We report the final CDF measurement of the $B^0_s$ mixing phase, mean
lifetime, and decay-width difference through the fit of the time evolution of
flavor-tagged $B^0_s \rightarrow J/\psi \phi$ decays. The measurement is based
on the full data set of 1.96 TeV $p\bar{p}$ collisions collected between
February 2002 and September 2011 by the CDF experiment. The results are
consistent with the standard model and other experimental determinations and
are amongst the most precise to date.Comment: 3 pages, 2 figures, conference IFAE 201

Leo, Sabato

English

arXiv

We report the final CDF measurement of the B0s mixing phase,

mean lifetime, and decay width difference through the fit of the time evolution of flavour-tagged B0s → J/ψφ decays. The measurement is based on the full data set of 1.96TeV pp collisions collected between February 2002 and September 2011 by the CDF experiment. The results are consistent with the standard model and other experimental determinations and are amongst the most precise to date

Leo, S.

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2013-11397-5Colloquia: IFAE 2012IL NUOVO CIMENTO Vol. 36 C, N. 1 Gennaio-Febbraio 2013Final measurement of B0s mixing phase in the full CDFRun II data setS. Leo on behalf of the CDF CollaborationUniversita` di Pisa and INFN, Sezione di Pisa - Pisa, Italyricevuto il 31 Agosto 2012Summary. — We report the final CDF measurement of the B0s mixing phase,mean lifetime, and decay width difference through the fit of the time evolution offlavour-tagged B0s→ J/ψφ decays. The measurement is based on the full data setof 1.96TeV pp collisions collected between February 2002 and September 2011 bythe CDF experiment. The results are consistent with the standard model and otherexperimental determinations and are amongst the most precise to date.PACS 11.30.Er – Charge conjugation, parity, time reversal, and other discretesymmetries.PACS 12.38.Qk – Experimental tests.PACS 13.25.Hw – Hadronic decays of mesons: Decays of bottom mesons.PACS 14.40.Nd – Bottom mesons.1. – IntroductionFlavor physics of quarks is considered one of the most promising sectors for indi-rect signs of new particles or interactions beyond the standard model (SM). The B0sdinamics in particular offers rich opportunities since its experimental exploration hasnot reached in extension and precision stringent constraints on the presence of NP incontrast to what happened in leading (and some subleading) processes involving chargedand neutral kaons and bottom mesons. The B0s oscillations are explained in terms ofsecond-order weak processes involving the the CKM matrix element Vts: a broad classof generic extensions of the SM is expected to affect the mixing amplitude, modifyingthe mixing “intensity”—that is the oscillation frequency—and the phase, βs. A non-SMenhancement of βs would also decrease the size of the decay-width difference between thelight and heavy-mass eigenstates of the B0s meson, ΔΓs [1]. While the oscillation inten-sity has been measured precisely [2], only loose constraints on the phase were availableuntil recently. The most effective determination of βs and ΔΓs is achieved through theanalysis of the time evolution of flavour–tagged B0s→J/ψφ decays.c© Societa` Italiana di Fisica 1516 S. LEO on behalf of the CDF COLLABORATION]2 Mass [GeV/c-K+ KψJ/5.28 5.3 5.32 5.34 5.36 5.38 5.4 5.42 5.44 5.462Candidates per 1 MeV/c0100200300400500600700800)-1Data (9.6 fbFitBackground[rad]sβ-1.5 -1 -0.5 0 0.5 1 1.5]-1[pssΓΔ-0.6-0.4-0.200.20.40.6 A. Lenz and U. Nierste, arXiv:1102.4274v1 (2011)168% CL95% CL1SM expectationSymmetry lineFig. 1. – Left: Distribution of J/ψK+K− mass with fit projection overlaid. Right: Confidenceregions at the 68% and 95% CL in the (βs,ΔΓs)-plane.The first such analysis was performed by the CDF experiment in 2008. Immediatelyafter D0 joined [3]. In 2010 the combination of CDF and D0 results suggested a mild devi-ation from the SM expectation. However, updated measurements [4-6] showed increasedconsistency with the SM, calling for additional experimental information to clarify thepicture.Here we report the new CDF update using the final data set of 10 fb−1.2. – AnalysisThe decays, collected by a low–momentum dimuon trigger, are fully reconstructedusing four tracks originating from a common displaced vertex, two matched to muonpairs consistent with a J/ψ decay (3.04 < mμμ < 3.14GeV/c2), and two consistent witha φ→K+K− decay (1.009 < mKK < 1.028GeV/c2). The dimuon mass constraint to theknown J/ψ mass combined with a transverse momentum resolution of σpT /pT ≈ 0.07%pT(pT in GeV/c) yield a mass resolution for our signals of about 9MeV/c2. The J/ψK+K−mass distribution (fig. 1, left), shows a signal of approximately 11000 decays, overlap-ping a similar amount of constant background dominated by the prompt combinatorialcomponent, and smaller contributions from mis-reconstructed B decays.The analysis relies on a joint fit to the time evolution of B0s mesons that resolve thefast oscillations by exploiting the 90 fs time resolution of the CDF micro-vertex detectorfor these final states. Because the B0s meson has spin zero and J/ψ and φ have spinone, the B0s→J/ψφ decay involve three independent amplitudes, each corresponding toone possible angular momentum state of the J/ψφ system which is also a CP -odd orCP -even eigenstate. To enhance the sensitivity to βs, the time-evolution of the decayamplitudes is fitted independently by exploiting differences in the distribution of thekaons’ and muons’ decay angles. Sensitivity to βs can be further enhanced accountingfor the difference in the time evolution of initially produced B0s and B¯0s . The flavour ofthe meson at the time of production is inferred by two independent classes of algorithms:the opposite-side flavour tag (OST) and the same-side kaon tag (SSKT) [2].The OST performances have been determined with 82000 B±→ J/ψ(→ μ+μ−)K±decays fully reconstructed in the same sample as the signal. We found an efficiency ofFINAL MEASUREMENT OF B0s MIXING PHASE IN THE FULL CDF RUN II DATA SET 17εOST = (92.8±0.1)%, an observed averaged dilution, DOST, equal to (12.3±0.4)% and aresulting effective tagging power of εOSTD2OST = (1.39± 0.05)%. The SSKT algorithmstag a smaller fraction of candidates with better precision. Its performances have beenpreviously determined [4] to be εSSKT = (52.2 ± 0.7)%, DSSKT = (21.8 ± 0.3)% andεSSKTD2SSKT = (3.2± 1.4)%.Since SSKT algorithm has been calibrated for early data only, we conservativelyrestrict its use to the events collected in that period. Simulation shows that this resultsin a modest degradation in βs resolution.The unbinned maximum likelihood joint fit uses 9 observables from each event todetermine 32 parameters including βs and ΔΓ, other physics parameters (B0s lifetime,decay amplitudes at t = 0 and phases, etc.), and several other (“nuisance”) parameters(experimental scale factors, etc.).3. – ResultsIf βs is fixed to its SM value, the fit shows unbiased estimates and gaussian uncer-tainties for ΔΓs, τs. We found ΔΓs = 0.068± 0.026(stat) ± 0.007(syst) ps−1, and meanB0s lifetime, τs = 1.528 ± 0.019(stat) ± 0.009(syst) ps. Systematic uncertainties includemismodeling of the signal mass model, lifetime resolution, acceptance description, andangular distribution of the background; a O(2%) contamination by B0→J/ψK∗(892)0decays misreconstructed as B0s→ J/ψφ decays; and the silicon detector misalignment.These results are among the most precise measurements from a single experiment.If βs is free to float in the fit, tests in statistical trials show that the maximumlikelihood estimate is biased for the parameters of interest, and the biases depend onthe true values of the parameters. Hence, we determine confidence regions in the βsand (βs,ΔΓs) spaces (fig. 1, right), by using a profile-likelihood ratio statistic as a χ2variable and considering all other likelihood variables as nuisance parameters. Con-fidence regions are corrected for non-Gaussian likelihood and systematic uncertaintiessuch as to ensure nominal coverage. By treating ΔΓs as a nuisance parameter, wealso obtain βs ∈ [−π/2,−1.51] ∪ [−0.06, 0.30] ∪ [1.26, π/2] rad at the 68% CL, andβs ∈ [−π/2,−1.36] ∪ [−0.21, 0.53] ∪ [1.04, π/2] rad at the 95% CL. Included in the fitis also the CP -odd component originated by non-resonant K+K− pair or by the f0(980)decays. The resulting S-wave decay amplitude is found to be negligible. All results areconsistent with the SM expectation and with determinations of the same quantities fromother experiments [5-7].REFERENCES[1] Faller S. et al., Phys. Rev. D, 79 (2009) 014005.[2] Abulencia A. et al. (CDF Collaboration), Phys. Rev. Lett., 97 (2006) 242003.[3] Aaltonen T. et al. (CDF Collaboration), Phys. Rev. Lett., 100 (2008) 161802; AbazovV. M. et al. (D0 Collaboration), Phys. Rev. Lett., 101 (2008) 241801.[4] Aaltonen T. et al. (CDF Collaboration), Phys. Rev. D, 85 (2012) 072002.[5] Abazov V. M. et al. (D0 Collaboration), Phys. Rev. D, 85 (2012) 032006; 84 (2011)052007.[6] Aaij R. et al. (LHCb Collaboration), Phys. Rev. Lett., 108 (2012) 101803.[7] Aaij R. et al. (LHCb Collaboration), arXiv:1202.4717 (2012).

Final measurement of $B^0_s$ mixing phase in the full CDF Run II data set

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Final measurement of $B^0_s$ mixing phase in the full CDF Run II data set