John W. Price N* October 2005 Cascade Physics A New Window on Baryon Spectroscopy John W. Price California State University, Dominguez Hills N* 2005 Workshop October 12-15, 2005

John W. Price N* October 2005 Outline ● Motivation for Ξ studies ● Description of Ξ program ● Summary and “call for help”

John W. Price N* October 2005 Why Cascades? ● N* physics getting hard to do – Need lots of data – Need complicated computer analysis – Interpretation difficult ● States broad, overlapping ● We need another type of particle that... –...is narrow & easy to isolate –...is otherwise similar to N*

John W. Price N* October 2005 Why Cascades? ● Cascade widths are ~9 times narrower than N* widths ● Identifiable in a missing mass plot – related to (# of light quarks) 2 in baryon (“9:4:1” ratio) [Riska, Eur. Phys. J. 17, 297 (2003)]

John W. Price N* October 2005 SU(3) F symmetry ● L QCD = L 0 + L m ● L 0 same for all quarks ● L 0 sets the mass scale for a given multiplet ● Lm=● Lm= ● L m sets the mass splitting within a multiplet

John W. Price N* October 2005 SU(3) F Multiplets With only qqq states, SU(3) F gives four multiplets one singlet: two octets: one decuplet: For every N*, a Ξ with “similar properties" For every Δ*, a Ξ with “similar properties”

John W. Price N* October 2005 Producing Cascades ● K - p → K + Ξ - – Get Cascade with missing mass or reconstruction – No suitable kaon beams available ● γp → K + K + Ξ - – Detect K + K +, infer cascade in missing mass plot – Small physics background; none below 1.6 GeV – Same technique used for all excited states – Hall B, CLAS suitable

John W. Price N* October 2005 Quality of Cascade Data ● Recent publication indicates σ t (γp → K + K + Ξ - )=3.5±0.5±1.5 nb 3.2<E γ <3.9 GeV 3.0<E γ <5.2 GeV Price et al., PRC 71, (2005) ● Careful analysis yields very clean signal

John W. Price N* October 2005 Cascades with higher statistics ● 70 pb -1 γp data ● 1.6 < E γ < 3.8 GeV ● Talk by Lei Guo – Session P2-C ● Large Ξ production rate – Sufficient for a Ξ program

John W. Price N* October 2005 Areas of study ● (Study of known/Search for missing) Ξ*'s ● Study of the Ξ 0 excited states ● Ξ production mechanism ● Ξ decay modes ● J P determination of the Ξ*'s ● s-d quark mass difference ● Ξp scattering

John W. Price N* October 2005 Known/Missing Ξ* study ● Recall from SU(3) F symmetry – Octets: each N* has a corresponding Ξ – Decuplets: each Δ* has a corresponding Ξ ● We're missing Ξ's ● Those we know aren't known well – Only have J P for three states ● Much to learn from a survey of the Ξ spectrum

John W. Price N* October 2005 The Ξ(1950) ● 3-star state ● J P unknown ● “something” seen by several groups ● Large disagreement about mass ● “The accumulated evidence for a Ξ near 1950 MeV seems strong enough to include a Ξ(1950) in the main Baryon Table, but not much can be said about its properties. In fact, there may be more than one Ξ near this mass.”

John W. Price N* October 2005 Looking for new Ξ states ● Look for new Ξ states based on existing baryons ● Consider: – N(1535)½ - → N(940)η – Λ(1670)½ - → Λ(1115)η – Σ(1750)½ - → Σ(1190)η ● Is there a Ξ(1870)½ - that decays to Ξ(1321)η?

John W. Price N* October 2005 Ξ 0 Properties ● Ξ 0 detected by requiring an additional π - ● Mass must be near Ξ - mass ● related to – Δm Ξ(1321) = +6.48±0.24 MeV – Δm Ξ(1530) = +3.2±0.6 MeV ● Only other measurements: N(940), Δ(1232) ● Related to medium modification

John W. Price N* October 2005 The Ξ Production Mechanism ● Ξ's must be produced via an intermediate state ● Ξ structure related to relative contributions of these and other mechanisms ● Different energy, angular dependence expected ● Much data, theoretical input needed to resolve

John W. Price N* October 2005 Decays of the Ξ*'s ● Ξ - (1321) and Ξ 0 (1321) decays well-known: Λπ ● All Ξ*'s decay strongly ● Ξ(1530) → Ξπ; BR[Ξ(1530) → Ξγ]<4% ● Ξππ channel opens up at 1585 MeV; ΛK at 1608 MeV, ΣK at 1682 MeV ● Can use BR's to distinguish nearby states

John W. Price N* October 2005 J P measurements of the Ξ*'s ● 8 of 11 Ξ's lack J P determination ● PWA technique not applicable ● Use Ξ* decay to determine J P – Method of Byers & Fenster (PRL 11, 52) ● Ξ* → Ξπ, Ξ → Λπ; used to find J P for Ξ(1530) – Method of J. Button-Shafer (PR 139, B607) ● Ξ* → Ξ(1530)π – Generalized by Minnaert (PL B66, 157) ● Ξ* → Ξπ and Ξ* → ΛK

John W. Price N* October 2005 m s -m d ● For octets:2(m N +m Ξ ) = 3m Λ +m Σ ● For decuplets:m Δ -m Σ = m Σ -m Ξ = m Ξ -m Ω ● Can only test with one decuplet and two octets ● Once J P is measured, place Ξ's into multiplets and compare with the above ● Issue: is m s -m d the same for excited baryons as it is for ground-state baryons?

John W. Price N* October 2005 Ξp scattering ● Very little work done on Ξp scattering – Useful for hypernuclear studies ● CERN data on σ t at GeV/c and GeV/c ● new KEK preprint; upper limit of 24 mb for elastic; 4.3(+6.3,-2.7) mb for Ξ - p → ΛΛ Ahn et al., nucl-ex/

John W. Price N* October 2005 Summary and Prognosis ● Cascade photoproduction has opened up a wide range of research areas ● Many things to keep experimentalists occupied ● We have a “new window” on baryon spectroscopy ● Lots of theoretical support needed – Production models – Lattice calculations (easier for cascades...) ● JLab Ξ workshop: 1-3 December 2005

John W. Price N* October 2005 Ξ Workshop ● December 1-3, ● Confirmed speakers: – A. Thomas – G. Miller – S. Capstick – F. Iachello ● Look forward to seeing you there!