51 research outputs found
A Prediction for the 4-Loop \beta Function
We predict that the four-loop contribution \beta_3 to the QCD \beta function
in the MS-bar prescription is given by
\beta_3\simeq 23,600(900) - 6,400(200) N_f + 350(70) N_f^2 + 1.5 N_f^3, where
N_f is the number of flavours and the coefficient of N_f^3 is an exact result
from large-N_f expansion. In the phenomenologically-interesting case N_f=3, we
estimate \beta_3 = (7.6 \pm 0.1) x 10^3. We discuss our estimates of the errors
in these QCD predictions, basing them on the demonstrated accuracy of our
method in test applications to the O(N) \Phi^4 theory, and on variations in the
details of our estimation method, which goes beyond conventional Pade
approximants by estimating and correcting for subasymptotic deviations from
exact results.Comment: 11 pages, LaTeX, including 2 figures in 3 ps files; requires
epsfig.sty; added comparison with recent exact result
Perturbative corrections to zero recoil inclusive decay sum rules
Comparing the result of inserting a complete set of physical states in a time
ordered product of decay currents with the operator product expansion gives
a class of zero recoil sum rules. They sum over physical states with excitation
energies less than , where is much greater than the QCD scale
and much less than the heavy charm and bottom quark masses. These sum rules
have been used to derive an upper bound on the zero recoil limit of the form-factor, and on the matrix element of the kinetic energy operator
between meson states. Perturbative corrections to the sum rules of order
have previously been computed. We
calculate the corrections of order and keeping all orders in , and show that these
perturbative QCD corrections suppressed by powers of
significantly weaken the upper bound on the zero recoil form-factor,
and also on the kinetic energy operator's matrix element.Comment: 13 pages revtex, four figures included; minor change
Remark on Charm Quark Fragmentation to Mesons
The observed mesons have flavor quantum numbers and
spin-parity of the light degrees of freedom . In
the limit the spin of the charm quark is conserved and
the fragmentation process is characterized by the
probability for the charm quark to fragment to a meson with a given
helicity for the light degrees of freedom. We consider the calculated fragmentation functions in the limit as a qualitative model for the fragmentation
functions. We find that in this model charm quark fragmentation to
light degrees of freedom with helicities is favored over fragmentation to light
degrees of freedom with helicities .Comment: 6 pages, CALT-68-192
Perturbative Strong Interaction Corrections to the Heavy Quark Semileptonic Decay Rate
We calculate the part of the order correction to the
semileptonic heavy quark decay rate proportional to the number of light quark
flavors, and use our result to set the scale for evaluating the strong coupling
in the order term according to the scheme of Brodsky, Lepage and
Mackenzie. Expressing the decay rate in terms of the heavy quark pole mass
, we find the scale for the strong coupling to be . If the decay rate is expressed in terms of the heavy
quark mass then the scale is . We use these
results along with the existing calculations for hadronic decay to
calculate the BLM scale for the nonleptonic decay width and the semileptonic
branching ratio. The implications for the value of extracted from
the inclusive semileptonic meson decay rate are discussed.Comment: 7 pages in Latex plus 1 uuencoded figure, uses epsf, UTPT-94-24,
CMU-HEP 94-29, CALT-68-1950 (previous results unchanged; we add a short
discussion of nonleptonic decays
Renormalization of Supersymmetric Gauge Theories on Orbifolds: Brane Gauge Couplings and Higher Derivative Operators
We consider supersymmetric gauge theories coupled to hyper multiplets on five
and six dimensional orbifolds and determine the bulk and local fixed point
renormalizations of the gauge couplings. We infer from a component analysis
that the hyper multiplet does not induce renormalization of the brane gauge
couplings on the five dimensional orbifold S^1/Z_2. This is not due to
supersymmetry, since the bosonic and fermionic contributions cancel separately.
We extend this investigation to T^2/Z_N orbifolds using supergraph techniques
in six dimensions. On general Z_N orbifolds the gauge couplings do renormalize
at the fixed points, except for the Z_2 fixed points of an even ordered
orbifold. To cancel the bulk one-loop divergences a dimension six higher
derivative operator is needed, in addition to the standard bulk gauge kinetic
term.Comment: 10 p
Enhanced subleading structure functions in semileptonic B decay
The charged lepton spectrum in semileptonic decay near
maximal lepton energy receives important corrections from subleading structure
functions that are formally suppressed by powers of but are
enhanced by numerical factors. We investigate the series of higher order terms
which smear over a region of width near the
endpoint the contributions proportional to times (i)
the matrix element of the chromomagnetic operator, and (ii) four-quark
operators. These contribute to the total rate at the few percent level, but
affect the endpoint region much more significantly. Implications for the
determination of are discussed.Comment: 12 page
Power Suppressed Corrections to Hadronic Event Shape Variables
For high energy processes () there are infrared safe
hadronic shape variables that have a calculable perturbative expansion in
. However, nonperturbative power suppressed corrections to these
variables are not well understood. We use the behavior of large orders of the
perturbation expansion to gain insight into the nonperturbative corrections.
Our results suggest that certain shape variables have nonperturbative
corrections suppressed by fractional powers of .Comment: (12 pages, 2 figures, uses harvmac and uufiles), UCSD/PTH 94-1
Constraints on Axion Models from
We explore a new class of axion models in which some, but not all, of the
left-handed quarks have a Peccei-Quinn symmetry. These models are potentially
afflicted by flavour changing neutral currents. We derive the bounds on the
Peccei-Quinn symmetry-breaking scale from bounds on the
branching ratio, showing that in some cases they are even stronger than the
astrophysical ones, but still not strong enough to kill off the models.Comment: 15pp RevTeX, 1 eps fig, uses graphics style. Expanded discussion on
massive scalars and pseudoscalars, typos. To appear in Physical Review
Constraints on Axion Models
Two particular classes of axion models are presented, each one yielding a
lower bound on the axion decay constant, based though on different
considerations. In the first class only some, and not all, of the right-handed
quarks have PQ charges, whereas in the second one the left-handed sector of the
same quarks is taken into account as well. In the first case we find that
bounds coming from astrophysics are significantly relaxed compared with those
for the DFSZ. As for the second class, the astrophysical constraints proved to
be less severe (with one exception), than those coming from FCNC processes.Comment: Talk given at the 5th IFT Workshop: Axions, U. of Florida,
Gainesville, Florida, USA, 13-15 Mar 1998. Based on hep-ph/9708281 and
hep-ph/9807363. 5pp, 1 eps fig, uses espcrc2 and graphics style
Constraints on Variant Axion Models
A particular class of variant axion models with two higgs doublets and a
singlet is studied. In these models the axion couples either to the -quark
or -quark or both, but not to , , , or . When the axion couples
to only one quark the models possess the desirable feature of having no domain
wall problem, which makes them viable candidates for a cosmological axion
string scenario. We calculate the axion couplings to leptons, photons and
nucleons, and the astrophysical constraints on the axion decay constant
are investigated and compared to the DFSZ axion model. We find that the most
restrictive lower bound on , that from SN1987a, is lowered by up to a
factor of about 30, depending on the model and also the ratio of the vacuum
expectation values of the higgs doublets. For scenarios with axionic strings,
the allowed window for in the quark model can be more than two orders
of magnitude. For inflationary scenarios, the cosmological upper bound on
, where is the QCD anomaly factor, is unaffected: however, the
variant models have either 3 or 6 times smaller than the DFSZ model.Comment: 21pp RevTeX, 1 eps fig, uses graphics style, typo corrected, and
corrected file sent this time. To appear in Physical Review
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