The low x Structure Function Data. Introduction. Brian Foster Bristol/DESY. Corfu Summer School, 4.9.01. Low x physics at HERA. Other probes of QCD dynamics @ HERA. Diffraction and its connection with low x DIS. Summary & Outlook. For low x , HERA ~ only game in town.
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The lowx Structure Function Data
Introduction
Brian Foster
Bristol/DESY
Corfu Summer
School, 4.9.01
Lowx physics at HERAOther probes of QCD dynamics @ HERA
Diffraction and its connection with lowx DIS
Summary & Outlook
Brian Foster  Corfu lectures
Factorization  hard processes can be regarded as convolution
of “subprocess” cross section with probability to find
participating partons in target & probe  subsequent
hadronisation ~ independent process
For DIS can (normally) consider
virtual photon as dfunction
=> s ~ f swhere
s is subprocess cross section
f is parton dist. function,
satisfying f/ m ~ f P
(m = renormalisation scale,
Pis a splitting function)
Brian Foster  Corfu lectures
where are AP splitting functions
QCD evolutionIn general Ps are perturbative expansions to particular
orders, keeping terms most important for particular regions:
Leading lnQ2 terms come, in axial gauge, from evolution
along parton chain strongly ordered in transverse momenta,
LO DGLAP sums up
terms  NLO sums
terms which arise when two adjacent
kts become comparable, losing factorlnQ2.
Brian Foster  Corfu lectures
In small x region, leading terms in ln 1/x must be summed
independent of Q2. This is done by the BFKL equation.
LO
terms arise from strong x ordering
Generally, however, QCD coherence angular ordering 
work in unintegrated f(x,kt2,m2)  2 hard scales more
complicated CCFM evolution equation. DGLAP/BFKL two
limits of angular ordering. DGLAP, q kt/kl, q grows since kt
grows; in BFKL, q grows because kl x falls.
QCD evolutionBrian Foster  Corfu lectures
e(k)
e'(k')
2
Q
Q2 = xys
g
*(q)
2
W
xP
p(P)
Lowx structure function data s = k+P=energy in the ep c.m.s.
Q2 = (kk')2 = q2 =virtuality of the exchanged
x = Q2/(2P•q)=fraction of proton momentum
carried by the struck quark
y = (P•q)/(P•k) =fraction of beam lepton energy
transferred to the photon
W 2 = ys ~ Q2/xenergy in the *p c.m.s.
Brian Foster  Corfu lectures
To reach lowest possibleQ2, some tricks needed!
As well as exquisite
understanding of
detector 
BPT F2(low Q2)from ZEUSBrian Foster  Corfu lectures
Since =>
sr =F
sr ~F2 for small y,
æ
sr ~F2  fory 1, so =
F2fit 
ç

F
F
F
(x,Q2)
ç
L
L
L
è
(x,Q2)
2
FLfrom QCDfitfrom H1Brian Foster  Corfu lectures
Brian Foster  Corfu lectures
Brian Foster  Corfu lectures
Brian Foster  Corfu lectures
Brian Foster  Corfu lectures
Brian Foster  Corfu lectures
There are many parameterisations of the structure function
data on the market  some more deeply based on physics
others rather just convenient functional forms.
e.g. DL fit ;
wheree0 is “hard Pomeron”
SinceW 2 ~ Q2/x , and ,
Regge theory, which governs highenergys s relevant for lowx
IV. Interpretation & modelsBrian Foster  Corfu lectures
Another model exploits the “doublelogarithmic” limit of QCD:
Haidt, coming from a different direction, uses
BallForte fit
LogarithmsBrian Foster  Corfu lectures
Several on the market  MRST, CTEQ essentially global NLO QCD:
QCD fits to all DIS data (HERA & fixed target) plus other
relevant channels; GRV attempts to generate structure functions
by evolution from “valencelike” gluon at very low Q . All give
excellent fits to the data, with many free parameters.
NLO QCD fitsGRV’98
CTEQ
Deviation of exp. data from CTEQ fit
Brian Foster  Corfu lectures
Although using a subset (DIS) of the data, recently “homegrown”
pdfs appeared which give errors on fitted pdfs as well as the
correlations  e.g. Botje
PDFs with errorsBotje
Brian Foster  Corfu lectures
NNLO estimates for “homegrown”
the splitting fns. now
becoming available 
(Van Neerven&Vogt)
MRS, CTEQ groups
using them.
Some strange effects!
“Premature”
(K.Ellis, DIS2000)
NNLO PDF fittingBrian Foster  Corfu lectures
Thorne achieves “homegrown”
interesting improvements
by incorporating
ln(1/x) terms in splitting
fns after NNLO BFKL
using running coupling
BFKL eq.
NNLO PDF fittingBrian Foster  Corfu lectures
ZEUS has very precise “homegrown” F2
data over 6 orders of
magnitudein (x, Q2).
What can it tell us?
Look at the log. derivative
since ~ LO gluon  most
sensitive to lowxdynamics
 fit x bins with form
F2 = A+B(ln Q2)+C(ln Q2)2
Plot derivative as fn. of x&
Q2in bins of constantW
F2 & its derivativesBrian Foster  Corfu lectures
Errors on “homegrown” F2
syst. +stat. in
quadrature
(correlations
ignored.
F2 & its derivativesBrian Foster  Corfu lectures
There is no turnover at constant “homegrown”
Q2
One can look at
3D surface of log. Slopes.
The fundamental point is that the precision and kinematic
range of the data is now opening up qualitatively new areas of
study. The question is  what does it mean?
F2 & its derivativesBrian Foster  Corfu lectures
As “homegrown” x falls, as we have seen, the gluon radiation drives a strong
increase in parton density and hence increase inF2.
At some point, the number of partons becomes so large
that they cannot “fit” inside the proton and their
wavefunctions overlap  this is known as parton saturation.
What is happening at low x?Brian Foster  Corfu lectures
Recently, great deal of interest in dipole models & saturation.
L.T.
Breit, mom.
prest
In principle offers unification of inclusive DIS, diffraction
+
1 g
exchange
2 g octet
exchange
2 g singlet
exchange
Diffraction
Dipole ModelsInclusive F2
Brian Foster  Corfu lectures
Example of this type of model: GolecBiernat & Wuesthoff predicts
Q2s0
Dipole ModelsBrian Foster  Corfu lectures
The GolecBiernat&Wusthoff model does a reasonable predicts
qualitative job  but so does QCD, and/or a variety of simple
parameterisations.
Fits to slopesBrian Foster  Corfu lectures
Although one can make QCD fit the logarithmic slopes, the predicts
resultant pdfs, as we saw earlier, are strange to say the least!
ZEUS
prel..
ZEUS
prel..
QCD fit pdfsBrian Foster  Corfu lectures
The agreement of the data with dipole models and the saturation
concept is intriguing  are we seeing the first departure from
linear evolution in QCD? Clearly premature to draw this
conclusion  NLO QCD can also reproduce the data to the same
level  although at the cost of producing pdfs that are very
difficult to interpret in a sensible way.
The fundamental point is that the precision and kinematic range
of the data is now opening up qualitatively new areas of study.
Perhaps we are seeing a qualitatively new behaviour of QCD 
but we can’t be certain. One of the problems is that the
interesting “critical line” is down atQ2 ~ 1 GeV2  we need to
measure at low x but higher Q2.  needs a higher energy than
HERA can achieve.
F2 derivatives  summaryBrian Foster  Corfu lectures
In diffraction, proton stays intact saturation
In great majority of DIS events,
proton breaks up into hadrons +
“remnant” in forward direction
We saw that, in dipole models, there was an intimate connection
between DIS & diffraction. Is this borne out by the data?
DiffractionBrian Foster  Corfu lectures
The most basic measurement is the total cross section for saturation
diffraction. Does it agree with our expectations?
}
GB
&W
No. It has sameW2dependence asstot  W0.4.
Contradicts optical theoremstot ~ Wa => sdiff ~ W2a; and
ifstot ~ g, sdiff ~g2; and Regge, from Pomeron traj. stot~ W0.16
DiffractionBrian Foster  Corfu lectures
What about the structure functions? The analogue to saturation F2 is F2D
DiffractionBrian Foster  Corfu lectures
Note steep rise in saturation W
dependence ofs 
indicative of hard
processes becoming
dominant.
The intimate link between diffraction & nondiffractive DIS
via dipole models & saturation also clearly applicable to vector
meson production.
Diffraction  vector mesonsBrian Foster  Corfu lectures
Inset shows saturation
fit with Wd.
For theJ/y, the charm mass seems to be large enough to
provide a hard scale even atQ2 = 0.
Diffraction  vector mesonsBrian Foster  Corfu lectures
Fit to saturation
measured
cross sections
with Wd.
For ther, theQ2provides a hard scale.
Diffraction  vector mesonsBrian Foster  Corfu lectures
Fit to saturation
measured
cross sections
with Wd.
For all vector mesons , Q2 + M2seems to be a common hard
scale.
Diffraction  vector mesonsBrian Foster  Corfu lectures
For saturation Q2 > 5 GeV2, lVM ~ 2* lDIS
Compare the W dependence for VM production and DIS.
Diffraction  vector mesonsBrian Foster  Corfu lectures
But the J/ saturation y wave
function needs to be
modelled so that
model dependence
enters extraction.
What else can we learn from VMs? Since J/y seems to always
be in the pQCD realm, we can in principle learn about proton
gluon distribution.
Diffraction  vector mesons
Brian Foster  Corfu lectures
By fitting crosssection saturation t dependence we can look at
Pomeron trajectory.
Diffraction  vector mesons
Brian Foster  Corfu lectures
What is the appropriate hard scale in VM production? saturation
Diffraction  vector mesons
Brian Foster  Corfu lectures
J/ saturation y
f/r
f
Production s ratios BFKL prediction (Forshaw et al.)
y/r
r
2g
BFKL
Diffraction  vector mesons
Brian Foster  Corfu lectures
Simplest final state in diffraction saturation
Deeply virtual Compton scattering
Measures Re part of a QCD amplitude
Measures “skewed” parton distributions 
generalisation on normal proton pdf’s.
Brian Foster  Corfu lectures
Data cf. QEDC only QEDC & DVCS MC saturation
Deeply virtual Compton scattering
DVCS process clearly necessary  extract crosssection
Brian Foster  Corfu lectures
Now cross section measured, can go onto to look at saturation
interference etc.
Deeply virtual Compton scattering
Brian Foster  Corfu lectures
The agreement of the data with dipole models and the saturation
concept is intriguing  are we seeing the first departure from
linear evolution in QCD? Clearly premature to draw this
conclusion  one of the problems is that the interesting “critical
line” is down at Q2~ 1 GeV2  we want to measure at low x
but higher Q2.
This I guess will have to wait
for THERA, LHC ep option,
….? Is there something else
we can do “now”? Yes,
possibly. Running HERA
with nuclei rather than p
gives access to highdensity
of partons at low x.
V Summary & OutlookBrian Foster  Corfu lectures
Many open questions  saturation
and of course
this is not only important
to those
interested in QCD!
If we want to useW,Z
production at LHC as
lumi. monitor, we had better
understand smallxat HERA!
h
Summary & OutlookBrian Foster  Corfu lectures
The quality & precision of the HERA data are driving studies saturation
of lowx physics.
Watching the Herculean labours of the F2 experts extracting the
9697 result tells me that we are nearing the end of the road for
improved precision in the standard inclusive F2 at low x 
from now one attention will turn to semiinclusive(particularly
F2charm) and rare processes
Much theoretical help required (as always) to tell us where/how
to look
The connection between diffraction and DIS is certainly a very
interesting one that can throw much light on lowx physics.
It may well justify a “HERAIII” programme  but all
this will depend on TESLA!
Summary & OutlookBrian Foster  Corfu lectures