Fermilab Tevatron Energies Research Articles

DirectJ/ψandψ′hadroproduction via fragmentation in the collinear parton model andkT-factorization approach

The ${p}_{T}$ spectra for direct $J/\ensuremath{\psi}$ and ${\ensuremath{\psi}}^{\ensuremath{'}}$ hadroproduction at Fermilab Tevatron energy are calculated within the framework of the NRQCD formalism and the fragmentation model in the collinear parton model as well as in the ${k}_{T}$-factorization approach. We describe the Collider Detector at Fermilab data and obtain a good agreement between the predictions of the parton model and the ${k}_{T}$-factorization approach. We perform the calculations using the relevant leading order in ${\ensuremath{\alpha}}_{s}$ hard amplitudes and taking equal values of the long-distance matrix elements for both models.

Diffractive hadroproduction of dijets andW’s at the Fermilab Tevatron collider and the Pomeron structure function

Results from a phenomenological analysis of dijet and W hard diffractive hadroproduction at Fermilab Tevatron energies are reported. The theoretical framework employed here is a modified version of the Ingelman-Schlein approach which includes DGLAP-evolved structure functions. Different from what has been achieved by the DESY ep HERA reactions, a reasonable overall description of such diffractive hadron processes is obtained only when a complex, quark-rich Pomeron structure function is employed in the calculation.

Rapidity gaps and production of minijets in high-energy hadronic collisions

High-energy hadronic interactions can produce a final state characterized by minijets separated by a large gap in the rapidity distribution of the produced secondary particles. We discuss the process by keeping in mind the possibility of having multiple parton collisions in the hadronic interaction. At Fermilab Tevatron energy the correction to the single scattering term induced by the presence of multiparton interactions is large for transverse momenta smaller than 6 GeV.

Signals for Double Parton Scattering at the Fermilab Tevatron.

Four double-parton scattering processes are examined at the Fermilab Tevatron energy. With optimized kinematical cuts and realistic parton level simulation for both signals and backgrounds, we find large samples of four-jet and three-jet+one-photon events with signal to background ratio being 20\%-30\%, and much cleaner signals from two-jet+two-photon and two-jet+$e^+e^-$ final states. The last channel may provide the first unambiguous observation of multiple parton interactions, even with the existing data sample accumulated by the Tevatron collider experiments.

Jet analysis by neural networks in high energy hadron-hadron collisions

We study the possibility to employ neural networks to simulate jet clustering procedures in high energy hadron-hadron collisions. We concentrate our analysis on the Fermilab Tevatron energy and on the k t] algorithm. We employ both supervised and unsupervised neural networks. In the first case we consider a multilayer feed-forward network trained by the backpropagation algorithm: our results show that these networks can satisfactorily simulate the relevant features of the k t] algorithm. We consider also unsupervised learning, where the neural network autonomously organizes the events in clusters. The results of this analysis are discussed and compared with the supervised approach.

Balitsky-Fadin-Kuraev-Lipatov approximation versus O( alpha s3) corrections to large-rapidity dijet production.

We examine dijet production at large rapidity intervals at Fermilab Tevatron energies by comparing an exact scrO(${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}^{3}$) calculation with the Balitsky-Fadin-Kuraev-Lipatov (BFKL) approximation, which resums the leading powers of the rapidity interval y to all orders in ${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}$. We analyze the dependence of the exact scrO(${\mathrm{\ensuremath{\alpha}}}_{\mathit{s}}^{3}$) calculation on the jet cone size as a function of y, and use this cross section to define an ``effective rapidity'' y^ which reduces the error that the largy-y approximation induces on the kinematics. Using y^ in the BFKL resummation, we reexamine jet production at large transverse momenta and the transverse momentum decorrelation of the tagging jets. We find less dramatic, but still significant, effects than found previously using the large-y approximation.

Heavy quark yields at high energies

It is shown that the absorption correction due to gluon rescattering changes drastically the value of the cross section for heavy quark production at s ⪆0.5 TeV and also the ratio of c and b cross sections. The cross section for b-quark production, estimated from the data on muon and J/ψ production should be increased by a factor of about 2–2.5 at Fermilab-Tevatron energies.

Measuring the WWZ coupling at the Fermilab Tevatron.

Hadroproduction of weak-boson pairs ${W}^{\ifmmode\pm\else\textpm\fi{}}Z$ and ${W}^{+}{W}^{\ensuremath{-}}$ is studied quantitatively at Fermilab Tevatron energies. Although the cross sections are expected to be small in the standard model, they grow substantially with nonstandard $\mathrm{WWZ}$ and $\mathrm{WW}\ensuremath{\gamma}$ couplings even after careful consideration of unitarity constraints. With 4.7 ${\mathrm{pb}}^{\ensuremath{-}1}$ of data at hand, the Collider Detector at Fermilab Collaboration may already be sensitive to nonstandard $\mathrm{WWZ}$ couplings if $|{\ensuremath{\kappa}}_{Z}\ensuremath{-}1|>1.3$ or $|{\ensuremath{\lambda}}_{Z}|>1.0 (90% \mathrm{C}.\mathrm{L}.)$ from $Z(\ensuremath{\rightarrow}{l}^{+}{l}^{\ensuremath{-}})$ plus dijet and $W(\ensuremath{\rightarrow}l\ensuremath{\nu})$ plus dijet event rates or from leptonic decays of both weak bosons. With 100 ${\mathrm{pb}}^{\ensuremath{-}1}$ of data, purely leptonic modes can be used to extend the sensitivity to $|{\ensuremath{\lambda}}_{Z}|>0.4 (90% \mathrm{C}.\mathrm{L}.)$, which is comparable and complementary to the ${e}^{+}{e}^{\ensuremath{-}}\ensuremath{\rightarrow}{W}^{+}{W}^{\ensuremath{-}}$ experiments at the CERN collider LEP II.

Weak-boson production at Fermilab Tevatron energies.

Weak bosons are produced in hadron collisions by quarks and antiquarks with average fractional momenta $x=\frac{{M}_{W,Z}}{\sqrt{s}}$. When $\sqrt{s}$ is increased by a factor of 3, the average value of $x$ shifts from about 0.15 at CERN $p\overline{p}$ collider energies to 0.05 at the Fermilab Tevatron, i.e., from a regime where valence quarks dominate to a regime where sea quarks dominate the production features of weak bosons. We investigate systematically the problem of calculating $W$ and $Z$ cross sections in this new regime, paying special attention to the ratio $\frac{\ensuremath{\sigma}(W)}{\ensuremath{\sigma}(Z)}$ which is necessary for determining the ratio of totals widths $\frac{\ensuremath{\Gamma}(W)}{\ensuremath{\Gamma}(Z)}$ from hadron-collider data. We emphasize that with the increased statistics in $p\overline{p}$ experiments, the parton fluxes responsible for the production of weak bosons can be controlled internally by a study of the asymmetry of the rapidity distribution of the $W$ boson. We derive a relation between the asymmetry in ${W}^{\ifmmode\pm\else\textpm\fi{}}$ rapidity distributions and the ratio $\frac{{F}_{2}^{n}}{{F}_{2}^{p}}$ of deep-inelastic lepton-hadron scattering structure functions. The production rates of weak bosons also become sensitive to production via the charm quark. This calculation raises some interesting theoretical issues. We discuss both problems in detail. We also study the implications of our results for the determination of the number of light neutrinos and the mass of the top quark via a measurement of the $W$-to-$Z$ event ratio.

Lepton-pair production in proton-nucleon collisions as a means of testing the standard electroweak model.

We use the standard Glashow-Salam-Weinberg SU(2)/sub L/ x U(1) electroweak theory to calculate the charge asymmetry expected for lepton-pair production in proton-nucleon collisions at various center-of-mass energies. Specifically, we discuss the prospect of measuring charge asymmetry as a means to determine the electroweak mixing parameter sin/sup 2/theta/sub W/, using a proton-beam energy of around 800 GeV. As motivated by the observation that an additional (nonstandard) gauge boson in a low-energy theory appears naturally in the E/sub 8/ x E/sub 8/ superstring studies, we also adopt an extended SU(2)/sub L/ x U(1) x U(1) electroweak model. To study manifestations of the nonstandard gauge boson, we consider a fixed-target experiment at a few TeV as well as for a proton-antiproton or proton-proton collider experiment in the few-TeV range. In particular, the behavior of charge asymmetry near the standard Z/sup 0/ pole is investigated as a possible means to determine the Z/sup 0/-Z'/sup 0/ mixing parameter sinphi. The possibility of a direct search for the nonstandard gauge boson Z'/sup 0/ at Fermilab Tevatron energies is also briefly discussed.

Testing couplings of three vector bosons through hadron collisions at Fermilab Tevatron energy.

A test for vector-boson trilinear couplings through hadron collisions at Fermilab Tevatron energy is investigated. The contributions from the standard model to the polarization cross section for ${W}^{+}$${W}^{\mathrm{\ensuremath{-}}}$ pair production process are given and deviations from the standard model are considered. It is pointed out that some effects may be observable in Tevatron collider experiments.

Gauginos as a signal for supersymmetry at pp-bar colliders.

We present a comprehensive analysis of the signals from the pair production of the mass eigenstates (W-italic-tilde, Z-italic-tilde, and gamma-tilde) of the gauge-Higgs-fermion system for both CERN and Fermilab Tevatron energies, assuming that gamma-tilde is relatively light (approx. ..W-italic-tildeZ-italic-tilde and Z/sup 0/..-->..W-tildeW-tilde-bar are kinematically accessible, there is a substantial level of (i) jet(s) + p-italic-dash-bar/sub T/ events from the hadronic decays of W-italic-tilde and Z-italic-tilde, (ii) a comparable level of background-free multilepton + p-italic-dash-bar/sub T/ events, essentially free from hadronic activity from the leptonic decay of W-italic-tilde and Z-italic-tilde, and (iii) jet(s) + lepton(s) + p-italic-dash-bar/sub T/ events from the leptonic decay of one of the gauginos and hadronic decay of the other. We present rates and distributions for these events and show that a conclusive absence of such amore » signal could lead to more stringent direct mass limits on masses of the W-italic-tilde and Z-italic-tilde than are currently available.« less

Order- alpha s QCD effects in technipion production and decay.

The three-body final-state decay modes of the technipion P/sup 0/' which contribute as order-..cap alpha../sub s/ corrections to two-body modes are calculated. The three-body modes are large, and the three-gluon final state is the dominant mode for M/sub P//sub 35 GeV. Related effects in large-p/sub perpendicular/ production of P/sup 0/' in p-p collisions are calculated, and the cross section at y = 0 is found to be comparable to those of W/sup +- / and Z production at Superconducting Super Collider energies but much smaller at Fermilab Tevatron energies. It is pointed out that similar results should be expected for scalars in other models, such as the standard Higgs or compositeness models.

Production of heavy quarks from W-gluon fusion.

We show that heavy-quark production via W-italic-gluon fusion in high-energy p-italicp-italic and p-barp-italic collisions is an important source of the heavier member of an SU(2)/sub L-italic/ doublet of quarks if the mass splitting within the doublet is large. W-italic-gluon fusion exceeds the strong production of heavy quarks for mass splittings greater than 300--350 GeV at ..sqrt..s-italic = 10 TeV and 400--450 GeV at ..sqrt..s-italic = 40 TeV. An alternative way to regard W-italic-gluon fusion is as the production of the heavier quark by fusing its light partner with a W-italic boson. We use a distribution function for the light partner to show that this process gives results which agree qualitatively with W-italic-gluon fusion. We also discuss the Drell-Yan production of an SU(2)/sub L-italic/ doublet of heavy quarks via a virtual W-italic boson and corrections to this process from initial gluons. We find that at the Fermilab Tevatron energy ..sqrt..s-italic = 2 TeV, W-italic-gluon fusion exceeds the Drell-Yan production of top quarks for masses above 100 GeV.

Asymmetry effects of supersymmetric QCD in dilepton production

A Callan-Gross-type relation between the structure functions determining dilepton production, which is satisfied by the basic O(1) and O(..cap alpha../sub s/) subprocesses of conventional QCD, is investigated in supersymmetric QCD (SQCD). It is found that SQCD subprocesses, mainly due to the presence of scalar quarks, strongly violate this relation, thus leading to sizable asymmetry effects in the angular distribution of dileptons. As an illustration, calculations are carried for p-barp..-->..l/sup +/l/sup -/+X at CERN collider and Fermilab Tevatron energies (..sqrt..s = 540 and 1600 GeV, respectively) and for pp..-->..l/sup +/l/sup -/+X at Tevatron energy; with scalar quarks of mass 20 GeV and light gluinos, asymmetries in the range of 20%--5% are found. The significance of the effects as a test of the short-distance structure of SQCD (valence part of the scalar-quark distribution) is discussed.

Direct photon production at CERN ISR, CERN collider, and Fermilab Tevatron energies

Predictions are presented for large-${p}_{T}$ direct photon production in $\overline{p}p$ collisions at CERN ISR ($\sqrt{s}=63$ GeV), CERN collider (540 GeV), and Fermilab Tevatron (1600 GeV) energies in perturbative QCD. Two sets of gluon and other parton distributions determined from deep-inelastic scattering are used. Higher-order corrections ($K$ factors) are approximately taken into account. Contributions from photon bremsstrahlung graphs and the related fractions of photon events accompanied by hadrons are presented and discussed in detail.