Rare B-decays - CERNRare decay observables • (Differential) branching ratios – q2dependence for...
Transcript of 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
1 September 2017 11L. Eklund
]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
1 September 2017 L. Eklund 12
)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)
1 September 2017 L. Eklund 13
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
1 September 2017 14L. Eklund
]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
1 September 2017 15L. Eklund
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
1 September 2017 16L. Eklund
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
1 September 2017 18L. Eklund
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
1 September 2017 L. Eklund 19
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
���
����
-� -� -� � � � �-�
-�
-�
�
�
�
�
�� �
�����
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[
1 September 2017 20L. Eklund
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^(∗)
1 September 2017 L. Eklund 21
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
1 September 2017 23L. Eklund
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
1 September 2017 24L. Eklund
-
Physics prospects
• Belle II: RD & RD*
• LHCb Phase I & II
1 September 2017 25L. Eklund
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
1 September 2017 26L. Eklund
-
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
1 September 2017 L. Eklund 29
]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
1 September 2017 L. Eklund 30
Result:𝑃[CD = −0.497 ± 0.013𝑃[ = −0.38 ± 0.510=.