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Miao Li

Publications and source records attributed to Miao Li.

301 records · Page 17Linked to original sources
arXiv1995

Doubling of Scattering Phase Shifts for Two Dimensional Strings

We discuss the origin of the leg factors appearing in 2D string theory. Computing in the world sheet framework we use the semiclassical method to study string amplitudes at high energy. We show that in the case of a simplest 2-point amplitude these factors correspond entirely to the time delay for reflection off the Liouville wall. Our semiclassical calculation reveals that the string longitudinal modes, although nonpropagating in 2D spacetime, have the effect of doubling the phase shift. Particular emphasis is put on comparison with the point particle (center of mass) case. A general method is then given for calculating an arbitrary amplitude semiclassically.

hep-th↗
arXiv1995

Large $N$ Solution of the 2D Supersymmetric Yang-Mills Theory

The Schwinger-Dyson equations of the Makeenko-Migdal type, when supplemented with some simple equations as consequence of supersymmetry, form a closed set of equations for Wilson loops and related quantities in the two dimensional super-gauge theory. We solve these equations. It appears that the planar Wilson loops are described by the Nambu string without folds. We also discuss how to put the model on a spatial lattice, where a peculiar gauge is chosen in order to keep one supersymmetry on the lattice. Supersymmetry is unbroken in this theory. We comment on possible generalization of these considerations to other models.

hep-th↗
arXiv1994

High energy Scattering in 2+1 QCD: A Dipole Picture

A dipole picture of high energy scattering is developed in the 2+1 dimensional QCD, following Mueller. A generalized integral equation for the dipole density with a given separation and center of mass position is derived, and meson-meson non-forward scattering amplitude is therefore calculated. We also calculate the amplitude due to two pomeron exchange, and the triple pomeron coupling. We compare the result obtained by this method to our previous result based on an effective action approach, and find the two results agree at the one pomeron exchange level.

hep-ph↗
arXiv1994

High energy scattering in 2+1 QCD

High energy scattering in 2+1 QCD is studied using the recent approach of Verlinde and Verlinde. We calculate the color singlet part of the quark-quark scattering exactly within this approach, and discuss some physical implication of this result. We also demonstrate, by two independent methods, that reggeization fails for the color singlet channel. We briefly comment on the problem in 3+1 QCD.

hep-th↗
arXiv1993

Dirichlet Strings

Strings propagating along surfaces with Dirichlet boundaries are studied in this paper. Such strings were originally proposed as a possible candidate for the QCD string. Our approach is different from previous ones and is simple and general enough, with which basic problems can be easily addressed. The Green function on a surface with Dirichlet boundaries is obtained through the Neumann Green function on the same surface, by employing a simple approach to Dirichlet conditions. An easy consequence of the simple calculation of the Green function is that in the simplest model, namely the bosonic Dirichlet string, the critical dimension is still 26, and the tachyon is still present in the spectrum, while the scattering amplitudes differ dramatically from those in the usual string theory. We discuss the high energy, fixed angle behavior of the four point scattering amplitudes on the disk and the annulus. We argue for general power-like behavior of arbitrary high energy, fixed angle scattering amplitudes. We also discuss the high temperature property of the finite temperature partition function on an arbitrary surface, and give an explicit formula of the one on the annulus.

hep-th↗
arXiv1993

Dirichlet String Theory and Singular Random Surfaces

We show that string theory with Dirichlet boundaries is equivalent to string theory containing surfaces with certain singular points. Surface curvature is singular at these points. A singular point is resolved in conformal coordinates to a circle with Dirichlet boundary conditions. We also show that moduli parameters of singular surfaces coincide with those of smooth surfaces with boundaries. Singular surfaces with saddle points indeed arise in the strong coupling expansion in lattice QCD. The kind of saddle point, which may be the origin of a singular point we need, is of infinite order.

hep-th↗
arXiv1993

Matter-ghost mixing and the exact 2d black hole metric

We study a revised version of Witten's 2d black hole, in which the matter and (b,c) ghosts are mixed. The level of the coset model is still 9/4. We show that this model is equivalent to that of Mukhi and Vafa, in which the level of the coset model is taken as 3, and the stress tensor is improved. We argue that the exact metric in such a model is just the semi-classical one, quite different from the exact metric in Witten's black hole, being studied by Dijkgraaf, Verlinde and Verlinde. In addition, there appear ghost-related terms as a part of the background in the world sheet action.

hep-th↗
arXiv1993

A proposal on the topological sector of 2d string

The field content of the two dimensional string theory consists of the dynamical tachyon field and some nonpropagating fields which consist in the topological sector of this theory. We propose in this paper to study this topological sector as a spacetime gauge theory with a simple centrally extended $w_\infty$ algebra. This $w_\infty$ algebra appears in both the world sheet BRST analysis and the matrix model approach. Since the two dimensional centrally extended Poincaré algebra is naturally embedded in the centrally extended $w_\infty$ algebra, the low energy action for the metric and dilaton appears naturally when the model is truncated at this level. We give a plausible explanation of emergence of discrete states in this formulation. This theory is again the effective theory at zero slope limit. To include higher order $α'$ corrections, we speculate that the whole theory is a gauge theory of a deformed $w_\infty$ algebra, and the deformation parameter is just $α'$.

hep-th↗
arXiv1992

Some remarks on tachyon action in 2d string theory

We discuss the effective action of tachyon in the two dimensional string theory at tree level. We show that already starting from the cubic terms the action is nonlocal and the usually assumed simplest cubic term does not give the correct amplitude. Four point 1PI terms are also discussed.

hep-th↗
arXiv1992

Driving Operators Relevant: A Feature of Chern-Simons Interaction

By computing anomalous dimensions of gauge invariant composite operators $(\barψψ)^n$ and $(ϕ^*ϕ)^n$ in Chern-Simons fermion and boson models, we address that Chern-Simons interactions make these operators more relevant or less irrelevant in the low energy region. We obtain a critical Chern-Simons fermion coupling, ${1\over κ_c^2} = {6\over 19}$, for a phase transition at which the leading irrelevant four-fermion operator $(\barψψ)^2$ becomes marginal, and a critical Chern-Simons boson coupling, ${1\over κ_c^2} = {6\over 34}$, for a similar phase transition for the leading irrelevant operator $(ϕ^*ϕ)^4$. We see this phenomenon also in the $1/N$ expansion.

hep-th↗
arXiv1992

Gauge Symmetries and Amplitudes in N=2 Strings

Picture changed operators are discussed in $N=2$ strings with space-time signature $(2,2)$. A gauge symmetry algebra is derived in a background of torus space-time $T^{2,2}$ and its simple representation on the picture changed operators is given. Simple Ward identities associated with the gauge algebra and their consequences for three and four point amplitudes of arbitrary loops are also discussed.

hep-th↗