TESTS OF LEPTON FLAVOUR UNIVERSALITY IN b c ν DECAYS AT THE LHCb EX- PERIMENT
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TESTS OF LEPTON FLAVOUR UNIVERSALITY IN b → c`ν DECAYS AT THE LHCb EX- PERIMENT Epiphany 2020, Cracow On behalf of the LHCb collaboration 9 January 2020 Simone Meloni simone.meloni@cern.ch
Lepton flavour universality
• Lepton flavour universality is an accidental symmetry of
the Standard Model
• Equality of the couplings of gauge bosons to leptons
(ge = gµ = gτ )
• Discrepancies between decays with the different lepton species in the SM originate only from their different masses
(Phase Space)
I Any deviation could be a clear signature of New Physics effects
• Some discrepancies between SM predictions and measurements have been reported in B meson decays
I Neutral current decays b → s`` (See talk from Jozef)
I Charged current decays b → c`ν (This talk)
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 2 / 15
Lepton Flavour Universality in b → c`ν transitions at the LHCb experiment
• Semileptonic transitions can benefit:
I Experimentally from high transition rates
I Theoretically from a simple description, through tree level
diagrams
B(B → Hc τ ν)
• Test variables are ratios of branching fractions R(Hc ) =
I
B(B → Hc µν)
Hadronic uncertainties partially cancel in the ratio
I theoretical uncertainty well under control, O(%) Hc = D, D ∗ , J/Ψ, Λc , Λ∗c
Decay Mode Branching ratio (%)
• Two decays are exploited at the LHCb experiment for
τ− → µ− ντ ν̄µ 17.39 ± 0.04
lepton universality tests in charged current decays τ − → π + π − π − ντ 9.00 ± 0.06
• The B momentum is not constrained in pp collisions
I The momentum of the B meson is reconstructed using approximated methods in semileptonic decays that exploit information
from its flight direction
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 3 / 15
The experimental status as of today
• All the R(D (∗) ) measurements exceed the Standard Model prediction (0.258 ± 0.005)
• Combining 6 measurements of R(D) and R(D ∗ ) from 3 experiments (LHCb, Belle and Babar)
I tension with the SM predictions of ≈ 3 σ (very likely to change, see talk at beauty)
I Theory uncertainty reduced to a level of % after updating SM calculations [arXiv:1908.09398]
• LHCb contributes with two measurements of R(D ∗ ) using semileptonic and hadronic decays of the τ lepton, and a
measurement of R(J/Ψ) (Run I dataset, 2 fb−1 )
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 4 / 15
R(D ∗ ) with τ → µνν (PRL 115, 11804)
• B 0 → (D ∗ → D + π − )`ν
• B → D ∗ τ ν separated from B 0 → D ∗ µν exploiting differences in kinematic
variables computed in the B rest frame
Event reconstruction strategy
• The B flight direction is measured from the position of the primary and decay
vertex
• The longitudinal momentum from collinear approximation
mB
(γβz )B ≈ (γβz )Hc µ ⇒ (pz )B = (pz )Hc µ
mHc µ
• Thanks to the excellent vertexing system of
LHCb, the resolution that can be achieved
on these variables (≈ 20%) is sufficient for a
good separation between the signal and
normalization channels
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 5 / 15
R(D ∗ ) with τ → µνν (PRL 115, 11804)
Main backgrounds
• Semileptonic decays with excited D ∗∗ ,
• 3D binned fits with templates that represent ≈ 12%
signal, normalization and backgrounds • Decays into double charm final states
B → D ∗ Hc X , Hc → µνX 0 ; ≈ 6 − 8%
• Combinatorial and MisID (h → µ); ≈ 1%
• The shape of the physical backgrounds is calibrated in control regions enriched in the presence of specific hadron
tracks that have not been associated to the decay vertex (anti-isolation)
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 6 / 15
R(D ∗ ) with τ → µνν (PRL 115, 11804)
Main backgrounds
• Semileptonic decays with excited D ∗∗ ,
• 3D binned fits with templates that represent ≈ 12%
signal, normalization and backgrounds • Decays into double charm final states
B → D ∗ Hc X , Hc → µνX 0 ; ≈ 6 − 8%
• Combinatorial and MisID (h → µ); ≈ 1%
R(D ∗ ) = 0.336 ± 0.027(stat.) ± 0.030(syst.)
2.1 σ higher than the Standard Model
• Main Systematic uncertainties
I MC Statistics
I Shape of the MisID background
• The shape of the physical backgrounds is calibrated in control regions enriched in the presence of specific hadron
tracks that have not been associated to the decay vertex (anti-isolation)
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 7 / 15
R(D ∗ ) with τ → πππν (PRL 120,171802 2018)(PRD 97,072013 2018)
• Very different background
contributions with respect to
τ → µνν analyses
• The decay is normalized to a similar
fully hadronic decay B → D ∗ 3π,
whose normalization is measured
through a fit to the mass of the
D ∗ 3π system
B(B 0 → D ∗− τ + νµ )
K(D ∗ ) =
B(B 0 → D ∗− 3π ± ) • Fit performed on τ lifetime, q 2 and
output of a BDT trained to reject
• Measurement converted to R(D ∗ ) double charm decays
through external measurements
B(B 0 → D ∗− 3π ± )
R(D ∗ ) = K(D ∗ )
B(B 0 → D ∗− µ+ νµ )
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 8 / 15
R(D ∗ ) with τ → πππν (PRL 120,171802 2018)(PRD 97,072013 2018)
• No background from semileptonic
decays
• The decay is normalized to a similar
fully hadronic decay B → D ∗ 3π,
whose normalization is measured
through a fit to the mass of the
D ∗ 3π system
•±
R(D ∗ ) = 0.283 Main background
0.019(stat.) is coming from
± 0.025(syst.) ± 0.013(ext.)
prompt D ∗ 3πX combinations,
1 σ higher than the Standard
−3 Model
reduced to O(10 ) cutting on τ
B(B 0 → D ∗− τ + νµ ) lifetime significance
∗
K(D ) = I Remaining dominant background
B(B 0 → D ∗− 3π ± ) ∗ (∗)
from B → D D(s) X
•
• Measurement converted Main Systematic uncertainties
toIR(D ∗ )
MC Statistics• Fit performed on τ lifetime, q 2 and
through external measurements
I External measurements (PDG averages)
output of a BDT trained to reject
I Background shapes and models
double charm decays
B(B 0 → D ∗− 3π ± )
R(D ∗ ) = K(D ∗ )
B(B 0 → D ∗− µ+ νµ )
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 9 / 15
R(J/Ψ) with τ → µνν (PRL 120, 121801)
B(Bc → J/Ψτ ν)
R(J/Ψ) = , J/Ψ → µ+ µ−
B(Bc → J/Ψµν)
• First observation Bc → J/Ψτ ν (3 σ) decay • Lower Bc production rate partially
• Bc channel not accessible at Υ(4S) compensated with the higher µ trigger
efficiency
• Number of signal events extracted with a
• Bc lifetime is much lower than the B 0,±
fit to
I m2
lifetime
miss I
I B decay time Cut on the Bc flight distance fully rejects
c
I combination of q 2 e Eµ semileptonic B 0,± backgrounds
Main backgrounds
• Mis-Identification (h → µ)
• Combinatorial
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 10 / 15R(J/Ψ) with τ → µνν (PRL 120, 121801)
B(Bc → J/Ψτ ν)
R(J/Ψ) = , J/Ψ → µ+ µ−
B(Bc → J/Ψµν)
• First observation Bc → J/Ψτ ν (3 σ) decay • Lower Bc production rate partially
• Bc channel not accessible at Υ(4S) compensated with the higher µ trigger
efficiency
• NumberR(J/Ψ) = 0.71
of signal ± 0.17(stat.)
events ± 0.18(syst.)
extracted with a • Bc lifetime is much lower than the B 0,±
fit to 2 σ higher than the Standard Model
I 2 lifetime
mmiss I
I Bc decay time Cut on the Bc flight distance fully rejects
I combination of q 2 e Eµ semileptonic B 0,± backgrounds
• Main Systematic uncertainties Main backgrounds
I MC Statistics • Mis-Identification (h → µ)
I MisID and Combinatorial background shape
I Hadronic Form Factors • Combinatorial
F New theoretical form factor calculations are available Talk at Beauty 2019
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 11 / 15Next steps: New analyses
• Many other analyses are ongoing at the LHCb • Especially interesting are baryonic decays, exclusive to
experiment to test Lepton Flavour Universality in hadron colliders
b → c`ν transitions (∗)
(∗) B(Λb → Λc τ ν)
I R(D + ),R(D 0 ),R(Ds(∗) ),R(J/Ψ) R(Λc ) =
I in both τ decay channels [arXiv:1808.08865] (∗)
B(Λb → Λc µν)
Experimental advantages Theoretical interest
• Λb production cross section is ≈ 40% the one of B • Different NP Lorentz structure tested
I Higher sensitivity to tensorial structure
mesons
• B(Λ0 → Λc µν) ≈ 3 × B(B → Dµν) • Very useful cross check for present discrepancies
b I Model independent relation [arXiv:1811.09603]
• Smaller Feed Down contribution due to isospin
conservation
I Λ∗∗ decays with at least two π R(Λc ) R(D) R(D ∗ )
c ≈ 0.262 + 0.738
I Simpler background to reject R(Λc )SM R(D)SM R(D ∗ )SM
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 12 / 15Next steps: larger data samples, larger MC samples
• In the coming years of data taking the integrated
luminosity will greatly increase
I Expected integrated luminosity after the UpgradeII is
≈ 300 fb−1
• Evaluation of systematic uncertainties assumes
uncertainties due to background modelling to scale with
luminosity
I The higher statistics in the control regions will help
constraining the background models
• Very large MC samples will be needed to keep under
control the systematics due to MC statistics that affects
all these analyses
I Fast simulations are already being used [arXiv:1810.10362]
[arXiv:1809.06229]
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 13 / 15Next steps: Further improvements in the analyses
• Up to now only the ratios of branching fractions have been used to
test LFU
• Very rich angular structure can be exploited and can give higher
sensitivity to NP Wilson coefficients [arXiv:1602.03030v2]
• B → D ∗ τ ν, τ → µνν
• With the high statistics collected it will be
• B → D ∗ µν
possible to perform angular analyses even
with a quite poor resolution on angular
observables [arXiv:1808.08865]
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 14 / 15Conclusions
• b → c`ν can be a very interesting laboratory to observe indirect effect of NP
• Some discrepancies have been reported with respect to the Standard Model predictions
• Very rich program in this area from the LHCb experiment
I Update of analyses with new data collected
I Both hadronic and leptonic decays of the τ lepton are being exploited
I New decays are now under consideration
• Very high statistics will be a challenge from the experimental side...
• ... it will enable to reduce the uncertainties to the measured parameters...
• ... it will enable us to explore new interesting ideas!
• STAY TUNED!
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 15 / 15Backup Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 16 / 15
R(D ∗ ) with τ → µνν (PRL 115, 11804)
R(D ∗ ) = 0.336 ± 0.027(stat.) ± 0.030(syst.)
2.1 σ higher than the Standard Model
systematic uncertainties
• MC statistics
• Shape of the Mis-ID background
• Shape of the MC derived background models
I Depend on the statistics in the control regions
I They will be reduced in the measurements performed with
the RunII data
• Hadronic form factors
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 17 / 15R(D ∗ ) with τ → πππν (PRL 120,171802 2018)(PRD 97,072013 2018)
R(D ∗ ) = 0.283 ± 0.019(stat.) ± 0.025(syst.) ± 0.013(ext.)
1 σ higher than the Standard Model
systematic uncertainties
• External measurements (PDG averages)
• Background shapes and models
I Extensive studies performed in dedicated control regions to
keep it under control
• MC statistics
• hadronic form factors
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 18 / 15R(J/Ψ) with τ → µνν (PRL 120, 121801)
R(J/Ψ) = 0.71 ± 0.17(stat.) ± 0.18(syst.)
2 σ higher than the Standard Model
systematic uncertainties
• MC Statistics
• Form Factors
I High theoretical uncertaintes
I New theoretical form factor calculations are available
Talk at Beauty 2019
• MisID and Combinatorial background shape
I Given the final state (µ+ µ± µ∓ ) no reliable Wrong Sign
control sample available
I Had to rely on MC simulation for estimating the shape of
this background
• Represents a complementary measurement for testing New Physics operators with a different Lorentz structure
from the one tested with B meson decays
Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 19 / 15Radiative corrections EPJ 79:744 • PHOTOS is used to simulate QED corrections • It does include interference effects between the emitted photons • It does not include Coulomb interactions • The Coulomb corrections depend on the kinematics of the particles, therefore selections • They can be included with dedicated, analysis dependent studies Simone Meloni (Università degli studi di Milano Bicocca) LFU in b → c`ν decays at LHCb 9 January 2020 20 / 15
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