Radiative Decays of the X(3872) in BaBar

Radiative Decays of the X(3872) in BaBar, SLAC Experimental SeminarBryan Fulsom, March 3, 2009

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Radiative Decays ofthe X(3872) in BaBar

Bryan Fulsom

University of British Columbia

SLAC Experimental Seminar

March 3, 2009


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Outline• Background Information

– Introduction to Charmonium and the X(3872)

– Recent Experimental Results

– Theoretical Predictions

• Analysis Description– The BaBar Experiment

– Event Reconstruction and Selection

– Fit Strategy and PDFs

• Results– Validation (MC and B c1,2 K)

– Systematic Uncertainties

– X(3872) Results and Implications


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Background Information


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Introduction• November Revolution: discovery of the J/ in 1974

• First evidence of the charm quark

– Strong confirmation of the quark model

• Charmonium is a bound state of cc

• Described by phenomenological models:

– Analogous to hydrogen, positronium

– Theory has enjoyed fairly good success

• Several recent results from the B Factories:

– Some discoveries fit naturally into the charmonium system

– Many others cannot be accommodated by the model

• QCD allows for more “exotic” possibilities

– hybrids, tetraquark states, four-quark molecules


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Basic Theoretical Description• 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.

• Many successful predictions on masses, widths, branching fractions

• Lattice QCD has made great advancements, but won’t be discussed here

...3

4)( br

rrV s


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Charmonium Production• Colour-suppressed b c decay

– Predominantly from B cc K

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

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

– JPC = 1--

• Double charmonium production

– Typically one J/ or (2S), plus second cc state

• Two-photon production

– Access to C = + 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

– J/ e+e-, +-, (2S) e+e-, +-

• Radiative:

– EM radiative transition emitting photon

– Emit gluons producing light quarks

– c1,2 J/ (2S) J/+-

• Strong interaction:

– Dominant above ~3.72 GeV (DD threshold)

– OZI-suppressed below this mass


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Exotic QCD• Meson Molecules

– Weakly bound state of two mesons

– Binding at long distance via pion exchange

– Mesons decay as though free

• “Tetraquarks”

– Diquark-antidiquark bound by spin-spin interactions

– Multiplets with non-zero charge and strangeness

– Decay via rearrangement and dissociation

• Charmonium hybrids

– Charmonium with excited gluon

– Doubly degenerate octet of states with exotic JPC

– Decays similar to charmonium, though predictions vary


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Charmonium SpectrumSpectroscopic

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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The X(3872)


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Discovery of the X(3872)• Belle discovered a signal in B J/+- K; confirmed by CDF, D0 and BaBar

• Narrow (<2.3MeV) particle with mass m(X)=3871.4+/-0.6 MeV/c2


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X(3872) J/ Dipion Mass

• Belle and CDF analysed the mass distribution from X J/

• Both seem to favour a “-like” shape, with J/- in an S-wave

• Shape in BaBar is similar, no attempt to fit


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X(3872) Angular Analyses• CDF analysed angular distribution of daughters,

and tested against various JPC assignments

• Angular analysis compatible with both 1++ and 2-+

– 2 prob.(1++)=27.8%, (2-+)=25.8%

Note: Angular analysis

from Belle favours 1++

disfavours 0++, 0-+, 1+-


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Evidence for X(3872) J/ • Both BaBar and Belle have found evidence for this radiative decay

• J/ and have charge parity C = –, implies X(3872) C = +

)0.4(10)1.06.08.1(

)/)((6

JXXKBBF

)4.3(10)3.00.13.3(

)/)((6

JXXKBBF


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X(3872) D0D0*K• Belle discovered X(3872) in B D0D00K

– Found mass 2.0 higher than W.A. for X(3872)

– Recent update confirms D0D0* decay (8.8)

– Compute m(X)=3872.60.50.4 MeV/c2

• BaBar search confirms X(3872) signal (4.9)

– Ratio of D0D00/D0D0 matches D0* expectation

– Mass ~4.5 above X(3872)

– Angular study inconclusive


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X(3872) Interpretation• Summary of X(3872) Properties:

– Narrow with mass m(X)=3871.4+/-0.6 MeV/c2

– Observed in X(3872) J/, dipion mass is “-like”

– Also seen in decays X(3872) D0D0* and X(3872) J/– Spin-parity identified as either JPC = 1++ or 2-+

• Charmonium Hybrid

– Lightest mass prediction m(ccg)>4.2GeV/c2

• Tetraquark State

– No evidence for charged/neutral mass splitting

– No evidence for charged partners


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X(3872) Interpretation• Conventional Charmonium

– Angular analysis: c1(23P1) (1++) or c2(11D1) (2-+)

– Not expected to violate isospin, X J/

– X(3872) is narrow and does not decay X DD

– Expect c2 J/ to be suppressed

– c1(2P) somewhat inconsistent with predicted mass

Study radiative decays as a diagnostic!

• D0D0* Molecular interpretation

– m(D0) + m(D0*) = 3871.8+/-0.4 MeV/c2

– Decays to X(3872) J/, D0D0*, J/ expected

– Compatible with JPC = 1++ assignment

– Successful predictions vary by model


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X(3872) ccTheoretical Predictions• Electromagnetic transitions for charmonium:

– c2 (11D1) (nS) forbidden (M2)

– c1 (23P1) [J/(2S)] allowed (E1)

– Predictions for relative rate varies, but are of similar order


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X(3872) ccTheoretical Predictions• Radiative decays of the D0D0* molecule:

– Decay to J/ is possible in vector meson dominance scenario

– (2S) proceeds via annihilation, highly disfavoured

• Caveat: Theory-dependent, few definite predictions exist


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X(3872) ccTheoretical Predictions• Radiative decays may discriminate between c2, c1(2P), and D0D0*

“Perhaps the most robust diagnostic is the ’ decay mode...Clearly a measurement of the J/ and ’ decay modes of the X(3872) will provide compelling clues to its internal structure.”


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Analysis Description


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The BaBar Experiment


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The BaBar Experiment


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Analysis Description• Search for radiative decays X(3872) [J/(2S)]

• Refit event with B mass constrained

• Variables of interest: mmiss = |pe+e- – pB|, mX, mK*

• Select candidate with best unconstrained mB

• Optimize selection cuts on MC using FOM:


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Event Selection• B Meson Variables

– B Vertex P(2)

– Reconstructed B mass

• Kinematic Variables

– mmiss, mX

• cc Masses

– Unconstrained J/, (2S) mass

• Event shape

– Isotropy of momenta

• Photon variables

– Energy and shower-related

• Kaon variables

– Mass selection

– Decay length and vertexing


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Signal Extraction Strategy• Fit strategy:

1. Unbinned 1(2)-D ML fit to mmiss (and mK* if applicable)

2. Use sPlot technique to get projection of signal events in mX

3. Fit to mX to extract number of signal events

• sPlot useful for isolating signal-like events for “bump-hunting”

• Test on well-known B c1,2 (J/) K decays first, then unblind for X(3872)

• Determine PDFs from truth-matched MC

• Fix most parameters except for normalization and background in mX fit


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PDF Definitions - Signal• Selected the following PDF shapes:

– mmiss: Crystal Ball ( ~ 5 MeV/c2)

– mX: Double Gaussian ( ~ 5, 10 MeV/c2)

– mK*: BW * Gaussian ( ~ 45, 10 MeV/c2)


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PDF Definition – Background• Bkgd. PDFs: mmiss: ARGUS + peak, mX: Linear (+ peak), mK*: Linear (+ peak)


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sPlot Procedure• sPlot technique uses information from discriminant variables (mmiss) to unfold the

distribution in uncorrelated control variable (mX) by species (signal, background)

1. Conduct ML fit using PDFs (F) in variable (y) to get N events of species (

2. For species (a), each event (i) is assigned weight:

where Va is the covariance matrix between two species (a, )

3. Produce a histogram of the control variable using the weights for species


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Signal Extraction Example• Begin with mixture of events

• Perform fit to mmiss to get

Nsig and Nbkgd

• Generate sPlots based on

the yield and distributions

• Can examine signal or

background species

• Fit sPlot for signal to

get X(3872) yield


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Fit Validation• Test fit with MC toys:

– Insert expected number of signal events from MC

– Generate background samples from distributions

– Repeat N times

• Truth-matched MC trials

– Measure of bias: PDFs determined on truth, “true” events should work

• Non-truth-matched MC trials

– Measure “fit efficiency” (ie: how well PDFs match data and extract results)

• Other trials

– X(3872) with null result, 8 times estimated BF ((2S)), varying number of c2 events, with and without NR backgrounds, crossfeed from other K and c modes, X(3872) J/ + background, etc.


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Efficiency• Determine signal extraction

efficiency in two stages

• Cut/Reconstruction efficiency

– Number of events from MC

reconstructed and passing all

selection cuts

• Fit efficiency from MC toy trials

– Described on previous slide

• General comment:

– decreases as N tracks increases

– Fit decreases as N increases and E decreases


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Results and Implications


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B c1,2 K Results• Observe signals in

all decay modes

• Consistent with PDG,

and best measurement for

K+, KS and K*+ modes

• Unexpected B0 c2 K*0

• Analysis of Run 1-3 subset

consistent with previous

BaBar result

• Set limits on other modes


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B c1,2 K Systematic Uncertainties• Primarily for B c1 K; statistics-dominated for B c2 K

• Systematic uncertainties on the B c1 K yield are small

• Generally dominated by effects beyond this analysis


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Summary of c1,2 K Results


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X(3872) J/ Results• Find ~3.6 evidence for

X(3872) J/

• No evidence in other

decay modes

• Measure:

BF(B+ X(3872) K+) x

(X(3872) J/) =

(2.8+/-0.8+/-0.2) x 10-6

• Consistent with previous:

(3.3+/-1.0+/-0.3) x 10-6


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X(3872) (2S) Results• First evidence for

X(3872) (2S)(~3.5 significance)

• Measure:

BF(B+ X(3872) K+) x

(X(3872) (2S) ) =

(9.5+/-2.7+/-0.9) x 10-6

• Ratio of BFs:

(X(3872) (2S) X(3872) J/


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X(3872) Systematic Uncertainties• Entirely statistics-dominated in the X(3872) modes

• Most effects are related to the signal yield, not the BF calculation

• Largest concern is uncertainties in MC/data modeling


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Summary of X(3872) Results


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Full mX Range


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Results and Implications• Results

– Best B c1K measurement, limits on c2, possible B c2 K*0 evidence

– Updated BF(B+ X(3872)K+)(X(3872) J/– First evidence for X(3872) (2S) – Accepted to PRL, to appear soon

• Hybrids and tetraquark already ruled out

• X(3872) (nS) disfavours 2-+ c2(1D) charmonium

• Large [(2S) J/ratio unexpected for D0D0*

• Consistent with 1++ c1(2P) charmonium

• Implications:


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Conclusions?• Can X(3872) = c1(2P)fit into the charmonium model?

– c1(2P) mass prediction is too high

– Explanation for isospin violation and c1(2P) DD suppression

• Can X(3872) (2S) be reconciled with molecular model?

– Theoretical work needed to explain large BF(D0D*0 (nS) )

• What do we learn regarding X(3872) structure from this?

– Perhaps neither purely molecular nor charmonium

– D0D*0 – c1(2P) mixing or other phenomena?

The X(3872) saga continues...


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Backup Slides


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Charmonium Spectrum


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X(3872) J/ Updates• Both Belle and BaBar have updated B X(3872)[XJ/] K results

• Belle finds no mass splitting, equal ratio between the B0 and B+ decay modes:

BaBar measurement confirms,

though suffers statistically

Sets limit on width: <3.3 MeV

2/)26.089.018.0( cMeVM X

05.022.082.0))3872((

))3872(( 00

KXBBR

KXBBRR

2/)4.06.17.2( cMeVM X

05.024.041.0))3872((

))3872(( 00

KXBBR

KXBBRR


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X(3872) J/ Conflict• Belle searched for B X(3872)[XJ/ • 3-invariant mass shows enhancement assumed to be

• Separate BaBar analysis of B KJ/• Observe Y(3940), no evidence for X(3872)

• ie: X J/ not necessarily X J/


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X(3872) Null Results• Upper limits and null results:

– X J/ = consistent with “molecule”

– X DD = rules out 0+, 1-, 2+, ...

– X c1,2 rules out charmonium possibilities

– X J/= no charged partner

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Radiative Decays of the X(3872) in BaBar