arXiv subjects

Richard F. Lebed

Publications and source records attributed to Richard F. Lebed.

At least 55 records · Page 3Linked to original sources

arXiv2015

Above-Threshold Poles in Model-Independent Form Factor Parametrizations

The model-independent parametrization for exclusive hadronic form factors commonly used for semileptonic decays is generalized to allow for the inclusion of above-threshold resonant poles of known mass and width. We discuss the interpretation of such poles, particularly with respect to the analytic structure of the relevant two-point Green's function in which they reside. Their presence has a remarkably small effect on the parametrization, as we show explicitly for the case of $D \to πe^+ ν_e$.

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arXiv2015

Diquark Substructure in $ϕ$ Photoproduction

Observed enhancements in the forward and backward directions for $ϕ$ meson photoproduction off nucleons are shown to be explainable by the production of a nonresonant recoiling $(su)$ diquark, $(\bar s ud)$ triquark pair. We show that the necessity of maintaining approximate collinearity of the quarks within these units constrains configurations with the minimum momentum transfer, and hence maximal amplitudes, to lie preferentially along the reaction axis.

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arXiv2015

A New Dynamical Picture for the Production and Decay of the $XY \! Z$ Mesons

I introduce an entirely new dynamical description for exotic charmoniumlike hadrons, based upon the competing effects of the strong attraction between quarks in a diquark, and the inability of the diquark to hadronize on its own due to being a color nonsinglet. This mechanism naturally explains, for example, the strong preference of the $Z(4475)$ to decay to $ψ(2S)$ rather than the $J/ψ$, the existence of a state $X(4630)$ that decays to $Λ_c$ baryon pairs, and why some but not all exotics lie near hadronic thresholds. Owing to high-energy constituent counting rules, the four-quark nature of the states produces major changes to both the high-$s$ scaling of cross sections for producing such states and to the potency of the cusp effect of attracting resonances to pair-production thresholds. The recently observed $P_c^+$ pentaquark candidates are seen to fit naturally into this scheme.

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arXiv2015

The Pentaquark Candidates in the Dynamical Diquark Picture

Starting with the dynamical picture of the exotic $c\bar c$-containing states $XY \! Z$ as the confinement-induced hadronization of a rapidly separating pair of a compact diquark and antidiquark, we describe the pentaquark candidates $P_c^+(4380)$ and $P_c^+(4450)$ in terms of a confined but rapidly separating color-antitriplet diquark $cu$ and color-triplet "triquark" $\bar c (ud)$. This separation explains the relatively small $P_c^+$ widths, despite these 5-quark systems lying far above both the $J/ψ\, p$ and $Λ_c \bar D^{(*)0}$ thresholds. The $P_c^+$ states are predicted to form isospin doublets with neutral partners $P_c^0$.

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arXiv2015

The QCD Dynamics of Tetraquark Production

We use the twist dimensions of the operators underlying the dynamical behavior of exclusive production processes as a tool for determining the structure of exotic heavy-quark states such as the $Z_c^+(4430)$ tetraquark. The resulting counting rules predict distinctive fall-offs of the cross sections in center-of-mass energy, thus distinguishing whether the tetraquarks are segregated into di-meson molecules, diquark-antidiquark pairs, or more democratically arranged four-quark states. In addition, we propose straightforward methods of experimentally producing additional exotic multiquark states.

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arXiv2015

Tetraquark Cusp Effects from Diquark Pair Production

We present the first study of the cusp effect (the movement of resonant poles due to the proximity of multiparticle thresholds) caused by the creation of diquark-antidiquark pairs. The cusp profile for such states is obtained from constituent counting rules. We compare the effectiveness of diquark cusps in moving resonant poles with that from a phenomenological form commonly used for meson-pair creation, and find that mesons tend to be more effective at lower energies $({\it e.g.}$, the $K\overline{K}$ threshold), while diquarks tend to be more effective at the charm [$X(3872)$] scales and above.

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arXiv2014

True Muonium $(μ^+ μ^-)$ on the Light Front

Applying Discretized Light Cone Quantization, we perform the first calculation of the spectrum of true muonium, the $μ^+ μ^-$ atom, as modified by the inclusion of an $|e\bar e >$ Fock component. The shift in the mass eigenvalue is found to be largest for triplet states. If $m_e$ is taken to be a substantial fraction of $m_μ$, the integrated probability of the electronic component of the $1 \, {}^3 /! S_1$ state is found to be as large as $O(10^{-2})$. Initial studies of the Lamb shift for the atom are performed. Directions for making the simulations fully realistic are discussed.

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arXiv2014

A New Picture for the Formation and Decay of the Exotic $XY \! Z$ Mesons

We present a new dynamical picture that identifies the formation of the exotic $\bar c c$-containing states $XY \! Z$ with the confinement-induced hadronization of a rapidly separating pair of a compact diquark and antidiquark. This picture combines the advantages of diquark-based models, which can accommodate much of the known $XY \! Z$ spectrum, with the experimental fact that such states are both relatively narrow and are produced promptly. It also naturally explains the preference of some of the exotic states to decay to $ψ(2S)$, rather than $J/ψ$, in terms of a simple wave-function overlap effect.

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arXiv2014

Are There Tetraquarks at Large $N_c$ in QCD(F)?

Weinberg recently pointed out a flaw in the standard argument that large $N_c$ QCD with color-fundamental quarks [QCD(F)] cannot yield narrow tetraquark states. In particular, he observed that the argument does not rule out narrow tetraquarks associated with the leading-order connected diagrams; such tetraquarks would have a width scaling as $N_c^{-1}$. It is shown here, however, that while the standard analysis of tetraquarks does not rule them out, a more thorough analysis rules out quantum-number exotic tetraquarks associated with the leading-order connected diagrams. This analysis is based entirely on conventional assumptions used in large $N_c$ physics applied to the analytic properties of meson-meson scattering. Our result implies that one of three possibilities must be true: i) quantum-number exotic tetraquarks do not exist at large $N_c$; ii) quantum-number exotic tetraquarks exist, but are associated with subleading connected diagrams and have anomalously small widths that scale as $N_c^{-2}$ or smaller; or iii) the conventional assumptions used in large $N_c$ analysis are inadequate.

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arXiv2014

Quantum-Number Exotic Tetraquarks at Large $N_c$ in QCD(AS)

It is shown that the large $N_c$ limit of QCD with quarks in the two-index antisymmetric color representation [QCD(AS)] has narrow tetraquark states with exotic flavor quantum numbers. They decay into mesons with a width that is parametrically $O(1/N_c^2)$. Tetraquarks with non-exotic quantum numbers mix at leading order with mesons of the same overall quantum numbers. QCD(AS) at $N_c=3$ corresponds to ordinary QCD; its large $N_c$ limit represents an alternative starting point for a $1/N_c$ expansion to the standard one with quarks in the fundamental color representation.

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arXiv2013

Large-N Structure of Tetraquark Mesons

Following Weinberg's argument that narrow tetraquark mesons are not precluded in large N_c QCD, we explore the flow of N_c factors needed for the consistency of this picture, and show that they must arise in a novel way, not simply through the usual combinatoric counting.

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arXiv2013

The Lee-Wick Standard Model at Finite Temperature

The Lee-Wick Standard Model at temperatures near electroweak scale is considered, with the aim of studying the electroweak phase transition. While Lee-Wick theories possess states of negative norm, they are not pathological but instead are treated by imposing particular boundary conditions and using particular integration contours in the calculation of S-matrix elements. It is not immediately clear how to extend this prescription to formulate the theory at finite temperature; we explore two different pictures of finite-temperature LW theories, and calculate the thermodynamic variables and the (one-loop) thermal effective potential. We apply these results to study the Lee-Wick Standard Model and find that the electroweak phase transition is a continuous crossover, much like in the Standard Model. However, the high-temperature behavior is modified due to cancellations between thermal corrections arising from the negative- and positive-norm states.

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arXiv2012

Precision Electroweak Constraints on the N=3 Lee-Wick Standard Model

The Lee-Wick (LW) formulation of higher-derivative theories can be extended from one in which the extra degrees of freedom are represented as a single heavy, negative-norm partner for each known particle (N=2), to one in which a second, positive-norm partner appears (N=3). We explore the extent to which the presence of these additional states in a LW Standard Model affect precision electroweak observables, and find that they tend either to have a marginal effect (e.g., quark partners on T), or a substantial beneficial effect (e.g., Higgs partners on the Zbb couplings). We find that precision constraints allow LW partners to exist in broad regions of mass parameter space accessible at the LHC, making LW theories a viable beyond-Standard Model candidate.

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arXiv2012

Collider Signatures of the N=3 Lee-Wick Standard Model

Inspired by the Lee-Wick higher-derivative approach to quantum field theory, Grinstein, O'Connell, and Wise have illustrated the utility of introducing into the Standard Model negative-norm states that cancel quadratic divergences in loop diagrams, thus posing a potential resolution of the hierarchy problem. Subsequent work has shown that consistency with electroweak precision parameters requires many of the partner states to be too massive to be detected at the LHC. We consider the phenomenology of a yet-higher derivative theory that exhibits three poles in its bare propagators (hence N=3), whose states alternate in norm. We examine the interference effects of W boson partners on LHC scattering cross sections, and find that the N=3 LWSM already makes verifiable predictions at 10 fb^(-1) of integrated luminosity.

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arXiv2012

Alternate 1/N_c Expansions and SU(3) Breaking from Baryon Lattice Results

A combined expansion in the number of QCD colors 1/N_c and SU(3) flavor breaking parameter epsilon has long been known to provide an excellent accounting for the mass spectrum of the lightest spin-1/2, 3/2 baryons when the quarks are taken to transform under the fundamental SU(N_c) representation, and in the final step N_c \to 3 and epsilon is set to its physical value ~0.3. Subsequent work shows that placing quarks in the two-index antisymmetric SU(N_c) representation leads to quantitatively equally successful mass relations. Recent lattice simulations allow for varying the value of epsilon and confirm the robustness of the original 1/N_c relations. In this paper we show that the same conclusion holds for the antisymmetric quarks, and demonstrate that the mass relations also hold under alternate prescriptions for identifying physical baryons with particular members of the large N_c multiplets.

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arXiv2011

Gauged Baryon and Lepton Number in MSSM_4 Brane Worlds

A recent D-brane model designed to accommodate a phenomenologically acceptable fourth generation of chiral fermions was noted to produce an unexpected additional unbroken nonanomalous U(1) gauge group at the string scale. We show that the corresponding charges acting on MSSM fields count baryon and lepton numbers. If broken spontaneously at lower scales, these U(1)_B and U(1)_L symmetries provide potential avenues for preserving baryogenesis while nonetheless explaining the suppression of proton decay (without the need for R parity), as well as the smallness of right-handed neutrino Majorana masses compared to the string scale.

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arXiv2011

Natural Four-Generation Mass Textures in MSSM Brane Worlds

A fourth generation of Standard Model (SM) fermions is usually considered unlikely due to constraints from direct searches, electroweak precision measurements, and perturbative unitarity. We show that fermion mass textures consistent with all constraints may be obtained naturally in a model with four generations constructed from intersecting D6 branes on a T^6/(Z_2 x Z_2) orientifold. The Yukawa matrices of the model are rank 2, so that only the third- and fourth-generation fermions obtain masses at the trilinear level. The first two generations obtain masses via higher-order couplings and are therefore naturally lighter. In addition, we find that the third and fourth generation automatically split in mass, but do not mix at leading order. Furthermore, the SM gauge couplings automatically unify at the string scale, and all the hidden-sector gauge groups become confining in the range 10^{13}--10^{16} GeV, so that the model becomes effectively a four-generation MSSM at low energies.

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arXiv2011

Realistic Four-Generation MSSM in Type II String Theory

We construct a four-generation MSSM with rank-4 Yukawa matrices from intersecting D6 branes on a T^6/(Z_2 x Z_2) orientifold. The Yukawa matrices obtained provide an example of Flavor Democracy (FD), where the Yukawa couplings are all nearly equal. Mass hierarchies may then be generated by slight perturbations away from FD. We find that it is possible to obtain hierarchical masses for the quarks and leptons of each generation and mixings between them. In addition, the tree-level gauge couplings are unified at the string scale. Finally, we also construct similar models with one, two, and three generations in which the rank of the Yukawa matrices is equal to the number of generations in each model.

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