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1Bernhard A. Mecking Bosen Student Workshop, September 2003
The Physics of CLAS
Bernhard A. MeckingJefferson Lab
20th Student Workshop on Electromagnetic InteractionsBosen, August 31 – September 5, 2003
CEBAF at Jefferson LabCLAS
technicalphysics programplanned upgrades
CEBAF upgrade plans
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2Bernhard A. Mecking Bosen Student Workshop, September 2003
CEBAF @ Jefferson Lab
• Main physics programs– nucleon electromagnetic form factors (incl. strange)– N N* electromagnetic transition form factors– spin structure functions of the nucleon– form factors and structure of light nuclei
• Superconducting recirculating electron accelerator– max. energy 5.7 GeV– max current 200 µA– e polarization 80%
• Experimental equipment in 3 halls (simultaneous operation) L[cm-2s-1]– 2 High Resolution Spectrometers (pmax=4 GeV/c) 1039
– 2 spectrometers (pmax=7 and 1.8 GeV/c) + special equipment 1039
– Large Acceptance Spectrometer for e and γ induced reactions 1034
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3Bernhard A. Mecking Bosen Student Workshop, September 2003
CEBAF Continuous Electron Beam Accelerator Facilityrecirculating
arcs
acceleratingstructures
CHL
RF separators
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4Bernhard A. Mecking Bosen Student Workshop, September 2003
CEBAF Site
Hall C
Hall B
Hall A
south linacnorth linac
injector
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5Bernhard A. Mecking Bosen Student Workshop, September 2003
Electron Beam Properties
Polarization• strained GaAs photocathode gives polarization up to 80%
• spin precession controlled by combination of Wien filter and linac energy setting (allows perfect spin alignment for two-hall operation, approximate for 3 halls)
Beam properties• injector produces 3 separate beams via 3 pulsed (500 MHz) lasers on a common
photocathode
• intensity ratio up to 106:1 (100µA in A/C vs. 100pA in B)
• energies in halls need to be a multiple of common linac energy setting
• max. beam power 1MW (e.g. 200µA at 5 GeV)
• typical beam spot size 100µm, momentum spread 10-4
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6Bernhard A. Mecking Bosen Student Workshop, September 2003
Electron Beam Profile
measurement performed moving 25µm wire through beam
using downstream photomultipliers as radiation detectors
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7Bernhard A. Mecking Bosen Student Workshop, September 2003
Hall B Physics Program
Areas Covered– excitation of N* resonances (elementary and off nuclei) – nucleon spin structure functions– hadronic final states in electron scattering off nuclei
Common Experimental Requirements– detection of >2 loosely correlated particles in the final state– high counting rates for experiments that have luminosity
limitations due to:• tagged photon beam (intensity limited by accidental coincidences)• polarized target operation (current limited by cooling + radiation
damage)
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8Bernhard A. Mecking Bosen Student Workshop, September 2003
Hall B Instrumentation
CEBAF Large Acceptance Spectrometer, CLAS–for operation with electron and photon beams–use missing mass technique -> good resolution for charged
particles–good particle identification–high luminosity operation
Trigger and Data Acquisition–programmable flexible trigger–high-speed data acquisition system
Beam Line Equipment–beam position, current, and polarization monitoring–polarized photon beam and bremsstrahlung tagging system
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9Bernhard A. Mecking Bosen Student Workshop, September 2003
Hall B Side View
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10Bernhard A. Mecking Bosen Student Workshop, September 2003
Bremsstrahlung Tagging System Layout
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11Bernhard A. Mecking Bosen Student Workshop, September 2003
CLAS 3-D View
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12Bernhard A. Mecking Bosen Student Workshop, September 2003
CLAS in Maintenance Position
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13Bernhard A. Mecking Bosen Student Workshop, September 2003
Characteristics of CLAS Components
• Charged particle tracking in six independent sectors– 3 drift chamber packages per sector– 34 layers (axial and stereo)– drift time recorded from 35,000 sense wires
• Threshold Cerenkov counters for e identification– C4F10 gas radiator– focussing mirror system– 250 PMT’s, time and charge recorded
• Scintillation time-of-flight counters– 5 cm thick scintillators with PMT’s at both ends– 600 PMT’s, time and charge recorded
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14Bernhard A. Mecking Bosen Student Workshop, September 2003
• Electromagnetic calorimeters– lead-scintillator sandwich construction, 39 layers– 1,300 PMT’s, time and charge recorded
• Beam line equipment– Moller polarimeter to measure electron polarization– bremsstrahlung tagging system with crystal radiator, 500 PMT’s– cryogenic (H, D, 3,4He) or polarized targets (H, D)
• Electronics and data acquisition– programmable two-level trigger system (custom design)– mostly commercial data conversion modules (18 FastBus, 5 VME crates)– parallel data readout into multi-processor on-line DAQ system
CLAS Components (cont’d.)
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15Bernhard A. Mecking Bosen Student Workshop, September 2003
CLAS Top View
Drift ChambersRegion 1Region 2Region 3
TOF Counters Cerenkov Counters
Large-angle CalorimeterElectromagnetic Calorimeter
1 m
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16Bernhard A. Mecking Bosen Student Workshop, September 2003
CLAS Rear ViewDrift Chambers
Region 1Region 2Region 3
TOF Counters
Main Torus Coils
Mini-torus Coils1 m
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17Bernhard A. Mecking Bosen Student Workshop, September 2003
Characteristics of CLAS Components
Super-conducting toroidal magnet with six kidney-shaped coils5 m diameter, 5 m long, 5 M-Amp-turns, max. field 2 Tesla
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18Bernhard A. Mecking Bosen Student Workshop, September 2003
CLAS Lines of Constant Field
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19Bernhard A. Mecking Bosen Student Workshop, September 2003
CLAS Forward Calorimeter Layout
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20Bernhard A. Mecking Bosen Student Workshop, September 2003
Forward Calorimeter Hit Pattern
1
2 3
4
56
31 4
6
2
5
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21Bernhard A. Mecking Bosen Student Workshop, September 2003
CLAS Single Event Display
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22Bernhard A. Mecking Bosen Student Workshop, September 2003
Trajectory Reconstruction Resolution
Pe (GeV)
0
0.0025
0.005
0.0075
0.01
1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25
∆Pe/Pe
Electrons
Monte Carlo (θ=35o)σ(3375 A)
σ(2250 A) / 1.5
Pe (GeV)
0
0.5
1
1.5
2
1.25 1.5 1.75 2 2.25 2.5 2.75 3 3.25
∆θe (
mra
d)
Monte Carlo (θ=35o)
σ(3375 A)σ(2250 A)
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23Bernhard A. Mecking Bosen Student Workshop, September 2003
Scintillation Counter Timing Resolution
0
20
40
60
80
100
120
140
160
180
200
0 50 100 150 200 250 300 350 400 450 500
Res
olut
ion
σ (p
s)
Scintillator Length (cm)
15 cm22 cm
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24Bernhard A. Mecking Bosen Student Workshop, September 2003
CLAS Luminosity for e-
0
0.5
1
1.5
2
2.5
3
3.5
0 0.5 1 1.5 2 2.5 3A / Z
Lum
inos
ity
(1034
nuc
leon
cm
-2 s
-1)
1H
3He
4He
12C
56Fe
NH3
(1H,2H)
2H
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25Bernhard A. Mecking Bosen Student Workshop, September 2003
Missing Mass Distribution
0 0.2 0.4 0.6 0.8 1 1.2 1.40
20
40
60
80
100
120
140
0π
η
0ρ/ω
’η
)2
MASS (GeV/c
)2C
OU
NT
S (
x10
0 0.2 0.4 0.6 0.8 1 1.2 1.40
20
40
60
80
100
120
140
γp pX
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26Bernhard A. Mecking Bosen Student Workshop, September 2003
1
10
10 2
10 3
10 4
10 5
0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5Mass by TOF, GeV
Even
ts π+
K+
1H
2H
3H
3He
Full spectra
∆E > 80 MeV
Mass/Z (GeV)
Mass Determination from p and β
all events
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27Bernhard A. Mecking Bosen Student Workshop, September 2003
z-Vertex Determination from Tracking
0
2000
4000
6000
8000
10000
12000
14000
-8 -6 -4 -2 0 Vertex (cm)
σ=0.2 cm
vertex determination used routinely for experiments with multiple targets in the beam:
• simultaneous LH2 and LD2
• multiple target foils
liquid hydrogen
heat shield
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28Bernhard A. Mecking Bosen Student Workshop, September 2003
Trajectory Reconstruction Distortions
Explored in e p e’π+ (n)Possible reasons
• drift chamber positions not known perfectly
• uncertainties in magnetic field
before corrections after momentum corrections
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29Bernhard A. Mecking Bosen Student Workshop, September 2003
Calorimeter Detection Efficiency for n
0 0.5 1 1.5 2 2.5 30
0.1
0.2
0.3
0.4
0.5
0.6
Forward Calorimeter
Large Angle Calorimeter
Neutron Momentum (GeV/c)
det
ecti
on
eff
icie
ncy
using tagged neutrons from e p e’π+ (n)
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30Bernhard A. Mecking Bosen Student Workshop, September 2003
Cross Section for e-p Scattering
d dσ Ω
*(
b/s
r)µ
d dσ Ω
*d d
σ Ω*
()
()
radi
ated
data
θc.m.(deg)
θc.m.(deg)
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31Bernhard A. Mecking Bosen Student Workshop, September 2003
CLAS Run and Analysis Conditions
Runluminosity 1034 cm-2 s-1
electromagnetic rate 109 / shadronic production rate 106 / strigger (Level I) on e- candidates (Cerenkov + calorimeter)trigger rate (max.) 4,000/sdata rate to disk (max.) 25 MB/s (~BaBar, CLEO, ½ RHIC STAR)data volume to silo (max.) 1 TeraByte/day)personnel 2 on shift,
on call: 7 system experts + engineering-on-call
Analysisfirst-pass analysis at JLab compute farm physics analysis at JLab for full data set
at outside institutions for filtered data
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32Bernhard A. Mecking Bosen Student Workshop, September 2003
W-Dependence of Selected Channels at 4 GeV
p(e,e’)X(trigger)
p(e,e’p)X
p(e,e’π+)X
p(e,e’pπ+)X
p(e,e’pπ+)X
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33Bernhard A. Mecking Bosen Student Workshop, September 2003
N* Program Physics Goals
• Understand QCD in the strong coupling regime– example: bound qqq systems
– mass spectrum, quantum numbers of nucleon excited states– what are the relevant degrees-of-freedom– wave function and interaction of the constituents
• Source of information – dominated by pion-induced reactions (mostly πN πN)
– advantage:• strong coupling large cross sections• simple spin structure• good quality beams
– disadvantage: no structure informationinsensitive to states with weak πN coupling
π π
N
N*
N
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34Bernhard A. Mecking Bosen Student Workshop, September 2003
hω1 hω2 hω3hω0
1
0
2
3
3q
(56,0+)(56,0+) (70,0+)
(70,1-) (20,1+) (56,1-) (70,1-)
(70,1-) (20,1-)
(70,2-)(70,2+)(56,2+)
(56,3-) (70,3-) (20,3-)
L
Particle Data Group
(1135 MeV) (1545 MeV) (1839 MeV) (2130 MeV)N
(Mass)
3/2
3/21/2
1/23/25/2
5/2
1/2
7/2
***
****
**
Lowest Baryon Supermultiplets
SU(6)xO(3) Symmetry
Quark Model Classification of N*
∆(1232)
“Missing”P13(1870)
Capstick and Roberts
D13(1520)S11(1535)
Roper P11(1440)S11(1790)
Non-quark Model State
D13(1895)Mart and Bennhold
?
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35Bernhard A. Mecking Bosen Student Workshop, September 2003
Theoretical Models for N* Resonances
• Constituent quark model– 3 constituent quarks – all 3 contribute to number of states – non-relativistic treatment (typically)
• Refinements of the constituent quark model– restore relativity– hadronic form factors– coupling between decay channels
• Lattice gauge calculations
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36Bernhard A. Mecking Bosen Student Workshop, September 2003
Electromagnetic Excitation
• helicity amplitudes very sensitive to the difference in wave functions of Nand N*
• can separate electric and magnetic parts of the transition amplitude
• varying Q2 allows to change the spatial resolution and enhances different multipoles
• sensitive to missing resonance states
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37Bernhard A. Mecking Bosen Student Workshop, September 2003
N* Program RequirementsExperiment
large high-quality data set for N* excitation covering- a broad kinematical range in Q2, W, decay angles- multiple decay modes (π, ππ, η, ρ, ω, K)- polarization information (sensitive to interference terms)
Analysis∆(1232): full Partial Wave Analysis possible
(isolated resonance, Watson theorem)
higher resonances- need to incorporate Born terms, unitarity, channel coupling- full PWA presently not possible due to lack of data (polarization)
(substitute by assuming energy dependence of resonance)- skills required at the boundary between experiment and theory
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38Bernhard A. Mecking Bosen Student Workshop, September 2003
Kinematics and Cross Sectionsexample:e p e’ p πo
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39Bernhard A. Mecking Bosen Student Workshop, September 2003
photo- and electro-
production data base
(mostly differential
cross sections)
photo- and electro-
production data base
(mostly differential
cross sections)
Standard Analysis Approach
known resonance parameters
(mass, width, quantum numbers, hadronic couplings)
known resonance parameters
(mass, width, quantum numbers, hadronic couplings)
AnalysisAnalysis
electromagnetic transition form
factors
electromagnetic transition form
factors
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40Bernhard A. Mecking Bosen Student Workshop, September 2003
e p e X at 4 GeV
CLAS
events
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41Bernhard A. Mecking Bosen Student Workshop, September 2003
CLAS Coverage for e p e’ X
2.01.0 1.5 2.5
2.0
4.0
0
1.0
3.0
5.0
CLAS
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42Bernhard A. Mecking Bosen Student Workshop, September 2003
CLAS Coverage for e p e’ p X, E=4 GeV
0. 0.5 1.0 1.5
1.0
1.5
2.0
mis
sing
stat
es
CLAS
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43Bernhard A. Mecking Bosen Student Workshop, September 2003
N ∆(1232) Transition Form Factors
SU(6): E1+=S1+=0
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44Bernhard A. Mecking Bosen Student Workshop, September 2003
Multipoles E1+/M1+, S1+/M1+ (before 2001)
Hall C
Hall C
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45Bernhard A. Mecking Bosen Student Workshop, September 2003
need broad coverage in pion decay angles cos(θ*) and Φ
Φ
cos(
θ*)
CLAS
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46Bernhard A. Mecking Bosen Student Workshop, September 2003
Multipole Analysis for γ*p p πο
Q2 = 0.9 GeV2
|M1+|2
Re(E1+M1+*) |M1+|2
Re(S1+M1+*)
CLAS
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47Bernhard A. Mecking Bosen Student Workshop, September 2003
Multipoles E1+/M1+, S1+/M1+ (2002)
Hall C
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48Bernhard A. Mecking Bosen Student Workshop, September 2003
Theoretical Interpretation of E1+/M1+, S1+/M1+Bonn(2002)
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49Bernhard A. Mecking Bosen Student Workshop, September 2003
N ∆ Transition, what’s next?• systematic uncertainties in extraction of E1+/M1+ from ep e’p πo
around 0.5%– differences in treatment of background terms (models not constrained)
– will become more severe for higher Q2 (∆ dropping faster)
• more experimental information in hand (analysis in progress)– cross sections e p e’p (πo) Q2 = (1.5 – 5.5) GeV2
– single-spin asymmetry σTL’ for e p e’p (πo) and e p e’ π+ (n)– polarization transfer in e p e’ p (πo)
– differential cross sections for e p e’ π+ n (∆ less important)
• experiments in the near future – extend Q2 range to 0.05 GeV2 (end of 2002)– extend Q2 range to ~7 GeV2 (1st half of 2003)
CLAS
CLAS
Hall A
CLAS
Hall C
CLAS
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50Bernhard A. Mecking Bosen Student Workshop, September 2003
Polarized Beam Observables
σLT’ response
e p e p πο
CLAS
function for
σLT’ = 0 if only a single diagram contributes (sensitive to the interference between ∆ and background)
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51Bernhard A. Mecking Bosen Student Workshop, September 2003
π+ ElectroproductionCLAS
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52Bernhard A. Mecking Bosen Student Workshop, September 2003
|q3>
|q2q>fewer degrees-of-freedomopen question: mechanism for q2 formation?
possible reason: decouple from πN-channel
been observed (in πN scattering) Problem: symmetric CQM predicts many more states than have
Two possible solutions:
1. di-quark model
2. not all states have been found
model calculations: missing states couple toNππ (∆π, Nρ), Nη, Nω, KY
γ coupling not suppressed electromagnetic excitation is ideal
“Missing” Resonances, where are they?
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53Bernhard A. Mecking Bosen Student Workshop, September 2003
Resonances in γ*p pπ+π-
extra strength
CLAS
Analysis performed by Genova-Moscow collaboration
step #1:
use the best information presently available
ΓNππ from PDGΓNγ AO/SQTM
W(GeV)
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54Bernhard A. Mecking Bosen Student Workshop, September 2003
Attempts to fit observed extra strength
P13
D13(1700)
CLAS
Analysis step #2:
- vary parametersof known D13
- introduce new P13
or
W(GeV)
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55Bernhard A. Mecking Bosen Student Workshop, September 2003
Summary of γ*p p π+ π- Analysis
M = 1.72 +/- 0.02 GeVΓT = 88 +/- 17 MeV∆ π : 0.41 +/- 0.13
Ν ρ : 0.17 +/- 0.10
(consistent with “missing” P13 state, but mass lower than predicted)
CLAS data at variance with N* information in PDG
Describing data requires • major modifications of the parameters of known resonances, or
• introduction of new P13 resonance with
Next steps:• more experimental data already in hand
• combined analysis with other decay channels: π Nη NK Λ
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56Bernhard A. Mecking Bosen Student Workshop, September 2003
cos θω +1-1
σ
p p
γ ωN*
ω
pp
γ
above resonance
region
inresonance
region
Resonance Contributions to γ*p pω ?CLAS
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57Bernhard A. Mecking Bosen Student Workshop, September 2003
Resonances in ω Photoproduction?
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58Bernhard A. Mecking Bosen Student Workshop, September 2003
Hyperon Photoproduction off the Proton
Technique: K identified by time-of-flight, hyperons via missing mass
Goal: Λ and Σ differential cross sections for 1.6 GeV < W < 2.3 GeV
Λ and Σ polarization measured via self-analyzing weak decay and proton detection
γ Κ+
Λ
N*
Npπ0
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59Bernhard A. Mecking Bosen Student Workshop, September 2003
Λ Photoproduction off the Proton
Dominant resonancesS11(1650)P11(1710)P13(1720)
Bump at 1.9 GeVD13(1895) ?
Carnegie Mellon
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60Bernhard A. Mecking Bosen Student Workshop, September 2003
Polarization of Photoproduced Λ
Model-t (Regge) has K and K* interference, misses at back angles
Model-s (hadrodynamic)Resonances, plus K and K* exchange
Carnegie Mellon
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61Bernhard A. Mecking Bosen Student Workshop, September 2003
preliminary
Resonances in Hyperon Electroproduction?
γ*p K+Y
backward hemisphereforward hemisphere
N* ?
CLAS
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62Bernhard A. Mecking Bosen Student Workshop, September 2003
• Needed: a coherent, consistent analysis of the data from a broad variety of channels, from photo- and electro-production, and for the available values of Q2
• Important to incorporate consistently available data obtained using hadronic beams
• JLab has requested support for an analysis center to be created as part of the theory group
Next Steps in Missing Resonance Search
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63Bernhard A. Mecking Bosen Student Workshop, September 2003
Integrals over Spin Structure Functions
Bjorken Sum Rule (Q2 → ∞):Basic assumptions: isospin symmetry,
current algebra or Operator Product Expansion within QCD
GDH Sum Rule (Q2 → 0):Basic assumptions: Lorentz invariance, gauge invariance, unitarity,
dispersion relation applied to forward Compton amplitude
= nucleon anomalous magnetic moment
[g1p(x)-g1
n(x)]dx = |gA/gV|16
0
1
[σ3/2(k) – σ1/2(k)] = dkk
kthres
2π2αm2 κ
κ
∞
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64Bernhard A. Mecking Bosen Student Workshop, September 2003
Q2 Dependence of the Moments
magnetic moment
(GDH SR)
pions, nucleon
constituentquarks, N*
multiple partons
single partons(Bjorken SR)
phot
on in
tera
cts
with
Q2 = 0
Q2 = ∞
DIS, pQCD
ChPT ?
GDH sum rule
twist expansion ?
quark modelsLQCD ?
Q2 (GeV2)1
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65Bernhard A. Mecking Bosen Student Workshop, September 2003
Polarized Solid State Target for CLAS
dynamically polarized NH3 and ND3
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66Bernhard A. Mecking Bosen Student Workshop, September 2003
CLAS: First Moment Γ1p = g1(x,Q2)dx
Q2 evolution of G1p reveals the importance of nucleon resonances.
Resonances are needed to explain
fall-off for Q2 < 1.5 GeV2
zero-crossing
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67Bernhard A. Mecking Bosen Student Workshop, September 2003
Generalized Parton Distributions (GPDs)
Proton form factors, transverse charge & current densities
Structure functions,quark longitudinalmomentum & helicitydistributions
developed by X. Ji, D. Mueller, A. Radyushkin (1994-1997)
Correlated quark momentum and helicity distributions in transverse space - GPDs
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68Bernhard A. Mecking Bosen Student Workshop, September 2003
GPDs & Deeply Virtual Exclusive Processes
x
e.g.: Deeply Virtual Compton Scattering (DVCS)
t
x+ξ x-ξ
H(x,ξ,t), E(x,ξ,t), . .
hard vertices
ξ – longitudinal momentum transfer
x – quark momentumfraction
–t – Fourier conjugateto transverse impact parameter
γ
“handbag” diagram
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69Bernhard A. Mecking Bosen Student Workshop, September 2003
Universality of Generalized Parton Distributions
Parton momentumdistributions
Elastic form factors
Real Comptonscattering at high t
Single SpinAsymmetries
Deeply Virtual Meson productionDeeply Virtual
Compton Scattering
GPDs
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70Bernhard A. Mecking Bosen Student Workshop, September 2003
Experimental Access to GPDs
Accessed by cross sections
Accessed by beam/target spin asymmetry
t=0
Quark distribution q(x)
-q(-x)
DIS only measures at x=0
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71Bernhard A. Mecking Bosen Student Workshop, September 2003
Access GPDs through DVCS
Helicity difference: ∆σ ~ sinφIm(F1H(ξ,ξ,t) +k1(F1+F2)H(ξ,ξ,t) +k2F2E(ξ,ξ,t)dφ
Eo = 11 GeV Eo = 6 GeV Eo = 4 GeV
BH
DVCSDVCS/BH comparable, allows asymmetry, cross section measurements
d4σdQ2dxBdtdφ ~ |TDVCS + TBH|2
TBH : determined by Dirac & Pauliform factors
TDVCS: determined by GPDs
DVCS BH
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72Bernhard A. Mecking Bosen Student Workshop, September 2003
ep e'pX - Missing Mass AnalysisCalibrate missing mass using radiative elastic and exclusive ep → e'pγγ events
ep → e'p(γ)
exclusive ep → e'pγ
ep → e'pπo
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73Bernhard A. Mecking Bosen Student Workshop, September 2003
S. Stepanyan et al. PRL 87, 2001
g α = 0.202 ± 0.028stat ± 0.013sys
Α(φ) = α sinφ + β sin2φ
Beam
Spi
n As
ymm
etry
1999 data, E=4.2GeV, <Q2>=1.3GeV2
g β = -0.024 ± 0.021stat ± 0.009sys
A. Belitsky et al.
Measurement of exclusive DVCS2001 data, E=5.75GeV, <Q2>=2.5GeV2
• Higher energy increases kinematics range.
• Higher statistics allows binning in Q2, t, ξ
preliminary
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74Bernhard A. Mecking Bosen Student Workshop, September 2003
Pentaquark: Baryon with five quarks
Goal: Determine quark content of colorless hadrons
Expectation from the quark model is that the properties of baryons are determined by three valence quarks (qqq)
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75Bernhard A. Mecking Bosen Student Workshop, September 2003
Hadron Multiplets
18810333 ⊕⊕⊕=⊗⊗
18810102788 ⊕⊕⊕⊕⊕=⊗
1833 ⊕=⊗
Baryons qqq
Mesons qq
Baryons built from meson-baryon basis
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76Bernhard A. Mecking Bosen Student Workshop, September 2003
What are Penta-Quarks?
• Minimum quark content is 5-quarks.• Anti-quark has different flavor than any of 4-quarks
( ).• Quantum numbers can not be defined by 3-quarks.
General idea of a five-quark states has been around since late 60’s.However, searches did not give any conclusive results.PDG dropped the discussion on pentaquark searches after 1988.
Qqqqq
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77Bernhard A. Mecking Bosen Student Workshop, September 2003
Exotic Baryon SearchThe chiral soliton model by D. Diakonov, M. Petrov, M. Polyakov predicts an anti-decuplet of penta-quark baryons.
The lightest state is predicted to be a baryon state with exotic quantum number S=+1, and M=1.53GeV, Γ=15MeV.
( )uud d d ss+
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78Bernhard A. Mecking Bosen Student Workshop, September 2003
Θ+ Photoproduction and Competing Reactions
pKnKnp
nKΘ+
pΘ+pn
−
+
+
K−
→Λ
Λ→
→
→
)1520()()1520()(
)()(
*
*γ
γn
γK
K+
nΘ+
p p
p
γK+
K
pΛ∗
n n
γN → φ(1020) N → K+K- N
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79Bernhard A. Mecking Bosen Student Workshop, September 2003
Exclusive γd Measurement
CLAS Collaboration(S. Stepanyan, K. Hicks, et al.), hep-ex/0307018requires FSI – both nucleons involved– no Fermi motion correction
necessary– FSI puts K- at larger lab
angles: better CLAS acceptance
– FSI not rare: in ~50% of Λ*(1520) events both nucleons detected with p > 0.15 GeV/c
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80Bernhard A. Mecking Bosen Student Workshop, September 2003
Kaon start times relative to the proton
ppπ- pπ+π-
22;
Kc
cK
mpp
cRtt
+=
⋅−=∆ β
β
∆t (p-K)(ns)
∆t (p-K+) (ns)
pK+K
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81Bernhard A. Mecking Bosen Student Workshop, September 2003
Reaction γd→pK+K-(X)
clear peak at neutron mass
15% non-pKK events within ±3σ of the peak
background under the neutron peak can be further reduced by tight timing cut
MM(pK+K-) [ GeV/c2 ]
Eve
nts
MM(pK+K-) [ GeV/c2 ]
Eve
nts
|∆tpK| ≤ 0.75 ns
0
100
200
300
400
500
0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2 1.25
0
50
100
150
200
250
300
350
0.8 0.9 1 1.1
reconstructed neutrons
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82Bernhard A. Mecking Bosen Student Workshop, September 2003
Removal of known resonances
• remove events with IM(K+K-) φ(1020) by IM > 1.07 GeV
• remove events with IM(pK-) Λ(1520)
• limit K+ momentum due to γ d p K− Θ + phase space pK+ < 1.0GeV/c
Cuts
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83Bernhard A. Mecking Bosen Student Workshop, September 2003
(nK+) Invariant Mass Distribution
MM(nK+) [ GeV/c2 ]
Eve
nts
NΘ = 43
MΘ = 1.542 GeV
σΘ = 0.009 GeV
NΘ/√NBg = 5.8σ
0
5
10
15
20
25
30
35
1.5 1.6 1.7 1.8 1.9
0)( PGGMF Bg ++= +Θ
distribution of Λ(1520) events
Θ+
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84Bernhard A. Mecking Bosen Student Workshop, September 2003
Θ+ Experimental Status
Experimental evidence for Θ+ has been reported by four groups:• LEPS at Spring-8 (Japan), January 2003 - peak in the invariant mass of the
nK+ at 1.54 GeV with statistical significance of 4.6σ
• DIANA at ITEP (Moscow), April 2003 – peak in the invariant mass of pKo at 1.538 GeV, statistical significance 4.4σ
• CLAS at JLAB, July 2003 – peak in the invariant mass of the nK+ at 1.542 GeV, statistical significance 5.3σ
• SAPHIR at ELSA, August 2003 – peak in the invariant mass of the nK+ at 1.54 GeV, statistical significance 4.8σ
All experiments observe a narrow width
Penta-Quark 2003 Workshop at JLab November 6-8, 2003
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85Bernhard A. Mecking Bosen Student Workshop, September 2003
Nucleon-Nucleon CorrelationsObservable: NN-pair with• large relative momentum• small total momentum
need to distinguish between Correlations and Currents
Correlations Currents
Two-Body Currents (MEC + IC)• not a Correlation• strongly enhance effect of correlation
NNN
*γ
NNN
*γ
NNN *γ
NNN
*γ
MEC
IC
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86Bernhard A. Mecking Bosen Student Workshop, September 2003
Three-Body Break-up of 3He
Two protons detected with p > 250 MeV/c > pFermi
Reconstruct neutron via missing mass
Select proton/neutron with almost all transferred energy (TN/ω ≈ 1)
Clear evidence of back-to-back excess over three-body absorption followed by phase-space decay simulation
3He(e,e’pp)n
Select proton/neutron with almost all transferred energy (TN/ω ≈ 1)
Clear evidence of back-to-back excess over three-body absorption followed by phase-space decay simulation
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87Bernhard A. Mecking Bosen Student Workshop, September 2003
Angular Distribution of Emitted NN-Pair
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88Bernhard A. Mecking Bosen Student Workshop, September 2003
Conclusions from 3He(e,e’pp)n Experiment
If one selects a fast leading nucleon in 3He(e,e’pp)nthen the remaining (fast) NN pair:
• is back-to-back • is isotropic with respect to momentum transfer q • has small momentum along q
Fast NN pair is not involved in the reactionTotal and relative momentum distributions similar for:
• pp and pn pairs• 0.5 < Q2 < 1 and 1 < Q2 < 2 (GeV/c)2
• WE ARE OBSERVING BOUND-STATE CORRELATIONS!
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89Bernhard A. Mecking Bosen Student Workshop, September 2003
Setup for Deeply Virtual Compton ScatteringPhysics Goalmeasure ξ, t, Q2 - dependence of ep e’p γ in a wide kinematics range to constrain GPD models.
Technical Problem– need to detect all final state particles to
identify process– double luminosity to 2x1034 cm-2 s-1
Technical solution– add forward calorimeter (436 lead tungstate crystals)
– readout via avalanche photodiodes (APD)
– SC 5Tesla solenoid Moller shield
PbWO4Electromagneticcalorimeter
s.c.solenoid
CLAS
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90Bernhard A. Mecking Bosen Student Workshop, September 2003
Bound Nucleon Structure (B O N U S)
pair spectrometer
Solution: - high pressure gas target - surrounded by radial drift chamber - GasElectronMultiplier gap
Technical problem:spectator protons have - low momentum and low range- isotropic angular distribution (no correlation) - high rate
CLAS coilPhysics issue:tag process off a neutron bound in deuterium by detecting the spectator proton in coincidence with the scattered e’
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91Bernhard A. Mecking Bosen Student Workshop, September 2003
Frozen Spin Target for CLAS
Solution:- frozen spin target
- temperature 50mK
- magnetic field 5kG
Status:
- design in progress at JLab- procurement started for
polarizing magnet
Technical problem:build polarized target for tagged photon beam - minimum obstruction of CLAS solid angle
- low distortion of particle trajectories in magnetic field
5 Tesla polarizing magnet
CLA
SBonn target at Mainz GDH experiment
JLab design
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92Bernhard A. Mecking Bosen Student Workshop, September 2003
Physics Drivers for CEBAF Upgrade
• New capabilities– search for origin of confinement (JPC exotic mesons)– determine parton distributions (high Q2 and W) via
• polarized and unpolarized inclusive scattering • semi-inclusive (tagged) structure functions• exclusive processes (DVCS, meson production)
• Push present program to higher Q2
– form factors of mesons, nucleons, and light nuclei
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93Bernhard A. Mecking Bosen Student Workshop, September 2003
CEBAF Upgrade Plan
• Upgrade accelerator to 12 GeV max. energy– maintain 100% duty cycle
– keep beam power constant (1MW) max. current 80µA
• Build new experimental hall for meson spectroscopy (Hall D)– polarized tagged photon beam (coherent bremsstrahlung)
– large acceptance detector for real photons only
• Upgrade existing 3 halls for higher beam energy
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94Bernhard A. Mecking Bosen Student Workshop, September 2003
CEBAF Accelerator Upgrade
• keep present accelerating system
• add ten new cryomodules at 100MeV energy gain – present cryomodules provide ~30 MeV– increased performance can be achieved by
• increased effective cavity length (5-cell ⇒ 7-cell)• Increased average gradient (7.5 MV/m ⇒ 17.5 MV/m)
• double cryogenic system capacity• upgrade recirculating arcs • add new beam line to Hall D
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95Bernhard A. Mecking Bosen Student Workshop, September 2003
6 GeV CEBAF
CHLCHL--22
Upgrade magnets Upgrade magnets and power and power suppliessupplies
12
Enhance equipment in Enhance equipment in existing hallsexisting halls
add Hall D (and beam line)
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96Bernhard A. Mecking Bosen Student Workshop, September 2003
Quark-Gluon Dynamics and Nucleon TomographyDeeply Virtual Compton Scattering (DVCS)Deeply Virtual Meson Production (DVMP)High-t DVCS and π0/η production
Valence Quark DistributionsProton and Neutron Spin Structure Neutron Structure Function F2n(x,Q2)Tagged Quark Distribution Functions Novel Quark Distribution Functions (tranversity, e(x),..)
Form Factors and Resonance Excitations The Magnetic Structure of the NeutronResonance Excitation Dynamics
Physics with quasi-real Photons
CLAS Physics Program at 12 GeV
Hadrons in the Nuclear MediumSpace-Time Characteristics of HadronizationColor transparency
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97Bernhard A. Mecking Bosen Student Workshop, September 2003
Upgraded CLAS (CLAS++)
Forward TOF
Preshower EC
Forward ECForward Cerenkov
Forward DC
Inner Cerenkov
Central Detector
Coil CalorimeterTorus Cold Ring
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98Bernhard A. Mecking Bosen Student Workshop, September 2003
CLAS++ - 2-dimensional Cut
~38o
~5o
repositioned torus coils
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99Bernhard A. Mecking Bosen Student Workshop, September 2003
12 GeV Upgrade Project Status
• Developed by User Community in collaboration with JLab
• Nuclear Science Advisory Committee, NSAC– plan presented during last 5-year Long Range Plan– recommended by NSAC for new construction
• Plan presented to Department of Energy– presently waiting for CD-0 (determination of ‘mission need’)
• Construction– estimated costs: $158M (in FY02$)– construction start expected in FY2007 (October 2006)– 3 year construction project
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100Bernhard A. Mecking Bosen Student Workshop, September 2003
Long-Term Future @ JLab
Study underway for an electron-light ion collider at JLab to investigate
inclusive and semi-inclusive DISdeep exclusive reactions (GPD’s)
Parameterselectrons 3 - 5 GeVions (p, d, 3He) 30-50 GeVluminosity ≤ 6x1034 cm-2 s-1
Design maintains fixed target capability with25 GeV external beam
luminosity ~1038 cm-2 s-1
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101Bernhard A. Mecking Bosen Student Workshop, September 2003
Ion Source RFQ
Injector
DTL CCL
IR IR
Beam Dump
SnakeSolenoids
Snake
5 GeV CEBAF with Energy Recovery
5 GeV electrons 50 GeV light ions
Electron-Light Ion Collider Layout
100 MV cryomodulesLia Merminga at EIC Workshop, BNL
02/27/2002