The Rise and Fall of the Pentaquarks in Experiments

1
The Rise and Fall of the Pentaquarks in
Experiments
Reinhard Schumacher Carnegie Mellon University
PANIC, Santa Fe, October 28, 2005
2
Overview

• Pentaquarks brief history predictions
• Experimental evidence (since PANIC Oct.02)
• The Q at LEPS, ITEP, CLAS, SAPHIR, ZEUS,
  HERMES
• The X - - at NA49/CERN
• The Q0c at H1/HERA
• The Q at STAR/RHIC
• Compare and evidence by reaction channel
• Theme Bandwagon effect led to overly-optimistic
  assessments of data by many groups

3
What are Pentaquarks?

• Objects with 4 quarks and 1 anti-quark
• Exotics contain an anti-quark different in
  flavor than the 4 quarks ? the anti-quark cant
  annihilate! The quantum numbers are impossible
  with just 3 quarks.
• Example uudds (exotic) The Q
• Baryon number 1/3 1/3 1/3 1/3 - 1/3 1
• Strangeness 0 0 0 0 1 1
• Early history
• Bag models R.L.Jaffe (76), deSwart(80)
• Soliton models Diakonov, Petrov (84)
  Chemtob(85) Praszalowicz(87), Walliser(92)
• PDG04 Q
• M 1539.2 1.6 MeV
• G 0.9 0.3 MeV
• Q? K n, K0 p

_
_
4
Anti-Decuplet in a Chiral-Soliton Model
PREDICTION D. Diakonov , V. Petrov, M. Polyakov,
Z. Phys. A359, 305 (1997).
G15 MeV very narrow!
5
Pentaquarks Three Model Descriptions
Quark description (Jaffe, Wilczek)
Chiral soliton model (Diakonov, Petrov,
Polyakov)
(ud)
L1
Pentaquarks in such models emerge as rotational
excitations of the soliton rigid core (q3)
surrounded by meson fields (qq) JP ½
s
(ud)
Two bosonic di-quarks (ud), plus s fermionic
state demands L1, giving JP ½ Lattice QCD
?existence and JP predictions are inconclusive.
6
Positive Evidence (most of it...)
JLab-d
SPring8
JLab-p
DIANA
SVD/IHEP
ITEP
SAPHIR
HERMES
H1
ZEUS
COSY-TOF
CERN/NA49
?0c
?5- - ?50
pp ? SQ.
7
The Data Without Fits
JLab-d
SPring8
JLab-p
DIANA
4.4?
7.8?
5.2?
HERMES
SAPHIR
ITEP
SVD/IHEP
5.6?
6.7?
4.8?
CERN/NA49
H1
COSY-TOF
ZEUS
?0c
?5- - ?50
5?
4.6?
5.5?
4.2?
8
Skeptics Comments

• Most of the positive evidence for pentaquark
  states is statistically very weak, despite claims
  of over 4 s significance.
• Without guides for the eye, many experimental
  spectra have low signal-to-noise, and large
  poorly-known backgrounds. No clean
  background-free peaks have been reported.
• There seems to have been a bandwagon effect in
  place from the start frantic attempt to get on
  board the list of positive sightings.

9
Anti-evidence for Pentaquarks
BABAR
BES
10
Pentaquark Expts Timeline
Photoproduction on Deuteron Q
Photoproduction on Proton pKs0
Photoproduction on Proton nKK-p
Exclusive K (N) ? pKs0
Inclusive lepton D, A ? p Ks0
p A ? pKs0 X
p p ? pKs0 S
Other Q Upper Limits



p p (or A) ? X - - X
Inclusive Q ? p K
Inclusive Q0c ? D() - p


9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12 1 2 3 4 5 6 7 8 9 10 11 12
LEPS-C
2002 2003
2004 2005
11
Impact on particle physics

• Since October 2002
• 50 experimental papers, both pro and con
• 550 theory papers

12
First Reported Observation of Q at LEPS/SPring-8
g C ? K- K X

• Inclusive measurement on Carbon target
• Detect only the two Ks
• Apply Fermi motion correction to MM (g,K-)

T. Nakano, PRL91, 012002, (2003)
13
First Reported Observation of Q at LEPS/SPring-8
Q
g C ? K- K X

• Solid signal sample
• Dashed background from protons in upstream H2
  target, normalized to signal above 1590 MeV
• Significance 4.6s
• Mass 1540 10 MeV
• Width lt 25 MeV

background
T. Nakano, PRL 91, 012002 (2003)
14
LEPS/SPring-8 g(n)?KK-n

• Deuteron target the proton is a spectator
  (undetected).
• Fermi motion is corrected to get the missing
  mass spectra.
• Tight ? exclusion cut is essential.
• Background is estimated by mixed events.

2004
Q
MMgK (GeV)
MMgK- (GeV)
15
Photoproduction at CLAS/JLab

• g D ? p K K- (n) exclusive channel
• No Fermi motion correction needed
• FSI puts K- at larger lab angles better CLAS
  acceptance
• FSI not rare in 50 of L(1520) events both
  nucleons detected with pgt0.2 GeV/c
• CLAS Collaboration (S. Stepanyan, K. Hicks, et
  al.), hep-ex/0307018, PRL 91 252001 (2003).

d
16
CLAS/JLab Q Exclusive Process I

Q
Events/10 MeV
MM(K-p) (GeV/c2)
17
2004 CLAS High-Stats Data
g p x10
g D x6
g p ? p w

• CLAS g11
• preliminary
• SAPHIR

g10 preliminary (3375A) World data
? n ? p ? - 1.05 lt Eg lt 1.15 GeV
ds/dWCM (mb/sr)
g p ? K L

• CLAS g11
• preliminary
• SAPHIR

s(mb)
cosqCM(p-)
Eg(GeV)
18
An Upper Limit on Q Production with CLAS.
(July 2005, Lepton-Photon Conf.)
no signal
Published result with background from new result

• In previous result, the background is
  underestimated. New estimate of
• the original data gives a significance of 3s,
  possibly due to fluctuations.

19
Newest LEPS/SPring-8 Claim
Q
Select on L(1520)--gt K- p Very forward c.m.
acceptance Strangeness of Q is tagged
T. Nakano, Presented at International Conference
on QCD and Hadronic Physics, Beijing, June 20,
2005.
(Pentaquark 2005, Oct. 21, CLAS g10 data dont
confirm this. (T. Mibe et al.), but for
larger L(1520) c.m. angles)
20
Exclusive Reaction on the Proton
?p ? ? K0 Ks0? ??-
? ? nK

• SAPHIR/ELSA at Bonn
• M 1540 4 MeV
• width lt 25 MeV
• Published 300nb cross section. Later statement
  signal smaller than shown, cross section of 50
  nb.
• N(Q)/N(L) 10

J. Barth, et al., Phys Lett B572, 127 (2004)
hep-ex/0307083.
21
New Upper Limit on the Q Cross Section
CLAS g p ? K0Kn (APS Tampa, 5-05,
hep-ex/0510061)
s g p ? Q K0 lt 0.8 nb _at_ 1.54 GeV/c2 N(Q)/N(L)
lt 0.2 (95CL)
Fit with a sum of smooth function and a Gaussian
with fixed width and centroid.
22
Exclusive Reaction on the Proton II

• g p ? p K- K (n)
• CLAS V. Kubarovsky et al.
• PRL 92 032001 (2004)
• Combined analysis of all CLAS data on protons for
  Eg lt5.2 GeV
• Cuts forward p, backward K
• indications of production from heavy N(2420)

Q
7.8s
M(nK)
Higher statistics run planned for 2006.
23
Semi-Exclusive K (N) ? pKs0
K Xe ? (Q) X ? K0s p X
850 MeV/c K beam
Q
DIANA/ITEP Bubble Chamber Experiment
Cuts applied to avoid hadron re- interaction in Xe
M15392 MeV G lt 9 MeV
Note No strangeness tag for the K0s p final
state
V.V. Barmin et al., Yad. Fiz. 66 1763 (2003)
hep-ex/304040
24
Belle/KEKB Low energy KN
ee- ? K/- X, K/-A ? pK0, pK-
momentum spectra of K and K-
1 / 50MeV
momentum, GeV/c
Momentum range possibly contributing to Q
formation.
gt Determine resonance width
25
Belle Limits on Q Width
GQ from KA ? pK0sX KD ? inclusive analysis
Belle limit 90CL
KA ? pK0s
397 fb-1
DIANA

no signal
Belle G lt 0.64 MeV (90 CL) _at_ M 1.539 GeV
G lt 1 MeV (90 CL) _at_ M 1.5251.545 GeV
Cahn,Trilling,PRD 69,11501 (2004).
Does not confirm previous DIANA signal.
26
Low Q2 , Quasi-real Photons
gv D ? K0 p X ? pp-p X
Q

• HERMES/HERA Inclusive on Deuterium
• Lepton scattering 27 GeV positron beam
• M 1526 2 2 MeV
• s 7.5 2.4 MeV
• A. Airapetian et al.
• Phys. Lett. B 585, 213 (2004).

Additional ? required
Airapetian et al., Intl Workshop on DIS,
Madison, 2005.
27
Scattering at High Q2 , ep ? e(K0sp)X
ZEUS/HERA
Fragmentation, Q2gt20GeV2
Q
S
Q/L 5 (independent of Q2)
gt Signal seen at medium Q2 and forward rapidity
in both pKs and pKs
S. Chekanov et al. Phys. Lett. B 591 (2004) 7.
28
Tomography with pKs0 Vertices
BaBar Preliminary233 fb1 ee data
Y cm
Y cm
Vetobeamspot R ? 2 cm
e
e
SVT
X cm
X cm
SVT support tube
DCH inner wall
Beampipe
z profile
gt Quasi-real photoproduction scattering on
nuclei (e- on BeO)
29
e-Be Electroproduction
Preliminary233 fb1 ee data
HERMES (ed?KS0p X)
HERMES
HERMES/BaBar comparison is valid. ZEUS High Q2,
no signal seen for Q2lt1 BaBar Quasi-real photons
Q20.
HERMES acceptance loss in low mass region? (PID
requires p(p) gt4.1 GeV/c p(KS0)gt3 GeV/c)
30
All of the Worlds Neutrino Scattering Data
( - )

• n A ? (K0 p) X
• ?(pp-p) X
• Data mining sum of 5 bubble-chamber experiments
  from CERN and Fermilab
• M(Q)1.533.005 GeV
• G lt 30 MeV
• A.E.Asratyan et al. (ITEP), Yad. Fiz. 67, 704
  (2004) hep-ex/0309042

Q
31
SVD-2 New Analysis
Two independent data set KS decays inside or
outside the Vertex Detector
Ep 70 GeV
M 1522 MeV s 12 MeV Nevnt
205
hep-ex/0509033

• New analysis improves Q signal
• by factor 8. Total significance 8s.

32
Cascade Pentaquarks Another Exotic Member of the
Anti-Decuplet ?
D.P.P., Z. Phys. A359, 305(1997).
X - -
R. Jaffe, F. Wilczek hep-ph/0307341
JM hep-ph/0308286
SZ hep-ph/0310270
Two bosonic di-quarks (ud), plus s fermionic
state symmetry demands L1, giving JP ½
Mass Prediction for X- - was 1.75 instead of 2.07
GeV
33
Cascade Pentaquarks X5- -(1862)

• NA49/CERN
• pp ? Xp X at 17 GeV c.m. energy
• Signal for Exotic S -2, Q-2
• Signal for Non-exotic S-2, Q0
• M1.862 0.002 GeV
• C. Alt et al . PRL 92 042003 (2004)
  hep-ex/0310014.

X--
X0
X(1530)
34
Cascade Pentaquarks X5- - (1862)
HERA-B (p A)
CDF

• State not produced in quark
• fragmentation or is severely suppressed.

g A
FOCUS
35
Charmed Pentaquark Q 0c(3100)
uuddc
hep-ex/0403017

• ep collisions at HERA
• Possible production
• mechanism photon-gluon
• fusion g()g ? cc

• FOCUS experiment also claims
• incompatibility with H1.

36
Q Evidence is it Isovector?

• STAR/RHIC, Huang et al.
• APS Tampa, April 2005 nucl-ex/0509037
• d-Au collisions, pK and pK- at 200 GeV/A
• but hardly at all in Au-Au (!?)
• DK coalescence?

STAR Preliminary
37
Present Pentaquark Candidates

• Best Survivors (pre-2005)
• CLAS g p ? QK-p
• COSY-TOF p-p ? QS
• ZEUS e p, Q2gt20 GeV2
• Newcomers (2005)
• STAR/RHIC d-Au ?Kso p
• LEPS gD ? QL(1520)
• SVD-2 p-A ?Kso p X
• refuted by WA89

38
Outlook future experiments

• Spring-8/LEPS g D
• COSY-2 p p
• JLab/CLAS g D
• JLab/Hall A g D
• KEK or JPARC K p
• HERA/ZEUS e p

39
Summary

• After 3 years of intense activity, pentaquarks
  have come and (nearly) gone...
• Most positive sightings have been contradicted or
  placed in doubt by better measurements
• Remaining candidates have no common
  phenomenological connection.
• No single truly convincing measurement has
  appeared
• Recent new candidates suffer (again) from low
  statistics and poorly-understood backgrounds
• Bandwagon effect in 2003/4 led to numerous
  over-optimistic positive results to be claimed.

40
Backup slides
41
Positive Sightings
?
Reaction searched
Claim Decay
?0c
Experiment
?5
publication






?
?
pA! pK0SX
? C12! K-Kn
LEPS
SVD
Kn
K0Sp
hep-ex/0401024
PRL 91 (2003) 012002
?
?
? d! KK- np ? p! ?K-Kn
? A! K0SpX
?
CLAS
BC at CERN FNAL hep-ex/0309042
K0Sp
Kn
PRL 91 (2003) 252001, PRL 92 (2004) 032001
?
?
ep! epK0SX
? p! K0SKn
HERMES
SAPHIR
(quasi-real photoproduction)
Kn
K0Sp
Phys.Lett.B585(2004) 213
Phys.Lett B572 (2003) 127
?
?
ep! epK0SX
pp! ?K0Sp
ZEUS
COSY
K0Sp
K0Sp
Phys.Lett.B595 (2004) 127
Phys.Lett.B592(2004)7
?
?5
pp! ?? X
KXe! K0SX
DIANA
NA49
??
K0Sp
Phys.Atom.Nucl.66(2003)1715
PRL 92(2004)042003
?0c
?
pC3H8! K0SpX
ep! epD-X
JINR
H1
D-p
K0Sp
Phys.Lett.B588(2004)17
hep-ex/0401024
(Table by Alex Dzierba)
42
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
?5
hep-ex/0408025,0410024
?5
Submitted to Phys. Lett. B
Hadronic Z decays
(?,K,p)Cu ! PX
?
?
hep-ex/0410027
hep-ex/0410080
?
?
(?,p,?)p! PX
Quark Confinement 2004
hep-ex/0410080
?
? p! PX
?-N! PX
?c0
?5
?5
DPF 2004
hep-ex/0410029
?
pp! PX
ep! PX
?c0
?5
?5
hep-ex/0407026
QNP2004 -
pA! PX
?
?
ee-! J/? (?(2S)
?5
PRD 70 (2004) 012004
Accepted in PRL
?
?
KN! PX
pC(N)! ? K X
?c0
hep-ex/0411005
hep-ex/0407026
?
ee-! U (4S)
?
AuAu! PX
hep-ex/0408064
?5
nuc-ex/0404001
?
?
Kp! K n ?
LASS
hep-ex/0412031
?5
(Table by Alex Dzierba)
43
Summary of LQCD
C. Alexandrou
Group Method of analysis/criterion Conclusion
Alexandrou and Tsapalis Correlation matrix, Scaling of weights Can not exclude a resonance state. Mass difference seen in positive channel of right order but mass too large
Chiu et al. Correlation matrix Evidence for resonance in the positive parity channel
Csikor et al. Correlation matrix, scaling of energies First paper supported a pentaquark , second paper with different interpolating fields produces a negative result
Holland and Juge Correlation matrix Negative result
Ishii et al. Hybrid boundary conditions Negative result in the negative parity channel
Lasscosk et al. Binding energy Negative result
Mathur et al. Scaling of weights Negative result
Sasaki Double plateau Evidence for a resonance state in the negative parity channel.
Takahashi et al. Correlation matrix, scaling of weights Evidence for a resonance state in the negative parity channel.
J. Negele, Lattice 2005 Correlation matrix, scaling of weights Maybe evidence for a resonance state?