High PT Inclusive Jets Study Manuk Zubin Mehta

High PT Inclusive Jets Study Manuk Zubin Mehta , Prof. Manjit kaur Panjab University, Chandigarh India CMS Meeting March , 2009 1

Introduction: Inclusive Jets - On the MC side : They are available for QCD jet with leading order (LO) matrix element - In many tests of QCD requires to use NLO calculations - But the absence of generator programs for QCD jet production in next-to-leading order -Necessary to apply additional corrections to account for the effects of multiple parton interactions and the hadronisation process. 2

Herwig++ , Herwig&Jimmy and Pythia 6 (The process of hadronisation is not at all understood , but there are several models that seem to give good approximation to what is occuring) Herwig and Pythia use different hadronisation model -Lund String Model in Pythia -Cluster Model in Herwig++ and Herwig&Jimmy And also having different Underlying events model (Underlying event is used to tune MC generators to data and measured in transverse region) - Tune D 6 T for Pythia 6 - Herwig++ used with default settings - Herwig+Jimmy tuned that developed in the Atlas Collaboration. - All three are tuned to Tevatron data 3

Analysis Setup - Herwig++ 2. 2, Pythia 6. 4 in CMSSW_2_1_4 Herwig 6+ jimmy in CMSSW_2_2_3 - Selection Criteria for Jets: pt > 50 Gev - Algorithm used : Incl kt , D = 0. 6 Sis. Cone , R = 0. 7 - Centre of mass energy : 10 Tev - The Phase space sliced into 17 pthat bins. - Number of event : 1 Million per bin (to reduce Statistical uncertainties ) 4

cont. . - With this coverage of 50 Ge. V to 4000 Ge. V is achieved. - Binning in rapidity is used according to detector geometry : 0. 00<|y|<=0. 55 , 0. 55<|y|<=1. 10 barrel region 1. 10<|y|<=1. 70 transition region 1. 70<|y|<=2. 50 endcap region 2. 50<|y|<=3. 20 transition region 3. 20<|y|<=5. 00 forward region 5

Comparison of Basic Observables Pythia Herwig+Jimmy Herwig++ 6

Number of Jets using three generators Herwig++ seems to gives a higher jet multiplicity. With k T D=0. 6 With Sis. Cone R= 0. 7 7

Inclusive Jet spectra from Herwig++, Herwig&Jimmy and Pythia 6 • Private Production : 17 Million events in 17 pthat bins. • CTEQ 6 L PDF for Pythia and Herwig&Jimmy • MRST 2001 PDF for Herwig++ 8

Comparision of inclusive jet crossection distribution With k T D=0. 6 These are weighted spectra of crossections in the • inner central barrel region • barrel endcaps • farward region Only Slight differences in the forward region are observed. 9 With Sis. Cone R =0. 7

Ratio of Herwig++ and Herwig+Jimmy crossections w. r. t Pythia in inner barrel region (0. 00 < |y| < 0. 55) With k T D=0. 6 Ratio for Herwig +Jimmy deviate slightly from one With Sis. Cone R =0. 7 10

Ratio of Herwig++ and Herwig+Jimmy crossections w. r. t Pythia in barrel endcaps region ( 1. 70 < |y| < 2. 50) With k T D=0. 6 Ratio exceeds one as rapidity increases. With Sis. Cone R =0. 7 11

Ratio of Herwig++ and Herwig+Jimmy crossections w. r. t Pythia in forward region ( 3. 20 < |y| < 5. 00) With k T D=0. 6 With Sis. Cone R =0. 7 12

Rapidity Distribution with k T D= 0. 6 Low p. That interval (80 -120 Ge. V) Medium p. That interval (10001400 Ge. V)

Highest p. That interval (35004000 Ge. V) The rapidity distribution for these intervals are found to be nearly identical for all generators. The Plots for Sis. Cone R =0. 7 also match very well. 14

Further Plan: Comparisons with simulated MC samples Comparison of generators and their models allows estimation of systematic uncertainties. Derive final hadronization+UE corrections including systematic uncertainties from the different MC for inclusive jets and document this in analysis note , we are preparing. 15

Back Up Slides

Hadronisation PYTHIA(string model) • It assumes colour charged strings between the partons coming from the parton shower step. • As they move appart, the strings break and new quark antiquark pairs appear at the breaking points. HERWIG(cluster model) • Thw colour neutral clusters are formed from neighbouring quarks, which then decay isotropically into hadrons. • To preserve energy conservation, a momentum transfer between the cluster is performed, If a cluster is lighter than the lightest hadron of the given quarks flavour

Multiple Parton Interactions Pythia and Herwig++ • Pythia and Herwig++ handle MPI by performing a number of minimum bias parton scatters above a certain threshold in transverse momentum exchange. • This threshold together with the chosen parton density function, defines a average number of additional interactions that take place in an event. • Other parameter include mattter distribution in the proton and variables that steer the amount of intial and final state radiation that is spread into the event. Herwig+Jimmy • Herwig 6 natively doesnot include MPI but only simple soft interaction model that is based on approach pioneered by UA 5 experiment. • The lacking functionality however be completed by linking with JIMMY package that also inspired the model in Herwig++.