Runion PLEPU : 16 nov. 2016 Olivier Deschamps LPC Clermont-F d - - PowerPoint PPT Presentation

Réunion PÔLEPU : 16 nov. 2016

Olivier Deschamps LPC Clermont-Fd UBP/CNRS/IN2P3

1

Radiative transition of b quarks

¤ Flavour Changing Neutral Current : b→qγ

¤ first observed at Cleo (1993) through B0,+→γK*0,+ ¤ Real photon (h=±1) implies the helicity flip on the quark lines

Photon polarization in radiative decays

¤ Standard Model : EW penguin dominates the b→qγ transition

¤ W coupling to (chirality) left-handed quarks ¤ Transition possible through helicity violation effect ~ mq

Photon polarisation

O7 ∝ mbsσ µνF

µν (1+γ5)b+ mssσ µνF µν (1−γ5)b

tanψ = AR(bL → sRγ R) / AL(bR → sLγ L) ≈ ms / mb

Leading (EM dipole) operator in the effective Hamiltonian approach : + other SM operator contributions (O2, …) : AR/AL ~ few %

• Angular analysis of the recoil 3-body in the B+→(K+π+π-)γ decay mode
• di-lepton angular analysis at low-q2 of the (virtual) photon decay in B→Ve+e-
• indirect extraction of the

real photon polarisation

• so far the most precise

extraction of a photon polarity-related quantities

• Angular analysis in the radiative transition of b-baryons
• Λb→Λ(pπ-)γ

very challenging at LHCb (expect an upper limit on BR)

• Λb→Λ*(pΚ-)γ

large signal at LHCb but very complicated (pK) mass spectrum

• rather small Λb polarisation measured at LHCb

Extracting the photon polarisation … from angular analysis

Photons from radiative decays are polarized @ 5.2σ significance First direct observation of photon polarization in b→sγ transition

PRL 112, 161801 (2014), LHCb

Would require theoretical amplitude prediction and (Kππ) amplitude analysis to extract λγ [similar study in neutral modes Ksπ+π-γ & K+π-π0γ is part of B.Quintana PhD]

JHEP 04(2015) 064, LHCb

Extracting the photon polarisation … from mixing

Direct access via the time-dependent decay rate of B→ ΦCPγ

• Bs decays : sensitive through the mixing term AΔ
• ΔΓs / Γs ~ 10%
• untagged analysis

untagged / tagged analysis required

• B0 decays : sensitive to the polarisation through the TD asymmetry term SCP

Out of LHCb reach for the main decay mode B0→ Ksπ0γ No sensitivity to AΔ (ΔΓ ΔΓd ~ 0) SM: AΔ=0.047±0.025±0.015 Muheim et al., PLB664(08)17 φ(s) is the mixing-decay weak phase Bs→φγ

: φs (SM)~2βs-2βs

∼ 0

Bd→K*0γ : φd (SM)~2β-2βs

∼ 2β

Bd→ρ0γ : φd (SM)~2β-2β ∼ 0

[B0→ Ksππγ is part of Boris PhD]

Radiative decays at LHCb

• Radiative decays reconstruction in LHCb
• mostly rely on high pT photons
• Due to trigger rate constraint and large

combinatorial

• L0-trigger (run1) : ET(γ) > 2.5-3.0 GeV
• Typical trigger efficiency . ~ 30-40%
• For comparison : (di)muon channel εtrg~80-90%
• Mass resolution driven by calorimeter :
• σM~ 90 MeV/c2
• For comparaison : B→J/ψV : σM ~ 10 MeV/c2
• No constraint on vertexing from γ + large

photon multiplicity + limited mass resolution :

• Large background contamination

→ Tight selections have to be applied

Propertime biasing cuts (IP, displ. vertex) cannot be avoided

7

Bs→φγ propertime analysis

¤ Analysis :

¤ Unbinned maximum likelihood fit of the Bs→φ(K+K-)γ propertime ¤ use B0→φ(K+π-)γ as control channel (simultaneous fit) ¤ Based on run1 data (2011-2012) : int. lum. 3fb-1

¤ Selection :

¤ Kinematical criteria : high-pT photon, invariant masses, track quality … ¤ Topological criteria : high-IP tracks, displaced 2ndary vertex, vertex quality, ¤ PID criteria : photon ID, Kaon/pion ID

¤ Large background contamination:

¤ Combinatorial ¤ Partially reconstructed : eg. B→K+hhγ ¤ "cross-feed" due to track misID (e.g. Λb→pK-γ) or π0/γ (B→hhπ0)

B(B0→K*0γ)=(4.33±0.15)x10-5 B(Bs→φγ)=(3.5±0.4)x10-5 (LHCb, 2011)

8

Bs→φγ signal extraction

9

Bs→φγ propertime analysis

10

Propertime fit

¤ Background-subtracted sample (sWeight method)

11

Interpretation

12

¤ Collaborative analysis ¤ more than 2 years work to achieve a good control of the acceptance ¤ was one of the 6 'LHCb key measurements' prioritized by LHCb in 2009 and the last to be published …

13

Prospective

Roadmap of selected key measurements of LHCb (2009)

)

2

(MeV/c

γ )

• π

+

(K

m )

2

Events / ( 10 MeV/c

500 1000 1500 2000 2500 3000 3500 0.60 ± = 5279.77

B

µ 0.66 ± = 95.64

B

σ 523 ± = 68797

γ

K →

B

N 4500 5000 5500 6000 5 − 5

)

2

(MeV/c

γ )

• K

+

(K

m )

2

Events / ( 10 MeV/c

100 200 300 400 500 600 1.3 ± = 5367.9

B

µ 1.3 ± = 92.9

B

σ 167 ± = 11338

γ φ →

#B

N 4500 5000 5500 6000 5 − 5

¤ Signal yield already increased by a factor 3, including run2 data

14

Prospective

Enhanced sensitivity in neutral B decays from tagged analysis giving access to Time-dependent asymmetries

For illustration :

LHCb tagging performance applied for the first time on a radiative decay

decay time

• Mixing asymmetry

unmixedmixed unmixedmixed

• LHCb

preliminary

¤ Analysis in development (Valencia)

15

16

Spare

Bs→φγ signal extraction

17

B+→ (Kππ ππ)+

resγ

Up-down photon asymmetry is proportional to the photon polarisation λγ Angular analysis of photon direction wrt to (Kππ)res decay plane in different mass bins

For a mixture of spin-parity Kres(1+,2+,1-) :

Helicity structure in B→ (Kππ ππ)resγ

L=3fb-1 N[B+→K+π-π+γ]=14x103

Helicity structure in B→ (Kππ ππ)resγ

LHCb-CONF-2013-009

as a by-product:

Photons from radiative decays are polarized @ 5.2σ significance First direct observation of photon polarization in b→sγ transition

[Phys. Rev. Lett. 112, 161801 (2014)]

Measuring the λγ value from the up-down asymmetry require to separate the (Kππ) resonances & theoretical determination of the helicity amplitude

e.g. for a single 1+ resonance

• Phys. Rev. Lett. 88 (2002) 051802