Lattice QCD (LQCD) is a non-perturbative formulation of Quantum Chromodynamics (QCD), the regnant theory of strong interactions, on a discrete Euclidian space-time grid. Large scale simulations of lattice QCD allow us to calculate the contributions of QCD to the properties, decays and interactions of hadrons composed of quarks and gluons. With the development of highly efficient numerical algorithms and peta-scale computing resources, we are able to calculate, from first principles, many quantities that are being used to both validate QCD and search for new physics beyond the standard model of elementary particles and interactions.

These simulations are extremely computationally intensive and require leadership class computing resources. They are being done at national supercomputer centers at NERSC and Oak Ridge, USQCD clusters at Fermilab and JLab, and institutional computing resources at Los Alamos.

The Los Alamos lattice QCD group has made many pioneering contributions since its inception in 1984. Current staff members include Tanmoy Bhattacharya, Vicenzo Cirigliano, Michael Graesser, Rajan Gupta, Emanuele Mereghetti, and Boram Yoon. They are working with postdoctoral fellows Yong-Chull Jang, Santanu Mondal and Sungwoo Park on five projects:

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Figure 1: The allowed parameter region of novel scalar and tensor couplings ϵS and ϵT that can arise in extensions of the standard model at the TeV Scale. The PNDME collaboration results are used for the low energy constraints from neutron beta decay and compared with analogous constraints from the Atlas and CMS experiments at the LHC. For details see Isovector charges of the nucleon from 2 + 1 + 1 -flavor lattice QCD


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Figure 2: Comparison of the tests of the partially Conserved Axial Current (PCAC) and the pion-pole dominance (PPD) hypothesis for the axial form factors. The traditional way of doing the analysis, S2pt, shows large violations, which are resolved by our new analysis strategy, SA4.  For perfect validation, the value should be unity up to O(a) corrections at all momenta Q2. (See Axial Vector Form Factors from Lattice QCD that Satisfy the PCAC Relation)


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Figure 3: Regions in the M2 − μ plane corresponding to various values of dn/de in split SUSY model, obtained by varying gTu,d,s within our estimated uncertainties. Using de ≤ 1.1 × 10−29 e cm and assuming maximal CP violation, the allowed region lies above the solid black line.
(See Flavor diagonal tensor charges of the nucleon from ( 2 + 1 + 1 )-flavor lattice QCD)


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Figure 4: Our (PNDME) results for the contributions of the up, down, and strange (Δu, Δd, and Δs) quarks to the spin of the proton are compared with other lattice and phenomenological estimates. (See Quark contribution to the proton spin from 2 + 1 + 1 -flavor lattice QCD)


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Figure 5: Lattice QCD data in the SIDIS limit, η → ∞, as a function of the Collins-Soper parameter ζ=0.83, are compared with the experimental extraction of the SIDIS generalized Sivers shift at ζ=0.83. In addition to lattice data for |bT| ≈ 0.35 fm, results from an earlier DWF-on-Asqtad study are also given. (See Nucleon transverse momentum-dependent parton distributions in lattice QCD: Renormalization patterns and discretization effects)


During 2007—2014, Gupta and Bhattacharya co-led the US wide HotQCD collaboration and investigated the behavior of QCD at finite temperature to elucidate the nature of the chiral transition to the quark-gluon plasma, determine the equation of state, and examine the restoration of U(1) axial symmetry. These results were published in six papers: 
(i) The equation of state in (2+1)-flavor QCD;
(ii) QCD Phase Transition with Chiral Quarks and Physical Quark Masses;
(iii) Fluctuations and Correlations of net baryon number, electric charge, and strangeness: A comparison of lattice QCD results with the hadron resonance gas model;
(iv) Chiral transition and U(1)_A symmetry restoration from lattice QCD using domain wall fermions;
(v) Chiral and deconfinement aspects of the QCD transition; and
(vi) Equation of state and QCD transition at finite temperature.

Their work showed that, for physical values of the up, down and strange quark masses, the transition from the hadron phase to the quark-gluon plasma is a crossover, with a crossover temperature T=154(9) MeV (See Chiral and deconfinement aspects of the QCD transition). The equation of state is shown in the Figure 6 below and compared with a similar calculation carried out by the Wuppertal-Budapest collaboration, who used a different discretized lattice action (Stout).


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Figure 6: The behavior of (energy – 3pressure), pressure (p) and entropy density (s) of QCD as a function of the temperature calculated by the HotQCD collaboration with the HISQ action and compared with the results from the Wuppertal-Budapest collaboration who used the Stout action (See The equation of state in (2+1)-flavor QCD). The equation of state of QCD, extracted from this data, is used in the analysis of experimental data produced in the collisions of relativistic heavy ions at Brookhaven National Lab and at the Large Hadron Collider.