682 research outputs found
Top Threshold Physics
Running a future Linear Collider at the top pair threshold allows for precise
measurements of the mass, the widths and the couplings of the top quark. I give
a nontechnical review on recent theoretical developments and the theory status
in top threshold physics concerning QCD corrections and top quark finite
lifetime and electroweak effects. I also discuss threshold physics in the
context of measurements of the top Yukawa coupling from
and of squark pair production.Comment: 13 pages, 6 figures, PoS style. Invited talk presented at the
International Workshop on Top Quark Physics, Coimbra, Portugal, 12-15 Jan
200
Two-Loop Ultrasoft Running of the O(v^2) QCD Quark Potentials
The two-loop ultrasoft contributions to the next-to-leading logarithmic (NLL)
running of the QCD potentials at order v^2 are determined. The results
represent an important step towards the next-to-next-to-leading logarithmic
(NNLL) description of heavy quark pair production and annihilation close to
threshold.Comment: 13 pages, 3 figures; typos corrected, reference added, information on
cross checks added on page 7; acknowledgments adde
Top Mass Measurements from Jets and the Tevatron Top-Quark Mass
Theoretical issues are discussed for the measurement of the top-mass using
jets, including perturbative and non-perturbative effects that relate
experimental observables to the Lagrangian mass, and appropriate choices for
mass schemes. Full account for these issues is given for e+e--> t-tbar using a
factorization theorem for event shapes for massive quarks. Implications for the
Tevatron top-mass measurement are discussed. A mass-scheme, the "MSR-mass", is
introduced which allows for a precise description of observables sensitive to
scales R << m, but at the same time does not introduce perturbative matching
uncertainties in conversion to the MSbar mass.Comment: 7 pages, proceedings for the International Workshop on Top Quark
Physics, and the 2nd Workshop on Theory, Phenomenology and Experiment in
Heavy Flavor Physics, 2008. v2: reference added, language in section 5
improve
Charm Quark Mass from Inclusive Semileptonic B Decays
The MSbar charm quark mass is determined to be m_c(m_c)=1224+-17+-54 MeV from
a global fit to inclusive B meson decay data, where the first error is
experimental, and includes the uncertainty in alpha_s, and the second is an
estimate of theoretical uncertainties in the computation. We discuss the
implications of the pole mass renormalon in the determination of m_c.Comment: 7 pages, 2 tables; revtex4. References added, minor changes; version
to appear in PL
The MSR Mass and the Renormalon Sum Rule
We provide a detailed description and analysis of a low-scale short-distance
mass scheme, called the MSR mass, that is useful for high-precision top quark
mass determinations, but can be applied for any heavy quark . In contrast to
earlier low-scale short-distance mass schemes, the MSR scheme has a direct
connection to the well known mass commonly used for
high-energy applications, and is determined by heavy quark on-shell self-energy
Feynman diagrams. Indeed, the MSR mass scheme can be viewed as the simplest
extension of the mass concept to renormalization scales
. The MSR mass depends on a scale that can be chosen freely, and
its renormalization group evolution has a linear dependence on , which is
known as R-evolution. Using R-evolution for the MSR mass we provide details of
the derivation of an analytic expression for the normalization of the renormalon asymptotic behavior of the pole mass in
perturbation theory. This is referred to as the
renormalon sum rule, and can be applied to any perturbative series. The
relations of the MSR mass scheme to other low-scale short-distance masses are
analyzed as well.Comment: 42 pages + appendices, 6 figures, v2: Refs and Appendix B added,
Fig.3 changed from nl=4 to nl=5, v3: journal versio
Two-Loop Massive Quark Jet Functions in SCET
We calculate the corrections to the primary massive
quark jet functions in Soft-Collinear Effective Theory (SCET). They are an
important ingredient in factorized predictions for inclusive jet mass cross
sections initiated by massive quarks emerging from a hard interaction with
smooth quark mass dependence. Due to the effects coming from the secondary
production of massive quark-antiquark pairs there are two options to define the
SCET jet function, which we call universal and mass mode jet functions. They
are related to whether or not a soft mass mode (zero) bin subtraction is
applied for the secondary massive quark contributions and differ in particular
concerning the infrared behavior for vanishing quark mass. We advocate that a
useful alternative to the common zero-bin subtraction concept is to define the
SCET jet functions through subtractions related to collinear-soft matrix
elements. This avoids the need to impose additional power counting arguments as
required for zero-bin subtractions. We demonstrate how the two SCET jet
function definitions may be used in the context of two recently developed
factorization approaches to treat secondary massive quark effects. We clarify
the relation between these approaches and in which way they are equivalent. Our
two-loop calculation involves interesting technical subtleties related to
spurious rapidity divergences and infrared regularization in the presence of
massive quarks.Comment: 51 pages + appendices, 8 figures, v2: journal versio
Electroweak Absorptive Parts in NRQCD Matching Conditions
Electroweak corrections associated with the instability of the top quark to
the next-to-next-to-leading logarithmic (NNLL) total top pair threshold cross
section in e+e- annihilation are determined. Our method is based on absorptive
parts in electroweak matching conditions of the NRQCD operators and the optical
theorem. The corrections lead to ultraviolet phase space divergences that have
to be renormalized and lead to NLL mixing effects. Numerically, the corrections
can amount to several percent and are comparable to the known NNLL QCD
corrections.Comment: 17 pages, revtex4, 4 postscript figures included; minor changes in
text and references, title modified in printed versio
Next-to-leading-logarithmic QCD Corrections to the Cross Section sigma(e+e- to t bar t H) at 500 GeV
We determine the next-to-leading logarithmic (NLL) QCD corrections to the
cross section sigma(e+e- to t bar t H) for center-of-mass energies up to 500
GeV. The dynamics is dominated by nonrelativistic effects, and the summation of
terms singular in the relative t bar t velocity is mandatory to all orders in
the strong coupling constant alpha_s using an effective theory. The summations
lead to an enhancement of the tree level predictions by about a factor of two
and are important for the determination of the top Yukawa coupling. We also
study the impact of polarization of the electron-positron beams and provide a
fast approximation formula for the known O(alpha_s) QCD fixed-order prediction.Comment: 18 pages, 3 tables, 5 figure
Phase Space Matching and Finite Lifetime Effects for Top-Pair Production Close to Threshold
The top-pair production cross section close to threshold in
collisions is strongly affected by the small lifetime of the top
quark. Since the cross section is defined through final states containing the
top decay products, a consistent definition of the cross section depends on
prescriptions how these final states are accounted for the cross section.
Experimentally, these prescriptions are implemented for example through cuts on
kinematic quantities such as the reconstructed top quark invariant masses. As
long as these cuts do not reject final states that can arise from the decay of
a top and an anti-top quark with a small off-shellness compatible with the
nonrelativistic power-counting, they can be implemented through imaginary phase
space matching conditions in NRQCD. The prescription-dependent cross section
can then be determined from the optical theorem using the forward
scattering amplitude. We compute the phase space matching conditions associated
to cuts on the top and anti-top invariant masses at next-to-next-to-leading
logarithmic (NNLL) order and partially at next-to-next-to-next-to-leading
logarithmic (NLL) order in the nonrelativistic expansion and, together
with finite lifetime and electroweak effects known from previous work, analyze
their numerical impact on the cross section. We show that the phase
space matching contributions are essential to make reliable NRQCD predictions,
particularly for energies below the peak region, where the cross section is
small. We find that irreducible background contributions associated to final
states that do not come from top decays are strongly suppressed and can be
neglected for the theoretical predictions.Comment: 62 pages, 21 figure
A Precise Determination of from the C-parameter Distribution
We present a global fit for , analyzing the available
C-parameter data measured at center-of-mass energies between and
GeV. The experimental data is compared to a NLL +
+ theoretical prediction (up to the
missing 4-loop cusp anomalous dimension), which includes power corrections
coming from a field theoretical nonperturbative soft function. The dominant
hadronic parameter is its first moment , which is defined in a scheme
which eliminates the renormalon ambiguity. The
resummation region plays a dominant role in the C-parameter spectrum, and in
this region a fit for and is sufficient. We find
and with
for bins of data. These results agree with the
prediction of universality for between thrust and C-parameter within
1-.Comment: 24 pages, 19 figure
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