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## Leading neutrons and protons

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**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**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**gap survival factor**S2 ~ 0.48 for photoproduction AddveQM:**0.48**Tel**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)**Photoproduction of leading n**S2 ~ 0.48 Prelim. ZEUS data (DIS2006)**Photoprod.**pt2 dep. ~ exp(-bpt2) All p-exch. models fail !! F(t) no help. led to include r, a2-exch. **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**cross section**pt2 slope now S2 ~ 0.4, instead of 0.48**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)**LHC**every bunch crossing !**~0.01N for 420m**~0.03N for 220m need to predict SD….see Misha Ryskin’s talk**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**Determination of triple-Pomeron coupling g3P**e.g. pppY, pppY…. 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)**Estimate of bare triple-Pomeron coupling g3P**cc system v. compact rescatt. suppressed so need gpJ/y Y data as a func. of large MY some info.exists as bkgd to gpJ/y pZEUS,H1**ZEUS,H1**0.05 0.2 PPP more compact than proton expected for PPP stronger abs. for r (old) (new)**first evidence that g3P(t)**does not vanish as t0. first exptal. evidence of “strong coupling” of Pomeron**factors ~0.5 ~0.5**from data in going W = 201800 GeV consistent with decrease of S2(s). Unlike limiting fragn. hypothesis, normalised s depends on energy due to S2**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 gpJ/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)**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 one or two even believe there is deep theoretical link between “soft” and “hard” physics.)