arXiv subjects

Christian Ehmann

Publications and source records attributed to Christian Ehmann.

6 recordsLinked to original sources

arXiv2011

Charmonium spectroscopy and mixing with light quark and open charm states from nF=2 lattice QCD

We study the charmonium spectrum including higher spin and gluonic excitations. We determine an upper limit on the mixing of the eta_c ground state with light pseudoscalar flavour-singlet mesons and investigate the mixing of charmonia near open charm thresholds with pairs of (excited) D and anti-D mesons. For charm and light valence quarks and nF=2 sea quarks, we employ the non-perturbatively improved Sheikholeslami-Wohlert (clover) action. Excited states are accessed using the variational technique, starting from a basis of suitably optimised operators. For some aspects of this study, the use of improved stochastic all-to-all propagators was essential.

hep-lat↗

arXiv2009

Mixing of S-Wave Charmonia with $\mathrm{D}\overline{\mathrm{D}}$ Molecule States

Charmonium states can decay into pairs of $D$ and $\overline{D}$ mesons if their masses are above the allowed decay thresholds. In general $c\bar{c}$ states near threshold will also undergo mixing with $D\overline{D}$ molecular (or tetraquark) states, by creation and annihilation of light quark-antiquark pairs. The investigation of such effects sheds light on the higher Fock state contributions to charmonium wavefunctions and on mass shifts, relative to a scenario where such mixing effects are neglected. A variational approach is applied to a mixing matrix between operators of both sectors, of $c\bar{c}$ and of $D\overline{D}$ molecular type. The efficient calculation of several diagrams appearing in this matrix requires all-to-all propagators, which are realized by sophisticated stochastic estimator techniques. The runs are performed on $n_F=2$ $24^3\times 48$ lattice volumes with $m_π \approx 280$ MeV, using the non-perturbatively improved clover Wilson action, both for valence and for sea quarks.

hep-lat↗

arXiv2009

$η'$-$η_c$-mixing with improved stochastic estimators

Charmonia are flavour singlet mesons and thus in principle contributions from disconnected quark line diagrams might affect their masses, either directly or via mixing with other flavour singlet channels. We present a first study that takes both effects into account. We employ improved stochastic all-to-all propagator techniques (including new methods) to calculate the diagrams that appear within the mixing matrix between the $η'$ and the $η_c$. The runs are initially performed on $N_f=2$ $16^3\times 32$ configurations with the non-perturbatively improved Sheikholeslami-Wilson action, both for valence and sea quarks.

hep-lat↗

arXiv2007

Charmonium spectrum including higher spin and exotic states

We study the charmonium spectrum including higher spin and exotic states. We use the Sheikholeslami-Wilson (clover) action for $N_f=2$ sea quarks as well as for the charm valence quark. In order to access excited states we apply a variational method with a basis of highly optimized operators.

hep-lat↗

arXiv2007

Excited mesons on dynamical clover-Wilson lattices

We present results for masses of excited mesons from dynamical clover-Wilson lattices provided by the CP-PACS collaboration at pion masses down to 500 MeV. Our analysis of the data is based on using a matrix of correlators from various source and sink operators. The spectroscopy results are discussed and compared to experimental values.

hep-lat↗

arXiv2007

Couplings of hybrid operators to ground and excited states of bottomonia

We analyze the overlap of local color-octet meson operators with the $Υ$ and the $η_b$ and their low-lying excited states, especially the first radial excitations. Our analysis is based on NRQCD and includes all terms up to order $v^4$. We use a variety of source and sink operators as a basis for the variational method, which enables us to clearly separate the mass eigenstates and hence to extract the desired amplitudes. The results show the usefulness of the variational method for determining couplings to excited hadronic states.

hep-lat↗