An NLO QCD Analysis of inclusive and jet data from ZEUS

1
An NLO QCD Analysisof inclusive and jet data
from ZEUS
Low-x Meeting Sinaia,
2005
Enrico Tassi (Calabria University and INFN)
on behalf of ZEUS
Based on Zeus Coll., DESY-05-050
(accepted by EPJC)
2
Proton PDFs and as(MZ)
A precise knowledge of the proton PDFs, is
crucial for an understanding of the proton
structure as well as necessary in any
calculation of cross sections at hadron
colliders (e.g. Standard Model tests and BSM
searches _at_ LHC)
QCD Factorisation
C. Glasman _at_DIS05
The strong coupling as determines the strength of
the interaction between coloured quanta and it is
the fundamental parameter of QCD. Among the
couplings of the Standard Model is, by far, the
less well known
3
Motivation

• Now, after the HERA I phase (1994-2000) of
  data-taking, the full set of ep and
• e-p inclusive Neutral Current (NC) and Charged
  Current (CC) cross sections are
• available for QCD analysis

96/97 ep NC L30 pb-1 2.7 lt Q2 lt 30000
GeV2 EPJ C21 443 (2001)
Nominal F2 94-97 ep CC L33 pb-1 280.lt
Q2 lt 17000 GeV2 EPJ C12 441 (2000) 98/99
e-p NC L16 pb-1 200 lt Q2 lt 30000 GeV2
EPJ C28 175 (2003) 98/99 e-p CC L16
pb-1 280 lt Q2 lt 17000 GeV2 PL B539 197
(2002) High-Q2/x data sets 99/00
ep NC L63 pb-1 200 lt Q2 lt 30000 GeV2
PR D70 052001 (2004) 99/00 ep CC L61
pb-1 280 lt Q2 lt 17000 GeV2 EPJ C32 16
(2003)

• We decided to include in the analysis, in a
  rigorous way, our precise jet cross
• sections (both in the DIS and ?-p regimes)
• gt with the full HERA I inclusive cross
  sections and the addition of the jet data we
• can perform now a QCD analysis based on
  ZEUS data only
• gt improved determinations of the gluon
  density and as
• Lets look at the various observables included in
  the analysis and
• discuss how they contribute to pdfs and as.

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Inclusive NC/CC DIS
Q2 -(k-k)2
xp
x momentum fraction of proton carried by
quark (HERA 10-6 1) Q2 resolving power of
probe
Double differential cross sections
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ZEUS F2

• Phase space
• 2.7 lt Q2 lt 30 000 GeV2
• 6.3 10-5 lt x lt 0.65
• F2 dominates cross section
• Precise determination of the
• low-x (xlt10-2) sea(xS) and gluon (xg)
  densities
• ZEUS high-x data still less precise than fixed
  target

BCDMS ?F2/F27
HERA ?F2/F230
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High-Q2 NC
Reduced cross section
Z0 exchange gives a new valence
structure function xF3 measurable from low- to
high-x on a pure proton target,
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High-Q2 CC
Reduced cross section
At LO
Used to constrain valence PDFs (instead of
Fixed-target) Still limited by statistics gt
need HERA II
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Inclusive jet production in NC DIS
High-ET jet production in the Breit Frame LO
contributions - boson-gluon fusion -
QCD-Compton
gt Directly sensitive to as and gluon/quark
density in the proton
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Inclusive Jet Cross Sections in ep NC DIS
ZEUS coll., PL B547 164 (2002)

• Phase space
• Q2 gt 125 GeV2
• EBT,jet gt 8 GeV and -2 lt ?Bjet lt 1.8
• Jets identified with the kT cluster
• algorithm in the Breit frame
• Small Experimental uncertainties
• ? jet energy scale (1 for ET,jetgt10 GeV)
• gt 5 on the cross sections
• Small theoretical uncertainties
• - higher order terms 5
• - Hadronic corrections
• (Chad lt10 and ?Chad 1)

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Dijet photoproduction
Measure dijet production in ?p collisions via ep
scattering for Q20
At LO two processes contribute gt Direct
sensitivity to as and gluon PDFs
Direct process
Resolved process
However pQCD cross sections
depends ,via the resolved process component, on
the photon PDFs. In order to suppress this
dependence the QCD analysis was restricted to
the direct-process-enriched region, xobs gt
0.75, where
is the fraction of the photons momentum
taking part in the hard process.
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Dijet ?p cross sections for x ?obs gt 0.75
ZEUS Coll., EPJ C23 615 (2002)

• Phase space
• ETjet1,(2), gt 14 (11) GeV and
• -1 lt ?jet1,2 lt 2.4 and x?obs gt 0.75
• and
• Q2lt1 GeV2 and 134 ltW?p2lt 277 GeV2
• Jets identified with the kT cluster
• algorithm in the Lab frame
• Small Experimental uncertainties
• ? jet energy scale (1 for ET,jetgt10 GeV)
• gt 5 on the cross sections
• Small theoretical uncertainties
• - higher order terms 10
• - Hadronic corrections
• (Chad lt10 and ?Chad 2-3)

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Global vs ZEUS DGLAP fit
Where does the information come from in a global
fit compared to a fit including only ZEUS data ?
ANALYSES FROM ZEUS ONLY ? Systematics well
understood ? measurements from our own
experiment ? No complications from heavy
target Fe or D corrections
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DGLAP Analysisdetails

• Parameterised the PDFs
• u valence (xuv), d valence (xdv), total sea
  (xS), gluon (xg) and x?x(d-u)
• at the starting scale Q02 7 GeV2 according
  to
• xf(x) p1xp2(1-x)p3
  (1p4x)
• Constraints on the parameters pi
• -momentum and number sum rules gt p1(g) ,
  p1(dv) , p1(uv)
• -no sensitive to the low-x behaviour of xuv
  and xdv gt p2(uv)p2(dv)
• -no sensitive to the flavour structure of the
  light sea gt fix x? (Gottfried sum rule,Drell Yan
  data)
• - suppression by factor 2 of the of the
  strange sea (CCFR NuTeV dimuon data)
• Evolve PDFs with Q2 using NLO DGLAP equations
• (MS scheme QCDNUM evolution program)
• Inclusive cross sections obtained convoluting the
  PDFs with
• coefficient functions (in the Roberts-Thorne
  Variable Flavour Number scheme).
• gt 11 free parameters ( as(MZ) when free,
  otherwise as(MZ) 0.118)

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Inclusion of Jet cross sections
Computation of NLO jet cross sections extremely
CPU intensive ( O(10) hours ) gt original
programs cannot be used directly in the fit so
we used this programs to compute as- and
PDFs-independent LO/NLO weights, s, obtained by
integrating the partonic cross sections over
the 3(2)-dimensional bins of the (?,µR,µF) space.
The NLO cross sections were then obtained by
folding these weights with the PDFs and as
according to

This procedure reproduces the exact NLO
predictions to better than 0.5
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Data sets

• A total of 577 data points
• Kinematic region
• 2.7 lt Q2 lt 30 000 GeV2
• 6.3 10-5 lt x lt 0.65
• W2 gt 20 GeV2
• Full account of the correlated
• systematic uncertainties
• A good description of the
• measured cross sections is
• obtained with

ZEUS-JETS and ZEUS-JETS-as fits
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QCD fit results Inclusive NC
Very good description of the low- medium-Q2 ep
NC reduced cross sections
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QCD fit results high-Q2 NC and CC
NC
CC
Good description - still limited by
statistics gt HERAII
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QCD fit results Jet cross sections
Inclusive jet cross sections in ep NC DIS
Dijet cross sections in ?p
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Valence distributions

• At high-x not as well constrained
• as when including fixed-target data
• -but becoming competitive
• being free from heavy-target
• corrections, isospin-symmetry
• assumptions etc.
• To further improve here we need
• precision high-Q2 ep CC/NC data
• from HERA II

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Glue sea

• Both are well determined at low-x (xlt10-2)
• -the sea distribution rises at low-x for all Q2
• the gluon density becomes valence-like
• at low Q2
• The gluon uncertainty has been reduced
• by the use of the jet data

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Jet data gluons
Comparing the gluon distribution obtained from
fits with and without jet data - no
significant change of shape no tension
between incl. and jet data - jet cross sections
help in constraining the gluon density in the
region 0.01 lt x lt 0.4 - Sizeable
reduction of the gluon unc e.g. from 17 to
10 at x0.06 and Q27 GeV2 ?similar reduction
by a factor two in the mid-x region over
the full Q2 region
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PDFs uncertainties
Checks

• Value of Q02 varied in the range
• 4lt Q02 lt 10 GeV2
• Parameterisation of the valence PDFs
• (1p4x) ? (1p4xp5vx)
• -Tighter cuts on the jet cross sections
• ET,jetgt10 GeV (DIS) , ET,jetgt17 GeV(?-p)
• -Hadronisation corrections
• Photon PDFs
• In general effects smaller than the experimental
  uncertainties

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Photon PDFs
Including the resolved component (excluded in the
ZEUS-JETS fit)
Sensitivity to different PDFs (direct-enriched
component)
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Comparing PDFs
Compatible with ZEUS-S/MRST2001/CTEQ6.1M
and H1
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Determination of as(MZ)
- Simultaneous determination of PDFs and as
(free parameter) from ZEUS data alone -
Inclusion of jet data greatly improves the
determination of as gt -
Exctracted value
as(MZ) 0.1183 0.0007(uncor.) 0.0027(corr.)
0.0008(model)
The theoretical uncertainty due to terms beyond
NLO is ?as(th) 0.0050 (limited by theory gt
NNLO)
In agreement with the world average as(MZ)
0.1182 0.0027 (Bethke,2004)
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PDFs with as free
The extracted as(MZ) value is very close to the
fixed value (0.180) used in the ZEUS-JET fit gt
there are no significant changes in the
central values of the PDFs parameters Uncertainti
es - The valence and sea unc. are unaffected -
The gluon unc. increases somewhat in the
low-Q2 region
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ZEUS/HERA as determinations
C. Glasman _at_ DIS05
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Summary

• ZEUS Inclusive and jet data used in a new NLO QCD
  analysis to determine the proton PDFs and as (MZ)
  
• Jet cross sections included rigorously in the
  DGLAP analysis
• The input of jet-production data constrains the
  gluon and allows
• an accurate determination of as (MZ)
• New PDFs available via HEPDATA-Durham web site

as(MZ) 0.1183 0.0007(uncor.) 0.0027(corr.)
0.0008(model)
more to come