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Beauty Physics at LHCb

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  1. Beauty Physics at LHCb AndreyGolutvin Vladimir Shevchenko ITEP & CERN 11th INTERNATIONAL MOSCOW SCHOOL OF PHYSICS Session «Particle Physics» February 8-16, 2008

  2. Outline • ABC of LHC • Flavor physics – informal introduction • The CKM matrix and Unitarity Triangle • LHCb detector • Search for New Physics in CP violation • Physics of loops • Rare decays at LHCb • Conclusions 1 2

  3. Jet d’ Eau 140 m Mont Blanc, 4808 m LHCb experiment: 700physicists 50 institutes 15countries CERN LHCb ATLAS CMS ALICE

  4. ABC of LHC • Tonnel length - 27 kilometers • Depth below ground - between 50 and 175 meters • p-p beams, 2808 bunches, 1.15×10 particles/bunch • v = 0.99999998 c • Energy • Nominal luminosity<L> ~ 1034 cм-2сек -1 11

  5. Energy of a proton in the beam = 7 TeV = 10-6 J It is about kinetic energy of a flying mosquito: Question: why not to use mosquitos in particle physics? Answer: because NAvogadro = 6.0221023 (mol)-1 Energy of a mosquito is distributed among ~ 1022 nucleons. On the other hand, total energy stored in each beam is 2808 bunches  1011 protons/bunch  7 TeV/proton = 360 MJ It is explosive energy of ~ 100 kg TNT or kinetic energy of “Admiral Kuznetsov” cruiser traveling at 8 knots.

  6. Particle acceleration • Charged particles influenced by applied electric and magnetic fields according to the Lorentz force: F = q (E + v  B) = dp/dt E field → energy gain, B field → curvature • CERN has a wide variety of accelerators, some dating back to 1950s • LHC machine re-uses the tunnel excavated for previous accelerator (LEP)Others (PS/SPS) used to accelerate protons before injection into the LHC Neutrino beam,low energy beamsand p fixed-target beams all running in parallel with LHC

  7. The LHC Reality Originalidea From an article in the CERN Courier

  8. Dipole magnets used to deflect the particlesRadius r [m] = 3.33 p [GeV] / B [T] • For the LHC, the machine has to fit in the existing 27 km tunnel, about 2/3 of which isused for active dipole field → r~ 2800 mSo to reach p = 7 TeV requires B = 8.3 T • Beams focused using quadrupole magnetsBy alternating Focusing and Defocusing quadrupoles, can focus in both x and y views The LHC has 1232 dipoles 392 quadrupoles

  9. View of LHC tonnel

  10. Flavor physics: informal introduction

  11. The Standard Model Zoo SU(3)SU(2)U(1) [ g; W, Z; ] Masses come out of interactions in the Standard Model and these interactions conserve (or do not conserve…) particular symmetries. Mass hierarchies (from hep-ph/0603118). The heaviest fermion of a given type has unit mass.

  12. Invariance properties with respect to transformations have been always important in physics • momentum • angular momentum • energy • translations in • rotations in • time translations invariance conservation Gauge symmetry – invariance with respectto transformations in «internal» space In the SM this space has structure ofU(1) × SU(2) × SU(3)

  13. U(1) × SU(2) × SU(3) gluon photon Z, W And gravity is everywhere leptons quarks Quarks are unique probes of the whole «internal space», hence flavor physics has to deal with weak, electromagnetic and strong interactions altogether

  14. Besides continuous symmetries of prime importance in high energy physics are discrete transformations • С – charge conjugation • P – space inversion • Т – time reflection Experimental fact: strong and electromagnetic interactions in the SM are C, P, T, CP, CT, PT and CPTinvariant.

  15. Maximal symmetry is not so interesting… Beauty slightly broken symmetry

  16. The breaking should not be too strong, however…

  17. СРТ theorem: Antiparticles and their interactions are indistinguishable from particles moving along the same world-lines but in opposite directions in 3+1 dimensional space-time. In particular, the mass of any particle is strictly equal to the mass of its antiparticle (experimentally checked in 1 part to 1018 in K-meson studies). The SM strictly conserves CPT. There are no however any theoretical reason why C, P and T should conserve separately. Often in physics if something can happen – it does.

  18. Weak interactions violate P-parity T.D.Lee, C.N.Yang, 1956 C.S.Wu, 1957

  19. L.D.Landau, 1959: hypothesis of combined CP-parity conservation J.Cronin, V.Fitch, 1964: CP-violation discovery in neutral K-mesons decays.

  20. CP violation In the world of elementary particles: (CPLEAR 1999) neutral kaon decay time distribution anti-neutral kaon decay time distribution

  21. Later CP-violation has been beautifully measured by experimentsBaBar and BELLE at the B factories These are machines (in the US and Japan) running on the (4S) resonance: e+e-(4S) B0B0 or B+B- • The CP asymmetry A(t) = G(B0 J/yKS) -G(B0 J/yKS) G(B0 J/yKS) +G(B0 J/yKS) A(t) = -sin2b sinDmt in the Standard Model • BABAR+BELLE measuresin2b = 0.674 ± 0.026 • This can be compared withthe indirect measurementfrom other constraints on theUnitarity Triangle

  22. M. Kobayashi, T.Maskawa, 1974: theoretical mechanism for CP-violation in the SM Idea: nontrivial superposition of non-interacting particles forms flavor eigenstate that interacts weakly In other words: it is impossible to diagonalize simultaneously the mass term and charged currents interaction term:

  23. It is easy to show that arbitrary complex unitaryN×N matrix can be parameterized by N(N-1)/2 generalized Euler angles and (N-1)(N-2)/2 complex phases. ForN<3 the matrix can always be rotated to an equivalent one which is real. But not for N=3. In other words, there exist 3×3 unitary matrices which cannot be made real whatever phases quark fields are chosen to have.

  24. Baryogenesis • Big Bang (~ 14 billion years ago) → matter and antimatter equally produced; followed by annihilation → nbaryon/ng ~ 10-10Why didn’t all the matter annihilate (luckily for us)? • No evidence found for an “antimatter world” elsewhere in the Universe • One of the requirements to produce an asymmetric final state (our world) from a symmetric matter/antimatter initial state (the Big Bang)is that CP symmetry must violated [Sakharov, 1967] • CP is violated in the Standard Model, through the weak mixing of quarksFor CP violation to occur there must be at least 3 generations of quarksSo problem of baryogenesis may be connected to why three generations exist, even though all normal matter is made up from the first (u, d, e, e) • However, the CP violation in the SM is not sufficient for baryogenesisOther sources of CP violation expected → good field to search for new physics

  25. CKM matrix can be parameterized by four parameters in many different ways. The so called «Wolfenstein parametrization» is based on expansion in powers of

  26. It is convenient to discuss the properties of CKM matrix in parametrization-invariant terms. Such invariant are absolute values of the matrix elements and «angles» between them If any of these angles is different from zero, it means that there is a complex phase in CKM matrix which cannot be rotated away. This violates CP. «Jarlskog invariant»

  27. Off-diagonal unitarity conditions can be represented as triangles on complex plane. The Unitarity triangle: All 6 unitarity triangles have equal area but only two of them are not degenerate. B-mesons decays are very sensitive to СР !   

  28. The Unitarity triangle : Bdmixing phase : Bsmixing phase : weak decay phase Im    0 1 Re Im   + Precise determination of parameters through B-decays study.  0 Re

  29. UT as a standard approach to test the consistency of SM Mean values of angles and sides of UT are consistent with SM predictions • Accuracy of sides is limited by theory: • Extraction of |Vub| • Lattice calculation of Accuracy of angles is limited by experiment: • = ±13° • b = ± 1° • = ± 25°

  30. Standard method to search for New Physics Define the apex of UT using at least 2 independent quantities out of 2 sides: and 3 angles: ,  and  Extract quantities Rb and  from the tree-mediated processes, that are expected to be unaffected by NP, and compare computed values for with direct measurements in the processes involving loop graphs. Interpret the difference as a signal of NP

  31. W– q1 b q b u, c, t q2 W− d, s mbγL+mqγR W − W − d (s) d (s) b u,c,t b u, c, t l+ q g Z, γ l− q Topologies in B decays Trees Penguins Boxes V*ib Viq q u,c,t b W− W+ u, c, t q b Viq V*ib

  32. Standard method to search for New Physics Define the apex of UT using at least 2 independent quantities out of 2 sides: and 3 angles: ,  and  Extract quantities Rb and  from the tree-mediated processes, that are expected to be unaffected by NP, and compare computed values for with direct measurements in the processes involving loop graphs. Interpret the difference as a signal of NP

  33. The sensitivity of standard approach is limited due to: - Geometry of UT (UT is almost rectangular) Comparison of precisely measured  with  is not meaningful due to error propagation: 3° window in  corresponds to (245)° window in 

  34. Precision comparison of the angle  and side Rt is very meaningful !!! However in many NP scenarios, in particular with MFV, short-distance contributions are cancelled out in the ratio of Md/Ms. So the length of the Rt side may happen to be not sensitive to NP Precision measurement of  will effectively constrain Rt and thus calibrate the lattice calculation of the parameter

  35. Complementary Strategy Compare observables and UT angles: ,  and  measured in different topologies: In trees: Theoretical uncertainty in Vub extraction Set of observables for (at the moment not theoretically clean): Theoretical input: improved precision of lattice calculations for fB , BB and B,,K* formfactors Experimental input: precision measurement of BR(BK*, )

  36. Search for NP comparing observables measured in tree and loop topologies (peng+tree) in B,, (peng+box) in B Ks (peng+box) in Bs  (tree+box) in B J/ Ks (tree) in many channels (tree+box) in Bs J/  New heavy particles, which may contribute to d- and s- penguins, could lead to some phase shifts in all three angles: (NP) = (peng+tree) - (tree) (NP) = (BKs) - (BJ/Ks) ≠ 0 (NP) = (Bs) - (BsJ/)

  37. Search for NP comparing observables measured in tree and loop topologies • Contribution of NP to processes mediated by loops • (present status) • to boxes: •  vs |Vub / Vcb | is limited by theory (~10% precision in |Vub|) (d-box) •  not measured with any accuracy (s-box) • to penguins: • ((NP)) ~ 30° (d-penguin) • ((NP)) ~8° (s-penguin) • ((NP)) not measured (s-penguin) • PS (NP) =  (NP) • (NP) measured in B and B decays may differ depending • on penguin contribution to  and  final states

  38. LHCb is aiming at search for New Physics in CP-violation and Rare Decays

  39. Large Hadron Collider - LHCb • Bunch crossing frequency: ~ 40 MHz • Number of reactions in unit of time: • since pp inelastic ~ 80 mbarn • for nominal LHC luminosity • N ~ 8108 • For LHCb L ~ 2 × 1032 cm-2s-1 • (local defocusing of the beam) • → multi-body interactions are • subdominant Inelastic pp reactions

  40. - b b bb angular distribution PT of B-hadron 100μb 230μb Pythia η of B-hadron b b • vertices and momenta reconstruction • effective particle identification(π, К, μ, е, γ) • triggers

  41. View of the LHCb cavern Calorimeters Magnet Muon detector RICH-2 OT RICH-1 VELO It’s full! Installation of major structures is essentially complete

  42. LHCb in its cavern Offset interaction point (to make best use of existing cavern) Shielding wall(against radiation) Electronics + CPU farm Detectors can be moved away from beam-line for access

  43. LHCb detector ~ 300 mrad p p 10 mrad  Forward spectrometer (running in pp collider mode)Inner acceptance 10 mrad from conical beryllium beam pipe

  44. LHCb detector  Vertex locator around the interaction region Silicon strip detector with ~ 30 mm impact-parameter resolution

  45. Vertex detector • Vertex detector has silicon microstrips with rf geometryapproaches to 8 mm from beam (inside complex secondary vacuum system) • Gives excellent proper time resolution of ~ 40 fs (important for Bs decays) Beam Vertex detector information is used in the trigger

  46. LHCb detector  Tracking system and dipole magnet to measure angles and momenta Dp/p ~ 0.4 %, mass resolution ~ 14 MeV (for Bs DsK)

  47. LHCb detector  Two RICH detectors for charged hadron identification

  48. LHCb detector e h  Calorimeter system to identify electrons, hadrons and neutrals. Important for the first level of the trigger

  49. LHCb detector m  Muon system to identify muons, also used in first level of trigger

  50. S: LHC prospects BsJ/ is the Bs counterpart of B0J/ KS • In SM S = - 2arg(Vts) = - 22 ~ - 0.04 • Sensitive to New Physics effects in the Bs-Bs system if NP in mixing S = S(SM) + S(NP) • 2 CP-even, 1 CP-odd amplitudes, angular analysis needed to separate, then fit to S, S, CP-odd fraction • LHCb yield in 2 fb-1 131k, B/S = 0.12 LHCb 0.021 0.021 ATLAS will reach s(s) ~ 0.08 (10/fb, ms=20/ps, 90k J/ evts)