B - Physics - University of Cincinnati

The Search for Time-DependentCP-Asymmetries in the NeutralB-Meson SystemMichael D. SokoloffPhysics DepartmentUniversity of CincinnatiPresented at theXX Encontro Nacional de Fisica de Particulas e Campos

The Nature of Particle Physics• As particle physicists, we study the fundamental constituents ofmatter and their interactions.• Our understanding of these issues is built upon certainfundamental principles– The laws of physics are the same everywhere– The laws of physics are the same at all times– The laws of physics are the same in all inertial referenceframes (the special theory of relativity)– The laws of physics should describe how the wave function ofa system evolves in time (quantum mechanics)• These principles do not tell us what types of fundamentalconstituents exist, or how they interact, but they restrict the typesof theories that are allowed.• In the past 30 years, we have developed a Standard Model ofparticle physics to describe the electromagnetic, weak nuclear,and strong nuclear interactions of constituents in terms ofquantum field theories.Physics 841, January 2001 Michael D. Sokoloff 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 of its decay products (its daughterparticles), albeit imperfectly.• The energy of the parent is exactly the sum of the energies of itsdaughters energies. Similarly, each component of the parent’smomentum is the sum of the corresponding components of thedaughters’ momenta.From the reconstructedenergy and momentum ofthe candidate parent, wecan calculate its invariantmassPhysics 841, January 2001 Michael D. Sokoloff 3

Classical Field Theory (E&M)Physics 841, January 2001 Michael D. Sokoloff 4

Fields and QuantaPhysics 841, January 2001 Michael D. Sokoloff 5

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Baryons and Mesons• Quarks are never observed as free particles.– baryons consist of three quarks, each with a differentcolor (strong nuclear) chargeproton = uudneutron = udd– mesons consist of 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 of strong interaction creates anattractive force which extends a short distance to bindnucleii together.Physics 841, January 2001 Michael D. Sokoloff 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 of the same generation. The StandardModel explains coupling between quark generations in termsof 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 ofsome particles and their antiparticles.Physics 841, January 2001 Michael D. Sokoloff 8

Particle-Antiparticle Mixing0B0BA second order weak charged current process, oftenreferred 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.Physics 841, January 2001 Michael D. Sokoloff 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 rates0of the and0B B to this final state, and that the asymmetrywill vary sinusoidally.B 0B 0B 0fCP≠B 0f CPPhysics 841, January 2001 Michael D. Sokoloff 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% of all hadronic events are BB.Physics 841, January 2001 Michael D. Sokoloff 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 collisionsPhysics 841, January 2001 Michael D. Sokoloff 19

The BABAR DetectorMeasure the trajectories and momenta of charged particlestraveling in a magnetic field.Measure the energy of photons and electrons.Identify muons which traverse large amounts of materialwithout interacting.Measure the speeds of 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’sPhysics 841, January 2001 Michael D. Sokoloff 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:Physics 841, January 2001 Michael D. Sokoloff 21

The Drift Chamber• 40 layers, alternating axial and stereo superlayers• Low density: 80%He, 20% Isobutane, Al wires• dE/dx resolution of 7%•

Particle Identification Using Ionization• Ionization (dE/dx) is measured in each of 40 layers in thedrift chamber.• A truncated mean value is used as the best estimate of theaverage ionization rate.• Recent improvements bring the average fractionalresolution for Bhabhas close to the design value of 7%.– improved feature extraction from the digitized signals;– improved understanding of the gas gain;– Software corrections for bias due to using truncatedmean, as a function of track dip angle.Physics 841, January 2001 Michael D. Sokoloff 23

The DIRCThe Detector of InternallyReflected CherenkovLight is used to identifycharged particles. TheCherenkov angle dependson the speed of 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 statusPhysics 841, January 2001 Michael D. Sokoloff 24

Electromagnetic Calorimeter• 7000 CsI crystals in barrel and forward endcap• Reconstruct photons above 20 MeV• Energy resolution of σ(E)/E =1% / 4 E(GeV) ⊕1.2%Physics 841, January 2001 Michael D. Sokoloff 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 regionPhysics 841, January 2001 Michael D. Sokoloff 26

A Sample EventB 0 → J/ψ K 0 s with K 0 s → π − π +Physics 841, January 2001 Michael D. Sokoloff 27

Summer 2000 ResultsB 0 → J/ψ K 0 s with K 0 s → π + πFlavor-tagged sample ofB 0 → J/ψ K 0 s used in sin2βanalysisCombined with analogoussample ofB 0 → J/ψ(2S) K 0 s for theOsaka result:sin2β = +0.12 ± 0.37 ± 0.09Physics 841, January 2001 Michael D. Sokoloff 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)Physics 841, January 2001 Michael D. Sokoloff 29

Summary and Conclusions[with December, 2000 updates]• The Standard Model of 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 of 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 asof 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 of 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 of the origin of the the mostobvious matter-antimatter asymmetry in the universe -- thepredominance of matter over antimatter.Physics 841, January 2001 Michael D. Sokoloff 30