arXiv subjects

Daniel Mohler

Publications and source records attributed to Daniel Mohler.

At least 37 records · Page 2Linked to original sources

arXiv2018

The hadronic vacuum polarization contribution to $(g-2)_μ$ from $2+1$ flavours of O($a$) improved Wilson quarks

We report on our ongoing project to determine the leading-order hadronic vacuum polarisation contribution to the muon $g-2$, using ensembles with $N_f=2+1$ flavours of O($a$) improved Wilson quarks generated by the CLS effort, with pion masses down to the physical value. We employ O($a$) improved versions of the local and conserved vector currents to compute the contributions of the light, strange and charm quarks to $(g-2)_μ$, using the time-momentum representation. We perform a detailed investigation of the systematic effects arising from constraining the long-distance regime of the vector correlator. To this end we make use of auxiliary calculations in the iso-vector channel using distillation and the Lüscher formalism. Our results are corrected for finite-volume effects by computing the timelike pion form factor in finite and infinite volume. For certain parameter choices, the corrections computed in this way can also be confronted with results determined on different volumes. Currently, the overall precision of our results is limited by the uncertainties in the lattice scale.

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arXiv2018

Splittings of low-lying charmonium masses at the physical point

We present high-precision results from lattice QCD for the mass splittings of the low-lying charmonium states. For the valence charm quark, the calculation uses Wilson-clover quarks in the Fermilab interpretation. The gauge-field ensembles are generated in the presence of up, down, and strange sea quarks, based on the improved staggered (asqtad) action, and gluon fields, based on the one-loop, tadpole-improved gauge action. We use five lattice spacings and two values of the light sea quark mass to extrapolate the results to the physical point. An enlarged set of interpolating operators is used for a variational analysis to improve the determination of the energies of the ground states in each channel. We present and implement a continuum extrapolation within the Fermilab interpretation, based on power-counting arguments, and thoroughly discuss all sources of systematic uncertainty. We compare our results for various mass splittings with their experimental values, namely, the 1S hyperfine splitting, the 1P-1S splitting and the P-wave spin-orbit and tensor splittings. Given the uncertainty related to the width of the resonances, we find excellent agreement.

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arXiv2017

CLS 2+1 flavor simulations at physical light- and strange-quark masses

We report recent efforts by CLS to generate an ensemble with physical light- and strange-quark masses in a lattice volume of 192x96^3 at $β=3.55$ corresponding to a lattice spacing of 0.064 fm. This ensemble is being generated as part of the CLS 2+1 flavor effort with improved Wilson fermions. Our simulations currently cover 5 lattice spacings ranging from 0.039 fm to 0.086 fm at various pion masses along chiral trajectories with either the sum of the quark masses kept fixed, or with the strange-quark mass at the physical value. The current status of simulations is briefly reviewed, including a short discussion of measured autocorrelation times and of the main features of the simulations. We then proceed to discuss the thermalization strategy employed for the generation of the physical quark-mass ensemble and present first results for some simple observables. Challenges encountered in the simulation are highlighted.

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arXiv2017

Towards extracting the timelike pion form factor on CLS two-flavour ensembles

Results are presented from an ongoing study of the $ρ$ resonance. The focus is on CLS 2-flavour ensembles generated using $\mathcal{O}(a)$ improved Wilson fermions with pion masses ranging from $265$ to $437$ $\mathrm{MeV}$. The energy levels are extracted by solving the GEVP of correlator matrices, created with the distillation approach involving $ρ$ and $ππ$ interpolators. The study is done in the centre-of-mass frame and several moving frames. One aim of this work is to extract the timelike pion form factor after applying the Lüscher formalism. We therefore plan to integrate this study with the existing Mainz programme for the calculation of the hadronic vacuum polarization contribution to the muon $g-2$.

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arXiv2017

Recent results on the meson and baryon spectrum from lattice QCD

Recent lattice results on the meson and baryon spectrum with a focus on the determination of hadronic resonance masses and widths using a combined basis of single-hadron and hadron-hadron interpolating fields are reviewed. These mostly exploratory calculations differ from traditional lattice QCD spectrum calculations for states stable under QCD, where calculations with a full uncertainty estimate are already routinely performed. Progress and challenges in these calculations are highlighted.

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arXiv2016

Towards extracting the timelike pion form factor on CLS 2-flavour ensembles

Results are presented from an ongoing study of the $ρ$ resonance. We use the distillation approach in order to create correlator matrices involving $ρ$ and $ππ$ interpolators. The study is done in a centre-of-mass frame and several moving frames. We are able to extract energy levels by solving the GEVP of those correlator matrices. The initial exploratory study is being done on a CLS 2-flavour lattice with a pion mass of $451$ $\mathrm{MeV}$ using $\mathcal{O}(a)$ improved Wilson fermions. One aim of this work is to extract the timelike pion form factor after applying the Lüscher formalism. We also plan to integrate this study with the existing Mainz programme for the calculation of the hadronic vacuum polarization contribution to the muon $g-2$ and will extend our study to lower pion masses and larger lattices in the future, including ensembles with $2+1$ flavours.

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arXiv2016

Prediction of positive parity $B_s$ mesons and search for the $X(5568)$

We use a combination of quark-antiquark and $B^{(*)}K$ interpolating fields to predict the mass of two QCD bound states below the $B^*K$ threshold in the quantum channels $J^P=0^+$ and $1^+$. The mesons correspond to the b-quark cousins of the $D_{s0}^*(2317)$ and $D_{s1}(2460)$ and have not yet been observed in experiment, even though they are expected to be found by LHCb. In addition to these predictions, we obtain excellent agreement of the remaining p-wave energy levels with the known $B_{s1}(5830)$ and $B_{s2}^*(5840)$ mesons. The results from our first principles calculation are compared to previous model-based estimates. More recently the D0 collaboration claimed the existence of an exotic resonance $X(5568)$ with exotic flavor content $\bar{b}s\bar{d}u$. If such a state with $J^P=0^+$ exists, only the decay into $B_sπ$ is open which makes a lattice search for this state much cleaner and simpler than for other exotic candidates involving heavy quarks. We conclude, however, that we do not find such a candidate in agreement with a recent LHCb result.

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arXiv2016

Positive parity $D_s$ mesons

We study the positive parity charmed strange mesons using lattice QCD, the only reliable ab initio method to study QCD at low energies. Especially the experimentally observed $D_{s0}^*(2317)$ and $D_{s1}(2460)$ have challenged theory for quite some time. The dynamical lattice QCD simulations are performed at two distinct pion masses, $m_π$ = 266 MeV and 156 MeV, using both $\bar{c}s$ as well as $DK$ and $D^*K$ scattering operators in the construction of the correlation matrix in order to take into the account threshold effects. While the $J^P = 0^+$ channel benefited most from the inclusion of scattering operators, it was also crucial for the case of the $D_{s1}(2460)$. Using the Lüscher method, which relates the finite volume spectrum to the infinite volume scattering matrix, we were able to determine the near threshold behavior of the scattering amplitude. From it we extracted the binding momenta and the masses of the below-threshold bound states $D_{s0}^*(2317)$ and $D_{s1}(2460)$ by determining the pole positions of the scattering amplitudes. Our results compare well with experiment, resolving a long standing discrepancy between theory and experiment.

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arXiv2015

Vector and scalar charmonium resonances with lattice QCD

We perform an exploratory lattice QCD simulation of $D \bar D$ scattering, aimed at determining the masses as well as the decay widths of charmonium resonances above open charm threshold. Neglecting coupling to other channels, the resulting phase shift for $D \bar D$ scattering in p-wave yields the well-known vector resonance $ψ(3770)$. For $m_π= 156$ MeV, the extracted resonance mass and the decay width agree with experiment within large statistical uncertainty. The scalar charmonium resonances present a puzzle, since only the ground state $χ_{c0}(1P)$ is well understood, while there is no commonly accepted candidate for its first excitation. We simulate $D \bar D$ scattering in s-wave in order to shed light on this puzzle. The resulting phase shift supports the existence of a yet-unobserved narrow resonance with a mass slightly below 4 GeV. A scenario with this narrow resonance and a pole at $χ_{c0}(1P)$ agrees with the energy-dependence of our phase shift. Further lattice QCD simulations and experimental efforts are needed to resolve the puzzle of the excited scalar charmonia.

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arXiv2015

Predicting positive parity $B_{s}$ mesons from lattice QCD

We determine the spectrum of $B_s$ 1P states using lattice QCD. For the $B_{s1}(5830)$ and $B_{s2}^*(5840)$ mesons, the results are in good agreement with the experimental values. Two further mesons are expected in the quantum channels $J^P=0^+$ and $1^+$ near the $BK$ and $B^{*}K$ thresholds. A combination of quark-antiquark and $B^{(*)}$ meson-Kaon interpolating fields are used to determine the mass of two QCD bound states below the $B^{(*)}K$ threshold, with the assumption that mixing with $B_s^{(*)}η$ and isospin-violating decays to $B_s^{(*)}π$ are negligible. We predict a $J^P=0^+$ bound state $B_{s0}$ with mass $m_{B_{s0}}=5.711(13)(19)$ GeV. With further assumptions motivated theoretically by the heavy quark limit, a bound state with $m_{B_{s1}}= 5.750(17)(19)$ GeV is predicted in the $J^P=1^+$ channel. The results from our first principles calculation are compared to previous model-based estimates.

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arXiv2015

Recent progress in lattice calculations of properties of open-charm mesons

Recent progress in lattice calculations of properties of open-charm mesons, both regular and exotic, is reviewed, with an emphasis on spectroscopy. After reviewing recent calculations of excited state energy levels I will discuss progress in extracting hadronic masses and widths of charmed states from Lattice QCD simulations including low-lying scattering channels directly, to determine phase shift data and bound state/ resonance properties. With regard to other properties results from recent calculations of the $DD^*π$ and $DDρ$, $D^*D^*ρ$ couplings are presented. Beyond regular mesons, searches for explicitly exotic (tetraquark) states are also reviewed.

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arXiv2015

Study of the $Z_c^+$ channel using lattice QCD

Recently experimentalists have discovered several charged charmonium-like hadrons $Z_c^+$ with unconventional quark content $\bar cc\bar d u$. We perform a search for $Z_c^+$ with mass below $4.2~$GeV in the channel $I^G(J^{PC})=1^+(1^{+-})$ using lattice QCD. The major challenge is presented by the two-meson states $J/ψ\, π$, $ψ_{2S}π$, $ψ_{1D}π$, $D\bar D^*$, $D^*\bar D^*$, $η_cρ$ that are inevitably present in this channel. The spectrum of eigenstates is extracted using a number of meson-meson and diquark-antidiquark interpolating fields. For our pion mass of 266~MeV we find all the expected two-meson states but no additional candidate for $Z_c^+$ below $4.2~$GeV. Possible reasons for not seeing an additional eigenstate related to $Z_c^+$ are discussed. We also illustrate how a simulation incorporating interpolators with a structure resembling low-lying two-mesons states seems to render a $Z_c^+$ candidate, which is however not robust after further two-meson states around $4.2~$GeV are implemented.

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arXiv2014

Low lying charmonium states at the physical point

We present results for the mass splittings of low-lying charmonium states from a calculation with Wilson clover valence quarks with the Fermilab interpretation on an asqtad sea. We use five lattice spacings and two values of the light sea quark mass to extrapolate our results to the physical point. Sources of systematic uncertainty in our calculation are discussed and we compare our results for the 1S hyperfine splitting, the 1P-1S splitting and the P-wave spin orbit and tensor splittings to experiment.

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arXiv2014

Study of the Zc+ channel in lattice QCD

Several charged charmonium-like hadrons called $Z_c$ have been recently discovered by different experiments. In contrast to conventional hadrons these contain at least two valence quarks and antiquarks ($\bar{c}c\bar{d}u$). We perform a lattice QCD simulation of the $I^G(J^{PC})=1^+(1^{+-})$ channel including all relevant two-meson operators under 4.3 GeV: $J/ψπ$, $ψ_{2S}π$, $ψ_{1D}π$, $D \bar{D}^*$, $D^* \bar{D}^*$, $η_c ρ$ as well as additional diquark anti-diquark operators. In our $N_f = 2$ simulation with pion mass at 266 MeV we are able to identify all two-meson levels within the energy region of interest. However we find no additional level identifiable as a candidate for $Z_c$.

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arXiv2014

Searching for the $X(3872)$ and $Z_c^+(3900)$ on HISQ lattices

We present preliminary simulation results for the I = 0 charmonium state $X(3872)(1^{++})$ and the I = 1 charmonium state $Z_c^+(3900)(1^{+-})$. The study is performed on gauge field configurations with 2+1+1 flavors of highly improved staggered sea quarks (HISQ) with clover (Fermilab interpretation) charm quarks and HISQ light valence quarks. Since the $X(3872)$ lies very close to the open charm $D \bar D^*$ threshold, we use a combination of $\bar c c$ and $D \bar D^* + \bar D D^*$ interpolating operators. For the $Z_c^+(3900)$ we use a combination of $J/ψπ$ and $D \bar D^* + \bar D D^*$ channels. This is the first such study with HISQ sea quarks and light valence quarks. To this end, we describe a variational method for treating staggered quarks that incorporates both oscillating and non-oscillating components.

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arXiv2014

$DK$ and $D^* K$ scattering near threshold

We study the three $D_s$ quantum channels $J^P = 0^+$, $1^+$ and $2^+$ where experiments have identified the charm-strange states $D^*_{s0} (2317)$, $D_{s1}(2460)$, $D_{s1}(2536)$ near the $DK$ and $D^*K$ thresholds, and $D^*_{s2}(2573)$. We consider correlation functions for sets of $\overline q q$ operators and, for $J^P = 0^+$, $1^+$, also the $DK$ and $D^*K$ meson-meson interpolators and determine for these cases values of the elastic scattering amplitude. Constructing the full set of correlators requires propagators which connect any pair of lattice sites. For one ensemble of gauge configurations ($32^3\times 64$, $m_π\approx 156$ MeV) a stochastic distillation variant is employed and for another ensemble ($16^3\times 32$, $m_π\approx 266$ MeV) we use the full distillation method. Both, $D^*_{s0} (2317)$ and $D_{s1}(2460)$, are found as bound states below threshold, whereas $D_{s1}(2536)$, and $D^*_{s2}(2573)$ are identified as narrow resonances close to the experimental masses.

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arXiv2014

K pi scattering and the K* decay width from lattice QCD

K* mesons and in particular the K*(892) were frequently addressed in lattice simulations, but always while ignoring that the K*(892) decays strongly. We present an exploratory extraction of the masses and widths for the K* resonances by simulating K pi scattering in p-wave with I = 1/2 on the lattice. The K pi system with total momenta P = 2*pi/L e_z, 2*pi/L (e_x + e_y) and 0, that allows the extraction of phase shifts at several values of K pi relative momenta, is studied. A Breit-Wigner fit of the phase renders a K*(892) resonance mass m^{lat} = 891 +/- 14 MeV and the K*(892) -> K pi coupling g^{lat} = 5.7 +/- 1.6 compared to the experimental values m^{exp} = 892 MeV and g^{exp} = 5.72 +/- 0.06, where g parametrizes the K* -> K pi width. When extracting the phase shift around the K*(1410) and K2*(1430) resonances we take into account the mixing of p-wave with d-wave and assume that the scattering is elastic in our simulation. This gives us an estimate of the K*(1410) resonance mass m^{lat} = 1.33 +/- 0.02 GeV compared to m^{exp} = 1.414 +/- 0.0015 GeV assuming the experimental K*(1410) -> K pi coupling. We contrast the resonant I = 1/2 channel with the repulsive non-resonant I = 3/2 channel, where the phase is found to be negative and small, in agreement with experiment.

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arXiv2014

Axial resonances a1(1260), b1(1235) and their decays from the lattice

The light axial-vector resonances $a_1(1260)$ and $b_1(1235)$ are explored in Nf=2 lattice QCD by simulating the corresponding scattering channels $ρπ$ and $ωπ$. Interpolating fields $\bar{q} q$ and $ρπ$ or $ωπ$ are used to extract the s-wave phase shifts for the first time. The $ρ$ and $ω$ are treated as stable and we argue that this is justified in the considered energy range and for our parameters $m_π\simeq 266~$MeV and $L\simeq 2~$fm. We neglect other channels that would be open when using physical masses in continuum. Assuming a resonance interpretation a Breit-Wigner fit to the phase shift gives the $a_1(1260)$ resonance mass $m_{a1}^{res}=1.435(53)(^{+0}_{-109})$ GeV compared to $m_{a1}^{exp}=1.230(40)$ GeV. The $a_1$ width $Γ_{a1}(s)=g^2 p/s$ is parametrized in terms of the coupling and we obtain $g_{a_1ρπ}=1.71(39)$ GeV compared to $g_{a_1ρπ}^{exp}=1.35(30)$ GeV derived from $Γ_{a1}^{exp}=425(175)$ MeV. In the $b_1$ channel, we find energy levels related to $π(0)ω(0)$ and $b_1(1235)$, and the lowest level is found at $E_1 \gtrsim m_ω+m_π$ but is within uncertainty also compatible with an attractive interaction. Assuming the coupling $g_{b_1ωπ}$ extracted from the experimental width we estimate $m_{b_1}^{res}=1.414(36)(^{+0}_{-83})$.

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