Entire Kinematic Region Research Articles

Revisiting evolution equations for generalised parton distributions

We revisit the evolution of generalised parton distributions (GPDs) in momentum space. We formulate the evolution kernels at one loop in perturbative quantum chromodynamics (pQCD) in a form that is suitable for numerical implementation and that allows for an accurate study of their properties. This leads to the first open-source implementation of GPD evolution equations able to cover the entire kinematic region and allowing for heavy-quark-threshold crossings. The numerical implementation of the GPD evolution equations is publicly accessible through the APFEL++ evolution library and is available within the PARTONS framework. Our formulation makes use of the operator definition of GPDs in light-cone gauge renormalised in the overline{text{ MS }} scheme. For the sake of clarity, we recompute the evolution kernels at one loop in pQCD, confirming previous calculations. We obtain general conditions on the evolution kernels derived from the GPD sum rules and show that our formulation obeys these conditions. We analytically show that our calculation reproduces the DGLAP and the ERBL equations in the appropriate limits and that it guarantees the continuity of GPDs. We numerically check that the evolved GPDs fulfil DGLAP and ERBL limits, continuity, and polynomiality. We benchmark our numerical implementation against analytical evolution in conformal space. Finally, we perform a numerical comparison to an existing implementation of GPD evolution, finding general good agreement on the kinematic region accessible to the latter. This work provides a pedagogical description of GPD evolution equations which benefits from a renewed interest as future colliders, such as the electron-ion colliders in the United States and in China, are being designed. It also paves the way for the extension of GPD evolution codes to higher accuracies in pQCD desirable for precision phenomenology at these facilities.

The fate of small classically stable Q-balls

The smallest classically stable Q-balls are, in fact, generically metastable: in quantum theory they decay into free particles via collective tunneling. We derive general semiclassical method to calculate the rate of this process in the entire kinematical region of Q-ball metastability. Our method uses Euclidean field-theoretical solutions resembling the Coleman’s bounce and fluctuations around them. As an application of the method, we numerically compute the decay rate to the leading semiclassical order in a particular one-field model. We shortly discuss cosmological implications of metastable Q-balls.

Perturbative corrections to B \u2192 D form factors in QCD

We compute perturbative QCD corrections to B → D form factors at leading power in Λ/mb, at large hadronic recoil, from the light-cone sum rules (LCSR) with B-meson distribution amplitudes in HQET. QCD factorization for the vacuum-to-B-meson correlation function with an interpolating current for the D-meson is demonstrated explicitly at one loop with the power counting scheme {m}_csim mathcal{O}left(sqrt{Lambda {m}_b}right) . The jet functions encoding information of the hard-collinear dynamics in the above-mentioned correlation function are complicated by the appearance of an additional hard-collinear scale mc, compared to the counterparts entering the factorization formula of the vacuum-to-B-meson correction function for the construction of B → π from factors. Inspecting the next-to-leading-logarithmic sum rules for the form factors of B → Dℓν indicates that perturbative corrections to the hard-collinear functions are more profound than that for the hard functions, with the default theory inputs, in the physical kinematic region. We further compute the subleading power correction induced by the three-particle quark-gluon distribution amplitudes of the B-meson at tree level employing the background gluon field approach. The LCSR predictions for the semileptonic B → Dℓν form factors are then extrapolated to the entire kinematic region with the z-series parametrization. Phenomenological implications of our determinations for the form factors fBD+,0(q2) are explored by investigating the (differential) branching fractions and the R(D) ratio of B → Dℓν and by determining the CKM matrix element |Vcb| from the total decay rate of B → Dμνμ.

Transverse-target-spin asymmetry in exclusive $$\omega $$-meson electroproduction

Hard exclusive electroproduction of omega mesons is studied with the HERMES spectrometer at the DESY laboratory by scattering 27.6 GeV positron and electron beams off a transversely polarized hydrogen target. The amplitudes of five azimuthal modulations of the single-spin asymmetry of the cross section with respect to the transverse proton polarization are measured. They are determined in the entire kinematic region as well as for two bins in photon virtuality and momentum transfer to the nucleon. Also, a separation of asymmetry amplitudes into longitudinal and transverse components is done. These results are compared to a phenomenological model that includes the pion pole contribution. Within this model, the data favor a positive pi omega transition form factor.

Study of Z production in PbPb and pp collisions at s N N = 2.76 $$ \sqrt{s_{\mathrm{NN}}}=2.76 $$ TeV in the dimuon and dielectron decay channels

The production of Z bosons is studied in the dimuon and dielectron decay channels in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV, using data collected by the CMS experiment at the LHC. The PbPb data sample corresponds to an integrated luminosity of about 150 inverse microbarns, while the pp data sample collected in 2013 at the same nucleon-nucleon centre-of-mass energy has an integrated luminosity of 5.4 inverse picobarns. The Z boson yield is measured as a function of rapidity, transverse momentum, and collision centrality. The ratio of PbPb to pp yields, scaled by the number of inelastic nucleon-nucleon collisions, is found to be 1.06 +/- 0.05 (stat) +/- 0.08 (syst) in the dimuon channel and 1.02 +/- 0.08 (stat) +/- 0.15 (syst) in the dielectron channel, for centrality-integrated Z boson production. This binary collision scaling is seen to hold in the entire kinematic region studied, as expected for a colourless probe that is unaffected by the hot and dense QCD medium produced in heavy ion collisions.

Non linear evolution: revisiting the solution in the saturation region

In this paper we revisit the problem of the solution to Balitsky-Kovchegov equation deeply in the saturation domain. We find that solution has the form of Levin-Tuchin solution but it depends on variable $\bar{z} = \ln(r^2 Q^2_s) + \mbox{Const}$ and the value of $\mbox{Const}$ is calculated in this paper. We propose the solution for full BFKL kernel at large $z$ in the entire kinematic region that satisfies the McLerram-Venugopalan initial condition

Nuclear modification of the transverse momentum distributions in high energy pA and AA collisions

We evaluate the transverse momentum spectrum of J / ψ (up to semi-hard momenta) in pA and AA collisions taking into account only the initial state effects, but resumming them to all orders in α s 2 A 1 / 3 . In our previous papers we noticed that cold nuclear matter effects alone could not explain the experimental data on rapidity and centrality dependencies of the J / ψ yield in AA collisions indicating the existence of an additional suppression mechanism. Our present calculations indicate that the discrepancy persists and even increases at semi-hard transverse momenta, implying a significant final state effect on J / ψ production in this kinematical domain. The QCD dipole model we employ is only marginally applicable for J / ψ production at mid-rapidity at RHIC energies but its use is justified in the forward rapidity region. At LHC energies we can quantitatively evaluate the magnitude of cold nuclear matter effects in the entire kinematical region of interest. We present our calculations of J / ψ transverse momentum spectra in pA and AA collisions at LHC and RHIC energies.

GR@PPA 2.8: Initial-state jet matching for weak-boson production processes at hadron collisions

The initial-state jet matching method introduced in our previous studies has been applied to the event generation of single W and Z production processes and diboson (W+W−, WZ and ZZ) production processes at hadron collisions in the framework of the GR@PPA event generator. The generated events reproduce the transverse momentum spectra of weak bosons continuously in the entire kinematical region. The matrix elements (ME) for hard interactions are still at the tree level. As in previous versions, the decays of weak bosons are included in the matrix elements. Therefore, spin correlations and phase-space effects in the decay of weak bosons are exact at the tree level. The program package includes custom-made parton shower programs as well as ME-based hard interaction generators in order to achieve self-consistent jet matching. The generated events can be passed to general-purpose event generators to make the simulation proceed down to the hadron level. Program summaryProgram title: GR@PPA 2.8Catalogue identifier: ADRH_v3_0Program summary URL:http://cpc.cs.qub.ac.uk/summaries/ADRH_v3_0.htmlProgram obtainable from: CPC Program Library, Queenʼs University, Belfast, N. IrelandLicensing provisions: Standard CPC licence, http://cpc.cs.qub.ac.uk/licence/licence.htmlNo. of lines in distributed program, including test data, etc.: 112 146No. of bytes in distributed program, including test data, etc.: 596 667Distribution format: tar.gzProgramming language: Fortran; with some included libraries coded in C and C++Computer: AllOperating system: Any UNIX-like systemRAM: 1.6 Mega bytes at minimumClassification: 11.2Catalogue identifier of previous version: ADRH_v2_0Journal reference of previous version: Comput. Phys. Comm. 175 (2006) 665External routines: Bash and Perl for the setup, and CERNLIB, ROOT, LHAPDF, PYTHIA according to the userʼs choice.Does the new version supersede the previous version?: No, this version supports only a part of the processes included in the previous versions.Nature of problem: We need to combine those processes including 0 jet and 1 jet in the matrix elements using an appropriate matching method, in order to simulate weak-boson production processes in the entire kinematical region.Solution method: The leading logarithmic components to be included in parton distribution functions and parton showers are subtracted from 1-jet matrix elements. Custom-made parton shower programs are provided to ensure satisfactory performance of the matching method.Reasons for new version: An initial-state jet matching method has been implemented.Summary of revisions: Weak-boson production processes associated with 0 jet and 1 jet can be consistently merged using the matching method.Restrictions: The built-in parton showers are not compatible with the PYTHIA new PS and the HERWIG PS.Unusual features: A large number of particles may be produced by the parton showers and passed to general-purpose event generators.Running time: About 10 min for initialization plus 25 s for every 1k-event generation for W production in the LHC condition, on a 3.0-GHz Intel Xeon processor with the default setting.

Leading-order electroweak corrections to the Drell-Yan process in the CMS experiment at LHC

The leading part of electroweak radiative corrections to the Drell-Yan process in the CMS (compact muon solenoid detector) experiment at LHC were calculated. Analytic and numerical tests indicate that the results are independent of all nonphysical parameters. A numerical analysis of radiative effects was performed by using the FORTRAN code READY with allowance for experimental constraints at the CMS detector. The results presented for the relative correction to the fully differential cross section for the Drell-Yan process over the entire kinematical region of the CMS detector would hopefully be helpful in handling experimental data.

Glauber-Gribov approach for DIS on nuclei in N = 4 SYM

In this paper the Glauber-Gribov approach for deep-inelastic scattering (DIS) with nuclei is developed in N=4 SYM. It is shown that the amplitude displays the same general properties, such as geometrical scaling, as is the case in the high density QCD approach. We found that the quantum effects leading to the graviton reggeization, give rise to an imaginary part of the nucleon amplitude, which makes the DIS in N=4 SYM almost identical to the one expected in high density QCD. We concluded that the impact parameter dependence of the nucleon amplitude is very essential for N=4 SYM, and the entire kinematic region can be divided into three regions which are discussed in the paper. We revisited the dipole description for DIS and proposed a new renormalized Lagrangian for the shock wave formalism which reproduces the Glauber-Gribov approach in a certain kinematic region. However the saturation momentum turns out to be independent of energy, as it has been discussed by Albacete, Kovchegov and Taliotis. We discuss the physical meaning of such a saturation momentum $Q_s(A)$ and argue that one can consider only $Q>Q_s(A)$ within the shock wave approximation.

Global study of nuclear structure functions

We present the results of a phenomenological study of unpolarized nuclear structure functions for a wide kinematical region of x and Q 2 . As a basis of our phenomenology we develop a model which takes into account a number of different nuclear effects including nuclear shadowing, Fermi motion and binding, nuclear pion excess and off-shell correction to bound nucleon structure functions. Within this approach we perform a statistical analysis of available data on the ratio of the nuclear structure functions F 2 for different nuclei in the range from the deuteron to the lead. We express the off-shell effect and the effective scattering amplitude describing nuclear shadowing in terms of few parameters which are common to all nuclei and have a clear physical interpretation. The parameters are then extracted from statistical analysis of data. As a result, we obtain an excellent overall agreement between our calculations and data in the entire kinematical region of x and Q 2 . We discuss a number of applications of our model which include the calculation of the deuteron structure functions, nuclear valence and sea quark distributions and nuclear structure functions for neutrino charged-current scattering.

Proton structure functions over the entire kinematical region

A new approach to describing inelastic charged-lepton scattering on a proton is proposed. The approach is based on combining the results obtained by theoretically studying the limiting cases of this process. The proton structure function is obtained in an analytic form that involves virtually no free parameters and which is in good agreement with experimental data over the entire kinematical region.

QCD saturation in the semi-classical approach

In this paper the semi-classical approach to the solution of non-linear evolution equation is developed. We found the solution in the entire kinematic region to the non-linear evolution equation that governs the dynamics in the high parton density QCD. The large impact parameter ( b t ) behaviour of the solution is discussed as well as the way how to include the non-perturbative QCD corrections in this region of b t . The geometrical scaling behaviour and other properties of the solution in the saturation (colour glass condensate) kinematic domain are analyzed. We obtain the asymptotic behaviour for the physical observables and found the unitarity bounds for them.

Theoretical investigation of tensor analyzing powers in deuteron fragmentation into pions

A detailed analysis of the fragmentation of tensorially polarized deuterons into pions in the kinematical region forbidden for free nucleon-nucleon collisions is performed. Within the relativistic impulse approximation, the inclusive spectrum of product pions and the tensor analyzing power T20 are explored by using various forms of the deuteron wave function. The effect of the P-wave component of the deuteron wave function on these observables is also investigated. It is shown that the invariant spectrum is more sensitive to the form of the NN → πX invariant amplitude than the tensor analyzing power T20. The inclusion of nonnucleon degrees of freedom in the deuteron makes it possible to reproduce the pion spectrum over the entire kinematical region and improves the description of experimental data on T20. According to present-day experimental data, T20 is positive and very small in magnitude, less than 0.2, which is at odds with the results of theoretical calculations disregarding nonnucleon degrees of freedom.

MEASUREMENT OF dσ/dy FOR HIGH MASS DRELL-YAN e+e-PAIRS AT CDF

We report on the first measurement of the rapidity distribution dσ/dy over nearly the entire kinematic region of rapidity for e+e-pairs in the Z-boson region of 66 < Mee< 116 GeV /c2and at higher mass Mee> 116 GeV /c2. The data sample consists of 108 pb-1of [Formula: see text] collisions at [Formula: see text] taken by the Collider Detector at Fermilab during 1992–1995. The total cross section in the Z-boson region is measured to be 252 ± 11 pb. The measured total cross section and dσ/dy are compared with quantum chromodynamics calculations in leading and higher orders. (CDF Note 5422)

Measurement ofdσ/dyfor high mass Drell-Yane+e−pairs frompp¯collisions ats=1.8TeV

We report on the first measurement of the rapidity distribution dsigma/dy over nearly the entire kinematic region of rapidity for e^+e^- pairs in the Z-boson region of 66<M_{ee}<116 GeV/c^2 and at higher mass M_{ee}>116 GeV/c^2. The data sample consists of 108 pb^{-1} of ppbar collisions at \sqrt{s}=1.8 TeV taken by the Collider Detector at Fermilab during 1992--1995. The total cross section in the $Z$-boson region is measured to be 252 +- 11 pb. The measured total cross section and d\sigma/dy are compared with quantum chromodynamics calculations in leading and higher orders.

Photon-pion charge asymmetry in e +e − reactions: A laboratory for perturbative QCD phases

The charge asymmetry in the production of a photon and a meson in e +e - annihilation is studied in perturbative QCD. This quantity measures the interference of amplitudes governed by different momentum scales. It is thus a powerful tool to probe strong interaction phases at high energy and in the context of Sudakov exponentiation and the chromo Coulomb phase. We find a null result at the lowest non-trivial order off α s in the entire kinematic region described by perturbative QCD.