Experimental Review of Pentaquarks:
Positive and Null Results
Forum on Pentaquarks (DESY)
February 1, 2005
Ken Hicks (Ohio University)
Outline
•
•
•
•
•
•
Preliminary Comments and Opinions
Evidence for the Q+
The Null Experiments
Some common “myths”
New Data (SPring-8)
Conclusions
February 1, 2005
K. Hicks, Ohio U.
Preliminary Remark
Congratulations to the ESA on a MAJOR success:
February 1, 2005
K. Hicks, Ohio U.
Opinions on Pentaquarks:
• There are valid criticisms for both
positive and null experimental results.
– A “scorecard” approach will not work. We
need better, higher-statistics, data.
• Science versus emotion
– There have been strong statements on
both sides of the existence question.
– Let’s make scientifically sound statements.
February 1, 2005
K. Hicks, Ohio U.
More Opinions
• If the Q+ exists, data suggests it likely
favors certain production mechanisms.
– This is an exotic baryon.
– It may have an exotic production mechanism.
• To make solid scientific statements:
– Calculate the expected rate of production.
– Understand the rate of the background.
– Compare with acceptance-corrected data.
February 1, 2005
K. Hicks, Ohio U.
If it exists, what is it?
• The first Q+ search was motivated by
the chiral soliton model of DPP.
– Is it is possible that there is another
interpretation of the Q+? We should not
be biased toward a particular theory.
• Lattice QCD suggests that the Q+ has
negative parity (opposite to DPP).
– But these are not “gold-plated” calculations
Diakonov, Petrov and Polyakov, Z. Phys. A359, 305 (1997).
February 1, 2005
K. Hicks, Ohio U.
Positive results
February 1, 2005
K. Hicks, Ohio U.
Comparison of Q+ Experiments
Where
Reaction
Mass
LEPS
gC  K+K- X
1540 +- 10 < 25
4.6
DIANA
CLAS
SAPHIR
ITEP
K+Xe K0p X
gd  K+K-p(n)
gp  K+K0(n)
nA  K0p X
1539 +1542 +1540 +1533 +-
4.4
5.2
4.8
6.7
CLAS
gp  p+K-K+(n)
HERMES e+d  K0p X
ZEUS
e+p  e’K0p X
COSY
pp  K0pS+
Width s’s*
2
5
6
5
<9
< 21
< 25
< 20
1555 +- 10 < 26
1526 +- 3 13 +- 9
1522 +- 3 8 +- 4
7.8
~5
~5
1530 +- 5 < 18
4-6
*Gaussian statistical significance: estimated background fluctuation
February 1, 2005
K. Hicks, Ohio U.
Evidence for Pentaquark States
Spring8
DIANA
JLab-d
SAPHIR
COSY-TOF
pp  S+Q+.
JLab-p
ITEP
SVD/IHEP
This is a lot
of evidence
HERMES
ZEUS
CERN/NA49
H1
February 1, 2005
K. Hicks, Ohio U.
Nomad
Critical Comments
• For many experiments, the background
shape is not clearly known.
• Some experiments have harsh angle
cuts that could affect the mass spectra.
• In all cases, the signal is weak
compared with standard resonances.
– Cuts are necessary to lower background.
February 1, 2005
K. Hicks, Ohio U.
CLAS: deuterium result
Q+
NQ = 43 events
Mass = 1.542 GeV
< 21 MeV
Significance 5.2±0.6 s
Significance = ?
Two different
background shapes
?
Events in the
L(1520) peak.
February 1, 2005
K. Hicks, Ohio U.
Official CLAS statement
• “Further analysis of the deuterium data find that
the significance of the observed peak may not be
as large as indicated.”
– We really need a calculation of the background before
the statistical significance of the peak can be known.
• Eventually the new experiment, with much
higher statistics, will settle the question.
– The g10 experiment (x10 statistics) is now complete,
and final results are expected at end of Feb. 2005.
– “Why is it taking so long?” --> It’s only 8 months!!
February 1, 2005
K. Hicks, Ohio U.
Results from ZEUS
NOTES:
1. Q+ peak is evident
only for Q2 > 20 GeV2.
--> ZEUS suggests that
this condition gives the
Q+ enough transverse
momentum to get into
their detector acceptance.
2. There is an assumption
of background shape.
--> A different background
changes the stat. signifig.
February 1, 2005
K. Hicks, Ohio U.
HERMES Q+ spectra
add additional p
signal / background:
1:3
February 1, 2005
K. Hicks, Ohio U.
signal / background
2:1
standard cuts applied
+ K* and L veto
Results from H1
(From Karin Daum)
Apply mass difference technique
M(D*p)=m(Kpp p)-m(Kpp)+MPDG(D*)
no enhancement in D* Monte Carlo
no enhancement in wrong charge D
Background well described by D* MC
and “wrong charge D” from data
narrow resonance at M=3099 3(stat.)  5 (syst.) MeV
• Signal is visible in different data taking periods
• But no signal seen in ZEUS data (question: different D* accep.?)
February 1, 2005
K. Hicks, Ohio U.
Null Results
February 1, 2005
K. Hicks, Ohio U.
Published Null Experiments
Group
Reaction
Limit
Sensitivity?
BES e+e-
J/Y --> QQ*
<1.1x10-5
No?
Belle e+e-
Y(2S) --> pK0
<0.6x10-5
??
BaBar e+e- U(4S) -->pKs0
<1.1x10-4
??
ALEPH
e+e- ->Z -> pKs0 <0.6x10-5
??
HERA-B
pA --> pKs0X
<0.02xL*
No?
CDF
pp* --> pKs0X
<0.03xL*
No?
HyperCP
pCu --> pKs0X
<0.3% K0p No?
PHENIX
AuAu -->n*K-
not given
??
Belle
K+Si -->pKs0X
<0.02xL*
Yes?
February 1, 2005
K. Hicks, Ohio U.
Critical Comments
• Inclusive versus Exclusive measurement
– inclusive has better resolution, but more
background (especially at higher energy)
• Backgrounds: combinatorial and from
other resonances. Can we estimate?
• Production mechanism: projectile or
target fragmentation?
– Is it calculable in some model?
February 1, 2005
K. Hicks, Ohio U.
Titov: inclusive production
(fragmentation region)
x
fast
slow
pq
pA

NN  ( N ...)(Q K ...)
NN  ( N ...)( LK ...)
Ratio: pentaquark to baryon production
ph
y
pq
Regge exchange dominates
(2 = diquarks as quasi-partons)
RQL  (1  z )
4( 2)
February 1, 2005
ph
;
; z p
h max
z  0.7
K. Hicks, Ohio U.
103  RQL  2.4 102
Slope for mesons
Slope for baryons
Slope for pentaquarks??
February 1, 2005
K. Hicks, Ohio U.
Hadron production in e+eSlope:
Pseudoscalar mesons:
~ 10-2/GeV/c2 (need
to generate one qq pair)
Slope for p.s.
mesons
Baryons:
~ 10-4 /GeV/c2
(need two more pairs)
Slope for
baryons
Pentaquarks:
~ 10-6 /GeV/c2 (?)
(need 4 more pairs)
Slope for
Pentaquark??
we don’t know the production mechanism!!
February 1, 2005
K. Hicks, Ohio U.
Some common “myths”
February 1, 2005
K. Hicks, Ohio U.
Myth #1
• “Kinematic reflection of the a2 and f2
tensor mesons explain the CLAS data”
Some people use a Regge
trajectory (p, p1, p2, etc.)
February 1, 2005
Near theshold (Eg<3 GeV)
pion exchange dominates
Regge exchange.
--> For T=(a20 and f2), the
g-p-T vertex violates C-parity!
--> calculations using diagrams
that do not violate C-parity
(Y. Oh et al., hep-ph/0412363)
give sT far too small to explain
CLAS data as a2/f2 “reflections”.
K. Hicks, Ohio U.
Myth #2
• “Ghost tracks could be responsible for
the peaks seen in the pK0 mass spectra”
This only can happen if
there is an error in the
tacking software.
--> The same track must
be used twice!
--> All pentaquark (pK0)
data analysis has been
checked, and no such
tracking error is found.
February 1, 2005
K. Hicks, Ohio U.
New Data
February 1, 2005
K. Hicks, Ohio U.
New data: LEPS deuterium*
Minimal cuts: vertex, MMgKK=MN, no f, Eg < 2.35 GeV
L(1520)
Q
MMgK (GeV)
February 1, 2005
MMgK (GeV)
*in collaboration with T. Nakano
K. Hicks, Ohio U.
LEPS: Fermi motion corrections
L(1520) resonance
MMgK (GeV)
MMgK (GeV)
•No large difference among
the three Fermi motion
correction methods
February 1, 2005
MMgK (GeV)
K. Hicks, Ohio U.
Fermi motion corrections: Q+
MMgK (GeV)
MMgK (GeV)
• No large differences
among the three Fermi
motion corrections.
February 1, 2005
MMgK (GeV)
K. Hicks, Ohio U.
LEPS: K－p detection mode
(New and Preliminary results)
• Inclusive production:
• Θ＋ is identified by K－p missing mass from
deuteron. ⇒ No Fermi correction is needed.
γ
p
n
February 1, 2005
Θ＋
K－
γ
Θ＋
L(1520)
p
K－
n
p
K. Hicks, Ohio U.
p
Event selections in K－p mode
Λ(1520)
K＋ mass
γp→K－pKπ
π－ mis-ID
as K－
MMp(γ,K－p) GeV/c2
Non-resonant
KKp
M(K－p) GeV/c2
Λ(1520) is tightly selected in 1.50–1.54 GeV/c2
February 1, 2005
K. Hicks, Ohio U.
K－p missing mass for
events in the L(1520) peak
Small enhancement
at 1.53 GeV.
But the statistics is
not large enough.
Hydrogen
target data
February 1, 2005
MMd(γ,K－p) GeV/c2
K. Hicks, Ohio U.
A possible reaction mechanism
• Q+ can be produced by re-scattering of K+.
• K momentum spectrum is soft for forward going
L(1520).
PK obtained by missing momentum
L(1520)
γ
K+/K0
p/n
Q
n/p
February 1, 2005
K. Hicks, Ohio U.
Formation
momentum
PK GeV/c
K－p missing mass for events with
missing momentum > 0.35 GeV/c
sideband regions
VERY PRELIMINARY!
MMd(γ,K－p) GeV/c2
MMd(γ,K－p) GeV/c2
select
February 1, 2005
K. Hicks, Ohio U.
Summary
• There is reason for caution about the
existence of the Q+.
– Need better experiments (pos. and null).
• Experiments need to have better control
over the background shape.
– Can backgrounds be calculated?
• The new LEPS data for the Q+ is
interesting, but not conclusive.
– CLAS data: internal review in ~1 month.
February 1, 2005
K. Hicks, Ohio U.
Outlook
• There are several new experiments that will
help settle the existence question:
–
–
–
–
SPring-8: LEPS (deuterium: higher statistics)
JLAB: CLAS (g10, g11, eg3)
COSY: TOF
DESY?
• We still need to understand the null
experiments:
– background? production mechanism?
February 1, 2005
K. Hicks, Ohio U.
Model-independent Parity
p
T=1
p
Q
K, or K*
S
At threshold
S-wave dominant
If S = 0, then Li = even, P = even
==> P(Q) = +
If S = 1, then Li = odd, P = odd
==> P(Q) = -
Thomas, Hosaka, KH, Prog. Theor. Phys. 111, 291 (2004).
See full calculation: C. Hanhart et al., hep-ph/0410293.
February 1, 2005
K. Hicks, Ohio U.
Width: Indirect Limits
•
•
•
•
•
•
Nussinov (hep-ph/0307357):
Q< 6 MeV
Arndt et al. (nucl-th/0308012):
Q< 1 MeV
Haidenbauer (hep-ph/0309243): Q< 5 MeV
Cahn, Trilling (hep-ph/0311245): Q~ 0.9 MeV
Sibertsev et al. (hep-ph/0405099): Q< 1 MeV
Gibbs (nucl-th/0405024):
Q~ 0.9 MeV
February 1, 2005
K. Hicks, Ohio U.
Width: Possible
Conclude:
width 
must be
~1 MeV
Gibbs, nucl-th/0405024

Q
Signal?
Input mass
Widths range:
0.6-1.2 MeV
0.9 MeV = solid
background
(non-reson.)
February 1, 2005
K. Hicks, Ohio U.
Comments: Width and Parity
• If the KN database is correct, it is likely
that the Q+ width is ~1 MeV.
• If the width is 1 MeV, the parity is
almost surely positive.
– negative parity width goes up by ~50.
• If the lattice results are correct, the
width is almost surely negative.
This problem of width/parity is the most
worrisome aspect to the existence of the Q+.
February 1, 2005
K. Hicks, Ohio U.
A di-quark model for pentaquarks
ud  ud  s
JW hep-ph/0307341
JM hep-ph/0308286
(ud)
L=1
s
L=1, one unit of orbital angular
momentum needed to get
J=1/2+ as in cSM
Uncorrelated quarks: JP = 1/2−
(ud)
Decay Width: ud ud  s uud  us 

1   200 MeV 
8 MeV
2
2 6
2 6
( )
Additional width suppression may come from w.f. overlap.
February 1, 2005
K. Hicks, Ohio U.