Parton Density Functions Research Articles

Cosmic ray interaction event generator SIBYLL 2.1

The cosmic ray interaction event generator Sibyll is widely used in extensive air shower simulations. We describe in detail the properties of Sibyll 2.1 and the differences with the original version 1.7. The major structural improvements are the possibility to have multiple soft interactions, introduction of new parton density functions, and an improved treatment of diffraction. Sibyll 2.1 gives better agreement with fixed target and collider data, especially for the inelastic cross sections and multiplicities of secondary particles. Shortcomings and suggestions for future improvements are also discussed.

Production of Jets and Photons at ATLAS

The measurement of the production of jets and photons in ATLAS will provide ideal tests of perturbative QCD in a kinematic regime never observed before. Gaining a precise quantitative understanding of perturbative QCD is essential to searches for new physics at the LHC. In addition, differential cross-section measurements of jet and photon production can be used to constrain parton density functions. The detection of jets and photons in ATLAS is achieved by using finely segmented calorimeters, which surround an inner tracking system enclosed in a 2T solenoid field.

Photon & Proton Structure – Overview

A brief overview is given of the current measurements of proton and photon stucture functions and other data relevant for the extraction of parton density functions (PDFs). Some comments are made on the outlook for the near future.

Proton parton-density function constraints from the Tevatron data on photon and jet production

Latest results from the Tevatron on photon and jet production are presented in this proceedings. The impact of the data is discussed from the point of view of proton parton-density constrains.

Quark-antiquark energy density function applied to Di-Gauge boson production at the LHC

In view of the start up of the 14 TeV pp Large Hadron Collider the quark anti-quark reactions leading to the final states W+W−, W±Z0 and Z0Z0 are studied, in the frame work of the Standard Model (SM), using helicity amplitudes. The differential and total cross sections are first evaluated in the parton anti-parton center of mass system. Subsequently they are transformed to their expected structure in pp collisions through the parton anti-parton Energy Density Functions which are here derived from the known Parton Density Functions of the proton. In particular the single and joint longitudinal polarizations of the di-boson final states are calculated. The effect on these reactions from the presence of s-channel heavy vector bosons, like the W' and Z', are evaluated to explore the possibility to utilize the gauge boson pair production as a probe for these 'Beyond the SM' phenomena.

An analysis on single and central diffractive heavy flavour production at hadron colliders

In this contribution results from a phenomenological analysis for the diffractive hadroproduction of heavy flavors at high energies are reported. Diffractive production of charm, bottom and top are calculated using the Regge factorization, taking into account recent experimental determination of the diffractive parton density functions in Pomeron by the H1 Collaboration at DESY-HERA. In addition, multiple-Pomeron corrections are considered through the rapidity gap survival probability factor. We give numerical predictions for single diffractive as well as double Pomeron exchange (DPE) cross sections, which agree with the available data for diffractive production of charm and beauty. We make estimates which could be compared to future measurements at the LHC.

Large-xResummation inQ2Evolution

The standard analytic solution to the Dokshitzer-Gribov-Lipatov-Altarelli-Parisi equation in Mellin space is improved by resumming the large-x divergences. Explicit results are given to next-to-leading order and next-to-leading logarithmic accuracy, which significantly reduce the parton density functions' theoretical uncertainties, more than the inclusion of next-to-next-to-leading order corrections in some cases, and is, therefore, of paramount importance for the reliable interpretation of ongoing and future experiments with hadron beams or targets, including those at the CERN Large Hadron Collider.

Measurement of the neutral current dis cross section at ZEUS

The ZEUS experiment at the ep collider HERA has performed a new measurement of the deep-inelastic scattering cross-sections with the data collected during the year 2006. The new measurement is optimised for the kinematic region of large values of the inelasticity y where several experimental challenges need to be overcome. The results were compared to the expectations of the Standard Model obtained with several parton density functions.

Inclusive jet and dijet production at HERA

The inclusive jet cross section in deep-inelastic scattering (DIS) and dijet cross-section in photo-production and DIS have been measured at s√ = 320 GeV at the electron-proton collider HERA by the H1 and ZEUS collaborations using data taken in 1998-2000 (HERA I) and in 2004-2007 (HERA II). Cross-sections are presented as a function of relevant kinematical variables such as boson virtuality Q2, proton (photon) momentum fraction carried by the interacting parton xp (xγ) or transverse energy ET of the jets. The data are compared to perturbative QCD (pQCD) predictions at next-to-leading order (NLO) and are in general in good agreement within the estimated theory error which exceeds the typical experimental uncertainty in most regions of phase-space. The sensitivity of these high-precision jet observables to QCD parameters such as the strong coupling αS and parameterisations of the parton density functions (PDF) of the proton and the photon are discussed.

Hard diffractive quarkonium hadroproduction at high energies

We present a study of heavy quarkonium production in hard diffractive processes by pomeron exchange for Tevatron and LHC energies. The numerical results are computed using a recent experimental determination of the diffractive parton density functions in the pomeron and these are corrected by unitarity corrections through the gap survival probability factor. We give predictions for single as well as central diffractive ratios. These processes are sensitive to the gluon content of the pomeron at small Bjorken variable x and may be particularly useful in studying small-xphysics. They may also be a good place to test the different available mechanisms for quarkonium production at hadron colliders.

Estimation of single and double diffractive heavy flavor production in hadron-hadron colliders

Results from a phenomenological analysis for diffractive hadroproduction of heavy flavors at high energies are reported. Diffractive production of charm, bottom, and top are calculated using Regge factorization, taking into account recent experimental determination of the diffractive parton density functions in the Pomeron by the H1 Collaboration at DESY-HERA. In addition, multiple-Pomeron corrections are considered through the rapidity gap survival probability factor. We give numerical predictions for single diffractive as well as double Pomeron exchange cross sections, which agree with the available data for diffractive production of charm and beauty. We make estimates which could be compared to future measurements at the CERN LHC.

Diffractive hadroproduction ofW±andZ0bosons at high energies

Results from a phenomenological analysis of $W$ and $Z$ hard diffractive hadroproduction at high energies are reported. Using the Regge factorization approach, we consider the recent diffractive parton density functions extracted by the H1 Collaboration at DESY-HERA. In addition, we take into account multiple Pomeron exchange corrections considering a gap survival probability factor. It is found that the ratio of diffractive to nondiffractive boson production is in good agreement with the CDF and D0 data. We make predictions which could be compared to future measurements at the LHC.

Effect of charged partons on black hole production at the Large Hadron Collider

The cross section for black hole production in hadron colliders is calculated using a factorization hypothesis in which the parton-level process is integrated over the parton density functions of the protons. The mass, spin, charge, colour, and finite size of the partons are usually ignored. We examine the effects of parton electric charge on black hole production using the trapped-surface approach of general relativity. Accounting for electric charge of the partons could reduce the black hole cross section by one to four orders of magnitude at the Large Hadron Collider. The cross section results are sensitive to the Standard Model brane thickness. Lower limits on the amount of energy trapped behind the event horizon in the collision of charged particles are also calculated.

Diffractive open charm production in deep-inelastic scattering and photoproduction at HERA

Measurements are presented of diffractive open charm production at HERA. The event topology is given by ep -> eX Y where the system X contains at least one charmed hadron and is well separated by a large rapidity gap from a leading low-mass proton remnant system Y. Two analysis techniques are used for the cross section measurements. In the first, the charm quark is tagged by the reconstruction of a D*(2010) meson. This technique is used in deep-inelastic scattering (DIS) and photoproduction. In the second, a method based on the displacement of tracks from the primary vertex is used to measure the open charm contribution to the inclusive diffractive cross section in DIS. The measurements are compared with next-to-leading order QCD predictions based on diffractive parton density functions previously obtained from a QCD analysis of the inclusive diffractive cross section at H1. A good agreement is observed in the full kinematic regime, which supports the validity of QCD factorization for open charm production in diffractive DIS and photoproduction.

Measurement of neutral current cross sections at high Bjorken-x with the ZEUS detector at HERA

A new method is employed to measure the neutral current cross section up to Bjorken-x values of one with the ZEUS detector at HERA using an integrated luminosity of 65.1 pb-1 for e+p collisions and 16.7 pb-1 for e-p collisions at $\sqrt{s}=318\,\mathrm{GeV}$ and 38.6 pb-1 for e+p collisions at $\sqrt{s}=300\,\mathrm{GeV}$ . Cross sections have been extracted for Q2≥648 GeV2 and are compared to predictions using different parton density functions. For the highest x bins, the data have a tendency to lie above the expectations using recent parton density function parametrizations.

Charmed-hadron fragmentation functions from CERN LEP1 revisited

In Phys. Rev. D 58, 014014 (1998) and 71, 094013 (2005), we determined non-perturbative D^0, D^+, D^{*+}, D_s^+, and Lambda_c^+ fragmentation functions, both at leading and next-to-leading order in the MS-bar factorization scheme, by fitting e^+e^- data taken by the OPAL Collaboration at CERN LEP1. The starting points for the evolution in the factorization scale mu were taken to be mu_0 = 2m_Q, where Q = c,b. For the reader's convenience, in this Addendum, we repeat this analysis for mu_0 = m_Q, where the flavor thresholds of modern sets of parton density functions are located.

Strong-flavor-changing effective operator contributions to single top quark production

We study the effects of dimension six effective operators on the production of single top quarks at the CERN LHC. The operator set considered includes terms with effective gluon interactions and four-fermion terms. Analytic expressions for the several partonic cross sections of single top production will be presented, as well as the results of their integration on the parton density functions.

Measurement of the photon structure function [formula omitted] with the L3 detector at LEP

The e+e−→e+e−hadrons reaction, where one of the two electrons is detected in a low polar-angle calorimeter, is analysed in order to measure the hadronic photon structure function F2γ. The full high-energy and high-luminosity data set, collected with the L3 detector at centre-of-mass energies 189 GeV⩽s⩽209 GeV, corresponding to an integrated luminosity of 608 pb−1 is used. The Q2 range 11 GeV2⩽Q2⩽34 GeV2 and the x range 0.006⩽x⩽0.556 are considered. The data are compared with recent parton density functions.

Quark asymmetries in the proton from a model for parton densities

Based on quantum fluctuations in momentum and of the proton into meson-baryon pairs, we develop a physical model for the nonperturbative $x$ shape of parton density functions in the proton. The model describes the proton structure function and gives a natural explanation of observed quark asymmetries, such as the difference between the anti-up and anti-down sea quark distributions and between the up and down valence distributions. An asymmetry in the momentum distribution of strange and antistrange quarks in the nucleon is found to reduce the NuTeV anomaly to a level which does not give a significant indication of physics beyond the standard model.

Interpretation of electron–proton scattering at low [formula omitted

Low-Q2 photons do not resolve partons in the proton, which gives problems when applying the deep inelastic scattering formalism, such as an unphysical, negative gluon density extracted from data. Considering instead hadronic fluctuations of the photon, we show that the generalised vector meson dominance model (GVDM) gives a good description of the measured cross section at low Q2, i.e., reproduces F2(x,Q2), using only few parameters with essentially known values. Combining GVDM and parton density functions makes it possible to obtain a good description of F2 data over the whole range of x and Q2.