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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 • Introduction • Baryon Resonance Transitions in Np, Nh • - ND(1232) • - The “Roper” P11(1440) and S11(1535) • Nucleon Spin Structure in the Resonance Region • Baryon States in pp+p- • Search for Pentaquark Baryons - Status • Summary & Outlook
resolution of probe π low N LQCD P.O. Bowman, et al., hep/lat0209129 e.m. probe high Why Hadronic Physics with e.m. Probes Central question:What are the relevant degrees of freedom at varying distance scales? q
CLAS JLab Site: The 6 GeV CW Electron Accelerator
CEBAFLarge Acceptance Spectrometer Torus magnet 6 superconducting coils Large angle calorimeters Lead/scintillator, 512 PMTs Liquid D2 (H2)target, NH3, ND3 g start counter; e minitorus Gas Cherenkov counters e/p separation, 216 PMTs Drift chambers argon/CO2 gas, 35,000 cells Electromagnetic calorimeters Lead/scintillator, 1296 PMTs Time-of-flight counters plastic scintillators, 684 PMTs
N* Program at CLAS e’ , K γv e N*,△ N’,△’, L N 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: 2p, w, r, K
ND(1232) Quadrupole Transition SU(6): E1+=S1+=0
Pion Electroproduction Structure Functions • Map out azimuthal and polar angle • dependence to extract structure functions • mnd mulipoles.
Q2=3GeV2 C L A S p r e l i m i n a r y cosqp f CLAS - ND(1232) transition Complete angular distributions in Qp and fp ,in full W & Q2 range.
Resonant Multipoles Non-Resonant Multipoles CLAS - Legendre Expansion of S.F. (M1+ dominance) Preliminary
1/3 of G*M at low Q2 is due • to vertex dressing and pion • cloud contributions. bare vertex dressed vertex ND(1232) Transition Form Factor G*M Sato-Lee pion cloud
CLAS -ND(1232) Transition • REM remains small also at • high Q2 with trend towards • REM ~ 0. No clear trend seen • towards 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 Np and ph channels. State suggested as KS dynamical resonance. • Q2 evolution of transition form factors allows stringent model tests.
UIM Global Fit – np+ response functions ep enp+ CLAS Q2=0.4
ep ep+n Polarized structure function sensitive to imaginary part of P11(1440) through interference with real Born background. Shift in S1/2 Shift in A1/2 sLT’ – Sensitivity to P11(1440) CLAS
Li LC-Capstick Cano rel.CQM-Warns nonrel. zero crossing rel. large longitudinal amplitude Meson contributions or relativity needed to describe data. Roper P11(1440) - Electrocoupling amplitudes UIM/DR - Analysis of CLAS data pp0, np+ PDG
hypCP Giannini GWU (p) rCQM Capstick, Keister nrCQM PDG rCQM - Warns no p/h discrepancy p/h discrepancy S11(1535) - Electrocoupling amplitudes UIM/DR - Analysis of CLAS data pp0, np+ ph
The Nucleon Spin Structure in the Resonance Region.
e + p e + X G1p parton hadron Structure function g1 andG1(Q2)Integral Expect rapid change of G1 in transition from the hadronic to the partonic regimes.
CLAS– 1st Moment of g1 (x,Q2) Proton Neutron GDH slope P r e l i m i n a r y Effect of D(1232) excitation
pQCD and OPE seem to work for Q2>0.7GeV2.Twist analysis underway. • HBcPT seems compatible for Q2<0.2GeV2 Bjorken IntegralG1p-n • First significant measurement • in range Q2 = 0.05–2.5 GeV2 Is a fundamental description of the Bjorken integral possible at all distances?
|q3> |q2q> Resonances in pp+p- • Symmetric CQM |q3> predicts many more states than are observed in elastic pN scattering analysis. • The quark-cluster model |q2q> has fewer degrees of freedom => fewer states. Accomodates all observed **** states. • For example, an additional P13 below 1900MeV would rule out the |q2q> model. • 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 Sample data Fit to high statistics photoproduction data requires inclusion of P33(1600) state. no P33(1600) with P33(1600)
no 3/2+ full calculation Background Resonances Interference Photo- and electroproduction comparsion no 3/2+ (1720) pp+p- full electroproduction photoproduction W(GeV) W(GeV)
Pentaquark Baryon Search Diakonov, Petrov, Polyakov, 1997 The anti-decuplet of 5-quark states in the cSM.
CLAS-d LEPS DIANA SAPHIR ITEP SVD/IHEP HERMES ZEUS COSY-TOF pp S+Q+. Evidence for the Q+(1540) CLAS-p
no cuts g p K+K-p+n CLAS– Production on Hydrogen 4.8 < Eg < 5.4 GeV • Further cuts are motivated by assumptions on production mechanism.
Select t-channel process • by tagging forward p+ • and reducing K+ from • t channel processes * • cosqp+ > 0.8 * • cosqK+ < 0.6 (in c.m. frame) Exclusive Production on Hydrogen Possible production mechanism
g p+ p- K- K+ N* proton Q+ n • Cut on Q+ mass, and plot M(nK+K-) cut CLAS- Q+(1540) on protons Eg = 3 - 5.4 GeV • Q+production through N* resonance decays? gpp+K+K- n M(nK+) V. Kubarovsky et al., PRL 92, 032001 (2004)
CLAS-Q+(1540)and N* ? cuts outside Q+ outside cuts g p+ p- K- N* ? K+ N* proton Q+ n • p-p cross section data in PDG have a gap in the mass range 2.3–2.43 GeV. • What do p-p scattering data say?
High statistics searches for the Q+ on hydrogen and deuterium targets in nK+ and pK0 channels CLAS - A Program for Pentaquark Physics • Solving the issues of the Q+(1540) • exact mass ? • spin = ½ ? • parity = + or - ? • production mechanism ? • Are there excited states of the Q+(1540)? • How are pentaquark states related to N* states? • Search for the other exotic members of the decuplet X5-- ,X5+, seen in NA49 but unconfirmed. • Where are the non-exotic pentaquarks, N*’s, S’s?
CLAS – Second generation experiments • G10 - Measurement on deuterium (under analysis) • Eg = 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) • Eg = 1.6 – 3.8 GeV • First high statistics run at lower energies • EG3 – Search for exotic X-- • decay reconstructiongD X X--p-X-p-Lp-p • (begin 12/2004) • in missing mass gp K+K+X- (g tagged) (run in 2005/6)
g p K0K0p simulation g p K+p+p- (n) simulation cos JCMK0 <- 0.35 N (1530) ~ 160 N (1575)~ 300 CLAS – G11 run on hydrogen target Simulated data for 1.6 < Eg < 2.2 GeV for ~10nb cross section for Q+(1530) and a hypothetical Q*(1575).
Summary & Outlook N* physics • ND(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 ph and Np. • New baryon resonances in Npp? • P33(1600) and a 3/2+(1720) needed to explain pp+p- data
Summary & Outlook, cont’d Spin Physics • g1(x,Q2) and G1(Q2) measured in large Q2 range for proton/deuterium. • pQCD Twist-2 description of G1(p-n) for Q2 > 0.7GeV2, • HBcPT for Q2 < 0.2 GeV2. Pentaquark Baryons • High statistics search for Q+ and excited states is underway • on deuterium and hydrogen. • - 6 GeV run in preparation in the search for X5-- 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 Q+(1540)? Several high energy experiments have analysed their data In the search for the Q+. 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 Q+ signal Do these results contradict experiments that have seen a signal?
Slope for p.s. mesons Slope for baryons Slope for Pentaquark?? 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 to generate 4 pairs) Pentaquark production in direct e+e-collisions likely requires orders of magnitudes higher rates than available.
s Q5+ d d u Q5+ u d e- e+ Current fragmentation e Pentaquarks suppressed Pentaquarks in Quark Fragmentation? Pentaquarks in ep ? (ZEUS, H1, HERMES) Pentaquarks in e+e- (BaBaR)? Target fragmentation Pentaquarks not suppressed Current fragmentation qqqqq q Pentaquark production suppressed
K+d X JP = ½- JP = ½- GQ = 0.9 +/-0.3 MeV (K+d X) What do we know about the width of Q+? W. Gibbs, nucl-th/0405024 (2004) Same width is obtained from analysis of DIANA results on K+Xe scattering. (R. Cahn and G. Trilling, PRD69, 11401(2004))
17cm Belle: The basic idea • 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. momentum spectra of K+ and K- 1 / 50MeV momentum, GeV/c
Belle: Distribution of Secondary pK- Vertices in Data barrel endcap Y, cm X, cm “Strange particle tomography” of the detector.
What should we have expected here? Belle: Mass Spectra of Secondary pK 155fb-1 1 / 5MeV pK- (1520) pKS m, GeV
momentum spectra of K+ and K- 1 / 50MeV momentum, GeV/c only narrow momentum bin can contribute to Q+ production if only 1 MeV wide and smeared by Fermi motion. Momentum range possibly contributing to Q+ production. K+ Q+ n stot: K+d Q+ width: 0.9+/-0.3 MeV