Parton Model Predictions Research Articles

Single and double charmed meson production at the LHC

We discuss production of charmed mesons in proton-proton collisions at the LHC. The cross section for inclusive production of cc pairs is calculated in the frame-work of the k ⊥ -factorization approach which effectively includes next-to-leading order corrections. Theoretical uncertainties of the model related to the choice of renormalization and factorization scales as well as due to the quark mass are discussed. Results of the k ⊥ -factorization approach are compared to NLO parton model predictions. The hadronization of charm quarks is included with the help of the Peterson fragmentation functions. Inclusive differential distributions in transverse momentum for several charmed mesons ( D 0 , D ± , D s ± ) are calculated and compared to recent results of the ALICE, ATLAS and LHCb collaborations. Furthermore, we also discuss production of two pairs cc of within a simple formalism of double-parton scattering (DPS). Surprisingly large cross sections, comparable to single-parton scattering (SPS) contribution, are predicted for LHC energies. We compare our predictions for double charm production ( DD meson-meson pairs) with recent results of the LHCb collaboration for azimuthal angle ϕ DD and rapidity distance between mesons Y DD . Meson-meson kinematical correlations are also confronted with those related to the standard meson-antimeson measurements. Our calculations clearly confirm the dominance of DPS in the production of events with double charm, however some strength seems to be still lacking. Possible missing contribution within the framework of single-ladder-splitting DPS mechanism is also discussed.

On the role of heavy flavor parton distributions at high energy colliders

We compare ‘fixed flavor number scheme’ (FFNS) and ‘variable flavor number scheme’ (VFNS) parton model predictions at high energy colliders. Based on our recent LO- and NLO-FFNS dynamical parton distributions, we generate radiatively two sets of VFNS parton distributions where also the heavy quark flavors h=c,b,t are considered as massless partons within the nucleon. By studying the role of these distributions in the production of heavy particles (hh¯, tb¯, hW±, Higgs–bosons, etc.) at high energy ep, pp¯ and pp colliders, we show that the VFNS predictions are compatible with the FFNS ones (to within about 10–20% at LHC, depending on the process) when the invariant mass of the produced system far exceeds the mass of the participating heavy quark flavor.

Quark-hadron duality and parity violating asymmetry of electroweak reactions in the Δ region

A dynamical model [T. Sato and T.-S. H. Lee, Phys. Rev. C 54, 2660 (1996); 63, 055201 (2001); T. Sato, D. Uno, and T.-S. H. Lee, ibid. 67, 065201 (2003)] of electroweak pion production reactions in the {delta}(1232) region has been extended to include the neutral current contributions for examining the local quark-hadron duality in neutrino-induced reactions and for investigating how the axial N-{delta} form factor can be determined by the parity violating asymmetry of N(e{sup {yields}},e{sup '}) reactions. We first show that the recent data of (e,e{sup '}) structure functions F{sub 1} and F{sub 2}, which exhibit the quark-hadron duality, are in good agreement with our predictions. For possible future experimental tests, we then predict that the structure functions F{sub 1},F{sub 2}, and F{sub 3} for ({nu},e) and ({nu},{nu}{sup '}) processes also show the similar quark-hadron duality. The spin-dependent structure functions g{sub 1} and g{sub 2} of (e,e{sup '}) have also been calculated from our model. It is found that the local quark-hadron duality is not seen in the calculated g{sub 1} and g{sub 2}, while our results for g{sub 1} and some polarization observables associated with the exclusive p(e{sup {yields}},e{sup '}{pi}) and p{sup {yields}}(e{sup {yields}},e{sup '}{pi}) reactions aremore » in reasonably good agreement with the recent data. In the study of parity violating asymmetry A of N(e{sup {yields}},e{sup '}) reactions, the relative importance between the nonresonant mechanisms and the {delta} excitation is investigated by taking into account the unitarity condition. Predictions are made for using the data of A to test the axial N-{delta} form factors determined previously in the studies of N({nu}{sub {mu}},{mu}{sup -}{pi}) reactions. The predicted asymmetry A are also compared with the parton model predictions for future experimental investigations of quark-hadron duality.« less

Electromagnetic response of confined Dirac particles

The eigenstates of a single massless Dirac particle confined in a linear potential are calculated exactly by direct solution of the Dirac equation. The electromagnetic structure functions are calculated from the Dirac wave functions of the ground and excited states of the particle by coupling to its conserved vector current. We obtain the longitudinal and transverse structure functions as a function of y=nu-q, where nu and q are the energy and momentum transferred to the target in its rest frame. At values of q>~2.5 GeV, much larger than the characteristic energy scale ~440 MeV of the confining potential, the response exhibits y scaling, a generalization of Bjorken scaling. We compare the exact structure functions with those obtained from the ground state wave functions in the plane wave impulse approximation. The deviation from the Callan-Gross relation is compared with the parton model prediction.

Parton distributions and heavy quark production on nucleon and nuclear targets

Abstract The perturbative QCD evolution equation and the respective parton distributions in a proton are discussed, mainly in small x region. The different possibilities of shadow corrections are considered which allow one to avoid the unitarity violation. The possibility of estimation of the quality of phenomenological sets of the structure functions on the base of perturbative QCD is presented. Two approximate methods to extract the gluon distributions directly from the F2(x, Q2) data are discussed together with the new HERA data. Charm and beauty hadroproduction and photoproduction processes are considered in the framework of perturbative QCD and the results are compared with conventional parton model predictions. The role of infrared region contribution to F2(xQ2) structure function is discussed. The case of nuclear targets is considered also and A-dependences of charmed and beauty hadron production cross sections are calculated accounting for the difference of parton distributions in free and bound n...

Quark model description of polarised deep inelastic scattering and the prediction of g2

We show how the operator product expansion evaluated in the approximation of ignoring gluons leads to the covariant formulation of the quark parton model. We discuss the connection with other formulations and show how the free quark model prediction, g 2 = 0, changes smoothly into the Wandzura-Wilczek (WW) relation for quark masses small relative to the nucleon mass. Previous contradictory parton model predictions are shown to follow from an inconsistent treatment of the mass shell conditions. The description is extended to include quark mass corrections.

Heavy quark production in parton model and in QCD

The cross sections of heavy quark production inpp collision and for their photo- and electroproduction are calculated in the framework of QCD. The virtual nature of the interacting gluons as well as their transverse motion and different polarizations are taken into account. The obtained cross sections exhibit more rapid growth with the initial energy than the parton model predictions and thepT distributions of produced heavy quarks are more smooth.

Third-family flavor physics in an SU(3)3 x SU(2)L x U(1)Y model.

We consider a model in which each family transforms under a different SU(3) color group. The low-energy effective theory is an extension of the Standard Model, with additional color octet gauge bosons $G_H$ with mass $M$ that couple preferentially to the third family quarks. We show that there are two distinct regions of the model's parameter space in which we can simultaneously evade all the current experimental constraints, one with $M \approx 250$ GeV and the other with $M \gtrsim 600$ GeV. Within each allowed region, we can obtain a correction to the $Zb\overline{b}$ vertex that is consistent with the slightly high value of $R_b$ observed at LEP. We show that there are $\Delta B = 1$ operators in our model that can suppress the $B$-meson semileptonic branching ratio $B_{SL}$ and the charm multiplicity per decay $n_c$ by enough to reconcile the spectator parton model predictions with the experimental data. In the non-supersymmetric version of our model, we can only obtain the desired corrections to $R_b$, $B_{SL}$ and $n_c$ in different regions of the allowed parameter space, while in the supersymmetric version, we can obtain all three corrections simultaneously. We also discuss a strong-coupling limit of our model in which the third-family quarks become composite.

Semi-exclusive pion production in deep-inelastic scattering

We calculate azimuthal asymmetries and the Callan-Gross R-ratio for semi-exclusive pion production in deep inelastic scattering taking into account higher twist effects. Our results are qualitatively different from the QCD-improved parton model predictions for semi-inclusive deep inelastic scattering.

Heavy quark production in parton model and in QCD

The cross sections of heavy quark production in $pp$ collision and for their photo- and electroproduction are calculated in the framework of QCD. The virtual nature of the interacting gluons as well as their transverse motion and different polarizations are taken into account. The obtained cross sections exhibit more rapid growth with the initial energy than the parton model predictions and the $p_T$ distributions of produced heavy quarks are more smooth.

Estimation of ΔΓ for the [formula omitted] system. Exclusive decays and the parton model

We estimate ΔΓ for the B s− B s system by making a sum over exclusive decay modes into ground state mesons, and compare our result to the parton model prediction. For the modes D s D s, D s D ∗ s + D ∗ s D s and D ∗ s D ∗ s (class I) which give the largest contribution to ΔΓ, we find duality between the exclusive calculation and the parton model in the heavy quark limit with 2 m c → m b and with the standard factorization assumption, which amounts to exact duality in the N c → ∞ limit. In this limit, for the dominant spectator contribution, we find the selection rule Γ[B 2(CP=−)→D s D ∗ s]=0 , so that the modes D s D ∗ s + D ∗ s D s contribute constructively to ΔΓ, with the same sign as D s D s (S wave) and D ∗ s D ∗ s ((S + D)-waves). Most of the modes of the modes of class II (ψη, η cϕ, ψϕ, …) contribute also constructively to ΔΓ but are suppressed by the effective color factor. On the other hand, from a purely theoretical point of view, we find a rather subtle and interesting difficulty as regards duality in the standard factorization assumption when considering together classes I and II. This unwanted violation of duality could correspond to the well known necessity of correcting this standard factorization assumption. On the basis of both the exclusive and the parton model calculations, we conclude that the sign of ΔΓ is well-defined and its magnitude is stable against theoretical uncertainties. Our estimate gives ΔΓ Γ of the order of 0.15, a value that could be measured at LEP.

The rule of discarding 1/ Nc in inclusive weak decays (I)

We continue to investigate the effects of the (dominant) subleading operator of the σH type in the 1/ N c parts of the weak nonleptonic amplitudes. If the previous work concentrated on exclusive decays now we analyse inclusive widths in a certain kinematic limit. Deviations from the parton model predictions are found due to soft-gluon color exchange. In the interference (i.e. 1/ N c) terms they are stronger than in the non-interferene terms. The sign is negative so that the 1/ N c parts tend to cancel. The absolute value of the effect is of order 1 in D-mesons and of order 0.25 in B-mesons.

Average rapidity shift of leading baryons at ultra-relativistic energies

We present the Dual Parton Model predictions for the average rapidity shiftexperienced by a baryon traveling through the nuclear matter as a function of the number of baryon-nucleon collisions. We show results for different values of the baryon-nucleon center-of-mass energy and compare them to the predictions of the Quark-Gluon String Model. Also the inelasticity parameter is shown for the same cases.

Space-time structure of deep-inelastic lepton-hadron scattering.

We discuss the space-time structure of deep-inelastic scattering in the target rest frame. At small Bjorken {ital x}, the process is dominated by quark pair production, and the Ioffe time'' between the production of the pair and its interaction in the target is long. We compute the leading logarithmic corrections to the parton model predictions for the virtual photoabsorption cross section, and analyze the transverse size of the pair and the Ioffe time as a function of the dynamical variables of the pair constituents. Both the transverse size and the Ioffe time depend significantly on the polarization (longitudinal or transverse) of the virtual photon. Hence one may expect that nuclear scattering corrections, including shadowing, may also be polarization dependent.

Inclusive D∗± production in photon-photon collisions

Abstract The TPC/Two-Gamma Collaboration has measured the inclusive cross section for production of charmed D∗± mesons in photon-photon collisions. The reaction utilized was e+e-→e+e-D∗±X, with D∗±→DOπ+-, DO→K-+π ±, and either zero or one outgoing e± detected. The result, σ(e + e - → e + e - D ∗± X) = 74±26±19 pb , is in agreement with the quark parton mo del prediction for e+e- → e+e-c c , combined with a Lund model for the hadronization of the charmed quarks.

When is a heavy quark not a parton? Charged Higgs production and heavy quark mass effects in the QCD-based parton model

Several issues pertaining to the application of the QCD-based parton model to new physics processes involving heavy partons are described and quantitatively studied using charged Higgs boson production as a prime example. The naive parton model predictions are found to over-estimate the actual cross section by a factor of 2 to 5, depending on the top-quark and Higgs masses. The role of the top quark as a “parton” is examined by a detailed study of the cancellation between the straight parton model contribution and a subtraction term required by QCD corrections. The accuracy of the zero-mass method for evaluating the first-order QCD correction is assessed (in light of the potentially large mass of the top quark) by a quantitative analysis of the cancellation of mass singularities between the correction terms. A pragmatic procedure for calculation based on a renormalization scheme without the heavy quark-parton is formulated and compared with the usual perturbative QCD formalism. The energy ranges over which heavy quarks (or other particles) should or should not be naturally treated as “partons” are delineated. Properly evolved parton distribution functions relevant to the specific renormalization schemes considered are employed for all the numerical studies in order to ensure consistency in the QCD framework.

Inclusive cross section ratios in highp T proton-proton scattering at ISR energies

Ratios of inclusive cross sections σ(π+)/σ(π++K++p) and\(\sigma (\pi ^ - )/\sigma (\pi ^ - + K^ - + \bar p)\) were measured for proton-proton interactions with a highp T hadron in the final state around c.m.s. scattering angles θ≅20°, 20° and 45° at two ISR energies\(\sqrt s = 31\) Gev and 62 GeV. Results are shown as functions of transverse and longitudinal momentum and are compared with parton model predictions. The different dependences of positive and negative pion fractions atp T ≅2–3 GeV/c on longitudinal momenta is similar to that observed in soft hadronic interactions at low values ofp T where the leading proton effect (diquark fragmentation) is known to contribute. The quantitative agreement of the data with diquark model predictions indicates the presence of diquark fragmentation also in highp T jets.

Why are perturbative corrections to highP T hadron production in photon-photon collisions very small?

TheO(αs) corrections to the single hadron inclusive cross-section at high transverse momentum has recently ben found to be very small. This is in sharp contrast to similar corrections to Drell-Yan cross-sections where the first perturbative terms can more than double the size of naive parton model predictions. In this paper we present an intuitive explanation of these results. By considering gauge symmetries of various sets of Feynman diagrams, we have obtained well defined relationships between photon-photon ande+e− annihilation cross-sections. These have been used to study dominant sectors of the phase-space for the two processes, resulting in a simple interpretation of the perturbative results. A by-product is a much simplified algorithm which has enabled us to extend our previous no-tag calculation to the case of single-tag events. In particular, by keeping one of the incomming photons off-shell by an arbitrary amount, we have exposed photon mass dependence of the cross-section right down to the region of deep inelastic scattering on a photon target.

Second order QCD prediction for direct photon production in e +e − annihilation

We compute the deep inelastic structure function for the decay of a heavy time-like photon into a quasi real photon and hadrons. We show that the QCD corrections imply important modifications to the parton model prediction in the whole x range. It is also found that the next to leading QCD contribution is quite large as compared to the leading one. We finally investigated the relationship between this structure function and its space-like analogue.

Data on the Gross-Llewellyn Smith Sum Rule as a Function ofq2

Data are presented on the Gross--Llewellyn Smith sum rule obtained from combined narrow-band neon and Freon bubble-chamber neutrino-antineutrino experiments. Remarkably no significant deviation from the parton-model prediction for the sum rule is observed at very low values of q/sup 2/< or approx. =1 GeV/sup 2/. Limits on the effective QCD scale parameter ..lambda.. and on the magnitude of the twist-4 correction are set. The best fit, neglecting higher-twist contributions, gives ..lambda.. = 92/sub -/36/sup +20/ MeV.