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Information from leading neutrons at HERA V.A. Khoze, A.D. Martin, M.G. Ryskin (also with A.B. Kaidalov) Alan Martin (Durham) 2 nd HERA-LHC Workshop CERN, 6-9 th June, 2006

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Q2Q2 x L, p t exchange dominates for x L > 0.6, but absorptive effects Nikolaev,Speth,Zakarov; Alesio,Pirner Leading neutron data (Q 2, x L, p t ) structure fn, F 2 (x,Q 2 ) at small x f q,g abs (qq-N) check of S 2 pion flux This study inspired by prelim. ZEUS data W Bill Schmidke Mara Soares Michele Arneodo

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gap survival factor S 2 ~ 0.48 for photoproduction Add ve QM:

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Calculation of survival factor, S 2 (x L,p t 2,Q 2 ) enhanced diag. need y i > 2-3: result ~ 15% (a) (b) (a) (b) normal eikonal diagrams space-time picture If enhanced diag were important, then n yield would be energy dep. Not seen in data.

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Photoproduction of leading n Prelim. ZEUS data (DIS2006) S 2 ~ 0.48

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gap survival factor expect S 2 =1 for high Q 2 Deep inelastic production expect R = photoprod / DIS(high Q 2 ) ~ 0.48 In practice, at finite Q 2 abs. corr. remain; use two-ch eikonal with qq-p ~ p ~ p ~ 31mb; compare with prelim. ZEUS data Add ve QM:

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R = photoprod / DIS incl ~ x - (1 - x L ) (0) - (Q2) precise data will yield information on both F 2 and S 2 …now the p t dep.

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p t 2 dep. ~ exp(-bp t 2 ) Photoprod. All -exch. models fail !! F(t) no help. led to include a -exch.

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Including and a 2 exchange , a 2 enlarges , more at larger p t leading to smaller slope, b Use , a 2 exch. degeneracy, additive QM Slope b now OK --- too large --- adjust parameters to attempt to simultaneously describe and b ~ S 2 r 2 now S 2 ~ 0.4 large flip Irving, Worden

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cross sectionp t 2 slope now S 2 ~ 0.4, instead of 0.48

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Slope b of p t 2 distrib. b = b( prod) - b(DIS) a 2

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pion structure function F 2 (x ,Q 2 ) “global” parton analysis of (data x >0.2) better description with Q 2 rescaled x =0.01 x =0.001 x =0.1

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Conclusions on leading neutrons at HERA Exploratory study of prelim. ZEUS data (Q 2, x L, p t, W) very informative exch (with abs.) describes , but not p t 2 slope b need also , a 2 exchange turnover of slope as x L 1 (t min 0) may be used to determine ,a 2 versus exchange contributions Absorptive corrections important S 2 ~ 0.4 Small contrib. from enhanced diagrams Simultaneous description all data (Q 2, x L, p t dep.) difficult Precise data should determine F 2 (x,Q 2 ) and S 2 (x L,p t,Q 2 ) important for LHC

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