Review of radiative B meson decays in LHCb

Review of radiative B meson decays in LHCb

Samuel Coquereau

X-CPAN Days, Salamanca, 29 - 31 October 2018

Introduction

I In the Standard Model (SM), the b→sγ process is forbidden at tree level.

^{W− u, c, t W−γ}

b s

B Using the effective field theory:

H_eff =−4G_F

√

2 VtbV_ts^∗(C7O₇+C₇⁰O₇⁰) I whereC7 describe the left-handed currents andC₇⁰ the right-handed

currents

I The photon polarization can be defined as:

Aγ = P(γ_L)−P(γ_R)

P(γL) +P(γR) A^LO_γ = 1− |^C_C⁷⁰

7|² 1 +|^C_C⁷⁰

7|²

Samuel Coquereau CPAN 2018 29^thOctober 2018 2 / 18

Introduction

I In the Standard Model (SM), the b→sγ process is forbidden at tree level.

^{W− u, c, t W−γ}

b s

B Using the effective field theory:

H_eff =−4G_F

√

2 VtbV_ts^∗(C7O₇+C₇⁰O₇⁰) I In the SM C₇⁰ = 0 due to absence of right-handed currents

I Leading to:

A_γ = P(γ_L)−XXP(γ_RXX)

P(γL) +^XXP(γR_XX)] = 1+O(m_s mb

) A^LO_γ = 1−^Z ZZ

|^C_C⁷⁰

7|² 1 +^Z

ZZ

|^C_C⁷⁰

7|²

= 1

The LHCb experiment

Interaction point

Velo

Tracking stations Tracking

RICH system

Muon chambers ECAL, HCAL Particle Id

Samuel Coquereau CPAN 2018 29^thOctober 2018 4 / 18

Analysis in radiative B meson decays at LHCb

1 Photon polarisation at LHCb

2 B_s →φγ

3 Multichannel

Photon polarization at LHCb

I First observation in B⁺→K⁺π⁺π⁻γ with 3fb⁻¹ (2011-2012) [Phys. Rev. Lett. 112, 161801 (2014)]

B Full amplitude analysis ongoing

I First measurement inB⁰→K^∗e⁺e⁻ lowq² with 3fb⁻¹: [JHEP04(2015)064]

A⁽²⁾_T =−0.23±0.23±0.05 A^Im_T = +0.14±0.22±0.05 B Update with run 1 + run 2

dataset ongoing

] c2

/ [MeV

−)

+e

−e π K+

m(

4800 5000 5200 5400

) 2cCandidates / (30 MeV/

0 5 10 15 20 25

30 Data

Model B⁰→K^*0e⁺e⁻ e− e+ ) X

*0 K

→( B Combinatorial

LHCb

Samuel Coquereau CPAN 2018 29^thOctober 2018 6 / 18

B

_s

→ φγ (see Clara’s talk)

I Time-dependent decay rate is sensitive to photon polarisation:

Γ(t)∝e^−Γst[cosh(∆Γ_st

2 )−A^∆sinh(∆Γ_st

2 )±Ccos(∆m_st)∓Ssin(∆m_st)]

I A^∆,C andS are dependent to the photon polarisation A^∆≈ 2Re(e^iφsC7C₇⁰)

|C₇|²+|C₇⁰|² S ≈ 2Im(e^iφsC7C₇⁰)

|C₇|²+|C₇⁰|² and C ≈0 I giving in the SM (C₇⁰ supressed):

A^∆_SM= 0.047^+0.029_−0.025 S_SM= 0±0.002 C_SM≈0.005±0.005

B

_s

→ φγ: untagged analysis

[Phys. Rev. Lett. 118 (2017) no2, 021801]

I Analysis performed using 3fb⁻¹ (Run 1) I 4200B_s →φγ signal events

A^∆=−0.98^+0.46_−0.52^+0.23_−0.20 B 2σ deviation from SM

B Uncertainty dominated by the statistics

B Systematic uncertainty could be reduce with more statistics

2] c ) [MeV/

γ φ ( m

5000 5500 6000

)2cCandidates / (25 MeV/

0 100 200 300 400

500 Data

Model Signal Peaking Missing kaon Combinatorial LHCb

γ φ

→

0

Bs

[ps]

t

0 5 10

Ratio of candidate yields

0 0.05 0.1 0.15 0.2 0.25 0.3

Data Fit SM LHCb

Samuel Coquereau CPAN 2018 29^thOctober 2018 8 / 18

B

_s

→ φγ: tagged analysis (see Clara’s talk)

I Analysis with Run 1 well advance (results still blinded) I Fit strategy :

B Simultaneous unbinned fit of Bs →φγ andB⁰→K^∗0γ (control channel) proper-time distribution

B Γ(t) PDF:

n e^−Γst0

h

cosh(^∆Γ₂^st0)− A^∆sinh(^∆Γ₂^st0) +

κ(1−2ω))Ccos(∆m_st⁰)− Ssin(∆m_st⁰) io

∗[A(ti)×R(t,t⁰|σ_t)]

B κ andω are the tagging decision and the mistag respectively B A(ti): binned version of the acceptance function

B R(t,t⁰|σ_t): 2-Gaussian model taken from MC

B

_s

→ φγ: tagged analysis (see Clara’s talk)

I Analysis with Run 1 well advance (results still blinded)

Samuel Coquereau CPAN 2018 29^thOctober 2018 10 / 18

B

_s

→ φγ: tagged analysis (see Clara’s talk)

I Analysis with Run 2:

B More statistics (∼ ×2 Run 1 yield) B Improved flavour tagging (∼10%) B Expected improvements in systematics

B

_s

→ φγ: tagged analysis (see Clara’s talk)

I Analysis with Run 2:

B More statistics (∼ ×2 Run 1 yield) B Improved flavour tagging (∼10%) B Expected improvements in systematics

I Expected constrains combining A^∆ andS (Run 1+ Run 2 Data) I σ(A^∆)'σ(S)'0.2

I S sensitive to Im(C₇⁰)

Samuel Coquereau CPAN 2018 29^thOctober 2018 12 / 18

Multichannel

I The goal:

B Measurement of B(Λ⁰_b→Λ^∗0γ) andB(B_s⁰ →φγ) relative to B(B⁰→K^∗γ)

B Measurement of A_CP for Λ_b →Λ^∗γ andB⁰ →K^∗γ I The strategy:

B Use the Run 1 dataset

B 5 simultaneous fits to extract the 5 yields: N(Λ_b→Λ^∗γ), N(Λ_b→Λ^∗γ), N(B⁰ →K^∗γ), N(B⁰ →K^∗γ) and N(B_s⁰→φγ) B Separate BDT’s for each channel to reduce combinatorial

background

B Particle identification also use to further reduce background

Mass fit strategy

I Signal:

B Double-sided Crystal Ball: αL,nL, αR,nR extracted from MC,σ andµfitted on data

I Background:

B Combinatorial: Chebychev polynomial, Comb(m;p0) = 1 +p0×m

B Peaking Background: Double sided Crystal Ball

B Partially reconstructed background : Argus convoluted with a gaussian,µand σ taken from signal andc andp fixed from MC

Samuel Coquereau CPAN 2018 29^thOctober 2018 14 / 18

Mass Fit

B⁰→K^∗0γ

2] c M(BK_B) [MeV/

Candidates / ( 27.5 )

0 200 400 600 800 1000 1200 1400 1600 1800

2000 A^BCP^d = -0.74271 ± 0.0087

0.022

± = 0.266

*γ

→ K B Ccomb

± 374 = 31547 γ K*

→ NB

0.017

± = 0.145 _B_2012 ρ D0

→ B+

C

0.044

± = 0.234 _B_2012 γ

→ K1 B+

C

0.043

± = 0.177 _B_2012 η K*

C

c2 0.85 MeV/

± = 5281.98

*γ

→ K µB

0.087

± ] = -0.3007 [MeV-1

*γ

→ K B p0

c2 0.82 MeV/

± = 87.47

*γ

→ K σB

0.0087

± = -0.74271 Bd

ACP

0.022

± = 0.266

*γ

→ K B Ccomb

± 374 = 31547 γ K*

→ NB

0.017

± = 0.145 _B_2012 ρ D0

→ B+

C

0.044

± = 0.234 _B_2012 γ

→ K1 B+

C

0.043

± = 0.177 _B_2012 η K*

C

c2 0.85 MeV/

± = 5281.98

*γ

→ K µB

0.087

± ] = -0.3007 [MeV-1

*γ

→ K B p0

c2 0.82 MeV/

± = 87.47

*γ

→ K σB

46005 4800 5000 5200 5400 5600 5800 6000 6200

− 0 5

B⁰ →K^∗0γ

2] c M(BK_Bbar) [MeV/

Candidates / ( 27.5 )

0 200 400 600 800 1000 1200 1400 1600 1800 2000 2200

0.0087

± = -0.74271 Bd

ACP

0.021

± = 0.232

*γ

→K B Ccomb

± 374 = 31547 γ K*

→ NB

0.016

± = 0.185 _Bbar_2012 ρ D0

→ B+

C

0.044

± = 0.102 _Bbar_2012 γ

→ K1 B+

C

0.043

± = 0.299 _Bbar_2012 η K*

C

c2 0.85 MeV/

± = 5281.98

*γ

→ K µB

0.11

± ] = -0.175 [MeV-1

*γ

→K B 0 p

c2 0.82 MeV/

± = 87.47

*γ

→ K σB

0.0087

± = -0.74271 Bd

ACP

0.021

± = 0.232

*γ

→K B Ccomb

± 374 = 31547 γ K*

→ NB

0.016

± = 0.185 _Bbar_2012 ρ D0

→ B+

C

0.044

± = 0.102 _Bbar_2012 γ

→ K1 B+

C

0.043

± = 0.299 _Bbar_2012 η K*

C

c2 0.85 MeV/

± = 5281.98

*γ

→ K µB

0.11

± ] = -0.175 [MeV-1

*γ

→K B 0 p

c2 0.82 MeV/

± = 87.47

*γ

→ K σB

46005 4800 5000 5200 5400 5600 5800 6000 6200

− 0 5

Mass Fit

] c2 M(Lambda_B) [MeV/

Candidates / ( 33.3333 )

0 100 200 300 400 500

0.017

± = -0.5966 Λb

ACP

0.14

± = 1.88

*γ Λ b→ Λ Ccomb

0.13

± = 5.60 γ Λ

→ RΛ

0.041

± = 0.105 _B_2012 γ

→ K1 B+

C

0.073

± = 0.045 _B_2012 γ φ K C

c2 1.6 MeV/

± = 5622.2 µΛ

c2 1.7 MeV/

± = 83.6 σΛ

0.000063

± = -0.0012936 γ Λ*

→ τΛ

0.017

± = -0.5966 Λb

ACP

0.14

± = 1.88

*γ Λ b→ Λ Ccomb

0.13

± = 5.60 γ Λ

→ RΛ

0.041

± = 0.105 _B_2012 γ

→ K1 B+

C

0.073

± = 0.045 _B_2012 γ φ K C

c2 1.6 MeV/

± = 5622.2 µΛ

c2 1.7 MeV/

± = 83.6 σΛ

0.000063

± = -0.0012936 γ Λ*

→ τΛ

4600 4800 5000 5200 5400 5600 5800 6000 6200 6400 66005

− 0 5

Λ_b→Λ^∗γ B In red: The signal

B In black: The combinatorial background

] c2 M(Lambda_Bbar) [MeV/

Candidates / ( 33.3333 )

0 100 200 300 400

500 A^Λ_CP^b = -0.5966 ± ^0.017

0.15

± = 1.97

*γ Λ b→ Λ Ccomb

0.13

± = 5.60 γ Λ

→ RΛ

0.045

± = 0.163 _Bbar_2012 γ

→ K1 B+

C

0.077

± = 0.045 _Bbar_2012 γ φ K C

c2 1.6 MeV/

± = 5622.2 µΛ

c2 1.7 MeV/

± = 83.6 σΛ

0.000065

± = -0.0012397

*γ Λ

→ τΛ

0.017

± = -0.5966 Λb

ACP

0.15

± = 1.97

*γ Λ b→ Λ Ccomb

0.13

± = 5.60 γ Λ

→ RΛ

0.045

± = 0.163 _Bbar_2012 γ

→ K1 B+

C

0.077

± = 0.045 _Bbar_2012 γ φ K C

c2 1.6 MeV/

± = 5622.2 µΛ

c2 1.7 MeV/

± = 83.6 σΛ

0.000065

± = -0.0012397

*γ Λ

→ τΛ

4600 4800 5000 5200 5400 5600 5800 6000 6200 6400 66005

− 0 5

Λ_b→Λ^∗γ B In green: B⁺→K₁⁺γ B In cyan: B⁺→φKγ

Samuel Coquereau CPAN 2018 29^thOctober 2018 16 / 18

First results

B ACP are still blind

B Cross feed contribution fixed in the fit

B Partial. reco. Bkg contribution let free in the fit

I Cross check onB(B⁰→K^∗0γ) and Branching ratio measured in agreement with SM (without syst.

uncert. yet)

I Using the PDG value for B(B⁰→K^∗0γ) we can extract:

2] c M(Bs) [MeV/

Candidates / ( 25.8333 )

0 100 200 300 400

500 ^{= 0.439 ± 0.026}

γ φ s→ B Ccomb

0.16

± = 7.16 γ φ s→ RB

0.0098

± = 0.0970 _2012 Kγ φ

→ B+

C

c2 1.7 MeV/

± = 5366.3 µBs

0.043

± ] = -0.5955 [MeV-1 γ φ

→ B p0

c2 1.7 MeV/

± = 88.4 σBs

0.026

± = 0.439 γ φ s→ B Ccomb

0.16

± = 7.16 γ φ s→ RB

0.0098

± = 0.0970 _2012 Kγ φ

→ B+

C

c2 1.7 MeV/

± = 5366.3 µBs

0.043

± ] = -0.5955 [MeV-1 γ φ

→ B p0

c2 1.7 MeV/

± = 88.4 σBs

46005 4800 5000 5200 5400 5600 5800 6000

− 0 5

B_s⁰→φγ

Conclusion

I Radiative decays are sensitive to the photon polarisation

B Allowing to constrain the C₇⁰ complex plane and to test the SM prediction

I Many analysis not shown here:

B B⁰ →K^∗γ isospin asymmetry, Search for Λ⁰_b→N^∗γ, ...

I Potential to improve the current results:

B Analysis using Run 2 dataset of LHCb are in progress B Improvement of reconstruction and analysis techniques I New measurement are coming:

B S(B_s⁰ →φγ),A_CP for Λ_b →Λ^∗γ B b-baryon decays (see Luis Miguel’s talk)

Samuel Coquereau CPAN 2018 29^thOctober 2018 18 / 18

Thank you for your attention

BACKUP

Samuel Coquereau CPAN 2018 29^thOctober 2018 18 / 18

Photon polarization in B

⁰

→ K

^∗

e

⁺

e

⁻

low q

²

I At lowq² B⁰ →K^∗e⁺e⁻ is dominated by the photon pole:

1 d(Γ + ¯Γ)/dq²

d³(Γ + ¯Γ) dcosθ_`dcosθ_Kdφ= 9

16π 3

4(1−F_L) sin²θ_K+F_Lcos²θ_K+ 1

4(1−F_L) sin²θ_K−F_Lcos²θ_K cos 2θ_`+ 1

2(1−F_L)A⁽²⁾_T sin²θ_Ksin²θ_{`</sub>cos 2φ+ (1−F<sub>L</sub>)A<sup>Re</sup><sub>T</sub> sin<sup>2</sup>θ<sub>`}cosθ_`+ 1

2(1−F_L)A^Im_T sin²θ_Ksin²θ_`sin 2φ

.

] c2 / [MeV

−) +e

−e π K+ m(

4800 5000 5200 5400

) 2cCandidates / (30 MeV/

0 5 10 15 20 25

30 Data

Model B⁰→K*⁰e+e⁻ e− e+ ) X

*0 K

→( B Combinatorial

LHCb

q²∈[0.002,1.120]

GeV²/c⁴

A⁽²⁾_T (q² →0) = 2Re(C₇C₇^0∗)

|C₇|²+|C₇⁰|², A^Im_T (q² →0) = 2Im(C7C₇^0∗)

|C₇|²+|C₇⁰|²