Study of W boson production in PbPb and pp collisions at sqrt(s[NN]) = 2.76 TeV

A measurement is presented of W-boson production in PbPb collisions carried out at a nucleon-nucleon (NN) centre-of-mass energy sqrt(s[NN]) of 2.76 TeV at the LHC using the CMS detector. In data corresponding to an integrated luminosity of 7.3 inverse microbarns, the number of W to mu mu-neutrino decays is extracted in the region of muon pseudorapidity abs(eta[mu])<2.1 and transverse momentum pt[mu]>25 GeV. Yields of muons found per unit of pseudorapidity correspond to (159 +/- 10 (stat.) +/- 12 (syst.)) 10E-8 W(plus) and (154 +/- 10 (stat.) +/- 12 (syst.)) 10E-8 W(minus) bosons per minimum-bias PbPb collision. The dependence of W production on the centrality of PbPb collisions is consistent with a scaling of the yield by the number of incoherent NN collisions. The yield of W bosons is also studied in a sample of pp interactions at sqrt(s)= 2.76 TeV corresponding to an integrated luminosity of 231 inverse nanobarns. The individual W(plus) and W(minus) yields in PbPb and pp collisions are found to agree, once the neutron and proton content in Pb nuclei is taken into account. Likewise, the difference observed in the dependence of the positive and negative muon production on pseudorapidity is consistent with next-to-leading order perturbative QCD calculations.Comment: Submitted to Physics Letters

Interfacial microstructure, element diffusion, mechanical properties and metallurgical bonding mechanism of 316L-AlSi10Mg multi-material parts fabricated by laser powder bed fusion

This work explored the interfacial microstructure, element diffusion, mechanical properties and metallurgical bonding mechanism of 316L-AlSi10Mg multi-material parts fabricated by laser powder bed fusion (LPBF). Experimental results revealed that insufficient volumetric energy density (VED) of the laser caused lack-fusion porosity in the 316L-AlSi10Mg transition zone, while too high VED produced keyhole-induced porosity defects. Using the optimal process parameters, multi-material parts can be produced with a good interface metallurgical bonding without significant defects. The partial Fe-FCC phase in 316L stainless steel changed into the Fe-BCC structure, and this shift has also changed the preferred orientation of the grains. The intermetallic compound Al5Fe2 and AlFe phases were found in the transition zone. In addition, Al–Fe icosahedral quasicrystals with five-fold symmetry were found at the boundary of the molten pool, which was caused by an extremely high cooling rate. The tensile strength of 316L-AlSi10Mg specimens is higher than that of AlSi10Mg but lower than that of 316L. In contrast to the 316L and AlSi10Mg regions, the fracture mechanism of multi-material fusion zone exhibits a quasi-cleavage fracture mode. The Vickers microhardness of the Al–Fe interface zone was higher than that of 316L with an average value of 235.57 HV0.2 and AlSi10Mg with 124.59 HV0.2, and the interfacial maximum hardness reached 526.68 HV0.2, which was caused by the very hard intermetallic compound Al5Fe2 and AlFe. The metallurgical bonding mechanism of multi-materials was that the dissimilar metals were mixed and in-situ alloyed in the molten pool by the Marangoni convection-induced strong circular flow during LPBF processing