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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-Q4

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.

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 are almost complete and preliminary results were presented by R. Gupta at Lattice 2017. The most time consuming of the ongoing calculations is 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. The analysis of electric, magnetic and axial vector form factors using 2-state fits was carried out for both the 2+1+1-flavor clover-on-HISQ and 2+1-flavor clover on clover lattice QCD formulations. These results have been extended to 3-state analysis and were presented by Yong-Chull Jang at Lattice 2017. Both talks at Lattice 2017 are being prepared for publication. Results for the axial vector form factors from the 2+1+1-flavor clover-on-HISQ have been published in arXiv:1705.06834 and are in the final stages of review by PRD. The final analysis of our 2+1+1-flavor clover-on-HISQ calculations is being carried out and the writing of three manuscripts for publication was initiated.

Relevant References:
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 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. Production runs for disconnected diagrams and the reweighting factor have started. Theoretical and numerical investigations of the operator mixing are being carried out. The status of these calculations was presented by Boram Yoon in the plenary talk on the calculation of matrix elements of novel CP violating operators and nEDM at Lattice 2017.

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 are preparing a manuscript describing the results in collaboration with Gambhir, Orginos and Stathopoulos at William and Mary. They are also continuing 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

Prepared and published results for the Sivers and Boer-Mulders shift, the transversity and the generalized worm-gear shift for two different fermion discretization schemes [arXiv:1706.03406]. Addressed the referee's comments. Paper accepted for publication in PRD.

Relevant References:

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 continuing his work with Tuhin Roy (TATA) and Arun Thalapillil (Pune) on improving the sensitivity of the LHC to new TeV-scale physics scenarios. Graesser continues to collaborate with D. Kang and B. Yoon (LANL) on applying deep learning methods to improve the sensitivity of the LHC to new 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

Graesser, together with Cirigliano, Dekens and Mereghetti (LANL) and de Vries (Niken) have been investigating the phenomenology of dimension-7 operators which can also contribute to a Delta L=2 neutrinoless double beta decay signal. Dimension-7 Delta L=2 operators are the next-leading order contribution after the dimension-5 Weinberg operator. They have classified the operators and evolved them to the electroweak scale where such operators are matched onto dimension-6, -7 and -9 operators. They then evolve the effective operators to the QCD scale and construct the chiral effective theory for such operators, and derive the two-nucleon neutrinoless double beta decay currents up to leading order in the chiral counting. They find the neutrinoless double beta decay amplitude to depend on only a relatively few number of nuclear matrix elements, which can be lifted from the existing literature. Some phenomenology of the dependence of the differential decay rate on the neutrino mass and dimension-7 operator Wilson coefficients, as well as different methods to compute the nuclear matrix elements, is discussed. This work has been submitted to the arxiv:1708.09390.

Relevant References:
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 recently joined the LANL HEP theory group and has been investigating the possibility of realizing the PQ mechanism (which offers a solution to the Strong CP Problem) at the GeV scale. Several interesting phenomenological consequences include an axion in the several MeV mass range, as well as new GeV states coupled hadronically, a possibility that has been under-explored in BSM physics. Her first paper on this subject (in collaboration with N. Weiner from NYU) was recently posted on the arXiv (arXiv:1710.03764) and submitted for publication. She is working on several follow-ups to this work.

Relevant References: