structure-function measurements at heramusashi.phys.se.tmu.ac.jp/nufactj03/nufactj03-kuze.pdf ·...
TRANSCRIPT
[email protected] 17/May/2003 NufactJ03 meeting 1
Structure-function measurementsat HERA
� Introduction
� F2 measurement and PDF fit
� Low-Q2 region
� High-Q2 region
� Current status and prospects
Masahiro Kuze
Department of PhysicsTokyo Institute of Technology
[email protected] 17/May/2003 NufactJ03 meeting 2
HERA ep Collider at DESY/Hamburg
• Ep=920GeV ⊗ Ee=27.5GeV (e+ or e−) ⇒• 2 colliding experiments and 2 fixed-target experiments• On-tape luminosity: ~110 pb-1 e+p, ~15 pb-1 e−p (‘98-’99) for H1 or ZEUS
HERA luminosity 1992 – 2000
10
20
30
40
50
60
70
10
20
30
40
50
60
70
Inte
grat
ed L
umin
osity
(pb
-1)
Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec
9293
94
95
1996
1997
98
1999 e-
1999 e+
2000
15.03.
s = 318GeV
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Deep Inelastic Scattering (DIS)
• Probe the proton (our most familiar micro-cosmos)with a point-like lepton probe. ‘giant electron-microscope’
• Bjorken x: Fractional momentum of a parton in the nucleon.
• 1/Q (momentum transfer) gives the spacial resolution.
• Neutral orCharged currentin t-channelpropagator
Q2=sxy
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Kinematic region probed
• > 100x larger kinematic reachcompared to fixed-target DISexperiments at CERN, DESY,FNAL, SLAC… (if proton is at rest,HERA CM energy means Ee=54TeV)
• At high Q2, probe the validity ofQCD at smallest distance →Quark structure? New particles?(Q2=40,000 GeV2 →1/Q=0.001 fm)
• At low Q2, probe the low-x region→ very soft constituents of proton;Saturation? Breakdown of standardDGLAP formalism (BFKL) ? log x
log
Q2
y =
1
y =
0.00
5
H1 + ZEUS
HERMES
E665
BCDMS
CCFR
SLAC
NMC
�
0
2
3
4
5
-6 -5 -4 -3 -2 -1
1
-1
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The Detectors
• ZEUS Detector– Uranium-Scintillator calorimeter
• for electrons• for hadrons
– Central tracking detector•
• H1 Detector– Liquid-Ar calorimeter
• for electrons• for hadrons
– Central tracking detector
σ(E) /E =18%/ Eσ(E) /E = 35%/ E
σ(pT ) / pT =
0.0058pT ⊕ 0.0065⊕ 0.0014 / pT
σ(E) /E =12%/ Eσ(E) /E = 50%/ E
2 out of (Ee, θe, Eh, θh)→ Reconstruction of (x, Q2)
e→
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Cross section & Structure functions
• NC differential cross section
[Y+F2(x, Q2) Y−xF3(x, Q2) − y2FL(x, Q2)]
F2 = Σ xqf+(x, Q2)[ef
2 − 2ef vf vePZ + (vf2+af
2)(ve2+ae
2)PZ2]
xF3 = Σ xqf−(x, Q2)[− 2ef af aePZ + 4vf af veaePZ
2] (f=u,d,c,s,b)
(Parton Distribution Functions)
PZ = sin−22θW⋅Q2/(Q2+MZ2) (Z-exchange & γ−Z interference)
Y± = 1 ± (1−y)2, ef: quark charge, vi/ai: EW couplings FL = F2 − 2xF1 (→0 in LO QCD, longitudinal Str. Function)
d2σ e ± p
dxdQ2 =2πα 2
xQ4
xqf± = xqf (x,Q
2) ± xq f (x,Q2)
+
[email protected] 17/May/2003 NufactJ03 meeting 7
Results of F2 Structure Function
• Strong rise of F2 as x decreases– Soft ‘sea’ of quarks in the proton
• Slope of rise gets steeper as Q2 ↑
softer parton smaller resol.
dynamics of quarks and gluons
• Good agreement with fixed-targetexperiments at middle - high x– Sea + valence quarks
HERA F2
0
1
2 Q2=2.7 GeV2 3.5 GeV2 4.5 GeV2 6.5 GeV2
0
1
2 8.5 GeV2 10 GeV2 12 GeV2 15 GeV2
0
1
2 18 GeV2
F2 em
22 GeV2 27 GeV2 35 GeV2
0
1
2 45 GeV2 60 GeV2
10-3
1
70 GeV2
10-3
1
90 GeV2
0
1
2
10-3
1
120 GeV2
10-3
1
150 GeV2
x
ZEUS NLO QCD fit
tot. error
H1 96/97ZEUS 96/97
BCDMSE665NMC
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F2 for fixed x, as a function of Q2
• At low x, strong scaling violationis seen.Large gluon density + splitting→ F2 increases
• At x ~ 0.1, approximate scaling.• At higher x, F2 decreases as Q2 ↑.
• Line = result of QCD fit (next slide)
– All data points well described.
g → qq
HERA F2
0
1
2
3
4
5
1 10 102
103
104
105
F2 em
-log
10(x
)
Q2(GeV2)
ZEUS NLO QCD fit
tot. error
H1 94-00 prelim.
H1 96/97
ZEUS 96/97
BCDMS
E665
NMC
x=6.32E-5 x=0.000102x=0.000161
x=0.000253
x=0.0004x=0.0005
x=0.000632x=0.0008
x=0.0013
x=0.0021
x=0.0032
x=0.005
x=0.008
x=0.013
x=0.021
x=0.032
x=0.05
x=0.08
x=0.13
x=0.18
x=0.25
x=0.4
x=0.65
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Perturbative-QCD fit of F2 data
• Example: ZEUS NLO DGLAP analysis PRD 67 (2003) 012007
– At Q2 = Q20, input functional form of PDF (Q2
0 =7GeV2)
• xf(x) = p1xp2(1-x)p3(1+p5x) for u-valence, d-valence, sea quarks and gluons
– ‘Q2 evolution’ is predicted byDGLAP (‘Altarelli-Parisi’) Equations
• �fi/�lnQ2 = (αs/2�)�fj⊗ Pij
Pqq Pgq Pqg Pgg
– Use world’s precision DIS data + ZEUS F2• BCDMS, NMC, E665, CCFR (µ-p, µ-D, ν-Fe)
– χ2 fit to determine p1 … p5
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PDFs obtained from the fitsZEUS
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
10-3
10-2
10-1
1
ZEUS NLO QCD fit
αs(MZ2) = 0.118
tot. error
CTEQ 6M
MRST2001
Q2=10 GeV2
xuv
xdv
xg(× 0.05)
xS(× 0.05)
x
xf
HERA: PDF determination
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
10-3
10-2
10-1
1
H1 2002 PDF Fit (prel.)�s(MZ
2) = 0.1185 fixeddata set: H1 (94/00) + BCDMS
ZEUS NLO QCD fit�s(MZ
2) = 0.1180 fixeddata set: ZEUS (96/97) + BCDMS, NMC, E665, CCFR
experimental errors only
Q2=1000 GeV2
xuv
xdv
xS( 0.05)
x
xf
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Low-x sea and gluon distributions• At Q2 ~ 1GeV2, gluon becomes valence-
like (and even tends to be negative)
• Sea quark is still rising
ZEUS
-2
0
2
4
6 Q2=1 GeV2
ZEUS NLO QCD fit
xg
xS
2.5 GeV2
xS
xg
0
10
207 GeV2
tot. error(αs free)
xS
xg
xf
20 GeV2
tot. error(αs fixed)
uncorr. error(αs fixed)
xS
xg
0
10
20
30
10-4
10-3
10-2
10-1
1
200 GeV2
xS
xg
10-4
10-3
10-2
10-1
1
2000 GeV2
x
xS
xg
ZEUS
0
5
10
15
20
1 10 102
103
104
xg
Q2(GeV2)
(a)
ZEUS NLO QCD fit
tot. error (αs-free)
tot. error (αs-fixed)
x=0.0001
x=0.001
x=0.01
x=0.1
[email protected] 17/May/2003 NufactJ03 meeting 12
Simultaneous extraction of αs and PDF
• Scaling violation: ∂F2/∂lnQ2 ~ αs•xg(x,Q2)Data at low x allow disentanglingcorrelation of αs and xg
• αs-free fit gives:H1: (additionally ±0.0005 from renormalization scale)
ZEUS: (additionally ±0.0004 from renormalization scale)
Difference in exp. error mainly from the treatment ofsystematic error and normalization of data pointsin the fitting procedure and error propagation.
α s = 0.1150 ± 0.0017(exp)−0.0005+0.0009(model)
α s = 0.1166 ± 0.0049(exp)± 0.0018(model)
HERA αs Measurements
[email protected] 17/May/2003 NufactJ03 meeting 13
What if there were no HERA data?
• HERA datadeterminethe low-xgluon andsea-quarkPDF.
• HERArevealed:F2 is verysteep.
ZEUS
0
1
2 Q2=2.7 GeV2 3.5 GeV2 4.5 GeV2 6.5 GeV2
0
1
2 8.5 GeV2 10 GeV2 12 GeV2 15 GeV2
0
1
2 18 GeV2
F2 em
22 GeV2 27 GeV2 35 GeV2
0
1
2 45 GeV2 60 GeV2
10-3
1
70 GeV2
10-3
1
90 GeV2
0
1
2
10-3
1
120 GeV2
10-3
1
150 GeV2
x
ZEUSNLO-QCD fit
tot. error
WITHOUT-ZEUS fittot. error
ZEUS 96/97BCDMSE665NMC
ZEUS
0
10
20 Q2=1 GeV2
ZEUS NLO QCD fit
tot. error
2.5 GeV2
WITHOUT-ZEUS fit
tot. error
0
10
20
xg
7 GeV2 20 GeV2
0
10
20
10-4
10-3
10-2
10-1
1
200 GeV2
10-4
10-3
10-2
10-1
1x
2000 GeV2
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Where is the lowest applicability of pQCD?
• Recall gluon becomes negativeat low Q2.
• It is not necessarily a problem,since PDF itself is not a physicalobservable.
• However, FL (observable)becomes also negative forQ2 < ~ 1 GeV2.
• Is this the lowest end of p-QCDapplicability?Look at the F2 data →
ZEUS
-0.2
0
0.2
0.4
0.6 Q2=0.3 GeV2 0.4 GeV2 0.5 GeV2
-0.2
0
0.2
0.4
0.6 0.585 GeV2
FL
0.65 GeV2 0.8 GeV2
-0.2
0
0.2
0.4
0.6 1.5 GeV2 2.7 GeV2
10-5
10-3
10-1
1
3.5 GeV2
-0.2
0
0.2
0.4
0.6
10-5
10-3
10-1
1
4.5 GeV2
10-5
10-3
10-1
1
6.5 GeV2
x
ZEUSNLO QCD fit
tot. error
[email protected] 17/May/2003 NufactJ03 meeting 15
Transition from perturbative to non-p. region
• At very low Q2, αs ↑ and non-perturbative effects important.
• ZEUS BPT data at very low Q2
available.
• Data cannot be described bypQCD fit below Q2 ~ 1 GeV2.(rather, it’s a surprise that pQCDworks as low as 1 GeV2 !)
• This is evident thanks to HERAdata at low x.
ZEUS
0
0.5
1
1.5
2 Q2=0.3 GeV2 0.4 GeV2 0.5 GeV2
0
0.5
1
1.5
2 0.585 GeV2
F2 em
0.65 GeV2 0.8 GeV2
0
0.5
1
1.5
2 1.5 GeV2 2.7 GeV2
10-5
10-3
10-1
1
3.5 GeV2
0
0.5
1
1.5
2
10-5
10-3
10-1
1
4.5 GeV2
10-5
10-3
10-1
1
6.5 GeV2
x
ZEUS NLO QCD fit
tot. error
ZEUS 96/97ZEUS BPT 97ZEUS SVX 95E665NMC
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F2 slope in low-x region
• For x<0.01, F2 can be wellparameterised by c•x−λ.
• λ fairly independent of x,linearly rises with logQ2 in‘DIS’ regime.
• Qualitative change happensaround Q2 < ~1GeV2, then λflattens in non-pert. region.
H1
Col
labo
ratio
n
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DIS in the hadronic picture
• At very low Q2, quasi-real photon acts likea hadron (vector-meson dominance)
• Plot shows F2 for fixed y, i.e.at fixed γ*-p CM energy W (W2~sy= Q2/x)
• σtot(γ*p)=(4π2α/Q2)F2(x=Q2/W2, Q2)so F2 must vanish as F2∝Q2 as Q2 → 0.
• Data show this behaviour (25<W<270 GeV)
and successfully fitted by a Regge-type fit.(consitent with Pomeron and Reggeon trajectoriesdetermined from hadron-hadron scattering)
[email protected] 17/May/2003 NufactJ03 meeting 18
Attempts to describe both regions
• γ dissociates into a qq pair (dipole) ofradius r=1/Q upstream of the proton.
• Its cross section with proton (σdipole)rises as r2 for small r (pQCD region).
• It saturates at large r, for r >~ R0.
• The critical radius R0 is characterisedby the parton density: R0~1/gluon~xλ
(low x = high-density environment).
• DIS cross section is a convolution ofσdipole and photon wave function Ψ.
Dipole models: a class of phenomenological modelsexample: Golec-Biernat & Wuesthoff DGLAP-like saturation
_
ˆ
ˆCartoon: R.Yoshida
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Does it describe the data?• This simple model gives a good
description of x<0.01 F2 data for both Q2
regions.
• Amazing thing is that the single modeldescribes F2, diffractive cross section andvector-meson production at the sametime (not discussed here).
Red:original GB&W
Blue:modified one (includesDGLAP evolution)Bartels, GB, KowalskiPRD66(2002)014001
Transition to low Q2
Q2 (GeV2)
F2(
x=Q
2 /sy,
Q2 )
y=0.007(x 1)
y=0.015(x 2)
y=0.025(x 4)
y=0.05(x 8)
y=0.12(x 16)
y=0.20(x 32)
y=0.26(x 64)
y=0.33(x 128)
y=0.4(x 256)
y=0.5(x 512)
y=0.6(x 1024)
y=0.7(x 2048)
y=0.8(x 4096)
10-1
1
10
10 2
10 3
10-2
10-1
1 10 102
Effective slopes
Q2(GeV2)
λ
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
10-2
10-1
1 10 102
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High-Q2 region − Electroweak effects• Pz = Q2/(Q2+MZ
2)At Q2 > ~5000 GeV2, effect ofZ-exchange clearly visible.
• σ(e−p) > σ(e+p) due to ±xF3
sensitive to valence quarks
• Statistically limited: more luminosity helps(esp. e−p)
ZEUS
0
0.5
1
1.5 Q2=200 GeV2 250 GeV2 350 GeV2 450 GeV2
0
0.25
0.5
0.75
1650 GeV2
σ∼ NC
800 GeV2 1200 GeV2 1500 GeV2
0
0.2
0.4
0.6
0.82000 GeV2 3000 GeV2 5000 GeV2
10-2
1
8000 GeV2
0
0.2
0.4
0.6
10-2
1
12000 GeV2
10-2
1
20000 GeV2
10-2
1
30000 GeV2
x
ZEUSNLO QCD fit
tot. error
ZEUS NC e- p98/99
ZEUS NC e+ p96/97
xF 3∝q(x,Q2) − q(x,Q2)
-0.2
0
0.2
0.4xF~
3
Q2 = 1500 GeV2 Q2 = 3000 GeV2 Q2 = 5000 GeV2
0
0.2
0.4
10-1
1
Q2 = 8000 GeV2
10-1
1
Q2 = 12000 GeV2
10-1
1x
Q2 = 30000 GeV2
ZEUSH1
SM
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Charged current − flavour decomposition
• e+p: only negative-charge quarks take part.
At high x, mainly d(x, Q2) probed.
• e−p: only positive-charge quarks take part.
At high x, mainly u(x, Q2) probed.
• In addition to u(x, Q2) > d(x, Q2), e+p getshelicity suppression (1-y)2. (y=Q2/sx)⇒ σ(e−p) >> σ(e+p) at large Q2.
• Data (not used in the fit) well described byQCD prediction.
ZEUS
0.5
1
1.5
2Q2=280 GeV2 530 GeV2 950 GeV2
ZEUSNLO QCD fit
tot. error
0.25
0.5
0.75
1 1700 GeV2
σ∼ CC
3000 GeV2 5300 GeV2
0.2
0.4
0.6
0.8
10-1
1
9500 GeV2
10-1
1
17000 GeV2
10-1
1
30000 GeV2
x
ZEUS CC e- p98/99
ZEUS CC e+ p94-97
d2σ e + p
dxdQ2 =GF
2πMW
2
MW2 +Q2
2
u + c + (1− y)2(d + s)[ ]
d2σ e − p
dxdQ2 =GF
2πMW
2
MW2 +Q2
2
u + c + (1− y)2(d + s)[ ]
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Can valence be determined by HERA only?
• ZEUS-only fit (all HERA-I NC/CC data, prel.) still a bit less precise thanglobal fit, but looks promising with future HERA-II data (e.g. d/u at high x).
• Using only ep data: no uncertainty from nuclear corrections (D, Fe, …).
ZEUS
10-2
10-1
1
0 0.2 0.4 0.6 0.8 1
xqV
Q2=1 GeV2
0 0.2 0.4 0.6 0.8 1
20 GeV2
ZEUS ONLY fit (prel.)
(94-00 data)
10-2
10-1
1
0 0.2 0.4 0.6 0.8 1
200 GeV2
tot. error
xuV
xdV
0 0.2 0.4 0.6 0.8 1
x
2000 GeV2
uncorr. error
ZEUS
10-2
10-1
1
0 0.2 0.4 0.6 0.8 1
xqV
Q2=1 GeV2
0 0.2 0.4 0.6 0.8 1
20 GeV2
ZEUS NLO QCD fit
10-2
10-1
1
0 0.2 0.4 0.6 0.8 1
200 GeV2
tot. error
xuV
xdV
0 0.2 0.4 0.6 0.8 1
x
2000 GeV2
uncorr. error
[email protected] 17/May/2003 NufactJ03 meeting 23
Evidence of Electro-Weak Unification
• At low Q2:NC ~ 1/Q4 (EM current)CC ~ GF
2 (Weak current)
• At high Q2 (> Mz2, MW
2):Both NC and CC mediated by unifiedEW current. σNC~σCC
• Dumping of σCC at high Q2
comes from Mw. Fit gives: (Z. e−p)
Mw = 80 ±2(stat)±1(syst)±1(PDF) GeV
• Space-like: q2(boson) << 0Completely different phase-space fromtime-like bosonsat LEP and Tevatron (q2 > 0).Complementary evidence/measurement. 10
-7
10-6
10-5
10-4
10-3
10-2
10-1
1
10
103
104
H1 e+p CC 94-00 prelim.
H1 e-p CCZEUS e+p CC 99-00 prelim.
ZEUS e-p CC 98-99 prelim.
SM e+p CC (CTEQ5D)
SM e-p CC (CTEQ5D)
H1 e+p NC 94-00 prelim.H1 e-p NC
ZEUS e+p NC 99-00 prelim.
ZEUS e-p NC 98-99 prelim.
SM e+p NC (CTEQ5D)
SM e-p NC (CTEQ5D)
y < 0.9
Q2 (GeV2)
dσ/d
Q2 (
pb/G
eV2 )
Interplay of EW and QCD physics
[email protected] 17/May/2003 NufactJ03 meeting 24
Limits on quark radius
• Repeat ‘form-factor measurement’ as we did for protons, butat Q2 ~ 40,000 GeV2 instead of 1 GeV2.– Resolution = 1/Q ~ 10-16cm = 0.001 proton radius
• If quark has a finite radius, cross section will decrease as theprobe ‘penetrates’ into it (sees less EW charge). σ = σSM(1−<Rq
2>Q2/6)2
• Limits on quark size(assuming electron is pointlike)ZEUS: Rq < 0.73*10-16 cm H1: Rq < 0.82*10-16 cm(95% CL)
Q2 [GeV2]
N/N
CT
EQ
5D
ZEUSZEUS (prel.) 94-00 e±p
Quark Radius Limit
Rq=0.73 ⋅10-16cm
1
10
103
104
103
104
0.8
1
1.2
High-Q2 NC data
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Large Extra Dimensions?• Arkani-Hamed, Dimopoulos and Dvali:
Assume n extra dimensions compactified to scale R,where only gravity propagates.Real GUT scale could be as low as TeV(RnMs
n+2 ~ M2Planck)
• Collider consequence: exchange of Kaluza-Kleinexcitations of gravitons would modify SM-particlescattering at high energy.
• HERA: eeqq CI formalism with λ/Ms4 as a parameter
• λ=+1: Ms > 0.83 TeV (H1), 0.81 TeV (ZEUS)λ=−1: Ms > 0.79 TeV (H1), 0.82 TeV (ZEUS)
• LEP, Tevatron limits ~ 1 TeV
0
1
2
3
103
104
���H1 data��
e−pNeutral Current√s¬=318 GeV
HERA I H1 preliminary
Q2 (GeV2)
dσ
/dQ
2 / dσ
SM
/dQ
2
Large Extra Dimension
Ms=580 GeV�� λ=+1Ms=610 GeV�� λ=−1
0
1
2
3
103
104
���H1 data��
e+pNeutral Current√s¬=318 GeV
HERA I H1 preliminary
Q2 (GeV2)
dσ
/dQ
2 / dσ
SM
/dQ
2
Large Extra Dimension
Ms=770 GeV�� λ=+1Ms=730 GeV�� λ=−1
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HERA luminosity upgrade
• 2000-2001: shutdown for lumi upgrade.– Final focussing magnets inside detector→ 5x luminosity than 2000 values
– Goal: Accumulate ~1 fb-1 in HERA-II run (till 2006)
– Longitudinal e± polarization for H1/ZEUS by spin rotators(was available only for HERMES)
• Also detector upgrades (ZEUS example)– Micro-vertex detector (b,c-tagging)
– Straw-tube tracker (forward tracking)
– New luminosity counters (calorimeter + spectrometer)
[email protected] 17/May/2003 NufactJ03 meeting 27
Current status
• New detector components successfully commissioned.
• New machine optics fine, achieved design specific luminosity
(luminosity per beam currents).
• 2002-03 run: with commissioning (modest) beam currents.
• Beam currents limited by backgrounds at detectors (chamber currents).
– Unexpectedly high double-scattered synchrotron radiation
– Vacuum conditioning of the new ring slower than expected
→ high proton beam-gas background in the detector
• Remedies will be taken in the shutdown March to June 2003.
• Achieved before shutdown: L=2.7*1031cm-2s-1 (design 7.0)
• Max 50% e+ polarization achieved with H1/HERMES/ZEUS rotators.
[email protected] 17/May/2003 NufactJ03 meeting 28
Summary
• Probing the proton with very high-energy probe showed us:– Precise measurement of sea-quark and gluon distributions
at low x → Crucial inputs for current and future experiments.
– Point of transition from perturbative to non-perturbative picture ofthe proton in QCD: Q2~1GeV2
– At high Q2, EW effects clearly visible and QCD evolution is validup to ~10,000GeV2.
• HERA-2 has started. High-luminosity run expected this fall.– Very precise determination of F2 and gluon distribution.
– Flavour decomposition using various tools.
– Polarized e±p to probe EW structure of DIS.
• Exciting future ahead with 10x more data and polarization.