Charm mixing and CP violation at LHCb

Transcription

1 Charm mixing and CP violation at LHCb Alex Pearce on behalf of the LHCb collaboration 13 th 19 th March 216 Moriond EW, La Thuile 1 / 15

2 Introduction Why charm? Mixing and CP violation established in kaon and B systems Only up-type quark where these effects may occur Expected to be very small effects in charm Sensitive to contributions from new physics 2 / 15

3 Introduction Why charm? Mixing and CP violation established in kaon and B systems Only up-type quark where these effects may occur Expected to be very small effects in charm Sensitive to contributions from new physics Why LHCb? It is a true charm factory Excellent vertex resolution and particle identification Charm cross-sections In the LHCb acceptance p T < 8 GeV, 2 < y < 4.5 σ(pp cc) = (1419 ± 134) 7 TeV [1] (294 ± 24) 13 TeV [2] 2 / 15

4 Mixing and CP violation formalism Two mass and two flavour eigenstates D L,H = p D ± q D Mass and width splitting parameterised as x = M 2 M 1, y = Γ 2 Γ 1 Γ 2Γ CP violation (CPV) manifests directly, indirectly, or as a superposition A f 1, Ā f q p 1, ( ) arg qaf pā f Tag D flavour with D + D π + or semileptonic decays of B hadrons 3 / 15

5 First observation of D D oscillations in D K + π π + π decays and measurement of the associated coherence parameters arxiv: Submitted to Phys. Rev. Lett.

6 D D oscillations in K + π π + π arxiv: CF D K π + π π + D DCS Right-sign amplitude 5 / 15

7 D D oscillations in K + π π + π arxiv: CF DCS D K π + π π + D K + π π + π D DCS Mixing D CF Right-sign amplitude Wrong-sign amplitude 5 / 15

8 D D oscillations in K + π π + π arxiv: CF DCS D K π + π π + D K + π π + π D DCS Mixing D CF Right-sign amplitude Wrong-sign amplitude R(t) rd 2 + R D r D y t τ + x 2 + y 2 ( ) t 2 4 τ x = x cos δ + y sin δ, y = y cos δ x sin δ R D e iδ cos δ + i sin δ 5 / 15

9 D D oscillations in K + π π + π arxiv: Using 3 fb 1 of luminosity collected in Run 1 Experimentally challenging Lower reconstruction efficiency Higher combinatorial background Five-dimensional phase space to parameterise 6 / 15

10 D D oscillations in K + π π + π arxiv: Using 3 fb 1 of luminosity collected in Run 1 Experimentally challenging Lower reconstruction efficiency Higher combinatorial background Five-dimensional phase space to parameterise Candidates / (.1 MeV/c 2 ) LHCb RS candidates Fit Background m [MeV/c 2 ] 6 / 15

11 D D oscillations in K + π π + π arxiv: Using 3 fb 1 of luminosity collected in Run 1 Experimentally challenging Lower reconstruction efficiency Higher combinatorial background Five-dimensional phase space to parameterise RS and WS signal candidates Candidates / (.1 MeV/c 2 ) Candidates / (.1 MeV/c 2 ) LHCb RS candidates Fit Background m [MeV/c 2 ] LHCb WS candidates 8 7 Fit 6 Background m [MeV/c 2 ] 6 / 15

12 D D oscillations in K + π π + π arxiv: R(t) rd 2 + R D r D y t τ + x 2 + y 2 ( ) t 2 τ LHCb 4 WS/RS Data Unconstrained fit No-mixing fit t / τ No-mixing hypothesis rejected at 8.2 σ 7 / 15

13 D D oscillations in K + π π + π arxiv: Can measure γ with measurements of B ± DK ± γ = arg( V ud V ub/v cd V cb) Compare rates of same final state with D and D Sensitivity increases with large strong phases differences between D /D decays This analysis is sensitive to this difference 8 / 15

14 D D oscillations in K + π π + π arxiv: Can measure γ with measurements of B ± DK ± γ = arg( V ud V ub/v cd V cb) Compare rates of same final state with D and D Sensitivity increases with large strong phases differences between D /D decays This analysis is sensitive to this difference Use HFAG inputs for x and y [ ] K3π δ D % CL 95.4% CL 99.7% CL LHCb K3π R D 8 / 15

15 D D oscillations in K + π π + π arxiv: Can measure γ with measurements of B ± DK ± γ = arg( V ud V ub/v cd V cb) Compare rates of same final state with D and D Sensitivity increases with large strong phases differences between D /D decays This analysis is sensitive to this difference Use HFAG inputs for x and y [ ] K3π δ D % CL 95.4% CL 99.7% CL LHCb K3π R D See Malcolm John s talk CKM angle γ at LHCb 8 / 15

16 Measurement of the difference of time-integrated CP asymmetries in D K K + and D π π + decays arxiv: Submitted to Phys. Rev. Lett.

17 Measurement of the difference of time-integrated CP asymmetries in D K K + and D π π + decays arxiv: Submitted to Phys. Rev. Lett. See Andreas Weiden s talk at YSF 1

18 Direct CP violation with A CP arxiv: Direct CPV if A f /Ā f 1 A CP (f ) = Γ(D f ) Γ(D f ) Γ(D f ) + Γ(D f ) Can measure difference in yields experimentally A Raw (f ) = N(D + (D f )π + ) N(D (D f )π ) N(D + (D f )π + ) + N(D (D f )π ) A CP (f ) + A Production (D + ) + A Detection (π + ) + A Detection (f ) Experimentally robust to measure a difference with CP eigenstates A CP = A Raw (K K + ) A Raw (π π + ) A CP (K K + ) A CP (π π + ) 1 / 15

19 Direct CP violation with A CP arxiv: Direct CPV if A f /Ā f 1 A CP (f ) = Γ(D f ) Γ(D f ) Γ(D f ) + Γ(D f ) Can measure difference in yields experimentally A Raw (f ) = N(D + (D f )π + ) N(D (D f )π ) N(D + (D f )π + ) + N(D (D f )π ) A CP (f ) + A Production (D + ) + A Detection (π + ) + A Detection (f ) Experimentally robust to measure a difference with CP eigenstates A CP = A Raw (K K + ) A Raw (π π + ) A CP (K K + ) A CP (π π + ) 1 / 15

20 Direct CP violation with A CP arxiv: Direct CPV if A f /Ā f 1 A CP (f ) = Γ(D f ) Γ(D f ) Γ(D f ) + Γ(D f ) Can measure difference in yields experimentally A Raw (f ) = N(D + (D f )π + ) N(D (D f )π ) N(D + (D f )π + ) + N(D (D f )π ) A CP (f ) + A Production (D + ) + A Detection (π + ) + A Detection (f ) Experimentally robust to measure a difference with CP eigenstates A CP = A Raw (K K + ) A Raw (π π + ) A CP (K K + ) A CP (π π + ) 1 / 15

21 Direct CP violation with A CP arxiv: First measurement using.6 fb 1 of D + -tagged D decays [1] A CP = (.82 ±.21 (stat) ±.11 (syst)) % Following measurements with B-tagged decays with 1 fb 1 and then 3 fb 1 [2] A CP = (.14 ±.16 (stat) ±.8 (syst)) % 11 / 15

22 Direct CP violation with A CP arxiv: New measurement using 3 fb 1 of D + -tagged D decays D K K + and D π π + signal candidates Candidates / (.5 MeV/c 2 ) LHCb + D K K 5 1 δm (MeV/c 2 ) Candidates / (.5 MeV/c 2 ) LHCb + D π π 5 1 δm (MeV/c 2 ) 12 / 15

23 Direct CP violation with A CP arxiv: New measurement using 3 fb 1 of D + -tagged D decays D K K + and D π π + signal candidates Candidates / (.5 MeV/c 2 ) LHCb + D K K 5 1 δm (MeV/c 2 ) Candidates / (.5 MeV/c 2 ) LHCb + D π π 5 1 δm (MeV/c 2 ) A CP = (.1 ±.8 (stat) ±.3 (syst)) % 12 / 15

24 Direct CP violation with A CP arxiv: Can parameterise A CP into direct and indirect components ( A CP = acp dir 1 + t ) y CP + t acp ind τ τ Also measure t, t Use LHCb measurements of A Γ a ind CP and y CP [1,2] 13 / 15

25 Direct CP violation with A CP arxiv: Can parameterise A CP into direct and indirect components ( A CP = acp dir 1 + t ) y CP + t acp ind τ τ Also measure t, t Use LHCb measurements of A Γ a ind CP and y CP [1,2] a dir CP A Γ SL K Κ + and π π A CP SL A CP prompt A Γ prompt π π + A Γ prompt K Κ + no CPV LHCb aind CP 13 / 15

26 Direct CP violation with A CP arxiv: Can parameterise A CP into direct and indirect components ( A CP = acp dir 1 + t ) y CP + t acp ind τ τ Also measure t, t Use LHCb measurements of A Γ a ind CP and y CP [1,2] a dir CP A Γ SL K Κ + and π π A CP SL A CP prompt A Γ prompt π π + A Γ prompt K Κ + no CPV LHCb aind CP Consistent with CP conservation hypothesis with p = / 15

27 Mixing parameters with D K Sπ + π arxiv: , submitted to JHEP Novel model-independent measurement of x and y Uses strong phase measurements from resonant ψ(377) production Method now validated on 1 fb 1 of data x = (.86 ±.53 (stat) ±.17 (syst)) %, y = (.3 ±.46 (stat) ±.13 (syst)) % Direct CP asymmetry with D K SK S JHEP 1 (215) 55 Measurement of yield asymmetry A CP Theory predicts O(1%) [1], single previous measurement A CP = (23 ± 19) % [2] Challenging: many KS decay outside the vertex locator A CP = ( 2.9 ± 5.2 (stat) ± 2.2 (syst)) % 14 / 15

28 Mixing parameters with D K Sπ + π arxiv: , submitted to JHEP Novel model-independent measurement of x and y Uses strong phase measurements from resonant ψ(377) production Method now validated on 1 fb 1 of data x = (.86 ±.53 (stat) ±.17 (syst)) %, y = (.3 ±.46 (stat) ±.13 (syst)) % Direct CP asymmetry with D K SK S JHEP 1 (215) 55 Measurement of yield asymmetry A CP Theory predicts O(1%) [1], single previous measurement A CP = (23 ± 19) % [2] Challenging: many KS decay outside the vertex locator A CP = ( 2.9 ± 5.2 (stat) ± 2.2 (syst)) % 14 / 15

29 Summary LHCb is making many interesting charm measurements Very high precision New techniques Still more to come from Run 1 Run 2 will bring a lot more data, a lot more interesting physics A CP = (.1 ±.8 (stat) ±.3 (syst)) % 15 / 15

30 Backup 1 / 5

31 D D oscillations ratio R. ratio (points) with the results of the unconstrained (solid line) and no-mixing (dashed line) fits superimposed. The bin centres are set to the decay-time where R(t) is equal to the bin integrated Table 1: Results of the decay-time dependent fits to the WS/RS ratio for the unconstrained and mixing-constrained fit configurations. The results include all systematic uncertainties. Fit Type Parameter Fit result Correlation coe cient 2 /ndf (p-value) rd K3 RD K3 y 1 K3 4 (x2 + y 2 ) Unconstrained rd K3 (5.67 ±.12) /7 (.35) RD K3 y K3 (.3 ± 1.8) (x2 + y 2 ) (4.8 ± 1.8) r K3 D R K3 D y K3 x y Mixing-constrained rd K3 (5.5 ±.7) /8 (.19) RD K3 y K3 ( 3. ±.7) x (4.1 ± 1.7) y (6.7 ±.8) Fit results with and without constraints from HFAG x and y consistent with the existing measurement from Belle [24], and has smaller uncertainties. Using the RS branching fraction, B(D! K + + )=(8.7±.23) 1 2 [2], the WS branching fraction, B(D! K + + ), is determined to be (2.66 ±.6 ±.8) 1 4 using the unconstrained result, and (2.6±.4±.7) 1 4 using the mixing-constrained result. Here the first uncertainty is propagated from RWS K3 and includes systematic e ects, and the second is from the knowledge of B(D! K + + ). In conclusion, the decay-time dependence of the ratio of D! K + + to D! K + + decay rates is observed, and the no-mixing hypothesis is excluded at a 2 / 5

32 Direct CP violation with A CP Table 1: Values of A CP measured in the disjoint data subsamples, according to magnet polarity (up, down), centre-of-mass energy of data taking ( p s = 7 and 8 TeV ) and trigger category (ntos, TOS). polarity trigger p s [ TeV ] ACP [%] up TOS 7.4 ±.35 up ntos 7.19 ±.29 down TOS 7.31 ±.29 down ntos 7.6 ±.24 up TOS 8.11 ±.21 up ntos 8.22 ±.17 down TOS 8.22 ±.21 down ntos ±.17 average.1 ±.8 Measurements of A CP on disjoint subsets of the data 3 / 5

33 Mixing parameters with D K Sπ + π Candidates per 1.7 MeV/c 2 Candidates per 95 kev/c D *+ LHCb m [GeV/c 2 D ] D *+ LHCb m [GeV/c 2 ] Candidates per 1.7 MeV/c 2 Candidates per 95 kev/c * D LHCb m [GeV/c 2 D ] * D LHCb m [GeV/c 2 ] Figure 3: Fitted (md, m) distributions. The upper row shows the md projection and the lower row m. The left column shows D + candidates and the right column D. The signal and background components are shown separately (signal as solid grey, D background dashed, combinatoric background dotted, and the sum as solid black). fits to each of the 32 subsamples are then carried out, with only the parameters of the combinatorial background shape, f3(md smooth), and the yield fractions P3(j) free t D [ps] Fits to the mass difference m and decay time t for D K S π+ π Candidates per.47 ps Candidates per.47 ps LHCb LHCb 4.6 Mixing parameters 2 4 / 5

34 Direct CP asymmetry with D K SK S Entries / (.33 MeV) 5 LHCb LL Data D*+ 4 Total fit Signal 3 Background m [MeV] Entries / (.33 MeV) 5 LHCb LL D* m [MeV] Entries / (.33 MeV) 45 LHCb LD 4 Data D*+ 35 Total fit 3 Signal 25 Background m [MeV] Entries / (.33 MeV) 45 LHCb LD 4 D* m [MeV] Entries / (.33 MeV) 7 LHCb LLtrig 6 D* m [MeV] Entries / (.33 MeV) 7 LHCb LLtrig 6 D* m [MeV] Figure 2: Distributions of m split into (left) D +, (right) D and (top) LL, (bottom) LLtrig, including the fit function. The solid (black) line corresponds to the total fit, the dashed (blue) line corresponds to the background, and the dash-dotted (red) line represents the signal contribution. Entries / (.33 MeV) 5 LHCb DD+ 45 D* m [MeV] Fits to the mass difference m for different K S Entries / (.33 MeV) 5 LHCb DD 45 D* m [MeV] Figure 3: Distributions of m split into (left) D +, (right) D and (top) LD, (bottom) DD, including the fit function. The solid (black) line corresponds to the total fit, the dashed (blue) line corresponds to the background, while the dash-dotted (red) line represents the signal contribution. vertex positions combined CP asymmetry a di erence of.19 is found, which is assigned as a systematic uncertainty. The systematic e ects that arise due to the slow pion charge asymmetry in the detector and a possible charge asymmetry of D production in pp collisions in the LHCb acceptance are determined using the control channel. However, the control channel contains charged kaons which introduce an additional detection asymmetry, as the interaction cross-sections systematic uncertainty due to production and detection asymmetries. Other checks have been performed but found to have statistically insignificant e ects. These tests include the split into di erent trigger types, di erent run periods, and di erent magnet polarities. Also the e ect of a possible di erence in contamination by charm from beauty decays between signal and the control channel has been checked and found to be negligible. The total systematic uncertainty is calculated from the quadratic sum of5 / 5