Pentaquarks

Pentaquarks Pentaquarks Valery KubarovskyValery Kubarovsky

Rensselaer Polytechnic Institute / Jefferson LabRensselaer Polytechnic Institute / Jefferson Lab

Valery Kubarovsky PIC-2005

IntroductionIntroduction More than 10 experiments have recently reported More than 10 experiments have recently reported

observation of the baryon with exotic quantum observation of the baryon with exotic quantum numbersnumbers

Light:Light: M=1525-1555 MeVM=1525-1555 MeV Narrow:Narrow: < 9-25 MeV (possibly ~1 MeV)< 9-25 MeV (possibly ~1 MeV) Strangeness Strangeness S=+1S=+1 (opposite to the strangeness of (opposite to the strangeness of

the usual baryons)the usual baryons) This new state was identified as the This new state was identified as the + + pentaquark pentaquark

baryon with quark contents baryon with quark contents

There is also evidence for two related states with the There is also evidence for two related states with the strangeness S=-2,strangeness S=-2,

And even charmed pentaquark And even charmed pentaquark (3100) with quark (3100) with quark

contents contents

suudd

),( 02/32/3

cuudd


Pentaquark in naïve quark Pentaquark in naïve quark modelmodel

u u d d s s Current massCurrent mass 4 MeV4 MeV 7 MeV7 MeV 150 150

MeVMeVConstituent Constituent massmass

350 350 MeVMeV

350 350 MeVMeV

470 470 MeVMeV

•Pentaquark mass = 4*350+470=1830 MeV•In addition there is some penalty for the p-wave (in case of the positive parity) •So the pentaquark mass must be more than 2 GeV in any constituent quark model•The predicted width is wide (>175 MeV) due to the allowed decay to the baryon and meson with mass well above the threshold •The ground state has negative parity.

The spontaneous breakdown of the chiral symmetry would produce nonzero constituent mass and the massless pseudoscaler Goldstone bosons


L=1, one unit of orbital angular momentum needed to obtain as in the SM

“Correlated” Qurk Model Jaffe, Wilczek

JP = ½+

L=0

(ud)

(ud)s

L=1

•The four quarks are bound into two spin zero , color and flavor 3 diquarks [ud], ds, [su] (Bose statistics).

•For identical diquarks , like [ud]2, the lightest state has negative space parity. So the q4q state has positive parity

•The narrow width is described by relatively week coupling to the K+n continuum from which it differs in color, spin and spatial wavefunctions.

_

_


Chiral Soliton Model•The extra QQ pair in the pentaquark is added in the form of a pseudo scalar Goldstone meson, which costs nearly zero energy.• In reality, to make the + from the nucleon, one has to create a quasi-Goldstone K-meson and to confine it inside the baryon of the size >1/M. It costs roughly 600 MeV. • So the + mass is near 1540 MeV.• = 15 MeV(D.Diakonov, V.Petrov and (D.Diakonov, V.Petrov and M.Polyakov, 1997)M.Polyakov, 1997)

_JP = ½+


Pentaquarks on the Pentaquarks on the latticelattice

The only known method to derive hadronic properties from The only known method to derive hadronic properties from first principles is lattice QCD.first principles is lattice QCD.

There have been a number of lattice studies to see if There have been a number of lattice studies to see if ++ can can be predicted from QCD. be predicted from QCD.

It was consistently found that the lightest isospin I=0 state It was consistently found that the lightest isospin I=0 state is below lightest isospin I=1 state but there was is below lightest isospin I=1 state but there was disagreement on the parity assignment of a possible disagreement on the parity assignment of a possible ++ baryonbaryon

There was also studies which did not find a pentaquark There was also studies which did not find a pentaquark resonance, only scattering states of weakly-interacting resonance, only scattering states of weakly-interacting Kaons and Nucleons.Kaons and Nucleons.

So the consensus has not been reached but as was written in So the consensus has not been reached but as was written in one of the articles one of the articles ““absence of evidence is not evidence absence of evidence is not evidence of absenceof absence”. ”.


The initial evidence for PentaquarksThe initial evidence for PentaquarksLEPS SAPHIR

CLAS-pHERMES Neutrino

pp ++.

COSY-TOF

DIANA

SVD

CLAS-D

ZEUS

4.6 4.4 5.2 4.8

7.8

~5

6.7

5.6~5

4.6

NA49

6.7 4.2

5-6

1862MeV


pK0

nK+

+ +

MassMass

World Average: 1532.5±2.4 MeV•The pK0 peaks have the mass positions systematically lower than for the nK+ decay mode.

•Shift could be due to different background shapes and interference effects

•Or it may indicate the serious concern about the existence of the+ baryon


What do we know about the + width?

JP = ½+

Widths seen in experimental analyses are dominated by resolution effects.More precise information is obtained in analyses with theoretical constraints.

DIANAPhys. Atom. Nucl. <9 MeVHERMES, PLB585, 213 (2004) = 17+/-9+/-3 MeVS. Nussinov et al., hep-ph/0307357 < 6 MeV (non-observation)R. Arndt et al., PRC68, 42201 (2003) < 1 MeV (non-observation)R. Cahn and G. Trilling, PRD69, 11401(2004)= 0.9 +/- 0.3MeV (from DIANA results)A. Sibirtsev, et al., hep-ph/0405099 (2004) < 1 MeV (K+d Kopp)W. Gibbs, nucl-th/0405024 (2004) = 0.9 +/-0.2 MeV (K+d X)

MeV


First Wave Experiments

Historical Overview


LEPS : LEPS : ++ published mass plot published mass plot

Background level is estimated by a fit in a mass region above 1.59 GeV.

Assumption:• Background is from non-resonant K+K- production off the neutron/nucleus• … is nearly identical to non-resonant K+K- production off the proton

1.540.01 MeV< 25 MeVGaussian significance 4.6

backgroundPhys.Rev.Lett. 91 (2003) 012002

hep-ex/0301020


DIANA/ITEP (Moscow)DIANA/ITEP (Moscow)

750 MeV K750 MeV K++ beam beam incident on 700 liters incident on 700 liters Xe Xe bubble chamberbubble chamber..

Interaction energy is Interaction energy is determined by the range determined by the range of the kaon.of the kaon.

Charged particles are Charged particles are identified by ionization, identified by ionization, momentum is measured momentum is measured by the range.by the range.

Charge exchange reaction

K+N+pK0S

K0S+-


DIANADIANA

Selecting forward going Selecting forward going protons and kaons - protons and kaons - kk and and pp < 100 < 100oo..

p and Kp and K00 are emitted back- are emitted back-to-back - costo-back - cospKpK < 0. < 0.

Peak at 1.539 GeV in the invariant mass of K0p.

Statistical significance 4.4. Measured width < 9 MeV

The best limit for width

pKXeXeK 0'


Experiments at JLABExperiments at JLAB

Experimental Hall BCLAS detector


Review of experiments: CLAS/JLAB Review of experiments: CLAS/JLAB

TOF counters

Drift chambers

Beam line and the target

Electromagnetic calorimeters

6 Superconducting toroidal coils

Cherenkov counters

CEBAF/Hall B CLAS DetectorCEBAF/Hall B CLAS Detector

Bremsstrahlung tagged photon facility, photon energy resolution ~0.2%


CLAS-CLAS-d: Exclusive reactiond: Exclusive reaction

Exclusive photoproduction onExclusive photoproduction ondeuterium:deuterium:

No correction for Fermi smearing is needed.

Aids significantly to reduce the background.

Possible reaction mechanism

Experimental data from 1999 run; Tagged photons with up to 3 GeV

energy; Target: 10 cm long liquid

deuterium;)(nKpKd


CLAS CLAS

Distribution of (1520) events

+

Simulated background

Gaussian background

nKpKd

M = 1.542 GeV < 21 MeV (M=9 MeV)Stat.sig. = 5.2


CLAS: Photoproduction onCLAS: Photoproduction on hydrogenhydrogen

n

After PID)(nKKp


CLAS-CLAS-p: p: with with forward going forward going ++

p

K-

N*

Cos*(+) > 0.8

M(nK+)

Fitted mass 1.555 GeV < 28 MeV consistent

with detector resolution

nK


preliminary

L(1520)

pK+K-p

preliminary

nK+K-n

LEPS/SPring-8 LEPS/SPring-8 ddKK++KK--n(p)n(p)

MM (GeV) MM (GeV)

• Dedicated experiment• Aimed for 4x statistics of 2003 result• Announced at N*2004

• A proton is a spectator (undetected).• Fermi motion is corrected to get the missing mass spectra.• Background is estimated by mixed events.


Conclusion of LEPS Conclusion of LEPS experimental groupexperimental group

LEPS high statistics experiment has LEPS high statistics experiment has reconfirmed the peak, very unlikely to be reconfirmed the peak, very unlikely to be due to statistical fluctuations.due to statistical fluctuations.

The preliminary study shows no indication The preliminary study shows no indication that the peak is generated by kinematical that the peak is generated by kinematical reflections, detector acceptance, Fermi-reflections, detector acceptance, Fermi-motion correction, nor cuts. motion correction, nor cuts.

““existence ranges from very likely to existence ranges from very likely to certain, but certain, but further confirmation is further confirmation is desirabledesirable” - “three-star” definition by PDG.” - “three-star” definition by PDG.


Second GenerationSecond GenerationDedicated ExperimentsDedicated Experiments


Search for Pentaquarks at JlabSearch for Pentaquarks at Jlab

g10 deuteron E ~ 1.0-3.5 GeV completed in 2004Hall-A Search for ++, o completed in 2004 Search for + planned for 2006/7g11 proton E ~ 1.6-3.8 GeV completed in 2004eg3 deuteron E ~ 4.0-5.4 GeV completed in 2005Super-g proton E~ 3.8 – 5.7 GeV planned for 2006/7

A comprehensive program to search for pentaquarks with high statistics and high resolution photoproduction experiments is in progress at Jefferson Lab


nKnK++ Mass Spectrum Mass Spectrum

M(nK+)(GeV)

Coun

ts/4

MeV

+(1540) ?

no structure is observed at a mass of ~1540 MeV

the nK+ mass spectrum is smooth

preliminary

nKKp 0


Upper Limit on theUpper Limit on the ++ Cross Cross SectionSectionmass dependencemass dependence

Upper limit (95% CL)p K0 < 2 nbpreli

minary

0Kp


Kinematics

Selection of forward angles of the K0 in the -p center of mass

Energy limited to 2.6 GeV

no hyperon rejection

p

+

CM

K0

Comparison with SAPHIR results

cosCM(K0) > 0.5

cosCM(K0) > 0.5

cosCM(K0) > 0.5

cosCM(K0) > 0.5

(1520)

SAPHIR

g11@CLAS

+(1540) ?

preliminary

M(nK+) (GeV)

Coun

ts

Coun

tsCo

unts Co

unts

M(nK+) (GeV)M(nK0) (GeV)

M(nK0) (GeV)

Observed YieldsSAPHIR N(+)/N(*) ~ 63/630 ~ 10%

CLAS N(+)/N(*) <110/53000 <0.2% (95%CL)

Cross SectionsSAPHIR p K0 ~ 300 nb reanalysis 50 nb

CLASp K0 < 2 nb (95%CL)


CLAS – 2nd Generation Experiment II

In previous result the background is underestimated. New estimate of the original data gives a significance of ~3possibly due to a fluctuation.

Set upper limit on cross section The new data show no signal

nosignal

corrected for r.s. with1520) as a guide.

Fermi momentum only

npKKd

< 5 nb (95% CL) model dependent.

Effective Lagrangian method (hep-ph/0505134)


totalTotal cross sections from proton and neutron do not show differences

Total cross section from proton and neutron

The new CLAS results do not exclude a state of < 1 MeV width.


Published Null ExperimentsPublished Null Experiments

ExperimentExperiment ReactionReaction LimitLimitBES eBES e++ee-- J/Y J/Y QQ* QQ* BR<1.1x10BR<1.1x10-5 -5

Belle eBelle e++ee-- Y(2S) Y(2S) pK pK00

KK++Si Si pK pKss00XX

BR<0.6x10BR<0.6x10-5-5

<0.02 L*<0.02 L*BaBar eBaBar e++ee-- U(4S) U(4S) pKpKss

00 BR<1.1x10BR<1.1x10-4-4

ALEPHALEPH ee++ee-- Z Z pK pKss00 <0.6x10<0.6x10-5-5

HERA-BHERA-B pA pA pK pKss00XX <0.02 L*<0.02 L*

CDFCDF pp* pp* pK pKss00XX <0.03 L*<0.03 L*

HyperCPHyperCP pCu pCu pK pKss00XX <0.3% K<0.3% K00pp

PHENIXPHENIX AuAu AuAu n*Kn*K-- not givennot givenSPHINXSPHINX pA pA K K00(pK(pK00)A, )A,

KK00(nK(nK++)A)A<0.02 L*<0.02 L*

- -


Belle – Limit onBelle – Limit on ++ WidthWidth

397 fb-1

LP2005 153K+A pK0

s Belle limit 90%CL

MeV (90% CL) @ M = 1.525–1.545 GeV

No

Not inconsistent with previous results.

from K+A pK0sX &

K+D inclusive analysis

no signal

Belle: < 0.64 MeV (90% CL) @ M = 1.539 GeV


Examples of collider searchesExamples of collider searches

(1520)

DELPHIK-p

K0sp

CDFJet eventsMin.bias events


Quark fragmentation as a source Quark fragmentation as a source ofof ++??

BaBar searches for + in quark fragmentation with high statistics LP2005 299

e+e- pK0s+X

e+

e-

q

q


Pentaquark in fragmentation?Quark fragmentation

e

q

uud

Pentaquark stronglysuppressed ?

+

q

Pentaquark less suppressed ?

Baryon fragmentation

s

e

dd

uud

s

Needs fewerquark pairs from the vacuum

+


High energy production mechanism

+ produced mostly at forward rapidity Lab > 0, and medium Q2 > 20 GeV2.

Consistent with + production in baryon fragmentation.

M(GeV)

ep eK0spX

ZEUS

Valery Kubarovsky PIC2005 Valery Kubarovsky PIC2005

Cascade PentaquarkCascade Pentaquark ----(1862) ?(1862) ?CDF

pp XHERA-B

State not produced in quark fragmentation or is severely suppressed.FOCUS A

FOCUS A < 0.0025/Brange


Charmed PentaquarkCharmed Pentaquark 00cc(3100) ?(3100) ?

LP2005 #16

Upper limit factor 4 lower than H1 results. Claim is that results are incompatible with H1.

Signal also in photoproduction Claim kinematical uniqueness.

FOCUS experiment claims incompatibility with H1.

LP2005 #391

FOCUS

FOCUS events

FOCUS events

H1 expected

H1 expected


Comments onComments on 0c

Each one observed by single experiment.

Strong evidence against both states from several other experiments with comparable kinematics and claimed higher sensitivity


New Positive ResultsNew Positive Results SVD2 SVD2 Spring8/LEPS Spring8/LEPS STARSTAR


SVD2 New Improved Analysis,SVD2 New Improved Analysis, Ep=70 GeV Ep=70 GeV pXKpA s

preliminary

M = 1522 MeV= 12 MeVNevnt = 205

M = 1523 MeV= 12 MeVNevnt = 165

Two independent data set: KS decays inside or outside the Vertex Detector

preliminary


Breaking News from LEPSBreaking News from LEPS

D K-p X ; M(K-p)=(1520)

MMd(γ,K － p) GeV/c2

Cou

nts/

5 M

eV

prel

imin

ary1.53

1.60 (?)


STAR: aSTAR: a++ ++ pentaquark?pentaquark?

STAR

M(pK+)

d-Au

++ ?STA

R Prelim

inary

KppK

d-Au

++?

KppKAu-Au

++?

Strong signal at M~1.53 GeV in both d-Au and Au-Au collisions.

“The STAR observed yield is so small such that many experiments would not have the sensitivity to see it.” (Huan Z. Huang).


Group Signal Backgr. Significancepubl. Comments

----------------------------------------------------------------------SPring8 19 17 4.6 3.2SPring8 56 162 ? 3.8SAPHIR 55 56 4.8 5.2 DIANA 29 44 4.4CLAS(d)** 43 54 5.2 4.4CLAS(p) 41 35 7.8 4.7 18 9 6.7 3.5 HERMES 51 150 4.3-6.2 3.6COSY 57 95 4-6 4.7ZEUS 230 1080 4.6 6.4SVD 41 87 5.6 3.6SVD-2 370 2000 7.5Improved analysis

NA49 38 43 4.2 4.2 H1 50.6 51.7 5-6 5.0

SVD-2 370 2000 ? 7.5s Improved analysisSPring8 200 285 5.0nSTAR 2,250 150,000 5.5candidate

Pentaquark Status Pentaquark Status

New CLAS-p

New CLAS-d

? HERA-B, CDF? ZEUS

s/ b+s

? New Clas-d? New Clas-d


ConclusionConclusion

Two high statistics experiments on protons and deuterium (CLAS) contradict results that observed ~5 signals with same kinematics. The new CLAS results do not exclude a state of < 1 MeV width. Sensitivity to + at high energy appears in baryon fragmentation not in quark fragmentation. This could explain some of the null results at high energies. New data from LEPS, SVD2, and STAR, support the observations.


OutlookOutlook

Analysis is continuing at Spring8, Jlab, COSY, HERMES, H1, ZEUS, SVD2, STAR, PHENIX

New measurements planned at SPring8, JLab H1, ZEUS, HERMES high luminosity run until 2007 Higher statistics data from STAR, PHENIX More statistics from B-factories

Focus moved from bump hunting to more quantitative estimations of cross sections or upper limits


Many labs are involved now in the search for pentaquarks. We have positive and negative results.There is also, in parallel much theoretical activity.

Do pentaquarks exist? The final conclusion is not straightforward.

Theoretical interest to pentaquarks will not disappear in any case. More over, “if high precision experiments will not find + and it’s partners, this may be even more difficult to understand than the + small mass and small width” (Praszalowicz, May 2005)

Concluding Remarks


QCD instantons QCD instantons DiQuarks are building blocks of multiquark states: pentaquarks and dibaryons

A nucleon is made of a quark and deeply bound scalar-isoscalar diquarks, absent in the decuplet.

In the instanton liquid model there are two kind of diquarks, the scalar and the tensor (with spin=1)

Mesons

Baryons ()

4Q Mesons

DiBaryons5Q Baryons

Baryons (8)


1% of 1% of

statisticsstatistics

g11@CLAS

X)Kπ(πM X )πM(π )M(nπ

)M(nπ )KM(n 0 X)(KM X

n

Λ(1116)

(1192)Σ0

0K (1189)Σ

(1197)Σ (1520)Λ*Λ(1116)

(1192)Σ0

(1520)Λ*(1385)Σ*

nKp


Hermes: Mass spectrum with additional

signal/background: 1:3

standard cuts applied + K* and veto• signal/background: 2:1

27.6 GeV positron beam is incident on a deuterium target in a search for + in quasi-real inclusive photoproduction.


++(1540)/L(1540)/L**(1520) Ratio(1520) Ratio

N(+) (95%CL) < 210

N(*) ~ 91000

preliminary

002.0)(

)(

1520

NN

95% CL


BelleBelle

m, GeV

1 / 5

MeV

pKS

pK-155fb-1

(1520)

This is approx. what we should have expected here! Assume that background events have same isospin structure as + events.

< 80 events

For I=0:nK+: pK0

s: pK0L

2 : 1 : 1


pKpKss00 inclusive mass inclusive mass

spectrumspectrum

No signal observed in the region of the + baryon.Note the statistics (20K events/2 MeV bin) andmass resolution ( 6 MeV )


Pentaquark production in direct e+e- collisions likelyrequires orders of magnitudes higher rates than available.

Hadron production in e+e-

Slope: Pseudoscalar mesons: ~ 10-2/GeV/c2 (need to generate one qq pair)

Baryons: ~ 10-4 /GeV/c2 (need to generate two pairs)

Pentaquarks: ~ 10-8 /GeV/c2 (?) (need to generate 4 pairs)

Slope for Pentaquark??

Slope forbaryons

Slope for p.s.mesons

Pentaquarks

Documents

Transcript of Pentaquarks