1. K. Hicks, Ohio Univesity CLAS Collaboration Meeting Nov. 4, 2016
Differential cross sections and polarization observables from K* Photoproduction and the Search for New N* States
K. Hicks, Ohio Univesity
CLAS Collaboration Meeting
Nov. 4, 2016

2. Outline Introduction: missing N* states
Reminder of the published K* data from Wei Tang’s PhD
The suggestion of “missing” high-mass resonances
The new results of spin-density matrix elements from Bonn
Interpretation using the Bonn-Gatchina PWA
Possible new high-mass N* resonances
What more needs to be done to establish these N*’s

3. The N* Spectrum (lattice QCD)
J.J. Dudek and R.G. Edwards, PRD85 (2012)
Hybrids

4. Physics of broad & overlapping resonances
Δ (1232)
Width: a few hundred MeV.
Resonances are highly overlapped
　　in energy except D(1232).
→Complex PWA is necessary
Width: ~10 keV to ~10 MeV
Each resonance peak is clearly separated.
From: H. Kamano, JAEA seminar 4

5. Missing Baryon States (2010)
Empty/Yellow boxes are missing/uncertain baryon states.

6. Missing Baryon States (2012)
Empty/Yellow boxes are missing/uncertain baryon states.

7. Evidence for new N* states and couplings
N((mass)JP
PDG 2010
PDG 2012 KΛ KΣ Nγ
N(1710)1/2+
***
(not seen in GW analysis) **
N(1880)1/2+ *
N(1895)1/2-
N(1900)3/2+
N(1875)3/2-
N(2150)3/2-
N(2000)5/2+
N(2060)5/2-
Bonn-Gatchina Analysis – A.V. Anisovich et al., EPJ A48, 15 (2012)
(First coupled-channel analysis that includes nearly all new photoproduction data)

8. Do new states fit into LQCD projections?
11/16/2018
R. Edwards et al., Phys.Rev. D84 (2011)
N(2060)5/2-
N(2120)3/2-
N(1875)3/2-
N(1895)1/2-
N(1860)5/2+
N(1900)3/2+
N(1880)1/2+
Known states:
N(1675)5/2-
N(1700)3/2-
N(1520)3/2-
N(1650)1/2-
N(1535)1/2-
mπ=396MeV
“Roper” state ~ 700 MeV too high
Lowest J- states ~ 300 MeV too high
Ignoring the mass scale, new candidate states fit with the JP values predicted from LQCD.
Slide borrowed from V. Burkert.

9. Excited Baryons in the history of the Universe
11/16/2018
Excited baryons are at
the transition between the quark-gluon liquid, described in hot QCD, and the confinement of quarks and gluons in nucleons, described in strong QCD. This period lasted ~ 10-6 seconds.
Do we understand this transition? N*
Slide borrowed from V. Burkert.

10. Slide borrowed from S. Mukherjee.

11. Slide borrowed from S. Mukherjee.

12. Slide borrowed from S. Mukherjee.

13. Slide borrowed from S. Mukherjee.

15. Summary of Introduction
Both Lattice QCD and the quark model predict more resonances than have been detected.
Some of these are expected to couple strongly to two-pion decay, to be measured at E45.
Theoretical tools to extract N*’s are improving
PWA from both ANL-Osaka and BoGa find new N*
Data from QCD freeze-out suggest more Y*’s
L(1405) decays asymmetric
Possible effect on n/p ratio in the early universe.

16. From: Wei Tang, Ph.D. Dissertation Defense, August, 2012
Theoretical models for the K* photoproduction
1. Isobar models:
To describe the physics process completely, all possible Feynman diagrams that could lead to the final state are required to be taken into account in the calculation.
evaluate tree-level Feynman diagrams that include resonant and non-resonant exchanges of baryons and mesons.
Advantage:
Identify the dominant contributions to the final states
Disadvantage:
Too many parameters, tuning and fixing of those parameters sometimes are tricky.
Mandelstam variables:
From: Wei Tang, Ph.D. Dissertation Defense, August, 2012

17. From: Wei Tang, Ph.D. Dissertation Defense, August, 2012
2. Regge-ized models:
Rather than focus on selecting of all possible s, t and u channel reaction processes, the reggeized models emphasis the t-channel.
The standard propagators in the Lagrangian are replaced by Regge propagators.
Originally applied to high energy hadron reactions
Might not be able to produce the results in detail, but at least it can tell us about how t-channel mesons exchanges affects the reaction
Sho Ozaki,
From: Wei Tang, Ph.D. Dissertation Defense, August, 2012

18. K*+L Differential Cross Sections
Curves are simple fits using 4th order
Legendra polynomials.

19. K*+S0 Differential Cross Sections

20. Total Cross Sections
Both data peak at about W=2.25 GeV. There are 3 well-known N*’s there: the N7/2-(2190), N9/2-(2250) and N9/2+(2250).
Note: the N9/2- is part of the L-forbidden [70,48] multiplet.

21. Comparison with theory
Cyan: Oh and Kim
Isobar Model
Blue: Kim, Nam, Oh, Kim
Regge Model
Dotted curves include additional s-channel N* with M<2.2 GeV and L<3.
Clearly, the currently available theoretical models cannot reproduce the data. This suggests that higher-mass and higher-L resonances are needed.

22. New analysis of K* L by the Bonn group
Used the CLAS K* skim of g11 done by Wei Tang
Measured K*+  K0 p+  p+ p+ p- final state
L identified using missing mass
Phase-space acceptance using fsgen with GSIM/GPP
Completely independent background subtraction
3 methods: 1) sideband, 2) Q-value to extract K*, 3) two-level Q-factor
All three methods give results consistent within statistical uncertainties
Analyzed angular distribution in the K* rest system
Extracted density matrix elements: r00, r10, r11 using Log Likelihood method
Fit using the BoGa PWA to search for new N* resonances

23. L and K* yield extraction

24. In the following plots:
Solid curves: final PWA fit (including possible new N* states)
Reduced c2: 0.84, 1.84, 0.76 for diff. x-sec, spin-density matrix, recoil pol.
Blue dotted curves: PWA fit without the new high-mass N*’s
Noticeable (but small) effect in the mass regions
Reduced c2: 1.92, 1.84, 0.61
Red dashed curves: t-channel contributions only
At higher photon energies, we expect t-channel to dominate

30. PDG: listed high-mass N* states plus new N*’s
Notes: 1) the photocoupling to the new N*’s (marked with *) are not known.
2) The N*(1880) and N*(1895) are very close to threshold—handled carefully.

31. Possible new high-mass resonances
Comments:
1) masses and widths in the fits are fairly stable.
2) These N*’s have significant B.R. to the K* final state
3) Reasonable PWA fits even with any two of the three states

32. Caveat Emptor The new N* states are seen only in this reaction
We need other final states at higher masses to confirm
It is likely that there is substantial coupling to two-pion decay
We need hadronic-beam data to separate photocouplings.
The spin-density matrix elements are useful to constrain PWA fits
There is significant interference in the N* amplitudes
Too much uncertainty in the PWA fits without the spin-density m.e.
More polarization data would be nice
This is, in fact, possible using g12 data

33. Summary This has been a fruitful collaboration between CLAS and Bonn
There are other opportunities, such as Nick Compton’s K0 L data
The spin-density matrix elements were relatively easy to extract, but this work would not have gotten done without the Bonn group
It is likely that the “missing” N* resonances are there, and just need to be “found”
Reactions with high-mass thresholds are useful to explore high-mass N*’s
A wide variety of final states can be explored by CLAS
Polarization observables are helpful to constrain the PWA
We have 2-3 possible new high-mass N*’s here (need confirmation)

34. Backup Slides

35. Moorhouse Selection Rule
The transition amplitudes for gp to all [70,48] are zero.
For gn, these transitions are allowed, e.g. the N5/2-(1675).
N1/2+
N3/2-
N5/2-
Data from N. Bianchi et al., PRC 54 (1996).

36. Lambda Selection Rule
Reference: Q. Zhao and F.E. Close, Phys. Rev. D 74 (2006)
The [70,48] resonances decouple from the KL and K*L channels.
This assumes the spectator approximation, where the [ud] quarks are coupled to s=0 in the L.
In the [70,48] N*’s, the [ud] are coupled to s=1.
The selection rule applies to both p and n targets.
This doesn’t apply to KS and K*S final states.
A study of KL, K*L, KS and K*S final states will test this spectator (diquark) hypothesis.