Study of excited baryonsat BESII

HongXun YangRepresenting BES Collaboration

IHEPyanghx@mail.ihep.ac.cn

January 25-26, 2007,

Outline

• Introduction

• FDC system for PWA

• N* in the decay of

• X(2075) and Nx in the decay of

• Summary

npJ /

pKJ /

L ~ ~51030 /cm2s at J/ peak

Ecm~2-5 GeV

The Beijing Electron Positron Collider

VC: xy = 100 m TOF: T = 180 ps counter: r= 3 cm MDC: xy = 220 m BSC: E/E= 22 % z = 5.5 cm dE/dx= 8.5 % = 7.9 mr B field: 0.4 T p/p=1.78%(1+p2) z = 3.1 cm

BESII Detector

Study of Excited Baryon States

Motivation• Probe the internal structure of light quark baryons

• Search for “missing” baryons predicted by quark model

• Obtain a better understanding of the strong interaction force in the non-perturbative regime

• Examples for FDC application

J/ decays

processes branching ratios(10-3) N* decays

1.10.1

6.00.5

2.00.1

2.10.2

0.90.4

1.30.3

0/ ppJ ppJ /

relatively large branching ratios (PDG2004)

npJ / ppJ /

'/ ppJ ppJ /

NN *

NN *NN *

NN *NN '* NN *

Pure isospin 1/2

Feynman diagram of the production of

For and , N and N systems are limited to be pure

isospin 1/2.

**,*,*, Np

NNJ / NNJ /

FDC system

Physics model

• Feynman rules

• Counterms

• physical parameters

Physical process

• Generate Feynman Diagram

• Manipulate Amplitude and generate fortran source

• Manipulate Kinematics and generate fortran source

• Compile files and link sources

Application of FDC

• Loglikelyhood

• MC integration

• Minimize lnL to get the parameters of each partial wave

2

1

||

iii

N

n MC

data

Acw

w

wL

MCN

kkMC w

Nw

1

1

from BES II data

Events selection

2 good charged tracks Q1+Q2 = 0 |cos| < 0.8 PID: TOF and dE/dx Mp > 1.15 GeV

0.88<Mmiss<1.0 GeV

Background < 8%

npπJ/ψ

N* in npπJ/ψ N*(1440)

N*(1520)

N*(1535)N*(1650)

N*(1675)

N*(1680)

?

L=0 limits it to be

3/2+ or 1/2+

N* in npπJ/ψ

Dalitz Plot:

Acceptance and other reasons make the plot asymmetry.

PWA with FDC

• Try to determine the JPC of the resonance around 2.0 GeV/c2

• Following resonances are consideredN(939), P11(1440), D13(1520), S11(1535),S11(1650), D15(1675), F15(1680), X

• For the background: phase space and sideband

Results of PWA (I)

data fit

Results of PWA (II)

• L = 0 is preferred due to the suppression of the centrifugal barrier factor for L 1

• For L = 0, JP is limited to be ½+ and 3/2+

• S decrease more than 400 if either ½+ or 3/2+ is included in the PWA fit

• S decrease more than 60 if one of them are added while another has been included

Results of PWA (III)

• The peak around 2.0 GeV/c2 cannot be reproduced by reflections of well-established N* resonances

• Jp=1/2+,3/2+ is preferred by PWA

conclusion

Events selection

• 4 good charged tracks

• PID: kaon and at least 1 proton ID2 (4C)<20

• Bg rate:1~2%

../ ccpKJ

• Two clear peaks at 1520, 1690 MeV/c2 in pK mass• N* in K mass

N*(1535)?

N*(1650)?*(1520) *(1690)

pKM KM

N* & * in

(I)

. . /c c pK J

• Bands for excited baryon states in Dalitz plot

X(2075) in

. . /c c pK J

*(1520)

*(1690)

N*(1535)?

N*(1650)?

Phase Space

X(2075)

Possible N* and Λ* in PWA

• mass(JP)

N*: 1535 (1/2-), 1650(1/2-), 1710(1/2+), 1720(3/2+),

1900(3/2+ or 3/2-), 2050(1/2+ or 3/2+)

Λ*: 1405(1/2-), 1520(3/2-), 1600(1/2+), 1670(1/2-),

1690(3/2-), 1800(1/2-), 1810(1/2+), 1890(3/2+)

Mathematical fit

• W/o any constrains, PWA with N* and Λ* can fit data:

(S= -997)

• However, it needs many unexpected big BRs and many large destructive interferences to cancel these big BRs.

Big BRs

Fraction of Ndata Nevent

N*(1900) 3/2+ 108% 5900

N*(2050) 3/2+ 33% 1800

Λ*(1890) 3/2+ 21% 1100

Λ*(1810) 1/2+ 9% 500

Λ*(1800) 1/2- 34% 1900

(1/2- is P-wave decay, which should be suppressed )

Estimated Nevent in Ndata

Nevent/2 (each decay mode, not include c.c.)

N*(1900) 3/2+ ~ 300 -400 N*(2050) 3/2+ ~ 100 -150 Λ*(1890) 3/2+ ~ 100 -150 Λ*(1810) 1/2+ ~ 150 -200 Λ*(1800) 1/2- ~ 20 - 30 (1/2- is P-wave decay, which should be strongly suppressed near threshold)

PWA with tight constrains and w/o X(2075)

• Constrain the Nevent of near threshold states ~ 100-200

• The PWA fit cannot reproduce the enhancement near pΛ threshold (S=-900).

• Adding X(2075), PWA fit can reproduce the enhancement near pΛ threshold.

• Significance of X(2075) >> 5 sigma. (S=-952)

PWA with tight constrains and with X(2075)

PWA with looser constrains and w/o X(2075)

Constrain the Nevent of near threshold states ~ 500-600

(already too big at least for Λ*(1810), Λ*(1890) as limited in )

The PWA fit can hardly reproduce the enhancement near pΛ threshold (S=-940).

../ ccJ

PWA with looser constrains and with X(2075)

• Adding X(2075), PWA fit can reproduce the enhancement near pΛ threshold.

• Significance of X(2075) > 5 sigma. (S=-962)

conclusion of pΛ enhancement

• Reproducing the pΛ threshold enhancement with pure N* and Λ* interferences needs many unexpected big BRs and large destructive interferences.

• PWA fits with pure N* and Λ* and with constrains can hardly reproduce the enhancement.

• PWA fit with X(2075) can easily reproduce the enhancement (independent of constrains) with high significance.

)(GeV/c2

ΛKM

PS, eff. corrected

MMM KΛK

A strong enhancement near the threshold of mass spectrum of ..ccK

(Arbitrary normalization)

BES II pKJ /

NX*

We perform PWA studies on the KΛ mass threshold structure:

The most important we want to study is its production BR

PWA is performed to pKJ /

possible N* and *states listed in PDG are fitted N(1720), N(1900), (1520), (1690), …

many different combinations are tried

different JP of Nx is tried

also tried N(1535) to fit Nx

An example of PWA fit

• Mass and Width scan

• Total fit (S=-952)

• Nevent:

Fraction Nevent

NX 14.7% 799

N(1720) 17.1% 929

N(1900) 13.2% 717

(1520) 4.8% 261

(1570) 21.8% 1184

(1690) 14.4% 782

(1890) 13.8% 750

X(2075) 11.3% 614

Mass scan(GeV/c2)

Width scan(GeV/c2)

• NXN(1535)

• Total fit (S=-932)

• Nevent:

Fraction Nevent

N(1535) 26.0% 1413

N(1720) 9.7% 527

N(1900) 11.4% 619

(1520) 4.8% 261

(1570) 22.2% 1026

(1690) 3.6% 739

(1890) 18.3% 994

X(2075) 11.2% 608

Dalitz plot (data)

Dalitz plot (PWA)

Eve

nts

/10M

eV Crosses: data

Hist.: PWA fit projection

JP check with various combinations

• JP ½- ½+ 3/2- 3/2+ non

• A -940 -848 -848 -930 -813

• B -845 -783 -806 -833 -752

• C -952 -841 -844 -916 -768

• D -880 -768 -752 -822 -650

• E -957 -889 -893 -944 -875

• F -970 -920 -925 -963 -919

• G -954 -925 -919 -944 -909

Fit results

Cases Mass(GeV) Width(MeV) Fraction(%) Nevent Log Likelyhood

a 1.52 ~ 1.62 110 22.3 1210 -940

b 1.56 80 ~110 44.4 2412 -845

c 1.62 70 14.7 799 -952

d 1.6 ~1.64 70 17.1 929 -880

e 1.57 90 20.6 1119 -957

f 1.62 70 ~ 90 19.9 1081 -970

g 1.58 80 15.6 845 -954

1.50 ~1.65 70 ~110 >14.7 >800

• A strong enhancement is observed near the mass threshold of MK at BES II.

• Preliminary PWA with various combinations of possible N* and Λ* in the fits —— The structure Nx*has:

Mass 1500~1650MeV

Width 70~110MeV

JP favors 1/2-

consistent with N*(1535)

The most important is:

It has large BR(J/ψ pNX*) BR(NX* KΛ) 2 X 10-4 ,

suggesting NX*has strong coupling to KΛ.

indicating it could be a KΛ molecular state

(5 - quark system).

Summary• FDC applied in the analysis of

and• Clear signals of excited baryons observed• Seemed to be “missing” states observed in

• Possible multi-quark states X(2075) and Nx in

• PWA on Nx, N* and * in

is still going on…

../ ccpKJ ../ ccnpJ

../ ccnpJ

../ ccpKJ

../ ccpKJ

Thank you!