776 research outputs found

### Renormalization Group Analysis in NRQCD for Colored Scalars

The vNRQCD Lagrangian for colored heavy scalar fields in the fundamental
representation of QCD and the renormalization group analysis of the
corresponding operators are presented. The results are an important ingredient
for renormalization group improved computations of scalar-antiscalar bound
state energies and production rates at next-to-next-to-leading-logarithmic
(NNLL) order.Comment: 19 pages, 8 figures; revtex4. References added; version to appear in
Phys. Rev.

### Perturbative corrections to the determination of Vub from the P+ spectrum in B->X_u l nu

We investigate the relation between the E_gamma spectrum in B->X_s gamma
decay and the P+ spectrum in semileptonic B->X_u l nu decay (P+ is the hadronic
energy minus the absolute value of the hadronic three-momentum), which provides
in principle the theoretically simplest determination of Vub from any of the
"shape function regions" of B->X_u l nu spectra. We calculate analytically the
P+ spectrum to order alpha_s^2 beta_0, and study its relation to the B->X_s
gamma photon spectrum to eliminate the leading dependence on nonperturbative
effects. We compare the result of fixed order perturbation theory to the
next-to-leading log renormalization group improved calculation, and argue that
fixed order perturbation theory is likely to be a more appropriate expansion.
Implications for the perturbative uncertainties in the determination of Vub
from the P+ spectrum are discussed.Comment: reference added, to appear in PR

### Charm and Bottom Masses from Sum Rules with a Convergence Test

In this talk we discuss results of a new extraction of the MS-bar charm quark
mass using relativistic QCD sum rules at O(as**3) based on moments of the
vector and the pseudoscalar current correlators and using the available
experimental measurements from e+e- collisions and lattice results,
respectively. The analysis of the perturbative uncertainties is based on
different implementations of the perturbative series and on independent
variations of the renormalization scales for the mass and the strong coupling
following a work we carried out earlier. Accounting for the perturbative series
that result from this double scale variation is crucial since some of the
series can exhibit extraordinarily small scale dependence, if the two scales
are set equal. The new aspect of the work reported here adresses the problem
that double scale variation might also lead to an overestimate of the
perturbative uncertainties. We supplement the analysis by a convergence test
that allows to quantify the overall convergence of QCD perturbation theory for
each moment and to discard series that are artificially spoiled by specific
choices of the renormalization scales. We also apply the new method to an
extraction of the MS-bar bottom quark mass using experimental moments that
account for a modeling uncertainty associated to the continuum region where no
experimental data is available. We obtain m_c(m_c) = 1.287 +- 0.020 GeV and
m_b(m_b) = 4.167 +- 0.023 GeV.Comment: 6 pages, 2 figures. Presented at the International Workshop on the
CKM Unitarity Triangle Vienna, Austria, September 8-12, 201

### Variable Flavor Number Scheme for Final State Jets

We discuss a variable flavor number scheme (VFNS) for final state jets which
can account for the effects of arbitrary finite quark masses in inclusive jet
observables. The scheme is a generalization of the VFNS scheme for PDFs applied
to setups with additional dynamical scales and relies on appropriate
renormalization conditions for the matrix elements in the factorization
theorem. We illustrate general properties by means of the example of
deep-inelastic scattering (DIS) in the endpoint region $x\rightarrow 1$ and
event shapes in the dijet limit, in particular the calculations of threshold
corrections, consistency conditions and relations to mass singularities found
in fixed-order massive calculations.Comment: 7 pages, 4 figures, Proceedings of the XXII. International Workshop
on Deep-Inelastic Scattering and Related Subjects, 28 April - 2 May 2014,
Warsaw, Polan

### On the Light Massive Flavor Dependence of the Large Order Asymptotic Behavior and the Ambiguity of the Pole Mass

We provide a systematic renormalization group formalism for the mass effects
in the relation of the pole mass $m_Q^{\rm pole}$ and short-distance masses
such as the $\overline{\rm MS}$ mass $\overline{m}_Q$ of a heavy quark $Q$,
coming from virtual loop insertions of massive quarks lighter than $Q$. The
formalism reflects the constraints from heavy quark symmetry and entails a
combined matching and evolution procedure that allows to disentangle and
successively integrate out the corrections coming from the lighter massive
quarks and the momentum regions between them and to precisely control the large
order asymptotic behavior. With the formalism we systematically sum logarithms
of ratios of the lighter quark masses and $m_Q$, relate the QCD corrections for
different external heavy quarks to each other, predict the ${\cal
O}(\alpha_s^4)$ virtual quark mass corrections in the pole-$\overline{\rm MS}$
mass relation, calculate the pole mass differences for the top, bottom and
charm quarks with a precision of around $20$ MeV and analyze the decoupling of
the lighter massive quark flavors at large orders. The summation of logarithms
is most relevant for the top quark pole mass $m_t^{\rm pole}$, where the
hierarchy to the bottom and charm quarks is large. We determine the ambiguity
of the pole mass for top, bottom and charm quarks in different scenarios with
massive or massless bottom and charm quarks in a way consistent with heavy
quark symmetry, and we find that it is $250$ MeV. The ambiguity is larger than
current projections for the precision of top quark mass measurements in the
high-luminosity phase of the LHC.Comment: 45 pages + appendix, 6 figures, v2: journal versio

### Bottom and Charm Mass Determinations with a Convergence Test

We present new determinations of the MS-bar charm quark mass using
relativistic QCD sum rules at O(alpha_s^3) from the moments of the vector and
the pseudoscalar current correlators. We use available experimental
measurements from e+e- collisions and lattice simulation results, respectively.
Our analysis of the theoretical uncertainties is based on different
implementations of the perturbative series and on independent variations of the
renormalization scales for the mass and the strong coupling. Taking into
account the resulting set of series to estimate perturbative uncertainties is
crucial, since some ways to treat the perturbative expansion can exhibit
extraordinarily small scale dependence when the two scales are set equal. As an
additional refinement, we address the issue that double scale variation could
overestimate the perturbative uncertainties. We supplement the analysis with a
test that quantifies the convergence rate of each perturbative series by a
single number. We find that this convergence test allows to determine an
overall and average convergence rate that is characteristic for the series
expansions of each moment, and to discard those series for which the
convergence rate is significantly worse. We obtain mc(mc) = 1.288 +- 0.020 GeV
from the vector correlator. The method is also applied to the extraction of the
MS-bar bottom quark mass from the vector correlator. We compute the
experimental moments including a modeling uncertainty associated to the
continuum region where no data is available. We obtain mb(mb) = 4.176 +- 0.023
GeV.Comment: 53 pages, 16 figures, 19 tables; v2 typos fixed, references added,
modification of section 6.3, results for bottom moments and bottom mass
updated, matches published versio

### Approximating the radiatively corrected Higgs mass in the Minimal Supersymmetric Model

To obtain the most accurate predictions for the Higgs masses in the minimal
supersymmetric model (MSSM), one should compute the full set of one-loop
radiative corrections, resum the large logarithms to all orders, and add the
dominant two-loop effects. A complete computation following this procedure
yields a complex set of formulae which must be analyzed numerically. We discuss
a very simple approximation scheme which includes the most important terms from
each of the three components mentioned above. We estimate that the Higgs masses
computed using our scheme lie within 2 GeV of their theoretically predicted
values over a very large fraction of MSSM parameter space.Comment: 31 pages, 10 embedded figures, latex with psfig.sty the complete
postscript file of this preprint, including figures, is available via
anonymous ftp at ftp://www-ttp.physik.uni-karlsruhe.de/ttp95-09/ttp95-09.ps
or via www at http://www-ttp.physik.uni-karlsruhe.de/cgi-bin/preprints

### B decays in the upsilon expansion

Theoretical predictions for B decay rates are rewritten in terms of the
Upsilon(1S) meson mass instead of the b quark mass, using a modified
perturbation expansion. The theoretical consistency of this expansion is shown
both at low and high orders. Our method improves the behavior of the
perturbation series for semileptonic and nonleptonic inclusive decay modes, as
well as for exclusive decay form factors. The results are applied to the
determination of the semileptonic B branching ratio, charm counting, the ratio
of B -> X tau nu and B -> X e nu decay rates, and form factor ratios in B -> D*
e nu decay. We also comment on why it is not possible to separate perturbative
and nonperturbative effects in QCD.Comment: 21 page

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