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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 2014-02

Tanmoy Bhattacharya, Alexander Friedland, Michael L. Graesser, Rajan Gupta, Emil Mottola, Michael S. Warren

The theory group is active in a number of diverse areas of research. Their primary areas of interest 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. Generally the questions pursued by this group relate to deep mysteries in our understanding of Nature at the boundaries of the Standard Model and the grammar we use to describe it–quantum field theory and General Relativity. The theory group continues to make advances at the forefront of research in these areas.

Lattice QCD

The Los Alamos Lattice QCD team and their collaborators are carrying out four precision studies investigating signatures of new physics at the TeV scale, elucidating the structure of the nucleon, and understanding QCD at finite temperature. Progress on these four projects is described below.

Nucleon charges and form-factors

Bhattacharya, Gupta, Yoon and their external collaborators (the PNDME collaboration) have reanalyzed the calculation of renormalization constants and the matrix elements of scalar and tensor operators to probe new physics at the TeV scale. They have revised the manuscript (arXiv:1306.5435 [hep-lat]) and submitted it for publication. They have submitted proposals for allocation of computer resources for FY15 to USQCD and XSEDE. The calculations on cluster and GPU computers at Los Alamos of the largest 64 3 × 144 lattices at the weakest coupling are continuing.

Latest Reference: Physical Review D85:5, (2012) 054512.

Matrix elements of novel CP violating operators and nEDM

Bhattacharya, Cirigliano, Gupta and Yoon are continuing to carry out the analysis of the mixing and renormalization of novel CP violating operators that contribute to the Neutron Electric Dipole Moment. They have determined 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 also started investigating the calculation of disconnected diagrams for the quark electric dipole matrix elements using clover fermions on HISQ lattices.

Latest References: Bhattacharya et al., arXiv:1212.4918 [hep-lat]; arXiv:1403.2445 [hep-lat].

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 are developing the final analysis tools using the free software package R to make simultaneous fits to data at different N_T to extrapolate to the continuum limit with full propagation of errors. These results will be presented by Bazavov at Quark Matter 2014 and by Bhattacharya at Lattice 2014 conferences. Bhattacharya and Gupta also contributed to the analysis and writing of the manuscript on the deconfinement transition and U(1) axial anomaly being prepared for publication in PRL using domain wall fermions.

Latest References: Physical Review D85, (2012) 054503; Physical Review D86, (2012) 034509; Physical Review D86, (2012) 094503.

Disconnected diagrams and Transverse Momentum Distribution Functions

Bhattacharya, Gupta and Yoon, in collaboration with Michael Engelhardt, have started production runs for to investigate the signal in both connected and disconnected diagrams that will be needed to evaluate the Sivers function and other transverse momentum distribution functions using computing resources provided by USQCD at JLab.

Improving searches for new particles at the LHC

Graesser and LANL post-doc Tuhin Roy began a collaboration to use Q-Jets to improve searches for new physics at the LHC. Tuhin is one of the originators of the Q-Jet idea. Previous Q-Jet studies have focused on improving the efficiency to tag a hadronically-decaying W boson through improving the mass resolution and cutting on the variance in the mass variable (called volatility by the Q-Jet authors). The new element here is to apply the Q-Jet idea to the whole event, that is, to consider multiple clustering interpretations of the whole event. They are specifically looking at improving the efficiency for identifying all-hadronic top quark pairs. Any significant improvements here will have implications for BSM searches. Computations for this project are being done on LANL's Institutional Computing cluster resources.

Precision Cosmology Simulations

Galaxy bias, the unknown relationship between the clustering of galaxies and the underlying dark matter density field is a major hurdle for cosmological inference from large-scale structure. While traditional analyses focus on the absolute clustering amplitude of high-density regions mapped out by galaxy surveys, Warren and collaborators propose a relative measurement that compares those to the underdense regions, cosmic voids. On the basis of realistic mock catalogs they demonstrate that cross correlating galaxies and voids opens up the possibility to calibrate galaxy bias and to define a static ruler thanks to the observable geometric nature of voids.

Reference: N. Hamaus, B.D. Wandelt, P.M. Sutter, G. Lavaux, M. S. Warren, Cosmology with Void-Galaxy Correlations, Phys. Rev. Lett., 112(4):041304, 2014.

Quantum Field Theory and Gravity

Instability of Global de Sitter space

We (Paul R. Anderson of Wake Forest Univ. and E. M.) have shown that global de Sitter space is unstable to particle creation, even for a massive free field theory with no self-interactions. The decay rate of de Sitter space into particles may be calculated in very much analogous manner as that of a uniform, constant electric field first found by Schwinger. For de Sitter space with H2=Λ/3 , the decay rate per unit volume to scalar particles of mass M is

Γ= 8H4 π2 ln [ coth ( π M2 H2 - 94 ) ] . (1)

We studied the particle creation process in real time, computed their energy density in global 𝕊3 spatial sections, and showed that in the contracting phase they lead to an exponentially large energy density, necessitating an inclusion of their backreaction effects, which lead to large deviation of the spacetime from de Sitter space before the expanding phase can begin. These results are quite general and can be understood as also following from the effective action of the quantum conformal anomaly (2) for fields of any spin in de Sitter space, viz.

Seff = b 2 d4x -g { - ( ϕ ) 2 +2 ( Rab - 13 Rgab ) ( aϕ ) ( bϕ ) + ( E - 23 R) ϕ } (2)

where the curvature invariants E and F are given in terms of the Riemann curvature Rabcd by

E Rabcd Rabcd - 4 Rab Rab + R2 , F Rabcd Rabcd - 2 Rab Rab + 13 R2 . (3)

The field ϕ is an additional scalar degrees of freedom in the low energy effective theory of gravity, not present in the classical Einstein theory, which describes the long distance quantum correlations due to the trace anomaly.

The stress tensor derived from this effective action shows that states invariant under the O(4 ) subgroup of the de Sitter group are also unstable to perturbations of lower spatial symmetry, implying that both the O( 4,1) isometry group and its O(4 ) subgroup are broken by quantum state fluctuations. In the expanding patch a small amplitude deviation of the state in sufficiently high k modes also produces large deviations of the stress tensor at early times, emphasizing the extreme sensitivity of inflation to its initial conditions.

The main conclusion of our analysis is that the most symmetric state usually assumed in inflation is not the stable vacuum state. These results suggest that spatially inhomogeneous and/or dynamical models of cosmological dark energy within a Hubble horizon volume possessing only rotational O( 3) symmetry are relevant for determining the vacuum state and magnitude of cosmological dark energy in the universe.

These two papers have now been published in:

During this quarter the following invited talks were given:

  • “New Horizons in Gravity: Dark Energy and Condensate Stars,” U. Miami, Physics Dept. seminar, Feb. 13, 2014.
  • “Instability of de Sitter Space, the Schwinger Effect and Dynamical Dark Energy,” FAUST seminar, Florida Atlantic Univ., Boca Raton, FL, Feb. 24, 2014
  • “What's the (Quantum) Matter with Black Holes?,” Barry Univ., Dept. of Physical Sciences, Miami Shores, FL, Feb. 26, 2014.