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

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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

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10

K

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Why Why

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

The name was clearly right!

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

charmonium resonances:• electron-positron annihilations:

• proton- antiproton annihilations:

• two-photon collisions:

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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.

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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

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• 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)

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

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cc )2(2

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

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Resonance scanResonance scan

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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

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• 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

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ORB

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

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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

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The charmonium spectrumThe charmonium spectrum

The fundamental stateThe fundamental state )( 01Sc

…and by E835 in the decay channel:

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The resonance observed by Crystal Ball…

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preliminary results

The fundamental stateThe fundamental state

Mass measurements Total width measurementspreliminary results

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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(

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0

The resonanceThe resonance)( 03

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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

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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

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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

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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.

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