69 research outputs found
Precision Thrust Cumulant Moments at N^3LL
We consider cumulant moments (cumulants) of the thrust distribution using
predictions of the full spectrum for thrust including O(alpha_s^3) fixed order
results, resummation of singular N^3LL logarithmic contributions, and a class
of leading power corrections in a renormalon-free scheme. From a global fit to
the first thrust moment we extract the strong coupling and the leading power
correction matrix element Omega_1. We obtain alpha_s(m_Z) = 0.1141 \pm
(0.0004)_exp \pm (0.0014)_hadr \pm (0.0007)_pert, where the 1-sigma
uncertainties are experimental, from hadronization (related to Omega_1) and
perturbative, respectively, and Omega_1 = 0.372 \pm (0.044)_exp \pm
(0.039)_pert GeV. The n-th thrust cumulants for n > 1 are completely
insensitive to Omega_1, and therefore a good instrument for extracting
information on higher order power corrections, Omega'_n/Q^n, from moment data.
We find (\tilde Omega'_2)^(1/2) = 0.74 \pm (0.11)_exp \pm (0.09)_pert GeV.Comment: 23 pages, 16 figures. v2: minor changes, references added, some data
added. v3: minor modifications to match published versio
Thrust at N^3LL with Power Corrections and a Precision Global Fit for alphas(mZ)
We give a factorization formula for the e+e- thrust distribution dsigma/dtau
with tau=1-T based on soft-collinear effective theory. The result is applicable
for all tau, i.e. in the peak, tail, and far-tail regions. The formula includes
O(alphas^3) fixed-order QCD results, resummation of singular partonic alphas^j
ln^k(tau)/tau terms with N^3LL accuracy, hadronization effects from fitting a
universal nonperturbative soft function defined in field theory, bottom quark
mass effects, QED corrections, and the dominant top mass dependent terms from
the axial anomaly. We do not rely on Monte Carlo generators to determine
nonperturbative effects since they are not compatible with higher order
perturbative analyses. Instead our treatment is based on fitting
nonperturbative matrix elements in field theory, which are moments Omega_i of a
nonperturbative soft function. We present a global analysis of all available
thrust data measured at center-of-mass energies Q=35 to 207 GeV in the tail
region, where a two parameter fit to and the first moment
Omega_1 suffices. We use a short distance scheme to define Omega_1, called the
R-gap scheme, thus ensuring that the perturbative dsigma/dtau does not suffer
from an O(Lambda_QCD) renormalon ambiguity. We find alphas(mZ)=0.1135 \pm
(0.0002)_{expt} \pm (0.0005)_{hadr} \pm (0.0009)_{pert}, with chi^2/dof=0.91,
where the displayed 1-sigma errors are the total experimental error, the
hadronization uncertainty, and the perturbative theory uncertainty,
respectively. The hadronization uncertainty in alphas is significantly
decreased compared to earlier analyses by our two parameter fit, which
determines Omega_1=0.323 GeV with 16% uncertainty.Comment: 45 pages, 21 figures, v2: added discussion of bin integration vs.
cumulant differences; comparison to Becher & Schwartz improved; typo in Eq.55
fixe
The pp -> K^+Sigma^+n cross section from missing mass spectra
We utilize existing inclusive data on K^+ meson momentum spectra of the
reaction pp -> K^+X at T_p = 2.3 - 2.85 GeV to deduce total cross sections for
pp -> K^+\Sigma^+n. The method used to extract those cross sections is
explained and discussed in detail. Our result for T_p = 2.85 GeV is consistent
with the data point from a direct measurement at the same beam energy. The
cross section obtained for T_p = 2.3 GeV is with 13.7\pm2.3 \mu b considerably
smaller than the value found in a recent experiment by the COSY-11
Collaboration at a somewhat lower beam energy, indicating that the pp ->
K^+\Sigma^+n reaction cross section could exhibit a rather unusual energy
dependence.Comment: 12 pages, 8 figure
Production of Lambda and Sigma^0 hyperons in proton-proton collisions
This paper reports results on simultaneous measurements of the reaction
channels pp -> pK+\Lambda and pp -> pK+\Sigma^0 at excess energies of 204, 239,
and 284 MeV (\Lambda) and 127, 162, and 207 MeV (\Sigma^0). Total and
differential cross sections are given for both reactions. It is concluded from
the measured total cross sections that the high energy limit of the cross
section ratio is almost reached at an excess energy of only about 200 MeV. From
the differential distributions observed in the overall CMS as well as in the
Jackson and helicity frames, a significant contribution of interfering nucleon
resonances to the \Lambda production mechanism is concluded while resonant
\Sigma^0-production seems to be of lesser importance and takes place only
through specific partial waves of the entrance channel. The data also indicate
that kaon exchange plays a minor role in the case of \Lambda- but an important
role for \Sigma^0-production. Thus the peculiar energy dependence of the
\Lambda-to-\Sigma^0 cross section ratio appears in a new light as its
explanation requires more than mere differences between the p\Lambda and the
p\Sigma^0 final state interaction. The data provide a benchmark for theoretical
models already available or yet to come.Comment: 18 pages, 10 figures; accepted by The European Physical Journal A
(EPJ A
Close-to-threshold Meson Production in Hadronic Interactions
Studies of meson production at threshold in the hadron--hadron interaction
began in the fifties when sufficient energies of accelerated protons were
available. A strong interdependence between developments in accelerator
physics, detector performance and theoretical understanding led to a unique
vivid field of physics. Early experiments performed with bubble chambers
revealed already typical ingredients of threshold studies, which were
superseded by more complete meson production investigations at the nucleon beam
facilities TRIUMF, LAMPF, PSI, LEAR and SATURNE. Currently, with the advent of
the new cooler rings as IUCF, CELSIUS and COSY the field is entering a new
domain of precision and the next step of further progress.
The analysis of this new data in the short range limit permits a more
fundamental consideration and a quantitative comparison of the production
processes for different mesons in the few--body final states. The
interpretation of the data take advantage of the fact that production reactions
close-to-threshold are characterized by only a few degrees of freedom between a
well defined combination of initial and exit channels. Deviations from
predictions of phase-space controlled one-meson-exchange models are indications
of new and exciting physics. Precision data on differential cross sections,
isospin and spin observables -- partly but by no means adequately available --
are presently turning up on the horizon. There is work for the next years and
excitement of the physics expected. Here we try to give a brief and at the same
time comprehensive overview of this field of hadronic threshold production
studies.Comment: 100 pages, Review article to be published in Prog. Part. Nucl. Phys.
Vol. 49, issue 1 (2002
Influence of N*-resonances on hyperon production in the channel pp->K+ Lambda p at 2.95, 3.20 and 3.30 GeV/c beam momentum
Hyperon production in the threshold region was studied in the reaction pp ->
K+Lp using the time-of-flight spectrometer COSY-TOF. Exclusive data, covering
the full phase-space, were taken at the three different beam momenta of
p_beam=2.95, 3.20 and 3.30 GeV/c, corresponding to excess energies of
epsilon=204, 285 and 316 MeV, respectively. Total cross-sections were deduced
for the three beam momenta to be 23.9+/-0.8 +/-2.0 ub, 28.4+/-1.3 +/-2.2 ub and
35.0+/-1.3 +/-3.0 ub. Differential observables including Dalitz plots were
obtained. The analysis of the Dalitz plots reveals a strong influence of the
N(1650)-resonance at p_beam=2.95 GeV/c, whereas for the higher momenta an
increasing relative contribution of the N(1710)- and/or of the
N(1720)-resonance was observed. In addition, the pL-final-state interaction
turned out to have a significant influence on the Dalitz plot distribution.Comment: accepted for publication at Physics Letters B; some minor text
changes were done; also the scale of the ordinates of figure 9 has been
changed
Towards the understanding of jet shapes and cross sections in heavy ion collisions using soft-collinear effective theory
- …