We study quantum spin correlations in top-quark pair production at future lepton colliders in the presence of a neutral gauge boson from anomaly-free general $U(1)$ extensions of the Standard Model. The process $\ell^+\ell^-\to t\bar t$, with $\ell=e,\mu$, is analyzed through the spin-density matrix including $\gamma$, $Z$ and $Z^\prime$ exchange and their interference. We focus on quantum-information observables such as the sufficient entanglement marker $\mathcal{D}_{\min}$, concurrence, purity and the maximal Clauser-Horne-Shimony-Holt (CHSH) parameter, and compare their behavior with conventional rate information. Within the $U(1)_X$ framework, we consider several representative charge assignments to investigate how different chiral structures influence these observables, with particular emphasis on the $Z^\prime$ resonance region and polarized $e^-e^+$ collisions, where the two allowed initial-state helicity configurations can be selectively enhanced. We show that electron-beam polarization provides a direct handle on the left- and right-handed lepton charges of various $U(1)_X$ scenarios. These results demonstrate that quantum spin observables provide information complementary to cross sections and angular distributions in searches for chiral neutral gauge interactions.
We investigate the discovery potential for the vector-like $X$-quark (VLX) at future muon--proton ($\mu p$) colliders through the process $\mu^+ p \to \bar{\nu}_\mu X \to \bar{\nu}_\mu t W^+$. A simplified effective model is adopted in which the production and decay of the VLX are governed by the coupling strength $g^{*}$, the generation-mixing parameter $R_{L}$, and the VLX mass $m_X$. A comprehensive Monte Carlo analysis is performed at $\sqrt{s}=5.29$, $6.48$, and $9.16\ \mathrm{TeV}$, considering four complementary decay channels: the Fully Leptonic (FL), Fully Hadronic (FH), and two Semi-Leptonic (SL1 and SL2) modes. An $80\%$ polarized muon beam together with boosted-object reconstruction based on fat-jet techniques is employed to improve the signal sensitivity. The expected exclusion and discovery reaches are evaluated using the Asimov significance. We find that the sensitivity can be improved substantially with increasing center-of-mass energy and larger values of $R_L$. Among the four channels, the FH mode provides the strongest sensitivity, reaching a $2\sigma$ exclusion limit of $m_X\simeq8.3\ \mathrm{TeV}$ with $g^* = 0.009 $ for $R_L=0.1$ at $\sqrt{s}=9.16\ \mathrm{TeV}$, whereas the FL mode gives the weakest reach because of its smallest branch ratio. These results demonstrate that future $\mu p$ colliders can offer significant sensitivity to heavy VLX over a broad region of parameter space.
Liangliang Shang, Shuting Zhao, Bingfang Yang et al.· 0 citations
A study of the production of the Higgs boson in association with either a top-quark pair ($t\bar{t}H$) or a single top quark ($tH$) in the $\tau$-lepton-pair decay channel is presented. The analysis relies on final states featuring fully hadronic decays of the top quarks and the $\tau$-leptons. It employs data samples of proton--proton collisions at $\sqrt{s}=13$ and 13.6 TeV recorded with the ATLAS detector at the CERN Large Hadron Collider and corresponding to integrated luminosities of 140 fb$^{-1}$ and 161 fb$^{-1}$, respectively. The measured signal strength ($\mu$), defined as the measured cross-section normalised to the Standard Model prediction, is $\mu_{t\bar{t}H}=1.51^{+0.71}_{-0.62}$ for $t\bar{t}H$ and $\mu_{tH}=-0.4^{+5.6}_{-5.1}$ for $tH$. Additionally, the $t\bar{t}H$ cross-section is measured differentially in three bins of the Higgs boson transverse momentum in the simplified template cross-section framework.
We study radiative double-charmonium production, $e^+e^-\to HH\gamma$ with $H=J/\psi$ and $\eta_c$, at $\sqrt{s}=10.58~{\rm GeV}$, including a possible tensor contribution from the fully charmed state $X(6900)$. The nonresonant amplitudes are calculated within the color-singlet non-relativistic QCD framework, while the $J^{PC}=2^{++}$ resonance is combined coherently with the continuum. We find that continuum--resonance interference significantly modifies the invariant-mass distributions near $M_{HH}\simeq M_X$. The $J/\psi J/\psi\gamma$ channel exhibits a pronounced phase-dependent resonant structure, with a near-resonance cross section of $0.038$--$0.111~{\rm fb}$. In contrast, the $\eta_c\eta_c\gamma$ channel is continuum dominated and shows only a localized interference effect with an extremely suppressed production rate, rendering its experimental observation extremely challenging. These channel-dependent features arise from the different effective decay structures and demonstrate the importance of an amplitude-level treatment of the $X(6900)$ contribution.
Meng Jia, Yi-Jie Li, Guanghua Xu et al.· 1 citation
A search is presented for the rare, loop-induced Higgs boson decay H $\to$ Z$\gamma$, where Z $\to$ $\ell^+\ell^-$ and $\ell$ = e or $\mu$. The search is performed using a sample of proton-proton (pp) collision data at the center-of-mass energies of 13 and 13.6 TeV, recorded by the CMS experiment at the LHC and corresponding to a total integrated luminosity of 200 fb$^{-1}$. The analysis design separately considers and optimizes sensitivity to Higgs boson production via gluon-gluon fusion, vector boson fusion, and associated-production processes. The signal is extracted from a simultaneous fit to the invariant mass distributions of the $\ell^+\ell^-\gamma$ system in the various production channels and event categories. The observed (expected) signal strength $\mu$, defined as the ratio of the measured product the of cross section and decay branching fraction value $\sigma$(pp $\to$ H)$\mathcal{B}$(H $\to$ Z$\gamma$) to the corresponding value predicted in the standard model, is found to be $\mu$ = 1.10$^{+0.52}_{-0.61}$ (1.00$^{+0.49}_{-0.46}$) for a Higgs boson mass of 125.38 GeV. The signal has an observed (expected) significance of 1.9 (2.3) standard deviations.
In this study, the sensitivity to anomalous quartic gauge couplings (aQGCs) is projected via $pp \rightarrow ZZ\gamma$ production in the 100 TeV proton-proton Future Circular Collider - hadron-hadron (FCC-hh) for an integrated luminosity of 30 ab$^{-1}$. The $\ell\ell\nu\nu\gamma$ final state under consideration consists of a same-flavor, opposite-sign lepton pair (electrons or muons) from one $Z$ boson, the invisible decay of the other $Z$ boson into neutrinos, and an accompanying photon. The FCC-hh detector response and its effects on the reconstructed objects are included through a realistic detector simulation. Three multivariate techniques are employed to separate the signal from the relevant SM backgrounds. Unitarity is preserved by a strict, operator-dependent bound on the total transverse mass ($M_T^{tot}$) of the system. The median expected significances are calculated within the Asimov approximation for one anomalous coupling varied at a time and for background systematic uncertainties between 0\% and 10\%. The highest separation power is obtained with the deep neural network method. The resulting 95\% confidence level limits on $|f_{T0}/\Lambda^{4}|$, $|f_{T8}/\Lambda^{4}|$, $|f_{T9}/\Lambda^{4}|$ and $|f_{M2}/\Lambda^{4}|$ in the combined $e+\mu$ channel without systematic uncertainties are $2.83\times 10^{-3}$, $1.65\times 10^{-3}$, $3.81\times 10^{-3}$ and $8.97\times 10^{-3}$ TeV$^{-4}$, respectively. We have an order of magnitude improvement when compared to current LHC limits with the assumption of 5\% systematic uncertainty.
A. Yilmaz, A. Senol, H. Denizli et al.· 0 citations
We investigate the discovery potential of a singly produced vector-like top quark $T$ via the $T\to tZ$ decay channel at future muon-proton ($\mu p$) colliders, considering three benchmark center-of-mass energies: $\sqrt{s}=5.29$, $6.48$, and $9.16$~TeV. The baseline signal final state consists of a semileptonic top decay $t\to bW\to b\ell\nu$ and a hadronic $Z\to q\bar{q}$ decay, where the collimated quark pair from the $Z$ boson is reconstructed as a single large-radius fat jet $J$. Full detector-level Monte Carlo simulations are performed for both the signal process and the dominant Standard Model backgrounds. We derive the $5\sigma$ discovery contours and the $95\%$ confidence level exclusion limits in the two-dimensional parameter space spanned by the VLQ mass $m_T$ and the effective coupling $g^*$. A comparison with the $T\to Wb$ channel and with searches at other collider facilities reveals that the $T\to tZ$ decay mode provides complementary sensitivity and offers unique advantages for probing high-mass vector-like $T$ quarks in the boosted jet topology. Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI. Article funded by SCOAP3 and published under licence by Chinese Physical Society and the Institute of High Energy Physics of the Chinese Academy of Science and the Institute of Modern Physics of the Chinese Academy of Sciences and IOP Publishing Ltd.
Ying-Hao Gao, Yao-Bei Liu· Chinese Physics C, High Ener...· 0 citations
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