Photoproduction of Cascade baryons

1
Photoproduction of Cascade baryons

• Yongseok Oh (UGA)
• H. Haberzettl (GWU)K. Nakayama (UGA)
• 
  nucl-th/0605169

2
What do we know about ???

• If all the particles can be classified as SU(3)
  flavor octet or decuplet, N(X) N(N) N(D)
• So far, only a dozen or so of X have been
  identified.
• Only X(1318) and X(1530) have four-star status.
• Even the quantum numbers of most of the X
  resonances are unknown.
• So, very little is known about the ? resonances.
  But this may offer a good opportunity to find
  many interesting physics.

PDG
possibility of being in part a pentaquark
?(1520)S11 (B.-S. Zou, this meeting).
3
Cascade (S-2) baryons
GS ?(1318)P11
1st ES??(1530)P13
4
Theory of ??baryons (spectrum and decays)

• Quark Models
• ? SU(3), NR, EME decay model
  (Chao, Isgur, Karl, PRD23, 81).
• ? SU(3), NR, OPE model
  (Glozman, Riska, PR268, 96).
• ? SU(3), semi-rel., OBE model
  (Glozman et al., PRD58, 98).
• ? SU(3), OBEOGE model (Valcarce,
  Garcilazo, Vijande, PRC72, 05).
• ? 1/Nc expansion of QCD
  (Schat, Goity, Scoccola, PRL88, 02).
• Other works in progress
• ? SU(3) quark model, relativistic
  (S. Capstick collaborators).
• ? (quenched) lattice QCD
  (N. Mathur, D. Richards).

5
??baryon spectrum (predictions and expt)
Extracted from S. Capstick, Cascades_at_Jlab, July
29 2006
6
An interesting feature of Cascades

• ?? ? ?? decays are suppressed with respect to
  ??N?.
• For example
• ??(1232) ? ?p?? 1205
  MeV ??
• ???????????????????????????????????????? 9-10
  MeV
• - Other channels involve K, which cuts down the
  available
• phase space.
• - Leads to the possibility of narrow excited
  states.
• - Why are they narrow?
• Some of this is phase space
• decay momentum for ???? (P-wave) is 227
  MeV
• 
  ?(1530)??? (P-wave) is 152 MeV.

7
? decay widths
Extracted from S. Capstick, Cascades_at_Jlab, July
29 2006
8
? baryons should be investigated

• Cascade baryons should be studied as an integral
  part of
• the baryon spectroscopy program
• ? being an S-2 baryons they are
  produced only indirectly and
• have relatively low production
  rates ( nb).
• ? it has received attention recently in
  connection with the search
• for pentaquark baryons
  (NA49 collab., PRL92, 04).
• ? the CLAS collaboration at JLab has
  initiated a cascade physics
• program recently cascade
  spectroscopy through X
• photoproduction off nucleons
  (J.Price et al., PRC71, 05 and refs. therein).
• ? only one early inclusive
  photoproduction of ??reported
• 
  (TAPS collab., NPB282, 87, at T?105
  GeV).

9
???gp?KK??
L. Guo D. P. Weygand, for CLAS collab.,
hep-ex/0601011, Proc. NSTAR05
preliminary CLAS data
10
Aim of the present work
(Exploratory) theoretical investigation of the
reaction gN?KKX?within a relativistic
meson-exchange model of hadronic
interactions. As a first step toward building a
reliable reaction model for analyzing the cascade
spectroscopy data, one needs to understand in
detail the production mechanism(s) of the well
established cascades (X(1318)P11, X(1530)P13).
To date, no
cascade photoproduction calculation is
available so far, except for the
hadronic model
calculation by Liu and Ko (PRC69, 04) in
connection with the
pentaquark cascade production in g??KKX5
includes only the
hyperon S(1193) in the intermediate
state.
?(1520)S11? (B.-S. Zou).
11
?N ? KKX (model)
K-exchange
N/N
X/X
contact current
Y Y resonance current Y?Y radiative decay
( K1(q1)?K2(q2) )
K-exchange
12
gN?KKX (model)
t-channel Drell-type processes
require an exotic meson (S2) exchange
therefore, they are not considered in the present
model
13
?N ? KKX (baryon resonances included)
L(1116), L(1405), L(1520) S(1193),
S(1385) X(1530) D(1232) ? negligible
all the model parameters fixed from the relevant
decay rates(PDG) and/or quark models and SU(3)
symmetry considerations.
no enough information to fix the parameters
of the model.
14
?N ? KKX (model parameters)
15
gN?KKX (free parameters of the model)
ps-pv mixing parameter BYK vertex (spin-1/2
baryons B and Y)
?? ps-pv mixing parameter) 0 ,
ps-coupling 1 , pv-coupling
signs of
gBLK 0.91 , L(1405), BN,X gLL'g
1.26 , L(1116), L'(1520) gSL'g 2.22 ,
S(1193), L'(1520)
? BLK vertex
? radiative transition vertex
16
?N ? KKX (hadronic form factors)
q
p
p'
F
LB n free parameters but the
same for all B
n?8 fB(p2) ? Gaussian with width LB
LK 1.3 GeV
LK 1.0 GeV
17
gN?KK? (preliminary CLAS data, L. Guo D. P.
Weygand, for CLAS collab.,
hep-ex/0601011, Proc. NSTAR05)
BYK (ps-coupling) (LB, n)(1.25GeV, 2)
BYK (pv-coupling) (LB, n)(1.38GeV, 8)
phase space
PRELIMINARY CLAS DATA
18
gN?KK? (dynamical content spin-3/2 hyperon
contributions)
Y?Y' (rad. decay)
YY' (res)

19
gN?KK? (preliminary CLAS data, L. Guo
D.P.Weygand , private
communication)
gp?KKX-
PRELIMINARY CLAS DATA
(x 15)
Y?Y' (rad. decay)
20
gN?KK? (higher mass resonances)
Consider spin-1/2 and -3/2 resonances ? gNYK
can be estimated from the partial decay widths. ?
unless g?YK is unrealistically large
JP1/2 and 3/2- are negligibly small !
on-shell
21
gN?KK? ( addition of higher mass resonances)
L(2000)3/2 (gNLKgXLK2.5)
L(1850)1/2- (gNLKgXLK2.0) L(1950)3/2
(gNLKgXLK2.0)
(LB,n) (1.23GeV,8) BYK (pv-coupling)
(LB,n) (1.25GeV,8) BYK (pv-coupling)
22
gN?KK? ( adding L(1850)1/2- L(1950)3/2 )
PRELIMINARY CLAS DATA
23
gN?KK? ( adding L(1850)1/2- L(1950)3/2 )
PRELIMINAY CLAS DATA (L.Guo D.Weygand, private
communication)
24
gN?KK? ( adding L(1800)1/2- , L(1890)3/2
L(2050)3/2 )
L(1800)1/2- (gNLKgXLK2.0) L(1890)3/2
(gNLKgXLK1.2) L(2050)3/2 (gNLKgXLK1.4)
25
gN?KK? ( adding L(1800)1/2- , L(1890)3/2
L(2050)3/2 )
PRELIMINARY CLAS DATA (L.Guo D. Weygand,
private communication)
26
gN?KK? (higher spin resonances in the 2.0-2.1 GeV
region)
? work in progress to include them ! ?
unidentified L(2050)3/2 simulating these high
spin states as far as the invariant mass
distribution is concerned .
27
Spin asymmetries

• Photon beam asymmetry target asymmetry
• Caution Spin asymmetries may be sensitive to
  production mechanisms and need careful and
  detailed analyses.
• What do we have in these simple models?

28
Beam Asymmetry ?B
Low-mass hyperons
higher-mass hyperons
pv coupling
ps coupling

• K-exchange ? ?? -1.
• pv and ps couplings give the similar beam
  asymmetry.
• beam asymmetry distinguishes the models with and
  without higher resonances.

29
Target Asymmetry ?T
with higher-mass hyperons
ps coupling
pv coupling

• Target symmetry has different sign depending on
  the coupling scheme.

30
Summary of our findings

• The dominant ?- production mechanism in ?p?KK?-
  is the t-channel K-exchange process which is
  crucial in describing the observed backward
  peaked ?- and forward peaked K angular
  distributions. Also, the beam asymmetry can
  possibly provide an independent test of the
  t-channel K-exchange dominance.
• Higher mass hyperons in the mass region of
  1.8-2.1 GeV (in particular, ?(1800)1/2- and
  ?(1890)3/2) are needed to possibly provide the
  required t-channel K-exchange dominance. Low mass
  hyperons instead give raise to a dominant
  radiative hyperon-hyperon transition processes
  which lead to a forward peaked ?- and backward
  peaked K angular distributions (just opposite to
  what is observed in the preliminary CLAS data).
• The target asymmetry can possibly impose a
  constraint on the ps-pv mixing parameter.

31
Summary of our findings

• The K?- invariant mass distribution data
  indicate a need for additional resonance(s) in
  the 2.0-2.1 GeV region. In fact, there are known
  spin-5/2 and -7/2 hyperons (with 3 and 4 stars
  status) precisely in this energy region. We are
  currently working to include these resonances
  into the model.
• (the unknown ?(2050)3/2 was introduced in
  the present calculation for illustration
• purposes to make this point)
• Measurements of other isospin channels would help
  disentangle the isoscalar ? and isovector ?
  hyperon resonance contributions.

32
Conclusion

• To our knowledge, this is the first quantitative
  calculation of the cascade photoproduction off
  nucleons.
• The basic features of the ?p?KK?-(1318)
  reaction could be understood. In particular, this
  reaction can be used to help extract information
  on higher mass hyperon resonances.
• The findings of the present work should serve as
  a basis for building more complete models of
  cascade photoproduction to help analyze the
  forthcoming cascade data.

33

The End
34
Resonance widths
,
,
,
R?Np
qiR qi (WmR )
R?Npp
35
?N ? KKX (phenomenological contact current)
q1
q2
p
p'
B
bare NBKg contact vertex
GmC
G1
NBK vertex
ei-eB-e10