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

2

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’

Low-x, 29/6/2006, A.Solano

3

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)

Absorption from additional pomeron exchange: recently Kaidalov, Khoze, Martin, Ryskin(Nikolaev, Speth, Zakharov, hep-ph/9708290) hep-ph/0602215; Khoze, Martin, Ryskin hep-ph/0606213

• 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

A

B

Low-x, 29/6/2006, A.Solano

4

Leading baryon detectorsy

Forward Neutron Calorimeter (FNC):

• 10 λl Pb-scintillator sandwich• σ/E = 0.65/√E, energy scale accuracy ±2%

Forward Neutron Tracker (FNT):

• scintillator hodoscope at 1 λl (position detector)• σX,Y = 0,23 cm σθ = 22 μrad

Z

yoY FNTxoX FNTEp nT 2

)( 2)( 222

(xo , yo): neutron 0o point

Leading Proton Spectrometer (LPS):

• 6 stations of silicon μstrips detectors (S1-S6)• σXL < 1%

pT resolution is dominated by pT spread of the proton beam (50-100 MeV)

Low-x, 29/6/2006, A.Solano

5

Data samples

For all samples: neutron: 0.2 < xL < 1

θn < 0.75 mrad pT2 < 0.476 xL

2 GeV2

(due to magnet apertures)

Deep Inelastic Scattering (DIS): 40 pb-1, Q2 > 2 GeV2

Photoproduction (PHP): 6 pb-1, Q2 < 0.02 GeV2

Dijets in photoproduction: 40 pb-1, Q2 < 1 GeV2, 130 < W < 280 GeV

ETjet1 > 7.5 GeV, ET

jet2 > 6.5 GeV

-1.5 < ηjet1,2 < 2.5

DIS and PHP have very different inclusive cross sections σinc

for sensible comparisons look at σLN/σinc

Additional benefit: systematic uncertainties of central ZEUS cancel

Low-x, 29/6/2006, A.Solano

6

xL distribution in DIS

Below xL ~ 0.7 yield drops due to decreasing pT

2 range

Systematic uncertainties from:

• 0o point• FNC energy scale• dead material before FNC

Low-x, 29/6/2006, A.Solano

7

pT2 distributions in DIS

• Note varying pT2 ranges in different xL bins

• Data are well described by exponential distributions: get intercepts and slopes

Low-x, 29/6/2006, A.Solano

8

pT2 distributions in DIS: intercepts & slopes

Intercepts a(xL) Slopes b(xL)

Low-x, 29/6/2006, A.Solano

9

b(xL) in DIS: comparison with OPE models

)'(),( 2/2

2

spxfdpdx

d eTLp

TL

Numerous parametrizations of the pion flux fπ/p(xL,pT

2) in literature

Best agreeing models shown here

More refinement needed: absorption, migration, more exchanges

A. Martin, 2nd HERA-LHC Workshop

Low-x, 29/6/2006, A.Solano

10

LN in DIS: comparison with non-OPE MCs

Compare with several popular MC models w/o OPE, all with default settings:

• LEPTO ~ ok in shape and magnitude for the xL distribution• All models fail in reproducing the slopes

Low-x, 29/6/2006, A.Solano

11

xL distribution in PHP vs. DIS

Curves from model by Kaidalov, Khoze, Martin, Ryskin

• Data in agreement with absorption hypothesis

• Gap survival factor from xL distribution:

(A. Martin, 2nd HERA-LHC Workshop)

ρ

S2 ~ 0.5 (~ 0.4 including ρ, a2)

Low-x, 29/6/2006, A.Solano

12

pT2 distributions in PHP vs. DIS

normalized to 1 at pT

2 = 0

π

π,ρ,a2

• Clear difference: b(PHP) > b(DIS) for 0.6 < xL < 0.9

• 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

Low-x, 29/6/2006, A.Solano

13

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

Low-x, 29/6/2006, A.Solano

14

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

Low-x, 29/6/2006, A.Solano

15

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