6,255 research outputs found
The pion form factor: Sudakov suppressions and intrinsic transverse momentum
It is demonstrated that any attempt to calculate the perturbative QCD
contribution to the pion form factor requires the inclusion of intrinsic
transverse momentum besides Sudakov form factors. For momentum transfers of the
order of a few GeV the intrinsic transverse momentum leads to a substantial
suppression of the perturbative QCD contribution.Comment: LaTeX, 12 pages, 3 figures included with epsf-style option
(postscript files added as uuencoded tar-compressed file), CERN-TH.6900, WU-B
93-1
The Overlap Representation of Skewed Quark and Gluon Distributions
Within the framework of light-cone quantisation we derive the complete and
exact overlap representation of skewed parton distributions for unpolarised and
polarised quarks and gluons. Symmetry properties and phenomenological
applications are discussed.Comment: LaTex, 36 pages. v2: incorrect paper attached originally. v3: erratum
adde
Two-vibron bound states in alpha-helix proteins : the interplay between the intramolecular anharmonicity and the strong vibron-phonon coupling
The influence of the intramolecular anharmonicity and the strong
vibron-phonon coupling on the two-vibron dynamics in an -helix protein
is studied within a modified Davydov model. The intramolecular anharmonicity of
each amide-I vibration is considered and the vibron dynamics is described
according to the small polaron approach. A unitary transformation is performed
to remove the intramolecular anharmonicity and a modified Lang-Firsov
transformation is applied to renormalize the vibron-phonon interaction. Then, a
mean field procedure is realized to obtain the dressed anharmonic vibron
Hamiltonian. It is shown that the anharmonicity modifies the vibron-phonon
interaction which results in an enhancement of the dressing effect. In
addition, both the anharmonicity and the dressing favor the occurrence of two
different bound states which the properties strongly depend on the interplay
between the anharmonicity and the dressing. Such a dependence was summarized in
a phase diagram which characterizes the number and the nature of the bound
states as a function of the relevant parameters of the problem. For a
significant anharmonicity, the low frequency bound states describe two vibrons
trapped onto the same amide-I vibration whereas the high frequency bound states
refer to the trapping of the two vibrons onto nearest neighbor amide-I
vibrations.Comment: may 2003 submitted to Phys. Rev.
Breakdown of disordered media by surface loads
We model an interface layer connecting two parts of a solid body by N
parallel elastic springs connecting two rigid blocks. We load the system by a
shear force acting on the top side. The springs have equal stiffness but are
ruptured randomly when the load reaches a critical value. For the considered
system, we calculate the shear modulus, G, as a function of the order
parameter, \phi, describing the state of damage, and also the ``spalled''
material (burst) size distribution. In particular, we evaluate the relation
between the damage parameter and the applied force and explore the behaviour in
the vicinity of material breakdown. Using this simple model for material
breakdown, we show that damage, caused by applied shear forces, is analogous to
a first-order phase transition. The scaling behaviour of G with \phi is
explored analytically and numerically, close to \phi=0 and \phi=1 and in the
vicinity of \phi_c, when the shear load is close but below the threshold force
that causes material breakdown. Our model calculation represents a first
approximation of a system subject to wear induced loads.Comment: 15 pages, 7 figure
Estimates for non-leading distribution functions
Estimates for leading and non-leading `twist' distribution functions are
obtained within the framework of a diquark spectator model using a non-local
operator representation.Comment: LaTeX, 10 pages, 14 ps figures included with epsfig.sty. Proceedings
of the DESY workshop, "Deep inelastic scattering off polarized targets:
Theory meets experiment", DESY-Zeuthen, September 1-5, 199
Leading asymmetries in two-hadron production in e^+e^- annihilation at the Z pole
We present the leading unpolarized and single spin asymmetries in inclusive
two-hadron production in electron-positron annihilation at the Z pole. The
azimuthal dependence in the unpolarized differential cross section of almost
back-to-back hadrons is a leading cos(2 phi) asymmetry, which arises solely due
to the intrinsic transverse momenta of the quarks. An extensive discussion on
how to measure this asymmetry and the accompanying time-reversal odd
fragmentation functions is given. A simple estimate indicates that the
asymmetry could be of the order of a percent.Comment: 8 pages, Revtex, 2 Postscript figures, uses epsf.sty. Some typos
corrected, minor clarifying changes, one reference added, including a comment
at the en
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