Leading neutrons and protons V.A. Khoze, A.D. Martin, M.G. Ryskin

Alan Martin (Durham)Forward Physics WorkshopManchester, Dec. 2006

For Forward Physics at LHC it is useful to start withleading neutrons observed at HERA --- prelimin. ZEUS data --- good example of problems to be faced.

Q2

xL, pt

exchange dominatesfor xL > 0.6,

but absorptive effects

Leading neutron data (Q2, xL, pt)

structure fn, F2

(x,Q2) at small x fq,g

W

Here, we study re-scatt. effects in photoprod:abs(qq-N) check of S2

gap survival factorS2 ~ 0.48 for photoproduction

AddveQM:

0.48

Tel

Calculation of survival factor, S2(xL,pt2,Q2)

enhanced diag.need yi > 2-3:result ~ 15%

(a)

(b)

(a)

(b)

normal eikonaldiagrams

space-timepicture

If enhanced diagwere important,then n yield wouldbe energy dep.Not seen in data.

Photoproduction of leading n

Prelim. ZEUS data (DIS2006)

S2 ~ 0.48

pt2 dep.

~ exp(-bpt2)

Photoprod.

All -exch.models fail !!F(t) no help.

led to includea-exch.

Including and a2 exchange

, a2

enlarges , more at larger pt

leading to smaller slope, b

Use , a2 exch. degeneracy, additive QM

Slope b now OK --- too large --- adjust parameters to attempt to simultaneously describe and b

S2 ~ 0.48

large flip

Irving,Worden

qq-p) ~ p) ~ p)~ 31mb1.3

now S2 ~ 0.41.6 34mbdiffve.

excit.

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 , but not pt2 slope b

need also , a2 exchange

turnover of slope as xL 1 (tmin 0) may be used todetermine ,a2 versus exchange contributions

Absorptive corrections important S2 ~ 0.4Small contrib. from enhanced diagrams

Simultaneous description all data (Q2, xL, pt dep.) difficult

This is good. Precise LN data should determine F2

(x,Q2) at small x and S2(xL,pt,Q2)

important for LHC

LHC

every bunch crossing !

need to predict SD….see Misha Ryskin’s talk

~0.01N for 420m~0.03N for 220m

Forward physics at LHC needs predictions for leading protons

s-channel unitarity generates a whole sequence ofmulti-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

Soft rescatt: leading hadron secondaries smaller xL

populate/destroy rapidity gap

old hadron data effective g3P (which embodies S2)

e.g. pppY, pY…. at large MY and s/MY2

Estimate of bare triple-Pomeron coupling g3P

cc system v. compact rescatt. suppressed

so need pJ/ Y data as a func. of large MYsome info.exists as bkgd to pJ/p ZEUS,H1

expected for PPP

ZEUS,H1

PPP morecompactthan proton

stronger abs. for

0.05

0.2

(new) (old)

first evidence that g3P(t) does not vanish as t0.

first exptal. evidence of “strong coupling” of Pomeron

Unlike limitingfragn. hypothesis,normalised dependson energy due to S2

factors ~0.5 ~0.5from data in goingW = 201800 GeV

consistent with decrease of S2(s).

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 pJ/ 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, sincefirst 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 with small signal on huge bkgd.Accurate mass is crucial – but what about 0’sfrom underlying/pile-up events !! Correlations.

(Moreover some believe there is deep theoretical link between “soft” and “hard” physics.)