27.5 GeV e± 920 (820) GeV p
HEP2005 International Europhysics Conference on High Energy Physics
EPS05 (July 21st-27th 2005), Lisboa, Portugal
Claire Gwenlan, University of Oxfordon behalf of the H1 and ZEUS collaborations
The structure of the proton & NLO QCD fits
Outline introduction & motivation
HERA kinematics & ep physics
HERA NLO QCD fits
new “proton-structure sensitive”
measurements from HERA - NC cross sections at high-x
- dijets in p collisions at high-ET
summary & outlook
Introduction & Motivation proton structure described by Parton Density Functions (PDFs) needed to make predictions for any process involving protons
must be known as precisely as possible to maximise potential for
discovery at current and future colliders (Tevatron, LHC)
HERA is most important source of information on proton structure data are very precise and cover wide
kinematic region also in relevant x-region for LHC
precise extraction of PDFs now
possible within one experiment
presented here:
latest NLO QCD fit to HERA data
two new HERA measurements: 1) NC cross sections at high-x
2) dijet p cross sections
that will provide further constraints on
PDFs in future QCD analyses2
BUT still regions where PDFs not
well known high-x quarks, gluon
xp
p’ = xp + q
2 2 2
2
2
kinematics:
Q =-q =-(k-k') : Virtuality of exchanged boson
Qx = : Bjorken scaling variable
2p.q
p.qy= : inelasticity
p.k
Q = sxy; s = centre-of-mass energy squared
Overview of HERA kinematics
deep inelastic scattering (Q2 > 1 GeV2)
/Z0 exchange neutral current (NC)
W± exchange charged current (CC)
__________________________________________
photoproduction (Q2 < 1 GeV2)
“quasi-real” exchange
/Z0, W± q = k-k’
3
4
INCLUSIVE NC/CC DIS
HERA inclusive data: directly sensitive to quarks in proton
indirectly sensitive to gluon through
QCD radiation (scaling violation at low-x)
2
23
2L
2222
2
y)(11Y where
)Q(x,xFY)Q(x,Fy)Q(x,FY~dxdQ
p)(ed
Inclusive cross sections and structure functions
2 i i
23 i i
2L s
STRUCTURE FUNCTIONS:
F ~ x(q+q ) dominant contribution
xF ~ x(q -q ) contributes at high-Q
F ~ .xg(x,Q ) contributes at high-y
CC e-p (e+p) sensitive to u (d) valence flavour separation
HERA neutral current F2
5
JET PRODUCTION AT HERA
Jet cross section measurements
Scaling violations may give rise to
distinct jets in final state lowest
order contributions from:
QCD Compton (*q qg)
Boson Gluon Fusion (*g qqbar)
jets directly sensitive to gluon density
in proton through BGF process
also directly sensitive to s through
both BGF and QCDC breaks correlation between s and gluon
BGF QCDC
6
QCD analysis of HERA data
Global HERA OnlyValence
Predominantly fixed target data (-Fe & D/p)
High-Q2 NC/CC e cross sections
Sea Low-x from HERA NC DIS High-x from fixed target Flavour from fixed target
Low-x from HERA NC DIS High-x ? (need HERA-II) Flavour ? (assumptions needed)
Gluon Low-x from HERA dF2/dlnQ2 Mid-to-high-x from Tevatron jet data (some fits) High-x from momentum sum
Low-x from HERA dF2/dlnQ2 Mid-to-high-x from HERA jet data High-x from momentum sum
now precise PDFs can be determined using only HERA data free from heavy-target corrections, isospin symmetry assumptions, …
avoids complications associated with combining data from different experiments
where does the information come from in a QCD fit ?
7
The ZEUS-JETS QCD fit
_____________________________________________________________________________________Eur. Phys. J 050364, hep-ph/05030274 – available in LHAPDF version 4
good description of data: 2/points = 471/577
EPS05 abstract 324, ZEUS Collaboration
data included (ZEUS): - 94-00 NC/CC inclusive e+p & e-p - 96-97 inclusive jets in NC DIS - 96-97 dijets in p
kinematic coverage and cuts: - 6.3 ·10-5 < x < 0.65 - 2.7 < Q2 < 30000 GeV2 - W2 > 20 GeV2 (higher twist)
xf(x) = p1 xp2 (1-x)p3 (1+p4x) at
starting scale Q02 = 7 GeV2
f(x) = uv, dv, sea, g, =(dbar-ubar) 11 free parameters
evolve PDFs in Q2 using NLO DGLAP
heavy quarks: Thorne-Roberts variable flavour number scheme
correlated experimental uncertainties evaluated using Offset Method
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Valence quark, sea and gluon
distributions
8
valence quarks:
- high-x becoming competitive with global fits
sea-quarks & gluon: - low-x as good as global fits (information comes from HERA anyway)
- high-x improved by jet data
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comparison of gluon distribution
from fits with and without jets:
no significant change in shape:
no tension between jet and
inclusive data QCD factorisation
Impact of the jet data on the gluon
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HERA jet cross sections constrain
gluon in range x = 0.01 – 0.4
reduction in gluon uncertainties
by factor of ~2 in mid-x region
over the full range of Q2
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determination of s(MZ) from ZEUS-JETS:
s(MZ) = 0.1183±0.0028 (exp.)
±0.0008 (model) ±0.0050 (theory)
first extraction using only HERA data
scale uncertainty would improve in NNLO fit
in agreement with world average :
s(MZ) = 0.1182 ± 0.0027 (Bethke, 2004)
and with other extractions from HERA
addition of jet data also allows a precise extraction of s(MZ)
HERA-Ony without jet data
HERA-Only with jet data
ZEUS-JETS PDF
global fit
Determination of s(MZ)
ZEUS
NC cross sections at high-x from HERA
motivation
PDFs decrease quickly at high-x and
PDF uncertainties are large
need constraints from data at high-x
highest measured points in DIS are
at x = 0.75 (BCDMS)
- data at higher x exist but are in
resonance region and cannot be easily
interpreted in terms of PDFs
highest measured points from
HERA (H1/ZEUS) are at x = 0.65
new measurement from ZEUS new technique developed to measure differential NC cross sections up to Bjorken x = 1
EPS05 abstract 331, ZEUS Collaboration
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12
ELECTRON + JET METHOD:
b. if JET near beam-pipe high-x jet not well reconstructed ZERO jet collect events in bin with xedge < x < 1
measure integral cross section up to x=1:
2. in each Q2 bin, define x bins:
a. if JET far from beam-pipe low-x x from EJet, Jet good resolution in x
1. use ELECTRON information for Q2
electron well reconstructed for Q2 > 450 GeV2
good resolution in Q2 for all-x
)cos(1E2EQ e'ee
2
Method & data selection
edge
1 2 2
x(d /dxdQ )dx
12Information up to x = 1
kinematic coverage of data
DATA SELECTION 98-99 e-p (16.7pb-1) & 99-00 e+p (66.1pb-1)- high energy electron with strict fiducial cuts
- 0 or 1 jet with ETJet>10 GeV and Jet>0.12
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ZEUS
e-p NC cross section results
highest x point is integrated cross section up to x=1:edge
221
x
x1
)/dxdQ(ddx edge
14
e+p NC cross section results
edge
221
x
x1
)/dxdQ(ddx edge
highest x point is integrated cross section up to x=1:
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generally good description by NLO QCD (using CTEQ6D, ZEUS-S PDFs) new direct constraints on PDFs at high-x (and lower-x through sum rules) similar for e-p ratios
e+p NC cross section ratios
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Photoproduction of dijets with high-ET
new analysis from H1: 99-00 e+p data (66.6 pb-1) new high-precision measurement of high-ET dijets in photoproduction
AIM include in combined QCD fit with DIS data to extract PDFs and s
photoproduction (Q2 ~ 0)
perturbatively calculable if ET of jets
used as hard scale
at O(s), two processes contribute
jet,i
2jet,iOBS
p Ti=1p
1x = E e
2E for strong constraints on proton PDF, reduce dependence on photon structure H1: direct-enriched x
OBS > 0.8
jet,i2
jet,iOBS -T
i=1
1x = E e
2E
resolved direct
EPS05 abstract 680, H1 Collaboration
measurements of dijet photoproduction:
direct sensitivity to s and gluon in proton
resolved processes also sensitive to both
gluon and quark densities in photon
longitudinal momentum fractions xp and x participating in hard scatter:
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High-ET differential cross sections in pHigh-ET dijet cross sections:
longitudinally invariant kT algorithm in lab.
Q2 < 1 GeV2, 0.1 < y < 0.9
pT,max > 25 GeV, pT,2 > 15 GeV, 0.5 < Jet < 2.75
QCD models: PYTHIA6.1 (CTEQ5L p, GRV-LO PDFs) Frixione-Ridolfi NLO QCD (CTEQ6M p, GRV-HO ) - hadronisation corrs. (PYTHIA+HERWIG) - yellow band: scale uncert. - green band: total uncert. (incl. PDFs,
hadronisation)
cross sections differential in xp and x:
longitudinal momentum fractions:
0.05 < xp < 0.7 0.1 < x < 1.0
• low-xp (< 0.1) and high-xp (> 0.1) regions
roughly distinguish g and q scattering
low-x (< 0.8) and high-x (> 0.8) regions
roughly distinguish between resolved and
direct photon events
generally good description by NLO QCD
g enriched
q enriched
resolved enriched
direct enriched
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Double differential cross sections
more detailed look in bins of
measurement divided into resolved-
(x < 0.8) and direct- (x > 0.8) enriched
generally good description by NLO QCD
(although data tends to lie below
prediction at high-xp when 1,2 > 1)
dominant uncertainties:
- experimental:
cal. e-scale, model (low-xp), stat. (high-xp)
- scale uncertainty smallest at high-xp
- PDFs better known at low- than high-xp
already constrained at low-x by
inclusive DIS data
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Double differential cross sections
more detailed look in bins of
similar conclusions for cross sections
in pT,max
direct-enhanced region (x > 0.8)
for cross sections in xp and pT,max
smaller scale uncertainties
reduced dependence on photon PDF
potentially strong constraints on
proton structure include in QCD fit
for PDFs and s
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Summary & OutlookHERA data now high precision and wide kinematic coverage new NLO
QCD combined fit to ZEUS inclusive DIS and jet data (ZEUS-JETS PDF)
simultaneous extraction of PDFs and s
rigorous inclusion of jet data for the first time in a QCD fit
significant reduction of gluon uncertainties at mid-to-high-x
precise determination of s from HERA data onlys(MZ) = 0.1183 ±0.0028 (exp.)±0.0008(model)±0.0050(theory)
new method developed by ZEUS to measure NC cross sections up to x = 1
first results in e+p (65 pb-1) and e-p (17 pb-1) data from HERA
new constraints on PDFs at high-x
under analysis within the framework of NLO QCD fitting
new high-ET dijet photoproduction cross sections measured by H1
67 pb-1 e+p data, reduced systematics compared to previous measurements
generally well described by NLO QCD
direct-enriched (x > 0.8) cross sections
smaller uncertainties and reduced dependence on structure
potentially strong constraints on gluon in proton
AIM: include data in combined NLO QCD analysis of inclusive DIS and jet data
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Extras …
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In contrast with evaluation of structure functions from evolved PDFs, the calculation of jet cross sections at NLO requires much CPU time: O(10 hours) per PDF set unaffordable in a fit of the proton PDFs
METHOD
deconvolve PDFs and s from matrix elements in the NLO calculation:
construct “grids” containing matrix element part of cross section such
that calculations for jet cross sections can be performed sufficiently fast
(and accurately) for any PDF and any value of s O(1 second) per PDF set
Jet cross section calculations can be performed for ANY PDF set and
ANY value of s in a fast way and with an accuracy better than 0.5%
2k ji,a,n, ,Q
2
2
grid of w eights in the "(x= ,Q )" plane for a given bin of the
cross section (i), parton species (a) and order (n) typical si ( ,Q
G
) 100×10e 0z in
2k ji,a,n
2 n 2jet a k j s j
n=1,2 a=q,q,g j k, ,Q
(i)= f ( ,Q ). ( GQ ).
ZEUS-JETS: inclusion of jet data
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ZEUS-JETS fit: comparison with other PDFs
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agreement with other PDF fits within
uncertainties
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ZEUS-S global fit: no jet data included
ZEUS-JETS fit: jet data included
Only small increase in uncertainties when s freed
jet cross sections directly sensitive to s via *g qqbar (coupled to
gluon) and via *q qg (NOT coupled to gluon)
extraction of s NOT strongly correlated to gluon
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ZEUS-JETS fit: correlation between gluon and s(MZ)
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migration from low-x is very small zero-jet events really are high-x events!!!
xtrue distribution of the highest x bin
NC at high-x: check x migrations
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kinematic quantities electron quantities
MC gives good description of data
NC at high-x: Control plots
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One-jet events kinematics in zero-jet events
NC at high-x: Control plots (cont.)
MC gives good description of data
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NC at high-x: e-p NC cross section ratios
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dijets at high-ET: angular distributions - cos*
cos*
Jet
Jet
beamline
sensitive to dynamics of p
- low MJJ, sensitive to jet ET cuts
- cut at MJJ > 65 GeV reduces bias
distribution follows form of
QCD matrix elements:
shape described by NLO QCD
but data tends to lie below
predictions
gluon propagator ~ |(1-cos*)|-2
quark propagator ~ |(1-cos*)|-1