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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 2014-04

Tanmoy Bhattacharya, Alexander Friedland, 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

A paper with describing the calculation of the tensor charges for probing novel tensor interactions at the TeV scale in neutron beta-decay has been published in PRD. Bhattacharya, Gupta and Yoon are continuing simulations using the All-Mode-Averaging (AMA) technique on finer lattices to quantify discretization errors and improve the extrapolation to the continuum limit. Gupta presented a detailed analysis of this work at Lattice 2015. On the cluster and GPU computers at Los Alamos, they are continuing to simulate the largest 643 x 14643 × 144 lattices at the weakest coupling. Bhattacharya, Gupta and Yoon started simulations on the Titan computer at Oakridge under the ALCC program. They are performing detailed tests of the efficacy of the variational method using multiple smeared sources.

Latest References: 
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

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. The paper describing these calculations has been accepted for publication 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 prepared and submitted to 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 a paper describing the numerical results is being prepared.

Latest References: Bhattacharya et al., 
Physical Review Letters 112:21 (2015) 212002 

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. Yoon presented results comparing estimates using two different lattice actions to understand systematic errors at Lattice 2015. 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

Graesser has been determining the phenomenological constraints on a natural SUSY scenario having a light stop and a light stau. This work is with former LANL post-doc Jinrui Huang. They are constraining this scenario using the LHC 8 TeV searches for stop production (CMS, Eur. Phys. J. C (2013) 73:2677, arXiv:1308.1586), stop production and decay to a stau (ATLAS-CONF-2014-014), CMS' measurements of the top quark production cross-section (Phys. Lett. B 739 (2014) 23, arxiv:1407.6643 (lepton-tau final state), and JHEP 11 (2012) 067, arXiv:1208.2671 (dilepton final state)), as well as CMS' search for leptoquarks decaying to top and tau final states (Phys. Lett. B 739 (2014) 229, arXiv:1408.0806), as well as direct-detection (LUX) and indirect detection (FERMI) constraints. Finally, they also determine the region in which the dark matter candidate can be a thermal relic, while satisfying all of the above constraints. They are finalizing their results and preparing for publication.

Graesser has also been investigating beyond-the-Standard Model scenarios violating lepton number that also contribute to a neutrinoless double-beta decay signal. Specifically, he has been classifying the dimension 9 operators that contribute to this process, and constraining their strength using LHC data.

Latest References: 
Physics Letters B749 (2014) 293 
Physical Review Letters111 (2013) 121802 
JHEP 1302(2013) 046 
JHEP 1210(2012) 025 
Physics Letters B714 (2012) 267 
Physics Review D85 (2012) 054512 
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. Carried out a higher resolution simulation on Titan at Oak Ridge with (10240**3) particles and 1/h Gpc box. 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.