116463245
The GPDs of The Photon
We report on a recent calculation of the generalized parton distributions (GPDs) of the photon when the momentum transfer in the transverse direction is non-zero. We use an overlap representation of the photon GPDs in terms of the photon light-front wave functions. We calculate the GPDs at leading order in electromagnetic coupling $\alpha$ and zeroth order in strong coupling $\alpha_s$. We consider also the case when the helicity of the photon is flipped. Fourier transform of the GPDs with respect to the transverse momentum transfer gives the parton distributions of the photon in impact parameter space.
119418615
Threshold resummation for W-boson production at BNL RHIC
We study the resummation of large logarithmic perturbative corrections to the partonic cross sections relevant for the process pp{yields}W{sup {+-}}X at the BNL Relativistic Heavy Ion Collider (RHIC). At RHIC, polarized protons are available, and spin asymmetries for this process will be used for precise measurements of the up and down quark and antiquark distributions in the proton. The corrections arise near the threshold for the partonic reaction and are associated with soft-gluon emission. We perform the resummation to next-to-leading logarithmic accuracy, for the rapidity-differential cross section. We find that resummation leads to relatively moderate effects on the cross sections and spin asymmetries.
119296844
Generalized parton distributions of the photon
Abstract We present a first calculation of the generalized parton distributions of the photon (both polarized and unpolarized) using overlaps of light-front wave functions at leading order in α and zeroth order in α s ; for non-zero transverse momentum transfer and zero skewness. We present the novel parton content of the photon in transverse position space.
118529485
Off-axis excitation of hydrogenlike atoms by twisted photons
We show that the twisted photon states, or photon states with large ($> \hbar$) angular momentum projection ($m_\gamma$) in the direction of motion, can photoexcite atomic levels for a hydrogen-like atom that are novel and distinct and are not restricted by $m_\gamma$, when the symmetry axis of the twisted-photon beam does not coincide with the center of the atomic target. Selection rules are given and interesting implications and observables for the above process are pointed out.
55544629
Universality of TMD correlators
In a high-energy scattering process with hadrons in the initial state, color is involved. Transverse momentum dependent distribution functions (TMDs) describe the quark and gluon distributions in these hadrons in momentum space with the inclusion of transverse directions. Apart from the (anti)-quarks and gluons that are involved in the hard scattering process, additional gluon emissions by the hadrons have to be taken into account as well, giving rise to Wilson lines or gauge links. The TMDs involved are sensitive to the process under consideration and hence potentially nonuniversal due to these Wilson line interactions with the hard process; different hard processes give rise to different Wilson line structures. We will show that in practice only a finite number of universal TMDs have to be considered, which come in different linear combinations depending on the hard process under consideration, ensuring a generalized universality. For quarks this gives rise to three Pretzelocity functions, whereas for gluons a richer structure of functions arises.
118544091
Excitation of an Atom by Twisted Photons
Twisted photon states, or photon states with large ($> \hbar$) angular momentum projection in the direction of motion, can photoexcite atomic final states of differing quantum numbers. If the photon symmetry axis coincides with the center of an atom, there are known selection rules that require exact matching between the quantum numbers of the photon and the photoexcited states. The more general case of arbitrarily positioned beams relaxes the selection rules but produces a distribution of quantum numbers of the final atomic states that is novel and distinct from final states produced by plane-wave photons. Numerical calculations are presented using a hydrogen atom as an example.
118408897
Metropolis Monte Carlo integration on the Lefschetz thimble: Application to a one-plaquette model
We propose a new algorithm based on the Metropolis sampling method to perform Monte Carlo integration for path integrals in the recently proposed formulation of quantum field theories on the Lefschetz thimble. The algorithm is based on a mapping between the curved manifold defined by the Lefschetz thimble of the full action and the flat manifold associated with the corresponding quadratic action. We discuss an explicit method to calculate the residual phase due to the curvature of the Lefschetz thimble. Finally, we apply this new algorithm to a simple one-plaquette model where our results are in perfect agreement with the analytic integration. We also show that for this system the residual phase does not represent a sign problem.
117551201
Quantum field theories on the Lefschetz thimble
In these proceedings, we summarize the Lefschetz thimble approach to the sign problem of Quantum Field Theories. In particular, we review its motivations, and we summarize the results of the application of two different algorithms to two test models.
118570813
Configuration-interaction Monte Carlo method and its application to the trapped unitary Fermi gas
We develop a quantum Monte Carlo method to estimate the ground-state energy of a fermionic many-particle system in the configuration-interaction shell model approach. The fermionic sign problem is circumvented by using a guiding wave function in Fock space. The method provides an upper bound on the ground-state energy whose tightness depends on the choice of the guiding wave function. We argue that the antisymmetric geminal product class of wave functions is a good choice for guiding wave functions. We demonstrate our method for the trapped two-species fermionic cold atom system in the unitary regime of infinite scattering length using the particle-number projected Hartree-Fock-Bogoliubov wave function as the guiding wave function. We estimate the ground-state energy and energy-staggering pairing gap as a function of the number of particles. Our results compare favorably with exact numerical diagonalization results and with previous coordinate-space Monte Carlo calculations.
118482889
Quantum Monte Carlo with Coupled-Cluster wave functions
ECT*, Villa Tambosi, I-38123 Villazzano (Trento), Italy(Dated: May 3, 2013)We introduce a novel many body method which combines two powerful many body techniques,viz., quantum Monte Carlo and coupled cluster theory. Coupled cluster wave functions are intro-duced as importance functions in a Monte Carlo method designed for the conﬁguration interactionframework to provide rigorous upper bounds to the ground state energy. We benchmark our methodon the homogeneous electron gas in momentum space. The importance function used is the coupledcluster doubles wave function. We show that the computational resources required in our methodscale polynomially with system size. Our energy upper bounds are in very good agreement withprevious calculations of similar accuracy, and they can be systematically improved by includinghigher order excitations in the coupled cluster wave function.
122546368
Quantum Monte Carlo with non-local chiral interactions
Quantum Monte Carlo methods have been used very successfully to calculate the ground state properties of a variety of nuclear and other strongly interacting many body systems. An exciting recent development in low energy nuclear physics is the formulation of nuclear Hamiltonians based on the symmetries of quantum chromodynamics via chiral effective field theory. However, the usual coordinate space based quantum Monte Carlo implementation is unsuitable for these Hamiltonians due to the presence of non-localites. Here we report on our recent work in developing a new quantum Monte Carlo algorithm suitable for these non-local Hamiltonians, and present some results for pure neutron matter.