rare b-decays - cernrare decay observables • (differential) branching ratios – q2dependence for...

Lars Eklund on behalf of the LHCb Collaboration

Rare B-decays

Experimental overview

Results from Babar, Belle, LHCb, ATLAS and CMS

Why study rare processes?

• Rare decays: SM is suppressed– Sensitive to BSM contributions

• Flavour Changing Neutral Currents (FCNC)– Forbidden at tree-level in the SM– Further suppression, e.g. helicity

• Probes mass scales beyond direct searches– Up to ~ 100 TeV

1 September 2017 2

B(s) → µ+µ-

b → s l+ l-

B(s) → µ+µ-

10-9

b → s l+ l-

10-7 10-2

B → D(*) l+ 𝝂 -

~10-50

lepton flavour

violation

semi-leptonicEW penguinshelicity suppressed

0

baryon number violation

branching ratio

lepton flavour universality tests

L. Eklund

FCNC: theoretical description

• Processes at many different energy scales

• Described by an effective Hamiltonian– Oi (Operators): long-distance, non-perturbative physics– Ci (Wilson coefficients): short distance, high energy physics

• BSM processes may modify these coefficients

1 September 2017 3

0.2 GeV ... 4 GeV … 80 GeV … 10-100 TeVΛQCD Λb ΛEW ΛBSM

(non-perturbative) b mass W mass BSM scale Example of SM terms

i = 7

i = 9

i = 10

L. Eklund

Rare decay observables

• (Differential) branching ratios– q2 dependence for b → s l+ l-

• E.g. B0 → K*µ+µ-

• Angular distributions– b → s l+ l- decays

• Effective lifetime– Bs decays where ΔΓ$ ≠ 0

• Measure of CP violation

• BSM processes can modify the effective Hamiltonian by– Modifying Wilson coefficients of operators present in SM– Making Wilson coefficients dependent on lepton flavour– Introducing new operators

1 September 2017 L. Eklund 4

Invariant mass2 of di-leptons

Reproduced from arXiv:1606.00916v2

Overview of results

• Fully leptonic decays– B(s) → µ+µ- & B(s) → 𝛕+𝛕 -

• Electroweak b → s l+ l- transitions– B0 → K*µ+µ- and friends– Differential branching ratios & angular analysis– Global fits to Wilson coefficients

• Lepton flavour universality test (l+ = e+, µ+ or 𝛕+) – B → K(*) l+ l- decay rates – B → D(*) l+ 𝝂 decay rates

• Outlook

1 September 2017 5L. Eklund

Fully leptonic final states

B(s) → µ+µ- and friends

6

B(s) → μ+μ- branching ratio

• Very precise predictions available– Only C10 contribute in the SM:

– BSM scalar & pseudo-scalar operators may contribute• Change in decay rate• Mixing induced CP violation

• LHCb & CMS: Run 1 dataset– Observation of Bs → µ+µ- (6.2 σ)– Evidence for B0 → µ+µ- (3.0 σ)

1 September 2017 7

𝐵𝑅(𝐵, → 𝑙/𝑙0) ∝ 𝑚45 1 −4𝑚45

𝑚895

�𝐶

B(s) → μ+μ- branching ratio

• ATLAS: 25 fb-1 run I– First ATLAS result on B(s) → µ+µ-

• LHCb 3+1.4 fb-1 run I+II– First single experiment observation

• 7.9σ significance Bs → µ+µ-

– Effective lifetime of Bs → µ+µ-

1 September 2017 8

]9− [10)− µ +µ → s0BB(

0 1 2 3 4 5 6 7

]9− [1

0)−

µ +µ

→ 0B

B(

0.2−

0

0.2

0.4

0.6

0.8

CMS & LHCb

68.2

7%

95.4

5%

99.7

3%

ATLAS-1 = 7 TeV, 4.9 fbs

-1 = 8 TeV, 20 fbs

ATLAS

SM

) = 2.3,Lln(∆Contours for -2 L6.2, 11.8 from maximum of

Significance: 1.4σ (3.0 expected from SM)

)−µ+µ → 0sBF(B0 2 4 6 8

9−10×)− µ+ µ

→ 0B

F(B

00.10.20.30.40.50.60.70.80.9

9−10×

68.27%

95.45%

99.73%

99.99%

SM

LHCb

L. Eklund

Eur. Phys. J. C (2016) 76:513

PRL 118, 191801 (2017)

The heavier cousin: B(s) → 𝛕+𝛕 -

• LHCb 3 fb-1 run 1– Larger branching ratio due to the larger mass

• Additional enhancement possible in BSM scenarios that violate LFU

– Experimentally challenging: multiple neutrinos in the final state • Reconstructed in the mode• Normalisation Bs→D-(→ K+𝜋-𝜋-)Ds(→ K-K+𝜋+)

1 September 2017 9

Can

dida

tes

1

10

210

310

410

LHCb

DataTotal

Signal×1 −Background

Neural network output0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Pull

5−05

Fit to B(s) → 𝛕+𝛕 - NN output

L. Eklund

PRL 118, 251802 (2017)

Electroweak b → s l+ l- transitionsB0 → K*µ+µ- and friends

b → s l+ l- differential branching ratios

• LHCb: differential branching fractions in several b → s l+ l- modes– Versus di-muon invariant mass squared: q2

• Discrepancy in the low di-muon invariant mass region


]4c/2 [GeV2q0 5 10 15 20

]2/G

eV4 c ×

-8 [1

02 q

/dBd 01

2

3

4

5

LCSR Lattice Data

−µ+µ0 K→0BLHCb

]4c/2 [GeV2q0 5 10 15 20

]2/G

eV4 c ×

-8 [1

02 q

/dBd 01

2

3

4

5

LCSR Lattice Data

LHCb−µ+µ+ K→+B

]4c/2 [GeV2q0 5 10 15 20

]2/G

eV4 c ×

-8 [1

02 q

/dBd 0

5

10

15

20LCSR Lattice Data

LHCb−µ+µ*+ K→+B

]4c/2 [GeV2q0 5 10 15

]2/G

eV4 c [2 q

/dB d

0

0.05

0.1

0.156−10×

LHCb

]4c/2 [GeV2q0 5 10 15 20

]-1 )4 c/2

(GeV

-7 [1

02 q

) / d

µ µ Λ

→ bΛ(Bd 0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

LHCb

SM prediction

Data

]4c/2 [GeV2q5 10 15

]4 c

-2G

eV-8

[10

2q

)/d

µµ

φ→

s0B

dB

( 0

1

2

3

4

5

6

7

8

9LHCb

SM pred.

Data

JHEP 06 (2014) 133: 1 fb-1

JHEP 11 (2016) 047: 3 fb-1 JHEP 06 (2015) 115: 3 fb-1 JHEP 09 (2015) 179: 3 fb-1

BS → ϕµ+µ-ΛB → Λµ+µ-

B0 → K*µ+µ-

b → s l+ l- differential branching ratios

• CMS: 20.5 fb-1 run I– B0 → K*µ+µ- differential branching ratio– Compatible with SM & LHCb results


)2 (GeV2q2 4 6 8 10 12 14 16 18

)2− G

eV× 8−

(10

2 q /

dΒd

0

2

4

6

8

10

12Data

〉 SM, LCSR 〈〉 SM, Lattice 〈

CMS (8 TeV)1−20.5 fb

Phys. Lett. B 753 (2016) 424

b → s l+ l- angular observables

• Angles defined in B meson rest frame– Angular variables: θl, θK & Φ– Di-lepton invariant mass2: q2

– Fit PDF for B0 → K*µ+µ-

• Form-factor uncertainties in the predictions of observables

– Alternative set of 7 observables with reduced uncertainties: 𝑃@

(A)


For instance:

B → K* μ+ μ– angular analysis

• LHCb: 3 fb-1 run I data set– Full angular analysis B → K* µ+ µ–

– Local SM deviation P5’• 2.8σ in 4 < q2 < 6 GeV2/c4

• 3.0σ in 6 < q2 < 8 GeV2/c4

– Fit to C9 3.4σ from SM

• Belle: 711 fb-1 full data set– Both e± and µ± modes– Local SM deviation P5’

• 2.6σ in 4 < q2 < 8 GeV2/c4 in the muon mode

• e± mode consistent with SM


]4c/2 [GeV2q0 5 10 15

5'P

-2

-1

0

1

2LHCb

SM from DHMV

PRL 118, 111801 (2017)

JHEP02 (2016) 104

B → K* μ+ μ – angular analysis

• ATLAS: 20.3 fb-1 run I data

• CMS: 20.5 fb-1 run 1 data

– Recent, preliminary result– Compatible with SM– Compatible with LHCb & Belle


ATLAS-CONF-2017-023

CMS PAS BPH-15-008

Global fits to Wilson coefficients

• Is there a pattern in these deviations?• Global fits of Wilson coefficients

– Approximately 100 observables• B(s) → µ+µ- and b → s ɣ BR• b → s l+ l- BR & angular observables

– Several global fits in literature• Fits prefer BSM contribution to C9• 3-5σ from SM depending on constraints

• Possible interpretations– BSM physics contributions

• e.g. Z’ of a few TeV– Limits in our understanding of QCD

• E.g. contributions from charm loops


Branching Ratios

Angular Observables HPiLAll

-3 -2 -1 0 1 2 3-3

-2

-1

0

1

2

3

C9NP

C 10NP

𝐶@ = 𝐶@CD + 𝐶@FGB → K* 𝛕 + 𝛕 – not yet observedBabar limit BR < 2.3 x 10-3PRL 118, 031802 (2017)

JHEP 06 (2016) 092

Lepton flavour universality

Ratios of branching ratios – comparing lepton flavour

1 September 2017 17

See talk by Anna Lupato:“Lepton flavour universality tests at LHCb”Sunday 15:40 in ‘Test of symmetries and conservation laws’

LFU: B → K(*) μ+ μ – vs. B → K(*) e+ e –

• Lepton flavour universality test– Close to unity in SM

• Measurements by Belle & Babar – Combines B0 & B+ ratios– R(K) and R(K*) for different q2 ranges

• Consistent with SM prediction

• LHCb 3 fb-1 run I: RK– Measured as a double ratio

• 2.6σ deviation from SM


𝑅H(∗) =ℬ(𝐵=// → 𝐾=// ∗ 𝜇/𝜇0)ℬ(𝐵=// → 𝐾=// ∗ 𝑒/𝑒0)

Summary plot from arXiv:1606.00916v2

𝑅H =ℬ(𝐵/ → 𝐾/𝜇/𝜇0)

ℬ(𝐵/ → 𝐾/ 𝐽 𝜓⁄ → 𝜇/𝜇0 )ℬ(𝐵/ → 𝐾/ 𝐽 𝜓⁄ → 𝑒/𝑒0 )

ℬ(𝐵/ → 𝐾/𝑒/𝑒0)S

PRL 113 (2014)151601

Babar: PRD 86 (2012) 032012Belle: PRL 103 (2009) 171801

LFU: B → K(*) μ+ μ – vs. B → K(*) e+ e –

• LHCb: 3 fb-1 run I: RK*– Tension with SM

• 2.1-2.3σ in 0.045 < q2 < 1.1 GeV2/c4

• 2.2-2.4σ in 1.1 < q2 < 6.0 GeV2/c4

• Fit to Wilson coefficients– Allowing for 𝐶@TFG ≠ 𝐶@UFG

• 3.5σ from SM • Preference for 𝐶VUFG ≠ 0


𝑅H∗ =ℬ(𝐵= → 𝐾=∗𝜇/𝜇0)ℬ(𝐵= → 𝐾=∗𝑒/𝑒0) 0 1 2 3 4 5 6

q2 [GeV2/c4]

0.0

0.2

0.4

0.6

0.8

1.0RK

⇤0

LHCb

LHCb

BIP

CDHMV

EOSflav.ioJC

JHEP01 (2017) 055

��

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PSI-PR-17-05a

rXiv:1704.05340 𝐶@ = 𝐶@CD + 𝐶@FG

R(D)0.2 0.3 0.4 0.5 0.6

R(D

*)0.2

0.25

0.3

0.35

0.4

0.45

0.5 BaBar, PRL109,101802(2012)Belle, PRD92,072014(2015)LHCb, PRL115,111803(2015)Belle, PRD94,072007(2016)Belle, PRL118,211801(2017)LHCb, FPCP2017Average

SM Predictions

= 1.0 contours2χ∆

R(D)=0.300(8) HPQCD (2015)R(D)=0.299(11) FNAL/MILC (2015)R(D*)=0.252(3) S. Fajfer et al. (2012)

HFLAV

FPCP 2017

) = 71.6%2χP(

σ4

σ2

HFLAVFPCP 2017

LFU: B → D(*) 𝛕+𝝂 vs. B → D(*) l+ 𝝂 - (l = e or μ)

• LFU ratios for semi-leptonic D(*) decays• RD measurements

– Belle & Babar• Hadronic tag, 𝜏0 → 𝑙0�̅�4𝜐[

• RD* measurements– Belle & Babar

• Hadronic tag, 𝜏0 → 𝑙0�̅�4𝜐[– Belle

• Hadronic tag, 𝜏0 → 𝜋0𝜐[, 𝜏0 → 𝜌0𝜐[• Semi-leptonic tag, 𝜏0 → 𝑙0�̅�4𝜐[

– LHCb 3 fb-1 (run I)• 𝜏0 → 𝑙0�̅�4𝜐[• 𝜏0 → 𝜋0𝜋/𝜋0𝜐[


𝑅^(∗) =ℬ(𝐵=/0 → 𝐷=/0 ∗ 𝜏/𝜈[)ℬ(𝐵=/0 → 𝐷=/0 ∗ 𝑙/𝜈4)

Tension with SM prediction• 2.3σ in RD• 3.4σ in RD*• 4.1σ combined

Interpretations of the 𝑅^(∗) results

• Example of BSM model that would influence 𝑅^(∗)– 2-Higgs-dublet model Type II

• Parametrised by tan(𝛽) 𝑚de⁄– Affects both measured and predicted 𝑅^(∗)


Comparison with Babar hadronic result Comparison with Belle hadronic result

Not consistent with 2HDM type II

Consistent with 2HDM type IIwith tan(𝛽) 𝑚de⁄ ~0.45

PRD 88 (2013) 072012

PRD 92 (2015) 072014

Outlook

Many exciting results in the years to come

1 September 2017 22

LHCb datasets

• LHCb results almost exclusively from 3 fb-1 Run I data– Run II doubles signal yield/fb-1

• Increased 𝑏𝑏j cross section

• LHCb Phase-I upgrade– Remove H/W trigger – 40 Mhz readout

• Doubles signal yield/fb-1 for hadronic channels• Construction progressing at full speed

• LHCb Phase-II upgrade– Preliminary studies ongoing

• Major detector upgrade• EoI submitted


CERN-LHCC-2017-003

SuperKEKB luminosity projection

Goal of Be!e II/SuperKEKB

9 months/year20 days/month

Inte

grat

ed lu

min

osity

(a

b-1 )

Peak

lum

inos

ity

(cm

-2s-

1 )

Calendar Year

Belle II, ATLAS & CMS

• Babar & Belle datasets– 550 + 1000 fb-1

• Belle II: 50 ab-1– Physics data taking from 2018

• See previous speaker (Mikihiko Nakao)– Dedicated talk by Jake Bennet

• Sunday, 14:50 (accelerators & detectors)

• ATLAS & CMS: results from 20-25 fb-1– ~ 80 fb-1 on tape

• Expect ~300 fb-1 by 2023– 3 ab-1 for sLHC upgrade era– Many important results are expected


Physics prospects

• Belle II: RD & RD*

• LHCb Phase I & II


Currentprecision

Belle II 5 ab-1

Belle II50 ab-1

RD 12% 5.6% 3.2%RD* 5.4% 3.9% 2.2%RK 12% 7% 2%

B(s) → µ+µ-Current

precisionPhase-I50 fb-1

Phase-II300 fb-1

σBR x10-9 0.7 0.15 0.07

Effective lifetime στ

25% 5% 2%

ATLAS & CMS are competitive in B(s) → µ+µ-

Prediction for B0 → K*µ+µ-CERN-LHCC-2017-003CERN-LHCC-2012-007 CERN

-LHC

C-2017-003

arXiv:1002.5012 andEPS-HEP 2017 talk by S. Falke

Summary

• Rare B-decays are an excellent probe for BSM physics

• B(s) → µ+µ- branching ratios consistent with SM– Within experimental precision

• Intriguing deviations from SM predictions– b → s l+ l- branching ratios & angular observables– Lepton flavour universality test: 𝑅H(∗) and 𝑅^(∗)

• Exciting results expected from– Data already recorded– Upgraded detectors


Backup slides

Bs → μ+μ- effective lifetime

• LHCb update: 3 fb-1 (run I) + 1.4 fb-1 (run 2)• New observable sensitive to CP violation

– AΔΓ = 1 in SM (i.e. no CPV)

1 September 2017 28

Decay time [ps]0 5 10

can

dida

tes /

(1 p

s)− µ+ µ

→ s0W

eigh

ted

B

0

2

4

6

8

Effective lifetime fit

LHCb

𝜏Tkk = 𝜏8l = 1.619 ± 0.009

De Bruyn et al., PRL 109, 041801 (2012)

Measurement for the sLHC era

L. Eklund

B0 → K* e+ e– angular analysis

• LHCb: 3 fb-1 run I– Particular interest in the low q2 region

• Sensitivity to photon polarisation• Wilson coefficients C7 & C7’

– Folded angular distributions• Increase sensitivity for low yields• 4 observables


]2c/ [MeV)−e+e−π+K(m4500 5000 5500 6000

) 2 cC

andi

date

s / (

40 M

eV/

0

20

40

60

80

100

DataModel

−e+e0*K → 0B−e+e)X0*K (→B

Combinatorial

LHCb

0.002 < q2 < 1.12 GeV2/c4

JHEP04 (2015) 064

Belle: 𝛕 polarisation in B → D* 𝛕+ 𝝂

• Belle full dataset– Complementary observable– Hadronic tag, 𝜏0 → 𝜋0𝜐[, 𝜏0 → 𝜌0𝜐[

• Polarisation of 𝛕-

– Γ±(D*): decay rate with 𝛕 helicity ±½– Differential BR


Result:𝑃[CD = −0.497 ± 0.013𝑃[ = −0.38 ± 0.510=.

rare b-decays - cernrare decay observables • (differential) branching ratios – q2dependence for...

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