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Experimental studies on kaonic atoms at DAΦNE
Nuclear Physics A 790 (2007) 667c–670c
Experimental studies on kaonic atoms at DAΦNE J. Zmeskal a , M. Bazzi b , G. Beerc , L. Bombelli d , A. M. Bragadirean be , M. Cargnellia , M. Catittib , C. Curceanu (Petrascu)b , C. Fiorinid , T. Frizzid , F. Ghiof , B. Girolamif , C. Guaraldob, M. Iliescub ,C. Curceanu(Petrascu)b e , T. Ishiwataria , P. Kienlea , P. Lechnerg , P. Levi Sandrib , V. Lucherinib , A. Longonid , J. Martona , D. Pietreanub , T. Pontae , D. L. Sirghibe , F. Sirghib , H. Soltaug , L. Str¨ uderh , E. Widmanna a
Stefan Meyer Insitut fuer subatomare Physik, Vienna, Austria
b
INFN, Laboratori Nazionali di Frascati, Frascati (Roma), Italy
c
Department of Physics and Astronomy, Univ. of Victoria, Victria B.C., Canada
d
Politecnico di Milano, Sez. Di Elettronica, Milano, Italy
e
IFIN-HH, Bucharest, Romania
f
INFN, Sez. Di RomaI and Istituto Superiore di Sanita, Roma, Italy
g
PNSensor GmbH, M¨ unchen, Germany
h
MPI for Extraterrestrial Physics, Garching, Germany
With precise X-ray spectroscopy on kaonic hydrogen at the DAΦNE electron-positron collider at Laboratori Natzionali di Frascati the chiral symmetry breaking scenario in the strangeness sector will be investigated by studying the K − p and K − d s-wave interaction at threshold. The strong interaction induced shift and width of the kaonic hydrogen 1s atomic state was measured with DEAR (DAΦNE Exotic Atom Research) and in future with SIDDHARTA (Silicon Drift Detector for Hadronic Atom Research with Timing Application) the kaonic hydrogen system will be measured with utmost precision. In addition, data for the K − d system will become available for the first time. Kaonic hydrogen together with precision measurements of kaonic helium will allow the study of the sub-threshold Λ(1405) resonance, which might act as a doorway for the formation of antikaon-mediated deeply bound nuclear states in light nuclei. Therefore, these precision measurements are a powerful tool to test chiral symmetry breaking in systems with strangeness. 1. INTRODUCTION One of the outstanding fundamental problems in hadron physics today is the question of the origin of the large masses of hadrons made up of light quarks. The current masses of the up (u) and down (d) quarks are two orders of magnitude smaller than the typical hadron mass of about 1 GeV. This extraordinary phenomenon is proposed to originate 0375-9474/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2007.03.012
668c
J. Zmeskal et al. / Nuclear Physics A 790 (2007) 667c–670c
from spontaneous breaking of chiral symmetry of massless quarks in strong interaction physics [1]. It results in a ground state – the vacuum state – with a finite expectation value for quark-antiquark pairs, the chiral quark condensate [2]. The hadrons are considered to be quasi particle excitations of this chiral condensate. Using precise X-ray spectroscopy on kaonic hydrogen the chiral symmetry breaking scenario in the strangeness sector will be investigated by studying the K − p and K − d swave interaction at threshold. That means the shift of the ground state 1s level due to the strong interaction (compared with the pure electromagnetic value) and the broadening of this level due to nuclear absorption of the kaon have to be measured. The DEAR [3–5] result confirmed, however with better precision, the finding of a previous experiment [6,7] showing that the strong interaction shift of the 1s state in kaonic hydrogen is repulsive, in agreement with scattering data and in disagreement with earlier results on kaonic atoms. The origin of this repulsive strong interaction at threshold can be traced back to the presence of the Λ(1405) resonance which leads on one hand to a repulsive K − p scattering length at threshold [8] and on the other hand to the possible existence of strongly bound kaonic states in light nuclei [9]. Moreover, apparent incompatibilities between DEAR kaonic hydrogen results and previous K − p scattering data underline the importance of even more precise kaonic hydrogen measurements as foreseen with SIDDHARTA. 2. THE DEAR RESULTS The DEAR experiment uses low momentum negative kaons, produced by the decay of φ mesons at DAΦNE, the φ-factory at Laboratori Nazionali di Frascati. The kaons are stopped in a specially designed light-weight cryogenic target cell, consisting of aluminum (top-plate and entrance ring), Kapton (side wall and entrance window) and a support structure of fiberglass reinforced epoxy to minimize fluorescence X-rays produced in the target material. The target cell is cooled to 25 K with a working pressure of 3 bars, which leads to a hydrogen gas density of 3.5 g/l (corresponding to 3% liquid hydrogen density). 16 CCD detector chips (Marconi Applied Technologies, CCD55-30) with a total area of 116 cm2 were placed around the cryogenic target cell. Each chip has 1242×1152 pixels with a pixel size of 22.5 μm × 22.5 μm and a depletion depth of about 30 μm. An energy resolution of 150 eV at 6 keV was achieved. The experimental challenge of DEAR was the extraction of a small signal in the presence of a large low-energy X-ray background mainly from electron gamma showers resulting from lost electrons and positrons due to either Touschek scattering or beam interactions with residual gas. The careful optimization of the shielding of our experimental setup and the improvements in the beam optics achieved by the machine crew made it possible to perform the most precise measurement to date of kaonic hydrogen X-ray transitions to the fundamental 1s level [3]. ε1s = −193 ± 37 (stat.) ± 6 (syst.) eV Γ1s = 249 ± 111 (stat.) ± 30 (syst.) eV
(1) (2)
New theoretical studies on kaonic hydrogen, triggered by the DEAR result, were recently published [10–12].
J. Zmeskal et al. / Nuclear Physics A 790 (2007) 667c–670c
669c
3. THE SIDDHARTA PROJECT – TOWARD KAONIC ATOM PRECISION SPECTROSCOPY To perform precision measurements of the shift and width of kaonic atoms at the percent level, the signal-to-background ratio has to be improved drastically (DEAR signal to background ratio ∼ 1:70). Therefore, a detector with good energy resolution (FWHM ∼ 150 eV at 6 keV) and in addition with good timing capability (better than 1 μs) is essential. Silicon Drift Detectors (SDDs) [13] fulfill all but one of the requirements – no large area devices were available at that time (available sizes up to 10 mm2 , first prototypes with an active area of 30 mm2 ). The SIDDHARTA (Silicon Drift Detector for Hadronic Atom Research by Timing Applications) collaboration was formed to develop large area SDD devices (active area 100 mm2 per chip) within the 6th-framework program of the EU (I3-Hadron Physics). The goal is to build SDD-chips with total active area of 3 cm2 , consisting of 3 individual elements on one chip and finally, a detector system with a total active area of more than 200 cm2 is foreseen. In the meantime the production of all the chips is finished. 10 chips are already mounted in ceramic holders and are currently under test. First results show a very good energy resolution of about 135 eV at 6 keV and a long-term stability of the order of 2-3 eV. With these SDD detectors providing timing capability, the X-ray signal from the kaonic atom transition of interest can be used together with the back-to-back K + K − pair in a triple coincidence. With this coincidence method we will suppress the continuous background as well as fluorescence X-rays by almost three orders of magnitude. 4. THE SIDDHARTA SETUP The SIDDHARTA setup follows the successful design strategy of DEAR [5]. The cryogenic target cell will be made only from selected materials, pure aluminum, Kapton and carbon reinforced epoxy. To achieve the required energy resolution the SDDs are cooled to ∼170 K and the preamplifier electronics has to be closely mounted at the backside of the SDD-chip. Two 3 cm2 SDD-chips are mounted in an aluminum case with three of aluminum cases connected to build up a sub-unit. Finally, twelve of these sub-units will be arranged around the target cell, with an active detector area of 216 cm2 . For a careful analysis of the material used in this setup, especially for the target cell, the SDD mounting and the structure material close to the SDD-chip, we have developed a PIXE (Proton Induced X-ray Emission) apparatus at VERA (Vienna Environmental Research Accelerator, University of Vienna, Institut f¨ ur Isotopenforschung und Kernphysik) to analyze the element content of the materials in use. The sensitivity for detecting Fe impurities is in the order of ppm. 5. THE SIDDHARTA PROGRAM With this setup, a percent level measurement of shift and width of kaonic hydrogen is feasible. With an average luminosity of 1032 cm−2 s−1 in about 30 days of beam time more than 30000 kaonic Kα events will be collected – enough to fulfill the goal of a percent level measurement.
670c
J. Zmeskal et al. / Nuclear Physics A 790 (2007) 667c–670c
For the deuterium case a Monte Carlo simulation, normalized to the measured DEAR continuous background events, shows that a kaonic deuterium measurement becomes possible. The following assumptions, coming from theory, were used in the simulation: shift = −300 eV, width = 600 eV, X-ray yield = 0.2% (a factor 10 less than in hydrogen) [14]. The result shows that in 30 days more than 3000 kaonic Kα events could be collected. In addition the strong interaction shift and width of the L-state of kaonic 4 He and of kaonic 3 He will be studied. 6. CONCLUSIONS The kaonic hydrogen measurement with DEAR at DAΦNE leads to an improved accuracy in the determination of the shift and width of the ground state of kaonic hydrogen and confirms the repulsive contribution of the strong interaction (as did the KpX experiment at KEK, Japan). Theoretical predictions based on chiral perturbation theory and a quantum field theoretical approach can now be tested with this new result. To go further in the direction of precision spectroscopy, we started to develop a new detector system – Large Area Silicon Drift Detectors – which will allow us to perform a percent level measurement of shift and width in kaonic hydrogen and to measure for the first time kaonic deuterium. In addition, measurements of kaonic 3 He and 4 He will be done with utmost precision. These measurements will lead to a deeper understanding of the kaon-nucleon interaction – in particular the chiral symmetry breaking in the strangeness sector is studied. These measurements are of course of great interest for the ongoing search for antikaon-mediated deeply bound nuclear states. ACKNOWLEDGMENTS Part of the work was supported by “Transnational Access to Research Infrastructure” (TARI), Hadron Physics I3, Contract No. RII3-CT-2004-506078. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Nambu and G. Jona-Lasinio, Phys. Rev. 122 (1961) 345. U. Vogl and W.Weise, Prog. Part. Nucl. Phys. 27 (1991) 195. G. Beer et al., Phys. Rev. Lett. 94 (2005) 212302. J. Zmeskal et al., Nucl. Phys. A. 754 (2005) 369. J. Zmeskal, Proceedings of EXA05, Austr. Acad. of Sci. Press, Vienna 2005, p. 139. M. Iwasaki et al., Phys. Rev. Lett. 78 (1997) 3067. T. M. Ito et al., Phys. Rev. C 58 (1998) 2366. N. Kaiser, P.B. Siegel and W. Weise, Nucl. Phys. A 594 (1995) 325. Y. Akaishi and T. Yamazaki, Phys. Rev. C 65 (2002) 44005. B. Borasoy, R. Nissler, W. Weise, Phys. Rev. Lett. 94 (2005) 213401. U.-G. Meißner, U. Raha, A. Rusetsky, Eur. Phys. J. C 35 (2004) 349. A. Ivanov et al., Eur. Phys. J. A21 (2004) 11, nucl-th/0310081. J. Kemmer, G. Lutz, Nucl. Instr. and Meth. A 253 (1987) 365. A. N. Ivanov et al., Eur. Phys. J. A 23 (2005) 79. J. Marton et al., Proceedings of the International Symposium on Detector Development, Stanford Linear Accelerator Center, Stanford, California, USA 2006.
Experimental studies on kaonic atoms at DAΦNE J. Zmeskal a , M. Bazzi b , G. Beerc , L. Bombelli d , A. M. Bragadirean be , M. Cargnellia , M. Catittib , C. Curceanu (Petrascu)b , C. Fiorinid , T. Frizzid , F. Ghiof , B. Girolamif , C. Guaraldob, M. Iliescub ,C. Curceanu(Petrascu)b e , T. Ishiwataria , P. Kienlea , P. Lechnerg , P. Levi Sandrib , V. Lucherinib , A. Longonid , J. Martona , D. Pietreanub , T. Pontae , D. L. Sirghibe , F. Sirghib , H. Soltaug , L. Str¨ uderh , E. Widmanna a
Stefan Meyer Insitut fuer subatomare Physik, Vienna, Austria
b
INFN, Laboratori Nazionali di Frascati, Frascati (Roma), Italy
c
Department of Physics and Astronomy, Univ. of Victoria, Victria B.C., Canada
d
Politecnico di Milano, Sez. Di Elettronica, Milano, Italy
e
IFIN-HH, Bucharest, Romania
f
INFN, Sez. Di RomaI and Istituto Superiore di Sanita, Roma, Italy
g
PNSensor GmbH, M¨ unchen, Germany
h
MPI for Extraterrestrial Physics, Garching, Germany
With precise X-ray spectroscopy on kaonic hydrogen at the DAΦNE electron-positron collider at Laboratori Natzionali di Frascati the chiral symmetry breaking scenario in the strangeness sector will be investigated by studying the K − p and K − d s-wave interaction at threshold. The strong interaction induced shift and width of the kaonic hydrogen 1s atomic state was measured with DEAR (DAΦNE Exotic Atom Research) and in future with SIDDHARTA (Silicon Drift Detector for Hadronic Atom Research with Timing Application) the kaonic hydrogen system will be measured with utmost precision. In addition, data for the K − d system will become available for the first time. Kaonic hydrogen together with precision measurements of kaonic helium will allow the study of the sub-threshold Λ(1405) resonance, which might act as a doorway for the formation of antikaon-mediated deeply bound nuclear states in light nuclei. Therefore, these precision measurements are a powerful tool to test chiral symmetry breaking in systems with strangeness. 1. INTRODUCTION One of the outstanding fundamental problems in hadron physics today is the question of the origin of the large masses of hadrons made up of light quarks. The current masses of the up (u) and down (d) quarks are two orders of magnitude smaller than the typical hadron mass of about 1 GeV. This extraordinary phenomenon is proposed to originate 0375-9474/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.nuclphysa.2007.03.012
668c
J. Zmeskal et al. / Nuclear Physics A 790 (2007) 667c–670c
from spontaneous breaking of chiral symmetry of massless quarks in strong interaction physics [1]. It results in a ground state – the vacuum state – with a finite expectation value for quark-antiquark pairs, the chiral quark condensate [2]. The hadrons are considered to be quasi particle excitations of this chiral condensate. Using precise X-ray spectroscopy on kaonic hydrogen the chiral symmetry breaking scenario in the strangeness sector will be investigated by studying the K − p and K − d swave interaction at threshold. That means the shift of the ground state 1s level due to the strong interaction (compared with the pure electromagnetic value) and the broadening of this level due to nuclear absorption of the kaon have to be measured. The DEAR [3–5] result confirmed, however with better precision, the finding of a previous experiment [6,7] showing that the strong interaction shift of the 1s state in kaonic hydrogen is repulsive, in agreement with scattering data and in disagreement with earlier results on kaonic atoms. The origin of this repulsive strong interaction at threshold can be traced back to the presence of the Λ(1405) resonance which leads on one hand to a repulsive K − p scattering length at threshold [8] and on the other hand to the possible existence of strongly bound kaonic states in light nuclei [9]. Moreover, apparent incompatibilities between DEAR kaonic hydrogen results and previous K − p scattering data underline the importance of even more precise kaonic hydrogen measurements as foreseen with SIDDHARTA. 2. THE DEAR RESULTS The DEAR experiment uses low momentum negative kaons, produced by the decay of φ mesons at DAΦNE, the φ-factory at Laboratori Nazionali di Frascati. The kaons are stopped in a specially designed light-weight cryogenic target cell, consisting of aluminum (top-plate and entrance ring), Kapton (side wall and entrance window) and a support structure of fiberglass reinforced epoxy to minimize fluorescence X-rays produced in the target material. The target cell is cooled to 25 K with a working pressure of 3 bars, which leads to a hydrogen gas density of 3.5 g/l (corresponding to 3% liquid hydrogen density). 16 CCD detector chips (Marconi Applied Technologies, CCD55-30) with a total area of 116 cm2 were placed around the cryogenic target cell. Each chip has 1242×1152 pixels with a pixel size of 22.5 μm × 22.5 μm and a depletion depth of about 30 μm. An energy resolution of 150 eV at 6 keV was achieved. The experimental challenge of DEAR was the extraction of a small signal in the presence of a large low-energy X-ray background mainly from electron gamma showers resulting from lost electrons and positrons due to either Touschek scattering or beam interactions with residual gas. The careful optimization of the shielding of our experimental setup and the improvements in the beam optics achieved by the machine crew made it possible to perform the most precise measurement to date of kaonic hydrogen X-ray transitions to the fundamental 1s level [3]. ε1s = −193 ± 37 (stat.) ± 6 (syst.) eV Γ1s = 249 ± 111 (stat.) ± 30 (syst.) eV
(1) (2)
New theoretical studies on kaonic hydrogen, triggered by the DEAR result, were recently published [10–12].
J. Zmeskal et al. / Nuclear Physics A 790 (2007) 667c–670c
669c
3. THE SIDDHARTA PROJECT – TOWARD KAONIC ATOM PRECISION SPECTROSCOPY To perform precision measurements of the shift and width of kaonic atoms at the percent level, the signal-to-background ratio has to be improved drastically (DEAR signal to background ratio ∼ 1:70). Therefore, a detector with good energy resolution (FWHM ∼ 150 eV at 6 keV) and in addition with good timing capability (better than 1 μs) is essential. Silicon Drift Detectors (SDDs) [13] fulfill all but one of the requirements – no large area devices were available at that time (available sizes up to 10 mm2 , first prototypes with an active area of 30 mm2 ). The SIDDHARTA (Silicon Drift Detector for Hadronic Atom Research by Timing Applications) collaboration was formed to develop large area SDD devices (active area 100 mm2 per chip) within the 6th-framework program of the EU (I3-Hadron Physics). The goal is to build SDD-chips with total active area of 3 cm2 , consisting of 3 individual elements on one chip and finally, a detector system with a total active area of more than 200 cm2 is foreseen. In the meantime the production of all the chips is finished. 10 chips are already mounted in ceramic holders and are currently under test. First results show a very good energy resolution of about 135 eV at 6 keV and a long-term stability of the order of 2-3 eV. With these SDD detectors providing timing capability, the X-ray signal from the kaonic atom transition of interest can be used together with the back-to-back K + K − pair in a triple coincidence. With this coincidence method we will suppress the continuous background as well as fluorescence X-rays by almost three orders of magnitude. 4. THE SIDDHARTA SETUP The SIDDHARTA setup follows the successful design strategy of DEAR [5]. The cryogenic target cell will be made only from selected materials, pure aluminum, Kapton and carbon reinforced epoxy. To achieve the required energy resolution the SDDs are cooled to ∼170 K and the preamplifier electronics has to be closely mounted at the backside of the SDD-chip. Two 3 cm2 SDD-chips are mounted in an aluminum case with three of aluminum cases connected to build up a sub-unit. Finally, twelve of these sub-units will be arranged around the target cell, with an active detector area of 216 cm2 . For a careful analysis of the material used in this setup, especially for the target cell, the SDD mounting and the structure material close to the SDD-chip, we have developed a PIXE (Proton Induced X-ray Emission) apparatus at VERA (Vienna Environmental Research Accelerator, University of Vienna, Institut f¨ ur Isotopenforschung und Kernphysik) to analyze the element content of the materials in use. The sensitivity for detecting Fe impurities is in the order of ppm. 5. THE SIDDHARTA PROGRAM With this setup, a percent level measurement of shift and width of kaonic hydrogen is feasible. With an average luminosity of 1032 cm−2 s−1 in about 30 days of beam time more than 30000 kaonic Kα events will be collected – enough to fulfill the goal of a percent level measurement.
670c
J. Zmeskal et al. / Nuclear Physics A 790 (2007) 667c–670c
For the deuterium case a Monte Carlo simulation, normalized to the measured DEAR continuous background events, shows that a kaonic deuterium measurement becomes possible. The following assumptions, coming from theory, were used in the simulation: shift = −300 eV, width = 600 eV, X-ray yield = 0.2% (a factor 10 less than in hydrogen) [14]. The result shows that in 30 days more than 3000 kaonic Kα events could be collected. In addition the strong interaction shift and width of the L-state of kaonic 4 He and of kaonic 3 He will be studied. 6. CONCLUSIONS The kaonic hydrogen measurement with DEAR at DAΦNE leads to an improved accuracy in the determination of the shift and width of the ground state of kaonic hydrogen and confirms the repulsive contribution of the strong interaction (as did the KpX experiment at KEK, Japan). Theoretical predictions based on chiral perturbation theory and a quantum field theoretical approach can now be tested with this new result. To go further in the direction of precision spectroscopy, we started to develop a new detector system – Large Area Silicon Drift Detectors – which will allow us to perform a percent level measurement of shift and width in kaonic hydrogen and to measure for the first time kaonic deuterium. In addition, measurements of kaonic 3 He and 4 He will be done with utmost precision. These measurements will lead to a deeper understanding of the kaon-nucleon interaction – in particular the chiral symmetry breaking in the strangeness sector is studied. These measurements are of course of great interest for the ongoing search for antikaon-mediated deeply bound nuclear states. ACKNOWLEDGMENTS Part of the work was supported by “Transnational Access to Research Infrastructure” (TARI), Hadron Physics I3, Contract No. RII3-CT-2004-506078. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
Nambu and G. Jona-Lasinio, Phys. Rev. 122 (1961) 345. U. Vogl and W.Weise, Prog. Part. Nucl. Phys. 27 (1991) 195. G. Beer et al., Phys. Rev. Lett. 94 (2005) 212302. J. Zmeskal et al., Nucl. Phys. A. 754 (2005) 369. J. Zmeskal, Proceedings of EXA05, Austr. Acad. of Sci. Press, Vienna 2005, p. 139. M. Iwasaki et al., Phys. Rev. Lett. 78 (1997) 3067. T. M. Ito et al., Phys. Rev. C 58 (1998) 2366. N. Kaiser, P.B. Siegel and W. Weise, Nucl. Phys. A 594 (1995) 325. Y. Akaishi and T. Yamazaki, Phys. Rev. C 65 (2002) 44005. B. Borasoy, R. Nissler, W. Weise, Phys. Rev. Lett. 94 (2005) 213401. U.-G. Meißner, U. Raha, A. Rusetsky, Eur. Phys. J. C 35 (2004) 349. A. Ivanov et al., Eur. Phys. J. A21 (2004) 11, nucl-th/0310081. J. Kemmer, G. Lutz, Nucl. Instr. and Meth. A 253 (1987) 365. A. N. Ivanov et al., Eur. Phys. J. A 23 (2005) 79. J. Marton et al., Proceedings of the International Symposium on Detector Development, Stanford Linear Accelerator Center, Stanford, California, USA 2006.