Download - LHCb results on rare leptonic decays of B-mesonsqfthep.sinp.msu.ru/talks2019/1569406784_Kurbatov_LHCb... · 2019. 9. 25. · Evgenii Kurbatov on behalf of the LHCb collaboration National

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.


LHCb detector

Int. J. Mod. Phys. A30 (2015) 1530022


LHC runs

Two datasets of different size with different kinematics


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


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


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.


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−


B0s → µ+µ−

]2c [MeV/−µ+µm5000 5200 5400 5600 5800 6000

)2C

andi

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/c

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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


B0s → µ+µ−

Decay time [ps]0 5 10

can

dida

tes

/ (1

ps)

− µ+ µ

→ s0W

eigh

ted

B

0

2

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Effective lifetime fit

LHCb

Phys. Rev. Lett. 118, 191801


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))


• 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 )


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


(Phys. Rev. Lett. 118 (2017) 251802)


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)


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


5

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15[0.25, 0.4]

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)2 cC

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]2c [MeV/±µ±em5000 5200 5400 5600 5800

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10[0.9, 1.0]

]2c [MeV/±µ±em5000 5200 5400 5600 5800

2

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10[0.9, 1.0]

DataTotalCombinatorial

ν−µp → b0Λ

ν+µ−π → 0B

±

µ±e → s0B ±

µ±e → 0B

LHCb

(JHEP 1803 (2018) 078)


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)


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.


)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

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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

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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

/

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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)


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


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.


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)


]2c [MeV/corrM4000 5000 6000 7000

)2 cC

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eV/

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2011-16LHCb

Total FitDataCombinatorialMisidentifiedPartially reconstructedPrediction fromPAN (2018) 81:347

Eur. Phys. J. C 79 (2019) 675)


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)


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


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