arXiv subjects

Sergey Syritsyn

Publications and source records attributed to Sergey Syritsyn.

At least 55 records · Page 3Linked to original sources

arXiv2019

The Role of Lattice QCD in Searches for Violations of Fundamental Symmetries and Signals for New Physics

This document is one of a series of whitepapers from the USQCD collaboration. Here, we discuss opportunities for Lattice Quantum Chromodynamics (LQCD) in the research frontier in fundamental symmetries and signals for new physics. LQCD, in synergy with effective field theories and nuclear many-body studies, provides theoretical support to ongoing and planned experimental programs in searches for electric dipole moments of the nucleon, nuclei and atoms, decay of the proton, $n$-$\overline{n}$ oscillations, neutrinoless double-$β$ decay of a nucleus, conversion of muon to electron, precision measurements of weak decays of the nucleon and of nuclei, precision isotope-shift spectroscopy, as well as direct dark matter detection experiments using nuclear targets. This whitepaper details the objectives of the LQCD program in the area of Fundamental Symmetries within the USQCD collaboration, identifies priorities that can be addressed within the next five years, and elaborates on the areas that will likely demand a high degree of innovation in both numerical and analytical frontiers of the LQCD research.

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arXiv2019

$K π$ scattering and the $K^*(892)$ resonance in 2+1 flavor QCD

In this project, we will compute the form factors relevant for $B \to K^*(\to K π)\ell^+\ell^-$ decays. To map the finite-volume matrix elements computed on the lattice to the infinite-volume $B \to K π$ matrix elements, the $K π$ scattering amplitude needs to be determined using Lüscher's method. Here we present preliminary results from our calculations with $2+1$ flavors of dynamical clover fermions. We extract the $P$-wave scattering phase shifts and determine the $K^*$ resonance mass and the $K^* K π$ coupling for two different ensembles with pion masses of $317(2)$ and $178(2)$ MeV.

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arXiv2018

Towards the P-wave nucleon-pion scattering amplitude in the $Δ(1232)$ channel

We use lattice QCD and the Lüscher method to study elastic pion-nucleon scattering in the isospin $I = 3/2$ channel, which couples to the $Δ(1232)$ resonance. Our $N_f=2+1$ flavor lattice setup features a pion mass of $m_π\approx 250$ MeV, such that the strong decay channel $Δ\rightarrow πN$ is close to the threshold. We present our method for constructing the required lattice correlation functions from single- and two-hadron interpolating fields and their projection to irreducible representations of the relevant symmetry group of the lattice. We show preliminary results for the energy spectra in selected moving frames and irreducible representations, and extract the scattering phase shifts. Using a Breit-Wigner fit, we also determine the resonance mass $m_Δ$ and the $g_{Δ-πN}$ coupling.

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arXiv2018

Proton decay matrix element on the lattice with physical pion mass

Proton decay is one of possible signatures of baryon number violation, which has to exist to explain the baryon asymmetry and the existence of nuclear matter. Proton decays must be mediated through effective low-energy baryon number violating operators made of three quarks and a lepton. We calculate matrix elements of these operators between the proton and various meson final states using the direct method. We report on preliminary results of matrix element calculation done with the 2+1 dynamical flavor domain wall fermions at the physical point for the first time.

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arXiv2018

Pion structure from Lattice QCD

We present preliminary study of parton distribution inside the pion using mixed action approach with HYP smeared valence clover quarks on HISQ sea within the framework of Large Momentum Effective Theory. We use 2+1 flavor $48^3 \times 64$ HISQ lattices with lattices spacing of a=0.06 fm and valence quark masses corresponding to pion mass of 300 MeV.

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arXiv2018

Calculating the $ρ$ radiative decay width with lattice QCD

We present the results of our lattice QCD study of the $πγ\toππ$ process, where the $ρ$ resonance appears as an enhancement in the transition amplitude. We use $N_f=2+1$ clover fermions on a lattice of $L=3.6$ fm and a pion mass of $320$ MeV. Using a combination of forward, stochastic, and sequential propagators, we calculate the two-point and three-point functions that allow us to determine the $πγ\toππ$ matrix elements for several values of the invariant mass $s$ and momentum transfer $q^2$. To fit the $q^2$ and $s$ dependence of the $πγ\toππ$ amplitude, we explore a set of general parametrizations based on a Taylor expansion. By analytic continuation to the complex pole corresponding to the $ρ$ resonance, we determine the resonant form factors and calculate the radiative decay width of the $ρ$.

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arXiv2018

Pion quasi parton distribution function on a fine lattice

We present a calculation of the bare quasi-PDF (qPDF) of the pion. We perform these calculations using the HotQCD HISQ gauge ensemble for our sea quarks along with a Wilson-Clover valence quark action. Our lattice size is $48^3\times64$, our lattice spacing is set at a = 0.06 fm, and our pion mass is tuned to 300 MeV. Utilizing momentum smearing techniques, we compute the bare qPDF boosted up to momentum 1.72 GeV. In addition we explore excited state contamination of the three-point correlator.

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arXiv2018

Renormalized quasi parton distribution function of pion

We present preliminary numerical results on the connected piece of the quasi-PDF of pion as determined using Wilson-Clover valence fermions on HISQ ensembles. We discuss its non-perturbative renormalization in RI/MOM scheme with and without removal of the divergent self-energy part, and compare its running with expectation from perturbation theory. We also discuss the matching of pion QPDF to PDF, and various systematic effects associated with it.

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arXiv2018

Nucleon electromagnetic form factors at high $Q^2$ from Wilson-clover fermions

We present results on the nucleon electromagnetic form factors from Lattice QCD at momentum transfer up to about $12$~GeV$^2$. We analyze two gauge ensembles with the Wilson-clover fermion action, a lattice spacing of $a\approx 0.09$~fm and pion masses $m_π\approx 170$~MeV and $m_π\approx 280$~MeV. In our analysis we employ momentum smearing as well as a set of techniques to investigate excited state effects. Good agreement with experiment and phenomenology is found for the ratios $G_E/G_M$ and $F_2/F_1$, whereas discrepancies are observed for the individual form factors $F_1$ and $F_2$. We discuss various systematics that may affect our calculation.

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arXiv2018

A Lattice QCD study of the $ρ$ resonance

We present a lattice QCD study of the $ρ$ resonance with $N_f=2+1$ clover fermions at a pion mass of approximately $320$ MeV and lattice size $3.6$ fm. We consider two processes involving the $ρ$. The first process is elastic scattering of two pions in P-wave with isospin $1$. Using the Lüscher method we determine the scattering phase shift, from which we obtain the $ρ$ resonance mass and decay width $Γ(ρ\toππ)$. The second process is the radiative transition $πγ\toππ$, where we follow the Briceño-Hansen-Walker-Loud approach to determine the transition amplitude in the invariant mass region near the $ρ$ resonance and for both space- and time-like photon momentum. This allows us to determine the coupling between the $ρ$, the pion and the photon, and the resulting $ρ$ radiative decay width.

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arXiv2017

Computing the nucleon charge and axial radii directly at $Q^2=0$ in lattice QCD

We describe a procedure for extracting momentum derivatives of nucleon matrix elements on the lattice directly at $Q^2=0$. This is based on the Rome method for computing momentum derivatives of quark propagators. We apply this procedure to extract the nucleon isovector magnetic moment and charge radius as well as the isovector induced pseudoscalar form factor at $Q^2=0$ and the axial radius. For comparison, we also determine these quantities with the traditional approach of computing the corresponding form factors, i.e. $G^v_E(Q^2)$ and $G_M^v(Q^2)$ for the case of the vector current and $G_P^v(Q^2)$ and $G_A^v(Q^2)$ for the axial current, at multiple $Q^2$ values followed by $z$-expansion fits. We perform our calculations at the physical pion mass using a 2HEX-smeared Wilson-clover action. To control the effects of excited-state contamination, the calculations were done at three source-sink separations and the summation method was used. The derivative method produces results consistent with those from the traditional approach but with larger statistical uncertainties especially for the isovector charge and axial radii.

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arXiv2017

$P$-wave $ππ$ scattering and the $ρ$ resonance from lattice QCD

We calculate the parameters describing elastic $I=1$, $P$-wave $ππ$ scattering using lattice QCD with $2+1$ flavors of clover fermions. Our calculation is performed with a pion mass of $m_π\approx 320\:\:{\rm MeV}$ and a lattice size of $L\approx 3.6$ fm. We construct the two-point correlation matrices with both quark-antiquark and two-hadron interpolating fields using a combination of smeared forward, sequential and stochastic propagators. The spectra in all relevant irreducible representations for total momenta $|\vec{P}| \leq \sqrt{3} \frac{2π}{L}$ are extracted with two alternative methods: a variational analysis as well as multi-exponential matrix fits. We perform an analysis using Lüscher's formalism for the energies below the inelastic thresholds, and investigate several phase shift models, including possible nonresonant contributions. We find that our data are well described by the minimal Breit-Wigner form, with no statistically significant nonresonant component. In determining the $ρ$ resonance mass and coupling we compare two different approaches: fitting the individually extracted phase shifts versus fitting the $t$-matrix model directly to the energy spectrum. We find that both methods give consistent results, and at a pion mass of $am_π=0.18295(36)_{stat}$ obtain $g_{ρππ} = 5.69(13)_{stat}(16)_{sys}$, $am_ρ= 0.4609(16)_{stat}(14)_{sys}$, and $am_ρ/am_{N} = 0.7476(38)_{stat}(23)_{sys} $, where the first uncertainty is statistical and the second is the systematic uncertainty due to the choice of fit ranges.

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arXiv2017

Up, down, and strange nucleon axial form factors from lattice QCD

We report a calculation of the nucleon axial form factors $G_A^q(Q^2)$ and $G_P^q(Q^2)$ for all three light quark flavors $q\in\{u,d,s\}$ in the range $0\leq Q^2\lesssim 1.2\text{ GeV}^2$ using lattice QCD. This work was done using a single ensemble with pion mass 317 MeV and made use of the hierarchical probing technique to efficiently evaluate the required disconnected loops. We perform nonperturbative renormalization of the axial current, including a nonperturbative treatment of the mixing between light and strange currents due to the singlet-nonsinglet difference caused by the axial anomaly. The form factor shapes are fit using the model-independent $z$ expansion. From $G_A^q(Q^2)$, we determine the quark contributions to the nucleon spin and axial radii. By extrapolating the isovector $G_P^{u-d}(Q^2)$, we obtain the induced pseudoscalar coupling relevant for ordinary muon capture and the pion-nucleon coupling constant. We find that the disconnected contributions to $G_P$ form factors are large, and give an interpretation based on the dominant influence of the pseudoscalar poles in these form factors.

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arXiv2017

Calculation of Nucleon Electric Dipole Moments Induced by Quark Chromo-Electric Dipole Moments

We present initial results of computing nucleon electric dipole moment induced by quark chromo-EDM, CP-violating quark-gluon coupling. Using chirally-symmetric domain wall and Möbius fermions with pion mass $m_π=172\text{ MeV}$, we calculate the connected part of the electric dipole form factor $F_3(Q^2)$. In addition, we perform an exploratory study of the method to calculate EDM using uniform background electric field on a lattice introduced without breaking the periodicity in the time direction.

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arXiv2017

Isovector charges of the nucleon from 2+1-flavor QCD with clover fermions

We present high-statistics estimates of the isovector charges of the nucleon from four 2+1-flavor ensembles generated using Wilson-clover fermions with stout smearing and tree-level tadpole improved Symanzik gauge action at lattice spacings $a=0.114$ and $0.080$ fm and with $M_π\approx 315$ and 200 MeV. The truncated solver method with bias correction and the coherent source sequential propagator construction are used to cost-effectively achieve $O(10^5)$ measurements on each ensemble. Using these data, the analysis of two-point correlation functions is extended to include four states in the fits and of three-point functions to three states. Control over excited-state contamination in the calculation of the nucleon mass, the mass gaps between excited states, and in the matrix elements is demonstrated by the consistency of estimates using this multistate analysis of the spectral decomposition of the correlation functions and from simulations of the three-point functions at multiple values of the source-sink separation. The results for all three charges, $g_A$, $g_S$ and $g_T$, are in good agreement with calculations done using the clover-on-HISQ lattice formulation with similar values of the lattice parameters.

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arXiv2017

Controlling Excited-State Contamination in Nucleon Matrix Elements

We present a detailed analysis of methods to reduce statistical errors and excited-state contamination in the calculation of matrix elements of quark bilinear operators in nucleon states. All the calculations were done on a 2+1 flavor ensemble with lattices of size $32^3 \times 64$ generated using the rational hybrid Monte Carlo algorithm at $a=0.081$~fm and with $M_π=312$ MeV. The statistical precision of the data is improved using the all-mode-averaging method. We compare two methods for reducing excited-state contamination: a variational analysis and a two-state fit to data at multiple values of the source-sink separation $t_{\rm sep}$. We show that both methods can be tuned to significantly reduce excited-state contamination and discuss their relative advantages and cost-effectiveness. A detailed analysis of the size of source smearing used in the calculation of quark propagators and the range of values of $t_{\rm sep}$ needed to demonstrate convergence of the isovector charges of the nucleon to the $t_{\rm sep} \to \infty $ estimates is presented.

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arXiv2016

Computing the nucleon Dirac radius directly at $Q^2=0$

We describe a lattice approach for directly computing momentum derivatives of nucleon matrix elements using the Rome method, which we apply to obtain the isovector magnetic moment and Dirac radius. We present preliminary results calculated at the physical pion mass using a 2HEX-smeared Wilson-clover action. For removing the effects of excited-state contamination, the calculations were done at three source-sink separations and the summation method was used.

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arXiv2016

Algorithms for Disconnected Diagrams in Lattice QCD

Computing disconnected diagrams in Lattice QCD (operator insertion in a quark loop) entails the computationally demanding problem of taking the trace of the all to all quark propagator. We first outline the basic algorithm used to compute a quark loop as well as improvements to this method. Then, we motivate and introduce an algorithm based on the synergy between hierarchical probing and singular value deflation. We present results for the chiral condensate using a 2+1-flavor clover ensemble and compare estimates of the nucleon charges with the basic algorithm.

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