# Presentation: 817916

## Recent Progress and Puzzles in Charmonium Physics

- Bryan Fulsom
- University of British Columbia
- BaBar Collaboration
- ISMD 2007 Berkeley
- August 9, 2007

## Outline

- Introduction to quarkonium spectroscopy
- The experiments
- The X(3872)
- More new states (X, Y, Z, ...)
- Future outlook

## 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

## 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):

## 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

## Charmonium Production

- Colour-suppressed bc 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

## 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

## The B Factories: BaBar and Belle

## Other Experiments – CLEO, CDF/D0

## Observed Charmonium Decays

## Discovery of the X(3872)

- In 2003, Belle discovered a new signal in B+X K+, XJ/+-
- 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

## X(3872) Highlights

- X(3872) has been extensively studied by BaBar and Belle
- XJ/ radiative decay confirmed
- 	by BaBar determines C = +1
- Belle/CDF dipion angular analysis
- 	in XJ/ favours JPC = 1++
- Decays via BD0D0(*)K
- No B0/B+ mass difference
- Mass 2.5 higher than W.A.
- No charged partners found, doesn’t decay to c1 or J/

## 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 XDD 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

## 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’

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

- Discovered by Belle the decay BKY(J/
- Recently measured by BaBar (preliminary)
- Reconstructed in same decay mode
- 	but parameter values differ
- Possible c1’ charmonium state
- Requires further investigation

## 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

## 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/00 (5.1)

## 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:

## 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

## 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)

## Charmonium Spectrum Revisited

## 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?