774 research outputs found
The photon structure function measurements from LEP
The present knowledge of the structure of the photon based on measurements of photon structure functions is discussed. This review covers QED structure functions and the hadronic structure function F_2^gamma.The present knowledge of the structure of the photon based on measurements of photon structure functions is discussed. This review covers QED structure functions and the hadronic structure function F_2^gamma
Signatures of HyperCharge Axions in Colliders
If in addition to the standard model fields, a new pseudoscalar field that
couples to hypercharge topological number density, the hypercharge axion,
exists, it can be produced in colliders in association with photons or Z
bosons, and detected by looking for its decay into photons or Z's. For a range
of masses below a TeV and coupling above a fraction of 1/TeV, existing data
from LEP II and the Tevatron can already put interesting constraints, and in
future colliders accessible detection range is increased significantly. The
hypercharge axion can help in explaining the matter-antimatter asymmetry in the
universe.Comment: 11 pages, 8 figures, uses axodraw.st
Spin alignment of vector meson in e+e- annihilation at Z0 pole
We calculate the spin density matrix of the vector meson produced in e+e-
annihilation at Z^0 pole. We show that the data imply a significant
polarization for the antiquark which is created in the fragmentation process of
the polarized initial quark and combines with the fragmenting quark to form the
vector meson. The direction of polarization is opposite to that of the
fragmenting quark and the magnitude is of the order of 0.5. A qualitative
explanation of this result based on the LUND string fragmentation model is
given.Comment: 15 pages, 2 fgiures; submitted to Phys. Rev.
Color Transparency via Coherent Exclusive rho Production
We examine the potential of the COMPASS experiment at CERN to study color
transparency via exclusive coherent vector meson production in hard
muon-nucleus scattering. It is demonstrated that COMPASS has high sensitivity
to test this important prediction of perturbative QCD.Comment: Feasibility study for COMPASS collaboration, 3 pages, no figures, 1
table. This contribution is based on talk presented at the Workshop on Spin
Physics, Trento, Italy, July 2001. The conference www site is
http://ECTstar.ect.it/contents.html The proceedings of the workshop will be
published as a special issue of Nuclear Physics B (proc suppl), eds. S Bass,
A De Roeck and A Deshpande. A more complete 26 page feasibility study with 7
figures and 3 tables is available as A. Sandacz et al., hep-ex/0106076
Revision of 26 Sept. for Ref. 11 modificatio
On the Role of Higher Twist in Polarized Deep Inelastic Scattering
The higher twist corrections to the spin dependent proton and
neutron structure functions are extracted in a model
independent way from experimental data on and found to be
non-negligible. It is shown that the NLO QCD polarized parton densities
determined from the data on g1, including higher twist effects, are in good
agreement with those found earlier from our analysis of the data on g1/F1 and
A1 where higher twist effects are negligible. On the contrary, the LO QCD
polarized parton densities obtained from the data on g1, including higher
twist, differ significantly from our previous results.Comment: 18 pages, latex, 6 figures, final version which will be published in
Phys. Rev. D, fig. 5 is changed, misprints in Table 2 are remove
K -> pi pi Electroweak Penguins in the Chiral Limit
We report on dispersive and finite energy sum rule analyses of the
electroweak penguin matrix elements in the chiral
limit. We accomplish the correct perturbative matching (scale and scheme
dependence) at NLO in alpha_s, and we describe two different strategies for
numerical evaluation.Comment: 4 pages, 2 figures. Contribution to QCD02, International Conference
on Quantum Chromo-Dynamics, Montpellier (France), July 2-9 200
t-channel unitarity and photon cross sections
We analyse the consequences of t-channel unitarity for the photon hadronic
cross sections. We show that gamma(*)p and gamma(*) gamma(*) elastic amplitudes
must include the complex-j plane singularities associated with hadronic elastic
amplitudes, and give the characteristics of possible new singularities. We also
show that several different models of hadronic total cross sections can be used
to predict the LEP data from the HERA measurements.Comment: 36 pages, LaTeX, 15 eps figures, needs elsart.cls (included). The
theoretical part has been reworked. The conclusions and applications remain
unaffecte
Constraints on R-parity violating couplings from LEP/SLD hadronic observables
We analyze the one loop corrections to hadronic Z decays in an R-parity
violating extension to the Minimal Supersymmetric Standard Model (MSSM).
Performing a global fit to all the hadronic observables at the Z-peak, we
obtain stringent constraints on the R-violating couplings constants lambda' and
lambda''. As a result of the strong constraints from the b asymmetry parameters
A_b and A_FB(b), we find that the couplings lambda'{i31}, lambda'{i32}, and
lambda''{321} are ruled out at the 1 sigma level, and that lambda'{i33} and
lambda''{33i} are ruled out at the 2 sigma level. We also obtain Bayesian
confidence limits for the R-violating couplings.Comment: 30 pages, 19 postscript figures, REVTeX, new section 8 on Bayesian
confidence limits adde
Models of Isospin Breaking in the Pion Form Factor: Consequences for the Determination of Pi_{\rho\omega}(m_rho^2) and (g-2)_mu/2
We study the implications of several recent high-precision measurements of
the pion form factor in the region of the rho-omega interference "shoulder" for
(i) the extraction of the rho-omega mixing matrix element,
Pi_{\rho\omega}(m_rho^2), and (ii) the evaluation of the isospin-breaking (IB)
correction needed to incorporate hadronic tau decay data into the determination
of the Standard Model expectation for the leading order hadronic contribution,
[a_mu]_{had}^{LO}, to the anomalous magnetic moment of the muon, focussing, in
the latter case, on the model-dependence of the rho-omega mixing component of
the IB correction. We consider a range of different models for the broad rho
contribution to the e^+e^- --> \pi\pi amplitude, applying these models to each
experimental data set, and find that the model dependence of the rho-omega
mixing correction is significantly larger than the uncertainty induced by
experimental errors for any individual model. We also find that, for each such
model, the recent data allows one to separate rho-omega mixing and direct omega
--> \pi\pi coupling contributions to the amplitude, and hence to obtain a
reasonably precise extraction of Pi_{\rho\omega}(m_rho^2), uncontaminated by
direct omega --> \pi\pi coupling effects, for use in meson exchange model
calculations of charge symmetry breaking in NN scattering.Comment: 25 pages, 4 figures. Final published version. Main change is
inclusion of a detailed discussion of the source of differences between the
present work and M. Davier et al. (arXiv:0906.5443v3
Constraints on Four Fermion Contact Interactions from Precise Electroweak Measurements
We establish constraints on a general four-fermion contact interaction from
precise measurements of electroweak parameters. We compute the one-loop
contribution for the leptonic width, anomalous magnetic, weak-magnetic,
electric and weak dipole moments of leptons in order to extract bounds on the
energy scale of these effective interactions.Comment: 16 pages, RevTeX, two figure
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