1. An Experimental Overview
Pentaquarks:
An Experimental Overview
Curtis A. Meyer
Based, in part, on work carried out with Alex Dzierba and Adam Szczepaniak
Before 2003 …. searches for flavor exotic
baryons showed no evidence for such
states.
Since 2003 …. Hadronic Physics has
been very interesting.
Pentaquarks

2. Spectacular Development
1997: Diakonov, Petrov and Polykov use a chiral
soliton model to predict a decuplet of pentaquark
baryons. The lightest has S=+1 and a mass of
1530 MeV and expected to be narrow.
Zeit. Phys. A359, 305 (1997).
+! K+n
2003: T. Nakano et al.  n ! K+K-n
on a Carbon target.
Phys. Rev. Lett. 91, , (2003)
The Dam Breaks ….
Pentaquarks

3. Positive Sightings + Experiment 0c 5 +  C12! K-K+n + LEPS SVD
Reaction searched +
Claim
Decay
Experiment
0c 5
publication +
pA! pK0SX
 C12! K-K+n +
LEPS
SVD
K+n
K0Sp
PRL 91 (2003)
hep-ex/
 d! K+K- np
 p! +K-K+n +
 A! K0SpX + 
CLAS
BC at CERN & FNAL
hep-ex/
K+n
K0Sp
PRL 91 (2003) , PRL 92 (2004)
 p! K0SK+n +
ep! e’pK0SX +
SAPHIR
HERMES
(quasi-real photoproduction)
K+n
K0Sp
Phys.Lett B572 (2003) 127
Phys.Lett.B585(2004) 213 +
ep! e’pK0SX +
pp! +K0Sp
COSY
ZEUS
K0Sp
Phys.Lett.B595 (2004) 127
K0Sp
Phys.Lett.B592(2004)7
K+Xe! K0SX’ +
pp!  X 5
DIANA
NA49
K0Sp
Phys.Atom.Nucl.66(2003)1715
PRL 92(2004)042003
p+C3H8! K0SpX +
ep! e’pD*-X
0c
JINR H1
K0Sp
hep-ex/
Phys.Lett.B588(2004)17
Pentaquarks
(Table by Alex Dzierba)

4. The Data 5.2 Reported Significance 4.6 4.8 5.2 7.8 5- - 50 4.4
5- - 50
4.4
6.7
~5
4.2
~5
4.6
5.6
4.3
~5.5
0c
Pentaquarks

5. Summary of Results + 5-- 50 c0 Na49: Mass: 1862 MeV
Width < Resolution
c0
H1: Mass : MeV
Width ~10 MeV
A narrow structure whose width is
less than experimental resolution
Old data constrain <1MeV
Pentaquarks

6. Statistics Experiment Signal Background Significance Publ. 1 2 3
Spring 
Spring
SPAHIR 
CLAS (d) 
CLAS (p) 
DIANA 
 
HERMES 
COSY 
ZEUS 
SVD 
NOMAD 
NA 
NA 
H 
Pentaquarks

7. Zeus Result Interesting Result ~6000  ~200 (1520) 230 +
Phys. Lett. B591 (2004) 7.
Fragmentation,
Q2>20GeV2
Mass=1465
Width=15
368 Events
~6000 
~200 (1520)
230 +
What does  look like?
(many !)
Interesting Result
fragmentation is
a good source of
+!
 ?
Observe: +
mass=1.521 GeV width=6.1 MeV
230 Events on 1080 Background
No Signal for: 5 , 0c
Pentaquarks
(U.Karshon, Pentaquark-04)

8. Negative Reports CDF ALEPH HyperCP DELPHI SELEX L3 FOCUS WA89 E690
ZEUS
BES
HERA-B
BELLE
SPHINX
BaBar
PHENIX
COMPASS +
Hadronic Z decays +
c0
c0
hep-ex/ , 5
Submitted to Phys. Lett. B 5
(+,K+,p)Cu ! PX +
Hadronic Z decays +
hep-ex/
hep-ex/
(,p,)p! PX + +
Quark Confinement 2004
hep-ex/
 p! PX +
-N! PX
c0
hep-ex/ 5
hep-ex/ 5
pp! PX +
ep! PX
c0
QNP2004 - 5
hep-ex/ 5
e+e-! J/ ((2S) +
pA! PX +
PRD 70 (2004)
PRL 93 (2004) 5
KN! PX +
pC(N)!  K X +
c0
hep-ep/
EPJ A21 (2004) 455
e+e-! U (4S) +
AuAu! PX +
hep-ex/ 5
J.Phys.G 30 (2004) S1201 + 5
Pentaquarks
(Table by Alex Dzierba)

9. NA49 5(1860) fixed target experiment at CERN - spectrometer
158 GeV/c proton beam
width is below detector resolution
Pentaquarks

10. Null Results
5(1860)
HERA-B
Pentaquarks

11. ALEPH and ZEUS also null result
5(1860)
ALEPH and ZEUS also null result
CDF
FOCUS
Pentaquarks

12. H1
at HERA
Pentaquarks

13. Null Results
ZEUS
CDF
FOCUS
Pentaquarks

14. Negative Results +(1540) HERA-B ALEPH BaBar CDF HyperCP SPHINX BELLE
pC! pK0SX
+(1540)
SPHINX
BELLE
(1520)
ALEPH
BaBar
e+e-! (p K0)X
pKS
HyperCP
CDF
(1520)
K*(892)
+?
Pentaquarks

15. Bubble Chamber No signals in the Dalitz Plots Pentaquarks
(Taken from the PDG, 2004)

16. Scattering Data K+n P-wave Phase Shifts 1MeV wide resonance at 1540
Pentaquarks

17. The Numbers Some Negative Results Positive Results
Experiment s b  (1520) 
____________________________________________
Spring
Spring
SAPHIR
CLAS(d)
CLAS(p)
DIANA 
HERMES
COSY
ZEUS *
SVD
NOMAD
________________________________________
s b  D* NA H
Experiment  (1520) 
____________________________________ E
ALEPH
CDF
BaBar
HERA-B
SPHINX
HYPERCP
COMPASS
BELLE
 (1530) D D*
________________________________________________________ E
ALEPH
CDF
BaBar
HERA-B
ZEUS WA
FOCUS
_____________________________________
* Estimate from cross section
Pentaquarks

18. Low Energy Experiments
Kinematic Reflections
+(1540)
a2(1320)! K+K-
Produce a spin-2 or spin-3 resonance that
decays to K+K- .
Have non-uniform populations of |m|=0,1,2,…
Produces a broad enhancement near 1.5
(1020)
a2(1320)
CLAS
Pentaquarks

19. Kinematic Reflection? The CLAS  d ! p n K+K- Data
Mass (K-n)
Mass (K+K-)
Mass (K+n)
Solid lines are predicted using K+K- resonances
Pentaquarks
(Dzierba, et al.)

20. Statistical Fluctuation
You need to understand your
background to claim a new
discovery!
CLAS Published
Simple Physics
Background
Naïve Background
Chance of the Background Fluctuating
into the observed signal
Dzierba
Background
Pentaquarks

21. Games Use Dzierba Background Generate 40 random spectra 3 are Fake
1 is CLAS
Pentaquarks

22. Severe Cuts  p! +K+K-(n) missing Uncut Spectrum
Mass (K+ n)
 p! +K+K-(n) missing
Uncut Spectrum
Mass (K+ n)
Mass (K- K+ n)
After Cuts
Design cuts to remove diagrams (b),
(c) and (d)
j t!-j < 0.28 GeV2
cos *K+ < 0.6
CLAS: Phys. Rev. Lett. 92, ,(2004)
Pentaquarks

23. Monte Carlo Study  p !  a2/f2 Y12 a2 or f2 ! K+K- j YML( cos, ) j2
Raw
j t!-j < 0.28 GeV2
and cos *K+ < 0.6
Pentaquarks
(Alex Dzierba)

24. Ghost Tracks  (p>2GeV/c)! p-
Mass(+-)
 (p>2GeV/c)! p- - p +
Select KS
Mass(pKS)
“Create a +”
It is easy to manufacture narrow peaks
in the data near 1.5GeV that appear to
decay to p KS .
Pentaquarks
(M. Longo QNP 2004)

25. 5 Pentaquark c Pentaquark One low statistics report by NA49
RIP
One low statistics report by NA49
Nine negative results.
Null results in both similar and different production mechanisms.
1-2 orders of magnitude more data in known resonances.
c Pentaquark
One low statistics report by H1
Five negative results.
Null results in both the same and different production mechanisms.
Factor of 10 to 1000 times for data in known resonances.
RIP
Pentaquarks

26. + Pentaquark 11 low-statistics reports near 1500 MeV
Low-energy reports suffer from some combination of the following:
(a) Fermi motion effects.
(b) Severe cuts whose effects may not have been adequately studied.
(c) Insufficiently constrained reactions.
(d) Kinematic reflections.
15 new high-statistics searches in a number of reactions with
excellent resolution that have come up empty.
Bubble chamber data from decades ago show no evidence.
KN scattering data severely limit this
The Zeus result is interesting.
It suggests that fragmentation is a good way to produce the +.
Not really consistent with ALEPH, BaBar, BELLE, CDF, …
What does H1 have to say on this?
Pentaquarks

27. PDG 2004
Pentaquarks

28. Conclusions The possible existence of pentaquarks is still very much
Pentaquark Stock Value
Jan.
Jun.
Dec.
2004
The possible existence of
pentaquarks is still very much
an experimental question, and
the data do not look very
convincing.
If they exist, they not only have
exotic quantum numbers, but
very exotic production and
decay modes.
I hope that the issue can be settled soon – but I am
not buying stock.
Pentaquarks