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Register a new userHyperon vector form factor from 2+1 flavor lattice QCD
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Shoichi Sasaki
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We present the first result for the hyperon vector form factor f_1 for Xi^0 -> Sigma^+ l bar{nu} and Sigma^- -> n l bar{nu} semileptonic decays from fully dynamical lattice QCD. The calculations are carried out with gauge configurations generated by the RBC and UKQCD collaborations with (2+1)-flavors of dynamical domain-wall fermions and the Iwasaki gauge action at beta=2.13, corresponding to a cutoff 1/a=1.73 GeV. Our results, which are calculated at the lighter three sea quark masses (the lightest pion mass down to approximately 330 MeV), show that a sign of the second-order correction of SU(3) breaking on the hyperon vector coupling f_1(0) is negative. The tendency of the SU(3) breaking correction observed in this work disagrees with predictions of both the latest baryon chiral perturbation theory result and large N_c analysis.
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We present results for the hyperon vector form factor f_1 for $Xi^0 rightarrow Sigma^+ lbar{ u}$ and $Sigma^- rightarrow n lbar{ u}$ semileptonic decays from dynamical lattice QCD with domain-wall quarks. Simulations are performed on the 2+1 flavor gauge configurations generated by the RBC and UKQCD Collaborations with a lattice cutoff of 1/a = 1.7 GeV. Our preliminary results, which are calculated at the lightest sea quark mass (pion mass down to approximately 330 MeV), show that a sign of the second-order correction of SU(3) breaking on hyperon vector coupling f_1(0) is likely negative.
We present the first results for the Kl3 form factor from simulations with 2+1 flavours of dynamical domain wall quarks. Combining our result, namely f_+(0)=0.964(5), with the latest experimental results for Kl3 decays leads to |V_{us}|=0.2249(14), reducing the uncertaintity in this important parameter. For the O(p^6) term in the chiral expansion we obtain Delta f=-0.013(5).
We determine the hyperon vector couplings $f_1(0)$ for $Sigma^{-}rightarrow nl^-bar{ u_l}$ and $Xi^0rightarrowSigma^{+}l^-bar{ u_l}$ semileptonic decays in the continuum limit with (2+1)-flavors of dynamical domain-wall fermions, using the Iwasaki gauge action at two different lattice spacings of $a$=0.114(2) and 0.086(2) fm. A theoretical estimation of flavor SU(3)-breaking effect on the vector coupling is required to extract $V_{us}$ from the experimental rate of hyperon beta decays. We obtain the vector couplings $f_1(0)$ for $Sigmarightarrow N$ and $Xirightarrow Sigma$ beta-decays with an accuracy of less than one percent. We then find that lattice results of $f_1(0)$ combined with the best estimate of $|V_{us}|$ with imposing Cabibbo-Kobayashi-Maskawa (CKM) unitarity are slightly deviated from the experimental result of $|V_{us}f_1(0)|$ for the $Sigmarightarrow N$ beta-decay. This discrepancy can be attributed to an assumption made in the experimental analysis on $|V_{us}f_1(0)|$, where the induced second-class form factor $g_2$ is set to be zero regardless of broken SU(3) symmetry. We report on this matter and then estimate the possible value of $g_2(0)$, which is evaluated from the experimental decay rate with our lattice result of $f_1(0)$ under the first-row CKM-unitarity condition.
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 $overline{ell}_6 = 16.2 ~ (1.0)$.
We present high statistics results for the isovector nucleon charges and form factors using seven ensembles of 2+1-flavor Wilson-clover fermions. The axial and pseudoscalar form factors obtained on each ensemble satisfy the PCAC relation once the lowest energy $Npi$ excited state is included in the spectral decomposition of the correlation functions used for extracting the ground state matrix elements. Similarly, we find evidence that the $Npipi $ excited state contributes to the correlation functions with the vector current, consistent with the vector meson dominance model. The resulting form factors are consistent with the Kelly parameterization of the experimental electric and magnetic data. Our final estimates for the isovector charges are $g_{A}^{u-d} = 1.31(06)(05)_{sys}$, $g_{S}^{u-d} = 1.06(10)(06)_{sys}$, and $g_{T}^{u-d} = 0.95(05)(02)_{sys}$, where the first error is the overall analysis uncertainty and the second is an additional combined systematic uncertainty. The form factors yield: (i) the axial charge radius squared, ${langle r_A^2 rangle}^{u-d}=0.428(53)(30)_{sys} {rm fm}^2$, (ii) the induced pseudoscalar charge, $g_P^ast=7.9(7)(9)_{sys}$, (iii) the pion-nucleon coupling $g_{pi {rm NN}} = 12.4(1.2)$, (iv) the electric charge radius squared, ${langle r_E^2 rangle}^{u-d} = 0.85(12)(19)_{sys} {rm fm}^2$, (v) the magnetic charge radius squared, ${langle r_M^2 rangle}^{u-d} = 0.71(19)(23)_{rm sys} {rm fm}^2$, and (vi) the magnetic moment $mu^{u-d} = 4.15(22)(10)_{rm sys}$. All our results are consistent with phenomenological/experimental values but with larger errors. Lastly, we present a Pade parameterization of the axial, electric and magnetic form factors over the range $0.04< Q^2 <1$ GeV${}^2$ for phenomenological studies.
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