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Updated measurement of decay-time-dependent CPasymmetries inD0 → K+K−D0 → K+K−D0 → K+K− andD0 → π+π−D0 → π+π−D0 → π+π− decays
Daniel Unverzagt*, Physikalisches Institut, Heidelberg University*on behalf of the LHCb Collaboration
1. Motivation• CP violation in charm decays very small
• Significant enhancements due to newphysics possible
• Charm hadrons: unique opportunity tomeasure CPV in systems containing up-type quark
• Discovery of direct CPV in charm decays
– (∼ 10−3)[1]
• No evidence of indirect CPV
– Expected scale (10−4 − 10−5)[2, 3]
• AΓ highest precision probe of CPV incharm mixing
AΓ(f) =τ̂(D̄0 → f)− τ̂(D0 → f)
τ̂(D̄0 → f) + τ̂(D0 → f)
2. How to measure AΓ?• Identifying flavor of the D-meson:
Semileptonic B → Dµ decaysare used to tag the flavor of theD-meson.
• Estimating time-dependent asymmetry:
ARAWCP (t) =N(D̄0(t)→ fCP )−N(D0(t)→ fCP )
N(D̄0(t)→ fCP ) +N(D0(t)→ fCP )= ACP (t) +
time−independent︷ ︸︸ ︷Aµ +Aprod +O(A3)
ACP (t) =Γ(D̄0(t)→ fCP )− Γ(D0(t)→ fCP )
Γ(D̄0(t)→ fCP ) + Γ(D0(t)→ fCP )≈ AdirCP −
t
τAΓ
3. Analysis strategy1. Select a clean B →
( )
D0µX sample
2. Split data in approximate equi-populateddecay-time bins
3. Simultaneous fit to the D0 and D̄0 massdistribution to determine the asymmetrybetween signal yields
4. Fit the yield asymmetry vs decay-time
• Validate analysis strategy onD0 → Kπ (no CPV, AΓ = 0)
5. Mass Fit
KK: ∼ 4.5× 105 events perdecay-time bin
ππ: ∼ 1.5× 105 events perdecay-time bin
1. Create D0 and D̄0 sub-samples by muon tag
2. Divide data in 20 decay-time bins
3. Estimate asymmetry bysimultaneous fit toD0 andD̄0 mass spectrum in eachdecay-time bin
6. SystematicsMistag: probability to wrongly associateunrelated muons with the D0 candidates liesbetween 1% and 3%.
Generate PE with realistic mistag fraction andassess impact on fit parameters.
Systematics are studied by large samples ofpseudoexperiments (PE).
Severalcrosschecksperformed
hello No additionalsystematics
were observed
4. Selection
Around 9 × 106 KK, 3 × 106 ππ and 76 × 106
Kπ (control channel) events after selection.
7. Final resultThis analysis[4]:
AΓ(K+K−) = (−4.3± 3.6± 0.5)× 10−4
AΓ(π+π−) = (2.2± 7.0± 0.8)× 10−4
Combination with previous LHCbmeasurements:
AΓ(K+K−) = (−4.4± 2.3± 0.6)× 10−4
AΓ(π+π−) = (2.5± 4.3± 0.7)× 10−4
No indication of CP violation in mixingor in the interference between mixing anddecay in neutral D-decays.
References[1] LHCb Collaboration. Observation of CP violation in
charm decays, Phys. Rev. Lett. 122, 211803[2] M. Bobrowski, A. Lenz, J. Riedl, and J. Rohrwild. How
large can the SM contribution to CP violation in D0 − D̄0
mixing be?, J. High Energ. Phys. 2010, 9 (2010)[3] Grossman, Yuval and Kagan, Alexander L. and Nir,
Yosef. New physics and CP violation in singly Cabibbosuppressed D decays, Phys. Rev. D 75, (2007) 036008.
[4] LHCb Collaboration. Updated measurement of decay-time-dependent CP asymmetries in D0 → K+K− andD0 → π+π− decays, Phys. Rev. D 101 (2020) no.1,012005
Alternative signal and background models are tested.
Flavor identifiedby muon charge