wg-a summary working group a structure functions, low x and diffraction robin devenish victor fadin...
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WG-A Summary
Working Group A
Structure Functions, Low x andDiffraction
Robin Devenish
Victor Fadin
(Yuri Kovchegov)
WG-A Summary
OUTLINE pt I (RD)
1. Inclusive DIS Data
2. QCD fits and parton densities
3. RHIC A-A collisions and low x
4. GVDM and Colour Dipole models
5. DVCS and vector mesons
6. Hard Diffraction
Cannot do justice to all the 60 talks and discussion….
WG-A Summary
Inclusive DIS Data
e-p
F2 new data at low Q2 and x > 0.001 from QED Comptons (Lenderman, H1)
FL first measurement at HERA using ISR events (J Cole, ZEUS) new extraction at high y and low Q2 (Lobodzinska, H1)
High Q2 NC CC – final HERA-I e+p data (Dubak, H1; Rautenberg, ZEUS)
-NF2, xF3 new data from high stats improved and -bar beams (Naples, NuTeV)
WG-A Summary
FL from ISR ~36 pb-1 (J Cole)
Need variable s and high y – lower E'e
WG-A Summary
QCD fits and parton densities
• Updates by ZEUS (Rautenberg) and H1 (Reisert) of NLO QCD fits• Detailed study (Portheault) on s-quark asymmetry and effect on NuTeV sin2W
• MRST (Martin) on uncertainties• CTEQ (Tung) on recent improvements• Beyond NLO (Alekhin) – very careful study using NNLO extensions, target mass and higher twist
• Technical improvement in fitting procedure - diagonalising ‘Hessian’ matrix – gives numerical stability and enables PDF uncertainties (from exp. sys errs) to be propagated to observables. Now well established thanks to work of CTEQ.
WG-A Summary
Problems with at low x and Q2
• Individual fits describe the data very well• There are well known worries about the partons at low x and Q2 – gluon rising less steeply than the sea and eventually going negative• MRST study of ‘stability’ against data fit (xmin, Q2
min)•NLO DGLAP not enough
- NNLO?- BFKL improvements?
WG-A Summary
Alekhin NNLO
WG-A Summary
Importance of FL
Summary of H1 extraction of FL – now extended to lower Q2
FL with a precision of present F2 would do wonders for low x physics!
(Lobodzinska)
WG-A Summary
• 8 orders of magnitude!
• Highest ET jets ever!
Comparison: prediction vs. first data from Tevatron IICDF – La Thuile and Moriond
(Tung)
WG-A Summary
– Cross-section is multiplicatively dependent on anti-quarks, i.e. u·u, d·d, …compared to additively dependent, i.e. u+d + u+d … in DIS.
– E605 (~1989, pA collision) has been the main source of experimental input to global analysis;
– pp/pd measurement of NA51 and E866 revolutionized the determination of d/u quark distribution ratio several years ago.
– Recent E866 data on separate pp and pd cross-sections will make the d/u discrimination much more quantitative.
The d/u ratio at large x is more directly probed by the CC e+p / e-p (and ep / ed) measurements at HERA.
- ---
Importance of D-Y data in tightening flavour constraints
(Tung)
WG-A Summary
RHIC A-A collisions and low x
Spectators
Impact parameter (b)
• Central collisions (small b) produce high density gluons• ‘Universality’ of low x gluon dynamics (input from HERA) • Produced particle properties (Nchg, ET) determined by gluons• Can this be measured and models tested? YES!
(B Cole)
WG-A Summary
Low-x Observables in PHENIX (B Cole)
Charged MultiplicityCharged MultiplicityPad Chambers:Pad Chambers:RRPC1PC1 = 2.5 m = 2.5 mRRPC3PC3 = 5.0 m = 5.0 m||<0.35, =
Transverse EnergyTransverse EnergyLead-Scintillator Lead-Scintillator EMCal:EMCal:RREMCEMC = 5.0 m = 5.0 m||<0.38, = (5/8)
Trigger & CentralityTrigger & Centrality Beam-Beam Counters:Beam-Beam Counters:3.0<||<3.9, = 20º Calorimeters:0º Calorimeters:|| > 6, |Z|=18.25 m
Collision Region (not to scale)
WG-A Summary
•Kharzeev, Levin, Nardi Model– Large gluon flux in highly boosted nucleus
– When probe w/ resolution Q2 “sees” multiple partons, twist expansion fails
• i.e. when >> 1
• New scale: Qs2
Q2 at which = 1
– Take cross section = s(Q2) / Q2
– Gluon area density in nucleus xG(x, Q2) nucleon
– Then solve: Qs2 = [constants] s (Qs
2) xG(x, Qs2) nucleon
• Observe: Qs depends explicitly on nucleon
– KLN obtain Qs2 = 2 GeV2 at center of Au nucleus.
– But gluon flux now can now be related to Qs
Qs2 / s (Qs
2)
Saturation in Heavy Ion Collisions
WG-A Summary
dN/d Saturation Model Comparisons
• Additional model “input”– x dependence of G(x) outside
saturation region• xG(x) ~ x- (1-x)4
– GLR formula for inclusive gluon emission:
• To evaluate yield when one of nuclei is out of saturation.
– Assumption of gluon mass (for y )
• M2 = Qs • 1 GeV
– Compare to PHOBOS data at 130 GeV.
– Incredible agreement ?!!
Kharzeev and Levin Phys. Lett. B523:79-87, 2001
dN
/d
per
part
. p
air
dN
/d
WG-A Summary
GVDM and Colour Dipole models
• At low x and Q2 a partonic approach may not make sense – return to an older picture for *p – vector dominance
• Talks by Ingelman and Schildknecht reviewed this approach.• Schildknecht emphasised the wider applicability of the GVDM approach – consistent picture of inclusive *p, *p to Vp and DVCS – and close connection to colour dipole models
• Dipole models - very flexible and powerful framework for describing low x low Q2 inclusive *p, *A, *p to Vp, DVCS - and allow for saturation• A combined GVDM + dipole model (Szczurek) gives a good description of low Q2 inclusive data – to Q2 = 0 limit
• Crucial extension from early GBW approach is to add transverse profile in form of impact parameter (Kowalski)
WG-A Summary
Proton
b – impact parameter
Impact Parameter Dipole Saturation Model
T(b) - proton shape 1)( 2
0
bdbT
well motivated:
Glauber- Mueller dipole cross section
))()/,(32
exp(12 ),( 2
022
2
2bTQrCxxgr
bd
rxds
(Kowalski)
WG-A Summary
d)-(2 parameter impact - d)-(2 momentum transv.- 2 bt
t-dependence of the diffractive cross sections determines the b distribution
22222
2*1
0
22 |),,())(),(32
exp(1),,(|16
1*
rzQbTxxgrrzQdzebdrddt
dsVM
bip
VM
)2/exp(~)(
)exp(~
2 BbbT
tBdt
d diff
)/()2/)(exp()(
)2/exp()(
022
2
EGGY
GG
wbKwbbbdbT
wbbT
(Kowalski)
WG-A Summary
DVCS and vector mesons
• New measurement of DVCS by ZEUS (Abramowicz)
• New results at large |t| for inelastic J/ from H1 (Beckingham)
• New results on elastic and J/ from H1 (Fleischmann)
• New results from ZEUS on and J/ (Tandler)
• First look at vector mesons with COMPASS (Korzenev)
• New data on gg to rr from L3 at LEP (Fedin)
• Issues• energy dependence W as function of MV and Q2
• choice of scale Q2+M2
WG-A Summary
Deeply Virtual Compton Scattering
The process
The background
Data and dipole models
(Abramowicz)
WG-A Summary
H1 VM results
(Beckingham)Change from exponential to (-t)-n
With n ~ 3
(Fleischmann)Energy dependence asFunction of Q2 – elastic
WG-A Summary
ZEUS VM (Tandler) - Ratio V/tot
• W independence for cannot be explained by pQCD or Regge
still unknown soft physics?
J/
pQCD:
V S/Q6 |xG(x,Q2)|2
V/tot W2/b
Regge:
V W4 (01)
V/tot W2 (01) /b
Q2+M2
4 GeV2
WG-A Summary
COMPASS experiment at CERN
WG-A Summary
Vector mesons with COMPASS
(Korzenev)
Will very soon be provinding a lot of high statistics results – with advantage of polarized beam and target
WG-A Summary
Cross section for
Broad enhancement near threshold of
L3 at LEP (Fedin)
WG-A Summary
Hard Diffraction
• New results on inclusive hard diffraction in DIS from H1 (Coppens) and ZEUS (Lim)
• New results on hard diffractive final state (Schaetzel)
• New results on leading baryons from ZEUS (Soares)
• First look at Diffraction at Tevatron Run II (Terashi, CDF) D0 showed first diffractive Z0 (Stevenson)
• Issues• Energy dependence (hard or soft Pomeron)• Factorisation within ep and between ep and ppbar (Schlein)
WG-A Summary
Ratio of hard diffraction to total DIS - ZEUS
(Lim)
WG-A Summary
Energy Dependence of Hard diffraction
(Lim)
WG-A Summary
Tests of factorization – Schaetzel H1
Factorisation works at large Q2 - but not for photoproduction
WG-A Summary
(Terashi CDF)
D0 has pots in quads as well
WG-A Summary
WG-A Summary
Erhan & Schlein test of factorization
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WG-A Summary
Summary so far…
• There is much still to learnt and measured…• QCD is good shape at large Q2 and hard scales• Partons are becoming a precision tool
• Prospect of connecting HERA to heavy ions is exciting – wider universality of gluon dynamics?
• But low x and low Q2 requires theoretical effort• Which leads into part II…