[PPT] - Probing proton structure at very high Q Masahiro Kuze Department of PowerPoint Presentation

SLIDE 1

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 1

Probing proton structure at very high Q

2

Introduction F2 measurement and PDF fit High-Q

2 region, NC and CC

Low-Q

2 region

Summary and prospects

Masahiro Kuze

Department of Physics Tokyo Institute of Technology

SLIDE 2

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 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

Integrated Luminosity (pb-1)

Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

92 93 94

95 1996 1997 98 1999 e- 1999 e+ 2000

15.03.

s = 318GeV

SLIDE 3

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 3

Deep Inelastic Scattering (DIS)

Probe the proton = our most familiar micro-cosmos

with a point-like lepton probe. ‘giant electron-microscope’

1/Q (momentum transfer) gives the spacial resolution.
Bjorken x: Fractional momentum of a parton in the nucleon.
y=(1-cosθ*)/2 (scattering angle in CM system) Q

2=sxy

Neutral or

Charged current in t-channel propagator

SLIDE 4

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 4

Kinematic region probed

> 100x larger kinematic reach

compared to fixed-target DIS experiments at CERN, DESY, FNAL, SLAC… (if proton is at rest,

HERA CM energy means Ee=54TeV)

At high Q

2, probe the validity of

SM/QCD at smallest distance → Quark structure? New particles?

(Q

2=40,000 GeV 2 →1/Q=0.001 fm)

At low Q

2, probe the low-x region

→ very soft constituents of proton; Saturation? Breakdown of standard DGLAP formalism (BFKL) ?

y=1 (HERA √s=320 GeV)

x Q2 (GeV2)

E665, SLAC CCFR, NMC, BCDMS, Fixed Target Experiments: ZEUS H1

10

1

1 10 10 2 10 3 10 4 10 5 10

6

10

5

10

4

10

3

10

2

10

1

1

SLIDE 5

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 5

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% / E σ ( E ) / E = 35% / E

σ ( p

T ) / p T =

0.0058 p

T ⊕ 0.0065 ⊕ 0.0014 / p T

σ ( E ) / E = 12% / E σ ( E ) / E = 50% / E

2 out of (Ee, θe, Eh, θh) → Reconstruction of (x, Q

2)

e→ ←p

SLIDE 6

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 6

Cross section & Structure functions

NC differential cross section for ep→eX reaction

[Y

+F2(x, Q 2) Y −xF3(x, Q 2) − y 2FL(x, Q 2)]

F2 = Σ xqf

+(x, Q 2)[ef 2 − 2ef vf vePZ + (vf 2+af 2)(ve 2+ae 2)PZ 2]

xF3 = Σ xqf

−(x, Q 2)[− 2ef af aePZ + 4vf af veaePZ 2]

(f=u,d,c,s,b)

(Parton Distribution Functions) PZ = sin

−22θW⋅Q 2/(Q 2+MZ 2) (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)

d

2

σ

e

±

p

dxdQ

2

= 2πα

2

xQ

4

xq

f ±

= xq

f ( x,Q 2

) ± xq

f ( x,Q 2

)

+

SLIDE 7

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 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 Q

2 ↑ softer parton smaller resol.

dynamics of quarks and gluons

Good agreement with fixed-target

experiments at middle - high x

(sea + valence quarks)

HERA F2

1 2

Q2=2.7 GeV2 3.5 GeV2 4.5 GeV2 6.5 GeV2

1 2

8.5 GeV2 10 GeV2 12 GeV2 15 GeV2

1 2

18 GeV2

F2

em

22 GeV2 27 GeV2 35 GeV2

1 2

45 GeV2 60 GeV2

10

3

1

70 GeV2

10

3

1

90 GeV2

1 2 10

3

1

120 GeV2

10

3

1

150 GeV2

x

ZEUS NLO QCD fit H1 PDF 2000 fit H1 96/97 ZEUS 96/97 BCDMS E665 NMC

SLIDE 8

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 8

F2 for fixed x, as a function of Q

2

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 Q

2 ↑.

Line = result of QCD fit (next slide)

– All data points well described. g → qq

HERA F2

1 2 3 4 5 1 10 10

2

10

3

10

4

10

5

F2

em

log10(x)

Q2(GeV2)

ZEUS NLO QCD fit H1 PDF 2000 fit H1 94-00 H1 (prel.) 99/00 ZEUS 96/97 BCDMS E665 NMC x=6.32E-5 x=0.000102 x=0.000161 x=0.000253 x=0.0004 x=0.0005 x=0.000632 x=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

SLIDE 9

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 9

Perturbative-QCD fit of F2 data

Example: ZEUS NLO DGLAP analysis PRD 67 (2003) 012007

– At Q

2 = Q 2 0, input functional form of PDF (Q

2 0 =7GeV 2)

xf(x) = p1x

p2(1-x) p3(1+p5x) for u-valence, d-valence, sea quarks and gluons

– ‘Q

2 evolution’ is predicted by

DGLAP (‘Altarelli-Parisi’) Equations

Żfi/ŻlnQ

2 = (αs/2ʞ)ʈfj⊗ Pij

Pqq Pgq Pqg Pgg – Use world’s precision DIS data + ZEUS F2

BCDMS, NMC, E665, CCFR (µ-p, µ-D, ν-Fe)

– χ

2 fit to determine p1 … p5

SLIDE 10

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 10

PDFs obtained from the fits

ZEUS

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(MZ

2) = 0.118

tot. error

CTEQ 6M MRST2001

Q2=10 GeV2

xuv xdv xg(× 0.05) xS(× 0.05)

x xf

(H1 PDF 2000: uses only H1 data)

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 10

4

10

3

10

2

10

1

x xf(x,Q2)

H1 PDF 2000 H1 ZEUS-S PDF ZEUS-S PDF

Q2=10 GeV2

xuV xdV xg(×0.05) xS(×0.05)

SLIDE 11

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 11

Low-x sea and gluon distributions

At Q

2 ~ 1GeV 2, gluon becomes valence-

like (and even tends to be negative)

Sea quark is still rising

ZEUS

2

2 4 6

Q2=1 GeV2 ZEUS NLO QCD fit

xg xS

2.5 GeV2

xS xg 10 20

7 GeV2

tot. error

(αs free)

xS xg

xf

20 GeV2

tot. error

(αs fixed)

uncorr. error

(αs fixed)

xS xg 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

5 10 15 20 1 10 10

2

10

3

10

4

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

“scaling violation”

f gluon

SLIDE 12

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 12

Results on high-Q

2 NC cross section

at low Q

2

At Q

2 > ~5000 GeV 2, effect of

Z-exchange clearly visible.

σ(e

−p) > σ(e +p) due to ±xF3

sensitive to valence quarks

ZEUS

0.5 1 1.5

Q2=200 GeV2 250 GeV2 350 GeV2 450 GeV2

0.25 0.5 0.75 1

650 GeV2

σ

∼ NC

800 GeV2 1200 GeV2 1500 GeV2

0.2 0.4 0.6 0.8

2000 GeV2 3000 GeV2 5000 GeV2

10

2

1

8000 GeV2

0.2 0.4 0.6 10

2

1

12000 GeV2

10

2

1

20000 GeV2

10

2

1

30000 GeV2

x

ZEUS NLO QCD fit

tot. error

ZEUS NC e- p 98/99 ZEUS NC e+ p 96/97

xF

3∝ q( x,Q 2

) − q( x,Q

2

)

0.2

0.2 0.4 0.6 0.2 0.4 10

1

10

1

10

1

Q2 = 1500 GeV2

xF3

H1 94-00 ZEUS 96-99 SM (CTEQ6D)

Q2 = 3000 GeV2 Q2 = 5000 GeV2 Q2 = 8000 GeV2 Q2 = 12000 GeV2 Q2 = 30000 GeV2

x

˜ σ ≡ xQ

4

2πα

2

Y

+ σ ( x,Q 2

) ≈ F

2

SLIDE 13

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 13

“Rutherford experiment”

n quarks
Single differential xsec in Q

2

Large Q

2 = large angle = smaller distance

1/Q

4 fall over 7 orders of magnitude

Analogy to nucleon form factor:

If finite ‘quark radius’ Rq, xsec will decrease as Q

2 grows.

σ = σSM(1−<Rq

2>Q 2/6) 2 Rq < 0.85×10

16cm

10

7

10

6

10

5

10

4

10

3

10

2

10

1

1 10 10

3

10

4

HERA Neutral Current

H1 e+p 94-00 ZEUS (prel.) e+p 99-00 SM e+p (CTEQ6D) H1 e-p ZEUS e-p 98-99 SM e-p (CTEQ6D) y < 0.9

Q2 (GeV2) dσ/dQ2 (pb/GeV2)

Q2 (GeV2) N/NCTEQ5D Quark Radius Limits

ZEUS

ZEUS 94-00 e±p Rq

2 = (0.85 ⋅10-16cm)2

Rq

2 = -(1.06 ⋅10-16cm)2

1 10 10

3

10

4

0.8 0.9 1 1.1 1.2 10

3

10

4

h/Q= 0.02fm 0.002fm

↓

↓

SLIDE 14

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 14

Charged current − flavour sensitive

e

+p: only negative-charge quarks

At high x, mainly d(x, Q

2) probed.

e

−p: only positive-charge quarks

At high x, mainly u(x, Q

2) probed.

In addition to u(x, Q

2) > d(x, Q 2),

helicity suppression (1-y)

2 in e +p

⇒ σ(e

−p) >> σ(e +p) at high Q 2

Data (not used in the fit) well described

by the QCD prediction.

d

2

σ

e

+

p

dxdQ

2

= G

F

2π M

W 2

M

W 2

+ Q

2

     

2

u + c + ( 1 − y)

2

(d + s)

[ ]

d

2

σ

e

−

p

dxdQ

2

= G

F

2π M

W 2

M

W 2

+ Q

2

     

2

u + c + ( 1 − y)

2

(d + s)

[ ]

1 2 0.5 1 0.25 0.5 0.75 1 10

2

10

1

10

2

10

1

10

2

10

1

HERA Charged Current

Q2 = 280 GeV2

σ

∼ H1 e-p ZEUS e-p 98-99 H1 e+p 94-00 ZEUS e+p 99-00 SM e-p (CTEQ6D) SM e+p (CTEQ6D)

Q2 = 530 GeV2 Q2 = 950 GeV2 Q2 = 1700 GeV2 Q2 = 3000 GeV2 Q2 = 5300 GeV2 Q2 = 9500 GeV2 Q2 = 17000 GeV2 Q2 = 30000 GeV2

x · u (1-y)2x · d

x

˜ σ ≡ G

F

2πx M

W 2

M

W 2

+ Q

2

     

2

       

−1

σ ( x,Q

2

)

SLIDE 15

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 15

Flavour decomposition of CC data

Neglecting Q

2 evolution of PDF

(approximate Bjorken scaling), σ shows linear dependence on (1-y)

2

e

−p: intercept = u+c

slope = dbar+sbar

e

+p: intercept = ubar+cbar

slope = d+s

˜ σ (e

+

p) = x u + c + ( 1 − y)

2

(d + s)

[ ]

ZEUS

0.5 1 x = 0.068

σ

~

0.5 1 x = 0.13 0.2 0.4 0.6 0.8 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 x = 0.24

(1-y)2

ZEUS e+p 99-00 ZEUS e−p 98-99 ZEUS-S x(u+c) x(u

_+c _)

˜ σ (e

−

p) = x u + c + ( 1 − y)

2

(d + s)

[ ]

~

u + c u + c

d + s

SLIDE 16

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 16

Electro-Weak Unification

At low Q

2:

NC ~ 1/Q

4 (EM current)

CC ~ GF

2 (Weak current)

At high Q

2 (> Mz 2, MW 2):

Both NC and CC mediated by unified EW current. σNC~σCC

High Q

2 = short distance

= high temperature = early universe

Can cover wide range of distance

in one experiment. (cf. fixed-target experiments Q

2 < ~ 500 GeV 2)

10

7

10

6

10

5

10

4

10

3

10

2

10

1

1 10 10

3

10

4

HERA

H1 e-p CC ZEUS e-p CC 98-99 SM e-p CC (CTEQ6D) H1 e-p NC ZEUS e-p NC 98-99 SM e-p NC (CTEQ6D) y < 0.9

Q2 (GeV2) dσ/dQ2 (pb/GeV2)

SLIDE 17

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 17

Low Q

2: applicability of pQCD?

Recall gluon becomes negative

at low Q

2.

It is not necessarily a problem,

since PDF itself is not a physical

bservable.
However, FL (observable)

becomes also negative for Q

2 < ~ 1 GeV 2.

Is this the lowest end of p-QCD

applicability? Look at the F2 data →

ZEUS

0.2

0.2 0.4 0.6

Q2=0.3 GeV2 0.4 GeV2 0.5 GeV2

0.2

0.2 0.4 0.6

0.585 GeV2

FL

0.65 GeV2 0.8 GeV2

0.2

0.2 0.4 0.6

1.5 GeV2 2.7 GeV2

10

5

10

3

10

1 1

3.5 GeV2

0.2

0.2 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

ZEUS NLO QCD fit

tot. error

SLIDE 18

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 18

Transition from perturbative to non-p. region

At very low Q

2, αs ↑ and non-

perturbative effects important.

ZEUS data at very low Q

2

available. (red data points)
Data cannot be described by

pQCD fit below Q

2 ~ 1 GeV 2.

(rather, it’s a surprise that pQCD works as low as 1 GeV

2 !)

Transition region seems to be

somewhere below 1 GeV

2.

ZEUS

0.5 1 1.5 2

Q2=0.3 GeV2 0.4 GeV2 0.5 GeV2

0.5 1 1.5 2

0.585 GeV2

F2

em

0.65 GeV2 0.8 GeV2

0.5 1 1.5 2

1.5 GeV2 2.7 GeV2

10

5

10

3

10

1 1

3.5 GeV2

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 96/97 ZEUS BPT 97 ZEUS SVX 95 E665 NMC

SLIDE 19

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 19

F2 slope in low-x region

For x<0.01, F2 can be well

parameterised by c•x

−λ.

λ fairly independent of x,

linearly rises with logQ

2 in

‘DIS’ regime.

Qualitative change happens

around Q

2 < ~1GeV 2, then λ

flattens in non-pert. region.

H1 Collaboration

SLIDE 20

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 20

ep scattering in hadronic picture

At very low Q

2, quasi-real photon acts like

a hadron (vector-meson dominance)

Plot shows F2/Q

2=(1/4π 2α) σtot(γ*p)

for fixed W(γ*-p CM energy;W

2~Q 2/x)

For Q

2→0, this should converge to

σtot(γp) (real photoproduction xsec.) W dependence etc. well described by soft hadronic (Regge) phenomenology.

High-Q

2 part well fitted by p-QCD.

Precise data in “transition region”

also available (shifted-vertex run etc.)

Many theoretical attempts to describe

all regions simultaneously (Dipole model, ALLM param., Fractal fit, …)

10

1

1 10 10 2 10 3 10

2

10

1

1 10 10

2

W2=75000GeV2

x2

W2=60000GeV2

x4

W2=36000GeV2

x8

W2=22600GeV2

x16

W2=13500GeV2

x32

W2=9000GeV2

x64

W2=6000GeV2

x128

W2=3600GeV2

x256

W2=2260GeV2

x512

W2=1350GeV2

x1024

W2=900GeV2

x2048

Q2[GeV2] F2(W2,Q2)/Q2 H1 Collaboration H1 svtx00 prel. H1 97 ZEUS BPT97

Fractal Fit (x<0.01) ALLM97 H1 QCD fit

Q2 min = 3.5 GeV2

SLIDE 21

kuze@phys.titech.ac.jp 20/Feb/2004 SQS04 21

Summary and prospects

Probing the proton with very high-energy probe showed us:

– At high Q

2, EW unification clearly visible and SM/QCD valid up to ~10,000GeV 2.

Quark still point-like: radius < 0.85×10

16cm

– Precise measurement of sea-quark and gluon distributions at low x (strong rise)

→ Crucial inputs for current and future experiments (e.g. LHC).

– Point of transition from perturbative to non-perturbative picture of the proton in QCD: Q

2~1GeV

2. Precise data available in this region.
HERA-2 programme: (all data shown here are from HERA-1: 1992-2000)

– Luminosity upgrade (5x higher). Goal: 1 fb

1 by 2007.

– High stat. in highest Q

2 (search for anomaly);

Measure b, c-quark distribution (detector upgrade: silicon microvertex). – Longitudinally polarised e

± available for H1/ZEUS (was only for HERMES).

EW physics in DIS (measure EW couplings of u,d; search right-handed currents, …) – Production run started in fall 2003.