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Lang, C. B.

Publications and source records attributed to Lang, C. B..

15 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2017

Lattice operators for scattering of particles with spin

We construct operators for simulating the scattering of two hadrons with spin on the lattice. Three methods are shown to give the consistent operators for P N, P V, V N and N N scattering, where P, V and N denote pseudoscalar, vector and nucleon. Explicit expressions for operators are given for all irreducible representations at lowest two relative momenta. Each hadron has a good helicity in the first method. The hadrons are in a certain partial wave L with total spin S in the second method. These enable the physics interpretations of the operators obtained from the general projection method. The correct transformation properties of the operators in all three methods are proven. Lastly, the total momentum of two hadrons is restricted to zero since parity is a good quantum number in this case.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

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, $m_{\pi}$ = 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 L\"uscher 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.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Vector and scalar charmonium resonances with lattice QCD

We perform an exploratory lattice QCD simulation of DD¯ 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 DD¯ 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 DD¯ 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. In addition, further lattice QCD simulations and experimental efforts are needed to resolve the puzzle of the excited scalar charmonia.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

$X(3873$ and $Y(4140)$ using diquark-antidiquark operators with lattice QCD

We perform a lattice study of charmonium-like mesons with $J^{PC}=1^{++}$ and three quark contents $\bar cc \bar du$, $\bar cc(\bar uu+\bar dd)$ and $\bar cc \bar ss$, where the later two can mix with $\bar cc$. This simulation with $N_f=2$ and $m_\pi=266$ MeV aims at the possible signatures of four-quark exotic states. We utilize a large basis of $\bar cc$, two-meson and diquark-antidiquark interpolating fields, with diquarks in both anti-triplet and sextet color representations. A lattice candidate for X(3872) with I=0 is observed very close to the experimental state only if both $\bar cc$ and $D\bar D^*$ interpolators are included; the candidate is not found if diquark-antidiquark and $D\bar D^*$ are used in the absence of $\bar cc$. No candidate for neutral or charged X(3872), or any other exotic candidates are found in the I=1 channel. We also do not find signatures of exotic $\bar cc\bar ss$ candidates below 4.3 GeV, such as Y(4140). Possible physics and methodology related reasons for that are discussed. Along the way, we present the diquark-antidiquark operators as linear combinations of the two-meson operators via the Fierz transformations.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

X(3872) and Y(4140) using diquark-antidiquark operators with lattice QCD

We discuss a recent lattice study of charmonium-like mesons with JPC = 1++ and three quark contents c¯cd¯u, c¯c(u¯u+d¯d) and c¯cs¯s, where the latter two can mix with c¯c. In this quantum channel, the long known exotic candidate, X(3872), resides. This simulation employs Nf = 2, mp = 266 MeV and a large basis of ¯ cc, two-meson and diquark-antidiquark interpolating fields, with diquarks in both anti-triplet and sextet color representations. It aims at the possible signatures of four-quark exotic states. Along the way, we discuss the relations between the diquark-antidiquark operators and the two-meson operators via the Fierz transformations.

Padmanath, M.↗
DOE OSTI2015

$DK$ 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 ($32^3\times 64$, $m_\pi\approx 156$ MeV) a stochastic distillation variant is employed and for another ensemble ($16^3\times 32$, $m_\pi\approx 266$ 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.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2015

Study of the $Z_c^+$ 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: $J/\psi \pi$, $\psi_{2S}\pi$, $\psi_{1D}\pi$, $D \bar{D}^*$, $D^* \bar{D}^*$, $\eta_c \rho$ 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 $Z_c$.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2014

Axial resonances a$_{1}$(1260), b$_{1}$(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 $\rho\pi$ and $\omega\pi$. Interpolating fields $\bar{q} q$ and $\rho\pi$ or $\omega\pi$ are used to extract the s-wave phase shifts for the first time. The $\rho$ and $\omega$ are treated as stable and we argue that this is justified in the considered energy range and for our parameters $m_\pi\simeq 266~$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 $\Gamma_{a1}(s)=g^2 p/s$ is parametrized in terms of the coupling and we obtain $g_{a_1\rho\pi}=1.71(39)$ GeV compared to $g_{a_1\rho\pi}^{exp}=1.35(30)$ GeV derived from $\Gamma_{a1}^{exp}=425(175)$ MeV. In the $b_1$ channel, we find energy levels related to $\pi(0)\omega(0)$ and $b_1(1235)$, and the lowest level is found at $E_1 \gtrsim m_\omega+m_\pi$ but is within uncertainty also compatible with an attractive interaction. Lastly, assuming the coupling $g_{b_1\omega\pi}$ extracted from the experimental width we estimate $m_{b_1}^{res}=1.414(36)(^{+0}_{-83})$.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗