1. Pentaquarks: Discovering new particles

2. Outline S=+1 B=+1 Mass = 1.54 GeV Historical introduction
Review of the Quark Model
Prediction of the Q+
Experimental evidence
Data from CLAS
What do we know about the Q+?
Other 5-quark states
Phys.Rev.Lett. 91 (2003)
S=+1
B=+1
Mass = 1.54 GeV

3. Families within families of matter
DNA
10-7 m
Molecule
10-9 m
Atom
10-10 m
Nucleus
10-14 m
Proton
10-15 m
Quark
<10-18 m

4. Families of atoms Gaps in table lead to predictions for
the properties of undiscovered atoms
Mendeleev (1869)

5. Properties of quarks u p d u n d s K− u s u Protons are made of (uud)
Quark Flavor
Charge (Q)
Baryon number
Strangeness (S) u
+2/3
+1/3 d
−1/3 s −1
− 2/3 +1 u d
+2/3
−1/3 p d u
+2/3
−1/3 n s
−1/3 u
−2/3 K− s u
+1/3
+2/3
Protons are made of (uud)
Neutrons are made of (ddu) K+

6. Families of quarks
S=+1
S= 0
I3 = Q ─ ½ (B+S) −½ +½
S=−1

7. Families of baryons
ms=150 MeV W− ?

8. Production and decay of W─ → Xo p─K0 W– K+ g L0 X0 p– K–
V.E. Barnes et. al., Phys. Rev. Lett. 8, 204 (1964)

9. Interactions understood in terms of quarks
Very high energy
“free” quarks not found, only
particles that
contain quarks

10. Electromagnetic and color forces
±1 charge
O(a) ~ 0.01 g
±3 “color” charges
O(as) ~ 1
QCD explains particle interactions

11. Quarks are confined inside colorless hadrons
Mystery remains:
Of the many possibilities for
combining quarks with color into
colorless hadrons, only two
configurations were found, till now… q
Particle Data Group 1986 reviewing evidence for exotic baryons states
“…The general prejudice against baryons not made of three quarks and the lack of any experimental activity in this area make it likely that it will be another 15 years before the issue is decided.
PDG dropped the discussion on pentaquark searches after 1988.

12. What are pentaquarks? Example: uudss, non-exotic
Minimum quark content is 4 quarks and 1 antiquark
“Exotic” pentaquarks are those where the antiquark has a different flavor than the other 4 quarks
Quantum numbers cannot be defined by 3 quarks alone.
Example: uudss, non-exotic
Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1
Strangeness = − = 0
Example: uudds, exotic
Baryon number = 1/3 + 1/3 + 1/3 + 1/3 – 1/3 = 1
Strangeness = = +1

13. Baryons built from meson-baryon, or qqqqq
Hadron multiplets K p
Mesons qq
Baryons qqq N S X W─
Baryons built from meson-baryon, or qqqqq Q+

14. The Anti-decuplet predicted by Diakonov et al.
In the Chiral Soliton Model, nucleons and Deltas are
rotational states of the same soliton field.
Z.Phys. A359, 305 (1997)
Rotational excitations include
G~15 MeV Z+
The mass splittings are
predicted to be equally
spaced
Anchor
G~140 MeV

15. Quark lines for the reactiong K− us us K+ Q+ n
ddu
ddu n
Q+ is composed of (uudds) quarks

16. Production mechanismsK+ K− g
pspec Q+ p
(p)
(K0) p K+ K− g
L*(1520)
Control Reaction

17. Experimental Evidence
Several experimental observations
No dedicated experiments to date
Walk through the analysis from CLAS

18. Exclusive measurement using gd reactions
CLAS Collaboration (S. Stepanyan, K. Hicks, et al.), hep-ex/
Requires FSI – both nucleons involved
No Fermi motion correction necessary
FSI puts K- at larger lab angles: better CLAS acceptance
FSI not rare: in ~50% of L*(1520) events both nucleons detected with
p > 0.15 GeV/c

19. JLab accelerator CEBAF

20. CEBAF Large Acceptance Spectrometer
Torus magnet
6 superconducting coils
Electromagnetic calorimeters
Lead/scintillator, 1296 photomultipliers
Liquid D2 (H2)target +
g start counter; e minitorus
Drift chambers
argon/CO2 gas, 35,000 cells
Gas Cherenkov counters
e/p separation, 256 PMTs
Time-of-flight counters
plastic scintillators, 684 photomultipliers

21. gd → p K+K─ (n) in CLAS K- K+ p

22. Particle identification by time-of-flight

23. “Kaon” vertex times relative to proton
ppp-
pp+p-
Dt (p-K─)
(ns)
pK+K─
Dt (p-K+) (ns)

24. Reaction gd→pK+K-(n) Reconstructed Neutrons
Clear peak at neutron mass.
15% non-pKK events within ±3s of the peak.
Almost no background under the neutron peak after event selection with tight timing cut.
Reconstructed Neutrons

25. Identification of known resonances
Remove events with IM(K+K-) → f(1020) by IM > 1.07 GeV
Remove events with IM(pK-) → L(1520)
Limit K+ momentum due to g d →p K- Q + phase space pK+ < 1.0GeV/c

26. Deuterium: nK+ invariant mass distributionQ+
Distribution of L*(1520) events
Mass = GeV
< 21 MeV
Significance 5.2±0.6 s

27. Searching for the Q+ on a proton target
Events with cosq(p+K-)>0.5
Mass(nK+) GeV
Mass = GeV n K+ K− p+ g p Q+ K0
K*0

28. First observation of Q+ at SPring-8
Phys.Rev.Lett. 91 (2003)
= 1.540.01 MeV
< 25 MeV
Gaussian significance 4.6s
background

29. DIANA at ITEP 850 MeV K+ beam
Cuts to suppress p and K0
reinteraction in Xe nucleus
K+ Xe  + N(K0p) N
hep-ex/
M=1539±2 MeV
G < 9 MeV
All Events

30. SAPHIR detector at ELSA
The reaction p  + Ks0, where Ks0 +- and +  nK+
M=1540±4 MeV
G < 25 MeV

31. Reanalysis of bubble chamber neutrino data
M = 1533±5 MeV,  < 20 MeV
Enlargement of signal region
n Ne,D  (Ks0p) X
ITEP group: hep-ex/

32. HERMES

33. What do we know about this S=+1 state?
LEPS : gC→(nK+) K−X
DIANA : K+Xe→(pK0) X
CLAS : gd→(nK+) K−p
SAPHIR : gp→(nK+) K0
Mass (GeV)
ITEP : n d,Ne→(pK0) K0
HERMES : e+d→(pK0) X
Parenthesis show Q+
decay products
Searches for isospin pK+
partner have found nothing
Number of Events in Peak

34. There is much more to learn
Spin, parity
Chiral soliton model predicts Jpc=½+ (p-wave)
Quark model naïve expectation is Jpc=½− (s-wave)
Isospin
Likely I=0, as predicted by the chiral soliton model.
Width (lifetime)
Measurements limited by experimental resolution.
Naïve estimates predict G~200 MeV.
Theoretical problem remains why the state is so narrow.
Analysis of existing K+d scattering data indicate that G < 1-2 MeV. (e.g. nucl-th/ )
Complete determination of the pentaquark multiplet

35. A di-quark model for pentaquarks
JW hep-ph/
JM hep-ph/
SZ hep-ph/
L=1
(ud) s
L=1, one unit of orbital angular momentum needed to get J=1/2+ as in cSM
Lattice QCD: JP = 1/2−
Decay Width: ( )
MeV 8 6 2
200 G
Mass Prediction for X−− is 1.75 instead of 2.07 GeV

36. A new cousin: observation of exotic X−−
M=1.862± GeV X0 Q+
X−−
X(1530)
CERN SPS hep-ex/

37. Current activities Workshop@ JLab Penta-Quark 2003, Nov 6-8
Approved experiment for 30 PAC days with deuterium target scheduled for early next year (spokespersons Ken Hicks, S. Stepanyan)
New proposals under discussion, for example to search for X−−…
Theory papers appearing daily on the preprint server.
Of course there is worldwide interest to continue to probe the nature of pentaquarks experimentally.

38. Summary
A key question in non-perturbative QCD is the structure of hadrons
We have presented evidence for an exotic baryon with
S = +1, which would have a minimal quark content of five quarks (uudds).
This baryon represents a new class of colorless hadrons.
The additional observation of a doubly negative S=−2 baryon (ddssu) establishes a second corner of the anti-decuplet the family of pentaquarks.
The observation of new members of the family of 5-quark states gives credibility to the existence of pentaquarks.
Hadronic physics is being driven by experimental discoveries.
Electromagnetic probes are playing a major role in the investigation of these new states.
Index of popular articles on the Pentaquark