Parton distributions and the LHC: W and Z production

W and Z bosons will be produced copiously at the LHC proton-proton collider. We study the parton distribution dependence of the total production cross sections and rapidity distributions, paying particular attention to the uncertainties arising from uncertainties in the parton distributions themselves. Variations in the gluon, the strong coupling, the sea quarks and the overall normalisation are shown to lead to small but non-negligible variations in the cross section predictions. Ultimately, therefore, the measurement of these cross sections will provide a powerful cross check on our knowledge of parton distributions and their evolution. Received: 23 November 1999 / Published online: 6 April 2000     

Soft and hard Pomeron in the structure function of the proton at low x and low Q^2

We study inclusive electroproduction on the proton at low x and low using a soft and a hard Pomeron. The contribution of the soft Pomeron is based on the Stochastic Vacuum Model, in which a nonperturbative dipole-dipole cross section can be calculated by means of a gauge invariant gluon field strength correlator. To model the hard Pomeron exchange we phenomenologically extend the leading order evolution of a power-behaved structure function, , proposed by López and Ynduráin. This extension allows to consider both the case and the region of higher on the basis of the same parametrization. A good simultaneous fit to the data on and on the cross section of real photoproduction is obtained for . With four parameters we achieve a for 222 data points. In addition, we use our model of the inclusive interaction to compute the longitudinal structure function . Received: 6 December 1998 / Revised version: 19 April 1999 / Published online: 18 June 1999     

Deep-inelastic inclusive ep scattering at low x and a determination of \alpha_s

A precise measurement of the inclusive deep-inelastic scattering cross section is reported in the kinematic range GeV and . The data were recorded with the H1 detector at HERA in 1996 and 1997, and correspond to an integrated luminosity of 20 pb. The double differential cross section, from which the proton structure function and the longitudinal structure function are extracted, is measured with typically 1% statistical and 3% systematic uncertainties. The measured derivative is observed to rise continuously towards small x for fixed . The cross section data are combined with published H1 measurements at high for a next-to-leading order DGLAP QCD analysis. The H1 data determine the gluon momentum distribution in the range to within an experimental accuracy of about 3% for GeV. A fit of the H1 measurements and the data of the BCDMS collaboration allows the strong coupling constant and the gluon distribution to be simultaneously determined. A value of is obtained in NLO, with an additional theoretical uncertainty of about , mainly due to the uncertainty of the renormalisation scale. Received: 21 December 2000 / Revised version: 10 May 2001 / Published online: 6 July 2001     

Photoproduction of top quarks in peripheral heavy ion collisions

In relativistic heavy ion collisions, top quarks can be produced by photon-gluon fusion when a photon from the Weizs?cker-Williams virtual photon field of one nucleus interacts with a gluon in the other nucleus. Photoproduction with heavy ions at the Large Hadron Collider (LHC) will be the first accessible non-hadronic top production channel. We calculate the photoproduction cross sections, pair mass and top quark rapidity distributions in peripheral lead-lead and oxygen-oxygen collisions. The cross sections are sensitive to the top quark charge and the large- gluon distribution in the nucleus. We find a cross section of 15 pb in oxygen-oxygen collisions, leading to 210 pairs in a one month ( s) LHC run. In pA collisions, the rate is higher, 1100 pairs per month for pO. A comparison of the AA and pA data might allow for a study of gluon shadowing at high Q . Received: 18 May 2001 / Revised version: 13 June 2001 / Published online: 24 August 2001     

$\mbox{Log}(1/x)$ gluon distribution and structure functionsin the loop-loop correlation model

We consider the interaction of the partonic fluctuation of a scalar photon with an external color field to calculate the leading and next-to-leading order gluon distribution of the proton following the work done by Dosch-Hebecker-Metz-Pirner. We relate these gluon distributions to the short and long distance behavior of the cross section of an adjoint dipole scattering off a proton. The leading order result is a constant, while the next-to-leading order result shows a enhancement at small x. To get numerical results for the gluon distributions at the initial scale Q²₀ = 1.8 GeV², we compute the adjoint dipole-proton cross section in the loop-loop correlation model. Quark distributions at the same initial scale are parameterized according to Regge theory. We evolve quark and gluon distributions to higher Q² values using the DGLAP equation and compute charm and proton structure functions in the small-x region for different Q² values.Received: 13 September 2003, Revised: 22 November 2003, Published online: 15 January 2003     

The unintegrated gluon distribution from the modified BK equation

We investigate the recently proposed non-linear equation for the unintegrated gluon distribution function which includes the subleading effects at small x. We obtained numerically the solution to this equation in (x,k) space, and also the integrated gluon density. The subleading effects affect strongly the normalization and the x and k dependence of the gluon distribution. We show that the saturation scale Q_s(x), which is obtained from this model, is consistent with the one used in the saturation model by Golec-Biernat and Wüsthoff. We also estimate the non-linear effects by looking at the relative normalization of the solutions to the linear and non-linear equations. It turns out that the differences are quite large even in the nominally dilute regime, that is when . Finally, we calculate the dipole-nucleon cross section.Received: 30 August 2004, Revised: 17 February 2005, Published online: 11 May 2005     

Proton-nucleus inelastic cross section at ultra high energy

Energy dependence of proton-nucleus reaction cross section at very high energy is discussed. It is stressed that depending on the gluon distribution near the nuclear surface, proton-nucleous total cross section increases much more rapidly compared to the usual Glauber independent nucleon estimate. The recent observation of smaller X _max than the expected value at UHECR domain can be an indication for such a mechanism.     

Semi-exclusive production of photons at HERA

We study the feasibility of measuring the semi-exclusive photon production at HERA. The cross section of photons produced at large transverse momenta, recoiling off an inclusive system Y of limited mass, can without photon isolation cuts simply be expressed in terms of hard PQCD subprocesses and standard target parton distributions. With the help of event generators we identify the kinematic region where quark and gluon fragmentation processes can be neglected. The cross section in this semi-exclusive region is large enough to be measured with an upgraded HERA luminosity of . The subprocesses of lowest order in are suppressed at low recoil masses , compared to higher order gluon exchange, i.e. BFKL contributions. The distinct -dependence makes it possible to determine experimentally the kinematic range where the higher order processes dominate. Received: 27 March 2000 / Revised version: 6 June 2000 / Published online: 9 August 2000     

微扰QCD演化过程中存在遮蔽效应的一个证据

核子的纵向结构函数F_L以及虚光子吸收的纵向和横向散射截面的比值R_L在小X区域主要取决于胶子的分布函数.而胶子的初始分布函数的形式决定了在演化过程中是否要考虑遮蔽效应.利用新的动力学演化方程分析了F_L和R_L,发现在部分子演化过程中存在遮蔽现象.     

Double meson production in ultraperipheral heavy-ion collisions

The double-meson production in ultraperipheral heavy-ions collisions is addressed, focusing on the particular case of from two-photon reactions. The cross section at photon level is obtained using distinct parameterizations for the gluon distribution on the light meson. The resulting estimates for the nuclear case are presented and discussed. As a by-product, we estimate the double production cross section using the pomeron-exchange factorization relations. Received: 20 March 2003 / Published online: 2 June 2003     

Role of anomalous chromomagnetic interaction in Pomeron and Odderon structures and in gluon distribution

We calculate the contribution arising from nonperturbative quark-gluon chromomagnetic interaction to the high-energy total quark-quark cross section and to gluon distributions in nucleon. The estimation obtained within the instanton model of QCD vacuum leads to the conclusion that this type of interaction gives the dominating contribution to the Pomeron coupling with the light quarks and to gluon distribution in light hadrons at small virtualities of quarks and gluons. We argue that the Odderon, which is the P = C = −1 partner of the Pomeron, is governed by the spin-flip component related to nonperturbative three-gluon exchange induced by anomalous quark-gluon chromomagnetic interaction.     

Nonperturbative gluon radiation and energy dependence of elastic scattering

The energy dependence of the total hadronic cross sections is caused by gluon bremsstrahlung which we treat nonperturbatively. It is located at small transverse distances about 0.3 fm from the valence quarks. The cross section of gluon radiation is predicted to exponentiate and rise with energy as s(delta) with delta = 0.17+/-0. 01. The total cross section also includes a large energy independent Born term which corresponds to no gluon radiation. The calculated total cross section and the slope of elastic scattering are in good agreement with the data.     

The NLO jet vertex for Mueller-Navelet and forward jets: The quark part

We calculate the next-to-leading corrections to the jet vertex which is relevant for the Mueller-Navelet-jets production in collisions and for the forward jet cross section in ep collisions. In this first part we present the results of the vertex for an incoming quark. Particular emphasis is given to the separation of the collinear divergent part and the central region of the produced gluon. Received: 16 January 2002 / Published online: 5 April 2002     

Soft gluons in logarithmic summations

We demonstrate that all the known single- and double-logarithm summations for a parton distribution function can be unified in the Collins–Soper resummation technique by applying soft approximations appropriate in different kinematic regions to real gluon emissions. Neglecting the gluon longitudinal momentum, we obtain the (double-logarithm) resummation for two-scale QCD processes, and the Balitsky–Fadin–Kuraev–Lipatov (single-logarithm) equation for one-scale processes. Neglecting the transverse momentum, we obtain the threshold (double-logarithm) resummation for two-scale processes, and the Dokshitzer–Gribov–Lipatov–Altarelli–Parisi (single-logarithm) equation for one-scale processes. If we keep the longitudinal and transverse momenta simultaneously, we derive a unified resummation for a large Bjorken variable x, and a unified evolution equation appropriate for both intermediate and small x. Received: 9 March 1999 / Revised version: 12 April 1999 / Published online: 3 August 1999     

One-loop factorization for inclusive hadron production in p-A collisions in the saturation formalism

We demonstrate the QCD factorization for inclusive hadron production in p-A collisions in the saturation formalism at one-loop order, with explicit calculation of both real and virtual gluon radiation diagrams. In particular, we find that the cross section can be written into a factorization form in the coordinate space at the next-to-leading order, while the naive form of the convolution in the transverse momentum space does not hold. The collinear divergences associated with the incoming parton distribution of the nucleon and the outgoing fragmentation function of the final-state hadron, as well as the rapidity divergence with small-x dipole gluon distribution of the nucleus are factorized into the splittings of the associated parton distribution and fragmentation functions and the energy evolution of the dipole gluon distribution function. The hard coefficient function is evaluated at one-loop order, and contains no divergence.     

Soft gluon radiation in Higgs boson production at the LHC

We examine the contributions of soft gluons to the Higgs production cross section at the LHC in the Standard Model and its minimal supersymmetric extension. The soft gluon radiation effects of this reaction share many features with the Drell-Yan process, but arise at lowest order from a purely gluonic initial state. We provide an extension of the conventional soft gluon resummation formalism to include a new class of contributions which we argue to be universal, and resum these and the usual Sudakov effects to all orders. The effect of these new terms is striking: only if they are included, does the expansion of the resummed cross section to next-to-leading order reproduce the exact result to within a few percent for the full range of Higgs boson masses. We use our resummed cross section to derive next-to-next-to-leading order results, and their scale dependence. Moreover, we demonstrate the importance of including the novel contributions in the resummed Drell-Yan process.     

On Higgs Boson Production via Gluon Fusion

A simple phenomenological formula for the gluon distribution function in a proton is proposed. The reported results of calculations of the cross section of Higgs boson production via gluon fusion with higher-order QCD radiative corrections are reproduced using this formula and the one-loop amplitude.     

A simple model for nuclear structure functions at small x in the dipole picture

A simple model for nuclear structure functions in the region of small x and small and moderate Q² is presented. It is a parameter-free extension, in the Glauber-Gribov approach to nuclear collisions, of a saturation model for the nucleon. A reasonable agreement with experimental data on the ratios of nuclear structure functions is obtained. Nuclear effects in the longitudinal-to-transverse cross section ratios are found to be small. Predictions of the model for values of x smaller than those available to present experiments are given. The unintegrated gluon distribution and the behaviour of the saturation scale which result from this model are shown and discussed. Received: 5 June 2002 / Revised version: 27 June 2002 / Published online: 16 October 2002