MENU 2004Institute of High Energy Physics

Beijing, China

August 29 – September 4, 2004

Highlights of Physics with CLAS at Highlights of Physics with CLAS at Jefferson LabJefferson Lab

Volker D. Burkert

Jefferson Lab

MENU 2004

August 29 – September 4, 2004

Outline

Baryon Resonance Transitions in N, N- N - The “Roper” P11(1440) and S11(1535)

Nucleon Spin Structure in the Resonance Region

Search for Pentaquark Baryons - Status

Summary & Outlook

Introduction

Baryon States in p

Why Hadronic Physics with e.m. Probes

resolution of probe

low

high

N

πCentral question: What are the relevant degrees of freedom at varying distance scales?

P.O. Bowman, et al., hep/lat0209129

LQCD

e.m. probe

q

CLAS

JLab Site: The 6 GeV CW Electron Accelerator

CEBAF Large Acceptance Spectrometer

Drift chambersargon/CO2 gas, 35,000 cells

Electromagnetic calorimetersLead/scintillator, 1296 PMTs

Torus magnet6 superconducting coils

Gas Cherenkov counterse/ separation, 216 PMTs

Time-of-flight countersplastic scintillators, 684 PMTs

Large angle calorimetersLead/scintillator, 512 PMTs

Liquid D2 (H2)target, NH3, ND3

start counter; e minitorus

N* Program at CLAS

Primary Goals

• Extract photo- and electrocoupling amplitudes for known , N* △resonances– Partial wave and isospin decomposition of hadronic decay– Helicity amplitudes A3/2, A1/2,S1/2 and their Q2 dependence

• Search for resonances expected from SU(6)xO(3) or other symmetries– More selective hadronic decays:

e

e’

γv

N N’,△’,

N*,△

, 2 , , ,

, , K

, K

N(1232) Quadrupole Transition

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

Pion Electroproduction Structure Functions

21M

*1 1Re( )E M

*1 1Re( )S M

*22 * * * *

* * 2 ( 1)( sin cos2 sin cos )L LT L TT LTpd

d k

/ 0( , )p e e p Map out azimuthal and polar angledependence to extract structure functionsmnd mulipoles.

CLAS - N(1232) transitionComplete angular distributions in and in full W & Q2 range.

Q2=3GeV2

C L A S p r e

l i m i n

a r y

cos

CLAS - Legendre Expansion of S.F.

0 1 1 2 2

0

0 1 1

LT L

TT

LT

A A P A P

C

D D P

2

1

*1 1 2 0

*1 1 1

/ 2

Re( ) 2 / 3 / 8

Re( ) / 6

oM A

E M A C

S M D

Resonant Multipoles

*0 1 1

*0 1 0

*1 1 2 0 0

Re( ) / 2

Re( )

Re( ) 2 / 8

E M A

S M D

M M A A C

Non-Resonant Multipoles

(M1+ dominance)

N(1232) Transition Form Factor G*M

1/3 of G*M at low Q2 is due

to vertex dressing and pion cloud contributions.

bare vertex

dressed vertex

pion cloud

Sato-Lee

CLAS - N(1232) Transition

REM remains small also at high Q2 with trend towardsREM ~ 0. No clear trend seentowards sign change.

RSM continues to rise in magnitude with Q2. No trend seen towards Q2- independent behavior .

Pion cloud models describe data well.

Structure of the P11(1440) & S11(1535).

Non-relativistic CQM’s have difficulties describing the properties of the P11(1440).

S11(1535) photo coupling amplitudes disagree for N and p channels. State suggested as K dynamical resonance.

Q2 evolution of transition form factors allows stringent model tests.

ep en+CLAS

UIM Global Fit – n+ response functions

Q2=0.4

LT’ – Sensitivity to P11(1440)

Shift in S1/2

Shift in A1/2

Polarized structure function sensitive to imaginary part of P11(1440) through interference with real Born background.

/LT

CLASep e+n

Roper P11(1440) - Electrocoupling amplitudes

UIM/DR - Analysis of CLAS datapnPDG

3q G Li

3q Cano

LC-Capstick

rel.CQM-Warns

nonrel.

rel.

Meson contributions or relativity needed to describe data.

zero crossing

large longitudinal amplitude

UIM/DR - Analysis of CLAS data

pn

p

S11(1535) - Electrocoupling amplitudes

PDG

GWU ()rCQM

nrCQM

rCQM - Warns

Capstick, Keister

hypCP Giannini

/ discrepancy

no discrepancy

The Nucleon Spin Structure in the Resonance Region.

Structure function g1 and 1(Q2) Integral

e + p e + X

Expect rapid change of 1 in transition from the hadronic to the partonic regimes.

partonhadron

1p

CLAS – Structure function g1p (x,Q2)

CLAS – Structure function g1d (x,Q2)

CLAS – 1st Moment of g1 (x,Q2)

Effect of (1232)excitation

NeutronGDH slope

Proton

P r e l i m i n a r y

Bjorken Integral p-n

Is a fundamental descriptionof the Bjorken integral possible at all distances?

HBPT seems compatiblefor Q2<0.2GeV2

pQCD and OPE seem to work for Q2>0.7GeV2. Twist analysis underway.

First significant measurement

in range Q2 = 0.05–2.5 GeV2

Resonances in p

|q3>

|q2q>

Symmetric CQM |q3> predicts many more states

than are observed in elastic N scattering analysis.

For example, an additional P13 below 1900MeV would

rule out the |q2q> model.

The quark-cluster model |q2q> has fewer degrees of freedom => fewer states. Accomodates all observed **** states.

Developed Dynamical Isobar Model to study the complex mass range near 1700 MeV and above. Includes all resonances < 2 GeV with hadronic and e.m. couplings.

Evidence for P33(1600) *** state

W=1.59 GeV

no P33(1600)

with P33(1600)

Fit to high statistics photoproduction datarequires inclusion ofP33(1600) state.

Sample data

Photo- and electroproduction comparsion

photoproductionelectroproduction

p

W(GeV) W(GeV)

Background

Resonances

Interference

full calculation

no 3/2+

no 3/2+ (1720)full

P33(1600) Mass

MeV

Width

MeV

A1/2*103

GeV-1/2

A3/2*103

GeV-1/2

This analysis 1686 +/- 10

338+/100 65+/-6 -30 +/- 10 -17 +/- 10

PDG (***) 1550 -1700 250 - 450 40 - 70 -29+/-20 -19+/-20

3/2+(1720) Mass

MeV

Width

MeV

N

New state? 1722 +/- 20 92 +/- 17 50 +/- 13 11+/- 10

PDG P13(1720)

1650 - 1750 100 - 200 --- 70-85

P states near 1700 MeV

The anti-decupletof 5-quark statesin the SM.

Diakonov, Petrov, Polyakov, 1997

Pentaquark Baryon Search

Evidence for the +(1540) LEPS SAPHIR

CLAS-p

HERMES

ITEP

pp ++.

COSY-TOF

DIANA

SVD/IHEP

CLAS-d

ZEUS

CLAS – Production on Hydrogen

4.8 < E < 5.4 GeV

p K+K-+n

Further cuts are motivated by assumptions on production mechanism.

no cuts

Select t-channel processby tagging forwardand reducing K+ from t channel processes

cos

cos

(in c.m. frame)

Exclusive Production on Hydrogen

Possible production mechanism

Cut on + mass, and plot M(nK+K-)

CLAS - +(1540) on protons

3 - 5.4 GeV

M(nK+)

p +K+K- n

cut

proton

+N* K+

n

K-

production through N* resonance decays?

V. Kubarovsky et al., PRL 92, 032001 (2004)

CLAS - +(1540) and N* ?

proton

+N* K+

n

K-

-p cross section data in PDG have a gap in the mass range 2.3–2.43 GeV.

What do -p scattering data say?

outside cuts

N* ?

cuts outside +

CLAS - A Program for Pentaquark Physics

Solving the issues of the +(1540)• exact mass ?• spin = ½ ? • parity = + or - ?• production mechanism ?

Are there excited states of the +(1540)?

How are pentaquark states related to N* states?

Search for the other exotic members of the decuplet -- ,+, seen in NA49 but unconfirmed.

Where are the non-exotic pentaquarks, *’s’s?

High statistics searches for the + on hydrogen and deuterium targets in nK+ and pK0 channels

CLAS – Second generation experiments

G10 - Measurement on deuterium (under analysis)• E = 1 – 3.6 GeV • > 10 times the statistics of published data.• Improved calibration of photon energy

G11 – Measurement on hydrogen (data taking finished July 26)• E = 1.6 – 3.8 GeV• First high statistics run at lower energies

EG3 – Search for exotic --

• decay reconstruction D X -- -- - -p (begin 12/2004) • in missing mass p K+K+- ( tagged) (run in 2005/6)

Simulated data for 1.6 < E < 2.2 GeV for ~10nb cross section for +(1530) and a hypothetical *(1575).

CLAS – G11 run on hydrogen target

p K+ + - (n)

cos CMK0

<- 0.35

N (1530) ~ 160

N (1575) ~ 300

simulatio

n

simulatio

n

p p

Summary & Outlook

N* physics - N(1232)

- Transition form factor measurements GM, REM, RSM for 0.1 < Q2 < 6 GeV2. Large meson effects at low to medium Q2

- 2nd resonance region

- First consistent P11(1440) electro couplings. Large meson cloud effects. - Consistent S11(1535) electro couplings for p and N.

- New baryon resonances in N- P33(1600) and a 3/2+(1720) needed to explain p data

Summary & Outlook, cont’d

Spin Physics- g1(x,Q2) and 1(Q2) measured in large Q2 range for proton/deuterium.

- pQCD Twist-2 description of 1(p-n) for Q2 > 0.7GeV2, HBPT for Q2 < 0.2 GeV2.

Pentaquark Baryons- High statistics search for + and excited states is underway on deuterium and hydrogen.

- 6 GeV run in preparation in the search for 5-- on deuterium.

Pentaquark Baryon Search

In the Pentaquark discussion someone said: “Extraordinary claims require extraordinary proof” This is the way we approach the second round of Pentaquark searches at JLab.

The CLAS collaboration has decided that results will only be shown when final.

Final results of the high-statistics runs are expected near the end of 2004.

Are the null experiments sensitive to +(1540)?

Several high energy experiments have analysed their data In the search for the +. In the following I examine two of them, BaBaR and Belle, both detectors to study e+e- interactions at high energy to study B mesons.

They use very different techniques and neither has seen a signal.

=> BaBaR studies particles produced in e+e- annihilations and subsequent quark fragmentation processes.

=> Belle uses K+ and K- produced in the fragmentation. They study K+-nucleus scattering in their silicon (?) tracking Detectors. This is similar to the DIANA experiment that measured K+Xe in a bubble chamber where they saw a + signal

Do these results contradict experiments that have seen a signal?

Pentaquark production in direct e+e- collisions likelyrequires orders of magnitudes higher rates than available.

Hadron production in e+e-

Slope: Pseudoscalar mesons: ~ 10-2/GeV/c2 (need to generate one qq pair)

Baryons: ~ 10-4 /GeV/c2 (need to generate two pairs)

Pentaquarks: ~ 10-8 /GeV/c2 (?) (need togenerate 4 pairs)

Slope for Pentaquark??

Slope forbaryons

Slope for p.s.mesons

Pentaquarks in Quark Fragmentation?

Pentaquarks in e+e- (BaBaR)?

q

qqqqq

5+

e- e+

Current fragmentation

Pentaquarkproduction suppressed

Pentaquarks in ep ? (ZEUS, H1,HERMES)

Target fragmentation

s

+

e

d

d

uu

d

Current fragmentation

Pentaquarkssuppressed

Pentaquarks not suppressed

What do we know about the width of +?

JP = ½-JP = ½-

K+d X

Same width is obtained from analysis of DIANA results on KXe scattering. (R. Cahn and G. Trilling, PRD69, 11401(2004))

= 0.9 +/-0.3 MeV (K+d X)

W. Gibbs, nucl-th/0405024 (2004)

Belle: The basic idea

momentum, GeV/c

1 /

50M

eV

momentum spectra of K+ and K-

• Small fraction of kaons interacts in the detector material. Select secondary pK pairs to search for the pentaquarks.

• Momentum spectrum of the projectile is soft. low energy regime.

17cm

Belle: Distribution of Secondary pK- Vertices in Data

Y, c

m

X, cm

barrel endcap

“Strange particle tomography” of the detector.

Belle: Mass Spectra of Secondary pK

m, GeV

1 /

5MeV

pKS

pK-155fb-1

(1520)

What should we have expected here?

tot: K+d

width: 0.9+/-0.3 MeV

momentum, GeV/c

1 /

50M

eV

momentum spectra of K+ and K-

only narrow momentum bincan contribute to + productionif only 1 MeV wide and smeared by Fermi motion.

K+

n+

Momentum range possibly contributing to + production.

Belle: Mass Spectra of Secondary pK

m, GeV

1 /

5MeV

pKS

pK-155fb-1

(1520)

This is approx. what we should have expected here! Assume that background events have same isospin structure as + events.

< 80 events

For I=0:nK+: pK0

s: pK0L

2 : 1 : 1

Principle of the DIANA Experiment

liquid Xe

Liquid Xenon Bubble Chamber

proton

Ks

850 MeV

K+

The K+ beam gets slowed down in the Xe bubble

chamber and comes to a stop if no interaction occurs. Every K+ has the chance to generate a + within a few MeV energy bin, unless it interacts before it is sufficiently slowed down. This is a much more efficient way of using K+ compared to using a broad band beam on a thin target.

DIANA