Topics in Baryon Spectroscopy and Structure Volker D. Burkert Jefferson Lab Scottish Universities Summer School in Physics August 2229, 2004, St. Andrews, UK Overview Introduction, Multiplets, SU(6)xO(3) Analysis Tools, Equipment Electromagnetic Excitation of the (1232) Structure of the Roper and other lower
mass resonances. Missing Resonances Exotic Baryons (Pentaquarks) I II III Why N*s are important Nathan Isgur, N*2000 Conference Nucleons represent the real world, they must be at the center of any discussion on why the world is the way it is
Nucleons represent the simplest system where the non-abelian character of QCD is manifest Gluon flux simulation of a 3-quark system. Nucleons are complex enough to reveal physics hidden from us in mesons Gell-Mann & Zweig - Quark Model: 3 x 3 x 3 = 10 + 8 + 8 + 1 O. Greenberg - The problem and color Phys. Rev. 85, 936 (1952) p X
p (1232) X An energy excitation spectrum indicates that the proton has a substructure. This was two years later confirmed in elastic ep scattering by Hofstadter. Total cross sections (PDG2004) p X p(GeV/c)
The (1232) leads to color ++ ++ uuu sflavorspin O. Greenberg introduces a new quantum number to get asymmetric w.f. p is the largest N cross section, but the state is not allowed in CQM w/o color.
++ as flavorspincolor ++ uuu Baryon multiplets Baryons qqq 3 3 3 10 8 8 1 Y=B+S 2 ++ -
N 1 I3 -1 -1/2
+1/2 1 Baryon Resonances and SU(6)xO(3) |Baryon> : |qqq> + |qqq(qq)| + |qqqG> + .. 3 Flavors: {u,d,s} 3 + 3 + {qqq}: +
+ 6 SU(3) 3 = 10 + 8 + 8 + 1 Quark spin sq = {qqq }: 6 Lectures by F. Close SU(2) 6 = 56 +70 + 70 + 20 SU(6) multiplets decompose into flavor multiplets:
56 = 410 + 28 70 = 210 + 48 + 28 + 21 20 = 28 + 41 O(3) Baryon spin: J = L + si parity: P = (-1)L SU(6)xO(3) Classification of Baryons Missing P13(1870) Capstick and Roberts D13(1520) S11(1535) (1232
) Roper P11(1440) Configuration Mixing in [70,1-] States with same I, Jp quantum numbers and different total quark spins Sq = 1/2 or Sq = 3/2, mix with mixing angle M. Sq = 1/2 Sq = 3/2 The pure quark states |N2, 1/2- > and |N4, 1/2- > in [70,1-] project onto physical states S11(1535) and S11(1650). |S11(1535)> = cos1|N2,1/2-> - sin1|N4, 1/2-> |S11(1650)> = sin1 |N ,1/2 > +cos1|N , 1/2 > 2 -
4 - 1 = 31o (measured in hadronic decays). Similarly for |N2,3/2- > and |N4,3/2- > |D13(1520)> = cos2|N2,3/2-> - sin2|N4, 3/2-> |D13(1700)> = sin2 |N2,3/2-> +cos2|N4, 3/2-> 2 = 6o The |N4,5/2- > quark state has no N2 partner, and cannot mix. |D15(1675) > = |N4,5/2- > p Notation: L2I,2J Analysis Tools
Simple searches for resonances For a 2-body decay one can search for resonance structures in the invariant mass distribution. p,m proton 1 1 P, M p2, m2 pion 4-vectors M2 = (pp + p)2 M
Rarely can resonances be observed just in mass distributions, e.g. if state is narrow, or if strongly excited. It also gives no information on quantum numbers other than isospin. Dalitz Plot for 3-body decay (e.g. p+K0) p1, m1 3-body decay p2, m2 P, M p3, m3 Resonance at: m12 = 1.8 GeV
Resonance at: m23 = 2.0 GeV A narrow resonance at m12 = 2.0 GeV may appear like a broad enhancement in m23 (kinematical reflection). Dalitz Plot: pK+K- p E = 1.6-3.5 GeV (1020)
Argand Diagram Elastic scattering amplitude of spinless particle with momentum k in cms: f(k,) = 1/k (2l+1)alPl(cos) l al = (l e2il 1)/2i , 0 < l < 1 , l : phase shift of lth partial wave For purely elastic scattering: l = 1, (e.g. N -> N) d/d = |f(k,)|2 Optical theorem: tot = 4/k[Im f(k,0)] Cross section for lth partial wave is bounded:
l = 4/k2(2l+1)|al|2 < 4(2l+1)/k2 Argand Diagram al : partial wave amplitude evolving with energy. The amplitude leaves the unitary circle where inelasticity sets in. al = (l e2il 1)/2i Im A 1 inelasticity sets in /2 1/2 -1/2
al +1/2 Re A Breit-Wigner Form B-W (non-relativistic) form for an elastic amplitude al with a resonance at cm energy ER and elastic width el and total width tot is el/2 al = ER E itot/2 Relativistic form: al = mel s m2 imtot
Many other B-W forms exist, dependent of process dynamics. Electromagnetic Excitation of Baryon Resonances Why electroexcitation of N*s ? resolution of probe Addresses the question: What are the relevant degrees of freedom at different distance scales? low N
LQCD q P.O. Bowman, et al., hep-lat/0209129 e.m. probe high Spatial resolution ~1/q => Constituent quark model with fixed quark masses only justified at photon point and low q. Reach of Current Accelerators
Spring-8 JLAB Large Acceptance Detectors for N* Physics. CLAS: (photon and electron reactions) Final states with mostly charged particles. Operates with electron beams and with energy-tagged photon beams. Coverage for photons limited to lab angles < 45o Crystal Barrel-ELSA: (photon reactions) CsI crystals with excellent photon detection, e.g. N SAPHIR-ELSA (photon reactions, detector dismantled) Charged particles in final state GRAAL (photon reactions): BGO crystals, with excellent photon detection, limited charged particle, polarized laser-backscattered tagged photon Crystal Ball MAMI (photon reactions) neutral final states with excellent resolution, limited W range BES (Beijing) N* in e+e- collisions.
Not included are setups for more specialized applications. JLab Site: The 6 GeV CW Electron Accelerator Emax ~ 6 GeV Imax ~ 200 A Duty Factor ~ 100% E/E ~ 2.5 10-5 Beam P ~ 80% E(tagged) tagged) ) ~ 0.85.5 GeV
CLAS CEBAF Large Acceptance Spectrometer Torus magnet 6 superconducting coils Liquid D2 (H2)target + start counter; e minitorus Drift chambers argon/CO2 gas, 35,000 cells Large angle calorimeters Lead/scintillator, 512 PMTs Gas Cherenkov counters e/ separation, 216 PMTs Electromagnetic calorimeters Lead/scintillator, 1296 PMTs
Time-of-flight counters plastic scintillators, 684 PMTs + Single Event d p K K X K- p K+ Missing Mass Distribution p
pX Super Photon ring-8 GeV SPring-8 Third-generation synchrotron radiation facility Circumference: 1436 m 8 GeV 100 mA 62 beamlines Laser Electron Photon facility at SPring-8 in operation since 2000
LEPS detector Aerogel Cerenkov (n=1.03) TOF wall Dipole Magnet (0.7 T) Start counter Liquid Hydrogen Target (50mm thick)
Silicon Vertex Detector MWDC 1 MWDC 3 MWDC 2 1m The GRAAL Experiment The Crystal Barrel @ ELSA CsI detector invariant mass
Electromagnetic Excitation of N*s e v e Primary Goals N*,,N* resonances N,, Extract photocoupling amplitudes for known ,N* resonances,N* resonances
N , 2 , , , , K Partial wave and isospin decomposition of hadronic decay Assume EM and strong interaction vertices factorize Helicity amplitudes A3/2 A1/2 S1/2 and their Q2 dependence Study quark wave function and symmetries Quark models: relativity, gluons vs. mesons. Identify missing resonances expected from SU(6)xO(3) More selective hadronic decays: 2, , K
Inclusive Electron Scattering p(e,e)X W-Dependence of Selected Channels at 4 GeV p(e,e)X (trigger) p(e,ep) p(e,e+)n p(e,ep+) p(e,ep+)X N(1232) Transition N-(1232) Quadrupole Transition
SU(6): E1+=S1+=0 N - Quadrupole transition in SQT N(938) N(938) (1232) (1232) C2 M1 Magnetic single quark Transition. C2
Coulomb single quark transition. Pion Electroproduction Structure Functions 2 d M 2 1 / Re(E 1*M 1 ) Re(S 1*M1 ) p (e , e p )
Longitudinal sensitivity w/o Rosenbluth separation. Measurement requires out-of-plane detection of hadronic decay. 0 Structure functions extracted from fits to * distributions for each (Q2 ,W, cos*) point. LT and TT interference sensitive to weak quadrupole and longitudinal multipoles.
The Power of Interference I Unpolarized structure function LT ~ Re(L*T) = Re(L)Re(T) + Im(L)Im(T) Amplify small resonance multipole by an interfering larger resonance multipole Im(S1+) Im(M1+) Large P33(1232) Small
Truncated Multipole Expansion in (1232) Region s, p waves only, Jmax= 3/2 , M1+ dominance, i.e. retain only terms containing M1+ 6 unknown terms remain, which can be determined uniquely by measuring the azimuthal and polar angle dependence of the cross section. N* program N(1232) transition Structure Functions - Invariant Mass W Structure Functions - cos * |M1+|2(1-3/5cos2)
-|M1+|2-2Re(M1+E1+*) A+6cosRe(M1+S1+*) Legendre Expansion of Structure Functions T L (M1+ dominance) Resonant Multipoles M Non-Resonant Multipoles
Re( E M Re(E1 0 Electroproduction of (1232)(1232) Im(M1+) => G*M Recent quark models still fall short at low Q2 Missing qq strength? Sea quarks? Multipole Ratios REM, RSM before 1999 Sign?
Q2 dependence? Data could not determine sign or Q2 dependence Multipole Ratios REM, RSM in 2002 Sign? <0! Q2 dependence Slope < 0 ! No trend towards zero crossing and pQCD behavior is observed for Q2 up to 4 GeV2.
REM, RSM in 2004 REM LQCD (unquenched) 0 Deviation from spherical symmetry of the (1232) in LQCD (unquenched). -5 0 RSM -5 -10
LQCD (unquenched) 10-1 1 Q2 (GeV2) 5 Dynamical models attribute the deformation to contributions of the pion cloud at low Q2. What does empirical E1+/M1+ ratio measure? e/ * Deformation of N,,N* resonances quark core? e e/
* 0 Shape of pion cloud? e Answer will depend on wavelength of probe. With increasing resolution, we are mapping out the shape of the vs. the distance scale. SU(6)xO(3) Classification of Baryons
D13(1520) S11(1535) Roper P11(1440) What are the issues? P11(1440): Poorly understood in nrCQMs Alternative models: - Light front kinematics (relativity) - Hybrid baryon with gluonic excitation |q 3G> - Quark core with large meson cloud |q 3m> - Nucleon-sigma molecule |Nm> - Dynamically generated resonance S11(1535):
Hard form factor Not a quark resonance, but Kdynamical system? D13(1520): Change of helicity structure with increasing Q 2 from =3/2 dominance to =1/2 dominance, predicted in nrCQMs, pQCD. CQM: - Photocoupling Amplitudes of the P11(1440) (status of 2003, data are from the 1970s & 80s, p0 cross sections only) LC |q3G> |q3G> nrCQM
nrCQM rCQM q3 G The failure of CQMs to describe the photocoupling amplitudes led to the development of the hybrid model |q3G> . In non-rel. approximation A1/2(Q2) , S1/2(Q2) behave like the (1232) amplitudes. Lattice calculations of P11(1440), S11(1535) F. Lee, N*2004 => Christine Davies Masses of both states well reproduced in quenched LQCD with
3 valence quarks. Resonance analyses above the Delta. Above the (1232) many multipoles can contribute. Resonance parameters are extracted in somewhat model-dependent fashion with approaches such as Unitary Isobar Models and Dispersion Relations, tuned to previous data. Parameterizations incorporate theoretical constraints such as known Born terms, unitarized amplitudes, and different isospin channels. A detailed discussion of analyses approaches is given in: V.Burkert, and T.S.-H. Lee, nucl-exp/0407020 (2004) Global Analysis of Nucleon Resonances Based on Unitary Isobar Model. Includes all resonances seen in photoproduction PWA Breit-Wigner resonant amplitudes:
Fixed background from nucleon pole diagrams, t-channel pion, - and -meson exchange. Regge behavior for W2 > 2.5 GeV2 with a smooth transition from UIM to Regge background: A (W ) Phase modifications to resonant P33 amplitudes to satisfy Watsons theorem below 2-pion threshold. Dispersion Relations Causality, analyticity constrain real and imaginary amplitudes: Born term is nucleon pole in s- and u-channels and meson-exchange in t-channel. Dispersion integrals summed over 3 energy regions: (
,0) i thr Re B ds / 2.2 Ge Integrals over resonance region saturated by known resonances (Breit-Wigner). P33(1232) amplitudes found by solving integral equations. Integrals over high energy region are calculated through ,,,b1,a1 Regge poles. However, these contributions were found negligible in Regions 1 and 2. For channel, contributions of Roper P11(1440) and S11(1535) to unphysical
region s<(m+mN)2 of dispersion integral included. thr Isospin Amplitudes Nucleon resonances are eigenstates of isospin, with I = 1/2 , 3/2. Final states in electromagnetic meson production are not eigenstates of isospin. The photon transfers I = 0, 1 resulting in 3 isospin amplitudes for production: Ts: Isoscalar, ImN = 1/2 T1v: Isovector, ImN = 1/2 T3v: Isovector, ImN = 3/2 For production from proton target: Examples: P(1232), I = 3/2 => T3v contributes => (+n/0p)2 = 1/2 P(1440), I = 1/2 => Ts, T1v contribute => (+n/0p)2 = 2
=> Need both channels to separate and N* states The Roper P11(1440) as a gluonic partner of the nucleon ? Because gluonic baryons do not have exotic quantum numbers they must be distinguished from ordinary baryons in different ways. ... electromagnetic transition form factors are a powerful tool in distinguishing regular |q3> states from |q3G> states. more complete data are needed to study the apparently strong Q2 dependence of A1/2 at small Q2, and to establish more accurate values for the longitudinal coupling. VB in: Czechoslovac Journal of Physics, Vol. 46, No. 7/8 (1996) Fit Summary # data points:15,447 , Ee = 1.515, 1.645 GeV Observable
Q2 Data points 2 data 2 data UIM DR 0.40
0.65 3530 3818 1.22 1.22 1.21 1.39 0.40 0.65 2308 1716
1.69 1.48 1.97 1.75 ALT / ( 0 ) 0.40 0.65 956 805 1.14 1.07 1.25 1.30
ALT / ( ) 0.40 0.65 918 812 1.18 1.18 1.63 1.15 d d
0.375 0.750 172 412 1.32 1.42 1.33 1.45 d d d d 0
Fits for ep en+ 2 d Fits to Structure Functions ep Q2=0.4 GeV2 en+ UIM Fits for ep en+ Polarized beam
Ae +-Ae= ++- / h 2 L (1 ) LT sin* sin * beam helicity UIM vs. DR Fits for ep en+ Q2=0.4 GeV2 W = 1.53 GeV UIM DR
Power of Interference II Unpolarized structure function LT ~ Re(L*T) = Re(L)Re(T) + Im(L)Im(T) Im(S1+) Im(M1+) P33(1232) Amplify small resonance multipole by an interfering larger resonance multipole Polarized structure function ~ Im(L*T) = Re(L)Im(T) + Im(L)Re(T) Amplify resonance multipole by a large background amplitude
LT Large Small Im(S1-) Re(E0+) Bkg P11(1440) Resonance Sensitivity to P11(1440) ep
Shift in S1/2 Shift in A1/2 e+n Polarized structure functions are sensitive to imaginary part of P11(1440) through interference with real Born background. Roper P11(1440) - Electrocoupling amplitudes q 3G Li CQM-Capstick
q 3 Cano rel.CQM-Warns nonrel. zero crossing rel. large longitudinal amplitude PDG pn UIM/DR - Analysis of CLAS data
Meson contribution or relativity are needed to describe data. Roper P11(1440) - Electrocoupling amplitudes q 3G Li CQM-Capstick q 3 Cano rel.CQM-Warns nonrel. rel.
PDG pn UIM/DR - Analysis of CLAS data Meson contribution or relativity are needed to describe data. previous results Comments on the Roper results LQCD shows a 3-quark component. Does it exclude a mesonnucleon resonance? Roper resonance transition formfactors not described in nonrelativistic CQM. If relativity (LC) is included the description is improved. Best description in model with large meson cloud. Gluonic excitation, i.e. a hybrid baryon, seems ruled out due to strong longitudinal coupling.
Other models need to predict transition form factors as a sensitive test of internal structure. The S11(1535) an isolated resonance S11 p (~55%) Q2=0 The S11(1535) an isolated resonance Use same approximation as for the (1232). |E0+|2 For lmax=2 There is no interference between the resonant multipoles E 0+ and S0+ in this approximation. Assume S0+ is small, use resonance approximation to
extract |E0+| => A1/2. S11(1535) - Electrocoupling amplitudes UIM/DR - Analysis of CLAS data GWU () PDG pn p hypCP Giannini rCQM nrCQM Capstick, Keister rCQM - Warns no discrepancy / discrepancy
no model comes close D13(1520) Electrocoupling amplitudes A3/2 A1/2 S1/2 CQM prediction: A1/2 dominance at high Q2. hypCP Giannini rCQM nrCQM Capstick, Keister rCQM - Warns
PDG average UIM/DR - Analysis of CLAS data pn A1/2/A3/2 ~ Q2 at large Q2, consistent with pQCD prediction. Single Quark Transition Model Basic process: q (F. Close, Quarks and Partons) q
In a frame where the process is collinear: z q q quark spin flipped along z boost z N* N q z = z q not collinear along z => z and Lz can be flipped Single Quark Transition Model
EM transitions between all members of two SU(6)xO(3) multiplets expressed as 4 reduced matrix elements A,B,C,D. J AL B Lz C z L D LL Lz 1 S z 1 Example: 56, 0 70,1 (D=0) Lz 1 S z 1
Lz 2 S z 1 A orbit flip B spin flip Fit A,B,C to D13(1535) and S11(1520) A3/2, A1/2 SU(6) ClebschGordon A,B,C,D
C spin-orbit Predicts 16 amplitudes of same supermultiplet Single Quark Transition Model Photocoupling amplitudes SQTM amplitudes (C-G coefficients and mixing angles) Single Quark Transition Model Predictions for [56,0+][70,1-] Transitions Proton Single Quark Transition Model Predictions for [56,0+][70,1-] Transitions Neutron A1/2=A3/2= 0 for
D15(1675) on protons Missing Baryon States Quark models with underlying SU(6)xO(3) symmetry predict many states, not observed in either hadronic experiments or in meson photo- and electro-production. Possible solutions: 1. States dont exist, e.g. di-quark model predicts fewer states, with different underlying symmetry group |q2q> 2. States exist but have not been found. Possible reason: they decouple from Nchannel. Model expectations: Hadronic couplings to N(, N) much larger, while photocouplings are more comparable to those for observed states. Other channels sensitive to missing states are: K, K, p
|q3> Evidence for new baryon states? - Is the P33(1600) is really there? - One more 3/2+(1720) state ? - A new N*(2000) ? - New resonances in p, K Search for Baryon States in p p Two methods: Isobar models (similar approach as in single pion analysis): energy-dependences of amplitudes are parameterized. fits to one-dimensional projections. Event-by event analysis: fit partial-wave content independently for every energy bin.
makes maximum use of all correlations in the multidimensional phase space. ambiguities can give multiple solutions. A variation of this method uses energy-dependent partial waves in isobar formulation. Search for Baryon States in p p Dynamical Isobar Model JLab-MSU Residual production mechanism SU(6)xO(3) Classification of Baryons P(1600) Evidence for P33(1600) *** state p
p Fit to high statistics photoproduction data requires inclusion of P33(1600) state. no P33(1600) with P33(1600) Sample data W=1.59 GeV P33(1600) state parameters this analysis world
Mass, MeV 1686 10 1550 - 1700 PDG 1687 44 Dytman 1706 10 Manley Total decay width, MeV 338 100 250 - 450 PDG 493 75 Dytman 430 75 Manley BF (
65 6 40 -70 PDG 59 10 Dytman 67 5 Manley A1/2 -30 10 - 29 20 PDG A3/2 -17 10 -19 20 PDG A1/2, A3/2 [GeV-1/2*100]
A new 3/2+(1720) baryon state? JLab-MSU Dynamical Model Analysis Contributions from conventional states only Fit with new 3/2+(1720) state M.Ripani et. al. Phys. Rev. Lett.91, 022002 (2003) Difference between curves due to signal from possible 3/2+(1720) state ep ep Photo- and electroproduction comparsion electroproduction p
photoproduction 2 Q2Q=0 =0 W(GeV) W(GeV) Photoexcitation of P13(1720) in p W=1.74 GeV P13(1720) state shows stronger presence in p data. PDG photocouplings Enhanced photocouplings fitted to the CLAS data
Total p p cross-section off protons. no 3/2+ Hadronic couplings and mass derived from the fit of virtual photon data, and 3/2+(1720) photocouplings fitted to the real photon data. Signal from 3/2+(1720) state present, but masked by large background and destructive N*/background interference. full calculation Background Resonances
Interference Parameters derived from combined analysis Mass and decays Mass, MeV Total width, MeV BF(), % BF(P), % New 3/2+ State
1722 92 50 11 PDG P13(1720) 1650-1750 100-200 not observed 70 85 Partial Wave Formalism for p
p Transition matrix: Tfi =
p = p ;f ||Ti|p;E
f V JP, M |> = |JP M,isobar,l,s,f > Decay amplitude f) calculated using isobar model: E.g. JP = 3/2+, M =+1/2 (l=1) , f =+ Production amplitude V (E) is fitted in unbinned
maximum likelihood procedure. Assume V (E) is independent of E in small energy range. No assumptions are made on intermediate resonances, only on quantum numbers. p L
p t-channel processes Waves used in the following analysis JP M Isobars Motivation 1/2+ 1/2
(={, o+}) P11(1440), P11(1710) 1/2- 1/2 , (p)(s=1/2 3/2+ 1/2, 3/2 ()(l=1) ,(p)(s=1/2) ,(p)(s=3/2;l=1,3) N*(1440) S11(1535), S11(1650),
S31(1620) P13(1720), P33(1600) 3/2- 1/2, 3/2 () (l=0,2) 5/2+ 1/2, 3/2 ()(l=1), pF15(1860) 5/2- 1/2, 3/2 ()(l=2) amplitudes)
Total of 35 waves (complex Diffractive production (t-channel) also included D13(1520), D13(1700) D33(1700) D15(1675) Partial wave fits to p data for W = 1.69 1.71 GeV 4 waves 37 waves Dalitz Plot for p Data Monte Carlo
Comparison with Isobar Model Fit shows good agreement between the two methods Can we discover new baryons with this technique? F15(1680) P13(1720) ? M ~ 1650 MeV, ~ 115 MeV M ~ 1770 MeV, ~ 85 MeV Mass shifts due to interference effects? Other searches for new baryon states.
New N* resonance in J/ decays ? New data from BEPC (e+e- collider in Beijing) suggest a new N* state at ~2068 MeV observed in: e+eJ/ NN Why is there no (1232) peak? p-n Isospin conservation in decay => IN = . p+n 2N*s Roper? 4 N*s 1360 2068 1360
MN 2068 Strangeness Photoproduction Dominant resonances S11(1650) P11(1710) P13(1720) D13(1895) ? Carnegie Mellon Strangeness Photoproduction Sample of data covering the
full kinematic range in energy and angles for K+ and K+, including recoil polarization Data indicate significant resonance contributions, interfering with each other and with non-resonant amplitudes. Extraction of resonance parameters requires a large effort in partial wave analysis and reaction theory. Strangeness in electroproduction CLAS forward hemisphere
*p K+ backward hemishere known N* new N*? Resonances in p Model: Y. Oh OPE + Pomeron N* Capstick model Sum p
p p p p p p
Pentaquark baryons are we discovering a new form of matter? From Meson & Baryons to Pentaquarks Mesons: quark-antiquark pair s 1/3 u K+ u 2/3 d
Baryons: three quarks (valence) 1/3 +2/3 d -1/3 d Pentaquarks: 4 quarks + 1 antiquark 1/3 u QCD requires that hadrons
must be colorless +2/3 s +1/ 3 u +2/3 + d 1/ 3
n Types of Pentaquarks Non-exotic pentaquarks The antiquark has the same flavor as one of the other quarks Difficult to distinguish from 3-quark baryons Example: uudss, same quantum numbers as uud Strangeness = 0 + 0 + 0 - 1 + 1 = 0 Exotic pentaquarks The antiquark has a flavor different from the other 4 quarks They have quantum numbers different from any 3-quark baryon Unique identification using experimental conservation laws Example: uudds Strangeness = 0 + 0 + 0 + 0 + 1 = +1 Hadron Multiplets
K Mesons qq - K Baryons qqq ++ N
Baryons built from qqqqq B+S + 2 1 I3 -1 -1/2 +1/2 1 --
+ The Anti-decuplet in the Chiral Soliton Model D. Diakonov, V. Petrov, M. Polyakov, Z.Phys.A359, 305 (1997) S = +1 Symmetries give an equal spacing between tiers S= 0 180MeV S = -1 S = -2 < 15 MeV
assumption in model The Anti-decuplet in the Chiral Soliton Model D. Diakonov, V. Petrov, M. Polyakov, Z.Phys.A359, 305 (1997) and in the Quark Model uudds udd (uu ss ) uud (d d ss ) dds (uu ss ) uds (uu d d ss ) ddssu dss (uu d d ) uss (uu d d )
uus (d d ss ) uussd Some quark descriptions of the + Pentaquark (qq)q description (Jaffe, Wilczek) (qqq)(qq) description (Karliner, Lipkin) two color non-singlets L=1 L=0 (ud) L=1 s
(ud) L=1, one unit of orbital angular momentum needed to obtain JP = + as in the SM (ud) (uds) distance > color magnetic force JP = + LQCD: JP = + no signal 2 groups 1 group
1 group Evidence for + Pentaquark Spring8 JLab DIANA JLab ITEP ELSA SVD/IHEP HERMES ZEUS
COSY-TOF pp ++. G. Rosner CEBAF Large Acceptance Spectrometer Torus magnet 6 superconducting coils Liquid D2 (H2)target + start counter; e minitorus Drift chambers argon/CO2 gas, 35,000 cells Large angle calorimeters Lead/scintillator, 512 PMTs Gas Cherenkov counters
e/ separation, 216 PMTs Electromagnetic calorimeters Lead/scintillator, 1296 PMTs Time-of-flight counters plastic scintillators, 684 PMTs CLAS -xclusive production from deuterium Photon beam on d) euterium E= 1 - 3 GeV TOF particle id t (p-K) (ns)
.p .k Kaon time relative to proton time K-pK+n D K-pK+event reconstruction tK t R p ; c c c p 2 mK2
pp- p+- .d .H pK+K t (p-K+) (ns) Process identification and event selection Missing mass technique D 3-body Dalitz plot
K pK n - + cut (1020) cut Neutrons mass +(1540) in CLAS S. Stepanyan et al., PRL91, 252001 (2003) Requires rescattering from proton to allow detection of proton in CLAS. CLAS ExclusiveProduction on Hydrogen
4.8 < E < 5.4 GeV p K+K-+n no cuts Further cuts are motivated by assumptions on production mechanism. ExclusiveProduction on Hydrogen Possible production mechanism Select t-channel process by tagging forward and reducing K+ from t channel processes cos
cos (in c.m. frame) + CLAS - (1540) on protons 3 - 5.4 GeV p M(nK+) +K+K- n Significance = 7.8
production through N* resonance decays? proton N* KK+ + n V. Kubarovsky et al., PRL 92, 032001 (2004)
cut + CLAS - production mechanism? E = 3 - 5.4 GeV p M(nK+) +K+K- n + production through N* resonance decays? 7.8significance ~ 35 MeV
proton N* KK+ + n cut Cut on + mass, and plot M(nK+K-) +
CLAS - (1540) and N* ? proton N* KK+ + n What do -p scattering data say? -p cross section data in PDG
have a gap in the mass range 2.32.43 GeV. Evidence for + Pentaquark Spring8 JLab DIANA JLab ITEP HERMES ZEUS ELSA
SVD/IHEP This is a lot of evidence! So, what isCOSY-TOF the problem? JP = pp ++. So, what is the problem? If Pentaquark baryons exist it is the most important finding in hadronic physics since the J/ discovery. It is absolutely necessary to obtain fully convincing experimental data. Many experiments see positive + signal with specific kinematical cuts, taken together they represent an impressive significance. However, few experiment have fully convincing results: - significance is often optimistically estimated ~46 - background estimates are not always justified - masses are not fully consistent (15251555) MeV - are kinematical reflections excluded?
Many high energy experiments present null results. This adds a level of uncertainty until we understand the sensitivities in various experiments. The very narrow width of ~1 MeV is not understood, although models have been developed that allow + widths of < 1 MeV. Reminder - Kinematical Reflection A narrow resonance in m12 near kinematical limit may appear like a broad enhancement in m23 (kinematical reflection). The +(1540) as a kinematical reflection ? Is this a more realistic background? + nK+
If kinematical reflections from M K+Kcan generate the + peak, they should show up in nK- as well, assume isospin symmetry. nK- Kinematic reflections do not seem to generate narrow nK - peak Nobody can seriously suggest that this is a kinematical reflection! p +K+K- n M(nK+) 7.8significance ~ 35 MeV
Is there a problem with the mass? K+d for + K d for + W. Gibbs (nucl-th/0405024) - IHEP ZEUS COSY HERMES
ITEP SAPHIR CLAS-p CLAS-d DIANA Spring-8 M ~ 12 MeV Mass shift could be due to different background shapes, final state interactions, and different interference effects in the two channels. Are the null experiments sensitive to +(1540)? Several high energy experiments have analyzed 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? Hadron production in e+e- Slope for p.s. mesons Slope:
Pseudoscalar mesons: ~ 10-2/GeV/c2 (need to generate one qq pair) Baryons: ~ 10-4 /GeV/c2 (need to generate two pairs) Slope for baryons Slope for Pentaquark?? 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. Pentaquarks in Quark Fragmentation? Pentaquarks in e+e- (BaBaR)? Pentaquarks in ep ? (ZEUS, H1, HERMES) e- e+ 5+ qqqqq Current fragmentation q Target fragmentation
Pentaquarks not suppressed s d + u u d d e Pentaquark production suppressed Current fragmentation Pentaquarks
suppressed What do we know about the width of +? K+d JP = - X W. Gibbs, nucl-th/0405024 (2004) JP = - = 0.9 +/-0.3 MeV (K+d X) Same width is obtained from analysis of DIANA results on KXe scattering. (R. Cahn and G. Trilling, PRD69, 11401(2004))
Belle: The basic idea Momentum spectrum of the projectile is soft. low energy regime. momentum spectra of K+ and K- 1 / 50MeV 17cm Small fraction of kaons interacts in the detector material. Select secondary pK pairs to search for the pentaquarks. momentum, GeV/c Belle: Distribution of Secondary pK- Vertices in
Data endcap Y, cm barrel X, cm Strange particle tomography of the detector. 1 / 5MeV Belle: Mass Spectra of Secondary pK 155fb-1 pK- (1520)
pKS m, GeV What should we have expected here? tot: K+d momentum spectra of K+ and K- 1 / 50MeV only narrow momentum bin can contribute to + production if only 1 MeV wide and smeared by Fermi motion. momentum, GeV/c
Momentum range possibly contributing to + production. width: 0.9+/-0.3 MeV K+ n + 1 / 5MeV Belle: Mass Spectra of Secondary pK 155fb-1 pK- For I=0: nK+: pK0s: pK0L 2 : 1 : 1
(1520) < 80 events pKS m, GeV This is approx. what we should have expected here! Assume that background events have same isospin structure as + events. Principle of the DIANA Experiment DIANA Liquid Xenon Bubble Chamber
850 MeV K+ Ks liquid Xe proton 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. Belle: Compare with DIANA
Kaon momentum range that may contribute to + excitation in nuclei ~ 50MeV/c. Note that this restriction is absent in the DIANA experiment where the K+ looses momentum continuously throughout the interaction region, i.e. every K+ has the chance to contribute to the + signal. 1 / 50MeV 17cm momentum spectra of K+ and K- momentum, GeV/c
K+Si/C (thin) K+Xe(thick) versus DIANA Belle Mom. spectrum 850MeV/c Summary of Existing Null Experiments need to prove their sensitivity to the + before they can claim anything. Proving a negative is, of course, difficult. The best is to reproduce the experiments that have seen the signal and repeat them with higher statistics, better systematics, etc.. This is what is happening at JLab. High energy experiments studying current fragmentation processes may not have sensitivity to see any signal. Sensitivity should be much higher in target fragmentation region (HERMES, ZEUS, H1). Experiments using broad band momentum spectrum in
secondary interaction (K+-nucleus) must compare with DIANA and K+D scattering results and prove sensitivity Whats next with CLAS? CLAS at JLab finished data taking with two runs - Statistics > 10 times with deuterium target - high statistics run on hydrogen target - Other high statistics runs at higher energy are in preparation CLAS - G10 online plots Fully exclusive processes: n d K-pK+n
d K-pKs()psp Poster by Bryan McKinnon Ks CLAS - G11 online plots p p p)
(calibration reaction) proton KsK+(n); KsKsp; K+K-p; K+K-+(n) K+ (n) p Ks The End of my Lectures
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