610 research outputs found
Two-loop matching coefficients for the strong coupling in the MSSM
When relating the strong coupling , measured at the scale of the
boson mass, to its numerical value at some higher energy, for example the
scale of Grand Unification, it is important to include higher order corrections
both in the running of and the decoupling of the heavy particles. We
compute the two-loop matching coefficients for within the Minimal
Supersymmetric Standard Model (MSSM) which are necessary for a consistent
three-loop evolution of the strong coupling constant. Different scenarios for
the hierarchy of the supersymmetric scales are considered and the numerical
effects are discussed. We find that the three-loop effects can be as large as
and sometimes even larger than the uncertainty induced by the current
experimental accuracy of .Comment: 22 pages, 8 figures (13 ps/eps-files
and mesons at finite temperature and density in the NJL model with dimensional regularization
Dynamical Symmetry breaking and meson masses are studied in the
Nambu-Jona-Lasinio (NJL) model at finite temperature and chemical potential
using the dimensional regularization. Since the model is not renormalizable in
four space-time dimensions, physical results and parameters depend on the
regularization method. Following the imaginary time formalism, we introduce the
temperature, and the chemical potential, . The parameters in the model
are fixed by calculating the pion mass and decay constant in the dimensional
regularization at .Comment: 28 pages, 9 figures, v2: a few points corrected and references adde
Cancellation of soft and collinear divergences in noncommutative QED
In this paper, we investigate the behavior of non-commutative IR divergences
and will also discuss their cancellation in the physical cross sections. The
commutative IR (soft) divergences existing in the non-planar diagrams will be
examined in order to prove an all order cancellation of these divergences using
the Weinberg's method. In non-commutative QED, collinear divergences due to
triple photon splitting vertex, were encountered, which are shown to be
canceled out by the non-commutative version of KLN theorem. This guarantees
that there is no mixing between the Collinear, soft and non-commutative IR
divergences
Covariant boost and structure functions of baryons in Gross-Neveu models
Baryons in the large N limit of two-dimensional Gross-Neveu models are
reconsidered. The time-dependent Dirac-Hartree-Fock approach is used to boost a
baryon to any inertial frame and shown to yield the covariant energy-momentum
relation. Momentum distributions are computed exactly in arbitrary frames and
used to interpolate between the rest frame and the infinite momentum frame,
where they are related to structure functions. Effects from the Dirac sea
depend sensitively on the occupation fraction of the valence level and the bare
fermion mass and do not vanish at infinite momentum. In the case of the kink
baryon, they even lead to divergent quark and antiquark structure functions at
x=0.Comment: 13 pages, 12 figures; v2: minor correction
The electromagnetic dipole operator effect on B -> Xs gamma at O(alpha_s^2)
The flavor-changing electromagnetic dipole operator O_7 gives the dominant
contribution to the B -> Xs gamma decay rate. We calculate two-loop QCD
corrections to its matrix element together with the corresponding
bremsstrahlung contributions. The optical theorem is applied, and the relevant
imaginary parts of three-loop diagrams are computed following the lines of our
recent t -> Xb W calculation. The complete result allows us to test the
validity of the naive non-abelianization (NNA) approximation that has been
previously applied to estimate the NNLO QCD correction to Gamma(B -> Xs
gamma)/Gamma(B -> Xu e nu). When both decay widths are normalized to m^5_{b,R}
in the same renormalization scheme R, the calculated O(alpha_s^2) correction is
sizeable (~ 6%), and the NNA estimate is about 1/3 too large. On the other
hand, when the ratio of the decay widths is written as
S*(m_b(m_b)/m_{b,pole})^2, the calculated O(alpha_s^2) correction to S is at
the level of 1% for both the complete and the NNA results.Comment: Corrected pi^2 terms in the description of intermediate steps in
Section II. Final results unchange
Conceptual aspects of QCD factorization in hadronic B decays
I review the meaning of ``QCD factorization'' in hadronic two-body B decays
and then discuss recent results of theoretical (rather than phenomenological)
nature: the proof of factorization at two loops; the identification of
``chirally enhanced'' power corrections; and the role of annihilation
contributions.Comment: 10 pages, LaTeX. Based on talks presented at the UK Phenomenology
Workshop on Heavy Flavour and CP Violation, 17 - 22 September 2000, Durham,
proceedings to appear in J. Phys. G; the 5th International Symposium on
Radiative Corrections (RADCOR2000), Carmel, California, September 11 - 15,
2000; the 4th Workshop on Continuous Advances in QCD, Minneapolis, 12-14 May
2000; the Vth International Workshop on Heavy Quark Physics, Dubna, 6-8 April
200
Higher-spin current multiplets in operator-product expansions
Various formulas for currents with arbitrary spin are worked out in general
space-time dimension, in the free field limit and, at the bare level, in
presence of interactions. As the n-dimensional generalization of the
(conformal) vector field, the (n/2-1)-form is used. The two-point functions and
the higher-spin central charges are evaluated at one loop. As an application,
the higher-spin hierarchies generated by the stress-tensor operator-product
expansion are computed in supersymmetric theories. The results exhibit an
interesting universality.Comment: 19 pages. Introductory paragraph, misprint corrected and updated
references. CQG in pres
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