The BaBar and Belle experiments, running mostly at the centerof-mass energy of about 10.58 GeV at the PEP-II e+e−

collider at SLAC and at the KEKB e+e− collider at KEK, have recorded integrated luminosities exceeding 0.5 ab−1 and 1.0 ab−1, respectively. The results of the two experiments have confirmed the success of the CKM mechanism for flavor mixing and CP violation and are in agreement with the Standard Model predictions. The SuperB experiment, a new generation flavor factory with very high luminosity (L = 1036 cm−2 s−1) will be able to record 75 ab−1 of data in five years of nominal data taking, which is about two orders of magnitude larger than the BaBar and Belle data samples. This new experiment has been proposed for searching for new physics effects in the flavor sector, mainly through precision measurements and searches for forbidden processes in B, D and τ decays. We report on recent results from the BaBar and Belle Collaborations, in particular on the measurement of the CKM angle γ and we present the physics reach of the SuperB experiment

Neri, N.

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DOI 10.1393/ncc/i2011-10741-1Colloquia: IFAE 2010IL NUOVO CIMENTO Vol. 33 C, N. 6 Novembre-Dicembre 2010Recent results from the B factories and perspectives on SuperBN. Neri on behalf of the BaBar and SuperB CollaborationsUniversita` di Pisa and INFN - Largo B. Pontecorvo 3, 56127 Pisa, Italy(ricevuto l’ 8 Ottobre 2010; pubblicato online il 18 Gennaio 2011)Summary. — The BaBar and Belle experiments, running mostly at the center-of-mass energy of about 10.58 GeV at the PEP-II e+e− collider at SLAC and atthe KEKB e+e− collider at KEK, have recorded integrated luminosities exceeding0.5 ab−1 and 1.0 ab−1, respectively. The results of the two experiments have con-firmed the success of the CKM mechanism for flavor mixing and CP violation andare in agreement with the Standard Model predictions. The SuperB experiment, anew generation flavor factory with very high luminosity (L = 1036 cm−2 s−1) willbe able to record 75 ab−1 of data in five years of nominal data taking, which isabout two orders of magnitude larger than the BaBar and Belle data samples. Thisnew experiment has been proposed for searching for new physics effects in the flavorsector, mainly through precision measurements and searches for forbidden processesin B, D and τ decays. We report on recent results from the BaBar and Belle Col-laborations, in particular on the measurement of the CKM angle γ and we presentthe physics reach of the SuperB experiment.PACS 12.15.Hh – Determination of Cabibbo-Kobayashi & Maskawa (CKM) matrixelements.PACS 13.25.Hw – Decays of bottom mesons.PACS 13.35.Dx – Decays of taus.1. – IntroductionThe BaBar and Belle experiments and the B factories (PEP-II and KEKB) at SLACand KEK were designed and built with the primary goal of performing the first precisiontests of the Cabibbo-Kobayashi-Maskawa (CKM) mechanism for flavor mixing and CPviolation with 3 generations of quarks.From the unitarity of the CKM quark mixing matrix we obtain the relation VudV ∗ub +VcdV∗cb + VtdV∗tb = 0, which is usually represented as a triangle in the complex plane(the “Unitarity Triangle”). BaBar and Belle have made fundamental contributions tothe measurements of the angles and sides of the Unitarity Triangle. The CP asymmetrymeasurements from BaBar and Belle can be directly related to the angles of the UnitarityTriangle (α, β, γ) with little theoretical uncertainty. The constraint on the angle β, fromthe amplitude of the time-dependent CP asymmetry of B0 → J/ΨK0S and other b → cc¯sc© Societa` Italiana di Fisica 6364 N. NERI on behalf of the BABAR and SUPERB COLLABORATIONSρ-1 -0.5 0 0.5 1η-1-0.500.51γβα)γ+βsin(2smΔdmΔ dmΔKεcbVubVηγγααdmΔKεKεsmΔ & dmΔSLubVν τubVβsin 2(excl. at CL > 0.95) < 0βsol. w/ cos 2excluded at CL > 0.95αβγρ−1.0 −0.5 0.0 0.5 1.0 1.5 2.0η−1.5−1.0−0.50.00.51.01.5excluded area has CL > 0.95Beauty 09CKMf i t t e rFig. 1. – Constraints in the (ρ¯, η¯)-plane from a global CKM fit from UTFit [1] (left plot) andCKMFitter [2] (right plot) Collaborations.decays, is the strongest, with a one standard deviation uncertainty of less than one degree.The most difficult angle to measure is γ and significant progress has been made in thepast years.The current experimental constraints on the Wolfenstein parameters ρ¯ and η¯, whichgive the coordinates of the apex of the Unitarity Triangle, are shown in fig. 1. The CPviolation parameters of the CKM matrix are overconstrained and the CKM theory hasbeen experimentally confirmed. Kobayashi and Maskawa were awarded of the 2008 NobelPrize in physics.In this paper I will focus on recent results from the B factories, such as the measure-ment of the CKM angle γ in B± → D(∗)K(∗)± decays. I will also discuss the perspectiveson SuperB, a next generation asymmetric e+e− flavor factory with very high peak lumi-nosity (L = 1036 cm−2 s−1) proposed to be built near Rome, whose main purpose is tosearch for evidence of physics beyond the SM and investigate its nature.2. – Recent results from B factories2.1. Measurement of the CKM angle γ in B± → D(∗)K(∗)± decays. – The mea-surement of the angle γ = arg(−(VudV ∗ub)/(VcdV ∗cb)) is an example of a measurementof unexpected success at the B factories. The methods [3-5] that currently give thestrongest constraints on γ use decays of the type B± → D(∗)K(∗)±(1), where the neutralD meson decays to a final state that is accessible to both D0 and D¯0. The final state canbe reached through two different quark-level processes, b → cu¯s and b → uc¯s (respec-tively B− → D(∗)0K(∗)− and B− → D¯(∗)0K(∗)−). The sensitivity to γ comes from theinterference between b → uc¯s and the b → cu¯s amplitudes which introduces a relativephase γ in the decay amplitude. These are both tree level b decays, so the interpretationof the measurements in terms of γ is theoretically extremely clean.(1) In what follows, the symbol D refers to either D0 or D¯0.RECENT RESULTS FROM THE B FACTORIES AND PERSPECTIVES ON SUPERB 65The most precise measurement of the angle γ comes from the analysis of B± →D(∗)K(∗)± decays where the D decays to three-body final states such as K0Sπ+π− orK0SK+K−. The main advantage of the method [5] is that it involves the entire resonantstructure of the three-body decay of the D meson, with interference between doubly-Cabibbo-suppressed, Cabibbo-allowed and CP -eigenstate amplitudes, all providing thesensitivity to γ. The price to pay is that it requires a detailed study of the resonancesand their interference through a Dalitz plot analysis technique, where a phenomenologicalparameterization of the D decay amplitude is assumed.Using a data sample of 435 fb−1 BaBar measures γ = (68 ± 14 ± 4 ± 3)◦ [6] ina combined fit of B± → DK±, B± → D∗K± and B± → DK∗± decays where Ddecays to K0Sπ+π− or K0SK+K−. Belle using a data sample of 605 fb−1 measures γ =(78.4+10.4−11.6 ± 3.6 ± 8.9)◦ [7] in a combined fit of B± → DK± and B± → D∗K± decayswith D → K0Sπ+π−. The first error is statistical, the second systematic and the third isdue to the D decay amplitude model.Both the BaBar and Belle measurements are statistics limited and would benefitfrom a larger data sample. At SuperB, with 75 ab−1, it is expected that Dalitz modelindependent approaches [5, 8] will be viable, rendering any controversy over the trueDalitz model systematic error on γ irrelevant and turning γ into a precision measurementwith an estimated sensitivity of about one degree.3. – The SuperB experimentSuperB is a next generation high-luminosity e+e− collider that will accumulate adata sample of 75 ab−1 with five years of nominal data taking. This experiment couldstart running as early as 2015, by which time the LHC will have accumulated a significantsample of data, and would be reporting the results of searches for or direct measurementsof New Physics (NP). Those results are limited in that they measure only flavor diagonalprocesses. In order to fully understand the nature of NP, one also has to measure theoff-diagonal terms, in analogy to the CKM mixing matrix. In the scenario that LHCdoes not find NP particles, SuperB has the possibility to explore NP scale beyond thereach of the LHC (up to 10TeV or more depending on NP models) looking for indirectsignals.In this sense the SuperB physics reach is complementary to the LHC one. In particularthere is also a large complementarity with the flavor physics potential of the LHCbexperiment. For example, rare decay modes with one or more neutrinos in the finalstate such as B+ → l+ν and B+ → K(∗)+νν¯, inclusive analyses of processes such asb → sγ and b → sl+l−, measurements of the CKM matrix elements |Vub| and |Vcb| andsearches for lepton flavor violation such as τ− → μ−γ are unique to SuperB, wherethe environment of the e+e− collider is clean and relatively simple compared to theevents at the hadronic machine [9]. In addition, the SuperB machine design includes thepolarization of the electron beam (85% polarization) for the production of polarized τpairs, allowing a better separation between signal and background events for τ decays,thus improving the experimental sensitivity in lepton flavor violating (LFV) processes.In table I are reported the expected experimental sensitivities (90% upper limits) for τLFV processes at SuperB with 75 ab−1 of data along with the present upper limits.Charm physics also plays an important role in the SuperB physics program. Con-straints on flavor-changing neutral currents from NP in the up-quark sector are muchweaker than in the down-quark sector. Thus, high-sensitivity studies of rare charm de-cays offer the possibility of isolating NP effects in D0 − D¯0 mixing, in CP violation and66 N. NERI on behalf of the BABAR and SUPERB COLLABORATIONSTable I. – Expected 90% CL upper limits on representative LFV τ lepton decays with 75 ab−1and current upper limits.Process Sensitivity at SuperB Current limitB(τ → μγ) 2× 10−9 4.5× 10−8B(τ → eγ) 2× 10−9 1.1× 10−7B(τ → μμμ) 2× 10−10 3.2× 10−8B(τ → eee) 2× 10−10 3.6× 10−8B(τ → μη) 4× 10−10 6.5× 10−8B(τ → eη) 6× 10−10 9.2× 10−8B(τ → eK0S) 2× 10−10 3.3× 10−8B(τ → μK0S) 2× 10−10 4.0× 10−8in rare decay branching fractions. The sensitivity on CP violation asymmetries in decaymodes such as D0 → K+K−, π+π− and D0 → K0Sπ+π− is expected to reach the levelof O(10−4) with 75 ab−1, which will probe the SM predictions. The machine design in-cludes the possibility to run at different center-of-mass energies, e.g. at the Ψ(3770) forexclusive DD¯ production and at the Υ(5S) for BsB¯s production, thus further enrichingthe physics program.The SuperB physics case is extensively discussed in refs. [10, 11]. In table II arereported, as an example, the golden modes for different NP scenarios. These decays arevery difficult or impossible to reconstruct at the LHC. Even at the clean environment ofSuperB the selection is experimentally challenging and to suppress backgrounds to anacceptable level the recoil technique is often necessary, in which the other B in the BB¯event is reconstructed in either a semileptonic or hadronic decay. Table III reports thecomparison of the experimental sensitivity today and with 75 ab−1, showing that in mostcases SuperB is able to measure the observables with a few percent accuracy.Table II. – Golden modes in different New Physics scenarios. An “X” indicates the goldenchannel of a given scenario. An “O” marks modes which are not the “golden” one of a givenscenario but can still display a measurable deviation from the Standard Model. The label CKMdenotes golden modes which require the high-precision determination of the CKM parametersachievable at SuperB.H+ Minimal Non-minimal NP Right-handedhigh tanβ FV FV Z-penguins currentsB(B → Xsγ) X O OACP (B → Xsγ) X OB(B → τν) X-CKMB(B → Xsl+l−) O O OB(B → Kνν) O XS(KSπ0γ) Xβ X-CKM ORECENT RESULTS FROM THE B FACTORIES AND PERSPECTIVES ON SUPERB 67Table III. – Comparison of current experimental sensitivities with those expected at SuperB(75 ab−1). Only a small selection of observables is shown. Quoted sensitivities are relativeuncertainties if given as a percentage, and absolute uncertainties otherwise. For more details,see refs. [9-11].Mode SensitivityCurrent Expected (75 ab−1)B(B → Xsγ) 7% 3%ACP (B → Xsγ) 0.037 0.004–0.005B(B+ → τ+ν) 30% 3–4%B(B+ → μ+ν) not measured 5–6%B(B → Xsl+l−) 23% 4–6%AFB(B → Xsl+l−)s0 not measured 4–6%B(B → Kνν) not measured 16–20%S(K0Sπ0γ) 0.24 0.02–0.034. – ConclusionsThe B factories have confirmed the success of the CKM mechanism for flavor mixingand CP violation with three generations of quark. Their results went well beyond theoriginal goal. As an example, the measurement of the CKM angle γ in B± → D(∗)K(∗)±decays has reached a precision which was not predicted at the beginning of the BaBar andBelle experiments: γ = (68±14±4±3)◦ (BaBar) and γ = (78.4+10.4−11.6±3.6±8.9)◦ (Belle).Flavor physics still offers a unique opportunity for searching for NP and understandingits origin. The most sensitive measurements to NP are reported in tables I and III andare currently statistics dominated. SuperB, a new generation super flavor factory, willaccumulate a data sample of 75 ab−1 with five years of nominal data taking. The physicsprogram is focused on the study of the flavor structure of NP by measuring the flavorcouplings and also by exploring the NP scale beyond the LHC reach (up to 10TeV ormore), by looking for indirect signals. The golden modes for different NP scenarios,reported in table II, are difficult or even impossible to reconstruct at LHC. In this sense,the SuperB experiment can be considered as an alternative path for discovering NP withrespect to the LHC.∗ ∗ ∗I would like to thank M. A. Giorgi, M. Rama and J. Walsh for useful discussions,the organizers for the invitation and also the INFN for its support.REFERENCES[1] Ciuchini M. et al. (UTFit Group), JHEP, 0107 (2001) 013.[2] Charles J. et al. (CKMfitter Group), Eur. Phys. J. C, 41 (2005) 1.[3] Gronau M. and London D., Phys. Lett. B, 253 (1991) 483.[4] Atwood D., Dunietz I. and Soni A., Phys. Rev. Lett., 78 (1997) 3257; Phys. Rev. D,63 (2001) 036005.68 N. NERI on behalf of the BABAR and SUPERB COLLABORATIONS[5] Giri A., Grossman Y., Soffer A. and Zupan J., Phys. Rev. D, 68 (2003) 054018.[6] del Amo Sanchez P. et al. (the BABAR Collaboration), Phys. Rev. Lett., 105(2010) 121801.[7] Poluektov A. et al. (the Belle Collaboration), Phys. Rev. D, 81 (2010) 112002.[8] Bondar A. and Poluektov A., Eur. Phys. J. C, 47 (2006) 347.[9] Browder T. E., Gershon T., Pirjol D., Soni A. and Zupan J., Rev. Mod. Phys., 81(2009) 1887.[10] Bona M. et al., arXiv:0709.0451 [hep-ex].[11] Hitlin D. G. et al., arXiv:0810.1312 [hep-ph].

Recent results from the B factories and perspectives on SuperB

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Recent results from the B factories and perspectives on SuperB

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