Exploiting the full Run II data sample collected by the CDF trigger on displaced vertices, we present a search for CP violation in neutral D mesons decays to hadronic final states. We use the strong D∗+ → D0π+ (and c.c.) decay to identify the flavor of the charmed meson at production time and exploit CPconserving strong c¯c pair-production in p¯p collisions. The results are the world’s most precise measurements to date and confirm the presence of sizable CP-violating effects in the charm sector as recently observed by the LHCb collaboration

Di Canto, A.

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arXiv

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DOI 10.1393/ncc/i2013-11400-3Colloquia: IFAE 2012IL NUOVO CIMENTO Vol. 36 C, N. 1 Gennaio-Febbraio 2013CP violation in charm decays at CDFA. Di Canto on behalf of the CDF CollaborationPhysikalisches Institut, Ruprecht-Karls-Universita¨t Heidelberg, Germany andINFN, Sezione di Pisa - Pisa, Italyricevuto il 31 Agosto 2012Summary. — Exploiting the full Run II data sample collected by the CDF triggeron displaced vertices, we present a search for CP violation in neutral D mesonsdecays to hadronic final states. We use the strong D∗+ → D0π+ (and c.c.) decayto identify the flavor of the charmed meson at production time and exploit CP -conserving strong cc¯ pair-production in pp¯ collisions. The results are the world’smost precise measurements to date and confirm the presence of sizable CP -violatingeffects in the charm sector as recently observed by the LHCb collaboration.PACS 11.30.Er – Charge conjugation, parity, time reversal, and other discretesymmetries.PACS 13.25.Ft – Decays of charmed mesons.While CP violation is well established for B and K mesons, this is not yet the case forcharm mesons. First evidence for CP violation in two-body singly Cabibbo-suppressedD0 decays has been recently reported by the LHCb Collaboration [1]. Whether this is ahint of possible new physics contributions to the decay amplitude or not is not yet clear.It is important to broaden our search for CP violation in further charmed meson decays.Owning to the large production cross-section available at the Tevatron collider andto the flexibility of a trigger for fully hadronic final states [2], the CDF experiment, ina decade of successful Run II operations, collected millions of D mesons decays, whichallow high-precision CP violation searches. Here we present two new measurements ofCP violation in neutral D mesons decays which are among the world’s most sensitive todate. In all cases, the production flavor of the neutral D meson is tagged by the chargeof the pion from the D∗+ → D0π+ decay (charge conjugated states are implied, unlessotherwise stated).1. – Time-integrated asymmetries in D0 → K0Sπ+π− decaysIn a data sample corresponding to an integrated luminosity of 6 fb−1, CDF searchesfor time-integrated CP asymmetries in the resonant substructure of the three-bodyD0 → K0Sπ+π− decay. As the Standard Model expectation of these CP asymmetriesc© Societa` Italiana di Fisica 26CP VIOLATION IN CHARM DECAYS AT CDF 27Table I. – Measured fit fraction asymmetries, AFF , for the considered intermediate resonancesof the D0 → K0Sπ+π− decay. The first uncertainties are statistical and the second systematic.Resonance AFF (%) Resonance AFF (%)K∗(892)− 0.36± 0.33± 0.40 K∗(892)+ 1.0± 5.7± 2.1K∗0 (1430)− 4.0± 2.4± 3.8 K∗0 (1430)+ 12± 11± 10K∗2 (1430)− 2.9± 4.0± 4.1 K∗2 (1430)+ −10± 14± 29K∗(1410)− −2.3± 5.7± 6.4 K∗(1680)− (not found)ρ(770) −0.05± 0.50± 0.08 ρ(1450) −4.1± 5.2± 8.1ω(782) −12.6± 6.0± 2.6 σ2 −6.8± 7.6± 3.8f0(980) −0.4± 2.2± 1.6 f0(1370) −0.5± 4.6± 7.7f2(1270) −4.0± 3.4± 3.0 f0(600) −2.7± 2.7± 3.6is O(10−6) [3], well below the experimental sensitivity, an observation of CP violationwould be a clear hint of new physics.We reconstruct approximately 350000 D∗-tagged D0 → K0S(→ π+π−)π+π− can-didates. Two complementary approaches are used: a full Dalitz fit and a model-independent bin-by-bin comparison of the D0 and D0 Dalitz plots. We briefly presenthere only the result of the first approach, a more comprehensive description of the analysiscan be found in ref. [4].For the first time at a hadron collider, a Dalitz amplitude analysis is applied for thedescription of the dynamics of the decay. We employ the isobar model and determinethe asymmetries between the different D0 and D0 sub-resonance fit fractions (table I) inorder to be insensitive to any global instrumental asymmetry in the reconstruction andidentification of the candidates of interest. The results represent a significant improve-ment in terms of precision with respect to previous determinations of these quantites [5],and no hints of any CP -violating effects are found. The measured value for the overallintegrated CP asymmetry isACP (D0 → K0Sπ+π−) =(−0.05± 0.57 (stat)± 0.54 (syst))%.Following the procedure described in ref. [6] and assuming no direct CP violation inthe D0 → K0Sπ+π− decay (AdirCP = 0), we can derive a measurement of time-integratedCP violation in D0 mixing (AindCP ) since the measured time-integrated asymmetry canbe approximately expressed as(1) ACP (D0 → f) ≈ AdirCP (D0 → f) +〈t〉τAindCP ,where f indicates a generic final state and 〈t〉/τ ≈ 2.28 is the observed average D0 decaytime of the sample in units of D0 lifetimes. We then findAindCP =(−0.02± 0.25 (stat)± 0.24 (syst))%.28 A. DI CANTO on behalf of the CDF COLLABORATION2. – Time-integrated asymmetries in D0 → K+K− and D0 → π+π− decaysBuilding upon the techniques developed for the previous measurement of individualasymmetries in D0 → h+h− (h = π or K) decays [6], CDF updated and optimized theanalysis toward the measurement of the difference of asymmetries, ΔACP = ACP (D0 →K+K−) − ACP (π+π−). The difference of asymmetries is here used as a tool to can-cel, to an excellent level of accuracy, the few percents detector-induced asymmetry inthe efficiencies for reconstructing the tagging-pion from the D∗ decay. The offline se-lection has been loosened with respect to the measurement of individual asymmetries,since their difference is much less sensitive to instrumental effects allowing for a moreinclusive selection, and we now use the full CDF Run II data sample, which correspondsto 9.7 fb−1 of integrated luminosity. Requirements on the minimum number of hits forreconstructing tracks are loosened, the pT threshold for D decay products is lowered from2.2 to 2.0GeV/c and ∼ 12% fraction of charmed mesons produced in B decays, whosepresence does not bias the difference of asymmetries, is now used in the analysis. As aresult of the improved selection, the D0 yield nearly doubles and the expected resolutionon ΔACP becomes competitive with LHCb’s [1]. In the following we briefly present theresult, more details can be found in ref. [7].Using the approximately 550 000 D∗-tagged D0 → π+π− and 1.21 · 106 D∗-taggedD0 → K+K− decays, we measureΔACP =(−0.62± 0.21 (stat)± 0.10 (syst))%,which is 2.7σ different from zero and consistent with the LHCb result [1], suggestingthat CDF data support CP violation in charm. By means of eq. (1) and using theobserved values of 〈t(K+K−)〉 − 〈t(π+π−)〉 = (0.27± 0.01) τ , the observed asymmetry iscombined with all other available measurements of CP violation in D0 → h+h− decaysto extract the values of AindCP and ΔAdirCP = AdirCP (D0 → K+K−) − AdirCP (D0 → π+π−).The combination yields ΔAdirCP = (−0.67 ± 0.16)% and AindCP = (−0.02 ± 0.22)%, whichdeviates by approximately 3.8σ from the no-CP violation point.Finally, the measured value of ΔACP from the subsample of additional events selectedby the new criteria is combined with the statistically independent results of ref. [6], toobtain a more precise determination of the individual asymmetries:ACP (D0 → π+π−) =(+0.31± 0.22 (stat. + syst.))%,ACP (D0 → K+K−) =(−0.32± 0.21 (stat. + syst.))%.These results are the world’s most precise to date; they improve and supersede theprevious corresponding results of ref. [6].REFERENCES[1] Aaij R. et al. (LHCb Collaboration), Phys. Rev. Lett., 108 (2012) 111602.[2] Ristori L. and Punzi G., Annu. Rev. Nucl. Part. Sci, 60 (2010) 595; Ashmanskas W. etal., Nucl. Instrum. Methods A, 518 (2004) 532.[3] Bigi I. I. and Yamamoto H., Phys. Lett. B, 349 (1995) 363; Xing Z.-Z., Phys. Lett. B,353 (1995) 313.[4] Aaltonen T. et al. (CDF Collaboration), arXiv:1207.0825 [hep-ex].[5] Asner D. M. et al. (CLEO Collaboration), Phys. Rev. D, 70 (2004) 091101(R).[6] Aaltonen T. et al. (CDF Collaboration), Phys. Rev. D, 85 (2012) 012009.[7] Aaltonen T. et al. (CDF Collaboration), arXiv:1207.2158 [hep-ex] and CDF PublicNote 10784.

CP violation in charm decays at CDF

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CP violation in charm decays at CDF

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