Mixing Improvement in a T-Shaped Micro-Junction through Small Rectangular Cavities

The T-shaped micro-junction is among the most used geometry in microfluidic applications, and many design modifications of the channel walls have been proposed to enhance mixing. In this work, we investigate through numerical simulations the introduction of one pair of small rectangular cavities in the lateral walls of the mixing channel just downstream of the confluence region. The aim is to preserve the simple geometry that has contributed to spread the practical use of the T-shaped micro-junction while suggesting a modification that should, in principle, work jointly with the vortical structures present in the mixing channel, further enhancing their efficiency in mixing without significant additional pressure drops. The performance is analyzed in the different flow regimes occurring by increasing the Reynolds number. The cavities are effective in the two highly-mixed flow regimes, viz., the steady engulfment and the periodic asymmetric regimes. This presence does not interfere with the formation of the vortical structures that promote mixing by convection in these two regimes, but it further enhances the mixing of the inlet streams in the near-wall region of the mixing channel without any additional cost, leading to better performance than the classical configuration

Where is the pseudoscalar glueball ?

The pseudoscalar mesons with the masses higher than 1 GeV are assumed to belong to the meson decuplet including the glueball as the basis state supplementing the standard $SU(3)_F$ nonet of light $q\bar{q}$ states $(u,d,s)$. The decuplet is investigated by means of an algebraic approach based on hypothesis of vanishing the exotic $SU(3)_F$ commutators of "charges" and their time derivatives. These commutators result in a system of equations determining contents of the isoscalar octet state in the physical isoscalar mesons as well as the mass formula including all masses of the decuplet: $\pi(1300)$, K(1460), $\eta(1295)$, $\eta(1405)$ and $\eta(1475)$. The physical isoscalar mesons $\eta_i$, are expressed as superpositions of the "ideal" $q\bar{q}$ states ($N$ and $S$) and the glueball $G$ with the mixing coefficient matrix following from the exotic commutator restrictions. Among four one-parameter families of the calculated mixing matrix (numerous solutions result from bad quality of data on the $\pi(1300)$ and K(1460) masses) there is one family attributing the glueball-dominant composition to the $\eta(1405)$ meson. Similarity between the pseudoscalar and scalar decuplets, analogy between the whole spectra of the $0^{-+}$ and $0^{++}$ mesons and affinity of the glueball with excited $q\bar{q}$ states are also noticed.Comment: 18 pp., 2. figs., 2 tabs.; Published version. One of the authors withdraws his nam

Exploring improved holographic theories for QCD: Part II

This paper is a continuation of ArXiv:0707.1324 where improved holographic theories for QCD were set up and explored. Here, the IR confining geometries are classified and analyzed. They all end in a "good" (repulsive) singularity in the IR. The glueball spectra are gaped and discrete, and they favorably compare to the lattice data. Quite generally, confinement and discrete spectra imply each other. Asymptotically linear glueball masses can also be achieved. Asymptotic mass ratios of various glueballs with different spin also turn out to be universal. Mesons dynamics is implemented via space filling D4-anti-D4 brane pairs. The associated tachyon dynamics is analyzed and chiral symmetry breaking is shown. The dynamics of the RR axion is analyzed, and the non-perturbative running of the QCD theta-angle is obtained. It is shown to always vanish in the IR.Comment: 65+18 pages, 20 figures (v3) Some modifications in Appendix E explaining in more detail the issue of initial conditions; typos corrected. Published versio

ϕ\phi Meson Production in In-In Collisions and the ϕ\phi Puzzle

The NA60 experiment measured dimuon production in In-In collisions at 158 AGeV. This paper presents a high statistics measurement of $\phi\to\mu\mu$ with the specific objective to provide insight on the $\phi$ puzzle, i.e. the difference in the inverse $T$ slopes and absolute yields measured by NA49 and NA50 in the kaon and lepton channel, respectively. Transverse momentum distributions were studied as a function of centrality. The slope parameter $T$ shows a rapid increase with centrality, followed by a saturation. Variations of $T$ with the fit range of the order of 15 MeV were observed, possibly as a consequence of radial flow. The $\phi$ meson yield normalized to the number of participants increases with centrality and is consistently higher than the yield measured by the NA49 experiment at any centrality.Comment: 4 Pages, 2 Figures. Proceedings of the 20$^{th}$ International Conference on Ultra-Relativistic Nucleus Nucleus Collision

Evidence for radial flow of thermal dileptons in high-energy nuclear collisions

The NA60 experiment at the CERN SPS has studied low-mass dimuon production in 158 AGeV In-In collisions. An excess of pairs above the known meson decays has been reported before. We now present precision results on the associated transverse momentum spectra. The slope parameter Teff extracted from the spectra rises with dimuon mass up to the rho, followed by a sudden decline above. While the initial rise is consistent with the expectations for radial flow of a hadronic decay source, the decline signals a transition to an emission source with much smaller flow. This may well represent the first direct evidence for thermal radiation of partonic origin in nuclear collisions.Comment: Accepted for publication in Physical Review Letter

Thermal dileptons at SPS energies

Clear signs of excess dileptons above the known sources were found at the SPS since long. However, a real clarification of these observations was only recently achieved by NA60, measuring dimuons with unprecedented precision in 158A GeV, In-In collisions. The excess mass spectrum in the region M<1 GeV is consistent with a dominant contribution from pi+pi- -> rho -> mu+mu- annihilation. The associated rho spectral function shows a strong broadening, but essentially no shift in mass. In the region M>1 GeV, the excess is found to be prompt, not due to enhanced charm production. The inverse slope parameter Teff associated with the transverse momentum spectra rises with mass up to the rho, followed by a sudden decline above. While the initial rise, coupled to a hierarchy in hadron freeze-out, points to radial flow of a hadronic decay source, the decline above signals a transition to a low-flow source, presumably of partonic origin. The mass spectra show at low transverse momenta the steep rise towards low masses characteristic for Planck-like radiation. The polarization of the excess referred to the Collins Soper frame is found to be isotropic. All observations are consistent with the interpretation of the excess as thermal radiation.Comment: Prepared for 20th International Conference on Ultra-Relativistic Nucleus-Nucleus Collisions: Quark Matter 2008 (QM2008), Jaipur, India, 4-10 Feb. 200

Study of the electromagnetic transition form-factors in \eta -> \mu^+\mu^-\gamma and \omega -> \mu^+\mu^-\pi^0 decays with NA60

The NA60 experiment at the CERN SPS has studied low-mass muon pairs in 158A GeV In-In collisions. The mass and pT spectra associated with peripheral collisions can quantitatively be described by the known neutral meson decays. The high data quality has allowed to remeasure the electromagnetic transition form factors of the Dalitz decays \eta -> \mu^+\mu^-\gamma and \omega -> \mu^+\mu^-\pi^0. Using the usual pole approximation F = (1-M_{\mu\mu}^{2}/\Lambda^{2})^{-1} for the form factors, we find \Lambda^{-2} (in GeV^{-2}) to be 1.95+-0.17(stat.)+-0.05(syst.) for the \eta and 2.24+-0.06(stat.)+-0.02(syst.) for the \omega. While the values agree with previous results from the Lepton-G experiment, the errors are greatly improved, confirming now on the level of 10\sigma the strong enhancement of the \omega form factor beyond the expectation from vector meson dominance. An improved value of the branching ratio BR(\omega -> \mu^+\mu^-\pi^0) = [1.73+-0.25(stat.)+-0.14(syst.)]*10^{-4} has been obtained as a byproduct.Comment: Submitted to Phys. Lett.

NA60 results on thermal dimuons

The NA60 experiment at the CERN SPS has measured muon pairs with unprecedented precision in 158A GeV In-In collisions. A strong excess of pairs above the known sources is observed in the whole mass region 0.2<M<2.6 GeV. The mass spectrum for M<1 GeV is consistent with a dominant contribution from pi+pi- -> rho -> mu+mu- annihilation. The associated rho spectral function shows a strong broadening, but essentially no shift in mass. For M>1 GeV, the excess is found to be prompt, not due to enhanced charm production, with pronounced differences to Drell-Yan pairs. The slope parameter Teff associated with the transverse momentum spectra rises with mass up to the rho, followed by a sudden decline above. The rise for M<1 GeV is consistent with radial flow of a hadronic emission source. The seeming absence of significant flow for M>1 GeV and its relation to parton-hadron duality is discussed in detail, suggesting a dominantly partonic emission source in this region. A comparison of the data to the present status of theoretical modeling is also contained. The accumulated empirical evidence, including also a Planck-like shape of the mass spectra at low pT and the lack of polarization, is consistent with a global interpretation of the excess dimuons as thermal radiation. We conclude with first results on omega in-medium effects.Comment: 10 pages, 12 figures, submitted to Eur. Phys. J.

J/psi azimuthal anisotropy relative to the reaction plane in Pb-Pb collisions at 158 GeV per nucleon

The J/$\psi$ azimuthal distribution relative to the reaction plane has been measured by the NA50 experiment in Pb-Pb collisions at 158 GeV/nucleon. Various physical mechanisms related to charmonium dissociation in the medium created in the heavy ion collision are expected to introduce an anisotropy in the azimuthal distribution of the observed J/$\psi$ mesons at SPS energies. Hence, the measurement of J/$\psi$ elliptic anisotropy, quantified by the Fourier coefficient v$_2$ of the J/$\psi$ azimuthal distribution relative to the reaction plane, is an important tool to constrain theoretical models aimed at explaining the anomalous J/$\psi$ suppression observed in Pb-Pb collisions. We present the measured J/$\psi$ yields in different bins of azimuthal angle relative to the reaction plane, as well as the resulting values of the Fourier coefficient v$_{2}$ as a function of the collision centrality and of the J/$\psi$ transverse momentum. The reaction plane has been estimated from the azimuthal distribution of the neutral transverse energy detected in an electromagnetic calorimeter. The analysis has been performed on a data sample of about 100 000 events, distributed in five centrality or p$_{\rm T}$ sub-samples. The extracted v$_{2}$ values are significantly larger than zero for non-central collisions and are seen to increase with p$_{\rm T}$.Comment: proceedings of HP08 conference corrected a typo in one equatio