Recent Progress and Puzzles in Charmonium Physics

Recent Charmonium Results, ISMD 2007Bryan Fulsom, Aug 9, 2007.

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Recent Progress and Puzzles in Charmonium Physics

Bryan Fulsom

University of British Columbia

BaBar Collaboration

ISMD 2007 Berkeley

August 9, 2007


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Outline

• Introduction to quarkonium spectroscopy

• The experiments

• The X(3872)

• More new states (X, Y, Z, ...)

• Future outlook


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In the beginning...

• November Revolution: simultaneous (SLAC/BNL)

discovery of the J/ in 1974

• Bound state of c-cbar quarks: “charmonium”

• First evidence of the charm quark

– Strong confirmation of the quark model

• Discovery of (2S)J/(e+e-) - soon followed


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Basic theoretical description

• Analogous to known two-particle bound systems (ie: hydrogen, positronium)

• Charmonium potential models (phenomenological):

– non-relativistic (charm quarks are “heavy” compared to binding energy)

– strong force potential via one gluon exchange (similar to Coulomb force)

– quark confinement (increases linearly with separation)

• Typical representation:

plus extensions to include spin-dependent terms, relativistic corrections, etc.

• NRQCD: Analytical calculations based on assumption of heavy quark mass

• Lattice QCD: Discrete computation of quark interactions

– NRQCD and Lattice are “real” QCD; now-mature methodologies

• Theory produces many experimentally testable predictions (mass, width, BF):

...3

4)( br

rrV s


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Charmonium Spectrum

Spectroscopic

notation:

N2S+1LJ

where L=S,P,D,F,...

Angular

momentum:

J = L+S

S(qq) = 0 or 1

Parity:

P = (-1)L+1

Charge

conjugation

C=(-1)L+S


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Charmonium Production

• Colour-suppressed bc decay

– Predominantly from B-meson decays

• e+e- annihilation/Initial State Radiation (ISR)

– e+e- collision below nominal c.m. energy

– JPC = 1--

• Double charmonium production

– Typically one J/ or , plus second ccbar state

• Two-photon production

– Access to C = +1 states

• pp annihilation

– All quantum numbers available


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Charmonium Decays

• Annihilation:

– Generally suppressed for bound state

– Decay to leptons is a clean experimental signal

• Strong interaction:

– Dominant above ~3.72 GeV (D mesons)

– Suppressed below this mass threshold

• Radiative:

– EM radiative transition emitting photon

– Emit gluons producing light quarks

• Features:

– Suppression of strong decays leads to (relatively) long lifetimes, narrow widths

– Radiative decays are competitive; often most accessible transitions


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The B Factories: BaBar and Belle


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Other Experiments – CLEO, CDF/D0


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Observed Charmonium Decays


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Discovery of the X(3872)

• In 2003, Belle discovered a new signal in B+X K+, XJ/+-

• Narrow (<2.3MeV) particle with mass m(X)=3871.2+/-0.6 MeV/c2

• Confirmed by CDF, D0 and BaBar

X(3872) now well established


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X(3872) Highlights

• X(3872) has been extensively studied by BaBar and Belle

• XJ/ radiative decay confirmed

by BaBar determines C = +1

• Belle/CDF dipion angular analysis

in XJ/ favours JPC = 1++

• Decays via BD0D0(*)K

– No B0/B+ mass difference

– Mass 2.5 higher than W.A.

• No charged partners found, doesn’t decay to c1 or J/


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X(3872) Interpretation

• X(3872) is puzzling

– Similar to charmonium, ie: narrow state decaying to J/

– However, above DD threshold expect to be wide and XDD dominant

– Quantum numbers established: 1++

– It does not fit into the charmonium model

• Note:

• Leading contender is that this could be a bound state of two D mesons

– i.e.: a D0D*0 molecule

– Supported by predictions of mass, decay modes, JPC, branching fractions

• Other exotic predictions:

– “Tetraquark” 4-quark bound state

– “Glueball” gluon bound state, charmonium-gluon hybrid

)()()( 0*DmDmXm


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The alphabet grows: X(3940), Z(3930)

• Belle discovered three new states in a similar mass range via distinct production methods and decay modes

• X(3940)

– Discovered by Belle in double-charmonium

production e+e-J/ X(3940)

– Decays to DD* but not DD

– Likely a charmonium state (c’’ or c1’)

• Z(3930)

– Discovered by the two-photon process

Z(3930)DDbar

– Predicted mass and width match

charmonium assignment of c2’


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...and grows...: the Y(3940)

• Discovered by Belle the decay BKY(J/

• Recently measured by BaBar (preliminary)

– Reconstructed in same decay mode

but parameter values differ

– Possible c1’ charmonium state

– Requires further investigation

2MeV/c)13113943()( Ym

MeV)262287()( Y

28.34.3 MeV/c)6.13.3914()(

YmMeV)133()( 12

8 Y


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BaBar Discovery of Y(4260)J/ -

• BaBar discovers a new peak in ISR events

• Found in the decay:

Y(4260)J/+-

• Fitted shape is a broad structure with

• e+e- requires quantum numbers JPC = 1--

• However, all of the 1-- charmonium states have already been discovered!

• Very difficult to accommodate as ccbar, unless previous assignments are wrong

226 MeV/c)84259()(

Ym

MeV)2388()( 64

Y


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CLEO confirms the Y(4260)

• After BaBar’s discovery of the Y(4260), CLEO performed an centre-of-mass energy scan and also collected data directly at the Y(4260) resonance

• They reconstructed 16 decay modes, confirming BaBar’s discovery of Y(4260)J/+- (11) and adding Y(4260)J/00 (5.1)


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Recent Y(4260) results from Belle

• Preliminary results presented at EPS in Manchester, mid-July 2007

• Confirm BaBar’s Y(4260) and finds an accompanying state

• For Y(4260)J/+-, they

reproduce BaBar’s signal:

but also claim a broad second shape:

21726 MeV/c)124247()(

YmMeV)19108()( 8

10 Y

27228 MeV/c)404008((?)

m

MeV)44226((?) 8779


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BaBar’s search for Y(4260)S+-

• Y(4260) decays to J/+-, natural to search for the decay to S

• Analysis in BaBar finds a new

peak that does not match the

Y(4260) mass/width!

• Seems to be a different structure:

• e+e- requires this state to be JPC = 1--, now overpopulated by two states

• Impossible to assign this as charmonium

2MeV/c)244324()( Ym

MeV)33172()( Y


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Belle’s latest results

• Also presented at EPS in Manchester, mid-July 2007

• For Y(4320)+-, BaBar result

confirmed:

and discovered a 5.8 new peak

• (Note: a single bin in BaBar’s Y(4320) analysis showed a statistically insignificant excess in this region, but the dataset was ~2.5 times smaller)

2MeV/c)994361()( Ym

MeV)101574()( Y

2MeV/c)5114664()( Ym

MeV)31548()( Y


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Charmonium Spectrum Revisited


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Conclusions

• The charmonium model has had great success below the DD threshold

• Above DD threshold, several states remain undiscovered or need further study

• A recent flood of experimental results from the B-factories is challenging our understanding of the strong force

• Is the X(3872) a meson molecule?

– If so, how well do we understand its mechanisms? If not, then what is it?

• What is the nature of the new “Y” states?

– Molecules? Tetraquarks? Hybrids? Glueballs? Something else?

• Is this the beginning of a rich new spectroscopy?

– Lattice and NRQCD theory takes the lead

• Experimental status:

– BaBar, Belle, and CLEO operational through 2008

– BES restarting, Super B factory potential for orders of magnitude more data

• What other exciting new states await to be discovered?

Recent Progress and Puzzles in Charmonium Physics

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