Recent Results from BaBar Adrian Bevan On the behalf of the BaBar collaboration HELLENIC SOCIETY FOR THE STUDY OF HIGH ENERGY PHYSICS Workshop on RECENT DEVELOPMENTS IN HIGH ENERGY PHYSICS AND COSMOLOGY Athens, April 2003 All results are preliminary unless otherwise stated

Athens, '032 Talk Outline Theoretical Motivation The PEP-II accelerator & BaBar Detector Experimental issues Vertex reconstruction Flavour tagging Particle Identification background suppression techniques Results sin2 , sin2 , direct CP Violation searches, rare decays Summary and Outlook

Athens, '033 Physics Background – CP Violation CP Violation is one of Sakharov’s conditions for obtaining a matter antimatter asymmetry in the universe. The observed CP Violation explained by the Standard Model of Particle Physics is 9 orders of magnitude too small to explain our universe There must be some kind of new physics to discover in the flavour sector Standard Model mechanism for CP Violation in weak interactions is given by a single complex phase in the CKM matrix. CP Violating phase present large uncertainty in magnitude and phase →related to a phase in the unitarity triangle A. D. Sakharov (1967) JTEP Letters 5, 24

Athens, '034 Experimental constraints before the B-factory era (CP Violation measurements prior to 1999) Observed effects have been entirely in the kaon system This defined our knowledge of CP Violation in the SM 1964, Christensen et al. discovered CP Violation in K 2 decay Following 37 years: measured several CP Violation effects in Semi-leptonic kaon decay,  ~10 -3 Non-leptonic kaon decay,  +  -,  0  0 : CP Violation in mixing:  ~10 -3 CP Violation in decay:  ’/  ~10 -3 (KTeV/NA48) Observed a CP Violating asymmetry in K L →  +  - e + e - (13% effect) (observed a T odd asymmetry in this decay as well)

Athens, '035 CP-LEAR measured T violation obtained an upper limit on CPT Violation from There are still important measurements to do with kaons over-constraining the CKM matrix is the aim With the advent of the B factories, we were able to test this description for the first time!

Athens, '036 Unitarity of the CKM matrix gives 6 triangles in the complex plane; 9 relations in all – few triangles have all sides with the same order in – kaon system is not one of these – interesting one for the B d/u system is “The Unitarity Triangle”: mixing The CKM Matrix in terms of B d/u decay (1,0) (0,0) (,)(,)

Athens, '037 For neutral K, B, D … mesons strong eigenstates are not CP eigenstates mass eigenstates are an admixture of different strong eigenstates particle  antiparticle (mixing),  f = 2 Neutral Meson Phenomenology (mixing is very suppressed in the case of D mesons) mass eigenstates=CP eigenstates if no CP Violation in mixing (q/p=1) if q/p  1 have CP Violation in mixing; e.g.  K =2.3x10 -3 CP even CP odd

Athens, '038 Direct CP Violation (  f = 1) need interference between diagrams with different strong (  i ) and weak phases (  i ): Direct CPV only seen in ;  ' ~ few theory predicts large asymmetries in B +/0 (few to ~80%) direct CPV

Athens, '039 Observing CP violation at the  (4S) Three observable interference effects: –CP violation in mixing (|q/p| ≠ 1) –(direct) CP violation in decay (|A/A| ≠ 1) –CP violation in interference of mixing and decay (Im ≠ 0) Analyse time evolution of B  B  system (assume  ) direct CP violation→ C ≠ 0 Indirect CP violation → S ≠ 0

Athens, '0310 s,d Searching for Direct CP Violation & probing New Physics with B u/d Large A CP requires amplitudes of similar order –b→u: suppressed tree: i.e. charmless decays large predicted A CP –b→s,d: penguins: radiative decays good for constraining BSM small predicted A CP Understand penguins Access to ,  and  Sensitive to New Physics effects via loops –minimal SUGRA: B→X s ,  K +, K 0  + … –R-parity Violating SUSY:  K S … –SUSY searches – K* 

Athens, '0311 Experimental Issues interesting modes have small branching fractions –large light quark continuum background qq –quarks below threshold are u,d,s & c –other B background Need good K/  separation Need to boost to do time dependent CP measurements require good vertex resolution for CP measurements need to ‘tag’ the flavour of the B (as a particle/anti particle) understand charge bias –detector: trigger, tracking; reconstruction –event selection, particle ID, analysis –physics: differences in (anti)particle interaction in matter (e.g. K  )

Athens, '0312 On the whole - very similar designs with many common features for both Belle and BaBar: e+e- colliders run on the  (4S) resonance – produce ~100% BB pairs run 40 MeV below resonance for qq background studies To measure CP violation in the B system, need to measure vertex difference of the evolving BB system (  t) silicon device need asymmetric beam energies The Accelerator & Experiment (PDG 2002)

Athens, '0313 E(e - ) = 9.0 GeV E(e + ) = 3.1 GeV   0.56 Design Achieved Luminosity (cm -2 s -1 ) 3 x x Int. Lum / day (pb -1 ) Int. Lum / month (fb -1 ) The PEP-II e+e- collider Most results use: 81 fb -1 on-resonance. 88 million BB events (as for ICHEP02). Most results use: 81 fb -1 on-resonance. 88 million BB events (as for ICHEP02).

Athens, '0314 Silicon Vertex Detector (SVT) Drift chamber (DCH) Detector for Internally reflected Cherenkov radiation (DIRC) Electromagnetic Calorimeter (EMC) 1.5 T Solenoid Instrumented Flux Return (IFR) SVT: 5 layers double-sided Si. Crucial for measuring  t. DCH: 40 layers in 10 super- layers, axial and stereo. DIRC: Array of precisely machined quartz bars. Excellent Kaon identification. EMC: Crystal calorimeter (CsI(Tl)) Very good energy resolution. Electron ID,  0 and  reco. IFR: Layers of RPCs within iron. Muon and neutral hadron (K L ) The B A B AR experiment

Athens, '0315 Measure angle of Cherenkov light –Transmitted by internal reflection –Detected by~10,000 PMTs –excellent K/  separation Particle Quartz bar Cherenkov light Active Detector Surface Particle Identification (PID) Detection of Internally Reflected Cherenkov Light (DIRC)

Athens, '0316 Measuring CP Violation from time dependence 1. Start with data sample of BB pairs 2. Reconstruct one B in a CP eigenstate decay mode 3. “Tag” the other B to make the matter/antimatter distinction 4. Determine the time between the two B decay vertices,  t 5. Plot  t distribution and do CP fit for S and C.

Athens, '0317 Event Selection Techniques Use beam energy to constrain mass & energy difference M ES  ~ 3 MeV B background signal  E  ~ MeV; larger with neutrals qq background

Athens, '0318 Event Selection Techniques B events are spherical u,d,s,c is jets shape variables u,d,s,c B event Signal u,d,s,c background Fisher Discriminant Arbitrary Units Maximum Likelihood fits or cut based analysis off-resonance &  E sidebands are used to parameterise light quark background flavour-tagging (e, , K, slow  from other B)

Athens, '0319 B Flavour Tagging Tagging algorithm with physics-based neural networks –Inputs include leptons, kaons, slow-  (from D*), and high-momentum tracks –Outputs combined and categorized by mistag probability (w) 5 mutually exclusive categories: Lepton – isolated high-momentum leptons Kaon I – high quality kaons or correlated K + and slow-  - Kaon II – lower quality kaons, or slow-  Inclusive – unidentified leptons, poor-quality kaons, high-momentum tracks Untagged – no flavor information is used b sc K-K- Q =  (1-2w) 2 = (28.1  0.7)% cleaner signal larger mistag prob

Athens, '0320 Tagging: example of Charmless B Decays Tagging efficiency is very different for signal and background Strong bkg suppression in categories with the lowest mistag prob (Lepton/Kaon) plots shown are for h + h -, a rare decay with significant backgrounds. 81/fb B→ h + h - sample split by tagging category

Athens, '0321 Vertex Reconstruction Resolution function parameters obtained from data for both signal and background –Signal from sample of fully reconstructed B decays to flavor eigenstates: D * ( , , a 1 ) –Background from data sideband sample Beam spot Interaction Point B REC Vertex B REC daughters Exclusive B rec reconstruction B TAG direction TAG tracks, V 0 s z B TAG Vertex B →  Example in B →  e + e - → qq  t (ps)  z resolution dominated by tag side (other B) Average  z resolution ~180  m Average  z ~260  m without the boost would have  z~20  m … e.g. CLEO without the boost would have  z~20  m … e.g. CLEO

Athens, '0322 Dominant amplitudes for b  ccs decay: Both amplitudes have the same weak phase: For B 0  J/  K s we obtain: Golden Mode for CP Violation in B decay (theoretically clean) b d c c s d W+W+ b d s c c d W+W+ c g TreePenguin Measuring  V cd V cb are almost real - only phase is from mixing B 0 mixing DecayK 0 mixing

Athens, '0323 sin 2  V tb * V td  V cb * V cd CP = +1 B  J/  K L 0 CP = -1 B  J/  K s 0, K s 0   +  -,  0  0 B   (2S) K s 0 B   c1 K s 0 B   c K s 0 B  J/  K *0, K *0  K s 0   mostly CP even  E=E b -E beam (GeV) J/  KL signal J/  X background other background

Athens, '0324 Latest result on 88M BB events K s modes K L modes hep-ex/ (PRL) sin2  =   | | =   tagged events (78% purity; 66% tagged ) tagging efficiency Q=(28.1  0.7)% Still statistics limited…

Athens, '0325 need more statistics to see if the SM holds out B0  KsB0  Ks pure penguin sensitive to new physics measured  in this mode should agree with In SM: 84 million BB pairs (supersedes ICHEP ‘02) 84 million BB pairs (supersedes ICHEP ‘02) c.f. world average: sin2  = 0.73 ± 0.06 >2  difference. Belle measures -0.73±0.64±0.18 Goodness-of-fit: 23% C  ~60 events

Athens, '0326 Also have results from D*D*,  ’K s, J/  0 but interpretation of the measurement of S and C in terms of the weak phase  is plagued by uncertainty … “reference” sin2 pure penguin mostly penguin? colour- & CKM-suppressed tree competing penguin CKM-suppressed tree small penguin pollution

Athens, '0327 Measuring  Interesting modes to measure  need to perform an Isospin analysis of branching ratios of B 0 and B 0 to 2  final states to determine shift in  due to the presence of penguin diagrams doing a ‘quasi 2 body analysis’ – will eventually have to analyse the whole Dalitz plot. In analogy to these modes one needs to analyse the time dependence of this decay and extract  from an isospin analysis of each of the three partial waves (L=0,1,2) in the final state (full angular analysis is required) The shift in  from penguin diagrams is expected to less than that in  Work is now in preparation for analysis of modes  the time evolution of     gives the shifted value of  need a neutral B meson decay with several contributing processes that interfere in order to be able to measure the phase

Athens, '0328 CP Violation in B  →     mixing decay Tree (T) Level: With Penguins (P): Need to measure the weak phase  can use isospin relations to extract shift penguins are significant: P/T~0.2

Athens, '0329 Can bound the shift on  using BR(B  →    0 ) and upper limit on B 0 →  0  0 The Isospin Analysis: Need to measure decay of B and B to final states in order to determine the shift Measure  ’  for B(B) from Already have Need to measure Grossman Quinn bound

Athens, '0330 qq + K  Fit projection in sample of  -selected events The first side of the Isospin triangle: B + →    - PRL fit , K  simultaneously  KK e+e→qq large qq background  /K separation

Athens, '0331 The Base of the Isospin Triangle: B  →     large qq background  /K separation Potential background from     –Minimize with tight cut on  E hep-ex/ , submitted to PRL Simultaneously fit for   /K  

Athens, '0332 Small signal; BR  few   background Background suppression –Event shape and flavour tagging to reduce qq –Cut on M(     ) and  E(       ) to reduce   background, then fix in the fit Significance including systematic errors = 2.5  The Missing Sides : B  →     e  e  → qq hep-ex/ , submitted to PRL

Athens, '0333 Bounding penguin pollution: Need ~20 the data to do a better isospin analysis than using a bound in B→  Use Grossman-Quinn Bound: assumes isospin most conservative of several bounds in literature account for correlated systematic uncertainties common to both analyses

Athens, '0334 Not a CP eigenstate related to  direct CPV  K is self tagging C  K, S  K,  S  K =0,  C  K =-1 large expected: A CP (  ) & A CP (  K) signal SXF+BBg

Athens, '0335 B0→+-B0→+- continuum B-background total fit ~88  10 6 B pairs Dilution from background 2.1 

Athens, '0336 Direct CP Violation Searches Looked for direct CP Violation in many modes so far no evidence most precise measurement ~4.6% charge asymmetry in  is 2.5  effect  is 2.1  effect look for updates using more data

Athens, '0337 Rare Decays: very rare process BR ~10 -7 penguin + FCNC process consistent with SM sensitive to new physics

Athens, '0338 s,d (Cabibbo favoured) (Cabibbo suppressed) radiative penguins new physics probes constrain |V td /V ts | (R t ) RtRt B 0  0  B +  +  B 0  signal region M ES (GeV)  E C.L. 78fb fb -1 Phys Rev Lett (2002)

Athens, '0339 Observation of a Narrow Meson Decaying to D + s  0 at a Mass of 2.32 GeV/c 2 hep-ex/ 00 D+sD+s

Athens, '0340 Other BaBar physics I’ve not covered BaBar has also produced results on B lifetime and mixing;  B,  m semi-leptonic B decays D mixing f B+B- /f B0B0 to name but a few in addition to B; have very large  and D samples to study produced in excess of 35 publications several more preprints have been submitted for publication

Athens, '0341 Summary Since the start up of BaBar and Belle we have learnt? SM description of CP Violation passed its first real test Observed CP Violation in the B System CP Violation in B system consistent with that in kaons Still not enough CP Violation to explain matter-antimatter asymmetry in the universe Expect ~500 fb -1 by 2006 (5x current data set) With this we will: start to test SM by doing alternate measurements of sin2  start to measure  with greater precision in several modes continue the search for direct CP Violation work towards over constraining the unitarity triangle can we break it? search for and constrain new physics in the flavour sector

Athens, '0342 Constraints on ( ,  ) Dominated by results from B physics! main constraint: sin2  fitting using Belle result and indirect constraints   ~1.5 0 measuring  is next test of SM CKM Fitter Group: ö A. Höcker, H. Lacker, S. Laplace, F. Le Diberder, Eur. Phys. Jour. C21 (2001) 225, [hep-ph/ ]