IR-improved DGLAP-CS theory: Kernels, parton distributions, reduced cross sections

It is shown that exact, amplitude-based resummation allows IR-improvement of the usual DGLAP-CS theory. This results in a new set of kernels, parton distributions and attendant reduced cross sections, so that the QCD perturbative result for the respective hadron-hadron or lepton-hadron cross section is unchanged order-by-order in α_s at large squared-momentum transfers. We compare these new objects with their usual counter-parts and illustrate the effects of the IR-improvement in some phenomenological cases of interest with an eye toward precision applications in LHC physics scenarios.     

Scalar-pseudoscalar pair production at the Large Hadron Collider at NLO+NLL accuracy in QCD

We thoroughly investigate both transverse momentum and threshold resummation effects on scalar-pseudoscalar pair production via quark-antiquark annihilation at the \begin{document}$ 13 \; \text{TeV}$\end{document} Large Hadron Collider at QCD NLO+NLL accuracy. A factorization method is introduced to properly supplement the soft-gluon (threshold) resummation contribution from parton distribution functions to the resummed results obtained by the Collins-Soper-Sterman resummation approach. We find that the impact of the threshold-resummation improved PDFs is comparable to the resummation effect of the partonic matrix element and can even predominate in high invariant mass regions. Moreover, the loop-induced gluon-gluon fusion channel in the type-I two-Higgs-doublet model is considered in our calculations. The numerical results show that the electroweak production via quark-antiquark annihilation dominates over the gluon-initiated QCD production by \begin{document}$ 1 \sim 2$\end{document} orders of magnitude.     

Charm production and parton distributions

Recent accurate data on F ₂(x,Q) and on F ₂ ^c (x,Q) from HERA at small-x require a more precise treatment of charm production in the global analysis of parton distributions. We improve on existing global QCD analyses by implementing the leptoproduction formalism of Aivazis et al. which represents a natural generalization of the conventional zero mass QCD parton framework to include heavy quark mass effects. We also perform analyses based on the fixed-flavor-number scheme, which is widely used in the literature, and demonstrate their uses and limitations. We discuss the implications of the improved treatment of heavy quark mass effect in practical applications of PQCD and compare our results with recent related works.     

Light bottom squark and gluino confront electroweak precision measurements

We address the compatibility of a light sbottom (mass approximately 2-5.5 GeV) and a light gluino (mass approximately 12-16 GeV) with electroweak precision measurements. Such light particles have been suggested to explain the observed excess in the b quark production cross section at the Tevatron. The electroweak observables may be affected by the sbottom and gluino through the supersymmetric-QCD (SUSY-QCD) corrections to the Zbb vertex. We examine, in addition to the SUSY-QCD corrections, the gauge boson propagator corrections from the stop which are allowed to be light from the SU(2)(L) symmetry. We find that this scenario is strongly disfavored from electroweak precision measurements.     

Small-x behavior of parton distributions from the observed Froissart energy dependence of the deep-inelastic-scattering cross sections

We fit the reduced cross section for deep-inelastic electron scattering data to a three parameter ln2s fit, A + beta ln2(s/s0), where s = (Q2/x)(1-x) + m2, and Q2 is the virtuality of the exchanged photon. Over a wide range in Q2 (0.11 < or = Q2 < or = 1200 GeV2) all of the fits satisfy the logarithmic energy dependence of the Froissart bound. We can use these results to extrapolate to very large energies and hence to very small values of Bjorken x-well beyond the range accessible experimentally. As Q2-->infinity, the structure function F2(p)(x,Q2) exhibits Bjorken scaling, within experimental errors. We obtain new constraints on the behavior of quark and antiquark distribution functions at small x.     

Heavy gluino and squark decays atp \bar p collider

The recent limits, \(m_{\tilde g} , m_{\tilde q} \gtrsim 40\) , GeV for gluino and squark masses obtained from experiments at the collider are based on jet +p _T analysis, in the hypothesis that the gluino or the squark decays into photino+quarks with a branching ratio near to one. We show that this hypothesis is generally not justified for higher masses of the gluino and the squarks, 50 GeV \( \lesssim m_{\tilde g,\tilde q} \lesssim \) 150 GeV, relevant to present and future \(\bar p\) colliders. In an interesting range of the parameters we study the different decay modes and the related signatures, among which isolated leptons or photons in the final states.     

Hard exclusive meson production and nonforward parton distributions

We present an analysis of twist-2, leading order QCD amplitudes for hard exclusive leptoproduction of mesons in terms of double/nonforward parton distribution functions. After reviewing some general features of nonforward nucleon matrix elements of twist-2 QCD string operators, we propose a phenomenological model for quark and gluon nonforward distribution functions. The corresponding QCD evolution equations are solved in the leading logarithmic approximation for flavor nonsinglet distributions. We derive explicit expressions for hard exclusive , , and neutral vector meson production amplitudes and discuss general features of the corresponding cross sections. Received: 12 November 1997 / Published online: 26 February 1998     

Light gluino,light bottom squark scenario,and LEP predictions

The scenario of light gluinos and light sbottoms was advocated to explain the discrepancy between the measured and theoretical production of b quarks at the Tevatron. This scenario will have model-independent predictions for Z-->qqgg at the Z0 pole, and e(+)e(-)-->qqgg at LEPII. We show that the data for Z-->qqg*-->qqbb; at LEPI cannot constrain the scenario, because the ratio Gamma(Z-->qqgg)/Gamma(Z-->qqg*-->qqbb)=0.15-0.04 for m(g)=12-16 GeV is smaller than the uncertainty of the data. However, at LEPII the ratio sigma(e(+)e(-)-->qqgg)/sigma(e(+)e(-)-->qqg*-->qqbb) approximately 0.4-0.2 for m(g)=12-16 GeV, which may give an observable excess in qqbb events; especially, the 4b events.     

Geometric scaling,multiplicity distributions and cross sections

From a geometric picture of hadrons as extended objects we arrive at some universal features of high energy collisions. In this approach the mean multiplicity, as a function of s, and the KNO scaling function are universal, and asymptotically the ratio σ_elastic/σ_total is expected to be the same for all processes.     

Single squark production at e+e- colliders

Single squark production is examined at present and next-generation e⁺e^- machines. In addition to total cross sections, missing energy and angular distributions are discussed which could allow detection of these events despite rather small rates.     

Polarized parton distributions from NLO QCD analysis of world DIS and SIDIS data

The combined analysis of polarized DIS and SIDIS data is performed in NLO QCD. The new parametrization on polarized PDFs is constructed. The uncertainties on PDFs and their first moments are estimated applying the modified Hessian method. Special attention is paid to the impact of novel SIDIS data on the polarized distributions of light sea and strange quarks. In particular, the important question of polarized sea symmetry is studied in comparison with the latest results on this subject.     

Theoretical spectral distributions and total cross sections for neutral subthreshold pion production in carbon-carbon collisions

With use of a recently developed particle-hole theory for pion production from coherent isobar formation and decay, calculations of total cross sections and kinetic-energy distributions for neutral subthreshold pions produced in carbon-carbon collisions at incident energies below 100 MeV/nucleon are made and compared with recent experimental data.     

Generalized parton distributions with helicity flip

We show that for both quarks and gluons there are eight generalized parton distributions in the proton: four which conserve parton helicity and four which do not. We explain why time reversal invariance does not reduce this number from eight to six, as previously assumed in the literature. Received: 7 February 2001 / Published online: 23 March 2001