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C. Alexandrou

Publications and source records attributed to C. Alexandrou.

At least 91 records · Page 5Linked to original sources
arXiv2009

The low-lying baryon spectrum with two dynamical twisted mass fermions

The masses of the low lying baryons are evaluated using two degenerate flavors of twisted mass sea quarks corresponding to pseudo scalar masses in the range of about 270-500 MeV. The strange valence quark mass is tuned to reproduce the mass of the kaon in the physical limit. The tree-level Symanzik improved gauge action is employed. We use lattices of spatial size 2.1 fm and 2.7 fm at two values of the lattice spacing with $r_0/a=5.22(2)$ and $r_0/a=6.61(3)$. We check for both finite volume and cut-off effects on the baryon masses. We performed a detailed study of the chiral extrapolation of the octet and decuplet masses using SU(2) $χ$PT. The lattice spacings determined using the nucleon mass at the physical point are consistent with the values extracted using the pion decay constant. We examine the issue of isospin symmetry breaking for the octet and decuplet baryons and its dependence on the lattice spacing. We show that in the continuum limit isospin breaking is consistent with zero, as expected. The baryon masses that we find after taking the continuum limit and extrapolating to the physical limit are in good agreement with experiment.

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arXiv2009

Partially quenched study of strange baryon with Nf = 2 twisted mass fermions

We present results on the mass of the baryon octet and decuplet using two flavors of light dynamical twisted mass fermions. The strange quark mass is fixed to its physical value from the kaon sector in a partially quenched set up. Calculations are performed for light quark masses corresponding to a pion mass in the range 270-500 MeV and lattice sizes of 2.1 fm and 2.7 fm. We check for cut-off effects and isospin breaking by evaluating the baryon masses at two different lattice spacings. We carry out a chiral extrapolation for the octet baryons and discuss results for the Omega.

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arXiv2009

Baryon structure from Lattice QCD

We present recent lattice results on the baryon spectrum, nucleon electromagnetic and axial form factors, nucleon to $Δ$ transition form factors as well as the $Δ$ electromagnetic form factors. The masses of the low lying baryons and the nucleon form factors are calculated using two degenerate flavors of twisted mass fermions down to pion mass of about 270 MeV. We compare to the results of other collaborations. The nucleon to $Δ$ transition and $Δ$ form factors are calculated in a hybrid scheme, which uses staggered sea quarks and domain wall valence quarks. The dominant dipole nucleon to $Δ$ transition form factor is also evaluated using dynamical domain wall fermions. The transverse density distributions of the $Δ$ in the infinite momentum frame are extracted using the form factors determined from lattice QCD.

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arXiv2008

A novel method for the determination of hadron excited states in Lattice QCD applied to the nucleon

A novel method for the precise identification and determination of the energies that contribute in the spectral decomposition of lattice correlators is presented. The method is based on statistical concepts and it relies heavily on simulation techniques. The $η_c$ correlator is analyzed within this method and the results obtained are compared to a previous analysis based on Bayesian statistics. An analysis of the nucleon local two-point correlators leads to the identification of the excited states in the positive and negative channels. A discussion on the Roper is included.

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arXiv2008

Delta-baryon electromagnetic form factors in lattice QCD

We develop techniques to calculate the four Delta electromagnetic form factors using lattice QCD, with particular emphasis on the sub-dominant electric quadrupole form factor that probes deformation of the Delta. Results are presented for pion masses down to approximately 350 MeV for three cases: quenched QCD, two flavors of dynamical Wilson quarks, and three flavors of quarks described by a mixed action combining domain wall valence quarks and dynamical staggered sea quarks. The magnetic moment of the Delta is chirally extrapolated to the physical point and the Delta charge density distributions are discussed.

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arXiv2008

Light baryon masses with dynamical twisted mass fermions

We present results on the mass of the nucleon and the Delta using two dynamical degenerate twisted mass quarks and the tree-level Symanzik improved gauge action. The evaluation is performed at four quark masses corresponding to a pion mass in the range of about 300-600 MeV on lattices of 2.1-2.7 fm. We check for cut-off effects by evaluating these baryon masses on lattices of spatial size 2.1 fm at beta=3.9 and beta=4.05 and on a lattice of 2.4 fm at beta=3.8. The values we find are compatible within our statistical errors. Lattice results are extrapolated to the physical limit using continuum chiral perturbation theory. Performing a combined fit to our lattice data at beta=3.9 and beta=4.05 we find a nucleon mass of 964\pm 28 (stat.) \pm 8 (syst.) MeV. The nucleon mass at the physical point provides an independent determination of the lattice spacing. Using heavy baryon chiral perturbation theory at O(p^3) we find a_{β=3.9}=0.0890\pm 0.0039(stat.) \pm 0.0014(syst.) fm, and a_{β=4.05}= 0.0691\pm 0.0034(stat.) \pm 0.0010(syst.) fm, in good agreement with the values determined from the pion decay constant. Isospin violating lattice artifacts in the Delta-system are found to be compatible with zero for the values of the lattice spacings used in this work. Performing a combined fit to our lattice data at beta=3.9 and beta=4.05 we find for the masses of the Delta^{++,-} and Delta^{+,0} 1316 \pm 60 (stat.) MeV and 1330 \pm 74 (stat.) MeV respectively. We confirm that in the continuum limit they are also degenerate.

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arXiv2007

The nucleon to Delta electromagnetic transition form factors in lattice QCD

The electromagnetic nucleon to Delta transition form factors are evaluated using two degenerate flavors of dynamical Wilson fermions and using dynamical sea staggered fermions with domain wall valence quarks. The two subdominant quadrupole form factors are evaluated for the first time in full QCD to sufficient accuracy to exclude a zero value, which is taken as a signal for deformation in the nucleon-Delta system. For the Coulomb quadrupole form factor the unquenched results show deviations from the quenched results at low q^2 bringing dynamical lattice results closer to experiment, thereby confirming the importance of pion cloud contributions on this quantity.

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arXiv2007

Nucleon and Nucleon to Delta Axial form factors from Lattice QCD

We present results on the nucleon axial vector form factors $G_A(q^2)$ and $G_p(q^2)$ in the quenched theory and using two degenerate flavors of dynamical Wilson fermions for momentum transfer squared from about 0.1 to about 2 GeV^2 and for pion masses in the range of 380 to 600 MeV. We also present results on the corresponding N to Delta axial vector transition form factors $C_5^A(q^2)$ and $C_6^A(q^2)$ using, in addition to Wilson fermions, domain wall valence quarks and dynamical staggered sea quarks provided by the MILC collaboration.

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arXiv2007

N to Delta electromagnetic and axial form factors in full QCD

Lattice results on the N to Delta electromagnetic, axial-vector and pseudoscalar form factors are evaluated using dynamical staggered sea quarks and domain wall valence quarks for pion masses in the range of 580-350 MeV, as well as, dynamical and quenched Wilson fermions for similar pion masses.

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arXiv2007

Axial Nucleon and Nucleon to Delta form fractors and the Goldberger-Treiman Relations from Lattice QCD

We evaluate the nucleon axial form factor, $G_A(q^2)$, and induced pseudoscalar form factor, $G_p(q^2)$, as well as the pion-nucleon form factor, $G_{πN N}(q^2)$, in lattice QCD. We also evaluate the corresponding nucleon to $Δ$ transition form factors, $C_5^A(q^2)$ and $C_6^A(q^2)$, and the pion-nucleon-$Δ$ form factor $G_{πNΔ}(q^2)$. The nucleon form factors are evaluated in the quenched theory and with two degenerate flavors of dynamical Wilson fermions. The nucleon to $Δ$ form factors, besides Wilson fermions, are evaluated using domain wall valence fermions with staggered sea quark configurations for pion masses as low as about 350 MeV. Using these form factors, together with an evaluation of the renormalized quark mass, we investigate the validity of the diagonal and non-diagonal Goldberger-Treiman relations. The ratios $G_{πNΔ}(q^2)/G_{πNN}(q^2)$ and $2C_5^A(q^2)/G_A(q^2)$ are constant as a function of the momentum transfer squared and show almost no dependence on the quark mass. We confirm equality of these two ratios consistent with the Goldberger-Treiman relations extracting a mean value of $1.61(2)$.

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arXiv2007

The axial N to Delta transition form factors from Lattice QCD

We evaluate the N to Delta axial transition form factors in lattice QCD in the quenched theory, with two degenerate flavors of dynamical Wilson fermions and using domain wall valence fermions with staggered sea quark configurations. We predict the ratio $C_5^A(q^2)/C_3^V(q^2)$ relevant to the parity violating asymmetry and check the validity of the off-diagonal Goldberger-Treiman relation.

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arXiv2006

Evidence for diquarks in lattice QCD

Diquarks may play an important role in hadron spectroscopy, baryon decays and color superconductivity. We investigate the existence of diquark correlations in lattice QCD by considering systematically all the lowest energy diquark channels in a color gauge-invariant setup. We measure mass differences between the various channels and show that the positive parity scalar diquark is the lightest. Quark-quark correlations inside the diquark are clearly seen in this channel, and yield a diquark size of order 1 fm.

hep-lat↗
arXiv2006

Hadron form factors using density-density correlators

Gauge invariant density-density correlators yield detailed information on hadron structure. Hadron deformation and form factors can be extracted for momentum transfers up to about 6 GeV$^2$. We use stochastic techniques and dilution to compute the all to all propagator required for the exact evaluation of density-density correlators. We present first results for the pion form factor.

hep-lat↗
arXiv2006

Hadron Deformation and Form Factors from Lattice QCD

We review the current status of lattice QCD studies of the nucleon system. In particular, we focus on the determination of the shape of the nucleon by probing its wave function as well as by evaluating the N to Delta transition form factors.

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arXiv2006

The nucleon electromagnetic form factors from Lattice QCD

We evaluate the isovector nucleon electromagnetic form factors in quenched and full QCD on the lattice using Wilson fermions. In the quenched theory we use a lattice of spatial size 3 fm at beta=6.0 enabling us to reach low momentum transfers and a lowest pion mass of about 400 MeV. In the full theory we use a lattice of spatial size 1.9 fm at beta=5.6 and lowest pion mass of about 380 MeV enabling comparison with the results obtained in the quenched theory. We compare our lattice results to the isovector part of the experimentally measured form factors.

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arXiv2005

A lattice study of the pentaquark states

We present a study of the pentaquark system in quenched lattice QCD using diquark-diquark and kaon-nucleon local and smeared interpolating fields. We examine the volume dependence of the spectral weights of local correlators on lattices of size $16^3\times 32$, $24^3\times32$ and $32^3\times 64$ at $β=6.0$. We find that a reliable evaluation of the volume dependence of the spectral weights requires accurate determination of the correlators at large time separations. Our main result from the spectral weight analysis in the pentaquark system is that within our variational basis and statistics we can not exclude a pentaquark resonance. However our data also do not allow a clear identification of a pentaquark state since only the spectral weights of the lowest state can be determined to sufficient accuracy to test for volume dependence. In the negative parity channel the mass extracted for this state is very close to the KN threshold whereas in the positive parity channel is about 60% above.

hep-lat↗
arXiv2005

Searching for diquarks in hadrons

Since the early days of QCD, it has been argued that inside hadrons quarks organise into substructures. Among those, popular pictures of various phenomena (including exotica and colour superconductivity) give a prominent role to diquarks, i.e. to colour antitriplet combinations of two quarks. Using a gauge-invariant setup, which combines the diquark with a static quark, we study spatial correlations of the two light quarks inside the diquark. After illustrating the setup, we discuss our first results for both the scalar (``good'') and the spin one (``bad'') diquark channels. In a regime in which $m_π \simeq 830$ MeV, our data show unambiguously that a scalar diquark forms with a size of $\simeq$ 0.9 fm. For the vector diquark we are able to put a lower bound of 4.1(7) fm on its size. We also investigate the mass splitting between the good and the bad diquark; our findings are compatible with phenomenologically inspired predictions for this quantity.

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