Simulation of Z->jets in CMS Outline –Introduction –Technique –Results –Conclusion

Analysis Processes Use Pythia to generate particles Use GEANT4 to simulate the response of Ecal and Hcal Digitization to ADC counts Convert ADC counts to energy in one calorimeter cell Combine ECAL and HCAL cells into projective towers Convert CaloTowers into standard objects Run clustering algorithm to produce jets

Source of the simulation Single particle: π + –Flat pt: 10 <pt< 70 GeV/c –Flat eta: -4.5<eta<4.5 –Flat phi: -π<phi< π –Fixed vertex: (vx,vy,vz)=(0,0,0) Single particle: Z0 –Pt=0; –Fixed vertex: (vx,vy,vz)=(0,0,0) –Decay in Pythia (Z->Q+Qbar) The MC data are big ~ 3MBytes/event Only the user defined tree is saved to disk events are analyzed in both cases.

Z->QQbar event in Pythia Event listing (summary) I particle/jet KS KF orig p_x p_y p_z E m 1 (Z0) (d) (dbar) (CMshower) (d) (dbar) (d) (g) (dbar) (g) (d) (g) (d) (dbar) (dbar) (g) (d) (g) (dbar) (g) (d) (g) (d) (g) (d) A (g) I (g) I (g) I (dbar) V (dbar) A (g) I (g) I (g) I (g) I (d) V (string) (K*0) (Kbar0) (rho-) pi pi pi (eta') (rho0) (eta) (string) (rho0) pi (rho-) (rho+) (pi0) (pi0) (rho-) (rho0) (rho0) (rho0) (eta) (Kbar0) (K*0) (K0) (pi0) K_L pi (pi0) gamma (rho0) pi pi ………

Jet reconstruction Input: towers (reco) or stable particles (MC) Output: Jets list, Jet direction, Jets energy Algorithms: Iterative Cone Midpoint Cone -- this analysis K T Seedless Infrared Safe cone

Jet reconstruction algorithms Iterative cone 1.Make an E T -ordered list for the inputs 2.Pick up the largest E T input (above a specified threshold) and create a cone (cone size is given R) 3.Use the objects in that cone to calculate the proto-jet direction and energy 4.The computed direction is used to seed a new proto-jet 5.Repeat step 3&4 until the energy of the proto-jet changes by less than 1% and the direction of the proto-jet changes by ΔR<0.01 between iterations 6.Put this stable proto-jet to the jet list. Remove all the objects in the stable proto-jet cone from the input list 7.Start from step 2 to find another jet until no more object has an E T above the threshold

Jet reconstruction algorithms Midpoint cone 1.Use all E T >threshold inputs to find stable proto-jets. In this procedure, no object is removed from the input list. 2.For every pair of proto-jets that are closer than R, a midpoint is calculated as the direction of the combined momentum. These midpoints are then used as additional seeds to find more proto-jets. 3.Starting with the highest E T proto-jet, if the proto-jet does not share objects with other proto-jets, it is defined as a jet and removed from the proto-jet list. 4.If there is shared part of two proto-jets, calculate factor f= E T _shared/ E T _neighbor 1.f>0.5, merge the 2 proto-jets to one 2.f<0.5, assign the shared part to the proto-jet that is closest 5.Repeat step 3&4 until no proto-jet left

Single π + jet events

Between Reco jets and Gen jets Match condition: ΔR<0.2

Jets from Z decay

Conclusions and todo list Code can run smoothly The first look of Z->QQbar events reco jets have spikes in a few certain eta and phi directions, need to figure out why move further to a more complicated case -- embed Z decay to real events Optimize the algorithm