Central Exclusive Di Muon Production Ronan Mc Nulty
- Slides: 24
Central Exclusive Di. Muon Production Ronan Mc. Nulty (UCD Dublin) On behalf of the LHCb collaboration SM@LHC, Durham, 11 th-14 th April 2011 Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 1
Motivation Usually proton collisions produce very many final state particle because the gluon is a coloured object. But if a colourless object is exchanged…. . • • • Results can be related to HERA and Tevatron Understand QCD in a clean environment Unambiguous evidence for pomeron. Search for odderon Constrain unintegrated parton distributions at very small x (2 x 10 -6) Search for saturation effects Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 2
Central Exclusive Production y=-10 y=-2 y=0 y=2 y=10 Elastic Scattering Single Diffraction Double Diffraction Central Exclusive Production (elastic) Central Exclusive Production (inelastic) Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 3
LHCb: a forward spectrometer y Interaction Point Tracking Stations Muon Chambers 250 mrad 10 mrad VELO Magnet RICH detectors ECAL HCAL Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 l 20 mz 4
Use of backwards tracks Clearly not exclusive Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 5
Trigger on two muons and <20 SPD hits Requiring a gap, there is evidence for central exclusive production decaying to two muons. Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 6
Simple Selection Criteria n n n No backward tracks (gap of ~2 units of rapidity) Precisely two forward muons No photons (for J/psi and diphoton process) One photon (for Chi. C analysis) p. T of dimuon <900 Me. V (<100 Me. V for pmmp). Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 7
Dimuon Mass Spectrum af te r tr ig ge r demand gap Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 8
J/ψ and ψ’ n n n No backward tracks Precisely two forward muons No photons Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 9
J/ψ and ψ’: Number of Photons J/ψ ψ’ Estimated feeddown from Chi_c Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 10
J/ψ and ψ’: Number of Tracks ψ’ J/ψ ψ’ feeddown Tempting to fit the background under the peak using straight line/exponential. Better if we can understand the physics giving background. Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 11
Inelastic backgrounds Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 12
Fit elastic and inelastic components Estimate purity below 900 Me. V to be 80+-3 % Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 13
J/ψ Non-resonant background & misid This is not background subtracted ! Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 14
ψ’: Non-resonant backgrounds & misid Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 15
pp->pmmp n n n No backward tracks Precisely two forward muons. mmm>2. 5 Ge. V No photons Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 16
Fit elastic and inelastic components Shape for inelastic events Fit to signal events Note: this time we have simulation that predicts the shape for the three contributions. Background shape from data Signal shape from simulation. Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 17
χc n n n No backward tracks Precisely two forward muons Precisely one photon Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 18
χc: Di. Muon Invariant Mass About half the background that was observed in the exclusive J/ψ analysis (since no continuum process). Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 19
χc: Di. Muon+Photon Invariant Mass (Caveat: Inelastic contribution appears to be much larger than for J/ψ) Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 20
Cross-section calculations n σ=(p. N/ε) / L n ε: Trigger, tracking, photon & selection efficiencies are estimated from simulation, with size of systematics taken from data/simulation agreement. n p: Feed-down for ψ’, χc subtracted. Uncertainty on fit to p. T spectrum taken as systematic on inelastic contribution. n L: Analysis only uses single-interaction events. Need to know average number of pile-up interactions. Currently translates into 20% uncertainty on L. Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 21
Summary Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 22
Comparison to Theory J/ψ: 474 +-103 pb Starlight (Klein & Nystrand) 292 pb Super. Chic (Harland-Lang, Khoze, Ryskin, Stirlin) 330 pb Motyka & Watt 330 pb Schäfer & Szczurek 710 pb ψ’: 12. 2 +- 3. 2 pb Starlight (Klein & Nystrand) 6 pb Schäfer & Szczurek ~ 17 pb σ(ψ’)/σ(J/ψ): 0. 20 +- 0. 03 Starlight (Klein & Nystrand) 0. 16 Schäfer & Szczurek ~ 0. 2 HERA: 0. 166 +- 0. 012 (lower √( p) ) CDF: 0. 14 +- 0. 05 (lower √( p) ) χ0: 9. 3 +- 4. 5 pb Super. Chic: 14 pb pμμp: 67 +- 19 pb χ1: 16. 4 +- 7. 1 pb 10 pb χ2: 28. 0 +-12. 3 pb LPAIR (J. Vermaseren) 42 pb Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 23
Summary n n First Observation of CEP at LHC First separation of χc spin states in CEP Good agreement with theory predictions Consistency with HERA and CDF results. Ronan Mc. Nulty, SM@LHC, Durham 11 -14 April 2011 24
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