Searches for LFU breaking at LHCbFlavour Physics Conference
Julián García PardiñasOn behalf of the LHCb Collaboration
Universidade de Santiago de Compostela (Spain)
XIII Rencontres du VietnamQuy Nhon (Vietnam), August 15, 2017
Table of contents
} Introduction
} Semileptonic decays◦ Muonic R(D∗)◦ Hadronic R(D∗)
} Rare b → sll decays◦ R(K)◦ R(K∗)
} Conclusions and outlook
1Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
Introduction
Overview of Lepton Flavour Universality (LFU)
The current situation
} In the SM, the electroweak couplings of the gauge bosons areindependent on the lepton flavour→ LFU.
} Some deviations have been measured in decays to τ/(µ, e)τ/(µ, e)τ/(µ, e) andto µ/eµ/eµ/e.
} Potential NP candidates: leptoquarks, H±, new Z′ bosons ...
Laboratory for LFU
} BBB decays allow to compare the three families, τ/µ/e.
} Ratios of branching fractions, e.g. R(D∗) � B(B→D∗τν)B(B→D∗µν) .
◦ Theoretically clean (∼cancellation of QCD effects).◦ Experimentally clean (∼cancellation of efficiency and
reconstruction effects).
2Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
Aim of this talk: two-front LFU
R(D(∗)
)� B(B→ D(∗)τ−ντ)/B(B→ D(∗)µ−νµ)R
(D(∗)
)� B(B→ D(∗)τ−ντ)/B(B→ D(∗)µ−νµ)R
(D(∗)
)� B(B→ D(∗)τ−ντ)/B(B→ D(∗)µ−νµ) [b → clν][b → clν][b → clν]
} Tree level in the SM.
} Abundant decay.
} Potential NP contributions thatcouple mainly to the third family.
R(K(∗)
)� B(B→ K(∗)µ+µ−)/B(B→ K(∗)e+e−)R
(K(∗)
)� B(B→ K(∗)µ+µ−)/B(B→ K(∗)e+e−)R
(K(∗)
)� B(B→ K(∗)µ+µ−)/B(B→ K(∗)e+e−) [b → sll][b → sll][b → sll]
} FCNC process, rare decay,forbidden at tree level in the SM.
} Very sensitive to either tree orloop NP contributions.
3Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
The LHCb detector
Advantages
} Excellent vertexing,tracking and PID.
} Large data statisticsfrom high bb̄ crosssection.
RICH detectorsCalorimeters
Challenges
} Missing neutrinos. �⇒ Unconstrained kinematics.◦ B-factories: use the information from the other B in Υ(4S) → BB.◦ LHCb: make use of approximations.
} High track multiplicity. �⇒ Significant amount of background, lowefficiency for electrons.
} High particle momenta. �⇒ Significant Bremsstrahlung for electrons.4
Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
Semileptonic decays
Results from the B factories
Before the first LHCb measurement (2015), BaBar and Belle had alreadyobserved some deviations in semileptonic B decays.
} Deviation of 2.0σ2.0σ2.0σ in R(D)R(D)R(D) and 2.7σ2.7σ2.7σ in R(D∗)R(D∗)R(D∗).Deviation of 3.4σ3.4σ3.4σ when combining R(D)R(D)R(D) and R(D∗)R(D∗)R(D∗).
[Phys. Rev. D 88 (2013), 072012]26
0.2 0.4 0.6
0.3
0.4
SM
σ 1σ 2σ 3σ 4σ 5
R(D)
R(D
∗ )
FIG. 17. (Color online). Representation of χ2 (Eq. 30) inthe R(D)–R(D∗) plane. The white cross corresponds to the
measured R(D(∗)), and the black cross to the SM predictions.The shaded bands represent one standard deviation each.
distribution in the R(D)–R(D∗) plane. The contours areellipses slightly rotated with respect to the R(D)–R(D∗)axes, due to the non-zero correlation.
For the assumption that R(D(∗))th = R(D(∗))SM, weobtain χ2 = 14.6, which corresponds to a probabilityof 6.9 × 10−4. This means that the possibility that themeasured R(D) and R(D∗) both agree with the SM pre-dictions is excluded at the 3.4σ level [42]. Recent calcu-lations [7, 8, 43, 44] have resulted in values of R(D)SM
that slightly exceed our estimate. For the largest of thosevalues, the significance of the observed excess decreasesto 3.2σ.
B. Search for a charged Higgs
To examine whether the excess in R(D(∗)) can be ex-plained by contributions from a charged Higgs boson inthe type II 2HDM, we study the dependence of the fitresults on tanβ/mH+ .
For 20 values of tanβ/mH+ , equally spaced in the[0.05, 1.00] GeV−1 range, we recalculate the eight signalPDFs, accounting for the charged Higgs contributions asdescribed in Sec. II. Figure 18 shows the m2
miss and |p∗ℓ |
projections of the D0τν ⇒ D0ℓ PDF for four values oftanβ/mH+ . The impact of charged Higgs contributionson the m2
miss distribution mirrors those in the q2 distri-
0 5 100
0.05
0.1
0.15
0.2
0 0.5 1 1.5 20
0.5
1
1.5
Pro
bability/G
eV2
Pro
bability/G
eV
m2miss (GeV2) |p∗
ℓ | (GeV)
SMtanβ/mH+ = 0.3GeV−1
tanβ/mH+ = 0.5GeV−1
tanβ/mH+ = 1GeV−1
FIG. 18. (Color online). m2miss and |p∗
ℓ | projections of theD0τν ⇒ D0ℓ PDF for various values of tanβ/mH+ .
0 0.5 1
100
105
110
0 0.5 1
100
105
110 0 0.5 1200
400
600
0 0.5 1800
900
1000
ε(B
→D
(∗)τ
−ν
τ)/
ε SM
(%)
B→
D(∗
)τ
−ν
τyie
ld
tanβ/mH+ (GeV−1)tanβ/mH+ (GeV−1)
FIG. 19. (Color online). Left: Variation of the B → Dτ−ντ
(top) and B → D∗τ−ντ (bottom) efficiency in the 2HDMwith respect to the SM efficiency. The band indicates theincrease on statistical uncertainty with respect to the SMvalue. Right: Variation of the fitted B → Dτ−ντ (top) andB → D∗τ−ντ (bottom) yields as a function of tanβ/mH+ .The band indicates the statistical uncertainty of the fit.
bution, see Fig. 3, because of the relation
m2miss =
!pe+e− − pBtag − pD(∗) − pℓ
"2= (q − pℓ)
2,
The changes in the |p∗ℓ | distribution are due to the change
in the τ polarization.We recalculate the value of the efficiency ratio
εsig/εnorm as a function of tanβ/mH+ (see Fig. 19).The efficiency increases up to 8% for large values oftanβ/mH+ , and, as we noted earlier, its uncertainty in-creases due to the larger dispersion of the weights in the2HDM reweighting.
The variation of the fitted signal yields as a functionof tanβ/mH+ is also shown in Fig. 19. The sharp drop inthe B → Dτ−ντ yield at tanβ/mH+ ≈ 0.4 GeV−1 is dueto the large shift in the m2
miss distribution which occurswhen the Higgs contribution begins to dominate the totalrate. This shift is also reflected in the q2 distribution and,as we will see in the next section, the data do not supportit. The change of the B → D∗τ−ντ yield, mostly causedby the correlation with the B → Dτ−ντ sample, is muchsmaller.
Figure 20 compares the measured values of R(D) andR(D∗) in the context of the type II 2HDM to the theoret-ical predictions as a function of tanβ/mH+ . The increasein the uncertainty on the signal PDFs and the efficiencyratio as a function of tanβ/mH+ are taken into account.Other sources of systematic uncertainty are kept constantin relative terms.
The measured values of R(D) and R(D∗) matchthe predictions of this particular Higgs model fortanβ/mH+ = 0.44±0.02 GeV−1 and tanβ/mH+ = 0.75±0.04 GeV−1, respectively. However, the combination ofR(D) and R(D∗) excludes the type II 2HDM chargedHiggs boson at 99.8% confidence level for any value oftanβ/mH+ , as illustrated in Fig. 21. This calculation isonly valid for values of mH+ greater than 15 GeV [5, 8].The region for mH+ ≤ 15 GeV has already been excluded
} BaBar studies disfavour Type-II 2HDM [Phys. Rev. D 88 (2013), 072012]} More recent Belle studies: compatibility with Type-II 2HDM in the
region around tan β/MH+ � 0.5 c2/GeV [Phys. Rev. D 92 (2015), 072014]and with leptoquarks [Phys. Rev. D 94 (2016), 072007].
5Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
First LHCb measurement of R(D∗)
Data: 3 fb−1 (Run 1), using τ and µ (no electrons), muonic decay of the τ.
R (D∗) � B(B→ D∗+τ−ντ)B(B→ D∗+µ−νµ)
with τ− → µ−νµντ
Measure semileptonic decays to the same visible final state D∗+µ−D∗+µ−D∗+µ−.
} Separate τ and µ components via a 3D binned template fit on them2
miss , E∗µ and q2 variables.Problem: missing neutrinos �⇒no analytical solution for pBpBpB .
Rest-frame approximation:B boost along z >> boost of decayproducts in B frame.
18% resolution on pB→ enough toseparate the components.
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Phys. Rev. Lett. 115, 111803 (2015)
Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
Results
Fit components:} τττ signal and µµµ normalisation.
} Backgrounds: feed-down from excited D states, double charm DD(where one D decays semileptonically), combinatorial,muon mis-ID.
R(D∗) � 0.336 ± 0.027(stat) ± 0.030(syst) 2.1σ from SM
(RSM(D∗) � 0.252 ± 0.003 [Phys.Rev.D85(2012) 094025])
- Comparable statistical and systematic uncertainties.- Dominant syst.: size of the simulated samples (template shape ...).
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Phys. Rev. Lett. 115, 111803 (2015)
Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
Hadronic R(D∗) in LHCb
Today: first LHCb measurement of R(D∗) using the hadronic τ decay,τ− → π−π+π−(π0)ντ , with Run 1 data.
Complementary to the muonic decay:} Explore a complementary data sample.
} No charged leptons in the final state:→ Zero bkg. from usual SL decays!
Khad(D∗) �B(B0 → D∗−τ+ντ)
B(B0 → D∗−π+π−π+), R(D∗)R(D∗)R(D∗) � Khad(D∗)×
B(B0 → D∗−π+π−π+)B(B0 → D∗−µ+νµ)
B(B0 → D∗−τ+ντ) from a 3D binned template fit.B(B0 → D∗−π+π−π+) from an un-binned fit to M(D∗−π+π−π+).B(B0 → D∗−π+π−π+) external, from BaBar [Phys. Rev. D94 (2016) 091101].B(B0 → D∗−µ+νµ) external, from the PDG.
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LHCb-PAPER-2017-017, in preparation
Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
Analysis strategy
} Main background:prompt D∗−π+π−π+X
(100x expected signal yield)→ Reduction of O(10−3) byrequiring a minimum τ flightdistance.
} Remaining background dominated by double-charm DD decays.→ Train a BDT against them, using the 3π dynamics, the D∗−3πdynamics and information of extra neutral tracks.
} Structure of the D∗Ds X bkg. extensively studied in control samples.
} Signal reconstruction: some approximations are done to reconstructthe B and τmomenta. Only 2ν missing �⇒ reasonably good precision.
} Fit variables: output from BDT against DD, τττ decay time (againstD∗D+X, due to the large D+ lifetime) and q2q2q2.
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LHCb-PAPER-2017-017, in preparation
Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
Results
R(D∗) � 0.285 ± 0.019(stat) ± 0.025(syst) ± 0.014(ext)- Dominant systematics: size of simulated samples.- Room for progress both in the internal and external sources.
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LHCb-PAPER-2017-017, in preparation
Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
Global picture of R(D(∗))
LHCb combination [FPCP 2017]:R(D∗) � 0.306 ± 0.016 ± 0.022 2.1σ above the SM
R(D*)0.2 0.3 0.4
BaBar had. tag 0.018± 0.024 ±0.332
Belle had. tag 0.015± 0.038 ±0.293
Belle sl.tag 0.011± 0.030 ±0.302
Belle (hadronic tau) 0.027± 0.035 ±0.270
LHCb 0.030± 0.027 ±0.336
LHCb (hadronic tau) 0.029± 0.019 ±0.285
Average 0.007± 0.013 ±0.304
S. Fajfer et al. (2012) 0.003±0.252
HFLAVFPCP 2017
/dof = 0.4/ 1 (CL = 52.00 %)2χ
World average [HFLAV]:
R(D∗) � 0.304 ± 0.013 ± 0.007(3.4σ from SM)R(D)R(D)R(D) ⊕⊕⊕ R(D∗)R(D∗)R(D∗) is 4.1σ4.1σ4.1σ from SM!
11Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
R(D)0.2 0.3 0.4 0.5 0.6
R(D
*)
0.2
0.25
0.3
0.35
0.4
0.45
0.5 BaBar, PRL109,101802(2012)Belle, PRD92,072014(2015)LHCb, PRL115,111803(2015)Belle, PRD94,072007(2016)Belle, PRL118,211801(2017)LHCb, FPCP2017Average
SM Predictions
= 1.0 contours2χ∆
R(D)=0.300(8) HPQCD (2015)R(D)=0.299(11) FNAL/MILC (2015)R(D*)=0.252(3) S. Fajfer et al. (2012)
HFLAV
FPCP 2017
) = 71.6%2χP(
σ4
σ2
HFLAVFPCP 2017
Rare b → sll decays
Measurement of R(K) in LHCb
R(K) �∫ [
dΓ(B+ → K+µ+µ−)/dq2] dq2∫ [dΓ(B+ → K+e+e−)/dq2
]dq2
q2 : (dilepton invariant mass)2
In 2014, LHCb measured R(K)R(K)R(K) for q2 ∈ [1 − 6]q2 ∈ [1 − 6]q2 ∈ [1 − 6] GeV2/c42/c42/c4, using Run 1 data.
} Challenges: low efficiency for electrons and Bremsstrahlung effects (energy loss).} Signal extracted via invariant mass fits.
RLHCb(K) � 0.745+0.090−0.074(stat) ± 0.036(syst) 2.6σ2.6σ2.6σ from RSM(K) � 1±O(10−3)
([Phys. Rev. Lett. 112, 149902] and others)
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Phys. Rev. Lett. 113, 151601 (2014)
Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
BaBar[PRD 86, 032012]
Belle[PRL 103, 171801]
Measurement of R(K∗) in LHCb
Today: first Run 1 LHCb measurement of R(K∗)R(K∗)R(K∗) in two q2q2q2 regions,[0.045 − 1.1] and [1.1 − 6] GeV2/c4, with K∗0 → K+π−.
To reduce systematics, R(K∗) is obtained from a double ratio:
R(K∗) �B(B0 → K∗0µ+µ−)
B(B0 → K∗0 J/ψ(→ (µ+µ−))/ B(B0 → K∗0e+e−)B(B0 → K∗0 J/ψ(→ (e+e−))
Bremsstrahlung effects:
} Worse B mass resolution.} Worse separation of partially reconstructed backgrounds.} Background from J/ψ and ψ(2S) leaking into the signal region.
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arXiv:1705.05802 (2017)
Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
J/ψJ/ψJ/ψ −→−→−→
ψ(2S)ψ(2S)ψ(2S) −→−→−→
Radiativetails
Analysis strategy
1. Maximise electron sample by combining 3 hardware (L0) trigger types:focused on electrons, on hadrons and signal independent.
2. To reduce Bremsstrahlung energy loss, recover as many γ as possible.
3. Similar selection for muon and electron modes: vetoes against peakingbkg., multivariate classifier, PID, removal of multiple candidates.
} Simultaneous M(K+π− l+ l−)M(K+π− l+ l−)M(K+π− l+ l−) fit to the J/ψ and non-resonant channels.
} The electron sample is separated in the 3 trigger categories and in 3Bremsstrahlung categories (0γ, 1γ and ≥ 2γ recovered).
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arXiv:1705.05802 (2017)
Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
Results and global picture
q2 bin RLHCb(K∗)RLHCb(K∗)RLHCb(K∗) σ from SM
Low 0.66+0.11−0.07 ± 0.03 2.1 − 2.3
Central 0.69+0.11−0.07 ± 0.05 2.4 − 2.5
The result is statistically dominated.
Dominant syst.: corrections to simulation+ residual background (for central q2). BaBar: [PRD 86 (2012), 032012]
Belle: [PRL 103 (2009), 171801]
Several groups have done global fits of the b → sllb → sllb → sll anomalies (using theeffective Hamiltonian framework). For example, from [arXiv:1704.05340 (2017)]:
} Some NP hypotheses favoured at 3σ3σ3σ when using only R(K)R(K)R(K) and R(K∗)R(K∗)R(K∗).} When adding other results from b → sll, b → µµ and b → sγ: several
NP hypotheses favoured at ∼ 5σ∼ 5σ∼ 5σ.
Similar findings from other groups [arXiv:1503.06199 (2015)] [ arXiv:1411.3161(2015)] [ arXiv:1603.00865 (2016)] .
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Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
Conclusions and outlook
Conclusions and outlook
LFU breaking is a very promising road to NP!From the B-factories + LHCb:
} Deviations of ∼ 4σ∼ 4σ∼ 4σ observed in semileptonic B decays.} Deviations of 2 − 3σ2 − 3σ2 − 3σ observed in b → sll decays.
LHCb will be playing the mayor role in the next years, both with newanalyses and with updates of the existing ones.
} Run 2 data (expect 5 fb−1 by 2018, at 13 TeV): significant improvementon both statistical and systematic uncertainties!
Ongoing and planned semileptonic analyses:
} R(D0), R(D+), R(Ds ), R(J/ψ), R(Λc), ...
Ongoing and planned rare decay analyses:
} R(φ), R(pK), R(Λ) ...
16Julián García Pardiñas (USC) Rencontres du Vietnam August 15, 2017
