Los Alamos HEP Theory Quarterly Report FY2016-Q3

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 submitted for publication. These 2+1+1-flavor clover-on-HISQ calculations are continuing on a second physical mass ensemble of lattices with size  96³×192963×192 at 0.06 fm on cluster and GPU computers at Los Alamos and using ERCAP allocations at NERSC. At the same time, the analysis of four ensembles of clover-on-clover simulations on the Titan computer at Oakridge under the ALCC program has been carried out. 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 published. These results were prepared for presentation at Lattice 2016. The analysis of electric, magnetic and axial form factors using 2-state fits has been carried out and these results were also presented at Lattice 2016 by post-doc Yong-Chull Jang. Manuscripts describing these results are being prepared for publication.

Latest References: 
arXiv:1606.07049 
Physical Review D93:11 (2016) 114506 
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

First calculations of the matrix elements of the quark chromo electric dipole moment operator were carried out to establish the signal and mixing with the pseudoscalar operator. Results were prepared for presentation at Lattice 2016 by Bhattacharya. Codes to calculate the disconnected diagrams were developed. The formalism for these calculations and new numerical techniques are being developed. 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 was also 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.

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 Letters 113 (2014) 082001 
Physical Review D86 (2012) 034509 
Physical Review D86 (2012) 094503 
Physical Review D85 (2012) 054503

Disconnected diagrams and Transverse Momentum Distribution Functions

A manuscript with the results for the Sivers and Boer-Mulders shift, the transversity and the generalized worm-gear shift for two different fermion discretization schemes is being prepared. 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. Bhattacharya, Gupta and Yoon are also investigating methods to speed up the calculation of disconnected diagrams and improve the signal.

Latest References: 
arXiv:1601.05717

Neutrinoless double beta decay

Graesser submitted a preprint to the arXiv during this quarter, in which he investigates possible Beyond the Standard Model contributions to neutrinoless double beta decay. These can appear in the form of higher dimension operators, where the scale is very roughly O(TeV) to be consistent with the null results from the GERDA experiment. A discovery of neutrinoless double-β decay would be profound, providing the first direct experimental evidence of overall lepton number violating processes. While a natural explanation is provided by an effective Majorana neutrino mass, in this preprint other new physics interpretations are carefully evaluated. At low--energies such new physics could manifest itself in the form of color and SU(2)L×U(1)Y invariant higher dimension operators. In this preprint Graesser determines a complete set of electroweak invariant dimension--9 operators, and this analysis supersedes those that only impose U(1)em invariance. Imposing electroweak invariance implies: 1) a significantly reduced set of leading order operators compared to only imposing U(1)em invariance; and 2) other collider signatures. Prior to imposing electroweak invariance there are 32 dimension-9 operators, which is reduced to 15 electroweak invariant operators at leading order in the expansion in the Higgs vacuum expectation value. Graesser establishes a systematic analysis of the hadronic realization of the 4-quark operators using chiral perturbation theory, and applies it to determine which of these operators have long-distance pion enhancements at leading order in the chiral expansion. Operators that vanish or are negligible at lowest chiral order are found to have non-zero two-pion couplings at NNLO chiral order. He also finds at dimension--11 and dimension--13, the electroweak invariant operators that after electroweak symmetry breaking produce the remaining ΔL=2 operators that would appear at dimension--9 if only U(1)em is imposed.

Latest References: 
arXiv:1606.04549 
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.