Person: Zevi Della Porta, G
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Zevi Della Porta
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Zevi Della Porta, G
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Publication Measurement of the Centrality and Pseudorapidity Dependence of the Integrated Elliptic Flow in Lead–lead Collisions at √sNN=2.76TeV With the ATLAS Detector(Springer Nature, 2014) Franklin, Melissa; Huth, John; Ippolito, Valerio; Lopez Mateos, David; Mercurio, Kevin; Morii, Masahiro; Spearman, William; Yen, Andy; Zevi Della Porta, GThe integrated elliptic flow of charged particles produced in Pb+Pb collisions at sNN=2.76 TeV has been measured with the ATLAS detector using data collected at the Large Hadron Collider. The anisotropy parameter, v2, was measured in the pseudorapidity range |η|≤2.5 with the event-plane method. In order to include tracks with very low transverse momentum pT, thus reducing the uncertainty in v2 integrated over pT, a 1~ b-1 data sample recorded without a magnetic field in the tracking detectors is used. The centrality dependence of the integrated v2 is compared to other measurements obtained with higher pT thresholds. The integrated elliptic flow is weakly decreasing with |η|. The integrated v2 transformed to the rest frame of one of the colliding nuclei is compared to the lower-energy RHIC data.Publication Search for Heavy Neutrinos and Right-Handed W Bosons in Events With Two Leptons and Jets in pp Collisions at √s =7 TeV With the ATLAS Detector(Springer Nature, 2012) Franklin, Melissa; Huth, John; Jeanty, L; Kagan, M; Lopez Mateos, David; Mercurio, Kevin; Morii, Masahiro; Zevi Della Porta, GThis letter reports on a search for hypothetical heavy neutrinos, N, and right-handed gauge bosons, WR, in events with high transverse momentum objects which include two reconstructed leptons and at least one hadronic jet. The results were obtained from data corresponding to an integrated luminosity of 2.1 fb⁻¹ collected in proton–proton collisions at √s = 7 TeV with the ATLAS detector at the CERN Large Hadron Collider. No excess above the Standard Model background expectation is observed. Excluded mass regions forMajorana and Dirac neutrinos are presented using two approaches for interactions that violate lepton and lepton-flavor numbers. One approach uses an effective operator framework, the other approach is guided by the Left– Right Symmetric Model. The results described in this letter represent the most stringent limits to date on the masses of heavy neutrinos and WR bosons obtained in direct searches.Publication Measurement of the Top-Quark Mass in the Fully Hadronic Decay Channel From ATLAS Data at √s= 7TeV(Springer Nature, 2015) Franklin, Melissa; Huth, John; Ippolito, Valerio; Lopez Mateos, David; Mercurio, Kevin; Spearman, William; Sun, Siyuan; Yen, Andy; Zevi Della Porta, GThe mass of the top quark is measured in a data set corresponding to 4.6 fb-1 of proton–proton collisions with centre-of-mass energy √ = 7TeV collected by the ATLAS detector at the LHC. Events consistent with hadronic decays of top–antitop quark pairs with at least six jets in the final state are selected. The substantial background from multijet production is modelled with data-driven methods that utilise the number of identified b-quark jets and the transverse momentum of the sixth leading jet, which have minimal correlation. The top-quark mass is obtained from template fits to the ratio of three-jet to dijet mass. The three-jet mass is calculated from the three jets produced in a top-quark decay. Using these three jets the dijet mass is obtained from the two jets produced in the W boson decay. The top-quark mass obtained from this fit is thus less sensitive to the uncertainty in the energy measurement of the jets. A binned likelihood fit yields a top-quark mass of mt = 175.1 ± 1.4 (stat.) ± 1.2 (syst.) GeV.Publication Search for Lepton Flavour Violation in the eμ Continuum With the ATLAS Detector in √s=7 TeV Pp Collisions at the LHC(Springer Nature, 2012) Franklin, Melissa; Huth, John; Jeanty, L; Kagan, M; Mercurio, Kevin; Morii, Masahiro; Zevi Della Porta, GThis paper presents a search for the t-channel exchange of an R-parity violating scalar top quark ( t~ ) in the e ± μ ∓ continuum using 2.1 fb−1 of data collected by the ATLAS detector in s√=7 TeV pp collisions at the Large Hadron Collider. Data are found to be consistent with the expectation from the Standard Model backgrounds. Limits on R-parity-violating couplings at 95 % C.L. are calculated as a function of the scalar top mass ( mt~ ). The upper limits on the production cross section for pp→eμX, through the t-channel exchange of a scalar top quark, ranges from 170 fb for mt~=95 GeV to 30 fb for mt~=1000 GeV .Publication Two-Particle Bose-Einstein Correlations in pp Collisions at √s = 0.9 and 7 TeV Measured With the ATLAS Detector(Springer Nature, 2015) Franklin, Melissa; Huth, John; Ippolito, Valerio; Lopez Mateos, David; Mercurio, Kevin; Morii, Masahiro; Spearman, William; Sun, Siyuan; Tolley, Emma; Yen, Andy; Zevi Della Porta, GThe paper presents studies of Bose–Einstein Correlations (BEC) for pairs of like-sign charged particles measured in the kinematic range pT > 100 MeV and |η| < 2.5 in proton collisions at centre-of-mass energies of 0.9 and 7 TeV with the ATLAS detector at the CERN Large Hadron Collider. The integrated luminosities are approximately 7 μb⁻¹, 190 μb⁻¹ and 12.4 nb⁻¹ for 0.9 TeV, 7 TeV minimum-bias and 7 TeV high-multiplicity data samples, respectively. The multiplicity dependence of the BEC parameters characterizing the correlation strength and the correlation source size are investigated for charged-particle multiplicities of up to 240. A saturation effect in the multiplicity dependence of the correlation source size parameter is observed using the high-multiplicity 7 TeV data sample. The dependence of the BEC parameters on the average transverse momentum of the particle pair is also investigated.Publication Measurement of Flow Harmonics With Multi-Particle Cumulants in Pb+Pb Collisions at √sNN =2.76 TeV With the ATLAS Detector(Springer Nature, 2014) Franklin, Melissa; Huth, John; Ippolito, Valerio; Lopez Mateos, David; Mercurio, Kevin; Morii, Masahiro; Spearman, William; Sun, Siyuan; Yen, Andy; Zevi Della Porta, G[en] ATLAS measurements of the azimuthal anisotropy in lead.lead collisions at √sNN = 2.76 TeV are shown using a dataset of approximately 7 μb-1 collected at the LHC in 2010. The measurements are performed for charged particles with transversemomenta 0.5 < pT < 20 GeV and in the pseudorapidity range vertical stroke η vertical stroke < 2.5. The anisotropy is characterized by the Fourier coefficients, vn, of the charged-particle azimuthal angle distribution for n = 2-4. The Fourier coefficients are evaluated using multi-particle cumulants calculated with the generating function method. Results on the transverse momentum, pseudorapidity and centrality dependence of the vn coefficients are presented. The elliptic flow, v2, is obtained from the two-, four-, six- and eight-particle cumulants while higher-order coefficients, v3 and v4, are determined with two- and four-particle cumulants. Flow harmonics vn measured with four-particle cumulants are significantly reduced compared to the measurement involving two-particle cumulants. A comparison to vn measurements obtained using different analysis methods and previously reported by the LHC experiments is also shown. Results of measurements of flow fluctuations evaluated with multiparticle cumulants are shown as a function of transverse momentum and the collision centrality. Models of the initial spatial geometry and its fluctuations fail to describe the flow fluctuations measurements. (orig.)Publication Measurement of the Forward-Backward Asymmetry of Electron and Muon Pair-Production in Pp Collisions at √S = 7 TeV With the ATLAS Detector(Springer Nature, 2015-09) Guimaraes da Costa, Joao; Butler, B.; Catastini, P.; Conti, G.; Franklin, Melissa; Huth, John; Ippolito, Valerio; Lopez, David; Mercurio, Kevin; Morii, Masahiro; Skottowe, H.P.; Spearman, William; Yen, Andy; Zevi Della Porta, GThis paper presents a measurement of the triple-differential cross section for the Drell-Yan process Z/γ* → ℓ+ℓ− where ℓ is an electron or a muon. The measurement is performed for invariant masses of the lepton pairs, m ℓℓ , between 46 and 200 GeV using a sample of 20.2 fb−1 of pp collisions data at a centre-of-mass energy of s√=8 TeV collected by the ATLAS detector at the LHC in 2012. The data are presented in bins of invariant mass, absolute dilepton rapidity, |yℓℓ|, and the angular variable cos θ* between the outgoing lepton and the incoming quark in the Collins-Soper frame. The measurements are performed in the range |y ℓℓ | < 2.4 in the muon channel, and extended to |y ℓℓ | < 3.6 in the electron channel. The cross sections are used to determine the Z boson forward-backward asymmetry as a function of |y ℓℓ | and m ℓℓ . The measurements achieve high-precision, below the percent level in the pole region, excluding the uncertainty in the integrated luminosity, and are in agreement with predictions. These precision data are sensitive to the parton distribution functions and the effective weak mixing angle.Publication Jet Energy Resolution in Proton-Proton Collisions at √s= 7 TeV Recorded in 2010 With the ATLAS Detector(Springer Nature, 2013) Huth, John; Jeanty, L; Kagan, M; Lopez Mateos, David; Mercurio, Kevin; Morii, Masahiro; Zevi Della Porta, GThe measurement of the jet energy resolution is presented using data recorded with the ATLAS detector in proton-proton collisions at √s=7 TeV . The sample corresponds to an integrated luminosity of 35 pb−1. Jets are reconstructed from energy deposits measured by the calorimeters and calibrated using different jet calibration schemes. The jet energy resolution is measured with two different in situ methods which are found to be in agreement within uncertainties. The total uncertainties on these measurements range from 20 % to 10 % for jets within |y|<2.8 and with transverse momenta increasing from 30 GeV to 500 GeV. Overall, the Monte Carlo simulation of the jet energy resolution agrees with the data within 10 %.Publication Jet energy measurement and its systematic uncertainty in proton–proton collisions at √s=7TeV with the ATLAS detector(Springer Nature, 2015) Franklin, Melissa; Huth, John; Jeanty, L; Lopez Mateos, David; Mercurio, Kevin; Morii, Masahiro; Spearman, William; Yen, Andy; Zevi Della Porta, GThe jet energy scale (JES) and its systematic uncertainty are determined for jets measured with the ATLAS detector using proton–proton collision data with a centre-of-mass energy of s√=7 TeV corresponding to an integrated luminosity of 4.7 fb −1 . Jets are reconstructed from energy deposits forming topological clusters of calorimeter cells using the anti- kt algorithm with distance parameters R=0.4 or R=0.6 , and are calibrated using MC simulations. A residual JES correction is applied to account for differences between data and MC simulations. This correction and its systematic uncertainty are estimated using a combination of in situ techniques exploiting the transverse momentum balance between a jet and a reference object such as a photon or a Z boson, for 20≤pjetT<1000 GeV and pseudorapidities |η|<4.5 . The effect of multiple proton–proton interactions is corrected for, and an uncertainty is evaluated using in situ techniques. The smallest JES uncertainty of less than 1 % is found in the central calorimeter region ( |η|<1.2 ) for jets with 55≤pjetT<500 GeV . For central jets at lower pT , the uncertainty is about 3 %. A consistent JES estimate is found using measurements of the calorimeter response of single hadrons in proton–proton collisions and test-beam data, which also provide the estimate for pjetT>1 TeV. The calibration of forward jets is derived from dijet pT balance measurements. The resulting uncertainty reaches its largest value of 6 % for low- pT jets at |η|=4.5 . Additional JES uncertainties due to specific event topologies, such as close-by jets or selections of event samples with an enhanced content of jets originating from light quarks or gluons, are also discussed. The magnitude of these uncertainties depends on the event sample used in a given physics analysis, but typically amounts to 0.5–3 %.Publication Measurement of the Cross-Section for W Boson Production in Association with b-Jets in Proton-Proton Collisions at \(\sqrt s = 7\) TeV at the LHC using the ATLAS detector(2013-08-21) Zevi Della Porta, G; Franklin, Melissa; Morii, Masahiro; Finkbeiner, DouglasThis dissertation presents a measurement of the W+b-jets \((pp → W + b(\bar{b}) + X)\) production cross-section in proton–proton collisions at a center-of-mass energy of 7 TeV at the LHC. The results are based on data corresponding to an integrated luminosity of \(4.6 fb^{−1}\), collected with the ATLAS detector. The measurement probes the QCD sector of the Standard Model at high energy, in a region where b-quark mass and double parton scattering play an important role. In addition, the measurement is relevant for searches for physics beyond the Standard Model in final states with a W boson and b-jets. The measurement relies on the leptonic decay modes of the W, and on the iden- tification of b-jets. The backgrounds to the W+b-jets process are estimated using Monte Carlo simulation and data-driven techniques. Cross-sections, corrected for all known detector effects and quoted in a limited kinematic range, are presented as a function of jet multiplicity and of the transverse momentum of the leading b-jet for both the muon and electron decay modes of the W boson.