Theory and Phenomenology of Strong Interactions

Project: Research project

Project Details


Project Summary __________________________________ 1 I. Extending the l/Ne Expansion. Lebed's previous work shows that the l/Ne expansion is essential to understand the observed static properties of the lightest baryons. Lattice simulations allow these results to be studied at variable levels of flavor SU(3) breaking, and the success of the l/Ne expansion has been shown to persist for the mass spectrum. With A. WalkerLoud (William& Mary), Lebed will extend this analysis to EM observables such as magnetic moments and charge radii. In addition, these results will be studied using an alternate l/Ne expansion in which the quarks carry not the fundamental color charge but live in a larger representation, which has been shown to work just as well in explaining baryon masses using lattice results. II. Generalized Parton Distributions. The upgraded Jefferson Lab facility will soon be online, and measurement of GPDs is one of its main goals. In recent years the theoretical predictions became solid enongh for robust extraction of the GPDs from experiment. However, since upgraded CEBAF will possess high luminosity only for the e- bearn, one must devise systematic extraction methods for the numerous functioIlB describing the golden reaction known as DVCS. We will elaborate on a technique similar to Rosenbluth separation to disentangle different Compton Thrm Factors from the data. We will address a similar question at low energies, traditionally described in terms of polarizabilities, but recently unified with CFFs to give a treatment holding in all kinematical regions, and will compute radiative corrections to hightwist amplitudes. III. Super Wilson loop and integrability. Maximally Supersymmetric Yang-Mills theory plays a distinguished role in attempts to solve QCD at strong coupling. After the N =4 SYM eigenspectrum problem was solved at all orders in coupling, attention was shifted to scattering amplitudes, which are akin to their QCD siblings. An important step was establishing a duality between the amplitudes and super Wilson loops on null polygonal contours. We will study the problem of their scheme dependence by using the light-cone OPE for correlation functions, and also use the recently uncovered open spin-chain integrable structures to solve the problem of multiparticle GPK excitations propagating on the loop, and develop a systematic method to find the super Wilson loop in the full kinematical domain at any coupling. IV. Correlation Functions and Event Shapes. Hadronic event shapes are infrared-safe observables measured at colliders to study strong interactions. They are the main quantities used to extract ex., and are sensitive to nonperturbative QCD dynamics at power-like accuracy, but are known analytically only at one loop, numerically at two. We recently developed a framework to use available Euclidean correlators either perturbatively or at strong coupling to predict these genuinely Lorentzian observables in a generic Conformal Field Theory. The maximal transcendentality principle identifies N =4 SYM expression with the "most complicated"part of that of QCD, and so is of direct phenomenological interest. We will use this framework to perform high-order studies of event shapes in fully analytic form, to understand the infrared safety of observables from the Regge behavior of string amplitudes. V. True Muonium on the Light Cone. A study by Lebed and S. Brodsky (SLAC) on the true mtlonitlm (1'+1'-) atom, the smallest state bound by and decaying via QED, was met with considerable experimental interest by groups aiming to be the first to observe and characterize the state. Lebed and his Ph.D. student H. Lanun will carry out a complete numerical analysis of this bound state using the light-front quantization formalism to obtain its Fock-component (I', e, 'Y) amplitudes that characterize the atom's internal structure and allow calculation of quantities such as its dissociation rate in passing through (fixed-target) matter. Further collaboration with Brodsky will address renormalization between e and I' energy scales. VI. Holography at Finite Temperature. The gauge-gravity ("holographic") correspondence for studying strongly-coupled systems exploits symmetries between certain gauge theories and gravity backgrounds on curved spacetimes. The extent of the correspondence has not yet been established at finite temperatures T (which introduce a mass scale that breaks conformal symmetry). Lebed, with C. Carone and J. Erlich (William& Mary), will calculate quark field-theory correlators at finite T versus amplitudes in gravity with Schwarzschild-type metrics, to discover what backgrounds support the holographic correspondence. VII. Broader Impact. In addition to the traditional broader impact of both investigators training junior researchers (proposed here at the postdoc, Ph.D. student, and undergraduate levels), Lebed is deeply involved in increasing public awareness of nuclear/particle physics through popular presentations. His Ph.D. student has joined an innovative undergraduate mentorship program, and as UG Program Director, Lebed's goal to increase the proportion of female physics majors includes bringing more women into nuclear/particle studies.
Effective start/end date9/1/148/31/18


  • National Science Foundation (NSF): $599,998.00

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