arXiv subjects

Randy Lewis

Publications and source records attributed to Randy Lewis.

At least 37 records · Page 2Linked to original sources

arXiv2018

Evidence for charm-bottom tetraquarks and the mass dependence of heavy-light tetraquark states from lattice QCD

We continue our study of heavy-light four-quark states and find evidence from lattice QCD for the existence of a strong-interaction-stable $I(J^P)=0(1^+)$ $ud\bar{c}\bar{b}$ tetraquark with mass in the range of 15 to 61 MeV below $\bar{D}B^*$ threshold. Since this range includes the electromagnetic $\bar{D}Bγ$ decay threshold, current uncertainties do not allow us to determine whether such a state would decay electromagnetically, or only weakly. We also perform a study at fixed pion mass, with NRQCD for the heavy quarks, simulating $qq^\prime \bar{b}^\prime \bar{b}$ and $q q^\prime \bar{b}^\prime\bar{b}^\prime$ tetraquarks with $q,\, q^\prime =ud$ or $\ell s$ and variable, unphysical $m_{b^\prime}$ in order to investigate the heavy mass-dependence of such tetraquark states. We find that the dependence of the binding energy follows a phenomenologically-expected form and that, though NRQCD breaks down before $m_{b^\prime}=m_c$ is reached, the results at higher $m_{b^\prime}$ clearly identify the $ud\bar{b}^\prime \bar{b}$ channel as the most likely to support a strong-interaction-stable tetraquark state at $m_{b^\prime}=m_c$. This observation serves to motivate the direct $ud\bar{c}\bar{b}$ simulation. Throughout we use dynamical $n_f=2+1$ ensembles with pion masses $m_π=$415, 299, and 164 MeV reaching down almost to the physical point, a relativistic heavy quark prescription for the charm quark, and NRQCD for the bottom quark(s).

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arXiv2018

Dark Matter from Strong Dynamics: The Minimal Theory of Dark Baryons

As a simple model for dark matter, we propose a QCD-like theory based on $\rm{SU}(2)$ gauge theory with one flavor of dark quark. The model is confining at low energy and we use lattice simulations to investigate the properties of the lowest-lying hadrons. Compared to QCD, the theory has several peculiar differences: there are no Goldstone bosons or chiral symmetry restoration when the dark quark becomes massless; the usual global baryon number symmetry is enlarged to $\rm{SU}(2)_B$, resembling isospin; and baryons and mesons are unified together in $\rm{SU}(2)_B$ iso-multiplets. We argue that the lightest baryon, a vector boson, is a stable dark matter candidate and is a composite realization of the hidden vector dark matter scenario. The model naturally includes a lighter state, the analog of the $η^\prime$ in QCD, for dark matter to annihilate into to set the relic density via thermal freeze-out. Dark matter baryons may also be asymmetric, strongly self-interacting, or have their relic density set via $3 \to 2$ cannibalizing transitions. We discuss some experimental implications of coupling dark baryons to the Higgs portal.

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arXiv2018

Photon operators for lattice gauge theory

Photon operators with the proper $J^{PC}$ quantum numbers are constructed, including one made of elementary plaquettes. In compact U(1) lattice gauge theory, these explicit photon operators are shown to permit direct confirmation of the massive and massless states on each side of the phase transition. In the abelian Higgs model, these explicit photon operators avoid some excited state contamination seen with the traditional composite operator, and allow more detailed future studies of the Higgs mechanism.

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arXiv2018

$α_s$ from the Lattice Hadronic Vacuum Polarisation

We present a determination of the QCD coupling constant, $α_s$, obtained by fitting lattice results for the flavour $ud$ hadronic vacuum polarisation function to continuum perturbation theory. We use $n_f=2+1$ flavours of Domain Wall fermions generated by the RBC/UKQCD collaboration and three lattice spacings $a^{-1}=1.79,\, 2.38$ and $3.15\ \text{GeV}$. Several sources of potential systematic error are identified and dealt with. After fitting and removing expected leading cut-off effects, we find for the five-flavour $\overline{\text{MS}}$ coupling the value $α_s(M_Z)=0.1181(27)^{+8}_{-22}$, where the first error is statistical and the second systematic.

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arXiv2018

A resolution of the inclusive flavor-breaking $τ$ $|V_{us}|$ puzzle

We revisit the puzzle of $|V_{us}|$ values obtained from the conventional implementation of hadronic-$τ$-decay-based flavor-breaking finite-energy sum rules lying $>3 σ$ below the expectations of three-family unitarity. Significant unphysical dependences of $|V_{us}|$ on the choice of weight, w, and upper limit, $s_0$, of the experimental spectral integrals entering the analysis are confirmed, and a breakdown of assumptions made in estimating higher dimension, $D>4$, OPE contributions is identified as the main source of these problems. A combination of continuum and lattice results is shown to suggest a new implementation of the flavor-breaking sum rule approach in which not only $|V_{us}|$, but also $D>4$ effective condensates, are fit to data. Lattice results are also used to clarify how to reliably treat the slowly converging D=2 OPE series. The new sum rule implementation is shown to cure the problems of the unphysical w- and $s_0$-dependence of $|V_{us}|$ and to produce results $\sim 0.0020$ higher than those of the conventional implementation. With B-factory input, and using, in addition, dispersively constrained results for the $Kπ$ branching fractions, we find $\vert V_{us}\vert =0.2231(27)_{exp}(4)_{th}$, in excellent agreement with the result from $K_{\ell 3}$, and compatible within errors with the expectations of three-family unitarity, thus resolving the long-standing inclusive $τ$ $|V_{us}|$ puzzle.

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arXiv2017

More on heavy tetraquarks in lattice QCD at almost physical pion mass

We report on our progress in studying exotic, heavy tetraquark states, $qq\prime \bar Q\bar Q\prime$. Using publicly available dynamical $n_f =2+1$ Wilson-Clover gauge configurations, generated by the PACS-CS collaboration, with pion masses $\simeq$164, 299 and 415 MeV, we extend our previous analysis to heavy quark components containing heavier than physical bottom quarks $\bar Q\bar Q\prime=\bar b\prime\bar b\prime$ or $\bar Q\bar Q\prime=\bar b\bar b\prime$, charm and bottom quarks $\bar c\bar b$ and also only charm quarks $\bar c\bar c$. Throughout we employ NRQCD and relativistic heavy quarks for the heavier than bottom, bottom and charm quarks. Using our previously established diquark-antidiquark and meson-meson operator basis we comment in particular on the dependence of the binding energy on the mass of the heavy quark component $\bar Q\bar Q$, with heavy quarks ranging from $m_Q=0.85\ldots 6.3\cdot m_b$. In the heavy flavor non-degenerate case, $\bar Q\bar Q\prime$, and especially for the tetraquark channel $ud\bar c\bar b$, we extend our work to utilize a $3\times 3$ GEVP to study the ground and threshold states thereby enabling a clear identification of possible binding. Finally, we present initial work on the $\bar Q\bar Q\prime=\bar c\bar c$ system where a much larger operator basis is available in comparison to flavor combinations with NRQCD quarks.

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arXiv2017

Lattice Prediction for Deeply Bound Doubly Heavy Tetraquarks

We investigate the possibility of $qq^\prime \bar b \bar b$ tetraquark bound states using $n_f=2+1$ lattice QCD ensembles with pion masses $\simeq 164$, $299$, and $415$ MeV. Motivated by observations from heavy baryon phenomenology, we consider two lattice interpolating operators both of which are expected to couple efficiently to tetraquark states: one with diquark-antidiquark and one with a meson-meson structure. Using nonrelativistic QCD to simulate the bottom quarks, we study the $ud\bar b \bar b$, $\ell s\bar b \bar b$ channels with $\ell=u,d$, and find unambiguous signals for strong-interaction-stable $J^P=1^+$ tetraquarks. These states are found to lie $189(10)$ and $98(7)$ MeV below the corresponding free two-meson thresholds.

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arXiv2016

Charmed-Bottom Mesons from Lattice QCD

We present ground state spectra of mesons containing a charm and a bottom quark. For the charm quark we use overlap valence quarks while a non-relativistic formulation is utilized for the bottom quark on a background of 2+1+1 flavors HISQ gauge configurations generated by the MILC collaboration. The hyperfine splitting between $1S$ states of $B_c$ mesons is found to be $56^{+4}_{-3}$ MeV. We also study the baryons containing only charm and bottom quarks and predict their ground state masses. Results are obtained at three lattice spacings.

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arXiv2016

Dark matter from one-flavor SU(2) gauge theory

SU(2) gauge theory with a single fermion in the fundamental representation is a minimal non-Abelian candidate for the dark matter sector, which is presently missing from the standard model. Having only a single flavor provides a natural mechanism for stabilizing dark matter on cosmological timescales. Preliminary lattice results are presented and discussed in the context of dark matter phenomenology.

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arXiv2015

Exploring free-form smearing for bottomonium and B meson spectroscopy

Free-form smearing was designed as a way to implement source operators of any desired shape. A variation of the method is introduced that reduces the computational cost by reducing the number of link multiplications to its absolute minimum. Practical utility is demonstrated through calculations of bottomonium and B meson masses.

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arXiv2015

Determining the QCD coupling from lattice vacuum polarization

The QCD coupling appears in the perturbative expansion of the current-current two-point (vacuum polarization) function. Any lattice calculation of vacuum polarization is plagued by several competing non-perturbative effects at small momenta and by discretization errors at large momenta. We work in an intermediate region, computing the vacuum polarization for many off-axis momentum directions on the lattice. Having many momentum directions provides a way to monitor and account for lattice artifacts. Our results are competitive with, and have certain systematic advantages over, the alternate phenomenological determination of the strong coupling from the same light quark vacuum polarization produced by sum rule analyses of hadronic tau decay data.

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arXiv2015

Low-energy constants from ALEPH hadronic tau decay data

We determined the NLO chiral low-energy constant $L_{10}$, and various combinations of NNLO chiral low-energy constants employing recently revised ALEPH results for the non-strange vector (V) and axial-vector (A) hadronic tau decay distributions and recently updated RBC/UKQCD lattice data for the non-strange V-A two-point function. In this talk, we explain the ingredients of this determination. Our errors are at or below the level expected for contributions of yet higher order in the chiral expansion, suggesting that our results exhaust the possibilities of what can be meaningfully achieved in an NNLO analysis.

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arXiv2015

Free-form smearing for bottomonium and B meson spectroscopy

To obtain high-quality results from lattice QCD, it is important to use operators that produce good signals for the quantities of interest. Free-form smearing is a powerful tool that helps to accomplish that goal. The present work introduces a new implementation of free-form smearing that maintains its usefulness and reduces its computational time dramatically. Applications to the mass spectrum of $B$, $B_s$, $B_c$ and bottomonium mesons show the effectiveness of the method. Results are compared with other lattice QCD studies and with experimental data where available. The present work includes the first lattice QCD exploration for some of these mesons.

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arXiv2015

Low-energy constants and condensates from ALEPH hadronic $τ$ decay data

We determine the NLO chiral low-energy constant $L_{10}^r$, and combinations $C_{12}^r\pm C_{61}^r+C_{80}^r$, $C_{13}^r-C_{62}^r+C_{81}^r$, $C_{61}^r$, and $C_{87}^r$, of the NNLO chiral low-energy constants incorporating recently revised ALEPH results for the non-strange vector ($V$) and axial-vector ($A$) hadronic $τ$ decay distributions and recently updated RBC/UKQCD lattice data for the non-strange $V-A$ two-point function. In the $\bar{\rm MS}$ scheme, at renormalization scale $μ=770\ {MeV}$, we find $L_{10}^r=-0.00350(17)$, $C_{12}^r+C_{61}^r+C_{80}^r=0.00237(16)\ {GeV}^{-2}$, $C_{12}^r-C_{61}^r+C_{80}^r=-0.00056(15)\ {GeV}^{-2}$, $C_{13}^r-C_{62}^r+C_{81}^r=0.00046(9)\ {GeV}^{-2}$, $C_{61}^r=0.00146(15)\ {GeV}^{-2}$, and $C_{87}^r=0.00510(22)\ {GeV}^{-2}$. With errors here at or below the level expected for contributions of yet higher order in the chiral expansion, the analysis exhausts the possibilities of what can be meaningfully achieved in an NNLO analysis. We also consider the dimension six and dimension eight coefficients in the operator product expansion in the $V-A$ channel.

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arXiv2014

Composite (Goldstone) Higgs Dynamics on the Lattice: Spectrum of SU(2) Gauge Theory with two Fundamental Fermions

We study the meson spectrum of the SU(2) gauge theory with two Wilson fermions in the fundamental representation. The theory unifies both Technicolor and composite Goldstone Boson Higgs models of electroweak symmetry breaking. We have calculated the masses of the lightest spin one vector and axial vector mesons. In addition, we have also obtained preliminary results for the mass of the lightest scalar (singlet) meson state. The simulations have been done with multiple masses and two different lattice spacings for chiral and continuum extrapolations. The spin one meson masses set lower limits for accelerator experiments, whereas the scalar meson will mix with a pGB of the theory and produce two scalar states. The lighter of the states is the 125 GeV Higgs boson, and the heavier would be a new yet unobserved scalar state.

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arXiv2014

Dark matter on the lattice

Several collaborations have recently performed lattice calculations aimed specifically at dark matter, including work with SU(2), SU(3), SU(4) and SO(4) gauge theories to represent the dark sector. Highlights of these studies are presented here, after a reminder of how lattice calculations in QCD itself are helping with the hunt for dark matter.

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arXiv2014

Free-form Smeared Bottomonium Correlation Functions

Gauge-invariant sources with a hydrogen wave function shape are constructed for bottomonium two-point correlation functions using the free-form smearing technique. The bottomonium spectrum, including a first lattice result for the D-wave first-excited state, is extracted from free-form smeared correlation functions. Results are compared with conventional smearing techniques and free-form smearing is found to have the advantage of reduced statistical errors.

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arXiv2014

Composite Goldstone Dark Matter: Experimental Predictions from the Lattice

We study, via first principles lattice simulations, the nonperturbative dynamics of $SU(2)$ gauge theory with two fundamental Dirac flavors. The model can be used simultaneously as a template for composite Goldstone boson dark matter and for breaking the electroweak symmetry dynamically. We compute the form factor, allowing us to estimate the associated electromagnetic charge radius. Interestingly we observe that the form factor obeys vector meson dominance even for the two color theory. We finally compare the model predictions with dark matter direct detection experiments. We find that the composite Goldstone boson dark matter cross sections is constrained by the most stringent direct-detection experiments. Our results are a foundation for quantitative new composite dynamics relevant for model building, and are of interest to current experiments.

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