The charmonium mass spectrum
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Transcript of The charmonium mass spectrum
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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
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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
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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)
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The discovery of the J/The discovery of the J/
The discovery of the J/ at SLAC…
…and at the Brookhaven National Laboratory
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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
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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!
)/( J
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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.
)1( PCJ
/ J
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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
c
• Angular coverage: 98% 4
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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
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• 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
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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
)0(),0( 0
cc )2(2
c• CLEO-II and LEP experiments are presently using this technique
)1(1
c
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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
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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
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The charmonium spectrumThe charmonium spectrum
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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
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The fundamental stateThe fundamental state
Mass measurements Total width measurementspreliminary results
)( 01Sc
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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
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The stateThe state
Search of the resonance in the decay channel:
c
0/ Jc
…and in the channel
/Jc
)2( 01Sc
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The resonanceThe resonance)(/ 13SJ
Mass measurements Total width measurements
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The resonanceThe resonance )2( 13S
Total Width measurementsMass measurements
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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
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0
The resonanceThe resonance)( 03
0 P
E835 is the first experiment which observed the resonance in annihilations
pp
eeJpp /0
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The stateThe state)( 03
0 P
Mass measurements Total width measurementspreliminary results, not yet in the PDG
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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
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The stateThe state)( 23
2 Pc
2cThe resonance excitation curve observed by E760
eeJpp c /2
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The stateThe state)( 23
2 Pc
mass measurements total width measurements
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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
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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 /
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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