Abstract
We present the first lattice-QCD calculation of the unpolarized strange and charm parton distribution functions using large-momentum effective theory. We use a lattice ensemble with ($2+1+1$) flavors of highly improved staggered quarks generated by the MILC Collaboration, with lattice spacing $a\ensuremath{\approx}0.12\text{ }\text{ }\mathrm{fm}$ and ${M}_{\ensuremath{\pi}}\ensuremath{\approx}310\text{ }\text{ }\mathrm{MeV}$, 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 ${P}_{z}=2.15\text{ }\text{ }\mathrm{GeV}$, and determine nonperturbative renormalization factors in regularization independent momentum subtraction scheme. We compare our lattice results with the matrix elements obtained from matching the parton distribution functions (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.
Highlights
Parton distribution functions (PDFs) provide a universal description of hadronic constituents as well as critical inputs for the discovery of the Higgs boson found at the Large Hadron Collider (LHC) through proton-proton collisions [1,2]
We compare our extrapolated matrix element results with those obtained from global fitting PDFs from CT18NNLO
The quasi-PDF matrix elements from both CT18 and NNPDF3.1 are consistent with our results within two standard deviations up to zPz ≈ 2, and deviate from our results at large zPz, suggesting deviations at moderate to small-x in the PDFs
Summary
Parton distribution functions (PDFs) provide a universal description of hadronic constituents as well as critical inputs for the discovery of the Higgs boson found at the Large Hadron Collider (LHC) through proton-proton collisions [1,2]. Among the known PDFs, the strange and charm PDFs have large uncertainties despite decades of experimental effort In addition to their applications to the energy frontier, PDFs reveal a nontrivial structure inside the nucleon, such as its momentum and spin distributions. We will be using the large-momentum effective theory (LaMET) framework [12,13] to provide information on the Bjorken-x dependence of the strange and charm PDFs. In the LaMET (or “quasi-PDF”) approach, timeindependent spatially displaced matrix elements that can be connected to PDFs are computed at finite hadron momentum Pz. A convenient choice for leading-twist PDFs is to take the hadron momentum and quark-antiquark separation to be along the z direction. This work is the first exploratory study to take on the challenges of the notorious “disconnected” contribution, an important step toward flavor-dependent PDFs from lattice QCD
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