arXiv subjects

Zong-Guo Si

Publications and source records attributed to Zong-Guo Si.

At least 19 recordsLinked to original sources

arXiv2026

Spin correlations and quantum entanglement in $γγ\to t\bar t$ at polarized photon colliders with NLO QCD corrections

We study spin correlations and quantum entanglement in the process $γγ\to t\bar t$ with the photons coming from Compton backscattered laser beam. We present predictions for the cross sections and spin observables including next-to-leading order (NLO) QCD corrections under various beam-polarization configurations. The NLO QCD corrections significantly enhance the total cross section while having only a minor impact on spin observables. Using the spin density matrix of the $t\bar t$ system, we further investigate quantum entanglement and Bell nonlocality. We find that the entanglement is the strongest near the $t\bar t$ invariant-mass threshold, while both entanglement and Bell nonlocality are highly sensitive to the initial beam polarization. Our results provide a theoretical basis for future studies of quantum correlations in top-quark pair production at photon colliders.

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arXiv2026

Searching for dark photons in $J/ψ$ decays

A dark photon is an Abelian gauge boson from a new $U(1)_D$ gauge symmetry, coupled to the Standard Model via kinetic mixing, with $ε$ inducing an effective coupling to the electromagnetic current and $g_χ$ to a stable dark matter particle $χ$. We study $J/ψ$ two-body and four-body decays via a light-mass dark photon ($m_U < 3.0$ GeV) in the framework of non-relativistic QCD (NRQCD), considering both visible and invisible decays of the dark photon into SM fermions or dark sector particles. We investigate the detection sensitivity to the dark photon mass $m_U$ and kinetic mixing parameter $ε$ at both the BESIII and STCF experiments.

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arXiv2026

Constraining the Higgs potential using multi-Higgs production

The Higgs self-couplings remain only weakly constrained by current Large Hadron Collider (LHC) measurements, leaving ample room for physics beyond the Standard Model that could modify the structure of the Higgs potential. Multi-Higgs production processes provide a particularly sensitive probe of deviations in both the Higgs trilinear and quartic self-couplings. In this note, we summarize the current status of next-to-leading-order electroweak (EW) corrections to double-Higgs production computed within the Standard Model Effective Field Theory and Higgs Effective Field Theory frameworks, emphasizing how these calculations introduce sensitivity to the Higgs self-couplings beyond what is accessible at leading order. We discuss the key conceptual and technical differences between the two effective field theory approaches, including their treatment of higher-dimensional operators, renormalization procedures, and the structure of EW~two-loop amplitudes. Despite these differences, both approaches yield broadly consistent constraints, illustrating the complementarity of double- and triple-Higgs measurements. With the high-luminosity LHC and future high-energy colliders on the horizon, these developments and further advances provide an essential foundation for extracting increasingly precise information on the dynamics of EW symmetry breaking.

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arXiv2026

Study $γγ\to τ^+τ^-$ process including $τ^+ τ^-$ spin information in Pb-Pb ultraperipheral collision and at Lepton collider

We study the $γγ\to τ^+τ^-$ process including full $τ^+ τ^-$ spin information in Pb--Pb ultraperipheral collision and at lepton colliders. We present the predictions for the corresponding cross sections and spin correlations at NLO electroweak precision, and find that the NLO electroweak contributions are numerically small for the observables considered. Additionally, we use the spin correlations obtained in this paper to analyze the quantum entanglement in the $τ^+τ^-$ system of the $γγ\to τ^+τ^-$ process. Our results show that there is a genuine entangled configuration near the $τ^+τ^-$ invariant mass threshold. This work is helpful for studying the $τ$-pair production induced by photon-photon collision at high energy colliders.

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arXiv2025

Planar Property and Long-range Azimuthal Correlation in $e^+e^-$ Annihilation

The $e^+e^-$ annihilation of unpolarized beams is free from initial hadron states or initial anisotropy around the azimuthal angle, hence ideal for studying the correlations of dynamical origin via final state jets. We investigate the planar properties of the multi-jet events employing the relevant event-shape observables at next-to-next-to-leading order ($\mathcal{O}$($α_{s}^{3}$)) in perturbative QCD; particularly, the azimuthal angle correlations on the long pseudo-rapidity (polar angle) range (Ridge correlation) between the inclusive jet momenta are calculated. We illustrate the significant planar properties and the strong correlations which are natural results of the energy-momentum conservation of the perturbative QCD radiation dynamics. Our study provides benchmarks of hard strong interaction background for the investigations on the collective and/or thermal effects via the Ridge-like correlation observables for complex scattering processes.

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arXiv2025

Doubly heavy hadron production in ultraperipheral collisions

The inclusive production of pseudoscalar heavy quarkonia ($η_c,\, η_b,\, B_c$), double heavy baryons $Ξ_{QQ^\prime}$ ($Q^{(\prime)}=c,\,b$ quarks) and tetraquarks $T_{QQ}$ in heavy ion ultraperipheral collisions (UPCs) is studied. Numerical results indicate that the experimental investigation of $η_c,\, Ξ_{cc}$, and $T_{cc}$ is feasible at the upcoming HL-LHC and future FCC. Heavy ion UPCs open another avenue for studying the production of these doubly heavy hadrons.

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arXiv2025

The exclusive production of a fully heavy tetraquark and a photon in electron-positron collision

The exclusive production of fully heavy tetraquark ($T(bb\bar{b}\bar{b})$, $T(cc\bar{c}\bar{c})$ and $T(bc\bar{b}\bar{c})$) in association with a hard photon in electron-positron collisions is calculated within the framework of non-relativistic QCD. Both inner structures of molecule-like state and compact state with $J=0,1,2$ for the fully heavy tetraquark are discussed. We find that it is dismal to observe any fully heavy tetraquarks in either the compact configuration or the molecule-like configuration through such exclusive processes at either Belle II or future Z factories like CEPC and FCC-ee.

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arXiv2025

Doubly-charmed pentaquark states in a mass splitting model

Concentrating on the mass differences relative to $P_ψ^{N}(4312)^+$, we systematically investigate the spectra of doubly-charmed pentaquark states in the compact $ccqq\bar{q}$ ($q=u, d, s$) configuration. The assumption that the observed $P_ψ^{N}(4312)^+$ is a compact hidden-charm pentaquark with $I(J^P)=\frac12(\frac32^-)$ is adopted. We also study the properties of strong decays within a simple rearrangement scheme. The results indicate that the $I(J^P)=\frac12(\frac12^-)$ $ccnn\bar{n}$ with $I_{nn}=0$ where $n$ denotes $u$ or $d$ quark, $I(J^P)=0(\frac12^-)$ $ccnn\bar{s}$, and $I(J^P)=0(\frac12^-)$ $ccns\bar{n}$ ground states should be stable.

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arXiv2025

QCD corrections to Higgs boson pair production and decay to the $b\bar{b}τ^+τ^-$ final state

We present a comprehensive investigation of next-to-leading order (NLO) quantum chromodynamics (QCD) corrections to Higgs boson pair production and the subsequent decay into $b\bar{b}τ^+ τ^-$. We adopt the narrow-width approximation to separate the production and decay contributions and employ the dipole subtraction method to address infrared divergences inherent in perturbative QCD corrections. After applying a set of typical experimental cuts, we show that NLO QCD corrections to the decay process induce a significant reduction of the fiducial cross section and reshape important kinematic distributions at the 13.6 TeV LHC, such as the invariant mass of the Higgs boson pair and the transverse momentum of the leading $b$-jet. We also investigate the dependence of the kinematic distributions on the Higgs self-coupling and provide the signal acceptance as a function of the Higgs self-coupling modifier with full QCD corrections.

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arXiv2025

Improved constraints on Higgs boson self-couplings with quartic and cubic power dependencies of the cross section

Precise determination of the Higgs boson self-couplings is essential for understanding the mechanism underlying electroweak symmetry breaking. However, owing to the limited number of Higgs boson pair events at the LHC, only loose constraints have been established to date. Current constraints are based on the assumption that the cross section is a quadratic function of the trilinear Higgs self-coupling within the $κ$ framework. Incorporating higher-order quantum corrections from virtual Higgs bosons would significantly alter this functional form, introducing new quartic and cubic power dependencies on the trilinear Higgs self-coupling. To derive this new functional form, we propose a specialized renormalization procedure that tracks all Higgs self-couplings at each calculation step. Additionally, we introduce renormalization constants for coupling modifiers within the $κ$ framework to ensure the cancellation of all ultraviolet divergences. With new functional forms of the cross sections in both the gluon-gluon fusion and vector boson fusion channels, the upper limit of $κ_{λ_{\rm 3H}}=λ_{\rm 3H}/λ_{\rm 3H}^{\rm SM}$ set by the ATLAS (CMS) collaboration is reduced from 6.6 (6.49) to 5.5 (5.39). However, extracting a meaningful constraint on the quartic Higgs self-coupling $λ_{\rm 4H}$ from Higgs boson pair production data remains challenging. We also present the invariant mass distributions of the Higgs boson pair at different values of $κ_λ$, which could assist in setting optimal cuts for experimental analysis.

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arXiv2025

Flavor Physics at the CEPC: a General Perspective

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 $Z$ pole, the expected production of 4 Tera $Z$ bosons will provide unique and highly precise measurements of $Z$ boson couplings, while the substantial number of boosted heavy-flavored quarks and leptons produced in clean $Z$ 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 $τ$ 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 $WW$ 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.

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arXiv2025

Probing the electromagnetic dipole moment of the $τ$ lepton in the $e^+e^- \to γ^*/Z \to τ^+ τ^-$ reaction

High-precision measurements of the lepton's electromagnetic dipole moments provide a powerful probe for testing the Standard Model. A non-zero value of the $τ$ lepton's electric(weak) dipole moment ($d_τ^γ$, $d_τ^{Z}$) would serve as a smoking gun of the new physics. On the one hand, current and future high energy colliders offer an ideal environment for such measurements. On the other hand, it is essential to investigate the optimal measurement method for extracting not only $d_τ^γ$ and $d_τ^{Z}$, but also the anomalous magnetic dipole moment ($a_τ$). In this work, we analyze the precision of observables, particularly optimal observables, for determining these physics quantities through the process $e^+e^- \to γ^*/Z \to τ^+ τ^-$ with $τ$ leptons undergoing (semi-)leptonic and hadronic decays at the centre-of-mass energies at the Z pole mass and significantly below $m_Z$. By considering the full kinematic information of the decay products, we find that the sensitivities to Im$d_τ$ and Re$d_τ$ can reach $10^{-21}$ $ecm$ at CEPC $\sqrt s = m_Z$, compared to $10^{-20}$ $ecm$ at Belle-II $Υ(4S)$ resonance and $10^{-17}$ $ecm$ at BEPCII $ψ(2S)$ resonance. For $a_τ$, we find that Im$a_τ$ and Re$a_τ$ with a precision of $10^{-5}$ at Belle-II $\sqrt s=10.58$ GeV can be attained.

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arXiv2025

Production of doubly charmed tetraquark $T_{cc}$ via photon-photon fusion at electron-positron colliders

Within a phenomenological diquark fragmentation model, we study the production of doubly charmed tetraquark $T_{cc}$ via photon-photon fusion at electron-positron colliders. The production of $T_{cc}$ is divided into two steps: the perturbative production of heavy $(cc)$-diquark and its nonperturbative hadronization. Two diquark configurations of $(cc)[^3S_1]_{\bar{3}}$ and $(cc)[^1S_0]_{6}$ are considered, and the $(cc)[^3S_1]_{\bar{3}}$ state dominates the produciotn of $T_{cc}$. We discuss two hadronization models of $(cc)[^3S_1]_{\bar{3}}$ intermediate state into the tetraquark $T_{cc}$. It is found that it is promising to observe the tetraquark $T_{cc}$ via photon-photon fusion process both at the Circular Electron Positron Collider (CEPC) and the International Linear Collider (ILC). We find that the cross sections are sensitive to constituent charm quark mass of diquark, and they also have strong dependence on the hadronization models.

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arXiv2025

Higgs boson pair production and decay at NLO in QCD: the $b\bar{b}γγ$ final state

The Higgs boson pair production at the LHC provides a probe to the Higgs boson self-coupling. The higher-order QCD corrections in this process are sizable and must be taken into account in comparison with data. Due to the small cross section, it is necessary to consider at least one of the Higgs bosons decaying to bottom quarks. The QCD corrections to the decay processes would also be important in such cases. We present a full calculation of the total and differential cross sections for the $b\bar{b}γγ$ final state with next-to-leading order (NLO) QCD corrections. After applying typical kinematic cuts in the final state, we find that QCD NLO corrections in the decay decrease the LO result by $19\%$ and reduce the scale uncertainties by a factor of two. The QCD corrections to the invariant mass $m_{jjγγ}$ distribution, the transverse momentum spectra of the leading bottom quark jet and photon are significant and can not be approximated by a constant factor.

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arXiv2025

Triply heavy tetraquark states in a mass-splitting model

In a modified chromomagnetic interaction model, assuming $X(4140)$ to be the lowest $1^{++}$ $cs\bar{c}\bar{s}$ tetraquark and treating it as the reference state, we systematically investigated the masses of the triply-heavy tetraquark states $QQ\bar{Q}\bar{q}$ ($Q=c,b;q=u,d,s$). Because of their higher masses, no stable tetraquarks were found. Using a simple scheme, we also estimated the partial widths of the rearrangement decay channels and relevant ratios. A compact triply heavy tetraquark candidate would be favored if its observed mass and partial width ratios were comparable with our predictions. We hope that the present work will be helpful for further studies.

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arXiv2025

NLO EW corrections to tau pair production via photon fusion in Pb-Pb ultraperipheral collision

We study the next-to-leading order (NLO) electroweak (EW) corrections to the $γγ\to τ^+ τ^-$ process in Pb-Pb ultraperipheral collision (UPC). We find that the EW correction $δσ_{\mathrm{EW}}$ decreases the total cross section $σ_{\mathrm{NLO}} = σ_{\mathrm{LO}} + δσ_{\mathrm{EW}}$ by -3\% at Pb-Pb center-of-mass energy $\sqrt{s_{NN}}=5.02$ TeV. The weak correction plays significant role whose contribution is about -4 times of that of QED. The CMS and ATLAS collaborations use the reaction $γγ\to τ^+ τ^-$ in Pb-Pb and proton-proton UPC to constrain tau's anomalous magnetic moment $a_τ$. By parameterizing the $γττ$ vertex with two form factors $F_{1,2}$, the cross section can be written as $σ_{a_τ} = σ_{\mathrm{LO}} + δσ_{a_τ}$, where $δσ_{a_τ}$ is proportional to $a_τ$. The impact of NLO EW corrections on $a_τ$ bounds in Pb-Pb UPC is limited, as the current experimental bounds are loose. We also find that various differential distributions of the two ratios $\mathrm{d} σ_{\mathrm{NLO}}/ \mathrm{d} σ_{\mathrm{LO}}$ and $\mathrm{d} σ_{a_τ}/ \mathrm{d} σ_{\mathrm{LO}}$ have different lineshapes. This work is significant to precisely study the interaction of $γττ$ via $γγ\to τ^+ τ^-$ process.

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arXiv2024

Doubly heavy tetraquark states in a mass splitting model

Treating the $X(4140)$ as a compact $J^{PC}=1^{++}$ $cs\bar{c}\bar{s}$ state and using its mass as a reference scale, we systematically estimate the masses of doubly heavy tetraquark states $QQ\bar{q}\bar{q}$ where $Q=c,b$ and $q=u,d,s$. Their decay properties are studied with a simple rearrangement scheme. Based on our results, the lowest $I(J^P)=0(1^+)$ $bb\bar{n}\bar{n}$ state is a stable tetraquark about 20 MeV below the $\bar{B}^*\bar{B}$ threshold. The mass and width of the low-mass $0(1^+)$ $cc\bar{n}\bar{n}$ ($n=u,d$) tetraquark are compatible with the $T_{cc}(3875)^+$ observed by the LHCb Collaboration. The location of the lowest $0(0^+)$ and $0(1^+)$ $bc\bar{n}\bar{n}$ states are found to be close to the $\bar{B}D$ and $\bar{B}^*D$ thresholds, respectively. We hope that the predicted ratios between partial widths of different channels may be helpful to identify compact tetraquark states from future measurements.

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arXiv2024

Pseudoscalar heavy quarkonium production in heavy ion ultraperipheral collision

The inclusive production of pseudoscalar heavy quarkoniua ($η_c,~η_b$ and $B_c$) via photon-photon fusion in heavy ion ultraperipheral collision (UPC) are calculated to QCD next-to-leading order in the framework of non-relativistic QCD (NRQCD). The total cross section of $η_c$ produced in Pb-Pb UPC is 194 $\mathrm{nb}$ and 1275 $\mathrm{nb}$ at nucleon-nucleon c.m. energies $\sqrt{S_{\mathrm{NN}}}=$ 5.52 TeV and 39.4 TeV, respectively. The cross sections for $η_b$ and $B_c$ mesons are more than two to three orders of magnitude smaller. We make a detailed phenomenological analysis on the $η_c$ production; the uncertainties caused by the renormalization scale and the charm quark mass, the cross sections in other ultraperipheral nucleon-nucleon colliding systems, and the transverse momentum distribution are discussed. At the coming HL-LHC and future FCC, the heavy ion UPC opens another door of the study on the production of heavy quarkonium.

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