The large luminosity and the increased kinematical domain accessible at the LHC will allow to isolate large numbers of events with multiple parton collisions. Interestingly, the hadron is probed in different points contemporarily in the process, which allows to obtain information on the correlations between partons in the hadron structure. The whole experimentally accessible information on multiple parton collisions consists in the probability distribution of the elementary interactions, while the inclusive cross sections, usually considered in large p_t processes, acquire a precise statistical meaning as moments of the multiplicity distribution. Conversely different and more "exclusive" cross sections become experimentally viable, providing complementary information on the interaction dynamics. The matter is discussed in details in the case of hadron-hadron collisions and the role of parton correlations is outlined both in the inclusive and in the "exclusive" cross sections.
New polarized fragmentation functions are introduced and justified, in addition to those conventional ones assumed to be independent of the helicity of the parent parton. It is demonstrated that due to our present ignorance concerning these new parton-spin dependent leading-twist fragmentation functions, it is impossible to utilize current experiments on spin-dependent semi-inclusive deep inelastic lepton nucleon scattering to disentangle the separate polarized parton distributions.
We investigate the role of exclusive channels in semi-inclusive electroproduction of pions and kaons. Using the QCD factorization theorem for hard exclusive processes we evaluate the cross sections for exclusive pseudoscalar and vector meson production in terms of generalized parton distributions and meson distribution amplitudes. We investigate the uncertainties arising from the modeling of the nonperturbative input quantities. Combining these results with available experimental data, we compare the cross sections for exclusive channels to that obtained from quark fragmentation in semi-inclus…
We argue that the collinear factorization of the fragmentation functions in high energy hadron and nuclei collisions breaks down at transverse momenta kT < Qs/g due to high parton densities in the colliding hadrons and/or nuclei. We calculate, at next-to-leading order in projectile parton density and to all orders in target parton density, the double-inclusive cross section for production of a pair of gluons in the scalar J^(PC)=0^(++) channel. Using the low energy theorems of QCD we find the inclusive cross section for pi-meson production.
The spatial distribution of partons in the proton can be probed in suitable exclusive scattering processes. I report on recent performance estimates for parton imaging at a proposed Electron-Ion Collider.
In a simplified model of Multiple Parton Interactions the inclusive cross sections, of processes with large momentum transfer exchange, acquire the statistical meaning of factorial moments of the distribution in multiplicity of interactions, while more exclusive cross sections, which can provide complementary information on the interaction dynamics, become experimentally viable. Inclusive and exclusive cross sections are linked by sum rules, which can be tested experimentally.
In the current region of semi-inclusive deep-inelastic scattering, most events are accompanied by intensive radiation of soft and collinear partons which cannot be reliably described at any fixed order of perturbative QCD. In this paper, a resummation formalism that describes such multiple parton radiation is compared to the HERA data on semi-inclusive DIS, including the distributions of energy flow and charged particle multiplicity. We show that the resummation of multiple parton radiation improves the agreement between the theory and the data. We make some suggestions on further experimental…
We propose an extension of matrix element plus parton shower merging at tree level to preserve inclusive cross sections obtained from the merged and showered sample. Implementing this constraint generates approximate next-to-leading order (NLO) contributions similar to the LoopSim approach. We then show how full NLO, or in principle even higher order, corrections can be added consistently, including constraints on inclusive cross sections to account for yet missing parton shower accuracy at higher logarithmic order. We also show how NLO accuracy below the merging scale can be obtained.
In this work we investigate the inclusive and exclusive photoproduction of dileptons, which is relevant for the physics programme to be studied in the proposed electron-proton collider, the LHeC. In the inclusive case, the process is sensitive to the parton distribution functions in the photon whereas the exclusive channel is connected to the small-$x$ QCD dynamics. For the latter, we investigate the role played by saturation physics at a very high energy scenario. The estimates for production cross sections and the number of events are presented.
In addition to the inclusive cross sections discussed within the QCD-parton model, in the regime of multiple parton interactions, different and more exclusive cross sections become experimentally viable and may be suitably measured. Indeed, in its study of double parton collisions, the quantity measured by CDF was an "exclusive" rather than an inclusive cross section. The non perturbative input to the "exclusive" cross sections is different with respect to the non perturbative input of the inclusive cross sections and involves correlation terms of the hadron structure already at the level of s…
We quantify uncertainties in the Monte-Carlo simulation of inclusive and dijet final states, which arise from using the MC@NLO technique for matching next-to-leading order parton level calculations and parton showers. We analyse a large variety of data from early measurements at the LHC. In regions of phase space where Sudakov logarithms dominate over high-energy effects, we observe that the main uncertainty can be ascribed to the free parameters of the parton shower. In complementary regions, the main uncertainty stems from the considerable freedom in the simulation of underlying events.
Some emerging difficulties in the theoretical description of exclusive semileptonic $\bar B$ decays are discussed in the context of the quark model. While there are no unambiguous problems at this time, I discuss physics beyond the valence quark model which should eventually be probed by precision measurements of $\bar B$ semileptonic decays.
We consider the effective Hamiltonian of four quark operators in the Standard Model in the exclusive and quasi-inclusive decays of the type $B\to K^{(*)} η^{\prime}$, $B\to η' X_s$, where $X_s$ contains a single Kaon. Working in the factorization assumption we find that the four quark operators can account for the recently measured exclusive decays $B\to η^{\prime}(η) K$ and $B\to K π$ for appropriate choice of form factors but cannot explain the large quasi-inclusive rate.
In the framework of the QCD parton model, we study unpolarized scattering of high energy real photons from a proton target into lepton pairs and a system of hadrons. For a given parametrization of parton distributions in the proton, we calculate the cross section of this process and show the cancellation of the interference terms.
We present theoretical evidence that the exclusive/inclusive ratio of semileptonic $Λ_b$-decays exceeds that of semileptonic B-decays where the experimental exclusive/inclusive ratio amounts to about 66%. We start from the observation that the spectator quark model provides a lower bound on the leading order Isgur-Wise function of the $Λ_b \to Λ_c$ transition in terms of the corresponding $B \to D,D^*$ mesonic Isgur-Wise function. Using experimental data for the $B \to D,D^*$ mesonic Isgur-Wise functions this bound is established. Applying a Bethe-Salpeter model including spectator quark inter…
We describe how unintegrated parton distributions can be calculated from conventional integrated distributions. We extend and improve the 'last-step' evolution approach, and explain why doubly-unintegrated parton distributions are necessary. We generalise k_t-factorisation to (z,k_t)-factorisation. We apply the formalism to inclusive jet production in deep-inelastic scattering, mainly at leading-order, but we also study the extension to next-to-leading order. We compare the predictions with recent HERA data.
We investigate the single-spin asymmetries in inclusive and exclusive processes of electron-proton scattering and electron-positron annihilation. In the decomposition of hadronic tensor, a Lorentz symmetric spin-dependent term is generally present. The existence of such a term implies single-spin asymmetries in these processes and these single-spin asymmetries for baryons can be understood in a unified manner. We argue that it is important to measure the single-spin asymmetries in both inclusive and exclusive processes for the $Λ$ production at the present B-factories. This will lead to the fi…
The overlap representation of skewed parton distributions is discussed and applications to wide-angle Compton scattering and electroproduction of mesons are presented. The amplitudes for these processes factorise into parton-level subprocess amplitudes and form factors representing 1/x-moments of skewed parton distributions.
We study the effect of absorptive corrections due to parton recombination on the parton distributions of the proton. A more precise version of the GLRMQ equations, which account for non-linear corrections to DGLAP evolution, is derived. An analysis of HERA F_2 data shows that the small-x gluon distribution is enhanced at low scales when the absorptive effects are included, such that a negative gluon distribution at 1 GeV is no longer required.
We discuss resummation of large logarithmic terms that appear in the cross-section of semi-inclusive DIS in the case when the final-state hadron follows the direction of the incoming electroweak vector boson in the c.m. frame of the vector boson and the initial-state proton.
The exclusive photon-photon production of the $K$-meson pairs has been analyzed within the partonic model of QCD for the kinematical conditions of BELLE experiment. The cross section dependences on partonic masses, charges, and a shape of $K$-meson wave function have been studied for the process under discussion.
The handbag mechanism for wide-angle exlusive scattering reactions is discussed and compared with other theoretical approaches. Its application to Compton scattering, meson photoproduction and two-photon annihilations into pairs of hadrons is reviewed.
We investigate the role of the choice of the upper phase space limit Q in the Curci-Furmanski-Petronzio (CFP) factorization scheme, which exploits dimensional regularization MS scheme. We examine how the choice of Q influences the evaluation of the standard DGLAP (inclusive) evolution kernels, gaining experience needed in the construction of the exclusive Monte Carlo modelling of the NLO DGLAP evolution. In particular, we uncover three types of mechanisms which assure the independence on Q of the inclusive DGLAP kernels calculated in the CFP scheme. We use the examples of three types of the Fe…
Recently dispersion relations have been applied to hard exclusive processes such as deeply virtual Compton scattering, and a holographic principle was proposed that maps out the generalized parton distributions entering the soft matrix elements for the processes from their values on a given kinematical ridge. We examine possible pitfalls associated with the implicit, direct identification in this approach of the physical hadronic states with colored partons, and suggest an improved treatment of this assumption.
In this talk we present theoretical evidence that the exclusive/inclusive ratio of semileptonic Lambda_b-decays exceeds that of semileptonic B-decays, where the experimental exclusive/inclusive ratio amounts to about 66%. We start from the observation that the spectator quark model provides a lower bound on the leading order Isgur-Wise function of the Lambda_b -> Lambda_c transition in terms of the corresponding B -> D,D^* mesonic Isgur-Wise function. Using experimental data for the B -> D,D^* mesonic Isgur-Wise functions this bound is established. Applying a QCD sum rule estimate of the Lambd…
Azimuthal angular correlations between produced hadrons/jets in high energy collisions are a sensitive probe of the dynamics of QCD at small x. Here we derive the triple differential cross section for inclusive production of 3 polarized partons in DIS at small x using the spinor helicity formalism. The target proton or nucleus is described using the Color Glass Condensate (CGC) formalism. The resulting expressions are used to study azimuthal angular correlations between produced partons in order to probe the gluon structure of the target hadron or nucleus. Our analytic expressions can also be …
Off-diagonal parton distributions occur in several hard exclusive reactions. They extend the study of hadron structure beyond what can be learned from ordinary distributions and have a particularly rich spin structure. The hard scattering subprocesses in electroproduction of mesons and of real photons satisfy helicity selection rules, which provide powerful tools to test leading-twist dominance at a given value of the hard scale.
We calculate for the first time cross sections for single and central diffractive as well as exclusive diffractive production of dilepton pairs in proton-proton collisions. Several differential distributions are shown. The inclusive diffractive processes are calculated using diffractive parton distributions extracted from the analysis of diffractive structure function and dijet production at HERA. We find that the inclusive single-diffractive Drell-Yan process is by about 2 orders of magnitude smaller than ordinary Drell-Yan process. The central-diffractive processes are smaller by one order o…
We studied the impact of future Electron Ion Collider inclusive and semi-inclusive polarized deep inelastic scattering data will have on the determination of the helicity parton distributions. Supplementing the Monte Carlo sampling variant of the DSSV14 analysis with pseudo-data on polarized inclusive and semi-inclusive electron-proton deep inelastic scattering with updated uncertainty estimates and for two different center-of-mass-system energies, $\sqrt{s}=44.7$ GeV and $\sqrt{s}=141.4$ GeV respectively, and on inclusive electron-helium collisions at $\sqrt{s}=115.2$ GeV, we find a remarkabl…
Exclusive $π^o$ electroproduction is suggested for extracting both the tensor charge and the transverse anomalous magnetic moment from experimental data. A connection between partonic degrees of freedom, given in terms of Generalized Parton Distributions, and Regge phenomenology is discussed. Calculations are performed using a physically motivated parametrization that is valid at values of the skewness, $ζ\neq 0$. Our method makes use of information from the nucleon form factor data, from deep inelastuc scattering parton distribution functions, and from lattice results on the Mellin moments of…
We report recent results in rare B decays. Our focus will be on b -> d gamma and b -> s l^+ l^- transitions. We discuss their impact on the CKM phenomenology and their role within our search for new physics. In particular, we analyse the impact of a recent lattice QCD estimate of the B -> K* form factor at zero recoil. We also briefly discuss the presently available optimal theoretical tools for the inclusive and exclusive modes.
An admixture of a right-handed $b \to c$ current in the semileptonic weak decays of the $B$ mesons would give a significantly different contribution to the inclusive rate of the decays $B \to l νX_c$ as compared to the exclusive decay $B \to D^* l ν$ at zero recoil. Thus a difference in the values of $|V_{cb}|$ extracted from the data on these two types of decay would measure such admixture. The present marginal mismatch in determination of $|V_{cb}|$ by the two methods can be interpreted as corresponding to $g_R/g_L \approx 0.14 \pm 0.18$.
We present an analysis of twist-2, leading order QCD amplitudes for hard exclusive leptoproduction of mesons in terms of double/nonforward parton distribution functions. After reviewing some general features of nonforward nucleon matrix elements of twist-2 QCD string operators, we propose a phenomenological model for quark and gluon nonforward distribution functions. The corresponding QCD evolution equations are solved in the leading logarithmic approximation for flavor nonsinglet distributions. We derive explicit expressions for hard exclusive pi^0, eta, and neutral vector meson production am…
Invariant inclusive single particle/jet cross sections in p-p collisions can be factorized in terms of two separable pT dependences, a [pT - sqrt(s)] sector and an [xR - pT - sqrt(s)] sector, where xR = E/Emax is the radial scaling variable. Here, we extend our earlier work by analyzing more extensive data to explore various s-dependent attributes and other systematics of inclusive jet, photon and single particle inclusive reactions. Approximate power laws in sqrt(s), pT and xR are found. Physical arguments are given which relate observations to the underlying physics of parton - parton hard s…
Within the Standard Model, CP rate asymmetries for $B\to K^-π^{+,0}$ could reach 10%. With strong final state phases, they could go up to 20--30%, even for $\bar K^0π^-$ mode which would have opposite sign. We can account for $K^-π^{+}$, $\bar K^0π^-$ and $ϕK$ rate data with new physics enhanced color dipole coupling and destructive interference. Asymmetries could reach 40--60% for $Kπ$ and $ϕK$ modes and are all of the same sign. We are unable to account for $K^-π^0$ rate. Our inclusive study supports our exclusive results.
Hard processes at the TeV scale exhibit enhanced (double log) EW corrections, that need resummation in view of the high level of precision of Next Linear Colliders. The fact that the weak sector is spontaneously broken causes some peculiarities with respect to unbroken theories like QCD. For observables that are exclusive with respect to $W,Z$ emission, some peculiar technical problems have yet to be solved. Surprisingly, double logarithmic enhancements are present even for inclusive observables like $e^+e^-\to$ hadrons, leading to violation of the Bloch-Nordsieck theorem. The last effect is p…
We propose a mechanism which can explain the large BR(B-> X_s eta')~ 10^(-3) observed by CLEO. In this mechanism, eta' is produced by the fusion of two gluons, one from QCD penguin b->sg and the other one emitted by the light quark inside the B meson. The inclusive decay rate which is calculated via using the factorization assumption, can easily account for the observed branching ratio. We also estimate the exclusive branching ratio BR(B->K eta')=7.0X10^(-5) which is in good agreement with the experimental data and present our prediction for K* mode BR(B->K* eta')=3.4X10^(-5).
We present a detailed study, performed in the framework of LO perturbative QCD with the inclusion of spin and k_T effects, of unpolarized cross sections for the Drell-Yan process and for inclusive pion and photon production in hadronic collisions, in different kinematical situations. We find a satisfactory agreement between theoretical predictions and experimental data. This supports the study of spin effects and transverse single spin asymmetries (SSA) within the same scheme. We then present results for SSA, generated by the so-called Sivers effect, in inclusive pion production in proton-prot…
We present a scheme for the generation of central exclusive final states in the Pythia 8 program. The implementation allows for the investigation of higher order corrections to such exclusive processes as approximated by the initial-state parton shower in Pythia 8. To achieve this, the spin and colour decomposition of the initial-state shower has been worked out, in order to determine the probability that a partonic state generated from an inclusive sub-process followed by a series of initial-state parton splittings can be considered as an approximation of an exclusive colour- and spin-singlet…
In our work, we describe two types of Compton processes. As an example of an inclusive process, we consider the high-energy photoproduction of massive muon pairs off the nucleon. We analyze the process in the framework of the QCD parton model, in which the usual parton distributions emerge as a tool to describe the nucleon in terms of quark and gluonic degrees of freedom. To study its exclusive version, a new class of phenomenological functions is required, namely, generalized parton distributions. They can be considered as a generalization of the usual parton distributions measured in deeply …
Exploring the nucleon's sea quark and gluon structure is a prime objective of a future electron-ion collider (EIC). Many of the key questions require accurate differential semi-inclusive (spin/flavor decomposition, orbital motion) and exclusive (spatial distributions of quarks/gluons) DIS measurements in the region 0.01 < x < 0.3 and Q^2 ~ few 10 GeV^2. Such measurements could ideally be performed with a high-luminosity collider of moderate CM energy, s ~ 10^3 GeV^2, and relatively symmetric configuration, e.g. E_e/E_p = 5/30-60 GeV. Specific examples are presented, showing the advantages of t…
The cross section for inclusive charm production by a low-energy antiproton beam is calculated using the parton model and next-to-leading order perturbative QCD. For an antiproton beam with a momentum of 15 GeV, the charm cross section at next-to-leading order in the QCD coupling constant changes by more than an order of magnitude as the charm quark mass is varied from 1.3 to 1.7 GeV. The variations can be reduced by demanding that the same value of the charm quark mass give the measured charm cross sections for fixed-target experiments with a proton beam. The resulting estimate for the charm …
B decay sum rules relate exclusive B semileptonic decay matrix elements to forward B-meson matrix elements of operators in HQET. At leading order the operators that occur are the b-quark kinetic energy $λ_1$ and chromomagnetic energy $λ_2$. The latter is determined by the measured $B^*-B$ mass splitting. The derivation of these sum rules is reviewed and perturbative QCD corrections are discussed. A determination of $λ_1$ and the energy of the light degrees of freedom in a B-meson, $\barΛ$, from semileptonic B decay data is presented. Future prospects for improving these sum rules are discussed…
Exclusive meson leptoproduction from nucleons in the deeply virtual exchanged boson limit can be described by generalized parton distributions (GPDs). Including spin dependence in the description requires 8 independent quark-parton and gluon-parton functions. The chiral even subset of 4 quark-nucleon GPDs are related to nucleon form factors and to parton distribution functions. The chiral odd set of 4 quark-nucleon GPDs are related to transversity, the tensor charge, and other quantities related to transversity. Different meson or photon production processes access different combinations of GP…
We consider the longitudinal polarization of Lambda and Lambda-bar produced in the current fragmentation region of polarized deep inelastic scattering. We show how the various cross sections can be used to test the underlying parton dynamics, and how one can extract information about certain parton densities which are poorly known, in particular the polarized strange density sum Delta-s(x) + Delta-s-bar(x), and about fragmentation functions which are totally unknown and which are difficult to access by other means. We show also how one can obtain information concerning the intriguing question …
The contribution of both inclusive and exclusive cross-section data from HERA to our knowledge of parton distribution functions is reviewed and future prospects are outlined.
I discuss the relation between inclusive and exclusive dynamics suggested by Bloom-Gilman duality. Duality implies the simultaneous applicability of two distinct limits, the standard DIS limit of hard inclusive processes taken at fixed xB and a limit where the hadronic mass is held fixed. I review experimental evidence for the relevance of the fixed mass limit in inclusive processes at high xF. Semi-local duality suggests that inclusive and exclusive processes occur on the same target Fock states. DIS scaling then implies that the Fock states contributing to hard exclusive processes have a lar…
We show for a first time ever a prototype of a fully exclusive QCD NLO parton shower for the initial state (albeit for a limited set of diagrams). It is based on the rigorous theorems of the collinear factorisation, however the standard DGLAP evolution kernels are replaced by their exclusive versions. Contrary to the standard DGLAP approach, the constructed parton shower provides fully exclusive events, i.e. four-momenta of all emitted partons. At the inclusive level it is identical to the MS-bar scheme of the DGLAP evolution.
The exclusive production of one $μ^+μ^-$ pair in collisions of two ultra-relativistic nuclei is considered. We present a simple method for the calculation of the Born cross section for this process based on an improved equivalent photon approximation. We find that the Coulomb corrections to this cross section (corresponding to multi-photon exchange of the produced $μ^{\pm}$ with the nuclei) are small while the unitarity corrections (corresponding to the exchange of light-by-light blocks between nuclei) are large. This is in sharp contrast to the exclusive $e^+e^-$ pair production where the Cou…
