179 research outputs found

### Ultrasoft Renormalization in Non-Relativistic QCD

For Non-Relativistic QCD the velocity renormalization group correlates the
renormalization scales for ultrasoft, potential and soft degrees of freedom.
Here we discuss the renormalization of operators by ultrasoft gluons. We show
that renormalization of soft vertices can induce new operators, and also
present a procedure for correctly subtracting divergences in mixed
potential-ultrasoft graphs. Our results affect the running of the
spin-independent potentials in QCD. The change for the NNLL t-tbar cross
section near threshold is very small, being at the 1% level and essentially
independent of the energy. We also discuss implications for analyzing
situations where mv^2 ~ Lambda_QCD.Comment: 31 pages, 11 fig

### Quarks in the Skyrme-'t Hooft-Witten Model

The three-flavor Skyrme-'t Hooft-Witten model is interpreted in terms of a
quark-like substructure, leading to a new model of explicitly confined
color-free ``quarks'' reminiscent of Gell-Mann's original pre-color quarks, but
with unexpected and significant differences.Comment: Latex, 6 pages, no figure

### Renormalization group improvement of the spectrum of Hydrogen-like atoms with massless fermions

We obtain the next-to-next-to-leading-log renormalization group improvement
of the spectrum of Hydrogen-like atoms with massless fermions by using
potential NRQED. These results can also be applied to the computation of the
muonic Hydrogen spectrum where we are able to reproduce some known double logs
at O(m\alpha^6). We compare with other formalisms dealing with log resummation
available in the literature.Comment: 9 pages, LaTeX. Minor changes, note added, final versio

### Double-Logarithmic Two-Loop Self-Energy Corrections to the Lamb Shift

Self-energy corrections involving logarithms of the parameter Zalpha can
often be derived within a simplified approach, avoiding calculational
difficulties typical of the problematic non-logarithmic corrections (as
customary in bound-state quantum electrodynamics, we denote by Z the nuclear
charge number, and by alpha the fine-structure constant). For some logarithmic
corrections, it is sufficient to consider internal properties of the electron
characterized by form factors. We provide a detailed derivation of related
self-energy ``potentials'' that give rise to the logarithmic corrections; these
potentials are local in coordinate space. We focus on the double-logarithmic
two-loop coefficient B_62 for P states and states with higher angular momenta
in hydrogenlike systems. We complement the discussion by a systematic
derivation of B_62 based on nonrelativistic quantum electrodynamics (NRQED). In
particular, we find that an additional double logarithm generated by the
loop-after-loop diagram cancels when the entire gauge-invariant set of two-loop
self-energy diagrams is considered. This double logarithm is not contained in
the effective-potential approach.Comment: 14 pages, 1 figure; references added and typographical errors
corrected; to appear in Phys. Rev.

### Heavy meson semileptonic differential decay rate in two dimensions in the large Nc

We study QCD in 1+1 dimensions in the large $N_c$ limit using light-front
Hamiltonian perturbation theory in the $1/N_c$ expansion. We use this formalism
to exactly compute hadronic transition matrix elements for arbitrary currents
at leading order in $1/N_c$. We compute the semileptonic differential decay
rate of a heavy meson, $d\Gamma/dx$, and its moments, $M_N$, using the hadronic
matrix elements obtained previously. We put some emphasis in trying to
understand parity invariance. We also study with special care the kinematic
region where the operator product expansion ($1/N \sim 1-x \sim 1$) or
non-local effective field theories ($1/N \sim 1-x \sim \Lambda_{QCD}/m_Q$) can
be applied. We then compare with the results obtained using an effective field
theory approach based on perturbative factorization, with the focus to better
understand quark-hadron duality. At the end of the day, using effective field
theories, we have been able to obtain expressions for the moments with relative
accuracy of $O(\Lambda_{QCD}^2/m_Q^2)$ in the kinematic region where the
operator product expansion can be applied, and with relative accuracy of
$O(\Lambda_{QCD}/m_Q)$ in the kinematic region where non-local effective field
theories can be applied. These expressions agree, within this precision, with
those obtained from the hadronic result using the layer-function approximation
plus Euler-McLaurin expansion. Very good numerical agreement for the moments is
obtained between the exact result and the result using effective field
theories.Comment: 52 pages, 30 figures, references added, small modifications, some
discussion of the four dimensional case changed, journal versio

### Chiral Perturbation Theory with tensor sources

We construct the most general chirally-invariant Lagrangian for mesons in the
presence of external sources coupled to the tensor current
\bar{\psi}\sigma_{\mu\nu}\psi. In order to have only even terms in the chiral
expansion, we consider the new source of O(p^2). With this choice, we build the
even-parity effective Lagrangian up to the p^6-order (NLO). While there are
only 4 new terms at the p^4-order, at p^6-order we find 78 terms for n_f=2 and
113 terms for n_f=3. We provide a detailed discussion on the different
mechanisms that ensure that our final set of operators is complete and
non-redundant. We also examine the odd-parity sector, to conclude that the
first operators appear at the p^8-order (NNLO).Comment: 23 pages, one figure; typos corrected, one paragraph added, new
section added, references added, published versio

### Enhanced Nonperturbative Effects in Z Decays to Hadrons

We use soft collinear effective field theory (SCET) to study nonperturbative
strong interaction effects in Z decays to hadronic final states that are
enhanced in corners of phase space. These occur, for example, in the jet energy
distribution for two jet events near E_J=M_Z/2, the thrust distribution near
unity and the jet invariant mass distribution near zero. The extent to which
such nonperturbative effects for different observables are related is
discussed.Comment: 17 pages. Paper reorganized, and more discussion and results include

### 1/N Expansion for Exotic Baryons

The 1/N expansion for exotic baryons is developed, and applied to the masses,
meson couplings and decay widths. Masses and widths of the 27 and 35 pentaquark
states in the same tower as the Theta+ are related by spin-flavor symmetry. The
27 and 35 states can decay within the pentaquark tower, as well as to normal
baryons, and so have larger decay widths than the lightest pentaquark Theta.
The 1/N expansion also is applied to baryon exotics containing a single heavy
antiquark. The decay widths of heavy pentaquarks via pion emission, and to
normal baryons plus heavy D^(*),B^(*) mesons are studied, and relations
following from large-N spin-flavor symmetry and from heavy quark symmetry are
derived.Comment: Major additions: plots of widths and branching ratios, discussion of
strong decays of heavy pentaquarks, including consequences of heavy quark
symmetr

### Power Counting in the Soft-Collinear Effective Theory

We describe in some detail the derivation of a power counting formula for the
soft-collinear effective theory (SCET). This formula constrains which operators
are required to correctly describe the infrared at any order in the
Lambda_QCD/Q expansion (lambda expansion). The result assigns a unique
lambda-dimension to graphs in SCET solely from vertices, is gauge independent,
and can be applied independent of the process. For processes with an OPE the
lambda-dimension has a correspondence with dynamical twist.Comment: 12 pages, 1 fig, journal versio

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