B - Physics - University of Cincinnati
B - Physics - University of Cincinnati
B - Physics - University of Cincinnati
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The Search for Time-DependentCP-Asymmetries in the NeutralB-Meson SystemMichael D. Sokol<strong>of</strong>f<strong>Physics</strong> Department<strong>University</strong> <strong>of</strong> <strong>Cincinnati</strong>Presented at theXX Encontro Nacional de Fisica de Particulas e Campos
The Nature <strong>of</strong> Particle <strong>Physics</strong>• As particle physicists, we study the fundamental constituents <strong>of</strong>matter and their interactions.• Our understanding <strong>of</strong> these issues is built upon certainfundamental principles– The laws <strong>of</strong> physics are the same everywhere– The laws <strong>of</strong> physics are the same at all times– The laws <strong>of</strong> physics are the same in all inertial referenceframes (the special theory <strong>of</strong> relativity)– The laws <strong>of</strong> physics should describe how the wave function <strong>of</strong>a system evolves in time (quantum mechanics)• These principles do not tell us what types <strong>of</strong> fundamentalconstituents exist, or how they interact, but they restrict the types<strong>of</strong> theories that are allowed.• In the past 30 years, we have developed a Standard Model <strong>of</strong>particle physics to describe the electromagnetic, weak nuclear,and strong nuclear interactions <strong>of</strong> constituents in terms <strong>of</strong>quantum field theories.<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 2
Special Relativity• Energy and momentum– Energy and momentum form a four-vector (t, x, y,z) . TheLorentz invariant quantity defined by energy and momentumis mass:E 2 − p2xc2 − py2 c2 − pz2 c2 = m2 c4– For the special case when an object is at rest so that itsmomentum is zeroE = mc 2• When a particle decays in laboratory, we can measure theenergy and momenta <strong>of</strong> its decay products (its daughterparticles), albeit imperfectly.• The energy <strong>of</strong> the parent is exactly the sum <strong>of</strong> the energies <strong>of</strong> itsdaughters energies. Similarly, each component <strong>of</strong> the parent’smomentum is the sum <strong>of</strong> the corresponding components <strong>of</strong> thedaughters’ momenta.From the reconstructedenergy and momentum <strong>of</strong>the candidate parent, wecan calculate its invariantmass<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 3
Classical Field Theory (E&M)<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 4
Fields and Quanta<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 5
<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 6
Baryons and Mesons• Quarks are never observed as free particles.– baryons consist <strong>of</strong> three quarks, each with a differentcolor (strong nuclear) chargeproton = uudneutron = udd– mesons consist <strong>of</strong> quark-antiquark pairs with cancelingcolor-anticolor chargesπ + = ud ;K − = su ;D 0 = cuB 0 = b dK 0 = s d; J / Ψ = cc0K S = (s d + sd )/ 2• Baryons and mesons (collectively called hadrons) have netcolor charge zero.• A Van der Waals-type <strong>of</strong> strong interaction creates anattractive force which extends a short distance to bindnucleii together.<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 7
Weak Charged Current Interactionsneutrino scatteringcharm decay⎛ u⎜ ⎞⎟⎝ d⎠⎛ c⎜ ⎞⎟⎝ s⎠⎛ t⎜ ⎞⎟⎝ b⎠⎛⎜⎝ν ee −⎞⎟⎠⎛⎜⎝ν µµ −⎞⎟⎠⎛⎜⎝ν ττ −⎞⎟⎠As a first approximation, the weak charged current inter-actioncouples fermions <strong>of</strong> the same generation. The StandardModel explains coupling between quark generations in terms<strong>of</strong> the Cabibbo-Kobayashi-Maskawa (CKM) matrix.⎛ d⎞⎛ d ′ ⎞ ⎛ V ud V us V ub⎞⎜ ⎟s⎜ ⎟ → ⎜ ⎟s ′⎜ ⎟ = ⎜⎟⎜ V cd V cs V cb ⎟⎜⎟⎝ b⎠⎝ b ′ ⎠ ⎝ V td V ts V tb ⎠⎛ d⎞⎜ ⎟s⎜ ⎟⎝ b⎠This matrix is approximately diagonal, but it allows formixing between generations and introduces a relativephase in the quantum mechanical amplitudes for decay <strong>of</strong>some particles and their antiparticles.<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 8
Particle-Antiparticle Mixing0B0BA second order weak charged current process, <strong>of</strong>tenreferred to as a box diagram amplitude, provides a00mechanism by which B particles oscillate into Bantiparticles, and vice versa.• Particles decay exponentially with characteristic timesN(t) = N 0 e −t/τ• Neutral B-mesons mix sinusoidally with characteristic timesN mix = N 0 e −t/τ sin (t/t mix )• Experimentally,t mix ≈ 1.4 τ ,which makes its observation relatively easy.<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 9
0CP Violation0• Both B particles and B antiparticle can decay to commonfinal states, as indicated below.0B0B} J/Ψ}}}K 0 → K S0J/ΨK 0 → K S0• The J/Ψ K0S final state is invariant under charge and parityconjugation; that is, it remains0J/Ψ K S.• The Standard Model predicts that the CKM phase willproduce a time-dependent asymmetry in the decay rates0<strong>of</strong> the and0B B to this final state, and that the asymmetrywill vary sinusoidally.B 0B 0B 0fCP≠B 0f CP<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 10
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Producing B-Mesons for CP Violation Studies• The B-factories at SLAC (California) and KEK (Japan)produce B-mesons via e+ e−annihilation.}}0B0B• At the upsilon(4s) resonance, e + e − → upsilon(4s)→ BB ,approximately 25% <strong>of</strong> all hadronic events are BB.<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 17
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The PEP-II Accelerator at SLACDesign Parameters:• = 3 x 10 33 cm −2 s −1• 9 GeV e − on 3.1 GeV e +• 0.75 A e − on 2.14 A e +• 1658 bunches in each ring• Head-on collisions<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 19
The BABAR DetectorMeasure the trajectories and momenta <strong>of</strong> charged particlestraveling in a magnetic field.Measure the energy <strong>of</strong> photons and electrons.Identify muons which traverse large amounts <strong>of</strong> materialwithout interacting.Measure the speeds <strong>of</strong> particle using Cherenkov radiationand ionization density.Silicon Vertex Tracker:tracking, ∆zDIRC PIDp/K/πDrift Chambertracking, dE/dxInstrumented Flux Returnµ, neutral hadron ID1.5T SolenoidElectromagnetic Calorimeter:good resolution, low energyreach for π 0 , γ’s<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 20
Silicon Vertex Tracker• 5 double -sided layers, φ/z• r = 32mm to 144mm• 15 µm (φ) to 19µm (z) resolution• 60 µm z vertex resolution• radiation hard to 2MRadφ resolution = 15µmz resolution = 19µmResolution measuredusing cosmic rays:<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 21
The Drift Chamber• 40 layers, alternating axial and stereo superlayers• Low density: 80%He, 20% Isobutane, Al wires• dE/dx resolution <strong>of</strong> 7%•
Particle Identification Using Ionization• Ionization (dE/dx) is measured in each <strong>of</strong> 40 layers in thedrift chamber.• A truncated mean value is used as the best estimate <strong>of</strong> theaverage ionization rate.• Recent improvements bring the average fractionalresolution for Bhabhas close to the design value <strong>of</strong> 7%.– improved feature extraction from the digitized signals;– improved understanding <strong>of</strong> the gas gain;– S<strong>of</strong>tware corrections for bias due to using truncatedmean, as a function <strong>of</strong> track dip angle.<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 23
The DIRCThe Detector <strong>of</strong> InternallyReflected CherenkovLight is used to identifycharged particles. TheCherenkov angle dependson the speed <strong>of</strong> theparticles.The Cherenkov angle difference for K and π at 4 GeV/c is ~6.5 mrad. The design specifies 3σ separation at this energy.Cherenkov angle resolution should be 2.6 mrad for backwardpositrons from Bhabha events . It is approaching the designspecification.July, 1999 status<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 24
Electromagnetic Calorimeter• 7000 CsI crystals in barrel and forward endcap• Reconstruct photons above 20 MeV• Energy resolution <strong>of</strong> σ(E)/E =1% / 4 E(GeV) ⊕1.2%<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 25
Finding the Constituents(July 2000 data)The first B-meson decay we willtry to study iswithorB 0 → J /ΨK S0J/Ψ → µ + µ -J /Ψ →e + e -andK S 0 →π + π -Signal region<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 26
A Sample EventB 0 → J/ψ K 0 s with K 0 s → π − π +<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 27
Summer 2000 ResultsB 0 → J/ψ K 0 s with K 0 s → π + πFlavor-tagged sample <strong>of</strong>B 0 → J/ψ K 0 s used in sin2βanalysisCombined with analogoussample <strong>of</strong>B 0 → J/ψ(2S) K 0 s for theOsaka result:sin2β = +0.12 ± 0.37 ± 0.09<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 28
Summer 2000 Results & Projectionsfor Full First RunSome projected results for the full 23 fb -1 sample(Estimated errors for combined results shown in brown)±0.014 (0.9%)±0.018 (1.1%)±0.010 (2.0%)(sin2β projection assumes additional modes will be used)<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 29
Summary and Conclusions[with December, 2000 updates]• The Standard Model <strong>of</strong> particle physics predicts timedependentasymmetries in the decay rates for B 0 → J/ψ K 0 sand its charge conjugate decay. .• B-factories (PEP-II at SLAC and KEK-B in Japan) aredesigned to produce 30 × 10 6 bb pairs per year. [3.6 x 106produced by PEP-II in the month <strong>of</strong> October, 2000]• The BABAR experiment at SLAC is able to detect B-mesondecays with good efficiency and good resolution. BABAR’’sdetectors are rapidly approaching design specifications.BABAR is on schedule to measure time-dependent CPviolationwithin a year ( ≈ 200 reconstructedwith very little background). [Approx. 140 reconstructed as<strong>of</strong> July, 2000.] Peak luminosity is already 10 33 cm -2 sec -1 .[It was greater than 3 x 10 33 cm -2 sec -1 as <strong>of</strong> October, 2000]• BABAR should be able to measure CP-violation in manydecay modes in the next few years, enough to test theStandard Model (thousands in B 0 → J/ψ K 0 s and thousands inadditional decay modes).• Understanding CP-violation in neutral B-meson decays mayprovide a better understanding <strong>of</strong> the origin <strong>of</strong> the the mostobvious matter-antimatter asymmetry in the universe -- thepredominance <strong>of</strong> matter over antimatter.<strong>Physics</strong> 841, January 2001 Michael D. Sokol<strong>of</strong>f 30