introduction the zeus pdf fit: an overview impact of future hera data on the zeus fit - end of...

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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HERA-I: statistics limited

HERA-I: statistics limited at high-ET and high-Q2

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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


sLL2

22NC

2

QxgF whereFyFY~dxdQ

p)(ed



20




provide tests of higher order QCD

test the need for extensions to DGLAP at low-x


sLL2

22NC

2

QxgF whereFyFY~dxdQ

p)(ed



Rob

ert T

horn

e

20

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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 %)




provide tests of higher order QCD

test the need for extensions to DGLAP at low-x


sLL2

22NC

2

QxgF whereFyFY~dxdQ

p)(ed



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


H1-Only PDF


H1-Only PDF

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H1+BCDMS fit


H1-Only PDF


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 )

13

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.


H1+BCDMS fit

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12

(F2p F2

n) F2p

x(16

dv 16

uv 13

d 13

u )

13

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.


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

33

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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

introduction the zeus pdf fit: an overview impact of future hera data on the zeus fit - end of...

Documents

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set of hera

end of hera

xcall hera

zeusjets pdf

zeus inclusive jets

zeus fit end

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