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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2016). Measurement of top quark pair differential cross sections in the dilepton channel in pp collisions at root s=7 and 8 TeV with ATLAS. Phys. Rev. D, 94(9), 092003–33pp.
Abstract: Measurements of normalized differential cross sections of top quark pair (t (t) over bar) production are presented as a function of the mass, the transverse momentum and the rapidity of the t (t) over bar system in proton-proton collisions at center-of-mass energies of root s = 7 and 8 TeV. The data set corresponds to an integrated luminosity of 4.6 fb(-1) at 7 TeV and 20.2 fb(-1) at 8 TeV, recorded with the ATLAS detector at the Large Hadron Collider. Events with top quark pair signatures are selected in the dilepton final state, requiring exactly two charged leptons and at least two jets with at least one of the jets identified as likely to contain a b hadron. The measured distributions are corrected for detector effects and selection efficiency to cross sections at the parton level. The differential cross sections are compared with different Monte Carlo generators and theoretical calculations of t (t) over bar production. The results are consistent with the majority of predictions in a wide kinematic range.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Search for new phenomena in dijet events using 37 fb(-1) of pp collision data collected at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 96(5), 052004–26pp.
Abstract: Dijet events are studied in the proton-proton collision dataset recorded at root s = 13 TeV with the ATLAS detector at the Large Hadron Collider in 2015 and 2016, corresponding to integrated luminosities of 3.5 fb(-1) and 33.5 fb(-1) respectively. Invariant mass and angular distributions are compared to background predictions and no significant deviation is observed. For resonance searches, a new method for fitting the background component of the invariant mass distribution is employed. The dataset is then used to set upper limits at a 95% confidence level on a range of new physics scenarios. Excited quarks with masses below 6.0 TeV are excluded, and limits are set on quantum black holes, heavy W' bosons, W* bosons, and a range of masses and couplings in a Z' dark matter mediator model. Model-independent limits on signals with a Gaussian shape are also set, using a new approach allowing factorization of physics and detector effects. From the angular distributions, a scale of new physics in contact interaction models is excluded for scenarios with either constructive or destructive interference. These results represent a substantial improvement over those obtained previously with lower integrated luminosity.
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Molina, F., Aguilera, P., Romero-Barrientos, J., Arellano, H. F., Agramunt, J., Medel, J., et al. (2017). Energy distribution of the neutron flux measurements at the Chilean Reactor RECH-1 using multi-foil neutron activation and the Expectation Maximization unfolding algorithm. Appl. Radiat. Isot., 129, 28–34.
Abstract: We present a methodology to obtain the energy distribution of the neutron flux of an experimental nuclear reactor, using multi-foil activation measurements and the Expectation Maximization unfolding algorithm, which is presented as an alternative to well known unfolding methods such as GRAVEL. Self-shielding flux corrections for energy bin groups were obtained using MCNP6 Monte Carlo simulations. We have made studies at the at the Dry Tube of RECH-1 obtaining fluxes of 1.5(4) x 10(13) cm(-2) s(-1) for the thermal neutron energy region, 1.9(5) x 10(12) cm(-2) s(-1) for the epithermal neutron energy region, and 4.3(11) x 10(11) cm(-2) s(-1) for the fast neutron energy region.
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ATLAS Collaboration(Aaboud, M. et al), Alvarez Piqueras, D., Barranco Navarro, L., Cabrera Urban, S., Castillo Gimenez, V., Cerda Alberich, L., et al. (2017). Jet energy scale measurements and their systematic uncertainties in proton-proton collisions at root s=13 TeV with the ATLAS detector. Phys. Rev. D, 96(7), 072002–36pp.
Abstract: Jet energy scale measurements and their systematic uncertainties are reported for jets measured with the ATLAS detector using proton-proton collision data with a center-of-mass energy of root s = 13 TeV, corresponding to an integrated luminosity of 3.2 fb(-1) collected during 2015 at the LHC. Jets are reconstructed from energy deposits forming topological clusters of calorimeter cells, using the anti-k(t) algorithm with radius parameter R = 0.4. Jets are calibrated with a series of simulation-based corrections and in situ techniques. In situ techniques exploit the transverse momentum balance between a jet and a reference object such as a photon, Z boson, or multijet system for jets with 20 < p(T) < 2000 GeV and pseudorapidities of vertical bar eta vertical bar < 4.5, using both data and simulation. An uncertainty in the jet energy scale of less than 1% is found in the central calorimeter region (vertical bar eta vertical bar < 1.2) for jets with 100 < p(T) < 500 GeV. An uncertainty of about 4.5% is found for low-p(T) jets with p(T) = 20 GeV in the central region, dominated by uncertainties in the corrections for multiple proton-proton interactions. The calibration of forward jets (vertical bar eta vertical bar > 0.8) is derived from dijet p(T) balance measurements. For jets of p(T) = 80 GeV, the additional uncertainty for the forward jet calibration reaches its largest value of about 2% in the range vertical bar eta vertical bar > 3.5 and in a narrow slice of 2.2 < vertical bar eta vertical bar < 2.4.
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Sanchis-Lozano, M. A., & Sarkisyan-Grinbaum, E. (2017). Ridge effect and three-particle correlations. Phys. Rev. D, 96(7), 074012–13pp.
Abstract: Pseudorapidity and azimuthal three-particle correlations are studied based on a correlated-cluster model of multiparticle production. The model provides a common framework for correlations in proton-proton and heavy-ion collisions allowing easy comparison with the measurements. It is shown that azimuthal cluster correlations are definitely required in order to understand three-particle correlations in the near-side ridge effect. This is similar to the explanation of the ridge phenomenon found in our previous analysis of two-particle correlations and generalizes the model to higher-order correlations.
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