The Search for Pentaquarks - Welcome to...

[email protected]/8/2004

The Search for Pentaquarks

Valerie Halyo SLAC


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


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


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.


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


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+


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”?


Pentaquark anti-decuplet

][5 suudd+θ

][5 duusd+Σ

][5 duuss+Ξ][5 uddss−−Ξ

][5 duuddN +

I3


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 +


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]


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


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+


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.


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


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


Now, let’s switch to the experimental view…


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.


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


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


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)


The CLAS detector at JLAB

CLAS has since accumulated and is now analyzing γd sample that is > 20 times larger.


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


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


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


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


Comparison with world data

Kn0sK p

Mean: 1531.5±2.627


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


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∗


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


Negative Results from High statistic experiments


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


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


Particle identification

Cherenkov angle from DIRCdE/dx from Drift Chamber

Distinguishing between p, π/µ, K


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


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*++


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


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.


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)


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+


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


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


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


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


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


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


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± ±


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.


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.


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


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


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]


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

+


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


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.


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.


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


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+)


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…

The Search for Pentaquarks - Welcome to...

Documents

Transcript of The Search for Pentaquarks - Welcome to...