Multi-parton interactions with CMS detector at LHC

Multi parton interactions (MPI) are experiencing a growing popularity and are widely invoked to account for observations that cannot be explained otherwise the activity of the Underlying Event, the rates for multiple heavy flavour production, the survival probability of large rapidity gaps in hard diffraction, etc.The definition, implementation and tuning of multi-parton interactions (MPI) models in Monte Carlo generators plays an important role for the LHC physics and provide a better definition of the collision dynamics and a better definition of background processes. CMS was involved into the MPI characterization from the beginning of the LHC data taking, starting from the Underlying Event measurements in Minimum Bias events. With the large integrated luminosity available, the Double Parton Scattering measurements, with two hard events in the same proton-proton collision, can be performed in different final states and at different energy scales.The proposed contribution is intended to review past and ongoing studies on MPI with the CMS detector providing a common interpretation. Presented at IFAE 2013 Incontri di Fisica delle Alte Energie IL NUOVO CIMENTO Vol. ? , N. Multi Parton Interactions with CMS detector at LHC D. Ciangottini(), on behalf of the CMS collaboration () Universitá degli Studi di Perugia, INFN Summary. — Multi parton interactions (MPI) are experiencing a growing popularity and are widely invoked to account for observations that cannot be explained otherwise: the activity of the Underlying Event, the rates for multiple heavy flavour production, the survival probability of large rapidity gaps in hard diffraction, etc. The definition, implementation and tuning of MPI models in Monte Carlo generators plays an important role for the LHC physics: a better definition of the collision dynamics and a better definition of background processes. CMS was involved into the MPI characterization from the beginning of the LHC data-taken, starting from the Underlying Event measurements in Minimum Bias events. With the large integrated luminosity available, the Double Parton Scattering (DPS) measurements (2 hard events in the same proton-proton collision) can be performed in different final states and at different energy scales. The proposed contribution is intended to review past and ongoing studies on MPI with the CMS detector, providing a common interpretation. 1. – Introduction In a proton-proton scattering the hadronic final state can be described as the superposition of different contributions. One of the most important contribution is due to the Multi Parton Interactions as shown in [1][2]. For this reason a correct modelling of MPI will result in more precise predictions in LHC physics searches. LHC probes small values of the momentum fraction x carried by the colliding partons and the large densities at small-x values increase the probability of having two simultaneous parton-parton scatterings producing two independently-identifiable hard scatterings in a single interaction. Double parton scattering can also contribute sizeably at larger x if the second process occurs with a large rate, as is the case in W + 2 jets [3]. Experimental searches for the occurrence of double parton scattering (DPS) processes have been proposed in double Drell-Yan, four jets, and same-sign WW productions, as well as in W production associated with jets. The large integrated luminosity available from 2012 data taking allows to study the double hard interactions. A preliminary overview on W+2jets final state will be presented, where specific kinematic observables, supposed to be sensitive to DPS contribution, have been analysed. c © Società Italiana di Fisica 1 2 D. CIANGOTTINI ETC. φ ∆ 0 0.5 1 1.5 2 2.5 3 D at a/ M C 0.5 1 1.5 2 2.5 Uncertainty φ ∆ 0 0.5 1 1.5 2 2.5 3 ) [p b] φ∆ / d( 0. 10 5* σd 1 10 MADGRAPH 5 + PYTHIA 6 (scaled to NLO) MADGRAPH 5 + PYTHIA 6, no MPI (scaled to NLO) PYTHIA 8 (scaled to NLO) Data -1 Ldt = 5 fb ∫ = 7 TeV, s W + jets, → CMS Preliminary, pp