Los Alamos HEP Theory Quarterly Report FY2016-Q2

Tanmoy Bhattacharya, Michael L. Graesser, Rajan Gupta, Michael S. Warren

The primary areas of activity of the theory group are in physics beyond the Standard Model, cosmology, dark matter, lattice quantum chromodynamics, neutrinos, the fundamentals of quantum field theory and gravity, and particle astrophysics. The questions pursued by this group relate to deep mysteries in our understanding of Nature at the level of the the Standard Model and beyond. The main tools we use are quantum field theory and General Relativity.

Lattice QCD

The Los Alamos Lattice QCD team and their collaborators are carrying out precision studies investigating signatures of new physics at the TeV scale, elucidating the structure of the nucleon, and understanding QCD at finite temperature. Progress during this quarter on the four projects being pursued is described below.

Nucleon charges and form-factors

The analysis of data of isovector charges using the All-Mode-Averaging (AMA) technique was completed and all the analysis and cross-checks for the manuscript for publication were completed. These these clover-on-HISQ calculations are continuing on the largest lattices 643 X 144643X144 lattices at the weakest coupling on the cluster and GPU computers at Los Alamos. Very significant progress on the clover-on-clover simulations on the Titan computer at Oakridge under the ALCC program was made. A manuscript with detailed tests of the efficacy of the variational method using multiple smeared sources versus a 2-state fit with multiple source-sink separations was prepared and submitted for publication. Gupta completed and submitted manuscript to the proceedings of lattice 2016 describing the clover-on-HISQ results. A proposal for a comprehensive suite of calculations of the hadron structure was prepared and submitted to the USQCD allocation committee.

Latest References: 
arXiv:1602.07737 
arXiv:1601.01730 
Physical Review D92:9 (2015) 094511 
Physical Review D89:9 (2014) 094502 
Physical Review D85:5 (2012) 054512.

Matrix elements of novel CP violating operators and nEDM

The paper with the 1-loop calculations of the mixing and renormalization of novel CP violating operators of dimension-5 that contribute to the Neutron Electric Dipole Moment was published in PRD. In this paper Bhattacharya, Cirigliano, Gupta and Yoon calculate the operator basis that allows for off-shell renormalization using external fixed momentum states. The paper describing the one-loop matching between MSbar and a renormalization independent scheme was completed and paper describing these calculations published in PRD. A second paper with calculations of the quark electric dipole moment (tensor charges of the up, down and strange quarks within the neutron), their contribution to the neutron electric dipole moment and implications for split SUSY models was published in PRL. Bhattacharya, Gupta, and Yoon continue to make progress on calculation of matrix elements involving disconnected diagrams for the quark electric dipole moment operator using the clover-on-HISQ formulation. Bhattacharya prepared and submitted the manuscript (proceedings of Lattice 2015) on the first results on the calculation strategy for quark chromo EDM operator and preliminary results to the arXiv.

Latest References: Bhattacharya et al., 
arXiv:1601.02264 
Physical Review D92:9 (2015) 114026 
Physical Review Letters 112:21 (2015) 212002 
arXiv:1502.07325 
arXiv:1403.2445 
arXiv:1212.4918

Behavior of QCD at finite temperature

The HotQCD collaboration is continuing to investigate fluctuations of conserved charges (electric charge, strangness, baryon number) around the transition temperature (140-160 MeV) to investigate the behavior of QCD near the possible critical end-point at finite chemical potential that will be probed by the Beam Energy Scan run II.

Latest References: 
Physical Review D90 (2014) 094503 
Physical Review Letters113 (2014) 082001 
Physical Review D86 (2012) 034509 
Physical Review D86 (2012) 094503 
Physical Review D85 (2012) 054503

Disconnected diagrams and Transverse Momentum Distribution Functions

Bhattacharya, Gupta, Yoon and collaborator Michael Engelhardt at NMSU, are continuing production runs for calculating matrix elements to evaluate the Sivers function and other transverse momentum distribution (TMD) functions using computing resources provided by USQCD at JLab. Results using two different lattice actions to compare and understand systematic errors were presented by Yoon at Lattice 2015. A manuscript with the results for the proceedings was prepared and submitted to the arXiv. A paper comparing estimates for TMDs using clover and domain-wall fermions is being prepared. Bhattacharya, Gupta and Yoon are investigating methods to speed up the calculation of disconnected diagrams and improve the signal.

Latest References: 
arXiv:1601.05717

Neutrinoless double beta decay

Graesser spent this quarter preparing a paper for submission to the arxiv. This work determines the leading short-distance operators that provide contributions to neutrinoless double beta decay, that are not due to an effective Majorana neutrino mass. These results supersede previous results in the literature which only imposes color and electromagnetic charge invariance of the operators. Graesser's work imposes full electroweak invariance, and the consequences are discussed. This work also sets up a systematic matching of such 4-quark operators to the chiral effective theory, and uses that formalism to determine the leading chiral operators that contribute to the pion-pion coupling. The pion-pion coupling is important for phenomenology since it can lead to an enhanced neutrinoless double beta decay rate compared to induced four-nucleon or pion-and-two-nucleon couplings.

Latest References: 
Physics Letters B749 (2014) 293 
arXiv:1311.2028 
Physical Review Letters111 (2013) 121802 
JHEP 1302(2013) 046 
JHEP 1210(2012) 025 
Physics Letters B714 (2012) 267 
Physics Review D85 (2012) 054512 
arXiv:1107.2666 
JHEP 1110(2011) 110

Precision Cosmology Simulations

The "Dark Sky Simulations: Early Data Release" paper by Skillman & Warren et al. (arXiv:1407.2600) is the first published N-body simulation results with over a trillion particles. The the data and analysis software made publicly available at http://darksky.slac.stanford.edu. Analysis of a higher resolution simulation on Titan at Oak Ridge with (10240**3) particles and 1/h Gpc box is underway. The calculation involves one zettaflops integrated and will generate one petabyte of data. It will be the highest resolution cosmological simulation of dark matter, mass function, power spectrum, galaxy halo merger history.