Supersymmetric μνSSM Model Impacts B Meson Decay

A theoretical study has investigated how new physics, specifically within the Supersymmetric Standard Model with neutrinos (μνSSM), influences the rare inclusive decay $B \to X_{\mathrm{s}} l^{+} l^{-}$. This decay is of particular interest because its rate and characteristics can be sensitive to particles and forces not accounted for in the Standard Model of particle physics. Researchers have identified the main contributions to the relevant Wilson coefficients, which are parameters describing the strength of interactions in the decay, and the particles associated with them within the μνSSM. The analysis focused on a systematic scan of the μνSSM parameter space, which allowed for the elucidation of the underlying physical mechanisms governing these dominant contributions. The obtained results are consistent with experimentally allowed regions, suggesting that the μνSSM could offer an explanation for potential future deviations observed in these decays. Experimental constraints from other relevant decays, such as $\bar{B} \to X_{\mathrm{s}}\gamma$, $B_{\mathrm{s}}^{0} \to \mu^{+} \mu^{-}$, and the 125 GeV Higgs boson mass, were also incorporated. A key part of the study was the systematic interference decomposition of the Wilson coefficient contributions to the forward-backward asymmetry (AFB). It was identified that the $C_7C_{10}$ and $C_9C_{10}$ interference terms are the dominant contributions governing the behavior of the AFB in both low- and high-$q^2$ (momentum transferred to the lepton pair) regions. Understanding these contributions is crucial for interpreting future measurements of the AFB, which is an observable sensitive to new physics and could reveal the existence of supersymmetric particles or other extensions of the Standard Model.

New Kinematic Dependencies in Collinear Scattering Amplitudes

Researchers have studied the spacelike-collinear limit of gauge-theory scattering amplitudes using the Method of Regions. They found that the resulting splitting amplitudes violate strict collinear factorization, showing a dependence on non-collinear partons. While the associated color dependence has long been known, starting at two loops, the splitting amplitude also acquires a dependence on the kinematics of these partons. This finding is crucial for understanding the complexity of particle interactions at high energies. The research reveals that this kinematic dependence originates from a unique hidden region, present in the asymptotic expansion of the five-point amplitude in the spacelike-collinear limit, but absent in the timelike limit. The authors propose that these hidden regions provide the mechanism by which crossing-related asymptotic limits cease to be analytically connected. They developed an algorithm for systematically identifying these hidden regions, applying it to the five-point amplitude in super Yang-Mills theory. By applying this algorithm, the scientists computed the hidden-region contributions to the complete set of basis integrals and recovered the exact kinematically dependent, factorization-violating splitting amplitude. In momentum space, the hidden region is characterized by soft and Glauber loop momenta. This discovery explains why the Wilson-line calculation captures the complete kinematic dependence, thereby accounting for the observed universality across gauge theories.

Quantum Spin Correlations to Search for Z' Bosons at Lepton Colliders

A theoretical study explores the potential of quantum spin correlations in top-antitop ($t\bar{t}$) pair production at future lepton colliders. The aim is to search for the existence of additional neutral gauge bosons, denoted $Z'$, which arise in extensions of the Standard Model with anomaly-free $U(1)$ symmetries. The research focuses on how the spin-density matrix, including the exchange of photons ($\gamma$), $Z$ bosons, and hypothetical $Z'$ bosons, as well as their interferences, can reveal the presence of these new particles. The researchers analyzed quantum-information observables such as the sufficient entanglement marker $\mathcal{D}_{\min}$, concurrence, purity, and the maximal Clauser-Horne-Shimony-Holt (CHSH) parameter. These were compared with information obtained from conventional production rates. Different charge assignments within the $U(1)_X$ framework were considered to investigate how chiral structures influence these observables, with particular emphasis on the $Z'$ resonance region and polarized $e^-e^+$ collisions. Electron-beam polarization provides a direct handle on the left- and right-handed lepton charges in various $U(1)_X$ scenarios. The results suggest that quantum spin observables provide information complementary to cross sections and angular distributions in searches for chiral neutral gauge interactions. This is crucial for identifying and characterizing potential $Z'$ bosons that could mediate fundamental forces yet unknown.