Measurement of charged-particle spectra in Pb+Pb collisions at ���sNN = 2.76TeV with the ATLAS detector at the LHC

Charged-particle spectra obtained in 0.15nb���1 of Pb+Pb interactions at sNN������������=2.76TeV and 4.2pb���1 of pp interactions at s���=2.76TeV with the ATLAS detector at the LHC are presented in a wide transverse momentum (0.5<pT<150GeV) and pseudorapidity (|��|<2) range. For Pb+Pb collisions, the spectra are presented as a function of collision centrality, which is determined by the response of the forward calorimeter located on both sides of the interaction point. The nuclear modification factors RAA and RCP are presented in detail as function of centrality, pT and ��. They show a distinct pT-dependence with a pronounced minimum at about 7GeV. Above 60GeV, RAA is consistent with a plateau at a centrality-dependent value, within the uncertainties. The value is 0.55��0.01(stat.)��0.04(syst.) in the most central collisions. The RAA distribution is consistent with flat |��| dependence over the whole transverse momentum range in all centrality classes.

Light-by-light scattering with intact protons at the LHC: from standard model to new physics

Funda����o de Amparo �� Pesquisa do Estado de S��o Paulo (FAPESP)

Probing quantum chromodynamics with the ATLAS detector : charged-particle event shape variables and the dijet cross-section

Die Quantenchromodynamik ist die zugrundeliegende Theorie der starken Wechselwirkung und kann in zwei Bereiche aufgeteilt werden. Harte Streuprozesse, wie zum Beispiel die Zwei-Jet-Produktion bei hohen invarianten Massen, k��nnen st��rungstheoretisch behandelt und berechnet werden. Bei Streuprozessen mit niedrigen Impuls��bertr��gen hingegen ist die St��rungstheorie nicht mehr anwendbar und ph��nemenologische Modelle werden f��r Vorhersagen benutzt. Das ATLAS Experiment am Large Hadron Collider am CERN erm��glicht es, QCD Prozesse bei hohen sowie niedrigen Impuls��bertr��gen zu untersuchen. In dieser Arbeit werden zwei Analysen vorgestellt, die jeweils ihren Schwerpunkt auf einen der beiden Regime der QCD legen:rnDie Messung von Ereignisformvariablen bei inelastischen Proton--Proton Ereignissen bei einer Schwerpunktsenergie von $sqrt{s} = unit{7}{TeV}$ misst den transversalen Energiefluss in hadronischen Ereignissen. rnDie Messung des zweifachdifferentiellen Zwei-Jet-Wirkungsquerschnittes als Funktion der invarianten Masse sowie der Rapidit��tsdifferenz der beiden Jets mit den h��chsten Transversalimpulsen kann genutzt werden um Theorievorhersagen zu ��berpr��fen. Proton--Proton Kollisionen bei $sqrt{s} = unit{8}{TeV}$, welche w��hrend der Datennahme im Jahr 2012 aufgezeichnet wurden, entsprechend einer integrierten Luminosit��t von $unit{20.3}{fb^{-1}}$, wurden analysiert.rn

Measurement of charged-particle event shape variables in inclusive ���(s)=7������TeV proton-proton interactions with the ATLAS detector

The measurement of charged-particle event shape variables is presented in inclusive inelastic pp collisions at a center-of-mass energy of 7 TeV using the ATLAS detector at the LHC. The observables studied are the transverse thrust, thrust minor, and transverse sphericity, each defined using the final-state charged particles' momentum components perpendicular to the beam direction. Events with at least six charged particles are selected by a minimum-bias trigger. In addition to the differential distributions, the evolution of each event shape variable as a function of the leading charged-particle transverse momentum, charged-particle multiplicity, and summed transverse momentum is presented. Predictions from several Monte Carlo models show significant deviations from data.

Measurement with the ATLAS detector of multi-particle azimuthal correlations in p+Pb collisions at sqrtsNN=5.02 TeV

In order to study further the long-range correlations ("ridge") observed recently in p+Pb collisions at sqrt(s_NN) =5.02 TeV, the second-order azimuthal anisotropy parameter of charged particles, v_2, has been measured with the cumulant method using the ATLAS detector at the LHC. In a data sample corresponding to an integrated luminosity of approximately 1 microb^(-1), the parameter v_2 has been obtained using two- and four-particle cumulants over the pseudorapidity range |eta|<2.5. The results are presented as a function of transverse momentum and the event activity, defined in terms of the transverse energy summed over 3.1<eta<4.9 in the direction of the Pb beam. They show features characteristic of collective anisotropic flow, similar to that observed in Pb+Pb collisions. A comparison is made to results obtained using two-particle correlation methods, and to predictions from hydrodynamic models of p+Pb collisions. Despite the small transverse spatial extent of the p+Pb collision system, the large magnitude of v_2 and its similarity to hydrodynamic predictions provide additional evidence for the importance of final-state effects in p+Pb reactions.

Measurement of flow harmonics with multi-particle cumulants in Pb+Pb collisions at ���sNN=2.76 TeV with the ATLAS detector

ATLAS measurements of the azimuthal anisotropy in lead���lead collisions at ���sNN = 2.76 TeV are shown using a dataset of approximately 7��b���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 |��| < 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.