2,153 research outputs found
RS-invariant resummations of QCD perturbation theory
No full textWe propose a renormalon-inspired resummation of QCD perturbation theory based on approximating the renormalization scheme (RS) invariant effective charge (EC) beta- function coefficients by the portion containing the highest power of b = (11N'-2N(_f))/6, the first beta-function coefficient, for SU(N) QCD with N(_f) quark flavours. This can be accomplished using exact large-N(_f) all-orders results. The resulting resummation is RS-invariant and the exact next-to-leading order (NLO) and next-to-NLO (NNLO) coefficients in any RS are included. This improves on a previously employed naive leading-6 resummation which is RS-dependent. The RS-invariant resummation is used to assess the reliability of fixed-order perturbation theory for the e(^+)e(^-) R-ratio, hadronic tau-decay ratio R(_r), and Deep Inelastic Scattering (DIS) sum rules, by comparing it with the exact NNLO results in the EC RS. For R and R(_r), where large-order perturbative behaviour is dominated by a leading ultra-violet renormalon singularity, the comparison indicates fixed-order perturbation theory to be very reliable. For DIS sum rules, which have a leading infra-red renormalon singularity, the performance is rather poor. We show that QCD Minkowski observables such as the R and R(_r) are completely determined by the EC beta-function, p(x), corresponding to the Euclidean QCD vacuum polarization Adler D-function, together with the NLO perturbative coefficient of D. An efficient numerical algorithm is given for evaluating R, Rr from a weighted contour integration of D(se(^10)) around a circle in the complex squared energy .s-plane, with p(x) used to evolve in s around the contour. The difference between the R, R(_r) constructed using the NNLO and leading-b resummed versions of pi(x) provides an estimate of the uncertainty due to the uncalculated higher order corrections. We estimate that at LEP energies ideal data on the R-ratio could determine a(_s)(M(_2)(_Z)) to three-significant figures. For R(_r) we estimate a theoretical uncertainty δa(_s)(m(^2)(_r)) ~ 0.001, corresponding to δa(_s)(m(^2)(_r)) ~ 0.002. This encouragingly small uncertainty is much less than has recently been deduced from comparison with the analogous naive all-orders resummation, which we demonstrate to be extremely RS dependent and hence misleading
Branching fraction and CP asymmetry of the decays B+→K0Sπ+ and B+→K0SK+
No full textAn analysis of B+ → K0
Sπ+ and B+ → K0
S K+ decays is performed with the LHCb experiment. The pp
collision data used correspond to integrated luminosities of 1 fb−1 and 2 fb−1 collected at centre-ofmass
energies of
√
s = 7 TeV and
√
s = 8 TeV, respectively. The ratio of branching fractions and the
direct CP asymmetries are measured to be B(B+ → K0
S K+
)/B(B+ → K0
Sπ+
) = 0.064 ± 0.009 (stat.) ±
0.004 (syst.), ACP(B+ → K0
Sπ+
) = −0.022 ± 0.025 (stat.) ± 0.010 (syst.) and ACP(B+ → K0
S K+
) =
−0.21 ± 0.14 (stat.) ± 0.01 (syst.). The data sample taken at
√
s = 7 TeV is used to search for
B+
c
→ K0
S K+ decays and results in the upper limit ( fc · B(B+
c
→ K0
S K+
))/( fu · B(B+ → K0
Sπ+
)) <
5.8 × 10−2 at 90% confidence level, where fc and fu denote the hadronisation fractions of a ¯b
quark
into a B+
c or a B+ meson, respectively
Effective lifetime measurements in the B-s(0) -> K+K-, B-0 -> K+pi(-) and B-s(0) -> pi K-+(-) decays
No full textMeasurements of the effective lifetimes in the View the MathML source, B0→K+π− and View the MathML source decays are presented using 1.0 fb−1 of pp collision data collected at a centre-of-mass energy of 7 TeV by the LHCb experiment. The analysis uses a data-driven approach to correct for the decay time acceptance.
This is the most precise determination to date of the effective lifetime in the View the MathML source decay and provides constraints on contributions from physics beyond the Standard Model to the View the MathML source mixing phase and the width difference ΔΓs
The perturbative QCD prediction to R revisited
No full text6 páginas, 3 figuras, 1 tabla.-- CERN-TH-6421-92 ; LAL-92-09.The perturbative QCD prediction to the total hadronic width of the tau lepton is re-examined. A more convergent perturbative expansion is proposed, which is associated with a smaller renormalization-scheme dependence and better-defined higher-order uncertainties.The work of A.P. has been partly supported by
CICYT, Spain, under grant No. AEN90-0040.Peer reviewe
Scale dependence and renormalon-inspired resummations for some QCD observables
No full textSince the advent of Quantum Field Theory (QFT) in the late 1940's, perturbation theory has become one of the most successful means of extracting phenomenologically useful information from QFT. In the ever-increasing enthusiasm for new phenomenological predictions, the mechanics of perturbation theory itself have taken a back seat. It is in this light that this thesis aims to investigate some of the more fundamental properties of perturbation theory. In the first part of this thesis, we develop the idea, suggested by C.J.Maxwell, that at any given order of Feynman diagram calculation for a QCD observable all renormalization group (RG)-predictable terms should be resummed to all-orders. This "complete" RG-improvement (CORGI) serves to separate the perturbation series into infinite subsets of terms which when summed are renormalization scheme (RS)-invariant. Crucially all ultraviolet logarithms involving the dimensionful parameter, Q, on which the observable depends are resummed, thereby building the correct Q-dependence. We extend this idea, and show for moments of leptoproduction structure functions that all dependence on the renormahzation and factorization scales disappears provided that all the ultraviolet logarithms involving the physical energy scale Q are completely resummed. The approach is closely related to Grunberg's method of Effective Charges. In the second part, we perform an all-orders resummation of the QCD Adler D-function for the vector correlator, in which the portion of perturbative coefficients containing the leading power of b, the first beta-function coefficient, is resummed to all-orders. To avoid a renormalization scale dependence when we match the resummation to the exactly known next-to-leading order (NLO), and next-NLO (NNLO) results, we employ the Complete Renormalization Group Improvement (CORGI) approach , removing all dependence on the renormalization scale. We can also obtain fixed-order CORGI results. Including suitable weight-functions we can numerically integrate these results for the D-function in the complex energy plane to obtain so-called "contour-improved" results for the ratio R and its tau decay analogue Rr. We use the difference between the all-orders and fixed-order (NNLO) results to estimate the uncertainty in αs(M2/z) extracted from Rr measurements, and find αs(M2/z) = 0.120±0.002. We also estimate the corresponding uncertainty in a{Ml) arising from hadronic corrections by considering the uncertainty in R(s), in the low-energy region, and compare with other estimates. Analogous resummations are also given for the scalar correlator. As an adjunct to these studies we show how fixed-order contour-improved results can be obtained analytically in closed form at the two-loop level in terms of the Lambert W-function and hypergeometric functions
Bibliographie sélective
No full textBoullier, D., La Télévision telle qu’on la parle, L’Harmattan, 2004. Brignier, J-M., Chavenon, H., Dupont-Ghestem, F., Dussaix, A-M., Haering, H., (préface d’Emmanuel Fraisse), Mesurer l’audience des médias. Du recueil des données au média planning, Paris, CESP, Dunod, 2002. Chaniac, R. et JezequeL, J.-P., La Télévision, Paris, La Découverte, Repères No 405, 2005. Katz, E. et Lazarsfeld, P. F., Influence personnelle (Post-face de Daniel Cefaï), Ina/Armand Colin, 2008. Le Diberder, A. et Coste..
Testing QCD with tau decays
No full text11 páginas, 2 tablas.-- CERN-TH-6422-92; LAL-92-10.The invariant-mass distribution of the hadronic state in τ decay can be used for testing fundamental aspects of the strong interactions. Using standard QCD techniques, it is possible to compute certain weighted integrals of the hadronic spectrum. We work out some QCD predictions which will be useful for analysing the data. They provide a direct way to simultaneously measure αs(mτ2) and the parameters characterizing the non-perturbative dynamics, allowing for a better control of the theoretical errors in the determination of αs(mτ2).This work has been supported in part by CICYT, Spain, under grant No. AEN90-0040.Peer reviewe
Measurement of relative branching fractions of B decays to ψ(2S) and J/ψ mesons
No full textThe relative rates of B-meson decays into J/ψ
and ψ(2S) mesons are measured for the three decay modes
in pp collisions recorded with the LHCb detector. The ratios
of branching fractions (B) are measured to be
B(B
+→ψ(2S)K
+
)
B(B+→J/ψK+)
= 0.594±0.006(stat)±0.016(syst)±0.015(Rψ),
B(B0→ψ(2S)K
∗0)
B(B0→J/ψK∗0)
= 0.476±0.014(stat)±0.010(syst)±0.012(Rψ),
B(B0
s
→ψ(2S)φ)
B(B0
s
→J/ψφ)
= 0.489±0.026(stat)±0.021(syst)±0.012(Rψ),
where the third uncertainty is from the ratio of the ψ(2S)
and J/ψ branching fractions to μ
+
μ
−.
1 Introductio
Evidence for the decay B0→J/ψω and measurement of the relative branching fractions of meson decays to J/ψη and J/ψη′
No full textFirst evidence of the B 0 → J / ψ ω decay is found and the B s 0 → J / ψ η and B s 0 → J / ψ η ′ decays are studied using a dataset corresponding to an integrated luminosity of 1.0 fb -1 collected by the LHCb experiment in proton-proton collisions at a centre-of-mass energy of sqrt(s) = 7 TeV. The branching fractions of these decays are measured relative to that of the B 0 → J / ψ ρ 0 decay:frac(B (B 0 → J / ψ ω), B (B 0 → J / ψ ρ 0)) = 0.89 ± 0.19 (stat) - 0.13 + 0.07 (syst),frac(B (B s 0 → J / ψ η), B (B 0 → J / ψ ρ 0)) = 14.0 ± 1.2 (stat) - 1.5 + 1.1 (syst) - 1.0 + 1.1 (frac(f d, f s)),frac(B (B s 0 → J / ψ η ′), B (B 0 → J / ψ ρ 0)) = 12.7 ± 1.1 (stat) - 1.3 + 0.5 (syst) - 0.9 + 1.0 (frac(f d, f s)), where the last uncertainty is due to the knowledge of f d / f s, the ratio of b-quark hadronization factors that accounts for the different production rate of B 0 and B s 0 mesons. The ratio of the branching fractions of B s 0 → J / ψ η ′ and B s 0 → J / ψ η decays is measured to befrac(B (B s 0 → J / ψ η ′), B (B s 0 → J / ψ η)) = 0.90 ± 0.09 (stat) - 0.02 + 0.06 (syst)
Measurement of the time-dependent CP asymmetry in B0 -> J/ψ KS0 decays
No full textThis Letter reports a measurement of the CP violation observables SJ/ψK0S and CJ/ψK0S in the decay channel B0→J/ψK0S performed with 1.0 fb−1 of pp collisions at s√=7 TeV collected by the LHCb experiment. The fit to the data yields SJ/ψK0S=0.73±0.07(stat)±0.04(syst) and CJ/ψK0S=0.03±0.09(stat)±0.01(syst). Both values are consistent with the current world averages and within
expectations from the Standard Model
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