The axial coupling of the nucleon, g, is the strength of its coupling to the weak axial current of the standard model of particle physics, in much the same way as the electric charge is the strength of the coupling to the electromagnetic current. This axial coupling dictates the rate at which neutrons decay to protons, the strength of the attractive long-range force between nucleons and other features of nuclear physics. Precision tests of the standard model in nuclear environments require a quantitative understanding of nuclear physics that is rooted in quantum chromodynamics, a pillar of the standard model. The importance of g makes it a benchmark quantity to determine theoretically-a difficult task because quantum chromodynamics is non-perturbative, precluding known analytical methods. Lattice quantum chromodynamics provides a rigorous, non-perturbative definition of quantum chromodynamics that can be implemented numerically. It has been estimated that a precision of two per cent would be possible by 2020 if two challenges are overcome: contamination of g from excited states must be controlled in the calculations and statistical precision must be improved markedly. Here we use an unconventional method inspired by the Feynman-Hellmann theorem that overcomes these challenges. We calculate a g value of 1.271 ± 0.013, which has a precision of about one per cent.

The lowest-lying glueball masses are computed in SU($N$) gauge theory on a spacetime lattice for constant value of the lattice spacing $a$ and for $N$ ranging from 3 to 8. The lattice spacing is fixed using the deconfinement temperature at temporal extension of the lattice $N_T = 6$. The calculation is conducted employing in each channel a variational ansatz performed on a large basis of operators that includes also torelon and (for the lightest states) scattering trial functions. This basis is constructed using an automatic algorithm that allows us to build operators of any size and shape in any irreducible representation of the cubic group. A good signal is extracted for the ground state and the first excitation in several symmetry channels. It is shown that all the observed states are well described by their large $N$ values, with modest ${\cal O}(1/N^2)$ corrections. In addition spurious states are identified that couple to torelon and scattering operators. As a byproduct of our calculation, the critical couplings for the deconfinement phase transition for N=5 and N=7 and temporal extension of the lattice $N_T=6$ are determined.Comment: 1+36 pages, 22 tables, 21 figures. Typos corrected, conclusions unchanged, matches the published versio

We present our lattice studies of SU(3) gauge theory with N f = 8 degenerate fermions in the fundamental representation. Using nHYP-smeared staggered fermions we study finite-temperature transitions on lattice volumes as large as L 3 ×Nt = 48 3 ×24, and the zero-temperature composite spectrum on lattice volumes up to 64 3 ×128. The spectrum indirectly indicates spontaneous chiral symmetry breaking, but finite-temperature transitions with fixed Nt ≤ 24 enter a strongly coupled lattice phase as the fermion mass decreases, which prevents a direct confirmation of spontaneous chiral symmetry breaking in the chiral limit. In addition to the connected spectrum we focus on the lightest flavor-singlet scalar particle. We find it to be degenerate with the pseudo-Goldstone states down to the lightest masses reached so far by non-perturbative lattice calculations. Using the same lattice approach, we study the behavior of the composite spectrum when the number of light fermions is changed from eight to four. A heavy flavor-singlet scalar in the 4-flavor theory affirms the contrast between QCD-like dynamics and the low-energy behavior of the 8-flavor theory. *

We present results for the spectrum of a strongly interacting SU(3) gauge theory with $N_f = 8$ light fermions in the fundamental representation. Carrying out non-perturbative lattice calculations at the lightest masses and largest volumes considered to date, we confirm the existence of a remarkably light singlet scalar particle. We explore the rich resonance spectrum of the 8-flavor theory in the context of the search for new physics beyond the standard model at the Large Hadron Collider (LHC). Connecting our results to models of dynamical electroweak symmetry breaking, we estimate the vector resonance mass to be about 2 TeV with a width of roughly 450 GeV, and predict additional resonances with masses below ~3 TeV.Comment: 6 pages, 6 figures. Added report number. Version submitted to journa

We use a variational technique to study heavy glueballs on gauge configurations generated with 2+1 flavours of ASQTAD improved staggered fermions. The variational technique includes glueball scattering states. The measurements were made using 2150 configurations at 0.092 fm with a pion mass of 360 MeV. We report masses for 10 glueball states. We discuss the prospects for unquenched lattice QCD calculations of the oddballs.Comment: 19 pages, 4 tables and 8 figures. One figure added. Now matches the published versio

We present the first observation of a flavor-singlet scalar meson as light as the pion in N f = 8 QCD on the lattice, using the Highly Improved Staggered Quark action. Such a light scalar meson can be regarded as a composite Higgs with mass 125 GeV. In accord with our previous lattice results showing that the theory exhibits walking behavior, the light scalar may be a technidilaton, a pseudo Nambu-Goldstone boson of the approximate scale symmetry in walking technicolor. PACS numbers: 11.15.Ha, 12.39.Mk, 12.60.Nz, 14.80.Tt Recently, a Higgs boson with mass around 125 GeV has been discovered at the Large Hadron Collider (LHC) [1,2]. While the current LHC data show good agreement with the Standard model Higgs boson, there exists a possibility that the Higgs boson is a composite particle in an underlying strongly coupled gauge theory. A typical example is the walking technicolor theory, featuring approximate scale invariance and a large anomalous dimension, γ m ≈ 1 [3] (see also similar works [4][5][6]). Such a theory predicts a light composite Higgs, "technidilaton" [3], emerging as a pseudo Nambu-Goldstone (NG) boson of the spontaneously broken approximate scale symmetry. It was shown [7,8] that the technidilaton is phenomenologically consistent with the current LHC data.Thus, the most urgent theoretical task to test walking technicolor theories would be to check whether or not such a light flavor-singlet scalar bound state exists from first-principle calculations with lattice gauge theory. Since the composite Higgs should be associated with the electroweak symmetry breaking, it must be predominantly a bound state of technifermions carrying electroweak charges, but not of technigluons having no electroweak charges (up to some mixings between them). Thus we look for a light flavor-singlet scalar meson in the correlator of fermionic operators on the lattice.One of the most popular candidates for walking technicolor theories is QCD with a large number of (massless) flavors (N f ) in the fundamental representation. For the past few years, we have studied the SU(3) gauge theory with N f = 4, 8, 12, and 16, in a common lattice setup [9][10][11]. (For reviews of lattice studies in search for candidates for walking technicolor theories, see [12][13][14][15].)In N f = 12 QCD we actually observed [11, 16] a flavorsinglet scalar meson (σ) lighter than the "pion" having the quantum numbers corresponding to the NG pion (π) in the broken phase. (Recently a light flavor-singlet scalar meson consistent with ours was also observed by another group [17] using a different lattice action.)We found [9] that N f = 12 QCD is consistent with a conformal theory. If it is a conformal theory, it should have no bound states ("unparticle") in the exact chiral limit, and hence a light bound state can only be formed in the presence of a fermion mass m f which explicitly (not spontaneously) breaks the scale/chiral/electroweak symmetry.Hence such a light scalar meson in N f = 12 QCD would not be a composite Higgs associated with the spontaneou...

We present a determination of nucleon-nucleon scattering phase shifts for ≥ 0. The S, P, D and F phase shifts for both the spin-triplet and spin-singlet channels are computed with lattice Quantum ChromoDynamics. For > 0, this is the first lattice QCD calculation using the Lüscher finitevolume formalism. This required the design and implementation of novel lattice methods involving displaced sources and momentum-space cubic sinks. To demonstrate the utility of our approach, the calculations were performed in the SU(3)-flavor limit where the light quark masses have been tuned to the physical strange quark mass, corresponding to mπ=mK ≈800 MeV. In this work, we have assumed that only the lowest partial waves contribute to each channel, ignoring the unphysical partial wave mixing that arises within the finite-volume formalism. This assumption is only valid for sufficiently low energies; we present evidence that it holds for our study using two different channels. Two spatial volumes of V ≈ (3.5fm) 3 and V ≈ (4.6fm) 3 were used. The finite-volume spectrum is extracted from the exponential falloff of the correlation functions. Said spectrum is mapped onto the infinite volume phase shifts using the generalization of the Lüscher formalism for two-nucleon systems. T + 1 n (t/b) J = 1, = 2, S = 1 J = 3, = 2, S = 1 J = 1, = 0, S = 1 J = 1, = 0, S = 1, |r| = 0 J = 1, = 0, S = 1 J = 1, = 0, S = 1, |r| = 0 FIG. 1: Effective masses for the energy splitting, ∆En = 2 m 2 N + q 2 n − 2mN , in lattice units for the second excited state in the spin triplet T + 1 channel at L/b = 32, showing operators having different [J S] labels (Eq.(1)): J = 1, = 2, S = 1 (black), J = 3, = 2, S = 1 (blue), J = 1, = 0, S = 1 (red), J = 1, = 0, S = 1, r = 0 (green). The dashed horizontal lines represent the energy levels of the nearest non-interacting two-nucleon states.

One intriguing beyond-the-Standard-Model particle is the QCD axion, which could simultaneously provide a solution to the Strong CP problem and account for some, if not all, of the dark matter density in the universe. This particle is a pseudo-Nambu-Goldstone boson of the conjectured Peccei-Quinn (PQ) symmetry of the Standard Model. Its mass and interactions are suppressed by a heavy symmetry breaking scale, f a , whose value is roughly greater than 10 9 GeV (or, conversely, the axion mass, m a , is roughly less than 10 4 µeV). The density of axions in the universe, which cannot exceed the relic dark matter density and is a quantity of great interest in axion experiments like ADMX, is a result of the early-universe interplay between cosmological evolution and the axion mass as a function of temperature. The latter quantity is proportional to the second derivative of the temperature-dependent QCD free energy with respect to the CP-violating phase, θ. However, this quantity is generically non-perturbative and previous calculations have only employed instanton models at the high temperatures of interest (roughly 1 GeV). In this and future works, we aim to calculate the temperature-dependent axion mass at small θ from firstprinciple lattice calculations, with controlled statistical and systematic errors. Once calculated, this temperature-dependent axion mass is input for the classical evolution equations of the axion density of the universe, which is required to be less than or equal to the dark matter density. Due to a variety of lattice systematic effects at the very high temperatures required, we perform a calculation of the leading small-θ cumulant of the theta vacua on large volume lattices for SU(3) Yang-Mills with high statistics as a first proof of concept, before attempting a full QCD calculation in the future. From these pure glue results, the misalignment mechanism yields the axion mass bound m a ≥ (14.6 ± 0.1) µeV when PQ-breaking occurs after inflation.