Lepton Flavour Universality tests with B decays at...

Johannes Albrecht (TU Dortmund)

Lepton Flavour Universality tests with B decays at LHCb

Johannes Albrecht (TU Dortmund) on behalf of the LHCb collaboration

19th of March 2018

Moriond QCD 2018

Lepton universality

•  In the SM, leptons couple universally to W± and Z0

à test this in ratios of semileptonic decays

•  Ratios differ from unity only by phase space à hadronic uncertainties cancel in the ratio

muons / electrons [b → s ] tau / muons [b → c ]

Capri 2012 MPA, CPV in charm and b-decays at LHCb

R(D*)=Β(B0"D*+τ-ντ)/Β(B0"D*+µ-ντ) with τ-"µ-νµντ

13

!  Ratio R(D*) sensitive to NP coupled dominantly to 3rd generation, e.g. a charged Higgs

!  Theoretically clean

– BaBar: R(D) and R(D*) combined "

3.4 σ tension (final data set)

RK =BR(B+ → K +µ+µ− )BR(B+ → K +e+e− )

RD* =BR(B0 →D*+ τ −ν )BR(B0 →D*+ µ−ν )

u,db

u,ds

Johannes Albrecht

Analogously: RK*

19. March 2018 1/15

LFU in b → s l+l- transitions

•  Some tensions seen in muon modes à see A. Crocombe’s talk

•  If effect is real: Does it only appear in muons or is it universal for all leptons?

•  Clean test: Compare same decay to muons and electrons à Form factors cancel à radiative corrections small (O(1%), Bordone, Isidori, Pattori: arXiv:1605.07633)

b → s µ+µ- base diagram

Johannes Albrecht 19. March 2018 2/15

Measurement of B → K* l+l-

Johannes Albrecht

•  Experimental challenge: Electron channel –  Bremsstrahlung: Photon recovery has limited efficiency and resolution

à wider mass windows –  Perform analysis in bins of q2, # recovered photons and trigger category

•  Analysis strategy as double ratio

19. March 2018 3/15

LHCb, 3fb-1 [JHEP 08 (2017) 055]

Mass distributions & yields

Johannes Albrecht

LHCb, 3fb-1 [JHEP 08 (2017) 055]

19. March 2018 4/15

BR(B0 → J /ψ(µ+µ− )K *0 )BR(B0 → J /ψ(e+e− )K *0 )

=1.043 ± 0.006(stat) ± 0.045(syst)

B0 → K*0 µ+µ-

Important cross check:

B0 → K*0 e+e-

RK* results

Johannes Albrecht

•  Compatible with the SM at 2.2 and 2.4 σ for the low and intermediate q2 region

19. March 2018 5/15

LHCb, 3fb-1 [JHEP 08 (2017) 055]

Reminder: Rk measurement at LHCb

T. Blake

RK result• In the run 1 dataset, LHCb

determines:

!

in the range 1 < q2 < 6 GeV2, which is consistent with the SM at 2.6!.

• Take double ratio with B+ → J/ѱ K+ to cancel possible sources of systematic uncertainty.

• Correct for migration of events in/out of the window due to Bremsstrahlung using MC (with PHOTOS).

32

]4c/2 [GeV2q0 5 10 15 20

KR

0

0.5

1

1.5

2

SM

LHCb BaBar Belle

LHCb

LHCb [PRL113 (2014) 151601 ]!BaBar [PRD 86 (2012) 032012]!Belle [PRL 103 (2009) 171801]

RK = 0.745+0.090�0.074

+0.036�0.036

RK < 1 implies a deficit of muons w.r.t. electrons.

T. Blake

RK result• In the run 1 dataset, LHCb

determines:

!

in the range 1 < q2 < 6 GeV2, which is consistent with the SM at 2.6!.

• Take double ratio with B+ → J/ѱ K+ to cancel possible sources of systematic uncertainty.

• Correct for migration of events in/out of the window due to Bremsstrahlung using MC (with PHOTOS).

32

]4c/2 [GeV2q0 5 10 15 20

KR

0

0.5

1

1.5

2

SM

LHCb BaBar Belle

LHCb

LHCb [PRL113 (2014) 151601 ]!BaBar [PRD 86 (2012) 032012]!Belle [PRL 103 (2009) 171801]

RK = 0.745+0.090�0.074

+0.036�0.036

RK < 1 implies a deficit of muons w.r.t. electrons.

(SM: RK=1.0, consistent at 2.6σ)

LHCb measures with 3fb-1

RK =BR(B+ → K +µ+µ− )BR(B+ → K +e+e− )

= 0.745 +0.090−0.074

(stat)± 0.036(syst)

Johannes Albrecht

LHCb, 3fb-1 [PRL113(2014)151601]

19. March 2018 6/15

LFU in b → s l+l- transitions: Next steps

•  Both RK and RK* use 3fb-1 of data –  More than double the dataset on tape, updates in the pipeline

•  Similar channels under investigation, e.g. –  Bs

→ φ l+l- : less events than RK, but very clean –  B+ → K+π-π+ l+l- expect O(100) events,

complex hadronic structure –  Λb

→ K+π- l+l-

•  Analysis of full LHCb dataset, Belle2 and LHCb upgrade will clarify the situation

–  If current tensions persist, they will be established with >5σ by LHCb (Run 2), and later also by Belle2

Johannes Albrecht

arxiv:1709.10308

19. March 2018 7/15

Sensitivity after (upgrade) running

•  Semileptonic tree level decays –  Differences for the decays in leptons should

originate only in different masses –  Clean probe for NP

•  Measure ratios of decay rates

•  First deviations seen by BaBar and Belle

•  Measurements challenging at LHCb –  Large production cross section, but B momentum unknown –  Missing momentum not measured, but B direction precisely known from vertexing

LFU in b → c l+ν transitions


RD* =BR(B0 →D*+ τ −ν )BR(B0 →D*+ µ−ν )

R(D*)SM = 0.252 ± 0.003 [PRD85(2012)094025]

RD* using muonic τ- decays


LHCb, 3fb-1 [PRL115(2015)11804]

•  The signal B0 → D* τ- (→ µ-νν) ν and the normalization mode B0 → D* µ-ν have the same visible final state

•  Signal separated in ML fit with 3D templates in kinematic variables

•  RD* = 0.336± 0.027(stat)± 0.030(syst)

Compatible with the SM at ~2.1σ


LHCb, 3fb-1 [1708.08856] à PRL and [1711.02505] à PRD

RD* using hadronic τ- decays

•  3-prong hadronic τ- decays, data sample complementary to muonic mode

•  Systematic uncertainty reduced normalizing

to a decay with similar final state

•  Measure RD* using the known muonic BR

•  Result:

–  Dominant systematic: simulated sample size Compatible with the SM at ~1σ

RD* = 0.286± 0.019(stat)± 0.025(syst)± 0.021(ext)

B0 → D0 3π

Summary of RD*

•  All measurements above SM prediction –  RD* exceeds SM by 3.4σ


Summary of RD* and RD

•  All measurements above SM prediction –  RD* exceeds SM by 3.4σ –  RD by 2.3σ –  Combination: around 4σ

•  Updates to the SM prediction for R(D*) slightly reduce the tension [1703.05330, 1707.09509, 1707.09977], 3-5% effect from QED, in MC? [1803.05881]


LFU in Bc decays

•  Test the same effective Feynman diagram in Bc decays:

•  Analysis similar to RD* –  Short Bc lifetime separates signal

from other B hadrons –  Efficient J/ψ trigger –  Main background: b→ J/ψ mis-ID hadron

•  First evidence (3σ) of Bc → J/ψ τ-ν


RJ /ψ =BR(Bc

+ → J /ψ τ +ν )BR(Bc

+ → J /ψ µ+ν )R(J/ψ)SM = 0.25 – 0.28 PLB452(1999)129, arXiv:0211021,

PRD73(2006)054024, PRD74(2006)074008

LFU in Bc decays: Analysis & results


LHCb, 3fb-1 [1711.05623]

•  3D templated binned ML fit to m2

miss, τµ, and Z=(Eµ, q2) •  Shapes derived from control samples or

simulation validated on data •  Result:

•  Dominant systematics: limited MC size, Bc → J/ψ form factors

RJ /ψ = 0.71± 0.17(stat)± 0.18(syst)Compatible with the SM at ~2σ

LFU in b → c l+ν transitions: Next steps

•  All shown analyses use 3fb-1 of data –  More than double the dataset on tape, updates in the pipeline –  MC statistics critical input

•  Similar channels under investigation, e.g. –  Λb

→ Λc (*)

l+ν : Baryonic modes with orthogonal systematics –  B →pp l+ν : Baryonic φ l+l- : tests new sector: b → u l+ν

•  Analysis of full LHCb dataset, Belle2 and LHCb upgrade will clarify the situation –  Expected final sensitivity on RD*

LHCb 3.6% Belle2 2.3% LHCb (upgrade) 1.6 %

19. March 2018 14/15 Johannes Albrecht

B ! D(⇤)⌧⌫ with Belle II & LHCbarXiv:1709.10308: J. Albrecht, F. U. Bernlochner, M. Kenzie, S. Reichert, D. M. Straub, A. Tully

Measurement SM Current World Current Projected Uncertainty1

prediction Average Uncertainty Belle II LHCb

5ab�1 50ab�1 8fb�1 22fb�1 50fb�1

2020 2024 2019 2024 2030

R(D) (0.299 ± 0.003) (0.403 ± 0.040 ± 0.024) 11.6% 5.6% 3.2% - - -

R(D⇤) (0.257 ± 0.003) (0.310 ± 0.015 ± 0.008) 5.5% 3.2% 2.2% 3.6% 2.1% 1.6%

R(D)

R(D

*)

0.3 0.35 0.4 0.450.24

0.26

0.28

0.3

0.32

0.34

LHCb Belle IIFuture WA SM predictionSM

σ1

σ3

σ5

σ7

σ9

-18fb

-122fb

-150fb

-15ab-150ab

1Projected uncertainties not including improvements in detectors and algorithms

P. Goldenzweig Belle II & correlation w/HL-LHC 31.10.2017 16 / 36

arxiv:1709.10308

Sensitivity after (upgrade) running

Summary


•  Tests of Lepton Flavour Universality show interesting tensions –  b → s l+l- : does a consistent picture with angular and BR analyses emerge? –  b → c τ-ν: less clear, but measurements consistently above prediction

•  Recorded data still holds many gems for us –  All presented analyses use only 3fb-1 of data

à LHCb dataset on tape today contains factor 3 in number of B hadrons –  Many modes still under analysis

•  The future is bright –  LHCb prepares for an upgrade in Run 3

à much improved trigger, e.g. ~ identical trigger for µ- and e- –  Belle 2 ramps up to cross check results

Run 2

The path is clear .. .. lets enjoy the way there!

RD* using muonic τ- decays


LHCb, 3fb-1 [PRL115(2015)11804]

•  The signal B0 → D* τ- (→ µ-νν) ν and the normalization mode B0 → D* µ-ν have the same visible final state

•  Signal separated exploiting differences in three key kinematic variables:

• 

•  Approximate pB à pB with ~18% resolution

*

*

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