Low x workshop Helsinki 2007 Joël Low x workshop Helsinki 2007 Joël FeltesseFeltesse

11

Inclusive FInclusive F22 at low x and F at low x and FLL measurement at HERAmeasurement at HERA

Joël Feltesse Desy/Hamburg/SaclayJoël Feltesse Desy/Hamburg/Saclay

On behalf of the H1 and ZEUS On behalf of the H1 and ZEUS CollaborationsCollaborations

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 22

OutlookOutlook

• Reminder on published resultsReminder on published results

• New preliminary H1/ZEUS combinationNew preliminary H1/ZEUS combination

• New preliminary dataNew preliminary data

• FFLL status and expectations from last status and expectations from last datadata

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 33

[Inte

grat

ed lu

min

osity

: 40

pb

-1]

quarksQxqQxqxeQxF ),(),(),( 2

_222

2

Impressive rise of F2 as x →0

Impressive rise of sea quarks density

The rise increases with Q2.

Excellent description of data by DGLAP evolution equations. No BFLKL terms required !

),( 222

QxFdxdQ

d

Inclusive cross section

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 44

No slow down of F2 riseNo saturation observed yet

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 55

Well known pattern of scaling violations :

•positive at x<0.08•negative at x>0.13

At Leading Order (DGLAP) :

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 66

Well ! But there is still room for Well ! But there is still room for improvement !improvement !

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 77

Combination of H1 and ZEUSCombination of H1 and ZEUS DIS Cross Section Measurements DIS Cross Section Measurements

• It should be the ultimate legacy from HERA.It should be the ultimate legacy from HERA.

• After a first simple combination of high QAfter a first simple combination of high Q2 2 data in data in 2006, dominated by statistical errors, a coherent 2006, dominated by statistical errors, a coherent approach taking into account systematic correlations approach taking into account systematic correlations and providing cross calibration has been developed.and providing cross calibration has been developed.

• To day : preliminary results on HERA I published data To day : preliminary results on HERA I published data taken between 1996-2000 at Qtaken between 1996-2000 at Q22 > 1.5 GeV > 1.5 GeV22

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 88

Input Data SetsInput Data Sets

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 99

MethodMethod

• Prior to combination, data are :Prior to combination, data are :

– Shifted to a (x,QShifted to a (x,Q22) common grid (~ no additional error)) common grid (~ no additional error)– Moved to 920 GeV Moved to 920 GeV

→ → Up to 5% uncertainty at high y. It should not be done in Up to 5% uncertainty at high y. It should not be done in the future.the future.

• Average cross section are determined in a Average cross section are determined in a simulaneous fit of data. The fit is not physics simulaneous fit of data. The fit is not physics model dependent !model dependent !

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 1010

Chi-2 definitionChi-2 definition

The Chi-2 is minimised with respect to Mi,true and αj. The correlated systematics uncertainties are floated coherently, such that each experiment calibrates the other one!

→ Significant reduction of some correlated systematics !

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 1111

Chi-2 definition : Chi-2 definition : additional (or multiplicative) subtletyadditional (or multiplicative) subtlety

Most systematic errors are usually estimated as relative error.But a smaller value of x-section has smaller absolute error.→ Bias towards smaller averages (checked with toy MC)Bias can be avoided by modifing Chi-2 definition.

In practice a reevaluation of the absolute error after one iteration is sufficient.The overall effect is small, except for normalisation which is finally taken as relative.

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 1212

Correlations between H1 and ZEUS Correlations between H1 and ZEUS data setsdata sets

Resulting deviations are small ~0.3 % except at low x high y, can be up to 2 %

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 1313

A visible improvement : reduction of A visible improvement : reduction of systematics at low Qsystematics at low Q22 and reduction of and reduction of

statistical errors at high Qstatistical errors at high Q22

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 1414

New low QNew low Q22 measurements from H1 measurements from H1

At this workshop,preliminary results on :

-Very low Q2 domain-High y (i.e. low x) domain

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 1515

Minimum Bias 1999MB’99

Shifted Vertex 2000SVX’00

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 1616

Preliminary results on reduced cross Preliminary results on reduced cross sectionssections

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 1717

MB’99 vs SVX’00 Comb of MB’99+SVX’00 vs renormalised MB’97

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 1818

Final (but still preliminary) combinationFinal (but still preliminary) combination

Data fill the transition region atQ2 ~ 1 GeV2.Good agreement with ZEUS.

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 1919

Combination improvementCombination improvement

A nice data set for future phenomenological fits

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 2020

New H1 measurement at low QNew H1 measurement at low Q22 high y high y12 < Q12 < Q2 2 < 50 Gev< 50 Gev22 and 0.75 < y < 0.9 and 0.75 < y < 0.9

at Ep= 920 GeVat Ep= 920 GeV

• It is part of the QIt is part of the Q22,y domain where the F,y domain where the FL L

measurement is done, but at lower proton beam measurement is done, but at lower proton beam energies !energies !

• The common experimental challenge is the low The common experimental challenge is the low scattered electron energy E’scattered electron energy E’ee > 3.4 GeV : > 3.4 GeV :– Calorimeter linerarity (Calorimeter linerarity (ππoo and and ψψ calibration) calibration)– Trigger efficiency (redundancy)Trigger efficiency (redundancy)– Photo production background (wrong charge track)Photo production background (wrong charge track)

– Radiative correction (measure 2 ERadiative correction (measure 2 Ebeambeam= = ΣΣ (E (Ehh – P – Pzzhh) + (E) + (Eee’-P’’-P’ee

zz))))

• HERA II data : 51 pbHERA II data : 51 pb-1-1 e e++p and 45 pbp and 45 pb-1-1 e e--pp

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 2121

Preliminary cross sections

Errors reduced bya factor 2 !

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 2222

New ZEUS measurement at high yNew ZEUS measurement at high y25 < Q25 < Q2 2 < 1300 Gev< 1300 Gev22 and 0.1 < y < 0.8 and 0.1 < y < 0.8

at Eat Epp= 920 GeV= 920 GeV

• Same experimental challenges as H1.Same experimental challenges as H1.

• Main differences: Main differences: – E’ > 5 GeVE’ > 5 GeV– Larger scattering angle (to the electron beam direction)Larger scattering angle (to the electron beam direction)– Photoproduction background subtraction mainly from e-Photoproduction background subtraction mainly from e-

tagger events (no charge measurement)tagger events (no charge measurement)

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 2323

Reduced cross sectionReduced cross section

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 2424

S.ShimizuDIS 07

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 2525

Direct measurement of FDirect measurement of FLL(Q(Q22,x) ,x)

At fixed (Q2,x), measure cross sections at several beam energies (minimum 2), i.e. at different y.

Perform straight line fit of σr to extract F2 and FL

FL is very sensitive to small relative shifts on cross sections.

A challenging high y measurement at low Ep.

xQ

Y

dQdx

d4

2

2

2 2 ),().(),( 222 QxFyfQxF L rxQ

Y 4

22

F2-FL

F2

Reduced cross section

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 2626

Successful runs of HERA at 460 GeV and Successful runs of HERA at 460 GeV and 575 GeV 575 GeV

460 GeV13.6 pb-1

575 GeV6.5 pb-1

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 2727

The 575 GeV run :The 575 GeV run : a unique tool to control systematics a unique tool to control systematics

Simulation

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 2828

Nice data on tapeNice data on tape

Be patient !

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 2929

SummarySummary

• New and improved results at low QNew and improved results at low Q22 (H1) and in the (H1) and in the high y domain (H1 and ZEUS) have been presented. high y domain (H1 and ZEUS) have been presented. New analysis of low QNew analysis of low Q22 data are still on going (H1). data are still on going (H1).

• In a fuitful collaboration H1 and ZEUS are well on In a fuitful collaboration H1 and ZEUS are well on track to provide high precision ( < 2 %) HERA Ftrack to provide high precision ( < 2 %) HERA F22 data at low x. data at low x. An important input to physics at LHC.An important input to physics at LHC.

• Successful last runs at low and medium proton Successful last runs at low and medium proton beam energies should provide a direct beam energies should provide a direct measurement of Fmeasurement of FLL with an unprecedented good with an unprecedented good precision. precision. A new handle to the gluon density at low A new handle to the gluon density at low x.x.

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 3030

EXTRASEXTRAS

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 3131

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 3232

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 3333

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 3434

High y HERA II compared to smaller y High y HERA II compared to smaller y

HERA IHERA I

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 3535

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 3636

Prospects at low β based on H1 parametrisation

Low x workshop Helsinki 2007 Joël FeltesseLow x workshop Helsinki 2007 Joël Feltesse 3737

FLD at high β

With the same assumptions on systematic sources as at low βmeasurement at the 3 σ level at high β looks difficult but feasible !

Assuming <FLD/FT

D > = 0.66 at 0.6< β <0.9

Preliminary expectations :

<Q2> = 13 GeV2

<xPom> = 0.0004 <β> = 0.75

<xPom.FLD> = 0.024± 0.004 ± 0.006