B. Blok

pQCD physics of multiparton interactions

We study production of two pairs of jets in hadron–hadron collisions in view of extracting contribution of double hard interactions of three and four partons (3→4, 4→4). Such interactions, in spite of being power suppressed at the level of the total cross section, become comparable with the standard hard collisions of two partons, 2→4, in the back-to-back kinematics when the transverse momentum imbalances of two pairing jets are relatively small.We express differential and total cross sections for two-dijet production in double parton collisions through the generalized two-parton distributions, 2GPDs (Block et al., Phys. Rev. D 83, 071501, 2011), and treat them in the leading logarithmic approximation of pQCD that resums collinear logarithms in all orders.A special emphasis is given to 3→4 double hard interaction processes which, being of the same order in

Four-jet production at LHC and Tevatron in QCD

\alpha _{\mathrm {s}}

Perturbative QCD correlations in multi-parton collisions

as the 4→4 process, turn out to be geometrically enhanced compared to the latter and should contribute significantly to four-jet production.The framework developed here takes into systematic consideration perturbative Q2 evolution of 2GPDs. It can be used as a basis for future analysis of NLO corrections to multiparton interactions (MPI) at LHC and Tevatron colliders, in particular for improving evaluation of QCD backgrounds to new physics searches.

Regge asymptotics and color suppressed heavy meson decays

By exploring the difference in scales between soft and hard QCD processes and space-time struc- ture of Feynman diagrams we derive within the parton model the general formulae for the two dijet production in pp collisions. Cross section resulting from collisions of four partons is calculated in terms of new generalized two-particle parton distributions (GPDs) in proton. We explain why the derived formulae hold modulo simple modifications even when the pQCD corrections are taken into account. Our derivation justifies the intuitive geometrical approach in the impact parameter space used before to describe the four-jet production. Comparison with CDF and D0 data shows that the independent parton approximation to light cone wave function of proton is insufficient to explain the data. We argue that the nonperturbative multi-parton correlations in the light cone wave functions of colliding protons play an important role. We explain kinematics where the discussed process is enhanced as compared to the four jet production in the hard collisions of two partons.

Calculation of 1/m(c)**3 terms in the total semileptonic width of D mesons

We examine the role played in double-parton interactions (DPI) by the parton–parton correlations originating from perturbative QCD parton splittings. Also presented are the results of the numerical analysis of the integrated DPI cross sections at Tevatron and LHC energies. To obtain the numerical results the knowledge of the single-parton GPDs gained by the HERA experiments was used to construct the non-perturbative input for generalized double-parton distributions. The perturbative two-parton correlations induced by three-parton interactions contribute significantly to a resolution of the longstanding puzzle of an excess of multi-jet production events in the back-to-back kinematics observed at the Tevatron.

Collectivity from interference

Abstract We discuss a possible generation of color suppressed B -decays amplitudes through a soft final state interaction. As a typical example, we consider in detail the decay B 0 → D 0 π 0 (and also B 0 → 2π 0 ). We show that in the approximation of the two particle unitarity and at zero order in α s this process can be related to the weak decay B 0 → D + π − followed by the strong charge exchange scattering in the Regge kinematics. We estimate the amplitude of this process using the light cone QCD sum rule technique and find that it is suppressed as a power of 1 m B in comparison to the amplitude generated by the effective non-leptonic Hamiltonian, but remains important for the physical value of m B

Hard four-jet production in pA collisions

We calculate the 1/{ital m}{sub {ital c}}{sup 3} corrections in the inclusive semileptonic widths of {ital D} mesons. We show that these are due to the novel penguin-type operators that appear at this level in the transition operator. Taking into account the nonperturbative corrections leads to the predicted value of the semileptonic width significantly lower than the experimental value. The 1/{ital m}{sub {ital c}}{sup 3} corrections worsen the situation or, at the very least, within uncertainty, give a small contribution. We indicate possible ways out of this problem. It seems most probable that violations of duality are noticeable in the energy range characteristic to the inclusive decays in the charm family. Theoretically these deviations are related to divergence of the high-order terms in the power expansion in the inverse heavy quark mass.

Open charm production in double parton scattering processes in the forward kinematics

A bstractIn hadronic collisions, interference between different production channels affects momentum distributions of multi-particle final states. As this QCD interference does not depend on the strong coupling constant αs, it is part of the no-interaction baseline that needs to be controlled prior to searching for other manifestations of collective dynamics, e.g., in the analysis of azimuthal anisostropy coefficients vn at the LHC. Here, we introduce a model that is based on the QCD theory of multi-parton interactions and that allows one to study interference effects in the production of m particles in hadronic collisions with N parton-parton interactions (“sources”). In an expansion in powers of 1/(Nc2 − 1) and to leading order in the number of sources N , we calculate interference effects in the m-particle spectra and we determine from them the second and fourth order cumulant momentum anisotropies vn{2} and vn{4}. Without invoking any azimuthal asymmetry and any density dependent non-linear dynamics in the incoming state, and without invoking any interaction in the final state, we find that QCD interference alone can give rise to values for vn{2} and vn{4}, n even, that persist unattenuated for increasing number of sources, that may increase with increasing multiplicity and that agree with measurements in proton-proton (pp) collisions in terms of the order of magnitude of the signal and the approximate shape of the transverse momentum dependence. We further find that the non-abelian features of QCD interference can give rise to odd harmonic anisotropies. These findings indicate that the no-interaction baseline including QCD interference effects can make a sizeable if not dominant contribution to the measured vn coefficients in pp collisions. Prospects for analyzing QCD interference contributions further and their possible relevance for proton-nucleus and nucleus-nucleus collisions are discussed shortly.

PARTON-HADRON DUALITY IN NONLEPTONIC B HADRON DECAYS

In a suitably chosen back-to-back kinematics, four-jet production in hadronic collisions is known to be dominated by contributions from two independent partonic scattering processes, thus giving experimental access to the structure of generalized two-parton distributions (2GPDs). Here, we show that a combined measurement of the double hard four-jet cross section in proton–proton and proton–nucleus collisions will allow one to disentangle different sources of two-parton correlations in the proton that cannot be disentangled with 4-jet measurements in proton–proton collisions alone. To this end, we analyze in detail the structure of 2GPDs in the nucleus (A), we calculate in the independent nucleon approximation all contributions to the double hard four-jet cross section in pA, and we determine corrections arising from the nuclear dependence of single parton distribution functions. We then outline an experimental strategy for determining the longitudinal two-parton correlations in the proton.

Large distance behavior of the light cone operator product in perturbative and nonperturbative QCD regimes

We calculate the rate of double open charm production in the forward kinematics studied recently in the LHCb experiment. We find that the mean field approximation for the double parton GPD (generalized parton distributions), which neglects parton–parton correlations, underestimates the rate by a factor of 2. The enhancement due to the perturbative QCD correlation