# Presentation: 542010

## The Physics of CLAS

- Bernhard A. Mecking
- Jefferson Lab
- 20th Student Workshop on Electromagnetic Interactions
- Bosen, August 31 – September 5, 2003
- CEBAF at Jefferson Lab
- CLAS
- 	technical
- 	physics program
- 	planned upgrades
- CEBAF upgrade plans

**Notes:**
My talk will be about the electroexcitation of nucleon resonances. 

This conferences is all about nucleons and baryons. 
Nathan Isgur has been a strong supporter of a nucleon research program at this laboratory.  He formulated the importance of studies of the nucleon in 3 remarks he made at the N*2000 conferences  in this auditorium:

## 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 A
- e polarization	80%
- Experimental equipment in 3 halls (simultaneous operation)          L[cm**-2**s**-1**]
- 2 High Resolution Spectrometers (pmax=4 GeV/c)			10**39**
- 2  spectrometers (pmax=7 and 1.8 GeV/c) + special equipment	10**39**
- Large Acceptance Spectrometer for e and   induced reactions	10**34**

## CEBAF  Continuous Electron Beam Accelerator Facility

- CEBAF  Continuous Electron Beam Accelerator Facility

- recirculating arcs

- accelerating

- structures

- CHL

- RF separators

## CEBAF Site

- Hall C

- Hall B

- Hall A

- south linac

- north linac

- injector

## 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 (100A 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. 200A at 5 GeV)
- typical beam spot size 100m,  momentum spread 10-4

## Electron Beam Profile

- measurement performed moving 25m wire through beam
- using downstream photomultipliers as radiation detectors

## 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)

## 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

## Hall B Side View

## Bremsstrahlung Tagging System Layout

## CLAS 3-D View

## CLAS in Maintenance Position

## 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

## 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

- 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.)

## CLAS Top View

## CLAS Rear View

## Characteristics of CLAS Components

- Super-conducting toroidal magnet with six kidney-shaped coils
- 5 m diameter, 5 m long, 5 M-Amp-turns, max. field 2 Tesla

## CLAS Lines of Constant Field

## CLAS Forward Calorimeter Layout

## Forward Calorimeter Hit Pattern

## CLAS Single Event Display

## Trajectory Reconstruction Resolution

## Scintillation Counter Timing Resolution

## CLAS Luminosity for e-

## Missing Mass Distribution

- p    pX

## Mass Determination from p and 

- Mass Determination from p and 

**all events**

## z-Vertex Determination from Tracking

- vertex determination used routinely for experiments with multiple targets in the beam:
-  simultaneous LH2 and LD2
-  multiple target foils

- liquid hydrogen

- heat shield

## Trajectory Reconstruction Distortions

- Explored in   e p    e’+ (n)
- Possible reasons
-  drift chamber positions not known perfectly
-  uncertainties in magnetic field

- before corrections

- after momentum corrections

## Calorimeter Detection Efficiency for n

- using tagged neutrons from  e p    e’+ (n)

## Cross Section for e-p Scattering

## CLAS Run and Analysis Conditions

**Run**
- luminosity	1034 cm-2 s-1
- electromagnetic rate	109 / s
- hadronic production rate	106 / s
- trigger (Level I)	on e- candidates (Cerenkov + calorimeter)
- trigger rate (max.)	4,000/s
- data rate to disk (max.)	25 MB/s (~BaBar, CLEO, 1⁄2 RHIC STAR)
- data volume to silo (max.)	1 TeraByte/day)
- personnel	2 on shift,
- 	on call: 7 system experts + engineering-on-call
**Analysis**
- first-pass analysis	at JLab compute farm
- physics analysis	at JLab for full data set
- 	at outside institutions for filtered data

## W-Dependence of Selected Channels at 4 GeV

- p(e,e’)X     (trigger)

- p(e,e’p)X

- p(e,e’+)X

- p(e,e’p+)X

- p(e,e’p+)X

## 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 ****N    ****N)
-  advantage:
- strong coupling     large cross sections
- simple spin structure
- good quality beams
- disadvantage: no structure information

- insensitive to states with weak N coupling

## Quark Model Classification of N*

**Quark Model Classification of N***

## 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

## Electromagnetic Excitation

- helicity amplitudes very sensitive to the difference in wave functions of N and 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

## N* Program Requirements

- Experiment
- 	large high-quality data set for N* excitation covering
- 		- a broad kinematical range in Q2, W, decay angles
- 		- multiple decay modes (K
- 		- polarization information (sensitive to interference terms)

- Analysis
- 	****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

## Kinematics and Cross Sections

- example:

- e p     e’ po

**Notes:**
Extracting the multipoles requires first to separate the relevant response function, by measureing the azimuthal and polar angle dependence of the diff. cross section, and fitting it to the expected phi dependence,  (next slide)
 as shown in this graph..... 

## Standard Analysis Approach

- (mostly differential cross sections)

- known resonance parameters
- (mass, width, quantum numbers, hadronic couplings)

- Analysis

- electromagnetic transition form factors

## e p    e X  at 4 GeV

- CLAS

**events**

##  CLAS Coverage for  e p     e’ X

**2.0**

**1.0**

**1.5**

**2.5**

**2.0**

**4.0**

**0**

**1.0**

**3.0**

**5.0**

- CLAS

**Notes:**
For electron scattering experiments CLAS is usually triggered on electrons only. 
The inclusive electron spectrum at 4 GeV beam energy,
shown here in the Q2 versus invariant mass W kinematics,  exhibits the elastic scattering band, the 3 resonance bands, and the transition to the deep inelastic regime.    
These lines indicate the region where many of the missing states are expected. 
(next slide)
The projection onto the W axis shows the elastic peak and the 
3 resonance bumps

## CLAS Coverage for  e p    e’ p X,  E=4 GeV 

- CLAS Coverage for  e p    e’ p X,  E=4 GeV** **

**0.**

**0.5**

**1.0**

**1.5**

**1.0**

**1.5**

**2.0**

**missing states**

- CLAS

**Notes:**
...... pi0, eta, or omega in the 
final state. The missing mass vs invariant mass scatter plot shows clear correlations of resonance excitations and specific final states.  
 Strong excitation near 1500 is correlated with the p-eta channel, and there is a lot of strenght in p-omega throughout the higher mass region.
The Delta sticks out in the p-pi0 channel, and can best be studied in this channel. 

## N    (1232) Transition Form Factors

- N    (1232) Transition Form Factors

- SU(6): E1+=S1+=0

**Notes:**
The Delta excitation has been studied experimentally for 3 decades, and the dominant magnetic dipole transition is well established, mediated by a simple spin flip. Electric or scalar quadrupole transitions, which are identical 0 in a symmetry SU(6) model, would indicate a deformation of the Delta and the nucleon.  
Dynamically, such contributions may arise through interaction with the pion cloud, or through the one gluon exchange. 
helicity conservation requires that the E/M asymptotically approaches +1. Qualitatively one expects such a behavior. The sign of the this ratio is related to the shape of the deformation of the Delta
in such a way.    

## Multipoles E1+/M1+, S1+/M1+  (before 2001)

- Multipoles E1+/M1+, S1+/M1+  (before 2001)

- Hall C

- Hall C

**Notes:**
This shows the data on the  electric quadrupole of 
magnetic dipole ratio and the scalar quadrupole to magnetic dipole ratio. The colored points are mostly from the 
seventies and exhibit large systematic discrepancies. 
The more recent data indicate that REM is small and negative, 
-2% to -3%, and has a weak Q2 dependence, while RSM may 
have a stronger Q2 dependence if one ignores this point.   

Today, the emphasis is on control of systematics.
While in spectrometer experiments the kinematics needs to be adjusted many times during a measurement  CLAS measures all kinematics simultaneously.     

## 

- cos(*)

- CLAS

## Multipole Analysis for   *p      p  

- Multipole Analysis for   *p      p 

- Q2 = 0.9 GeV2

- |M1+|2

- Re(E1+M1+*) |M1+|2

- Re(S1+M1+*)

- CLAS

**Notes:**
and with some approximation analysed in terms of multipoles.
The first terms is dominated by the transverse component 
and strongly dependent on the dominant magnetic dipole transition, while the TT interference is most sensitive to the 
electric quadrupole, and the LT interference to the scalar quadrupole.
(new graph)
  Overlaying the new results on the old data.... 

## Multipoles E1+/M1+, S1+/M1+  (2002)

- Multipoles E1+/M1+, S1+/M1+  (2002)

- Hall C

**Notes:**
bridging the photon point for REM and the high Q2 results from earlier JLAB measurements. The REM ratio remains small and negative around -2% with little Q2 dependence.

The RSM ratio shows a strong Q2 dependence and becomes increasingly negative at higher Q2. 

(next graph)
For comparison with some selected models I eliminate 
old data and blow up the vertical scale....

##  Theoretical Interpretation of  E1+/M1+, S1+/M1+ 

** **Theoretical Interpretation of  E1+/M1+, S1+/M1+

- Bonn(2002)

**Notes:**
Data are compared with two relatized CQMs, a chiral quark soliton model, and a dynamical model with hadronic degrees of freedom. Non of the pure quark models is successful in describing both ratios, simultaneously.  

The comparison strengthens the claim that most of the quadrupole strength is due to non-resonant meson exchange, implemented in the dynamical models. 

A dynamical model by Kamalov and Yang, using a different unitarization procedure, gives similiarly good agreement with the data.
One should note that these models have been fitted to the photon point and the high Q2 data. 
 (next slide)  
Ultimately, we want to come to a QCD description of these quantities. 
Currently there is only one calculation in quenched QCD 
for REM with a large error bar....

## N    Transition, what’s next?

- systematic uncertainties in extraction of E1+/M1+ from ep   e’p o around 0.5%
- differences in treatment of background terms (models not constrained)
- will become more severe for higher Q**2**  (**** dropping faster)
- more experimental information in hand (analysis in progress)
- cross sections  e p    e’p (******o**)   Q2 = (1.5 – 5.5) GeV2
- single-spin asymmetry **TL’** for  e p    e’p (******o**)  and   e p    e’ ******+** (n)
- polarization transfer in  e p    e’ p (******o**)
- differential cross sections for  e p    e’ ******+** n  (**** less important)
- experiments in the near future
- extend Q**2** range to 0.05 GeV**2**   (end of 2002)
- extend Q**2** range to ~7 GeV**2**   (1st half of 2003)

- CLAS

- CLAS

- Hall A

- CLAS

- Hall C

- CLAS

## Polarized Beam Observables

- LT’  response

- e p    e p 

- CLAS

- function for

- LT’ = 0  if only a single diagram contributes   (sensitive to the interference between  and background)

**Notes:**
The Mami beam asymmetry data at large cms angles show a strong sensitivity to model descriptions is. The dotted line is a rescaled MAID model. 
The CLAS data on the sigma_LT' response function cover the full angle range and show similar sensitivity to model descriptions. 
Since the resonance contributions are nearly the same in either model, the discrepancies are largely due to the different description of non-resonant terms. 

## + Electroproduction 

- + Electroproduction** **

- CLAS

**Notes:**
allows ectraction of resonant amplitudes in the 2nd resonance region.  
(next slide) 
the next graph shows the preliminary results for one Q2 point on the Roper resonance....   

## “Missing” Resonances, where are they?

- |q3>

- |q2q>

-  fewer degrees-of-freedom
-  open question: mechanism for q2  formation?

- possible reason: decouple from N-channel

- been observed (inN 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 to   		N, N,  N, N,  KY

-  coupling not suppressed      electromagnetic excitation is ideal

- “Missing” Resonances, where are they?

**Notes:**
In recent years a number of alternatives to the "standard" constituent quark model have been proposed, in large measures   to fix some of the real or perceived shortcomings of that model, especially to explain the absence of many states predicted in that model. Several talks at this conference are dedicated to these new approaches. One of the oldest alternative models is the quark-diquark model, where two of the quarks form an elementary object. This model has the advantage that due to the fewer excitation degrees of freedom a much smaller number of states are predicted which corresponds more to what we see in the data...
The various alternatives correspond to quite different underlying quark symmetries and interactions, search for at least some of the states predicted in the standard model, but not in the alternative models is imperative. 
A series of P13 states with coupling to the omega channel are predicted in the standard model but not in the quark-diquark model..... . 
Electromagnetic production of omegas suffers from strong diffractive or t-channel contributions, leading to strong forward peaking which complicates the analysis. However, diffractive production is reduced with increasing Q2.  

While the diffractive or t-channel contribution gives the forward peaking, s-channel resonance production produces a more symmetric angular dependence with significant large angle production. 
(next) The data exhibit ..

## Resonances in  *p      p+-

- Resonances in  *p      p+-

- extra strength

- CLAS

- Analysis performed by Genova-Moscow collaboration
- step #1:

- use the best information presently available
- N   from PDG
- N    AO/SQTM

- W(GeV)

**Notes:**
These are the first high statistics electroproduction data in the 2-pi channel.
The data can be best fitted by a new P13 state with properties that are very different from the properties of the known 
P13 located in the same mass range. 

## Attempts to fit observed extra strength

- P13

- D13(1700)

- CLAS

- Analysis step #2:

- -	vary parameters
-    of known D13
- introduce new P13

- or

- W(GeV)

**Notes:**
alternate possibilities, e.g. an increase in D13(1700) strength  give poorer fits especially to the hadronic distributions, ...

## Summary of   *p      p + -  Analysis

- Summary of   *p      p + -  Analysis

- M = 1.72 +/- 0.02 GeV

- T = 88 +/- 17 MeV

-  : 0.41 +/- 0.13

-   : 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:

-  N
-  N
- K

**Notes:**
has been seen in hadronic analyses ...

A new P13 would be consistent with a "missing state" by Capstick & Roberts, except for a 130MeV lower mass....  

(next slide, omega) 
While many resonances are known to decay into the 2-pion channel not a single resonance listed in the RPP is known to couple to the N-omega channel. Therefore, any state seen in that channel would be a discovery, either of a new decay, or of a missing state. 
(next) Why is the search for at least some of the missing states so important for the understanding of baryon structure? ... 

## Resonance Contributions to  *p     p

- cos 

- +1

- -1

- 

- above resonance
- region

- in
- resonance region

- Resonance Contributions to  *p     p

- CLAS

##  Resonances in  Photoproduction? 

** **Resonances in  Photoproduction?

## Hyperon Photoproduction off the Proton

- Technique: K identified by time-of-flight, hyperons via missing mass

- Goal: and  differential cross sections for 1.6 GeV < W < 2.3 GeV

-  and  polarization measured via self-analyzing weak decay and proton detection

##  Photoproduction off the Proton

- Dominant resonances
- S11(1650)
- P11(1710)
- P13(1720)

- Carnegie Mellon

## Polarization of Photoproduced 

- Model-t (Regge) has K and K* interference, misses at back angles

- Model-s (hadrodynamic)
- Resonances, plus
- K and K* exchange

- Carnegie Mellon

## preliminary

- Resonances in Hyperon Electroproduction?

-     *p        K+Y

- backward hemisphere

- forward hemisphere

- N* ?

- CLAS

## 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

- 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

## 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

## Q2 Dependence of the Moments

- Q2 Dependence of the Moments

-  **magnetic **
** ****moment**
**(GDH SR) **

**pions, nucleon**

**constituent**
**quarks, N***

**multiple partons**

**single partons**
**(Bjorken SR)**

**photon interacts with**

- Q2 = 0

- Q2 = ****

## Polarized Solid State Target for CLAS

**dynamically polarized NH****3**** and ND****3**

## CLAS:  First Moment   1p =   g1(x,Q2)dx

- CLAS:  First Moment   1p =   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

## Generalized Parton Distributions (GPDs)

- Correlated quark momentum
- and helicity distributions in
- transverse space - GPDs

- GPDs & Deeply Virtual Exclusive Processes

- x

-  e.g.: Deeply Virtual Compton Scattering (DVCS)

- t

- x+

- x-

-  – longitudinal
- momentum transfer

- x – quark momentum
- fraction

-  –t – Fourier conjugate
- to transverse impact
- parameter

- 

- “handbag” diagram

## GPDs

- Parton momentum
- distributions

- Elastic form factors

- Real Compton
- scattering at high t

- Single Spin
- Asymmetries

- Deeply Virtual Meson
- production

- Deeply Virtual
- Compton Scattering

## Experimental Access to GPDs

**DIS only measures at x=0**

## Access GPDs through DVCS

- Helicity difference:~ sinIm{(F1H(,t) +k1(F1+F2)H(,t) +k2F2E(,t)}d

**T****BH**** ****:** determined by Dirac & Pauli
-         form factors
**T****DVCS**: determined by GPDs

- DVCS

- BH

## ep    e'pX - Missing Mass Analysis

- ep    e'pX - Missing Mass Analysis

- Calibrate missing mass using radiative elastic and  exclusive ep   e'p events

- ep  e'p()

- exclusive  ep   e'p

- ep  e'po

##  Measurement of exclusive DVCS 

- S. Stepanyan et al. PRL **87**, 2001

-    =  0.202 ± 0.028stat ± 0.013sys

- sin + sin2

- Beam Spin Asymmetry

-  1999 data, E=4.2GeV, <Q2>=1.3GeV2

-    = -0.024 ± 0.021stat ± 0.009sys

- A. Belitsky et al.

-  Measurement of exclusive DVCS

-  2001 data, E=5.75GeV, <Q2>=2.5GeV2

-   Higher energy increases kinematics range.
-   Higher statistics allows binning in Q2, t, 

- preliminary

## 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)

## Hadron Multiplets

- Baryons qqq

- Baryons built from meson-baryon basis

## 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.

## Exotic Baryon Search   

- Exotic Baryon Search**   **

-  The 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, =15MeV.

## + Photoproduction and Competing Reactions 

*p*

*K*

*n*

*K*

*n*

*p*

*n*

*K*

****

*p*

****

*p*

*n*

- 

- 

- 

- 

- 

- 

- 

- 

- 

- )

- 1520

- (

- )

- (

- )

- 1520

- (

- )

- (

- )

- (

- )

- (

- *

- *

**

**

- *N* (1020) *N* *K**+**K**- **N*****

## Exclusive  d  Measurement

-      CLAS Collaboration
- 	(S. Stepanyan, K. Hicks, *et al.*),
- 	hep-ex/0307018
- 	requires 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 *(1520) events both nucleons detected with p > 0.15 GeV/c

## Kaon start times relative to the proton

- t (p-K─)
- (ns)

- t (p-K+) (ns)

- pK+K─

## Reaction d→pK+K-(X)

- clear peak at neutron mass
- 15% non-pKK events within ±3 of the peak
- background under the neutron peak can be further reduced by tight timing cut

- reconstructed neutrons

## Removal of known resonances

- remove events with IM(K+K-)* *(1020) by IM > 1.07 GeV
- remove events with IM(pK-) (1520)
- limit K+ momentum due to dp K phase space **p****K****+**** < 1.0GeV/c**

- Cuts

## (nK+) Invariant Mass Distribution

- distribution of (1520) events

******+**

## + Experimental Status

- Experimental evidence for + 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 4.6
- DIANA at ITEP (Moscow), April 2003 – peak in the invariant mass of pKo at 1.538 GeV, statistical significance 4.4
- CLAS at JLAB, July 2003 – peak in the invariant mass of the nK+ at 1.542 GeV, statistical significance 5.3
- SAPHIR at ELSA, August 2003 – peak in the invariant mass of the nK+ at 1.54 GeV, statistical significance 4.8
- All experiments observe a narrow width

- Penta-Quark 2003 Workshop at JLab
- November 6-8, 2003

## Nucleon-Nucleon Correlations

- Observable: 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

- MEC

- IC

## 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/ ≈ 1)
- Clear evidence of back-to-back excess over three-body absorption followed by phase-space decay simulation

*3**He(e,e’pp)n*

- Select proton/neutron with almost all transferred energy (TN/ ≈ 1)
- Clear evidence of back-to-back excess over three-body absorption followed by phase-space decay simulation

## Angular Distribution of Emitted NN-Pair

## Conclusions from 3He(e,e’pp)n Experiment 

- If one selects a fast leading nucleon in 3He(e,e’pp)n
- then 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 reaction
- Total 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!

**Notes:**
Thus, because when we select a quasifree leading nucleon the fast NN pairs are:
1 Back to Back,
2 Isotropic and
3 Have small average momentum along q
we conclude that the fast NN pair is not involved in absorbing the virtual photon.  
Because we measure similar total and relative momentum distributions for
pp and pn pairs and
0.5 < Q2 < 1 (Ebeam = 2.2 GeV) and 
1 < Q2 < 2 (GeV/c)^2  (Ebeam = 4.4 GeV) 
we conclude that we have measured bound state NN correlations.

## Setup for Deeply Virtual Compton Scattering

**Physics Goal**
- measure** ******, t, Q2 - dependence of ep     e’p 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

## 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 coil

**Physics issue:**
- tag process off a neutron bound in deuterium by detecting the spectator proton in coincidence with the scattered e’

## 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

- CLAS

- Bonn target at Mainz GDH experiment

- JLab design

## Physics Drivers for CEBAF Upgrade  

- New capabilities
- search for origin of confinement (J**PC** exotic mesons)
- determine parton distributions (high Q**2** and W) via
- polarized and unpolarized inclusive scattering
- semi-inclusive (tagged) structure functions
- exclusive processes (DVCS, meson production)
- Push present program to higher Q**2**
- form factors of mesons, nucleons, and light nuclei

## CEBAF Upgrade Plan 

- Upgrade accelerator to 12 GeV max. energy
- maintain 100% duty cycle
- keep beam power constant (1MW)        max. current  80A
- 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

## 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

## Slide 95

## CLAS Physics Program at 12 GeV

-    Deeply Virtual Compton Scattering (DVCS)
-    Deeply Virtual Meson Production (DVMP)
-    High-t DVCS and  production

-  **Valence Quark Distributions**
-    Proton 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 Neutron
-    Resonance Excitation Dynamics

-  **Physics with** **quasi-real Photons**

-  **Hadrons in the** **Nuclear Medium**
-    Space-Time Characteristics of Hadronization
-    Color transparency

## Upgraded CLAS  (CLAS++)

- Upgraded CLAS  (CLAS++)

*Forward TOF*

*Preshower EC*

*Forward EC*

*Forward Cerenkov*

*Forward DC*

*Inner Cerenkov*

*Central Detector*

*Coil Calorimeter*

*Torus Cold Ring*

## CLAS++ - 2-dimensional Cut 

- CLAS++ - 2-dimensional Cut** **

- ~38o

- ~5o

*repositioned *
*torus coils*

## 12 GeV Upgrade Project Status 

- 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

## Long-Term Future @ JLab

- Study underway for an electron-light ion collider at JLab to investigate** **
**	****	**inclusive and semi-inclusive DIS
- 		deep exclusive reactions (GPD’s)
- Parameters
- 		electrons		  3 - 5 GeV
- 		ions (p, d, 3He)		30-50 GeV
- 		luminosity 		 6x1034 cm-2 s-1
**	****	**** **
- Design maintains fixed target capability with
- 		25 GeV external beam
- 		luminosity ~1038 cm-2 s-1

## Electron-Light Ion Collider Layout

- Ion Source

- RFQ

- Injector

- DTL

- CCL

- IR

- IR

- Beam Dump

- Snake
- Solenoids

- Snake

- 5 GeV CEBAF with Energy Recovery

- 5 GeV electrons

- 50 GeV light ions

- 100 MV cryomodules

- Lia Merminga at EIC Workshop, BNL 02/27/2002*** ***