Cascade Physics A New Window on Baryon Spectroscopy

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Cascade PhysicsA New Window on Baryon
Spectroscopy

• John W. Price
• California State University, Dominguez Hills
• N 2005 Workshop
• October 12-15, 2005

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Outline

• Motivation for ? studies
• Description of ? program
• Summary and call for help

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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

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Why Cascades?

• Cascade widths are 9 times narrower than N
  widths

• related to ( of light quarks)2 in baryon
  (941 ratio) Riska, Eur. Phys. J. 17, 297
  (2003)

• Identifiable in a missing mass plot

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SU(3)F symmetry

• LQCDL0Lm
• L0 same for all quarks
• L0 sets the mass scale for a given multiplet

• Lm sets the mass splitting within a multiplet

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SU(3)F Multiplets
With only qqq states, SU(3)F gives four multiplets
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Producing Cascades

• K-p?K?-
• Get Cascade with missing mass or reconstruction
• No suitable kaon beams available
• ?p?KK?-
• Detect KK, 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

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Quality of Cascade Data

• Recent publication indicates st(?p?KK?-)3.50.5
  1.5 nb

• Careful analysis yields very clean signal

Price et al., PRC 71, 058201 (2005)
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Cascades with higher statistics

• 70 pb-1 ?p data
• 1.6 lt E? lt 3.8 GeV

• Large ? production rate
• Sufficient for a ? program

• Talk by Lei Guo
• Session P2-C

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Areas of study

• (Study of known/Search for missing) ?'s
• Study of the ?0 excited states
• ? production mechanism
• ? decay modes
• JP determination of the ?'s
• s-d quark mass difference
• ?p scattering

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Known/Missing ? study

• Recall from SU(3)F symmetry

• Octets each N has a corresponding ?
• Decuplets each ? has a corresponding ?
• We're missing 14-33 ?'s
• Those we know aren't known well
• Only have JP for three states
• Much to learn from a survey of the ? spectrum

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The ?(1950)

• 3-star state
• JP unknown
• something seen by several groups
• Large disagreement about mass

• The accumulated evidence for a ? near 1950 MeV
  seems strong enough to include a ?(1950) in the
  main Baryon Table, but not much can be said about
  its properties. In fact, there may be more than
  one ? near this mass.

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Looking for new ? states

• Look for new ? states based on existing baryons
• Consider
• N(1535)½-?N(940)?
• ?(1670)½-??(1115)?
• S(1750)½-?S(1190)?
• Is there a ?(1870)½- that decays to ?(1321)??

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?0 Properties

• ?0 detected by requiring an additional p-
• Mass must be near ?- mass

• ?m?(1321) 6.480.24 MeV
• ?m?(1530) 3.20.6 MeV
• Only other measurements N(940), ?(1232)
• Related to medium modification

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The ? Production Mechanism

• ?'s must be produced via an intermediate state

• ? structure related to relative contributions of
  these and other mechanisms
• Different energy, angular dependence expected
• Much data, theoretical input needed to resolve

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Decays of the ?'s

• ?-(1321) and ?0(1321) decays well-known ?p
• All ?'s decay strongly
• ?(1530)??p BR?(1530)???lt4
• ?pp channel opens up at 1585 MeV ?K at 1608 MeV,
  SK at 1682 MeV
• Can use BR's to distinguish nearby states

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JP measurements of the ?'s

• 8 of 11 ?'s lack JP determination
• PWA technique not applicable
• Use ? decay to determine JP
• Method of Byers Fenster (PRL 11, 52)
• ???p, ???p used to find JP for ?(1530)
• Method of J. Button-Shafer (PR 139, B607)
• ???(1530)p
• Generalized by Minnaert (PL B66, 157)
• ???p and ???K

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ms-md

• For octets 2(mNm?) 3m?mS
• For decuplets m?-mS mS-m? m?-mO
• Can only test with one decuplet and two octets
• Once JP is measured, place ?'s into multiplets
  and compare with the above
• Issue is ms-md the same for excited baryons as
  it is for ground-state baryons?

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?p scattering

• Very little work done on ?p scattering
• Useful for hypernuclear studies
• CERN data on st 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 ?-p???

Ahn et al., nucl-ex/0502010
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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 ? workshop 1-3 December 2005

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? Workshop

• December 1-3, 2005 _at_JLab
• Confirmed speakers
• A. Thomas
• G. Miller
• S. Capstick
• F. Iachello

• Look forward to seeing you there!