Volker D' Burkert

1
Have Pentaquark States been seen?
Volker D. Burkert Jefferson Lab

• Outline
• Brief introduction
• Original evidence for against pentaquarks
• How can results be compared?
• 2nd generation experiments and implications
• Status of pentaquarks October 2005?
• Conclusions

N2005 Tallahassee, Florida, October 12-15,
2005
2
Pentaquark history

• General idea of five-quark states has been around
  since late 60s. Predicted masses range from 1500
  to 1800 MeV, widths hundreds MeV.
• Bag models R.L. Jaffe 76, J. De Swart 80,
  lightest pentaquark Jp 1/2-
• Soliton models Diakonov, Petrov 84,
  Chemtob85, Praszalowicz 87. Walliser92

• A new wave of experimental searches was motivated
  by predictions in cSM model Diakonov, Petrov,
  Polyakov. Z.Physics A359 (1997).

.
3
The initial evidence for the Q
CLAS-D
DIANA
4.4?
5.2?
4.8?
4.6?
CLAS-p
Neutrino
SVD
HERMES
7.8?
6.7?
5.6?
5?
ZEUS
KN -gtX
5?
4.6?
3?
4
The Q width

• Measured width dominated by
• experimental resolution

• Analysis of KA data give limit of
• few MeV.

KD -gt X
KD , KXe X
Sibirtsev et al. GQ lt 1 MeV

• Very narrow for a hadronically
• decaying particle with mass 100 MeV
• above threshold
• Q(1540) -gt nK, pK0

From selected data sample

• What can we say about event rates?
• Kn -gt Q gt G1 MeV,
• K-p -gt L gt G16 MeV.
• s(Q)/s(L) 1.4 for formation.

GQ 0.9 /-0.3 MeV
W. Gibbs, Phys.Rev.C70, 054208 (2004)
5
Non-evidence for Pentaquarks
BABAR
BES
X--
DELPHI
X--
Q0c
6
Are the results on Q consistent?
Low energy effective degrees of freedom,
meson-baryon coupling High energy quark
degrees of freedom, fragmentation

• Can the positive low statistics results be
  reproduced with
• high statistics, e.g. LEPS, CLAS_d, CLAS_p,
  SAPHIR, COSY,
• HERMES,..?

• Is fragmentation at high energy an efficient way
  of generating
• pentaquarks? Why do some experiment (ZEUS,
  SVD)
• see a signal while others dont?

7
Upper limit on the Q cross section (95c.l.)
s g p ? Q K0 lt 1.25 nb _at_ 1.54 GeV/c2
CLAS gp -gt K0Kn
Fit with a sum of smooth function and a Gaussian
with fixed width and centroid.
g
K0
Results put stringent limits on possible
production mechanism, e.g. implies very small
coupling to K
K
n
p
Q
K
8
Comparison with SAPHIR results
SAPHIR
Observed Yields SAPHIR N(Q)/N(L)
10 CLAS N(Q)/N(L) lt 0.2
(95CL) Cross Sections SAPHIR sg p ? Q K0
300 nb CLAS sg p ? Q K0 lt 2 nb
Counts
Counts
cosqCM(K0) gt 0.5
cosqCM(K0) gt 0.5
M(nK) (GeV)
M(nK0) (GeV)
g11_at_CLAS
Counts
Counts
L(1520)
cosqCM(K0) gt 0.5
preliminary
cosqCM(K0) gt 0.5
Q(1540) ?
M(nK) (GeV)
M(nK0) (GeV)
9
Upper limit on Q production with CLAS.
G10
10
Upper limit on GQ from CLAS Results
Theoretical cross sections are for GQ1 MeV
( ) with K exchange
11
Old and New Signal from LEPS/SPring8
gD -gt K-KX

• Mix K, K-, ? from different events in LH2 data.
• ?(1520) contribution was removed from the sample.

?
?(1520)
preliminary
preliminary
MMC(?,K) GeV/c2
MMC(?,K-) GeV/c2
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K-p missing mass spectrum
? fitted in MMlt1.52 GeV/c2
LEPS
5s excess at 1.53 GeV/c2 corresponding to
? 1.6 GeV bump was also seen.
?
preliminary
1.6 GeV bump
Counts/5 MeV
preliminary
Counts/5 MeV
?
from sidebands
MMd(?,K-p) GeV/c2
MMd(?,K-p) GeV/c2
Signal emerges with selection of L(1520) in
M(K-p)
13
Pentaquark Studies at HERA
ep -gt eK0spX
High energy production mechanism?
ZEUS
gt Signal seen at medium Q2 and forward rapidity
in both pKs and pKs
14
ZEUS Cross section on Q
M(GeV)
Q/L 5 (independent of Q2)
15
Belle Low energy KN scattering
ee- -gt K/- X, K/-A -gt pK0, pK-
Detector Tomography
momentum spectra of K and K-
1 / 50MeV
momentum, GeV/c
Momentum range possibly contributing to Q
formation.
gt Determine resonance width
16
Belle Limit on Q Width
GQ from KA -gt pK0sX KD -gt inclusive analysis
KA -gt pK0s
Belle limit 90CL
397 fb-1
DIANA
No
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,PRD69,11501 (2004).
Not inconsistent with previous results.
17
Q/L cross section ratios
Consistent with lt1 MeV width.
Inconsistent with lt 1 MeV width?
18
Hadron production in ee- collisions
Baryons 10-4 /GeV/c2 (generate two pairs)
Pentaquarks 10-8 /GeV/c2 (?) (generate four
pairs)
G 1Mev
G 1MeV BR(X-- ?X-p-)0.5
19
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 experiments on
nuclei
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e-Be Electroproduction
Preliminary233 fb1 ee data
(quasi-real photoproduction)
HERMES (ed?KS0p X)
HERMES
HERMES/BaBar comparison disturbing. ZEUS High
Q2, no signal for Q2lt1 BaBar Quasi-real photons
HERMES acceptance loss in low mass region? (PID
requires p(p) gt4.1 GeV/c p(KS0)gt3 GeV/c)
21
STAR d-Au results
Possible mechanism for production of multi-q/g
states Coalescence of out of QGP?
STAR Preliminary
?
M (GeV/c2)
Also a Q candidate ??
Is this a Q candidate ?
22
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

• New analysis improves Q signal
• by factor 8. Total significance 7.5s.

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Cascade Pentaquark X5--(1862) ?
HERA-B
CDF

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

gA
FOCUS
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Charmed Pentaquark Q0c(3100) ?

• Signal also in photo-
• production
• Claim kinematical
• uniqueness.
• Possible production
• mechanism - PGF.

• FOCUS experiment also claims
• incompatibility with H1.

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Pentaquark Status _at_ N 2005

• Group Signal Backgr. Significance
• publ. Comments
• --------------------------------------------------
  --------------------
• SPring8 19 17 4.6s 3.2s
• SPring8 90 260 4.8s
• SAPHIR 55 56 4.8s 5.2s
• DIANA 29 44 4.4s 3.4s
• CLAS(d) 43 54 5.2s 4.4s
• CLAS(p) 41 35 7.8s 4.7s
• ? 18 9
  6.7? 3.5s
• HERMES 51 150 4.3-6.2? 3.6s
• ZEUS 230 1080 4.6?
  6.4s
• COSY 57 95
  4-6? 4.7s
• SVD 41 87
  5.6? 3.6s
• SVD-2 370 2000 7.5s
• NA49 38 43
  4.2? 4.2s
• H1 50.6 51.7
  5-6? 5.0s

? BELLE
Q
BABAR
WA89
X5
Q0c
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The evidence for the Q in 10/2005
DIANA
4.4?
4.6?
CLAS-p
Neutrino
7.8?
6.7?
ZEUS
KN -gtX
5?
4.6?
3?
27
Concluding comments
X- - (1862), Q 0c(3100)

• Each one observed by single experiment.
• Strong evidence against both states from
  several other
• experiments with comparable kinematics and
  claimed
• higher sensitivity
• The collaborations observing the states should
  show that
• other measurements lack sensitivity, or
  re-examine their
• own results.

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Concluding Comments
Q(1540)

• Two high statistics experiments on protons and
  deuterium (CLAS)
• contradict results that observed 5s signals
  with same kinematics.
• Within hadronic models the new CLAS results put
  limits on the widths.
• For most models GQ lt 0.5 MeV. New limit from
  Belle not (yet?) in
• contradiction with 1 MeV limit. Increase in
  statistics could change that.
• Need to examine if gD and gA experiments with
  lower statistics
• are consistent with CLAS cross section limit
  on neutrons .
• Babar is challenging the HERMES results on Q
  with very high statistics.
• New data/analysis from LEPS, SVD, and STAR,
  support Q observations.
• WA89 claims incompatibility with SVD results

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Outlook

• The narrow Q pentaquark is not in good health,
  but it is
• too early to pronounce it dead.
• Ongoing efforts to further test previous results
  
• - new analyses underway from COSY, SPring8
• - measurements planned at SPring8, JLab (CLAS,
  Hall A)
• - H1, ZEUS, HERMES high luminosity run until
  2007
• - higher statistics data from STAR, PHENIX
• - more statistics and analyses from B-factories
  (Belle, BaBar)
• Even if the Q pentaquark will not survive the
  next years of
• intense scrutiny, given the huge theoretical
  effort, we will
• still will have learned a lot about hadron
  structure and strong
• QCD.

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SVD-2 New Analysis
A. Aleev et al., hep-ex/0509033
Ep 70 GeV
Two independent data set KS decays inside or
outside the Vertex Detector
M 1522 MeV s 12 MeV Nevnt
205
M 1523 MeV s 12 MeV Nevnt
165

• New analysis improves Q signal
• by factor 8.

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Summary of LQCD
C. Alexandrou