information from leading neutrons at hera v.a. khoze, a.d. martin, m.g. ryskin (also with a.b....

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