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Theoretical Physics 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 FY2018-Q2

Daniele Alves, 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. The team spent considerable time working on Quantum Information and Computing and are PI/members of four proposals responding to the DOE call "Quantum Information Science Enabled Discovery For High Energy Physics" was developed and submitted. 

Nucleon charges and form-factors

Extensive additional simulations of isovector and flavor-diagonal charges gA, gS and gT from the 2+1+1-flavor clover-on-HISQ calculations were completed, including the analysis of the very time consuming 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. The final analysis of nucleon charges, electric, magnetic and axial vector form factors using up to 3-state fits for both the 2+1+1-flavor clover-on-HISQ and 2+1-flavor clover on clover lattice QCD formulations is almost complete and five manuscripts describing the results are in final draft form. Preliminary results, presented by Rajan Gupta and Yong-Chull Jang at Lattice 2017, have been published as part of the conference proceedings. First results for the axial vector form factors from 2+1+1-flavor clover-on-HISQ have been published in PhysRevD.96.114503.

Relevant References:
Physical Review D96 (2017) 114503 arXiv:1705.06834
Physical Review D95:5 (2017) 074508
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 (cEDM) and mixing with the pseudoscalar operator are ongoing. The formalism for these calculations and new numerical techniques are being developed. Status of results was presented at Lattice 2017 by Boram Yoon in a plenary talk titled "neutron electric moment on the lattice". The gradient flow method for controlling the mixing and renormalizing issues for the cEDM operator is being investigated.

Relevant References:
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, and the interaction of dark matter with nucleons. These matrix elements get contributions from disconnected diagrams, that are computationally challenging to compute with high precision. Bhattacharya, Gupta and Yoon worked on the draft manuscript describing the results of extensive simulations carried out over the last three years for publication. In these calculations they analyze the chiral and continuum extrapolation for the flavor diagonal charges for the first time. They are continuing to collaborate with Gambhir, Orginos and Stathopoulos at William and Mary to develop a new method based on deflation and hierarchical probing to speed up the calculation of disconnected diagrams.

Relevant References:

Transverse Momentum Distribution Functions

Results for the Sivers and Boer-Mulders shift, the transversity and the generalized worm-gear shift for two different fermion discretization schemes were finalized and published in PRD [arXiv:1706.03406]. New simulations, in collaboration with the Regensburg group, have been initiated.

Relevant References:
Physical Review D96 (2017) 094508 arXiv:1706.03406

Behavior of QCD at finite temperature

Investigated the properties of QCD at finite temperature, including the nature of the transition, the equation of state and the fluctuations of conserved charges (electric charge, strangeness, baryon number) around the transition temperature (140–160 MeV) as part of the HOTQCD collaboration. The five publications have over 2100 citations.

Relevant 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

Graesser is revisiting the topological dark matter scenario and generalizing it in several directions. He is collaborating with J. Osinski, a University of New Mexico graduate student visiting Los Alamos for 6 months on a DOE Office of Science Graduate Student Research Award. They are exploring how the predictions for the relic dark matter density in such a scenario depends on the cosmological history prior to Big Bang Nucleosynthesis. A draft is in preparation for posting to the arXiv.

He is also continuing his work with Tuhin Roy (TATA) and Arun Thalapillil (Pune) on improving the sensitivity of the LHC to new TeV-scale physics scenarios.

Relevant 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

During this quarter Graesser has been revisiting the contribution of Majorana neutrinos to the neutrinoless double beta decay process, using chiral effective field theory. He and collaborators (Cirigliano, Dekens, de Vries, Mereghetti, Pastore, van Kolck) posted a preprint ``A new leading contribution to neutrinoless double beta decay”, arXiv:1802.10097, which has been accepted for publication in Physical Review Letters. In this paper, the authors identify a new contact operator involving 4 nucleons - i.e., short range - which occurs at the same order as the neutrino potential induced by Majorana neutrinos. The evidence is based on renormalization and using chiral symmetry to relate this process to other isospin breaking observables involving two nucleons. This effect had been missed in all previous computations. The authors speculate on the impact this new contribution will have on nuclear matrix elements of relevance to experimental searches.

Graesser and collaborators (Cirigliano, Dekens, de Vries, Mereghetti) have developed a "master decay" formula that computes the neutrinoless double decay process induced by Majorana neutrinos, as well as any other Beyond the Standard Model physics described at low energies by dimension-7 and/or dimension-9 Delta L=2 violating operators. This master formula is derived using chiral effective field theory, and includes all leading order effects. This formula supersedes all previous formulae for NDBD 0+ -> 0+ transitions, as it includes a rigorous derivation of all leading order chiral dynamics (leading order perturbative and non-perturbative renormalization effects), all relevant low-energy constants (known and unknown), a larger set of gauge invariant operators, and includes all possible interferences in the amplitude. It can be used to match to specific lepton number violating models, a point illustrated by matching the effective theory to the left-right symmetric model. A paper is in preparation.

Relevant References:
Journal of High Energy Physics12 (2017) 82 arXiv:1708.09390
Journal of High Energy Physics (2017)99
Physics Letters B769 (2017) 460

MeV Axions and New Physics at the GeV Scale

Daniele S. M. Alves has worked on the following topics: (1) phenomenology of the QCD axion in the MeV mass range, and associated new physics at the GeV scale; (2) phenomenology of sterile neutrinos with BSM interactions, including a "sterile neutrino MSW effect", and associated implications for short baseline anomalies; and (3) generalizations of Tensor Networks and Entanglement Renormalization methods to continuous field theories. A proposal responding to the DOE call "Quantum Information Science Enabled Discovery For High Energy Physics" was developed and submitted.

Relevant References: