- Email: [email protected]
KLOE-2
Available online at www.sciencedirect.com
Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 32–37 www.elsevier.com/locate/npbps
Results and prospects on light meson spectroscopy with KLOE/KLOE-2 Federico Nguyena , for the KLOE and KLOE-2 Collaborations a INFN
“Roma TRE”, Via della Vasca Navale 84, I-00146, Rome (Italy)
Abstract The most recent KLOE results on light hadron studies are presented, with emphasis on: the rare η → e+ e− e+ e− + − decay observed for the first time, the dynamics √ of the η → π π γ decay, the search for the U boson produced with η mesons and the σe+ e− →ηγ measurement at s = 1 GeV. We also outlook the prospects on light meson spectroscopy with the KLOE-2 project, aiming to extend the KLOE harvest with increased statistics and detector upgrades. Keywords: e+ e− collisions, light mesons, exotic particles
1. Introduction: KLOE and DAΦNE DAΦNE is an e+ e− collider √ designed to operate at the center of mass energy s 1.02 GeV, namely the φ meson mass Mφ . It has provided to the KLOE experiment an integrated luminosity of about 2.5 √ fb−1 on peak −1 of the φ meson and also about 240 pb at s 1 GeV. The KLOE detector consists of a large volume cylindrical drift chamber [1], 3.3 m length and 2 m radius, surrounded by a calorimeter [2] made of lead and scintillating fibers. The detector is inserted in a superconducting coil producing an axial field B=0.52 T. Charged particle momenta are reconstructed with resolution σ p /p 0.4% for large angle tracks coming from the collision point. Energy clusters are reconstructed from calorimeter cells close in space and in √ time with /E = 5.7%/ E(GeV) energy and time resolution of σ E √ and σt = 57 ps/ E(GeV) ⊕ 100 ps. The trigger system [3] requires at least two energy deposits above threshold in the calorimeter: the threshold is 50 MeV for the barrel and 150 MeV for the end caps. 2. First measurement of the BR(η → e+ e− e+ e− ) The η → e+ e− e+ e− decay has been observed for the first time [4]. The predicted branching ratio for this decay is based on the triangle anomaly [5], with values Email address: [email protected] (Federico Nguyen)
0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.02.008
Figure 1: Feynman diagram for the η → e+ e− e+ e− decay.
in the range 2.4-2.6 × 10−5 . Figure 1 shows the Feynman diagram for this decay, where the e+ e− pairs are created by two virtual photons coupled to the η meson. This coupling is parametrized [6] by the transition form factor Fη (m21 , m22 ), with m21,2 being the invariant mass square of the e+ e− pairs. The measurement is based on an integrated luminosity of 1.7 fb−1 φ decays analyzing the chain φ → η(→ e+ e− e+ e− )γ, with the monochromatic photon having energy Eγ 363 MeV. Therefore, the analysis requirements are following: • one high energy photon, Eγ > 250 MeV, • at least four tracks coming from the collision point. Background from η Dalitz decays with photon conversion, η → e+ e− γ(→ e+ e− ), are reduced studying the e+ e− invariant mass and the distance between e+ e− tracks at the beam pipe or at the drift chamber wall. Processes with one π+ π− pair misidentified as e+ e− (e.g. φ → π+ π− π0 (→ e+ e− γ)) are reduced by measuring the time of flight of the charged particles with
33
F. Nguyen / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 32–37
the calorimeter. The main residual background is made up of radiative Bhabha events with photon conversion, e+ e− → e+ e− γγ(→ e+ e− ). Figure 2 shows the two com-
10 4
300 10 3
200 10 2
100
0
0
500
525
550
575 600 Meeee [MeV]
Figure 2: Fit of the e+ e− e+ e− invariant mass, after selection.
ponents fit of the data spectrum: background and signal are respectively parametrized by a third order polynomial and by two Gauss functions. The fit yields 362 signal events, with χ2 /nd f = 43.9/34, corresponding to a confidence level of 12%. Accounting for a signal efficiency of 20% together with other corrections we have: BR(η → e+ e− e+ e− ) = 2.4(2)stat (1)syst × 10−5 , where the main systematic errors are due to photon conversion effects and stability of the result against fit bin variation.
3. Search for light dark bosons Recent astrophysical observations [7] can be interpreted assuming the existence of a light hidden sector interacting with Standard Model particles through the mixing between a new gauge vector boson U, with mass lighter than O(GeV), and the photon. If the mixing parameter is of the order 10−4 -10−2 , the U boson can be produced [8] at low energy e+ e− colliders, in association with conventional vector or pseudoscalar mesons. We analyzed the process φ → ηU, where the η meson is tagged by the η → π+ π− π0 channel and we searched [9] for the U boson in the e+ e− invariant mass, Mee . The irreducible background is the decay φ → ηγ∗ → ηe+ e− , having the same signature. The Mee spectrum is studied from an integrated luminosity of 1.5 fb−1 of φ decays, where a clean sample of η → π+ π− π0 (→ γγ) is selected. This results in about 14000 φ → ηγ∗ (→ e+ e− ) events, whose Mee distribution is well described with the form factor Fη (M2φ , M2ee ) ∝ (1 − M2ee /Λ2 )−1 , as shown in Figure 3. The upper limit determination for φ → ηU events proceeds in the following way:
50 100 150 200 250 300 350 400 450 500
Mee (MeV)
Figure 3: Fit of the e+ e− invariant mass with the Fη (M2φ , M2ee ) form factor describing the φ → ηγ∗ → ηe+ e− background process.
• for each MU value in the range 5 < MU < 470 MeV, Monte Carlo, MC, events are generated in subsamples of 1 MeV width, • for each subsample, the average value of the ηγ∗ background is obtained fitting the Mee spectrum with 5 MeV binning, removing five bins centered at the MU nominal subsample and the signal hypothesis is tested comparing observed data, background and MC signal exactly in these five bins. The exclusion plot is obtained with the CLs method [10] accounting for the uncertainty in the Fη (M2φ , M2ee ) parametrization. Figure 4 shows the smoothed exclusion plot at 90% CL on α /α compared with existing limits from other experiments, with α ∝ 2 and α being the fine structure constant: a value α /α > 2 × 10−5 is excluded at 90% CL for 50 < MU < 420 MeV. The U boson can also be produced with different mechanisms at e+ e− colliders: e+ e− → γU(→ + − ) and e+ e− → U(→ + − )h , where h is a Higgs-like particle, related to the breaking of the hidden symmetry. Assuming [11] the h mass mh < MU , the signature of this latter process – also known as h -strahlung – is an
+ − pair recoiling against missing energy. In particular, our present event classification criteria are highly efficient for U → μ+ μ− events, when mh < 300 MeV, whatever mass for the U with mh < MU . We performed feasibility studies of the search for e+ e− → U h → μ+ μ− X/ events, where X/ is the missing energy associated to the long-lived h . The selection requirements are: • two tracks of opposite charge identified as muons
34
F. Nguyen / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 32–37
Main motivations for studying the η, η → π+ π− γ decays reside on their sensitivity to the box anomaly diagram [13]. Figure 5 shows the two diagrams contributing to the η, η → π+ π− γ decays. Particular interests in the η case rise from the following arguments: • the contact term, CT, due to the box anomaly has the same size of the vector meson dominance, VMD, contribution from ρ exchange, and should be visible in the ππ spectrum, • the recent CLEO measurement [14] of the ratio Rη = Γη→π+ π− γ /Γη→π+ π− π0 = 0.175(7)stat (6)syst is lower by more than two standard deviations from the past measurements [15].
Figure 4: Exclusion plot at 90% CL in the plane coupling α /α vs. U boson mass MU , compared with other experimental constraints.
by means of a neural network algorithm based on energy, time and position of the clusters, • missing momentum computed from tracks pointing towards the barrel calorimeter, • no energy deposit in the calorimeter, except for the clusters associated to tracks. The efficiency for this selection ranges from 15 to 40% depending upon the set of mh , MU masses. Residual background is composed of e+ e− → μ+ μ− γ and e+ e− → π+ π− γ events for MU > 800 MeV and φ → K + K − → μ+ μ− ν¯ν decays for MU ∼ 550 MeV, not suppressed even enforcing the requirement of the μ+ μ− vertex close to the collision point. The background from K ± decays gets negligible√when analyzing data taken off the φ resonance peak at s = 1 GeV, still keeping the same sensitivity on the mh , MU parameter space [12].
The measurement [16] is performed using an integrated luminosity of about 560 pb−1 , analyzing φ → ηγ → π+ π− 2γ decays tagged by the monochromatic photon. The following items are addressed: • major background processes are φ → π+ π− π0 (→ 2γ) that has the same signal signature and η → π+ π− π0 (→ 2γ) events with one lost photon, • these are suppressed by means of kinematic cuts, resulting in a background-to-signal ratio of 10%, • at the end of the selection, the signal yield is about 205 000 events with an efficiency of 21.3%. Figure 6 shows the Erecoil − | precoil | distribution, where the quantities refer to energy and momentum of the pho-
4. Study of the η → π+ π− γ decay
Figure 5: Feynman diagrams contributing to the η/η → π+ π− γ decay.
Figure 6: Erecoil − | precoil | distribution after selection for data, signal an background MC samples.
35
F. Nguyen / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 32–37
The normalization is provided by a sample of 1.2 × 106 events of η → π+ π− π0 decays, from the same data set, selected with 22.8% efficiency and backgroundto-signal ratio of 0.7%. This sample yields a branching ratio BRη→π+ π− π0 = 22.41(3)stat (35)syst % consistent with the PDG [15] result, BRη→π+ π− π0 = 22.74(28)%. The value of the ratio from the present measurement, Rη = 0.1838(5)stat (30)syst , is consistent with the CLEO value and confirms the discrepancy with older results. The fit to the ππ invariant mass and determination of the parameters related to the box anomaly are in progress. 5. Measurement of σ(e+ e− → ηγ) at
√
s = 1 GeV
• charged particles are recognized as pions from energy and time of their clusters in the calorimeter, • two out of the three photons originate from a neutral pion with good pairing probability, • a kinematic fit is applied, imposing energy and momentum conservation and fitting the output improved quantities, to obtain the ηγ events number. Figure 7 shows the π+ π− γγ invariant mass evaluated with the momenta improved in resolution, as a result of the kinematic fit, for data and MC spectra for signal and relevant background processes. From the fit to this distribution, the preliminary result is σe+ e− →ηγ (1 GeV) = 0.866 (9)stat (93)syst nb, where the systematic uncertainty is given by residual e+ e− → ωπ0 → π+ π− π0 π0 background. Figure 8 shows that this value is more accurate σ (nb)
ton recoiling against the system made up of charged pions and monochromatic photon, γφ , from the φ decay: √ Erecoil = s − Eπ+ − Eπ− − Eγφ | precoil | = | pπ+ + pπ− + pγφ |
4 3.5 3 2.5 2 1.5 1 0.5 0 950
960
970
980
990
1000
Figure 8: Values of σe+ e− →ηγ measurements around
1010 1020 E(MeV)
√
s = 1 GeV.
than the closer data points [19]. Figure 7: Fit to the π+ π− γγ invariant mass, evaluated with improved resolution momenta, with signal and background MC components. + −
+ − 0
The study of e e → η(→ π π π )γ is motivated [17] by the measurement of the γ∗ γ∗ fusion [18] process e+ e− → e+ e− η, with decay channel η → π+ π− π0 . In fact, both final states have the same experimental signature if only the three pions are fully reconstructed. The analyzed sample consists of data taken at √ s = 1 GeV with an integrated luminosity of 240 pb−1 . The selection consists of the requirement of two opposite charge tracks coming from the collision region and three photons. In particular:
6. From KLOE towards KLOE-2 A new beam crossing scheme, allowing for a reduced beam size, has been succesfully tested [20] at DAΦNE enabling to reach a peak luminosity of a factor three larger than previously obtained. The KLOE-2 physics project [21] aims to collect 20 fb−1 in about three years. This luminosity increase already allows for significant tests in the dynamics of light meson decays, such for example systematic studies of η decays: • the box anomaly effects in η → π+ π− γ decays,
36
F. Nguyen / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 32–37
and structure. With an integrated luminosity of 2.2 fb−1 we performed [23] the search for KS KS γ → 2π+ 2π− γ ¯ < events, resulting in the upper limit BR(φ → K Kγ) 1.9 × 10−8 at 90% CL. Figure 10 shows that this result is ¯ prediction computed consistent with the BR(φ → K Kγ) with a0 (980) [24], f0 (980) [25, 26] couplings measured by KLOE and shows that with an integrated luminosity O(10 fb−1 ) the 1 × 10−8 sensitivity is within reach, and that the inner tracker allows to reach the 5 × 10−9 level with the same statistics, thanks to an increased efficiency for pion tracks. 7. Conclusions
Figure 9: Comparison between ππ spectra with and without the scalar meson exchange described in the inset Feynman diagram.
• the sensitivity to the scalar meson exchange in the invariant ππ mass, mππ , of η → ηππ decays. Figure 9 shows the mππ spectrum with (right-centered, in red) and without (left-centered, in blue) the scalar meson contribution described in the inset Feynman diagram. The golden signature is η → η(→ γγ)π+ π− , already measured [22] at KLOE with 22.8% efficiency and about 10% background contamination. Moreover, the KLOE-2 project also includes the installation of the following detector upgrades: • e+ e− taggers for γ∗ γ∗ fusion processes [18], • an inner tracker made up of four layers of cylindrical triple gas electron multipliers, to provide a better vertex reconstruction for charged particles coming from the interaction point and to increase the acceptance of low transverse momentum tracks, • quadrupole sampling calorimeters, made of tungsten and scintillator tiles, read by silicon photomultipliers, to enhance coverage for photons coming from the KL decaying anywhere in the apparatus, • LYSO crystal calorimeters read by avalanche photodetectors, to increase the prompt photons acceptance from a minimum angle of 21◦ to about 10◦ . These improvements have a significant impact on the KLOE-2 physics reach in light meson spectroscopy and in the search for rare decays. For example, the yet un¯ has observed decay φ → [a0 (980) + f0 (980)]γ → K Kγ a large sensitivity to the a0 (980)- f0 (980) interference
¯ comFigure 10: Esclusion limit at 90% CL on the BR(φ → K Kγ) pared with the range of values evaluated using parameters from KLOE analyses on a0 (980) and f0 (980). The sensitivities with an integrated luminosity O(10 fb−1 ) and also with an inner tracker are shown.
At the moment, the KLOE-2 detector re-started to take collision data events. Novel data acquisition and slow-control systems are tested and long automatic runs of data taking are successfully operating. We published more than 20 publications on light hadron physics: scalar meson studies, η/η dynamics and tests of discrete symmetries, as well as hadron cross section measurements. The analysis of the high statistics collected is going on, and highlights have been presented: first measurement of the BRη→e+ e− e+ e− , upper limit on the U boson production in a parameter space not yet excluded and the η → π+ π− γ decay. With the plan of at least 20 fb−1 in the next three years and upgraded detectors, KLOE-2 will provide with: further precision study of scalar mesons (e.g. in η de-
F. Nguyen / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 32–37
cays), measurement of the η/η transition form factors and search for extremely rare phenomena such as exotic bosons or the a0 (980)/ f0 (980) → K K¯ decay. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
M. Adinolfi et al., Nucl. Instrum. Meth. A 488 (2002) 51. M. Adinolfi et al., Nucl. Instrum. Meth. A 482 (2002) 364. M. Adinolfi et al., Nucl. Instrum. Meth. A 492 (2002) 134. F. Ambrosino et al., [KLOE & KLOE-2 Collaborations], Phys. Lett. B 702 (2011) 324 [arXiv:1105.6067 [hep-ex]]. L. G. Landsberg, Phys. Rept. 128 (1985) 301. J. Bijnens and F. Perrsson, hep-ph/0106130. N. Arkani-Hamed, D. P. Finkbeiner, T. R. Slatyer and N. Weiner, Phys. Rev. D 79 (2009) 015014 [arXiv:0810.0713 [hep-ph]]. R. Essig, P. Schuster and N. Toro, Phys. Rev. D 80 (2009) 015003 [arXiv:0903.3941 [hep-ph]]. F. Archilli et al., [KLOE-2 Collaboration], arXiv:1110.0411 [hep-ex], accepted for publication on Physics Letters B. T. Junk, Nucl. Instrum. Meth. A 434 (1999) 435 [hepex/9902006]. B. Batell, M. Pospelov and A. Ritz, Phys. Rev. D 79, 115008 (2009) [arXiv:0903.0363 [hep-ph]]. F. Archilli et al., [KLOE-2 Collaboration], arXiv:1107.2531 [hep-ex]. M. Benayoun, P. David, L. DelBuono, P. Leruste and H. B. O’Connell, Eur. Phys. J. C 31 (2003) 525 [nuclth/0306078]. A. Lopez et al. [CLEO Collaboration], Phys. Rev. Lett. 99 (2007) 122001 [arXiv:0707.1601 [hep-ex]]. K. Nakamura et al. [Particle Data Group Collaboration], J. Phys. G 37 (2010) 075021. F. Ambrosino et al. [KLOE & KLOE-2 Collaborations], arXiv:1107.5733 [hep-ex]. F. Archilli et al. [KLOE-2 Collaboration], arXiv:1107.3782 [hep-ex]. F. Nguyen, for the KLOE & KLOE-2 Collaborations, “Twophoton physics at KLOE/KLOE-2”, these Proceedings. M. N. Achasov et al. [SND Collaboration], Phys. Rev. D 76 (2007) 077101 [arXiv:0709.1007 [hep-ex]]. M. Zobov et al., Phys. Rev. Lett. 104 (2010) 174801. G. Amelino-Camelia et al., Eur. Phys. J. C 68 (2010) 619 [arXiv:1003.3868 [hep-ex]]. A. Aloisio et al. [KLOE Collaboration], Phys. Lett. B 541 (2002) 45 [hep-ex/0206010]. F. Ambrosino et al. [KLOE Collaboration], Phys. Lett. B 679 (2009) 10 [arXiv:0903.4115 [hep-ex]]. F. Ambrosino et al. [KLOE Collaboration], Phys. Lett. B 681 (2009) 5 [arXiv:0904.2539 [hep-ex]]. F. Ambrosino et al. [KLOE Collaboration], Phys. Lett. B 634 (2006) 148 [hep-ex/0511031]. F. Ambrosino et al. [KLOE Collaboration], Eur. Phys. J. C 49 (2007) 473 [hep-ex/0609009].
37
Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 32–37 www.elsevier.com/locate/npbps
Results and prospects on light meson spectroscopy with KLOE/KLOE-2 Federico Nguyena , for the KLOE and KLOE-2 Collaborations a INFN
“Roma TRE”, Via della Vasca Navale 84, I-00146, Rome (Italy)
Abstract The most recent KLOE results on light hadron studies are presented, with emphasis on: the rare η → e+ e− e+ e− + − decay observed for the first time, the dynamics √ of the η → π π γ decay, the search for the U boson produced with η mesons and the σe+ e− →ηγ measurement at s = 1 GeV. We also outlook the prospects on light meson spectroscopy with the KLOE-2 project, aiming to extend the KLOE harvest with increased statistics and detector upgrades. Keywords: e+ e− collisions, light mesons, exotic particles
1. Introduction: KLOE and DAΦNE DAΦNE is an e+ e− collider √ designed to operate at the center of mass energy s 1.02 GeV, namely the φ meson mass Mφ . It has provided to the KLOE experiment an integrated luminosity of about 2.5 √ fb−1 on peak −1 of the φ meson and also about 240 pb at s 1 GeV. The KLOE detector consists of a large volume cylindrical drift chamber [1], 3.3 m length and 2 m radius, surrounded by a calorimeter [2] made of lead and scintillating fibers. The detector is inserted in a superconducting coil producing an axial field B=0.52 T. Charged particle momenta are reconstructed with resolution σ p /p 0.4% for large angle tracks coming from the collision point. Energy clusters are reconstructed from calorimeter cells close in space and in √ time with /E = 5.7%/ E(GeV) energy and time resolution of σ E √ and σt = 57 ps/ E(GeV) ⊕ 100 ps. The trigger system [3] requires at least two energy deposits above threshold in the calorimeter: the threshold is 50 MeV for the barrel and 150 MeV for the end caps. 2. First measurement of the BR(η → e+ e− e+ e− ) The η → e+ e− e+ e− decay has been observed for the first time [4]. The predicted branching ratio for this decay is based on the triangle anomaly [5], with values Email address: [email protected] (Federico Nguyen)
0920-5632/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysbps.2012.02.008
Figure 1: Feynman diagram for the η → e+ e− e+ e− decay.
in the range 2.4-2.6 × 10−5 . Figure 1 shows the Feynman diagram for this decay, where the e+ e− pairs are created by two virtual photons coupled to the η meson. This coupling is parametrized [6] by the transition form factor Fη (m21 , m22 ), with m21,2 being the invariant mass square of the e+ e− pairs. The measurement is based on an integrated luminosity of 1.7 fb−1 φ decays analyzing the chain φ → η(→ e+ e− e+ e− )γ, with the monochromatic photon having energy Eγ 363 MeV. Therefore, the analysis requirements are following: • one high energy photon, Eγ > 250 MeV, • at least four tracks coming from the collision point. Background from η Dalitz decays with photon conversion, η → e+ e− γ(→ e+ e− ), are reduced studying the e+ e− invariant mass and the distance between e+ e− tracks at the beam pipe or at the drift chamber wall. Processes with one π+ π− pair misidentified as e+ e− (e.g. φ → π+ π− π0 (→ e+ e− γ)) are reduced by measuring the time of flight of the charged particles with
33
F. Nguyen / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 32–37
the calorimeter. The main residual background is made up of radiative Bhabha events with photon conversion, e+ e− → e+ e− γγ(→ e+ e− ). Figure 2 shows the two com-
10 4
300 10 3
200 10 2
100
0
0
500
525
550
575 600 Meeee [MeV]
Figure 2: Fit of the e+ e− e+ e− invariant mass, after selection.
ponents fit of the data spectrum: background and signal are respectively parametrized by a third order polynomial and by two Gauss functions. The fit yields 362 signal events, with χ2 /nd f = 43.9/34, corresponding to a confidence level of 12%. Accounting for a signal efficiency of 20% together with other corrections we have: BR(η → e+ e− e+ e− ) = 2.4(2)stat (1)syst × 10−5 , where the main systematic errors are due to photon conversion effects and stability of the result against fit bin variation.
3. Search for light dark bosons Recent astrophysical observations [7] can be interpreted assuming the existence of a light hidden sector interacting with Standard Model particles through the mixing between a new gauge vector boson U, with mass lighter than O(GeV), and the photon. If the mixing parameter is of the order 10−4 -10−2 , the U boson can be produced [8] at low energy e+ e− colliders, in association with conventional vector or pseudoscalar mesons. We analyzed the process φ → ηU, where the η meson is tagged by the η → π+ π− π0 channel and we searched [9] for the U boson in the e+ e− invariant mass, Mee . The irreducible background is the decay φ → ηγ∗ → ηe+ e− , having the same signature. The Mee spectrum is studied from an integrated luminosity of 1.5 fb−1 of φ decays, where a clean sample of η → π+ π− π0 (→ γγ) is selected. This results in about 14000 φ → ηγ∗ (→ e+ e− ) events, whose Mee distribution is well described with the form factor Fη (M2φ , M2ee ) ∝ (1 − M2ee /Λ2 )−1 , as shown in Figure 3. The upper limit determination for φ → ηU events proceeds in the following way:
50 100 150 200 250 300 350 400 450 500
Mee (MeV)
Figure 3: Fit of the e+ e− invariant mass with the Fη (M2φ , M2ee ) form factor describing the φ → ηγ∗ → ηe+ e− background process.
• for each MU value in the range 5 < MU < 470 MeV, Monte Carlo, MC, events are generated in subsamples of 1 MeV width, • for each subsample, the average value of the ηγ∗ background is obtained fitting the Mee spectrum with 5 MeV binning, removing five bins centered at the MU nominal subsample and the signal hypothesis is tested comparing observed data, background and MC signal exactly in these five bins. The exclusion plot is obtained with the CLs method [10] accounting for the uncertainty in the Fη (M2φ , M2ee ) parametrization. Figure 4 shows the smoothed exclusion plot at 90% CL on α /α compared with existing limits from other experiments, with α ∝ 2 and α being the fine structure constant: a value α /α > 2 × 10−5 is excluded at 90% CL for 50 < MU < 420 MeV. The U boson can also be produced with different mechanisms at e+ e− colliders: e+ e− → γU(→ + − ) and e+ e− → U(→ + − )h , where h is a Higgs-like particle, related to the breaking of the hidden symmetry. Assuming [11] the h mass mh < MU , the signature of this latter process – also known as h -strahlung – is an
+ − pair recoiling against missing energy. In particular, our present event classification criteria are highly efficient for U → μ+ μ− events, when mh < 300 MeV, whatever mass for the U with mh < MU . We performed feasibility studies of the search for e+ e− → U h → μ+ μ− X/ events, where X/ is the missing energy associated to the long-lived h . The selection requirements are: • two tracks of opposite charge identified as muons
34
F. Nguyen / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 32–37
Main motivations for studying the η, η → π+ π− γ decays reside on their sensitivity to the box anomaly diagram [13]. Figure 5 shows the two diagrams contributing to the η, η → π+ π− γ decays. Particular interests in the η case rise from the following arguments: • the contact term, CT, due to the box anomaly has the same size of the vector meson dominance, VMD, contribution from ρ exchange, and should be visible in the ππ spectrum, • the recent CLEO measurement [14] of the ratio Rη = Γη→π+ π− γ /Γη→π+ π− π0 = 0.175(7)stat (6)syst is lower by more than two standard deviations from the past measurements [15].
Figure 4: Exclusion plot at 90% CL in the plane coupling α /α vs. U boson mass MU , compared with other experimental constraints.
by means of a neural network algorithm based on energy, time and position of the clusters, • missing momentum computed from tracks pointing towards the barrel calorimeter, • no energy deposit in the calorimeter, except for the clusters associated to tracks. The efficiency for this selection ranges from 15 to 40% depending upon the set of mh , MU masses. Residual background is composed of e+ e− → μ+ μ− γ and e+ e− → π+ π− γ events for MU > 800 MeV and φ → K + K − → μ+ μ− ν¯ν decays for MU ∼ 550 MeV, not suppressed even enforcing the requirement of the μ+ μ− vertex close to the collision point. The background from K ± decays gets negligible√when analyzing data taken off the φ resonance peak at s = 1 GeV, still keeping the same sensitivity on the mh , MU parameter space [12].
The measurement [16] is performed using an integrated luminosity of about 560 pb−1 , analyzing φ → ηγ → π+ π− 2γ decays tagged by the monochromatic photon. The following items are addressed: • major background processes are φ → π+ π− π0 (→ 2γ) that has the same signal signature and η → π+ π− π0 (→ 2γ) events with one lost photon, • these are suppressed by means of kinematic cuts, resulting in a background-to-signal ratio of 10%, • at the end of the selection, the signal yield is about 205 000 events with an efficiency of 21.3%. Figure 6 shows the Erecoil − | precoil | distribution, where the quantities refer to energy and momentum of the pho-
4. Study of the η → π+ π− γ decay
Figure 5: Feynman diagrams contributing to the η/η → π+ π− γ decay.
Figure 6: Erecoil − | precoil | distribution after selection for data, signal an background MC samples.
35
F. Nguyen / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 32–37
The normalization is provided by a sample of 1.2 × 106 events of η → π+ π− π0 decays, from the same data set, selected with 22.8% efficiency and backgroundto-signal ratio of 0.7%. This sample yields a branching ratio BRη→π+ π− π0 = 22.41(3)stat (35)syst % consistent with the PDG [15] result, BRη→π+ π− π0 = 22.74(28)%. The value of the ratio from the present measurement, Rη = 0.1838(5)stat (30)syst , is consistent with the CLEO value and confirms the discrepancy with older results. The fit to the ππ invariant mass and determination of the parameters related to the box anomaly are in progress. 5. Measurement of σ(e+ e− → ηγ) at
√
s = 1 GeV
• charged particles are recognized as pions from energy and time of their clusters in the calorimeter, • two out of the three photons originate from a neutral pion with good pairing probability, • a kinematic fit is applied, imposing energy and momentum conservation and fitting the output improved quantities, to obtain the ηγ events number. Figure 7 shows the π+ π− γγ invariant mass evaluated with the momenta improved in resolution, as a result of the kinematic fit, for data and MC spectra for signal and relevant background processes. From the fit to this distribution, the preliminary result is σe+ e− →ηγ (1 GeV) = 0.866 (9)stat (93)syst nb, where the systematic uncertainty is given by residual e+ e− → ωπ0 → π+ π− π0 π0 background. Figure 8 shows that this value is more accurate σ (nb)
ton recoiling against the system made up of charged pions and monochromatic photon, γφ , from the φ decay: √ Erecoil = s − Eπ+ − Eπ− − Eγφ | precoil | = | pπ+ + pπ− + pγφ |
4 3.5 3 2.5 2 1.5 1 0.5 0 950
960
970
980
990
1000
Figure 8: Values of σe+ e− →ηγ measurements around
1010 1020 E(MeV)
√
s = 1 GeV.
than the closer data points [19]. Figure 7: Fit to the π+ π− γγ invariant mass, evaluated with improved resolution momenta, with signal and background MC components. + −
+ − 0
The study of e e → η(→ π π π )γ is motivated [17] by the measurement of the γ∗ γ∗ fusion [18] process e+ e− → e+ e− η, with decay channel η → π+ π− π0 . In fact, both final states have the same experimental signature if only the three pions are fully reconstructed. The analyzed sample consists of data taken at √ s = 1 GeV with an integrated luminosity of 240 pb−1 . The selection consists of the requirement of two opposite charge tracks coming from the collision region and three photons. In particular:
6. From KLOE towards KLOE-2 A new beam crossing scheme, allowing for a reduced beam size, has been succesfully tested [20] at DAΦNE enabling to reach a peak luminosity of a factor three larger than previously obtained. The KLOE-2 physics project [21] aims to collect 20 fb−1 in about three years. This luminosity increase already allows for significant tests in the dynamics of light meson decays, such for example systematic studies of η decays: • the box anomaly effects in η → π+ π− γ decays,
36
F. Nguyen / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 32–37
and structure. With an integrated luminosity of 2.2 fb−1 we performed [23] the search for KS KS γ → 2π+ 2π− γ ¯ < events, resulting in the upper limit BR(φ → K Kγ) 1.9 × 10−8 at 90% CL. Figure 10 shows that this result is ¯ prediction computed consistent with the BR(φ → K Kγ) with a0 (980) [24], f0 (980) [25, 26] couplings measured by KLOE and shows that with an integrated luminosity O(10 fb−1 ) the 1 × 10−8 sensitivity is within reach, and that the inner tracker allows to reach the 5 × 10−9 level with the same statistics, thanks to an increased efficiency for pion tracks. 7. Conclusions
Figure 9: Comparison between ππ spectra with and without the scalar meson exchange described in the inset Feynman diagram.
• the sensitivity to the scalar meson exchange in the invariant ππ mass, mππ , of η → ηππ decays. Figure 9 shows the mππ spectrum with (right-centered, in red) and without (left-centered, in blue) the scalar meson contribution described in the inset Feynman diagram. The golden signature is η → η(→ γγ)π+ π− , already measured [22] at KLOE with 22.8% efficiency and about 10% background contamination. Moreover, the KLOE-2 project also includes the installation of the following detector upgrades: • e+ e− taggers for γ∗ γ∗ fusion processes [18], • an inner tracker made up of four layers of cylindrical triple gas electron multipliers, to provide a better vertex reconstruction for charged particles coming from the interaction point and to increase the acceptance of low transverse momentum tracks, • quadrupole sampling calorimeters, made of tungsten and scintillator tiles, read by silicon photomultipliers, to enhance coverage for photons coming from the KL decaying anywhere in the apparatus, • LYSO crystal calorimeters read by avalanche photodetectors, to increase the prompt photons acceptance from a minimum angle of 21◦ to about 10◦ . These improvements have a significant impact on the KLOE-2 physics reach in light meson spectroscopy and in the search for rare decays. For example, the yet un¯ has observed decay φ → [a0 (980) + f0 (980)]γ → K Kγ a large sensitivity to the a0 (980)- f0 (980) interference
¯ comFigure 10: Esclusion limit at 90% CL on the BR(φ → K Kγ) pared with the range of values evaluated using parameters from KLOE analyses on a0 (980) and f0 (980). The sensitivities with an integrated luminosity O(10 fb−1 ) and also with an inner tracker are shown.
At the moment, the KLOE-2 detector re-started to take collision data events. Novel data acquisition and slow-control systems are tested and long automatic runs of data taking are successfully operating. We published more than 20 publications on light hadron physics: scalar meson studies, η/η dynamics and tests of discrete symmetries, as well as hadron cross section measurements. The analysis of the high statistics collected is going on, and highlights have been presented: first measurement of the BRη→e+ e− e+ e− , upper limit on the U boson production in a parameter space not yet excluded and the η → π+ π− γ decay. With the plan of at least 20 fb−1 in the next three years and upgraded detectors, KLOE-2 will provide with: further precision study of scalar mesons (e.g. in η de-
F. Nguyen / Nuclear Physics B (Proc. Suppl.) 225–227 (2012) 32–37
cays), measurement of the η/η transition form factors and search for extremely rare phenomena such as exotic bosons or the a0 (980)/ f0 (980) → K K¯ decay. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13]
[14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26]
M. Adinolfi et al., Nucl. Instrum. Meth. A 488 (2002) 51. M. Adinolfi et al., Nucl. Instrum. Meth. A 482 (2002) 364. M. Adinolfi et al., Nucl. Instrum. Meth. A 492 (2002) 134. F. Ambrosino et al., [KLOE & KLOE-2 Collaborations], Phys. Lett. B 702 (2011) 324 [arXiv:1105.6067 [hep-ex]]. L. G. Landsberg, Phys. Rept. 128 (1985) 301. J. Bijnens and F. Perrsson, hep-ph/0106130. N. Arkani-Hamed, D. P. Finkbeiner, T. R. Slatyer and N. Weiner, Phys. Rev. D 79 (2009) 015014 [arXiv:0810.0713 [hep-ph]]. R. Essig, P. Schuster and N. Toro, Phys. Rev. D 80 (2009) 015003 [arXiv:0903.3941 [hep-ph]]. F. Archilli et al., [KLOE-2 Collaboration], arXiv:1110.0411 [hep-ex], accepted for publication on Physics Letters B. T. Junk, Nucl. Instrum. Meth. A 434 (1999) 435 [hepex/9902006]. B. Batell, M. Pospelov and A. Ritz, Phys. Rev. D 79, 115008 (2009) [arXiv:0903.0363 [hep-ph]]. F. Archilli et al., [KLOE-2 Collaboration], arXiv:1107.2531 [hep-ex]. M. Benayoun, P. David, L. DelBuono, P. Leruste and H. B. O’Connell, Eur. Phys. J. C 31 (2003) 525 [nuclth/0306078]. A. Lopez et al. [CLEO Collaboration], Phys. Rev. Lett. 99 (2007) 122001 [arXiv:0707.1601 [hep-ex]]. K. Nakamura et al. [Particle Data Group Collaboration], J. Phys. G 37 (2010) 075021. F. Ambrosino et al. [KLOE & KLOE-2 Collaborations], arXiv:1107.5733 [hep-ex]. F. Archilli et al. [KLOE-2 Collaboration], arXiv:1107.3782 [hep-ex]. F. Nguyen, for the KLOE & KLOE-2 Collaborations, “Twophoton physics at KLOE/KLOE-2”, these Proceedings. M. N. Achasov et al. [SND Collaboration], Phys. Rev. D 76 (2007) 077101 [arXiv:0709.1007 [hep-ex]]. M. Zobov et al., Phys. Rev. Lett. 104 (2010) 174801. G. Amelino-Camelia et al., Eur. Phys. J. C 68 (2010) 619 [arXiv:1003.3868 [hep-ex]]. A. Aloisio et al. [KLOE Collaboration], Phys. Lett. B 541 (2002) 45 [hep-ex/0206010]. F. Ambrosino et al. [KLOE Collaboration], Phys. Lett. B 679 (2009) 10 [arXiv:0903.4115 [hep-ex]]. F. Ambrosino et al. [KLOE Collaboration], Phys. Lett. B 681 (2009) 5 [arXiv:0904.2539 [hep-ex]]. F. Ambrosino et al. [KLOE Collaboration], Phys. Lett. B 634 (2006) 148 [hep-ex/0511031]. F. Ambrosino et al. [KLOE Collaboration], Eur. Phys. J. C 49 (2007) 473 [hep-ex/0609009].
37