MENU 2004 Institute of High Energy Physics Beijing, China August 29 – September 4, 2004

Highlights of Physics with CLAS at Jefferson Lab Volker D. Burkert Jefferson Lab MENU 2004 August 29 – September 4, 2004

Outline  Baryon Resonance Transitions in N , N  - N  - The “Roper” P 11 (1440) and S 11 (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 π Central question: What are the relevant degrees of freedom at varying distance scales? P.O. Bowman, et al., hep/lat LQCD e.m. probe q

CLAS JLab Site: The 6 GeV CW Electron Accelerator

C EBAF L arge A cceptance S pectrometer Drift chambers argon/CO 2 gas, 35,000 cells Electromagnetic calorimeters Lead/scintillator, 1296 PMTs Torus magnet 6 superconducting coils Gas Cherenkov counters e/  separation, 216 PMTs Time-of-flight counters plastic scintillators, 684 PMTs Large angle calorimeters Lead/scintillator, 512 PMTs Liquid D 2 (H 2 )target, NH3, ND3  start counter; e minitorus

N* Program at C LAS Primary Goals Extract photo- and electrocoupling amplitudes for known △, N* resonances –Partial wave and isospin decomposition of hadronic decay –Helicity amplitudes A 3/2, A 1/2,S 1/2 and their Q 2 dependence Search for resonances expected from SU(6)xO(3) or other symmetries –More selective hadronic decays: e e’ γvγv N N’,△’,  N*,△ , , K, K

Inclusive Electron Scattering p(e,e’)X

W-Dependence of Selected Channels at 4 GeV p(e,e’)X p(e,e’p)   p(e,e’  + )n p(e,e’p  + )  

N  (1232) Quadrupole Transition SU(6): E 1+ =S 1+ =0

Pion Electroproduction Structure Functions  Map out azimuthal and polar angle dependence to extract structure functions mnd mulipoles.

CLAS - N  (1232) transition Complete angular distributions in   and    in full W & Q 2 range.  Q 2 =3GeV 2 C L A S p r e l i m i n a r y cos  

CLAS - Legendre Expansion of S.F. Resonant Multipoles Non-Resonant Multipoles (M 1+ dominance)

N  (1232) Transition Form Factor G * M  1/3 of G * M at low Q 2 is due to vertex dressing and pion cloud contributions. bare vertex dressed vertex pion cloud Sato-Lee

CLAS - N  (1232) Transition  R EM remains small also at high Q 2 with trend towards R EM ~ 0. No clear trend seen towards sign change.  R SM continues to rise in magnitude with Q 2. No trend seen towards Q 2 - independent behavior.  Pion cloud models describe data well.

Structure of the P 11 (1440) & S 11 (1535).  Non-relativistic CQM’s have difficulties describing the properties of the P 11 (1440).  S 11 (1535) photo coupling amplitudes disagree for N  and p  channels. State suggested as K  dynamical resonance.  Q 2 evolution of transition form factors allows stringent model tests.

ep en  + CLAS UIM Global Fit – n  + response functions Q 2 =0.4

 LT’ – Sensitivity to P 11 (1440) Shift in S 1/2 Shift in A 1/2 Polarized structure function sensitive to imaginary part of P 11 (1440) through interference with real Born background. CLAS ep e  + n

Roper P 11 (1440) - Electrocoupling amplitudes UIM/DR - Analysis of CLAS data p    n   PDG Li 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 p    n   pp S 11 (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 g 1 and  1 (Q 2 ) Integral e + p e + X Expect rapid change of  1 in transition from the hadronic to the partonic regimes. parton hadron  1p

CLAS – Structure function g 1p (x,Q 2 )

CLAS – Structure function g 1d (x,Q 2 )

CLAS - A 1 at large x Proton Deuteron Data are consistent with an approach to A 1 = 1 as x  1 as required by pQCD.

CLAS – 1 st Moment of g 1 (x,Q 2 ) Effect of  (1232) excitation Neutron GDH slope Proton P r e l i m i n a r y

Bjorken Integral   p-n Is a fundamental description of the Bjorken integral possible at all distances?  HB  PT seems compatible for Q 2 <0.2GeV 2  pQCD and OPE seem to work for Q 2 >0.7GeV 2. Twist analysis underway.  First significant measurement in range Q 2 = 0.05–2.5 GeV 2

Resonances in p     |q 3 > |q 2 q>  Symmetric CQM |q 3 > predicts many more states than are observed in elastic  N scattering analysis.  For example, an additional P 13 below 1900MeV would rule out the |q 2 q> model.  The quark-cluster model |q 2 q> 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 P 33 (1600) *** state W=1.59 GeV no P 33 (1600) with P 33 (1600) Fit to high statistics photoproduction data requires inclusion of P 33 (1600) state. Sample data

Photoexcitation of P 13 (1720) in p     W=1.74 GeV PDG photocouplings Enhanced photocouplings fitted to the CLAS data P 13 (1720) state shows stronger presence in  p data. Sample data

Photo- and electroproduction comparsion photoproductionelectroproduction pp W(GeV) Background Resonances Interference full calculation no 3/2 + no 3/2 + (1720) full

Data fitted in mass range GeV and for photon virtualities 0, 0.65, 0.95, 1.30 GeV 2, assuming various J  assignment for a possible new state. JJ 3/2 + 1/2 - 3/2 - 1/2 + 7/2 + 5/2 - 5/2 + 2 Best description obtained with J  =3/2 + Spin-parity analysis of real & virtual photon data

P 33 (1600)Mass MeV Width MeV   A 1/2 *10 3 GeV -1/2 A 3/2 *10 3 GeV -1/2 This analysis1686 +/ /10065+/ / /- 10 PDG (***) / /-20 3/2 + (1720)Mass MeV Width MeV  N  New state?1722 +/ / / /- 10 PDG P 13 (1720) P states near 1700 MeV

The anti-decuplet of 5-quark states in 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

Spring-8 DIANA CLAS-dCLAS-p SAPHIR ITEP HERMES COSY ZEUS IHEP Summary of Experimental Masses K + d for ½ - K + d for ½ +  M ~ 12 MeV Shift could be due to different background shapes, second scattering, and interference effects.

CLAS – Production on Hydrogen 4.8 < E g < 5.4 GeV  p K + K -  + n  Further cuts are motivated by assumptions on production mechanism. no cuts

 Select t-channel process by tagging forward   and 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 - ) C LAS -  + (1540) on protons  GeV M(nK + )  p  + K + K - n cut   proton  ++ N* K+K+ n K-K-     production through N * resonance decays? V. Kubarovsky et al., PRL 92, (2004)

CLAS -  + (1540) and N* ?   proton  ++ N* K+K+ n K-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?

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 pK 0 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 g < 2.2 GeV for ~10nb cross section for  + (1530) and a hypothetical  * (1575). CLAS – G11 run on hydrogen target  p  K +  +  - (n) cos CM K 0 < N (1530) ~ 160 N (1575) ~ 300 simulation  p     p

Summary & Outlook N* physics - N  (1232) - Transition form factor measurements G M, R EM, R SM for 0.1 < Q 2 < 6 GeV 2. Large meson effects at low to medium Q nd resonance region - First consistent P 11 (1440) electro couplings. Large meson cloud effects. - Consistent S 11 (1535) electro couplings for p  and N . - New baryon resonances in N  - P 33 (1600) and a 3/2 + (1720) needed to explain p     data

Summary & Outlook, cont’d Spin Physics - g 1 (x,Q 2 ) and  1 (Q 2 ) measured in large Q 2 range for proton/deuterium. - pQCD Twist-2 description of  1 (p-n) for Q 2 > 0.7GeV 2, HB  PT for Q 2 < 0.2 GeV 2. 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 likely requires orders of magnitudes higher rates than available. Hadron production in e + e - Slope: Pseudoscalar mesons: ~ /GeV/c 2 (need to generate one qq pair) Baryons: ~ /GeV/c 2 (need to generate two pairs) Pentaquarks: ~ 10-8 /GeV/c 2 (?) (need to generate 4 pairs) Slope for Pentaquark?? Slope for baryons Slope for p.s. mesons

Pentaquarks in Quark Fragmentation? Pentaquarks in e + e - (BaBaR)? q qqqqq 5+5+ e-e- e+e+ Current fragmentation Pentaquark production suppressed Pentaquarks in ep ? (ZEUS, H1, HERMES) Target fragmentation s ++ e d d u u d Current fragmentation Pentaquarks suppressed Pentaquarks not suppressed

What do we know about the width of  + ? J P = ½ - K + d X Same width is obtained from analysis of DIANA results on K  Xe scattering. ( R. Cahn and G. Trilling, PRD69, 11401(2004))   = 0.9 +/-0.3 MeV (K + d X) W. Gibbs, nucl-th/ (2004)

Belle: The basic idea momentum, GeV/c 1 / 50MeV 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, cm X, cm barrelendcap “Strange particle tomography” of the detector.

Belle: Mass Spectra of Secondary pK m, GeV 1 / 5MeV pK S pK - 155fb -1  (1520) What should we have expected here?

 tot : K + d   width: 0.9+/-0.3 MeV momentum, GeV/c 1 / 50MeV momentum spectra of K + and K - only narrow momentum bin can contribute to  + production if only 1 MeV wide and smeared by Fermi motion. K+K+ n ++ Momentum range possibly contributing to  + production.

Belle: Mass Spectra of Secondary pK m, GeV 1 / 5MeV pK S 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 + : pK 0 s : pK 0 L 2 : 1 : 1

Principle of the DIANA Experiment liquid Xe Liquid Xenon Bubble Chamber proton     KsKs 850 MeV K+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