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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 fourth quarter of 2014.

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

Gupta completed the analysis and write-up of the calculations of the matrix elements of scalar and tensor operators to probe new physics at the TeV scale, presented by him at Lattice 2014. Boram wrote up his analysis of disconnected diagrams for nucleon matrix elements presented at Lattice 2014. Bhattacharya, Gupta and Yoon are continuing simulations 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. They have started writing the paper analyzing the tensor charge.

Latest References: 
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 continue to make progress on 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 have finished the determination of the operator basis that allows for off-shell renormalization using external fixed momentum states. Significant progres has been made on writing the paper describing the one-loop matching between MSbar and a renormalization independent scheme is in progress. The numerical calculations of the relevant matrix elements are being done in collaboration with the RBC group using resources provided by the national USQCD initiative. Bhattacharya, Gupta, and Yoon have made progress on calculation of matrix elements involving disconnected diagrams for the quark electric dipole moment operator using clover fermions on HISQ lattices and preliminary results were presented by Yoon at Lattice 2014.

Latest References: Bhattacharya et al., 

Behavior of QCD at finite temperature

Bhattacharya wrote up the talk he presented at Lattice 2014 conference on the determination of the equation of state based on work done with the HotQCD collaboration and published in Physical Review D. They are continuing to investigate the fluctuation 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

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. Yoon wrote up the results he presented at lattice 2014, which include the first results for the quark electric dipole moment operator that contributes to nEDM. A paper on the calculations of TMDs is being prepared.

Top Quark, Dark Matter and the LHC

Graesser is continuing to work on several projects with Jessie Shelton, Tuhin Roy (TATA) 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 the University of Maryland in October.

Latest References: 
Physics Letters B749 (2014) 293 
Snowmass 2013 Top quark working group report 
Physical Review Letters111 (2013) 121802 
JHEP 1302(2013) 046 
JHEP 1210(2012) 025 
Physics Letters B714 (2012) 267 
Physics Review D85 (2012) 054512 
A dark force for baryons 
JHEP 1110(2011) 110

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 c