Timon Schmelzer FPCP 2018 | On behalf of the LHCb Collaboration

Measurement of CPV in Bs→ϕϕ decays at LHCb

Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

Introduction

‣ Reasonable BR(Bs→ϕϕ) of ‣ Bs→ΦΦ is dominated by gluonic-penguin diagram ‣ Quark transitions of b→sss ‣ Could be sensitive to NP effects within the loop

‣ Decay of pseudo scalar into two vector mesons ‣ Final state is mixture of CP-even and CP-odd eigenstates ‣ Time-dependent angular analysis is required

‣ CP violating phase predicted <1° in SM calculations ‣ Dependence on Vts in mixing and decay amplitudes

leads to cancellation of Bs–mixing phase

‣ Previous measurement:

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arXiv:0810.0249 arXiv:0910.5237

Dominant penguin diagram

Mixing diagrams

Vts

Vts

Vts

Phys. Rev. Lett. 89, 231803

(1.87 ± 0.15) × 10−5

ϕssss

Phys. Rev. D. 90.052011ϕssss = (−0.17 ± 0.15(stat.) ± 0.03(syst.))rad

Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

The Bs→ΦΦ analysis in a nutshell

‣ Selection ‣ Mass model and peaking backgrounds ‣ Triple products asymmetries ‣ Time dependent fit model ‣ Angular acceptance ‣ Decay time acceptance ‣ Decay time resolution ‣ Flavour tagging ‣ Systematics ‣ Results/Summary

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https://bit.ly/2m3Q

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Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

The Bs→ΦΦ analysis in a nutshell

‣ Selection ‣ Mass model and peaking backgrounds ‣ Triple products asymmetries ‣ Time dependent fit model ‣ Angular acceptance ‣ Decay time acceptance ‣ Decay time resolution ‣ Flavour tagging ‣ Systematics ‣ Results/Summary

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https://bit.ly/2m3Q

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Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

The LHCb experiment

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RICHs

VeLoTracking

Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

‣ Loose preselection (aka Stripping in LHCb) ‣ Combine two oppositely charged K to ϕ mesons ‣ Combine two phi mesons to Bs mesons ‣ Combination must satisfy certain conditions: ‣ Track qualities, mass windows, vertex qualities…

‣ To remove combinatorial background: Use MVA method (MLP) with topological features ‣ Signal proxy: Signal MC of Bs→ϕϕ ‣ Background proxy: Upper and lower mass sideband

‣ Afterwards: Cut point optimisation based onsignificance figure of merit

Selection

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FoM =S

S + BS = Number of signal candidatesB = Expected number of background candidates in signal region

with

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Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

‣ Signal model: Crystal Ball + Student-T distribution with shared mean ‣ Background model: Exponential ‣ B0→ϕK*0 (π–K misidentification)

‣ Vetoed via mass window cuts in K+π- and K+K-K+π-

‣ Λb→ϕK–p (p–K misidentification) ‣ Yield estimated from data, shape from MC

‣ Yields after unbinned extended maximum likelihood fit: ‣ nsig: ~8500 ‣ nbkg,comb: ~3500 ‣ nΛb: ~200

Mass fit description

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LHCb-CONF-2018-001

Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

Decay angle definitions

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‣ Θ1,2: angle between K+ momentum in ϕ1,2 meson rest frame and parent ϕ1,2 in Bs rest frame

‣ Φ: angle betweentwo ϕ meson decay planes

‣ n1,2: normalization vectors to the decay planes of the ϕ mesons^

Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

‣ Bs→ϕϕ is P→VV decay ‣ Selection is unable to differentiate between ϕ and f0(980)

or non-resonant pair of ϕ ‣ Total decay amplitude A(t, Θ1, Θ1, Φ) contains P→VS decays (and even P→SS) ‣ Ai(t): time–dependent decay amplitudes

‣ Differential decay rate:

Time-dependent angular description

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Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

Complete fit model

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Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

Anatomy of time-dependent term & external inputs

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‣ Bs decay rates ‣ Gaussian constraints to values measured in Bs→J/ψΦ and Bs→J/ψππ

‣ Bs oscillation frequency ‣ Also taken from the channels mentioned above

‣ Dilution (Flavour tagging) ‣ ω = mistag probability ‣ ω can be reduced by combining different tagging algorithms

ΓH , ΓL , Γs (Γs =ΓH + ΓL

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Δms

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Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

Flavour tagging and decay topology

�12

PV

SV

�

Bs

K+

K+

�

s

s̄

K+

SS Kaon

Same Side (SS)Opposite Side (OS)

q̄Bq

��

b � c

b � X��

OS MuonOS Electron

c � s K� OS Kaon

K–

K–~1cm

OS Vertex ChargeOS Charm

b

u

b̄

Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

‣ Reason: IP requirements on final–state particles ‣ Corrected using similar decay chains (Run differences due to Trigger changes) ‣ Run 1: ‣ Run 2:

‣ Described via combination of six cubic splines

Decay Time Acceptance

�13

B0s → D−

s ( → K+K−π−)π+

B0 → J/ψ( → μ+μ−)K*0(K+π−)

ε(t) =6

∑i=0

cibi(t)

B0s → Dsπ+ B0 → J/ψK*0

LHCb-CONF-2018-001

Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

‣ Assumption: CPV is polarisation independent

Fit results

�14

Complete model CP-even P-wave CP-odd P-wave S-wave

LHCb-CONF-2018-001

Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

Summary & Conclusion

�15

‣ Measuring CPV in Bs→ϕϕ is challenging ‣ Presented the CPV measurement of Bs→ϕϕ

on 5fb-1 @ LHCb ‣ Omitted some important parts of this analysis ‣ Decay time acceptance/resolution ‣ Flavour tagging calibration ‣ Systematics…

‣ No evidence of CPV is found ‣ Most parameter uncertainties are statistically

dominated ‣ Stay tuned!

Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb �16

BACKUP

Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

‣ Selection and acceptance of LHCb introduce decay–angle–dependent efficiency ‣ Write angular coefficient in same basis as efficiency parameterisation

Angular acceptance

�17

legendre polynomials

spherical harmonics

Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

‣ Can probe T-violation via AU and AV ‣ Uses time–integrated, untagged method

‣ Counting experiment

‣ Measurements are compatible with 0

Triple Product Asymmetries

�18

AU/V =Γ(U/V > 0) − Γ(U/V < 0)Γ(U/V > 0) + Γ(U/V < 0)

U = sin(Φ)cos(Φ)V = sign(cos(θ1)cos(θ2))sin(Φ)

with

AU = (0.0 ± 1.2(stat.) ± 0.4(syst.)) %AV = (−0.3 ± 1.2(stat.) ± 0.4(syst.)) %

Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

‣ Bs oscillation frequency: ‣ Per-event decay-time resolution must be smaller ‣ Resolution from MC, then data-driven calibration: ‣ 1. Use sample of prompt data. Choose good quality tracks, originate from PV ‣ 2. Choose one random track and assign the mass of a ϕ–meson to it ‣ 3. Add two other tracks, where in [Bs – 250 MeV/c2; Bs + 250 MeV/c2] ‣ 4. Fit the decay time on this data to calibrate MC:

‣ After calibration: (depending on dataset)

Decay Time Resolution

�19

Δms ≈ 20 ps−1

σti

m3 tracks

σcali = 41 − 44 fs

σcali = q0 + q1σt

i

Timon Schmelzer | FPCP 2018 | CPV in Bs→ϕϕ @ LHCb

Systematics

�20

‣ AA: Angular acceptance ‣ TA: Time acceptance ‣ TR: Time resolution ‣ Largest impact on : Mass model and Fit Bias ϕsss

s