Measurements of Vub and B0-B0 mixing by Belle and Babar

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Nuclear Physics B (Proc. Suppl.) 115 (2003) 227-231

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Measurements of Vub and B°-B ° Mixing by Belle and BaBar L. E. Piilonen a* ~Department of Physics, Virginia Polytechnic Institute and State University, Blacksburg VA 24061-0435, USA We summarize recent results on measurements of the quark mixing matrix element V~,band the mixing between neutral Ba mesons by the Belle and BaBar experiments at the Japanese High Energy Accelerator Research Organization (KEK) and the Stanford Linear Accelerator Center (SLAC), respectively.

1. I n t r o d u c t i o n Within the Standard Model of strong and electroweak interactions, the Cabibbo-KobayashiMaskawa (CKM) matrix[l] encodes the unitary transformation between the weak interaction eigenstates and the full Hamiltonian eigenstates of the charge 1/3 quarks. The element Vtd determines the value of the mass difference m?Ttd between the mass eigenstates of the neutral Bd meson, a parameter that is of central concern to the measurements of time-dependent indirect CP nonconservation in the decays of neutral Bd mesons to CP eigenstates. The element V~b is the most poorly determined, leaving open the possibility that the three generation CKM matrix is non-unitary because it is incomplete. The Belle[2] and BaBar[3] experiments at the B factories[4,5] at the Japanese High Energy Accelerator Research Organization (KEK) and the Stanford Linear Accelerator Center (SLAC), respectively, t~ave reported new results on measurements of V~b and Amd by a variety of techniques, and these will be summarized in this paper. All results presented here are PRELIMINARY unless accompanied by a citation to a published result. 2. Vub M e a s u r e m e n t s

The most fruitful technique to date for measuring the magnitude of V~b has been to select exclusive semileptonic decays of the form B -+ Xugu, *representing the Belle Collaboration

where Xu is a pion, a rho meson, or an omega meson and e is a muon or an electron, and then determine the partial branching fraction for events where the charged lepton momentum is above the charm production threshold. This method, unfortunately, leads to a large theoretical uncertainty in the calculation of the hadronic form factor when converting the partial branching fraction to a value of IVubl. (See, for example, the paper by M. Luke elsewhere in these proceedings.)

]Vub[ f r o m B --+ ~ u In this measurement, events containing a single lepton and an oppositely charged pion are extracted from the hadronic data set. The lepton momentum is restricted to the range 1.2 G e V / c < p~ < 2.8GeV/c, and the sum of the magnitudes of the lepton and pion is constrained to lie above 3.1 GeV/c. Backgrounds from other X~gu decays, unrelated B B decays, and continuum (non-resonant e+e - ~ q~) production are suppressed by reconstructing the missing neutrino using the measured charged and neutral energy in the event. The neutrino's momentum must lie within the acceptance of the detector, and the magnitude of the cosine of the angle between the B meson and the pion-lepton combination must be less than one. The latter quantity sometimes lies outside this physical range for background events but only rarely for signal events. The remaining events are subjected to a twodimensional maximum likelihood fit in the plane of residual missing energy A E (after accounting for the visible pion and charged lepton energy 2.1. Belle:

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and the reconstructed neutrino energy) and the charged lepton momentum p~. This fit provides the most likely yields of signal and each background category enumerated above (see Fig. 1). The preliminary result for the branching fraction of B ° -~ 7r-g+u, using data corresponding to 29.4fb -1 of integrated luminosity, is B = (1.91 + 0.15 + 0.30 + 0.09) x 10 -4, where the uncertainties are statistical, systematic, and theoretical, respectively. The corresponding value of the associated CKM matrix element is IV~bl = (3.90 + 0.16 + 0.31 + 0.74) x 10 -3. The theoretical uncertainties are determined from an average of the predictions of the LCSR[6] and UKQCD[7] models. 2.2. Belle: ]V~b] f r o m B --+ pgt,

A similar study by Belle of the decay B + --+ p°g+u gives a branching ratio of B = (1.44 40.18) x 10 -4 and a corresponding value of the CKM matrix element IV~bl = (3.50±0.20-4-0.28) x 10 -3, based on 29.4fb -1 of data. The theoretical uncertainties are as yet undetermined, although they are likely to be comparable to those in B --+ 7rgu. [V=b[ f r o m B --+ peu BaBar's strategy is to extract the branching fractions for B --+ :ret,, B --+ pea, and B ~ wet, using data both above and below the charm production threshold. The pion decay branching fraction is extracted with a fit to the A E distribution; the other two are extracted using a 2D fit to A E vs. p~. The maximum sensitivity comes from the data above the charm production threshold for the decay B ° --+ p - e + u , for which the measured branching fraction is B = (3.26 + 0.65 + 0°:~43+ 0.44) x 10 -4 and the corresponding CKM matrix element is ]V~b[ = (3.57 + 0.36 :t:°:333+0.60) x 10 -3, from a 20.2 fb -1 data sample. 2.3. B a B a r :

3. Amd M e a s u r e m e n t s In B factories, Bd° and ~dd mesons are produced in a coherent P-wave state from T(4S) decay and each is able to oscillate into its antiparticle via Standard Model processes (typically involving intermediate t quarks). There-

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Figure 1. Belle: projections onto the axes of residual missing energy A E and charged lepton momentum Pe. The dots represent the data; the unshaded, hatched, shaded, and cross-hatched histograms represent the two-dimensional maximum likelihood fit contributions from signal B --+ ~gu decays, B -+ X~gu backgrounds, other B B decays, and continuum production, respectively.

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fore, the mass difference Amd between the neutral B d m a s s eigenstates can be measured by observing the flavour of one of these B mesons at time ttag through its decay products (an energetic lepton or a kaon, typically) or their topology, and then observing the flavour of the other B meson at time ttag + At. The time difference At is measured by the boosted distance Az along the beam axis between the two B decay vertices. The probability of observing an oppositeflavour meson pair (OF) or a same-flavour meson pair (SF) is proportional to 1 + cos(AmdAt) or 1 - cos(AmdAt), respectively--if the flavour of each meson can be determined correctly. Imperfect tagging, cascade charm decays of the tagging B meson, and finite vertexing resolution tend to combine, smear, and shift in time these ideal distributions so that the measured asymmetry A(At) = (NoF - NSF)/(NoF + NSF) is not as simple as cos(AmdAt). Nevertheless, the value of Amd can be extracted from the asymmetry measurements with proper knowledge of these distorting effects. 3.1. B a B a r : Amd f r o m D i l e p t o n E v e n t s Here, each B meson's flavour is tagged by the charge of a high-momentum lepton in dilepton events. (Leptons that are consistent with 7 conversion, two-photon or Bhabha events, or daughters of J/¢ or ~' mesons are discarded.) The value of At is obtained from the distance between the intersections of the two leptons with the beam interaction point profile. Using 20.7 fb -1 of data, BaBar obtains a value of Amd = (0.493 ± 0.012 ± 0.009) ps -1, where the first uncertainty is statistical and the second is systematic. [8] See Fig. 2. 3.2. Belle: A m d f r o m D i l e p t o n E v e n t s Belle has a comparable measurement using dilepton events: the result is A m d -= (0.463 + 0.008 4- 0.016) ps -1 from 5.9 f b - 1 of data.[9] 3.3. B a B a r : Amd f r o m B --+ H a d r o n s In this method, one B meson's fiavour is tagged using its daughters, as described earlier, and the other's by a full reconstruction to the final states D * - h +, D - h + or J/~b K *°, where h represents a pion, a p meson, or an al meson. These decays are very clean: the mode-dependent purity is be-

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tween 82% and 95%. The maximum likelihood fit to the opposite- and same-flavour events in 30.7 fb -1 of data gives the asymmetry plot shown in Fig. 3, corresponding to a mass difference of Amd = (0.516 ± 0.016 ± 0.010) ps -1.[10] 3.4. Belle: Amd f r o m B --+ H a d r o n s Belle does a similar analysis, although decays to final states containing the al meson are not included. The best fit to the opposite- and sameflavour distributions gives the asymmetry curve of Fig. 4 and a mass difference of A T n d -~- (0.521 ± 0.017 ± 0.011 o.o14) P s - l , using 29.1 f b - 1 of data.[11] 3.5. Belle: A m d f r o m B --+ D*Tr D e c a y s Belle also does a partial reconstruction of the decay chain ~ o -+ D * % r - and D *+ -+ D°Tr + by looking for the two oppositely charged pi-

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Figure 3. BaBar: A s y m m e t r y of opposite- and same-flavour events vs. decay time difference, where one B meson decays to a fully reconstructed hadronic final state. The curve arises from the best fit to the individual OF and SF distributions.

ons, the former being energetic and the latter quite soft. The D o meson is not explicitly reconstructed from its decay products. The pions emerge nearly back-to-back in the centre-of-mass frame. The a s y m m e t r y between the oppositeand same-flavour distributions is shown in Fig. 5 for 29.1 fb - I of data, and the extracted mass difference is A m d = (0.505 + 0.017 + 0.020) ps -1.

3.6. BaBar: A m d f r o m B ~ D*g~ D e c a y s Here, one B meson's flavour is tagged as described earlier and the other's is determined by the semileptonic decay chain ~0 ~ D,+t?v ' + D o --+ K-Tr + or K-~r+7c ° or D *+ -+ D o 7rsoft, K-Tr+7~+~r - or KsTr+~r - . Backgrounds are suppressed with cuts on the m o m e n t a of the D* and lepton in the centre-of-mass frame. The distribution of the mass difference between the D* and D o is used to determine the amount of signal and residual background in the final sample. Although the fitting procedure has been determined, the final result for A m d has not yet been unblinded. 3.7. Belle: A m d f r o m B --+ D*gv D e c a y s Belle uses substantially the same decay chains as BaBar in using the semileptonic decay of the

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IAtl (ps) Figure 4. Belle: A s y m m e t r y of opposite- and same-flavour events vs. decay time difference, where one B meson decays to a fully reconstructed hadronic final state. The curve represents the best fit to the individual OF and SF distributions.

second B meson to determine its flavour. The neutrino in Belle is reconstructed from the missing m o m e n t u m and energy and is required to lie within the detector acceptance; also, the cosine of the angle between the B meson and the D * lepton pair is required to lie within the physical region. The fit to the distributions of oppositeand same-flavour events vs. A t (see Fig. 6) gives a value of A m d = 0.489 + 0.012 ~4-o o.oll . o 1 4 P s - 1 using 29.1fb -1 of data.J12]

4. C o n c l u s i o n and A c k n o w l e d g e m e n t s Several recent measurements of the CKM matrix element Vub and of the mass difference Amd between the eigenstates of the neutral Bd m e s o n have been reported recently by the Belle and BaBar collaborations, and have been presented here. I am indebted to the following people for assistance in gathering this information: A. We-

L.E. Piilonen / Nuclear Physics B (Proc. Suppl.) 115 (2003) 227-231

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instein, B. Brau, C. H. Cheng, and T. Meyer of the BaBar Collaboration, and J. W. Nam, C. Schwanda, Y. Zheng, and T. Tomura of the Belle Collaboration. REFERENCES

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Figure 6. Belle: Opposite- and same-flavour asymmetry as a function of At for events where one B meson decays semileptonically. The curve shows the result of the fit to the individual SF and OF distributions.

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