1. - Structure of Matter and QCD
HERA
- Structure of Matter and QCD e p
Contents
15 year running
of HERA and H1/ZEUS
Electroweak results
Structure of the proton
Katsuo Tokushuku
(KEK, ZEUS)
@ YongPyong APCTP2007
22/Feb/2007

2. （27.5GeV electron 920GeVproton） the world largest electron microscope
DESY/HERA
HERA H1
ZEUS
HERA:
（27.5GeV　electron 　920GeVproton）
　　the world largest electron microscope

3. (27.5GeV electron(positron) vs. 920 GeV proton)
nuclei
Progress in accelerator enables us to investigate the smaller structure.
HERA:
(27.5GeV　electron(positron)
　vs. 920 GeV　proton)
Q2max=s=4EeEp~10000GeV2
cf. in the rest frame
　 s=2EeMp
In order to obtain the same CMS energy
as HERA in a fixed target experiment,
it requires 54TeV electron beam.
No sub-parton structure
down to 0.67x10-18m
(HERA-I/II result, prelim.)

4. Tunnel Construction started
A view of the HERA ring tunnel
Tunnel Construction started
in 1984
Proton ring
Electron ring

5. Experiments started in 1992

6. HERA will stop at the end of June in 2007
HERA History ( )
15 years
3 year
break
HERA-I
HERA-II
HERA will stop at the end of June in 2007

7. Neutral Current (NC) Charged Current (CC) Positron Proton Jet Jet
Scattered positron
Positron
Proton
Jet
Jet
Scattered positron
Anti-neutrino
Proton 920GeV
Proton 920GeV
　γ、Z W
quark
（u,d,s,c..)
d,s-quark
positron
27GeV
positron
27GeV
Scattered quark
（jet）
u,c-quark （jet）

8. Introduction: Deep Inelastic Scatteringq
e 27.5GeV
p 920GeV g
current jet
remnant jet Q2 x W p k k’ p
Described by 2 kinematic variables
photon virtuality
Bjorken x
Inelasticity
Q2=-q2
x = Q2/2p.q
y = p.q/p.k
s=Q2xy
FL : Longitudinal Str. Ft. (0 in QPM)
F3 : Small at Q2 << Mz2
quark distribution function

9. pQCD view of F2 PDF depends on Q2
DGLAP evolution　（Dokshitzer, Gribov, Lipatov, Altarelli, Parisi）
splitting function (known from pQCD)
Q2→larger：
　high-x q and g are split into low x q and g. y x
(y-x) y x
(y-x) x F2
~e2(q+q)

10. Kinematical region for HERA structure function measurements
2 order higher
region in Q2,
2 order lower
region in x
Wide (O(106)) span
in Q2:
Precise measurements
for Q2 evolution

11. Early HERA data showed rapid increase of F2 at low x.
Predictions of F2
Gluck, Reya and Vogt
“pQCD” : parton evolution
Precise F2 determination
( data samples)
HERA Kinematic Limit 1
Fixed
target
data
Early HERA data showed rapid
increase of F2 at low x.
Donnachie & Landshoff
“Hadronic”: Regge theory behavior of γp total cross section

12. 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 goes up
Good agreement with fixed-target experiments at middle - high x
Sea + valence quarks

13. F2 for fixed x, as a function of Q2
At low x, strong scaling violation is seen.
Large gluon density splitting
 F2 increases
At x ~ 0.1, approximate scaling.
At higher x, F2 decreases as Q2  Quark radiates off gluon: q→qg
Line = result of QCD fit (comming slides)
All data points well described. y x
(y-x) y x
(y-x)

14. Kinematical region for HERA structure function measurements
s=Q2xy
2 order higher
region in Q2,
2 order lower
region in x
Wide (O(106)) span in Q2:
Precise measurements for Q2 evolution

15. γ e q ee eq 2 Z
ve,ae
vq,aq
In SM,
parity +
parity - W e q ν q’

16. Measurements of NC/CC Cross sections
HERA-I　Final Results
Neutral Current
Charged Current
At high Q2 (Q2~MW,Z2),
σNC　～　σCC
　　　　　　 αNC~αCC　　　
 Electroweak unification
Good agreement with the SM
Mw = 80.3 2.1(stat)1.2(syst)1.0(PDF) GeV (from ZEUS ep data)
NC(e+p) < NC(e-p)
　　　　　  γZ interference
CC(e+p)<CC(e-p)
　　　　　 u,d-quark distribution in the proton

17. Seaches of BSM Good agreement with the SM e e
“softer” scattering
If the quark is not point-like
e e
Good agreement with the SM
　Quark Radius＜0.67×10-16cm (prelim.)
　No signal for Leptoquarks so far
Excess in data (e+p)

18. HERA I  II
Longitudinal polarization of lepton beam :  Direct EW sensitivity
Sokolov-Ternov effect  Lepton beam has transverse polarization +
Spin rotator before/after the H1/ZEUS/HERMES detectors.
Polarization build-up at HERA
Luminosity Upgrade :
 High-Q2 requires large luminosity
Final focusing magnets in the detector
30 ~40% on average

19. asymmetry in CC in this energy region.
Charged Current Scattering
Proton
820GeV
ｄ、s-quark
Left handed
electron
Ｗ（left handed）
electron neutrino (left handed)
u,c-quark
(Jet）
Proton
820GeV
ｄ、s-quark
Right handed
electron
Ｗ(right handed）
Right handed electron neutrino
u,c-quark
（Jet）
left handed
right handed
σR=0.2±1.8±1.6 pb (H1 and ZEUS combined preliminary result) -> WR mass limit ~ 200 GeV
The first measurement of Left/Right
asymmetry in CC in this energy region.

20. Polarized Neutral Current Cross section
Very subtle effect from g-Z interference
-> H1+ZEUS combined results
Pol=70%

21. Measurement of q-Z coupling
u-quark
d-quark
Polarized data improves the vector couplings.
HERA-II data makes a significant impact on the quark couplings

22. In SM, Quark-charge weighted valence quark distribution
cf. np: F3 : valence quark
ep :F2 : charge-square weighted
In SM,

23. PDF determination from HERA
A fit (almost) exclusively with HERA data
sea, gluon
valence
QCD + EW physics
Jets cross sections  gluon
Z0 exchange
 • sea + valence
• valence only
W± exchange
 u or d quark
γexchange
 sea
scaling violation
 gluon
Fixed target experiments
Standard fits (a la CTEQ, MRST…) use data from various (fixed-target and collider) experiments. Why HERA only fits?
Single Experiment: Well known systematic uncertainties including correlation.
Proton Only: No nuclear effects.
High Q2：　No Higher twist
Still we need following information from the other experiments:
Strange quark information
Ubar-dbar asymmetry

24. ~577 data points NC: high Q2 CC: high Q2 Jet NC: low Q2
PDF determination from HERA
A fit (almost) exclusively with HERA data
NC: low Q2
sea, gluon
valence
QCD + EW physics
Jets cross sections  gluon
Z0 exchange
 • sea + valence
• valence only
W± exchange
 u or d quark
CC: high Q2
Jet
γexchange
 sea
scaling violation
 gluon
~577 data points
Fixed target experiments
DGLAP equation
Standard fits (a la CTEQ, MRST…) use data from various (fixed-target and collider) experiments. Why HERA only fits?
Single Experiment: Well known systematic uncertainties including correlation.
Proton Only: No nuclear effects.
High Q2：　No Higher twist
xf(x) = p1xp2(1-x)p3(1+p5x) at Q2=Q02
Still we need following information from the other experiments:
Strange quark information
Ubar-dbar asymmetry

25. PDFs obtained from the fits
Gluon uncertainty
~100GeV Higgs
@LHC
As seen in the F2 rise at low-x, many sea quarks.
Gluons are dominant at low-x
Gluon density is determined at ~ 5% level, in the “LHC-Higgs” region.
H1/ZEUS comparison:
Reasonable agreement between the experiment
The main difference comes from
Initial Parameter
Handling of sys. errors
Selection of low energy experiments

26. Simultaneous extraction of s and PDF
Scaling violation:
F2/lnQ2 ~ sxg(x,Q2)
Data at low x allow disentangling correlation of s and xg
s-free fit gives:
H1+fixed target:
(additionally  from renormalization scale)
ZEUS+fixed target:
(additionally  from renormalization scale)
ZEUS only:
Difference in exp. error mainly from the treatment of systematic error and normalization of data points in the fitting procedure and error propagation.
Various as measurements
at HERA. : All consistent
-> Universality of QCD

27. Summary HERA and ZEUS/H1 experiments
Collider = x100 extended region in Q2 and x.
High- Q2 NC and CC: electroweak effects
NC: effect of Z exchange (q-Z coupling, valence quark)
CC: flavor-specific (sees positive and negative quarks differently)
Measurements with the polarized electron beam are more sensitive to the EM parameters.
F2 measurement and PDF determination
Very steep rise of sea and gluon at low x.
pQCD (DGLAP) gives a fairly good description of the data
for Q2= 1 ~ GeV2
Proton PDF is determined with HERA data. Gluon distribution is well constrained.