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Quarterly Progress Reports

Investigating the field of high energy physics through experiments that strengthen our fundamental understanding of matter, energy, space, and time.

Los Alamos HEP Theory Quarterly Report FY2016-Q1

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 a manuscript for publication was written as well as the proceedings summarizing this work that was presented at Lattice 2015. They are continuing these clover-on-HISQ calculations on the largest lattices 643×144 lattices at the weakest coupling on the cluster and GPU computers at Los Alamos. Bhattacharya, Gupta and Yoon started clover-on-clover simulations on the Titan computer at Oakridge under the ALCC program. They carried out detailed tests of the efficacy of the variational method using multiple smeared sources versus a 2-state fit with multiple source-sink separations. A manuscript for publication describing these calculations and high precision results is being prepared.

Latest References: 
Physical Review D92:9 (2015094511 
Physical Review D89:9 (2014094502 
Physical Review D85:5 (2012054512.

Matrix elements of novel CP violating operators and nEDM

Bhattacharya, Cirigliano, Gupta and Yoon finished 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. They determined 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 presented the first results on the calculation strategy for quark chromo EDM operator at Lattice 2015. The data are being analyzed and the proceedings describing the method and preliminary results was prepared and submitted to the arXiv.

Latest References: Bhattacharya et al., 
Physical Review D92:9 (2015114026 
Physical Review Letters 112:21 (2015212002 

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 (2014094503 
Physical Review Letters113 (2014082001 
Physical Review D86 (2012034509 
Physical Review D86 (2012094503 
Physical Review D85 (2012054503

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.

Top Quark, Dark Matter and the LHC

In this quarter, Graesser investigated beyond-the-Standard Model scenarios violating lepton number (Delta L=2) that also contribute to a neutrinoless double-beta decay signal. Specifically, he has been classifying the higher dimension operators that contribute to this process, and constraining their strength using LHC data. At low energies 14 operators (at dimension 9) are known to contribute to a neutrinoless double beta decay. However, when constrained by SU(2)_L x U(1)_Y gauge invariance, only 6 of the 14 operators appear at dimension 9. Continuing to dimension 11, one finds 7 of the remaining operators. To obtain all 14 of the low-energy operators one must go to dimension 13 in an electroweak invariant theory. Since it is unlikely in general for a UV theory to not generate any of the 6 dimension 9 operators in the electroweak invariant theory, these findings indicate that generically one does not expect to generate at the UV scale all of the 14 operators that could appear in principle in the low energy theory. (Although electroweak RGE evolution of these operators will in general populate them all.) Graesser has been constraining these operators using run I LHC data collected at 8 TeV, with preliminary results indicating that such operators must be suppressed by a UV scale of Lambda ~ 1 TeV or larger. Graesser is investigating the competitiveness of these LHC bounds with bounds from the GERDA experiment.

Latest References: 
Physics Letters B749 (2014293 
Physical Review Letters111 (2013121802 
JHEP 1302(2013046 
JHEP 1210(2012025 
Physics Letters B714 (2012267 
Physics Review D85 (2012054512 
JHEP 1110(2011110

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.