The SuperB update

Nuclear Physics B (Proc. Suppl.) 209 (2010) 64–69 www.elsevier.com/locate/npbps

The SuperB update Marcello A. Giorgi a , on behalf of the SuperB collaboration a

INFN Sezione di Pisa and Universit`a di Pisa, Dipartimento di Fisica “Enrico Fermi” Largo B. Pontecorvo, 3 Pisa, Italy The update of the SuperB project is presented in the contest of the Capri round table. It includes a discussion of the specifications characteristic of SuperB as luminosity in excess of 1036 cm−2 s−1 to integrate more than 75ab−1 in 5 years and a beam polarization higher than 80 % . The advantage of running at open Charm threshold is also elucidated in view of looking for CP violation in Charm sector.

1. Introduction The search for new physics through the use of very high luminosity machines, leading to high sensitivities for rare processes is complementary with the choice of pursuing new physics by opening new energy thresholds, as done at LHC. Understanding the new physics (NP) flavor structure during LHC operations by means of a Super Flavor Factory is described in various papers (see for example SuperB CDR [1] and the SuperB physics document [2]). Since then and namely in 2010 during the preparation of the SuperB TDR, intermediate reports, so called white papers, have been prepared with the updating on physics [3], accelerator [4] and detector [5]. In what follows the sensitivities achievable at SuperB for some selected channels will be shown considering samples consisting of integrated luminosities L ≥ 75ab−1, corresponding to 5 years run of an e+ e− asymmetric machine running with a peak luminosity L = 1036 cm−2 s−1 . The purpose of this choice is to elucidate the special capability of SuperB due to the beam polarization and the option to run at charm threshold, a more detailed description of the general physics program is in [3], part of this program as for b physics is also possible in hadron experiments as in LHCb and, perhaps with a lower design luminosity at SuperKEKB. SuperB is somehow the real specie evolution of the two asymmetric B Factories, PEP-II [6] and KEKB [7], and their companion detectors, BABAR [8] and Belle [9], 0920-5632/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2010.12.011

that having started their operations in 1999 have produced a wealth of flavour physics results, subjecting the quark and lepton sectors of the Standard Model (SM) to a series of stringent tests, all of which have been passed. The asymmetric BFactories PEP-II and KEKB e+ e− operating at the center of mass energy corresponding to the mass of the Υ (4S), started their successful era with the discovery by BABAR and BELLE collaboration of the indirect (2001) and the direct (2004) CP violation [10–13] in the b sector. After almost a decade of measurements at B Factories thanks to the precise measurements of the sides, of sin2β and of the angles α and γ, the Unitarity Triangle is now beginning to be constrained. B Factories have in fact proved to be a unique tool for studying symmetry violations in heavy quarks and leptons. The continuing success of the two machines and experiments have demonstrated that the B sector results are fully compatible with the CKM paradigma [14,15]. Some physics examples are shown here to highlight the specific aspects of SuperB , we first remind the machine characteristics whose parameters are presented in table 1. 2. Polarization The option of having in SuperB one beam polarized at the level of 80% opens a window on a new physics chapter, it makes easier the search for Lepton Flavor Violation (LFV), the search for CP violation in τ sector, the investigation of τ

65

M.A. Giorgi / Nuclear Physics B (Proc. Suppl.) 209 (2010) 64–69

Table 1 Machine parameters for SuperB Parameter Energy (HER/LER) Circumference Luminosity Beam Current (HER/LER) Nbunches εy (HER/LER) εx (withIBS) (HER/LER) βy (HER/LER) βx (HER/LER) Crossing angle χ One beam Polarization RF power (AC line) hor. tune shift (HER/LER) ver. tune shift (HER/LER)

units GeV m 1036 cm− 2s−1 A pm nm μm cm mrad % MW % %

magnetic structure in addition to a new way of measuring the Standard Model (SM) parameters. 2.1. Tau lepton Physics As extensively discussed in the Proceedings of the VI SuperB Workshop of Valencia [2] and in the SuperB White Paper [3], the τ physics will assume great importance to probe new physics beyond SM. LFV in τ decay is one of the most promising benchmarks for discovery of New Physics, at the moment limits between few 10−8 and few 10−7 to branching ratios of the radiative channel τ → μ γ and τ → 3 leptons have been established by BABAR [22,23] and BELLE [24,25]. The radiative channels at the moment present sources of irreducible background coming from τ → μ ν ν γ radiative decays. BABAR has measured the following upper limits at 90 %CL: with 515 fb−1 UL (τ → μ γ) = 4.4 10−8 with 468 fb−1 UL(τ → lll) = 1.8-3.3 10−8 and BELLE: with 535 fb−1 UL(τ → μ γ) = 4.5 10−8 with 782 fb−1 UL( τ → lll) = 1.5-2.7 10−8 . A high statistics as with 75 ab−1 can allow a better tagging of τ in the final state and strongly reduce or eliminate the first of the two background sources mentioned above. Since the analysis optimization depends on the size of the analyzed sample and on the amount of expected backgrounds, one must re-optimize the B-factory

Baseline 6.7/4.18 1258 1.0 1.89/2.41 978 5/5.8 2/2.41 252/206 2.6/3.2 60 85 17.1 0.0021/0.0033 0.0978/0.0901

Low Emittance 6.7/4.18 1258 1.0 1.46/1.80 978 2.5/2.9 1/1.21 179/146 2.6/3.2 60 85 12.7 0.0016/0.0025 0.0901/0.0901

Tau-Charm 2.58/1.61 1258 0.1 1.36/1.70 1956 13/15.75 5.2/6.3 658/536 6.76/8.32 60 1.68 0.0052/0.0060 0.0914/0.0915

analyses for the SuperB luminosity, especially for the low background searches. In the following, we extrapolate from the most recent results from BABAR by re-optimizing the analysis for √ τ → . A conservative 1/ L scaling is generally expected for τ → μγ, polarization in SuperB will help in reducing the ”irreducible” background mainly due to the allowed decay τ → μννγ. The experimental reach is expressed in terms of the expected 90 % CL upper limit (UL) in absence of signal events, as well as in terms of branching fraction at the evidence level of 3σ in the presence of projected backgrounds; a minimum of 5 expected signal events is required to affirm evidence for a signal. By re-optimizing the BABAR analysis for 75 ab−1 and assuming the same detector performance as in BABAR the expected 90 % confidence ULs are in the range 2.3 × 10−10 to 8.2 × 10−10 , depending on the channel. The 3σ evidence of a branching fraction are between 1.2 × 10−9 and 4.0 × 10−9 . Based on the refined analysis the extrapolation from BABAR to SuperB is shown in Fig. 1. In addition SuperB includes in the baseline design an 80% longitudinally polarized electron beam and spin rotators to facilitate the production of polarized τ pairs. As shown in Fig. 2 the polarization will give another tool to reduce by means of angular analysis the background contribution of τ → μ ν ν γ, improving the (signal)/(background) ratio by almost a factor of 2. If

66

M.A. Giorgi / Nuclear Physics B (Proc. Suppl.) 209 (2010) 64–69

Figure 1. The limits for various LFV channels versus the integrated luminosity are not strongly affected  by systematic errors, they scale better than (L)

LFV is found in decays as τ → μ γ or τ → μ μ μ, the polarization will be the tool for determining the helicity structure of the violating coupling. This polarization as discussed since a long time is the key to the search for a τ EDM, or for CP violation in τ decay and for the measurement of τ magnetic form factors or tau g-2. The use of polarized beams, as in the baseline design of SuperB , would help reducing backgrounds to τ → μγ decay, which is expected to be the most sensible to new physics, in fact polarized beams would allow to reduce backgrounds coming from e+ e− → μμγ processes. In fact, when considering single hadron decay in one of the two τ ’s, the signal and backgrounds could be clearly separated exploiting the correlation between the angle formed by the tracks and their respective τ parent. The correlation can be clearly seen in Fig. 2. The sensitivities achieved after few years of data taking with SuperB would be as high as 2 × 10−9 for τ → μγ and 2×10−10 for τ → μμμ. Due to the lack of polarization option in SuperKEKB the angular distribution of muons coming from τ → μγ can not be used to reject backgrounds leading to sensitivities worse by a factor of 2.5. The other hint for New Physics come from g-2

Figure 2. The helicity angle is defined as the angle between the flight direction of the track in the τ rest frame and the τ direction in the lab frame. The correlation between signal and tag helicity angles is shown for ρ-tagged events both in the case of backgrounds (on top) and signal (bottom figure). An angular analysis could improve the precision of a factor ∼ 4.

measurement : at present muon g-2 is measured to be Δaμ = aSM − aexp = (3 ± 1) × 10−9 and μ μ any effect on τ ’s would at least scale with the ratio between the tau and muon mass, making the effect within reach of future flavor factories SuperB and SuperKEKB. The two machine have different design, only SuperB will have a high polarized beam (≥ 80%) and the capability of running at charm threshold. The polarization and an integrated luminosity ≥ 75ab−1 will allow to investigate the magnetic structure of τ ,

M.A. Giorgi / Nuclear Physics B (Proc. Suppl.) 209 (2010) 64–69

combining the measurements of total cross section angular distribution and Forward-Backward asymmetry with a sensitivity up to 0.6 × 10−6 , equivalent to the sensitivity for muons in g-2 experiments. 2.2. Measurements of Standard Model Parameters Longitudinal e− beam polarization could be used also to make precise test in the SM electroweak sector. SLD and LEP experiments measured both sin2 θW and the neutral current coupling Z − b¯b, by looking at the lepton production asymmetries. SuperB will be able to record ∼ 30 × 106 hadronic Z decays with polarized electrons, which should be compared to the 0.5 × 106 recorded by SLC, with the large improvement in statistics, considering the polarization to be under control at the order of the 0.5%, the error on sin2 θW will be reduced of almost a factor 2, and exploring energy regions far below the Z pole, as shown in Fig. 3.

Figure 3. Results from previous precision measurement on sin2 θW as a function of the transfered momentum, the shaded area represent the SM prediction, SuperB investigated region is pointed by the arrow.

Along with sin2 θW SuperB would have an un-

67

precedented reach for the study of Z-b¯b coupling, with a reduction of the error on gVb of a factor 8. The unprecedented sensitivities reachable by SuperB would be such that deviation observed by LEP and SLD from SM may be confirmed (hence implying NP) or rejected (proving the robustness of the SM). 3. Charm physics Major improvements are foreseen in the charm sector as well. The recent observation of large D0 D 0 mixing raises the exciting possibility of finding CP violation in charm decay, which would almost certainly indicate physics beyond the Standard Model. A large set of measurements made mainly in the experiments BABAR [17] and BELLE [18] at B-Factories but also CDF [19] and data at ψ(3770) show clear evidence of Charm mixing, still with present statistics no evidence is shown of CP violation as can be clearly seen from the HFAG [20] average. HFAG fits to the measured oservables for x, y, | pq | , Arg qp , δKπ , δKππ0 , RD using data from D0 → K + l ν, D0 → h+ h− , D0 → K + π − , D0 → K + π − π 0 , D0 → KS0 π + π − , D0 → K + π − π + π − and branching fractions measured at ψ(3770) [21]. The fits exclude the nomixing point (x=y=0) at 6.7 σ and don’t show any evidence for CP violation. One-dimensional likelihood functions for parameters are obtained by keeping, for any value of the parameter, all other parameters free to choose the preferred value. The resulting central values at 68.3 % C.L. and the intervals 95 % C.L., are: +0.27 x = 0.97−0.29 (0.39-1.48 at 95 % CL) +0.18 y = 0.78−0.19 (0.41-1.13 at 95 % CL) From mixing parameters x and y can be deduced that: being y positive, the lifetime of the CP even state is shorter, as in the kaon sector, but since also x appears to be positive, the CP even state should be heavier, unlike in the kaon sector. No evidence of CP violation in D0 − D 0 mixing. In SuperB with 75 ab−1 the expected overall uncertainties on the mixing parameters ( combining statistic, systematic and model uncertainties) are 4.9 10−4 for x and 3.5 10−4 for y and on the

68

M.A. Giorgi / Nuclear Physics B (Proc. Suppl.) 209 (2010) 64–69

CP parameters 1.9 10−4 for  and 1.9 degrees for Arg pq as shown in Fig. 4.

a factor 3) suffers from lower luminosities. Figure 5 shows the situation with SuperB with the statistics accumulated by running at Υ (4S) for five years in three cases: with only SuperB running at Υ (4S), combined with the expected Chinese τ -charm factory, and SuperB run at charm threshold for less than one year. 4. Conclusions

Figure 4. Two dimensional contours Charm mixing and CP violation as a function of pq a strong phase δ that doesn’t change sign, at present (top) and after 5 years of datataking from one of the Super Flavor Factories under design (bottom).

The design of SuperB is strengthened by the results of the test of LNF for DAΦN E upgrade. It contains the two options: the polarization for τ physics, and run as asymmetric factory at charm threshold (4.0 GeV). The latter option with a possible upgrade of vertex detector would allow the study of CP violation with time dependent analysis analogous to the BABAR and Belle extraction of sin2β. The SuperB machine is expected to produce physics before the mid of next decade, integrated luminosity is expected to reach 75 ab−1 in five years of running. The activity on the detector is going on and various options are considered for the optimal apparatus. It will be a derived from the concepts of BABAR and BELLE apparata that have demonstrated a high level of robustness in running at the asymmetric Bfactories. SuperB will take profit from the experience of the BABAR experimenters and many components of BABAR detector will be reused in SuperB . The SuperB project has received many encouraging signals coming from the Italian Government. Still it is not funded, the project and the machine design are now in the TDR phase and the document will be delivered in 2011, its validity has received so far important recognition. The management structure of SuperB is in place and the program for recruiting the personnel for the machine and detector construction has started. REFERENCES

The run at threshold offers lower background and access to the measurement of both direct and indirect CPV, it comes at the expense of statistics, and although having larger cross section (by

1. M. Bona et al., arXiv:0709.0451 [hep-ex]. 2. Proceeding of SuperB Workshop VI: arXiv:0810.1312v1 [hep.ex] 3. B.O’Leary et al. arXiv:1008.1541v1

M.A. Giorgi / Nuclear Physics B (Proc. Suppl.) 209 (2010) 64–69

69

Figure 5. Discovery potential of SuperB in charm sector, where the expected (x,y) from SuperB data collected at Υ (4S) (a), SuperB data with BES III expected data (b), and SuperB with data both collected at Υ (4S) and 500 fb−1 collected at charm threshold.

4. M.E.Biagini et al. arXiv:1009.6178v1 5. E.Grauges et al. arXiv:1007.4241v1 6. “PEP-II: An Asymmetric B Factory. Conceptual Design Report. June 1993,” SLAC-R418. 7. “KEKB B-Factory Design Report”, KEK Report 95-7, 1995. 8. B. Aubert et al. [BABAR Collaboration], Nucl. Instrum. Meth. A 479, 1 (2002) 9. Nucl. Instrum. Meth. A 479, 117 (2002). 10. B. Aubert et al. [BABAR Collaboration], Phys. Rev. Lett. 87, 091801 (2001) 11. K. Abe et al. [Belle Collaboration], Phys. Rev. Lett. 87, 091802 (2001) 12. B. Aubert et al. [BaBar Collaboration], Phys. Rev. Lett. 93, 131801 (2004) 13. K. Abe et al. [Belle Collaboration], Phys. Rev. Lett. 93, 021601 (2004) 14. N. Cabibbo, Phys. ReV. Lett. 10, 531 (1963) 15. M. Kobayashi and T. Maskawa, Prog. Theor. Phys. 49, 652 (1973) 16. B. C. Allanach et al., arXiv:hep-ph/0202233. 17. B. Aubert et al. [BaBar Collaboration] Phys. Rev. Lett. 98, 211802 (2007). B. Aubert et al. [BaBar Collaboration],arXiv:0712.2249 [hepex]- submitted to Phys. Rev. D. 18. M.Staric et al. [Belle Collaboraton] Phys. Rev. Lett. 98, 211803 (2007). 19. T.Aaltonenet al. [CDF Collaboraton] Phys.

Rev. Lett. 100, 121802 (2008). 20. A.J.Schwartz,Proceedings of 5th Flavor Physics and CP Violation Conference (FPCP2007), Bled, Slovenia,12-16 May 2007, pp 024 arXiv:0708.4225 [hep-ex]. A.J.Schwartz,Proccedings of BES - BELLECLEO BABAR Workshop on Charm Physics, IHEP, Beijing, 26-27 November 2007. arXiv:0803.0082 [hep-ex]. 21. HFAG Heavy Flavor Averaging Group (charm subgroup) http://www.slac.stanford.edu/xorg/hfag/charm/. 22. B. Aubert et al. [BaBar Collaboration] Phys. Rev. Lett. 95, 041802 (2005) 23. B. Aubert et al. [BaBar Collaboration] Phys. Rev. Lett. 98, 251803 (2007) 24. K. Hayasaka et al. [Belle Collaboraton] arXiv:0705.0650v1[hep-ex] 25. Y.Miyazaki et al. [Belle Collaboraton] Phys. Lett. 660, 154 (2008)