Measurements of CPV and mixing in charm decays

Transcription

1 Measurements of CPV and mixing in charm decays on behalf of LHCb Collaboration Moriond QCD La Thuile, Italy March 21 28, 2015

2 Charm and New Physics In indirect searches for new physics, charm furnish a unique probe of flavour physics in the up-quark sector complementary to strange and bottom physics Indirect searches for NP with charm gives complementary constraints to the direct searches at the Energy Frontier Precision measurements in charm are necessary as inputs for B physics (B! DK, B! Dπ ) and the measurement of the CKM angle γ Many "null-tests" available, one of them is the search for CP violation, which is expected to be small in SM (but not zero)... but SM predictions are difficult to be calculated Moriond QCD, La Thuile March

3 CP Violation in Charm D 0 -D 0 Mixing parameters 3 modes of observing CP violation: in decay: amplitudes for a process and its conjugate differ direct CPV in mixing: rates of D 0!D 0 and D 0!D 0 differ in interference between mixing and decay diagrams indirect CPV Moriond QCD, La Thuile March

4 CP Violation in Charm D 0 -D 0 Mixing parameters 3 modes of observing CP violation: CP violation is not yet observed in decay: amplitudes for a process and its conjugate differ direct CPV in mixing: rates of D 0!D 0 and D 0!D 0 differ in interference between mixing and decay diagrams indirect CPV Moriond QCD, La Thuile March

5 Charm physics at LHCb Unprecedented charm yields at LHC produced world best measurements: Mixing and CPV in D 0 Kπ [PRL 111 (2013) ] Direct CPV with A CP [JHEP 07 (2014) 041, LHCb-CONF ] Indirect CPV in A Γ [PRL 112 (2014) ] (only 2011 data!) CP violation searches in multibody decays [PLB 726 (2013) 623, PLB 728 (2014) 585, JHEP 10 (2014) ] Still statistically dominated in core measurements Experimentally we can tag D 0 flavour at production: TODAY: Measurement of indirect CP asymmetry in D 0!K - K + and D 0!π - π + [arxiv: , submitted to JHEP] Search for CP violation in D 0!π - π + π 0 [LHCB-PAPER , submitted to Phys. Lett. B] Moriond QCD, La Thuile March

6 Indirect CP violation Measurement of indirect CP violation in D 0! K - K + and D 0!π - π + decays [arxiv: , sub. to JHEP] The D 0 mesons are produced in semileptonic b- hadron decays, where the charge of the accompanying muon is used to determine the initial state as D 0 or D 0 Moriond QCD, La Thuile March

7 Yields with L = 3fb -1 Latest LHCb measurement using D 0 from semi-leptonic B decays Full Run1 dataset, L=3 fb 1 D0! K π + used as control channel N(D 0 )= [arxiv: , sub. to JHEP] N(D 0 )= N(D 0 )= Moriond QCD, La Thuile March

8 How A Γ is measured The time-dependent CP asymmetry for D 0 decay decaying into a final state CP eingenstate (i.e. K - K + or π - π + ) is defined as where, Γ is the effective D 0 lifetime. A Γ can be approximated in terms of D 0 mixing parameter x, y, φ, A Γ What we measure is A raw f,[t i,t j ] in 50 bins of D 0 decay time. Then we fit the time evolution of the asymmetries to extract A Γ CP t A RAW ( t) A 0 A Γ τ The A raw is affected by muon detection asymmetry and D 0 production asymmetry, which introduces a bias on A dir CP but not on A Γ. ( ) = N(B D0 µ X) N(B D 0 µ + X) N(B D 0 µ X)+ N(B D 0 µ + X) Moriond QCD, La Thuile March

9 Results [arxiv: , sub. to JHEP] Fits to the time evolution of the asymmetries give In agreement with previous LHCb measurements using prompt D 0 (1fb -1 ) [PRL 112 (2014) ] A Γ ( KK) = ( ± ± 0.012)% ( ππ ) = ( 0.033± ± 0.014)% A Γ Statistically dominated uncertainty Largest systematic contribution coming from the mistag asymmetry Moriond QCD, La Thuile March

10 A Γ average Assuming indirect CP violation to be independent from the D 0 final state, the overall LHCb average, including pion-tagged measurement is [ arxiv: , sub. to JHEP] Update of the most precise measurement (pion-tagged) with 3/fb is ongoing Moriond QCD, La Thuile March

11 Direct CP violation Search for CPV in D 0!π - π + π 0 decays [Phys. Lett. B 740 (2015) 158] The D 0 mesons are produced in D *+ decays, where the charge of the accompanying pion is used to determine the initial state as D 0 or D 0 π - π + π 0 Moriond QCD, La Thuile March

12 Yields with 2fb -1 Time integrated CP asymmetry search in D 0!π - π + π 0 using the energy test method [Phys. Rev. D, 84 (2011), p ] One of the first CPV analysis with π 0 s in LHCb (a) Resolved π 0, two ECAL cluster, better mass resolution, low p T (b) Merged π 0, single ECAL cluster, worse mass resolution, hig p T N S = N S = Decay dominated by ρ(770) resonances: ρ 0 π 0, ρ + π -, ρ-π+ [Phys. Lett. B 740 (2015) 158] Moriond QCD, La Thuile March

13 Strategy Model independent and unbinned method to search for local CP asymmetry in the Dalitz plane [Phys. Rev. D, 84 (2011), p ] Define test statistic T, which depends on the distance between pair of events in the Dalitz plane If the distributions of events in both flavour samples are identical, T will fluctuate around a value close to zero Larger CP asymmetries lead to larger value of T Moriond QCD, La Thuile March

14 Results From data A corresponding p-value is calculated The nominal T value observed in data is compared to a distribution of T values obtained from permutation samples where the flavour of each candidate is randomly reassigned to simulate samples without CP violation World s best sensitivity Final result for CPV in D 0!π - π + π 0 [Phys. Lett. B 740 (2015) 158] Moriond QCD, La Thuile March

15 Visualization of local asymmetries A visualisation of regions of significant asymmetry is obtained by assigning an asymmetry significance to each event The positive (negative) asymmetry significance is set for the D 0 candidates having positive (negative) contribution to the measured T value, respectively Moriond QCD, La Thuile March

16 Conclusions Precision CPV measurements in the charm sector at LHCb First application of the energy test for direct CPV searches in the multibody D 0!π - π + π 0 (2 fb -1 ) Using a novel unbinned model-independent technique One of the first CPV measurement using π 0 s at LHCb The analysis has the best sensitivity from a single experiment to CP violation in this decay. Measurement of A Γ using muon tagged D 0 decays corresponding the full Run 1 data sample (3 fb-1) All searches consistent with no direct or indirect CPV Still several LHCb Run 1 analyses ongoing Moriond QCD, La Thuile March

17 Backup M. Gersabeck Moriond QCD, La Thuile March