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.)