lepton flavour universality tests with b decays at...
TRANSCRIPT
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
Johannes Albrecht 19. March 2018 8/15
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
Johannes Albrecht 19. March 2018 9/15
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
Johannes Albrecht 19. March 2018 10/15
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
Johannes Albrecht 19. March 2018 11/15
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]
Johannes Albrecht 19. March 2018 11/15
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
Johannes Albrecht 19. March 2018 12/15
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
Johannes Albrecht 19. March 2018 13/15
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
Johannes Albrecht 19. March 2018 15/15
• 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
Johannes Albrecht 19. March 2018 9/15
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
*
*