introduction the zeus pdf fit: an overview impact of future hera data on the zeus fit - end of...
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Introduction The ZEUS PDF fit: an overview Impact of future HERA data on the ZEUS fit - end of current HERA-II running scenario - additional studies (FL, sea quark asymmetry)
Conclusions
Impact of HERA-II data on the ZEUS PDF fits
13th International Workshop on Deep Inelastic Scattering (DIS05)
Madison, Wisconsin, USA
27th April – 1st May 2005
Claire Gwenlan
with the help of M. Klein (DESY),
C. Targett-Adams (UCL), R. Thorne (Cambridge), A. Tricoli (Oxford)
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Besides being interesting in their own right, it is essential to know theparton density functions (PDFs) of the proton as precisely as possible in order to maximise the physics potential at both current and future colliderse.g. high-x gluon is dominating uncertainty in several LHC processes
Introduction_______________________________
Presented here are the results of studies that give a first look at thepotential impact of future HERA measurements on the proton PDFs:1. within current HERA-II running scenario
- increased luminosity- cross sections optimised for sensitivity to PDFs
2. Other possibilities (FL, sea quark asymmetry)
HERA data are now very precise and cover a wide range in (x,Q2) determination of proton PDFs now possible within one experiment
Most recently, ZEUS have performed a NLO QCD analyses on their full set of HERA-I e+ and e- structure function data and high precision jet data the ZEUS-JETS PDF
With future measurements at HERA, hope to be able to do even better …
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Observables used in QCD fits to determine PDFs:
Inclusive NC/CC DIS ep cross sections
direct sensitivity to quarks
only indirect sensitivity to gluon (scaling violation)
Jet cross sections: directly sensitive to gluon through boson-gluon-fusion
xp2 i i
23 i i
2L s
F ~ x(q +q ) Dominates
xF ~ x(q -q ) High Q
F ~ .xg(x,Q ) High y ( NLO)
23L
222
2
y)(11 Y; xFYFyFY~dxdQ
p)(ed
Determination of proton PDFs at HERA_______________________________
Q2 = -q2 = -(k-k’)2
Factorisation: observable = short range interaction PDFs
Gluon in proton
Jet
Jet
, Z0, W
Now, after the HERA-I (94-00) phase of data-taking, the full set of inclusive NC/CC e+/e- data, and high precision jet data are available for QCD analysis…
ZEUS-JETS NLO QCD fit _______________________________
parameter constraints: momentum and quark number sum rules
low-x behaviour of uv and dv set equal
flavour structure of light quark sea set
consistent with Gottfried sum and Drell-Yan
heavy quarks treated in variable flavour
number scheme of Thorne and Roberts
PDF Param. at Q02 =7 GeV2
u-val. (xuv) Auv xbuv (1-x)cuv (1+duvx)
d-val. (xdv) Adv xbdv (1-x)cdv (1+ddvx)
total sea (xS) AS xbS (1-x)cS
gluon (xg) Ag xbg (1-x)cg (1+dgx)
dbar-ubar (x) A xb (1-x)c
For more details on the ZEUS-JETS fit (hep-ph/05030274), and the data included, see cont. to this workshop, “Proton PDFs using Structure Functions and Jet Data from ZEUS”, Juan Terron.
all HERA-I ZEUS incl. NC/CC e+/e- (94-00)
ZEUS inclusive jets in DIS (96-97)
ZEUS dijets in photoproduction (96-97)
11 free parameters in total
Data:
Parameterisation:
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Current HERA-II running scenario_______________________________ HERA-II is running efficiently...
“ 700pb-1 integrated luminosity, equally divided between e+/e-,
expected by the end of HERA-II running in mid-2007 ”
We are here…
2004 2005 2006 2007
Current running scenario
HERA-I (92-00) HERA-II (02 )
e+ 165 90
e- 27 > 50
HERA delivered luminosity (pb-1)
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Valence High Q2 inclusive NC/CC e cross sections
Sea Low-x from inclusive NC DIS High-x ? Flavour ? (assumptions needed)
Gluon Low-x from HERA dF2/dlnQ2
Mid-to-high-x from HERA jet dataHigh-x from momentum sum rule
Where does the information come from in a HERA-Only fit ?
What impact will future HERA measurements have on the PDFs?_______________________________
HERA-I: statistics limited
HERA-I: statistics limited at high-ET and high-Q2
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Example: high-Q2 NC and CC data_______________________________
F2 dominates NC cross section, HERA-I: F2/F2~30%
e+ and e- needed for flavour separation,but high-Q2 CC statistically limited at HERA-I, especially e-p data
HERA-II will provide greatly increased luminosity
+ 2CC
- 2CC
σ =x [u+c+(1-y) (d+s)]
σ =x [u+c+(1-y) (d+s)]
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Valence High Q2 inclusive NC/CC e cross sections
Sea Low-x from inclusive NC DIS High-x ? Flavour ? (assumptions needed)
Gluon Low-x from HERA dF2/dlnQ2
Mid-to-high-x from HERA jet dataHigh-x from momentum sum rule
Where does the information come from in a HERA-Only fit ?
What impact will future HERA measurements have on the PDFs?_______________________________
HERA-I: statistics limited
HERA-I: statistics limited at high-ET and high-Q2
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Valence High Q2 inclusive NC/CC e cross sections
Sea Low-x from inclusive NC DIS High-x ? Flavour ? (assumptions needed)
Gluon Low-x from HERA dF2/dlnQ2
Mid-to-high-x from HERA jet dataHigh-x from momentum sum rule
Where does the information come from in a HERA-Only fit ?
What impact will future HERA measurements have on the PDFs?_______________________________
HERA-I: measurements in only certain kinematic regions – potential to optimise cuts for
sensitivity to gluon
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Measure jet cross sections in kinematic regions “optimised” for sensitivity to gluon
- ongoing ZEUS study: dijets in photoproduction (Q2 < 1 GeV2) data simulated using NLO QCD (Frixione-Ridolfi) and CTEQ5M1 proton PDF (500 pb-1)
Optimised jet cross sections_______________________________
OBSγresolved-photon enhanced: x <0.75 OBS
γdirect-photon enhanced: x >0.75
jet1,2T
jet1 jet2
E >20,15 GeV1<η <2, 2<η <3
jet1,2T
jet1 jet2
E >30,25 GeV1<η ,η <2
jet1,2T
jet1 jet2
E >35,30 GeV-1<η ,η <2.5
jet1,2T
jet1,2 jet1,2
E >20,15 GeV{2<η <3} / {2.5<η <3}
jet1,2T
jet1,2 jet1,2
E >20,15 GeV{1.5<η <3} / {2.5<η <3}
jet1,2T
jet1 jet2
E >20,15 GeV1<η <2, 2<η <3
Christopher Targett-Adams (UCL)
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Impact of HERA-II in current running scenario:- case study_______________________________
Data sample L of HERA-I measurement
(pb-1)
assumed L of HERA-II
measurement (pb-1)
Central values taken from…
Systematic uncertaintie
s taken from…
High-Q2 NC e+ 63 700 existing data
existing data
High-Q2 NC e- 16 700 existing data
existing data
High-Q2 CC e+ 61 700 existing data
existing data
High-Q2 CC e- 16 700 existing data
existing data
Inclusive DIS jets 37 500 existing data
existing data
Dijets in p 37 500 existing data
existing data
Optimised dijets in p
- 500 NLO QCD NOT INCLUDED
data exists for HERA-I running
data does not yet exist for HERA-I running
Impact of the projected HERA-II measurements has been studied in the
context of the ZEUS-JETS fit
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HERA-II projected fit_______________________________ The HERA-II projected PDF
compared to the ZEUS-JETS fit
Impact from increased statistics on high-Q2 NC/CC e+/e- data
Impact from optimised cross sections (mostly) and increased statistics on jet data
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u-valence uncertainties _______________________________log-x scale (low-x region) linear-x scale (high-x region)
uncertainties on u-valence distribution significantly reduced over visible x range
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d-valence uncertainties _______________________________log-x scale (low-x region) linear-x scale (high-x region)
uncertainties on d-valence distribution significantly reduced over visible x range
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log-x scale (low-x region) linear-x scale (high-x region)
uncertainties on sea-quark distribution significantly reduced at high-x most significant improvement from increased statistics
Sea-quark uncertainties _______________________________
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gluon uncertainties _______________________________log-x scale (low-x region)
uncertainties on mid-to-high-x gluon significantly reduced most significant improvement comes from optimised cross sections
linear-x scale (high-x region)
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HERA Kinematic (x,Q2) Range_______________________________ HERA data covers large region in (x,Q2)
also relevant x-region for LHC
HERA-II projected PDFs should have
significant impact on e.g. high-pT
jets, heavy particle searches etc.
sensitive to improvement to high-x partons
LHC cross sections under study
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Impact of a HERA measurement of FL _______________________________),(. 2
sLL2
22NC
2
QxgF whereFyFY~dxdQ
p)(ed
FL contributes at NLO (and HO) and is directly sensitive to
the gluon density in the proton
measured at fixed target exps. (x > 10-3)
precision FL measurement at HERA
requires low-Ep running vary y at
fixed (x,Q2)
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precision measurement of FL at HERA-II:
pin down gluon density at low-x
reduce uncertainties on gluon PDF
Impact of a HERA measurement of FL _______________________________),(. 2
sLL2
22NC
2
QxgF whereFyFY~dxdQ
p)(ed
FL contributes at NLO (and HO) and is directly sensitive to
the gluon density in the proton
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precision measurement of FL at HERA-II:
pin down gluon density at low-x
reduce uncertainties on gluon PDF
provide tests of higher order QCD
test the need for extensions to DGLAP at low-x
Impact of a HERA measurement of FL _______________________________),(. 2
sLL2
22NC
2
QxgF whereFyFY~dxdQ
p)(ed
FL contributes at NLO (and HO) and is directly sensitive to
the gluon density in the proton
Rob
ert T
horn
e
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Simulation of HERA-II FL: Max Klein (DESY)
FL simulated using GRV94 NLO PDF
statistical uncertainties correspond to:
systematic uncertainties from current
H1 analysis of 99-00 data (few %)
precision measurement of FL at HERA-II:
pin down gluon density at low-x
reduce uncertainties on gluon PDF
provide tests of higher order QCD
test the need for extensions to DGLAP at low-x
Impact of a HERA measurement of FL _______________________________),(. 2
sLL2
22NC
2
QxgF whereFyFY~dxdQ
p)(ed
FL contributes at NLO (and HO) and is directly sensitive to
the gluon density in the proton
Ep (GeV) 920 575 465 400
L (pb-1) 10 5 3 2
For further details, see “On the future measurement of FL at low-x at HERA”, Max Klein, in proceedings, DIS04.
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Impact on gluon distribution_______________________________
Fit including simulated FL data compared
to the ZEUS-JETS PDF Gluon uncertainties reduced at low-x
and low-Q2 (not relevant for LHC) 22
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FL predictions very sensitive to underlying theory
choice of PDF, order of QCD calculation etc.
How sensitive is the NLO QCD fit to inclusion of “extreme” sets of simulated FL data
Sensitivity of the NLO QCD fit_______________________________
PDF QCD theory
Max. FL MRSG95 NNLO*
Mid. FL GRV94 NLO
Min. FL MRST2003 NLO
ZEUS fit relatively stable to inclusion of
extreme FL data-sets
fit cannot describe “just any data”
an FL measurement of this precision should
have power to discriminate between
theoretical models
Simulated FL data extremes provided by Robert Thorne (Cambridge)
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Summary _______________________________
HERA-I has provided precision measurements over large region in (x,Q2)
PDFs of the proton already determined to high precision using HERA data only
most recently: ZEUS-JETS fit to HERA-I inclusive NC/CC and jet data
In current running scenario:
HERA-II will provide significantly increased statistical precision
- high-Q2 NC/CC data significant improvement to valence distributions
- high-Q2 and high-ET jet data improvement to high-x gluon
potential for measuring cross sections optimised for sensitivity to gluon
- significant improvement to high-x gluon
In low-Ep running scenario a precise measurement of FL at low-x possible
inclusion in fit indicates improvement in gluon uncertainties at low-x and low-Q2
stability of fit to extreme FL sets indicates ability of HERA FL to discriminate
between theory
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Bonus Extra: - sea quark asymmetry, a study by Max Klein, Burkard Reisert_______________________________
ū=đ was a natural assumption for long time,
until E866, HERMES found a difference
at x ~ 0.1 all global fits followed Indications for strange-anti-strange asymmetry
What causes rise of F2 at low x?
Have measured 4ū+đ but ū and đ are unknown at low x would be accessible via deuteron (eD) running at HERA-II
2F = 4/9 x(U+U) + 1/9 x(D+D)
Are sea and anti-quarks equal ?Are up and down quarks equal at low x?
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No information on sea flavour composition
- only one measurement at low-x:
Assume quark and anti-quark distributions
are equal at low-x and u=d
The H1 NLO QCD fit _______________________________
PDF Param. at Q02 =4 GeV2
xU=x(u+c) AU xbU (1-x)cU (1+dUx+eUx3)
xUbar AUbar xbUbar (1-x)cUbar
xD=x(d+s) AD xbD (1-x)cD (1+dDx)
xDbar ADbar xbDbar (1-x)cDbar
xg Ag xbg (1-x)cg (1+dgx)
0x as 1ud that
means whichf1f1A- A
BBBBB-
csDU
qDUDU
/
),/().(
HERA-I H1 incl. NC/CC e+/e- (94-00): H1-Only
+ BCDMS (p,D data): H1+BCDMS
2F = 4/9 x(U+U) + 1/9 x(D+D)
experimental errorsmodel uncertainties
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Releasing the dbar-ubar constraint_______________________________
H1-Only fit (default assumptions) b) A,on constraint (no fitOnly-H1
experimental errorsmodel uncertainties
H1-Only PDF
experimental errorsmodel uncertainties
H1-Only PDF
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Releasing the dbar-ubar constraint_______________________________
H1+BCDMS fit
experimental errorsmodel uncertainties
H1-Only PDF
experimental errorsmodel uncertainties
H1-Only PDF
b) A,on constraint (no fitBCDMSH1
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(F2p F2
n) F2p
x(16
dv 16
uv 13
d 13
u )
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x(d u ) at low x
simulated accuracy (20pb-1 e-D, 40pb-1 e-p)
Q2 =5 GeV2
The light sea quark asymmetry is expected and hasbeen assumed to vanish at low x. However, F2 risesstrongly towards low x which deserves to be studied.
Releasing the dbar-ubar constraint_______________________________
H1+BCDMS fit
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12
(F2p F2
n) F2p
x(16
dv 16
uv 13
d 13
u )
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x(d u ) at low x
simulated accuracy (20pb-1 e-D, 40pb-1 e-p)
Q2 =5 GeV2
The light sea quark asymmetry is expected and hasbeen assumed to vanish at low x. However, F2 risesstrongly towards low x which deserves to be studied.
Releasing the dbar-ubar constraint_______________________________
H1+BCDMS fit
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Mini-summary (sea-quark asymmtery)_______________________________
So far HERA has not resolved the light sea quarks at low x QCD fits employ (“reasonable”) assumption that ū=đ and u=ū and d=đ at low-x
- this reasonable assumption was proven to be wrong at larger x ~ 0.1
without these requirements the fits become unstable
deuteron data from fixed target experiments (e.g. BCDMS) do help but cannot
solve the problem since data lie at higher-x
Would need eD running at HERA-II to resolve this issue
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Extras …_______________________________
33 Uncertainties with full HERA-II inclusive data-set comparable to global fits
HERA-II Projected PDF
Comparison with global fit (u-val.)_______________________________ZEUS-S global PDF
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Comparison with global fit (d-val.)_______________________________
Uncertainties comparable to or better than current global fit
HERA-II Projected PDFZEUS-S global PDF
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Impact on sea/gluon uncertainties _______________________________
already at HERA-I, measurement of optimised jet cross sections would have a
significant impact on the high-x gluon