DOUBLE PARTON SCATTERING STUDIES VIA DIBOSON PROCESSES USING CMS DETECTOR AT LHC

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1 DOUBLE PARTON SCATTERING STUDIES VIA DIBOSON PROCESSES USING CMS DETECTOR AT LHC XXI DAE-BRNS High Energy Physics Symposium December 2014, Indian Institute of Technology, Guwahati Ankita Mehta 1, Ramandeep Kumar 1, V.Bhatnagar 1, K.Mazumdar 2, J.B.Singh 1 1 Panjab University Chandigarh 2 TIFR,Mumbai 11 December 2014 Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

2 Outline Introduction MultiParton Interactions (MPI) Double Parton Scattering (DPS) and Effective Cross section Motivation for using same-sign WW Generator Level Studies DPS Sensitive variables Summary Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

3 Multi-Parton Interactions A hadron hadron collision is described in terms of single hard scattering between the partons of colliding hadrons. Large parton densities and small x probability to have more than one hard scatterings between partons. MPI is accompained by huge hadronic activity, are usually soft. Harder MPI s include two simultaneous hard scatterings: Double Parton Scattering. Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

4 Double Parton Scattering Single Parton Scattering (SPS) A single pair of partons from colliding hadrons, produce a single hard scattering. pp W ± W ± jj + X q q W ± W ± jj l 1 + l 2 + ν 1 + ν 2 Double Parton Scattering (DPS) Two independent hard scatterings are produced by two pairs of partons from colliding hadrons. pp W ± W ± + X q q W ± l 1 + ν 1 q q W ± l 2 + ν 2 SPS DPS Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

5 Effective Cross section Cross section of two processes X and Y occuring simultaneously can be written as: σ(x + Y ) = m σ(x ) σ(y ) σ eff where σ(x ) and σ(y ) are cross section for processes X and Y, m is the symmetry factor m = 1, if processes X and Y are identical otherwise one. 2 Measurement of Effective area parameter for Double Parton Scattering (σ eff ) provides access to information about hadron structure in transverse plane, understanding of potential background to the new Physics searches. Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

6 Recent Measuremments of σ eff σ eff Is expected to be independent of process type and collision energy. Has been measured using different high rate processes i.e. W /Z + 2 jets, γ + 3 jet etc. Need to verify these predictions using experimental measurements. Different experiments predict different values of σ eff, results from ATLAS and CMS agree within uncertainity limits. [mb] σ eff 40 CMS (W + 2 jets) ATLAS (W + 2 jets) CDF (4 jets) 35 CDF (γ + 3 jets) Corrected CDF (γ + 3 jets) D0 (γ + 3 jets) D0 (γ + 3 jets) 2014 D0 (γ + b/c jet + 2 jets) UA2 (4 jets - lower limit) 25 AFS (4 jets - no errors given) s [TeV] Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

7 Motivation for using Same Sign WW W boson production: benchmark process at LHC in single channel. σ for single boson production at LHC is five orders of magnitude larger than that of double boson production. σ for DPS via same sign WW is smaller than that for opposite sign WW. σ for same sign WW production is almost same for DPS and SPS processes. Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

8 Sgnal and Background Processes 1 Signal: Our signal consist of two same sign leptons (with their associated neutrinos), either of same or different flavor (depending on the final state considered either ee, eµ or µµ), produced from the decay of two W bosons 2 SPS: This background has two leptons and jets in the final state but originatng from single hard scattering. 3 Top: This background has as signature leptons, missing E T and at least one jet. The contribution to the signal can occur when top decays leptonically and jets coming from the b quarks are lost in the event. 4 WZ and ZZ: The contribution of these backgrounds occurs when both bosons decay leptonically and one of the leptons escapes detection. 5 W +Jets: This background originates when one jet is misidentied as a lepton in the detector. 6 Drell-Yan: This backgroud has two leptons in the final state but not genuine missing E T. 7 QCD Multijets: This background has a huge cross section as compared to signal process. Data driven techniques are needed to estimate this background. Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

9 Generator Level Studies Selection at Generator level Monte-Carlo samples for DPS and SPS processes are used to perform these studies. W candidate is selected with appropriate status, decaying either to a muon or an electron with its respective neutrino. Depending on the flavor of final state lepton, three possible final states are considered viz. ee, µµ and eµ. Kinematics of W are studied by selecting the only those events having two same-sign W bosons, both decaying either to a muon or an electron with their associated neutrino. Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

10 Kinematics of W p (W ) T leading η(w ) leading φ(w ) leading Figure : Transverse Momentum,eta and phi distributions for leading W for SPS and DPS samples W bosons coming out of DPS processes have lower values of transverse momentum as compared to SPS processes. Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium 2014 /17

11 Kinematics of Jets In the processes involving two same sign W bosons via double parton scattering, jets are mainly produced from underlying events. Jets in the case of of SPS process involving two W bosons,are produced during primary hard scattering p (jet ) T leading η(jet ) leading φ(jet ) leading Figure : Transverse momentum, eta and phi distributions for leading jet for SPS and DPS samples Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

12 Kinematics of Jets(II) p (jet ) T sub-leading η(jet ) sub-leading φ(jet ) sub-leading Figure : Transverse momentum, eta and phi distributions for sub-leading jet for SPS and DPS samples Jets in the case of DPS have lower transverse momentum as compared to jets in SPS processes. Jets in the case of DPS are present in forward η regions where as for SPS events jets are mostly present in central η regions. Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

13 Study of DPS processes in µµ final state Transverse momentum, eta and phi distributions for leading and sub-leading muon for SPS and DPS samples p (µ ) T leading η(µ ) leading φ(µ ) leading p (µ ) T sub-leading η(µ ) sub-leading φ(µ ) sub-leading Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

14 Study of DPS processes in ee final state Transverse momentum, eta and phi distributions for Leading and sub-leading electron for SPS and DPS samples p (el ) T leading η(el ) leading φ(el ) leading p (el ) T sub-leading η(el ) sub-leading φ(el ) sub-leading Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

15 Descriminating Variable Studies Descriminating Variables As observed from previous distributions, dilepton transsverse momentum can be used to discriminate DPS from background processes. In case of DPS, two final state leptons are produced in independent hard interactions, they are expected to be randomly distributed in azimuthal plane. Di-lepton invariant transverse mass may also show some discrimination between DPS and other backgrounds. Depending on the study of kinematical variable distributions for leptons, following variable could help us discrminate DPS processes from other background processes. Scalar sum of transverse momentum of leptons ( + p T (l 1, l 2 )) Dilepton Invariant mass ( M Inv (l 1, l 2 )) Azimuthal separation between two leptons ( φ(l 1, l 2 )) Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

16 Descriminating Variable Studies(II) + p T (l 1, l 1 ), φ(l 1, l 2 ), Mass inv (l 1, l 1 ) for ee and µµchannel - DPS(W W,W W - ) p (elel) T φ(elel) M Inv (elel) - DPS(W W,W W - ) p (µµ) T φ(µµ) M Inv (µµ) Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17

17 Summary Generator level sudies have been performed for DPS and SPS processes. Basic kinematics of W bosons, leptons and jets has been studied for ee and µµ final state. Variables sensitive to DPS processes have been investigated. Variables depending on the Missing transverse energy can also be explored to show some discrimination towards DPS processes. THANKS Ramandeep Kumar (Ankita Mehta) XXI DAE-BRNS High Energy Physics Symposium /17