1. 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

2. 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

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

4. 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.

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

6. 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:

7. 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.

8. p t 2 dep. ~ exp(-bp t 2 ) Photoprod. All  -exch. models fail !! F(t) no help. led to include  a  -exch. 

9. 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

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

11. Slope b of p t 2 distrib.  b = b(  prod) - b(DIS)   a 2

12. 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

13. 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