Search Results (31 total)

We study nuclear effects in charged current deep inelastic neutrino-iron scattering in the framework of a χ² analysis of parton distribution functions (PDFs). We extract a set of iron PDFs and show that under reasonable assumptions it is possible to constrain the valence, light sea, and strange quark distributions. Our iron PDFs are used to compute x_Bj-dependent and Q²-dependent nuclear correction factors for iron structure functions which are required in global analyses of free nucleon PDFs. We compare our results with nuclear correction factors from neutrino-nucleus scattering models and correction factors for ℓ^±-iron scattering. We find that, except for very high x_Bj, our correction factors differ in both shape and magnitude from the correction factors of the models and charged-lepton scattering.

New data sets have recently become available for neutrino and antineutrino deep inelastic scattering on nuclear targets and for inclusive dimuon production in pp and pd interactions. These data sets are sensitive to different combinations of parton distribution functions in the large-x region and, therefore, provide different constraints when incorporated into global parton distribution function fits. We compare and contrast the effects of these new data on parton distribution fits, with special emphasis on the effects at large x. The effects of the use of nuclear targets in the neutrino and antineutrino data sets are also investigated.

We evaluate the impact of heavy-quark masses on transverse momentum (q_T) distributions of W, Z, and supersymmetric neutral Higgs bosons at the Tevatron and LHC. The masses of charm and bottom quarks act as non-negligible momentum scales at small q_T and affect resummation of soft and collinear radiation. We point out inconsistencies in the treatment of heavy-flavor channels at small q_T in massless and fixed-flavor number factorization schemes, and formulate small-q_T resummation in a general-mass variable flavor number factorization scheme. The improved treatment of the quark-mass dependence leads to non-negligible effects in precision measurements of the W boson mass at the LHC and may cause observable modifications in production of Higgs bosons and other particles in heavy-quark scattering.

When scattering amplitudes are calculated in the double-logarithmic approximation, it is possible to relate the double-logarithmic on-shell and off-shell amplitudes. Explicit relations are obtained for scattering amplitudes in QED, QCD, and the electroweak standard model. The off-shell amplitudes are considered in the hard and the Regge kinematic limits. We compare our results in both the Feynman and Coulomb gauges.

Analysis of semi-inclusive hadroproduction in deep inelastic scattering suggests broadening of transverse momentum distributions at x below a few 10^-3, which can be modeled in the Collins-Soper-Sterman formalism by a modification of impact-parameter-dependent parton densities. We discuss the consequences of such a modification for the production of electroweak bosons at hadron-hadron colliders. If substantial small-x broadening is observed in forward Z⁰ boson production in the Tevatron Run-2, it will strongly affect predicted q_T distributions for W^±, Z⁰, and Higgs boson production at the Large Hadron Collider.

Light strongly interacting supersymmetric particles may be treated as partonic constituents of nucleons in high energy scattering processes. We construct parton distribution functions for protons in which a light gluino is included along with standard model quark, antiquark, and gluon constituents. A global analysis is performed of a large set of data from deep-inelastic lepton scattering, massive lepton pair and vector boson production, and hadron jet production at large values of transverse momentum. Constraints are obtained on the allowed range of gluino mass as a function of the value of the strong coupling strength α_s(M_Z) determined at the scale of the Z boson mass. We find that gluino masses as small as 10 GeV are admissible provided that α_s(M_Z)≥0.12. Current hadron scattering data are insensitive to the presence of gluinos heavier than ∼100–150  GeV.

A next-to-leading-order (NLO) calculation of neutrino cross sections, including power-suppressed mass terms, is used to evaluate the Paschos-Wolfenstein ratio, in order to better assess the validity and significance of the NuTeV anomaly. We study the shift of sin⁡²θ_W obtained in calculations with parton distribution function sets that allow s(x)≠s̄(x), enabled by recent neutrino dimuon data from CCFR and NuTeV. The extracted value of sin⁡²θ_W is closely correlated with the strangeness asymmetry. Taken together with recent developments of possible isospin violation and electroweak effects, our results suggest that the new dimuon data, the Weinberg angle measurement, and other data sets used in global QCD parton structure analysis can all be consistent within the standard model. A full NLO analysis of the actual experimental measurement will help to clarify this issue further.

Previously published CTEQ6 parton distributions adopt the conventional zero-mass parton scheme since the corresponding hard cross sections are universally available. For precision observables which are sensitive to charm and bottom quark mass effects, we provide in this paper an improved CTEQ6HQ parton distribution set determined in the more general variable flavor number scheme that incorporates heavy flavor mass effects. We describe in detail the QCD scheme and analysis procedure used, examine the predominant features of the new distributions, and compare them with previous distributions.

Differential distributions for heavy quark production depend on the heavy quark mass and other momentum scales, which can yield additional large logarithms and inhibit accurate predictions. Logarithms involving the heavy quark mass can be summed in heavy quark parton distribution functions in the Aivazis-Collins-Olness-Tung (ACOT) factorization scheme. A second class of logarithms involving the heavy-quark transverse momentum can be summed using an extension of the Collins-Soper-Sterman (CSS) formalism. We perform a systematic summation of logarithms of both types, thereby obtaining an accurate description of heavy-quark differential distributions at all energies. Our method essentially combines the ACOT and CSS approaches. As an example, we present angular distributions for bottom quarks produced in parity-conserving events at large momentum transfers Q at the DESY ep collider HERA.

Experimental analyses of charged current deep inelastic charm production—as observed through dimuon events in neutrino-iron scattering—measure the strangeness component of the nucleon sea. A complete analysis requires a Monte Carlo simulation to account for experimental detector acceptance effects; therefore, a fully differential theoretical calculation is necessary to provide complete kinematic information. We investigate the theoretical issues involved in calculating these differential distributions at next-leading order (NLO). Numerical results are presented for typical fixed-target kinematics. We present a corresponding FORTRAN code suitable for experimental NLO analysis.

The first measurements of ΔxF₃ are higher than current theoretical predictions. We investigate the sensitivity of these theoretical predictions upon a variety of factors including the renormalization scheme and scale, quark mass effects, higher twist, isospin violation, and PDF uncertainties.

We investigate a simplified version of the Aivazis, Collins, Olness, and Tung (ACOT) prescription for calculating deeply inelastic scattering from Q² values near the squared mass M_H² of a heavy quark to Q² much larger than M_H².

We consider laboratory experiments that can detect stable, neutral strongly interacting massive particles (SIMPs). We explore the SIMP annihilation cross section from its minimum value (restricted by cosmological bounds) to the barn range, and vary the mass values from a GeV to a TeV. We calculate, as a function of the SIMP-nucleon cross section, the minimum nucleon number A for which there should be binding in a nucleus. We consider accelerator mass spectrometry with a gold (A=200) target, and compute the likely abundance of anomalous gold nuclei if stable neutral SIMPs exist. We also consider the prospects and problems of detecting such particles at the Fermilab Tevatron. We estimate optimistically that such detection might be possible for SIMPs with SIMP-nucleon cross sections larger than 0.1 mbarn and masses between 25 and 50 GeV.

We compute the deeply virtual Compton scattering (DVCS) amplitude for forward and backward scattering in the asymptotic limit. Since this calculation does not assume ordering of the transverse momenta, it includes important logarithmic contributions that are beyond those summed by the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi evolution. These contributions lead to a powerlike behavior for the forward DVCS amplitude.

Existing calculations of heavy quark hadroproduction in perturbative QCD are either based on the conventional zero-mass perturbative QCD theory, valid for energy scales much higher than the quark mass, or on a next-to-leading order (NLO) fixed-flavor-number (FFN) scheme which is appropriate for energy scales comparable to the quark mass. We formulate this problem in the general mass variable-flavor-number scheme which incorporates initial (final) state heavy quark parton distribution (fragmentation) functions as well as exact mass dependence in the hard cross section. This formalism has the built-in feature of reducing to the FFN scheme near threshold and to the conventional zero-mass parton picture in the very high energy limit. Making use of existing calculations in the NLO FFN scheme, we obtain more complete results on bottom quark production in the general scheme to order α_s³ both for current accelerator energies and for CERN LHC. Modest improvement over the FFN results is observed in the reduced scale dependence of the cross section and in the increased magnitude of the cross section, in the direction of experimental measurement. It is shown that the general scheme represents a more efficient way of organizing the perturbation series, since the bulk of the large NLO (α_s³) FFN contribution to the single heavy-quark inclusive cross section is already contained in the (resummed) order α_s² “heavy flavor excitation” term in this scheme. Remaining limitations of the present calculation and possible improvements are discussed.

The uncertainty in the calculation of many important new processes at the Fermilab Tevatron and CERN LHC is dominated by that concerning the gluon distribution function. We investigate the uncertainty in the gluon distribution of the proton by systematically varying the gluon parameters in the global QCD analysis of parton distributions. The results depend critically on the parton momentum fraction x and the QCD scale Q². The uncertainties are presented for integrated gluon-gluon and gluon-quark luminosities for both the Tevatron and LHC as a function of sqrt[τ]=sqrt[x₁x₂]=sqrt[ŝ/s], the most relevant quantity for new particle production. The uncertainties are reasonably small, except for large x.

In a consistently formulated PQCD framework incorporating nonzero mass heavy quark partons, there is still the freedom to define parton distributions obeying either mass-independent or mass-dependent evolution equations, contrary to statements made in a recent paper. With properly matched hard cross sections, different choices merely correspond to different factorization schemes; they yield the same physical cross sections. We demonstrate this principle in a concrete order α_s calculation of the Deeply Inelastic Scattering charm structure function. We also examine the proper matching between parton definitions and subtractions in the hard cross section near threshold where the calculation is particularly sensitive to mass effects of the heavy quark. The results obtained from the general-mass formalism are quite stable against different choices of scale and exhibit a smooth transition in the threshold region (using either mass-independent or mass-dependent evolution), in contrast with results of another recently proposed scheme.

The impact of recent precision measurements of DIS structure functions and inclusive jet production at the Fermilab Tevatron on the global QCD analysis of parton distribution functions is studied in detail. Particular emphasis is placed on exploring the range of variation of the gluon distribution G(x,Q) allowed by these new data. The strong coupling of G(x,Q) with α_s is fully taken into account. A new generation of CTEQ parton distributions, CTEQ4, is presented. It consists of the three standard sets [modified minimal subtraction (MS̅ ), deep inelastic scattering (DIS), and leading order (LO)], a series that gives a range of parton distributions with corresponding α_s’s, and a set with a low starting value of Q. Previously obtained gluon distributions that are consistent with the high E_t jet cross section are also discussed in the context of this new global analysis.

Measurement of the distribution of hadronic energy in the final state in deeply inelastic electron scattering at DESY HERA can provide a good test of our understanding of perturbative QCD. For this purpose, we consider the energy distribution function, which can be computed without needing final state parton fragmentation functions. We compute this distribution function for finite transverse momentum q_T at order α_s, and use the results to sum the perturbation series to obtain a result valid for both large and small values of transverse momentum.

We compare the results of the fixed-flavor scheme calculation of Laenen, Riemersma, Smith, and van Neerven with the variable-flavor scheme calculation of Aivazis, Collins, Olness, and Tung for the case of neutral-current (photon-mediated) heavy-flavor (charm and bottom) production. Specifically, we examine the features of both calculations throughout phase space and compare the structure function F₂(x,Q²). We also analyze the dependence of F₂ on the mass factorization scale μ. We find that the former is most applicable near threshold, while the latter works well for asymptotic Q². The validity of each calculation in the intermediate region is dependent upon the x and Q² values chosen.

The elements, theoretical basis, and experimental status of perturbative quantum chromodynamics are presented. Relevant field-theoretic methods are introduced at a nonspecialist level, along with a review of the basic ideas and methods of the parton model. This is followed by an account of the fundamental theorems of quantum chromodynamics, which generalize the parton model. Summaries of the theoretical and experimental status of the most important hard-scattering processes are then given, including electron-positron annihilation, deeply inelastic scattering, and hard hadron-hadron scattering, as induced both by electoweak interactions and by quantum chromodynamics directly. In addition, a discussion is presented of the global fitting approach to the determination of parton distributions in nucleons.

A unified QCD formulation of leptoproduction of massive quarks in charged current and neutral current processes is described. This involves adopting consistent factorization and renormalization schemes which encompass both vector-boson-gluon-fusion (‘‘flavor creation’’) and vector-boson-massive-quark-scattering (‘‘flavor excitation’’) production mechanisms. It provides a framework which is valid from the threshold for producing the massive quark (where gluon fusion is dominant) to the very high energy regime when the typical energy scale μ is much larger than the quark mass m_Q (where the quark scattering should be prevalent). This approach effectively resums all large logarithms of the type [α_s(μ)ln(μ²/m_Q²)]ⁿ which limit the validity of existing fixed-order calculations to the region μ∼O(m_Q). We show that the (massive) quark-scattering contribution (after subtraction of overlaps) is important in most parts of the (x,Q) plane except near the threshold region. We demonstrate that the factorization scale dependence of the structure functions calculated in this approach is substantially less than those obtained in the fixed-order calculations, as one would expect from a more consistent formulation.

Existing calculations of heavy quark production in charged current and neutral current lepton-hadron scattering are formulated differently because of the artificial distinction of ‘‘light’’ and ‘‘heavy’’ quarks made in the traditional approach. A proper QCD formalism valid for a wide kinematic range from near threshold to energies much higher then the quark mass should treat these processes in a uniform way. We formulate a unified approach to both types of leptoproduction processes based on the conventional factorization theorem. In this paper, we present the general framework with complete kinematics appropriate for arbitrary masses, emphasizing the simplifications provided by the helicity formalism. We illustrate this approach with an explicit calculation of the leading-order contribution to the quark structure functions with general masses. This provides the basis for a complete QCD analysis of charged-current and neutral current leptoproduction of charm and bottom quarks to be presented in subsequent papers.

We examine the predictions of the conventional SU(2)_L⊗SU(2)_R⊗U(1)_B-L left-right-symmetric model in the case where the minimal Higgs sector (containing one bidoublet, one L-triplet, and one R-triplet Higgs field) and the standard lepton representations (incorporating right-handed partners for the observed neutrinos) are adopted. We show that a complete analysis of spontaneous symmetry breaking for the Higgs sector leads to a highly restrictive range of possibilities for global minima that are simultaneously consistent with all experimental observations (such as lepton masses, K_L-K_S mixing, etc.). As a result, the possible phenomenologies for the gauge and Higgs bosons of the model are very limited. For instance, we demonstrate that in the absence of explicit CP violation in the Higgs potential, spontaneous CP violation does not arise and the fermion couplings exhibit ‘‘manifest’’ left-right symmetry. Further, we find no entirely natural solutions other than ones in which all of the extra (non-standard-model) gauge and Higgs bosons associated with the left-right-symmetric extension are extremely heavy (typically, more massive than 10⁷ GeV). Only by ‘‘fine-tuning’’ certain parameters of the Higgs potential is it possible to bring these extra particles down to an observable mass scale. Alternatively, symmetries can be introduced to eliminate the terms in the Higgs potential associated with these parameters, but only at the sacrifice of introducing undesirable consequences for fermion masses. Many of the pitfalls and problems are illustrated using a simplified model. Overall, we emphasize the necessity of performing a complete minimization of the Higgs sector before extracting phenomenology.

Gluon-initiated contributions to deep-inelastic-scattering processes, such as charm production, can be comparable in magnitude to the ‘‘leading-order’’ sea-quark processes. A proper next-to-leading-order calculation in QCD confirms this and yields distinct dependences of these two contributions on the kinematic variables and on the charm-quark mass. These results imply that previous analyses of charm-production data to extract the strange and charm content of the nucleon, as well as the precise determination of standard-model parameters based on these analyses, need to be reassessed.