Charmonium Physics at BaBar

1
Charmonium Physics at BaBar

• Erich W. Varnes
• Princeton University
• High-Energy Physics Seminar
• Brookhaven National Laboratory
• August 23, 2001

2
The BaBar Collaboration

• Italy 12/89
• INFN, Bari
• INFN, Ferrara
• Lab. Nazionali di Frascati dell' INFN
• INFN, Genova
• INFN, Milano
• INFN, Napoli
• INFN, Padova
• INFN, Pavia
• INF, Pisa
• INFNN, Roma and U "La Sapienza"
• INFN, Torino
• INFN, Trieste
• Norway 1/3
• U of Bergen
• Russia 1/13
• Budker Institute, Novosibirsk

3
Topics Covered

• Inclusive charmonium production
• In BB events
• PRD in preparation
• In the continuum
• SLAC-PUB-8854, hep-ex/0106044, to appear in PRL
• Exclusive B to charmonium production
• SLAC-PUB-8909, hep-ex/0107025, submitted to PRD
• Measurement of B ? J/yK decay amplitudes
• SLAC-PUB-8898, hep-ex/0107049, submitted to PRL
• Measurement of sin2b
• SLAC-PUB-8904, hep-ex/0107013, to appear in PRL

4
MotivationQCD Studies

• Inclusive charmonium (J/y, y(2S), and cC)
  production rate and kinematics
• Inclusive production rate (both from B meson
  decay and continuum), polarization, and momentum
  distributions provide tests of non-relativistic
  QCD
• Branching fractions of B decays to exclusive
  charmonium states
• Decay has both penguin and tree contributions
• Calculation of amplitudes requires solving
  non-purturbative QCD
• Existing models depend on simplifying
  assumptions, such as the factorization
  hypothesis.

5
MotivationCP Violation

• Some of the neutral exclusive charmonium modes we
  reconstruct are CP eigenstates (e.g.
  J/yKS)certqain
• Both the tree and penguin diagrams involve same
  combination of CKM matrix elements and are
  sensitive to parameter sin2b
• If CKM matrix is sole source of CP-violation,
  there must be large asymmetries in the B meson
  system
• Magnitude of asymmetry is sin2b, with very small
  theoretical uncertainty (from subleading penguin
  amplitudes)

(r,h)
a
b
g
1
6
Measuring CP Violation at an ee- Accelerator

• Primary reaction is ee- ? U(4S)?
• If the B's are neutral, they will mix between B
  and as they propagate
• State is coherent until one B decays
• After the first B decays, survivor will continue
  to mix
• If one B decays to a CP eigenstate F, CP
  violation appears in the ratio
• where
• Dt is the proper time difference between the
  decay of the CP B and the other B in the event
• f(-) (Dt) is the decay rate for a state which
  was B to go to F at time Dt
• If decay to F is dominated by a single diagram,
  
• AF ? -hsin2fW sin DmdDt
• where fW is the weak phase of the decay
  amplitude, and h is the CP eigenvalue of F

7
CP Violation at an ee- Accelerator cont.

• AF vanishes when integrated over all times need
  to measure Dt for each event
• However, B's are nearly at rest in U(4S) frame
• Solution is to boost lab frame with respect to
  U(4S)
• Then in lab one sees
• Requires an accelerator with asymmetric beam
  energies
• Measurement of AF also requires
• Precision z vertexing
• Good particle ID for leptons and charged hadrons
  (to isolate CP eigenstate decaysy and tag the
  flavor of the other B )
• These considerations were of primary importance
  to the design of the SLAC B Factory

BCP
Btag
8
The SLAC B Factory
9
The PEP-II Accelerator

• PEP-II consists of two accelerator rings built in
  the existing PEP tunnel
• The High-Energy Ring (HER) stores electrons at
  9.1 GeV
• The Low-Energy Ring (LER) stores positrons at 3.0
  GeV
• bg 0.56
• Bs travel about 250 mm in lab
• Some parameters

10
The BaBar Detector
Instrumented flux return
DIRC standoff box
Silicon Tracker
Drift Chamber
DIRC Bars
1.5 T Solenoid
EM Calorimeter
11
BaBar Run History

• Most analyses use data taken between October 1999
  and October 2000
• Run 1 of BaBar
• 20.7 fb-1
• For sin2b, an additional 9 fb-1 recorded in 2001
  is used
• Total data sample is already about 3x that
  recorded by CLEO
• Allows improvements to entire suite of charmonium
  measurements

Start of Run 2
End of Run 1
12
Counting the number of U(4S) Decays

• All B branching fraction measurements require an
  accurate count of the number of events in
  our data sample
• This is done by comparing the ratio of mm to
  multihadron events in on- and off-resonance data

Ratio of cross sections, efficiencies on/off
resonance (close to 1)

• Multihadron events selected on basis of track
  multiplicity, total energy, and event shape
• Selection also suppresses continuum
• NU(4S) in our data sample
• Run 1 22.72 0.36 million
• sin2b sample 32 million
• Largest uncertainty arises from the knowledge of
  the relative mm to multihadron selection
  efficiency

13
Inclusive Charmonium Production

• Charmonium states are identified by the decays
• J/y ? ??- where ? is e or m
• y(2S) ? ??- or J/y pp-
• cc1 ? J/y g
• Cuts used to suppress background
• Same event selection as used in U(4S) counting
• J/y mass cuts (when reconstructing y(2S) ? J/y
  pp-)
• Particle identification (leptons and photons)

14
Particle Identification

• Lepton and photon identification is crucial for
  all charmonium analyses
• We use the following criteria
• For each type we have a set of standard
  selections with different tradeoffs between
  efficiency and purity (VeryLoose, Loose, Tight,
  VeryTight)
• Each analysis uses the selection which minimizes
  the expected error

• Muons
• EMC energy
• Number of IFR layers hit
• Expected/measured range in IFR
• Average IFR multiplicity

• Photons
• EMC cluster shape

• Electrons
• E/p ratio
• Number of EMC crystals in cluster
• dE/dx from DCH
• EMC cluster shape
• Cerenkov angle in DIRC

• Kaons
• Cerenkov angle in DIRC
• dE/dx from DCH

15
Particle ID Performance

• Electron ID Muon ID
  Kaon ID
• VeryTight Loose
  VeryTight

16
Tracking Efficiency

• Tracks used for lepton candidates are required to
  have at least 12 hits in the DCH
• Ensures that momentum and dE/dx are well-measured
• The fact that the SVT can reconstruct tracks
  independent of the DCH allows a precise
  measurement of the efficiency for reconstructing
  such tracks
• Look at sample of tracks that could have been
  found in the SVT alone (i.e., have at least 8 SVT
  hits)
• Find fraction with 12 or more DCH hits
• Provides infinite statistics in the same type
  of events used for analysis
• Limiting systematic is fraction of tracks formed
  from random combinations of noise hits in SVT
• We assign a systematic of 1.2 per track

17
Tracking EfficiencyResults

• For first part of Run 1, drift chamber HV was at
  1900V rather than design value of 1960V
• Reduced after sparks observed in early part of
  run
• Led to lower efficiency in central region
• Adding water to gas mix allowed HV to be restored
  to 1960V
• New reconstruction software gives higher
  efficiency and significantly reduces dependence
  on HV

18
Photon Selection

• Photons appear as isolated calorimeter clusters
  that are not matched to a charged track
• To discriminate against neutral hadrons, the
  shape of the cluster is quantified using two
  variables

Lateral energy distribution (LAT)
Zernike moment A42
19
Fiducial Cuts

• PEPII design requires final-focus quadrupoles
  well within detector volume
• For rate measurements, we avoid this region by
  requiring 23.5o lt q lt 138o for electrons
• inclusive rate measurements use the same range
  for muons
• For exclusive rate measurements the range is
  expanded to 17o lt q lt 155o
• Limited by reach of control samples used to
  cross-check muon ID

• Means that part of DCH coverage is obscured by
  silicon electronics and magnet material
• Simulation overestimates electron detection
  efficiency in these regions

20
Fitting for Signal Yield

• Background mass distribution is fit with a
  3rd-order Chebyshev polynomial
• One needs accurate model for distribution of
  signal events
• This is done using MC simulation, corrected for
• Bias in momentum measurement
• Taken from comparison of J/y ? mm peak position
  in data and MC
• Differences in mass resolution
• Taken from comparison of J/y ? mm widths in data
  and MC
• Differences in rate of bremsstrahlung (material
  model)
• Derived by producing separate pdfs for MC events
  that produced a brem photon and those that
  didnt, and allowing the fraction of each to
  float in fit to data

21
Inclusive Charmonium BFs

• ee- mm-

• J/y
• BF (1.04 0.01 (stat) 0.03 (syst))
• PDG (1.15 0.06)
• y(2S)
• BF (0.28 0.02 (stat) 0.03 (syst))
• PDG (0.35 0.05)
• cc1
• BF (0.38 0.03 (stat) 0.03 (syst))
• PDG (0.42 0.07)

22
J/y Production in Continuum

• Clear J/y signal seen in data taken below U(4S)
  resonance
• And in on-resonance data in kinematic region
  inaccessible to B ? J/yX

23
J/y in Continuum Angular Distributions

• Helicity
• Production angle

Distribution at high p favors models with
intermediate color octet state
24
Exclusive Charmonium Modes

• We are interested primarily in fully
  reconstructing decays of the type
• Leads to the following menu of reconstructed
  modes (C.C. implied)

Used to measure sin2b
25
Light Meson Selection

• po reconstructed as pairs of isolated or merged
  photons (only for KS ? popo)
• K any good track is taken as candidate for most
  analyses
• PID not used to allow smaller systematics
• Exception is cc1K0, which needs PID to see
  signal
• KS reconstructed from pp- or popo pairs (only
  for B ? J/yKS)
• pp- vertex required to be separated from
  charmonium vertex
• p0p0 vertex defined as position along flight
  direction that gives best po masses
• K0 and K are formed from K, KS, p, and po
  candidates

26
Light Meson Mass Resolutions
po ? gg
KS ? pp-
s 3.5 MeV
Our selection
s 6.9 MeV
KS ? popo

• 55 MeV
• Dominated
• by intrinsic
• K width

s 15 MeV
27
Reconstruction of Exclusive Charmonium Decays

• Mass-constrained charmonium candidates combined
  with Ks, KL, K, or K candidates to form B
  candidates
• Signal candidates should have mass consistent
  with B and energy in the CM frame consistent with
  beam energy
• We form two variables to measure these quantities
• Difference between beam and B candidate energies
• Resolution typically 10 MeV for charged modes, 30
  MeV for modes with neutrals
• Beam energy substituted mass
• mES has much better resolution (3 MeV/c2) than
  invariant mass of B candidate
• If more than one candidate is found in an events,
  best DE is chosen

28
Reconstruction of B ? J/yKL

• Neutral hadrons are detected either from a non-EM
  deposit in the calorimeter or from a cluster of
  hits in the IFR
• Neither signature gives a meaningful constraint
  on the energy of the hadron
• Line from J/y vertex to cluster centroid gives KL
  direction
• Can still reconstruct the decay by assuming
  neutral hadron was a KL, and using either the B
  mass or energy to assign the KL energy
• Only one independent variable remains
• We choose to fix the B mass and use DE to
  discriminate signal from background

29
Bremsstrahlung Recovery

• Selection of charmonium mesons is nearly
  identical to that in inclusive analyses
• To increase efficiency for ee- modes, we combine
  electrons with nearby photons to recover some of
  the loss due to bremsstralung

30
Background Suppression

• Many of the variables used to suppress backgound
  have already been mentioned
• Meson masses, lepton ID, mES and DE
• For two-body charmonium decay, the helicity angle
  q? is a powerful discriminant against continnum
  background
• For y(2S)? J/y pp and cc ? J/y g, the angle
  between the thrust axes of the B candidate and
  the rest of the event is used

Events in DE sideband
Events in signal region
31
Signals for J/yK Modes
CP modes
32
Other J/y Modes
33
Non-J/y Modes
34
Estimation of Efficiency

• To first order, efficiency is given by Monte
  Carlo with GEANT detector simulation
• However, we know the MC is too optimistic in some
  cases, so we make the following corrections
• Lepton ID efficiencies are taken from inclusive
  J/y data
• Tracking efficiency is corrected by relative
  data/MC efficiency
• MC momentum and energy measurements are smeared
  to match data resolution
• Photon reconstruction efficiency is taken from tt
  data sample
• KL detection efficiency taken from ee- ? Fg
  control sample
• Though this is a long list of corrections, the
  largest total correction to the efficiency of
  any mode is 16

35
Extraction of Signal

• Signal is estimated by
• Counting events in the signal region
• Subtracting integral of combinatoric background
  fit
• Subtracting estimated background from other
  charmonium modes (which may peak in the signal
  region of mES

36
Systematic Uncertainties
37
Exclusive Charmonium Branching Fractions

• Comparison of our measurements to PDG
  values/limits

• In calculating the branching fractions, we assume
• U(4S) always decays to BB
• U(4S) produces equal numbers of charged and
  neutral B pairs

38
Ratios of Branching Fractions

• Some interesting information is obtained by
  taking ratios or our branching fractions
• Cancellation of some systematics is an added
  benefit
• For example, we find
• But isospin requires decay amplitudes to be
  identical
• Implies ratio of charged to neutral B pairs in
  sample is
• 1.10 0.06 0.05
• In addition, some B decay models predict ratios
  of vector to scalar branching fractions. We
  measure

39
Measurement of sin2bEvent selection

• There are three types of final states used to
  measure sin2b
• CP 1 eigenstates J/yKS, y(2S)KS and cCKS
• CP 1 eigenstates J/yKL
• Provides consistency check on analysis,
• since CP asymmetry should be opposite
• Mixed eigenstates J/yK0
• Requires amplitude analysis to determine fraction
  of L1 (CP 1) component
• The selection of CP 1 and mixed-CP candidates
  for the CP analysis is nearly identical to that
  for the branching fraction analysis, expect that
  the fiducial cuts are removed
• Having a high efficiency is more important than
  having a well-understood efficiency for these
  analyses
• We also add 9 fb-1 of data from 2001 to the
  sample

Newly added to our measurement
40
CP 1 Event Selection

• This channel is unique in having a large
  background dominated by other B? J/yX decays
• Background may well be CP-asymmetric
• Means that the optimal selection for CP analysis
  is different than that for branching fraction
  measurement.
• Rather than S/(SB)1/2 one needs to optimize
• Result

Signal
Inclusive J/y background
Other backgrounds
41
Angular Analysis of J/yK Events

• J/yK0 decays have the proper CKM structure for
  measuring sin2b
• However, since there are two vector particles,
  both CP eigenstates are present in this decay
• One needs to do an angular analysis to separate
  the CP 1 and CP 1 amplitudes
• Result
• CP 1 component is 16 4

42
Efficiency in Run2 vs. Run1

• Although added Run2 sample increases the
  luminosity of the data sample by lt 50, our
  efficiency for reconstructing CP events was
  significantly increased

• One indication is
• D 0? 3KS signal in three samples of equal
  luminosity
• Primarily due to better reconstruction code
• Reprocessing of Run1 data is underway

Run2 1930V
Run1 1960V
Run1 1900V
43
A Tagged J/yKS Event
44
Closeup View of the Event Vertex
45
Vertex Reconstruction

• Vertex of CP B is easy to reconstruct
  (intersection of lepton tracks with beam axis)
• Resolution dominated by tag B
• Tag vertex found by
• Starting with all tracks not from CP B
• Beam spot postion provides additional constraint
  (not used in our previous analyses)
• Combining pairs which form a displaced V0 vertex
• Eliminating tracks with a large contribution to
  c2
• Iterating until vertex is stable
• Algorithm is 97 efficient

46
B Flavor Tagging

• Measuring the flavor of the non-CP B is also
  crucial
• Power of any tagging algorithm is determined by
  its efficiency for giving an answer (e), and
  probability of giving the wrong answer (w)
• Often quoted in terms of dilution D (1-2w)
• Uncertainty on sin2b goes as

• BaBar uses charge of identified lepton and kaon
  tracks as primary tag mechanism
• In addition, a neural net tag algorithm is used
  to
• Resolve cases where kaon and lepton tag conflict
• Pick up additional information from tracks that
  fail standard PID selection
• Use additional information from slow pion charge
  and total "jet charge

• Tagging methods
• Generic B decay

b
c
s
W
W-
?-
n
47
Measurement of Tagging Performance

• Measured by analyzing B mixing in
  fully-reconstructed hadronic B decays
• Tagging efficiency given by the fraction of
  events with a tag opposite the reconstructed B
• Wrong-tag probability given by fraction of events
  in which both Bs appear to have same flavor at
  same decay time

• Total eD2 26.1 ? 1.2

48
Extraction of sin2b

• sin2b is extracted using a maximum likelihood fit
• PDG values of Dmd and tB are used in the fit
• CP sample and sample of non-CP hadronic B decays
  are fit simultaneously to extract
• Dilutions for each tagging category (4
  parameters)
• Difference in dilution between
  tags (4)
• Vertex resolution parameters for signal (16, 8
  each for 2000 and 2001 data)
• Background vertex resolution (3)
• Background time dependence (9)
• Background mistag fractions (8)
• sin2b (1)
• Total of 45 parameters fit

49
Our Result

• sin2b 0.59 ? 0.14 (stat.) ? 0.05 (syst.)

f-
f
Opposite amplitudes!
50
Systematic Uncertainties

• Dominant systematics are dsin2b
• Parameterization of Dz resolution function 0.03
• Largely due to residual uncertainties in SVT
  alignment
• Possible differences in mistag rates for CP
  events and control samples 0.03
• Level and CP composition of background 0.02
• Total systematic 0.05
• Systematics are small, but we are nearing the
  point where well need to pay attention to them
  in order to reduce the total error

51
Crosschecks

• Check for consistency among decay modes

• and tagging categories (KL sample not included)

Consistent at 8 level
52
Crosschecks

• Also fit for apparent sin2b on non-CP
  eigenstate samples (expect to get 0)
• No evidence of spurious asymmetries

53
Run1 vs. Run2 Data

• Current value is somewhat higher than our
  previous result
• 0.34 0.20
• Difference is consistent with statistics (Run2
  data is 1.3s higher than Run1)
• Applying current analysis to previous sample
  gives 0.32 0.18

Run1
Run1
54
Comparison to Standard Model

• Heres how our results compares to the standard
  model expectation
• No hint of new physics here.

55
(Unofficial) Combined Measurement

• Globally, CP violation in the B system is
  established at over 7s
• World average value is consistent with Standard
  Model constraints
• BaBar and Belle results differ by 1.8s
• No cause for concern, but bears watching as
  errors improve

56
Outlook

• PEPII has an aggressive plan for improving the
  luminosity, leading to the following projection
• This sample is over twice as large as was
  originally anticipated
• Statistical error on sin2b will be 0.03

57
Summary

• The SLAC B Factory had a very successful first
  run, collecting the worlds largest sample of
  U(4S) decays
• Belle also has had an excellent year of running,
  and have a comparable sample
• We have measured inclusive and exclusive
  charmonium branching fractions with precision
  superior to previous experiments
• We have also published the first observation of
  CP violation outside of the K meson system by
  measuring
• sin2b 0.59 0.14 (stat) 0.05 (syst)
• But were just at the beginning of our program
• The greatest challenges lie ahead
• Measuring sin2b in penguin-dominated modes
• Measuring (or at least constraining) the other CP
  angles
• and many more