John W. Price N* 2005 12-15 October 2005 Cascade Physics A New Window on Baryon Spectroscopy John W....

John W. Price N* 2005 12-15 October 2005

Cascade PhysicsA New Window on Baryon Spectroscopy

John W. PriceCalifornia State University, Dominguez Hills

N* 2005 WorkshopOctober 12-15, 2005


Outline

● Motivation for Ξ studies

● Description of Ξ program

● Summary and “call for help”


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*


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)]


SU(3)F symmetry

● LQCD=L0+Lm

● L0 same for all quarks

● L0 sets the mass scale for a given multiplet

q

mq q

i

qi● Lm=

● Lm sets the mass splitting within a multiplet


SU(3)F MultipletsWith only qqq states, SU(3)F gives four multiplets

one

singlet: N 0 N0

0two octets:

0

0

0one

decuplet:

For every N*, a Ξ with

“similar properties"

For every Δ*,

a Ξ with

“similar properties”


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


Quality of Cascade Data

● Recent publication indicates σt(γp→K+K+Ξ-)=3.5±0.5

±1.5 nb

3.2<Eγ<3.9 GeV

3.0<Eγ<5.2 GeV

Price et al., PRC 71, 058201

(2005)

● Careful analysis yields very clean signal


Cascades with higher statistics

● 70 pb-1 γp data

● 1.6 < Eγ < 3.8 GeV

● Talk by Lei Guo

– Session P2-C

● Large Ξ production rate

– Sufficient for a Ξ program


Areas of study

● (Study of known/Search for missing) Ξ*'s

● Study of the Ξ0 excited states

● Ξ production mechanism

● Ξ decay modes

● JP determination of the Ξ*'s

● s-d quark mass difference

● Ξp scattering


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 14-33 Ξ's

● Those we know aren't known well

– Only have JP for three states

● Much to learn from a survey of the Ξ spectrum


The Ξ(1950)● 3-star state● JP 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.”


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?


Ξ0 Properties

● Ξ0 detected by requiring an additional π-

● Mass must be near Ξ- mass

● related to m m m 0 mq

md

mu

– ΔmΞ(1321) = +6.48±0.24 MeV

– ΔmΞ(1530) = +3.2±0.6 MeV

● Only other measurements: N(940), Δ(1232)

● Related to medium modification


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


Decays of the Ξ*'s

● Ξ-(1321) and Ξ0(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


JP measurements of the Ξ*'s● 8 of 11 Ξ's lack JP determination

● PWA technique not applicable

● Use Ξ* decay to determine JP

– Method of Byers & Fenster (PRL 11, 52)

● Ξ*→Ξπ, Ξ→Λπ; used to find JP 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


ms-md

● For octets: 2(mN+mΞ) = 3mΛ+mΣ

● For decuplets: mΔ-mΣ = mΣ-mΞ = mΞ-mΩ

● Can only test with one decuplet and two octets

● Once JP is measured, place Ξ's into multiplets and compare with the above

● Issue: is ms-md the same for excited baryons

as it is for ground-state baryons?


Ξp scattering● Very little work done on Ξp

scattering

– Useful for hypernuclear studies

● CERN data on σt at 101.5

GeV/c and 133.8 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/0502010


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


Ξ Workshop● December 1-3, 2005

@JLab

● Confirmed speakers:

– A. Thomas

– G. Miller

– S. Capstick

– F. Iachello

● Look forward to seeing you there!

John W. Price N* 2005 12-15 October 2005 Cascade Physics A New Window on Baryon Spectroscopy John W....

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