Richard Kass 1 OSU Feb The BABAR EXPERIMENT Physics motivation: CP violation The accelerator: PEPII at SLAC The detector: BABAR Physics results Richard Kass 2 OSU Feb C and P Symmetry Continuous symmetries have been key in our understanding and discovery of the laws of nature: Symmetry OperationConserved Quantity Translation in spaceLinear momentum Rotation in spaceAngular momentum Translation in timeEnergy Change of phaseElectrical charge WikipediA: Noethers theorem is a central result in theoretical physics that shows that conservation laws can be derived from any continuous symmetry. Discrete Symmetries are important also: Parity: (x, y, z) (-x, -y, -y) vectors (mom.) change sign but axial vectors (ang. mom.) do not Charge Conjugation: particles turn into anti-particles (and visa versa) proton anti-proton, electron positron C and P are good (conserved) symmetries for EM and the nuclear force. So, they must be good for all the forces right ? Richard Kass 3 OSU Feb P and CP Violation WRONG Parity is violated by Weak Interaction (e.g. -decay) Discovered in 1957 (Wu, Co 60 ) Big effect, maximal violation! Even though Parity was violated it was thought that the combination of Parity & Charge Conjugation would be conserved in Weak Interaction Cronin and Fitch discovered the violation of CP in the decay of the long-lived, CP-odd neutral K meson into a CP-even final state: Br(K L + - ) ~ 0.2% instead of zero. The laws of physics are different for matter and anti-matter! Fitch Cronin For ~ 40 years the only way to study CP violation was to use KAONS We now can study CP violation with B-MESONS C. S. Wu Richard Kass 4 OSU Feb What is CP Violation?? Are the laws of physics the same for particles & anti-particles? IS CP conserved in nature? P=parity operation: (x,y,z)-> (-x,-y,-z) C=charge conjugation: turn a particle into anti-particle ALMOST YES for the weak force CP violation in kaons ~2/1000 So the answer is NO BUT this is a very small effect. So, whats the problem? Richard Kass 5 OSU Feb The Early Universe had lots of matter and anti-matter. We all know about matter since it is the stuff we are made of. But what is anti-matter? Dirac (1928) Invents relativistic quantum mechanics Has extra solution and predicts anti-matter Anti-matter is like matter but opposite electric charge e.g. a negatively charged proton Ideas of Einstein and Dirac lead to lots of possibilities for anti-matter! Why not an anti-electron (= positron =e + )? Einstein (1905) Matter and energy are equivalent and can transform into each other. Richard Kass 6 OSU Feb Anti-Matter Found! The positron (e + ) was discovered in 1932 in cosmic rays by Carl Anderson at Caltech The photograph shows how positrons were first identified in cosmic rays using a cloud chamber, magnetic field and lead plate A bubble chamber photo showing examples of e + e - Why not a photon converting into matter + anti-matter? Anti-proton found in 1955 C.D. Anderson, Phys. Rev. 43, 491 (1933). e + bending in B-field e+e+ e+e+ e-e- e-e- e-e- Richard Kass 7 OSU Feb Matter-AntiMatter Symmetry In our current view of nature the fundamental building blocks are quarks and leptons: An electron is a lepton and a proton is a bound state of 3 quarks (2 us and a d) There is symmetry between building blocks: For every type of quark/lepton there is an anti-quark/anti-lepton anti-proton = uud Bound states of quark anti-quark pairs are MESONS lots of mesons are possible: + = u d, K + =u s, B + =u b Anti-matter is routinely produced on Earth! Accelerator laboratories: Fermilab: anti-protons Cornell/SLAC/KEK: e + Hospitals: Positron Emission Tomography Looks good on earth, what about rest of the universe? 3 generations of quarks & leptons Richard Kass 8 OSU Feb Anti-Matter in the Universe In the Big Bang particle-antiparticle pairs were created from pure energy in a spontaneous explosion BUT today we cannot detect significant amounts of antimatter in the universe - why not? Since matter and antimatter can annihilate into photons how did an amount of matter survive? When we look into the night sky we only see MATTER! Anti-proton/proton ratio~10 -4 in cosmic rays No evidence for annihilation, e + e - , from intergalactic clouds Predict: n Matter /n Photon ~ 0 Experiment: n b /n ~ (6.1 0.3) x (W ilkinson M icrowave A nisotropy P robe ) Richard Kass 9 OSU Feb CP( ) = CP Violation in the Standard Model Under a CP Operation we have: q q W-W- g q W+W+ g* Mirror coupling To incorporate CP violation: g g* (coupling has to be complex) In the SM a quark turns into another quark by coupling to a W-boson e.g. a neutron (udd) decays to proton (uud) via: duW - It turns out that with 3 generations of quarks we can easily incorporate CP violation into the Standard Model: The Kobayashi-Maskawa Matrix (1973) Richard Kass 10 OSU Feb CP violation & B mesons By ~1987 there was enough info known about the KM matrix and the standard model for theorists to make believable predictions about observing CP violation with B-mesons. 5 quarks were observed, top quark thought to be heavy W and Z bosons observed & masses measured lifetime of B mesons measured mixing in B mesons measured (a B 0 can turn into a B 0 ) some B meson branching fractions measured THEORY IS IN GOOD SHAPE KM model is believable Richard Kass 11 OSU Feb CP violation & B mesons EXPERIMENTS are in BAD SHAPE Theory says we will need ~10 8 B mesons to observe CPV the decay modes that exhibit CPV dont occur very often Current (1987) experiments can collect~10 5 B mesons/year Theory says that the easiest way to observe CPV involves measuring the distance (~30 m) a B meson travels before it decays. But the best an experiment can do is ~100 m Also, since % of B meson decays are NOT useful for studying CPV the detector must be highly efficient at eliminating these unwanted decays. Richard Kass 12 OSU Feb Steps to observing CPV with B mesons Produce B-mesons pairs using the reaction e + e - (4S) B 0 B 0 (must build an asymmetric energy collider) Reconstruct the decay of one of the B-mesonss into a CP eigenstate example CP: B 0 K S and B 0 K S Reconstruct the decay of the other B-meson to determine its flavor (tag) use high momentum leptons: B 0 e + or + )X and B 0 e - or - )X flavor of CP eigenstate also determined at time of the tag decay. Measure the distance (L) between the two B meson decays and convert to proper time must reconstruct the position of both B decay vertices t=L/( c) Fit the decay time difference between B 0 s and B 0 s to the functional form: dN/dt e - |t| [1 cp (sin2 sin( mt)] Determine sin2 cp = 1 CP of final state = for B 0 K S CP violating phase m=difference between B mass eigenstates m=0.47x10 12 h/s -B 0,+ B 0 Richard Kass 13 OSU Feb Example of Golden Signature of CPV with B mesons Decay rate is not the same for B 0 and B 0 tag. Richard Kass 14 OSU Feb t =0 How to Measure Time Dependent Decay Rates We need to know the flavor of the B at a reference t=0. B 0 (4S) The two mesons oscillate coherently : at any given time, if one is a B 0 the other is necessarily a B 0 In this example, the tag- side meson decays first. It decays semi-leptonically and the charge of the lepton gives the flavour of the tag-side meson : l = B 0 l = B 0. Kaon tags also used. tag B 0 l (e-, -) =0.56 z = t c rec B 0 t picoseconds later, the B 0 (or perhaps it is now a B 0 ) decays. At t=0 we know this meson is B 0 Richard Kass 15 OSU Feb The Y(4S) - a copious, clean source of B meson pairs 1 of every 4 hadronic events is a B B pair No other particles produced in Y(4S) decay Equal amounts of matter and anti-matter produced Use e + e - annihilations at Y(4S) to get a clean sample of B mesons At Y(4S) produce B - /B + (b u / b u) and B 0 B 0 ( b d/b d ) mesons m B 0 ~ m B - ~ 5.28 GeV BB Threshold How to Get the Data Sample Richard Kass 16 OSU Feb Why Do We Need an Asymmetric Energy e+e- Collider? In order to measure time we must measure distance: t=L/v. How far do B mesons travel after being produced by the Y(4S) (at rest) at a symmetric e + e - collider? At a symmetric collider we have for the B mesons from Y(4S) decay: p lab =0.3 GeV, m B =5.28 GeV Average flight distance = ( )c B = (p/m)(468 m)=(0.3/5.28)(468 m)=(27 m) This is too small to measure!! If the beams have unequal energies then the entire system is Lorentz Boosted: = p lab /E cm =(p high -p low )/E cm SLAC: 9 GeV+3.1 GeV = 0.55 = 257 m KEK:8 GeV+3.5 GeV = 0.42 = 197 m We can measure these decay distances ! Because of the boost and the small p lab the time measurement is a z measurment. symmetric CESR asymmetric SLAC, KEK z-axis B =1.6x sec Richard Kass 17 OSU Feb PEPII-Asymmetric e + e - Collider Stanford Linear Accelerator Center, Stanford, California PEPII is an asymmetric e + e collider : 9 GeV (e - )/3.1 GeV (e + ) A B-meson travels a measurable distance before decay: =0.56 < c ~260 m PEP-II Peak Luminosity 1.2 x cm -2 s -1 (about 50X better than previous accelerators) BaBar recorded 424 fb -1 at Y(4S) ~4.65 x 10 8 (4S)B B events PEP-II Peak Luminosity 1.2 x cm -2 s -1 (about 50X better than previous accelerators) BaBar recorded 424 fb -1 at Y(4S) ~4.65 x 10 8 (4S)B B events Richard Kass 18 OSU Feb Detector Requirments-I Measure momentum of charged particles charged particle bend in B-field Measure the energy of neutral particles (mostly -rays) electromagnetic calorimeter Measure the decay length of unstable particles decay lengths vary from ~100 m to 10 cm Measure the identity of produced particles tell protons from kaons from pions from muons Trigger the experiment on (almost) every type of event some events have only 2 particles, some have 20. NO deadtime want to collect data whenever the accelerator is running Detector related effects understood at ~1% level e.g. do K + s behave differently than K - s? (could fake CPV!) Custom Electronics Cant just use commercially available stuff, must design chips, etc Richard Kass 19 OSU Feb Detector Requirments-II Useable software. ~million lines of code, hundreds of users distributed over the world need a realistic computer model of how the detector will work take raw data and turn it into 4-vectors Must be able to repair and maintain detector components things break, wear out, accidents. Must take 5-6 years to design and construct Must be designed, built, & operated within a budget ~100 Million $$$$ Must find several hundred physicists to work for > 10 years collaboration formed in 1994 physicists from North America, Europe, Asia. Richard Kass 20 OSU Feb T Solenoid Electromagnetic Calorimeter (EMC) Detector of Internally Recflected Cherenkov Light (DIRC) Instrumented Flux Return (IFR) Silicon Vertex Tracker (SVT) Drift Chamber (DCH) e - (9 GeV) e + (3.1 GeV) BaBar Detector SVT, DCH: charged particle tracking: vertex & mom. resolution, K 0 s / EMC: electromagnetic calorimeter: /e/ 0 / DIRC, IFR, DCH: charged particle ID: //K/p Highly efficient trigger for B mesons Richard Kass 21 OSU Feb 5 Layers of double-sided, AC-coupled silicon 0.94 m 2 of Si and z strips Inner 3: Precision Vertexing Outer 2: Pattern recognition, Low P t tracking Custom rad-hard readout IC (the AToM chip). 140k channels Low-mass design (Kevlar/carbon fiber mechanical support) Be Beam Pipe Magnet The BaBar SVT Richard Kass 22 OSU Feb SVT Performance Average hit efficiency 97% Slow pion efficiency 70% for P T >50 MeV Average z hit resolution m z-side Si wafers upilex fanout -side Si wafers upilex fanout Richard Kass 23 OSU Feb layer small-cell chamber 7104 drift cells formed from hexagonal field wire pattern 80 & 120 m Aluminum field wires and 20 m tungsten sense wires Layers organized into superlayers Wire directions in axial-u-v pattern Allows fast Level 1 trigger based on segments 80:20 helium:isobutane gas mixture Low-mass gas to minimize multiple scattering Small Lorentz angle results in simple t-to-d relation Mechanical Structure Thin aluminum endplates 30K precision holes locate feedthroughs with crimp pins Forward endplate reduced to 12 mm thickness in acceptance region Load-bearing cylindrical walls 1-mm thick beryllium inner wall (40% load) Nomex-carbon fiber composite outer wall assembled in two halves The BaBar Drift Chamber Measures position (relative to wire) Measures ionization which helps ID pions, kaons, protons Richard Kass 24 OSU Feb Display of an Event Display Richard Kass 25 OSU Feb The DIRC PID: Need to tell a pion from a kaon from a proton Richard Kass 26 OSU Feb The DIRC-Performance 3 S.D. means the probability of calling a kaon a pion is ~1 in 300. Richard Kass 27 OSU Feb Electromagnetic Calorimeter Measure the energy of photons and electrons/positrons CsI(TL) scintillates: Energy in => Light out Richard Kass 28 OSU Feb Muon detector Anything that goes through the entire detector is a muon The magnet iron is filled with charged particle tracking devices Use special type of proportional chambers Resistive Plate Chambers (RPC) Limited Streamer tubes (LST) Richard Kass 29 OSU Feb Muon Detector full scale LST Worked in Smith Lab Inside an LST 8 cells per LST Me installing last LST into BABAR We have some LSTs in PRB. Richard Kass 30 OSU Feb Threshold kinematics: we know the initial energy (E* beam ) of the Y(4S) system Therefore we know the energy & magnitude of momentum of each B meson Background (spherical) (jet-structure) Signal Analysis Technique Also, use neural networks + unbinned maximum likelihood fits Event topology Richard Kass 31 OSU Feb CPV Results for Sin2 Measurement of sin2 with: B J/ K 0, J/ K*, (2S)K S, c K S, & c1 K S Summer 2009 HUGE success, Just as theorists predicted Richard Kass 32 OSU Feb BaBar Status Data taking with BaBar ended April 2008 We are currently in the intensive analysis phase > 400 published articles to date, lots more to come. BaBar was more successful than anyone imagined.. Discovered and measured CPV using B mesons First observation of mixing with D mesons Discovered several new particles (e.g. eta_b) Limits on existence/mass of SUSY particles New software tools for data analysis BaBar (and Belle) showed that the KM model works really well! Richard Kass 33 OSU Feb BaBar & Belle confirm matter-antimatter asymmetry; leads to 2008 Nobel Prize in Physics Makoto Kobayashi Toshihide Maskawa But CPV is still a big mystery/problem The CPV in the KM model is way too small to explain the matter-anti-matter asymmetry we see in the universe Richard Kass 34 OSU Feb Extra Slides Richard Kass 35 OSU Feb How Can This Happen? 1.Baryon number violation (Proton Decay) 2.Thermal non-equilibrium 3.C and CP violation (Asymmetry between particle and anti-particle) In 1967 Sakharov showed that the generation of the net baryon number in the universe requires: Richard Kass 36 OSU Feb Visualizing CKM information from B-meson decays The Unitarity Triangle The CKM matrix V ij is unitary with 4 independent fundamental parameters Unitarity constraint from 1st and 3rd columns: i V * i3 V i1 =0 To test the Standard Model: Measure angles, sides in as many ways possible Area of triangle proportional to amount of CP violation CKM phases ( in Wolfenstein convention ) u d t c bs V ud V us V ub V cd V cs V cb V td V ts V tb Richard Kass 37 OSU Feb Three Types of CP Violation I) Indirect CP violation/CP violation in mixing K K l expected to be small (SM: ) for B 0 s II) Direct CP violation: Prob( B f ) Prob( B f ) in K Br( B 0 ) Br(B 0 ) III) Interference of mixing & decay: Prob(B(t) f CP ) Prob( B (t) f CP ) B 0 s (CKM angle ) B 0 (CKM angle ) In this talk we will be discussing type III) CP violation Due to quantum numbers of Y(4S) and B meson we must measure time dependant quantities to see this CP violation Only CP violation possible for charged Bs Richard Kass 38 OSU Feb CP Violation at the Y(4S) mixing q/p CPV from the interference between two decay paths: with and without mixing Measure time dependent decay rates & m from B 0 B 0 mixing S f and C f depend on CKM angles Direct CP Violation: C |B L >=p|B 0 >+q| B 0 > |B H >=p|B 0 >- q| B 0 > | A f /A f |1 direct CP violation |q/p|1 CP violation in mixing Richard Kass 39 OSU Feb Why do we need an asymmetric collider? The source of B mesons is the (4S), which has J PC =1 --. The (4S) decays to two bosons with J P =0 -. Quantum Mechanics (application of the Einstein-Rosen-Podosky Effect) tells us that for a C=- initial state ( (4S)) the rate asymmetry: N=number of events f CP = CP eigenstate (e.g. B 0 K S ) f fl = flavor state (particle or anti-particle) (e.g. B 0 e + X) However, if we measure the time dependence of A we find: Need to measure the time dependence of decays to see CP violation using the Bs produced at the (4S).