A. Oyanguren (IFIC U. Valencia/CSIC)

Transcription

1 A. Oyanguren (IFIC U. Valencia/CSIC)

2 Outline Radiative b decays B s φγ B 0 K *0 e + e - [PRL 118(2017)021801] [JHEP04(2015)064] b-baryons: Λ b Λγ, Ξ b Ξ γ, Ω b Ωγ Conclusions A. Oyanguren 2

3 The LHCb experiment A. Oyanguren 3

4 The LHCb experiment [INT.J.MOD.PHYSA 30 (2015) ] [JINST 3 (2008) S08005] A. Oyanguren 4

5 Radiative b decays Photons in b sγ are predicted to be left-handed in the SM (small corrections of order m s /m b ~ 2%) b u,c,t γ s,d Some new physics models, particularly Left-Right Symmetric Models, predict an anomalous component of polarized photons W - γ [D. Atwood, M. Gronau and A. Soni, PRL79(97)185] [M. Gronau, D. Pirjol, PRD66(02)054008] [F. Yu, E. Kou, C. Lü, JHEP12(2013)102] b H -, χ -,g, χ 0 ~ s,d Expressed in terms of Wilson coefficients: C 7( ) A. Oyanguren 5

6 Radiative b decays How to access the photon polarization in b-hadron decays? Time dependent analyses, using B-B interference of mixing and decay: - Final common state for neutral B and B: B (s) Vγ, V KK, ππ - B s more profitable ( Γ s > > Γ d ) LHCb: V to charged tracks, better no π 0 s, no K s s (Ex: B d K* 0 (K s π 0 )γ) - B s φγ, B d ργ, B d ωγ - Observables: time dependent decay widths, CP asymmetries - Need the use of flavour tagging, which reduces a lot the statistics (ε eff ~5%) Angular analyses: - B (s) to three-body + γ decays (B + K - π + π + γ) - Decays of b-baryons (Λ b Λγ ) - Decays with an electron pair in the final state γ e - e + with γ real: radiative decays with converted photons (B (s) Vγ( e - e + )) or virtual: B K*e + e - analyzed in the low q 2 region A. Oyanguren 6

7 Radiative b decays A difficult task in pp collisions! Huge amount of combinatorial photons in the calorimeter: ~ 10 γ s/event Large calorimeter occupancy, large background High energy photons (p T > 2.5GeV) from π 0 γγ merge in the same calorimeter cells. Photons give low constraints in radiative B decays (no origin vertex) η γγ π 0 γγ [LHCb-PUB ] A. Oyanguren 7

8 Photon LHCb Radiative b decays B s φ ( KK) γ π 0 γγ reconstruction σ~8mev m=134.9± 0.1MeV Calorimetric photons: converted to a e + e - pair after the magnet or unconverted High energy photons from B decays: B mass resolution σ~92 MeV/c 2 Converted photons: γ e + e - materialization Small fraction ~ 20% Better B mass resolution, σ~30 MeV/c 2 (From B J/ψK ( * ) samples) [CERN-LHCb-PROC ] A. Oyanguren 8

9 Radiative b decays First LHCb: [Nuc. Phys. B 867 (2013) 1-18] 5279 ± 93 B d K*γ 691 ± 36 B s φγ B 0 B 0 (this with 1fb -1, update with 3fb -1 soon) [Sam Coquereau] A. Oyanguren 9

10 Measuring the photon polarization with: B s φγ

11 B s φγ The time dependent decay rate for a B (s) Vγ a and B (s) Vγ: Γ B,B (t) A C S Only accessible for B s decays Γ d ~ 0 Γ s ~ 0.081±0.011 ps -1 Need to tag the flavour of the initial B (s) A. Oyanguren 11

12 B s φγ At present only information from S and C parameters in the B d system Results in agreement with the SM predictions A. Oyanguren 12

13 B s φγ The time dependent decay rate for a B (s) Vγ a and B (s) Vγ: Γ B,B (t) A C S For the B s φγ decay channel the SM predictions are: [Muheim, Xie, Zwicky, PLB664(2008)174] A SM S SM Left-Right Symmetric models: A up to ~ 0.7 [Atwood, Gronau, Soni PRL79(1997)185] Fraction of anomalous polarized photons ~ 40% A. Oyanguren 13

14 B s φγ Untagged analysis C and S terms cancel, access to the A parameter Use B 0 K*γ (with K* 0 K + π - ) as control channel of B s φγ (with φ K + K - ) Experimentally: Similar topology: similar trigger, reconstruction and selection a high-energy photon and two tracks of opposite sign (K + K - /π - ) Theoretically: PV B (s) SV γ h + h - B 0 K* 0 γ is flavour specific Γ d ~ 0 Γ d ~ Γ s Not affected by new physics: Key to control the experimental effects A. Oyanguren 14

15 B s φγ The decay time is measured from the B momentum and flight distance, after a full kinematical fit of the decay channel Γ Bs (t r ) measured = A(t) Γ Bs (t; A ) R(t,t r ) Untagged decay time distribution: Run 1 3 fb events A(t) low acceptance: trigger + tracking (Impact Parameter requeriments) A(t) high acceptance: VELO detector acceptance True τ distribution Selected events Need to control the proper time acceptance A(t) = Effect of proper time resolution negligible (σ t ~ fs) at n 1+at n x (1+βt) A. Oyanguren 15

16 The results: B s φγ 4000 candidates candidates 3fb -1 [PRL 118(2017)021801] Compatible with the SM within 2σ A. Oyanguren 16

17 B s φγ In terms of the Wilson coefficients C 7 and C 7 : A. Oyanguren 17

18 B s φγ Tagging the flavour of the initial B meson (B or B): Γ B,B (t) A C S access to C and S parameters Tagging algorithms: Opposite side (OS): From the opposite B: - e, µ from semilept. B decays, - kaons from b c s, - opposite B vertex Same side (SS): From fragmentation of the signal b (π for B, K for B s ) N evts x ε tag (1-2ω) 2 Tagging efficiency, ε tag ~ 75% Mistag probability, ω ~ 36% Effective efficiency: ε eff ~5.44% 18

19 B s φγ Expected sensitivity: Dilution of the oscillation (lost of sensitivity in the C and S parameters) due to reconstruction effects 19

20 Expected results (unofficial!, LHCb values set to SM) B s φγ [Thesis C. Sánchez Mayordomo] Run1 Run1 A. Oyanguren 20

21 B s φγ First test of the good performance of the flavour tagging for radiative B (s) decays (ε eff > 5%) σ s, σ A ~0.2 Important constraints on new physics when including the S parameter! 21

22 Measuring the photon polarization with: B 0 K* 0 e - e + 22

23 B 0 K* 0 e - e + Measurement of angular observables of the B 0 K*e - e + in the low q 2 < 1GeV 2 [JHEP04(2015)064] (3fb -1 ) Virtual γ decaying in an observable - + pair Sensitive to the photon polarization due to the photon pole (for B V - + ) Requires to go very low in the q 2 region electrons A. Oyanguren 23

24 B 0 K* 0 e - e + The differential decay rate depends on three angles: θ, θ K and φ access to the photon polarization information [D. Becirevic and E. Schneider Nucl. Phys. B 854 (2012) 321] : longitudinal polarization of the K* (expected small at low q 2, γ polarized ) 24

25 B 0 K* 0 e - e + Electrons are difficult to reconstruct since they loose energy by radiation: need bremsstrahlung recovery adding neutral clusters from the ECAL, with E T > 75MeV Long radiative tail in the B mass distribution: controlled from B K*γ events (γ e - e +, with bremsstrahlung emission) A. Oyanguren 25

26 B 0 K* 0 e - e + 4-dimensional fit to m(k + π - e - e + ) and the three angles θ, θ K and φ: 150±17

27 B 0 K* 0 e - e + The results of the fitted parameters (A T im and A T (2) being sensitive to the γ polarization): Compatible with the SM predictions*: [Adapted from Jäger and Camalich arxiv: ] The best sensitivity to C 7( ) up to date! [JHEP04(2015)064] [M. Borsato, CERN-THESIS ] *leading order estimation, 5% accuracy for SM value 27

28 B 0 K* 0 e - e + Prospects for Run2: Statistics of Run1 x (1 + ~ 4) (with same performance): Run1 + Run2 ~ 750 B K*e - e + events (Marie-Helene Schune) projections The photon polarization could be measured to about 5 to 7 %! A. Oyanguren 28

29 Measuring the photon polarization with: b-baryons (Λ b, Ξ b, Ω b )

30 b-baryons: Λ b Λγ Exploiting the angular correlations between the polarized initial state and the final state: [Mannel, Recksiegel, J.Phys. G24 (1998) ; Hiller, Kagan, PRD 65, (2002)] For Λ b decaying into Λ 0 (1115) with J=1/2: P Λ b is the Λ b polarization α p, 1/2 is the weak decay parameter α γ is the photon polarization The Λ b transverse production polarization has been found to be small: P Λ b =0.06 ± 0.07 ± 0.02 [PLB724 (2013)27] No sensitivity in cosθ γ α p1/2 = ± [PDG2016] access to α γ via the angular distribution of the proton 30

31 b-baryons: Λ b Λγ Experimental challenge: the Λ b decay vertex cannot be reconstructed due to the long lifetime of the Λ 0 baryon (cτ = 7.89 cm) p Λ b Λ 0 π - γ Another possibility: to use stranger b-baryons: Ξ b - Ξ - γ, Ω b- Ω - γ [L. Oliver, J.-C. Raynal, and R. Sinha, Phys.Rev. D82 (2010) ] Ξ b -, Ω b - Ξ - (Ω - ) π - (K - ) Heavy baryon track + one pion or kaon track to constraint the vertex (cτ (Ξ - )= 4.9 cm) (cτ (Ω - )= 2.5 cm) 31

32 b-baryons: Ξ b- Ξ - γ Angular distribution for the Ξ b - Ξ - γ : Integrating over cosθ p : α Λ =-0.458± [PDG2016] Similar sensitivity to the photon polarization as the Λ b Λγ channel Number of events Toy simulations with 1000 events 32

33 Expected number of events: b-baryons: Ξ b- Ξ - γ Trigger (HLT2) for Λ b Λγ, Ξ b Ξγ, Ω b Ωγ Good angular resolution at LHCb, effect on σ αγ small Other experimental effects (acceptance, background, etc ) already studied A. Oyanguren 33

34 b-baryons, the challenge 80% of the tracks hitting only downstream detectors The high track combinatorics doesn t allow to trigger them (HLT1)

35 Conclusion The photon polarization is being measured at LHCb using several channels and different observables Important to constrain the Wilson coefficient C 7 ( ) Difficult analyses due to the γ/e reconstruction in pp collisions, but we did it NP constraints more precise than the ones from B-factories! Working hard in new and improved measurements Run 2 results already coming New Physics Stay tuned, there is much to come! A. Oyanguren 35

36 Merci!

37 Measuring the photon polarization with: B + K + π - π + γ 37

38 B + K + π - π + γ The photon polarization parameter λ γ It can be extracted by studying the three body decay of a K J (J P ) resonant state in B K res ( Kππ) γ radiative decays [Kou et al, PRD83 (2011) ; Gronau et al, PRL88 (2002) ] the decay plane defines the direction of the photon: The Up-down asymmetry A UD λ γ Need to count the number of events with photon emitted above/below the Kπ-plane and subtract them. A. Oyanguren 38

39 B + K + π - π + γ Reconstruct a kaon resonance from three charged tracks: two pions of opposite sign and a kaon, plus a high E T photon. [PRL 112, (2014) ] B 3fb ±153 Background substracted Kππ spectrum: Many kaon resonances with different properties are expected to contribute 39

40 B + K + π - π + γ A UD studied in several m(kππ) regions, fitting m B and the cosθ distribution. λ γ differs from 0 at 5.2σ First evidence of photon polarization in b s transitions! [PRL 112, (2014) ] (but this cannot be translated easily in R, L amplitudes ) [G. Veneziano, CERN-THESIS ] 40