Physics program at the DAΦNE collider upgraded in luminosity

Nuclear Physics B (Proc. Suppl.) 181–182 (2008) 390–395 www.elsevierphysics.com

Physics program at the DAΦNE collider upgraded in luminosity C. Bloisea a

Laboratori Nazionali di Frascati dell’INFN, P.O. Box 13, 00044 Frascati, Italy. We review the physics program at the Frascati e+ e− collider, whose upgrade in luminosity is currently underway, giving a summary of the proposals for experiments at the upgraded collider, and an outlook on the improvement of the measurements of interest for precision tests of the Standard Model, CP and CPT symmetries, and low-energy QCD interactions.

1. Proposal for experiments at DAFNE2 The Frascati φ-factory, DAΦNE is a high luminosity collider realized at the end of ninenties. Three detectors have been built and have taken data at DAΦNE in 2001-2007: a general purpose detector for particle physics, KLOE, a hypernuclear physics detector, FINUDA, and a detector for the measurement of the X-ray spectra from kaonic atoms, DEAR. At the end of 2007 one of the beam-crossing regions (IR1) has been modified changing the beam pipe and installing two sextupoles for obtaining a new beam-crossing scheme. With the new scheme [1], in which the machine is brought to operate at larger crossing angle and reduced beam size in the interaction region, an increase of the luminosity by a factor of 3-4 is expected, keeping constant the circulating currents. Peak luminosities of about 5×1032 cm−2 s−1 can be translated to 800 pb−1 of integrated luminosity per month, and even more when the injection line will be upgraded to operate the machine in a continuousinjection regime. SIDDHARTA[2], currently running at DAΦNE, is a new experiment for the measurement of the kaonic Hydrogen and Deuterium, based on large area silicon drift detectors (SDDs), for improving on the DEAR results [3], thanks to the much better signal-to-background ratio that can be obtained exploiting the time resolution of the new devices. A first experimental setup with a Nitrogen gas target and a reduced number of SDDs has been installed on the new interaction 0920-5632/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2008.09.071

region at DAΦNE for preparing the run with the final detector. The Nitrogen target allow measurements in a short time of the X-ray spectra of the kaonic atom and is used to determine the background conditions and to choose the operation setting accordingly. Besides SIDDHARTA, that will integrate a total luminosity of the order of 1 fb−1 , and can complete the data taking in middle 2009, there are three proposals for experiments at DAΦNE, briefly summarized in the following paragraphs 1 . 1.1. The AMADEUS proposal The proposal of the AMADEUS Collaboration [6] is to complement the KLOE detector with a cryogenic target close to the interaction region to perform an exhaustive search for deeplybound kaonic states. The target system could be installed around the beam pipe and inside the KLOE drift chamber (central region diameter: 50 cm). The system is complemented by a scintillator (or scintillating fiber) detector placed before the target to bring kaons at rest and to provide the information for triggering on the backto-back topology of the kaon pairs generated from φ-decay. Reactions induced in the target by K− at rest are studied from particle identification and from the measurement of their momenta and energies in the large-acceptance, high-resolution 1 The DANTE[4] and the KLOE-2[5] Collaborations have also presented a physics program to be pursued at energy higher than the φ peak, from 1-2.4 GeV, which is not included in this paper.

C. Bloise / Nuclear Physics B (Proc. Suppl.) 181–182 (2008) 390–395

environment provided by the KLOE detector. Masses and decay widths of kaonic clusters are obtained from the measurement of proton and neutron distributions in missing mass spectra, and additional information are provided from the study of the kinematical distributions of particles in specific final states. A part of the experimental programme is focused on the dibaryonic (with 3 He target) and tribaryonic states (with 4 He target). The integrated luminosity needed for semi-exclusive measurements of masses and total widths is about 2 fb−1 , and the extension of the programme to include more specific final states is obtained with 6 fb−1 . 1.2. The FINUDA proposal The FINUDA Collaboration has presented the proposal for a new run at the upgraded collider to complement and extend the results on hypernuclei spectroscopy and deeply bound kaonic states obtained from the analysis of the data sample of about 1 fb−1 collected in 2006-07 at DAΦNE. In particular, with 3 fb−1 of new data, FINUDA can, among other topics, better resolve the observed structures of the 12 Λ C hypernucleus [7]; extend spectroscopy studies to include hypernucleus formation on the Silicon modules of the tracking system; improve on knowledge of the low-energy part of the proton spectra from Λ decay inside the 6 Li, 4 He, 12 C hypernuclei, which has been measured for the first time by FINUDA [8] down to 15 MeV of kinetic energy; improve the results of the search for deeplybound states and nuclear kaon cluster formation on 6 Li and 12 C [9][10][11], by deuteron (triton) detection and the study of the kinematic correlations with Λ decay products. 1.3. The KLOE-2 proposal The KLOE-2 Collaboration has proposed to prepare the KLOE apparatus with minimal upgrades for a new run at DAΦNE aiming to integrate 5 fb−1 , mostly to: test the machine performance with the KLOE solenoidal magnetic field; improve on Kl3 , Kμ2 channels, for lepton uni-

391

versality and Vus ; set the best limits on several QM-CPTviolating parameters; improve on measurement of the Ke2 branching ratio, down to few per mil precision, thus testing LFV at this level; improve on several KS , η and η’ rare decays; observe for the first time f0 , a0 → KK decays; measure the part from threshold to about 200 MeV of the Mππ invariant mass spectrum for events of γ − γ fusion, e+ e− → e+ e− ππ. In the proposal, this phase of the KLOE-2 experiment is followed by the installation of the detector upgrades currently under study, for improving on tracking capability, on detection of those photons coming from decays close to the interaction region, and on identification of the γ − γ processes. In a longer data taking campaign, KLOE-2 can cover the physics program reported in [5], improving on systematics, thanks to a better detector, and on statistics, thanks to an integrated luminosity ≥ 20 fb−1 . 2. Improving on Standard Model tests 2.1. CKM unitarity Using the KLOE measurements [12] of the kaon semileptonic decay rates, the values of f+ (0) Vus ± ± for KLe3 , KLμ3 , KSe3 , Ke3 , and Kμ3 decay modes have been obtained. It is worth noting that the only experimental input to this analysis not provided by KLOE is the KS lifetime [13], measured by NA48 with 0.08% precision. The average of the five different results on f+ (0) Vus is f+ (0) Vus = 0.2157 ± 0.0006, with χ2 /ndf = 7.0/4 ( 13%). Lattice evaluations of f+ (0) are rapidly improving in precision. The RBC and UKQCD Collaborations have recently obtained f+ (0) = 0.9644(49) [14]. This value and the KLOE measurements give Vus = 0.2237(13), with 6 per mil accuracy, dominated by the error on f+ (0) . The availability of precise lattice evaluations for the pion- and kaon-decay constants, fπ and fK , leads to a competitive determination of Vus . The relation between Γ(Kμ2 )/Γ(πμ2 ) and 2 2 |Vus | / |Vud | is exploited in this case, with the advantage that lattice-scale uncertainties and radiative corrections largely cancel out in the ratio

392

C. Bloise / Nuclear Physics B (Proc. Suppl.) 181–182 (2008) 390–395

[15]. The KLOE measurements of BR(Kμ2 ) and τ± , together with Γ(πμ2 ) from [16], give: Vus /Vud × fK /fπ = 0.2766 ± 0.0009.

(1)

Using fK /fπ = 1.189(7) [17], from a recent lattice calculation from the HPQCD/UKQCD Collaboration, Vus /Vud = 0.2326 ± 0.0015. To test the unitarity of the quark mixing matrix, all the information from KLOE measurements (Kμ2 , Ke3 , Kμ3 ) can be combined, together with superallowed 0+ → 0+ nuclear β decays. The best estimate of |Vus |2 and |Vud |2 from a fit to KLOE results, Vus = 0.2237(13) and Vus /Vud = 0.2326(15), and to Vud = 0.97418(26) [18], is: |Vus |2 = 0.0506(4) and |Vud |2 = 0.9490(5), with χ2 /ndf = 2.34/1 (13% probability) and a correlation of 3%. These values confirm the unitarity of the first row of the CKM quark mixing matrix: |Vus |2 +|Vud |2 −1 = −0.0004±0.0007 (∼ 0.6σ)(2) Improved precision on the lattice parameters relevant for the CKM unitarity test in the first row is expected soon, calling for further experimental work to match the new precision frontier that KLOE-2 can provide. As an example peculiar to KLOE-2, we take the case of the KS0 semileptonic decays. KLOE has published the best determination of this branching ratio [19] BR(KS0 → πeν) = (7.046 ± 0.091) · 10−4. Most of the systematics scale with statistics, thus one can expect to improve the precision approximately with the square root of the integrated luminosity, reaching the per mil level at 10 fb−1 . In the KS case, final accuracy on Vus can not be spoiled by the precision on the lifetime, which is already known with relative error of eight parts in ten thousands [13]. 2.2. Lepton universality The KLOE-2 experimental program for improving on Vus reflects on the opportunity for giving a more stringent test of lepton universality and for testing lepton-flavor violating phenomena which can be induced by processes beyond the Standard Model, as presented in the Sec. 2.3. The comparison of the values of f+ (0) Vus for Ke3 and Kμ3 modes provides, in fact, a test of lepton

universality. Specifically, 2

rμe =

(f+ (0) Vus )μ3,exp (f+ (0) Vus )2e3,exp

=

Γμ3 Ie3 (1 + δKe )2 (3) Γe3 Iμ3 (1 + δKμ )2

where δKl is a correction due to SU (2) breaking and photon radiation. The ratio rμe is equal to the ratio gμ2 /ge2 of the coupling costants, gμ2 /ge2 , which are equal in the SM. Averaging charged and neutral modes, rμe = 1.000 ± 0.008. This has to be compared with the sensitivity obtained in π → ν decays, (rμe )π = 1.0042(33), and in τ leptonic decays, (rμe )τ = 1.000(4) [16]. 2.3. Lepton flavor violation The ratio of the Vus values obtained from helicity-suppressed and helicity-allowed modes, R23 = |Vus (K2 )/Vus (K3 ) × Vud (0+ → 0+ )/Vud (πμ2 )|, which is equal to 1 in the SM, can be affected by the presence of scalar currents like those due to charged Higgs, H + , exchange. In this case the value of R23 could be lower [20]:       m2π+ m2 + tan2 β (4) 1 − R23 = 1 − K 2 2 mH + mK + 1 + 0.01 tan β  with tan β the ratio of vacuum expectations of the two Higgs fields. In this scenario both, 0+ → 0+ nuclear beta decays, and K3 , are not affected by Higgs exchange diagrams, so that for these processes the unitarity constraint is still valid. R23 is evaluated from a fit to the experimental data on Kμ2 and K3 using as external inputs the most recent lattice determinations of f+ (0) and fK /fπ , and the value of Vud from [18], R23 = 1.008 ± 0.008. This measurement can be used to set bounds in the mH + - tan β plane, complementing the information from the BR(B → τ ν) measurements [21]. A particularly sensitive probe of new physics in leptonic kaon decays is the ratio RK =

Γ(K ± → e± νe ) Γ(K ± → μ± νμ )

(5)

This observable is predicted to 0.04% accuracy within the SM [22], while deviations up to the per cent level are possible in some MSSM scenario,

C. Bloise / Nuclear Physics B (Proc. Suppl.) 181–182 (2008) 390–395

due to lepton-flavor violating contributions with τ -neutrinos in the final state [23]. The current world average on RK has a relative error of  6%, and the preliminary results from NA48/2 and KLOE are improving the sensitivity down to the percent level. The KLOE preliminary result is [24]: RK = (2.55 ± 0.05 ± 0.05) × 10−5 . In the supersymmetric framework of Ref.[23] this result leads to significant constraints in the mH + − tanβ plane, depending on the lepton flavour violating (LFV) coupling, Δ13 [25]. The KLOE-2 goal is to reach 0.5-0.7% precision with the first 5 fb−1 of integrated luminosity. 3. Testing CP and CPT symmetries A precision on Re (  / ) [26] of a few parts in ten thousand can be obtained with KLOE-2 both via the measurement of the four separate branching ratios and via interferometry (which allows also the measurement of Im (  / )). KLOE has already measured the ratio of the charged to neutral two-pion decays of the KS0 with a precision of 0.2% [27], and the BR(KL0 → π + π − ) with a precision of 1% [28]. The key missing ingredient for the measurement of Re (  / ) is BR(KL0 → π 0 π 0 ) for which one can obtain an accuracy of few per mil with an integrated luminosity ≥10 fb−1 . Recently, it has been argued [29] that some unconventional CP-violation mechanism could induce an angular asymmetry of the production plane of the e+ e− pair with respect to that of the π + π − pair, for the decay η → e+ e− π + π − . This asymmetry, Aη , can be as large as ∼1%, while in the Standard Model it is negligible. KLOE has started an analysis of this decay channel, with very promising results. A signal of several hundreds events is clearly seen in a subsample of about 600 pb−1 , to be compared with the two previous measurements, which are based on 7 and 16 events [30]. One can estimate a sensitivity on Aη of order few percent with the present statistics. Therefore, at KLOE-2, one can reach a sensitivity down to the per mil level. Due to the low average momentum of the four tracks, acceptance in this case is a key issue. The insertion of the inner tracker would therefore be extremely beneficial. CPT invariance is a theorem in the frame-

393

work of quantum field theory (QFT), as a consequence of Lorentz invariance and locality. In several quantum gravity (QG) models, however, CPT can be violated via some mechanism which in general violates also standard Quantum Mechanics (QM). As an example we take the EHNS model [31], suggesting observable effects in the behaviour of entagled neutral meson systems [32]. KLOE has already published competitive upper limits on the EHNS parameters [33], from the analysis of the correlated KL0 − KS0 pairs, by measuring the relative distance of the decay point when both the kaons decay into a pair of charged pions. KLOE-2 can substantially improve the sensitivity of these tests, reaching the parameter window corresponding to effects below the Planck limit. This is especially true in case the new inner tracker will be installed to improve the vertex resolution in the central region close to the interaction zone, from 0.9 to 0.1 τS . 4. Improving on low-energy QCD Several techinques have been developed to systematically perform QCD inspired calculations on the strong and electromagnetic interactions of pseudoscalar mesons, in the framework of an effective theory, the Chiral Perturbation Theory (ChPT) [34]. Such a theory is based on a perturbative expansion in terms of the momenta of the involved mesons. The price to pay to reach higher precisions considering diagrams at higher order in the expansion, is the rapid increase in the number of free parameters, to be determined experimentally. For instance, KLOE has mea+0.03 sured BR(KS0 → γγ) = (2.27±0.13−0.04 ) ×10−6 [12], a result in disagreement with NA48 determination by more than 3σ, suggesting, differently from NA48, small contribution from O(p6 ) ChPT terms (see Fig.1). The KLOE measurement is limited by statistics and by the presence of a large background of KS0 → 2π 0 events with two lost photons. A large improvement on background rejection (almost a factor of three) is expected by a dedicated photon-veto detector in the low-θ region. The new photon-veto is also effective in the case of another controversial measurement of interest

394

3.50 3.25

C. Bloise / Nuclear Physics B (Proc. Suppl.) 181–182 (2008) 390–395

BR´10

6

NA31 NA48/99 NA48/03

3.00 2.75 2.50 2.00

cPT O(p4)

KLOE

1,75 1,50 1.25

Figure 1. Comparison of BR(KS → γγ) measurements and χPT predictions.

for ChPT, the η → π 0 γγ decay. This is particularly interesting since in ChPT all lower order contributions are suppressed, so that it provides a window for O(p6 ) effects. Recent results from Crystal Ball [35] and KLOE are in good agreement, although different from previous measurements [36]. Furthermore, the Mγγ spectrum, which is of interest for the theory, has never been measured, because of the large background coming from η → 3π 0 decays. The nature of the lowest mass scalar mesons is a long standing question. Evidence of the lowest mass states is still experimentally weak, and the quark content of the f0 and the a0 is not fully understood. KLOE has measured the radiative decays φ → π + π − γ, π 0 π 0 γ, ηπ 0 γ [37]. The couplings of the f0 and the a0 with kaons (gf kk , gakk ), have been determined indirectly, using some phenomenological model. A direct measurement of the two couplings can be obtained searching for the much rarer decay chains φ → (f0 , a0 )γ → KKγ. KLOE performed this search and set the preliminary upper limit B(f0 , a0 → KK) < 1.8×10−8, very close to the theoretically expected value. Therefore KLOE-2 can obtain the first measurement of this process already with few fb−1 of integrated luminosity. Radiative decays, φ → ππγ, have also been

used to search for the σ meson. While the σ meson contribution is required in the framework of the Kaon-loop model to obtain a good fit to the KLOE data for the neutral channel, the ISR background to the charged decay mode makes the Mππ spectrum quite insensitive to this term. However, a more direct evidence for the σ meson could come from γγ → π 0 π 0 processes at low energy [38][39]. KLOE-2 can provide data on the cross section in the region of interest [40], where the present situation is really poor. On this purpose, the feasibility of a tagger system for γ − γ events to improve on background suppression, is under study. KTeV has recently published the measurement of BR(π 0 → e+ e− ) [41] showing 3 − σ disagreement with the theoretical predictions [42][43]. The (π 0 → e+ e− ) process is an helicitysuppressed decay probing the π 0 γ  γ  coupling, which could be affected to a detectable level by contributions beyond the SM [44]. These contributions could also show up from the measurement of the branching fraction of the η meson to lepton pairs. With the first KLOE-2 run, 6% relative precision could be reached in the measurement of both, the BR(π 0 → e+ e− ), using π 0 ’s from φ → π + π − π 0 , K → ππ 0 , KS → π 0 π 0 , KL → π 0 π 0 π 0 , and the BR(η → μ+ μ− ). 5. Conclusions The scientific case for continuing the experimentation at DAΦNE upgraded in luminosity is compelling. The expertise developed in the field during the last ten years constitutes a key element to meet the challenge of improving systematics at the level demanded by the increase of the data sample. Moreover, the experimental readiness of the project calls for pursuing with strenght the realization of the machine upgrade for the new phase of experimentation at DAΦNE.

Acknowledgments It is a pleasure for me to thank the organizers for all the efforts to ensure a stimulating, informative workshop.

C. Bloise / Nuclear Physics B (Proc. Suppl.) 181–182 (2008) 390–395

REFERENCES 1. D.Alesini et al., LNF-06/33 (IR) (2006). 2. www.lnf.infn.it/esperimenti/siddharta/ 3. S.Okada et al. (DEAR Collab.), Phys. Lett. B653 (2007) 387. 4. www.lnf.infn.it/conference/nucleon05/FF/ 5. R.Beck et al. (KLOE-2 Collab.), LNF-07/19 (IR) (2007). 6. P.Buehler et al. (AMADEUS Collab.), LNF07/24 (IR) (2007). 7. M.Agnello et al. (FINUDA Collab.), Phys. Lett. B622 (2005) . 8. M.Agnello et al. (FINUDA Collab.), arXiv: 0803.1915 [nucl-ex] (2008). 9. M.Agnello et al. (FINUDA Collab.), Phys. Rev. Lett. 94 (2005) 212303. 10. M.Agnello et al. (FINUDA Collab.), Phys. Lett. B649 (2007) 25. 11. M.Agnello et al. (FINUDA Collab.), Eur. Phys. J. A33 (2007) 251. 12. A.Passeri, in these Proceedings. 13. A.Lai et al. (NA48 Collab.), Phys. Lett. B537 (2002) 28. 14. P.A. Boyle et al. (RBC/UKQCD Collab.), arXiv:0710.5136 (2007). 15. W.Marciano, Phys. Rev. Lett. 93 (2004) 231803. 16. W.M. Yao et al., J. Phys. G33 (2006) and update [pdg.web.cern.ch/pdg]. 17. E.Follana et al. (HPQCD/UKQCD Collab.), arXiv:0706.1726 (2007). 18. I.S. Towner and J.C. Hardy, arXiv:0710.3181 (2007). 19. F.Ambrosino et al. (KLOE Collab.), Phys. Lett. B636 (2006) 173. 20. G.Isidori and P.Paradisi, Phys. Lett. B639 (2006) 499. 21. K.Ikado et al. (Belle Collab.), Phys. Rev. Lett. 97 (2006) 251802; B.Aubert et al. (BaBar Collab.), arXiv:0705.1820 (2007). 22. M.Finkemeier, Phys. Lett. B 387 (1996) 391. 23. A.Masiero, P.Paradisi and R.Petronzio, Phys. Rev. D74 (2006) 011701. 24. F.Ambrosino et al. (KLOE Collab.), PoS(KAON)050 (2007). 25. F.Ambrosino et al. (KLOE Collab.), J. High Energy Phys. 04 (2008) 059.

395

26. A.J. Buras and M.Jamin, J. High Energy Phys. 0401 (2004) 048. 27. F.Ambrosino et al. (KLOE Collab.), Eur. Phys. J. C48 (2006) 767. 28. F.Ambrosino et al. (KLOE Collab.), Phys. Lett. B638 (2006) 140. 29. Dao-Neng Gao, Mod. Phys. Lett. A17 (2002) 1583. 30. R.R. Akhmetshin et al. (CMD-2 Collab.), Phys. Lett. B501 (2001) 191; Chr. Bargholtz et al. (WASA Collab.), Phys. Lett. B644 (2006) 299. 31. J.Ellis, J.S. Hagelin, D.V. Nanopoulos, M.Srednicki, Nucl. Phys. B241 (1984) 381. 32. P.Huet, M.Peskin, Nucl. Phys. B434 (1995) 3. 33. F.Ambrosino et al. (KLOE Collab.), Phys. Lett. B642 (2006) 315. 34. J.Gasser and H.Leutwyler, Annals Phys. 158 (1984) 142. 35. S.Prakhov et al. (BNL Crystal Ball Collab.), Phys. Rev. C72 (2005) 025201. 36. D.Alde et al. (GAMS2000 Collab.), Z. Phys. C25 (1984) 225. 37. A.Aloisio et al. (KLOE Collab.), Phys. Lett. B536 (2002) 209; F.Ambrosino et al.(KLOE Collab.), Phys. Lett. B634 (2006) 148. 38. J.Gasser et al., Eur. Phys. J. C40 (2005) 205. 39. F.Nguyen, F.Piccinini and A.Polosa, Eur. Phys. J. C47 (2006) 65. 40. H.Marsiske et al. (Crystal Ball Collab.), Phys. Rev. D41 (1990) 3324. 41. E.Abouzaid et al. (KTeV Collab.), Phys. Rev. D75 (2007) 012004. 42. G.D’Ambrosio, and D.Espriu, Phys. Lett. B175 (1986) 237. 43. A.E. Dorokhov, and M.A. Ivanov, Phys. Rev. D75 (2007) 114007; A.E. Dorokhov, in these Proceedings. 44. Y.Kahn, M.Schimtt, and T.M.P. Tait, arXiv: 0712.0007 [hep-ph] (2007).