Discovery of the Pentaquark: Experimental Status - … · Discovery of the Pentaquark: Experimental...

Discovery of the Pentaquark: Experimental Status

Moskov Amarian

(DESY-Zeuthen)

Pentaquark ForumDESY

November 25, 2003

Introduction

Experimental results (chronologically):

SPRING-8

DIANA

CLAS

SAPHIR

ITEP-2

Search for Θ+ → pKs at HERMES

... continued

New weird Ξ−− from NA49

Summary and Outlook

Moskov Amarian, Discovery of the Pentaquark : Experimental Status – p.1/22

Historical Lesson on S = +1 Baryon System

PDG 1986; Phys. Lett. B170, 289 wrote:The evidence for strangeness +1 baryon resonance was reviewedin our 1976 edition,1 and more recently by Kelly2 and by Oades.3

Two new partial-wave analyses4 have appeared since our 1984edition. Both claim that P13 and perhaps other waves resonate.

and then continued:However, the results permit no definite conclusion- the same storyheard for 15 years. The standards of proof must simply be muchmore severe here than in a channel in which many resonancesare already known to exist. The general prejudice against baryonsnot made of three quarks and the lack of any experimental activityin this area make it likely that it will be another 15 years before theissue is decided.


Experimental Evidence from LEPS at SPRING-8

LEPS Collaboration at SPRING-8:(T.Nakano et al.,PRL 91,012002(2003))

Tagged photons → energy range (1.5– 2.4) GeV, (σ = 15 MeV) :(Compton back-scattering from 8 GeV electrons)

Targets:0.5 cm thick plastic scintillator(SC) (C:H ' 1 : 1)

5 cm thick liquid-hydrogen (LH2) 9.5 cm upstream of (SC) target

Particle Identification: Time-of-Flight

Search for resonance Θ+ → K+n in exclusive reaction:γn(12C) → K+K−n


The Θ+ resonance from LEPS

a)

MMcγK

+ (GeV/c2)

Eve

nts/

(0.0

2 G

eV/c

2 )

b)

MMcγK

− (GeV/c2)E

vent

s/(0

.02

GeV

/c2 )

0

5

10

15

20

1.5 1.6 1.7 1.80

5

10

15

1.5 1.6 1.7 1.8

a) solid histogram: K−n missing mass (no res. expected)dashed histogram: K−p missing mass (Λ(1520) expected)

b) solid histogram: K+n missing mass (Θ+ expected ?)dashed histogram: LH2 spectrum of K+p (no res. expected)

Conclusion: M=1540 ± 10 MeV, Γ <25 MeV, σ = Ns√

Nb

= 4.6 ±1.


Experimental Evidence from DIANA at ITEPDIANA collaboration at ITEP:Xenon bubble chamber

K+Xe → K0s pX (Pk+ ∼ 480MeV )

(Quasi-free K+n → K0p)

PID via ionization and momenta fromranges (no magnet)

a) solid histgram: K0p invariant masswithout cutsdashed histogram: background dueto charge-exchange K+Xe → K0Xe

b) solid histogram: K0p spectrumwith cuts imposed:K0 and p goingforward and back-to-backConclusion: (hep-ex/0303040)M = 1539±2 MeV, Γ ≤ 9 MeV, σ=4.4


Experimental Evidence from CLAS at JLAB

d

γ Θ+n

K+

K-p

K-p

n

exclusive γd → pK+K−n

CLAS Collaboration (S.Stepanyan etal., hep-ex/0307018)

MM(pK+K-) [ GeV/c2 ]

Eve

nts

MM(pK+K-) [ GeV/c2 ]

Eve

nts

|∆tpK| ≤ 0.75 ns

0

100

200

300

400

500

0.8 0.85 0.9 0.95 1 1.05 1.1 1.15 1.2 1.25

0

50

100

150

200

250

300

350

0.8 0.9 1 1.1

Tagged photon beam fromEe = (2.5–3.1) GeV,with ∆Eγ = (3–5) MeV

Particle Identification with TOFMoskov Amarian, Discovery of the Pentaquark : Experimental Status – p.6/22

The Θ+ resonance from CLAS

M(K+K-) [GeV/c2]

Eve

nts

φ Nφ = 126Mφ = 1.02 GeV/c2

σφ = 0.004 GeV/c2

M(pK-) [GeV/c2]

Eve

nts

Λ(1520) NΛ = 212MΛ = 1.518 GeV/c2

σΛ = 0.011 GeV/c2

01020304050607080

0.9 0.95 1 1.05 1.1 1.15 1.2 1.25 1.3 1.35 1.4

010203040506070

1.4 1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8 1.85 1.9

Production of φ and Λ(1520) in thereaction γd → pK+K−n

They are removed in the analysis.

M(nK+) [ GeV/c2 ]

Eve

nts

0

5

10

15

20

25

30

35

1.5 1.6 1.7 1.8 1.9

solid histogram: final M(nK+)solid line: arbitrary fit + bkgddashed histogram: Λ(1520) events

Conclusion: M=1542±5 MeV, FWHM=21 Mev, σ=5.3±0.5Moskov Amarian, Discovery of the Pentaquark : Experimental Status – p.7/22

Experimental Evidence from SAPHIR at ELSA

SAPHIR detector at ELSA : 1.–2.7 GeV tagged photon beam

π+, π−, K+ identified with TOF

Exclusive γp → K0Θ+ → K0K+n → π+π−K+n

��

��

�

� �

� ��

detect K0K+→ reconstruct K+n Missing Mass (hep-ex/0307083)


The Θ+ resonance from SAPHIR

0

5

10

15

20

25

30

35

40

1.4 1.6 1.8mass(nK+)/GeV

Θ+ (1540)co

unts

a

0

100

200

300

400

500

600

700

800

900

1.4 1.6 1.8mass(nπ+π-)/GeV

Λ (1520)coun

ts

b

a) K+n Missing Mass with the cut ΘcmKs

> 0.5

b) π+π−n Missing Mass with (lower histogram)without (upper histogram) Θcm

Ks> 0.5 cut

Conclusion: M=1540±4 ± 2MeV, Γ < 25MeV, σ=4.8


The Θ+ Isospin from SAPHIR

0

20

40

60

80

100

1.45 1.5 1.55 1.6 1.65 1.7 1.75 1.8mass(pK+)/GeV

no Θ++

coun

ts

K+p Missing Mass

from Clebsh-Gordan and acceptance: expected 5000 Θ++

→ far above experimental value

Conclusion: Θ+ is an isoscalar


Experimental Evidence from Neutrinos

1.4 1.6 1.8 2 2.2 2.4 2.60

2

4

6

8

10

12

14

16

18

20

22

24

p) , GeVS0

m(K

Neon plus Deuterium

co

mb

. / 10 M

eV

1.5 1.6 1.7 1.8 1.90

2

4

6

8

10

12

14

16

18Chi2 / ndf = 22.33 / 21

0.004737 ±mass = 1.533 0.002043 ±sigma = 0.008379

6.417 ±excess = 25.56 0.5881 ±p3 = 1.214 4.732 ±p4 = 17.71

p) , GeVS0

m(K

co

mb

. / 10 M

eV

Chi2 / ndf = 22.33 / 21 0.004737 ±mass = 1.533

0.002043 ±sigma = 0.008379 6.417 ±excess = 25.56

0.5881 ±p3 = 1.214 4.732 ±p4 = 17.71

Reanalysis of existing bubblechamber data from CERN, FNAL

Inclusive K0p production by highenergy ν and ν, Eν,ν '(40–140) GeV, Neon plus Deuterium

Upper panel: dots depict random starbackground

Lower panel curve : Gaussian pluslinear, width consistent with resolution

Conclusion (A.E.Asratyan et al.,hep-ex/0309042):M = 1533±5MeV, Γ < 20MeV, σ=6.7


The HERMES Spectrometer

1

0

2

-1

-2

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

� � � � �

m

LUMINOSITY

CHAMBERSDRIFT

FC 1/2

TARGETCELL

DVC

MC 1-3

HODOSCOPE H0

MONITOR

BC 1/2

BC 3/4 TRD

PROP.CHAMBERS

FIELD CLAMPS

PRESHOWER (H2)

STEEL PLATE CALORIMETER

DRIFT CHAMBERS

TRIGGER HODOSCOPE H1

0 1 2 3 4 5 6 7 8 9 10

RICH270 mrad

270 mrad

MUON HODOSCOPEWIDE ANGLE

FRONTMUONHODO

MAGNET

m

IRON WALL

e+

27.5 GeV

140 mrad

170 mrad

170 mrad

140 mrad

MUON HODOSCOPES

SILICON

Resolution: δp/p = 1.4 . . . 2.5% , δΘ . 1 mrad

Particle Identification: TRD, Preshower, Calorimeter,RICH (dual radiator)


The HERMES RICH

0

0.05

0.1

0.15

0.2

0.25

0 2 4 6 8 10 12 14 16 18 20p [GeV]

ΘC [r

ad]

π

K

p

Particle Identification:cosΘ = 1

n

detection efficiencies


The Θ+ Pentaquark Search at HERA-HERMES

e + D → Θ+ + X → Ksp + X (Ee+=27.5 GeV)

Protons and pions identified with RICH

Ks reconstructed using decay length

pπ− events from Λ(1116) ± σ range excluded


The Θ+ from HERMES: Penta or not Penta ?

M(π+π-p) [GeV]

Eve

nts/

8 M

eV e+D → Ks + p + XNs/Nb = 71.7/163.6in 2σ intervalFit Gauss+P4Chi2=1.4

HERMES preliminary

M=1526 ± 2(stat) ±2(sys) MeVσ = 7.5 ± 2.4(stat) MeV

0

10

20

30

40

50

60

70

80

1.45 1.5 1.55 1.6 1.65 1.7

Resonance is observed at1526 ± 2(stat.) ± 2(syst.)MeVin Ksp invariant mass distribution

the width:σ = 7.5 ± 2.4(stat.) MeVdominated by experimentalresolution


The Θ+ Monte Carlo

14.90 / 11Constant 240.5 13.30Mean 1.540 0.2692E-03Sigma 0.6994E-02 0.2385E-03

Invariant Mass (π+π-p) (GeV)

0

50

100

150

200

250

1.4 1.45 1.5 1.55 1.6 1.65 1.7

The Θ+ generated with:M=1540MeV and σ = 2MeV

Reconstructed values:M=1540MeV and σ = 7MeV(due to resolution)


PYTHIA6 and Mixed Event Background

1480

→ 1560

→

1580

→

1620

→

1660

→16

70→

M(π+π-p) [GeV]

Eve

nts

/ 8 M

eV

0

10

20

30

40

50

60

70

80

1.5 1.6 1.7

Hatched histogram – PYTHIA6MC simulation (lumi normalized):No resonance structure fromreflections of known mesonic orbaryonic resonances

Solid histogram — mixed eventbackground normalized toPYTHIA6:well reproduces the shape ofPYTHIA6 simulation

Excited Σ∗ hyperons not includedin PYTHIA6 lie below 1500MeVand above 1550MeV


Θ+ Isospin from HERMES (prelim.)

MΛ=1522±2.0(stat) MeVσΛ=9.4±2.0(stat) MeV

pk- →

pk+ →

M(pk-), M(pk+) [GeV]

Eve

nts/

(4 M

eV)

0

100

200

300

400

500

600

700

1.45 1.5 1.55 1.6 1.65 1.7

Clear signal for Λ(1520)

No peak structure in pK+

Gell-Mann-Nishidjima relation:Q = I3 + Y/2, where Y=B+S

if no Θ++ then:

→ Θ+ is an isosinglet


Hunting Other Members

uudds-

uussd-ddssu-

0pK or nK+

Ξ π0 + or Σ Κ0+Ξ (2070)

Θ+ (1530)

N(1710)

Σ(1890)

Ξ π- - or Σ Κ- -

PENTAQUARKS: hiding in the corners


Exotic Ξ−− from NA49 at CERN

NA49 Coll.: evidence for exotic Ξ−−(dsdsu) in pp at√

s=17 GeVS = −2 and I = 3/2 with M = 1862 ± 2MeV and width < 18 MeV

Xi- pi- inv mass

0

10

20

30

40

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

Entri

es /

7.5

MeV

/c2

M(Ξπ) [GeV/c2]

a) Ξ-π-

Xi- pi- inv mass

0

5

10

15

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

b) Ξ-π+

Xi- pi- inv mass

0

5

10

15

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

c) Ξ– +π-

Xi- pi- inv mass

0

2

4

6

8

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

d) Ξ– +π+

Xi- pi- inv mass

0

10

20

30

40

50

60

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

a)

Entr

ies

/ 7.5

MeV

/c2

M(Ξπ) [GeV/c2]

Xi- pi- inv mass

-10

0

10

20

30

1.4 1.6 1.8 2 2.2 2.4 2.6 2.8

b)


Mass Values and Experimental Widths of Θ+

0

1

2

3

4

5

6

7

1520 1540Mass value [MeV]

Sequ

ence

SPring-8

DIANA

CLAS

SAPHIR

ITEP (υ,s)

HERMES

FWHM [MeV]

0

1

2

3

4

5

6

7

0 25 50

Left panel: measured massesThe weighted average of allexperiments:M=1535±2.5MeV (hatchedband)

reduced χ2 a la PDGcorresponds to confidencelevel of 0.1

Right panel: measured widths

ZEUS →Welcome on Board

HERA-B: strong controversyor affordable result?


Summary

Exotic baryon resonance Θ+ is observed in different experimentswith very narrow width and PENTAQUARK structure

Quantum numbers are undefined except of isospin → Θ+ is anISOSINGLET

New member of anti-decuplet Ξ−− is observed at NA49 (needsconfirmation)

Further theoretical and experimental work is needed to establishquantum numbers like SPIN and PARITY of observedresonances

Pentaquarks open NEW window for our understanding of quarksand hadrons in the extreme conditions and possibly also in thestellar objects

HERA experiments contribute to the world efforts on this subjectMoskov Amarian, Discovery of the Pentaquark : Experimental Status – p.22/22

Discovery of the Pentaquark: Experimental Status - … · Discovery of the Pentaquark: Experimental...

Documents

Transcript of Discovery of the Pentaquark: Experimental Status - … · Discovery of the Pentaquark: Experimental...