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 2014-04

Tanmoy Bhattacharya, Alexander Friedland, 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. The theory group participated in the DOE on-site review of the HEP program at Los Alamos during April 2015 and was judged to be performing excellent high impact research.

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, elucidating the structure of the nucleon, and understanding QCD at finite temperature. Progress during this quarter on the four projects being pursued is described below.

Nucleon charges and form-factors

Final analysis and a paper describing the calculation of the tensor charges for probing novel tensor interactions at the TeV scale in neutron beta-decay was prepared and submitted for publication to PRD. Bhattacharya, Gupta and Yoon are continuing simulations using the All-Mode-Averaging (AMA) technique on finer lattices to quantify discretization errors and improve the extrapolation to the continuum limit. Gupta presented this detailed analysis at Lattice 2015. On the cluster and GPU computers at Los Alamos, they are continuing to simulate the largest 643×144 lattices at the weakest coupling. Bhattacharya, Gupta and Yoon started setting up codes and analysis routines for doing calculations on the Titan computer at Oakridge under the ALCC program.

Latest References: 
arXiv:1506.06411 
Physical Review D89:9 (2014094502 
Physical Review D85:5 (2012054512.

Matrix elements of novel CP violating operators and nEDM

Bhattacharya, Cirigliano, Gupta and Yoon finished 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. They determined 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. The paper describing these calculations has been submitted for publication to PRD. A second paper analyzing the calculation of the quark electric dipole moment (tensor charges of the up, down and strange quarks within the neutron) and their contribution to the neutron electric dipole moment and implications for splitSUSY models was prepared and submitted to PRL. Bhattacharya, Gupta, and Yoon continue to make progress on calculation of matrix elements involving disconnected diagrams for the quark electric dipole moment operator using the clover-on-HISQ formulation. Bhattacharya presented the first results on the calculation strategy for quark chromo EDM operator at Lattice 2015. The data are being analyzed and a paper describing the numerical results is being prepared.

Latest References: Bhattacharya et al., 
arXiv:1506.04196 
arXiv:1502.07325 
arXiv:1403.2445 
arXiv:1212.4918

Behavior of QCD at finite temperature

The HotQCD collaboration is continuing to investigate fluctuations of conserved charges (electric charge, strangness, baryon number) near the transition temperature to investigate the behavior of QCD near the possible critical end-point at finite chemical potential.

Latest References: 
Physical Review D90 (2014094503 
Physical Review Letters113 (2014082001 
Physical Review D86 (2012034509 
Physical Review D86 (2012094503 
Physical Review D85 (2012054503

Disconnected diagrams and Transverse Momentum Distribution Functions

Bhattacharya, Gupta, Yoon and collaborator Michael Engelhardt at NMSU, are continuing production runs for calculating matrix elements to evaluate the Sivers function and other transverse momentum distribution (TMD) functions using computing resources provided by USQCD at JLab. Yoon presented results comparing estimates using two different lattice actions to understand systematic errors at Lattice 2015. A paper comparing estimates for TMDs using clover and domain-wall fermions is being prepared. Bhattacharya, Gupta and Yoon are investigating methods to speed up the calculation of disconnected diagrams and improve the signal.

Top Quark, Dark Matter and the LHC

Graesser spent this quarter on two projects. He has been developing improvements to Graesser and Shelton's previous topness variable, in which the previous estimate for the top quark center-of-mass energy, 2 times the top quark mass, is replaced with a better estimate. He also rewrote from scratch the minimization routine (Nelder-Mead) used in their topness variable, obtaining, for the same accuracy, a factor of approximately 7 improvement in running time compared to the previous routine. This new version has been released for use in CMS and will soon be made publicly available. Graesser also spent time on determining the phenomenological constraints on a natural SUSY scenario having a light stop and a light stau. This work is with LANL post-doc Jinrui Huang. They are constraining this scenario using the LHC 8 TeV searches for stop production (CMS, Eur. Phys. J. C (2013) 73:2677, arXiv:1308.1586), stop production and decay to a stau (ATLAS-CONF-2014-014), CMS' measurements of the top quark production cross-section (Phys. Lett. B 739 (2014) 23, arxiv:1407.6643 (lepton-tau final state), and JHEP 11 (2012) 067, arXiv:1208.2671 (dilepton final state)), as well as CMS' search for leptoquarks decaying to top and tau final states (Phys. Lett. B 739 (2014) 229, arXiv:1408.0806), as well as direct-detection (LUX) and indirect detection (FERMI) constraints.

Latest References: 
Physics Letters B749 (2014293 
arXiv:1311.2028 
Physical Review Letters111 (2013121802 
JHEP 1302(2013046 
JHEP 1210(2012025 
Physics Letters B714 (2012267 
Physics Review D85 (2012054512 
arXiv:1107.2666 
JHEP 1110(2011110

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. Carried out a higher resolution simulation on Titan at Oak Ridge with (10240**3) particles and 1/h Gpc box. 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.

Supernova neutrino oscillations

Friedland has started studying experimental and theoretical constraints on neutrino-nucleus scattering cross sections at the GeV energies. Both parity-violating electron scattering data and recent neutrino scattering data at MiniBooNE and Minerva are being analyzed.

New physics in the neutrino sector

Friedland in collaboration with postdocs Cherry and Shoemaker has completed the first study of the "neutrino portal" framework. In this framework, neutrinos couple through mixing to a hidden sector containing also the dark matter particles. The coupling evades direct laboratory detection, but leads to significant cosmological effects. In particular, the observed deficit of dark matter structure on small scales can be naturally explained. Friedland et al observed that in this framework a novel experimental signature is possible. Ultra-high-energy neutrinos traveling traveling over cosmological distances could be scattered on the neutrinos in the relic neutrino background. This phenomenon can have observable consequences for the Icecube data. These consequences are outlines in a recent paper arXiv:1411.1071.

Friedland, in collaboration with K. Babu and P. Machado, has been studying experimental implications of a new gauge group in the third generation of quarks and leptons. Numerous bounds coming from precision measurements in the quark and charged lepton sector have been derived. The focus is to understand what values of nonstandard neutrino interactions are allowed in this scenario.


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