arXiv subjects

Boram Yoon

Publications and source records attributed to Boram Yoon.

At least 55 records · Page 3Linked to original sources

arXiv2020

Nucleon Momentum Fraction, Helicity and Transversity from 2+1-flavor Lattice QCD

High statistics results for the isovector momentum fraction, $\langle x \rangle_{u-d}$, helicity moment, $\langle x \rangle_{Δu-Δd}$, and the transversity moment, $\langle x\rangle_{δu-δd}$, of the nucleon are presented using seven ensembles of gauge configurations generated by the JLab/W&M/LANL/MIT collaborations using $2+1$-flavors of dynamical Wilson-clover quarks. Attention is given to understanding and controlling the contributions of excited states. The final results are obtained using a simultaneous fit in the lattice spacing $a$, pion mass $M_π$ and the finite volume parameter $M_πL$ keeping leading order corrections. The data show no significant dependence on the lattice spacing and some evidence for finite-volume corrections. The main variation is with $M_π$, whose magnitude depends on the mass gap of the first excited state used in the analysis. Our final results, in the $\overline{\rm MS}$ scheme at 2 GeV, are $\langle x \rangle_{u-d} = 0.160(16)(20)$, $\langle x \rangle_{Δu-Δd} = 0.192(13)(20)$ and $\langle x \rangle_{δu-δd} = 0.215(17)(20)$, where the first error is the overall analysis uncertainty assuming excited-state contributions have been removed, and the second is an additional systematic uncertainty due to possible residual excited-state contributions. These results are consistent with other recent lattice calculations and phenomenological global fit values.

hep-lat↗

arXiv2020

A machine learning approach for efficient multi-dimensional integration

We propose a novel multi-dimensional integration algorithm using a machine learning (ML) technique. After training a ML regression model to mimic a target integrand, the regression model is used to evaluate an approximation of the integral. Then, the difference between the approximation and the true answer is calculated to correct the bias in the approximation of the integral induced by a ML prediction error. Because of the bias correction, the final estimate of the integral is unbiased and has a statistically correct error estimation. The performance of the proposed algorithm is demonstrated on six different types of integrands at various dimensions and integrand difficulties. The results show that, for the same total number of integrand evaluations, the new algorithm provides integral estimates with more than an order of magnitude smaller uncertainties than those of the VEGAS algorithm in most of the test cases.

physics.comp-ph↗

arXiv2020

Probing nucleon strange and charm distributions with lattice QCD

We present the first lattice-QCD calculation of the unpolarized strange and charm parton distribution functions using large-momentum effective theory (LaMET). We use a lattice ensemble with 2+1+1 flavors of highly improved staggered quarks (HISQ) generated by MILC collaboration, with lattice spacing $a\approx 0.12$ fm and $M_π\approx 310$ MeV, and clover valence fermions with two valence pion masses: 310 and 690 MeV. We use momentum-smeared sources to improve the signal up to nucleon boost momentum $P_z=2.18$ GeV, and determine nonperturbative renormalization factors in RI/MOM scheme. We compare our lattice results with the matrix elements obtained from matching the PDFs from CT18NNLO and NNPDF3.1NNLO global fits. Our data support the assumptions of strange-antistrange and charm-anticharm symmetry that are commonly used in global PDF fits, and we find smaller than expected parton distribution at mid to small $x$.

hep-lat↗

arXiv2020

Recent results of nucleon structure & matrix element calculations

A review of recent lattice calculations of nucleon structure and matrix elements of operators in nucleons is presented. It primarily covers developments in the calculation of the matrix elements of the scalar, tensor, pseudo-scalar, axial-vector and vector operators in the ground state of neutrons and protons in the isospin symmetric limit. Some preliminary calculations of the electric dipole moment, the gravitational moments and stress-energy distribution, and the magnetic polarizability are briefly described.

hep-lat↗

arXiv2020

Neutron Electric Dipole Moments with Clover Fermions

We present preliminary results for the contributions to the neutron EDM arising from the QCD $θ$-term, the Weinberg three-gluon and the quark chromo-EDM operators from our ongoing lattice calculations using clover valence quarks on the MILC HISQ lattices. We use the gradient-flow technique to smooth the lattices and renormalize the gluonic operators, and use the Schwinger source method to incorporate the quark chromo-EDM interactions in the quark propagator. For the QCD $θ$-term and the Weinberg three-gluon operator, we report results in the gradient-flow scheme from 8 ensembles at four lattice spacings and three pion masses, including 2 physical pion mass ensembles. For the quark chromo-EDM, unrenormalized results are presented at two lattice spacings, $a=0.12$ and $0.09$ fm, and two pion masses, $M_π= 310$ MeV and $220$ MeV.

hep-lat↗

arXiv2020

Machine-Learning Prediction for Quasi-PDF Matrix Elements

There have been rapid developments in the direct calculation in lattice QCD (LQCD) of the Bjorken-$x$ dependence of hadron structure through large-momentum effective theory (LaMET). LaMET overcomes the previous limitation of LQCD to moments (that is, integrals over Bjorken-$x$) of hadron structure, allowing LQCD to directly provide the kinematic regions where the experimental values are least known. LaMET requires large-momentum hadron states to minimize its systematics and allow us to reach small-$x$ reliably. This means that very fine lattice spacing to minimize lattice artifacts at order $(P_z a)^n$ will become crucial for next-generation LaMET-like structure calculations. Furthermore, such calculations require operators with long Wilson-link displacements (in finer lattice units), increasing the communication costs relative to that of the propagator inversion. In this work, we explore whether machine-learning (ML) algorithms can make correlator predictions to reduce the computational cost of these LQCD calculations. We consider two algorithms, gradient-boosting decision tree and linear models, applied to LaMET data, the matrix elements needed to determine the kaon and $η_s$ unpolarized parton distribution functions (PDFs), meson distribution amplitude (DA), and the nucleon gluon PDF. We find that both algorithms can reliably predict the target observables with different fit quality and systematic errors. The predictions from smaller displacement $z$ to larger ones work better than those for momentum $p$ due to the higher correlation among the data.

hep-lat↗

arXiv2020

A New Heavy Flavor Program for the Future Electron-Ion Collider

The proposed high-energy and high-luminosity Electron-Ion Collider (EIC) will provide one of the cleanest environments to precisely determine the nuclear parton distribution functions (nPDFs) in a wide $x$-$Q^{2}$ range. Heavy flavor production at the EIC provides access to nPDFs in the poorly constrained high Bjorken-$x$ region, allows us to study the quark and gluon fragmentation processes, and constrains parton energy loss in cold nuclear matter. Scientists at the Los Alamos National Laboratory are developing a new physics program to study heavy flavor production, flavor tagged jets, and heavy flavor hadron-jet correlations in the nucleon/nucleus going direction at the future EIC. The proposed measurements will provide a unique way to explore the flavor dependent fragmentation functions and energy loss in a heavy nucleus. They will constrain the initial-state effects that are critical for the interpretation of previous and ongoing heavy ion measurements at the Relativistic Heavy Ion Collider and the Large Hadron Collider. We show an initial conceptual design of the proposed Forward Silicon Tracking (FST) detector at the EIC, which is essential to carry out the heavy flavor measurements. We further present initial feasibility studies/simulations of heavy flavor hadron reconstruction using the proposed FST.

nucl-ex↗

arXiv2020

Nucleon charges and form factors using clover and HISQ ensembles

We present high statistics ($\mathcal{O}(2\times 10^5)$ measurements) preliminary results on (i) the isovector charges, $g^{u-d}_{A,S,T}$, and form factors, $G^{u-d}_E(Q^2)$, $G^{u-d}_M(Q^2)$, $G^{u-d}_A(Q^2)$, $\widetilde G^{u-d}_P(Q^2)$, $G^{u-d}_P(Q^2)$, on six 2+1-flavor Wilson-clover ensembles generated by the JLab/W&M/LANL/MIT collaboration with lattice parameters given in Table 1. Examples of the impact of using different estimates of the excited state spectra are given for the clover-on-clover data, and as discussed in [1], the biggest difference on including the lower energy (close to $Nπ$ and $Nππ$) states is in the axial channel. (ii) Flavor diagonal axial, tensor and scalar charges, $g^{u,d,s}_{A,S,T}$, are calculated with the clover-on-HISQ formulation using nine 2+1+1-flavor HISQ ensembles generated by the MILC collaboration [2] with lattice parameters given in Table 2. Once finished, the calculations of $g^{u,d,s}_{A,T}$ will update the results given in Refs.[3,4]. The estimates for $g^{u,d,s}_{S}$ and $σ_{Nπ}$ are new. Overall, a large part of the focus is on understanding the excited state contamination (ESC), and the results discussed provide a partial status report on developing defensible analyses strategies that include contributions of possible low-lying excited states to individual nucleon matrix elements.

hep-lat↗

arXiv2020

Nucleon Axial Form Factors from Clover Fermion on 2+1+1-flavor HISQ Lattice

The nucleon axial form factors -- axial $G_A$, induced pseudoscalar $\widetilde{G}_P$ and pseudoscalar $G_P$ -- have displayed large systematics in lattice QCD calculations. The major symptoms were the violation of the partially conserved axial current (PCAC) relation between the three form factors, and the underestimation of the induced pseudoscalar coupling $g_P^\ast$ and the axial charge radius $r_A$ compared to phenomenological estimates. The small $g_P^\ast$ was a consequence of the failure of the pion-pole dominance (PPD) hypothesis, especially at low $M_π^2$. The small charge radius $r_A$ and the underestimate of $g_A$ were related. The dominant systematic responsible is the lack of inclusion of low-energy ($N π$) states that are not manifest in the multiexponential fit to the nucleon two-point correlator. We show that this low-energy state can be determined from the three-point correlator $\langle N A_4 N \rangle $ with the insertion of the temporal component of the axial current $A_4$ within the nucleon state, ie, the strategy labeled $S_{A4}$ [1]. Including this low-energy state in fits to control excited-state contamination (ESC) gives results for $g_A$, $r_A$, and $g_P^\ast$ that are consistent with experimental/phenomenological values. However, the systematic uncertainties, especially in data at small $Q^2$, are now much larger.

hep-lat↗

arXiv2019

Axial Vector Form Factors from Lattice QCD that Satisfy the PCAC Relation

Previous lattice QCD calculations of axial vector and pseudoscalar form factors show significant deviation from the partially conserved axial current (PCAC) relation between them. Since the original correlation functions satisfy PCAC, the observed deviations from the operator identity cast doubt on whether all the systematics in the extraction of form factors from the correlation functions are under control. We identify the problematic systematic as a missed excited state, whose energy as a function of the momentum transfer squared, $Q^2$, is determined from the analysis of the 3-point functions themselves. Its mass is much smaller than those of the excited states previously considered and including it impacts the extraction of all the ground state matrix elements. The form factors extracted using these mass/energy gaps satisfy PCAC and other consistency conditions, and validate the pion-pole dominance hypothesis. We also show that the extraction of the axial charge $g_A$ is very sensitive to the value of the mass gaps of the excited states used and current lattice data do not provide an unambiguous determination of these, unlike the $Q^2 \neq 0$ case. To highlight the differences and improvement between the conventional versus the new analysis strategy, we present a comparison of results obtained on a physical pion mass ensemble at $a\approx 0.0871\,\mathrm{fm}$. With the new strategy, we find $g_A = 1.30(6)$. A very significant improvement over previous lattice results is found for the axial charge radius $r_A = 0.74(6)\,\mathrm{fm}$, extracted using the $z$-expansion to parameterize the $Q^2$ behavior of $G_A(Q^2)$, and $g_P^\ast = 8.06(44)$ obtained using the pion pole-dominance ansatz to fit the $Q^2$ behavior of the induced pseudoscalar form factor $\widetilde{G}_P(Q^2)$.

hep-lat↗

arXiv2019

Machine Learning Estimators for Lattice QCD Observables

A novel technique using machine learning (ML) to reduce the computational cost of evaluating lattice quantum chromodynamics (QCD) observables is presented. The ML is trained on a subset of background gauge field configurations, called the labeled set, to predict an observable $O$ from the values of correlated, but less compute-intensive, observables $\mathbf{X}$ calculated on the full sample. By using a second subset, also part of the labeled set, we estimate the bias in the result predicted by the trained ML algorithm. A reduction in the computational cost by about $7\%-38\%$ is demonstrated for two different lattice QCD calculations using the Boosted decision tree regression ML algorithm: (1) prediction of the nucleon three-point correlation functions that yield isovector charges from the two-point correlation functions, and (2) prediction of the phase acquired by the neutron mass when a small Charge-Parity (CP) violating interaction, the quark chromoelectric dipole moment interaction, is added to QCD, again from the two-point correlation functions calculated without CP violation.

hep-lat↗

arXiv2019

Nucleon Electromagnetic Form Factors in the Continuum Limit from 2+1+1-flavor Lattice QCD

Results are presented for the nucleon isovector electromagnetic form factors using 11 ensembles generated by the MILC collaboration using the 2+1+1-flavors HISQ action. They span 4 lattice spacings $a \sim$ 0.06, 0.09, 0.12 and 0.15~fm and 3 values of $M_π\sim 135, 225$ and 315 MeV. High-statistics estimates are used to perform a simultaneous extrapolation in the lattice spacing, lattice volume and light-quark masses. The $Q^2$ dependence over the range 0.05-1.4 ${\rm GeV}^2$ is investigated using both the $z$-expansion and the dipole form. Final $z$-expansion estimates for the isovector r.m.s. radius are $r_E = 0.769(27)(30)$ fm $r_M = 0.671(48)(76)$ fm and $μ^{p-n} = 3.939(86)(138)$ Bohr magneton. The first error is the combined uncertainty from the leading-order analysis, and the second is an estimate of the additional uncertainty due to using the leading order chiral-continuum-finite-volume fits. The dipole estimates, $r_E = 0.765(11)(8)$ fm, $r_M = 0.704(21)(29)$ fm and $μ^{p-n} = 3.975(84)(125)$, are consistent with those from the $z$-expansion but with smaller errors. Our analysis highlights three points. First, all data from the eleven ensembles and existing lattice data on, or close to, physical mass ensembles from other collaborations collapses more clearly onto a single curve when plotted versus $Q^2/M_N^2$ as compared to $Q^2$ with the scale set by quantities other than $M_N$. The difference between these two analyses is indicative of discretization errors, some of which presumably cancel when the data are plotted versus $Q^2/M_N^2$. Second, the size of the remaining deviation of this common curve from the Kelly curve is small and can be accounted for by statistical and possible systematic uncertainties. Third, to improve lattice estimates, high statistics data for $Q^2 < 0.1$ ${\rm GeV}^2$ are needed.

hep-lat↗

arXiv2019

A complete set of in-medium splitting functions to any order in opacity

In this Letter we report the first calculation of all ${\cal O}(α_s)$ medium-induced branching processes to any order in opacity. Our splitting functions results are presented as iterative solutions to matrix equations with initial conditions set by the leading order branchings in the vacuum. The flavor and quark mass dependence of the in-medium $q \rightarrow qg$, $g\rightarrow gg$, $q \rightarrow g q$, $g \rightarrow q\bar{q}$ processes is fully captured by the light-front wavefunction formalism and the color representation of the parent and daughter partons. We include the explicit solutions to second order in opacity as supplementary material and present numerical results in a realistic strongly-interacting medium produced in high center-of-mass energy heavy ion collisions at the Large Hadron Collider. Our numerical simulations show that the second order in opacity corrections can change the energy dependence of the in-medium shower intensity. We further find corrections to the longitudinal and angular distributions of the in-medium splitting kernels that may have important implications for jet substructure phenomenology.

hep-ph↗

arXiv2019

Light and heavy flavor dijet production and dijet mass modification in heavy ion collisions

Back-to-back light and heavy flavor dijet measurements are promising experimental channels to accurately study the physics of jet production and propagation in a dense QCD medium. They can provide new insights into the path length, color charge, and mass dependence of quark and gluon energy loss in the quark-gluon plasma produced in reactions of ultra-relativistic nuclei. To this end, we perform a comprehensive study of both light and heavy flavor dijet production in heavy ion collisions. We propose the modification of dijet invariant mass distributions in such reactions as a novel observable that shows enhanced sensitivity to the QGP transport properties and heavy quark mass effects on in-medium parton showers. This is achieved through the addition of the jet quenching effects on the individual jets as opposed to their subtraction. The latter drives the subtle effects on more conventional observables, such as the dijet momentum imbalance shifts, which we also calculate here. Results are presented in Pb+Pb collisions at $\sqrt{s_{NN}}$ = 5.02 TeV for comparison to data at the Large Hadron Collider and in Au+Au collisions at $\sqrt{s_{NN}}$ = 200 GeV to guide the future sPHENIX program at the Relativistic Heavy Ion Collider.

hep-ph↗

arXiv2018

Updates on Nucleon Form Factors from Clover-on-HISQ Lattice Formulation

Updates on results for the electric, magnetic and axial vector form factors are presented. The data analyzed cover high statistics measurements on 11 ensembles generated with 2+1+1 flavors of HISQ fermions by the MILC collaboration. The data cover the range $0.057\;\mathrm{fm} < a < 0.15\;\mathrm{fm}$ in lattice spacing, $135\;\mathrm{MeV} < M_π< 320\;\mathrm{MeV}$ in the pion mass and $3.3 < M_πL < 5.5 $ in the lattice size. Fits to control excited-state contamination use up to 3-states in the spectral decomposition of the 3-point functions. The simultaneous chiral-continuum-finite-volume fits include leading order corrections in each of the three variables.

hep-lat↗

arXiv2018

Neutron Electric Dipole Moment from Beyond the Standard Model

We present an update on our calculations of the matrix elements of the CP violating quark and gluon chromo-EDM operators, as well as the operators these mix with, such as the QCD Theta-term. Their contribution to the neutron EDM is obtained by extrapolating the $F_3$ form factor of a vector current to zero momentum transfer. The calculation is being done using valence Wilson-clover quarks on HISQ background configurations generated by the MILC collaboration.

hep-lat↗

arXiv2018

Isovector and flavor diagonal charges of the nucleon from 2+1+1 flavor QCD

We present high-statistics results for the isovector and flavor diagonal charges of the proton using 11 ensembles of 2+1+1 flavor HISQ fermions. In the isospin symmetric limit, results for the neutron are given by the $u \leftrightarrow d$ interchange. A chiral-continuum fit with leading order corrections was made to extract the connected and disconnected contributions in the continuum limit and at $M_π=135$ MeV. All results are given in the $\overline{MS}$ scheme at 2 GeV. The isovector charges, $g_A^{u-d} = 1.218(25)(30)$, $g_S^{u-d} = 1.022(80)(60) $ and $g_T^{u-d} = 0.989(32)(10)$, are used to obtain low-energy constraints on novel scalar and tensor interactions, $ε_{S}$ and $ε_{T}$, at the TeV scale. The flavor diagonal axial charges are: $g_A^u \equiv Δu \equiv \langle 1 \rangle_{Δu^+} = 0.777(25)(30)$, $g_A^d \equiv Δd \equiv \langle 1 \rangle_{Δd^+} = -0.438(18)(30)$, and $g_A^s \equiv Δs \equiv \langle 1 \rangle_{Δs^+} = -0.053(8)$. Their sum gives the total quark contribution to the proton spin, $\sum_{q=u,d,s} (\frac{1}{2} Δq) = 0.143(31)(36)$. This result is in good agreement with the recent COMPASS analysis $0.13 < \frac{1}{2} ΔΣ< 0.18$. Implications of results for the flavor diagonal tensor charges, $g_T^u = 0.784(28)(10)$, $g_T^d = -0.204(11)(10)$ and $g_T^s = -0.0027(16)$ for constraining the quark electric dipole moments and their contributions to the neutron electric dipole moment are discussed. These flavor diagonal charges also give the strength of the interaction of dark matter with nucleons via axial and tensor mediators.

hep-lat↗

arXiv2018

Isovector Charges of the Nucleon from 2+1+1-flavor Lattice QCD

We present high statistics results for the isovector charges $g^{u-d}_A$, $g^{u-d}_S$ and $g^{u-d}_T$ of the nucleon. Calculations were carried out on eleven ensembles of gauge configurations generated by the MILC collaboration using highly improved staggered quarks (HISQ) action with 2+1+1 dynamical flavors. These ensembles span four lattice spacings $a \approx$ 0.06, 0.09, 0.12 and 0.15 fm and light-quark masses corresponding to $M_π\approx$ 135, 225 and 315 MeV. Excited-state contamination in the nucleon 3-point correlation functions is controlled by including up to three-states in the spectral decomposition. Remaining systematic uncertainties associated with lattice discretization, lattice volume and light-quark masses are controlled using a simultaneous fit in these three variables. Our final estimates of the isovector charges in the $\overline{\text{MS}}$ scheme at 2 GeV are $g_A^{u-d} = 1.218(25)(30)$, $g_S^{u-d} = 1.022(80)(60) $ and $g_T^{u-d} = 0.989(32)(10)$. The first error includes statistical and all systematic uncertainties except that due to the extrapolation ansatz, which is given by the second error estimate. We provide a detailed comparison with the recent result of $g_A^{u-d} = 1.271(13)$ by the CalLat collaboration and argue that our error estimate is more realistic. Combining our estimate for $g_S^{u-d}$ with the difference of light quarks masses $(m_d-m_u)^{\rm QCD}=2.572(66)$ MeV given by the MILC/Fermilab/TUMQCD collaboration for 2+1+1-flavor theory, we obtain $(M_N-M_P)^{\rm QCD} = 2.63(27)$ MeV. We update the low-energy constraints on novel scalar and tensor interactions, $ε_{S}$ and $ε_{T}$, at the TeV scale by combining our new estimates for $g^{u-d}_S$ and $g^{u-d}_T$ with precision low-energy nuclear experiments, and find them comparable to those from the ATLAS and the CMS experiments at the LHC.

hep-lat↗