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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. A number of advances have been made by the theory group in different areas during the second quarter of 2015.

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

Significant progress has been made in the analysis of tensor charges for probing novel tensor interactions at the TeV scale in neutron beta-decay. 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. On the cluster and GPU computers at Los Alamos, they are simulating the largest 643 ×144 lattices at the weakest coupling. A paper analyzing the tensor charge with control over all systematic errors is being prepared. Three proposals requesting computer allocation from ALCC and USQCD for doing these calculations were prepared.

Latest References: Bhattacharya et al., 
Physical Review D89:9 (2014) 094502 
Physical Review D85:5 (2012) 054512.

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 to the arXiv.org. Bhattacharya, Gupta, and Yoon have made progress on calculation of matrix elements involving disconnected diagrams for the quark electric dipole moment operator using the clover-on-HISQ formulation. The data are being analyzed and a paper describing the numerical results is being prepared.

Latest References: Bhattacharya et al., 

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 possible critical end-point at finite chemical potential.

Latest References: 
Physical Review D90 (2014) 094503 
Physical Review Letters113 (2014) 082001 
Physical Review D86 (2012) 034509 
Physical Review D86 (2012) 094503 
Physical Review D85 (2012) 054503 
Physical Review D80 (2009) 014504

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. Bhattacharya, Gupta and Yoon are investigating methods to speed up the calculation of disconnected diagrams and improve the signal. A paper comparing estimates for TMDs using clover and domain-wall fermions is being prepared.

Top Quark, Dark Matter and the LHC

Graesser is continuing to work on several projects with Jessie Shelton, Tuhin Roy (TIFR, India) and LANL post-doc Jinrui Huang. Graesser and Shelton are continuing their investigations of their topness kinematic variable which is now being used by ATLAS. Using this and other kinematic variables they are performing projections of the LHC's expected sensitivity to the "natural susy" scenario in the next run at 14 TeV. Graesser and Roy are generalizing the "QJet" idea originally developed by Roy and collaborators, to a "QEvent" concept that could be used to improve the sensitivity of LHC searches to new physics. Graesser and Huang are determining the phenomenological constraints on a natural SUSY scenario having a light stop and also a light stau, the latter motivated by obtaining a thermal relic, and the former by naturalness. They are applying a number of LHC constraints obtained during the run at 8 TeV to this scenario, the most important being direct searches for light stops, the measurements of the top quark pair production cross-section and mass, the measurements of the Higgs branching ratios, as well as direct searches for light charginos, neutralinos or staus. Much of this work uses LANL's cluster computing resources.

Graesser gave a talk on the topness work at Caltech in February.

Latest References: 
Physics Letters B749 (2014293 
Physical Review Letters111 (2013121802 
JHEP 1302(2013046 
JHEP 1210(2012025 
Physics Letters B714 (2012267 
Physics Review D85 (2012054512 
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.