Recent results from rare decays

Transcription

1 Recent results from rare decays Jeroen van Tilburg (Physikalisches Institut Heidelberg) Don t worry about the number of slides: Only half of them is new Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 1/38

2 Recap of last week What we have learned last week: Neutral mesons (K,D,B d,b s ) mix and oscillate. Beautiful example of (fast) B s oscillations: m s =17.77 ± 0.10(stat) ± 0.07 (sys) ps 1 Requires good proper time resolution and tagging of B flavour at production. There are three types of CP violation: CPV in mixing Small in SM (<1%), only observed so far in kaon decays CPV in decay Difficult to extract weak phases due to unknown strong phases and T/P ratio. CPV in interference between mixing and decay Large effects and clean determination of weak angles possible. Example: LHCb s measurement of sin(2β s ) Measured value: φ s =-0.03 ± 0.16(stat) ± 0.07 (sys) SM value: φ s = No large phase from new physics Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 2/38

3 Probes for New Physics searches The aim of heavy flavour physics is to study B and D decays and to look for anomalous effects beyond the Standard Model. Requirements to look for New Physics effects: Should not be ruled out by existing measurements. Prediction from SM should be well known. These requirements are fulfilled for these processes: CP violation Rare decays CP violation and rare decays of B and D hadrons are the main focus of LHCb. Today: Rare decays Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 3/38

4 Introduction to rare decays Flavour changing neutral currents (FCNC) forbidden in SM at tree level. Suppressed at higher order due to GIM mechanism FCNC decays good testing ground for SM. Corresponding decays are always rare (B-mesons < 10-5 ) New particles can appear as virtual particles in box and penguin diagrams. Indirect searches have a high sensitivity to effects from new particles. Good testing ground: b s transitions. B s oscillations box diagram B s φ γ B d,s µ + µ Penguin diagrams B d K * µ + µ b s s b b s b s b s Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 4/38

5 Example: Box diagrams (recap) New particles could enter in the B s box diagram `?? Could affect both amplitude and phase: Δm s = Δm s SM + Δm s NP φ s =φ s SM + φ s NP LHCb s s measurements: Δm s = ± 0.041(stat) ± (sys) ps 1 φ s = ± 0.16(stat) ± 0.07 (sys) Preliminary SM: Δm s =17.3 ± 2.6 ps 1 SM: φ s = ± No hints (yet) for new physics in box diagrams, but still some room left. Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 5/38

6 But there are penguins on the horizon! Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 6/38

7 Quo$ng John Ellis (Wikipedia): The story about penguins Nucl. Phys. B131: Don t try to beat Melissa Franklin at darts Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 7/38

8 Rare decays Just as in the box diagram, new particles can easily enter in the penguin diagram. We can measure branching ratio, polarization, angular distributions. compare with theoretical prediction from SM (if deviation: NP) No problem to calculate the SM Feynman diagrams for the individual quarks, so what is the problem? We don t measure the individual quarks: we measure only hadrons. cannot use perturbation theory to calculate the (soft) QCD effects (hadronic effects) Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 8/38

9 OPE Theoretical approach: Operator Product Expansion + renormalization group equations Basic idea: Energy scale of weak decays is low compared to mass of W (propagator). Absorb W exchange in effective Fermi theory (expansion of W propagator) Full theory (W exchange) Fermi theory Effective fourfermion operator Allows to separate low-energy effects (non-pertubative QCD) and high-energy effects (pertubative QCD + weak interactions + new physics). Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 9/38

10 Effective Hamiltonian CKM elements: for b s : V ts * V tb Wilson coefficients (high energy) Low-energy operators Renormalization scale (µ) (Unphysical) border between the two regimes for B decays: a few GeV (around b-quark mass) Energy scales: Wilson coefficients Operators: decay constants, form factors (large theory uncertainties) Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 10/38

11 Ordering the diagrams Current-current operators: Q 1, Q 2 QCD penguins: Q 3, Q 4, Q 5, Q 6 small Electroweak penguins: Q 7, Q 8, Q 9, Q 10 Electro/Chromo-magnetic penguins: Q 7γ, Q 8 g FCNC operators Box operators: Q ΔS=2, Q ΔB=2 Semi-leptonic operators: Q 9V, Q 10A Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 11/38

12 New physics in b s New physics could show up as: Modified Wilson coefficients new particles in the penguin loop New operators e.g. right-handed currents Three interesting channels: SM operators BR (SM) BR LHCb B s φ γ B d K * µ + µ Q 7γ Q 7 γ,q 9,Q 10 Large theory uncertainties O(20%) (5.7±2.0)x10-5 (1.05±0.15)x10-6 γ polarisation Angular distributions B s µ + µ Q 10 (3.2±0.2)x10-9 < 1.1x10-9 (95%) BR Focus of today Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 12/38

13 Search for B d,s µ + µ Expected BRs Numbers not up-to-date Increasing BR Why are the B d decays suppressed relative to B s? Top quark is dominating in the loop. Cabibbo suppression by factor V td /V ts 2 Why is the BR for taus so much larger than for electrons? Decays are helicity suppressed. Spin of B d,s is zero. One lepton needs helicity flip. Why is the search for B d,s µ + µ most popular? Muons are easiest to reconstruct (taus always give a neutrino) µ R µ + R B µ d,s (s=0) L µ + L Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 13/38

14 Search for B d,s µ + µ The decay B d,s µ + µ provides sensitive probe for New Physics SM diagrams: Only semi-leptonic operator Q 10 (C S and C P are suppressed): b µ + C 10 d,s µ New physics could modify Wilson coefficients C 10, C S, C P (or introduce new operators). NP example: MSSM Operators Q S, Q P will enhance BR: BR( B d, s # µ + µ " ) ~ tan M 6 4 A! Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 14/38

15 Search for B d,s µ + µ Recent excitement from CDF measurement (2011): BR(B s µ + µ +11 ) = (18 )x10 9 arxiv: Compare BR(B s µ + µ ) SM = (3.2 ± 0.2)x10 9 Slight excess of events in two bins Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 15/38

16 Search for B d,s µ + µ Back to the analysis in LHCb: Evaluate signal/background in a 2D-space of Invariant mass m µµ MVA classifier BDT combining kinematic and geometrical variables [LHCb-CONF ] Next: Look at invariant mass spectrum in each of the 4 bins Least sensitive bin Most sensitive bin Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 16/38

17 Search for B d,s µ + µ Expected mass resolution obtained from interpolation of dimuon resonances Procedure verified with B hh events ϒ(1S) ϒ(3S) ϒ(2S) [LHCb-CONF ] ψ(2s) J/ψ Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 17/38

18 Search for B d,s µ + µ M µµ for signal region in 4 bins of BDT background from bb µµx and misidentified B h + h Expect ~ 1 event in each bin from SM. [LHCb-CONF ] Small excess (2 events) in most sensitive bin, compatible with SM. Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 18/38

19 Search for B d,s µ + µ [LHCb-CONF ] Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 19/38

20 Search for B d,s µ + µ Normalization B BR( B + " " 0 q + " K #! The final branching ratio can be calculated as: % f 0 +! +! cal cal Bq " µ µ " µ µ ) = BR cal # # = $ cal # N 0 +! Bq " µ µ % sig f 0 N B cal q Three complementary normalization channels with very different systematics: B B s 0 + J /% ( µ µ +! J /% ( µ µ ) $ ( K! ) K + + K! ) N [LHCb-CONF ] Values for α very compatible Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 20/38

21 Search for B d,s µ + µ No significant excess observed in 0.3 fb -1 Upper limits: arxiv: BR(B s µ + µ ) < 1.5 x 10 8 (95% CL) BR(B d µ + µ ) < 5.2 x 10 9 (95% CL) CMS also set a limit this Summer with 1.1 fb -1 BR(B s µ + µ ) < 1.9 x 10 8 (95% CL) arxiv: LHCb + CMS analyses combined (preliminary) BR(B s µ + µ ) < 1.1 x 10 8 (95% CL) This is ~ 3.4 SM value Most probable value ~ Excess over SM not confirmed. Future prospects LHCb-CONF arxiv: Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 21/38

22 Angular distributions in B d µ + µ K * B d µ + µ K * rare decay in the SM. BR (B d l + l K * ) ~ 1.0 x 10 6 Example of SM diagram: W exchange Example of NP diagram: Charged Higgs exchange W ± is spin 1 particle, while H ± is spin 0. Modifies the angular distributions of the muons. Generally, angular distributions contain a lot of information. Sensitive to SUSY, graviton exchanges, extra dimensions Many observables which probe helicity structure of NP Most popular A FB (see next slide) Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 22/38

23 Angular distributions in B d µ + µ K * A FB : µ forward-backward asymmetry Definition: Idea: Measure A FB as a function of invariant mass of muon pair (q 2 ). Zero crossing point of A FB (q 2 ) well predicted in SM Hadronic uncertainties are minimized Measures ratio Wilson coefficients C 9 /C 7. C 7γ constrained by B s φ γ but not its sign. Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 23/38

24 Angular distributions in B d µ + µ K * Previous results from CDF & B-factories show intriguing behaviour at low q 2 : however, precision is limited. [arxiv: ] SM C 7 =-C 7 SM Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 24/38

25 Angular distributions in B d µ + µ K * B d mass window Event selection 309 pb -1 in 2011 ψ(2s) veto J/ψ veto 302 signal events [LHCb-CONF ] Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 25/38

26 Angular distributions in B d µ + µ K * Already effective in constraining NP arxiv: Data consistent with SM predictions at present sensitivity and indicate that A FB is changing sign as predicted by the SM Next steps Determine zero-crossing point in A FB (q 2 ) Include full 2011 data set. With > 2 fb -1 do full angular analysis (most recent CDF result also has negative first bin: arxiv: ) Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 26/38

27 Cartoon Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 27/38

28 Concluding slides What is the minimum you should take home from these 4 lectures? Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 28/38

29 LHCb detector RICH1+2 Muon LHCb made for Heavy Flavour physics Good vertex resolution Time-dependent measurements. Suppress background from prompt decays. Good particle identification Important for trigger, flavour tagging Suppress background. Good momentum resolution Mass resolution of heavy flavours. Suppress background. Calo Velo Tracking stations Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 29/38

30 The power of indirect searches Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 30/38

31 C, P and CP in weak interactions The weak interaction violates C and P maximally. But CP was thought to be a good symmetry, until 1964 when it was experimentally found to be broken. Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 31/38

32 Where did we see that before? Color P C C anti-color P Left right Escher s (Dutch artist) impression of C, P and CP violation. Where is the CP violation? Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 32/38

33 CP violation in the weak interaction CP violation requires complex matrix elements. Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 33/38

34 Mixing of neutral mesons x y B 0 meson # m $ " #" $ 2"! 0.7! 0 B s meson # m x $ " #" y $ 2"! 26! 0.1 The 4 different neutral meson systems have very different mixing properties. B s system: very fast mixing K 0 meson $ m x % # $# y % 2# " 1 "! 1 D 0 meson # m x $ " #" y $ 2"! 0.01! 0.01 Kaon system: large decay time difference. Charm system: very slow mixing Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 34/38

35 Overview: Types of CP violation Three types of CP viola$on (always two amplitudes!): 1. CP viola$on in mixing ( indirect CP viola$on): q p! 1 Note that in the SM all these effects Af are caused by a single complex parameter δ in the CKM matrix! 2. CP viola$on in decay ( direct CP viola$on):! A f Tree Penguin 3. CP viola$on in the interference: arg! + arg! " 0 f f Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 35/38

36 FCNC penguin decays In FCNC decays new particles can enter at same level as SM particles. Sensitive probes for new physics. Two examples of quantities which can be well-predicted in SM: BR(B s µ + µ ) Zero crossing point of A FB (q 2 ) Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 36/38

37 It s s all about imaginary numbers Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 37/38

38 Conclusion LHCb has just collected 1.1 fb -1 of data. Waiting for you to be analysed! Advanced topics in Particle Physics: LHC physics, 2011 Jeroen van Tilburg 38/38