51 research outputs found
Re-evaluation of the Gottfried sum using neural networks
We provide a determination of the Gottfried sum from all available data,
based on a neural network parametrization of the nonsinglet structure function
F_2. We find S_G=0.244 +- 0.045, closer to the quark model expectation S_G=1/3
than previous results. We show that the uncertainty from the small x region is
somewhat underestimated in previous determinations.Comment: 4 pages, 3 figures, latex with revte
Precision determination of the strong coupling constant
Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Physics, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 213-223).In this thesis we study the event shapes variable thrust. Event shape variables are observables that characterize the shape of the distribution of the final state particles of a reaction. We take advantage of the formalism of Soft Collinear Effective Theory (SCET), an effective theory of the strong interactions appropriate for describing energetic jets. We give a factorization theorem for the process e+e- to hadrons, valid in the whole range of thrust values. This factorization theorem resums large logarithms at the N3LL accuracy and contains the full O(a) result for the fixed order cross section. In order to be able to describe the whole range of thrust values, we define the profile functions, which are thrust-dependent factorization scales which smoothly interpolate between regions where resummation of large logarithms is important and where it is not. To determine non perturbative effects, we fit renormalon-free nonperturbative matrix elements of operators defined in field theory, Q1. We perform a global analysis to all available thrust data in the tail region, where a two parameter fit to a,(mz) and the first power correction Q1 suffices. We find cr(mz) = 0.1135 i (0.0002)expt ± (0.0005)hadr ± (0.0009)pert, with X2 /dof(= 485) = 0.91, where the displayed 1-sigma errors are the total experimental error, the hadronization uncertainty, and the perturbative theory uncertainty, respectively. Furthermore, we perform a global analysis to all available data on the first moment of the thrust distribution. This analysis is a partially independent check of the tail fit, in fact it probes different regions of the thrust distribution and the analysis of experimental systematic uncertainties was conducted independently with respect to the data for the distribution. We find a,(mz) = 0.1141 i (0.0004)exp ± (0.0014)hadr ± (0.0007)pert with X2/dof(= 45) = 1.33. We also consider pp collisions, in particular the Drell-Yan process. Here we calculate analytically the beam thrust logarithms of the relevant beam functions and of the coefficient function at O(a2). This is a necessary ingredient for the calculation of the nonsingular terms in resummed predictions.by Riccardo Abbate.Ph.D
A new prescription for soft gluon resummation
We present a new prescription for the resummation of the divergent series of
perturbative corrections, due to soft gluon emission, to hard processes near
threshold in perturbative QCD (threshold resummation). This prescription is
based on Borel resummation, and contrary to the commonly used minimal
prescription, it does not introduce a dependence of resummed physical
observables on the kinematically unaccessible x->0 region of parton
distributions. We compare results for resummed deep-inelastic scattering
obtained using the Borel prescription and the minimal prescription and exploit
the comparison to discuss the ambiguities related to the resummation procedure.Comment: 15 pages, 3 figures. Final version, to be published in Phys. Lett. B.
Several typos corrected (in particular eqs 26,41,53); fig. 3 adde
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
Synthesis and Stereochemical Characterization of a Novel Chiral α-Tetrazole Binaphthylazepine Organocatalyst
A novel α-tetrazole-substituted 1,1'-binaphthylazepine chiral catalyst has been synthesized and its absolute configuration has been determined by DFT computational analysis of the vibrational circular dichroism (VCD) spectrum of its precursor. The VCD analysis, carried out through the model averaging method, allowed to assign the absolute configuration of a benzylic stereocenter in the presence of a chiral binaphthyl moiety. The 1,1'-binaphthylazepine tetrazole and the nitrile its immediate synthetic precursor, have been preliminarily tested as chiral organocatalysts in the asymmetric intramolecular oxa-Michael cyclization of 2-hydroxy chalcones for the synthesis of chiral flavanones obtaining low enantioselectivity
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
Strength of Correlations in a Silver Based Cuprate Analogue
AgF2 has been proposed as a cuprate analogue which requires strong
correlation and marked covalence. On the other hand, fluorides are usually
quite ionic and 4d transition metals tend to be less correlated than their 3d
counterparts, which calls for further scrutiny. We combine valence band
photoemission and Auger-Meitner spectroscopy of AgF and AgF2 together with
computations in small clusters to estimate values of the Ag 4d Coulomb
interaction U 4d and charge-transfer energy. Based on these values, AgF2 can be
classified as a charge-transfer correlated insulator according to the
Zaanen-Sawatzky-Allen classification scheme. Thus, we confirm that the material
is a cuprate analogue from the point of view of correlations, suggesting that
it should become a high-temperature superconductor if metallization is achieved
by doping. We present also a computation of the Hubbard U in density functional
"+U" methods and discuss its relation to the Hubbard U in spectroscopies.Comment: 12 pages, 10 figure
Case Report - Multinodular goiter in a patient with Congenital Hypothyroidism and Bannayan-Riley-Ruvalcaba syndrome: the possible synergic role of TPO and PTEN mutation
We report the case of a paediatric female patient affected by Bannayan-Riley-Ruvalcaba syndrome (BRRS) and congenital hypothyroidism (CH) with homozygous mutation of the TPO gene. She underwent total thyroidectomy at the age of seven years because of the development of a multinodular goiter. BRRS patients present an increased risk of benign and malignant thyroid disease since childhood because of inactivating mutation of PTEN, an onco-suppressor gene. Instead, homozygous mutations in the TPO gene can be associated with severe forms of hypothyroidism with goiter; previous studies have described cases of follicular and papillary thyroid cancer in CH patients with TPO mutation despite a perfectly controlled thyroid function with Levothyroxine therapy. To our knowledge, this is the first case that describes the possible synergic role of coexisting mutation of both TPO and PTEN in the development of multinodular goiter underlining the importance of a tailored surveillance program in these patients, especially during childhood
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