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. 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 are described below.

Nucleon charges and form-factors

Results of the calculation of renormalization constants and quantification of systematic errors in calculations of the matrix elements of scalar and tensor operators to probe new physics at the TeV scale were presented by Gupta at Lattice 2014 and Solvay Conference in Brussels in Sept 2014. Bhattacharya, Gupta and Yoon are extending the simulations to finer lattices to quantify discretization errors and improve the extrapolation to the continuum limit. The collaboration was awarded 20.75 M core hours on clusters at FNAL for FY15 by USQCD which are being used to increase the statistics. On the cluster and GPU computers at Los Alamos, they are simulating the largest 643×144 lattices at the weakest coupling.

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

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 established an operator basis that allows for off-shell renormalization using external fixed momentum states, and a 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., arXiv:1212.4918arXiv:1403.2445

Behavior of QCD at finite temperature

Bhattacharya and Gupta carried out the statistical analysis of the entire data set generated by the full HotQCD collaboration to determine the equation of state. They also developed the final analysis tools using the free software package R to make simultaneous fits to data at different NT to extrapolate to the continuum limit with full propagation of errors. These results were presented by Bazavov at Quark Matter 2014 and by Bhattacharya at Lattice 2014 conferences. The final paper has been accepted for publication to Physical Review D. Bhattacharya and Gupta also contributed to the analysis and writing of the manuscript on the deconfinement transition and U(1) axial anomaly using domain wall fermions that was published in Physical Review Letters 113 (2014) 082001. They are now investigating the fluctuation of conserved charges (Electric charge, strangness, baryon number) near the transition to probe the possible critical end-point at finite chemical potential.

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

Disconnected diagrams and Transverse Momentum Distribution Functions

Bhattacharya, Gupta, Yoon and collaborator Michael Engelhardt at NMSU, are carrying out production runs for calculating matrix elements to evaluate the Sivers function and other transverse momentum distribution functions using computing resources provided by USQCD at JLab. Engelhardt presented these results at Lattice 2014. Bhattacharya, Gupta and Yoon have investigated methods to speed up the calculation of disconnected diagrams and improve the signal. Yoon presented these results at lattice 2014, which included the first results for the quark electric dipole moment operator that contributes to nEDM.

Dark matter and the LHC

The preprint arXiv:1311.5886 [hep-ph], by Cirigliano, Graesser, Ovanesyan, and Shoemaker, has now being accepted for publication in Phys. Lett. B.

Work by Graesser and Jessie Shelton presented in Phys. Rev. Lett. 111 (2013) 121802 proposed a new kinematic variable (topness) designed to improve the sensitivity of the LHC experiments to top squark production, with top squarks decaying to tb+MET and tt+MET final states. Their topness variable is now being used by the ATLAS experiment in their search for top squark production (arXiv:1407.0583 [hep-ex]), with the top squarks decaying to those final states. The ATLAS search (arXiv:1410.4031 [hep-ex]) looking for production of dark matter that predominately couples to top quarks also uses the topness variable.

Graesser continues to work on several projects with Jessie Shelton, Tuhin Roy (now at TATA) and LANL post-doc Jinrui Huang, using LANL's cluster computing resources. These projects are aimed at improving the sensitivity of the LHC to beyond-the-Standard Model physics appearing in final states having significant top quark backgrounds.

Huang and collaborators Tao, Yu, and Wang continue their exploration of the nearly Peccei-Quinn symmetric limit shared by common singlet extensions of the Minimal Supersymmetric Standard Model, that was initiated in arXiv:1309.6633 [hep-ph]Phys. Rev. Lett. 112 (2014) 221803. This limit has been established as a viable framework for studying sub-electroweak scale dark matter phenomenology and has interesting and direct connections to new exotic Higgs decay physics. In arXiv:1407.0038 [hep-ph] they discuss benchmark models in this framework that accommodate the Galactic Center gamma ray excess. They emphasize connections between the phenomenology of dark matter direct detection, indirect detection, and new exotic decay channels for the 125 GeV Higgs boson decays to h→ττMET and h→bbMET final states. They estimate the sensitivity at the LHC for rare Higgs boson decay modes. This preprint has been accepted for publication in Phys. Rev. D.

Precision Cosmology Simulations

The "Dark Sky Simulations: Early Data Release" paper by Skillman & Warren et al. was submitted to Computational Science & Discovery, and the data and analysis software was made publicly available at http://darksky.slac.stanford.edu. Work continues on Titan at Oak Ridge testing independent timestep code modifications to support the high resolution ds14b simulation.

Supernova neutrino oscillations

Friedland and collaborators have continued studying the rich and complicated dynamics of neutrino flavor transformations inside a supernova and its implications for the design of future neutrino detectors.

On the theoretical front, it was realized that, while all existing calculations of collective oscillations in a supernova assume flavor universality, small deviations from universality could in fact have a big impact on the outcome of the calculations. To quantify this effect, Friedland and E. Passemar (LANL postdoc, now faculty at Indiana U) have computed non-universal contributions to the MSW potentials induced by loop effects. The computation is completed and two papers – one on the physics of the calculations and one on its impact on the oscillation dynamics – are in preparation.

On the experimental front, the spectra obtained in the simulations were put in the simulation software for the future liquid argon and water Cherenkov detectors. This work, done in collaboration with Kate Scholberg (Duke U) and Michael Smy (UC Irvine), demonstrated that prominent nonthermal spectral features can indeed be seen a large liquid argon detector, while in a water-Cherenkov detectors they are completely washed out due to the inherently poor energy resolution. The results were presented at the August LBNE collaboration meeting at Fermilab.

New physics in the neutrino sector

In collaboration with postdocs J. Cherry and I. Shoemaker, Friedland has been developing a framework of nonstandard neutrino interactions that arise from neutrino mixing with a new secluded sector. It has been found that the presence of this new physics could be probed with the Ultra-High-Energy neutrinos recently observed by the Icecube collaboration. It was further noted that the scale of the masses in the secluded sector naturally coincides with the values favored by models of self-interacting dark matter. The signatures of this mechanism and its possible connection to the dark matter sector are currently being explored. Preliminary results were presented by J. Cherry at the MIAPP neutrino workshop in Garching, Germany.

In addition to the LBNE collaboration meeting presentation, Friedland has also given a lecture at The 2014 International Summer School on AstroComputing (UCSD, July 2014) and an overview talk on supernova neutrino oscillations at the MIAPP neutrino workshop in Garching, Germany


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