18 research outputs found
pi N to Multi-pi N Scattering in the 1/N_c Expansion
We extend the 1/N_c meson-baryon scattering formalism to pi N to multi-pi N
case. We first show that the leading-order large N_c processes proceed through
resonant intermediate states (e.g., rho N or pi Delta). We find that the pole
structure of baryon resonances can be uniquely identified by their
(non)appearance in eta N or mixed partial-wave pi Delta final states.Comment: Invited talk at Exclusive Reactions at High Momentum Transfer
workshop, JLAB, Newport News, VA, May 21-24, 2007, 4 pages, 3 figure
1/N_c Corrections in Meson-Baryon Scattering
Corrections to meson/ground-state baryon scattering amplitudes in the 1/N_c
expansion of QCD have previously been shown to be controlled by the t-channel
difference |I_t - J_t| of isospin and angular momentum and by the change of
hypercharge Y_t. Here we derive the corresponding expressions in the original
scattering s channel, allowing for nonzero meson spin and nontrivial SU(3)
flavor quantum numbers, and provide explicit examples of the crossing relevant
for pi N --> rho N and K N scattering.Comment: Addition of a physical example, minor changes to clarify certain
issues and add one reference. 18 pages, no figure
Positive Parity Pentaquarks Pragmatically Predicted
We consider the possibility that the lightest pentaquark is a parity even
state, with one unit of orbital angular momentum. Working within the framework
of a constituent quark model, we show that dominant spin-flavor interactions
render certain parity-even states lighter than any pentaquark with all quarks
in the spatial ground state. For such states, we focus on predicting the mass
and decays of other members of the same SU(3) flavor multiplet. Specifically,
we consider the strangeness -2 cascade pentaquarks, which are relatively immune
to mixing. We take into account flavor SU(3) breaking effects originating from
the strange quark mass as well as from the structure of the spin-flavor
exchange interactions themselves. We predict the lightest cascade pentaquarks
at approximately 1906 MeV, with a full width about 3 times larger than that of
the Theta^+.Comment: 13 pages, 1 figure, 4 tables, Revtex
Unusual High-Energy Phenomenology of Lorentz-Invariant Noncommutative Field Theories
It has been suggested that one may construct a Lorentz-invariant
noncommutative field theory by extending the coordinate algebra to additional,
fictitious coordinates that transform nontrivially under the Lorentz group.
Integration over these coordinates in the action produces a four-dimensional
effective theory with Lorentz invariance intact. Previous applications of this
approach, in particular to a specific construction of noncommutative QED, have
been studied only in a low-momentum approximation. Here we discuss
Lorentz-invariant field theories in which the relevant physics can be studied
without requiring an expansion in the inverse scale of noncommutativity.
Qualitatively, we find that tree-level scattering cross sections are
dramatically suppressed as the center-of-mass energy exceeds the scale of
noncommutativity, that cross sections that are isotropic in the commutative
limit can develop a pronounced angular dependence, and that nonrelativistic
potentials (for example, the Coloumb potential) become nonsingular at the
origin. We consider a number of processes in noncommutative QED that may be
studied at a future linear collider. We also give an example of scattering via
a four-fermion operator in which the noncommutative modifications of the
interaction can unitarize the tree-level amplitude, without requiring any other
new physics in the ultraviolet.Comment: 24 pages LaTeX, 4 eps figures (v2: reference added, v3: minor
clarifications
Pion Form Factors in Holographic QCD
Using a holographic dual model of QCD, we compute the pion electromagnetic
form factor F_pi(Q^2) in the spacelike momentum transfer region, as well as
pion couplings to vector mesons g_rho^(n) pi pi. Spontaneous and explicit
chiral symmetry breaking are intrinsic features of this particular holographic
model. We consider variants with both ``hard-wall'' and ``soft-wall'' infrared
cutoffs, and find that the F_pi(Q^2) data tend to lie closer to the hard-wall
model predictions, although both are too shallow for large Q^2. By allowing the
parameters of the soft-wall model (originally fixed by observables such as
m_rho) to vary, one finds fits that tend to agree better with F_pi(Q^2). We
also compute the pion charge radius for a variety of parameter
choices, and use the values of f^(n)_rho, g_{rho^(n) pi pi} and m^(n)_rho to
observe the saturation of F_pi(0) by rho poles.Comment: 17 pages, 2 figures, revised fits using consistent normalization of
f_pi. References update
pi N --> Multi-pi N Scattering in the 1/N_c Expansion
We extend the 1/N_c expansion meson-baryon scattering formalism to cases in
which the final state contains more than two particles. We first show that the
leading-order large N_c processes proceed through resonant intermediate states
(e.g., rho N or pi Delta). We then tabulate linear amplitude expressions for
relevant processes and find that the pole structure of baryon resonances can be
uniquely identified by their (non)appearance in eta N or mixed partial-wave pi
Delta final states. We also show that quantitative predictions of pi N to pi
Delta branching ratios predicted at leading order alone do not agree with
measurements, but the inclusion of 1/N_c corrections is ample to explain the
discrepancies.Comment: 23 pages, 3 eps figures, ReVTeX4, added reference and discussion,
identical to PRD versio
Bulk Majorons at Colliders
Lepton number violation may arise via the spontaneous breakdown of a global
symmetry. In extra dimensions, spontaneous lepton number violation in the bulk
implies the existence of a Goldstone boson, the majoron J^(0), as well as an
accompanying tower of Kaluza-Klein (KK) excitations, J^(n). Even if the
zero-mode majoron is very weakly interacting, so that detection in low-energy
processes is difficult, the sum over the tower of KK modes may partially
compensate in processes of relevance at high-energy colliders. Here we consider
the inclusive differential and total cross sections for e^- e^- --> W^- W^- J,
where J represents a sum over KK modes. We show that allowed parameter choices
exist for which this process may be accessible to a TeV-scale electron
collider.Comment: 11 pages LaTeX, 3 eps figures (references added