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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
20
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
October, 1999W. Smith, U. WisconsinW. Smith, U. Wisconsin
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
October, 1999W. Smith, U. WisconsinW. Smith, U. Wisconsin
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
October, 1999W. Smith, U. WisconsinW. Smith, U. Wisconsin
Two High -Q2 DIS NC events
W. Smith, U. Wisconsin October, 1999October, 1999
Low Q2 Measurements
+e
BPC
Zeus Beampipe Calorimeter:
H1 & Zeus Shifted Vertex:
H1 & Zeus Initial State Radiation
W. Smith, U. Wisconsin October, 1999October, 1999
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
W. Smith, U. Wisconsin October, 1999October, 1999
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
W. Smith, U. Wisconsin October, 1999October, 1999
0
10000
20000
30000
40000
50000
0 10 20 30
Data
MC
Photo MC
Rapgap MC
Positron Energy (GeV)
Eve
nts
0
5000
10000
15000
20000
25000
30000
35000
40000
30 40 50 60 70
E-Pz (GeV)
Eve
nts
1
10
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
W. Smith, U. Wisconsin October, 1999October, 1999
0
2500
5000
7500
10000
12500
15000
17500
20000
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
1
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
W. Smith, U. Wisconsin October, 1999October, 1999
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)
W. Smith, U. Wisconsin October, 1999October, 1999
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
W. Smith, U. Wisconsin October, 1999October, 1999
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
W. Smith, U. Wisconsin October, 1999October, 1999
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
W. Smith, U. Wisconsin October, 1999October, 1999
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
W. Smith, U. Wisconsin October, 1999October, 1999
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
W. Smith, U. Wisconsin October, 1999October, 1999
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
October, 1999W. Smith, U. WisconsinW. Smith, U. Wisconsin
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
October, 1999W. Smith, U. WisconsinW. Smith, U. Wisconsin
Charged Current Events
October, 1999W. Smith, U. WisconsinW. Smith, U. Wisconsin
Jets in photoproduction
W. Smith, U. Wisconsin October, 1999October, 1999
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)
W. Smith, U. Wisconsin October, 1999October, 1999
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)
W. Smith, U. Wisconsin October, 1999October, 1999
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?
W. Smith, U. Wisconsin October, 1999October, 1999
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 (
W. Smith, U. Wisconsin October, 1999October, 1999
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
W. Smith, U. Wisconsin October, 1999October, 1999
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 (