A search for new heavy particles manifested as resonances in two-jet final states is presented. The data were produced in 7 TeV proton-proton collisions by the LHC and correspond to an integrated luminosity of 315  nb⁻¹ collected by the ATLAS detector. No resonances were observed. Upper limits were set on the product of cross section and signal acceptance for excited-quark (q*) production as a function of q* mass. These exclude at the 95% C.L. the q* mass interval 0.30

More than half a million minimum-bias events of LHC collision data were collected by the ATLAS experiment in December 2009 at centre-of-mass energies of 0.9 TeV and 2.36 TeV. This paper reports on studies of the initial performance of the ATLAS detector from these data. Comparisons between data and Monte Carlo predictions are shown for distributions of several track-and calorimeter-based quantities. The good performance of the ATLAS detector in these first data gives confidence for successful running at higher energies.

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The studies presented in this thesis led to the following publication:• Measurement of Inclusive Jet Cross Sections in Z/γ * (→ e + e − ) + jets Production in pp Collisions at √ s = 1.96 TeV, Phys. Rev. Lett. 100, 102001 (2008). AcknowledgmentsThe work reported in this Ph.D. thesis involved 3 years of my life, 2 continents, 3 notebooks, several terabytes of disk space, and interacting with many people.Undoubtedly, the person that most influenced this thesis is my supervisor Mario Martínez. His broad knowledge of physics and QCD in particular brought endless inputs to this thesis, and his commitment to the well-done work was a constant guidance in the processes of the analysis. Monica is the person who day by day helped me in every step of the analysis. I would like to thank her for creating such a great and friendly working environment. Alon has also helped me in the final stages of this work providing useful ideas and cross checks. Thanks also to Regis for asking and answering so many questions, his aim for perfection surely had an impact in my work.All this was possible thanks to Enrique Fernández who gave me the opportunity to work at IFAE. Special thanks go to Matteo Cavalli-Sforza who warmly welcomed me at IFAE, for sharing his neverending desire for learning and passion for physics. I would also like to express my gratitude to Ilya Korolkov, my Master Thesis supervisor, who had to deal with my learning curve in the research world and always supported and encouraged me.I would like to thank all the CDF collaborators, specially the spokespeople and the QCD conveners, for their help and inputs to the analysis, and for the success of the CDF experiment.Tota aquesta feina no hagués estat possible sense els companys i amics de l'IFAE. Primer de tot al Xavi i l'Olga, perésser els meus amfitrions a l'IFAE, que amb el temps han esdevingut més que amics. Ells em van ensenyar tot el que calia saber per sobreviure fora de la nostra estimda terra, primer al CERN, i després als Estat Units. La seva facilitat per conèixer nova gent i fer amics allí on van han fet que sempre em sentís acompanyat. I també a la Carolina, amb qui després de conèixer-nos durant la carrera vam tornar a trobar-nos a l'IFAE, el seu esperit sempre alegre i la seva rialla encomanadissa han ajudat a passar el moments més durs. We had a lot of physics and non-physics discussions with Gianluca, we shared problems and solutions, and he has been a really good friend and office-mate. I overlapped some months with Stefano in Barcelona. We did not have much time to know each other but I wish him good luck with his thesis.The friends that I met at Fermilab: Ernest,Óscar, Jónatan, Cristóbal, Kike, Miguel, Bárbara, Bruno, Arán, Ignacio, Roberto y alguno más que seguro que me olvido. Gracias a todos por crear un ambiente tan familiar y acogedor que me ha hecho sentir como en casa. Gracias sobretodo por los buenos momentos pasados fuera del trabajo, por las partidas de Go, las maratones cinematográficas, el volei, la piscina y un sinfín de cosas que nos permit...