Pertubative QCD Research Articles

Removing phase-space restrictions in factorized cross sections

Factorization in gauge theories holds at the amplitude or amplitude-squared level for states of given soft or collinear momenta. When performing phase-space integrals over such states, one would generally like to avoid putting in explicit cuts to separate soft from collinear momenta. Removing these cuts induces an overcounting of the soft-collinear region and adds new infrared-ultraviolet divergences in the collinear region. In this paper, we first present a regulator-independent subtraction algorithm for removing soft-collinear overlap at the amplitude level which may be useful in pertubative QCD. We then discuss how both the soft-collinear and infrared-ultraviolet overlap can be undone for certain observables in a way which respects factorization. Our discussion clarifies some of the subtleties in phase-space subtractions and includes a proof of the infrared finiteness of a suitably subtracted jet function. These results complete the connection between factorized QCD and Soft-Collinear Effective Theory.

Analysis of BFormula -> J/ a1(1260)+ in a perturbative QCD approach

In this paper, we analyse the hadronic decay of B to J a1(1260) in pertubative QCD approach (pQCD), where a1(1260) is a axial-vector meson and J{psi} is a vector meson.

Pertubative QCD in Bs $ \rightarrow$ $ \phi$ $ \phi$

In this research, the hadronic decay of \( B_{s}\rightarrow \phi \phi\) is analyzed by using the QCD factorization (QCDF) and pertubative QCD (pQCD). The experimental branching ratio of this decay is \( 1.9\times 10^{-5}\) and our results according to the QCDF method and pQCD are \( (0.05\pm 0.02)\times 10^{-5}\) and \( (1.7\pm 0.01)\times 10^{-5}\) , respectively. We consider that the branching ratio of pQCD is in agreement with available experimental data.

Combination of measurements of the top-quark pair production cross section from the Tevatron Collider

We combine six measurements of the inclusive top-quark pair (ttbar) production cross section (sigma_ttbar) from data collected with the CDF and D0 detectors at the Fermilab Tevatron with proton anti-proton collisions at sqrt{s} = 1.96 TeV. The data correspond to integrated luminosities of up to 8.8 fb^{-1}. We obtain a value of sigma_ttbar = 7.60 \pm 0.41 pb for a top-quark mass of m_t=172.5 GeV. The contributions to the uncertainty are 0.20 pb from statistical sources, 0.29 pb from systematic sources, and 0.21 pb from the uncertainty on the integrated luminosity. The result is in good agreement with the standard model expectation of 7.35^{+0.28}_{-0.33} pb at NNLO+NNLL in pertubative QCD.

THE HEART OF FACTORIZATION

Factorization is at the heart of nearly any calculation in pertubative QCD. It follows from the universal behavior of gauge theories in soft and collinear limits. This talk gives a summary of recent progress on producing a more transparent understanding of factorization and connecting traditional approaches to those of Soft-Collinear Effective Theory. The main result is the formulation and proof, at tree-level, of a factorization formula in QCD. The proof exploits the many advantages of spinor helicity methods, but does not use any effective field theory tricks. Once the factorization formula is proven, the transition to an effective theory description is effortless.

Relativistic corrections to double charmonium production in high energy proton-proton interaction

On the basis of pertubative QCD and the relativistic quark model we calculate relativistic corrections to the process of pair $J/\psi$ production in proton-proton collisions at LHC energy $\sqrt S=7$ TeV. Relativistic terms in the production amplitude connected with the relative motion of heavy quarks and the transformation law of the bound state wave functions to the reference frame of moving $J/\psi$ mesons are taken into account. For the gluon and quark propagators entering the amplitude we use a truncated expansion in relative quark momenta up to the second order. Relativistic corrections to the quark bound state wave functions are considered by means of the Breit-like potential. It turns out that the examined effects decrease initial nonrelativistic cross section more than two times. The final result lies below the experimental value measured by LHCb.

Open heavy flavour production at HERA

Abstract Selected results from open heavy flavour production at HERA are presented. The production of charm and bottom quarks is discussed and compared with predictions from pertubative QCD. The fragmentation fractions of charm quarks into several charmed mesons are compared between ep and e+e− collisions.

STUDY OF DIRECT-PHOTON AND PION PRODUCTION

We present comparisons of high-pT photon and pion production in hadronic interactions with expectations from next-to-leading order pertubative QCD (NLO pQCD). We also comment on phenomenological models of kT smearing (which approximate effects of additional soft-gluon emission) and on the status of resummation calculations.

The weak parity-violating pion-nucleon coupling

We use QCD sum rules to obtain the weak parity-violating pion-nucleon coupling constant f πNN . We find that f πNN ≈ 2 × 10 −8, about an order of magnitude smaller than the “best estimates” based on quark models. This result follows from the cancellation between pertubative and nonperturbative QCD processes not found in quark models, but explicit in the QCD sum rule method. Our result is consistent with the experimental upper limit found from 18F parity-violating measurements.

Testing J/ psi production and decay properties in e+e- annihilation.

Within the framework of pertubative QCD we calculate inclusive [ital J]/[psi] production using the color singlet model.'' Predictions for the total cross section including the leading corrections from initial state radiation and the [psi][prime] contribution are given. We analyze the production and decay properties by considering suitable lepton-hadron correlations. It is shown that the production and the decay process of a [ital J]/[psi] are described by four structure functions, respectively. We give analytical expressions for these structure functions and suggest energy-dependent coefficients of angular distributions as a test for the model. Finally we give detailed predictions for an energy of 10.5 GeV that could be tested at the CLEO-II experiment. The effect of arbitrarily polarized electron and/or positron beams is included.

JET CHARGE CROSS SECTION METHOD

We propose in this paper a seheme ealled Jet Charge Gross Section Method. In this method, the results of ealculations in pertubative QCD a.re not only less affected by the sea quark and gluon distribution functions in hadrons, just like in the calculations of cross section difference of hadrons: dσ(AB→h+X)-dσ(AB→h-X), but also independent of any fragmentation functions of partons. Furthermore, it is possible to measure the mean electric charges of various quark jets indirectly without distinguishing the quark flavors.