Academic literature on the topic 'Multi-hadron operators'

An extended multi-hadron operator is developed to extract the spectra of irreducible representations in the finite volume. The irreducible representations of the cubic group are projected using a coordinate-space operator. The correlation function of this operator is computationally effcient to extract lattice spectra. In particular, this new formulation only requires propagator inversions from two distinct locations, at fixed physical separation. We perform a proof-of-principle study on a 243 × 48 lattice volume with mπ ≈ 900 MeV by isolating the spectra of A+1, E+ and T+2 of the ππ system with isospin-2 in the rest frame.

Highlights from recent computations in lattice QCD involving baryons are presented. Calcula tions of the proton mass and spin decompositions are discussed, a percent level determination of the nucleon axial coupling is described, and determinations of the proton and neutron electromagnetic form factors and light-cone parton distribution functions are outlined. Recent results applying the so-called Luscher method to meson-baryon systems are presented. Key points emphasized are that much better precision with disconnected diagrams is being achieved, incorporating multi-hadron operators is now feasible, and more and more studies are being done with physical quark masses.

The use of remote robotic systems for inspection and maintenance in hazardous environments is a priority for all tasks potentially dangerous for humans. However, currently available robotic systems lack that level of usability which would allow inexperienced operators to accomplish complex tasks. Moreover, the task’s complexity increases drastically when a single operator is required to control multiple remote agents (for example, when picking up and transporting big objects). In this paper, a system allowing an operator to prepare and configure cooperative behaviours for multiple remote agents is presented. The system is part of a human–robot interface that was designed at CERN, the European Center for Nuclear Research, to perform remote interventions in its particle accelerator complex, as part of the CERNTAURO project. In this paper, the modalities of interaction with the remote robots are presented in detail. The multimodal user interface enables the user to activate assisted cooperative behaviours according to a mission plan. The multi-robot interface has been validated at CERN in its Large Hadron Collider (LHC) mockup using a team of two mobile robotic platforms, each one equipped with a robotic manipulator. Moreover, great similarities were identified between the CERNTAURO and the TWINBOT projects, which aim to create usable robotic systems for underwater manipulations. Therefore, the cooperative behaviours were validated within a multi-robot pipe transport scenario in a simulated underwater environment, experimenting more advanced vision techniques. The cooperative teleoperation can be coupled with additional assisted tools such as vision-based tracking and grasping determination of metallic objects, and communication protocols design. The results show that the cooperative behaviours enable a single user to face a robotic intervention with more than one robot in a safer way.

There are many outstanding problems in nuclear physics which require input and guidance from lattice QCD calculations of few baryons systems. However, these calculations suffer from an exponentially bad signal-to-noise problem which has prevented a controlled extrapolation to the physical point. The variational method has been applied very successfully to two-meson systems, allowing for the extraction of the two-meson states very early in Euclidean time through the use of improved single hadron operators. The sheer numerical cost of using the same techniques in two-baryon systems has so far been prohibitive. We present an alternate strategy which offers some of the same advantages as the variational method while being significantly less numerically expensive. We first use the Matrix Prony method to form an optimal linear combination of single baryon interpolating fields generated from the same source and different sink interpolating fields. Very early in Euclidean time this optimal linear combination is numerically free of excited state contamination, so we coin it a calm baryon. This calm baryon operator is then used in the construction of the two-baryon correlation functions.To test this method, we perform calculations on the WM/JLab iso-clover gauge configurations at the SU(3) flavor symmetric point with mπ~ 800 MeV — the same configurations we have previously used for the calculation of two-nucleon correlation functions. We observe the calm baryon significantly removes the excited state contamination from the two-nucleon correlation function to as early a time as the single-nucleon is improved, provided non-local (displaced nucleon) sources are used. For the local two-nucleon correlation function (where both nucleons are created from the same space-time location) there is still improvement, but there is significant excited state contamination in the region the single calm baryon displays no excited state contamination.

Abstract Modern analysis on parton distribution functions (PDFs) requires calculations of the log-likelihood functions from thousands of experimental data points, and scans of multi-dimensional parameter space with tens of degrees of freedom. In conventional analysis the Hessian approximation has been widely used for the estimation of the PDF uncertainties. The Lagrange Multiplier (LM) scan while being a more faithful method is less used due to computational limitations, and is the main focus of this study. We propose to use Neural Networks (NNs) and machine learning techniques to model the profile of the log-likelihood functions or cross sections for multi-dimensional parameter space in order to overcome those limitations which work beyond the quadratic approximations and meanwhile ensures efficient scans of the full parameter space. We demonstrate the efficiency of the new approach in the framework of the CT18 global analysis of PDFs by constructing NNs for various target functions, and performing LM scans on PDFs and cross sections at hadron colliders. We further study the impact of the NOMAD dimuon data on constraining PDFs with the new approach, and find enhanced strange-quark distributions and reduced PDF uncertainties. Moreover, we show how the approach can be used to constrain new physics beyond the Standard Model (BSM) by a joint fit of both PDFs and Wilson coefficients of operators in the SM effective field theory.

Abstract The microscopic dynamics of particle collisions is imprinted into the statistical properties of asymptotic energy flux, much like the dynamics of inflation is imprinted into the cosmic microwave background. This energy flux is characterized by correlation functions $$ \left\langle \mathcal{E}\left({\overrightarrow{n}}_1\right)\cdots \mathcal{E}\left({\overrightarrow{n}}_k\right)\right\rangle $$ E n → 1 ⋯ E n → k of energy flow operators $$ \mathcal{E}\left(\overrightarrow{n}\right) $$ E n → . There has been significant recent progress in studying energy flux, including the calculation of multi-point correlation functions and their direct measurement inside high-energy jets at the Large Hadron Collider (LHC). In this paper, we build on these advances by defining a notion of “celestial non-gaussianity” as a ratio of the three-point function to a product of two-point functions. We show that this celestial non-gaussianity is under perturbative control within jets at the LHC, allowing us to cleanly access the non-gaussian interactions of quarks and gluons. We find good agreement between perturbative calculations of the non-gaussianity and a charged-particle-based analysis using CMS Open Data, and we observe a strong non-gaussianity peaked in the “flattened triangle” regime. The ability to robustly study three-point correlations is a significant step in advancing our understanding of jet substructure at the LHC. We anticipate that the celestial non-gaussianity, and its generalizations, will play an important role in the development of higher-order parton showers simulations and in the hunt for ever more subtle signals of potential new physics within jets.

Since the inception of lattice QCD, significant effort has been invested into exploring hadronic spectra, both to shed light upon the nature and properties of various states, and to test the validity of the methodology itself. Critical challenges in this endeavour are the judicious selection of interpolating operators, and the choice of calculation paradigm within which these operators are utilised to extract observables. In this thesis both of these challenges are addressed. Focusing on the topical nucleon sector, various local five-quark interpolating fields are introduced and spectroscopic calculations are performed with them. These local multi-hadron operators of interest give rise to diagrams that contain loop propagators that necessarily require a different calculation recipe. Stochastic estimation techniques are utilised to evaluate these propagation amplitudes, and a method to smear these propagators is developed. The variational method for extracting hadronic excitations is then examined by producing spectra with a variety of operator bases. Fitting a single-state ansatz to the eigenstate-projected correlators is demonstrated to provide robust energies for the low-lying spectrum that are essentially invariant despite originating from qualitatively different bases. In the negative-parity nucleon sector, the introduction of local five-quark operators permits the extraction of a state consistent with the S-wave πN scattering threshold, while in the positive-parity channel the excited state spectrum remains essentially unchanged under the addition of the local five-quark operators. Despite the use of multiple five-quark operators with qualitatively different quark, γ-matrix and parity structures, the overlap of local five-quark operators with five-quark scattering states is found to be low. Non-local five-quark interpolating fields are then introduced, and stochastic noise minimisation techniques are developed in order to combat the computational difficulties introduced by these operators. Explicitly projecting momenta onto single-hadron pieces of these non-local multi-hadron operators is known to provide significantly enhanced overlap with scattering states and as such we perform this projection enabling a presentation of a proof of principle calculation in the negative parity nucleon sector. Furthermore, the calculation methodology and associated algorithms to evaluate correlators directly from n-quark operators are developed with a high degree of generality, forming the basis for a rich spectrum of future work in a wide variety of channels.
Thesis (Ph.D.) -- University of Adelaide, School of Physical Sciences, 2017.