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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 FY2017-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, Novel CP violating operator's contribution to nEDM, elucidating the structure of the nucleon, and understanding QCD at finite temperature. Progress during this quarter on these projects is described below.

Nucleon charges and form-factors

Results of isovector charges gA, gS and gT from the 2+1+1-flavor clover-on-HISQ calculations were published in PRD. The analysis of isovector charges using the 2+1-flavor clover-on-clover lattice QCD formulation was completed, a manuscript for publication was prepared and submitted to the arXiv. The four ensembles of clover-on-clover were simulated on the Titan computer at Oak Ridge under the ALCC program. The clover-on-clover calculation has the advantage of being based on a unitary lattice formulation, and provides an important check on the 2+1+1-flavor clover-on-HISQ calculations. We are continuing the analysis of a second physical mass HISQ ensemble with lattice size 963 ×192 at 0.06 fm on cluster and GPU computers at Los Alamos and using ERCAP allocations at NERSC. Proceedings of talks given at Lattice 2016 were prepared for publication. 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.

Recent References:
Physical Review D94:5 (2016) 054508
Physical Review D93:11 (2016) 114506
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

Calculations of the matrix elements of the quark chromo electric dipole moment operator and mixing with the pseudoscalar operator are ongoing. The formalism for these calculations and new numerical techniques are being developed. Status of results were presented at Lattice 2016 by Bhattacharya and at CONF12 by Gupta. Codes to calculate the disconnected diagrams and the reweighting factor were developed and production runs started. 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.,
Physical Review D92:9 (2015) 114026
Physical Review Letters 112:21 (2015) 212002

Disconnected diagrams

The matrix elements of iso-scalar and flavor diagonal operators are needed for the analysis of a number of interesting qualities such as the nucleon electric dipole moment, the nucleon sigma term, and the strangeness content of the proton. These matrix elements get contributions from disconnected diagrams, that are computationally challenging to compute with high precision. Bhattacharya, Gupta and Yoon, in collaboration with Gambhir, Orginos and Stathopoulos at William and Mary are using a new method based on deflation and hierarchical probing to speed up the calculation of disconnected diagrams. First results were presented by Gambhir at Lattice2016.


Transverse Momentum Distribution Functions

A manuscript with results for the Sivers and Boer-Mulders shift, the transversity and the generalized worm-gear shift for two different fermion discretization schemes is in final stages of preparation. Gupta presented these results at the POETIC7, Philadelphia, November, 2016 meeting. 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.

Latest References:

Behavior of QCD at finite temperature

The HotQCD collaboration is continuing to investigate fluctuations of conserved charges (electric charge, strangeness, 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

Dark Matter and LHC Physics

In this quarter Graesser worked on a number of ongoing projects and started several new ones. He and Yue Zhang (Northwestern U.) are investigating the phenomenological implications of scenario of dark matter in which the dark matter is light, i.e., less than a GeV, and the mediator is a pseudo-scalar. That the mediator is a pseudo-scalar leads to a qualitatively different phenomenology than previously discussed in the literature. They have been investigating the phenomenological effect on such interactions on the MiniBoone experiment. Graesser and Shashank Shalgar (Los Alamos) are investigating the impact that interactions between dark matter and either photons or neutrinos can have on the dark matter power spectrum and the cosmic microwave background.

Graesser is also working on a number of projects using 8 and 13 TeV LHC data to constrain models of TeV scale physics. These projects includes studying models having light stops and models that lead at low-energies to novel signatures of neutrinoless double beta decay. Graesser is also collaborating with Tuhin Roy (TATA Institute, Mumbai, India) on a new method to improve the sensitivity of the LHC to new TeV-scale physics. Graesser is collaborating with D. Kang and B. Yoon on applying deep learning methods, used in image classification, to improve the sensitivity of the LHC to a boosted W'.

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

Neutrinoless double beta decay

Graesser is collaborating with V. Cirigliano, W. Dekens and E. Mereghetti on using chiral SU(3) to estimate the size of Delta L=2 hadronic matrix elements between two pions, that are relevant to short-distance Beyond-the-Standard-Model contributions to a neutrinoless double beta decay signal. Using chiral SU(3) perturbation theory, these matrix elements can be related to kaon-anti-kaon and K->pi pi matrix elements, which have been computed using lattice QCD.

Latest References:

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.