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

Réunion PÔLEPU : 16 nov. 2016 Olivier Deschamps LPC Clermont-F d UBP/CNRS/IN2P3 1

Radiative transition of b quarks ¤ Flavour Changing Neutral Current : b → q γ ¤ first observed at Cleo (1993) through B 0,+ →γ K* 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 → q γ transition ¤ W coupling to (chirality) left-handed quarks ¤ Transition possible through helicity violation effect ~ m q

Photon polarisation Leading (EM dipole) operator in the effective Hamiltonian approach : O 7 ∝ m b s σ µ ν F µ ν (1 + γ 5 ) b + m s s σ µ ν F µ ν (1 − γ 5 ) b tan ψ = A R ( b L → s R γ R ) / A L ( b R → s L γ L ) ≈ m s / m b + other SM operator contributions (O 2 , …) : A R /A L ~ few %

Extracting the photon polarisation … from angular analysis • Angular analysis of the recoil 3-body in the B + → (K + π + π - ) γ decay mode PRL 112, 161801 (2014), LHCb Photons from radiative decays are polarized @ 5.2 σ significance First direct observation of photon Would require theoretical amplitude prediction polarization in b → s γ transition and (K ππ ) amplitude analysis to extract λ γ [similar study in neutral modes K s π + π - γ & K + π - π 0 γ is part of B.Quintana PhD] • di-lepton angular analysis at low-q 2 of the (virtual) photon decay in B → Ve + e - • indirect extraction of the JHEP 04(2015) 064, LHCb real photon polarisation • so far the most precise extraction of a photon polarity-related quantities • Angular analysis in the radiative transition of b-baryons • very challenging at LHCb (expect an upper limit on BR) Λ b →Λ (p π - ) γ • Λ b →Λ * (p Κ - ) γ large signal at LHCb but very complicated (pK) mass spectrum • rather small Λ b polarisation measured at LHCb

Extracting the photon polarisation … from mixing Direct access via the time-dependent decay rate of B → Φ CP γ tagged analysis required untagged / φ (s) is the mixing-decay weak phase B s →φγ : φ s (SM)~2 β s - 2 β s ∼ 0 B d → K *0 γ : φ d (SM)~2 β - 2 β s ∼ 2 β B d →ρ 0 γ : φ d (SM)~2 β - 2 β ∼ 0 • B 0 decays : sensitive to the polarisation through the TD asymmetry term S CP Out of LHCb reach for the main decay mode B 0 → K s π 0 γ No sensitivity to A Δ ( ΔΓ ΔΓ d ~ 0) [ B 0 → K s ππγ is part of Boris PhD] • B s decays : sensitive through the mixing term A Δ • ΔΓ s / Γ s ~ 10% • untagged analysis Muheim et al., PLB664(08)17 SM: A Δ =0.047±0.025±0.015

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) : E T ( γ ) > 2.5-3.0 GeV • Typical trigger efficiency . ~ 30-40% • For comparison : (di)muon channel ε trg ~80-90% • Mass resolution driven by calorimeter : • σ M ~ 90 MeV/c 2 • For comparaison : B → J/ ψ V : σ M ~ 10 MeV/c 2 • 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

B s →φγ propertime analysis ¤ Analysis : ¤ Unbinned maximum likelihood fit of the B s →φ (K + K - ) γ propertime ¤ use B 0 →φ (K + π - ) γ as control channel (simultaneous fit) ¤ Based on run1 data (2011-2012) : int. lum. 3fb -1 B (B 0 → K* 0 γ )=(4.33±0.15)x10 -5 B (B s →φγ )=(3.5±0.4)x10 -5 (LHCb, 2011) ¤ Selection : ¤ Kinematical criteria : high-pT photon, invariant masses, track quality … ¤ Topological criteria : high-IP tracks, displaced 2 ndary vertex, vertex quality, ¤ PID criteria : photon ID, Kaon/pion ID ¤ Large background contamination: ¤ Combinatorial ¤ Partially reconstructed : eg. B → K + hh γ ¤ "cross-feed" due to track misID (e.g. Λ b → pK - γ ) or π 0 / γ (B → hh π 0 ) 8

B s →φγ signal extraction 9

B s →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 ¤ Signal yield already increased by a factor 3, including run2 data ) ) 3500 2 2 Events / ( 10 MeV/c Events / ( 10 MeV/c 600 = 5367.9 1.3 = 5279.77 0.60 µ ± µ ± B B d s = 95.64 0.66 = 92.9 1.3 σ ± σ ± 3000 B B s d N = 68797 ± 523 N = 11338 167 500 ± *0 B K #B → φ γ → γ s d 2500 400 2000 300 1500 200 1000 100 500 2 m (MeV/c ) 2 m (MeV/c ) + - (K ) π γ + - (K K ) γ 5 5 0 0 5 − − 5 4500 5000 5500 6000 4500 5000 5500 6000 Roadmap of selected key measurements of LHCb (2009) 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 � unmixed �� mixed � unmixed �� mixed LHCb preliminary � � � � �� � Mixing asymmetry ���� � � � decay time ���� ¤ Analysis in development (Valencia) 15

Spare 16

B s →φγ signal extraction 17

Helicity structure in B → (K ππ ππ ) res γ B + → (K ππ ππ ) + res γ L =3fb -1 N [B + → K + π - π + γ ] =14x10 3 For a mixture of spin-parity K res (1 + ,2 + ,1 - ) : Up-down photon asymmetry is proportional to the photon polarisation λ γ Angular analysis of photon direction wrt to (K ππ ) res decay plane in different mass bins

Helicity structure in B → (K ππ ππ ) res γ [Phys. Rev. Lett. 112, 161801 (2014)] Photons from radiative decays are polarized @ 5.2 σ significance First direct observation of photon polarization in b → s γ transition as a by-product: LHCb-CONF-2013-009 Measuring the λ γ value from the up-down asymmetry require to separate the (K ππ ) resonances & theoretical determination of the helicity amplitude Phys. Rev. Lett. 88 (2002) 051802 e.g. for a single 1+ resonance