On theQ 2-extrapolations of AF-corrected Drell-Yan processes

TheQ ²-dependence of the integrated Drell-Yan cross sections with asymptotic freedom (AF) corrections is investigated forQ ²≦ 10⁷ GeV². Existing AF-parametrizations lead to widely differing predictions. Lowest order QCD indicates a strong dependence on thex-behaviour of the input momentum distributions (atQ ²=Q ₀ ² ) which for sea quarks and gluons are guesses. Consequently, AF-corrected Drell-Yan extrapolations toW-energies and beyond involve considerable uncertainties.     

Higher-order QCD corrections in prompt photon production

We exhibit a method for simultaneously treating recoil and threshold corrections in single-photon inclusive cross sections, working within the formalism of collinear factorization. This approach conserves both the energy and transverse momentum of resummed radiation. At moderate p(T), we find the potential for substantial enhancements from higher-order perturbative and power-law nonperturbative corrections.     

Measurement of the inclusive jet cross section using the kT algorithm in pp collisions at (square root s)=1.96 TeV

We report on a measurement of the inclusive jet production cross section as a function of the jet transverse momentum in pp collisions at (square root s)=1.96 TeV using data collected with the upgraded Collider Detector at Fermilab in run II, corresponding to an integrated luminosity of 385 pb-1. The measurement is carried out for jets with rapidity 0.1相似文献   

The dilepton-production cross section in principal value resummation

Using a recent calculation of the perturbative hard part for dilepton production that sums large threshold corrections to all orders in perturbative QCD, we compute the corresponding cross sections. The hard part has been evaluated using principal value resummation and contains all singular momentum-dependent corrections. We also include a resummation of large Sudakov terms, which are independent of parton momenta. We give predictions for the dilepton-mass distribution, the rapidity distribution and the rapidity-integrated K-factor at fixed-target energies and compare with various experimental results in several kinematic regimes. We find that principal value resummation produces cross sections that are finite and well-behaved. For both protons and anti-protons on fixed targets, the resummed cross sections are, in general, in excellent agreement with the data.     

QCD corrections at the Z0

All QCD corrections in hadron-initiated lepton-pair production are questionable for Q²?100 GeV² since a perturbative expansion is not justified in this kinematical range. Application of the renormalization group (RG) program at Z₀ energies is meaningful. We exhibit the size of the non-log correction terms, introduce experimental tests, and point to the assumptiona inherent in Drell-Yan (DY) type analyses.     

Diffractive open charm production at HERA

We use perturbative QCD to calculate the cross sections σ^LT for the diffractive production of open charm (cc) from longitudinally and transversely polarised photons (of virtuality Q2) incident at high energy (√S) on a proton target. We study both the Q ² and M ² dependence of the cross sections, where M is the invariant mass of the cc pair. Surprisingly, the result for σ^T, as well as for σ^L, is perturbatively stable. We estimate higher-order corrections and find a sizeable enhancement of the cross sections. The cross sections depend on the square of the gluon density g(x, K²), and we show that the observation of open charm at the HERA electron-proton collider can act as a sensitive probe of the gluon distribution for x = (Q ² + M ²)/s and scale K2 = (M ²c + )(1 + Q ²/M ²) where the average quark transverse momentum squared 〈k ₂ ^t ~ (M ²c . As compared to diffractive J/#x03C8; production, open charm has the advantage that it is independent of the non-perturbative ambiguities arising from the J/#x03C8; wave function. We estimate the fraction of diffractive events that arise from cc¯ production.     

Vector boson production at colliders: A theoretical reappraisal

We study the production of vector bosons in hadron-hadron collisions via the Drell-Yan mechanism in QCD. Our treatment of the transverse momentum and rapidity distributions of the produced bosons takes all available theoretical principles and results into account in a systematic way. The resulting q_T distribution reduces to the perturbative limit for large q_T, includes the summation of soft gluons and reproduces the known results for the total cross section. A full numerical analysis of W and Z cross sections at collider and tevatron energies is made.     

What lattice calculations can tell us about the gluon gas

Higher order perturbative and nonperturbative corrections to the grand potential of hot QCD are considered qualitatively Comparing with lattice results, it is argued that the nonperturbative parts are small but that the O(g⁴) term in $\text{Ω}$ is large and positive.     

Large perturbative corrections to the Drell-Yan process in QCD

The total cross section $\text{d}\text{σ}\text{d}\text{Q}{}^2$ for the production of a muon pair of invariant mass Q²via the Drell-Yan mechanism and the Feynman x_F differential cross section $\text{d}{}^2\text{σ}\text{d}\text{Q}{}^2\text{d}\text{x}{}_{\mathrm{<mtext>F</mtext>}}$ are calculated in QCD retaining all terms up to order α_s(Q². The calculations are performed using dimensional regularisation of the intermediary infrared and collinear singularities, but we present our results in a form independent of such details. The corrections to both these cross sections coming from radiative corrections to the lowest-order $\text{q}\text{q}$ annihilation diagram are found to be large at present values of Q² and S when the cross section is expressed in terms of parton densities derived from leptonproduction, for all Drell-Yan processes of practical interest. Numerical calculations are presented which show, for any reasonable parametrisation of the parton densities, that the neglect of higher-order terms in α_s(Q²) is not justifiable. The quark-gluon diagrams on the other hand give small corrections in this order and are only important for PP scattering.     

Perturbative QCD corrections in Drell-Yan processes

The influence of lowest-order QCD corrections on the Drell-Yan cross section $\text{Q}{}^4\text{(}\text{dσ}\text{dQ}{}^2\text{)(τ, Q}{}^2\text{)}$ is determined and compared with the asymptotic freedom (AF) corrections. The perturbative calculation exhibits the AF-characteristics of a (strongly) rising Q²-dependence for √τ?0.1 (qg-scattering) and falling for √τ?0.2 (qq?-annihilation). Qualitative agreement between the two calculation methods in the entire √τ-range is obtained with α_s = 0.3.     

How to regularize the infrared and mass singularities in QCD

The parton cross sections in perturbative QCD are mass (M) and infrared (IR) divergent. The singularities are prevented by on/off-shell parton masses or by dimensional regularization. All mass assignments must be chosen in agreement with Kinoshita's double-cut rule. The “off-shell” assignment, often used in the past, is incorrect. We demonstrate that Drell-Yan mass factorization in order α_S is independent of all regularizatiohns; the coffcient function is unique.     

Anomalous quark-gluon chromomagnetic interaction and high-energy ρ-meson electroproduction

It is shown that existence of a large anomalous chromomagnetic moment of quark induced by nonperturbative structure of QCD leads to the additional contribution to exclusive ρ-meson electroproduction off proton target. The contribution coming from new type of quark-gluon interaction to the ρ-meson production cross section for both transversal and longitudinal polarization of virtual photon is found. Such nonperturbative contribution together with conventional perturbative two-gluon exchange allows us to describe the experimental data at low Q ² for transversal polarization. However, in the longitudinal polarization case there is still some discrepancy with the data. The possible source of this deviation is discussed.     

QCD at lowQ 2—A correspondence relation for moments of structure functions

The precocious validity of QCD predictions in deep inelastic lepton nucleon scattering ande ⁺ e ^? annihilation is interpreted as a signal for an underlying “correspondence principle” relating perturbative and nonperturbative physics on theQ ² average. Correspondence relations for nonsinglet moments of deep inelastic structure functions are formulated, discussed and successfully tested against experiment. The relations provide an independent determination of the QCD ?-parameter from lowQ ² data in perfect agreement with results from largeQ ² analyses.     

A unified description of the perturbative photon structure function inx space

We give here in detail the derivation of the various contributions to the photon structure function in the perturbative region. The lowx region problem is pointed out and the QCD corrections to the hadronic part are discussed, including the quasi perturbative region of higher twist effects. The transition from highQ ² to lowQ ² is examined and the basis for a realistic and consistent simulation of higher order QCD processes is given.     

Heavy quark correlations ine + e − annihilations

In heavy quark jets the quark mass acts as a regulator of collinear singularities, making the quark momentum an infra-red safe variable in perturbative QCD. This allows a direct comparison of measured heavy hadron momentum spectra with perturbative calculations. We exploit the factorisation of heavy quark fragmentation to derive QCD predictions for momentum correlations between heavy hadrons produced ine ⁺ e ^? annihilations. We study the practical feasibility and model sensitivity of our approach using Monte Carlo simulations. Higher order perturbative corrections and contributions from non-perturbative effects are found to be at the level of 10%.     

Factorization for one-loop corrections in the Drell-Yan process

We study all the one-loop corrections to the Drell-Yan process in QCD, including gluon exchanges involving spectator partons. Our method demonstrates weak factorization at measured dimuon Q^T, and is suitable for generalization to higher orders.     

Light-Front Holography,Color Confinement,and Supersymmetric Features of QCD

Light-Front Quantization—Dirac’s “Front Form”—provides a physical, frame-independent formalism for hadron dynamics and structure. Observables such as structure functions, transverse momentum distributions, and distribution amplitudes are defined from the hadronic light-front wavefunctions. One obtains new insights into the hadronic spectrum, light-front wavefunctions, and the functional form of the QCD running coupling in the nonperturbative domain using light-front holography—the duality between the front form and AdS₅, the space of isometries of the conformal group. In addition, superconformal algebra leads to remarkable supersymmetric relations between mesons and baryons of the same parity. The mass scale \({\kappa}\) underlying confinement and hadron masses can be connected to the parameter \({\Lambda_{\overline {MS}}}\) in the QCD running coupling by matching the nonperturbative dynamics, as described by the effective conformal theory mapped to the light-front and its embedding in AdS space, to the perturbative QCD regime. The result is an effective coupling defined at all momenta. This matching of the high and low momentum transfer regimes determines a scale Q₀ which sets the interface between perturbative and nonperturbative hadron dynamics. The use of Q₀ to resolve the factorization scale uncertainty for structure functions and distribution amplitudes, in combination with the principle of maximal conformality for setting the renormalization scales, can greatly improve the precision of perturbative QCD predictions for collider phenomenology. The absence of vacuum excitations of the causal, frame-independent front form vacuum has important consequences for the cosmological constant. I also discuss evidence that the antishadowing of nuclear structure functions is non-universal; i.e., flavor dependent, and why shadowing and antishadowing phenomena may be incompatible with the momentum and other sum rules for nuclear parton distribution functions.     

Quark and gluon jets in the breit frame of lepton-nucleon scattering

Gluon bremsstrahlung,q→Gq, and quark pair production from gluons,laggy, in deep inelastic reactions is investigated in the Breit frame (moving alongQ in the laboratory). These QCD effects diminish the overall forward momentum. There are also events with asingle largep _⊥ forward jet. One spectacular class of events is predicted in which no forward going hadrons emerge, in the Breit frame. These effects are not mimicked by nonperturbative (limitedp _⊥) parton jets at large but attainableQ ².