150 research outputs found
Ultrasoft Renormalization in Non-Relativistic QCD
For Non-Relativistic QCD the velocity renormalization group correlates the
renormalization scales for ultrasoft, potential and soft degrees of freedom.
Here we discuss the renormalization of operators by ultrasoft gluons. We show
that renormalization of soft vertices can induce new operators, and also
present a procedure for correctly subtracting divergences in mixed
potential-ultrasoft graphs. Our results affect the running of the
spin-independent potentials in QCD. The change for the NNLL t-tbar cross
section near threshold is very small, being at the 1% level and essentially
independent of the energy. We also discuss implications for analyzing
situations where mv^2 ~ Lambda_QCD.Comment: 31 pages, 11 fig
Uncertainties in the MSbar bottom quark mass from relativistic sum rules
A detailed compilation of uncertainties in the MSbar bottom quark mass
m_b(m_b) obtained from low-n spectral sum rules at order alpha_s^2 is given
including charm mass effects and secondary b production. The experimental
continuum region above 11.1 GeV is treated conservatively. An inconsistency of
the PDG averages for the electronic partial widths of Upsilon(4S) and
Upsilon(5S) is pointed out. From our analysis we obtain m_b(m_b)=4.20\pm 0.09
GeV. The impact of future CLEO data is discussed.Comment: 11 pages, late
Double Bubble Corrections to Heavy Quark Production
Second order corrections to the heavy quark production
cross-section due to massless quarks and coloured scalars are calculated for
all energies above threshold. Based on the method introduced in this letter
also the gauge non-invariant second order corrections due to the pure gluonic
selfenergy insertion and a certain class of and
corrections are determined. For the special choice of
the gauge parameter, , the leading threshold and high energy behaviour
of the pure second order gluonic corrections to the cross-section are governed
by the gluonic self energy insertion.Comment: 9 pages, latex, 17 figures, the complete postscript file of this
preprint, including figures, is available via anonymous ftp at
ftp://www-ttp.physik.uni-karlsruhe.de/ttp96-04/ttp96-04.ps (129.13.102.139)
or via www at http://www-ttp.physik.uni-karlsruhe.de/cgi-bin/preprints
Bottom Quark Mass Determination from low-n Sum Rules
We study the uncertainties in the MSbar bottom quark mass determination using
relativistic sum rules to O(alpha_S^2). We include charm mass effects and
secondary b bbar production and treat the experimental continuum region more
conservatively than previous analyses. The PDG treatment of the region between
the resonances Upsilon (4S) and Upsilon (5S) is reconsidered. Our final result
reads: m_b(m_b)=(4.20 \pm 0.09) GeV.Comment: 4 pages, Talk given at QCD 03, Montpellier, France, 2-9 July 200
Charm Effects in the Bottom Quark Mass from Mesons
We study the shift in the mass due to a non-zero charm quark mass.
This shift affects the value of the -quark mass extracted
from the system by about -20 MeV, due to an incomplete cancellation
of terms that are non-analytic in the charm quark mass. The precise size of the
shift depends on unknown higher order corrections, and might have a
considerable uncertainty if they are large.Comment: 7 pages, revtex, 2 postscript figure
Hadron Radiation in Leptonic Decays
The rate for the final state radiation of hadrons in leptonic decays is
evaluated, using as input experimental data for in
the low energy region. Configurations with a lepton pair of large and a
hadronic state of low invariant mass are dominant. A relative rate
is calculated. This
result is about twice the prediction based on a parton model calculation with a
quark mass of MeV. The rate for secondary production of heavy quarks is
calculated in the same formalism.Comment: 7 pages, LaTex (uses epsf.sty, 2 figures appended as uuencoded eps
files). Errors of some numerical values corrected. Conclusions are not
affected by these modification
Pade/renormalization-group improvement of inclusive semileptonic B decay rates
Renormalization Group (RG) and optimized Pade-approximant methods are used to
estimate the three-loop perturbative contributions to the inclusive
semileptonic b \to u and b \to c decay rates. It is noted that the \bar{MS}
scheme works favorably in the b \to u case whereas the pole mass scheme shows
better convergence in the b \to c case. Upon the inclusion of the estimated
three-loop contribution, we find the full perturbative decay rate to be
192\pi^3\Gamma(b\to u\bar\nu_\ell\ell^-)/(G_F^2| V_{ub}|^2) = 2065 \pm 290{\rm
GeV^5} and 192\pi^3\Gamma(b\to c\ell^-\bar\nu_\ell)/(G_F^2|V_{cb}|^2)= 992 \pm
198 {\rm GeV^5}, respectively. The errors are inclusive of theoretical
uncertainties and non-perturbative effects. Ultimately, these perturbative
contributions reduce the theoretical uncertainty in the extraction of the CKM
matrix elements |V_{ub}| and |V_{cb}| from their respective measured inclusive
semileptonic branching ratio(s).Comment: 3 pages, latex using espcrc2.sty. Write-up of talk given at BEACH
2002, UBC, Vancouve
Reducing theoretical uncertainties in mb and lambda1
We calculate general moments of the lepton energy spectrum in inclusive
semileptonic B -> X_c l \nu decay. Moments which allow the determination of
mb^{1S} and lambda1 with theoretical uncertainties Delta(mb^{1S}) ~ 0.04 GeV
and Delta(lambda1) ~ 0.05 GeV^2 are presented. The short distance 1S mass is
used to extract a mass parameter free of renormalon ambiguities. Moments which
are insensitive to mb and lambda1 and therefore test the size of the 1/mb^3
matrix elements and the validity of the OPE are also presented. Finally, we
give an expression for the total branching ratio with a lower cut on the lepton
energy, which allows one to eliminate a source of model dependence in current
determinations of |Vcb| from B -> X_c l \nu decay.Comment: 8 pages, one figur
Selection and reconstruction of the top quarks in the all-hadronic decays at a Linear Collider
A method of reconstruction of the top quarks produced in the process E+E- ->
t\bar{t} -> 6 jets at a Linear Collider (LC) is proposed. The approach does not
involve a kinematic fit, as well as assumptions on the invariant masses of the
dijets originating from the decays of W bosons and, therefore, the method is
expected to be less sensitive to theoretical and experimental uncertainties on
the top-mass measurement than traditional reconstruction methods. For the first
time, the reconstruction of the top quarks was investigated using the full LC
detector simulation after taking into account the background arising from QCD
multi-jet production.Comment: 22 pages, including 13 figures and 3 table
The QCD heavy-quark potential to order v^2: one loop matching conditions
The one-loop QCD heavy quark potential is computed to order v^2 in the color
singlet and octet channels. Several errors in the previous literature are
corrected. To be consistent with the velocity power counting, the full
dependence on |p' + p|/|p' - p| is kept. The matching conditions for the NRQCD
one-loop potential are computed by comparing the QCD calculation with that in
the effective theory. The graphs in the effective theory are also compared to
terms from the hard, soft, potential, and ultrasoft regimes in the threshold
expansion. The issue of off-shell versus on-shell matching and gauge dependence
is discussed in detail for the 1/(m k) term in the potential. Matching on-shell
gives a 1/(m k) potential that is gauge independent and does not vanish for
QED.Comment: 28 pages, References added and minor changes to section III, results
unchange
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