Many low-energy, particle-physics experiments seek to reveal new fundamental physics by searching for very rare scattering events on atomic nuclei. The interpretation of their results requires quantifying the non-linear effects of the strong interact
ion on the spin-independent couplings of this new physics to protons and neutrons. Here we present a fully-controlled, ab-initio calculation of these couplings to the quarks within those constituents of nuclei. We use lattice quantum chromodynamics computations for the four lightest species of quarks and heavy-quark expansions for the remaining two. We determine each of the six quark contributions with an accuracy better than 15%. Our results are especially important for guiding and interpreting experimental searches for our universes dark matter.
In a previous letter (arXiv:1306.2287) we determined the isospin mass splittings of the baryon octet from a lattice calculation based on quenched QED and $N_f{=}2{+}1$ QCD simulations with 5 lattice spacings down to $0.054~mathrm{fm}$, lattice sizes
up to $6~mathrm{fm}$ and average up-down quark masses all the way down to their physical value. Using the same data we determine here the corrections to Dashens theorem and the individual up and down quark masses. For the parameter which quantifies violations to Dashenss theorem, we obtain $epsilon=0.73(2)(5)(17)$, where the first error is statistical, the second is systematic, and the third is an estimate of the QED quenching error. For the light quark masses we obtain, $m_u=2.27(6)(5)(4)~mathrm{MeV}$ and $m_d=4.67(6)(5)(4)~mathrm{MeV}$ in the $bar{mathrm{MS}}$ scheme at $2~mathrm{GeV}$ and the isospin breaking ratios $m_u/m_d=0.485(11)(8)(14)$, $R=38.2(1.1)(0.8)(1.4)$ and $Q=23.4(0.4)(0.3)(0.4)$. Our results exclude the $m_u=0$ solution to the strong CP problem by more than $24$ standard deviations.
We present a QCD calculation of the $u$, $d$ and $s$ scalar quark contents of nucleons based on $47$ lattice ensembles with $N_f = 2+1$ dynamical sea quarks, $5$ lattice spacings down to $0.054,text{fm}$, lattice sizes up to $6,text{fm}$ and pion mas
ses down to $120,text{MeV}$. Using the Feynman-Hellmann theorem, we obtain $f^N_{ud} = 0.0405(40)(35)$ and $f^N_s = 0.113(45)(40)$, which translates into $sigma_{pi N}=38(3)(3),text{MeV}$, $sigma_{sN}=105(41)(37),text{MeV}$ and $y_N=0.20(8)(8)$ for the sigma terms and the related ratio, where the first errors are statistical and the second are systematic. Using isospin relations, we also compute the individual up and down quark contents of the proton and neutron (results in the main text).
Electromagnetic effects are increasingly being accounted for in lattice quantum chromodynamics computations. Because of their long-range nature, they lead to large finite-size effects over which it is important to gain analytical control. Nonrelativi
stic effective field theories provide an efficient tool to describe these effects. Here we argue that some care has to be taken when applying these methods to quantum electrodynamics in a finite volume.
Indirect CP violation in K rightarrow {pi}{pi} decays plays a central role in constraining the flavor structure of the Standard Model (SM) and in the search for new physics. For many years the leading uncertainty in the SM prediction of this phenomen
on was the one associated with the nonperturbative strong interaction dynamics in this process. Here we present a fully controlled lattice QCD calculation of these effects, which are described by the neutral kaon mixing parameter B_K . We use a two step HEX smeared clover-improved Wilson action, with four lattice spacings from aapprox0.054 fm to aapprox0.093 fm and pion masses at and even below the physical value. Nonperturbative renormalization is performed in the RI-MOM scheme, where we find that operator mixing induced by chiral symmetry breaking is very small. Using fully nonperturbative continuum running, we obtain our main result B_K^{RI}(3.5GeV)=0.531(6)_{stat}(2)_{sys}. A perturbative 2-loop conversion yields B_K^{MSbar-NDR}(2GeV)=0.564(6)_{stat}(3)_{sys}(6)_{PT}, which is in good agreement with current results from fits to experimental data.
Some algorithmic details of our $N_f=2+1$ QCD mixed action simulations with overlap valence and improved Wilson sea quarks are presented.
