recent results from h1/zeus on structure functions
DESCRIPTION
Recent results from H1/ZEUS on structure functions. Masahiro Kuze KEK/IPNS (TIT from April). Introduction F 2 measurement and PDF fit Low- Q 2 region High- Q 2 region Current status and prospects. HERA ep Collider at DESY/Hamburg. - PowerPoint PPT PresentationTRANSCRIPT
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)
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 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 ep (‘98-’99) for H1 or ZEUS
€
s = 318GeV
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 3
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
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 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 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) ?
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 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( pT ) / pT =
0.0058pT ⊕0.0065⊕0.0014 / pT
€
σ(E) / E =12% / E
σ (E) / E = 50% / E
2 out of (Ee, e, Eh, h) Reconstruction of (x, Q2)
e
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 6
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)
€
d2σe ± p
dxdQ2=
2πα 2
xQ4
€
xq f± = xq f (x,Q2) ± xq f (x,Q2)
€
+
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 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-target experiments at middle - high x– Sea + valence quarks
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 8
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.
€
g → qq
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 9
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
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 10
PDFs obtained from the fits
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 11
Low-x sea and gluon distributions• At Q2 ~ 1GeV2, gluon becomes valence-
like (and even tends to be negative)
• Sea quark is still rising
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 12
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.
€
s = 0.1150 ± 0.0017(exp)−0.0005+0.0009(model)
€
s = 0.1166 ± 0.0049(exp) ± 0.0018(model)
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 13
What if there were no HERA data?
• HERA data determine the low-x gluon and sea-quark PDF.
• HERA revealed: F2 is very steep.
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 14
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
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 15
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
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 16
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.
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 17
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.
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 18
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)
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 19
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
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 20
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
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 21
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)
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 22
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.
€
d2σe + p
dxdQ2=
GF
2π
MW2
MW2 + Q2
⎛
⎝ ⎜
⎞
⎠ ⎟
2
u + c + (1− y)2(d + s)[ ]
€
d2σe − p
dxdQ2=
GF
2π
MW2
MW2 + Q2
⎛
⎝ ⎜
⎞
⎠ ⎟
2
u + c + (1− y)2(d + s)[ ]
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 23
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, …).
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 24
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
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 25
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)
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 26
ZEUS detector upgrade
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 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.
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 28
S.F.measurements with 1 fb-1
• Expected precision in F2 and gluon determination
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 29
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
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 30
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
Masahiro Kuze (KEK) 29/Mar/2003 JPS Symposium 31
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.