Direct Production Mechanism Research Articles

Bottomonium production and polarization in the NRQCD with k_T-factorization. III: Upsilon (1S) and chi _b(1P) mesons

The Upsilon (1S) meson production and polarization at high energies is studied in the framework of the k_T-factorization approach. Our consideration is based on the non-relativistic QCD formalism for a bound states formation and off-shell production amplitudes for hard partonic subprocesses. The direct production mechanism, feed-down contributions from radiative chi _b(mP) decays and contributions from Upsilon (3S) and Upsilon (2S) decays are taken into account. The transverse momentum dependent (TMD) gluon densities in a proton were derived from the Ciafaloni–Catani–Fiorani–Marchesini evolution equation and the Kimber-Martin–Ryskin prescription. Treating the non-perturbative color octet transitions in terms of multipole radiation theory, we extract the corresponding non-perturbative matrix elements for Upsilon (1S) and chi _b(1P) mesons from a combined fit to transverse momenta distributions measured at various LHC experiments. Then we apply the extracted values to investigate the polarization parameters lambda _theta , lambda _phi and lambda _{theta phi }, which determine the Upsilon (1S) spin density matrix. Our predictions have a reasonably good agreement with the currently available Tevatron and LHC data within the theoretical and experimental uncertainties.

Bottomonia production and polarization in the NRQCD with k_T-factorization. II: Upsilon (2S) and chi _b(2P) mesons

The Upsilon (2S) production and polarization at high energies is studied in the framework of k_T-factorization approach. Our consideration is based on the non-relativistic QCD formalism for bound states formation and off-shell production amplitudes for hard partonic subprocesses. The direct production mechanism, feed-down contributions from radiative chi _b(3P) and chi _b(2P) decays and contributions from Upsilon (3S) decays are taken into account. The transverse momentum dependent gluon densities in a proton were derived from the Ciafaloni–Catani–Fiorani–Marchesini evolution equation, Kimber-Martin-Ryskin prescription and Parton Branching method. Treating the non-perturbative color octet transitions in terms of the mulitpole radiation theory, we extract the corresponding non-perturbative matrix elements for Upsilon (2S) and chi _b(2P) mesons from a combined fit to Upsilon (2S) transverse momenta distributions measured by the CMS and ATLAS Collaborations at the LHC energies sqrt{s} = 7 and 13 TeV and from the relative production rate R^{chi _b(2P)}_{Upsilon (2S)} measured by the LHCb Collaboration at sqrt{s} = 7 and 8 TeV. Then we apply the extracted values to investigate the polarization parameters lambda _theta , lambda _phi and lambda _{theta phi }, which determine the Upsilon (2S) spin density matrix. Our predictions have a good agreement with the currently available data within the theoretical and experimental uncertainties.

Prompt charmonia production and polarization at the LHC in the NRQCD with kT -factorization. III. J/ψ meson

In the framework of $k_T$-factorization approach, the production and polarization of prompt $J/\psi$ mesons at the LHC energies is studied. Our consideration is based on the non-relativistic QCD formalism for bound states and off-shell amplitudes for hard partonic subprocesses. Both the direct production mechanism and feed-down contributions from $\chi_c$ and $\psi(2S)$ decays are taken into account. The transverse momentum dependent (or unintegrated) gluon densities in a proton were derived from Ciafaloni-Catani-Fiorani-Marchesini evolution equation or, alternatively, were chosen in accordance with Kimber-Martin-Ryskin prescription. The non-perturbative color-octet matrix elements were first deduced from the fits to the latest CMS data on $J/\psi$ transverse momentum distributions and then applied to describe the ATLAS and LHCb data on $J/\psi$ production and polarization at $\sqrt s = 7$, $8$ and $13$ TeV. We perform an estimation of polarization parameters $\lambda_\theta$, $\lambda_\phi$ and $\lambda_{\theta \phi}$ which determine $J/\psi$ spin density matrix and demonstrate that treating the soft gluon emission as a series of explicit color-electric dipole transitions within NRQCD leads to unpolarized $J/\psi$ production at high transverse momenta, that is in qualitative agreement with the LHC data.

Differentiating the production mechanisms of the Higgs-like resonance using inclusive observables at hadron colliders

We present a study on differentiating direct production mechanisms of the newly discovered Higgs-like boson at the LHC based on several inclusive observables. The ratios introduced reveal the parton constituents or initial state radiations involved in the production mechanisms, and are directly sensitive to fractions of contributions from different channels. We select three benchmark models, including the SM Higgs boson, to illustrate how the theoretical predictions of the above ratios are different for the $gg$, $b\bar b(c\bar c)$, and $q\bar q$ (flavor universal) initial states in the direct production. We study implications of current Tevatron and LHC measurements. We also show expectations from further LHC measurements with high luminosities.

PromptJ/ψproduction at the LHC: New evidence for thekTfactorization

In the framework of the kt-factorization approach, the production and polarization of prompt J/Psi mesons in pp collisions at the LHC energy 7 TeV is studied. Both the direct production mechanism as well as feed-down contributions from chic1, chic2 and psi' decays are taken into account. Our consideration is based on the color singlet model supplemented with the off-shell matrix elements for the corresponding partonic subprocesses. The unintegrated gluon densities in a proton are determined using the CCFM evolution equation as well the Kimber-Martin-Ryskin prescription. We compare our numerical predictions with the first experimental data taken by the CMS, ATLAS and LHCb collaborations. The estimation of all polarization parameters which determine J/Psi spin density matrix is performed.

Charged- and neutral-pion production in theS-matrix approach

The $S$-matrix approach is used to calculate both charged- and neutral-pion production in nucleon-nucleon ($\mathit{NN}$) scattering near threshold. The irreducible pion-rescattering diagram, direct production mechanism, \ensuremath{\Delta} isobars in intermediate states, and $Z$ diagrams mediated by heavy isoscalar mesons are included in the calculation. For the $\mathit{NN}$ distortions, we considered a realistic interaction, within the Bonn family of potentials, which describes the nucleonic inelasticities above the pion production energy threshold.

Charged Higgs boson production from supersymmetric particle cascade decays at the CERN LHC

We analyze the cascade decays of the scalar quarks and gluinos of the Minimal Supersymmetric extension of the Standard Model, which are abundantly produced at the Large Hadron Collider, into heavier charginos and neutralinos which then decay into the lighter ones and charged Higgs particles, and show that they can have substantial branching fractions. The production rates of these Higgs bosons can be much larger than those from the direct production mechanisms, in particular for intermediate values of the parameter $\tan \beta$, and could therefore allow for the detection of these particles. We also discuss charged Higgs boson production from direct two-body top and bottom squark decays as well as from two- and three-body gluino decays.

Turbulence measurement in a separated and reattaching flow over a backward-facing step with the aid of three-dimensional particle tracking velocimetry

With a three-dimensional particle tracking velocimeter, detailed turbulent flow measurement was made in a plane channel with a one-sided 50% abrupt expansion, which acted as a backward-facing step. The plane channel flow reached a fully developed state well upstream of the step. The Reynolds number based on the center-line velocity, U c , and the step height, H, was about 5540. The measurement region ranged from −2H upstream to 12H downstream of the step, and it was divided into about 300 × 120 sub-regions when the turbulence statistics were calculated. The mean reattachment position, where the forward flow fraction near the wall equaled to 0.5, was found to be located at 6.51H downstream of the step. All the distributions of mean and fluctuating velocities well demonstrated characteristics of the separated flow. As in the previous experimental investigations, the Reynolds normal and shear stresses had the maximum values upstream of the reattachment. The anisotropy tensor, which was calculated from the Reynolds stress tensor, revealed a peculiar phenomenon near the reattachment point that, even without any direct production mechanism, the spanwise normal stress was the largest among the three normal stresses. The ratio between the Reynolds shear stress and the turbulent kinetic energy showed considerably large values in the free shear layer downstream of the step shoulder.

Turbulence measurement in a separated and reattaching flow over a backward-facing step with the aid of three-dimensional particle tracking velocimetry

With a three-dimensional particle tracking velocimeter, detailed turbulent flow measurement was made in a plane channel with a one-sided 50% abrupt expansion, which acted as a backward-facing step. The plane channel flow reached a fully developed state well upstream of the step. The Reynolds number based on the center-line velocity, U c, and the step height, H, was about 5540. The measurement region ranged from −2H upstream to 12H downstream of the step, and it was divided into about 300 × 120 sub-regions when the turbulence statistics were calculated. The mean reattachment position, where the forward flow fraction near the wall equaled to 0.5, was found to be located at 6.51H downstream of the step. All the distributions of mean and fluctuating velocities well demonstrated characteristics of the separated flow. As in the previous experimental investigations, the Reynolds normal and shear stresses had the maximum values upstream of the reattachment. The anisotropy tensor, which was calculated from the Reynolds stress tensor, revealed a peculiar phenomenon near the reattachment point that, even without any direct production mechanism, the spanwise normal stress was the largest among the three normal stresses. The ratio between the Reynolds shear stress and the turbulent kinetic energy showed considerably large values in the free shear layer downstream of the step shoulder.

On the interactions between large-scale structure and fine-grained turbulence in a free shear flow. III. A numerical solution

We present the results of a numerical computation of the interactions between the horizontally periodic monochromatic component of a large-scale coherent structure and the fine-grained turbulence in a mixing layer. The dependent variable characterizing the coherent structure is thetotalflow quantity which includes both the Reynolds-averaged (here the horizontal average) mean flow and the large-scale coherent structure, this being distinguished from the fine-grained turbulence by a conditional average linked to the periodicity of the coherent structure. The dependent variables characterizing the fine-grained turbulence are the conditionally averaged turbulent stresses, which are themselvestotalquantities comprising the Reynoldsaveraged mean stresses and the coherent-structure induced or modulated stresses. The transport equations for such conditionally averaged quantities are identical in form to the Reynolds-mean equations. Closure approximations for the Reynolds-mean stresses due to Launderet al. (1975) are directly extended to the conditionally averaged stresses. These approximations include the pressure-velocity strain redistribution which accounts for rapid distortions due to the large-scale coherent structure, a simple diffusional approximation to the triple correlations and an approximate transport equation for the rate of viscous dissipation. The large-scale structure is two dimensional with its vorticity axis perpendicular to the mean flow direction, the fine-grained turbulence being three dimensional. The initial conditions are made self-consistent by numerically solving the horizontally homogeneous, time-dependent, Reynolds-mean problem in the absence of coherent structure until the mean Reynolds stresses and viscous dissipation become self-similar as the shear layer growth rate becomes constant. The solution to the Rayleigh equation with this self-similar mean velocity profile is then obtained. The wavenumber mode which corresponds to the most-amplified case is thensuddenlyimposed, together with the Reynolds mean velocity, as the initial condition for the large-scale structure. It is argued that, since the coherent structure-induced stresses and viscous dissipation function develop in finite time, the initial conditions for the conditionally averaged turbulence quantities are precisely those for the initial self-similar Reynolds-mean quantities, consistent with the sudden imposition of a large-scale structure upon the flow. The structural details are presented in terms of the time evolution of the conditionally averaged streamfunction and vorticity contours. While the streamline patterns resemble Kelvin’s cats’ eyes, the vorticity patterns display large non-uniformities within the cats’ eyes; thus it is not possible to construct an analytical nonlinear critical layer theory for this problem consistent with the numerical results. The physical interpretation is presented, after Reynolds (horizontal) averaging, in terms of the time evolution of global energy-exchange mechanisms between the Reynolds-averaged mean flow, the large-scale coherent structure and the fine-grained turbulence. The mean flow loses energy to both components of the oscillations, the large-scale structure gains energy from the mean flow and loses energy to the fine-grained turbulence, and the fine-grained turbulence gains energy from both the mean flow and the large-scale structure and loses kinetic energy through viscous dissipation of the small eddies. The possibility that the large-scale structure transfers energy back to the mean flow (like ‘damped’ disturbances in hydrodynamic stability interpretations) is also shown. Owing to the nonlinear interaction between the mean flow and large-scale structure, through the action of the large-scale Reynolds stresses, the global energy of the large-scale structure first increases and then decays with time. Such a growth and decay reflects the inability, at some time, of the largescale to effectively transfer energy from the mean flow and to subsequently transfer energy back to the mean flow through the action of the large-scale Reynolds stresses. The simultaneous evolution of the global fine-grained turbulent energy exhibits an increase from its initial self-preservation level to a final, higher one, in response to the new mean flow condition caused by the evolution of the large-scale structure. This physical picture of the global energy evolution is consistent with previous much-simplified analyses of the temporal problem (Liu & Merkine 1976a) and approximately consistent with results for the spatial problem as well (Alper & Liu 1978). The structural details of the direct production mechanisms of the conditionally averaged horizontal and vertical contributions to the fine-grained turbulence energy from the two dimensional, large-scale structure are shown in terms of contour plots for the cats’ eyes regions; of these, the dominant production mechanisms essentially resemble the contours of the horizontal and vertical contributions to the turbulence energy. Since there is no direct production mechanism for the spanwise contribution to the turbulence energy, this contribution must arise entirely from the pressure-velocity strain redistribution mechanism obtained from the closure approximation. The structural details of this redistribution mechanism essentially resemble those of the spanwise contribution to the turbulence energy. Certain apsects of available experimental observations are interpreted in terms of the present considerations. Finally, the importance of well-controlled experiments to further study the large-scale coherent structures in free turbulent shear flows is re-emphasized.