1. Cascade Physics with CLAS
Aleksandr Starostin
Ben Nefkens
UCLA

2. Outline Motivation Measurements with CLAS6 Cascades with CLAS12
Summary
February 26, 2009
A.Starostin,

3. Cascades: Physics Motivation
Review of Cascade physics:
Workshop “Cascade Physics: A New Window on Baryon Spectroscopy”, December 1-3, 2005, JLab
Presentations available at
February 26, 2009
A.Starostin,

4. What is known about cascades?
Ξ0 = uss, Ξ-= dss
Only 11 Ξ* known; 6 “well-established”
Those we know aren't known well: only have JP for three states and a guess of a fourth
SU(3) symmetry requires one Ξ I=1/2 per octet and per decuplet:
n(Ξ*)=n(N*) +n(Δ*)
Adamovich et al., EPJ C5, 621
Hemingway et al., PLB 68, 1972
February 26, 2009
A.Starostin,

5. Width of the Cascades
N* and Δ* resonances overlap, because of that the spectroscopy of N* and Δ* requires complicated (and ambiguous) phase-shift analysis
If cascades are as narrow as expected, the parameters of the cascade excited states can be extracted directly from the mass spectra
February 26, 2009
A.Starostin,

6. Cascades in quark models
45 cascades states with masses below 2.5 GeV
S.Capstick and N. Isgur PRD (1986)
February 26, 2009
A.Starostin,

7. Cascades in algebraic model
33 cascades states with masses below 2.5 GeV
A.R. Bijker, F.Iachello, and A. Leviatan
Ann. Phys (2000)
February 26, 2009
A.Starostin,

8. Cascades in the Constituent Quark Model
S.Capstick and N.Isgur
Reasons for Ξ* to be narrow:
Phase-space factor for the decay Ξ*->Ξπ compared to N*->Nπ and Δ*->Δπ
Flavor overlap factor
Riska relation:
Γ(N*,Δ*) : Γ(Λ*,Σ*) : Γ(Ξ*) ≈ 32: 22: 12
February 26, 2009
A.Starostin,

9. Physics with Cascades
Spectroscopy of cascade exited states (complementary to N* and Δ*)
d-u quark mass difference (m(Ξ-)-m(Ξ0) ~ 6.5 MeV, requires good energy resolution), s-d quark mass difference, test of octet-decuplet mass relations
Search for exotic states: Ξ+ and Ξ--; ΞΞ bound state
February 26, 2009
A.Starostin,

10. Search for parity doublets
1/2
3/2
5/2
7/2
9/2 - + N*
1650
( )
1710
( )
1700
( )
1720
( )
1675
( )
1680
( )
2250
( )
2220
( )
February 26, 2009
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11. Search for parity doublets
The high-mass light baryon spectrum exhibits occurrence of parity doublets. Glozman suggested to explain the mass degeneracy by assuming that chiral symmetry is restored in highly excited hadronic states.
L.Y. Glozman, PL B 475, 329 (2000)
T.D. Cohen and L.Y. Glozman PR D 65, (2001)
R.L. Jaffe et al. Phys. Rep. 435, 157 (2006)
M. Shifman and A. Vainshtein PR D 77, (2008)
“Evidence for a Parity Doublet Δ(1920)P33 and Δ(1940)D33
from γp->pπ0η”
I.Horn et al., PRL 101, (2008)
February 26, 2009
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12. Photoproduction of cascades
Cascades in CLAS g11:
Number of events: ground state – 7678, first exited state – 658
maximum energy: 3.8 GeV for most of the run, some data at 4.8 GeV (production threshold ~2.4 GeV)
σtotal(γp -> 2K+Ξ-) ~ 10nb, flat between 3.5 GeV and 4.8 GeV (estimated by Denis Weygand).
New data – run g12:
The data were collected in spring 2007
Cooking process is in progress
J.W. Price et al. PRC 71, (2005), L. Guo et al. PRC 76, (2008)
February 26, 2009
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13. Detector requirements
Very good K/π separation – RICH detector can be great help
Two detached vertices – high resolution tracking device is essential
Hall-B: tagged photons with energies below 7.5 GeV, 12 GeV electron beam
CLAS12: very good momentum resolution, good particle ID in forward direction, central tracker
February 26, 2009
A.Starostin,

14. Photoproduction vs. Electroproduction
First stage: photoproduction at energies below 7.5 GeV. The energy range covers cascades with masses below 2.85 GeV
Second stage: electroproduction at energies above 7.5 GeV. Will cover masses from 2.5 to 3.8 GeV. LowQ2 spectrometer is required.
February 26, 2009
A.Starostin,

15. Acceptance Estimates
Kinematics: γp->Ξ-K+K+, followed by Ξ-->π-Λ and Λ->π-p+
Resolutions: FD >1%, CD>5%
PID: both kaons in FD
Momentum cutoffs: 300 MeV/c and 500 MeV/c
February 26, 2009
A.Starostin,

16. Acceptance Estimates for γp->Ξ0K+K0
γp->Ξ0K+K0, followed by Ξ0->π0Λ and K0->π-π+, Λ->π-p+
February 26, 2009
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17. Decay modes of excited cascades
Ξ* will be “tagged” using KK in the forward detector
The background can be suppressed by detecting products of the decays
States with similar masses but different JP can be separated using different decay modes
February 26, 2009
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18. Reactions to detect – charged cascades
γp->Ξ*K+K+:
γp->(Ξ-π0)K+K+, or γp->(Ξ0π-)K+K+
γp->(ΛK-)K+K+
γp->(Σ0K-)K+K+, or γp->(Σ-K0)K+K+
Assuming decays Ξ-->Λπ-(100%), Ξ0->Λπ0(100%), Λ->pπ-(64%), Σ-->nπ-(100%), Σ0->Λγ(100%) and K0s->π+π- all these decay modes can be detected, some with one missing particles
February 26, 2009
A.Starostin,

19. Reactions to detect – neutral cascades
γp->Ξ*K0K+:
γp->(Ξ0π0)K0K+, or γp->(Ξ-π+)K0K+
γp->(ΛK0)K0K+
γp->(Σ0K0)K0K+, or γp->(Σ+K-)K+K+, or γp->(Σ-K+)K+K+
Assuming decays Ξ-->Λπ-(100%), Ξ0->Λπ0(100%), Λ->pπ-(64%), Σ-->nπ-(100%), Σ0->Λγ(100%), Σ+->pπ0(52%), and K0s->π+π- all these decay modes except for γp->(Ξ0π0)K0K+, can be detected, some with one missing particles
February 26, 2009
A.Starostin,

20. Summary
CLAS12 will provide almost 4π geometrical acceptance, very good momentum resolution and PID capabilities combined with 7.5GeV tagged photon beam. All this makes cascade program with CLAS12 feasible. Both, charged and neutral cascades can be detected using missing mass and invariant mass techniques.
The objectives of the cascade program will be to establish spectrum of the excited states and their spin-parity. The data can also be used to investigate existence of the cascade parity doublets, calculate d-u quark mass difference and search for the exotic cascade states.
February 26, 2009
A.Starostin,

21. February 26, 2009
A.Starostin,

22. Cascades in 1/Nc expansion
70-plet:
7 masses in range MeV
Two pairs of same J states nearly degenerate
L=2 56-plet:
6 masses in range MeV
Same J states separated by >140 MeV
Larger errors in predictions than in 70-plet
Parameter free mass relations rather well tested.
Presented by J L Goity
February 26, 2009
A.Starostin,

23. Search for “Cascaderium”
NN scattering length a =-17.3 fm.
If ΞΞ 1S0 potential is same as for NN:
predicted a BOUND STATE:
dibaryon S=-4
decays weakly to ΞΛπ->NN4π
ΞΞ binding predicted by many potential models
γd -> 2K+ 2K0 [Ξ0 Ξ-]
γα -> 2K+ 2K0 [Ξ0 Ξ-] n p
Neutral cascades are preferred to avoid repulsion
(J.Miller, Cascade workshop)
Figure by R.Jones
February 26, 2009
A.Starostin,

24. Search for exotic Ξ+ and Ξ--
Ξ+ and Ξ–- are unique signatures for a five-quark state, member of the antidecupled
Evidences for existance of Ξ–- were found by CERN NA49 (PRL, 92, (2004))
Search was also conducted by CLAS (run g12?)
February 26, 2009
A.Starostin,