The BABAR EXPERIMENT Physics motivation CP violation The

The BABAR EXPERIMENT Physics motivation: CP violation The accelerator: PEPII at SLAC The detector: BABAR Physics results Richard Kass OSU Feb 10 2010 1

C and P Symmetry Continuous symmetries have been key in our understanding and discovery of the laws of nature: Wikipedi. A: “Noether’s theorem is a central result in theoretical physics that shows that conservation laws can be derived from any continuous symmetry. ” Symmetry Operation Conserved Quantity Translation in space Linear momentum Rotation in space Angular momentum Translation in time Energy Change of phase Electrical charge Discrete Symmetries are important also: Parity: (x, y, z) ↔ (-x, -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 OSU Feb 10 2010 2

P and CP Violation WRONG Parity is violated by Weak Interaction (e. g. b-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. C. S. Wu 1964 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(KL→π+π-) ~ 0. 2% instead of zero. The laws of physics are different for matter and anti-matter! Cronin Fitch For ~ 40 years the only way to study CP violation was to use KAONS We now can study CP violation with B-MESONS Richard Kass OSU Feb 10 2010 3

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, what’s the problem? Richard Kass OSU Feb 10 2010 4

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? Einstein (1905) Matter and energy are equivalent and can transform into each other. 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+)? Richard Kass OSU Feb 10 2010 5

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 C. D. Anderson, Phys. Rev. 43, 491 (1933). e+ bending in B-field g Why not a photon converting into matter + anti-matter? e- eg e+ A bubble chamber photo showing examples of γ→e+e- Anti-proton found in 1955… e- e+ Richard Kass OSU Feb 10 2010 6

Matter-Anti. Matter 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 u’s 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: π+= ud, K+=us, B+=ub Anti-matter is routinely produced on Earth! Accelerator laboratories: Fermilab: anti-protons Cornell/SLAC/KEK: e+ 3 generations of quarks & leptons Hospitals: Positron Emission Tomography Richard Kass Looks good on earth, what about rest of the universe? OSU Feb 10 2010 7

Anti-Matter in the Universe 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-→γ, from intergalactic clouds 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? Predict: n. Matter/n. Photon~ 0 Experiment: nb/ng~ (6. 1 ± 0. 3) x 10 -10 (Wilkinson. Microwave. Anisotropy. Probe) Richard Kass OSU Feb 10 2010 8

CP Violation in the Standard Model 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: d→u. W- Under a CP Operation we have: coupling CP( g q q’ ) = W- g* q q’ W+ Mirror To incorporate CP violation: g ≠ g* (coupling has to be complex) 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 OSU Feb 10 2010 9

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 OSU Feb 10 2010 10

CP violation & B mesons EXPERIMENTS are in BAD SHAPE Theory says we will need ~108 B mesons to observe CPV the decay modes that exhibit CPV don’t occur very often Current (1987) experiments can collect~105 B mesons/year Theory says that the “easiest” way to observe CPV involves measuring the distance (~30 mm) a B meson travels before it decays. But the best an experiment can do is ~100 mm Also, since 99. 9999% of B meson decays are NOT useful for studying CPV the detector must be highly efficient at eliminating these unwanted decays. Richard Kass OSU Feb 10 2010 11

Steps to observing CPV with B mesons Produce B-mesons pairs using the reaction e+e- ®¡ (4 S) ® B 0 B 0 (must build an asymmetric energy collider) Reconstruct the decay of one of the B-mesons’s into a CP eigenstate example CP: B 0 ®y. KS and B 0 ®y. KS Reconstruct the decay of the other B-meson to determine its flavor (“tag”) use high momentum leptons: B 0 ®(e+ or m+ )X and B 0 ®(e- or m- )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/(bgc) Fit the decay time difference between B 0’s and B 0’s to the functional form: d. N/dtµe-G|t| [1±hcp(sin 2 b)sin(Dmt)] -B 0, +B 0 hcp = ± 1 CP of final state = -1 for B 0 ®y. KS CP violating phase Dm=difference between B mass eigenstates Dm=0. 47 x 1012 h/s Determine sin 2 b Richard Kass OSU Feb 10 2010 12

Example of “Golden” Signature of CPV with B mesons Decay rate is not the same for B 0 and B 0 tag. Richard Kass OSU Feb 10 2010 13

How to Measure Time Dependent Decay Rates t =0 We need to know the flavor of the B at a reference t=0. Dz = Dt gbc 0 At t=0 we B 0 know this meson is B 0 B rec (4 S) bg =0. 56 B 0 The two mesons oscillate coherently : at any given time, if one is a B 0 the other is necessarily a B 0 Richard Kass tag l - (e-, m -) In this example, the tagside 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. OSU Feb 10 2010 Dt picoseconds later, the B 0 (or perhaps it is now a B 0) decays. 14

How to Get the Data Sample Use e+e- annihilations at Y(4 S) to get a clean sample of B mesons At Y(4 S) produce B-/B+ (bu/bu) and B 0 B 0 (bd/bd) mesons m. B ~ 5. 28 Ge. V - BB Threshold 0 The Y(4 S) - a copious, clean source of B meson pairs 1 of every 4 hadronic events is a BB pair No other particles produced in Y(4 S) decay Equal amounts of matter and anti-matter produced Richard Kass OSU Feb 10 2010 15

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(4 S) (at rest) at a symmetric e+e- collider? t. B=1. 6 x 10 -12 sec At a symmetric collider we have for the B mesons from Y(4 S) decay: plab =0. 3 Ge. V, m. B=5. 28 Ge. V Average flight distance <L>= (bg)ct. B= (p/m)(468 mm)=(0. 3/5. 28)(468 mm)=(27 mm) This is too small to measure!! If the beams have unequal energies then the entire system is Lorentz Boosted: bg = plab /Ecm=(phigh-plow)/Ecm SLAC: 9 Ge. V+3. 1 Ge. V bg = 0. 55 <L>= 257 mm KEK: 8 Ge. V+3. 5 Ge. V bg = 0. 42 <L>= 197 mm We can measure these decay distances ! Because of the boost and the small plab the time measurement is a z measurment. z-axis symmetric asymmetric CESR Richard Kass SLAC, KEK OSU Feb 10 2010 16

PEPII-Asymmetric e+e- Collider PEP-II Peak Luminosity 1. 2 x 1034 cm-2 s-1 (about 50 X better than previous accelerators) Ba. Bar recorded 424 fb-1 at Y(4 S) ~4. 65 x 108 U(4 S)→BB events Stanford Linear Accelerator Center, Stanford, California PEPII is an asymmetric e+e− collider: 9 Ge. V (e-)/3. 1 Ge. V (e+) A B-meson travels a measurable distance before decay: bg=0. 56 → <bgct>~260 mm Richard Kass OSU Feb 10 2010 17

Detector Requirments-I Measure momentum of charged particles charged particle bend in B-field Measure the energy of neutral particles (mostly g-rays) electromagnetic calorimeter Measure the decay length of unstable particles decay lengths vary from ~100 mm 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 Can’t just use commercially available stuff, must design chips, etc Richard Kass OSU Feb 10 2010 18

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 OSU Feb 10 2010 19

Ba. Bar Detector Electromagnetic Calorimeter (EMC) 1. 5 T Solenoid Detector of Internally Recflected Cherenkov Light (DIRC) ) e. V + (3. 1 G e Ge. V) e (9 Instrumented Flux Return (IFR) Drift Chamber (DCH) Silicon Vertex Tracker (SVT) SVT, DCH: charged particle tracking: vertex & mom. resolution, K 0 s/Λ EMC: electromagnetic calorimeter: g/e/π0/η DIRC, IFR, DCH: charged particle ID: π/μ/K/p Highly efficient trigger for B mesons Richard Kass OSU Feb 10 20

The Ba. Bar SVT § 5 Layers of double-sided, AC-coupled silicon § 0. 94 m 2 of Si § Φ and z strips § Inner 3: Precision Vertexing § Outer 2: Pattern recognition, Low Pt tracking § Custom rad-hard readout IC (the ATo. M chip). 140 k channels § Low-mass design (Kevlar/carbon fiber mechanical support) Richard Kass OSU Feb 10 2010 Magnet Be Beam Pipe 21

SVT Performance z-side upilex fanout Si wafers φ-side Average hit efficiency 97% Slow pion efficiency 70% for PT>50 Me. V Average z hit resolution 10 - 40 μm Richard Kass OSU Feb 10 2010 22

The Ba. Bar Drift Chamber 40 -layer small-cell chamber 7104 drift cells formed from hexagonal field wire pattern 80 & 120 mm Aluminum field wires and 20 mm 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 30 K 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 Measures position (relative to wire) Measures ionization which helps ID pions, kaons, protons Richard Kass OSU Feb 10 2010 23

Display of an Event Display Richard Kass OSU Feb 10 2010 24

The DIRC PID: Need to tell a pion from a kaon from a proton Richard Kass OSU Feb 10 2010 25

The DIRC-Performance 3 S. D. means the probability of calling a kaon a pion is ~1 in 300. Richard Kass OSU Feb 10 2010 26

Electromagnetic Calorimeter Measure the energy of photons and electrons/positrons Cs. I(TL) scintillates: Energy in => Light out Richard Kass OSU Feb 10 2010 27

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 OSU Feb 10 2010 28

Muon Detector full scale LST We have some LSTs in PRB. Inside an LST 8 cells per LST Worked in Smith Lab Me installing last LST into BABAR Richard Kass OSU Feb 10 2010 29

Analysis Technique Threshold kinematics: we know the initial energy (E*beam) of the Y(4 S) system Therefore we know the energy & magnitude of momentum of each B meson Event topology Signal (spherical) Background (jet-structure) Also, use neural networks + unbinned maximum likelihood fits Richard Kass OSU Feb 10 2010 30

CPV Results for Sin 2 b Measurement of sin 2β with: B → J/ψ K 0, J/ψ K*, ψ(2 S)KS, ηc. KS, & χc 1 KS Summer 2009 HUGE success, Just as theorists predicted Richard Kass OSU Feb 10 2010 31

Ba. Bar Status Data taking with Ba. Bar ended April 2008 We are currently in the “intensive analysis” phase > 400 published articles to date, lots more to come. Ba. Bar 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 Ba. Bar (and Belle) showed that the KM model works really well! Richard Kass OSU Feb 10 2010 32

Ba. Bar & 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 OSU Feb 10 2010 33

Extra Slides Richard Kass OSU Feb 10 2010 34

How Can This Happen? In 1967 Sakharov showed that the generation of the net baryon number in the universe requires: 1. Baryon number violation (Proton Decay) 2. Thermal non-equilibrium 3. C and CP violation (Asymmetry between particle and anti-particle) Richard Kass OSU Feb 10 2010 35

Visualizing CKM information from B-meson decays The Unitarity Triangle The CKM matrix Vij is unitary with 4 independent fundamental parameters Unitarity constraint from 1 st and 3 rd columns: i V*i 3 Vi 1=0 d s b u Vud Vus Vub c Vcd Vcs Vcb t Vtd Vts Vtb CKM phases (in Wolfenstein convention) To test the Standard Model: Measure angles, sides in as many ways possible Area of triangle proportional to amount of CP violation Richard Kass OSU Feb 10 2010 36

Three Types of CP Violation I) Indirect CP violation/CP violation in mixing K®pp, K®pln, expected to be small (SM: 10 -3) for B 0’s II) Direct CP violation: Prob(B®f) ¹ Prob(B®f) Only CP violation possible for e¢/e in K®pp charged B’s Br(B 0®K-p+) ¹ Br(B 0®K+p-) III) Interference of mixing & decay: Prob(B(t)®f. CP) ¹ Prob(B(t)®f. CP) B 0®y. Ks B 0®p+p- (CKM angle b) (CKM angle a) Due to quantum numbers of Y(4 S) and B meson we must measure time dependant quantities to see this CP violation In this talk we will be discussing type III) CP violation Richard Kass OSU Feb 10 2010 37

CP Violation at the Y(4 S) CPV from the interference between two decay paths: with and without mixing q/p mixing |BL>=p|B 0>+q|B 0> |BH>=p|B 0>- q|B 0> Measure time dependent decay rates & Dm from B 0 B 0 mixing Direct CP Violation: C¹ 0 |Af/Af|≠ 1→ direct CP violation |q/p|≠ 1→ CP violation in mixing Richard Kass Sf and Cf depend on CKM angles OSU Feb 10 2010 38

Why do we need an asymmetric collider? N=number of events f. CP= CP eigenstate (e. g. B 0®y. KS) ffl= flavor state (particle or anti-particle) (e. g. B 0®e+X) The source of B mesons is the U(4 S), which has JPC =1 --. The U(4 S) decays to two bosons with JP =0 -. Quantum Mechanics (application of the Einstein-Rosen-Podosky Effect) tells us that for a C=- initial state (U(4 S)) the rate asymmetry: 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 B’s produced at the U(4 S). Richard Kass OSU Feb 10 2010 39