e+e− → γ + Hadrons

We give the spectrum in momentum and angle of directly produced γ's in e⁺e^? → γ + hadrons, and show that in QCD if p_⊥² of the recoil hadron jet relative to the photon is large, the leading logarithmic corrections to the lowest order result are absent. This process is therefore of great value in learning whether QCD actually governs quark dynamics.     

Finite corrections to quark fragmentation functions in perturbative QCD

The finite corrections of order α _s in perturbative QCD to the cross-sections for semi-inclusive hadron production from deep inelastic leptonhadron scattering and electron-positron annihilation are calculated. We define the effective quark fragmentation functions viae ⁺ e ^? → hadron +X including the finite terms in order to estimate these corrections for the reactions lepton + hadron → lepton + hadron + anything. Contrary to the leading term the next-to-leading order term does not factorize into parts depending on the target and the fragment, respectively. For the processese +p →e + π^± +X andv +p →μ ^? + π^± +X the finite corrections of order α _s turn out to be at most 20% in the range of momenta covered by present experiments.     

Jets in pp collisions: Radiation zeroes and the electric charges of colored quarks

Measurements involving more than one photon (e.g., γγ → hadrons, e⁺e^- → q$\text{q}$γ) can determine the electric charge of colored quarks rather than the color-averaged value measured, for example, by the R-value in e⁺e^- → hadrons. We draw attention to the fact that this measurement of the absolute quark charges can be performed in hadron collisions via the process qq → qqγ. The charge signature is dramatic due to the appearance of radiation zeroes whose position depends on the values of the electric charges. This can be exploited to perform such measurements with a detector lacking γ/π⁰ separation capability. Such experiments also study the interface of color and electromagnetic amplitudes and can further elucidate the QCD origin of jets.     

Hadron structure in the ladder model (II)

The (flavor non-singlet) probability Φ(k) to find a far-off-shell quark in a hadron is obtained in the renormalization group improved ladder model for QCD in the space-like axial gauge in the region k_T²??2k·P, extending an earlier result for the region k_T²≈?2k·P. The resulting Drell-Yan cross section at measured Q_T agrees in the appropriate limit with that given by Parisi and Petronzio (and disagrees with the DDT form). By using a soft photon method in an abelian gauge theory, I argue that ladder diagrams with strong ordering of gluon q· P's in fact dominate Φ(k) in the high-energy limit considered.     

High-twist correction for inclusive hadron production in photon-photon collisions

The contribution of the QCD high-twist subprocess γq→πq′ to the inclusive cross section for e⁺ e^? → e⁺ e^? + π^± + X is calculated both in the PETRA/PEP and the LEP energy range, in the equivalent-photon approximation. Superposed on the Born cross section and its O(α_s) correction, it gives agreement with the data at relatively low transverse momenta. It is suggested that this high-twist contribution may be further isolated experimentally by triggering simultaneously on both a fast hadron and the opposite-side jet.     

Correlation with large transverse momentum photons and the gluon structure function

The inclusive two-particle cross section for the production of largep _T photons and opposite side charged hadrons in proton-proton collisions is examined in detail in the framework of QCD. The model, with the parametrization as in our previous work, agrees with the measured single photon spectra. Quantitative predictions are made for two-particle distributions to be measured at the ISR. The contributions due to the photon bremsstrahlung and the effects coming from the intrinsic constituent motion are estimated. It is shown that indeed the direct subprocess gluon+quark→photon+quark gives the dominant contribution in the experimentally relevant region of phase space, and that direct information about the gluon structure function can be obtained from such measurements.     

Quark jet as a trigger for gluon jet in deep inelastic muon scattering

We calculate, to first order in QCD, the probability for a fast hadron observed at laboratory angle θ _h from the virtual photon directionq, to originate from the quark of a quark-gluon final state. Given this probability, the average gluon energy and angle with respect toq are then calculated. This approach provides a method for gluon jet direction in forthcoming experiments.     

Hadron resonance gas and nonperturbative QCD vacuum at finite temperature

Nonperturbative QCD vacuum with two light quarks at finite temperature was studied in a hadron resonance-gas model. Temperature dependences of the quark and gluon condensates in the confined phase were obtained. It is shown that the quark condensate and one-half (chromoelectric component) of the gluon condensate are evaporated at the same temperature corresponding to the quark-hadron phase transition. With allowance for the temperature shift of hadron masses, the critical temperature was found to be T _c ?190 MeV.     

Kaon and pion ratio probes of jet quenching in nuclear collisions

Non-Abelian energy loss in quark gluon plasmas is shown to lead to novel hadron ratio suppression patterns in ultrarelativistic nuclear collisions. Here we investigate pion and kaon production in pp and AA collisions in a perturbative QCD frame, suppression pattern and hadron ratios. The K^?/K⁺ and K⁺/π⁺ ratios are found to be most sensitive to the opacity (density) of the plasma. Experimental data indicate that the fragmentation dominated pQCD region will be reached only at higher p _T; in an intermediate p _Tregion other particle production mechanisms dominate the K/π ratios.     

Electroproduction of η-meson at the S11 (1535) resonance

The reaction e^? + p → e^? + p + η has been studied in the region of the S₁₁ (1535)-resonance by detecting the recoil proton in coincidence with the scattered electron. The reaction has been observed at three four-momentum transfers of the virtual photon: q² = 0.2, 0.28 and 0.4 (GeV/c). First results of the differential cross section measurements are given and compared with quark model calculations.     

Radiative corrections to high-energy neutrino scattering

Motivated by precise neutrino experiments, we reconsider the electromagnetic radiative corrections to the data. We investigate the usefulness and demonstrate the simplicity of the “leading log” approximation: the calculation to order α ln (Q/μ), α ln (Q/m_q). Here Q is an energy scale of the overall process, μ is the lepton mass and m_q is a hadronic mass, the effective quark mass in a parton model. We identify those questions the answers to which do not depend on unknown hadron parameters like quark masses. The leading log radiative corrections to dδ/dy distributions and to suitably interpreted dδ/dx distributions are quark-mass independent. We improve upon the conventional leading log approximation and compute explicitly the largest terms that lie beyond the leading log level. In practice this means that our model-independent formulae, though approximate, are likely to be excellent estimates everywhere except at low energy or very large y. We point out that radiative corrections to measurements of deviations from the Callan-Gross relation and to measurements of the “sea” constituency of nucleons are gigantic. The QCD inspired study of deviations from scaling is of particular interest. We compute, beyond the leading log level, the radiative corrections to the QCD predictions.     

QCD calculation of large pT hadronic collision jet structure

We define an analogue of ”thrust” which enables the investigation of the jet structure of large p_T hadronic collisions via a perturbative QCD calculation. In this paper we present the result for the single subprocess qq → qq + X, where the quark flavours are distinct, to O(g⁶) in the strong coupling constant.     

Electron-positron pair production as a probe of chiral symmetry in a hot QCD plasma

Chiral symmetry in the high-temperature plasma phase of QCD is consistent with at least three qualitatively different quark dispersion relations. The rate for $\text{q}\text{q}\text{→}\text{e}{}^{\mathrm{+}}\text{e}{}^{\mathrm{?}}$, with zero total momentum of the e⁺e^? pair in the plasma rest frame is direct measure of the quark dispersion relation. For the three plausible dispersion relations the rates differ by orders of magnitude when the e⁺e^? invariant mass M < 2T.     

A new method to determine the electroweak couplings of individual light flavours at LEP

A method is presented for determining the yields and properties of individual light quark flavours in Z ⁰ decays that is essentially free of detailed assumptions about hadronisation. The method uses an equation system with the number of events which are single and double tagged by high energy hadrons as inputs. In addition, SU(2) isospin symmetry and the flavour independence of QCD are used to derive general relations between hadron production from the various primary light quarks. Assuming the branching fractions R _q of the Z ⁰ into down and strange quarks to be the same, five million hadronic Z ⁰ decays may allow precisions of δ(R _d = R _s)/(R _d = R _s) ～ 0.05 and δA _FB(d = s) ～ δA _fb(u) ～ 0.015 for the corresponding asymmetries. The method can be extended to include somewhat more model dependent symmetries of hadron production, which then allows the electroweak observables for each of the individual light quarks to be determined.     

Soft gluon corrections to large-pT direct photon and dilepton production

QCD O(α_s²) corrections to large-p_T real and virtual photon production from scattering in the quark-gluon channel are studied in the soft gluon limit. In this limit we carefully preserve the analytic structure of amplitudes in the small-momenta integration regions. The result is a rather large correction to the Born term which significantly improves the agreement with data on p + p → γ + X. The application of the approach to other processes is discussed and the dominant terms of the O(α_s²) correction to e⁺e^? → three jets are reproduced.     

Deep-inelastic one-particle inclusive processes in the parton model

We study the processes e + N → e′ + h + anything, e⁺ + e^? → h + anything, within the parton model of Landshoff, Polkinghorne and Short. Definite and peculiar predictions are obtained, also common to the Drell, Levy and Yan cut-off theory and to the λφ³ ladder model. These predictions can be summarized as follows: (i) in the photon fragmentation region the cross section factorizes after integration over transverse momentum of h; (ii) in the target fragmentation region Regge behaviour, valid at large ω, gives rise to double Regge exchange for small values of the Feynman variable y, but only up to $\text{y ??1/ω}$; (iii) the structure functions vanish at y = 0; (iv) total multiplicities in the first process increase like 1n ω for large ω, and in the second reach a finite limit at large q².In the case where only one SU(3) multiplet of partons is present (as in the quark parton model) unique relations, valid in the limit of exact SU(3), are found between the cross section and multiplicities in the first process and the analogous quantities for the forward region of the second process.     

ω Meson photoproduction from deuterium at 3.9 GeV photon energy

Measurements are presented of the cross sections for ω meson photoproduction from deuterium at an average photon energy of 3.9 GeV. The cross section for the coherent process γd → dω is found to be 1.4 ± 0.5 μb and that for the incoherent process γd → npω is 3.3 ± 0.7 μb. The forward differential cross section for γd → dω is estimated to be (dσ/dt)_t=0 = 54 ± 20 μb/GeV². This result indicates that the value of the A₂ exchange amplitude in ω photoproduction from hydrogen is less than that predicted from the measurements of the difference between the total hadron photoproduction cross sections from neutrons and protons. We note that the world data tend to show that the A₂ exchange amplitude is more consistent with zero than with the value needed to explain these differences. Measurements are also presented of the ratio of the coupling constants of the photon to the ? and ω mesons which tend to favour the vector dominance model proposed by Das, Mathur and Okubo with exact SU(3) symmetry at infinite energies.