charmless b-hadron decays - cern · 2017-08-18 · c vetoes, no signiÞcant signal is observed in...

Charmless b-hadron decays

Giulio Dujany

on behalf of the LHCb collaboration

13-19/08/2017 XIIIe Rencontres du Vietnam

Overview

1 Introduction

2 Observation of the B0s → pΛK− decay [Phys. Rev. Lett. 119, 041802

(2017)]

3 Observation of the B0(s)→ pph+h′− decays [arXiv:1704.08497, Accepted

by PRD]

4 First observation of the B0→ pp decay

5 Update of the branching fraction measurements of the B0(s)→ K0

S h+h′−

decays [arXiv:1707.01665, Submitted to JHEP]

6 Conclusions

Giulio Dujany (LPNHE, CNRS) Charmless b-hadron decays Flavour 2017 2 / 25

https://arxiv.org/abs/1704.07908





Baryonic B decays – motivationsMany baryonic B decays still to be studied / observedMulti-body final state tend to have larger BF that 2-body ones

B(B0 → Λ−c pπ+π−) ∼ 10−3 � B(B0 → Λ−

c pπ0) ∼ 10−4

� B(B0 → Λ−c p) ∼ 10−5

B(B0 → ppK0) ∼ 10−6 � B(B0 → pp) ∼ 10−8

Threshold enhancement in baryon-antibaryon systems observed inmany decay modes (see eg. [Eur. Phys. J. C74 (2014) 3026])

Threshold enhancement observed forthe first time by Belle in B± → ppK±

[Phys. Rev. Lett. 88 (2002) 181803]0

20

40

60

80

100

120

2 2.5 3 3.5 4 4.5Mpp

_ (GeV/c2)

dN /

dMpp_

(Eve

nts

/ (G

eV/c

2 ))

Even

ts /

(5 M

eV/c

2 )

0

5

10

3 3.05 3.1 3.15

FIG. 2. The fitted yield divided by the bin size forB+ → pp̄K+ as a function of pp̄ mass. The charm veto isapplied. The distribution from non-resonant B+ → pp̄K+

MC simulation is superimposed. The inset shows the pp̄ massdistribution for the J/ψK+ signal region.

the ECL shower width distribution is consistent with MCexpectations for the proton and anti-proton candidates.In addition, we check the branching fraction as the cutson the proton and anti-proton probabilities and likeli-hood ratio are varied. We do not observe any systematictrends beyond statistics.

To verify the analysis procedure and branching frac-tion determination, we remove the J/ψ veto and ex-amine the decay chain B+ → J/ψK+, J/ψ → pp̄. Aclear signal of 26.4 ± 5.2 events is then observed in the∆E spectrum. We also observe 25.9 ± 5.1 events inthe Mbc distribution. The pp̄ invariant mass spectrumfor J/ψK+ signal candidates is shown as an inset inFig. 2. We use the ∆E yield and the MC detectionefficiency of 0.30 to determine the branching fractionB(B+ → J/ψK+) = (13.1 ± 2.6) × 10−4. This is ingood agreement with the PDG world average, B(B+ →J/ψK+) = (10.0±1.0)×10−4 [12], which was obtained byexperiments that reconstruct the J/ψ in dilepton modes.

We also examined two related decay modes B0 →pp̄KS and B+ → pp̄π+ that may help clarify the inter-pretation of the signal. Measurement of B0 → pp̄KS willhelp to determine the role of the spectator quark in b → sdecays with baryons, while observation of B+ → pp̄π+

will constrain the ratio of the b → u tree and b → spenguin diagrams in decays with baryons.

For B0 → pp̄KS, after the application of the charm

and Λc vetoes, no significant signal is observed in ei-ther the ∆E or Mbc distribution. A fit to the ∆E dis-tribution gives 6.4+4.4

−3.7 events. Applying the Feldman-Cousins procedure [13], we obtain an upper limit ofless than 16 events at the 90% confidence level (C.L.).After reducing the detection efficiency by the system-atic error, we obtain an upper limit at 90% C.L. ofB(B0 → pp̄K0) < 7.2 × 10−6.

In the B+ → pp̄π+ mode, after the application ofthe charm veto we perform a fit to the ∆E distributionthat allows for B+ → pp̄π+ signal and a reflection frommisidentified B+ → pp̄K+ decays. This fit gives a signalyield of 16.2+8.6

−8.0 events and a significance of 2.1σ. Theexcess in the ∆E fit corresponds to a branching fractionB(B+ → pp̄π+) = (1.9+1.0

−0.9±0.3)×10−6 or an upper limitof B(B+ → pp̄π+) < 3.7 × 10−6 at 90% C.L. after takinginto account the systematic error.

We have observed a significant signal (5.6σ) for thedecay B+ → pp̄K+. This is the first b → s decay modewith baryons in the final state. In the future, this modecan be used to search for direct CP violation [2]. Wefind that its pp̄ mass spectrum is inconsistent with phasespace and is peaked toward low mass. This feature is sug-gestive of quasi two-body decay. It is also possible thatthe decay is a genuine three-body process and that thisfeature of the M(pp̄) spectrum is a baryon form factoreffect [14,15].

We wish to thank the KEKB accelerator group for theexcellent operation of the KEKB accelerator. We ac-knowledge support from the Ministry of Education, Cul-ture, Sports, Science, and Technology of Japan and theJapan Society for the Promotion of Science; the Aus-tralian Research Council and the Australian Departmentof Industry, Science and Resources; the National Sci-ence Foundation of China under contract No. 10175071;the Department of Science and Technology of India; theBK21 program of the Ministry of Education of Koreaand the CHEP SRC program of the Korea Science andEngineering Foundation; the Polish State Committee forScientific Research under contract No. 2P03B 17017; theMinistry of Science and Technology of the Russian Fed-eration; the Ministry of Education, Science and Sportof Slovenia; the National Science Council and the Min-istry of Education of Taiwan; and the U.S. Departmentof Energy.

[1] Hereafter, the inclusion of the charge conjugate mode isimplied.

[2] G. Eilam, M. Gronau, and J. Rosner, Phys. Rev. D 39,819 (1989).

[3] X. Fu et al. (CLEO Collaboration), Phys. Rev. Lett. 79,

5



https://arxiv.org/abs/hep-ex/0202017

Baryonic B decays – short historyB factories

2002: First observation of a baryonic B decay [Phys. Rev. Lett. 88 (2002)181803]Observation and study of many B0 and B+ baryonic decays, both withcharm in the final state and charmlessObserved threshold enhancement in baryon-antibaryon invariant massof several decays

LHCb2013: First observation of 2-body charmless baryonic modeB+ → pΛ(1520) and first evidence of CP violation in a baryonic B decayseen in B+ → ppK+ [Phys. Rev. Lett. 113, 141801 (2014)]2013: First evidence of the very suppressed B0 → pp [J. High Energy Phys.10 (2013) 005]2014: First observation of a baryonic B+

c decay (B+c → J/ψppπ+) [Phys.

Rev. Lett. 113, 152003 (2014)]









Some aspects common to the following analyses

DatasetLHCb Run I dataset (3 fb−1)

Selection strategyTrigger + loose preselectionMultivariate classifier (BDT or neural network) against combinatorialbackgroundParticle identification criteria on charged hadrons (each candidate onlyin one final state sample)Remove explicitly charm mesons, baryons and charmonia (D0,Λc,J/ψ , . . . ) with a veto on the concerned mass window


Observation of theB0

s → pΛK− decay

[Phys. Rev. Lett. 119, 041802 (2017)]



Analysis strategy [Phys. Rev. Lett. 119, 041802 (2017)]

Goal: Observe first B0s baryonic decay

I Look for three-body decay as larger BF expected than 2-body

Signal: B0s → pΛK−

Normalisation mode: B0 → pΛπ−

Bsgn

Bnorm=

fd

fs

Nsgn

Nnorm

εnorm

εsgn

Categories: different years andreconstruction categories

[JINST 10 (2015) P02007]




Fit [Phys. Rev. Lett. 119, 041802 (2017)]

Perform a simultaneous fit to signal (B0s → pΛK−) and normalisation

mode (B0 → pΛπ−) for all categoriesI Share shape parametersI Constrain cross-feed background from different final states

Backgrounds considered: combinatorial, B0s → pΣK−, B0 → pΣπ−

m(pΛK−) [ MeV/c2]

4900 5000 5100 5200 5300 5400 5500 5600Can

did

ates/

(10

MeV/c

2)

LHCb

0

20

40

60

80

100Total fitB0 → pΛπ−

B0s → pΛK−

B0→ pΣ0π−

B0s → pΣ

0K−

Comb. bkg.

m(pΛπ−) [MeV/c2]

4900 5000 5100 5200 5300 5400 5500 5600Can

didates/(10MeV/c

2)

LHCb

020406080100120140160180200220

Observe B0s → pΛK− with statistical significance> 15σ

(234± 29 signal candidates from the fit to all the categories)



Results [Phys. Rev. Lett. 119, 041802 (2017)]

First observation of a baryonic B0s decay!

Branching fraction measured to be

B(B0s → pΛK−) +B(B0

s → pΛK+) = [ 5.46±0.61(stat)±0.57(syst)

±0.50(Bnorm) ±0.32(fs/fd)]× 10−6

Observed threshold enhancement in pp mass spectra (efficiencycorrected and background subtracted)

m(pΛ) [MeV/c2]

2500 3000 3500 4000 4500 5000

Normalized

Weigh

ts

LHCb

0

0.1

0.2

0.3

0.4

0.5

B0→ pΛπ−

m(pΛ) [MeV/c2]

2500 3000 3500 4000 4500

Normalized

Weigh

ts

LHCb

0

0.1

0.2

0.3

0.4

0.5

B0s → pΛK−



Observation of theB0(s)→ pph+h′− decays

[arXiv:1704.08497, Accepted by PRD]



Analysis strategy [arXiv:1704.08497, Accepted by PRD]

Goal: Observe and measure BF of charmless B0(s)→ pph+h′− modes

I Only known B(B0→ ppK∗0) = (1.24+0.28−0.25)× 10−6 (BaBar and Belle)

Signal: B0(s)→ pph+h′− (h(′) = K or π)

Normalisation mode: B0→ (J/ψ→ pp)(K∗0→ K+π−), same selectionas signal except charm vetoesSimultaneous fit to invariant mass of 3 final states3-D fit in m(ppK±π∓), m(pp) and m(K±π∓) for normalisation mode



Simultaneous signal fits [arXiv:1704.08497, Accepted by PRD]

]2c) [MeV/KKpp(m5200 5300 5400 5500

)2 cC

andi

date

s / (

8 M

eV/

0

20

40

60

80

100

120

140

160

180DataTotal fit

KKp p→ 0B

KKp p→ s0B

πKp p→ 0BComb. bkgd.

LHCb

]2c) [MeV/πKpp(m5200 5300 5400 5500

)2 cC

andi

date

s / (

5 M

eV/

0

100

200

300

400

500

600

700 DataTotal fit

πKp p→ 0B

πKp p→ s0B

ππp p→ 0BComb. bkgd.

LHCb

nppK+K−

B = 68± 17 nppK+K−

Bs= 635± 32

nppK±π∓

B = 4155± 83 nppK±π∓

Bs= 246± 39

nppπ+π−

B = 902± 35 nppπ+π−

Bs= 39± 16

First strong evidence First observations

]2c) [MeV/ππpp(m5200 5300 5400 5500

)2 cC

andi

date

s / (

10

MeV

/

0

50

100

150

200

250

300DataTotal fit

ππp p→ 0B

ππp p→ s0B

πKp p→ 0BComb. bkgd.

LHCb



Substructures in m(h+h′−) [arXiv:1704.08497, Accepted by PRD]

B0s → ppK+K−

]2c) [MeV/KK(m1000 2000 3000

)2 c N

orm

. Ent

ries

/ (4

0 M

eV/

0

0.01

0.02

0.03

0.04

0.05LHCbφ(1020)

B0→ ppK±π∓

]2c) [MeV/πK(m1000 2000 3000

)2 c N

orm

. Ent

ries

/ (8

0 M

eV/

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09 LHCbK∗(892)

Efficiency-corrected andbackground-subtractedVector mesons visible(φ(1020), K∗(892)0, ρ0(770))

B0→ ppπ+π−

]2c) [MeV/ππ(m1000 2000 3000

)2 c N

orm

. Ent

ries

/ (8

0 M

eV/

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08LHCb

ρ0(770)



Threshold enhancement in m(pp) [arXiv:1704.08497]

B0s → ppK+K−

]2c) [MeV/pp(m2000 2500 3000

)2 c N

orm

. Ent

ries

/ (5

0 M

eV/

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

LHCb

B0→ ppK±π∓

]2c) [MeV/pp(m2000 2500 3000

)2 c N

orm

. Ent

ries

/ (5

0 M

eV/

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

LHCb

Efficiency-corrected andbackground-subtractedClear threshold enhancement

B0→ ppπ+π−

]2c) [MeV/pp(m2000 2500 3000

)2 c N

orm

. Ent

ries

/ (5

0 M

eV/

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

LHCb



Results [arXiv:1704.08497, Accepted by PRD]

3 first observations and a strong evidence

Decay channel Significance [σ] Branching fraction / 10−6

B0→ ppK+K− 4.1 0.113±0.028±0.011±0.008B0→ ppK±π∓ > 25 5.9 ±0.3 ±0.3 ±0.4B0→ ppπ+π− > 25 2.7 ±0.1 ±0.1 ±0.2B0

s → ppK+K− > 25 4.2 ±0.3 ±0.2 ±0.3 ±0.2B0

s → ppK±π∓ 6.5 1.30 ±0.21 ±0.11 ±0.09 ±0.08B0

s → ppπ+π− 2.6 0.41 ±0.17 ±0.04 ±0.03 ±0.02

val± stat± syst± σ(B)± σ(fs/fd)

Upper limit set on B(B0s → ppπ+π−)

B(B0s → ppπ+π−) < 6.6× 10−7 at 90% CL



First observation of theB0→ pp decay

[LHCb-PAPER-2017-022, In preparation]


Motivations [LHCb-PAPER-2017-022, In preparation]

Two-body baryonic B decays rather suppressedNot seen at the B factoriesFirst two-body mode observed: B+ → pΛ(1520) [Phys. Rev. Lett. 113,141801 (2014)]

First evidence of B0→ pp with 1 fb−1, no B0s signal [J. High Energy Phys.

10 (2013) 005]

B(B0→ pp) = (1.47+0.62+0.35−0.51−0.14 )× 10−8 at 68.3% CL

B(B0s → pp) = (2.84+3.57+2.00

−2.12−0.21 )× 10−8 at 90% CL

Most recent calculations in agreement with branching fractionO(10−8) for B0→ pp [Phys. Rev. D91, 077501 (2015), Phys. Rev. D 91,036003 (2015)]

Important to improve knowledge of these very rare decays B0(s)→ pp









Analysis strategy [LHCb-PAPER-2017-022, In preparation]

Signal: B0(s)→ pp

Normalisation mode:B0→ K+π−

]2c) [MeV/πK(m5200 5400 5600

)2C

andi

date

s / (

19 M

eV/c

10

210

310

410 DataTotal fit

-π+K → 0B-K+π → 0

sB

-K+K → 0sB

-π+π → 0B-πp → 0

bΛ-pK → 0

bΛPart. reco.Comb. bkg.

LHCb

Various sources of background consideredHb → hh′, B→ hh′h′′

Baryonic B decays and many-bodyΛ0b decays

Semi-leptonic decays with B+ → ppl+ν


Results [LHCb-PAPER-2017-022, In preparation]

]2c) [MeV/pp(m5100 5200 5300 5400 5500

)2C

andi

date

s / (

19 M

eV/c

0

5

10

15

20

25Data

pp → 0Bpp → 0

sBComb. bkg.

LHCb

signal yieldpp → 0B0 20 40 60

L ln

∆

-2

0

10

20

30

40

50

60

70

LHCbstat. only

also syst.

N(B0→ pp) = 39± 8 N(B0s → pp) = 2± 4

First observation of B0→ pp at 5.3 σ

B(B0→ pp) = (1.25±0.27±0.18)× 10−8

Use Feldman-Cousin method to set upper limit on B0s → pp

B(B0s → pp) < 1.5× 10−8 at 90% CL


Update of the branchingfraction measurements of

the B0(s)→ K0

S h+h′−

decays

[arXiv:1707.01665, Submitted to JHEP]



Motivations [arXiv:1707.01665, Submitted to JHEP]

Update with 3 fb−1 of previous LHCb analysis on 1 fb−1 [J. High EnergyPhys. 10 (2013) 143]

Search still unobserved for B0s → K0

S K+K−

Prepare for Dalitz-plot analysis

Family of modes with great physics potentialPossibility to perform Dalitz-plot analysisContribution to extractions of angle γ, weak phase of B0 mixing





Analysis strategy [arXiv:1707.01665, Submitted to JHEP]

Measure branching fraction of differentB0(s)→ K0

S h+h′− decays with respect toB0→ K0

S π+π−

Categories: different data-taking periodsand K0

S decaying inside or outside the VELOSelection:Optimise selection separately foreach decay mode (two different optimisedselections for each final state)Fit: Simultaneous fit to 3 final states

I Signal: B0, B0s

I combinatorial backgroundI partially reconstructed backgroundI mis-identified backgrounds

(B0→ K0S π

+π− and B0→ K0S K+K− in

m(K0S K±π∓) and B0→ K0

S K±π∓ andB0

s → K0S K±π∓ in the other two)

]2c) [MeV/−K+K0SK(m

5200 5400 5600 5800

)2 cC

andi

date

s / (

16.

25 M

eV/

1

10

LHCbLong

]2c) [MeV/

±

π±K0SK(m

5200 5400 5600 5800

)2 cC

andi

date

s / (

16.

25 M

eV/

1

10

LHCbLong

]2c) [MeV/−π+π0SK(m

5200 5400 5600 5800 )2 c

Can

dida

tes

/ ( 1

6.25

MeV

/

1

10

210LHCbLong



Results [arXiv:1707.01665, Submitted to JHEP]

Branching fraction ratio relative to B0→ K0S π

+π−

B(B0→ K0S K±π∓)

B(B0→ K0S π

+π−)= 0.123±0.009 (stat) ±0.015 (syst) ,

B(B0→ K0S K+K−)

B(B0→ K0S π

+π−)= 0.549±0.018 (stat) ±0.033 (syst) ,

B(B0s → K0

S π+π−)

B(B0→ K0S π

+π−)= 0.191±0.027 (stat) ±0.031 (syst) ±0.011 (fs/fd) ,

B(B0s → K0

S K±π∓)B(B0→ K0

S π+π−)

= 1.70 ±0.07 (stat) ±0.11 (syst) ±0.10 (fs/fd) ,

B(B0s → K0

S K+K−)

B(B0→ K0S π

+π−)= 0.026±0.011 (stat) ±0.007 (syst) ±0.002 (fs/fd) .

Use Feldman-Cousin method to set upper limit for B0s → K0

S K+K−

B(B0s → K0

S K+K−)

B(B0→ K0S π

+π−)∈ [0.008 − 0.051] at 90% C.L.



Results [arXiv:1707.01665, Submitted to JHEP]

From World average (omitting LHCb previous result) [PDG]B(B0→ K0π+π−) = (4.96±0.20)× 10−5:

B(B0→( )

K0K±π∓) = (6.1±0.5±0.7±0.3)× 10−6 ,

B(B0→ K0K+K−) = (27.2±0.9± 1.6± 1.1)× 10−6 ,

B(B0s → K0π+π−) = (9.5± 1.3± 1.5±0.4)× 10−6 ,

B(B0s →

( )

K0K±π∓) = (84.3± 3.5± 7.4± 3.4)× 10−6 ,

B(B0s → K0K+K−) ∈ [0.4 − 2.5]× 10−6 at 90% C.L. ,

Compatible with previous analysis [J. High Energy Phys. 10 (2013) 143]

Dalitz-plot analyses under way





Conclusion

LHCb is very active in the study of charmless b-hadron decaysIn particular many results with baryonic final statesIn this talk presented several recent results from the LHCbcollaboration:

I First observation of baryonic Bs decayI First observation of 3 new B0

(s)→ pph+h′− modes and evidence of afourth one

I First observation of the B0→ pp decay: the rarest hadronic B decayobserved so far

I Updated branching fraction of B0(s)→ K0

S h+h′− decays

Run II is providing a lot more statisticsExpect in the near future to perform more sofisticated analyses withthe newly observed decay modes (CP-violation studies, amplitudeanalyses, . . . )


BACKUP

LHCb [Int. J. Mod. Phys. A 30 (2015) 1530022]

Acceptance: 2 < η < 5∼ 1/4 of produced bb pairs.

Decay timeresolution ∼ 45 fs

IP resolution ∼ 20µm

Vertex resolution ∼ 13µmin x y (25 tracks)

∆p/p ∼ 0.5 − 1.0%

ε(µ) ∼ 97%,misID(π→ µ) ∼ 1 − 3%

ε(K) ∼ 95%,misID(π→ K) ∼ 5%

Giulio Dujany (LPNHE, CNRS) Charmless b-hadron decays Flavour 2017 27

http://www.worldscientific.com/doi/abs/10.1142/S0217751X15300227

ReconstructingΛ and K0s [Int. J. Mod. Phys. A 30 (2015) 1530022]

Long

Downstream

Giulio Dujany (LPNHE, CNRS) Charmless b-hadron decays Flavour 2017 28

http://www.worldscientific.com/doi/abs/10.1142/S0217751X15300227

charmless b-hadron decays - cern · 2017-08-18 · c vetoes, no signiÞcant signal is observed in...

Documents