14 Search Results

The parity-violating cross-section asymmetry in the elastic scattering of polarized electrons from unpolarized protons has been measured at a four-momentum transfer squared Q² = 0.624 GeV² and beam energy E_b = 3.48 GeV to be A_PV = -23.80 ± 0.78 (stat) ± 0.36 (syst) parts per million. This result is consistent with zero contribution of strange quarks to the combination of electric and magnetic form factors G_E^s + 0.517 G_M^s = 0.003 ± 0.010 (stat) ± 0.004 (syst) ± 0.009 (ff), where the third error is due to the limits of precision on the electromagnetic form factors and radiative corrections.more » With this measurement, the world data on strange contributions to nucleon form factors are seen to be consistent with zero and not more than a few percent of the proton form factors.« less

We measure the mass dependence of the forward-backward charge asymmetry in 157,553 pp = Z/γ* = e⁺e^- interactions, corresponding to 5.0 fb^-1 of integrated luminosity collected by the D0 experiment at the Fermilab Tevatron Collider at √s = 1.96 TeV. The effective weak mixing angle (θ^ℓ_eff) from this process involving predominantly the first generation of quarks is extracted as sin² θ^ℓ_eff = 0.2309 ± 0.0008 (stat.) ± 0.0006 (syst.). We also present the most precise direct measurement of the vector and axial-vector couplings of u and d quarks to the Z boson.

We describe an experiment to search for a new vector boson A' with weak coupling alpha' > 6 x 10^–8 α to electrons (α' = e²/4π) in the mass range 65 MeV < m_A' < 550 MeV. New vector bosons with such small couplings arise naturally from a small kinetic mixing of the "dark photon" A' with the photon -- one of the very few ways in which new forces can couple to the Standard Model -- and have received considerable attention as an explanation of various dark matter related anomalies. A' bosons are produced by radiation off an electronmore » beam, and could appear as narrow resonances with small production cross-section in the trident e⁺e^- spectrum. We summarize the experimental approach described in a proposal submitted to Jefferson Laboratory's PAC35, PR-10-009. This experiment, the A' Experiment (APEX), uses the electron beam of the Continuous Electron Beam Accelerator Facility at Jefferson Laboratory (CEBAF) at energies of ~1-4 GeV incident on 0.5-10% radiation length Tungsten wire mesh targets, and measures the resulting e+e- pairs to search for the A' using the High Resolution Spectrometer and the septum magnet in Hall A. With a ~1 month run, APEX will achieve very good sensitivity because the statistics of e+e- pairs will be ~10,000 times larger in the explored mass range than any previous search for the A' boson. These statistics and the excellent mass resolution of the spectrometers allow sensitivity to α'/α one to three orders of magnitude below current limits, in a region of parameter space of great theoretical and phenomenological interest. Similar experiments could also be performed at other facilities, such as the Mainz Microtron.« less

Around $$\theta_{\pi}=$$90$$^\circ$$, the coupling to the $$\rho^\circ N$$ channel leads to a good accounting of the charged pion exclusive photoproduction cross section in the energy range 3 < E_γ < 10 GeV, where experimental data exist. Starting from a Regge Pole approach that successfully describes vector meson production, the singular part of the corresponding box diagrams (where the intermediate vector meson-baryon pair propagates on-shell) is evaluated without any further assumptions (unitarity). Such a treatment provides an explanation of the $$s^{-7}$$ scaling of the cross section. Furthermore, elastic rescattering of the charged pion improves the basic Regge pole model at forwardmore » and backward angles.« less

We compute, for the first time using lattice QCD methods, radiative transition rates involving excited charmonium states, states of high spin and exotics. Utilizing a large basis of interpolating fields we are able to project out various excited state contributions to three-point correlators computed on quenched anisotropic lattices. In the first lattice QCD calculation of the exotic $$1^{-+}$$ $$\eta_{c1}$$ radiative decay, we find a large partial width $$\Gamma(\eta_{c1} \to J/\psi \gamma) \sim 100 \,\mathrm{keV}$$. We find clear signals for electric dipole and magnetic quadrupole transition form factors in $$\chi_{c2} \to J/\psi \gamma$$, calculated for the first time in this framework,more » and study transitions involving excited $$\psi$$ and $$\chi_{c1,2}$$ states. We calculate hindered magnetic dipole transition widths without the sensitivity to assumptions made in model studies and find statistically significant signals, including a non-exotic vector hybrid candidate $Y_{\mathrm{hyb?}} \to \et« less

A conceptual model of movement ecology has recently been advanced to explain all movement by considering the interaction of four elements: internal state, motion capacity, navigation capacities,and external factors. We modified this framework togenerate predictions for species richness dynamics of fragmented plant communities and tested them in experimental landscapes across a 7-year time series. We found that two external factors, dispersal vectors and habitat features, affected species colonization and recolonization in habitat fragments and their effects varied and depended on motion capacity. Bird-dispersed species richness showed connectivity effects that reached an asymptote over time, but no edge effects, whereas wind-dispersedmore » species richness showed steadily accumulating edge and connectivity effects, with no indication of an asymptote. Unassisted species also showed increasing differences caused by connectivity over time,whereas edges had no effect. Our limited use of proxies for movement ecology (e.g., dispersal mode as a proxy for motion capacity) resulted in moderate predictive power for communities and, in some cases, highlighted the importance of a more complete understanding of movement ecology for predicting how landscape conservation actions affect plant community dynamics.« less

Here an explicit operator construction of the axial vector current in the boson σ model is given in perturbation theory. This current is finite and satisfies the PCAC condition with the renormalized pion field. It also (at least formally) satisfies the SU₂ × SU₂ equal-time time-component current algebra.

Here we examine the medium strong mass splittings for pseudo-scalar mesons, vector mesons and spin-2/3 baryons in the framework of the «fifth interaction» proposed by Ne’eman. In addition to the resonance contributions from the low-lying states, we write an expression for the contribution from f₈ Regge exchange in terms of contributions to electromagnetic mass differences from $$\text{A}^0_2$$ Regge exchange. Assuming that the $$\text{A}^0_2$$ Regge pole gives the correct contribution to the e.m. mass differences, we estimate the contribution of the f₈-meson. In the approximation of neglecting the continuum spectrum, the ratio of tadpole contributions is given by the ratio ofmore » coupling constants of the f₈-meson with hadrons. The results of the tadpole model are in agreement with the subtraction scheme of Segrè. We also find that in the presence of such anSU₃-symmetry-breaking term, the renormalization of the hadronic weak-strangeness-changing vector current vertex, to second order in such a term, is quadratically divergent.« less

Using asymptotic SU(3) and superconvergence relations, the ω-φ mixing angle is determined and mass formulas for pseudoscalar and vector mesons are derived.

The πω problem, as the simplest case of the pseudoscalar-meson and vector-meson system, is discussed from the standpoint of the S-matrix approach. A general procedure of constructing invariant amplitudes in spin and isospin space of the pseudoscalar-vector system is given, and for πω scattering, a set of invariant amplitudes are conveniently chosen and their crossing properties are discussed. These amplitudes are expressed by one-dimensional representations which are derived from the Mandelstam representations by the Cini-Fubini technique. Partial-wave expansions as well as projections are done by the use of the Jacob-Wick helicity amplitudes. A prescription for calculating the driving forces frommore » the exchange of particles is presented and applied to the exchange of the ρ and B mesons in states of the two possible quantum numbers J^P =1⁺ and 2^–. The procedure consists of a zero-width approximation to the transition amplitudes in states of given J and L, crossing-symmetric relations, and the one-dimensional dispersion representations of the invariant amplitudes. The relationship between the invariant amplitudes and the helicity amplitudes greatly facilitates this procedure. The t-channel reaction is also analyzed. A method of solution of the partial-wave dispersion relations is discussed based on a recently developed formalism, and is extended further to avoid the difficulty associated with the zeros of the driving forces. A systematic program to understand the quantum numbers of the B meson as a πω resonance is also discussed. The qualitative nature of the forces due to the B exchange in states of each possible quantum number is briefly sketched. Finally, a model calculation which favors a 2^– state of the B meson is presented.« less