Low-x, 29/6/2006, A. Solano 1 Leading Neutron Energy and p T Distributions from ZEUS A. Solano Univ. of Torino and INFN On behalf of the ZEUS Collaboration Outline: • Introduction and models • Data-sets • Leading neutrons in DIS • Leading neutrons in photoproduction • Leading neutrons & leading protons • Summary
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Leading Neutron Energy and p T Distributions from ZEUS
Leading Neutron Energy and p T Distributions from ZEUS. A. Solano Univ. of Torino and INFN On behalf of the ZEUS Collaboration. Outline: Introduction and models Data-sets Leading neutrons in DIS Leading neutrons in photoproduction Leading neutrons & leading protons Summary. - PowerPoint PPT Presentation
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Low-x, 29/6/2006, A.Solano
1
Leading Neutron Energy and pT
Distributions from ZEUS
A. SolanoUniv. of Torino and INFN
On behalf of the ZEUS Collaboration
Outline:
• Introduction and models
• Data-sets
• Leading neutrons in DIS
• Leading neutrons in photoproduction
• Leading neutrons & leading protons
• Summary
Low-x, 29/6/2006, A.Solano
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Leading neutron production
Neutrons often carry a large fraction of the proton beam energy xL = En/Ep
Test of production models:
• `Standard` fragmentation:
compare to usual MC generators
• Particle exchange:
compare to One Pion Exchange (OPE) models
probe the structure function of the exchange particle
test vertex factorization: many models predict factorization violation due to rescattering on the photon (neutron absorption)
e’
e’
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Absorption
p
d’Alesio and Pirner, EPJ A7 (2000) 109
• the larger the photon, the fewer the n’s detected (more absorption in PHP than DIS)• the smaller the nπ system, the fewer the n’s detected (more absorption at high pT in PHP vs. DIS)
• evaluate correction due to enhanced absorptive diagrams B (~ 15%)• show importance of migration due to rescattering for xL< 0.8
• include effects of ρ, a2 exchange
• estimate the gap survival factor (important for LHC!) which takes into account that rescattering may populate the rapidity gap with secondary particles carrying away energy from the leading neutron
• Qualitatively consistent with absorption: more abs. at small rnπ , i.e. large pT, in PHP
Comparison with model by A. Martin, 2nd HERA-LHC Workshop
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LN in dijet photoproduction vs. DIS
Neutron energy spectra suggest phase space limitation:with energetic dijets in the final state little room is left for leading neutron
Slopes have similar magnitude, statistics limits any conclusion on possible differences
Hard to draw any conclusion on absorption
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Leading neutrons & leading protons in DIS
• Clear different trends
• LN: main contribution is π exchange
• LP: contribution of other trajectories
• Similar magnitude for xL~ 0.7-0.8
π exchange LN ~ LP
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Summary
• Precise measurements of leading neutron xL and pT2 distributions were presented
• Pure OPE models do not describe the data
• MCs with `standard` fragmentation do not describe the data, LEPTO is promising
• The b-slopes in DIS are better reproduced including ρ, a2 exchanges
• Comparing PHP with DIS shows that leading neutron production is suppressed in photoproduction at low xL, high pT, in agreement with absorption hypothesis
• The neutron energy spectrum in photoproduction is compatible with effects of absorption and migration as calculated by Kaidalov, Khoze, Martin and Ryskin, by which a gap survival factor S2 ~ 0.4 has been evaluated
• Leading neutrons in dijet photoproduction have similar slopes but a different energy spectrum than in DIS
•
The b-slopes of protons and neutrons have different behaviours but agree at
xL ~ 0.7-0.8 where π exchange is dominant in both cases