We introduce a novel level crossing phenomenon in the mass mixing between QCD axions: one canonical QCD axion and one $ Z_{{\cal{N}}} $ axion. The level crossing can occur at or slightly before the QCD phase transition critical temperature, depending on the ratio of the axion decay constants, $ \sim 1.69 $ . The cosmological evolution of the mass eigenvalues in these two scenarios is similar; however, the transition of axion energy density differs significantly. Finally, we estimate the relic density of the QCD axion dark matter in this context. Additionally, this level crossing may have some interesting cosmological implications.

China

Hai-Jun Li

Yu-Feng Zhou

Mass mixing between QCD axions * Supported by the CAS Project for Young Scientists in Basic Research YSBR-006, the National Key R&D Program of China (2017YFA0402204), and the National Natural Science Foundation of China (11821505, 11825506, 12047503)

Motivated by the gradual increase in experimental data, we revisit the couplings of axion-like particle (ALP) to electroweak gauge bosons across the ALP mass range from MeV to 100 GeV. We extend both current and projected experimental limits on these couplings, including the ALP couplings with $\textit{W}$-boson $ g_{aW} $ and hypercharge $\textit{B}$-boson $ g_{aB} $ . To clarify the contributions from electroweak boson couplings, we analyze and compare the resulting effects for various values of $ g_{aW} $ and $ g_{aB} $ . The couplings induce flavor-conserving ALP interactions with standard model fermions at the one-loop level, while $ g_{aW} $ additionally results in flavor-changing ALP-quark couplings. These phenomena warrant further investigation through rare meson decays and neutral meson mixing processes, particularly in light of recent results from $ B^+\to K^+\nu\bar{\nu} $ and $ K^+\to \pi^+\nu\bar{\nu} $ . We find that the rare two-body decays of pseudoscalar mesons offer the most sensitive probes below the kinematic threshold. In the high-mass region, complementary bounds arise from Pb-Pb collision and $\textit{Z}$-boson measurements, including the invisible decay $ Z\to a\gamma $ with subsequent ALP decays and constraints from oblique parameters ($\textit{S}$, $\textit{T}$, $\textit{U}$). Future lepton colliders, such as CEPC and FCC-ee operating at the $\textit{Z}$-pole, along with the search for hidden particles, provide further opportunities to probe ALP couplings to electroweak gauge bosons.

Jin Sun

Zhi-Peng Xing

Seokhoon Yun

Revisiting axion-like particle couplings to electroweak gauge bosons * Supported by IBS under the project code, (IBS-R018-D1) and NSFC (12375088, 12335003, 12405113)

We present an investigation on cosmological implications resulting from spontaneous dark $ S U(2)_d $ symmetry breaking in conjunction with the Witten effect. Furthermore, we examine the mass of hidden monopoles generated by phase transition as well as the impact of axion-monopole interactions on the axion mass. Following spontaneous breaking of the PQ symmetry, axions emitted by cosmic strings or domain walls as well as hidden monopoles generated during this transition may collectively contribute to the observed dark matter relic density. Additionally, the stochastic gravitational wave background generated by sub-GeV phase transitions and by the decay of axionic domain walls formed during sub-ZeV phase transitions could account for the low-frequency gravitational wave signals detected by PTA experiments.

Ruiyu Zhou

Ligong Bian

Gravitational waves and dark matter with Witten effect * Supported by the National Key Research and Development Program of China (2021YFC2203004). Ruiyu Zhou is supported by the National Natural Science Foundation of China (12305109, 12147102), Chongqing Natural Science Foundation project (CSTB2022NSCQ-MSX0534), and Science and Technology Research Project of Chongqing Municipal Education Commission (KJQN202300614). Ligong Bian is supported by the National Natural Science Foundation of China (12322505, 12347101). Ligong Bian also acknowledges Chongqing Natural Science Foundation (CSTB2024NSCQ-JQX0022) and Chongqing Talents: Exceptional Young Talents Project (cstc2024ycjhbgzxm0020).

Recent anomalies observed in $ e^+e^- $ nuclear transitions of $ ^8 {{\rm{Be}}} $ , $ ^4 {\rm{He}} $ , and $ ^{12} {\rm{C}} $ by the ATOMKI collaboration may hint at the existence of a vector boson with a mass around 17 MeV, which is referred to as X17. If it exists, this boson would affect similar processes in particle physics, including the Dalitz decays of vector mesons. Recently, the BESIII collaboration measured the Dalitz decay $ D^{*0}\to D^0e^+e^- $ for the first time and reported a $ 3.5\sigma $ excess over the theoretical prediction based on the vector meson dominance (VMD) model. This excess may be another signal of X17. In this study, we investigate the possible effects of X17 on the Dalitz decays $ D^*_{(s)}\to D_{(s)}e^+e^- $ , $ B^*_{(s)}\to B_{(s)}e^+e^- $ , and $ J/\psi\to \eta_ce^+e^- $ . The required hadronic form factors are calculated within the framework of our covariant confined quark model without relying on heavy quark effective theory or the VMD model. We present predictions for the Dalitz decay widths and the ratios $ R_{ee}(V)\equiv \Gamma(V\to Pe^+e^-)/\Gamma(V\to P\gamma) $ within the standard model and in several new physics scenarios involving modifications attributed to X17. Our results are compared with other theoretical calculations.

Vietnam

C.T. Tran

Russia

M.A. Ivanov

A.T.T. Nguyen

Probing the ATOMKI X17 vector boson using Dalitz decays $\textit{V}$ → $\textit{Pe}$ $^{+}$ $\textit{e}$ $^{−}$ * Supported by Ho Chi Minh City University of Technology and Education (T2024-65)

Vector-like quarks (VLQs) feature in new physics models beyond the standard model (SM) to address certain problems it faces. In this study, we investigate the pair production of TeV-scale vector-like $\textit{B}$ quark (VLQ-$\textit{B}$) at the proposed 3 TeV compact linear collider (CLIC) using a simplified effective lagrangian framework. We consider the decay modes of $ B\rightarrow bZ $ and $ B\rightarrow bh $ , coupled with the hadronic decay of $\textit{Z}$ and $\textit{h}$ bosons. The large mass of VLQ-$\textit{B}$ results in highly boosted $\textit{Z}$ or $\textit{h}$ bosons, which tend to form fat-jets. Using detector simulation of the signal and background events, along with jet clustering employing a large radius $\textit{R}$, we conduct signal-background analyses. Exclusion limits at the 95% confidence level and 5 $ \sigma $ discovery prospects are obtained assuming an integrated luminosity of 5 ab$^{−1}$.

Shuo Yang

Yi-Hang Wang

Peng-Bo Zhao

Ji-Long Ma

Search for heavy vector-like $\textit{B}$ quark via pair production in fully hadronic channels at CLIC * Supported in part by the National Natural Science Foundation of China (12147214, 11905093), Basic Research Project of Liaoning Provincial Department of Education for Universities (LJKMZ20221431), and Teaching Reform Research Project for graduates of Liaoning Normal University

The rigorous calculation of the spin-orbit terms in a three-quark system is realized based on the Gaussian expansion method and the infinitesimally-shifted Gaussian basis functions in the framework of the relativized quark model, by ignoring the mixing between different excited states. Then, the complete mass spectra of the singly heavy baryons are obtained rigorously, under the heavy-quark dominance mechanism. Accordingly, systematical analyses are performed for the reliability and predictive power of the model, the fine structure of the singly heavy baryon spectra, the assignments of the excited baryons, and some important topics about heavy baryon spectroscopy, such as the missing states, 'spin-orbit puzzle,' and clustering effect. The results confirm that, under the heavy-quark dominance mechanism, the relativized quark model can describe the excitation spectra and the fine structures of the singly heavy baryons correctly and precisely.

Zhen-Yu Li

Guo-Liang Yu

Zhi-Gang Wang

Jian-Zhong Gu

Hong-Tao Shen

Mass spectra of singly heavy baryons in the relativized quark model with heavy-quark dominance * Supported by the Open Project of Guangxi Key Lab of Nuclear Physics and Technology (NLK2023-04), the Central Government Guidance Funds for Local Scientific and Technological Development in China (Guike ZY22096024), the Natural Science Foundation of Guizhou Province-ZK[2024](General Project)650, the National Natural Science Foundation of China (11675265, 12175068), the Continuous Basic Scientific Research Project (WDJC-2019-13) and the Leading Innovation Project (LC 192209000701)

Motivated by the first observation of $\rm CP $ violation in baryon decays, we study the topological amplitudes of bottom baryon decays in the $ S U(3)_{\rm F} $ limit. The topological diagrams of the charmless two-body decays of bottom baryons are presented in detail. The linear relations between topologies and $ S U(3) $ irreducible amplitudes are derived through tensor contraction and $ S U(3) $ decomposition. Four amplitudes among the 13 independent amplitudes are critical to the $\rm CP$ asymmetries. The small $ \rm CP $ asymmetries might indicate small relative strong phases between amplitudes $ A_2 $ and $ A_{12,14}^\prime $ . To avoid them, we suggest measuring $\rm CP $ asymmetries in the $ \Xi^0_b\to pK^- $ and $ \Xi^-_b\to \Lambda^0 K^- $ decays. Furthermore, the Körner-Pati-Woo theorem can be tested by measuring the branching fractions of the $ \Lambda_b^0\to\Sigma^0K^0_S $ and $ \Lambda_b^0\to\Sigma^-K^+ $ modes.

Di Wang

Wei-Chen Fu

Topological amplitudes of bottom baryon decays in the $\textit{SU}$(3)$_{F}$ limit * Supported in part by the National Natural Science Foundation of China (12105099)

The magnitude of the direct CP asymmetry generated during the weak decay of hadrons is attributed to the weak phase and certain strong phases. The weak phase originates from the CKM matrix, while a strong phase may result from the resonance effect produced by the mixing of vector mesons $ V\left\{\rho^{0}(770),\,\omega(782),\,\phi(1020)\right\} $ to $ \pi^+ \pi^- $ meson pairs. $ \rho^{0}(770) $ can decay directly into $ \pi^+ \pi^- $ meson pairs; both $ \omega(782) $ and $ \phi(1020) $ can also decay into $ \pi^+ \pi^- $ meson pairs, with a small contribution from isospin symmetry breaking. The main contribution for the middle state vector meson $ \rho^{0}(770)-\omega(782)-\phi(1020) $ interference is the mix of $ \rho^{0}(770) $ , $ \omega(782)-\rho^{0}(770) $ , and $ \phi(1020)-\rho^{0}(770) $ . We calculated the CP asymmetry and decay branching ratio for $ \bar{B}^0_{s} \rightarrow \pi^+ \pi^- \pi^0 (\bar K ^{0}) $ within the framework of QCD factorization and compared them with previous studies. We also analyzed the $ \bar{B}^0_{s} \rightarrow \pi^+ \pi^- \eta(\eta^{(')}) $ decay process. The results show that the CP asymmetry of these four decay processes is significantly enhanced, especially for the $ \bar{B}^0_{s} \rightarrow \pi^+ \pi^- \bar K ^{0} $ decay process. Moreover, the decay branching ratio also changes under the resonance effect. These results might provide support for the experimental analysis of the $ \bar B^{0}_s $ meson.

Xi-Liang Yuan

Chao Wang

Zhuang-Dong Bai

Gang Lü

Role and contribution of the resonance effect for the decay process of ${ \bar {\boldsymbol B}^{\bf 0}_{\boldsymbol s} \boldsymbol\rightarrow \boldsymbol\pi^{+}\boldsymbol\pi^{-}{\boldsymbol P}}$ * Supported by National Natural Science Foundation of China (Project No.11805153) and Natural Science Foundation of Henan Province (Project No.252300420319)

This study investigates the intrinsic electric dipole moment (EDM) of the $\textit{τ}$ lepton, which is an important quantity in the search for physics beyond the Standard Model (BSM). In preparation for future measurements at the Super Tau-Charm Facility (STCF), we employ Monte Carlo simulations of the $ e^+e^- \rightarrow \tau^+\tau^- $ process and optimize the analysis methodology for EDM extraction. Machine learning techniques are implemented to efficiently identify signal events ( $ \tau^\pm\rightarrow \pi^\pm\pi^0\nu_\tau $ ), which result in a significant improvement in signal-to-noise ratio. Our optimized event selection algorithm achieves 80.0% signal purity with 6.3% efficiency. We develop an analytical approach for $\textit{τ}$ lepton momentum reconstruction and derive the squared spin density matrix along with optimal observables, which maximize the sensitivity to $ d_\tau $ . The relationship between these observables and the EDM is established with the estimated sensitivity of $ |d_\tau| \lt 3.89\times 10^{-18}\,e\cdot\mathrm{cm} $ at a 68% confidence level. These results provide a foundation for future experimental measurements of the $\textit{τ}$ lepton EDM in STCF experiments.

Xulei Sun

Yongcheng Wu

Xiaorong Zhou

Sensitivity study of the tau lepton electric dipole moment at the Super Tau-Charm Facility * Supported by the National Key R&D Program of China (2022YFA1602200, 2023YFA1607200); the National Natural Science Foundation of China (NSFC) (12341501, 12341503, 12341504); the international partnership program of the Chinese Academy of Sciences (211134KYSB20200057). Xulei Sun is supported by the Undergraduate Research Program of the University of Science and Technology of China and the Xinhe Scholar Program of the School of the Gifted Young

Inspired by the latest experimental progress, we systematically study the Okubo-Zweig-Iizuka (OZI)-allowed two-body strong decay properties of 1$\textit{P}$-, 1$\textit{D}$-, 2$\textit{S}$-, and 2$\textit{P}$-wave $ \Lambda_c $ baryons within the $\textit{j}$-$\textit{j}$ coupling scheme in the framework of the quark pair creation model. The calculations indicate the following. (i) Taking the observed states $ \Lambda_c(2595)^+ $ and $ \Lambda_c(2625)^+ $ as the 1$\textit{P}$-wave $\textit{λ}$-mode states $ \Lambda_c|J^P=1/2^-,1\rangle_{\lambda} $ and $ \Lambda_c|J^P=3/2^-,1\rangle_{\lambda} $ , respectively, we can reproduce the experimental data well in theory. (ii) Combined with the measured mass and decay properties of $ \Lambda_c(2860)^+ $ , this excited state can be explained as the 1$\textit{D}$-wave $\textit{λ}$-mode state $ \Lambda_c|J^P=3/2^+,1\rangle_{\lambda\lambda} $ . (iii) The newly observed state $ \Lambda_c(2910)^+ $ may be assigned as one of the 1$\textit{P}$-wave $\textit{ρ}$-mode states $ \Lambda_c|J^P=3/2^-,2\rangle_{\rho} $ or $ \Lambda_c|J^P=5/2^-,2\rangle_{\rho} $ . Meanwhile, we notice that the partial decay width ratio between $ \Sigma_c\pi $ and $ \Sigma_c^*\pi $ for the two candidates is significantly different. Hence, experimental progress in this ratio measurement may elucidate the nature of $ \Lambda_c(2910)^+ $ . (iv) According to the properties of $ \Lambda_c(2765)^+ $ , we find that the 2$\textit{S}$-wave $\textit{λ}$-mode state $ \Lambda_{c1}|J^P=1/2^+,0\rangle_{\lambda} $ is a potential candidate. (v) The 2$\textit{P}$-wave $\textit{λ}$-mode state $ \Lambda_{c1}|J^P=3/2^-,1\rangle_{\lambda} $ is most likely to be a good assignment of the controversial state $ \Lambda_c(2940)^+ $ . Both the total decay width and partial decay ratio between $ pD^0 $ and $ \Sigma_c\pi $ are in good agreement with observations. (vi) In addition, for the missing $ \Lambda_c $ excitations, we obtain their strong decay properties and hope these are useful for future experimental exploration.

Yu-Bin Zhang

Li-Ye Xiao

Xian-Hui Zhong

Possible explanations of the observed Λ $_{c}$ resonances * Supported by the National Natural Science Foundation of China (12005013, 12175065, 12235018, 11947048)