A measurement of the underlying event (UE) activity in proton-proton collisions is performed using events with charged-particle jets produced in the central pseudorapidity region).

Hadronic event shapes have been measured in proton-proton collisions at sqrt(s)=7 TeV, with a data sample collected with the CMS detector at the LHC. The sample corresponds to an integrated luminosity of 3.2 inverse picobarns. Event-shape distributions, corrected for detector response, are compared with five models of QCD multijet production.

Measurements are presented of the production of primary K(S)0 and Lambda particles in proton-proton collisions at sqrt(s) = 7 TeV in the region transverse to the leading charged-particle jet in each event. The average multiplicity and average scalar transverse momentum sum of K(S)0 and Lambda particles measured at pseudorapidities abs(eta)

This thesis addresses in a very new and elegant way several measurements and the extraction of so-called double parton scattering. The new and elegant way lies in the combination of measurements and a very smart extraction of double parton scattering results, which is easy to apply and overcomes many of the technical difficulties of older methods. Many new phenomena in particle physics can be observed when particles are collided at the highest energies; one of the highlights in recent years was the discovery of the Higgs boson at the Large Hadron Collider at CERN. Understanding the production mechanism of the Higgs boson at the LHC requires detailed knowledge of the physics of proton-proton collisions. When the density of partons in the protons becomes large, there is a non-negligible probability that more than one parton participates in the interaction and the so-called double parton scattering becomes important. In some cases very particular final state signatures can be observed, which can be regarded as an indication of such double partonic scattering and where the different interactions can be separated. Such multiple partonic interactions play an important role when precise predictions from known processes are required.

ABSTRACT: This dissertation presents a measurement of event shape variables in proton-antiproton collisions at a center of momentum energy of 1.96 TeV. The observables studied are the transverse thrust and thrust minor, both defined over final state momenta perpendicular to the beam direction. These variables are calculated using unclustered calorimeter energy measured using the CDF-II detector at the Fermi National Accelerator Lab. In addition to differential distributions, we present the evolution of event shape mean values as a function of the leading jet Et. The data is compared to dedicated theoretical predictions (NLO+NLL) and subsequently to PYTHIA Monte Carlo with and without multiple parton interactions. In the presence of an underlying event the observables are found to significantly depart from the predictions of perturbative QCD.

A measurement of the activity accompanying the hard collision is performed in proton-proton collisions at two center of mass energies of 0.9 TeV and 7 TeV at CMS. The direction of the track with the highest transverse momentum is used to divide the azimuthal plane to three regions: ... Toward ... Away ..., and ... Transverse ... The data are corrected to the hadronic level and compared with two Monte-Carlo Generator tunes. The results are compared with other experiments at the LHC and the Tevatron. These results can be used to add further constrains on the Monte-Carlo generators used to simulate these collisions. It will also enable us to give better predictions for hadronic collisions at the higher energies the LHC is planning to reach in the future.

We present a new measurement of the Z/?* transverse momentum distribution in the range 0-330 GeV, in proton anti-proton collisions at √s = 1.96 TeV. The measurement uses 0.97 fb−1 of integrated luminosity recorded by the D0 experiment and is the first using the Z/?* → ??− + X channel at this center-of-mass energy. This is also the first measurement of the Z/?* transverse momentum distribution that presents the result at the level of particles entering the detector, minimizing dependence on theoretical models. As any momentum of the Z/?* in the plane transverse to the incoming beams must be balanced by some recoiling system, primarily the result of QCD radiation in the initial state, this variable is an excellent probe of the underlying process. Tests of the predictions of QCD calculations and current event generators show they have varied success in describing the data. Using this measurement as an input to theoretical predictions will allow for a better description of hadron collider data and hence it will increase experimental sensitivity to rare signals.

Pseudorapidity ($\eta $) distributions of charged particles produced in proton-proton collisions at a centre-of-mass energy of 8 $~\text {TeV}$ are measured in the ranges $

This thesis is based on the first data from the Large Hadron Collider (LHC) at CERN. Its theme can be described as the classical Rutherford scattering experiment adapted to the LHC: measurement of scattering angles to search for new physics and substructure. At the LHC, colliding quarks and gluons exit the proton collisions as collimated particle showers, or jets. The thesis presents studies of the scattering angles of these jets. It includes a phenomenological study at the LHC design energy of 14 TeV, where a model of so-called large extra dimensions is used as a benchmark process for the sensitivity to new physics. The experimental result is the first measurement, made in 2010, by ATLAS, operating at the LHC start-up energy of 7 TeV. The result is compatible with the Standard Model and demonstrates how well the physics and the apparatus are understood. The first data is a tiny fraction of what will be accumulated in the coming years, and this study has set the stage for performing these measurements with confidence as the LHC accumulates luminosity and increases its energy, thereby probing smaller length scales.