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ATLAS Collaboration(Aad, G. et al), Ahuja, S., Aikot, A., Cabrera Urban, S., Cantero, J., Carrion Martinez, C., et al. (2026). Observation of a cross-section enhancement near the t(t)over-bar production threshold in √s=13 TeV pp collisions with the ATLAS detector. Rep. Prog. Phys., 89(5), 057801–45pp.
Abstract: A measurement of t (t) over bar production is presented in the invariant-mass region near the pair production threshold, m(t (t) over bar) similar to 345 GeV, in final states with two charged leptons and multiple jets. The measurement is based on 140 fb(-1) of proton-proton collision data collected at root s = 13 TeV with the ATLAS detector at the Large Hadron Collider. The data are compared to two models of t (t) over bar production: a baseline model including only perturbative quantum chromodynamics (pQCD) predictions for the hard process at approximate next-to-next-to leading order accuracy in the strong coupling, and an extended model that, in addition, incorporates non-relativistic QCD simulations that also include the formation of colour-singlet quasi-bound-states near the t (t) over bar threshold. The agreement between the data and the models is quantified via a profile-likelihood fit to the reconstructed m(t (t) over bar) distributions, in bins of two angular observables sensitive to spin-correlations in the t (t) over bar system. An excess of events is observed over the baseline pQCD prediction, with an observed significance over 8 standard deviations. This excess is consistent with the formation of colour-singlet and spinsinglet S-wave quasi-bound t((t) over bar) states, as predicted by non-relativistic QCD, and corresponds to an observed cross-section of 9.3- (+1.4)(1.3) pb.
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Barbat, M. F., Nieves, J., & Tolos, L. (2026). Scattering and femtoscopic correlation functions of the Σ++cπ+,Σ0cπ- and Σ+bπ+ systems. Phys. Lett. B, 878, 140529–10pp.
Abstract: We present predictions for scattering observables and femtoscopic correlation functions (CFs) of the I=2 Sigma(++)(c)(pi+) , Sigma(0)(c)pi systems and its heavy-flavor counterpart . In both heavy-quark sectors, the strong interaction is formulated within two distinct theoretical frameworks, each constrained to reproduce the lowest-lying odd-parity isoscalar spin-1/2 resonances, Lambda(c)(2595) and Lambda(b)(5912), respectively. While the Sigma(0)(c)pi (_)pair is governed solely by the strong interaction, electrostatic contributions are included in the other two channels involving charged particles through relativistic Coulomb wave functions. We show that the differences observed in the scattering observables between the two strong-interaction models arise mainly from the specific ultraviolet regularization schemes employed. The inclusion of Coulomb effects induces only a very small increase in both the scattering length and the effective range. The resulting CFs in the charm and bottom sectors display analogous global features, in agreement with expectations from heavy-quark flavor symmetry. Both, the Sigma(++)(c)pi(+) and Sigma(+)(b)pi(+) CFs, when computed including only the strong interaction, exhibits substantial discriminating power among the different models. However, once Coulomb effects are incorporated, the CFs become largely affected by the repulsive electrostatic interaction, which diminishes their sensitivity to the details of the underlying strong dynamics, thereby reducing the capability to differentiate between theoretical descriptions. Thus, the Sigma(0)(c)pi CF-being free from Coulomb effects-provides the most suitable observable for constraining the strong dynamics of the isotensor Sigma(c)pi system.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Lucio Martinez, M., et al. (2026). First Evidence of the Bs0 → K- π+ γ Decay. Phys. Rev. Lett., 136(17), 171801–15pp.
Abstract: The first search for the B-s(0\) -> K-pi(+gamma) decay in the range 796 < m(K-pi(+)) < 1800 MeV/c(2) is performed using data from proton-proton collisions collected by the LHCb experiment at center-of-mass energies of 7, 8, and 13 TeV, corresponding to an integrated luminosity of 9 fb(-1). The photons are reconstructed through their conversion into an electron-positron pair, which significantly improves the mass resolution of the reconstructed decays with respect to decays with an unconverted photon. A signal excess with a significance of 3.5 standard deviations is measured, constituting the first experimental evidence for this decay. In the range 796 < m(K-pi(+)) < 996 MeV/c(2), the ratio R between the branching fractions of the signal decay and the favored (B) over bar (0) -> K-pi(+)gamma decay is measured to be R = (3.7 +/- 1.2 +/- 0.4) x 10(-2) where the first uncertainty is statistical and the second is systematic. This measurement is consistent with the value predicted in the Standard Model. In the range 996 < m(K-pi(+)) < 1800 MeV/c(2), the ratio R = (0.2 +/- 2.7 +/- 1.3) x 10(-2) is measured.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Lucio Martinez, M., et al. (2026). Precision Measurement of CP Violation and Branching Fractions in B± → KS0h± (h = π, K) Decays and Search for the Rare Decay Bc± → KS0K±. Phys. Rev. Lett., 136(16), 161801–14pp.
Abstract: The decay B-+/- -> K-S(0)pi(+/-), with a CP asymmetry expected to be close to zero in the standard model, is theoretically clean and sensitive to potential new physics. An analysis of the decays B-+/- -> K-S(0)pi(+/-) and B-+/- -> (KSK +/-)-K-0 is performed using proton-proton collision data collected by the LHCb experiment at a center-of-mass energy of 13 TeV, corresponding to an integrated luminosity of 5.4 fb(-1). The CP asymmetries are determined to be A(CP)(B-+/- -> K-S(0)pi(+/-)) = -0.028 +/- 0.009 +/- 0.009 and A(CP)(B-+/- -> (KSK +/-)-K-0) = 0.118 +/- 0.062 +/- 0.031, and the branching fraction ratio is measured to be B(B-+/- -> (KSK +/-)-K-0)/B(B-+/- -> K-S(0)pi(+/-)) = 0.055 +/- 0.004 +/- 0.002, where the first uncertainties are statistical and the second are systematic. These results are the most precise measurements of these quantities to date. A search for the rare decay B-c(+/-) -> (KSK +/-)-K-0 is also performed. No significant signal is observed, and the upper limit on the product of the branching fraction ratio B(Bc(+/-) -> (KSK +/-)-K-0)/B(B-+/- -> K-S(0)pi(+/-)) and the fragmentation-fraction ratio f(c)/f(u) is set to be 0.015 (0.016) at the 90% (95%) confidence level.
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LHCb Collaboration(Aaij, R. et al), Fernandez Casani, A., Jaimes Elles, S. J., Jashal, B. K., Libralon, S., Lucio Martinez, M., et al. (2026). Observation of the decay χc1 (3872) → J/ψ μ+ μ-. Phys. Rev. D, 113(7), 072019–14pp.
Abstract: The first observation of the chi(c1) (3872) -> J/psi mu(+)mu(-) decay is reported using proton-proton collision data recorded with the LHCb detector corresponding to an integrated luminosity of 9 fb(-1). The decay mode is observed for the first time, with a significance of 6.5s. Its branching fraction is measured relative to the chi(c1) (3872) -> J/psi pi(+)pi(-) decay mode B(chi(c1)(3872) -> J/psi mu(+)mu(-))/B(chi(c1)(3872) -> J/psi pi(+)pi(-)) = (1.68 +/- 0.32 +/- 0.05) x 10(-3), where the first uncertainty includes both statistical contributions and systematic contributions which are uncorrelated between data-taking periods, and the second represents the systematic contributions that are correlated between data- taking periods.
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