ep collisions with the zeus detector at...

October, 1999W. Smith, U. Wisconsin

ep Collisions with the Zeus Detector at HERA

Wesley Smith,U. Wisconsin

Outline:• Proton Structure• Photon Structure• Conclusions

W. Smith, U. Wisconsin October, 1999

H1

ZEUS

p

e

North Hall

South Hall

EastHall

HERMESHERA-B

WestHall

Collider:•√s= 300 GeV

•Equivalent to 47 TeV fixed target

Experiments:•2 general purpose detectors

•H1 & Zeus

•Dedicated Fixed Target •HERMES:

•Polarized electrons on polarized H target•HERA-B

•Proton Halo on wire target

820 GeV p x 27 GeV e±(920 GeV p in 1999)

HERA

W. Smith, U. Wisconsin October, 1999

ZEUS CollaborationManitoba, McGill,Toronto, York

CANADABonn, DESY, DESY-Zeuthen, Freiburg,Hamburg I,

Hamburg II, Julich, SiegenGERMANY

Tel Aviv, WeizmannISRAEL

Bologna, Cosenza, Florence, Frascati-Rome, Padua,La Sapienza-Rome, Turin

ITALYTokyo-INS, Tokyo-Metropolitan

JAPANSeoul

KOREANIKHEF-AmsterdamThe NETHERLANDS

Cracow, WarsawPOLANDMoscowRUSSIAMadridSPAIN

Bristol, London(I.C.), London(U.C.),Oxford, RutherfordUNITED KINGDOM

Andrews, Argonne, Brookhaven, Columbia, Iowa,Ohio State, Pennsylvania State,

U.C. Santa Cruz, Wisconsin, Yale U.S.A.

50 Total Institutions420 Total Physicists

W. Smith, U. Wisconsin October, 1999October, 1999

HERA luminosity 1992 – 99

5

10

15

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25

30

35

5

10

15

20

25

30

35

Inte

grat

ed L

umin

osity

(pb

-1)

Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec

19921993

1994

1995

19961997

1998

1999 1999

18.10.

HERA Data Runs ZEUS Data Samples:

•48 pb-1 e+p (1994-1997)•22 pb-1 e-p (1998-1999)•1999: 15.8 pb-1 e+p (as of Oct. 18)

October, 1999W. Smith, U. WisconsinW. Smith, U. Wisconsin

Deep Inelastic Scattering

x = the fraction of the proton's momentum carried by the struck parton

y = the fraction of the electron's energy lost in the proton rest frame

s = (k + P)2 = center of mass energy

Q2 = -q2 = -(k-k')2 = (momentum transferred)2

Q2 = sxyy = P•qP•kx = Q

2

2P•q

e±(k')e±(k)

(q)xP

p(P)

The DIS process

Measuring DIS at HERA:

electron (27 GeV) proton (820 GeV)

electron

jet

proton remnant

Forward Rear

beampipe


Photoproduction

photon remnant

Proton Remnant

Jet

Jetelectron goes down the

beampipe (low Q2)

photon remnant

q

q

q

q

Q2 < 4 GeV2

Almost real photon

θ > 170°

Jet

Jet

Proton RemnantProton Remnant

Direct: Resolved:

(Jet)

(Jet)

(Jet)

(Jet)(Jet)

γ γ

gg

(down beampipe)

(towards rear)

Background for DIS


DIS cross section

DIS differential cross section:

γ* is longitudinally or transversely polarizedF2(x,Q

2) = Structure function = interaction btw. transversely polarized photons & spin 1/2 partons = charge weighted sum of the quark distributions.

FL(x,Q2) = Structure function = cross section due

to longitudinally polarized photons that interact with the proton. The partons that interact have transverse momentum. (Important at high y).

F3(x,Q2) = Parity-violating structure function from

Z0 exchange. (Important at high Q2).

e±(k')e±(k)

p(P)

α

α

γ∗(q)

JetJet

Y± = 1± (1− y)2

dσ NC (e± p)dxdQ2

= 2πα 2xQ4

Y+ F2 −y2

Y+FL m

Y−Y+

xF3


Two High -Q2 DIS NC events


Low Q2 Measurements

+e

BPC

Zeus Beampipe Calorimeter:

H1 & Zeus Shifted Vertex:

H1 & Zeus Initial State Radiation


HERA Kinematic Range

F2 measurements span large region not accessible by fixed target experiments, from soft to hard scattering, with some overlap

10-1

1

10

10 2

10 3

10 4

10 5

10-6

10-5

10-4

10-3

10-2

10-1

1x

Q2

(GeV

2 )

kine

mat

ic lim

it y=

1

y = 0.

005

H1,ZEUS 1996+97

H1,ZEUS 1994

H1,ZEUS SVX 1995

ZEUS BPC 1995

ZEUS BPT 1997

E665

BCDMS

CCFR

SLAC

NMC


Kinematic reconstruction

Two Kinematic Variables• x,Q2

Four Measured Quantities• Ee’,θe’,Eh,γh

Energy Conservation• Σ = E-Pz = 2Ee - this quantity summed over all calorimeter cells yields a measure less sensitive to noise, jet fluctuations and lost proton remnant.

Multiple Reconstruction Methods• Electron Method ( Ee’,θe’)

•Poor resolution at low y•Classic Fixed Target Technique

• Double Angle Method (θe’,γh)•Less sensitive to energy scale•Subject to calorimeter noise at low Q2

•Commonly used by ZEUS• Jaquet-Blondel Method (Eh,γh)

•Hadronic energy mis-measurement results in large systematic error.

•Only possibility for Charged Current events

E’e,θ’e

Eh,γh


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10000

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30000

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50000

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Data

MC

Photo MC

Rapgap MC

Positron Energy (GeV)

Eve

nts

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20000

25000

30000

35000

40000

30 40 50 60 70

E-Pz (GeV)

Eve

nts

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10 2

10 3

10 4

10 5

0 50 100 150

Positron Theta (deg)

Eve

nts

1

10

10 2

10 3

10 4

-200 -100 0 100 200

Vertex Z (cm)

Eve

nts

Kinematic Reconstruction

Electron energy, E-Pz , electron angle & Z vertex used to measure F2

Data shown vs. DIS, Photoprod., Diffractive MC


0

2500

5000

7500

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17500

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22500

-3 -2 -1 0

Data

MC

Photo MC

Rapgap MC

log10(yPT)

Eve

nts

0

2000

4000

6000

8000

10000

12000

14000

-5 -4 -3 -2 -1 0

log10(xPT)

Eve

nts

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10

10 2

10 3

10 4

0 1 2 3 4 5

log10(Q2 PT GeV2)

Eve

nts

0

5000

10000

15000

20000

25000

0 50 100 150

γPT (deg)

Eve

nts

Reconstructed Kinematics

Reconstructed x, y, Q2, & hadron shower angle (γh)• Photoproduction bkgd. contributes mostly at high y• x & Q2 used for binning & unfolding of F2


In the naive parton model the structure function F2 is given by the charge weighted sum of parton momentum densities that depends only on x (scaling).Perturbative QCD provides a scheme to characterize the Q2 dependence (scaling violations):

Perturbative QCD

Dokshitzer-Gribov-Lipatov-Altarelli-Parisi equations describe evolution of parton densities to higher Q2

Calculation of DIS cross section requires FL:

The parameterization of gluon density can be determined by fitting QCD evolution to DIS data.

dqi (x,Q2 )

d lnQ2= αs (Q

2 )2π

dwwx

1∫ qi (w,Q

2 )Pqqxw

+ g(w,Q

2 )Pqgxw

dg(x,Q2 )d lnQ2

= αs (Q2 )

2πdwwx

1∫ qi (w,Q

2 )i∑ Pgq

xw

+ g(w,Q

2 )Pggxw

F2 (x,Q2 ) = ei

2xqi (x,Q2 )

i∑

FL (x,Q2 ) = αs (Q

2 )π

dww

xw

2

x

1∫

43

F2 (w,Q2 ) + 2 ei

2

i∑ 1− x

w

wg(w,Q

2 )

Given an empirical parameterization for parton densities at Q2=Q0

2 ,e.g.:

xg(x) = Agxδg (1− x)ηg (1+ γ gx)


0

0.25

0.5

0.75

1

Q2 = 1.5ZEUS 96,97ZEUS 94NMCZEUS 96,97 NLOCTEQ4D

F2

Q2 = 2.7 Q2 = 3.5

0

0.5

1

1.5

Q2 = 4.5

F2

Q2 = 6.5 Q2 = 8.5

0

0.5

1

1.5

2

-4 -2 0

Q2 = 10.

log10(x)

F2

-4 -2 0

Q2 = 12.

log10(x)

-4 -2 0

Q2 = 15.

log10(x)

New F2 vs. x at Lower Q2

Can reach lower Q2 than 1994 data Steep rise in F2 with decreasing x seen at low Q2


0

0.25

0.5

0.75

Q2 = 2000.0ZEUS 96,97ZEUS 94NMCZEUS 96,97 NLOCTEQ4D

F2

Q2 = 3000.0 Q2 = 5000.0

0

0.25

0.5

0.75

Q2 = 8000.0

F2

-2 -1 0

Q2 = 12000.0

log10(x)

-2 -1 0

Q2 = 20000.0

log10(x)

0

0.25

0.5

0.75

-2 -1 0

Q2 = 30000.0

log10(x)

F2

New F2 vs. x at Higher Q2

Extension of measurement to higher Q2 Steep rise in F2 with decreasing x at high Q2 confirmed

Now exploring valencequark region


F2 vs. Q2 from '96-'97 Data

Scaling violation increasing at low x QCD fits to scaling violation yields gluon

ZEUS Preliminary

x = 0.000102

01

x = 0.000161

01 x = 0.000253

01 x = 0.000400

01 x = 0.000500

01 x = 0.000632

01 x = 0.000800

01 x = 0.001020

01

x = 0.001300

01

x = 0.001610

01

x = 0.002100

01

x = 0.002530

01

x = 0.003200

01

x = 0.005000

01

x = 0.00800001

x = 0.01300001

x = 0.02100001

x = 0.03200001

x = 0.05000001

x = 0.08000001

x = 0.13000001

x = 0.18000001

x = 0.25000001

x = 0.40000001

x = 0.65000001

1 10 102

103

104

105

Log10(Q2 GeV2)

F2

ZEUS 96,97

NMC

ZEUS Fit 96,97

CTEQ4D


Charm Production in DIS...a more direct probe of the gluon

c

c-

e’e

pg

X

γ

Boson-gluon fusion process:

F2charm is the charm contribution to F2 Charm almost exclusively from boson-gluon fusion From F2charm, gluon content can be extracted


Zeus Structure Functions

Medium to High Q2:• NLO-pQCD provides a self consistent description of the data from Q2 = 1 to 20,000 GeV2.

• Strong rise of F2 towards low x seen across kinematic range

• New precise (1% stat + 3% syst) higher statistics data

• Measurement of F2 provides additional consistency check

• Good agreement between HERA and fixed target• Clear scaling violations enable calculation of gluons with 10-15% precision

• Gluon vs. sea density at Q2 = 1?

Low Q2:• Observed transition from perturbative to non perturbative regime

• Regge models can describe the data• At lowest Q2 a soft Pomeron is sufficient• At higher Q2, need a hard/variable α Pomeron• Need to understand coexistence of high Q2 pQCD and low Q2 Regge Theory

c


γ*p Total Cross Section

HERA is a photon-proton collider

At low Q2 , a source of almost real photons:

•Can measure cross section over a wide range of γ*p center of mass energies, W

• ...also study transition region between non-perturbative and perturbative QCD in the range 0.3


Charged Current DIS

d u

ν_

W

e+

+

Cross Section for e�p �� X �

d��CCdxdQ�

�G�F

��

�

�� Q��m�W��

��u� �c� �� y��d � s�

�

� For e�p scattering the dominating contributionto the cross section comes from the d quark

� Largest theoretical error arises fromuncertainty of the d quark density

� The main experimental uncertainty is thehadronic energy scale of the calorimeter


Charged Current Events


Jets in photoproduction


0

5

10

15

20

25

30

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Jet Production (Real Photons)

xγOBS < 0.75 - Resolved Enrichedxγ

OBS > 0.75 - Direct Enriched

Use jets to probe photon structure•Scale ~E

T2 of jets

HERA can probe higher scales than e+e-•e+e- scale < 400 GeV2

•HERA scale ~30 - ~1000 GeV2 (Higher w/ more luminosity)


Jet Production (Virtual Photons)

Probe virtual photon structure using jets•Large virtuality range available

DIS (Q2>0) usually assumes all structure is from proton (pointlike photon)

•When ET

2 > Q2 photon structure is important (small part of phase space)


Future Outlook1998-1999:

• Modifications to HERA• e-p running• Expect similar luminosity as e+p

2000-2005:• Major HERA upgrade• New Silicon Vertex Detector

•Charm tags: F2(charm)• Factor of 7 increase in luminosity

•1 x 10 31 to 7 x 1031

• Higher Proton energy: > 900 GeV?• Polarized electrons & positrons• Goal of 1 fb-1 by 2005• Beyond: polarized protons?

Physics expectations by 2005:• αs(Mz) measured to .001• xg(x) measured to 1%

• F2: does the rise at low-x continue?, xF

3• Quark couplings from NC, CC polarized e+/-

• WWγ couplings• Leptoquarks?


Conclusions - INew x,Q2 range in Deep Inelastic Scattering

• 0.11 < Q2 < 5000 GeV2 & down to x ~10-5 & growing • F2 observed to rise rapidly as x decreases• F2(charm) contributes large fraction (~25%) to F2

pQCD fits to the F2 data, with DGLAP evolution• fit the data well even down to Q2~1.5 GeV2

F2 measured at low Q2

• Transition between perturbative and soft regime • γ*p cross section measured vs. W

The gluon distribution has been extracted• Steep rise of gluon with decreasing x is observed

High Q2 Neutral Current data:• Typical systematic error ~ 2-3%• Statistically limited for Q2 > 1000 GeV2

• Consistent w/SM up to Q2 ~ 30000 GeV2

• 2 exceptional events at Q2 > 35000 GeV2 ( 10000 GeV2

• Consistent w/SM up to Q2 ~ 10000 GeV2

• 1 exceptional event at Q2 > 30000 GeV2 (


Conclusions - II Jets in Photoproduction:

• Photon structure probed at scales up to ~103 GeV2 and higher

Real Photons• Jet cross sections sensitive to different photon parton density parametrisations

• Jet data give new constraints on photon parton distribution functions (higher scale)

Virtual Photons• Virtual photon structure probed across large range of virtuality

• Virtual photon structure observed to be important when Q2


Zeus Photoproduction

Dijets• General agreement with models

•worse at high Et • Resolved reduced but remainspresent at higher Et

Isolated High-Et (Prompt) Photons• Consistent with LO QCD expectations• Measured cross section and η-distribution consistent with NLO QCD prediction

•σ = 15.3 +/- 3.8 (stat) +/- 1.8 sys pb•GRV favored over GS photon PDF

Virtual Photon Structure• Resolved (low xγ) processes are suppressed with increasing photon virtuality

• LO Resolved needed to describe the data at the highest photon virtuality measured(Q2 = 4.5 GeV2)

ep Collisions with the Zeus Detector at HERAOutline:Proton StructurePhoton StructureConclusions

HERACollider:Ãs= 300 GeVEquivalent to 47 TeV fixed target

Experiments:2 general purpose detectors H1 & Zeus

Dedicated Fixed Target HERMES:Polarized electrons on polarized H target

HERA-BProton Halo on wire target

ZEUS CollaborationManitoba, McGill,Toronto, YorkCANADABonn, DESY, DESY-Zeuthen, Freiburg,Hamburg I,Hamburg II, Julich, SiegenGERMANYTel Aviv, WeizmannISRAELBologna, Cosenza, Florence, Frascati-Rome, Padua,La Sapienza-Rome, TurinITALYTokyo-INS, Tokyo-MetropolitanJAPANSeoulKOREANIKHEF-AmsterdamThe NETHERLANDSCracow, WarsawPOLANDMoscowRUSSIAMadridSPAINBristol, London(I.C.), London(U.C.),Oxford, RutherfordUNITED KINGDOMAndrews, Argonne, Brookhaven, Columbia, Iowa,Ohio State, Pennsylvania State, U.C. Santa Cruz, Wisconsin, Yale U.S.A.50 Total Institutions420 Total Physicists

HERA Data RunsZEUS Data Samples:48 pb-1 e+p (1994-1997)22 pb-1 e-p (1998-1999)1999: 15.8 pb-1 e+p (as of Oct. 18)

Deep Inelastic Scatteringx = the fraction of the proton's momentum carried by the struck parton

PhotoproductionAlmost real photon

DIS cross sectionTwo High -Q2 DIS NC eventsLow Q2 MeasurementsZeus Beampipe Calorimeter:

HERA Kinematic Range Kinematic reconstructionTwo Kinematic Variablesx,Q2

Four Measured QuantitiesEeÕ,qeÕ,Eh,gh

Energy ConservationS = E-Pz = 2Ee - this quantity summed over all calorimeter cells yields a measure less sensitive to noise, jet fluctuations and lost proton remnant.

Multiple Reconstruction MethodsElectron Method ( EeÕ,qeÕ)Poor resolution at low yClassic Fixed Target Technique

Double Angle Method (qeÕ,gh)Less sensitive to energy scaleSubject to calorimeter noise at low Q2Commonly used by ZEUS

Jaquet-Blondel Method (Eh,gh)Hadronic energy mis-measurement results in large systematic error.Only possibility for Charged Current events

Kinematic ReconstructionElectron energy, E-Pz , electron angle & Z vertex used to measure F2Data shown vs. DIS, Photoprod., Diffractive MC

Reconstructed Kinematics Reconstructed x, y, Q2, & hadron shower angle (gh)Photoproduction bkgd. contributes mostly at high yx & Q2 used for binning & unfolding of F2

Perturbative QCDNew F2 vs. x at Lower Q2Can reach lower Q2 than 1994 dataSteep rise in F2 with decreasing x seen at low Q2

New F2 vs. x at Higher Q2Extension of measurement to higher Q2Steep rise in F2 with decreasing x at high Q2 confirmed

F2 vs. Q2 from '96-'97 DataScaling violation increasing at low xQCD fits to scaling violation yields gluon

Charm Production in DISF2charm is the charm contribution to F2Charm almost exclusively from boson-gluon fusionFrom F2charm, gluon content can be extracted

Zeus Structure FunctionsMedium to High Q2:NLO-pQCD provides a self consistent description of the data from Q2 = 1 to 20,000 GeV2. Strong rise of F2 towards low x seen across kinematic range New precise (1% stat + 3% syst) higher statistics dataMeasurement of F2 provides additional consistency checkGood agreement between HERA and fixed targetClear scaling violations enable calculation of gluons with 10-15% precisionGluon vs. sea density at Q2 = 1?

Low Q2:Observed transition from perturbative to non perturbative regimeRegge models can describe the dataAt lowest Q2 a soft Pomeron is sufficientAt higher Q2, need a hard/variable a PomeronNeed to understand coexistence of high Q2 pQCD and low Q2 Regge Theory

g*p Total Cross SectionHERA is a photon-proton collider

At low Q2 , a source of almost real photons:Can measure cross section over a wide range of g*p center of mass energies, W...also study transition region between non-perturbative and perturbative QCD in the range 0.3 Q2 photon structure is important (small part of phase space)

Future Outlook1998-1999:Modifications to HERAe-p runningExpect similar luminosity as e+p

2000-2005:Major HERA upgradeNew Silicon Vertex DetectorCharm tags: F2(charm)

Factor of 7 increase in luminosity1 x 10 31 to 7 x 1031

Higher Proton energy: > 900 GeV?Polarized electrons & positronsGoal of 1 fb-1 by 2005Beyond: polarized protons?

Physics expectations by 2005:as(Mz) measured to .001xg(x) measured to 1%F2: does the rise at low-x continue?, xF3Quark couplings from NC, CC polarized e+/-WWg couplingsLeptoquarks?

Conclusions - INew x,Q2 range in Deep Inelastic Scattering0.11 < Q2 < 5000 GeV2 & down to x ~10-5 & growing F2 observed to rise rapidly as x decreasesF2(charm) contributes large fraction (~25%) to F2

pQCD fits to the F2 data, with DGLAP evolutionfit the data well even down to Q2~1.5 GeV2

F2 measured at low Q2 Transition between perturbative and soft regime g*p cross section measured vs. W

The gluon distribution has been extractedSteep rise of gluon with decreasing x is observed

High Q2 Neutral Current data:Typical systematic error ~ 2-3%Statistically limited for Q2 > 1000 GeV2Consistent w/SM up to Q2 ~ 30000 GeV22 exceptional events at Q2 > 35000 GeV2 ( 10000 GeV2Consistent w/SM up to Q2 ~ 10000 GeV21 exceptional event at Q2 > 30000 GeV2 (

ep collisions with the zeus detector at...

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