230 research outputs found
Classical sum rules and spin correlations in photoabsorption and photoproduction processes
In this paper we study the possibility of generalizing the classical
photoabsorption () sum rules, to processes
and crossed helicity amplitudes. In the first case, using detailed balance, the
sum rule is written as where is a kinematical constant which depends only
on the mass of the particles and the center of mass energy. For other crossed
helicity amplitudes, we show that there is a range of values of and for
which the differential cross section for the process or in which the helicities of the photon and particle have
specific values, is equal to the differential cross section for the process in
which one of these two helicities is reversed (parallel-antiparallel spin
correlation).Comment: 9 pages, 2 figure
Kaon photoproduction on the nucleon: Contributions of kaon-hyperon final states to the magnetic moment of the nucleon
By using the Gerasimov-Drell-Hearn (GDH) sum rule and an isobaric model of
kaon photoproduction, we calculate contributions of kaon-hyperon final states
to the magnetic moment of the proton and the neutron. We find that the
contributions are small. The approximation of sigma_{TT'} by sigma_{T} clearly
overestimates the value of the GDH integral. We find a smaller upper bound for
the contributions of kaon-hyperon final states to the proton's anomalous
magnetic moment in kaon photoproduction, and a positive contribution for the
square of the neutron's magnetic moment.Comment: 6 pages, revtex, 1 postscript figure, to appear in Phys. Rev.
Scaling laws in hadronic processes and string theory
We propose a possible scheme for getting the known QCD scaling laws within
string theory. In particular, we consider amplitudes for exclusive scattering
of hadrons at large momentum transfer, hadronic form factors and distribution
functions.Comment: 13 pages, 2 figures, a comment and a reference added, a final version
to appear in Physical Review
Angular Conditions,Relations between Breit and Light-Front Frames, and Subleading Power Corrections
We analyze the current matrix elements in the general collinear (Breit)
frames and find the relation between the ordinary (or canonical) helicity
amplitudes and the light-front helicity amplitudes. Using the conservation of
angular momentum, we derive a general angular condition which should be
satisfied by the light-front helicity amplitudes for any spin system. In
addition, we obtain the light-front parity and time-reversal relations for the
light-front helicity amplitudes. Applying these relations to the spin-1 form
factor analysis, we note that the general angular condition relating the five
helicity amplitudes is reduced to the usual angular condition relating the four
helicity amplitudes due to the light-front time-reversal condition. We make
some comments on the consequences of the angular condition for the analysis of
the high- deuteron electromagnetic form factors, and we further apply the
general angular condition to the electromagnetic transition between spin-1/2
and spin-3/2 systems and find a relation useful for the analysis of the
N- transition form factors. We also discuss the scaling law and the
subleading power corrections in the Breit and light-front frames.Comment: 24 pages,2 figure
Parton model versus color dipole formulation of the Drell-Yan process
In the kinematical region where the center of mass energy is much larger than
all other scales, the Drell-Yan process can be formulated in the target rest
frame in terms of the same color dipole cross section as low Bjorken-x deep
inelastic scattering. Since the mechanisms for heavy dilepton production appear
very different in the dipole approach and in the conventional parton model, one
may wonder whether these two formulations really represent the same physics. We
perform a comparison of numerical calculations in the color dipole approach
with calculations in the next-to-leading order parton model. For proton-proton
scattering, the results are very similar at low x_2 from fixed target to RHIC
energies, confirming the close connection between these two very different
approaches. We also compare the transverse momentum distributions of Drell-Yan
dileptons predicted in both formulations. The range of applicability of the
dipole formulation and the impact of future Drell-Yan data from RHIC for
determining the color dipole cross section are discussed. A detailed derivation
of the dipole formulation of the Drell-Yan process is also included.Comment: 20 pages, 5 figure
Frame-Independence of Exclusive Amplitudes in the Light-Front Quantization
While the particle-number-conserving convolution formalism established in the
Drell-Yan-West reference frame is frequently used to compute exclusive
amplitudes in the light-front quantization, this formalism is limited to only
those frames where the light-front helicities are not changed and the good
(plus) component of the current remains unmixed. For an explicit demonstration
of such criteria, we present the relations between the current matrix elements
in the two typical reference frames used for calculations of the exclusive
amplitudes, i.e. the Drell-Yan-West and Breit frames and investigate both
pseudoscalar and vector electromagnetic currents in detail. We find that the
light-front helicities are unchanged and the good component of the current does
not mix with the other components of the current under the transformation
between these two frames. Thus, the pseudoscalar and vector form factors
obtained by the diagonal convolution formalism in both frames must indeed be
identical. However, such coincidence between the Drell-Yan-West and Breit
frames does not hold in general. We give an explicit example in which the
light-front helicities are changed and the plus component of the current is
mixed with other components under the change of reference frame. In such a
case, the relationship between the frames should be carefully analyzed before
the established convolution formalism in the Drell-Yan-West frame is used.Comment: 14 pages, 4 figure
Structure Functions are not Parton Probabilities
The common view that structure functions measured in deep inelastic lepton
scattering are determined by the probability of finding quarks and gluons in
the target is not correct in gauge theory. We show that gluon exchange between
the fast, outgoing partons and target spectators, which is usually assumed to
be an irrelevant gauge artifact, affects the leading twist structure functions
in a profound way. This observation removes the apparent contradiction between
the projectile (eikonal) and target (parton model) views of diffractive and
small x_{Bjorken} phenomena. The diffractive scattering of the fast outgoing
quarks on spectators in the target causes shadowing in the DIS cross section.
Thus the depletion of the nuclear structure functions is not intrinsic to the
wave function of the nucleus, but is a coherent effect arising from the
destructive interference of diffractive channels induced by final state
interactions. This is consistent with the Glauber-Gribov interpretation of
shadowing as a rescattering effect.Comment: 35 pages, 8 figures. Discussion of physical consequences of final
state interactions amplified. Material on light-cone gauge choices adde
Spin Structure of the Pion in a Light-Cone Representation
The spin structure of the pion is discussed by transforming the wave function
for the pion in the naive quark model into a light-cone representation. It is
shown that there are higher helicity () states in
the full light-cone wave function for the pion besides the ordinary helicity
() component wave functions as a consequence from
the Melosh rotation relating spin states in light-front dynamics and those in
instant-form dynamics. Some low energy properties of the pion, such as the
electromagnetic form factor, the charged mean square radius, and the weak decay
constant, could be interrelated in this representation with reasonable
parameters.Comment: 15 Latex pages, 2 figures upon reques
Photon and Z induced heavy charged lepton pair production at a hadron supercollider
We investigate the pair production of charged heavy leptons via
photon-induced processes at the proposed CERN Large Hadron Collider (LHC).
Using effective photon and Z approximations, rates are given for
production due to fusion and fusion for the cases of
inelastic, elastic and semi-elastic collisions. These are compared with
the corresponding rates for production via the gluon fusion and Drell-Yan
mechanisms. Various and differential luminosities
for collisions are also presented.Comment: 22 pages, RevTex 3.0, 6 uuencoded and compressed postscript figures
included. Reference to one paper changed from the original preprint number to
the published version. Everything else unchange
Block-Spin Approach to Electron Correlations
We consider an expansion of the ground state wavefunction of quantum lattice
many-body systems in a basis whose states are tensor products of block-spin
wavefunctions. We demonstrate by applying the method to the antiferromagnetic
spin-1/2 chain that by selecting the most important many-body states the
technique affords a severe truncation of the Hilbert space while maintaining
high accuracy.Comment: 17 pages, 3 Postscript figure
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