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

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 United Kingdom [10/80] U of Birmingham U of Bristol Brunel University U of Edinburgh U of Liverpool Imperial College Queen Mary & Westfield College Royal Holloway, University of London U of Manchester Rutherford Appleton Laboratory 9 Countries 72 Institutions 554 Physicists USA[35/276] California Institute of Technology UC, Irvine UC, Los Angeles UC, San Diego UC, Santa Barbara UC, Santa Cruz U of Cincinnati U of Colorado Colorado State Florida A&M U of Iowa Iowa State U LBNL LLNL U of Louisville U of Maryland U of Massachusetts, Amherst MIT U of Mississippi Mount Holyoke College Northern Kentucky U U of Notre Dame ORNL/Y-12 U of Oregon U of Pennsylvania Prairie View A&M Princeton SLAC U of South Carolina Stanford U U of Tennessee U of Texas at Dallas Vanderbilt U of Wisconsin Yale Canada[4/16] U of British Columbia McGill U U de Montr₫al U of Victoria China[1/6] Inst. of High Energy Physics, Beijing France[5/50] LAPP, Annecy LAL Orsay LPNHE des Universit₫s Paris 6/7 Ecole Polytechnique CEA, DAPNIA, CE-Saclay Germany[3/21] U Rostock Ruhr U Bochum Technische U Dresden

Topics Covered Inclusive charmonium production –In BB events  PRD in preparation –In the continuum  SLAC-PUB-8854, hep-ex/ , to appear in PRL Exclusive B to charmonium production –SLAC-PUB-8909, hep-ex/ , submitted to PRD Measurement of B  J/  K* decay amplitudes –SLAC-PUB-8898, hep-ex/ , submitted to PRL Measurement of sin2  –SLAC-PUB-8904, hep-ex/ , to appear in PRL

Motivation QCD Studies Inclusive charmonium (J/  (2S), and  C )  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.

Motivation CP Violation Some of the neutral exclusive charmonium modes we reconstruct are CP eigenstates (e.g. J/  K S )certqain Both the tree and penguin diagrams involve same combination of CKM matrix elements and are sensitive to parameter sin2  –If CKM matrix is sole source of CP-violation, there must be large asymmetries in the B meson system –Magnitude of asymmetry is sin2  with very small theoretical uncertainty (from subleading penguin amplitudes) 1  (   

Measuring CP Violation at an e + e - Accelerator Primary reaction is e + e -   (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:  t is the proper time difference between the decay of the CP B and the other B in the event f +(-) (  t) is the decay rate for a state which was B to go to F at time  t If decay to F is dominated by a single diagram, A F  -  sin2  W · sin  m d  t where  W is the weak phase of the decay amplitude, and  is the CP eigenvalue of F

CP Violation at an e + e - Accelerator cont. A F vanishes when integrated over all times  need to measure  t for each event However, B's are nearly at rest in  (4S) frame Solution is to boost lab frame with respect to  (4S) Then in lab one sees: Requires an accelerator with asymmetric beam energies Measurement of A F 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 B CP B tag

The SLAC B Factory

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  = 0.56 B’s travel about 250  m in lab Some parameters ParameterDesignAchieved Luminosity3 x cm -2 s x cm -2 s -1 Number of bunches LER current2140 mA HER current750 mA920 mA LER lifetime240 min280 min HER lifetime240 min600 min Horizontal beam size 222  m180  m Vertical beam size 6.7  m5.1  m

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

BaBar Run History Most analyses use data taken between October 1999 and October 2000 –“Run 1” of BaBar –20.7 fb -1 For sin2 , 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 End of Run 1 Start of Run 2

Counting the number of  (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  to multihadron events in on- and off-resonance data: Multihadron events selected on basis of track multiplicity, total energy, and event shape –Selection also suppresses continuum N  (4S) in our data sample: Run 1: ± 0.36 million sin2  sample: 32 million Largest uncertainty arises from the knowledge of the relative  to multihadron selection efficiency Ratio of cross sections, efficiencies on/off resonance (close to 1)

Inclusive Charmonium Production Charmonium states are identified by the decays: –J/     where   is e or   (2S)     or J/      c1  J/  Cuts used to suppress background: –Same event selection as used in  (4S) counting –J/  mass cuts (when reconstructing  (2S)  J/     ) –Particle identification (leptons and photons)

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 Electrons: E/p ratio Number of EMC crystals in cluster dE/dx from DCH EMC cluster shape Cerenkov angle in DIRC Muons: EMC energy Number of IFR layers hit Expected/measured range in IFR Average IFR multiplicity Photons: EMC cluster shape Kaons: Čerenkov angle in DIRC dE/dx from DCH

Particle ID Performance Electron ID Muon ID Kaon ID VeryTight Loose VeryTight

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

Tracking Efficiency Results 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

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 A 42

Fiducial Cuts PEPII design requires final-focus quadrupoles well within detector volume: For rate measurements, we avoid this region by requiring 23.5 o <   for electrons –inclusive rate measurements use the same range for muons –For exclusive rate measurements the range is expanded to 17 o <  < 155 o  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

Fitting for Signal Yield Background mass distribution is fit with a 3 rd -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/  peak position in data and MC –Differences in mass resolution  Taken from comparison of J/  widths in data and MC –Differences in rate of bremsstrahlung (material model)  Derived by producing separate pdf’s for MC events that produced a brem photon and those that didn’t, and allowing the fraction of each to float in fit to data

Inclusive Charmonium BFs J/  BF = (1.04 ± 0.01 (stat) ± 0.03 (syst))% PDG: (1.15 ± 0.06)%  (2S) BF = (0.28 ± 0.02 (stat) ± 0.03 (syst))% PDG: (0.35 ± 0.05)%  c1 BF = (0.38 ± 0.03 (stat) ± 0.03 (syst))% PDG: (0.42 ± 0.07)% e + e -    

J/  Production in Continuum Clear J/  signal seen in data taken below  (4S) resonance: And in on-resonance data in kinematic region inaccessible to B  J/  X:

J/  in Continuum Angular Distributions Helicity Production angle Distribution at high p* favors models with intermediate color octet state

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): B decay modeSecondary decay mode (J/  always reconstructed in mode) J/  K 0 K 0  K L or K S ; K S      or     J/  K + -- J/  K* 0 K* 0  K +  - or K S    K S     J/  K* + K* +  K +  0 or K S    K S     J/       (2S)K S  (2S)  or J/      K S      (2S)K +  (2S)  or J/      c1 K S  c1  J/  ; K S       c1 K +  c1  J/   c1 K* 0  c1  J/  ; K* 0      Used to measure sin2 

Light Meson Selection    reconstructed as pairs of isolated or merged photons (only for K S      ) K + : any good track is taken as candidate for most analyses –PID not used to allow smaller systematics –Exception is  c1 K* 0  which needs PID to see signal K S : reconstructed from      or     pairs (only for B  J  K S ) –      vertex required to be separated from charmonium vertex –      vertex defined as position along flight direction that gives best   masses K* 0 and K* + are formed from K +, K S,    and    candidates

Light Meson Mass Resolutions    Our selection  = 6.9 MeV K S     K S      = 15 MeV  = 3.5 MeV  = 55 MeV Dominated by intrinsic K* width

Reconstruction of Exclusive Charmonium Decays Mass-constrained charmonium candidates combined with K s, K L, 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:  m ES has much better resolution (~3 MeV/c 2 ) than invariant mass of B candidate If more than one candidate is found in an events, best  E is chosen

Reconstruction of B  J/  K L 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/  vertex to cluster centroid gives K L direction Can still reconstruct the decay by assuming neutral hadron was a K L, and using either the B mass or energy to assign the K L energy –Only one independent variable remains We choose to fix the B mass and use  E to discriminate signal from background

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

Background Suppression Many of the variables used to suppress backgound have already been mentioned –Meson masses, lepton ID, m ES and  E For two-body charmonium decay, the helicity angle   is a powerful discriminant against continnum background For  (2S)  J/  and  c   J/ , the angle between the thrust axes of the B candidate and the rest of the event is used Events in  E sideband Events in signal region

Signals for J/  Modes CP modes

Other J/  Modes

Non-J/  Modes

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/  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  data sample –K L detection efficiency taken from e + e -   control sample Though this is a long list of corrections, the largest total correction to the efficiency of any mode is 16%

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 m ES

Systematic Uncertainties SourceMagnitude Number of  (4S) decays 1.6% Secondary branching fractions1.7 – 9.6% Detector resolution0.4 – 1.3% Tracking efficiency2.4 – 7.9% Photon efficiency0.2 – 5.0% Background estimation1.1 – 14.5% Signal MC statistics0.1 – 2.4% Model of kinematic distributions 2.2 – 13.2%

Exclusive Charmonium Branching Fractions Comparison of our measurements to PDG values/limits: In calculating the branching fractions, we assume  (4S) always decays to BB  4S) produces equal numbers of charged and neutral B pairs

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:

Measurement of sin2  Event selection There are three types of final states used to measure sin2  –CP –1 eigenstates: J/  K S,  (2S)K S and  C K S –CP +1 eigenstates: J/  K L  Provides consistency check on analysis, since CP asymmetry should be opposite –Mixed eigenstates: J/  K* 0  Requires amplitude analysis to determine fraction of L=1 (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

CP +1 Event Selection This channel is unique in having a large background dominated by other B  J/  X 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/(S+B) 1/2 one needs to optimize Result: Signal Inclusive J/  background Other backgrounds

Angular Analysis of J/  K* Events J/  K* 0 decays have the proper CKM structure for measuring sin2  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%

Efficiency in Run2 vs. Run1 Although added Run2 sample increases the luminosity of the data sample by < 50%, our efficiency for reconstructing CP events was significantly increased One indication is D 0  3K S signal in three samples of equal luminosity Primarily due to better reconstruction code Reprocessing of Run1 data is underway Run1 1900V Run1 1960V Run2 1930V

A Tagged J/  K S Event

Closeup View of the Event Vertex

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 V 0 vertex –Eliminating tracks with a large contribution to  2 –Iterating until vertex is stable Algorithm is 97% efficient B tag B CP zz - +

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  and probability of giving the wrong answer (w)  Often quoted in terms of dilution D = (1-2w) –Uncertainty on sin2  goes as Tagging methods: –Generic B decay: b W-W- c s - W+W+ –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

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 B’s appear to have same flavor at same decay time Total  D 2 = 26.1  1.2 %

Extraction of sin2   sin2  is extracted using a maximum likelihood fit PDG values of  m d and   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) – sin2  Total of 45 parameters fit

Our Result sin2  0.59  0.14 (stat.)  0.05 (syst.) f-f- f+f+ Opposite amplitudes!

Systematic Uncertainties Dominant systematics are:  sin2  –Parameterization of  z 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 we’ll need to pay attention to them in order to reduce the total error

Crosschecks Check for consistency among decay modes… …and tagging categories (K L sample not included): Consistent at 8% level

Crosschecks Also fit for “apparent sin2  ” on non-CP eigenstate samples (expect to get 0) No evidence of spurious asymmetries Sample Apparent sin2  Reconstructed B 0 (excl. CP modes) 0.02 ± 0.04 Reconstructed B ± (excl. CP modes) 0.03 ± 0.04 B 0  J/  K* 0 (non-CP) 0.12 ± 0.12 B ±  Charmonium X 0.07 ± 0.07

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 1.3  higher than Run1): Applying current analysis to previous sample gives 0.32 ± 0.18 Run1

Comparison to Standard Model Here’s how our results compares to the standard model expectation No hint of new physics here….

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

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 sin2  will be 0.03 Year Peak Luminosity (x10 33 cm -2 s -1 ) Integrated Luminosity (fb -1 ) Total605

Summary The SLAC B Factory had a very successful first run, collecting the world’s largest sample of  (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 sin2  0.59 ± 0.14 (stat) ± 0.05 (syst) But we’re just at the beginning of our program –The greatest challenges lie ahead:  Measuring sin2  in penguin-dominated modes  Measuring (or at least constraining) the other CP angles  …and many more