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QQ photoproduction in ultraperipheral PbPb collisions in CMS

Summer Student Session CERN, 16 th August 2006 Aurélien Hees. QQ photoproduction in ultraperipheral PbPb collisions in CMS. _. Very strong E. g. J/ Y , ¡. J/ Y  e + e - ¡  m + m -. g + Pb . ℓ -. g. ℓ +. Phys ics process.

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QQ photoproduction in ultraperipheral PbPb collisions in CMS

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  1. Summer Student Session CERN, 16th August 2006 Aurélien Hees QQ photoproduction inultraperipheral PbPb collisionsin CMS _

  2. Very strong E g J/Y, ¡ J/Y e+e- ¡  m+m- g + Pb  ℓ- g ℓ+ Physics process • ultra-relativistic beam of Pb produces a very strong localized EM field (due to high Pb nucleus charge) • This strong EM produces a flux of photons • This g flux will collide another Pb nucleus and this collision produces quarkonia : J/Y (cc) and ¡ (bb)  decay into 2 leptons _ _

  3. Pb* Pb J/,¡ Pb Pb* Physics motivation • The goal : to study the structure inside the nucleus : • what are the building blocks of the nucleus ? • how much energy and momentum do they carry ? • Parton Distribution Function (pdf) : probability density for finding a parton with a certain momentum fraction x • gPb collisions allow to study PDF in a range of (x,Q2) not probed so far ZEUS X=fraction of the proton momentum carried by the parton

  4. Compact Muon Solenoid

  5. Detection in CMS J/Y e+e- : tracker and Electromagnetic Calorimeter (ECAL) ¡  m+m- : tracker and Muon chambers TRACKER ECAL HCAL Muon Chambers

  6. ¡ m- g m+ Input Monte-Carlo signal • Input signal : Monte-Carlo simulation done by : Starlight [J.Nystrand, S.Klein, NPA752(2005)470] • Centred at mid-rapidity • Peaked at very low pT • ¡ distributions : ~ CMS acceptance Single m Rapidity distribution ¡ distributions(m pairs)

  7. g g Input background • g + g collisions  ℓ+ℓ- • Generated by Starlight (again) • Mixed to ¡ (J/Y) events according to cross-sections ratios : Single m rapidity distribution More data’s OUT of CMS acceptance m pairs :

  8. Detector’s simulation (Geant4) Mixing simulated data’s (background + signal) Digitization Data’s Reconstruction CMS Simulation & Reconstruction • e+e- : tracker + ECAL • m+m- : tracker + muon chambers • reconstructed pT distribution • reconstructed rapidity distribution

  9. Results : Acceptance x Efficiency • Acceptance x Efficiency = • Acc x Eff (y) = ~ 0.8 within |y|<1 ~ 0 beyond |y|>2.5 • Acc x Eff (pT) = ~ 0.65 ~ CMS acceptance

  10. ¡ Background Results : mass distribution • Peak at the right energy ~ 9.46 GeV • With good resolution ~ 70 MeV • Signal observable since most of continuum pairs go comparatively more forward • We assume perfect trigger • Polynomial + gaussian fit Background substraction

  11. Conclusion • Simulations studies of J/Y and ¡ in CMS (@ = 5.5 TeV) • Signal observable (Signal/Background ~ 2) • Good mass resolution σJ/ψ= 60 MeV/c2 andσ¡ = 68 MeV/c2 • High rates expected (for L = 1 pb-1 ) : J/ ~ 80 Mevents,  ~ 500 kevents (assuming perfect trigger) • Acceptance x Efficiency pT ~ 0.35 (J/) and 0.65 (¡) • Studies of the parton distribution function at low x possible !

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