H1 and ZEUS

1
Structure function measurements and what they
tell us about PDFs and the strong coupling
constant
Tomá Latovicka (H1 collaboration) DESY Zeuthen,
Charles University Prague at LLWI2003, Lake
Louise, Canada

• H1 and ZEUS
• Structure functions
• at HERA
• as and PDFs
• Summary/Outlook

2
H1 and ZEUS at HERA

• HERA at DESY, Hamburg
• ep accelerator ring, 27.5 x 920 GeV
• circumference 6.3km
• 4 experiment halls

ZEUS
H1
3
H1 and ZEUS experiments

• nearly aparatus
• delivering data since early 90ies

2

• recently both experiments upgraded

4
Deep Inelastic Scattering
Four-momentum transfer squared
Bjorken scaling variable
Inelasticity scaling variable
Centre of mass energy squared
5
Cross-sections and Structure Functions
NC Cross Section
NC Reduced cross section
CC Cross Section
6
DIS slang reminder
low Q2 - transition domain from non-perturbative
domain to deep inelastic domain
strong coupling is large, limits
of pQCD
high Q2 - strong coupling is relatively small
pQCD calculations reliable xF3
and CC enter the game at large Q2
low x (high y) - driven by gluons sea
(confinement) region longitudinal str.
function FL unique acceptance by H1
high x (low y) - driven by quarks
valence quark region fixed target experiments
7
Kinematic plane coverage

• covered almost to the limits of phase space
• compared to fixed target experiments,
  measurements extend to high Q2 and high y by more
  than 2 orders of magnitude

8
The Structure Function F2

• F2 is not calculable from the first principles
• Various theoretical predictions a decade ago
• during its running HERA made an impressive
  progress
• - directly
  related to quark densities

1993
2000
9
The Structure Function F2
10
Scaling violations of F2

• at low x driven by gluons
• described by QCD
• DGLAP evolution equations
• many open questions
• does F2 saturate?
• when gluon density is
• large -gt gluon fusion
• low Q2 region

• QCD analysis digests quarks and gluon densities
• from fits to F2
• (parametrised at an initial scale, then DGLAP
  evolution takes over)
• e.g.

11
CC and NC measurements

• Standard model describes both NC and CC very well
  over a large range of Q2
• Electroweak unification at about MZ2 scale
• e and e- cross sections different due to
  different quark contribution and helicity
  structure of EW interactions

12
xF3 and FL measurements

• xF3 errors dominated by stat. errors, higher
  luminosity needed
• FL not measured directly, runs at different beam
  energies needed

13
PDF fits and the strong coupling
QCD analyses require many choices to be
made Should be reflected in PDF uncertainty

• Choice of data sets used
• Treatment of experimental systematic
  uncertainties
• Q02 starting scale
• Q2min of data included in fit
• Renormalisation / factorisation scales
• Choice of densities to parameterise
• Allowed functional form of PDF parameterisation
• Cuts to limit analysis to perturbative phase
  space
• Treatment of heavy quarks
• etc...

14
Results on the strong coupling constant

• H1

0.1150 -0.0017(exp) 0.0009 -0.0007(model)
EPJ C21(01)33
H1 BCDMS precise data if systematíc errors
are not fitted 0.0005 NMC replaces BCDMS
0.116-0.003 (exp) 4 light flavours 0.0003
BCDMS deuteron data added 0.1158 - 0.0016
(exp)

• ZEUS

0.1166 -0.0008(unc) -0.0032(corr)
-0.0036(norm) -0.0018(model)
pBCDMS,NMC,E665 dNMC,E665 d/p NMC xF3
CCFR systematíc errors are not allowed to vary in
chi2 minimisation Q2gt2.5 GeV2, W2gt20GeV2,
RT-VFNS, b(uv)1/2, b(dv)1/2 fit alphas, xg, uv,
dv, sea, dbar-ubar (MRST) if fixed flavour scheme
is used 0.0010
Phys.Rev. D67(03)
large chi2 variations if Q2 ? (¼ .. 4) Q2
renormalisation scale-0.005 (H1)
? (½ .. 2) - 0.004 (ZEUS) not
included in error
15
Parton distribution functions from NLO QCD fits
(ZEUS)
uv
dv
Sea, g

• xg and Sea distributions determined by low x /
  Q2 HERA F2 data
• xuv determined from high x NC data
• xdv determined from high x CC e data

16
Parton distribution functions from NLO QCD fits
(H1)

• CC and NC cross sections
• are sensitive only to
• U, antiU, D,antiD
• (F2N has c-s admixture)
• H1BCDMS H1 only
• uv, dv, sea are replaced by the observables
• ! possible to determine PDFs with H1 data alone
  (with assumption on sea symmetry)

H1 ICHEP02
17
Parton distribution functions from NLO QCD fits
H1 vs ZEUS comparison of PDFs independent
fits, different approaches only experimental
errors! (matter of choice)
3
10
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NLO gluon momentum density

• fixed flavour number scheme
• Laenen, Riemersma, Smith, vanNeerven
• needs precision data at low Q2, all x!
• needs precision FL measurement!

H1 EPJ C21(01)33
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New low Q2 measurements
low Q2 1999 dedicated run
low Q2 x-section and F2 determination at low x
shifted vertex 2000 run
Transition region between perturbative and
non-perturbative kinematic range
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New measurements at low Q2, small x

• Extended phase space
• Data fill the gap in pre-
• cision measurements
• around Q21GeV2
• and extend phase space
• to higher y at all Q2
• (till 12 GeV2)
• sr changes behavior
• at high y (small x)
• g FL signature
• (even at very low Q2)
• Data with Q2 g 0
• Valuable for studying
• underlying dynamics
• of DIS ? ?p

21
Rise of F2 towards low x

• H1 / ZEUS / NMC data used to fit Q2 dependencies
  for xlt0.01

• Behaviour is changing at around 1 GeV2
• Theory expects ? to reach value of 0.08 for Q2
  ? 0
• No sign of saturation at small x observed (yet)

22
Summary (concluding remarks)

• First phase of HERA running delivered many
  interesting results
• Precision of 2-3 achieved for F2
• The inclusive DIS data at HERA are confronted
  with NLO QCD analyses QCD (the DGLAP equations)
  is able to describe
• all the cross section data ep, e-p, NC,
  CC in a wide kinematical range both Q2 x
  covering 5 orders of magnitude
• extracted from DIS data is competetive with
  the world average
• Fits allow HERA data to constrain PDFs
  
• HERA 2 (after luminosity upgrade) high
  precision xF3 direct measurement of FL
• The next important step e-d runs (dv/uv at high
  x), HERA 3 ?!