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[email protected]/8/2004
Outline
• What is a pentaquark?• Experimental motivation
– Θ5(1540), Ξ5(1850), Θc(3100)• Theoretical motivation and predictions• Examples of positive results• Negative searches for pentaquarks:
– Two examples: BaBar and E690– Summary of positive and negative searches
• Conclusions and Future Plans
[email protected]/8/2004
What is a pentaquark?
• Bound state of 4 quarks and 1 antiquark• “Exotic” pentaquarks are those for which the
antiquark has a different flavor than the other 4 quarks.
Example: , non-exoticBaryon number = 1 Strangeness = 0
Example: , exoticBaryon number = 1 Strangeness = +1
][ qqqqq
][ suuds
][ suudd
[email protected]/8/2004
Claims of exotic narrow baryons
Μ(Ξ−−) = Μ(Ξ−π−)=Μ(θs+) = (nK,pKs)=
MeVMeVsysstat
312)(5)(33099
±<±±
σ
Μ(θc+) = Μ(D*-p) =
MeVFWHMGeV
21005.0542.1
<±
MeVFWHMGeV
18002.0862.1
<±
θs+ Claimed by LEPS(γn),
DIANA(Kxe), CLAS (γp), SAPHIR(γp), HERMES(ed), νn
12 claims for θs+
θc+ Claimed by H1
Collaboration at DESYΞ−−/ Ξ0 Claimed by
NA49 Experiment at CERN
Some of these results are not yet confirmed.
[email protected]/8/2004
Recent activity
Early 2003, claims of K+n resonance at ~1540 MeV.
proliferation of theory papers
followed by reports of negative results
Figures from “Pentaquarks--Facts and Mysteries or Sisyphus at Work”, Josef Pochodzalla, hep-ex/0406077
~0.5 paper/day
12 sightings
10 negative results
[email protected]/8/2004
Color singlet combinations of quarksand antiquarks
3f ⊗ 3f = 1f ⊕ 8fMesons: q q
Baryons: q q q 3c ⊗ 3c ⊗ 3c = 1c ⊕ 8c ⊕ 8c ⊕ 10c3f ⊗ 3f ⊗ 3f = 1f ⊕ 8f ⊕ 8f ⊕ 10f
K
π
K
∆Σ∗
YY
ΣΛpn
JP=1/2+
I3I3Ξ∗ Ξ∗
Ω−
JP=3/2+
[email protected]/8/2004
How is Θ put together?
Θ n K decay, strangeness = 1, can be [ududs] pentaquark
Flavor representation of q4q:
3 ⊗ 3 ⊗ 3 ⊗ 3 ⊗ 3 = (3)1 ⊕ (8)8 ⊕ (4)10 ⊕ (2)10 ⊕ (3)27 ⊕ 35
Exotics must be in 10 27 35:
8 ⊗ 8 = 1 ⊕ (2)8 ⊕ 10 ⊕ 10 ⊕ 27 Uncorrelated quarks or “Molecule”?
[email protected]/8/2004
Pentaquark anti-decuplet
][5 suudd+θ
][5 duusd+Σ
][5 duuss+Ξ][5 uddss−−Ξ
][5 duuddN +
I3
[email protected]/8/2004
Chiral soliton model of Diakanov et al.• The two lowest-lying baryon multiplets are JP = 1/2+
octet and JP = 3/2+ decuplet
• The third excitation is exotic anti-decuplet with JP = 1/2+
• Equal mass splittings within anti-decuplet
• Width < 30 MeVY
I3)(5 suudd+θ
Dia
kano
v, P
etro
v, P
olya
kov
Z.Ph
ys.A
359
, 305
(199
7) 1530 MeV
1710 MeV (input)
1890 MeV
2070 MeV
)]([5 ssdduudN ++
)]([5 ssdduus +Σ+
][5 duuss+Ξ)]([ dduuuss +
[email protected]/8/2004
Diquark model of Jaffe and Wilczek
JW hep-ph/0307341
sudud ]][[6f ⊗ 3f = 8 f ⊕ 10fJM hep-ph/0308286
SZ hep-ph/0310270
L=1
(ud)
(ud)
s
Correlated quark pairs Q form flavor, color and spin antisymmetric states
Pentaquark is made of (QQ)sq for example θ+ [ud][ud]s
Bose-Einstein statistics requires (QQ)s for identical diquark pairs to be in an L=1 state & anti-q => J=1/2+
Motivation:
light-quark scalars seem to have significant component of [qq][qq]
[email protected]/8/2004
Diquark model of Jaffe and Wilczek
• Accommodates the Roper N(1400) P11• Predicts N(1700) with hidden strangeness• Predicts light Ξ• Predicts Σ and Λ at 1600 MeV• Predicts positive parity• Predicts possible charm and bottom analog• Ideal mixing 8 f 10f
[email protected]/8/2004
Diquark-triquark model of Karliner and Lipkin
• Coupling of 5 quarks into a diquark-triquark system in a nonstandard color representation and in a relative P-wave
• I=0, JP = 1/2+ and in an anti-decuplet of SU(3)f f
• Predicts M(Θ5+) = 1592 MeV
but close agreement to experimentally claimed mass (~1540 MeV) and M(Ξ5) = 1722 MeV
JP = 1/2+
[email protected]/8/2004
Essential distinctions
The three models make different predictions for the parity of the θ+
Chiral soliton ModelDiquark ModelDiquark-Triquark Model
Uncorrelated Quarks
Chiral Soliton Model Heavy exotic cascades
Diquark Model Light exotic cascades
+Parity
Original spectrum
The new DDP version has θ+ and Ξ−− N(1650-1690), Σ(1760-1810) and Σ(1770) is a candidate.
[email protected]/8/2004
Mass spectrum of the anti-decuplet
Ξ5
Mas
s (A
rbitr
ary
Uni
ts)
Chiral Soliton Model
Σ5
Ξ5, Ξ5, S
Λ5, Σ5
Diquark Model (Jaffe, Wilczek)
Σ5, S
Θ5
N5
N5, S Accommodate two JP=1/2+ N states:-N(1710) and Roper resonance N(1440)?- N(1710) should then couple to states with hidden strangeness, like KΛ.
Predictions for doubly-strange Ξ5masses are very different.
N5
Θ5
[email protected]/8/2004
Lattice gauge theory and QCD sum rules
• A number of lattice gauge theory and QCD sum rule calculations have been done for pentaquarks but results are not in agreement
• No conclusive lattice predictions for– the existence of a bound pentaquark state– its parity– its mass– its width
[email protected]/8/2004
Now, let’s switch to the experimental view…
[email protected]/8/2004
Note on the S=+1 baryon system
Claim for Z* S=+1 at 1780, 1865 MeV in the late 80s
PDG 1986; Phys. Lett. B170, 289
The evidence for strangeness +1 baryon resonances was reviewed in our 1976 edition, and more recently by Kelly and by Oades. Two new partial-wave analyses have appeared since our 1984 edition. Both claim that the P13 and perhaps other waves resonate. However, the results permit no definite conclusion -- the same story heard for 15 years. The standards of proof must simply be much more severe here than in a channel in which many resonances are already known to exist. The general prejudice against baryons not made of three 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.
Particle listings were dropped for these S=+1 baryon states after 1986.
[email protected]/8/2004
First claim for the Θ5+
Super Photon ring-8 GeV SPring-8
•Third-generation synchrotron radiation facility
•Circumference: 1436 m
•8 GeV
•100 mA
•62 beamlines
• photons produced in Compton backscattering of laser photons from 8-GeV e-
• look for γ n K- (K+ n) by measuring missing mass due to n• signal for Θ5
+ K+ n in missing mass• first announced in October 2002• Jan. 14, 2003: submitted evidence for a K+ n peak at 1540 MeV.
349 citations as of 11/29/04
[email protected]/8/2004
LEPS missing mass plots for γN → K+K-N events:
Λ(1520) →p K-
Θ5(1540)Γ< 25 MeV4.6 σsolid line: n K
dashed line: p K
n
γK
K+
nΘ+
p pp
γK+
K
pΛ∗
n n
[email protected]/8/2004
CEBAF Large Acceptance Spectrometer
Electromagnetic calorimetersLead/scintillator, 1296 photomultipliers
Gas Cerenkov counterse/p separation, 256 PMTs
Time-of-flight countersplastic scintillators, 684 photomultipliers
Drift chambersargon/CO2 gas, 35,000 cells
Torus magnet6 superconducting coils
Liquid D2 (H2)target +γ start counter; e mini-torus
Bremsstrahlung photon tagger2.47-3.11 GeV e Eγ = (20% - 95%) Ee
Three experiments:
γ p → π+ K- K+ n (Eγ = 3.2 - 5.5 GeV
γ d → p K- K+ n (Eγ = 1.5 - 3 GeV)
γ He3 → ΛΘ+p (Eγ = 0.53 – 1.55 GeV)
γ d → ΛΘ+ (Eγ = 0.3 – 3.6 GeV)
[email protected]/8/2004
The CLAS detector at JLAB
CLAS has since accumulated and is now analyzing γd sample that is > 20 times larger.
[email protected]/8/2004
CLAS missing mass distributions
285 citations
deuteron targetΘ5+
Both nucleons involved no Fermi motion is necessary
42 events
Mass = 1542 ± 5 MeVWidth < 21 MeV
Significance = 5.2 σ −6.0 σ
[email protected]/8/2004
CLAS missing mass distributions
146 citations
proton targetΘ5+
π+
π−
proton
γ
Θ+N* K+
n
K-
Mass = 1555 ± 10 MeVWidth < 26 MeV
Significance = 7.8 σ
[email protected]/8/2004
First 9 Θ5+[uudds]->KN “sightings”
Data replotted by Josef Pochodzalla with statistical errors and no fits. From “Pentaquarks -- Facts and Mysteries or Sisyphus at Work”, hep-ex/0406077
γn K+Xe γd
γpγp
pnpp
νA
e+d
+ ZEUS + JINR-BC + H2BC
HERMES HERA
SAPHIR ELSA
LEPS Spring-8
DIANA ITEP
CLAS JLAB
CLAS JLAB
COSY TOF
SVD-2 IHEP
CERN + FNAL bubble chambers
[email protected]/8/2004
First 9 Θ5+[uudds]->KN “sightings”
Data replotted by Josef Pochodzalla with statistical errors and no fits. From “Pentaquarks -- Facts and Mysteries or Sisyphus at Work”, hep-ex/0406077
HERMES HERA
SAPHIR ELSA
LEPS Spring-8
DIANA ITEP
CLAS JLAB
CLAS JLAB
COSY TOF
SVD-2 IHEP
CERN + FNAL bubble chambers
4.6σ 4.4σ4.6 -5.8σ
4.8σ
4.2-6.1σ
6.7σ
5.6σ
7.8σ
4 - 6σ
[email protected]/8/2004
Discovery of Ξ5−− /Ξ5
0
Narrow resonances in the Ξ- π− and Ξ- π+ invariant massNA49 experiment pentaquarks S=-2, I=3/2
Ξ-- /Ξ0 [dsdsu]
158GeV protonsTarget: Liquid H2
≈
GeVs 2.17=
Μ=1.862+/−0.002 GeV/c2
Μσ < 18 MeV /c2
[email protected]/8/2004
Ξ old and new data
J.M. Gago et al., “ Ξ∗ Production in K- p Interactions at 14.3 GeV/c”Ecole-Polytechnique-Saclay-RHEL Collaboration,(CERN/EP/PHYS 76-50)
J.W.Price,J.Ducote,J.Goetz,B.M.K.Nefkens,“Photoproduction of the double-strange Ξ hyperons”For the CLAS Collaboration,(arXiv:nucl-ex/0402006)
Ξ−∗Ξ0∗
[email protected]/8/2004
H1 candidates for Θc(3100)
28-GeV electrons on 820-GeV and 920-GeV protons
Θc0[ududc]: H1
Mass = (3099 ± 3 ± 5) MeV
s = (12 ± 3) MeV
51 ± 11 events
[email protected]/8/2004
Negative Results from High statistic experiments
[email protected]/8/2004
Revisiting old dataDalitz plots from old bubble chamber experiments for K+N→KNπ, compiled by George Trilling for RPP2004 “A Possible Exotic Baryon Resonance”
Diagonal bands show where one would expect to see the Θ5(1540)+.
1.54 GeV
[email protected]/8/2004
The BaBar detector
SVT: 5 double side layers, 97% efficiency, 15 mm z hit resolution
DCH : 40 axial and stereo layers
Tracking: σ(pT)/pT = 0.13 % × pT + 0.45 %, σ(z0) = 65m @ 1 GeV/c
DIRC: 144 quartz bars
EMC: 6580 CsI(Tl) crystals σE/E = 2.3 %⋅E-1/4 ⊕ 1.9 %
IFR: 19 RPC layers, muonand KL id
[email protected]/8/2004
Particle identification
Cherenkov angle from DIRCdE/dx from Drift Chamber
Distinguishing between p, π/µ, K
[email protected]/8/2004
Motivation for pentaquark searches
If pentaquark production is the same as other baryon production then we expect:~ 8x10-4 θs
+ per event~ 4x10-5 Ξ5
-- per event
[email protected]/8/2004
The BaBar search strategy
5
5 5
5
5
• Pentaquark theory expects θ5s+,
Ξ5 to be members of the 105+85
• Dedicated searches for claimed states: θ5s
+, Ξ5--, Ξ5
0, θ5c0
• Inclusive searches for other states in the 105+85
• Specific searches for selected states from other multiplets: θ*++
[email protected]/8/2004
Search for θ5+(1540) p Ks
0
Λc+(2285)
1540
No Signal of θ5+
• Clean Ks0 π+π− reconstruction using geometrical cuts
• Clean protons using dE/dx and Cherenkov angle• Clear Λc
+ sample with 52000 events• No structure near 1540 MeV/c2
• No narrow structure except Λc+
Many additional/different cuts tried; same results
[email protected]/8/2004
Λc+ p Ks
0 control sample
• Fit signal in bins of p*• Check mass scale• Shape of resolution function same
as simulation• Width of resolution function same
as simulation• Yield consistent with expectations
(Efficiency ~30%)
Now fit for θ5+:
• P-wave Breit-Wigner signal xFthreshold resolution function
• Resolution ~2 MeV/c2 due toproximity to threshold
•Assume Γ=8 MeV/c2 best experimentalupper limit
•Assume Γ=1 MeV/c2 very narrow state
⊗
MC simulation of mass resolution tracksdata very well.
[email protected]/8/2004
θ+ cross section and upper limit
• Fitted yields consistent with zero• We derived @ 95% C.L.
a model independent cross section upper limit as a function of momentum
Model independent limits on the total production cross section @ 95% C.L. :
Γ= 1 Γ = 8 MeV/c2
< 182.8 < 363.1 fb
< 5.4x10-5 < 10.7x10-5 per qq event
< 17.9x10-5 < 35.0x10-5 per Υ(4s)
[email protected]/8/2004
Search for Ξ5-- Ξ-π- and Ξ5
0 Ξ-π+
We look for inclusive production of :
Xee −−−+ Ξ⎯→⎯ 5−−−− Ξ⎯→⎯Ξ π5
Xee 05Ξ⎯→⎯−+
+−Ξ⎯→⎯Ξ π05
−−−Ξ
π
−Λ0π
0Λ
p
IP
−−Ξ
π
• Geometrical selection of two displaced vertices• Loose particle ID• Select masses near the nominal Λ0 and Ξ- masses• Control particles: Ξ*0(1530) Ξ-π+, Ξc(2470) Ξ-π+
[email protected]/8/2004
Ξ−π−, Ξ−π+ invariant mass
No Signal of Ξ5--
No Signal of Ξ50
95% C.L. U.L. on Ξ−−
prod. cross section:< 33.7 fb< 1.0x10-5 per qq< 3.2x10-5 per Υ(4s)(Γ = 18 MeV/c2)
~5K events
~2K events
[email protected]/8/2004
Search for Ξ50 Λ0 Κs
0
• Analysis method similar to above• Ξc
0 Λ0Ks0 seen, ~2K events
<
No Signal of Ξ50
95% C.L. U.L. on Ξ50 production
cross-section x ΒΦ :
< 82.8 fb Γ=1MeV/c2
<204.7 fb Γ=18MeV/c2
[email protected]/8/2004
Search for Ξ- Λ0 Κ−
−− Λ⎯→⎯Ξ kdduudss 05 ),(
No Signal of Ξ5-
• Λ0 selection as above• Combine with identified K-
• Control particle Ω-, ~8K events
95% C.L. U.L. on production cross-section x ΒΦ :
< 83.6 fb Γ=1MeV/c2
<181.0 fb Γ=18MeV/c2
[email protected]/8/2004
Additional searches
M(p Ks) in [1.60 – 1.8 ] GeV/c2
• Also Σ0K+, Σ0K-, Σ0Ks0
• No unexpected narrow structure
N5+(uudss) Λ0K+ N5
0(uddss) Λ0Ks
Σ5+(uudss) pKs
[email protected]/8/2004
Results in perspective
• Assume pentaquarks have J= ½
• ΒΦ(θ+ p Ks0) = 25%
• ΒΦ(Ξ5-- Ξ-π-) = 50%
• ΒΦ for Ξ5-, Ξ5
0 very unclear
• Limits are below expectations for baryons:
θ5+ by a factor of 8-15
Ξ-- by a factor of 4-6
[email protected]/8/2004
Exclusive search for θ∗++ (pK+)
• Prediction for a θ*++[uuuds] pentaquark• I=1, I3=1• m(θ*++) = [1.43—1.7] GeV/c2
• Γ = [37—80] MeV/c2
• We searched in the exclusiveB+ θ*++p pK+p decay.
• 81 fb-1 data sample was used; 89M BBevents
• “Standard” B decay analysis
[email protected]/8/2004
B-> ppK+ measurement
5.276 < mES < 5.286 GeV
∆E < 29 MeV • 212 events in the signal region • ~68 events have charmonium decays to pp• ~40 background events
ΒΦ(B ppK+) = (6.7 0.9(stat.) 0.6(syst.))x10-6± ±
[email protected]/8/2004
Upper limit on θ∗++
Events in theCharmonium region
Events outside theCharmonium region
• No entries for m θ*++ < 1850 MeV/c2
ΒΦ(B+ θ*++(pK+)p) < 1.49x10-7 @ 90% C.L.
[email protected]/8/2004
Summary of BaBar results
• Many Λ0, Ξ−, Ξ∗0, Ω−, Ξc0, Λc
+ baryons found in our data.
• No signal for any pentaquark seen:
Production of θ+(1540), Ξ--(1860) well below ordinary baryons of similar mass.
No narrow peaks for hypothetical 105+85 states in several search modes.
• No signal for the hypothetical θ∗++ pK+;
ΒΦ(B+ θ∗++p pK+p) < 1.5x10-7 @ 90% C.L.
[email protected]/8/2004
Pentaquark searches from E690• Fermilab fixed-target experiment E690• 800 GeV proton beam (√s = 38.8 GeV) on Liquid H2
• 5 billion events recorded in 1991• hundreds of thousands of clean reconstructed hyperons.
Key Features of E690:• Liquid hydrogen target• Magnetic spectrometer• Open geometry• Large geometric acceptance• Thin-gap drift chambers• Helium between chambers• Excellent momentum resolution• Excellent mass resolution• Particle identification was not used for the exotic baryon search (not needed)• Same apparatus used in BNL E766 for L and S mass measurements: PRL 72, 1322 (1994) and
PRD 56, 2544 (1997).
1 meter
HodoscopeMiddle
HodoscopeRear
CherenkovCounter
SpectrometerMagnet
Drift Chambers
Veto Counters
ProtonBeam
H Target2
[email protected]/8/2004
E690 search for Θ5+ pKs in
pp pKsK-π+p
1540 MeV
Monte Carlo pKpKss mass
resolution (σ) at 1540 MeV is 1.5 MeV.
Even
ts p
er 2
MeV
m (p Ks) for Ks K- π+ events
m (p K-) for Ks K- π+ events
Λ(1520), ~5k eventsFWHM ~ 14 MeV
[email protected]/8/2004
E690 search for Ξ±π± in pp p Ξ±π±X N
umbe
r per
2 M
eV
Num
ber p
er 2
MeV
Ξ+π- Effective Mass Ξ+π+ Effective Mass
Ξ-π- Effective MassΞ-π+ Effective Mass
Ξ(1530)
Ξ(1530)
Arrows at1750 MeV &1860 MeV
Monte Carlo mass resolution (σ) for Ξπ:
3.3 MeV at 1750 MeV;4.5 MeV at 1862 MeV.
m(Ξ + π - ) [GeV]
m(Ξ - π - ) [GeV]
m(Ξ + π + ) [GeV]
m(Ξ - π + ) [GeV]
[email protected]/8/2004
Is θ5+[uudds] → K N signal real?
LEPS ~25 / 160 S/B~2/1
CLAS 212 events S/B~2
HERA-B (p nucleus) ~3k events S/B~1/3
CDF (pp) ~3k events S/B~1/6
ALEPH (e+e-) ~3k events S/B~1/6
E690 (pp) ~5k S/B~2
BABAR (e+e-) ~40k S/B~1
Experiment # of Λ(1520) → K− p signal events
Control Mode
See no evidence for θ+
Claim evidence for θ5
+
[email protected]/8/2004
Is Ξ5--[ddssu] → Ξ- π- signal real?
NA49 (pp) ~ 45 evts, S/B~1
ALEPH (e+e-) No signal
ZEUS (e p) No signal
HERA-B (p nucleus) No signal
CDF (pp) No signal
BABAR (e+e-) No signal
E690 (pp) No signal
WA89 (Σ- C/Cu) No signal
Experiment Ξ5−−(1860) # Ξ− # Ξ(1530)0
→ Ξ−π+
~1,600
~1,800
~2,600
~19,000
~36,000
~258,000
~512,000
~676,000
~20, S/B~2/3
~300, S/B~1
~160, S/B~1
~2k, S/B~1
~5k, S/B~1
~15k, S/B~2
~20k, S/B~1
Control Mode
[email protected]/8/2004
Is θc[uuddc] → D*- p signal real?
H1: • 3.4k D* events (S/B ~ 2.5)• Number of events in narrow peak in D*p mass spectrum
near 3.1 GeV: 51 ± 11⇒ (1.5±0.3)% of D*’s come from pentaquark decays?!
ZEUS: No signal ⇒ < 1% of D*’s from pentaquarks.FOCUS: No signal with 36k very clean D* signal events. CDF: No signal with 540k very clean D* signal events.
[email protected]/8/2004
Future experimental outlook
• more γd data was analyzed from LEPS; still see signal.
• factor of 20 more γd data from CLAS being analyzed.
• more pp data from COSY-TOF (x few) will be accumulated next year.
• proposal at KEK has been approved.– K+ p → π+ Θ+
– 1 MeV mass resolution ⇒ precise measurement of mass and width.
[email protected]/8/2004
[email protected]/8/2004
The γ3He→ΛΘ+p reaction
• Kinematical reflections not possible, only one K in final state• S=+1 both for nK+ and pK0, thanks to Λ (S=-1)• No background channels to remove to identify final state
π −
π +Κ+Θ+
Κ0He3
p
p
p
p
n
Λ π −
γ
(n)
(K+)
Decay modes studied: Θ+→K0p Λ→pπ- K0 → π+π- 6 charged particlesΘ+→K+n Λ→pπ- 4 charged, 1 neutral
[email protected]/8/2004
CLAS G10: γd→ΛΘ+
• G10: first CLAS experiment dedicated to Θ+
• Eγ = 0.8 – 3.59 GeV • data taking finished end of May• 10 billion of events collected• data are being processed
Topologies to be analyzed:1) ppπ−π+π− (all detected)2) pπ−π+ (missing Λ,Σ)3) pπ−π+π− (missing p)4) ppπ+π− (missing π-)5) ppπ- (missing K0)6) pπ- (missing Θ+)7) pπ-K+ (missing n)
Also search for Θ++ in the reaction:γd→pK+ π- (missing Λ,Σ)
(n)d
(K+)
[email protected]/8/2004
Summary and outlook
• γ3He→ΛpK0p and γ3He→ΛpK+n measured for the first time, with CLAS • good channel to verify existence of Θ+:
no kinematical reflections are possible final state reconstructed with no need of removal cuts
• 3 independent topologies under study • low statistics in all topologies• in all 3 analyses: hints of an enhancement in the NK invariant mass spectrum at M≈1.55 GeV/c2
analysis of high-statistics dedicated CLAS data on deuteron is underway…