Information fromleading neutrons at HERA

V.A. Khoze, A.D. Martin, M.G. Ryskin

(also with A.B. Kaidalov)

Alan Martin (Durham)2nd HERA-LHC WorkshopCERN, 6-9th June, 2006

Q2

xL, pt

exchange dominatesfor xL > 0.6,

but absorptive effects

Nikolaev,Speth,Zakarov;Alesio,Pirner

Leading neutron data (Q2, xL, pt)

structure fn, F2

(x,Q2) at small x fq,g

abs(qq-N) check of S2

pion flux This study inspired by prelim. ZEUS data

W

Bill SchmidkeMara SoaresMichele Arneodo

gap survival factorS2 ~ 0.48 for photoproduction

AddveQM:

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

gap survival factorexpect S2=1 for high Q2

Deep inelastic production

expect R = photoprod / DIS(high Q2) ~ 0.48

In practice, at finite Q2 abs. corr. remain;use two-ch eikonal with qq-p ~ p ~ p ~ 31mb; compare with prelim. ZEUS data

AddveQM:

R = photoprod / DISincl~ x -

(1 - xL) (0) - (Q2)

precise data willyield informationon both F2

and S2

…now the pt dep.

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 r2

now S2 ~ 0.4

large flip

Irving,Worden

cross section pt2 slope

now S2 ~ 0.4, instead of 0.48

Slope b of pt2 distrib.

b = b(prod) - b(DIS)

a2

pion structure function F2 (x,Q2)

“global” parton analysis of (data x>0.2)

better description with Q2 rescaled

x=0.01

x=0.001

x=0.1

Conclusions on leading neutrons at HERA

Exploratory study of prelim. ZEUS data (Q2, xL, pt, W)very informative

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

Precise data should determine F2(x,Q2) and S2(xL,pt,Q2)

important for LHC