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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2012). ATLAS search for a heavy gauge boson decaying to a charged lepton and a neutrino in pp collisions at root s=7 TeV. Eur. Phys. J. C, 72(12), 2241–23pp.
Abstract: The ATLAS detector at the LHC is used to search for high-mass states, such as heavy charged gauge bosons (W'), decaying to a charged lepton (electron or muon) and a neutrino. Results are presented based on the analysis of pp collisions at a center-of-mass energy of 7 TeV corresponding to an integrated luminosity of 4.7 fb(-1). No excess beyond Standard Model expectations is observed. A W' with Sequential Standard Model couplings is excluded at the 95 % credibility level for masses up to 2.55 TeV. Excited chiral bosons (W*) with equivalent coupling strength are excluded for masses up to 2.42 TeV.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Fassi, F., Ferrer, A., et al. (2013). Search for pair production of heavy top-like quarks decaying to a high-p(T) W boson and a b quark in the lepton plus jets final state at root s=7 TeV with the ATLAS detector. Phys. Lett. B, 718(4-5), 1284–1302.
Abstract: A search is presented for production of a heavy up-type quark (t') together with its antiparticle, assuming a significant branching ratio for subsequent decay into a W boson and a b quark. The search is based on 4.7 fb(-1) of pp collisions root s = 7 TeV recorded in 2011 with the ATLAS detector at the CERN Large Hadron Collider. Data are analyzed in the lepton + jets final state, characterized by a high-transverse-momentum isolated electron or muon, large missing transverse momentum and at least three jets. The analysis strategy relies on the substantial boost of the W bosons in the t'(t') over bar signal when m(t') greater than or similar to 400 GeV. No significant excess of events above the Standard Model expectation is observed and the result of the search is interpreted in the context of fourth-generation and vector-like quark models. Under the assumption of a branching ratio BR(t' -> W b) = I, a fourth-generation t' quark with mass lower than 656 GeV is excluded at 95% confidence level. In addition, in light of the recent discovery of a new boson of mass similar to 126 GeV at the LHC, upper limits are derived in the two-dimensional plane of BR(t' -> Wb) versus BR(t' -> Ht), where H is the Standard Model Higgs boson, for vector-like quarks of various masses.
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Llosa, G., Barrillon, P., Barrio, J., Bisogni, M. G., Cabello, J., Del Guerra, A., et al. (2013). High performance detector head for PET and PET/MR with continuous crystals and SiPMs. Nucl. Instrum. Methods Phys. Res. A, 702, 3–5.
Abstract: A high resolution PET detector head for small animal PET applications has been developed. The detector is composed of a 12 mm x 12 mm continuous LYSO crystal coupled to a 64-channel monolithic SiPM matrix from FBK-irst. Crystal thicknesses of 5 mm and 10 mm have been tested, both yielding an intrinsic spatial resolution around 0.7 mm FWHM with a position determination algorithm that can also provide depth-of-interaction information. The detectors have been tested in a rotating system that makes it possible to acquire tomographic data and reconstruct images of Na-22 sources. An image reconstruction method specifically adapted for continuous crystals has been employed. The Full Width at Half Maximum measured from a point source reconstructed with ML-EM was 0.7 mm with the 5 mm crystal and 0.8 mm with the 10 mm crystal.
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Studen, A., Brzezinski, K., Chesi, E., Cindro, V., Clinthorne, N. H., Cochran, E., et al. (2013). Silicon detectors for combined MR-PET and MR-SPECT imaging. Nucl. Instrum. Methods Phys. Res. A, 702, 88–90.
Abstract: Silicon based devices can extend PET-MR and SPECT-MR imaging to applications, where their advantages in performance outweigh benefits of high statistical counts. Silicon is in many ways an excellent detector material with numerous advantages, among others: excellent energy and spatial resolution, mature processing technology, large signal to noise ratio, relatively low price, availability, versatility and malleability. The signal in silicon is also immune to effects of magnetic field at the level normally used in MR devices. Tests in fields up to 7 T were performed in a study to determine effects of magnetic field on positron range in a silicon PET device. The curvature of positron tracks in direction perpendicular to the field's orientation shortens the distance between emission and annihilation point of the positron. The effect can be fully appreciated for a rotation of the sample for a fixed field direction, compressing range in all dimensions. A popular Ga-68 source was used showing a factor of 2 improvement in image noise compared to zero field operation. There was also a little increase in noise as the reconstructed resolution varied between 2.5 and 1.5 mm. A speculative applications can be recognized in both emission modalities, SPECT and PET. Compton camera is a subspecies of SPECT, where a silicon based scatter as a MR compatible part could inserted into the MR bore and the secondary detector could operate in less constrained environment away from the magnet. Introducing a Compton camera also relaxes requirements of the radiotracers used, extending the range of conceivable photon energies beyond 140.5 keV of the Tc-99m. In PET, one could exploit the compressed sub-millimeter range of positrons in the magnetic field. To exploit the advantage, detectors with spatial resolution commensurate to the effect must be used with silicon being an excellent candidate. Measurements performed outside of the MR achieving spatial resolution below 1 mm are reported.
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ATLAS Collaboration(Aad, G. et al), Cabrera Urban, S., Castillo Gimenez, V., Costa, M. J., Ferrer, A., Fiorini, L., et al. (2013). Measurement of the Lambda(0)(b) lifetime and mass in the ATLAS experiment. Phys. Rev. D, 87(3), 032002–19pp.
Abstract: A measurement of the Lambda(0)(b) lifetime and mass in the decay channel Lambda(0)(b) -> J/psi (mu(+) mu(-))Lambda(0)(p pi(-)) is presented. The analysis uses a signal sample of about 2200 Lambda(0)(b) and (Lambda) over bar (0)(b) decays that are reconstructed in 4.9 fb(-1) of ATLAS pp collision data collected in 2011 at the LHC center-of-mass energy of 7 TeV. A simultaneous mass and decay time maximum likelihood fit is used to extract the Lambda(0)(b) lifetime and mass. They are measured to be tau(Lambda b) = 1.449 +/- 0.036(stat) +/- 0.017(syst) ps and m(Lambda b) = 5619.7 +/- 0.7(stat) +/- 1.1(syst) MeV.
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