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-Q2

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-flavor clover-on-clover calculations were published in PRD. 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. The analysis of electric, magnetic and axial form factors using 2-state fits was carried out and manuscripts for publication were drafted and are in their final stages for publication.

Recent References:
Physical Review D95:5 (2017) 074508
Physical Review D94:5 (2016) 054508
Physical Review D93:11 (2016) 114506
arXiv:1601.01730
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.,
arXiv:1701.04132
arXiv:1612.08438
arXiv:1601.02264
Physical Review D92:9 (2015) 114026
Physical Review Letters 112:21 (2015) 212002
arXiv:1502.07325
arXiv:1403.2445
arXiv:1212.4918

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. First results were presented by Gambhir at Lattice2016. A manuscript describing the results is being prepared for publication.

References:
arXiv:1611.01193

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.

Latest References:
arXiv:1611.01193
arXiv:1601.05717

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.

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 has been 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.

Latest References:
Physics Letters B749 (2014) 293
arXiv:1311.2028
Physical Review Letters111 (2013) 121802
JHEP 1302(2013) 046
JHEP 1210(2012) 025
Physics Letters B714 (2012) 267
Physics Review D85 (2012) 054512
arXiv:1107.2666
JHEP 1110(2011) 110

Neutrinoless double beta decay

In this quarter Graesser has been working with Vincenzo Cirigliano, Wouter Dekens and Emanuele Mereghetti (LANL) on a couple of projects related to phenomenological signals of neutrinoless double beta decay. In 1701.01442 (Phys. Lett B769 (2017) 460-464), the authors use SU(3) chiral perturbation theory to provide a first estimate of the two-pion matrix element of all Delta I=2 Lorentz scalar 4-quark operators that contribute to a Delta L=2 neutrinoless double beta decay signal, arising from short distance physics. The authors relate the two-pion matrix element to the kaon-anti-kaon matrix elements, as well as K-> 2 pi, which have been computed accurately using lattice QCD. SU(3) breaking effects due to meson mass splitting occurs at next-to-leading chiral order and based on a one-loop computation the authors estimate the size of those corrections to be of O(20%-30%). Long distance pion exchange can give the leading contribution to neutrinoless double beta signal arising from short distance physics scenarios, and the two - pion matrix element is an important input to that process.

Graesser, together with Cirigliano, Dekens and Mereghetti are investigating the phenomenology of dimension-7 operators which can also contribute to a Delta L=2 neutrinoless double beta decay signal.

Latest References:
Physics Letters B769 (2017) 460
arXiv:1606.04549

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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