The charmonium mass spectrum

The charmonium mass spectrumThe charmonium mass spectrum

Presented by: Wander BaldiniWander Baldini

Ferrara University and INFNFerrara University and INFN

Informal workshop on charmonium spectroscopy Genova June 7th-8th 2001

OutlineOutline

A little bit of history: the November revolution. Main experimental techniques for the study of

charmonium:

• The charmonium spectrum: present status

)( ccee

)( ccpp )(** cceeeeee

The November revolutionThe November revolution

From ”The Rise of the Standard Model:”

"As I look back to the first three years at SPEAR, I consider this one of the most revolutionary, or perhaps the most revolutionary, experiment in the history of particle physics....."

G.Goldhaber

The discovery of the J/The discovery of the J/

• On November 10-th 1974, at SLAC and BNL an extremely narrow resonance was discovered at an energy of about 3100 MeV

• The resonance was immediately confirmed at Frascati

• Its small width couldn't be explained in terms of the known quarks u,d or s

• This resonance was called J at BNL and at SLAC (for a reason that I will explain soon)

The discovery of the J/The discovery of the J/

The discovery of the J/ at SLAC…

…and at the Brookhaven National Laboratory

What is this resonance made of?What is this resonance made of?

• A few years before, Glashow, Iliopoulos and Maiani proposed a model to explain the absence of the S=1 neutral weak currents:

• This model predicted the existence of a new quark “charm” with charge +2/3

• The discovery of the J/confirmed this prediction and was actually the definitive confirmation of the existence of quarks

50

10

K

K

Why Why

You may wonder why this resonance was called look at the picture of this event….

The name was clearly right!

)/( J

Experimental techniquesExperimental techniques • Three main methods are used to study the

charmonium resonances:• electron-positron annihilations:

• proton- antiproton annihilations:

• two-photon collisions:

)( ccee

)( ccpp

)(** cceeeeee

Electron-Positron annihilationsElectron-Positron annihilations

• This method is one of the first exploited • It allows the direct formation of the charmonium

states with the same quantum number of the photon

• All the other states are studied through radiative decay of and

• It provides a low background method for the identification of charmonium states

• MarkI,II,III and Crystal Ball at SLAC are some

of the experiments that exploited this technique.

)1( PCJ

/ J

Crystal BallCrystal Ball Crystal ball is a non magnetic detector designed to study the charmonium states mainly through the detection of photons emitted in radiative transitions:

llJc /

• energy resolution:4 )(

%8.2)(

GeVEE

E

• Main detector made of 672 pyramidal NaI(Tl) blocks

• Each block is 15.7 radiation lengths thick

c

• Angular coverage: 98% 4

Proton-antiproton annihilationsProton-antiproton annihilations

• This method allows the direct formation of all the charmonium resonances

• The mass and width of the resonance are obtained from beam parameters and do not depend on the detector energy resolution

• The charmonium signal can be clearly selected over the large hadronic background by studying the electromagnetic decays

• This technique has been pioneered by R704 at the Intersecting Storage Ring at CERN and extensively used by E760/E835 at theFermilab Antiproton Accumulator

• Non magnetic spectrometer designed to study the charmonium resonances through their e.m. decays:

• Angular and energy resolutions: from 1.5 to 5 mrad

mrad10

%2.1)(

%6)(

GeVEE

E

• Angular coverage: 33% 4

E760/E835 at FermilabE760/E835 at Fermilab

eeJpp c / cpp

Two photon collisionsTwo photon collisions • With this technique C-even charmonium states can be produced through the fusion of two quasi-real photons emitted by e+ and e- :

• The e+ and e- usually go undetected along the beam pipe (untagged events)

)(** cceeeeee

• The and resonances can be produced, forbidden by Young theorem

)0(),0( 0

cc )2(2

c• CLEO-II and LEP experiments are presently using this technique

)1(1

c

Resonance scanResonance scan

44

12)( 2

2

2

2R

R

RBW

ME

fRBRppB

k

JE

• Each charmonium resonance is studied by changing the c.m. energy in small steps

)250( keV

• The charmonium is detected through its e.m. decays

• The mass and the width of the resonance are extracted from the excitation curve with a maximum likelihood fit

BBWB sdEsEfLdt

ev

)(.#

0

• The measured excitation curve is the convolution of the resonance cross section (Breit-Wigner) and of the beam energy distribution

Beam energy measurementBeam energy measurement

21

22232

ORB

ORB

LL

ff

ppcpmpE

• The beam energy is calculated from the orbit length (Lorb.) and from the revolution frequency (f)

21

2

cLf

cpmpE

ORB

• The uncertainty on the energy measurement is dominated by

orb L )10/( 7 ff

• The reference orbit length Lref is calculated at the energy with a precision of

)100( keVm mm67.0

• The orbit length at all the other energies is calculated thanks to 48 Beam Position Monitors, which provide the orbit length difference :L

)( LLL REForb

The charmonium spectrumThe charmonium spectrum

The fundamental stateThe fundamental state )( 01Sc

…and by E835 in the decay channel:

cpp

The resonance observed by Crystal Ball…

c

cee

cJee /

preliminary results

The fundamental stateThe fundamental state

Mass measurements Total width measurementspreliminary results

)( 01Sc

The stateThe state• Crystal ball is the only experiment which saw an evidence of this resonance

MeV)50.3594( • Mass:

• Total width: MeV8

• E760/E835 searched for this resonance in the energy region: Ecm=(3570-3660) MeV, in the decay channel: but no evidence of a signal was found

c

Crystal Ball

)2( 01Sc

The stateThe state

Search of the resonance in the decay channel:

c

0/ Jc

…and in the channel

/Jc

)2( 01Sc

The resonanceThe resonance)(/ 13SJ

Mass measurements Total width measurements

The resonanceThe resonance )2( 13S

Total Width measurementsMass measurements

The P wave singlet state hThe P wave singlet state hcc

• The only experiment which observed this resonance is E760, in the decay channel:

eeJPpp 01

1 /

• Mass:

• Total width: < 1.1 MeV

MeV)2.015.02.3526(

11P

0

The resonanceThe resonance)( 03

0 P

E835 is the first experiment which observed the resonance in annihilations

pp

eeJpp /0

The stateThe state)( 03

0 P

Mass measurements Total width measurementspreliminary results, not yet in the PDG

The stateThe state)( 13

1 Pc

mass measurements E760 is the only experiment which

precisely measured the total width:

1cMeV

c)08.011.088.0(

1

eeJpp c /1

The stateThe state)( 23

2 Pc

2cThe resonance excitation curve observed by E760

eeJpp c /2

The stateThe state)( 23

2 Pc

mass measurements total width measurements

The D wave statesThe D wave states

• The charmonium “D states” are above the open charm threshold (3730 MeV ) but the widths of the J= 2 states and are expected to be small:

DDD 23,1

forbidden by parity conservation*

23,1 DDD forbidden by energy conservation

21D2

3D

• Only the , considered to be largely state, has been clearly observed

)3770(1

3D

The D wave statesThe D wave states

• The only evidence of another D state has been observed at Fermilab by experiment E705 at an energy of 3836 MeV, in the reaction:

XJLi /

• This evidence was not confirmed by the same experiment in the reaction and more recently by BES

XJpLi /

ConclusionsConclusions

After almost 30 years since its discovery we havelearned a lot about charmonium. But, still, manyquestions, like the non observation of the , the confirmation of the resonance and the poor knowledge of the D states are still open questionsand efforts should be put to solve them.

c1

1P