Rare Decays at LHCb Mitesh Patel CERN CERN

Rare Decays at LHCb Mitesh Patel (CERN) CERN Theory Institute LHC b Focus week Wednesday 28 th May 2008

Introduction • New physics can give effective Hamiltonian, H, new operators Oi or modified Wilson coefficients Ci • Rare B decays give a number of opportunities to constrain these contributions: From G. Hiller [hep-ph/0308180] 2

Bs→mm 3

Bs→mm • Bs→mm helicity suppressed • Well predicted in SM: – BR(Bs→mm) = (3. 35± 0. 32)× 10 -9 [1] NUHM • Sensitive to (pseudo) scalar operators – MSSM: tan 6 β/MA 4 enhancement – NUHM: favours large tan b (~30) • Current limits from Tevatron: – CDF BR < 4. 7× 10 -8 – D 0 BR < 7. 5× 10 -8 90% CL [2] [3] • J. Ellis et al. , ar. Xiv: 0709. 0098 v 1 [hep-ph] [1] hep-ph/06040507 v 5 [2] ar. Xiv: 0712. 1708 v 1 [hep-ex] [3] ar. Xiv: 0705. 300 v 1 [hep-ex] 4

Bs→mm at LHCb • Searching for Bs→mm with LHCb: – Large prodn x-secn for b’s at high h, low p. T → At L=2 x 1032 cm 2 s-1, 1012 bb pairs in 107 s – Trigger has m p. T threshold >~1 Ge. V → ~1. 5 k. Hz inclusive μ, di-μ – Small event size → Can write this rate out, open analysis – can retain max. efficiency – High precision magnetic spectrometer → Bs mass resolution ~20 Me. V (c. f. CMS ~40 Me. V, ATLAS ~80 Me. V) – Vertex detector very close to LHC beams → Excellent vtx, impact parameter resolution 100 mb 230 mb Bs μμ Bs KK • Mass (Me. V) 5

Bs→mm at LHCb • Events classified according to geometrical likelihood, PID and Bs invariant mass: Invariant Mass – Geometric likelihood: • Bs Lifetime • m SIPS: Mu Impact Parameter Significance • DOCA: Distance of closest approach • Bs IP: Bs impact parameter to prim. vtx • Isolation: No. of good secondary vtx that can be made with m candidates – PID: • Calibration muons (MIPs in calorimeter, J/y muons) – Bs Invariant Mass PID Geometry Arbitrary normalisation Red: signal Blue: bb inc. Black: b μ Green: Bc+ J/Ψμν 6

Bs→mm at LHCb • Analysis: – Signal description: B→hh (~200 k events/2 fb-1) – Background estimation from mass sidebands – Normalisation: B+→J/y. K+ (2 M events/2 fb-1) – Dominant uncertainty on BR from relative Bs, B+ hadronisation fraction ~13% Red: signal Blue: bb inc. Black: b μ Green: Bc+ J/Ψμν 7

Bs→mm at LHCb • Background: – Dominated by b→μ, b→c→μ also contributes – Mis-id (B→hh), insignificant – Dominant exclusive bkgrd Bc+→J/Ymn, tiny cf. b→μ, b→μ – Drell-yan insignificant at these masses Background Signal • Total efficiency for all geometric likelihood values ~10% • Taking events with GL>0. 5, assuming SM BR, with 2 fb-1: Geometric likelihood – Signal ~30 events – Bkgrd ~83 events 8

BR (x 10– 9) Bs→mm at LHCb 90% CL limit on BR (only bkgrd is observed) Uncertainty in background prediction 10 -7 2 x 10 -8 (~0. 05 fb-1) Expected final CDF+D 0 limit SM prediction 5 x 10 -9 (~ 0. 4 fb-1) • J. Ellis et al. , ar. Xiv: 0709. 0098 v 1 [hep-ph] Integrated luminosity (fb– 1) Exclusion: 0. 1 fb– 1 BR < 10 -8 0. 5 fb– 1 < SM With SM Branching ratio 2 fb– 1 3 evidence 6 fb– 1 5 observation With 0. 1 fb-1 can measure BR 9 (15)× 10 -9 at 3 (5) With 0. 5 fb-1 can measure BR 5 (9)× 10 -9 at 3 (5) 9

Bd→K*mm 10

Bd→K*mm • BR measured at B-factories, in agreement with SM: BR(Bd→K*μμ)= (1. 22+0. 38 -0. 32)× 10 -6 [1] • Decay described by three angles (ql, f, q. K*) • Angular distributions as function of q 2 gives sensitivity to NP contributions • Forward-backward asymmetry AFB in ql angle has received particular theoretical attention – predicted in a number of different models [1] PDG 2006 Ali et al, Phys. Rev. D 61: 074024, 2000 11

• B-factories each collected O(100) signal events AFB Bd→K*mm at LHCb BABAR ’ 08 • CDF has ~35 signal events • With L=2 x 1032 cm 2 s-1, LHCb will observe this no. of events with ~0. 25 fb-1 integrated luminosity Mmm 2 (Ge. V 2) AFB • Given projected total datasets these experiments, a total of <1000 events might be observed at all facilities SM BELLE ’ 06 Wrong sign C 9, C 10 excluded ? Mmm 2 (Ge. V 2) 12

• AFB Bd→K*mm at LHCb Signal selection: An example 0. 1 fb-1 experiment – Total selection efficiency ~1% → 7200 signal events /2 fb-1 (~50% below m. J/Y) • Full AFB spectrum of interest but zero-crossing point often computed: – s 0 SM = 4. 39+0. 38 -0. 35 Ge. V 2 [1] (older value used in model →) An example 0. 5 fb-1 experiment Simple linear fit suggests precision: 0. 5 fb-1 σ(s 0) • Mmm 2 (Ge. V 2) AFB • 2 fb-1 10 fb-1 0. 8 Ge. V 2 0. 5 Ge. V 2 0. 3 Ge. V 2 Looking at extended beyond linear fit Mmm 2 (Ge. V 2) [1] ar. Xiv: 0106067 v 2 [hep-ph] 13

Background: – b→μ, b→μ dominant contribution, symmetric distribution in ql – scales AFB observed – b→μ, b→c→μ significant contribution, asymmetric ql distribution – effect on AFB depends on ql shape – As for Bs→μμ, don’t observe any significant background from m mis-id – Non-resonant Kpmm events not yet observed – Bkgrd rejection dependent on Bd mass resoln: (m. Bd) ~15 Me. V (c. f. ATLAS 50 Me. V) – B/S ~0. 5 • Analysis issues: – In order to correct AFB value measured, require knowledge relative angular efficiency: • p. T cuts on muons (in e. g. trigger), remove events with ql ~0, p • muon reconstruction requirements distort momentum spectrum bkgrd from: b→m ql / rad No. of Events • No. of Events Bd→K*mm at LHCb bkgrd from: b→μ, b→c→μ ql / rad 14

Bd→K*mm at LHCb 2 fb-1 • Decays contain much more information than ql, AFB distributions FL • Fitting projections of ql, f, q. K* angular distributions: Mmm (Ge. V 2) 2 fb-1 AT(2) → fraction of longitudinal polarization, FL, and transverse asymmetry AT 2 Kruger & Matias, Phys. Rev. D 71: 094009, 2500 Mmm (Ge. V 2) 15

Bd→K*mm at LHCb • Full angular fit also under investigation: AFB (SM, 2. 2 – 4. 1 Ge. V 2) Full ang. fit (AFB) = 0. 02 • Parameterised in terms of transversity amplitudes – A 0 L, R, A┴L, R, A║L, R, 6 complex numbers • Correlations give access to helicities AFB (SM, 2. 2 – 4. 1 Ge. V 2) Fit of proj. angles (AFB) = 0. 04 – Probe chiral structure NP operators • Once have enough events in each q 2 bin for fit to converge → better precision on AFB, FL, and AT 2 (but require full acceptance correction) • Can form any observable once have fitted all amplitudes – new theoretically clean observables with good NP sensitivity sought! 16

Bs→fg 17

Bd→K*γ, Bs→fg • BR(Bd→Xsg) measured by B-factories, rate in agreement with SM • B-factories measured CP asymmetry ACP in Bd→K*(Ksπ0)γ : In SM, C=0 (direct CPV) S=sin 2 y sin f AΔ=sin 2 y cos f where y fraction of “wrong” polarization → C=-0. 03± 0. 14, S=-0. 19± 0. 23 [HFAG] • LHCb can perform analogous measurement in Bs→fg – As DGs≠ 0, Bs→fg decay probes AΔ as well as C and S 18

Bs→fg at LHCb • Signal Selection: – ET > 2. 7 Ge. V – Mass resoln ~90 Me. V – Proper time resoln ~80 fs (not critical for measuring AD) – Total Efficiency ~0. 3% – Yield: • Bs→fg – 11 k / 2 fb-1 with B/S<0. 55 Equivalent of 13 mins of simulated BB events – already see a peak! True K*g signal events • Bd→K*(K+p-)g – 68 k / 2 fb-1 with B/S~0. 60 Combinatorial background 19

Bs→fg at LHCb • Analysis issues: – Acceptance function a(t) (Bd→K*g) – (t) as function of topology • Precision on ACP parameters with Bs→fg decays from 0. 5 fb-1 – (AD) = 0. 3 (no tagging required) – (S, C) = 0. 2 (require tagging) • With 2 fb-1: – (AD) = 0. 22 – (S, C) = 0. 11 20

Conclusions • Rare B decays in LHCb will find NP or constrain extensions of SM • With the first data: – Bs→μμ excluded at SM value with 0. 5 fb-1 – Bd→K*μμ measure AFB spectrum, σ(s 0) ~0. 8 Ge. V 2 with 0. 5 fb-1 • With 2 fb-1 integrated luminosity: – Bs→μμ evidence if SM BR (observation with 6 fb-1 data) – Bd→K*μμ measure AFB spectrum, σ(s 0) ~0. 5 Ge. V 2, new observables with more complex fits (A(2)T, …) – Bs →fγ CP asymmetry ACP → fraction of “wrong” polarization • Host of other channels will be accessible: – – Radiative : Lb→Lγ, Lb→ L*γ, B →r 0 g, B→wγ, mmg b→sll : B+→K+ll (RK), Bs→ fmm LFV : Bq→ll’ … 21

Other Channels • Preliminary study of Bs→ fmm: – Expect ~1000 signal events from 2 fb-1 data with B/S<0. 9 @ 90% CL – Factor 4 reduction in production rate Bs cf. Bd – The f does not tag the B → need flavour tagging, factor ~15 reduction → expect √ 60 worse resolution than Bd→K*mm – Can make CP-averaged measurement of AFB (if non-zero CPV) • Study of b→d transition Bs→K*mm also planned: – Again, factor 4 reduction in production rate Bs cf. Bd – Rate reduced by |Vtd/Vts|2=0. 2082 ~ 1/25 – Given these reductions, expect will have to work harder to reduce background → ~< 700 events/ 2 fb-1 22

PID “Robustness” of mis-id bkg estimation: Bs KK 1. 0 fb -1 2 evt in s. r bb inclusive above GL = 0. 2: 19 b dimuon 3 other muons 2 muon + mis-id single mis-id probability needs to increase a factor ~10 to be of the same order as di-muon bkgrd Double mis-id: - dominated by B hh -~4 evts/fb-1 a factor ~50 less than dimuon - Mis-id needs to increase by a factor ~7 to be of the same order as dimuon bkgrd 23

RK in B+→K+ll • RK theoretically well controlled in SM : • Effect of extensions to SM can be O(10%) e. g. from neutral Higgs boson exchange Ba. Bar/Belle 95% CL limit on RK • Related to BR(Bs→mm) • LHCb sensitivity with B+→K+ll has been investigated – can also be done with the K* decay CDF 95% CL limit on Bs mm 24

RK in B+→K+ll (cont’d) • From 10 fb-1 data : – Bd ee. K – Bd mm. K ~ 10 k ~ 19 k • Gives RK = 1 (fixed) ± 0. 043 • Possible status with 10 fb-1 data : – BR(Bs→mm) ~ 3× 10 -9 • RK ~ 1 – compatible with MSSM with small tan b • RK ≠ 1 – NP : right handed currents or broken lepton universality – BR(Bs→mm) ≠ 3× 10 -9 • RK ~ 1 – as above • RK = 1+e – MFV 25

Bd→K*mm – non-resonant bkgrd • Presently neglecting non-resonant background • Limit can crudely be derived from Ba. Bar data → expect ~2000 events/2 fb-1 (→ B/S=0. 5± 0. 2) • Has been suggested that, under certain kinematic conditions, these can be treated as signal [Grinstein, Pirjol, hep-ph/0505155] : Drawn for Mmm = 2 Ge. V • Region I: soft pion, energetic kaon – Shifts zero of AFB and larger theory errors • Region II: energetic Kp pair – Can be treated as B Xmm and X Kp • Region III: soft kaon, energetic pion – Amplitude suppressed so very few events… Defined by kinematics 26

Bd→K*mm – non-resonant bkgrd (cont’d) • Presently neglecting non-resonant background • Limit can crudely be derived from Ba. Bar data → expect ~2000 events/2 fb-1 (→ B/S=0. 5± 0. 2) • Has been suggested that, under certain kinematic conditions, these can be treated as signal [Grinstein, Pirjol, hep-ph/0505155] : Drawn for Mmm = 2 Ge. V • Region I: soft pion, energetic kaon – Shifts zero of AFB and larger theory errors • Region II: energetic Kp pair – Can be treated as B Xmm and X Kp • Region III: soft kaon, energetic pion – Amplitude suppressed so very few events… Defined by kinematics 27

Bd→K*mm – non-resonant bkgrd (cont’d) • Presently neglecting non-resonant background • Limit can crudely be derived from Ba. Bar data → expect ~2000 events/2 fb-1 (→ B/S=0. 5± 0. 2) • Has been suggested that, under certain kinematic conditions, these can be treated as signal [Grinstein, Pirjol, hep-ph/0505155] : Drawn for Mmm = 2 Ge. V • Region I: soft pion, energetic kaon – Shifts zero of AFB and larger theory errors • Region II: energetic Kp pair – Can be treated as B Xmm and X Kp • Region III: soft kaon, energetic pion – Amplitude suppressed so very few events… Defined by kinematics 28

Bd→K*mm – non-resonant bkgrd (cont’d) • However, isolating region II has a large effect on the signal yield: K*mm signal Mmm ~ 2 Ge. V Kpmm (non-resonant) Mmm ~ 2 Ge. V Have relaxed the K* mass cut for signal and NR events • E. g. separating regions at Ep=600 Me. V : find 27% signal events and 44% NR events in region II 29

Bd→K*mm – non-resonant bkgrd (cont’d) • However, isolating region II has a large effect on the signal yield: K*mm signal Mmm ~ 2 Ge. V Kpmm (non-resonant) Mmm ~ 2 Ge. V Have relaxed the K* mass cut for signal and NR events • E. g. separating regions at Ep=600 Me. V : find 27% signal events and 44% NR events in region II 30

Bd→K*mm – non-resonant bkgrd (cont’d) • However, isolating region II has a large effect on the signal yield: K*mm signal Whole Mmm range Kpmm (non-resonant) Whole Mmm range Have relaxed the K* mass cut for signal and NR events • E. g. separating regions at Ep=600 Me. V : find 27% signal events and 44% NR events in region II • Plan to measure d. G/dm. Kp 31