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| Super B Factories |
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Super B Factories
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Although experiments at B factories have shown no significant derivations from the Cabibbo-Kobayashi-Maskawa theory (CKM theory), they still leave enough room for new physics. With the Belle detector, about 1 ab-1 of statistics will have been collected up to its scheduled shutdown in summer 2009, with BaBar about half of this was achieved. To make a material step forward here and put the standard model to the test, the B factories' luminosity and with it the measurements' precision has to be increased drastically („Super B Factories“). For this cause two proposals have been put forward so far: An upgrade („SuperKEKB“) has been suggested for the KEKB machine, which is in full operating mode and holds the world record in luminosity, to lead in its final stage to an instantaneous luminosity which is 50 to 80 times higher (L = 0,8·1036 cm-2s-1). A further proposal for a new super B factory in Europe has been devised as well. [As a side-note, it should be said that the SLAC-B-physics program has been cancelled, but there is a proposition for a new machine called SuperB, for which a luminosity of 1·1036; cm-2s-1 is targeted. There exists, however, no site for this detector yet.]
While the SuperKEKB project is highly ambitious and some more difficulties will have to be overcome until design luminosity is reached, the vast experience and impressive track record of the machine physicists at KEK can be trusted and it is to be expected that the machine will, as scheduled, reach its high-luminosity phase in the year 2013. With an integrated luminosity of more than 50 ab-1, which can be reached after some years, completely new regions of precision tests will be approached. The intensive search for New Physics will coincide with data taking at the LHC and complement it. Measurements at SuperKEKB will concentrate on the resonances Υ(4S) and Υ(5S) which, among others, produce the quantum-mechanically entagled neutral B meson pairs with d quarks (Υ(4S)) and s quarks (Υ(5S)) and play an important role in the examination of CP violation.
Physics potential of Super B Factories
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Fig. 1: Unitarity triangle, composed from the ratios of the elements of the CKM matrix shown. |
With integrated luminosities of more than 50 ab-1, experimental uncertainties will go down by almost an order of magnitude compared to recently published data. In this way, measurents of a completely new kind and especially sensitive to New Physics are possible. Only some examples from the numbers of possible reactions sensitive to new physics at a super B factory will be mentioned. Experimental sensitivities of a super B factory can roughly be divided into two categories: Precision measurements of the CKM mixing matrix (measurements of the unity triangle, see fig. 1) and the search for New Physics.
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Fig. 2: Present status (August 2008) of the determination of the CKM mixing angle (see text) |
The CKM matrix transforms the matter eigenstates of the down quarks (d, s, b) into the flavor eigenstates (d’, s’, b’), which transform into the up quarks (u, c, t) under weak interaction (and vice versa). The matrix elements of the CKM matrix are the (complex) couplings of down quarks to up quarks. From the unitarity of the CKM matrix the triangle in the complex plane shown in fig. 2 can be derived. In the case of CP violation this triangle has a non-vanishing area and consequently finite angles (φ₁=β, φ₂=α, φ₃=γ). The present status in determinign the elements of the unitarity triangle is shown in fig. 2. The plot shows 95% CL contours of the measurements and the apex of the unitarity triangle under the assumption that all measuremens are described by the standard model. The measurements generally are - as mentioned above - in concordance with the standard model, but there still are some deviations („tensions“ in the B-physics community jargon), such as the branching ratio B→τν or the derivation of sin(2φ₁) from tree and loop ("penguin") graphs.
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Fig. 3: Comparison of precision measurements at a super B factory. Left: standard model, right: central values of the observables as measured presently. |
A comparison of th expected precision of a super B factory (>50 ab-1) unter the two assumptions that the Kobayashi-Maskawa theory (standard model) is correct or that the central values for the observables keep their current values is shown in fig. 3. In the second case the tensions mentioned above are real and point to physics beyomd the standard model. The regions shown are 95% confidence intervals, which can be joined on the left-hand sinde (standard model) in a common apex of the unitarity triangle, on the right-hand side ("New Physics"), on the contrary, show several such areas with high significance which substantiate an extension of the standard model.
A comparison to fig. 2 very clearly shows the potential of a super B factory. One can immediately recognize that the sensitivity for New Physics (right-hand side) beyond the standard model is extremely high.
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Fig. 5: Simulation of a lepton-number violating decay of a τ lepton, τ→μγ with a branching ratio of 10⁻⁸ at a super B factory (>50 ab⁻ⁱ). The invariant mass distribution shown of a muon with a photon. |
Measuremts of observables that are predicted as vanishingly small in the standard model ("nil tests") should be mentioned as a further example. Among those are lepton-number violating τ decays (for example τ→µγ) and CP violation in the mixing of neutral charm mesons, which cannot occurr in the standard model. The sensitivity of experiments for lepton-number violationg decays (Lepton-Flavor Violation, LFV, forbidden in the standard model) at B factories which has steadily increased over the years is shown in fig. 5. The limits of about 10-7 presently reached are close to predicitions of different expansions of the standard model. With a super B factory, the τ→µγ decay can be observed down to a braching ratio lower than 10-8, as shown in the plot. The significance for such a decay is higher than 5σ.
Super B Factories and LHCb
A super B factory is going to be taking data contemporarily to the LHC, especially the LHCb experiment. A comparison of physics potentials of both experiments shows them to be mainly complementary. LHCb's forte is the higher statistics for selected exclusive decay channels of B mesons, especially Bs, with charged particles. With the high boost of B mesons, additionally CP-violating oscillations of the Bs system can be monitored, which could be difficult for super B factories. Furthermore, there are some rare decays accessible by LHCb, such as Bs→µµ, which are only measurable at a super B factory for high statistics (> 80 ab-1) due to the branching ratios expected.
Still, the super B factories has evident advantages for all decays of B mesons into final states with neutral particles, such as B→Kνν or B→ ρ+ρ- or solely leptonic final states like B→τν.
An important characteristic of B factories is that due to the production mechanism of B meson pairs at the threshold there are no further particles produced. Thus, the kinematics allow for a complete reconstruction of B mesons with several neutral particles. In this way it becomes possible to measure inclusive braching ratios and the corresponding CP-violating final states containing several neutral particles as well. Such measurements are impossible with LHCb. Inclusive obervables (e.g. b → sγ) have the additional advantage of often being theoretically much better understood than the corresponding exclusive channels and consequently observations are not limited by theoretical uncertainties in comparisons with the standard model.
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