Inclusive DIS Results from HERAbib-pubdb1.desy.de/record/79250/files/kretzschmardiff06.pdf · 2017. 9. 11. · Inclusive DIS Results from HERA Jan Kretzschmar 1. Introduction The

Inclusive DIS Results from HERA

Jan Kretzschmar∗†DESY Zeuthen, Plantanenallee 6, 15738 Zeuthen, GermanyE-mail: [email protected]

Deep inelasticep-scattering results and prospects for the two HERA experiments H1 and ZEUS

are presented. The topics covered are low and highQ2 inclusive structure function measurements,

inclusive tagging of heavy quark production in DIS, and QCD fits and parton distribution func-

tions extracted in NLO DGLAP fits. At lowQ2 improvements and new results are expected due

to the better understanding of systematic errors, the lowEp run, and combination of ZEUS and

H1 data. The highQ2 domain profits especially from the increased luminosity andthe beam po-

larisation available after the HERA upgrade. Also for the heavy quark sector improvements are

expected due to better statistics and new silicon vertex detectors. A NNLO QCD analysis may

lead to better determined parton distribution functions for the LHC and test our understanding of

QCD.

DIFFRACTION 2006 - International Workshop on Diffraction in High-Energy PhysicsSeptember 5-10 2006Adamantas, Milos island, Greece

∗Speaker.†On behalf of the H1 and ZEUS Collaborations

c© Copyright owned by the author(s) under the terms of the Creative Commons Attribution-NonCommercial-ShareAlike Licence. http://pos.sissa.it/

mailto:[email protected]

Inclusive DIS Results from HERA Jan Kretzschmar

1. Introduction

The HERA collider facility in Hamburg, Germany, is a unique tool for electron-proton scat-tering at highest energies. It consists of two accelerators: one for protons, which are acceleratedto 920 GeV, and one for electrons or positrons1, which are accelerated to 27.6 GeV. For the twocolliding beam experiments H1 and ZEUS this leads to a centre of mass energy of

√s = 320 GeV.

In Deep Inelastic Scattering (DIS) of leptons off nucleons the substructure of the nucleons wasdiscovered [1]. DIS continues to be a tool for exploring the substructure of the nucleons with thehighest precision. The leading order Feynman diagram for neutral current (NC) scattering is givenin figure 1.

The kinematics of the scattering are described in terms of the Lorentz invariant quantitiesBjorken scaling variablex, inelasticityy, and virtualityQ2, which are related byQ2 = xys. Figure 1shows the kinematic(x,Q2)-plane, where the HERA experiments and the fixed target experimentshave made measurements of the proton structure.Q2 values of up to 50000 GeV2 andx valuesdown to 10−5 are reached at HERA.

x

Q2 /

GeV

2

y =

1

y = 0.

004

HERA Experiments:

H1 1994-2000

ZEUS 1994-2000

Fixed Target Experiments:

NMC

BCDMS

E665

SLAC

10-1

1

10

10 2

10 3

10 4

10-6

10-5

10-4

10-3

10-2

10-1

1

Figure 1: Lowest order Feynman diagram for NC DIS (left). Kinematic plane in(x,Q2), where mea-surements of the proton structure functionF2 have been performed by the HERA collider and fixed targetexperiments, respectively (right).

One of the most fundamental measurements to be performed is that of the inclusive NC crosssection for the reactionep → e′X , which can be expressed in the form

d2σe±pNC

dxdQ2 =2πα2Y+

xQ4

(

F2(x,Q2)− y2

Y+FL(x,Q

2)∓ Y−Y+

xF3(x,Q2)

)

, (1.1)

with Y± = 1± (1− y)2 and the structure functionsF2, FL, andxF3.The structure functionF2 gives the dominant contribution to the cross section in most of the

kinematic domain and therefore has been measured with the best accuracy.Figure 2 gives anexample of the progress made with the HERA collider: from the very first dataand the discoveryof the strong rise ofF2 towards lowx to the currently reached 2−3% precision in the bulk region.

1In the following the term electron will usually apply to both lepton charges, if not stated otherwise explicitly.

2


Figure 2: Measurement of the proton structure functionF2 at Q2 = 15 GeV2 with the very first HERA dataand with much improved precision today [2].

The longitudinal structure functionFL contributes mostly at high inelasticityy, andxF3 appearsdue to neutral weak currents mediated by theZ boson and is only important at highQ2.

2. Results and Prospects for the LowQ2 Region

In the lowQ2 bulk region (10< Q2/GeV2 < 100) the structure functionF2 has been measuredwith an overall precision of 2−3%. In the very lowQ2 region (Q2 < 10 GeV2) H1 has obtainedpreliminary results with special experimental techniques [3, 4]:

• runs with shifted vertex to enhance the acceptance of H1 for lower angle and hence lowerQ2

scattering,

• using events with untagged initial state radiation (ISR) to reach higherx at low Q2,

• and runs with a special minimum bias trigger setup.

The preliminary results for the lowestQ2 region reached together with measurements from ZEUSand fixed target experiments are shown in figure 3.

The rise ofF2 towards lowx can be described asF2(x,Q2) ∝ x−λ (Q2). The slopeλ (Q2) wasextracted from these new results as well as other available data, see figure 3. At Q2 . 1 GeV2 achange of the slope is observed, as one leaves the DIS regime and entersthe domain of nonpertur-bative QCD effects.

The longitudinal structure functionFL contributes to the cross section weighted by the kine-matic factory2/Y+. Furthermore it is bound by 0≤ FL ≤ F2 due to the relations ofF2 andFL tothe absorption cross sections for longitudinal and transversely polarised photons. With data takenat the nominal HERA centre of mass energy, H1 has performed a model dependant simultaneousextraction ofF2 andFL from their preliminary lowQ2 data reaching to high inelasticityy [4], seefigure 4. The reduced cross section is parameterised as

σr = c(Q2) · x−λ (Q2)− y2/Y+ ·FL , (2.1)

3


Figure 3: Measurements of the proton structure functionF2 at Q2 < 1 GeV2 by ZEUS and recently by H1from shifted vertex runs and using untagged ISR events (left) and the rise ofF2 towards lowx quantified byλ (Q2) (right) [3].

and the decrease ofσr at low x, i.e. highy, is attributed to the behaviour of the kinematic factor.OneFL value for everyQ2 bin is thus determined.

H1

Col

labo

ratio

n

H1

Col

labo

ratio

n

Figure 4: Reduced cross section measurements in the range 1. Q2/GeV2 < 10 from special runs (left) andFL determination by the shape method [4] (right).

To separate theF2 andFL contributions to the cross sections without further assumptions, oneneeds a variation ofy at fixedx andQ2, which requires a change of the centre of mass energy.For this direct determination ofFL a run period of HERA with reduced proton beam energyEp

is foreseen in 2007. The simultaneous extraction of the structure functionsis illustrated in figure5 (left). For the accuracy and control of systematics it would be optimal to measure at severalcentre of mass energies and to cover the range of the kinematic factor 0≤ f (y) = y2/Y+ ≤ 1 asevenly as possible. This also means measuring up to the highest inelasticityy and therefore lowestscattered electron energies. On the other hand each beam energy may require long setup time by theHERA machine group and because the achievable luminosity scales about asL ∝ E2

p, the amountof different beam energies has to be limited.

One likely scenario at the moment is, that only one additional beam energy ofEp = 460 GeV,i.e. half of the nominal proton beam energy, will be used and aboutL = 10 pb−1 will be collected.

4


Figure 5: The direct measurement of the structure functionsFL and F2 from the reduced cross sectionmeasurementsσr at fixedx,Q2 for different inelasticitiesy (left). Simulated measurement ofFL (middle)andFD

L (right) with H1 for the case of one additional lowEp = 460 GeV run.

Both experiments, H1 and ZEUS, have studied their capabilities for the measurement. A simula-tion of the expected measurement of the longitudinal structure function for the inclusive and thediffractive case (FL andFD

L respectively) by H1 is given in figure 5. Significant measurement ofH1, towards lowest possibleQ2, and of ZEUS can be expected. Studies to prepare the directFL

measurements are ongoing in H1 and in ZEUS, in close contact with HERA.H1 has in the last years invested much work to control the systematics at lowQ2, which is the

limiting factor for preciseF2 andFL measurements. One advantage of H1 is the upgraded BackwardSilicon Tracker (BST), so precise tracking and charge determination areavailable not only at higherQ2 with the central tracking chamber, but also at lowerQ2. Using the charge determination ofbackward going particles,γp background can be separated from the genuine DIS signal at lowenergies of the scattered lepton. An illustration using data taken in a special run in 1999 is given infigure 6.

Figure 6: The ratioE/p of the energy measured in the calorimeterE (4< E/GeV< 10) and the signed mo-mentum determined with the BSTp. The signal positrons are visible at+1, while the shadedγp backgroundis about charge symmetric and visible at+1 and−1.

Detailed calibration and cross checks withπ0 → γγ, J/ψ → e+e−, and Compton eventsep →epγ have limited the residual systematic uncertainty of the lepton energy scale in the backward

5


calorimeter of H1 to 0.2% at the kinematic peak and 1% at the lowest energies, see figure 7. Usingthis knowledge H1 plans to improve the precision of theF2 structure function measurement furtherin the low Q2 bulk and highy region. A further step in precision may be reached by combiningZEUS and H1 results [5].

0

1000

2000

3000

4000

5000

6000

7000

8000

22 24 26 28

dataMC

GeV

-3

-2

-1

0

1

2

3

0 5 10 15 20 25 30

π0

J/ψkin. peak

QED Compton

E, GeV

Dat

a-M

C E

-sca

le, p

erce

nt

Figure 7: Current preliminary understanding of the lepton energy scale in the backward calorimeter of H1.

3. Results and Prospects for the HighQ2 Region

In figure 8 a collection ofF2 measurements by H1 and ZEUS in HERA I, and by fixed targetexperiments is shown. The errors in the highestQ2 domain are still dominated by the statistics.With the data from the current HERA II running phase these errors will besignificantly reduced.Also the lepton beams are longitudinally polarised, where a typical polarisationof 35% is achieved.The spectrum of measurements at highQ2 has thus now broadened to charged and neutral currents(CC and NC) with both charge and helicity states of the lepton. Combining these results, tests ofthe electroweak theory and parton flavour decomposition within one experiment will be possible.First results were obtained already and will be highlighted here.

3.1 Neutral Currents at High Q2

For polarised beams the structure functions need to be generalised, while the form of thereduced NC inclusive cross section stays the same. The structure function xF3 can now no longerbe ignored:

σ±NC =

d2σe±pNC

dxdQ2

xQ4

2πα2Y+= F2−

y2

Y+FL ∓

Y−Y+

xF3 . (3.1)

The generalised structure functions receive contributions fromγ and Z exchange andγZ inter-ference. While at leading order theF2 contributions are given by sums of quark and antiquarkdistributions times the appropriate quark-lepton couplings,xF3 is proportional to the difference ofquarks and antiquarks and hence sensitive to valence quarks and possible differences of the sea andanti-quark distributions.

The new HERA II data is being analysed by both collaborations. Both single and doubledifferential results have been obtained [6, 7]. One example for double differential NC cross sections

6


10-3

10-2

10-1

1

10

10 2

10 3

10 4

10 5

10 6

1 10 102

103

104

105

Q2 / GeV2

F 2 ⋅

2i

x = 0.65, i = 0

x = 0.40, i = 1

x = 0.25, i = 2

x = 0.18, i = 3

x = 0.13, i = 4

x = 0.080, i = 5

x = 0.050, i = 6

x = 0.032, i = 7

x = 0.020, i = 8

x = 0.013, i = 9

x = 0.0080, i = 10

x = 0.0050, i = 11

x = 0.0032, i = 12

x = 0.0020, i = 13

x = 0.0013, i = 14

x = 0.00080, i = 15

x = 0.00050, i = 16

x = 0.00032, i = 17

x = 0.00020, i = 18

x = 0.00013, i = 19

x = 0.000080, i = 20x = 0.000050, i = 21 H1 e+p

ZEUS e+p

BCDMS

NMC

H1 PDF 2000

extrapolation

H1

Col

labo

ratio

n

ZEUS

00.20.40.60.8

1

2 = 200 GeV2Q

00.20.40.60.8

1

2 = 450 GeV2Q

00.20.40.60.8

1

2 = 1200 GeV2Q

00.20.40.60.8

1 2 = 3000 GeV2Q

-210 -1100

0.20.40.60.8

1 2 = 12000 GeV2Q

2 = 250 GeV2Q

2 = 650 GeV2Q

2 = 1500 GeV2Q

2 = 5000 GeV2Q

-210 -110

2 = 20000 GeV2Q ZEUS NC

)-1p (63.2 pb+ e

SM (ZEUS-JETS)

= 0e P

2 = 350 GeV2Q

2 = 800 GeV2Q

2 = 2000 GeV2Q

2 = 8000 GeV2Q

-210 -110

2 = 30000 GeV2Q ZEUS NC (prel.)

)-1p (122 pb- e

SM (ZEUS-JETS)

= 0 (corrected)e P

x

σ~

Figure 8: Left: A collection of F2 points measured by H1, ZEUS, and fixed target experiments measuredso far. Right: New measurements of the unpolarised NC doubledifferential cross sections fore+ p ande−pscattering [6].

in e+p ande−p scattering is given in figure 8 (right). These new results have been usedto extract thestructure functionxF3 and the polarisation asymmetryA±. To increase the statistical significance,H1 and ZEUS have obtained for the first time combined results using a total luminosity of 480 pb−1

[8]. These are summarised in the following.

The structure functionxF3 is determined from the difference of unpolarisede+p ande−p NCcross sections:

xF3 = − Y+

2Y−

(

σ+NC − σ−

NC

)

. (3.2)

For measurements performed with polarised beams, data sets with both polarisations are com-bined and the small residual polarisation is corrected for. TheγZ interference is the dominantprocess, hence further results are given only forxFγZ

3 . Figure 9 shows the results obtained by H1,ZEUS and their combination. A significant reduction of errors can be seen. As xFγZ

3 is only weaklyQ2 dependent, all measurements have been combined atQ2 = 1500 GeV2. The interpretation interms of quark densities

xFγZ3 =

x3(2uv +dv +∆) (3.3)

shows the sensitivity to the valence quarksuv anddv, and a (small) sea asymmetry∆ = 2(usea −u+ c− c)+(dsea − d + s− s), which conventionally is assumed to vanish.

Similar one can determine a polarisation asymmetryA± using the polarisede±RL p NC crosssections:

A± =2

PR −PL· σ±(PR)−σ±(PL)

σ±(PR)+σ±(PL). (3.4)

The asymmetriesA± directly measure the parity violation as they are proportional to a product of

7


HERA

0

0.2

0.4

0.6

0.8

1

10-2

10-1

1x

xF3γZ

Q2=1500 GeV2

H1 (prel.)

0

0.2

0.4

0.6

0.8

1

10-2

10-1

1x

xF3γZ

Q2=1500 GeV2

ZEUS (prel.)

0

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0.6

0.8

1

10-2

10-1

1x

xF3γZ

Q2=1500 GeV2

H1+ZEUS Combined (prel.)

H1 2000 PDFZEUS-JETS PDF

HERA

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

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1

103

104

Q2 (GeV2)

A

H1 (prel.)

A+

A-

H1 2000 PDF

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

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103

104

Q2 (GeV2)

A

ZEUS (prel.)

A+

A-

ZEUS-JETSPDF

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

103

104

Q2 (GeV2)

A

H1+ZEUS Combined (prel.)

A+

A-

H1 2000 PDFZEUS-JETS PDF

Figure 9: Determination ofxFγZ3 (left) and the polarisation asymmetriesA± (right) by H1, ZEUS, and the

combination of both [8].

vector and axial vector couplings

A± ≃ kaeFγZ

2

F2∝ aevq . (3.5)

The combined result, see figure 9, is the most significant confirmation of parity violation in DIS todate.

3.2 Charged Currents at HighQ2

The CC cross section is sensitive to the lepton beam charge and depends linearly on the longi-tudinal lepton polarisationPl:

d2σe±pCC

dxdQ2 = (1±Pl)G2

F

2πx

(

M2W

Q2 +M2W

)

φ±CC . (3.6)

The functionsφ±CC contain the quark density functions and are sensitive to the down-type and

up-type quarks in the proton withe+ and e− beams respectively. The total, single, and doubledifferential CC cross sections have been measured using the latest data from HERA II [9, 10, 11].The total cross section dependence on the lepton beam polarisation shownin figure 10, confirms thechiral structure of the Standard Model, where theW boson only couples to left handed fermions.

In the future improvements for all the measurements at highQ2 are expected, once the fullHERA I and II data sets are analysed. As of August 2006 the luminosities withe+ ande− beams

8


2 = 280 GeV2Q 2 = 530 GeV2Q 2 = 950 GeV2Q

2 = 1700 GeV2Q 2 = 3000 GeV2Q 2 = 5300 GeV2Q

2 = 9500 GeV2Q 2 = 17000 GeV2Q

ZEUS CC (prel.))-1p (78.8 pb- e

ZEUS-pol (prel.) = -0.27e P

2 = 30000 GeV2Q

-210 -110 -210 -110 -210 -1100

0.2

0.4

0.6

0.8

1

1.2

0.2

0.4

0.6

0.81

1.2

1.4

1.6

0.5

1

1.5

2

2.5

x

σ~

ZEUS

Figure 10: A collection of total CC cross sections as a function of the beam polarisation as measured by H1and ZEUS (left). New polarised double differential CC results by the ZEUS collaboration (right).

are about balanced with∼170 pb−1 physics data per beam charge and experiment. At least further100 pb−1 of high energye+p data are expected before the low energy run will be carried out. TheHERA program will then be terminated in the middle of 2007.

4. Heavy Quark Production in DIS

The production of heavy flavours in DIS can be calculated using perturbative QCD (pQCD).The problem is well suitable for perturbative calculations as there are multiplehard scales available,i. e. the heavy quark massmq, the transverse momentum of the produced jetpt , and of courseQ2.It is however not clear, which of these scales is most relevant or if some combination should bechosen. First NNLO calculations have become available recently [12]. Various schemes for themass treatment of the heavy quarks especially in the threshold region are performed and theirvalidity must be checked by comparison to experiment. As the Boson-Gluon fusion process isthe dominant heavy quark production process at HERA, the measurementgives a complementaryaccess to the gluon density.

The heavy flavour tagging can be done using exclusive states likeD∗ mesons produced inthe charm fragmentation or muons from the semileptonic decays of the heavy quarks. A moreinclusive approach is available with high resolution silicon detectors. H1 hasrecently publishedresults obtained with the Central Silicon Tracker (CST) [13, 14]. For each track in the hadronicfinal state a signed impact parameterδ is determined. Large values are rejected as they are likelyto be caused by long lived strange particles. Then the significance distributionsSi = δi/∆δi of thetrack with thei-th highest significance are evaluated, see figure 11. An enhancementof the heavyquark contributions can be seen at high significance values due to the decay length of the charmand beauty hadrons. The light flavours background distribution on the other hand is smeared dueto resolution effects nearly symmetric around zero and can be reduced bysubtracting the negative

9


from the positive part of the distribution. The fractional contributions of the heavy quarks are thendetermined by scaling the MC predictions for the light and heavy quark processes to fit the data.

2S-10 -5 0 5 10

Eve

nts

1

10

210

310

410

510H1 DataTotal MCudscb

(b)

2S-10 -5 0 5 10

Eve

nts

1

10

210

310

410

510

2S0 2 4 6 8 10

) 2 )

- n

( -S

2 n

( S

1

10

210

310

410H1 Data

Total MCudsc

b

(b)

Figure 11: Significance of the track with the second highest significance S2 together with the scaled MCdistributions for light flavour (uds), charm (c), and beauty (b) contributions on the left. The right hand sideis obtained by subtracting the negative from the positive part of the distribution to reduce the influence ofresolution effects.

From the reduced cross sections of the reactionep → eccX the structure functionsFcc2 andFcc

L

are defined analogous to the inclusive case as

σccr = Fcc

2 − y2/Y+ ·FccL , (4.1)

and equivalent for the beauty contribution (bb). Up to now the published results are based onHERA I data taken by H1. TheFcc

2 measurements obtained by this technique and exclusive meth-ods and the first determination ofFbb

2 are shown in figure 12.

10-2

10-1

1

10

10 2

1 10 102

103

x=0.0002i=5

x=0.0005i=4

x=0.002i=3

x=0.005i=2

x=0.013i=1

x=0.032i=0

H1 DataH1 Data (High Q2)H1 D✽

ZEUS D✽

MRST04MRST NNLOCTEQ6HQ

Q2 /GeV2

F 2cc_ × 4

i

10-3

10-2

10-1

1

10

10 2

10 102

103

x=0.0002i=5

x=0.0005i=4

x=0.002i=3

x=0.005i=2

x=0.013i=1

x=0.032i=0

Q2 /GeV2

F 2bb_ × 8

i

H1 DataH1 Data (High Q2)MRST04MRST NNLOCTEQ6HQ

Figure 12: Determination of the heavy quark contributions to DIS with the impact parameter method by H1and quantified by the structure functionsFcc

2 andFbb2 [13, 14].

10


The charm contribution at lowx andQ2 is usually of the order 25% of the total DIS crosssection. The beauty contribution is smaller with only about 1% and depends strongly onQ2. Theproduction of heavy flavours is described by pQCD within the present measurement accuracy. Forthe HERA II phase ZEUS has installed a “Micro Vertex Detector” and H1 hasupgraded theirsilicon tracking in the forward and backward region. Both collaborations are in the process ofunderstanding the alignment of their new detectors and analysing the new data. An improvedprecision is expected from the measurements performed using the data collected at HERA II.

5. QCD Fits and Parton Distribution Functions

Measuring DIS inclusive cross sections always serves the twofold purpose of determiningthe parton distribution functions (PDFs), and testing our understanding ofthe theory of stronginteraction, QCD. For these goals it is necessary to analyse the structure functions in the frameworkof QCD fits. The scaling violations are predicted by the DGLAP equations andthe flavours canbe separated using fixed target deuteron data. HighQ2 CC data from the HERA experiments alsoprobe the flavour content.

Both H1 and ZEUS collaborations have performed this analyses with specialemphasis ontheir own data. The results are shortly reviewed here and differences inthe assumptions, inputdata, and goals are emphasised. All are based on solving NLO DGLAP equations and all give agood description of their input data in terms ofχ2/n.d. f .

5.1 H1 Gluon andαs fit using inclusive DIS HERA I + BCDMS Data

The goal of this fit [2] was to determine the gluon distributionxg(x,Q2) using H1 data (HERA I,Q2 < 3000 GeV2) alone from the scaling violations. A simultaneous determination of flavour sep-arated parton distributions was not aimed at in order to keep the number of fitparameters small.Therefore the fitted distributions were chosen asxg, V = 9

4uv − 32dv, andA = u− 1

4(uv −2dv) (va-lence and sea-like distributions). Together with additional BCDMS data, a competitive determina-tion of the strong coupling constantαs is possible and yields:

αs(M2Z) = 0.1150±0.0017(exp)+0.0009

−0.0005(model)±0.005(ren.scale) .

The extracted gluon distribution and theχ2 − χ2min distribution as a function ofαs are shown in

figure 13.

5.2 PDF Determination by H1 using inclusive DIS HERA I Data

Another fit subsequently performed by H1 is the so called H1PDF 2000 [15], using HERA Iinclusive DIS data. The goal is to determine the flavour separated parton distributions of the proton.The fitted distributions were chosen different from the conventional global analyses:xg, xU , xU ,xD, andxD, where e.g.xU = x(u + c) is the sum of all up-type quarks and similar for the otherdistributions. This choice of parametrisation reflects directly the quark distribution decompositionof the measured NC and CC cross sections, and it weakens the influence of assumptions about theflavour decomposition of the sea. The obtained results together with fits by theCTEQ and MRSTcollaborations are shown in figure 14.

11


H1

Col

labo

ratio

n

H1

Col

labo

ratio

n

Figure 13: The gluon distributionxg(x,Q2) and the strong coupling constantαs as determined by H1 by aNLO QCD fit to DIS data of H1 and BCDMS [2].

0

1

2

0

1

2

0

10

20

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10-2

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1 10-2

10-1

1

Q2=10 GeV2

xP(x

)

xU

xU

H1 PDF 2000MRST2001CTEQ6

Q2=1000 GeV2

xU

xU

xD

xD

xD

xD

xg

x

xg

H1

Col

labo

ratio

n

Figure 14: Results of the H1PDF2000 fit [15] in comparison with global fits.

5.3 ZEUS-S and ZEUS-JETS fits

The ZEUS collaboration has also published fit results to their DIS data. For the ZEUS-S fit[16] HERA I inclusive DIS data was used together with various fixed target data. In a more recentanalysis, the ZEUS-JETS fit [17], jet production data has been added,while the inclusive DIS datafrom other experiments is not used any more. The fitted distributions in both cases are the moreconventional setxuv, xdv, xS, xg, andx(d − u). Results of both fits are given in figure 15.

The addition of the jet data has the advantage, that the gluon evolution can beseparated betterfrom the value of the strong coupling constantαs. Therefore the fractional uncertainty of the gluondistribution is reduced and a competitive determination ofαs is achieved, see figure 16. The resultfor αs is

αs(M2Z) = 0.1182±0.0028(exp)± (0.0008)(model)±0.005(ren.scale) .

12


0

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uncorr. uncert. total exp. uncert.

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vxu

vxd

0.05)×xS (

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ZEUS−S PDF

ZEUS

xf

Figure 15: Fit results of the ZEUS-S and ZEUS-JETS analyses [17].

0.1 0.105 0.11 0.115 0.12 0.125 0.13

0

5

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20

25

ZEUS fit

with jet data without jet data

)Z(Msα

min

2 χ -

2 χ

tota

l

world average(Bethke 2004)

ZEUS

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Q

without jet data with jet data

2 = 2.5 GeV2

Q

2 = 7 GeV2Q 2 = 20 GeV2Q

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Figure 16: Results of the ZEUS-JETS fit:χ2 − χ2min distributions for theαs determination (right) and

fractional errors of the gluon distribution for fits with andwithout the use of jet data [17].

5.4 ZEUS-pol fit using inclusive DIS + Jet HERA I+II data

In a preliminary analysis [18], ZEUS has updated the JETS-fit by with polarised highQ2

HERA II e−p data to the ZEUS-pol fit. This leads to improved precision for the determination ofthe valence quark distributions at highx, see figure 17. Also fits with two of the electroweak (light)quark couplingsau,vu,ad,vd as free parameters were performed. The results presented in figure17 are compatible with the Standard Model and are competitive with determinationsfrom othercollider experiments. They show an improved precision over the HERA I results published by H1[19].

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−1

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Figure 17: Results of the ZEUS-Pol fits: improved accuracy for the valence quarks at highx (left) anddetermination of the light quark couplings (right) [18].

5.5 Status and Prospects for the PDFs

A precise knowledge of parton distribution functions is essential for predictions and under-standing the measurements at the LHC. As can be seen from figure 18, central (|η | . 2.5) pro-duction of heavy (M ∼ 100 GeV) particles will use all the PDF information obtained in HERAexperiments in the range 10−4 . x . 0.1. Up to now there has been fair agreement between thePDFs extracted with different approaches. The level of uncertainties can be judged from a com-parison of the “standard fits” by H1 and ZEUS, see figure 18. A reasonable agreement as well assome differences can be observed. For further discussion see also [5].

xU

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Figure 18: The kinematic plane in(x,Q2) extended by the measurements of the Tevatron experiments andthe region of interest of the LHC.

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To reduce the uncertainties, two things are important. First, further improvements of experi-mental input is possible in a number of areas. These include the ultimate precision results in thelow Q2 bulk and highy domain and the directFL measurement with the run at reduced centreof mass energy. Also the highQ2 DIS measurements with high luminosity will enable flavourdecomposition within one experiment.

Following the highest experimental precision, further steps have to be taken to fully exploitthese measurements. This should include a combination of all HERA data, where a proper treat-ment of the correlated errors may lead to reduction of systematic uncertainties. A NNLO QCDanalysis of the combined data, and cross checks and improvements with semi-inclusive data (e. g.jets and heavy quarks) would be necessary from the theoretical side.

6. Conclusion and Outlook

HERA has still potential at lowQ2 and improved results are expected soon due to a betterunderstanding of the systematic uncertainties. The run at lower proton beam energyEp expectedfor next year, which will allow to measure the fundamental structure function FL directly for thefirst time in the HERA domain. With increased luminosity and using the polarised electron andpositron beams, new and improved measurements will be possible at highQ2. The new silicondetectors will help to determine the heavy quark content of the proton better.Finally a reduction ofthe PDF uncertainties with improved experimental input, HERA combined data, and NNLO QCDanalysis will lead to valuable input for the LHC and will test our understanding of QCD.

References

[1] M. Breidenbachet al., “Observed Behavior Of Highly Inelastic Electron - Proton Scattering”, Phys.Rev. Lett.23 (1969) 935.

[2] C. Adloff et al. [H1 Collaboration], “Deep-inelastic inclusive e p scattering at low x and adetermination ofα (s)”, Eur. Phys. J. C21 (2001) 33 [arXiv:hep-ex/0012053].

[3] A. Petrukhin, “New measurement of the structure function F2(x,Q2) at low Q2 with initial stateradiation data”, H1prelim-04-042,Prepared for 12th International Workshop on Deep InelasticScattering (DIS 2004), Strbske Pleso, Slovakia, 14-18 Apr 2004

[4] E. M. Lobodzinska, “Determination of the longitudinal proton structure functionFL at low Q2 atHERA”, [arXiv:hep-ph/0311180].

[5] S. Alekhinet al., “HERA and the LHC - A workshop on the implications of HERA forLHC physics:Proceedings Part A”, 75-113, [arXiv:hep-ph/0601012].

[6] ZEUS Collaboration, “Polarised neutral current DIS cross sections with 2004/05e−p data andxF3”,ZEUS-prel-06-001,To appear in the proceedings of 33rd International Conference on High EnergyPhysics (ICHEP 06), Moscow, Russia, 26 Jul - 2 Aug 2006.

[7] H1 Collaboration, “Neutral Current Interactions ine±p Scattering with Longitudinally PolarisedLeptons”, H1prelim-06-042,To appear in the proceedings of 33rd International Conference on HighEnergy Physics (ICHEP 06), Moscow, Russia, 26 Jul - 2 Aug 2006.

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[8] H1 and ZEUS Collaborations, “Electroweak Neutral Currents at HERA”, H1prelim-06-142,ZEUS-prel-06-022,To appear in the proceedings of 33rd International Conference on High EnergyPhysics (ICHEP 06), Moscow, Russia, 26 Jul - 2 Aug 2006.

[9] ZEUS Collaboration, “Polarised charged current DIS cross sections with 2004/05 e-p data”,ZEUS-prel-06-002,To appear in the proceedings of 33rd International Conference on High EnergyPhysics (ICHEP 06), Moscow, Russia, 26 Jul - 2 Aug 2006.

[10] A. Aktaset al. [H1 Collaboration], “First measurement of charged currentcross sections at HERAwith longitudinally polarised positrons”, Phys. Lett. B634(2006) 173 [arXiv:hep-ex/0512060].

[11] H1 Collaboration, “ HighQ2 Charged Current in polarisedep collisions”, H1prelim-06-041,Toappear in the proceedings of 14th International Workshop on Deep Inelastic Scattering (DIS 2006),Tsukuba, Japan, 20-24 Apr 2006.

[12] R. S. Thorne, AIP Conf. Proc.792(2005) 847 [arXiv:hep-ph/0506251].

[13] A. Aktaset al. [H1 Collaboration], “Measurement ofFcc2 andFbb

2 at highQ2 using the H1 vertexdetector at HERA”, Eur. Phys. J. C40 (2005) 349 [arXiv:hep-ex/0411046].

[14] A. Aktaset al. [H1 Collaboration], “Measurement ofFcc2 andFbb

2 at low Q2 andx using the H1 vertexdetector at HERA”, Eur. Phys. J. C45 (2006) 23 [arXiv:hep-ex/0507081].

[15] C. Adloff et al. [H1 Collaboration], “Measurement and QCD analysis of neutral and charged currentcross sections at HERA”, Eur. Phys. J. C30 (2003) 1 [arXiv:hep-ex/0304003].

[16] S. Chekanovet al. [ZEUS Collaboration], “A ZEUS next-to-leading-order QCD analysis of data ondeep inelastic scattering”, Phys. Rev. D67 (2003) 012007 [arXiv:hep-ex/0208023].

[17] S. Chekanovet al. [ZEUS Collaboration], “An NLO QCD analysis of inclusive cross-section andjet-production data from the ZEUS experiment”, Eur. Phys. J. C 42 (2005) 1 [arXiv:hep-ph/0503274].

[18] ZEUS Collaboration, “QCD and Electroweak analysis of the ZEUS NC and CC inclusive and jetcross sections”, ZEUS-prel-06-003,To appear in the proceedings of 14th International Workshop onDeep Inelastic Scattering (DIS 2006), Tsukuba, Japan, 20-24 Apr 2006.

[19] A. Aktaset al. [H1 Collaboration], “A determination of electroweak parameters at HERA”, Phys. Lett.B 632(2006) 35 [arXiv:hep-ex/0507080].

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Inclusive DIS Results from HERAbib-pubdb1.desy.de/record/79250/files/kretzschmardiff06.pdf · 2017. 9. 11. · Inclusive DIS Results from HERA Jan Kretzschmar 1. Introduction The