Pentaquarks: review of the experimental evidence

1
Pentaquarks review of the experimental
evidence 8th International Conference on
Strangeness in Quark Matter Cape Town, 15-20
September 2004
Sonia Kabana University of Bern, Switzerland
Contents 1. Introduction 2. Review of
pentaquark candidates observed ?, ?, ?,
?0c, ?--, ?-, ?0, N0/ ?0 I1/2 3. Is the
variation of m(?) as expected for a real
particle? 4. How do the observations compare to
theory ? 5. Non-observations and discussion of
systematic errors 6. Conclusions and outlook

2
1 Introduction
QCD predicts beyond qqq or q q also multiquark
states (6q), (2q 2q), (4q q), (5q 2q) etc,
hybrids (qq gg), glueballs (gg)
What are pentaquarks Minimum quark content
is 4-quarks and 1-antiquark (qqqq q) What are
exotic pentaquarks states with the q having
different flavour than the other four
quarks Quantum numbers cannot be defined by 3
quarks alone Example uudd s Baryon number
1/31/31/31/3-1/3 1 Strangeness 00001 1
3

• Earlier searches for pentaquarks
• Lead to some candidates e.g.
• Z0(1780) and Z0(1865) (PR D 46 (1992) 961)
  --gt Not confirmed
• Sigma(3170) state, Width lt 20 MeV (1980),
  S/sqrt(B) 9, using 8.25 GeV K- p
• K-p --gt ? KKpis, ?KKpis, XiKpis (J
  Amirzadeh et al. PLB 89B 1 (1979) 125)
• Confirmed by 6.5 GeV k- p, (Cambridge-Michigan
  state Collaboration)
• No non-observation has been reported
• Was taken out of PDG because no other experiment
  published results on this

PDG 1986 Phys.Lett. B170, 289 Note on the
S1 baryon system The evidence for strangeness
1 baryon resonances was reviewed in our 1976
edition However the results permit no definite
conclusion.. The general prejudice against
baryons not made of 3 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.
4
First ever mentioned 1964, M. Gell-Mann,
PLB 8 (1964) 214. H.J. Lipkin, PLB45 (1973) 267,
R.L. Jaffe et al, PLB60 (1976) 201 etc Soliton
model prediction (1984) octet (J1/2,
P), decuplet (J3/2,P), antidecuplet
(J1/2, P), 27-plet (J3/2,P),
e.g. A. Manohar, Nucl.Ph.B248(1984)19, M.
Chemtob,Nucl.Ph.B256(1985)600

• The chiral soliton model (CSM) describes the
  established 8 and 10 baryons (e.g.
• mass splittings) within 1 (E
  Guadagnini Nucl Phys B236 (1984)35)
• - Critics CSM pentaquark predictions not
  robust T. Cohen, I. Klebanov etc

5
First prediction of the mass 1987, M
Preszalowicz, Chiral Soliton Model, World
Scient. 87, p. 112, hep-ph/0308114
? Mass Z (uudds) 1530 MeV J. Ellis
et al, JHEP 0405002, 2004 newest calcul.--gt
m(?) 1432-1657 MeV ! First prediction of
the width 1997, M. Polyakov, D. Diakonov, V.
Petrov, Z. Phys. A 359(1997)305, (assuming the
N(1710) is a pentaquark), Chiral Soliton Model
? Mass of Z (uudds) 1530
MeV Spin1/2, Isospin0, S1
Width lt 15MeV The width prediction
was the key to the experimental discovery of a
theta candidate in 2003 ? First
observation of Z renamed to ? (1540-10 MeV)
by LEPS coll., T Nakano et al, PRL91 (2003) 012002
6
Th. models Soliton model, Corr. Quark model,
Uncor. Quark model, lattice, etc
Correlated diquark model (B Jaffe, F Wilzcek)
octet and anti-10 are degenerate (Diakonov et al)
two octets (1/2) below 2 GeV mass, PDG or
new
N0(1440)
N(1440)
N(1710)
N0(1710)
?0(1660) ?0(1600)
?0(1880) ?01810)
?(1880)
?-(1880)
?-(1660)
?(1660)
?0(1690)
?-(1690)
?-(1950)
?0(1950)
7
2. Review of pentaquark candidates observed
?, ?, ?, ?0c, ?--, ?-, ?0, N0/ ?0 I1/2
8
?
First observation of Z renamed to ? by LEPS
coll., T Nakano et al, PRL91 (2003) 012002,
hep-ex/0301020 ? on 12C and p target,
E(?)1.5-2.4 GeV Search for ? n -gt ? K- --gt K
K- n
Veto protons to avoid ? p -gt K L(1520) -gt K
K- p
require p detection -gt L(1520) peak (dashed line)
Signal sample veto p -gt no L(1520)
peak (solid line)
Signal sample shows a peak at 1540 MeV
--gt signal sample is dominant g n interactions
Significance S/?(B) 19/ ?(17) 4.6 Mass 1540
- 10 MeV - 5 MeV (syst) Width lt 25 MeV
LEPS preliminary results confirmed the ?
Candidate peak with 90 counts T Nakano, LEPS,
Pentaquarks2004 worksop
9
DIANA Collab. (bubble chamber exp.) at ITEP V.
Barmin et al, Phys. Atom. Nucl. 66, (2003) 1715,
hep-ex/0304040 K beam (850 MeV) on Xe --gt K0s p
X , K0s-gt pi pi- or pi0 pi0 Look for
K n --gt K0 p (events 41000 incident K)

• Mass theta 1539-2 MeV
• Width lt 9 MeV
• S/sqrt(B) 29/sqrt(44) 4.4

Inv mass(K0sp) without cuts to reduce
rescattering S/sqrt(B) of peak at 1539
MeV2.6 Inv mass(K0sp) with cuts to reduce
rescattering (?(p,K0s)lt100o and Pts of p,K0s
back to back ) S/sqrt(B) of peak at 1539
MeV29/sqrt(44)4.4
10
CLAS, S. Stepanyan et al, PRL91 (2003) 252001,
hep-ex/0307018 exclusive reaction ? d
--gt K K- p n trigger on p, K, K-
M(?) 1542 - 5 MeV, Widthlt 21 MeV, S/sqrt(B)
5.2 - 0.6
?--gt KK- ?(1520) --gt pK-
? --gt K n
Solid line fit to S (gauss) B 9gaussconst
term) --gt S/sqrt(B)5.8 Dotted line MC simulated
background --gt S/sqrt(B)4.8 Dashed-dotted shape
of events excluded by a ?(1520) rejection cut (m
?1485-1551 MeV)
11
CLAS, V. Kubarovsky et al., hep-ex/0311046, ? p
-gt pi K-K n E(? ) 3-5.47 GeV
Assumed production of ?
Pi-
Pi
?
Cos ?(pi)gt0.8 Cos ?(K)lt0.6

• ? production through N(2400) --gt ? K- ?
• Mass(theta)1555-10 MeV, Width lt 26 MeV,
  
• S/sqrt(B)7.8-1 --gt highest stat. Significance
  obtained
• No theta -gt p K seen

12

• M. Battaglieri et al, CLAS Coll., Trento feb.
  2004
• Gamma p -gt Theta K0s
• Excited theta ? Cut on cos theta(K0s) improves
  S/B ratio

• Lowest peak below published CLAS masses
• However preliminary
• Splitting of about 50 MeV is as expected for the
  ground and an excited theta state

13
SAPHIR coll. At ELSA PLB572 (2003)127 ? p --gt
? K0s -gt n K K0s, K0s-gt pi pi- Photon
energies 31-94 of 2.8 GeV , 108 events
Target Hydrogen
cos?(K0s) gt 0.5 reduces the ?(1520)

• Mass 1540 -4 - 2 MeV
• Width lt 25 MeV
• S/sqrt( B)4.8 (5.2)

background is well understood to be mainly due
to ?(1520) K
14
HERMES Coll. hep-ex 0312044 27.6 GeV e beam on
deuterium gas target e d --gt p K0s
?(1520) --gtpK- seen No ? in pK inv mass

• Mass of theta 1528 -2.6-2.1 MeV
• Width 17 - 9 - 3 MeV
• resolution ?(pK0s) 4.3-6.2 MeV
• S/sqrt(B) ranges from 4.2 to 6.3
• S/(error of S) 3.4 - 4.3

15
COSY-TOF hep-ex/0403011 Pp-gt Sigma K0 p,
(Sigma-gt n pi) (theta -gt K0 p)
(K0s-gtpipi-) The Sigma tags the s quark,
exclusive reaction Proton beam on hydrogen,
Momentum(p)2.85-3.3 GeV

• S/sqrt(B)5.9
• Mass1530-5 MeV
• Width lt 18-4 MeV
• Cross section 0.4 -0.1 -0.1 (syst) mikrobarn
• -gt in rough agreement with th. predictions of
  0.1-1 mikrobarn for pp, pn near threshold
• (Polyakov, Ko et al)

16
ZEUS ep --gt K0s p and K0s anti-p (first
observation) v(s)300-318 GeV
Trento workshop, Feb. 2004 A. Raval
Zeus Coll., hep-ex/0403051
M(theta)1521.5-1.5(stat)2.8-1.7 Width6.1-1.6
2.0-1.4 MeV Resolution width2.0-0.5
MeV S/sqrt(B)3.9-4.6
17
A. Asratyan et al, hep-ex/0309042 Analysis of
data from several ?A (Ap, d, Ne) experiments
(WA25, WA21, WA59, E180, E638 at BEBC-CERN and
Fermilab) ltE(?)gt34-137 GeV M(?)1533-5
MeV Widthlt20 MeV S/sqrt(B)6.7
Enhanc. In 1650-1850 Sigma --gt K0bar p?
Shifted bins
18
SVD-2 at IHEP, A. Aleev et al.,
hep-ex/0401024 pA --gt p K0s X, E(p)70 GeV
Fritjof MC does not reproduce the background,
probably due to missing ? states in 1560-1800
MeV range of m(pK0s) After applying cut P(K0s)
P(p) to reduce the ? component, while reducing
the ? by 10, Fritjof background agrees with data
M()1526-3-3 MeV Widthlt24 MeV S/sqrt(B)5.6 Cros
s sec30-120 mikrobarn A dependence A 0.7
19
D. Barna et al., NA49 Coll. pp --gt p K0s X, at
vs17 GeV
K-
N(2400)
?
Assume CLAS suggested production mechanism
K. Kadija et al, NA49 Coll., pentaquark2004
workshop, July 2004 reported also a ? peak in
pp-gtp K0s at 1531 MeV
20
NOMAD Collaboration, L. Camilleri,
Neutrino2004 Conference, Paris, June 2004 ? A
? pK0s X Large acceptance detector, low
multiplicity events
m(?)1528.7 - 2.5 MeV Width lt a few MeV Mass
resolution 9 MeV S/sqrt(B) 4.3 (S33)
21
Experiments observing ? candidates
22
? searches
- Some experiments reported non observation of a
peak in p K for the theta e.g. Saphir,
Hermes, CLAS hep-ex/0311046
STAR dAu 200 GeV preliminary
CLAS, hep-ex/0312019
Saphir theta
Peak due to Phi /Hyperon Reflexions

• - CLAS reported a preliminary hint for a theta
  candidate peak in pK near 1579 MeV
  (Pentaquark2004 workshop)
• STAR reported a preliminary hint for possible
  peaks in pK and antip K- near 1530 MeV (J.
  Ma, APS meeting 05/01/2004)
• Width 8.7 MeV , S/sqrt(B) 3.8
• 15 M events

23
?0c
Accepted for publication in PLB

• First evidence for e p --gt
• theta_c0 (uuddc) -gt D- p -gt (anti-D0 pi-) p
  -gt (Kpi-pi-) p
• anti-theta_c0 (uuddc) -gt D anti-p

However not confirmed by Zeus using almost same
cuts and larger statistics ICHEP2004 and new
SLAC preprint
24

• Mass 3099 - 3 - 5 (syst) MeV
• Width lt 12-3 MeV
• Very thorough study of syst. Errors

25

• N0/Xi

Evidence for Xi--, Xi0, Xi- pentaquarks from
NA49 in pp collisions at sqrt(s)17 GeV
NA49 coll., C. Alt et al, PRL92(2004)042003,
hep-ex/0310014
K. Kadija NA49, JLAB2003

• Xi- (1850) --gt Xi pi- (preliminary) while
  Xi (1860) --gt Xi pi Not seen
• Xi--(1862-2) --gt Xi- pi-
• Xi0 (1864-5)--gt Xi- pi Widthlt18 MeV
• --gt Xi- (and Xi0) pentaquarks could be from
  anti-10 or from 8

26

• N0/Xi0 I1/2

AuAu(min.bias, vs200 GeV)--gt ? K0s STAR
Coll., S. Kabana et al, hep-ex/0406032 Observation
of a possible narrow peak at 1733.6 -0.5 - 5
MeV Cut out upper 10 of ?(tot) to suppress
background --gt S/?(B)30.6/ ?( 35.4) 5.15,
? lt4.6 -2.4 MeV
STAR AuAu coll. 200 GeV preliminary
Bin size 3 MeV Blue line mixed event background

Best Significance obtained in semiperipheral ev.
S/?(B)19.36/ ?(10.64)5.93
27
No cut on centrality
STAR Coll., S. Kabana et al, hep-ex/0406032
STAR AuAu coll. 200 GeV preliminary
S/?(B)40.55/ ?(83.45) 4.44
28

• What could the Lambda K0s peak at 1734 MeV be ?
• - PDG states nearby N(1710) N(1730) ? has large
  width 100 MeV
• - Partial wave analysis suggests two new narrow N
  states at
• 1680 and/or 1730 MeV width lt 30 MeV
  (nucl-th/0312126, R Arndt et al)
• Cannot be the ?0 I3/2 pentaquark because ? ? K0s
  decay violates isospin
• It is a candidate for two pentaquark states
• ?0 I1/2 ? ? anti-K0 (octet,
  expected m1700 or 1860 MeV)
• N0 ? ? K0 (octet, expected
  m1730MeV, or anti-10)
• The ? K0s channel allows to separate the ? I1/2
  (octet) from ? I3/2 (anti-10)
• - No peak near 1860 MeV? disfavours picture of
  degenerate 8 and anti-10 however a low BR can
  prevent observation
• Is there a way to resolve the ambiguity ?(ussdd)
  or N(uddss) ?
• Yes
• Through the measurement of their isospin
  partners
• N ? ? K and ?- I1/2 ? ? K-

29
Graal experiment evidence for a state in 1670
MeV ? A ? eta neutron Near lower mass 1680
width lt 30 MeV found by (nucl-th/0312126, R
Arndt et al)
S. Kouznetsov, NSTAR2004
30
Graal experiment evidence for a state in 1727
MeV Seen in 2 decay channels? A --gt K0s Lambda
and Sigma- K
S. Kouznetsov, Trento2004
31
3. Is the variation of m(?) as expected for a
real particle?
All lt m(?)
gt1.533-0.023 GeV Lines ? ? pK0s lt
m(?) gt1.529-0.011 GeV ? ? K n
lt m(?) gt1.540-0.020 GeV
32

• All ? lt m(?) gt1.533-0.023 GeV, chi2/dof3.92
• ? ? pK0s lt m(?) gt1.529-0.011 GeV,
  chi2/dof3.76
• ? ? K n lt m(?) gt1.540-0.020 GeV,
  chi2/dof0.94
• (Note we assumed an error of 5 MeV for Graal as
  no error was given
• For the prel. CLAS result we assume syst. Error
  10 MeV as quoted previously by CLAS)
• After adding a syst. Error of 0.5 m (ca. 8 MeV)
  when no syst. Error is given
• All ? lt m(?) gt1.533-0.031 GeV, chi2/dof2.1
• ? ? pK0s lt m(?) gt1.529-0.022 GeV,
  chi2/dof0.95
• ? ? K n lt m(?) gt1.540-0.022 GeV,
  chi2/dof0.91
• When syst. Errors are added for experiments which
  dont give them, he chi2/dof for the mass(theta)
  from theta?pK0s and nK separately is very good
• The overall bad chi2/dof of 2.1 is mainly due
  to a syst. Deviation between the Kn and K0s p

• Kn, K0s p diff. due to syst. errors in nK ?
  Fermi-mom. corr, neutron not dir. meas.
• gt Need direct measurement of K n ? E.g. Phenix (
  anti-?) and Graal (K n)
• Syst. Err. by all exp. needed ? maybe no
  discrepancy but understimation of syst. errors ?

33
5. Non-observations and discussion of syst. errors

• Many experiments reported non observation of
  pentaquark candidates
• (HERA-B, CDF, Belle, BES, LEP-exp., D0, E690,
  Zeus(theta0c), BABAR, etc)
• Non observations mainly in inclusive, ee-,
  hadron reactions at large energy
• Could the non-observations and the observations
  be consistent ?
• M Karliner H Lipkin, PLB597(2004)309, Azimov et
  al production mechanism studies
• - Suppression factor in ee- (as no sbar,
  baryon in initial state)
• - Theta may be produced mainly through decay
  of N(2400)
• Titov et al., Decrease of theta cross section
  with increasing energy
• Bicudo et al., theta maybe a heptaquark --gt
  additional suppression factor
• --gt More measurements of pentaquark candidate
  observations (also cross sections, spin, parity)
  and upper limits from non-observations needed
  (no rumors..)
• --gt Experiments should explore assumtpions for
  prod. Mechanisms in their cuts

34

• Few papers critically discussing the pentaquark
  evidence
• Dzjerba et al theta peak from kinematic
  reflexions from decays of resonances
• S Wenig H G Fischer (hep ph 0401014) Previous
  non observations etc.
• Another paper (hep ph 0311250) discuss that the
  theta peak can be possibly reproduced by effects
  like missidentification
• How to address these questions
• experiments must estimate all sources of syst.
  Errors and prove the credibility of their results
• More data statistics will be very helpful to
  enhance the significance and help also for better
  syst. studies
• (Dzierba et al) study stability of mass(theta)
  vs incid. beam momentum
• Search for pentaquarks in different collision
  systems and decay channels

35
F Close ICHEP2004
One is real (CLAS) Three are fake Can you tell
which?
Dzierba Szczepaniak Teige
36
5. How do the observations compare to theory ?

• - Theta not an s-wave K-N molecule or
  pentaquark, can be an s-wave heptaquark K-pi-N
  (borromean binding)
• If s-wave heptaquark, its decay into p-wave K-N
  is suppressed by angular momentum leading to
  narrow width (P. Bicudo, hep-ph/0405254)
• ? do the expected cross sections for heptaquarks
  agree with the observations ?
• - Theta not a p wave K-N, due to large width
  expected O(100 MeV) (e.g. Capstick et al,
  hep-ph/0307019)
• Theta could be a K-N molecular resonance with
  L3, assuming a special potential (D.E. Kahana et
  al, hep-ph/0310026)
• In chiral soliton model one expects a theta
  isospin singlet (anti-10, S1/2), (e.g. M
  Polyakov et al, 1997), and a triplet (?,?,
  ?0) of I1 (27-plet, S3/2) (J Ellis et al,
  hep-ph)
• In quark models theta can be an isotensor I2
  (?, ?, ?, ?0, ?-) ( uuuus, uuuds,
  uudds, uddds, dddds)
• If ? exists, QCD at large Nc predicts several
  excited states with similar mass (100 MeV
  range), with possibly different widths (T. Cohen
  et al, hep-ph/0309150).

37
Limit on width of theta to explain non
observations (Arndt et al, Cahn et al , etc)
approx. 1 MeV In agreement with direct
measurements (Hermes 17-9-3 somewhat larger
than the exp. Resolution of 4.3-6.2 MeV , Zeus
width (sigma 6.1-1.62.0-1.4) consistent with
exp. Res. of 2sigma MeV) --gt Need higher stat.
to improve the errors Cross section(theta)
agrees with assumption of spin 1/2 --gt
preliminary indication that spin of theta 1/2
(B Jaffe, hep-ph/0409065) Xi masses (NA49) do
agree well with Chiral Soliton Model latest
calculations (J Ellis et al) N(1730) agrees with
expectations of the corr. Diquark model
(Jaffe/Wilzcek) for N(s) 1710 CSM expects
1650-1690 (Diakonov et al), resp. 1650-100 MeV
(J Ellis et al) for N N(1680) is also in this
range of masses. N(1730) could be octet.
38
Theoretical models Soliton model, Correl. Quark
model, Uncor. Quark model etc
Seen (1535)
?
And Theta0c, theta
N(1650-90)
?(1750-1800)
Seen (1860)
Correlated diquark model (B Jaffe, F Wilzcek)
octet and anti-10 are degenerate (Diakonov et al)
two octets (1/2) below 2 GeV mass, PDG or
new
N0(1440)
N(1440)
N(1710)
N0(1710)
?0(1660) ?0(1600)
?0(1880) ?01810)
?(1880)
?-(1880)
?-(1660)
?(1660)
?0(1690)
?-(1690)
?-(1950)
?0(1950)
39
4 Conclusions and outlook

• Experimental results on pentaquark candidates
• Theta(1530) seen by many experiments in two
  decay channels.
• However individual significances no more
  than 7.8 --gt Confirmation with higher statistics
  needed
• - mass(theta) differs in Kn and K0sp channels
  --gt Direct measurement of Kn (K- anti-n) needed,
  better study of syst. of Fermi motion
  corrections, estimate syst. Errors
• - Cross section agrees with predictions,
  and suggests spin(theta)1/2
• - Width order 1 MeV --gt Need more precise
  measurements
• - Spin, parity, cross section measurements
  needed
• - mass vs beam momentum to check reflexions
• 2. Theta(1573) excited state?,
  Theta(1530,1579) preliminary hints

40

• 3. Xi--(1862), Xi0 (1864) and (prelim.) Xi-
  (1854) candidates
• 4. N0 or Xi0 I1/2 (1727/1734) in two decay
  channels (if same state then is N0) and N0(1670)
  candidates (prelim.)
• Agree with (Arndt et al) --gt 1680,1730 narrow
  states
• 5. Theta_c0(3099) candidate
• 6. Non observations ee-, inclusive reactions,
  high energy --gt to be understood
• Outlook
• Need confirmation with higher Significance
  allowing also better systematic Studies
• If pentaquarks exist, many more states to be
  found
• Expect factors up to 10 increase in statistics
  from CLAS, LEPS
• STAR, COSY-TOF, ZEUS, H1, within 2005 etc.
• Several experimental proposals approved in Japan
  and JLAB
• to measure spin, parity, mass, width of theta

41

• Are there pentaquarks ? Which is their parity,
  mass, widths
• and spins ? Why are they narrow?
• Focused experimental and theoretical efforts
  should allow us to
• answer these questions in the next few years
• this is work in progress !