Los Alamos National Laboratory

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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-Q3

Tanmoy Bhattacharya, Michael L. Graesser, Rajan Gupta, Michael S. Warren

Pedro Machada from Fermilab and Daniele Alvez from New York University have accepted LANL's offer for staff scientist positions and will join us in Fall, 2017.

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

New results of isovector and flavor-diagonal charges gA, gS and gT from the 2+1+1-flavor clover-on-HISQ calculations were obtained and presented at Lattice 2017 by R. Gupta. These are being prepared for publication. We are continuing increasing the statistics and improving 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. The results were presented by Yong-Chull Jang at Lattice 2017. Results for the axial vector form factors from the 2+1+1-flavor clover-on-HISQ have been published [arXiv:1705.06834]. Three additional manuscripts for publication are being drafted and final stages of the analysis is being carried out.

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. 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. The status of these calculations was presented by Boram Yoon at a plenary talk 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 quatities 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, 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. Results from a comprehensive analysis were presented by Gupta at Lattice2017. A manuscript describing the results is being prepared for publication.

Relevant References:

Transverse Momentum Distribution Functions

The manuscript with results for the Sivers and Boer-Mulders shift, the transversity and the generalized worm-gear shift for two different fermion discretization schemes has been submitted for publication [arXiv:1706.03406]. Gupta has presented these results at the POETIC7, Philadelphia, November, 2016 meeting.

Relevant References:

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.

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. 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

Relevant References:
Physics Letters B769 (2017) 460

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.

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