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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 FY2019-Q1

Daniele Alves, Tanmoy Bhattacharya, Michael L. Graesser, Rajan Gupta

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 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 neutrino-nucleon interactions. Progress during this quarter on these projects is described below. The team is also actively working on Quantum Information and Computing under the DOE call "Quantum Information Science Enabled Discovery For High Energy Physics".

Nucleon charges and form-factors

The analysis of isovector charges gA, gS and gT from the 2+1+1-flavor clover-on-HISQ calculations was completed and published in PRD. A paper describing the contribution of quark spin to the proton spin was published in PRD. A paper describing the calculation of the flavor diagonal tensor charges was also published in PRD. The analysis of 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 was carried out and manuscripts describing the results were worked on. Gupta is serving as a co-author on the FLAG 2019 review on nucleon matrix elements.

Relevant References:
Physical Review D98 (2018) 094512 arXiv:1806.10604
Physical Review D98 (2018) 091501 arXiv:1808.07597
Physical Review D98 (2018) 034503 arXiv:1806.09006
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. Investigations of gradient flow method to deal with the divergent renormalization and mixing problem of the cEDM and Weinberg operators are continuing. Results presented at Lattice 2018 by Tanmoy Bhattacharya were written up for the conference proceedings.

Relevant References:
Physical Review D92:9 (2015) 114026
Physical Review Letters 112:21 (2015) 212002

Contributions from Disconnected diagrams

The matrix elements of flavor diagonal operators are needed for the analysis of a number of interesting qualities such as the nucleon electric dipole moment, the quark contribution to the nucleon spin, the nucleon sigma term and the strangeness content of the proton, and the interaction of dark matter with nucleons. These matrix elements also get contributions from disconnected diagrams, that are computationally challenging to compute with high precision. Bhattacharya, Gupta and Yoon published two manuscript describing the results of extensive simulations carried out over the last three years. These analyses included chiral and continuum extrapolation for the disconnected contribution of the flavor diagonal charges for the first time.

Relevant References:
Physical Review D98 (2018) 094512 arXiv:1806.10604
Physical Review D98 (2018) 091501 arXiv:1808.07597

Transverse Momentum Distribution Functions

New simulations, in collaboration with the Regensburg group, are being done.

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

Dark Matter and LHC Physics

Graesser and UNM graduate student Jacek Osinski explored topological dark matter, in the form of hidden sector magnetic monopoles, produced in the early Universe during a second order phase transition. They generalized results in the prior literature by considering the impact of a non-standard cosmological history on the relic abundance of the monopoles. They found a monopole mass of order (1-100) PeV to be generic for the cosmological histories they considered, if monopoles are to entirely reproduce the current abundance of dark matter. Their results verify the robustness of the O(PeV) scale for the monopole mass that is needed if the early Universe is radiation-dominated around the time of production. A draft is in preparation.

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

Michael Graesser: Neutrinoless double beta decay

The neutrinoless double beta decay process (NDBD) is an important probe of any beyond-the-Standard-Model physics that violates lepton number by two units. NDBD provides a critical constraint on Majorana neutrino masses for active neutrinos, interactions of sterile/right-handed neutrinos, and on general classes of multi-TeV physics violating lepton number by two units. An important theoretical program is to use chiral effective theory to systematically describe how such high-energy physics, including Majorana neutrino masses, contribute to the NDBD process.

While there is the well-known long-range component of a Majorana neutrino mass to the NDBD matrix element, in Phys. Rev. Lett. 120 (2018) no.20, 202001, Graesser and collaborators found that there is also a short-range component. The numerical size of this new unknown low-energy constant (LEC) can only be computed using lattice QCD, but its size can be estimated in at least two ways. Non-perturbative renormalization of the long-range component -- which demonstrates a need for the new LEC -- also suggests the new contact interaction is suppressed only by $1/F^2_\pi$ rather than the nucleon mass$^2$, thereby providing a sizable effect on top of the long-range component. Next, the above reference shows that the LEC in NDBD is related to one of two LECs that describe charge and isospin breaking effects in nucleon scattering. The sizable charge and isospin breaking observed in nucleon-nucleon scattering lengths requires similarly large LECs, further suggesting that a large LEC occurs in NDBD.

He and his collaborators have now extended the above analysis in NDBD to next-leading-order in the chiral expansion, and a paper is in preparation.

Relevant References:
Journal of High Energy Physics12 (2018) 097 arXiv:1806.02780
Physical Review Letters 120:20 (2018) 202001 arXiv:1802.10097
Journal of High Energy Physics12 (2017) 82 arXiv:1708.09390
Journal of High Energy Physics (2017)99
Physics Letters B769 (2017) 460

Daniel S. M. Alves

During the past quarter, Daniele Alves has made progress on two projects: (1) models of sterile neutrinos with BSM interactions with matter via light mediators, specifically, on how these models are constrained by solar neutrinos, and how they could potentially address the distortion of the 8B solar neutrino spectrum, if confirmed; (2) generalizing Tensor Networks and Entanglement Renormalization to continuous effective field theories, specifically, using the Sine-Gordon/Thirring model as a testing ground for these methods.

Relevant References:
Journal of High Energy Physics 07 (2018) 92 arXiv:1710.03764