Leading neutrons and protons

1. Leading neutrons and protons V.A. Khoze, A.D. Martin, M.G. Ryskin For Forward Physics at LHC it is useful to start with leading neutrons observed at HERA --- prelimin. ZEUS data --- good example of problems to be faced. Alan Martin (Durham) Forward Physics Workshop Manchester, Dec. 2006

2. Q2 p exchange dominates for xL > 0.6, but absorptive effects W xL, pt Leading neutron data (Q2, xL, pt)  p structure fn, F2p(x,Q2) at small x  fpq,g Here, we study re-scatt. effects in photoprod: sabs(qq-N) check of S2

3. gap survival factor S2 ~ 0.48 for photoproduction AddveQM:

4. 0.48 Tel

5. Calculation of survival factor, S2(xL,pt2,Q2) g g g g g g (a) normal eikonal diagrams g g g g g g enhanced diag. need yi > 2-3: result ~ 15% (b) If enhanced diag were important, then n yield would be energy dep. Not seen in data. g space-time picture (a) (b)

6. Photoproduction of leading n S2 ~ 0.48 Prelim. ZEUS data (DIS2006)

7. Photoprod. pt2 dep. ~ exp(-bpt2) All p-exch. models fail !! F(t) no help. led to include r, a2-exch. 

8. Including r and a2 exchange r, a2 Irving, Worden large flip enlarges s, more at larger pt leading to smaller slope, b Use r, a2 exch. degeneracy, additive QM Slope b now OK --- s too large --- adjust parameters to attempt to simultaneously describe s and b 1.3 s(qq-p) ~ s(rp) ~ s(pp)~ 31mb S2 ~ 0.48 diffve. excit. 1.6 34mb now S2 ~ 0.4

9. cross section pt2 slope now S2 ~ 0.4, instead of 0.48

10. Conclusions on leading neutrons at HERA Exploratory study of prelim. ZEUS data (Q2, xL, pt, W) informative for forward physics at LHC • exch (with abs.) describes s, but not pt2 slope b • need also r, a2 exchange turnover of slope as xL 1 (tmin 0) may be used to determine r,a2 versus p exchange contributions important for LHC Absorptive corrections important S2 ~ 0.4 Small contrib. from enhanced diagrams Simultaneous description all data (Q2, xL, pt dep.) difficult This is good. Precise LN data should determine F2p(x,Q2) at small x and S2(xL,pt,Q2)

11. LHC every bunch crossing !

12. ~0.01N for 420m ~0.03N for 220m need to predict SD….see Misha Ryskin’s talk

13. Forward physics at LHC needs predictions for leading protons s-channel unitarity generates a whole sequence of multi-Pomeron diagrams:  (low-mass) SD, DD (multi-ch eikonal) Also have non-eikonal high-mass SD, DD : need triple-Pomeron coupling g3P

14. Determination of triple-Pomeron coupling g3P e.g. pppY, pppY…. at large MY and s/MY2 Soft rescatt: leading hadron  secondaries  smaller xL  populate/destroy rapidity gap old hadron data  effective g3P (which embodies S2)

15. Estimate of bare triple-Pomeron coupling g3P cc system v. compact  rescatt. suppressed so need gpJ/y Y data as a func. of large MY some info.exists as bkgd to gpJ/y pZEUS,H1

16. ZEUS,H1 0.05 0.2 PPP more compact than proton expected for PPP stronger abs. for r (old) (new)

17. first evidence that g3P(t) does not vanish as t0. first exptal. evidence of “strong coupling” of Pomeron

18. factors ~0.5 ~0.5 from data in going W = 201800 GeV consistent with decrease of S2(s). Unlike limiting fragn. hypothesis, normalised s depends on energy due to S2

19. Conclusions Leading neutrons/forward physics: S2 is important but no effect from enhanced diagrams Forward physics at LHC needs prediction of SD to quantify “pile-up” backgrounds see following talks From inclusive gpJ/y Y photoprod. we estimate the bare g3P coupling to be about 3 times larger than old hadron estimate of the effective g3P. (already anticipated---but is a direct estimate)

20. Conclusions continued… Is “soft” physics a speciality at the LHC, since first interest is in high pt leptons, photons and jets ?? No --- perhaps it is generally important ! At LHC need good knowledge of underlying event. e.g. consider H  gg with small signal on huge bkgd. Accurate gg mass is crucial – but what about p0’s from underlying/pile-up events !! Correlations. (Moreover some believe there is deep theoretical link between “soft” and “hard” physics.)