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LHCb results on rare leptonic decaysof B-mesonsQFTHEP, Sochi Russia, 25 september 2019Evgenii Kurbatov on behalf of the LHCb collaborationNational Reseasrch University Higher School of Economics
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 1
LHC
Arpad Horvath c©
The Large Hadron Collider (LHC) is the world’s largest and most powerful
particle collider. It lies in a tunnel 27 kilometres in circumference and as
deep as 175 metres beneath the France–Switzerland border near Geneva.
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 2
LHCb detector
Int. J. Mod. Phys. A30 (2015) 1530022
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 3
LHC runs
Two datasets of different size with different kinematics
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 4
Motivation
• Their time-integrated branching fractions are predicted in
the SM with small uncertainty due to absence of hadrons
in final state
• The decays are sensitive probes for physics beyond the SM
• Within the Standard Model (SM) of particle physics, fully
leptonic decays of B mesons are very rare
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 5
Decays overview:
• Measurements of the B0s → µ+µ− branching fraction andeffective lifetime and search for B0 → µ+µ− decays
• Search for the decays B0s → τ+τ− and B0 → τ+τ−
• Search for the lepton-flavour violating decays
B0(s) → e±µ∓
• Search for the lepton-flavour-violating decays
B0s → τ±µ∓ and B0 → τ±µ∓
• Search for the rare decay B+ → µ+µ−µ+νµ
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 6
Measurements of the B0s → µ+µ−branching fraction and effectivelifetime and search for B0 → µ+µ−decays
Results are based on data collected with the LHCb detector,
corresponding to an integrated luminosity of 1 fb−1 of pp
collisions at a centre-of-mass energy√s = 7 TeV, 2 fb−1 at√
s = 8 TeV and 1.4 fb−1 recorded at√s = 13 TeV.
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 7
Normalization
Normalization channels are: B+ → J/ψK+ and B0 → K+π−
B(B0(s) → µ+µ−) =Bnorm �norm fnormNnorm �sig fd(s)
×NB0(s)→µ+µ− ≡
≡ αnormB0(s)→µ+µ−
×NB0(s)→µ+µ−
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 8
B0s → µ+µ−
]2c [MeV/−µ+µm5000 5200 5400 5600 5800 6000
)2C
andi
date
s / (
50
MeV
/c
0
5
10
15
20
25
30
35 Total−µ+µ → s0B−µ+µ → 0B
Combinatorial−
h'+ h→ (s)0B
µν+µ)−(K−π → (s)
0B−µ+µ0(+)π → 0(+)B
µν−µ p→ b
0Λ
µν+µψ J/→ +cB
LHCb
BDT > 0.5
Phys. Rev. Lett. 118, 191801
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 9
B0s → µ+µ−
Decay time [ps]0 5 10
can
dida
tes
/ (1
ps)
− µ+ µ
→ s0W
eigh
ted
B
0
2
4
6
8
Effective lifetime fit
LHCb
Phys. Rev. Lett. 118, 191801
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 10
B0s → µ+µ−• B0s → µ+µ− signal is seen with a significance of 7.8
standard deviations
• The B0s → µ+µ− branching fraction is measured to be(3.0± 0.6+0.3−0.2)× 10−9, where the first uncertainty isstatistical and the second systematic (Phys. Rev. Lett.
118, 191801)
• SM prediction: B(B0s → µ+µ−) = (3.65± 0.23)× 10−9(PRL 112(2014)101801)
• Previous results from LHCb + CMS data analysis:
B(B0s → µ+µ−) = 2.8+0.7−0.6 × 10−9 (Nature 522, 68–72 (04 June2015))
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 11
• B0s → µ+µ− effective lifetimeτ(B0s → µ+µ−) = 2.04± 0.44± 0.05ps , where the firstuncertainty is statistical and the second systematic
• No evidence for a B0 → µ+µ− signal is found,B(B0 → µ+µ−) < 3.4× 10−10 at 95% confidence
• Previous results from LHCb + CMS data analysis:
B(B0 → µ+µ−) = 3.9+1.6−1.4 × 10−10 (Nature 522, 68–72 (04June 2015))
• All results are in agreement with the Standard Model
expectations
(Phys. Rev. Lett. 118, 191801 )
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 12
Search for the decays B0s → τ+τ− andB0 → τ+τ−
The analysis is performed with proton–proton collision data
corresponding to integrated luminosities of 1.0 fb−1 and 2.0
fb−1 recorded with the LHCb detector at centre-of-mass
energies of 7 and 8 TeV, respectively. The τ leptons are
reconstructed through the decay τ → π−π+π−ντ
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 13
(Phys. Rev. Lett. 118 (2017) 251802)
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 14
B0(s) → τ+τ−• Assuming no contribution from B0 → τ+τ− ,
B(B0s → τ+τ−) < 6.8× 10−3 at 95% CL• If no contribution from B0s → τ+τ− is assumed,
B(B0 → τ+τ−) < 2.1× 10−3 at 95% CL• Previous limit (BaBar collaboration): B(B0 → τ+τ−)< 4.10× 10−3 at 90% CL(Phys. Rev. Lett. 96 (2006) 241802)
• These results correspond to the first direct limit on
B(B0s → τ+τ−) and the world’s best limit onB(B0 → τ+τ−)
(Phys. Rev. Lett. 118 (2017) 251802)
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 15
Search for the lepton-flavour violatingdecays B0(s) → e±µ∓
A search for the lepton-flavour violating (LFV) decays
B0(s) → e±µ∓ was performed using pp collision data collectedat centre-of-mass energies of 7 and 8TeV, corresponding to a
total integrated luminosity of 3fb−1.
Two normalisation channels were used: the B0 → K+π−decay which has a similar topology to that of the signal, and
the B+ → J/Ψ K+ decay, with J/Ψ→ µ+µ− ,which has anabundant yield and a similar purity and trigger selection
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 16
5
10
15[0.25, 0.4]
5
10
15[0.25, 0.4]
5
10
15[0.4, 0.5]
5
10
15[0.4, 0.5]
5
10
15[0.5, 0.6]
5
10
15[0.5, 0.6]
)2 cC
andi
date
s / (
50
MeV
/
2
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8
10[0.6, 0.7]
2
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8
10[0.6, 0.7]
2
4
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8
10[0.7, 0.8]
2
4
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8
10[0.7, 0.8]
2
4
6
8
10[0.8, 0.9]
2
4
6
8
10[0.8, 0.9]
]2c [MeV/±µ±em5000 5200 5400 5600 5800
2
4
6
8
10[0.9, 1.0]
]2c [MeV/±µ±em5000 5200 5400 5600 5800
2
4
6
8
10[0.9, 1.0]
DataTotalCombinatorial
ν−µp → b0Λ
ν+µ−π → 0B
±
µ±e → s0B ±
µ±e → 0B
LHCb
(JHEP 1803 (2018) 078)
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 17
B0(s) → e±µ∓
• No excesses are observed and upper limits on the branchingfractions are set to B(B0s → e±µ∓)< 6.3× 10−9 and(B0 → e±µ∓)< 1.3× 10−9 at 95% CL
• These results represent the best upper limits to date and are afactor 2 to 3 better than the previous results from LHCb(B(B0s → e±µ∓)< 1.4× 10−8 and B(B0 → e±µ∓)< 3.7× 10−9 at95% CL) (Phys. Rev. Lett. 111 (2013) 141801)
(JHEP 1803 (2018) 078)
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 18
Search for the lepton-flavour-violatingdecays B0s → τ±µ∓ and B0 → τ±µ∓
A search for B0s → τ±µ∓ and B0 → τ±µ∓ decays isperformed using data corresponding to an integrated
luminosity of 3 fb−1 of proton-proton collisions, recorded with
the LHCb detector in 2011 and 2012.
The τ lepton is reconstructed in the τ → π−π+π−ντ channel.
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 19
)2cB_BDFplus_M_1 (GeV/
)2 cC
and.
/ (0
.05
GeV
/
0200
400
600800
1000
12001400
16001800
2000
BDT bin 1LHCb
]2c [GeV/BM4.6 4.8 5 5.2 5.4 5.6 5.8
Pull
3−2−1−0123 )2cB_BDFplus_M_1 (GeV/
)2 cC
and.
/ (0
.05
GeV
/
0
100
200
300
400
500
BDT bin 2LHCb
]2c [GeV/BM4.6 4.8 5 5.2 5.4 5.6 5.8
Pull
3−2−1−0123
)2cB_BDFplus_M_1 (GeV/
)2 cC
and.
/ (0
.05
GeV
/
0
50
100
150
200
250
300
BDT bin 3LHCb
]2c [GeV/BM4.6 4.8 5 5.2 5.4 5.6 5.8
Pull
3−2−1−0123 )2cB_BDFplus_M_1 (GeV/
)2 cC
and.
/ (0
.05
GeV
/
0
20
40
60
80
100
120
BDT bin 4LHCb
]2c [GeV/BM4.6 4.8 5 5.2 5.4 5.6 5.8
Pull
3−2−1−0123
(arXiv:1905.06614)
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 20
B0(s) → τ±µ∓
• B(B0s → τ±µ±) < 4.2×10−5 at 95% CL (Assuming nocontribution from B0 → τ±µ±)
• B(B0 → τ±µ±) < 1.4×10−5 at 95% CL (Assuming nocontribution from B0s → τ±µ±)
• These are the first limit on B(B0s → τ±µ±) and theworld’s best limit on B(B0 → τ±µ±)
arXiv:1905.06614
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 21
Search for the rare decayB+ → µ+µ−µ+νµ
A search for the rare leptonic decay B+ → µ+µ−µ+νµ isperformed using proton-proton collision data corresponding to
an integrated luminosity of 4.7 fb−1 collected by the LHCb
experiment. The search is carried out in the region where the
lowest of the two µ+µ− mass combinations is below 980
MeV/c2. The branching fraction of a B+ → µ+µ−µ+νµsignal is obtained by normalising to the
B+ → J/Ψ(→ µ+µ−)K+ decays.
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 22
u
b̄ b̄
B∗+
ν
µ+
µ+
µ−γ∗
W+
B+
u
b̄ u
ρ, ω
µ−
µ+
µ+
νW+
γ∗
B+
ν
µ+
µ+
µ−
γ∗
W+B+
(Eur. Phys. J. C 79 (2019)
675)
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 23
]2c [MeV/corrM4000 5000 6000 7000
)2 cC
andi
date
s / (
50 M
eV/
0
20
40
60
80
100
120
140
160
2011-16LHCb
Total FitDataCombinatorialMisidentifiedPartially reconstructedPrediction fromPAN (2018) 81:347
Eur. Phys. J. C 79 (2019) 675)
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 24
B+ → µ+µ−µ+νµ• No signal is observed for the B+ → µ+µ−µ+νµ decay• An upper limit of 1.6× 10−8 at 95% confidence level is
set on the branching fraction, where the lowest of the two
µ+µ− mass combinations is below 980MeV/c2.
• The limit for the full kinematic region stays the same
under the assumption that the decay is dominated by
intermediate vector mesons.
Eur. Phys. J. C 79 (2019) 675)
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 25
Conclusions
• Lot of rare B mesons decays were studied by the LHCb
team
• All results are consistent with the Standard Model
• Nearly all results presented are either unique or the most
accurate for the time
September 25, 2019 LHCb results on rare leptonic decays of B-mesons 26
home.cern
http://home.cern
IntroductionMeasurements of the B0s+- branching fraction and effective lifetime and search for B0+- decaysSearch for the decays B0s+- and B0+-Search for the lepton-flavour violating decays B0(s) eSearch for the lepton-flavour-violating decays B0s and B0