Light Partons Research Articles

Heavy quark radiative energy loss in QCD matter

Heavy quark medium induced radiative energy loss is derived to all orders in opacity, ( L/ λ g ) n . The analytic expression generalizes the GLV opacity expansion for massless quanta to heavy quarks with mass M in a QCD plasma with a gluon dispersion characterized by an asymptotic plasmon mass, m g=gT/ 2 . Remarkably, we find that the general result is obtained by simply shifting all frequencies in the GLV series by ( m g 2+ x 2 M 2)/(2 xE). Numerical evaluation of the first order in opacity energy loss shows that both charm and bottom energy losses are much closer to the incoherent radiation limit than light partons in nuclear collisions at both RHIC and LHC energies. However, the radiation lengths of heavy quarks remain large compared to nuclear dimensions and hence high p T heavy quark production is volume rather than surface dominated.

Bproduction asymmetries in perturbativeQCD

This paper explores a new mechanism for B production in which a b quark combines with a light parton from the hard-scattering process before hadronizing into the B hadron. This recombination mechanism can be calculated within perturbative QCD up to a few nonperturbative constants. Though suppressed at large transverse momentum by a factor Lambda_QCD m_b/p_t^2 relative to b quark fragmentation production, it can be important at large rapidities. A signature for this heavy-quark recombination mechanism in proton-antiproton colliders is the presence of rapidity asymmetries in B cross sections. Given reasonable assumptions about the size of nonperturbative parameters entering the calculation, we find that the asymmetries are only significant for rapidities larger than those currently probed by collider experiments.

Ordered analysis of heavy flavor production in deep-inelastic scattering

At low ${Q}^{2},$ charm production in deep-inelastic scattering is adequately described by assuming generation in electroweak boson-light parton scattering (dominantly boson-gluon fusion), which naturally incorporates the correct threshold behavior. At high ${Q}^{2}$ this description is inadequate, since it does not sum logs in ${Q}^{2}{/m}_{c}^{2},$ and is replaced by the treatment of the charm quark as a light parton. We show how the problem of going from one description to the other can be solved in a satisfactory manner to all orders. The key ingredient is the constraint of matching the evolution of the physical structure function ${F}_{2}$ order by order in ${\ensuremath{\alpha}}_{s}{(Q}^{2}),$ in addition, to the matching of the value of ${F}_{2}$ itself. This leads to new expressions for the coefficient functions associated with the charm parton, which are unique in incorporating both the correct threshold and asymptotic behaviors at each order in perturbation theory. The use of these improved coefficients leads to an improvement in global fits and an excellent description of the observed ${F}_{2,\mathrm{charm}}.$

Next-to-leading order photon structure functions

We present a complete next-to-leading order (NLO) QCD analysis of the photon structure functions F 2 γ ( x, Q 2 ) and F L γ ( x, Q 2 ). All contributions due to light partons and heavy quarks have been included.

Heavy-quark correlations in hadron collisions at next-to-leading order

We present a calculation of the fully exclusive parton cross sections for heavy-quark production at order O( α S 3) in QCD. Our result includes the Born cross section for producing a Q Q pair, of order O( α S 2), the virtual corrections to the Born cross section, of order O( α S 3), and the cross section for producing a Q Q pair plus a light parton, of order O( α S 3). We can therefore compute distributions in which correlations among the heavy quarks (and, if present, the associated jet) are correctly taken into account up to order O( α S 3). We present some applications of phenomenological interest to top, bottom and charm production at hadron colliders.

Squark pair production mechanisms in [formula omitted] collisions

Through a phenomenological analysis in the transverse plane of a p p collision, we study different production mechanisms of squark pairs, decaying into a quark and a photino (missing energy): excitation of squarks inside the proton, fusion of light partons into squark pairs, and decay of heavier gluinos into squarks. The rates for missing transverse momentum are calculated and confronted with the monojet events observed at the CERN collider. In the framework of “minimal” supergravity models the excitation and fusion of squark pairs are disfavoured, whereas the dominant processes involve heavier gluinos and squarks cascading into photinos through intermediate squarks. The agreement with experimental data is better if heavier squarks and gluinos decay through an intermediate b-squark. We remark that energy dissipating effects during the cascade towards the final state tend to improve the comparison between theory and experiment.

Fragmentation of hadrons from heavy quark partons

A model is presented to describe hadron fragmentation off light and heavy partons. Fragmentation functions are parametrized by one variable. When a heavy parton of a new flavour fragments, a heavy hadron tends to carry away most of the parton momentum, leaving light hadron ( π and K) spectra softer than those from light partons.

DEEP INELASTIC PROCESSES WITH HADRONIC INITIAL STATES*

The application to hadron-hadron collisions of what is presumed to have been learned from elastic and inelastic electron scattering is discussed. Two subjects which perhaps typify the applications of deep inelastic physics to hadronic reactions: pp ..-->.. ..mu../sup +/..mu../sup -/(Q/sup 2/) + anything at high energy and large Q/sup 2/--from both the parton model and light cone viewpoint and Parton models of large angle elastic scattering, especially the interchange model of Blankenbecler, Brodsky, and Gunion are emphasized. Also covered briefly is inclusive production of hadrons at high momenta transverse to the beams at ISR. (JFP)

Hadron production in virtual photon-photon annihilation

In this article, we systematically study the amplitudes for annihilation of two virtual photons into hadron resonances and continuum states, a process measurable with colliding electron-positron beams. The asymtotic behavior of resonance form factors and their absolute scale follows from light cone or parton model considerations; definite predictions for inclusive two-photon annihilation are derived by the same methods.

Duality in the current propagator

The idea of local duality between generalized vector dominance and light cone algebra or parton model is applied to the propagator of vector currents. A set of sum rules reminiscent of the generalized first Weinberg sum rules but involving certain unknown parameters are derived. These parameters are fixed from the leptonic decay modes of ϱ, ω, ϕ. The sum rules are used to calculate the ω-ϕ mixing angle and thereby predict Γ(ϕ→ K K ) = 3.24±0.34 MeV , in agreement with experiment. Simple mass mixing or current mixing model for the vector mesons seem to be inconsistent with data. The total cross section for e + e −→hadrons is calculated in the scaling region. If scaling is achieved at s ⋍ 9–16 GeV 2 then it is argued that about 22% of the events in this energy region will include strange particles.