Four-momentum Transfer Research Articles

Lattice study of $J/\Psi$J/Ψ form factors with twisted boundary conditions

We present a lattice calculation for the electromagnetic form factors of $J/\Psi$ by using $N_{f}=2$ twisted mass gauge field configurations. We apply twisted boundary conditions to compute correlation functions at small values of momentum transfer that are difficult to access with periodic boundary conditions. The electromagnetic form factors with more small four-momentum transfer are determined, and further the charge radius and the quadrupole moment for $J/\Psi$ are fitted, respectively.

The P2 experiment

This article describes the future P2 parity-violating electron scattering facility at the upcoming MESA accelerator in Mainz. The physics program of the facility comprises indirect, high precision search for physics beyond the Standard Model, measurement of the neutron distribution in nuclear physics, single-spin asymmetries stemming from two-photon exchange and a possible future extension to the measurement of hadronic parity violation. The first measurement of the P2 experiment aims for a high precision determination of the weak mixing angle to a precision of 0.14% at a four-momentum transfer of Q^2 = 4.5 10^{-3} GeV^2. The accuracy is comparable to existing measurements at the Z pole. It comprises a sensitive test of the standard model up to a mass scale of 50 TeV, extendable to 70 TeV. This requires a measurement of the parity violating cross section asymmetry -39.94 10^{-9} in the elastic electron-proton scattering with a total accuracy of 0.56 10^-9 (1.4 %) in 10,000 h of 150 \micro A polarized electron beam impinging on a 60 cm liquid H_2 target allowing for an extraction of the weak charge of the proton which is directly connected to the weak mixing angle. Contributions from gamma Z-box graphs become small at the small beam energy of 155 MeV. The size of the asymmetry is the smallest asymmetry ever measured in electron scattering with an unprecedented goal for the accuracy. We report here on the conceptual design of the P2 spectrometer, its Cherenkov detectors, the integrating read-out electronics as well as the ultra-thin, fast tracking detectors. There has been substantial theory work done in preparation of the determination of the weak mixing angle. The further physics program in particle and nuclear physics is described as well.

Measurement of the Branching Fraction For the Semileptonic Decay D^{0(+)}→π^{-(0)}μ^{+}ν_{μ} and Test of Lepton Flavor Universality.

Using a data sample corresponding to an integrated luminosity of 2.93 fb^{-1} taken at a center-of-mass energy of 3.773GeV with the BESIII detector operated at the BEPCII collider, we perform an analysis of the semileptonic decays D^{0(+)}→π^{-(0)}μ^{+}ν_{μ}. The branching fractions of D^{0}→π^{-}μ^{+}ν_{μ} and D^{+}→π^{0}μ^{+}ν_{μ} are measured to be (0.272±0.008_{stat}±0.006_{syst})% and (0.350±0.011_{stat}±0.010_{syst})%, respectively, where the former is of much improved precision compared to previous results and the latter is determined for the first time. Using these results along with previous BESIII measurements of D^{0(+)}→π^{-(0)}e^{+}ν_{e}, we calculate the branching fraction ratios to be R^{0}≡B_{D^{0}→π^{-}μ^{+}ν_{μ}}/B_{D^{0}→π^{-}e^{+}ν_{e}}=0.922±0.030_{stat}±0.022_{syst} and R^{+}≡B_{D^{+}→π^{0}μ^{+}ν_{μ}}/B_{D^{+}→π^{0}e^{+}ν_{e}}=0.964±0.037_{stat}±0.026_{syst}, which are compatible with the theoretical expectation of lepton flavor universality within 1.7σ and 0.5σ, respectively. We also examine the branching fraction ratios in different four-momentum transfer square regions, and find no significant deviations from the standard model predictions.

Analysis of the semileptonic Brightarrow K_1 ell ^+ ell ^-transitions and non-leptonic B rightarrow K_1 gamma decay in the AdS/QCD correspondence

We consider the axial-vector mesons K_1(1270) and K_1(1400) as a mixture of two |^3P_1rangle and |^1P_1rangle states with the mixing angle theta that equal to (-34pm 13)^circ . We calculate the light-front distribution amplitudes (LFDAs) and decay constant formulas for both the axial-vector mesons K_1 in the AdS/QCD correspondence. The transition form factors of the semileptonic Brightarrow K_1 decays are derived in terms of the LFDAs for K_1 mesons. Using these form factors and decay constant values, the differential branching ratios of Brightarrow K_1 (1270, 1400) ell ^+ ell ^-, ell =mu ,tau transitions are plotted with respect to the four-momentum transfer squared, q^2. In addition, the branching ratio values of these decays and the non-leptonic B rightarrow K_1(1270, 1400) gamma decays are estimated. A comparison is made between our results for the branching ratios of Brightarrow K_1(1270, 1400) gamma decays in the AdS/QCD model and predictions obtained from the light-cone sum rules (LCSR) as well as the experimental values. Finally, the forward-backward asymmetries for the aforementioned semileptonic decays are plotted on q^2 in both the AdS/QCD correspondence and two Higgs doublet model (2HDM) in order to test the standard model (SM) and search for the new physics (NP).

Tensor form factor of D→π(K)ℓν and D→π(K)ℓℓ decays with Nf=2+1+1 twisted-mass fermions

We present the first lattice Nf=2+1+1 determination of the tensor form factor $f_T^{D \pi(K)}(q^2)$ corresponding to the semileptonic and rare $D \to \pi(K)$ decays as a function of the squared 4-momentum transfer $q^2$. Together with our recent determination of the vector and scalar form factors we complete the set of hadronic matrix elements regulating the semileptonic and rare $D \to \pi(K)$ transitions within and beyond the Standard Model, when a non-zero tensor coupling is possible. Our analysis is based on the gauge configurations produced by ETMC with Nf=2+1+1 flavors of dynamical quarks, which include in the sea, besides two light mass-degenerate quarks, also the strange and charm quarks with masses close to their physical values. We simulated at three different values of the lattice spacing and with pion masses as small as 220 MeV. The matrix elements of the tensor current are determined for plenty of kinematical conditions in which parent and child mesons are either moving or at rest. As in the case of the vector and scalar form factors, Lorentz symmetry breaking due to hypercubic effects is clearly observed also in the data for the tensor form factor and included in the decomposition of the current matrix elements in terms of additional form factors. After the extrapolations to the physical pion mass and to the continuum and infinite volume limits we determine the tensor form factor in the whole kinematical region accessible in the experiments. A set of synthetic data points, representing our results for $f_T^{D \pi(K)}(q^2)$ for several selected values of $q^2$, is provided and the corresponding covariance matrix is also available. At zero four-momentum transfer we get $f_T^{D \pi}(0) = 0.506 (79)$ and $f_T^{D K}(0) = 0.687 (54)$, which correspond to $f_T^{D \pi}(0)/f_+^{D \pi}(0) = 0.827 (114)$ and $f_T^{D K}(0)/f_+^{D K}(0)= 0.898 (50)$.

Semi-inclusive π0 target and beam-target asymmetries from 6 GeV electron scattering with CLAS

We present precision measurements of the target and beam-target spin asymmetries from neutral pion electroproduction in deep-inelastic scattering (DIS) using the CEBAF Large Acceptance Spectrometer (CLAS) at Jefferson Lab. We scattered 6-GeV, longitudinally polarized electrons off longitudinally polarized protons in a cryogenic 14NH3 target, and extracted double and single target spin asymmetries for ep→e′π0X in multidimensional bins in four-momentum transfer (1.0<Q2<3.2 GeV2), Bjorken-x (0.12<x<0.48), hadron energy fraction (0.4<z<0.7), transverse pion momentum (0<PT<1.0 GeV), and azimuthal angle ϕh between the lepton scattering and hadron production planes. We extracted asymmetries as a function of both x and PT, which provide access to transverse-momentum distributions of longitudinally polarized quarks. The double spin asymmetries depend weakly on PT. The sin⁡2ϕh moments are zero within uncertainties, which is consistent with the expected suppression of the Collins fragmentation function. The observed sin⁡ϕh moments suggest that quark gluon correlations are significant at large x.

Combination and QCD analysis of charm and beauty production cross-section measurements in deep inelastic ep scattering at HERA

Measurements of open charm and beauty production cross sections in deep inelastic ep scattering at HERA from the H1 and ZEUS Collaborations are combined. Reduced cross sections are obtained in the kinematic range of negative four-momentum transfer squared of the photon 2.5~hbox {GeV}^2le Q^2 le 2000, hbox {GeV}^2 and Bjorken scaling variable 3 cdot 10^{-5} le x_mathrm{Bj} le 5 cdot 10^{-2}. The combination method accounts for the correlations of the statistical and systematic uncertainties among the different datasets. Perturbative QCD calculations are compared to the combined data. A next-to-leading order QCD analysis is performed using these data together with the combined inclusive deep inelastic scattering cross sections from HERA. The running charm- and beauty-quark masses are determined as m_c(m_c) = 1.290^{+0.046}_{-0.041} mathrm{(exp/fit)}{}^{+0.062}_{-0.014} mathrm{(model)}{}^{+0.003}_{-0.031} mathrm{(parameterisation)} GeV and m_b(m_b) = 4.049^{+0.104}_{-0.109} mathrm{(exp/fit)}{}^{+0.090}_{-0.032} mathrm{(model)}{}^{+0.001}_{-0.031} mathrm{(parameterisation)}~mathrm{GeV}.

Separated kaon electroproduction cross section and the kaon form factor from 6 GeV JLab data

The $^{1}H$($e,e^\prime K^+$)$\Lambda$ reaction was studied as a function of the Mandelstam variable $-t$ using data from the E01-004 (FPI-2) and E93-018 experiments that were carried out in Hall C at the 6 GeV Jefferson Lab. The cross section was fully separated into longitudinal and transverse components, and two interference terms at four-momentum transfers $Q^2$ of 1.00, 1.36 and 2.07 GeV$^2$. The kaon form factor was extracted from the longitudinal cross section using the Regge model by Vanderhaeghen, Guidal, and Laget. The results establish the method, previously used successfully for pion analyses, for extracting the kaon form factor. Data from 12 GeV Jefferson Lab experiments are expected to have sufficient precision to distinguish between theoretical predictions, for example recent perturbative QCD calculations with modern parton distribution amplitudes. The leading-twist behavior for light mesons is predicted to set in for values of $Q^2$ between 5-10 GeV$^2$, which makes data in the few GeV regime particularly interesting. The $Q^2$ dependence at fixed $x$ and $-t$ of the longitudinal cross section we extracted seems consistent with the QCD factorization prediction within the experimental uncertainty.

Pion vector form factor from lattice QCD at the physical point

We present an investigation of the electromagnetic pion form factor, $F_\pi(Q^2)$, at small values of the four-momentum transfer $Q^2$ ($\lesssim 0.25$ GeV$^2$), based on the gauge configurations generated by European Twisted Mass Collaboration with $N_f = 2$ twisted-mass quarks at maximal twist including a clover term. Momentum is injected using non-periodic boundary conditions and the calculations are carried out at a fixed lattice spacing ($a \simeq 0.09$ fm) and with pion masses equal to its physical value, 240 MeV and 340 MeV. Our data are successfully analyzed using Chiral Perturbation Theory at next-to-leading order in the light-quark mass. For each pion mass two different lattice volumes are used to take care of finite size effects. Our final result for the squared charge radius is $\langle r^2 \rangle_\pi = 0.443~(29)$ fm$^2$, where the error includes several sources of systematic errors except the uncertainty related to discretization effects. The corresponding value of the SU(2) chiral low-energy constant $\overline{\ell}_6$ is equal to $\overline{\ell}_6 = 16.2 ~ (1.0)$.

Recent results from COMPASS on the GPD program

The dedicated COMPASS GPD program started in 2012 with a short pilot ’DVCS run’ and the first results from this run on DVCS, exclusive π0 and exclusive ω channels have been obtained. From the sum of cross sections measured with positive and negative beam polarities the DVCS cross-section and its dependence on the squared four-momentum transfer t have been extracted. It allowed a first estimate of the transverse size of the partonic distribution of the nucleon √〈r2⊥〉 i in a range of intermediate xBj with a mean xBj value of 0.056. We report also on the absolute value of the cross section for exclusive π0 production and its dependence on t and azimuthal angle ϕ between the scattering plane and π0 production plane. These measurements are aiming to constrain the GPDs, in particular chiral-odd (transversity) GPDs. Further, we present results on the spin density matrix elements for exclusive ω meson production and discuss the contributions from unnaturaly parity exchanges and from transitions that do not conserve helicity.

Study of the in-medium nucleon electromagnetic form factors using a light-front nucleon wave function combined with the quark-meson coupling model

We study the nucleon electromagnetic (EM) form factors in symmetric nuclear matter as well as in vacuum within a light-front approach using the in-medium inputs calculated by the quark-meson coupling model. The same in-medium quark properties are used as those used for the study of in-medium pion properties. The zero of the proton EM form factor ratio in vacuum, the electric to magnetic form factor ratio μpGEp(Q2)/GMp(Q2) (Q2=−q2>0 with q being the four-momentum transfer), is determined including the latest experimental data by implementing a hard constituent quark component in the nucleon wave function. A reasonable fit is achieved for the ratio μpGEp(Q2)/GMp(Q2) in vacuum, and we predict that the Q02 value to cross the zero of the ratio to be about 15 GeV2. In addition the double ratio data of the proton EM form factors in 4He and H nuclei, [GEp4He(Q2)/G4HeMp(Q2)]/[GEp1H(Q2)/GMp1H(Q2)], extracted by the polarized (e→,e′p→) scattering experiment on 4He at JLab, are well described. We also predict that the Q02 value satisfying μpGEp(Q02)/GMp(Q02)=0 in symmetric nuclear matter, shifts to a smaller value as increasing nuclear matter density, which reflects the facts that the faster falloff of GEp(Q2) as increasing Q2 and the increase of the proton mean-square charge radius. Furthermore, we calculate the neutron EM form factor double ratio in symmetric nuclear matter for 0.1<Q2<1.0 GeV2. The result shows that the neutron double ratio is enhanced relative to that in vacuum, while for the proton it is quenched, and agrees with an existing theoretical prediction.

Extraction of the neutron electric form factor from measurements of inclusive double spin asymmetries

Background: Measurements of the neutron charge form factor, GEn, are challenging because the neutron has no net charge. In addition, measurements of the neutron form factors must use nuclear targets which require accurately accounting for nuclear effects. Extracting GEn with different targets and techniques provides an important test of our handling of these effects. Purpose: The goal of the measurement was to use an inclusive asymmetry measurement technique to extract the neutron charge form factor at a four-momentum transfer of 1(GeV/c)2. This technique has very different systematic uncertainties than traditional exclusive measurements and thus serves as an independent check of whether nuclear effects have been taken into account correctly. Method: The inclusive quasielastic reaction 3He⃗(e⃗,e′) was measured at Jefferson Laboratory. The neutron electric form factor, GEn, was extracted at Q2=0.98(GeV/c)2 from ratios of electron-polarization asymmetries measured for two orthogonal target spin orientations. This Q2 is high enough that the sensitivity to GEn is not overwhelmed by the neutron magnetic contribution, and yet low enough that explicit neutron detection is not required to suppress pion production. Results: The neutron electric form factor, GEn, was determined to be 0.0414±0.0077(stat)±0.0022(syst), providing the first high-precision inclusive extraction of the neutron's charge form factor. Conclusions: The use of the inclusive quasielastic 3He⃗(e⃗,e′) with a four-momentum transfer near 1(GeV/c)2 has been used to provide a unique measurement of GEn. This new result provides a systematically independent validation of the exclusive extraction technique results and implies that the nuclear corrections are understood. This is contrary to the proton form factor where asymmetry and differential cross section measurements have been shown to have large systematic differences.Received 24 June 2017DOI:https://doi.org/10.1103/PhysRevC.96.065206©2017 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasPhotonuclear reactionsPhysical SystemsNeutronsPropertiesForm factorsNuclear PhysicsParticles & Fields

Design and performance of the spin asymmetries of the nucleon experiment

The Spin Asymmetries of the Nucleon Experiment (SANE) performed inclusive, double-polarized electron scattering measurements of the proton at the Continuous Electron Beam Accelerator Facility at Jefferson Lab. A novel detector array observed scattered electrons of four-momentum transfer 2.5<Q2<6.5GeV2 and Bjorken scaling 0.3<x<0.8 from initial beam energies of 4.7 and 5.9 GeV. Employing a polarized proton target whose magnetic field direction could be rotated with respect to the incident electron beam, both parallel and near perpendicular spin asymmetries were measured, allowing model-independent access to transverse polarization observables A1, A2, g1, g2 and moment d2 of the proton. This document summarizes the operation and performance of the polarized target, polarized electron beam, and novel detector systems used during the course of the experiment, and describes analysis techniques utilized to access the physics observables of interest.

Measurement of the shape of the Λb0→Λc+μ−ν¯μ differential decay rate

A measurement of the shape of the differential decay rate and the associated Isgur-Wise function for the decay Λb0→Λc+μ−ν¯μ is reported, using data corresponding to 3 fb−1 collected with the LHCb detector in proton-proton collisions. The Λc+μ−ν¯μ(+anything) final states are reconstructed through the detection of a muon and a Λc+ baryon decaying into pK−π+, and the decays Λb0→Λc+π+π−μ−ν¯μ are used to determine contributions from Λb0→Λc*+μ−ν¯μ decays. The measured dependence of the differential decay rate upon the squared four-momentum transfer between the heavy baryons, q2, is compared with expectations from heavy-quark effective theory and from unquenched lattice QCD predictions.Received 7 September 2017DOI:https://doi.org/10.1103/PhysRevD.96.112005Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.© 2017 CERN, for the LHCb CollaborationPhysics Subject Headings (PhySH)Research AreasForm factorsParticle decaysQuantum chromodynamicsParticles & Fields

COMPASS results on DVCS and exclusive π0 production

The high energy polarised muon beam available at CERN, with positive or negative charge, makes COMPASS a unique place for studies of General Parton Distributions (GPDs). The dedicated COMPASS GPD program started in 2012 with commissioning of a new long liquid hydrogen target and new detectors, such as the large recoil particle detector CAMERA and the large-angle electromagnetic calorimeter. It was followed by a short pilot ‘DVCS run’. The first results on DVCS and exclusive π0 channels have been obtained. From the sum of cross sections measured with positive and negative beam polarities the pure DVCS cross-section and its dependence on the squared four-momentum transfer t have been extracted. It allowed a first estimate of the transverse size of the partonic distribution of the nucleon in a range of intermediate xBj with a mean xBj value of 0.056. We report also on the absolute value of the cross section for exclusive π0 production and its dependence on t and azimuthal angle ϕ between the scattering plane and π0 production plane. These measurements are aiming to constrain the GPDs, in particular chiral-odd (transversity) GPDs. The full data taking for the GPD program approved within the COMPASS-II proposal continues in 2016 and 2017.

Slope and curvature of the hadronic vacuum polarization at vanishing virtuality from lattice QCD

We compute the slope and curvature, at vanishing four-momentum transfer squared, of the leading order hadron vacuum polarization function, using lattice QCD. Calculations are performed with 2+1+1 flavors of staggered fermions directly at the physical values of the quark masses and in volumes of linear extent larger than 6fm. The continuum limit is carried out using six different lattice spacings. All connected and disconnected contributions are calculated, up to and including those of the charm.

Measurement of νμ charged-current single π0 production on hydrocarbon in the few-GeV region using MINERvA

The semi-exclusive channel $\nu_{\mu}+\textrm{CH}\rightarrow\mu^{-}\pi^{0}+\textrm{nucleon(s)}$ is analyzed using MINERvA exposed to the low-energy NuMI $\nu_{\mu}$ beam with spectral peak at $E_{\nu} \simeq 3$ GeV. Differential cross sections for muon momentum and production angle, $\pi^{0}$ kinetic energy and production angle, and for squared four-momentum transfer are reported, and the cross section $\sigma(E_{\nu})$ is obtained over the range 1.5 GeV $\leq E_{\nu} <$ 20 GeV. Results are compared to GENIE and NuWro predictions and to published MINERvA cross sections for $\nu_{\mu}\textrm{-CC}(\pi^{+})$ and $\bar{\nu}_{\mu}\textrm{-CC}(\pi^{0})$. Disagreements between data and simulation are observed at very low and relatively high values for muon angle and for $Q^2$ that may reflect shortfalls in modeling of interactions on carbon. For $\pi^{0}$ kinematic distributions however, the data are consistent with the simulation and provide support for generator treatments of pion intranuclear scattering. Using signal-event subsamples that have reconstructed protons as well as $\pi^{0}$ mesons, the $p\pi^{0}$ invariant mass distribution is obtained, and the decay polar and azimuthal angle distributions in the rest frame of the $p\pi^{0}$ system are measured in the region of $\Delta(1232)^+$ production, $W < 1.4$ GeV.

Beam helicity asymmetries in $K^{+} \Lambda$ K + Λ electroproduction off the proton at low Q2

Beam helicity asymmetries in the $\overrightarrow{e}p \rightarrow e' K^{+} \Lambda $ reaction have been measured at unprecedentedly low four-momentum transfers $\langle Q^{2} \rangle = 0.05$ (GeV/c)2. At the Mainz Microtron (MAMI), the experiment was performed with a longitudinally polarized beam and an out-of-plane detection of the scattered electron. This experiment probed the associated helicity-dependent structure function ${\rm d} \sigma_{LT'}/{\rm d} \Omega_K^{c.m.}$ , which is sensitive to the details of resonances of the proton. The results were compared to models for kaon electroproduction using effective Lagrangians. The MAMI data is not supporting the Kaon-Maid isobar model, which uses strong longitudinal couplings of the virtual photon to nucleon resonances and predicts a strong peaking of the structure function at forward angles and low $Q^{2}$ . The data is also in disagreement with a Regge-plus-resonance model that predicts the incorrect sign of the structure function. The combination of the MAMI results wih data taken at higher four-momentum transfers measured at Jefferson Lab indicates a smooth transition in $Q^{2}$ without significant changes of the interference pattern in the electroproduction process.

Scalar and vector form factors of D→π(K)ℓν decays with Nf=2+1+1 twisted fermions

We present a lattice determination of the vector and scalar form factors of the $D\ensuremath{\rightarrow}\ensuremath{\pi}\ensuremath{\ell}\ensuremath{\nu}$ and $D\ensuremath{\rightarrow}K\ensuremath{\ell}\ensuremath{\nu}$ semileptonic decays, which are relevant for the extraction of the CKM matrix elements $|{V}_{cd}|$ and $|{V}_{cs}|$ from experimental data. Our analysis is based on the gauge configurations produced by the European Twisted Mass Collaboration with ${N}_{f}=2+1+1$ flavors of dynamical quarks, at three different values of the lattice spacing ($a\ensuremath{\simeq}0.062,0.082,0.089\text{ }\text{ }\mathrm{fm}$) and with pion masses as small as 210 MeV. Quark momenta are injected on the lattice using nonperiodic boundary conditions. The matrix elements of both vector and scalar currents are determined for plenty of kinematical conditions in which parent and child mesons are either moving or at rest. Lorentz symmetry breaking due to hypercubic effects is clearly observed in the data and included in the decomposition of the current matrix elements in terms of additional form factors. After the extrapolations to the physical pion mass and to the continuum limit, we determine the vector and scalar form factors in the whole kinematical region from ${q}^{2}=0$ up to ${q}_{\mathrm{max}}^{2}=({M}_{D}\ensuremath{-}{M}_{\ensuremath{\pi}(K)}{)}^{2}$ accessible in the experiments, obtaining a good overall agreement with experiments, except in the region at high values of ${q}^{2}$ where some deviations are visible. A set of synthetic data points, representing our results for ${f}_{+}^{D\ensuremath{\pi}(K)}({q}^{2})$ and ${f}_{0}^{D\ensuremath{\pi}(K)}({q}^{2})$ for several selected values of ${q}^{2}$, is provided and also the corresponding covariance matrix is available. At zero four-momentum transfer, we get ${f}_{+}^{D\ensuremath{\rightarrow}\ensuremath{\pi}}(0)=0.612(35)$ and ${f}_{+}^{D\ensuremath{\rightarrow}K}(0)=0.765(31)$. Using the experimental averages for $|{V}_{cd}|{f}_{+}^{D\ensuremath{\rightarrow}\ensuremath{\pi}}(0)$ and $|{V}_{cs}|{f}_{+}^{D\ensuremath{\rightarrow}K}(0)$, we extract $|{V}_{cd}|=0.2330(137)$ and $|{V}_{cs}|=0.945(38)$, respectively. The second row of the CKM matrix is found to be in agreement with unitarity within the current uncertainties: $|{V}_{cd}{|}^{2}+|{V}_{cs}{|}^{2}+|{V}_{cb}{|}^{2}=0.949(78)$.

Beam-Recoil Polarization Measurement of π^{0} Electroproduction on the Proton in the Region of the Roper Resonance.

The helicity-dependent recoil proton polarizations P_{x}^{'} and P_{z}^{'} as well as the helicity-independent component P_{y} have been measured in the p(e[over →],e^{'}p[over →])π^{0} reaction at four-momentum transfer Q^{2}≃0.1 GeV^{2}, center-of-mass proton emission angle θ_{p}^{*}≃90°, and invariant mass W≃1440 MeV. This first precise measurement of double-polarization observables in the energy domain of the Roper resonance P_{11}(1440) by exploiting recoil polarimetry has allowed for the extraction of its scalar electroexcitation amplitude at an unprecedentedly low value of Q^{2}, establishing a powerful instrument for probing the interplay of quark and meson degrees of freedom in the nucleon.