58 Search Results

Here, we report the first evidence of spin interference in exclusive 𝐽/𝜓 → 𝑒⁺⁢𝑒⁻ photoproduction in ultraperipheral heavy-ion collisions at STAR at $$\sqrt{𝑠_{𝑁⁢𝑁}}$$ = 200  GeV. In Au + Au collisions, a negative cos⁡(2⁢𝜙) modulation is found for 𝑝_𝑇 < 120  MeV/𝑐 with a significance of 3.2⁢𝜎, while the isobar data (Ru + Ru, Zr + Zr) show a consistent negative modulation with a significance of 1.9⁢𝜎, opposite in sign to that in 𝜌⁰ → 𝜋⁺⁢𝜋⁻ photoproduction. This establishes for the first time that the interference sign is controlled by the spin structure of the final-state daughters, resolving the ambiguity present inmore » the all-boson 𝜌⁰ channel. The compact 𝐽/𝜓 probes gluon distributions at perturbative scales, resulting in a weaker modulation and providing stringent constraints on color glass condensate calculations. These findings demonstrate that spin-dependent interference in heavy vector mesons provides a new, experimentally accessible handle on gluon structure beyond traditional cross-section measurements.« less

Chiral molecules, ubiquitous in chemistry and biology, can differentiate electrons by their spin, a phenomenon known as chirality-induced spin selectivity (CISS). Despite its robustness and technological relevance, CISS has resisted conventional explanation: Spin-orbit coupling (SOC) models cannot fully account for the observed magnitude, room-temperature persistence, or equilibrium signatures. Here, we argue that structural chirality enforces a twin-pair exchange mechanism via the indistinguishability principle, which intrinsically couples spin and spatial degrees of freedom such that wave functions cannot be factorized into spin and spatial components. We derive an effective Hamiltonian that describes both transport and equilibrium CISS phenomena and is non-Hermitian.more » However, the inherent pseudo-Hermiticity, with $$\mathscr{PT}$$ symmetry as a special case, ensures real eigenvalues and thermodynamic consistency. We demonstrate that our framework is a step toward resolving long-standing anomalies of CISS. It situates CISS alongside equilibrium symmetry-breaking phenomena such as ferromagnetism and superconductivity, with implications for spintronics, catalysis, and the origins of biological homochirality.« less

The STAR experiment reports new, high-precision measurements of the transverse single-spin asymmetries for 𝜋^± within jets, namely the Collins asymmetries, from transversely polarized 𝑝^↑⁢𝑝 collisions at $$\sqrt{s}$$ = 510 GeV. The energy-scaled distribution of jet transverse momentum, 𝑥_T = 2⁢𝑝_T,jet/$$\sqrt{s}$$, shows a remarkable consistency for Collins asymmetries of 𝜋^± in jets between $$\sqrt{s}$$ = 200 GeV and 510 GeV. This indicates that the Collins asymmetries are nearly energy independent, with, at most, a very weak scale dependence in 𝑝^↑⁢𝑝 collisions. These results extend to high-momentum scales (𝑄² ≤ 3400 GeV²) and enable unique tests of evolution and universality in themore » transverse-momentum-dependent formalism, thus providing important constraints for the Collins fragmentation functions.« less

The conservation of lepton flavor is a prediction of the Standard Model and is still an excellent approximate symmetry despite our observation of neutrino oscillations. Lepton flavor violation by one or two units has been discussed for decades, with several dedicated experiments exploring the vast model landscape but no discoveries so far. Here, we explore operators and processes that violate at least one lepton flavor by three units and identify testable signatures. In the Standard Model effective field theory, such operators already arise at mass dimension 7 and can be tested through their contributions to Michel parameters in leptonic decays.more » True neutrinoless charged-lepton flavor violation arises at mass dimension 10 and can realistically only be seen in the tau decay channels 𝜏 → $$𝑒⁢𝑒⁢𝑒⁢\bar{𝜇}⁢\bar{𝜇}$$ or 𝜏 → $$𝜇⁢𝜇⁢𝜇\bar{𝑒}\bar{𝑒}$$, for example in Belle II. Testable rates for these tau decays require light new particles and subsequently predict an avalanche of remarkably clean but so-far unconstrained collider signatures.« less

The hadron physics community has been actively debating the interpretation of so-called mechanical properties of hadrons. Nonrelativistic quantum-mechanical systems like the hydrogen atom have been appealed to in these debates as analogies. Since such appeals are likely to continue, it is important to have Galilei-covariant expressions for matrix elements of the energy-momentum tensor. In this work, I obtain Galilei-covariant breakdowns of such matrix elements into mechanical form factors, with a special focus on spin-half states. I additionally study the spatial densities associated with these form factors, using the pilot wave interpretation to guide their breakdown into contributions from internal structuremore » and from quantum-mechanical effects such as wave packet dispersion. For completeness, I also obtain nonrelativistic Breit frame densities.« less

Combining the synthetic tunability of molecular compounds with the optical selection rules of transition metal dichalcogenides (TMDCs) that derive from spin-valley coupling could provide interesting opportunities for the readout of quantum information. However, little is known about the electronic and spin interactions at such interfaces and the influence on spin-valley relaxation. Here, in this work, vanadyl phthalocyanine (VOPc) molecular layers are thermally evaporated on WSe₂ to explore the effect of molecular layer thickness on excited-state spin-valley polarization. The thinnest molecular layer supports an interfacial state which destroys the spin-valley polarization almost instantaneously, whereas a thicker molecular layer results in longer-livedmore » spin-valley polarization than the WSe₂ monolayer alone. The mechanism appears to involve a tightly bound species at the molecule/TMDC interface that strengthens exchange interactions and is largely avoided in thicker VOPc layers that isolate electrons from WSe₂ holes.« less

A study of resonant structures in 𝐵⁺→𝐷*^+⁢𝐷⁻⁢𝐾⁺ and 𝐵⁺→𝐷*^−⁢𝐷⁺⁢𝐾⁺ decays is performed, using proton-proton collision data at center-of-mass energies of √𝑠 =7, 8, and 13 TeV recorded by the LHCb experiment, corresponding to an integrated luminosity of 9 fb⁻¹. A simultaneous amplitude fit is performed to the two channels with contributions from resonances decaying to 𝐷*⁻⁢𝐷⁺ and 𝐷*⁺⁢𝐷⁻ states linked by 𝐶 parity. This procedure allows the 𝐶 parities of resonances in the 𝐷*^±⁢𝐷^∓ mass spectra to be determined. Four charmonium or charmoniumlike states are observed decaying into 𝐷*^±⁢𝐷^∓: 𝜂_𝑐⁡(3945), ℎ_𝑐⁡(4000), 𝜒_𝑐⁢1⁡(4010), and ℎ_𝑐⁡(4300), with quantum numbers 𝐽^𝑃⁢𝐶 equal tomore » 0⁻⁺, 1⁺⁻, 1⁺⁺, and 1⁺⁻, respectively. At least three of these states have not been observed previously. In addition, the existence of the 𝑇$$^*_{\bar{c}\bar{s}0}$$⁢(2870)⁰ and 𝑇$$^*_{\bar{c}\bar{s}1}$$(2900)⁰ resonances in the 𝐷⁻⁢𝐾⁺ mass spectrum, already observed in the 𝐵⁺→𝐷⁺⁢𝐷⁻⁢𝐾⁺ decay, is confirmed in a different production channel.« less

Spin accumulation in semiconductor structures at room temperature and without magnetic fields is key to enable a broader range of optoelectronic functionality. Current efforts are limited owing to inherent inefficiencies associated with spin injection across semiconductor interfaces. Here we demonstrate spin injection across chiral halide perovskite/III-V interfaces achieving spin accumulation in a standard semiconductor III-V (Al_xGa_1-x)_0.5In_0.5P multiple quantum well light-emitting diode. The spin accumulation in the multiple quantum well is detected through emission of circularly polarized light with a degree of polarization of up to 15 ± 4%. The chiral perovskite/III-V interface was characterized with X-ray photoelectron spectroscopy, cross-sectional scanningmore » Kelvin probe force microscopy and cross-sectional transmission electron microscopy imaging, showing a clean semiconductor/semiconductor interface at which the Fermi level can equilibrate. Finally, these findings demonstrate that chiral perovskite semiconductors can transform well-developed semiconductor platforms into ones that can also control spin.« less

In a previous publication [Benić , ], we have computed the perturbative QCD contribution to transverse single spin asymmetries (SSAs) in semi-inclusive deep inelastic scattering (SIDIS) involving the $g_T\left(x\right)$ distribution. In this paper, we first present a more efficient derivation of the asymmetries which is applicable to both transverse and longitudinal SSAs and correct some inconsistencies in our previous calculation. We then adapt the method to compute transverse SSAs in the Drell-Yan process proportional to $g_T\left(x\right)$ and its gluonic counterpart and discuss the crossingmore » symmetry between the results for SIDIS and the Drell-Yan process. Finally, we present numerical results for various asymmetries measurable at the EIC, RHIC, COMPASS, and Fermilab (SpinQuest), including also part of the genuine twist-three corrections to $g_T\left(x\right)$ from a recent global analysis. We find that the asymmetries can reach percent-level magnitude, if the kinematics predominantly probes the large- $x$ (valence) region of the polarized proton, but remain at subpercent levels otherwise. Published by the American Physical Society 2024« less

Abstract Carbon‐based quantum dots (QDs) enable flexible manipulation of electronic behavior at the nanoscale, but controlling their magnetic properties requires atomically precise structural control. While magnetism is observed in organic molecules and graphene nanoribbons (GNRs), GNR precursors enabling bottom‐up fabrication of QDs with various spin ground states have not yet been reported. Here the development of a new GNR precursor that results in magnetic QD structures embedded in semiconducting GNRs is reported. Inserting one such molecule into the GNR backbone and graphitizing it results in a QD region hosting one unpaired electron. QDs composed of two precursor molecules exhibit nonmagnetic,more » antiferromagnetic, or antiferromagnetic ground states, depending on the structural details that determine the coupling behavior of the spins originating from each molecule. The synthesis of these QDs and the emergence of localized states are demonstrated through high‐resolution atomic force microscopy (HR‐AFM), scanning tunneling microscopy (STM) imaging, and spectroscopy, and the relationship between QD atomic structure and magnetic properties is uncovered. GNR QDs provide a useful platform for controlling the spin‐degree of freedom in carbon‐based nanostructures.« less