC: a general perspective * We acknowledge financial support from the National Natural Science Foundation of China (NSFC) (12125507, 12047503, 12035008, 2211530479, 12475094, 12135006, 12075097, 12375086, 2022YFA1601903, 12061141007, 12375091, 12342502, 12235018, 12335003, 12105100, 12475106, 11961141015, 12188102, 12175245, 12205171, 12321005, tsqn202312052, 2024HWYQ-005, 12405121, 12447167, 12061141006, 12405102, 12125503,12305115, 12075213, 12335005, 12235008); the Chinese Academy of Sciences (YSBR-101, XDB34030000); the National Key R&D Program of China (2022YFE0116900, 2023YFA1606703, 2022YFA1601901); the National Key Research and Development Program of China (2023YFA1606300); the Excellent Postdoctoral Program of Jiangsu Province (2023ZB891); the Shenzhen Science and Technology Program (202206193000001, 20220816094256002); the Natural Science Foundation for Distinguished Young Scholars of Henan Province (242300421046); the Beijing Municipal Natural Science Foundation (JQ22002); the Area of Excellence (AoE/P-404/18-3) and the General Research Fund (16304321) (both grants are issued by the Research Grants Council of Hong Kong S.A.R); the MOST National Key R&D Program (2023YFA1606303); the Shanghai Key Laboratory for Particle Physics and Cosmology, Key Laboratory for Particle Astrophysics and Cosmology (Ministry of Education), Shanghai Jiao Tong University

Neutrinophilic mediators are well-motivated messenger particles that can probe some of the least known sectors of fundamental physics involving nonstandard interactions of neutrinos with themselves and potentially with dark matter. In particular, light mediators coupling to the active neutrinos will induce new decay modes of the Standard Model mesons (e.g., <math altimg='si1.svg'><msup><mrow><mi>&#960;</mi></mrow><mrow><mo>&#177;</mo></mrow></msup><mo>,</mo><msup><mrow><mi>K</mi></mrow><mrow><mo>&#177;</mo></mrow></msup><mo stretchy='false'>&#8594;</mo><msup><mrow><mi>&#8467;</mi></mrow><mrow><mo>&#177;</mo></mrow></msup><mo linebreak='goodbreak' linebreakstyle='after'>+</mo><mover accent='true'><mi>&#957;</mi><mrow><mo stretchy='false'>(</mo><mo linebreak='badbreak' linebreakstyle='after'>&#8722;</mo><mo stretchy='false'>)</mo></mrow></mover><mo linebreak='goodbreak' linebreakstyle='after'>+</mo><mi>&#981;</mi></math>), charged leptons (e.g., <math altimg='si18.svg'><msup><mrow><mi>&#964;</mi></mrow><mrow><mo>&#177;</mo></mrow></msup><mo stretchy='false'>&#8594;</mo><msup><mrow><mi>&#960;</mi></mrow><mrow><mo>&#177;</mo></mrow></msup><mo linebreak='goodbreak' linebreakstyle='after'>+</mo><mover accent='true'><mi>&#957;</mi><mrow><mo stretchy='false'>(</mo><mo linebreak='badbreak' linebreakstyle='after'>&#8722;</mo><mo stretchy='false'>)</mo></mrow></mover><mo linebreak='goodbreak' linebreakstyle='after'>+</mo><mi>&#981;</mi></math>), and gauge bosons (e.g., <math altimg='si19.svg'><mi>Z</mi><mo stretchy='false'>&#8594;</mo><mi>&#957;</mi><mo linebreak='goodbreak' linebreakstyle='after'>+</mo><mover accent='true'><mrow><mi>&#957;</mi></mrow><mrow><mo stretchy='false'>&#175;</mo></mrow></mover><mo linebreak='goodbreak' linebreakstyle='after'>+</mo><mi>&#981;</mi></math>). A common lore is that these decays suffer from infrared divergences in the limit of the vanishing mediator mass, i.e., <math altimg='si107.svg'><msub><mrow><mi>m</mi></mrow><mrow><mi>&#981;</mi></mrow></msub><mo stretchy='false'>&#8594;</mo><mn>0</mn></math>. Here, we show that including the 1-loop contributions of these mediators to the standard 2-body decays (e.g., <math altimg='si5.svg'><msup><mrow><mi>&#960;</mi></mrow><mrow><mo>&#177;</mo></mrow></msup><mo>,</mo><mspace width='0.2em' /><msup><mrow><mi>K</mi></mrow><mrow><mo>&#177;</mo></mrow></msup><mo stretchy='false'>&#8594;</mo><msup><mrow><mi>&#8467;</mi></mrow><mrow><mo>&#177;</mo></mrow></msup><mo linebreak='goodbreak' linebreakstyle='after'>+</mo><mover accent='true'><mi>&#957;</mi><mrow><mo stretchy='false'>(</mo><mo linebreak='badbreak' linebreakstyle='after'>&#8722;</mo><mo stretchy='false'>)</mo></mrow></mover></math>, etc.), the infrared divergence from the 3-body decay cancels out exactly by virtue of the Kinoshita&#8211;Lee&#8211;Nauenberg theorem. Including these cancellation effects, we then update the existing laboratory constraints on neutrinophilic scalar mediators, thereby extending the limits far beyond the decaying parent particle mass and excluding a wider range of parameter space. These new &#8220;physical&#8221; limits derived here have significant implications for the future detection prospects of nonstandard neutrino (self-)interactions.

P.S. Bhupal Dev

Doojin Kim

Deepak Sathyan

Kuver Sinha

Yongchao Zhang

New laboratory constraints on neutrinophilic mediators

We use multimessenger observations of the neutron star merger event GW170817 to derive new constraints on axionlike particles (ALPs) coupling to photons. ALPs are produced via Primakoff and photon coalescence processes in the merger, escape the remnant, and decay back into two photons, giving rise to a photon signal approximately along the line of sight to the merger. We analyze the spectral and temporal information of the ALP-induced photon signal and use the Fermi Large Area Telescope (Fermi-LAT) observations of GW170817 to derive our new ALP constraints. We also show the improved prospects with future MeV <math><mi>γ</mi></math>-ray missions, taking the spectral and temporal coverage of Fermi-LAT as an example.

P. S. Bhupal Dev

Canada

Jean-François Fortin

Steven P. Harris

First Constraints on the Photon Coupling of Axionlike Particles from Multimessenger Studies of the Neutron Star Merger GW170817

We propose a simple interpretation of the γγ excesses reported by both CMS and ATLAS groups at 95 GeV together with the LEP excess in the <math><mi>Z</mi><mi>b</mi><mover><mrow><mi>b</mi></mrow><mrow><mo>¯</mo></mrow></mover></math> channel around the same mass in terms of a neutral scalar field in the minimal left-right symmetric model (LRSM). We point out that the scalar field which implements the seesaw mechanism for neutrino masses has all the right properties to explain these observations, without introducing any extra fields. The key point is that this scalar particle is hardly constrained because it couples only to heavy right-handed particles. As a result, the diphoton decay mode receives contributions from both mixing with the Standard Model (SM) Higgs and the heavy charged bosons in the LRSM. The latter depends on the <math><mi>S</mi><mi>U</mi><msub><mrow><mo>(</mo><mn>2</mn><mo>)</mo></mrow><mrow><mi>R</mi></mrow></msub><mo>×</mo><mi>U</mi><msub><mrow><mo>(</mo><mn>1</mn><mo>)</mo></mrow><mrow><mi>B</mi><mo>−</mo><mi>L</mi></mrow></msub></math> symmetry breaking scale <math><msub><mrow><mi>v</mi></mrow><mrow><mi>R</mi></mrow></msub></math>. The complete allowed parameter space for explaining the 95 GeV excesses in this model can be probed with the high-precision measurements of the SM Higgs mixing with other scalars at the high-luminosity LHC and future Higgs factories.

Rabindra N. Mohapatra

Explanation of the 95 GeV And Excesses in the Minimal Left-right Symmetric Model

The recent astrophysical transient Swift <math><mrow><mi>J</mi><mn>1913.1</mn><mo>+</mo><mn>1946</mn></mrow></math> may be associated with the <math><mi>γ</mi></math>-ray burst GRB 221009A. The redshift of this event is <math><mi>z</mi><mo>≃</mo><mn>0.151</mn></math>. Very high-energy <math><mi>γ</mi></math> rays (up to 18 TeV) followed the transient and were observed by the Large High Altitude Air Shower Observatory (LHAASO); additionally, Carpet-2 detected a photonlike air shower of 251 TeV. Photons of such high energy are expected to readily annihilate with the diffuse extragalactic background light (EBL) before reaching Earth. If the <math><mi>γ</mi></math>-ray identification and redshift measurements are correct, new physics could be necessary to explain these measurements. This paper provides the first <math><mi>C</mi><mi>P</mi></math>-even scalar explanation of the most energetic 18 TeV event reported by LHAASO. In this minimal scenario, the light scalar singlet <math><mi>S</mi></math> mixes with the Standard Model Higgs boson <math><mi>h</mi></math>. The highly boosted <math><mi>S</mi></math> particles are produced in the GRB and then undergo the radiative decay diphoton <math><mi>S</mi><mo>→</mo><mi>γ</mi><mi>γ</mi></math> while propagating to Earth. The resulting photons may thus be produced at a remote region without being nullified by the EBL. Hence, the usual exponential reduction of <math><mi>γ</mi></math> rays is lifted due to an attenuation that is inverse in the optical depth, which becomes much larger due to the scalar carriers.

Shyam Balaji

Japan

Mexico

Maura E. Ramirez-Quezada

Joseph Silk

Light scalar explanation for the 18 TeV GRB 221009A

We address a method of limiting neutron–mirror neutron mixing ( $$\epsilon _{nn'}$$  <math> <msub> <mi>ϵ</mi> <mrow> <mi>n</mi> <msup> <mi>n</mi> <mo>′</mo> </msup> </mrow> </msub> </math>  ) by analyzing its effect on neutron star (NS) heating. This method employs observational bounds on the surface temperature of NSs to constrain  $$\epsilon _{nn'}$$  <math> <msub> <mi>ϵ</mi> <mrow> <mi>n</mi> <msup> <mi>n</mi> <mo>′</mo> </msup> </mrow> </msub> </math>  . It has been suggested that the bound obtained this way is so stringent that it would exclude any discovery of  $$n-n'$$  <math> <mrow> <mi>n</mi> <mo>-</mo> <msup> <mi>n</mi> <mo>′</mo> </msup> </mrow> </math>   oscillation in the currently planned terrestrial experiments at various laboratories. This conclusion motivated us to critically analyze this suggestion in more detail. In this note, we point out a very interesting new effect present in nearly exact mirror models, which can significantly affect this bound. The new element is that in nearly exact mirror models there is the mirror analog of  $$\beta $$  <math> <mi>β</mi> </math>   decay, i.e.  $$n' \rightarrow p' + e' + {\bar{\nu }}'_e$$  <math> <mrow> <msup> <mi>n</mi> <mo>′</mo> </msup> <mo>→</mo> <msup> <mi>p</mi> <mo>′</mo> </msup> <mo>+</mo> <msup> <mi>e</mi> <mo>′</mo> </msup> <mo>+</mo> <msubsup> <mrow> <mover> <mrow> <mi>ν</mi> </mrow> <mrow> <mo>¯</mo> </mrow> </mover> </mrow> <mi>e</mi> <mo>′</mo> </msubsup> </mrow> </math>  , which creates a cloud of mirror particles  $$n'$$  <math> <msup> <mi>n</mi> <mo>′</mo> </msup> </math>  ,  $$p'$$  <math> <msup> <mi>p</mi> <mo>′</mo> </msup> </math>  ,  $$e'$$  <math> <msup> <mi>e</mi> <mo>′</mo> </msup> </math>  ,  $$D'$$  <math> <msup> <mi>D</mi> <mo>′</mo> </msup> </math>   and He $$'$$  <math> <msup> <mrow></mrow> <mo>′</mo> </msup> </math>   inside the NS. The resulting  $$e'$$  <math> <msup> <mi>e</mi> <mo>′</mo> </msup> </math>   can “rob” the energy generated by the  $$n \rightarrow n'$$  <math> <mrow> <mi>n</mi> <mo>→</mo> <msup> <mi>n</mi> <mo>′</mo> </msup> </mrow> </math>   transition from the NS, via  $$e-e'$$  <math> <mrow> <mi>e</mi> <mo>-</mo> <msup> <mi>e</mi> <mo>′</mo> </msup> </mrow> </math>   scattering enabled by the presence of a (minute) millicharge in mirror particles. Such a tiny millicharge on mirror particles is highly likely in these models. This results in energy being emitted as unobserved mirror photons via fast mirror bremsstrahlung. whose effect is to relax the stringent bounds on  $$\epsilon _{nn'}$$  <math> <msub> <mi>ϵ</mi> <mrow> <mi>n</mi> <msup> <mi>n</mi> <mo>′</mo> </msup> </mrow> </msub> </math>  .

Israel

Itzhak Goldman

Rabindra Mohapatra

Shmuel Nussinov

Constraints on neutron–mirror-neutron oscillation from neutron star cooling

In this work, by using the machine learning methods, we study the sensitivities of heavy pseudo-Dirac neutrino N in the inverse seesaw at the high-energy hadron colliders. The production process for the signal is pp → ℓ → 3ℓ +   <math> <msubsup> <mi>E</mi> <mi>T</mi> <mtext>miss</mtext> </msubsup> </math>  $$ {E}_T^{\mathrm{miss}} $$ , while the dominant background is pp → WZ → 3ℓ +   <math> <msubsup> <mi>E</mi> <mi>T</mi> <mtext>miss</mtext> </msubsup> </math>  $$ {E}_T^{\mathrm{miss}} $$ . We use either the Multi-Layer Perceptron or the Boosted Decision Tree with Gradient Boosting to analyse the kinematic observables and optimize the discrimination of background and signal events. It is found that the reconstructed Z boson mass and heavy neutrino mass from the charged leptons and missing transverse energy play crucial roles in separating the signal from backgrounds. The prospects of heavy-light neutrino mixing |V  ℓN  | 2 (with ℓ = e, μ) are estimated by using machine learning at the hadron colliders with   <math> <msqrt> <mi>s</mi> </msqrt> </math>  $$ \sqrt{s} $$  = 14 TeV, 27 TeV, and 100 TeV, and it is found that |V  ℓN  | 2 can be improved up to   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (10 −6) for heavy neutrino mass m  N  = 100 GeV and   <math> <mi>O</mi> </math>  $$ \mathcal{O} $$ (10 −4) for m  N  = 1 TeV.

Jie Feng

Mingqiu Li

Qi-Shu Yan

Yu-Pan Zeng

Hong-Hao Zhang

Zhijie Zhao

Improving heavy Dirac neutrino prospects at future hadron colliders using machine learning

It was pointed out in a recent paper that the observed cooling rate of old, cold neutron stars (NS) can provide an upper limit on the transition rate of neutron to mirror neutron (<math><mrow><mi>n</mi><mo>−</mo><msup><mrow><mi>n</mi></mrow><mrow><mo>′</mo></mrow></msup></mrow></math>). This limit is so stringent that it would preclude any discovery of <math><mi>n</mi><mo>→</mo><msup><mi>n</mi><mo>′</mo></msup></math> oscillation in the current round of terrestrial searches for the process. Motivated by this crucially important conclusion, we critically analyze this suggestion and note an interesting new effect present in nearly exact mirror models for <math><mi>n</mi><mo>→</mo><msup><mi>n</mi><mo>′</mo></msup></math> oscillation, which significantly affects this bound. The new element is the <math><mi>β</mi></math> decay <math><mrow><msup><mrow><mi>n</mi></mrow><mrow><mo>′</mo></mrow></msup><mo>→</mo><msup><mrow><mi>p</mi></mrow><mrow><mo>′</mo></mrow></msup><mo>+</mo><msup><mrow><mi>e</mi></mrow><mrow><mo>′</mo></mrow></msup><mo>+</mo><msubsup><mrow><mover><mrow><mi>ν</mi></mrow><mrow><mo>¯</mo></mrow></mover></mrow><mrow><mi>e</mi></mrow><mrow><mo>′</mo></mrow></msubsup></mrow></math>, which creates a cloud of mirror particles <math><msup><mi>n</mi><mo>′</mo></msup></math>, <math><msup><mi>p</mi><mo>′</mo></msup></math>, <math><msup><mi>e</mi><mo>′</mo></msup></math>, and <math><msup><mi>D</mi><mo>′</mo></msup></math> inside the NS core. The <math><msup><mi>e</mi><mo>′</mo></msup></math> can “rob” the energy generated by the <math><mi>n</mi><mo>→</mo><msup><mi>n</mi><mo>′</mo></msup></math> transition via <math><mi>e</mi><mo>−</mo><msup><mi>e</mi><mo>′</mo></msup></math> scattering enabled by the presence of a (minute) millicharge in mirror particles. This energy is emitted as unobserved mirror photons via fast mirror bremsstrahlung leading to a relaxation of this upper limit.

Neutron–Mirror-Neutron Oscillation and Neutron Star Cooling

The P2 experiment aims at high-precision measurements of the parity-violating asymmetry in elastic electron-proton and electron-<math><mrow><mmultiscripts><mrow><mi>C</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>12</mn></mrow></mmultiscripts></mrow></math> scatterings with longitudinally polarized electrons. We discuss here the sensitivity of P2 to leptoquarks, which within the P2 energy range can be described in the language of Standard Model effective field theory (SMEFT). We give the expected P2 limits on the SMEFT operators and on the leptoquark parameters, which will test energy scales up to 15 TeV. In many cases those limits exceed current constraints from LHC and atomic parity violation (APV) experiments. We also demonstrate that degeneracies of different SMEFT operators can partially be resolved by use of APV experiments and different targets (protons and <math><mrow><mmultiscripts><mrow><mi>C</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>12</mn></mrow></mmultiscripts></mrow></math>) at P2. Moreover, we show that P2 could confirm or resolve potential tensions between the theoretical and experimental determinations of the weak charge of <math><mrow><mmultiscripts><mrow><mi>Cs</mi></mrow><mprescripts></mprescripts><none></none><mrow><mn>133</mn></mrow></mmultiscripts></mrow></math>.

Ingolf Bischer

Werner Rodejohann

Xun-Jie Xu

Searching for new physics from SMEFT and leptoquarks at the P2 experiment

We study the non-standard interactions of neutrinos with light leptonic scalars (ϕ) in a global (B − L)-conserved ultraviolet (UV)-complete model. The model utilizes Type-II seesaw motivated neutrino interactions with an SU(2) L -triplet scalar, along with an additional singlet in the scalar sector. This UV-completion leads to an enriched spectrum and consequently new observable signatures. We examine the low-energy lepton flavor violation constraints, as well as the perturbativity and unitarity constraints on the model parameters. Then we lay out a search strategy for the unique signature of the model resulting from the leptonic scalars at the hadron colliders via the processes H  ±±  → W  ±  W  ±  ϕ and H  ±  → W  ±  ϕ for both small and large leptonic Yukawa coupling cases. We find that via these associated production processes at the HL-LHC, the prospects of doubly-charged scalar H  ±±  can reach up to 800 (500) GeV and 1.1 (0.8) TeV at the 2σ (5σ) significance for small and large Yukawa couplings, respectively. A future 100 TeV hadron collider will further increase the mass reaches up to 3.8 (2.6) TeV and 4 (2.7) TeV, at the 2σ (5σ) significance, respectively. We also demonstrate that the mass of ϕ can be determined at about 10% accuracy at the LHC for the large Yukawa coupling case even though it escapes as missing energy from the detectors.