recent results from h1/zeus on structure functions

Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 1

Recent results from H1/ZEUSon structure functions

Introduction F2 measurement and PDF fit Low-Q2 region High-Q2 region Current status and prospects

Masahiro Kuze

KEK/IPNS (TIT from April)


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 ep (‘98-’99) for H1 or ZEUS

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s = 318GeV


Deep Inelastic Scattering (DIS)

• Probe the proton = our most familiar micro-cosmoswith 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 or

Charged currentin t-channelpropagator

Q2=sxy


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 Q2, probe the validity of QCD 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 standard DGLAP formalism (BFKL) ?


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

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σ(E) / E =18% / E

σ (E) / E = 35% / E

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σ( pT ) / pT =

0.0058pT ⊕0.0065⊕0.0014 / pT

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σ(E) / E =12% / E

σ (E) / E = 50% / E

2 out of (Ee, e, Eh, h) Reconstruction of (x, Q2)

e


Cross section & Structure functions

• NC differential cross section

[Y+F2(x, Q2) YxF3(x, Q2) y2FL(x, Q2)]

F2 = xqf+(x, Q2)[ef

2 2ef vf vePZ + (vf2+af

2)(ve2+ae

2)PZ2]

xF3 = xqf(x, Q2)[ 2ef af aePZ + 4vf af veaePZ

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

(Parton Distribution Functions)

PZ = sin22WQ2/(Q2+MZ2) (Z-exchange & Z interference)

Y = 1 (1y)2, ef: quark charge, vi/ai: EW couplings

FL = F2 2xF1 (0 in LO QCD, longitudinal Str. Function)

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d2σe ± p

dxdQ2=

2πα 2

xQ4

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xq f± = xq f (x,Q2) ± xq f (x,Q2)

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+


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-target experiments at middle - high x– Sea + valence quarks


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 .

• Line = result of QCD fit (next slide)

– All data points well described.

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g → qq


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 by DGLAP (‘Altarelli-Parisi’) Equations

• ∂fi/∂lnQ2 = (s/2π)∑fj Pij

Pqq Pgq Pqg Pgg

– Use world’s precision DIS data + ZEUS F2

• BCDMS, NMC, E665, CCFR (-p, -D, -Fe)

2 fit to determine p1 … p5


PDFs obtained from the fits


Low-x sea and gluon distributions• At Q2 ~ 1GeV2, gluon becomes valence-

like (and even tends to be negative)

• Sea quark is still rising


Simultaneous extraction of s and PDF

• Scaling violation: F2/lnQ2 ~ sxg(x,Q2)Data at low x allow disentangling correlation of s and xg

s-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.

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s = 0.1150 ± 0.0017(exp)−0.0005+0.0009(model)

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s = 0.1166 ± 0.0049(exp) ± 0.0018(model)


What if there were no HERA data?

• HERA data determine the low-x gluon and sea-quark PDF.

• HERA revealed: F2 is very steep.


Relevance for future hadron colliders

• Low-x PDF only measured at HERA: x > as low as 10-4 important for 100GeV<M<1TeV in ‘central’ region at LHC.

• High-x PDF: Q2 evolution to <~104 GeV2 checked at HERA. QCD still works.

• Crucial inputs for signal cross sectionand QCD-background estimation.– Higgs, SUSY, top-pair, W/Z (luminosity)…

Martin et al., EPJC 14 (2000) 133

Q2


Where is the lowest applicability of pQCD?

• Recall gluon becomes negative at low Q2.

• It is not necessarily a problem, since PDF itself is not a physical observable.

• However, FL (observable) becomes also negative forQ2 < ~ 1 GeV2.

• Is this the lowest end of p-QCD applicability?Look at the F2 data


Transition from perturbative to non-p. region

• At very low Q2, s and non-perturbative effects important.

• ZEUS BPT data at very low Q2 available.

• Data cannot be described by pQCD fit below Q2 ~ 1 GeV2.(rather, it’s a surprise that pQCD works as low as 1 GeV2 !)

• This is evident thanks to HERA data at low x.


F2 slope in low-x region

• For x<0.01, F2 can be well parameterised by cx.

fairly independent of x, linearly rises with logQ2 in ‘DIS’ regime.

• Qualitative change happens around Q2 < ~1GeV2, then flattens in non-pert. region.


DIS in the hadronic picture

• At very low Q2, quasi-real photon acts like a hadron (vector-meson dominance)

• Plot shows F2 for fixed y, i.e.at fixed *-p CM energy W (W2~sy= Q2/x)

σtot(*p)=(42/Q2)F2(x=Q2/W2, Q2)so F2 must vanish as F2Q2 as Q2 0.

• Data show this behaviour (25<W<270 GeV) and successfully fitted by a Regge-type fit. (consitent with Pomeron and Reggeon trajectories determined from hadron-hadron scattering)


Attempts to describe both regions

dissociates into a qq pair (dipole) of radius 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 characterised by the parton density: R0~1/gluon~x

(low x = high-density environment).

• DIS cross section is a convolution of σdipole and photon wave function .

Dipole models: a class of phenomenological modelsexample: Golec-Biernat & Wuesthoff DGLAP-like saturation

_

ˆ

ˆCartoon: R.Yoshida


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 model describes F2, diffractive cross section and vector-meson production at the same time (not discussed here).

Red:original GB&W

Blue:modified one (includesDGLAP evolution)Bartels, GB, KowalskiPRD66(2002)014001


High-Q2 region Electroweak effects• Pz = Q2/(Q2+MZ

2)At Q2 > ~5000 GeV2, effect of Z-exchange clearly visible.

σ(ep) > σ(e+p) due to ±xF3

sensitive to valence quarks

• Statistically limited: more luminosity helps (esp. ep)€

xF 3∝ q(x,Q2) − q(x,Q2)


Charged current flavour decomposition

• e+p: only negative-charge quarks take part.

At high x, mainly d(x, Q2) probed.

• ep: 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(ep) >> σ(e+p) at large Q2.

• Data (not used in the fit) well described by QCD prediction.

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d2σe + p

dxdQ2=

GF

2π

MW2

MW2 + Q2

⎛

⎝ ⎜

⎞

⎠ ⎟

2

u + c + (1− y)2(d + s)[ ]

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d2σe − p

dxdQ2=

GF

2π

MW2

MW2 + Q2

⎛

⎝ ⎜

⎞

⎠ ⎟

2

u + c + (1− y)2(d + s)[ ]


Can valence be determined by HERA only?

• ZEUS-only fit (all HERA-I NC/CC data, prel.) still a bit less precise than global 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, …).


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 unified EW current. σNC~σCC

• Dumping of σCC at high Q2 comes from Mw. Fit gives: (Z. ep)

Mw = 80 2(stat)1(syst)1(PDF) GeV

• Space-like: q2(boson) << 0Completely different phase-space from time-like bosonsat LEP and Tevatron (q2 > 0).Complementary evidence/measurement.

Interplay of EW and QCD physics


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)


ZEUS detector upgrade


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.


S.F.measurements with 1 fb-1

• Expected precision in F2 and gluon determination


Flavour-specific measurements

• Complete ‘mapping’ of the proton…– d/u at high x: charged current

– Strange: charm in CC and/or leading particle

– Charm and bottom: improved tagging with micro-vertex

Charm 500 pb-1 Bottom/charm 500 pb-1


Physics with polarized e

• Lepton polarization probes EW structure– Direct measurement of helicity dependence of

CC DIS (Right-handed currents?)– Measure light-quark EW couplings

500 pb-1 = for e+p, ~5xfor ep, ~30xmore statistics


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 of the proton in QCD: Q2~1GeV2

– At high Q2, EW effects clearly visible and QCD evolution is valid up 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 ep to probe EW structure of DIS.

• Exciting future ahead with 10x more data and polarization.

recent results from h1/zeus on structure functions

Documents

q2 data

high q2

q2 yxf3x

q2 y2flx

fixed x

fmat low q2

lowx region

f2 increasesat x