65 research outputs found

### Axial charges of hyperons and charmed baryons using $N_f=2+1+1$ twisted mass fermions

The axial couplings of the low lying baryons are evaluated using a total of
five ensembles of dynamical twisted mass fermion gauge configurations. The
simulations are performed using the Iwasaki gauge action and two degenerate
flavors of light quarks, and a strange and a charm quark fixed to approximately
their physical values at two values of the coupling constant. The lattice
spacings, determined using the nucleon mass, are $a=0.082$ fm and $a=0.065$ fm
and the simulations cover a pion mass in the range of about 210 MeV to 430 MeV.
We study the dependence of the axial couplings on the pion mass in the range of
about 210 MeV to 430 MeV as well as the $SU(3)$ breaking effects as we decrease
the light quark mass towards its physical value.Comment: 34 pages, 17 figure

### Sigma-terms and axial charges for hyperons and charmed baryons

We present results for the $\sigma$-terms and axial charges for various
hyperons and charmed baryons using $N_f=2+1+1$ twisted mass fermions. For the
computation of the three-point function we use the fixed current method. For
one of the $N_f=2+1+1$ ensembles with pion mass of 373 MeV we compare the
results of the fixed current method with those obtained with a stochastic
method for computing the all-to-all propagator involved in the evaluation of
the three point functions.Comment: Talk presented at 31st International Symposium on Lattice Field
Theory LATTICE 2013, July 29 - August 3, 2013, Mainz, Germany, PoS(LATTICE
2013)279. 7 pages and6 figure

### The nucleon spin and momentum decomposition using lattice QCD simulations

We determine within lattice QCD, the nucleon spin carried by valence and sea
quarks, and gluons. The calculation is performed using an ensemble of gauge
configurations with two degenerate light quarks with mass fixed to
approximately reproduce the physical pion mass. We find that the total angular
momentum carried by the quarks in the nucleon is $J_{u+d+s}{=}0.408(61)_{\rm
stat.}(48)_{\rm syst.}$ and the gluon contribution is $J_g {=}0.133(11)_{\rm
stat.}(14)_{\rm syst.}$ giving a total of $J_N{=}0.54(6)_{\rm stat.}(5)_{\rm
syst.}$ consistent with the spin sum. For the quark intrinsic spin contribution
we obtain $\frac{1}{2}\Delta \Sigma_{u+d+s}{=}0.201(17)_{\rm stat.}(5)_{\rm
syst.}$. All quantities are given in the $\overline{\textrm{MS}}$ scheme at
2~GeV. The quark and gluon momentum fractions are also computed and add up to
$\langle x\rangle_{u+d+s}+\langle x\rangle_g{=}0.804(121)_{\rm stat.}(95)_{\rm
syst.}+0.267(12)_{\rm stat.}(10)_{\rm syst.}{=}1.07(12)_{\rm stat.}(10)_{\rm
syst.}$ satisfying the momentum sum.Comment: Version published in PR

### Quark flavor decomposition of the nucleon axial form factors

We present results on the isoscalar form factors including the disconnected
contributions, as well as on the strange and charm quark form factors. Using
previous results on the isovector form factors, we determine the flavor
decomposition of the nucleon axial form factors. These are computed using an
ensemble of $N_f=2+1+1$ twisted mass fermions simulated with physical values of
quark masses. We investigate the SU(3) flavor symmetry and show that there is
up to 10\% breaking for the axial and up to 50\% for the induced pseudoscalar
form factors. By fitting the $Q^2$-dependence, we determined the corresponding
root mean square radii. The pseudoscalar coupling of the $\eta$ meson and the
nucleon is found to be $g_{\eta NN}=3.7(1.0)(0.7)$, and the Goldberger-Treiman
discrepancy for the octet combination about 50\%

### Strange nucleon electromagnetic form factors from lattice QCD

We evaluate the strange nucleon electromagnetic form factors using an
ensemble of gauge configurations generated with two degenerate maximally
twisted mass clover-improved fermions with mass tuned to approximately
reproduce the physical pion mass. In addition, we present results for the
disconnected light quark contributions to the nucleon electromagnetic form
factors. Improved stochastic methods are employed leading to high-precision
results. The momentum dependence of the disconnected contributions is fitted
using the model-independent z-expansion. We extract the magnetic moment and the
electric and magnetic radii of the proton and neutron by including both
connected and disconnected contributions. We find that the disconnected light
quark contributions to both electric and magnetic form factors are non-zero and
at the few percent level as compared to the connected. The strange form factors
are also at the percent level but more noisy yielding statistical errors that
are typically within one standard deviation from a zero value.Comment: 10 pages, 11 figure

### Nucleon scalar and tensor charges using lattice QCD simulations at the physical value of the pion mass

We present results on the light, strange and charm nucleon scalar and tensor
charges from lattice QCD, using simulations with $N_f=2$ flavors of twisted
mass Clover-improved fermions with a physical value of the pion mass. Both
connected and disconnected contributions are included, enabling us to extract
the isoscalar, strange and charm charges for the first time directly at the
physical point. Furthermore, the renormalization is computed non-perturbatively
for both isovector and isoscalar quantities. We investigate excited state
effects by analyzing several sink-source time separations and by employing a
set of methods to probe ground state dominance. Our final results for the
scalar charges are $g_S^u = 5.20(42)(15)(12)$, $g_S^d = 4.27(26)(15)(12)$,
$g_S^s=0.33(7)(1)(4)$, $g_S^c=0.062(13)(3)(5)$ and for the tensor charges
$g_T^u = 0.782(16)(2)(13)$, $g_T^d = -0.219(10)(2)(13)$,
$g_T^s=-0.00319(69)(2)(22)$, $g_T^c=-0.00263(269)(2)(37)$ in the $\overline{\rm
MS}$ scheme at 2~GeV. The first error is statistical, the second is the
systematic error due to the renormalization and the third the systematic
arising from possible contamination due to the excited states.Comment: 20 pages and 13 figure

### Complete flavor decomposition of the spin and momentum fraction of the proton using lattice QCD simulations at physical pion mass

We evaluate the gluon and quark contributions to the spin of the proton using
an ensemble of gauge configuration generated at physical pion mass. We compute
all valence and sea quark contributions to high accuracy. We perform a
non-perturbative renormalization for both quark and gluon matrix elements. We
find that the contribution of the up, down, strange and charm quarks to the
proton intrinsic spin is
$\frac{1}{2}\sum_{q=u,d,s,c}\Delta\Sigma^{q^+}=0.191(15)$ and to the total spin
$\sum_{q=u,d,s,c}J^{q^+}=0.285(45)$. The gluon contribution to the spin is
$J^g=0.187(46)$ yielding $J=J^q+J^g=0.473(71)$ confirming the spin sum. The
momentum fraction carried by quarks in the proton is found to be $0.618(60)$
and by gluons $0.427(92)$, the sum of which gives $1.045(118)$ confirming the
momentum sum rule. All scale and scheme dependent quantities are given in the
$\mathrm{ \overline{MS}}$ scheme at 2 GeV

### Nucleon axial, tensor and scalar charges and $\sigma$-terms in lattice QCD

We determine the nucleon axial, scalar and tensor charges within lattice
Quantum Chromodynamics including all contributions from valence and sea quarks.
We analyze three gauge ensembles simulated within the twisted mass
formulation at approximately physical value of the pion mass. Two of these
ensembles are simulated with two dynamical light quarks and lattice spacing
$a=0.094$~fm and the third with $a=0.08$~fm includes in addition the strange
and charm quarks in the sea. After comparing the results among these three
ensembles, we quote as final values our most accurate analysis using the latter
ensemble.
For the nucleon isovector axial charge we find $1.286(23)$ in agreement with
the experimental value. We provide the flavor decomposition of the intrinsic
spin $\frac{1}{2}\Delta\Sigma^q$ carried by quarks in the nucleon obtaining for
the up, down, strange and charm quarks $\frac{1}{2}\Delta\Sigma^{u}=0.431(8)$,
$\frac{1}{2}\Delta\Sigma^{d}=-0.212(8)$,
$\frac{1}{2}\Delta\Sigma^{s}=-0.023(4)$ and
$\frac{1}{2}\Delta\Sigma^{c}=-0.005(2)$, respectively. The corresponding values
of the tensor and scalar charges for each quark flavor are also evaluated
providing valuable input for experimental searches for beyond the standard
model physics.
In addition, we extract the nucleon $\sigma$-terms and find for the light
quark content $\sigma_{\pi N}=41.6(3.8)$~MeV and for the strange
$\sigma_{s}=45.6(6.2)$~MeV. The y-parameter that is used in phenomenological
studies we find $y=0.078(7)$.Comment: Expanded version as accepted in Phys. Rev. D.20 pages and 20 figure

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