51�������

Abstract:

The first measurement of the magnitude of the CKM quark mixing matrix element |Vcb| using on-shell W-boson decays is presented. The measurement is made with tt¯$$ t\overline{t} $$ events from pp collision data corresponding to an integrated luminosity of 140 fb−1 at a centre-of-mass energy of 13 TeV collected by the ATLAS experiment at the Large Hadron Collider. A value of:Vcb=50−14+11×10−3$$ \left|{V}_{cb}\right|=\left({50}_{-14}^{+11}\right)\times {10}^{-3} $$is measured, where the uncertainties have roughly equal contributions from the limited size of the data sample and from systematic effects. This is consistent with existing measurements made at much lower energy scales in B hadron decays. While the sensitivity does not yet rival that of previous determinations, this novel approach probes a very different physical situation, namely the rare hadronic t→bb¯c$$ t\to b\overline{b}c $$ decay at high momentum-transfer, rather than semileptonic B decays at low momentum-transfer.

Abstract:

ABSTRACT We present the discovery and multiwavelength follow-up of gamma-ray bursts (GRB) 250818B, detected by the Space Variable Objects Monitor (SVOM) and localized by the Gravitational-wave Optical Transient Observer. We compile X-ray, optical/near-infrared, and radio data into broad-band light curves and spectral energy distributions. Despite being initially reported as short, the public prompt light-curve figure suggests a duration of $\sim 5$ s, arguing against a secure short-duration classification. The afterglow is unusually bright relative to short-GRB samples: in X-rays and optical it is broadly consistent with the long-GRB population, while in the radio it is among the most luminous events in the short-GRB comparison sample and remains within the long-GRB range. MeerKAT detects the source at 3.1 GHz, while the Atacama Large Millimetre/submillimetre Array provides deep higher-frequency limits. Keck/LRIS spectroscopy shows continuum and metal absorption (Fe ii, Mg ii, Mg i), giving $z=1.216$. Synchrotron forward-shock modelling favours a constant-density medium and refreshed (energy-injection) emission, well described by a two-component jet with core isotropic-equivalent kinetic energy $E_{0}\sim 8\times 10^{52}$ erg, ambient number density $n_0\sim 12\, \mathrm{cm^{-3}}$, jet half-opening angle $\theta _j\simeq 0.13$ rad ($\sim 7.4^\circ$), and electron index $p\simeq 1.67$. We do not identify a host conclusively: the data admit either a faint LS DR10 galaxy at $\sim 4\, {\rm arcsec}$ ($\sim 34$ kpc) as an offset candidate, or a fainter, near-coincident uncatalogued host. GRB 250818B highlights the power of rapid counterpart identification in the SVOM era, while uncertain prompt classification and inconclusive host association still limit progenitor interpretation.

Abstract:

This paper presents differential cross-section measurements of events containing a charged lepton, missing transverse momentum, two b-jets, and two light jets, consistent with the W+W−bb¯$$ {W}^{+}{W}^{-}b\overline{b} $$ final state. The analysis is based on 140 fb−1 of proton-proton collision data at s=13$$ \sqrt{s}=13 $$ TeV recorded with the ATLAS detector during Run 2 of the Large Hadron Collider (2015–2018). Differential cross-sections are unfolded at particle level for a range of kinematic observables and in three kinematic regions. Both regions and observables are optimised for sensitivity to the interference between W+W−bb¯$$ {W}^{+}{W}^{-}b\overline{b} $$ produced with singly and doubly resonant top quarks. Results are compared with various theoretical predictions and deviations are observed in regions sensitive to the interference between singly and doubly resonant top-quark production. The unfolded results provide new information about the modelling of interference and off-shell effects of the W+W−bb¯$$ {W}^{+}{W}^{-}b\overline{b} $$ final state, improving the understanding of top-quark production mechanisms and providing new inputs for future Monte Carlo generator development.