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CP VIOLATION (B-factories)

CP VIOLATION (B-factories). P. Pakhlov (ITEP). The major experiments to explore CP. K + → π + νν. K L → π 0 νν. Kaon system:. Indirect CP Violation. Direct CP Violation. Not useful to constrain CKM matrix parameters (too large hadronic uncertainties). Rare K decays to πνν.

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CP VIOLATION (B-factories)

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  1. CP VIOLATION (B-factories) P. Pakhlov (ITEP)

  2. The major experiments to explore CP K+→π+νν KL→π0νν Kaon system: Indirect CP Violation Direct CP Violation Not useful to constrain CKM matrix parameters (too large hadronic uncertainties) Rare K decays to πνν Theoretically very clean modes, but a nightmare for experimentalists: Br ~ 10–11, two neitrinos.

  3. The major experiments to explore CP • D-meson system? • Tiny CP violation, due to degenerated unitarity triangle and GIM/CKM suppression • Rare η decays? • UL for CP violation in strong interaction Difficult to observe the SM effect, test physics beyond the SM • EDM of n, p, nuclei? • The present ULs are much higher than the SM predictions (however, they are close to many models beyond SM) • B-meson system? • Large CP violation, • Many independent measurements, • Simple hadron dynamics, because of heavy b-quark • Hadronic uncertainties can be estimated or cancel in appropriate observables.

  4. B-mesons b q • What are B mesons? • B0 = d b • B+ = u b • JPC = 0 – + • τ = 1.5 × 10-12 s (ct  450 μm) • How are they produced? • e+e–  (4S)  B B is the cleanest process (large BB/other cross section; no extra particles) • Also at hadron machines: pp B + B + anything • How are they decay? • Usually to charm b  c, e.g. B  D • Much rarely to light quarks |bc|2|bu|2  100

  5. ARGUS and CLEO – pioneers in B-physics • Large mixing is observed by ARGUS in 1987 • Measurements of |Vcb|, |Vub|, |Vtd| and |Vts|: the UT has comparable sides and therefore angles are not 0 or 180º. • Large Br(B  J/KS) ~ 10–3 – very attractive final state • All these were good news for physicists: • Large mixing – easy to measure CP violation, as interference occurs before B decays • CP violation in B can be large • Convenient final state • The Nature is more favorable to us than we could expect

  6. Neutral meson mixing from CKM matrix Hamiltonian is non-hermitian due to the decay; Equal from CPT invariance It is just a numerical (complex) matrix 2×2: contributes to off-diagonal elements “Box diagram”

  7. Peculiarity of B-meson system Common CP final states for B0 and B0 Box diagram Thus, mass (width)-differences are approximated by where Contains weak phase

  8. CP violation in B mesons • No “KL” methods applicable! • Lifetime difference is tiny ((BH)- (BL)/(B) ~1%): no way to work with a beam of long lived B’s. • Semileptonic asymmetry also vanishes. • New ideas required! • Sanda & Carter (1980): consider a final state f common for both B0 and B0: • We arrive at decay rate asymmetry for the B0(t=0) and B0(t=0) because of interference of two amplitudes with different weak phases • The effect is large! Sanda & Carter estimated the asymmetry ~ 0.1 (compare with 0.002 CP violating effects in KL)

  9. × A + × A Remember: |A|=|A|, |p|=|q| × A + × A For B(t=0) = B0 Interfere B fCP with B  B  fCP tree diagram (A) Sanda, Bigi & Carter: For B(t=0) = B0 box + tree diagram Calculate t-dependent rates:

  10. B0 J/ KS b c b t d d KS J/ψ s c J/ψ KS c d d c t s d b V*td + Vtd taking into account Penguin diagram is difficult to estimate. But we are lucky: it’s amplitude is collinear to those of the tree one. Why?

  11. B0 ππ V*td b d d π– π– t Vub u u b π+ π+ u d d d u d • In this case the penguin diagram is not small and has different weak phase: • The indirect CP violation • ~ S sin(Δm t), where S≠ sin 2α, but sin(2α + some not-negligible phase). • There will be direct CP asymmetry ~ A cos(Δm t), How to take into account this? Wait for the next lecture.

  12. (4S) resonance • (4S)  B0B0 / B+B–~ 50:50 + no extra particles! • Coherent BB production in P-wave • B-energy is known (B momentum is very low ~ 340MeV • bb bound state • JPC=1– – (≡JPCof photon) • (e+e–(4S))  1nb • Good signal/background ~ 1:3 e+e–  (4S)  B B A very convenient process to study CP violation in B!

  13. How to measure CPV at e+e–collider? The source of B mesons is the (4S), which has JPC = 1– –. The (4S) decays to two bosons with JP = 0–. Quantum Mechanics (application of the Einstein-Rosen-Podosky Effect) tells us that for a C = –1 initial state (Υ(4S)) the rate asymmetry: N = number of events fCP = CP eigenstate (e.g. B0→J/ψKS) ffl = flavor state (particle or anti-particle) (e.g. B0→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 B’s produced at the (4S).

  14. Asymmetric e+e– collaider • CP violation asymmetry vanishes if integrated over Δt from – to +  kills good idea? • No! but requires new idea: • Need to reconstruct B-decay vertex: Impossible at symmetric B-factory – we don’t know B’s production point! • But possible if (4S) has a sizeable boost in lab frame • We can measure t-dependent asymmetry! Flavor-tag decay (B0 or B0?) Asymmetric energies e J/ e KS z t=0

  15. What‘s required to discover CPV? • Produce B mesons! Need accelerator • Produce a lot of B mesons! Need good accelerator • Produce a huge number of B mesons! Need accelerator with record luminosity • Effectively reconstruct B mesons • Correctly determine the flavor of second B • Precisely reconstruct the decay vertices very Need good detector with excellent PID and Vertex

  16. Two B-factories were approved in 1990

  17. e+e– Asymmetric B-factories Mt. Tsukuba PEP-II KEKB Belle ~1 km in diameter BaBar SLAC 3.1 x 9GeV 3.5 x 8 GeV stop Apr-2008 World highest luminosities L = 2.1 (KEKB) & 1.2 (PEP-II) × 10 34 cm–2 s–1 775(Belle) & 465(BaBar) millions BB-pairs Also tau- and charm- factories: 109ττ / cc pairs

  18. PEP-II at SLAC KEKB at KEK 9GeV (e–)  3.1GeV (e+) designed luminosity: 3.5  1033cm-2s-1 achieved 10.2  1033cm-2s-1 (3 times larger!) Belle 13 countries, 57 institutes, ~ 400 persons BaBar 8GeV (e–)  3.5GeV (e+) designed luminosity: 10.0  1033cm-2s-1 achieved 21.2  1033cm-2s-1 (2 times larger!) 11 countries, 80 institutes, ~ 600 persons

  19. History of 10 years running

  20. How to measure CPV at B-factories? • Reconstruct the decay of one of the B-mesons’s into a CP eigenstate • for example: B  J/ KS • Reconstruct the decay of the other B-meson to determine its flavor (“tag”) • Partial reconstruction is sufficient • Measure the distance (L) between the two B meson decays and convert to proper time • need to reconstruct the positions of both B decay vertices t = L/(c) • Correct for the wrong tag and not perfect vertex resolution • Extract CP asymmetry from the dN /d t distribution: dN/d t ~ e -|t| [1 ± cp sin2 sin(m t)]

  21. Step 1: Select BJ/KS • Reconstruct BCP long lived daughter: B  J/ KS  ℓℓ • Check the intermediate masses: M(ℓℓ) ~ M(J/); M() ~ M(KS) KS decay vertex • Check the mass and ENERGY (a big advantage of B-factories – we know B energy = Ebeam in the CM system) of J/KS combination

  22. B charmonium KS BJ/KS • Use many other decays B to charmonium (ηc, χc1, ψ’) + KS to increase statistics: • These final states have the same (odd) CP eigenvalue • They are equally theoretically clean (no penguin uncertainties) • They can be reconstructed with the similar high purity B-candidate CM energy B-candidate CM momentum

  23. B  J/KL Important to check if the asymmetry flip the sign for the opposite CP eigen value Difficult to detect KL: cτ ~ 15m; only nuclear interactions. Detect nuclear shower in iron: measure direction but not momentum. Use known J/KL = Ebeam energy to calculate momentum. Purity 97 % CP odd Purity 59 % CP even pK L information is poor → lower purity

  24. Step2: Flavor tagging In ~99% of B0 decays: B0 and B0 are distinguishable by their decay products Semileptonic decays X ℓ+ν X ℓ–ν B0 B0 Hadronic decays D X D X B0 B0 |Δt| (ps) All charged tracks (not associated with the reconstructed BCP) are from the second Btag in the event: ℓ, K and even  charge provides the information of Btag flavor.

  25. Step 3: Vertex reconstruction Use tracks from both BCP and Btag to find out z-coordinate of the two B-decay vertices.

  26. _ _ Take into account detector effects B0 tag B0 tag S = sin 2β = 0.65 A=0 B0 tag B0 tag Detector smeared True Need to solve inverse problem to get true value R : detector resolution w : wrong tag fraction (misidentification of flavor)  (1-2w) quality of flavor tagging They are well determined by using control sample D*lν, D(*)π etc…

  27. First Observation: CPV in B 2001 1137 events B0 tag _ B0 tag Events ( ) J/ψ K*0 Asymmetry 32M BB-pairs Asymmetry 31M BB-pairs [PRL 87,091801(2001)] [PRL 87,091802(2001)] sin 2β = 0.99 ± 0.14 ± 0.06 sin 2β = 0.59 ± 0.14 ± 0.05

  28. The recent Belle result B0 tag _ B0 tag J/ψ KS Nsig= 7482 J/ψ KL Nsig= 6512 sin 2β = 0.642 ± 0.031 ± 0.017 A = 0.018 ± 0.021 ± 0.014 Phys.Rev.Lett., 98, 031802(2007)

  29. B0 tag B0 tag _ _ B0 tag B0 tag Compare CP odd and even final states Asymmetry= –ξCP sin 2β sin(Δm Δt) sin 2β = + 0.643 ± 0.038 A = – 0.001 ± 0.028 sin 2β = + 0.641 ± 0.057 A = – 0.045 ± 0.033

  30. The recent BaBar result sin 2β = 0.687 ± 0.028 ± 0.012 A = 0.024 ± 0.020 ± 0.016 Phys.Rev. D79, 072009 (2009)

  31. There are two solutions for β How to avoid ambiguity? In some B decays the asymmetry is related to cos2β. It is difficult to achieve good accuracy, but even rough measurement allows to exclude the second solution.

  32. Other modes that measure sin2β b c D+ J/ψ d b c d D– π0 d c d c d d CP even

  33. We have done a great job: • CPV violation is observed in the system different from the neutral kaon system. The CPV large (~70%) compared to 0.2% in K0 decays. • The parameter of CPV is measured with great precision (~ 3%) and related to KM parameters without theoretical uncertainties. • The angle of UT triangle is measured (without ambiguity) with the precision better than 1º. Can we relax now? No, because we have not yet proved that KM anzatz works well. Yes, because the time for this lecture is almost over.

  34. The CM+KM test One way to test the Standard Model is to measure the 3 sides & 3 angles and check if the triangles closes! How to measure other UT angles? How to measure UT sides? VtdV*tb VudV*ub α experimentally easy γ β VcdV*cb sin2β: sin2α: sin2γ: hard

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