Search for pair-produced resonances decaying to quark pairs in proton-proton collisions at root s=13 TeV

A general search for the pair production of resonances, each decaying to two quarks, is reported. The search is conducted separately for heavier resonances (masses above 400 GeV), where each of the four final-state quarks generates a hadronic jet resulting in a four-jet signature, and for lighter resonances (masses between 80 and 400 GeV), where the pair of quarks from each resonance is collimated and reconstructed as a single jet resulting in a two-jet signature. In addition, a b-tagged selection is applied to target resonances with a bottom quark in the final state. The analysis uses data collected with the CMS detector at the CERN LHC, corresponding to an integrated luminosity of 35.9 fb(-1), from proton-proton collisions at a center-of-mass energy of 13 TeV. The mass spectra are analyzed for the presence of new resonances, and are found to be consistent with standard model expectations. The results are interpreted in the framework of R-parity-violating supersymmetry assuming the pair production of scalar top quarks decaying via the hadronic coupling lambda ''(312) or lambda ''(323) and upper limits on the cross section as a function of the top squark mass are set. These results probe a wider range of masses than previously explored at the LHC, and extend the top squark mass limits in the (t) over tilde -> qq' scenario.Peer reviewe

Operation and performance of the ATLAS Tile Calorimeter in Run 1

The Tile Calorimeter is the hadron calorimeter covering the central region of the ATLAS experiment at the Large Hadron Collider. Approximately 10,000 photomultipliers collect light from scintillating tiles acting as the active material sandwiched between slabs of steel absorber. This paper gives an overview of the calorimeter’s performance during the years 2008–2012 using cosmic-ray muon events and proton–proton collision data at centre-of-mass energies of 7 and 8TeV with a total integrated luminosity of nearly 30 fb−1. The signal reconstruction methods, calibration systems as well as the detector operation status are presented. The energy and time calibration methods performed excellently, resulting in good stability of the calorimeter response under varying conditions during the LHC Run 1. Finally, the Tile Calorimeter response to isolated muons and hadrons as well as to jets from proton–proton collisions is presented. The results demonstrate excellent performance in accord with specifications mentioned in the Technical Design Report

Activated Carbon Fibers with a High Heteroatom Content by Chemical Activation of PBO with Phosphoric Acid

The preparation of activated carbon fibers (ACFs) by phosphoric acid activation of poly­(<i>p</i>-phenylene benzobisoxazole) (PBO) fibers was studied, with particular attention to the effects of impregnation ratio and carbonization temperature on porous texture. Phosphoric acid has a strong effect on PBO degradation, lowering the temperature range at which the decomposition takes place and changing the number of mass loss steps. Chemical analysis results indicated that activation with phosphoric acid increases the concentration of oxygenated surface groups; the resulting materials also exhibiting high nitrogen content. ACFs are obtained with extremely high yields; they have well-developed porosity restricted to the micropore and narrow mesopore range and with a significant concentration of phosphorus incorporated homogeneously in the form of functional groups. An increase in the impregnation ratio leads to increases in both pore volume and pore size, maximum values of surface area (1250 m²/g) and total pore volume (0.67 cm³/g) being attained at the highest impregnation ratio (210 wt % H₃PO₄) and lowest activation temperature (650 °C) used; the corresponding yield was as large as 83 wt %. The obtained surface areas and pore volumes were higher than those achieved in previous works by physical activation with CO₂ of PBO chars

Molecular Transport in Thin Thermoresponsive Poly(<i>N</i>-isopropylacrylamide) Brushes with Varying Grafting Density

The effect of the grafting density on the molecular transport through thermoresponsive brushes of poly­(<i>N</i>-isopropylacrylamide) (PNIPAM) grafted onto flat gold substrates was investigated using voltammetry and impedance spectroscopy. PNIPAM brush layers were synthesized at four different grafting densities using surface-initiated atom transfer radical polymerization (SI-ATRP) from mixed self-assembled monolayers of ω-mercaptoundecyl bromoisobutyrate and undecanethiol chemisorbed on gold surfaces. Tethered PNIPAM layers with grafting densities resulting from initiator concentrations lower than 25% in the thiol monolayer show the same transport properties as the initial self-assembled monolayer before brush synthesis. For higher grafting densities, the diffusion coefficients, <i>D</i>, of the K₃[Fe­(CN)₆]/K₄[Fe­(CN)₆] redox probe is 7 orders of magnitude smaller than those typically measured in aqueous solutions and independent of whether the brush is collapsed or swollen. The collapse of the PNIPAM brush drives a hydrophilic/hydrophobic transition in addition to structural/conformational transformations of the grafted layers, resulting in still smaller values of <i>D</i>. However, these changes do not lead to a blocking effect on the active area of the gold surface, which is only determined by pinholes or discontinuities in the thiol initiator monolayer. These results are only observed for thin PNIPAM brush layers

Novel Near-Room-Temperature Type I Multiferroic: Pb(Fe_0.5Ti_0.25W_0.25)O₃ with Coexistence of Ferroelectricity and Weak Ferromagnetism

We report on the crystal structure evolution and the physical properties of the complex perovskite Pb­(Fe_0.5Ti_0.25W_0.25)­O₃. It presents a paraelectric to ferroelectric transition at <i>T</i>_C = 293 K, determined by permittivity measurements. The room-temperature neutron powder diffraction pattern (NPD) shows an admixture of the ferroelectric phase (34%, <i>P</i>4<i>mm</i> space group) and the paraelectric polymorph (66%, <i>Pm</i>3̅<i>m</i> space group). In both polymorphs, the perovskite crystal structure contains the three B cations (Fe, Ti, W) distributed at random at the octahedral sites, and Pb is shifted away from the center of the cubic (sub)­cell. On the other hand, the presence of iron drives the appearance of magnetic interactions above room temperature. This is related to the existence of Fe-rich islands where the strong Fe³⁺–O–Fe³⁺ superexchange interactions govern the magnetic behavior. The magnetic structure has been determined from low-temperature NPD experiments as a G-type antiferromagnetic (AFM) cell. Furthermore, there is a net magnetization in the entire range of temperature, which is related to the existence of noncompensated spins in each island. The coexistence of ferroelectricity and a magnetically ordered state and the observation of a possible coupling between both phenomena allow us to suggest the multiferroic-magnetoelectric nature of the sample

Influence of sp³–sp² Carbon Nanodomains on Metal/Support Interaction, Catalyst Durability, and Catalytic Activity for the Oxygen Reduction Reaction

In this work, platinum nanoparticles were impregnated by two different techniques, namely the carbonyl chemical route and photodeposition, onto systematically surface-modified multiwalled carbon nanotubes. The different interactions between platinum nanoparticles with sp²–sp³ carbon nanodomains were investigated. The oxidation of an adsorbed monolayer of carbon monoxide, used to probe electronic catalytic modification, suggests a selective nucleation of platinum nanoparticles onto sp² carbon nanodomains when photodeposition synthesis is carried out. XPS attests the catalytic center electronic modification obtained by photodeposition. DFT calculations were used to determine the interaction energy of a Pt cluster with sp² and sp³ carbon surfaces as well as with oxidized ones. The interaction energy and electronic structure of the platinum cluster presents dramatic changes as a function of the support surface chemistry, which also modifies its catalytic properties evaluated by the interaction with CO. The interaction energy was calculated to be 8-fold higher on sp³ and oxidized surfaces in comparison to sp² domains. Accelerated Stability Test (AST) was applied only on the electronic-modified materials to evaluate the active phase degradation and their activity toward oxygen reduction reaction (ORR). The stability of photodeposited materials is correlated with the surface chemical nature of supports indicating that platinum nanoparticles supported onto multiwalled carbon nanotubes with the highest sp² character show the higher stability and activity toward ORR