A new experimental search for muonium-antimuonium conversion was conducted at
the Paul Scherrer Institute, Villigen, Switzerland. The preliminary analysis
yielded one event fulfilling all required criteria at an expected background of
1.7(2) events due to accidental coincidences. An upper limit for the conversion
probability in 0.1 T magnetic field is extracted as $8 \cdot 10^{-11}$ (90%
CL).Comment: 2 figure

Abela, R.

Bagaturia, J.

Bertl, W.

Engfer, R .

Grossmann, A.

Hughes, V. W.

Jungmann, K.

Karpuchin, V.

Kisel, I.

Korenchenko, A.

Korenchenko, S.

Kravchuk, N.

Kuchinsky, N.

Merkel, J.

Meyer, V.

Moiseenko, A.

Mzavia, D.

Putlitz, G. zu

Reinhard, I.

Renker, D.

Sakhelashvili, T.

Schmidt, P. V.

Träger, K.

Walter, H. K.

Willmann, L.

Wirtz, H. P.

English

arXiv

University of Groningen Research Database

  
 University of Groningen
Increased Sensitivity to Possible Muonium to Antimuonium Conversion
Meyer, V.; Grossmann, A.; Jungmann, Klaus-Peter; Merkel, J.; Putlitz, G. zu; Reinhard, I.;
Träger, K.; Schmidt, P. V.; Willmann, Lorenz; Engfer, R .
Published in:
AIP Conference Proceedings
DOI:
10.1063/1.54250
IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from
it. Please check the document version below.
Publication date:
1998
Link to publication in University of Groningen/UMCG research database
Citation for published version (APA):
Meyer, V., Grossmann, A., Jungmann, K., Merkel, J., Putlitz, G. Z., Reinhard, I., ... Hughes, V. W. (1998).
Increased Sensitivity to Possible Muonium to Antimuonium Conversion. AIP Conference Proceedings. DOI:
10.1063/1.54250
Copyright
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Increased Sensitivity to Possible
Muonium to Antimuonium Conversion
V. Meyer1, A. Grossmann1, K. Jungmann1, J. Merkel1, G. zu
Putlitz1, I. Reinhard1, K. Tra¨ger1, P.V. Schmidt1, L. Willmann1, R .
Engfer2, H.P. Wirtz2, R. Abela3, W. Bertl3, D. Renker3, H.K.
Walter3, V. Karpuchin4, I. Kisel4, A. Korenchenko4, S.
Korenchenko4, N. Kravchuk4, N. Kuchinsky4, A. Moiseenko4, J.
Bagaturia5, D. Mzavia5, T. Sakhelashvili5, V.W. Hughes6
1Physikalisches Institut, Universita¨t Heidelberg, D-69120 Heidelberg, Germany; 2Physik Institut,
Universita¨t Zu¨rich, CH-8057 Zu¨rich, Switzerland; 3Paul Scherrer Institut, CH-5232 Villigen,
Switzerland; 4Joint Institute of Nuclear Research, RU-141980 Dubna, Russia; 5Tbilisi State
University, GUS-380086 Tbilisi, Georgia; 6Physics Department, Yale University, New Haven
CT 06520, USA
Abstract. A new experimental search for muonium-antimuonium conversion was con-
ducted at the Paul Scherrer Institute, Villigen, Switzerland. The preliminary analysis
yielded one event fulfilling all required criteria at an expected background of 1.7(2)
events due to accidental coincidences. An upper limit for the conversion probability in
0.1 T magnetic field is extracted as 8 · 10−11 (90% CL).
The hydrogen like muonium atom (M = µ+e−) consists of two leptons from dif-
ferent generations. The close confinement of the bound state offers excellent oppor-
tunities to explore precisely fundamental electron-muon interaction. The dominant
part of the binding in this system is electromagnetic and can be calculated to very
high accuracy in the framework of quantum electromagnetics (QED). Indeed, pre-
cision experiments on electromagnetic transitions in muonium have been employed
both to verify bound state QED calculations and for determining most accurate
values of fundamental constants [1].
Since the effects of all known fundamental forces in muonium are calculable very
well, it renders the possibility to search sensitively for yet unknown interactions
between both particles. A conversion of muonium into its antiatom (M = µ−e+)
would violate additive lepton family number conservation and is not provided in
standard theory. However, muonium-antimuonium conversion appears to be natu-
ral in many speculative theories, which try to extend the standard model in order
to explain some of its yet not well understood features like parity violation in weak
interaction and particle mass spectra. The interaction could be mediated by a dou-
bly charged Higgs boson [2], heavy Majorana neutrinos [3], a neutral scalar [4], e.g.
a supersymmetric τ -sneutrino [5] or a dileptonic gauge boson [6].
An experiment had been set up to search for spontaneous muonium-antimuonium
conversion at the Paul Scherrer Institute (PSI) in Villigen, Switzerland [7]. It
uses the powerful signature developed in an experiment at the Los Alamos Meson
Physics Facility (LAMPF) USA, which requires the coincident identification of both
constituents of the antiatom in its decay [8].
Muonium atoms were produced by stopping a beam of surface muons in a SiO2
powder target, where a fraction of them forms muonium by electron capture, some
of which diffuse through the target surface with thermal energies into vacuum.
Energetic electrons from the decay of the µ− in the antiatom can be observed in a
magnetic spectrometer at 0.1 T magnetic field consisting of five concentric multiwire
proportional chambers and a 64 fold segmented hodoscope. The positron in the
atomic shell of the antiatom is left behind after the decay with 13.5 eV average
kinetic energy. It can be electrostatically accelerated to 8 keV and guided in a
magnetic transport system onto a position sensitive microchannel plate detector
(MCP). Annihilation radiation can be observed in a 12 fold segmented pure CsI
calorimeter surrounding the MCP.
The muonium production was monitored regularly by reversing all electric and
magnetic fields of the instrument every five hours for a duration of 20 minutes.
Targets had to be replaced twice a week because of observed deterioration of muo-
nium production on a one week time scale. In the course of the experiment 5.7 ·1010
muonium atoms were observed in the interaction volume for antimuonium decays.
There was one event which passed all required criteria, i.e. fell into a 99% con-
fidence interval of each relevant distribution. The expected background due to
accidental coincidences is 1.7(2) events.
The preliminary combination of all data recorded in the experiment between 1993
and 1996 [7,9,10] results in an upper limit for the conversion probability in 0.1 T
magnetic field of P
MM
≤ 8 ·10−11 (90 % CL). For an assumed effective (V-A)x(V-A)
type four fermion interaction this corresponds to an upper limit for the coupling
constant of G
MM
≤ 3 · 10−3GF(90 % CL), where GF is the weak interaction Fermi
coupling constant [9].
This new result allows to rule out definitively a certain Z8 model with more
than three particle generations [4] and to set a new lower limit of 2.6 TeV/c2
* g3l (g3l depends on model, of order unity) on the mass of a dileptonic gauge
boson in GUT models - well beyond the value extracted from high energy Bhabha
scattering [6]. It can be further shown in the framework of minimal left right
symmetric and supersymmetric models [11] that lepton number violating muon
decay (µ+ → e+ + νµ + νe) is not an option for explaining the excess neutrino
counts in the LSND neutrino experiment at Los Alamos [12].
This work was supported by the Bundesminister fu¨r Bildung und Forschung
(BMBF) of Germany, the Schweizer Nationalfond, the Russian Federation for Fun-
damental Research and a NATO research grant.
-5
-2.5
0
2.5
5
-20 -10 0 10 20
TOF - TOFexpected [ns]
R
dc
a 
[c
m
]
-5
-2.5
0
2.5
5
-20 -10 0 10 20
TOF - TOFexpected [ns]
R
dc
a 
[c
m
]
R
dc
a 
[c
m
]
FIGURE 1. The distribution of the distance of closest approach (Rdca) between a track from
an energetic particle in the magnetic spectrometer and the back projection of the position on
the MCP detector versus the time of flight (TOF) of the atomic shell particle for a muonium
measurement (left) and for all data recorded in 1996 while searching for antimuonium (right).
One single event falls within 3 standard deviations region of the expected TOF and Rdca which
is indicated by the ellipse. The events concentrated at early times and low Rdcacorrespond to a
background signal from the allowed decay µ→ 3e+ 2ν.
REFERENCES
1. Hughes V W and zu Putlitz G, in: Quantum Electrodynamics, Singapore, World
Scientific Kinoshita T (ed.) p. 822 (1990); Jungmann K, in: Atomic Physics 14, New
York, AIP Press, Wineland D et al. (ed.) p. 102 (1994)
2. Herczeg P and Mohapatra R N, Phys.Rev.Lett. 69 2475 (1992)
3. Halprin A, Phys.Rev.Lett. 48 1313 (1982)
4. Hou W S and Wong G G, Phys.Rev. D53 1537 (1996)
5. Mohapatra R N, Z.Phys. C56 117 (1992); Halprin A and Masiero A, Phys.Rev.D48,
2987 (1993)
6. Horrikawa K and Sasaki K, Phys.Rev. D53 560 (1994)
7. Abela R et al., Phys.Rev.Lett. 77 1951 (1996)
8. Matthias B et al., Phys.Rev.Lett. 66 2716 (1991)
9. Schmidt P, doctoral dissertation, University of Heidelberg, unpublished (1997)
10. Wirtz H P, doctoral dissertation, University of Zu¨rich, unpublished (1997)
11. Herczeg P, conference ”Beyond the Dessert”, Castle Ringberg, Germany (1997)
12. Athanassopoulos C et al. Phys.Rev. C54 2685 (1996)


Increased Sensitivity to Possible Muonium to Antimuonium Conversion

A new experimental search for muonium-antimuonium conversion was conducted at the Paul Scherrer Institute, Villigen, Switzerland. The preliminary analysis yielded one event fulfilling all required criteria at an expected background of 1.7(2) events due to accidental coincidences. An upper limit for the conversion probability in 0.1 T magnetic field is extracted as $8 \cdot 10^{-11}$ (90% CL)

ARTS repository - University of Groningen

University of GroningenIncreased Sensitivity to Possible Muonium to Antimuonium ConversionMeyer, V.; Grossmann, A.; Jungmann, K.; Merkel, J.; Putlitz, G. zu; Reinhard, I.; Träger, K.;Schmidt, P. V.; Willmann, L.; Engfer, R .Published in:AIP Conference ProceedingsDOI:10.1063/1.54250IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionFinal author's version (accepted by publisher, after peer review)Publication date:1998Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Meyer, V., Grossmann, A., Jungmann, K., Merkel, J., Putlitz, G. Z., Reinhard, I., Träger, K., Schmidt, P. V.,Willmann, L., Engfer, R. ., Wirtz, H. P., Abela, R., Bertl, W., Renker, D., Walter, H. K., Karpuchin, V., Kisel,I., Korenchenko, A., Korenchenko, S., ... Hughes, V. W. (1998). Increased Sensitivity to Possible Muoniumto Antimuonium Conversion. AIP Conference Proceedings, 412(1). https://doi.org/10.1063/1.54250CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 23-12-2025arXiv:hep-ex/9805012v1  14 May 1998Increased Sensitivity to PossibleMuonium to Antimuonium ConversionV. Meyer1, A. Grossmann1, K. Jungmann1, J. Merkel1, G. zuPutlitz1, I. Reinhard1, K. Träger1, P.V. Schmidt1, L. Willmann1, R .Engfer2, H.P. Wirtz2, R. Abela3, W. Bertl3, D. Renker3, H.K.Walter3, V. Karpuchin4, I. Kisel4, A. Korenchenko4, S.Korenchenko4, N. Kravchuk4, N. Kuchinsky4, A. Moiseenko4, J.Bagaturia5, D. Mzavia5, T. Sakhelashvili5, V.W. Hughes61Physikalisches Institut, Universität Heidelberg, D-69120 Heidelberg, Germany; 2Physik Institut,Universität Zürich, CH-8057 Zürich, Switzerland; 3Paul Scherrer Institut, CH-5232 Villigen,Switzerland; 4Joint Institute of Nuclear Research, RU-141980 Dubna, Russia; 5Tbilisi StateUniversity, GUS-380086 Tbilisi, Georgia; 6Physics Department, Yale University, New HavenCT 06520, USAAbstract. A new experimental search for muonium-antimuonium conversion was con-ducted at the Paul Scherrer Institute, Villigen, Switzerland. The preliminary analysisyielded one event fulfilling all required criteria at an expected background of 1.7(2)events due to accidental coincidences. An upper limit for the conversion probability in0.1 T magnetic field is extracted as 8 · 10−11 (90% CL).The hydrogen like muonium atom (M = µ+e−) consists of two leptons from dif-ferent generations. The close confinement of the bound state offers excellent oppor-tunities to explore precisely fundamental electron-muon interaction. The dominantpart of the binding in this system is electromagnetic and can be calculated to veryhigh accuracy in the framework of quantum electromagnetics (QED). Indeed, pre-cision experiments on electromagnetic transitions in muonium have been employedboth to verify bound state QED calculations and for determining most accuratevalues of fundamental constants [1].Since the effects of all known fundamental forces in muonium are calculable verywell, it renders the possibility to search sensitively for yet unknown interactionsbetween both particles. A conversion of muonium into its antiatom (M = µ−e+)would violate additive lepton family number conservation and is not provided instandard theory. However, muonium-antimuonium conversion appears to be natu-ral in many speculative theories, which try to extend the standard model in orderto explain some of its yet not well understood features like parity violation in weakinteraction and particle mass spectra. The interaction could be mediated by a dou-bly charged Higgs boson [2], heavy Majorana neutrinos [3], a neutral scalar [4], e.g.a supersymmetric τ -sneutrino [5] or a dileptonic gauge boson [6].An experiment had been set up to search for spontaneous muonium-antimuoniumconversion at the Paul Scherrer Institute (PSI) in Villigen, Switzerland [7]. Ituses the powerful signature developed in an experiment at the Los Alamos MesonPhysics Facility (LAMPF) USA, which requires the coincident identification of bothconstituents of the antiatom in its decay [8].Muonium atoms were produced by stopping a beam of surface muons in a SiO2powder target, where a fraction of them forms muonium by electron capture, someof which diffuse through the target surface with thermal energies into vacuum.Energetic electrons from the decay of the µ− in the antiatom can be observed in amagnetic spectrometer at 0.1 T magnetic field consisting of five concentric multiwireproportional chambers and a 64 fold segmented hodoscope. The positron in theatomic shell of the antiatom is left behind after the decay with 13.5 eV averagekinetic energy. It can be electrostatically accelerated to 8 keV and guided in amagnetic transport system onto a position sensitive microchannel plate detector(MCP). Annihilation radiation can be observed in a 12 fold segmented pure CsIcalorimeter surrounding the MCP.The muonium production was monitored regularly by reversing all electric andmagnetic fields of the instrument every five hours for a duration of 20 minutes.Targets had to be replaced twice a week because of observed deterioration of muo-nium production on a one week time scale. In the course of the experiment 5.7 ·1010muonium atoms were observed in the interaction volume for antimuonium decays.There was one event which passed all required criteria, i.e. fell into a 99% con-fidence interval of each relevant distribution. The expected background due toaccidental coincidences is 1.7(2) events.The preliminary combination of all data recorded in the experiment between 1993and 1996 [7,9,10] results in an upper limit for the conversion probability in 0.1 Tmagnetic field of PMM≤ 8 ·10−11 (90 % CL). For an assumed effective (V-A)x(V-A)type four fermion interaction this corresponds to an upper limit for the couplingconstant of GMM≤ 3 · 10−3GF(90 % CL), where GF is the weak interaction Fermicoupling constant [9].This new result allows to rule out definitively a certain Z8 model with morethan three particle generations [4] and to set a new lower limit of 2.6 TeV/c2* g3l (g3l depends on model, of order unity) on the mass of a dileptonic gaugeboson in GUT models - well beyond the value extracted from high energy Bhabhascattering [6]. It can be further shown in the framework of minimal left rightsymmetric and supersymmetric models [11] that lepton number violating muondecay (µ+ → e+ + νµ + νe) is not an option for explaining the excess neutrinocounts in the LSND neutrino experiment at Los Alamos [12].This work was supported by the Bundesminister für Bildung und Forschung(BMBF) of Germany, the Schweizer Nationalfond, the Russian Federation for Fun-damental Research and a NATO research grant.-5-2.502.55-20 -10 0 10 20TOF - TOFexpected [ns]Rdca [cm]-5-2.502.55-20 -10 0 10 20TOF - TOFexpected [ns]Rdca [cm]Rdca [cm]FIGURE 1. The distribution of the distance of closest approach (Rdca) between a track froman energetic particle in the magnetic spectrometer and the back projection of the position onthe MCP detector versus the time of flight (TOF) of the atomic shell particle for a muoniummeasurement (left) and for all data recorded in 1996 while searching for antimuonium (right).One single event falls within 3 standard deviations region of the expected TOF and Rdca whichis indicated by the ellipse. The events concentrated at early times and low Rdcacorrespond to abackground signal from the allowed decay µ → 3e + 2ν.REFERENCES1. Hughes V W and zu Putlitz G, in: Quantum Electrodynamics, Singapore, WorldScientific Kinoshita T (ed.) p. 822 (1990); Jungmann K, in: Atomic Physics 14, NewYork, AIP Press, Wineland D et al. (ed.) p. 102 (1994)2. Herczeg P and Mohapatra R N, Phys.Rev.Lett. 69 2475 (1992)3. Halprin A, Phys.Rev.Lett. 48 1313 (1982)4. Hou W S and Wong G G, Phys.Rev. D53 1537 (1996)5. Mohapatra R N, Z.Phys. C56 117 (1992); Halprin A and Masiero A, Phys.Rev.D48,2987 (1993)6. Horrikawa K and Sasaki K, Phys.Rev. D53 560 (1994)7. Abela R et al., Phys.Rev.Lett. 77 1951 (1996)8. Matthias B et al., Phys.Rev.Lett. 66 2716 (1991)9. Schmidt P, doctoral dissertation, University of Heidelberg, unpublished (1997)10. Wirtz H P, doctoral dissertation, University of Zürich, unpublished (1997)11. Herczeg P, conference ”Beyond the Dessert”, Castle Ringberg, Germany (1997)12. Athanassopoulos C et al. Phys.Rev. C54 2685 (1996)

University of Groningen

   University of GroningenIncreased Sensitivity to Possible Muonium to Antimuonium ConversionMeyer, V.; Grossmann, A.; Jungmann, K.; Merkel, J.; Putlitz, G. zu; Reinhard, I.; Träger, K.;Schmidt, P. V.; Willmann, L.; Engfer, R .Published in:AIP Conference ProceedingsDOI:10.1063/1.54250IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionFinal author's version (accepted by publisher, after peer review)Publication date:1998Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Meyer, V., Grossmann, A., Jungmann, K., Merkel, J., Putlitz, G. Z., Reinhard, I., Träger, K., Schmidt, P. V.,Willmann, L., Engfer, R. ., Wirtz, H. P., Abela, R., Bertl, W., Renker, D., Walter, H. K., Karpuchin, V., Kisel,I., Korenchenko, A., Korenchenko, S., ... Hughes, V. W. (1998). Increased Sensitivity to Possible Muoniumto Antimuonium Conversion. AIP Conference Proceedings, 412(1). https://doi.org/10.1063/1.54250CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 28-04-2023arXiv:hep-ex/9805012v1  14 May 1998Increased Sensitivity to PossibleMuonium to Antimuonium ConversionV. Meyer1, A. Grossmann1, K. Jungmann1, J. Merkel1, G. zuPutlitz1, I. Reinhard1, K. Träger1, P.V. Schmidt1, L. Willmann1, R .Engfer2, H.P. Wirtz2, R. Abela3, W. Bertl3, D. Renker3, H.K.Walter3, V. Karpuchin4, I. Kisel4, A. Korenchenko4, S.Korenchenko4, N. Kravchuk4, N. Kuchinsky4, A. Moiseenko4, J.Bagaturia5, D. Mzavia5, T. Sakhelashvili5, V.W. Hughes61Physikalisches Institut, Universität Heidelberg, D-69120 Heidelberg, Germany; 2Physik Institut,Universität Zürich, CH-8057 Zürich, Switzerland; 3Paul Scherrer Institut, CH-5232 Villigen,Switzerland; 4Joint Institute of Nuclear Research, RU-141980 Dubna, Russia; 5Tbilisi StateUniversity, GUS-380086 Tbilisi, Georgia; 6Physics Department, Yale University, New HavenCT 06520, USAAbstract. A new experimental search for muonium-antimuonium conversion was con-ducted at the Paul Scherrer Institute, Villigen, Switzerland. The preliminary analysisyielded one event fulfilling all required criteria at an expected background of 1.7(2)events due to accidental coincidences. An upper limit for the conversion probability in0.1 T magnetic field is extracted as 8 · 10−11 (90% CL).The hydrogen like muonium atom (M = µ+e−) consists of two leptons from dif-ferent generations. The close confinement of the bound state offers excellent oppor-tunities to explore precisely fundamental electron-muon interaction. The dominantpart of the binding in this system is electromagnetic and can be calculated to veryhigh accuracy in the framework of quantum electromagnetics (QED). Indeed, pre-cision experiments on electromagnetic transitions in muonium have been employedboth to verify bound state QED calculations and for determining most accuratevalues of fundamental constants [1].Since the effects of all known fundamental forces in muonium are calculable verywell, it renders the possibility to search sensitively for yet unknown interactionsbetween both particles. A conversion of muonium into its antiatom (M = µ−e+)would violate additive lepton family number conservation and is not provided instandard theory. However, muonium-antimuonium conversion appears to be natu-ral in many speculative theories, which try to extend the standard model in orderto explain some of its yet not well understood features like parity violation in weakinteraction and particle mass spectra. The interaction could be mediated by a dou-bly charged Higgs boson [2], heavy Majorana neutrinos [3], a neutral scalar [4], e.g.a supersymmetric τ -sneutrino [5] or a dileptonic gauge boson [6].An experiment had been set up to search for spontaneous muonium-antimuoniumconversion at the Paul Scherrer Institute (PSI) in Villigen, Switzerland [7]. Ituses the powerful signature developed in an experiment at the Los Alamos MesonPhysics Facility (LAMPF) USA, which requires the coincident identification of bothconstituents of the antiatom in its decay [8].Muonium atoms were produced by stopping a beam of surface muons in a SiO2powder target, where a fraction of them forms muonium by electron capture, someof which diffuse through the target surface with thermal energies into vacuum.Energetic electrons from the decay of the µ− in the antiatom can be observed in amagnetic spectrometer at 0.1 T magnetic field consisting of five concentric multiwireproportional chambers and a 64 fold segmented hodoscope. The positron in theatomic shell of the antiatom is left behind after the decay with 13.5 eV averagekinetic energy. It can be electrostatically accelerated to 8 keV and guided in amagnetic transport system onto a position sensitive microchannel plate detector(MCP). Annihilation radiation can be observed in a 12 fold segmented pure CsIcalorimeter surrounding the MCP.The muonium production was monitored regularly by reversing all electric andmagnetic fields of the instrument every five hours for a duration of 20 minutes.Targets had to be replaced twice a week because of observed deterioration of muo-nium production on a one week time scale. In the course of the experiment 5.7 ·1010muonium atoms were observed in the interaction volume for antimuonium decays.There was one event which passed all required criteria, i.e. fell into a 99% con-fidence interval of each relevant distribution. The expected background due toaccidental coincidences is 1.7(2) events.The preliminary combination of all data recorded in the experiment between 1993and 1996 [7,9,10] results in an upper limit for the conversion probability in 0.1 Tmagnetic field of PMM≤ 8 ·10−11 (90 % CL). For an assumed effective (V-A)x(V-A)type four fermion interaction this corresponds to an upper limit for the couplingconstant of GMM≤ 3 · 10−3GF(90 % CL), where GF is the weak interaction Fermicoupling constant [9].This new result allows to rule out definitively a certain Z8 model with morethan three particle generations [4] and to set a new lower limit of 2.6 TeV/c2* g3l (g3l depends on model, of order unity) on the mass of a dileptonic gaugeboson in GUT models - well beyond the value extracted from high energy Bhabhascattering [6]. It can be further shown in the framework of minimal left rightsymmetric and supersymmetric models [11] that lepton number violating muondecay (µ+ → e+ + νµ + νe) is not an option for explaining the excess neutrinocounts in the LSND neutrino experiment at Los Alamos [12].This work was supported by the Bundesminister für Bildung und Forschung(BMBF) of Germany, the Schweizer Nationalfond, the Russian Federation for Fun-damental Research and a NATO research grant.-5-2.502.55-20 -10 0 10 20TOF - TOFexpected [ns]Rdca [cm]-5-2.502.55-20 -10 0 10 20TOF - TOFexpected [ns]Rdca [cm]Rdca [cm]FIGURE 1. The distribution of the distance of closest approach (Rdca) between a track froman energetic particle in the magnetic spectrometer and the back projection of the position onthe MCP detector versus the time of flight (TOF) of the atomic shell particle for a muoniummeasurement (left) and for all data recorded in 1996 while searching for antimuonium (right).One single event falls within 3 standard deviations region of the expected TOF and Rdca whichis indicated by the ellipse. The events concentrated at early times and low Rdcacorrespond to abackground signal from the allowed decay µ → 3e + 2ν.REFERENCES1. Hughes V W and zu Putlitz G, in: Quantum Electrodynamics, Singapore, WorldScientific Kinoshita T (ed.) p. 822 (1990); Jungmann K, in: Atomic Physics 14, NewYork, AIP Press, Wineland D et al. (ed.) p. 102 (1994)2. Herczeg P and Mohapatra R N, Phys.Rev.Lett. 69 2475 (1992)3. Halprin A, Phys.Rev.Lett. 48 1313 (1982)4. Hou W S and Wong G G, Phys.Rev. D53 1537 (1996)5. Mohapatra R N, Z.Phys. C56 117 (1992); Halprin A and Masiero A, Phys.Rev.D48,2987 (1993)6. Horrikawa K and Sasaki K, Phys.Rev. D53 560 (1994)7. Abela R et al., Phys.Rev.Lett. 77 1951 (1996)8. Matthias B et al., Phys.Rev.Lett. 66 2716 (1991)9. Schmidt P, doctoral dissertation, University of Heidelberg, unpublished (1997)10. Wirtz H P, doctoral dissertation, University of Zürich, unpublished (1997)11. Herczeg P, conference ”Beyond the Dessert”, Castle Ringberg, Germany (1997)12. Athanassopoulos C et al. Phys.Rev. C54 2685 (1996)

   University of GroningenIncreased Sensitivity to Possible Muonium to Antimuonium ConversionMeyer, V.; Grossmann, A.; Jungmann, K.; Merkel, J.; Putlitz, G. zu; Reinhard, I.; Träger, K.;Schmidt, P. V.; Willmann, L.; Engfer, R .Published in:AIP Conference ProceedingsDOI:10.1063/1.54250IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionFinal author's version (accepted by publisher, after peer review)Publication date:1998Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Meyer, V., Grossmann, A., Jungmann, K., Merkel, J., Putlitz, G. Z., Reinhard, I., Träger, K., Schmidt, P. V.,Willmann, L., Engfer, R. ., Wirtz, H. P., Abela, R., Bertl, W., Renker, D., Walter, H. K., Karpuchin, V., Kisel,I., Korenchenko, A., Korenchenko, S., ... Hughes, V. W. (1998). Increased Sensitivity to Possible Muoniumto Antimuonium Conversion. AIP Conference Proceedings, 412(1). https://doi.org/10.1063/1.54250CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 31-10-2023arXiv:hep-ex/9805012v1  14 May 1998Increased Sensitivity to PossibleMuonium to Antimuonium ConversionV. Meyer1, A. Grossmann1, K. Jungmann1, J. Merkel1, G. zuPutlitz1, I. Reinhard1, K. Träger1, P.V. Schmidt1, L. Willmann1, R .Engfer2, H.P. Wirtz2, R. Abela3, W. Bertl3, D. Renker3, H.K.Walter3, V. Karpuchin4, I. Kisel4, A. Korenchenko4, S.Korenchenko4, N. Kravchuk4, N. Kuchinsky4, A. Moiseenko4, J.Bagaturia5, D. Mzavia5, T. Sakhelashvili5, V.W. Hughes61Physikalisches Institut, Universität Heidelberg, D-69120 Heidelberg, Germany; 2Physik Institut,Universität Zürich, CH-8057 Zürich, Switzerland; 3Paul Scherrer Institut, CH-5232 Villigen,Switzerland; 4Joint Institute of Nuclear Research, RU-141980 Dubna, Russia; 5Tbilisi StateUniversity, GUS-380086 Tbilisi, Georgia; 6Physics Department, Yale University, New HavenCT 06520, USAAbstract. A new experimental search for muonium-antimuonium conversion was con-ducted at the Paul Scherrer Institute, Villigen, Switzerland. The preliminary analysisyielded one event fulfilling all required criteria at an expected background of 1.7(2)events due to accidental coincidences. An upper limit for the conversion probability in0.1 T magnetic field is extracted as 8 · 10−11 (90% CL).The hydrogen like muonium atom (M = µ+e−) consists of two leptons from dif-ferent generations. The close confinement of the bound state offers excellent oppor-tunities to explore precisely fundamental electron-muon interaction. The dominantpart of the binding in this system is electromagnetic and can be calculated to veryhigh accuracy in the framework of quantum electromagnetics (QED). Indeed, pre-cision experiments on electromagnetic transitions in muonium have been employedboth to verify bound state QED calculations and for determining most accuratevalues of fundamental constants [1].Since the effects of all known fundamental forces in muonium are calculable verywell, it renders the possibility to search sensitively for yet unknown interactionsbetween both particles. A conversion of muonium into its antiatom (M = µ−e+)would violate additive lepton family number conservation and is not provided instandard theory. However, muonium-antimuonium conversion appears to be natu-ral in many speculative theories, which try to extend the standard model in orderto explain some of its yet not well understood features like parity violation in weakinteraction and particle mass spectra. The interaction could be mediated by a dou-bly charged Higgs boson [2], heavy Majorana neutrinos [3], a neutral scalar [4], e.g.a supersymmetric τ -sneutrino [5] or a dileptonic gauge boson [6].An experiment had been set up to search for spontaneous muonium-antimuoniumconversion at the Paul Scherrer Institute (PSI) in Villigen, Switzerland [7]. Ituses the powerful signature developed in an experiment at the Los Alamos MesonPhysics Facility (LAMPF) USA, which requires the coincident identification of bothconstituents of the antiatom in its decay [8].Muonium atoms were produced by stopping a beam of surface muons in a SiO2powder target, where a fraction of them forms muonium by electron capture, someof which diffuse through the target surface with thermal energies into vacuum.Energetic electrons from the decay of the µ− in the antiatom can be observed in amagnetic spectrometer at 0.1 T magnetic field consisting of five concentric multiwireproportional chambers and a 64 fold segmented hodoscope. The positron in theatomic shell of the antiatom is left behind after the decay with 13.5 eV averagekinetic energy. It can be electrostatically accelerated to 8 keV and guided in amagnetic transport system onto a position sensitive microchannel plate detector(MCP). Annihilation radiation can be observed in a 12 fold segmented pure CsIcalorimeter surrounding the MCP.The muonium production was monitored regularly by reversing all electric andmagnetic fields of the instrument every five hours for a duration of 20 minutes.Targets had to be replaced twice a week because of observed deterioration of muo-nium production on a one week time scale. In the course of the experiment 5.7 ·1010muonium atoms were observed in the interaction volume for antimuonium decays.There was one event which passed all required criteria, i.e. fell into a 99% con-fidence interval of each relevant distribution. The expected background due toaccidental coincidences is 1.7(2) events.The preliminary combination of all data recorded in the experiment between 1993and 1996 [7,9,10] results in an upper limit for the conversion probability in 0.1 Tmagnetic field of PMM≤ 8 ·10−11 (90 % CL). For an assumed effective (V-A)x(V-A)type four fermion interaction this corresponds to an upper limit for the couplingconstant of GMM≤ 3 · 10−3GF(90 % CL), where GF is the weak interaction Fermicoupling constant [9].This new result allows to rule out definitively a certain Z8 model with morethan three particle generations [4] and to set a new lower limit of 2.6 TeV/c2* g3l (g3l depends on model, of order unity) on the mass of a dileptonic gaugeboson in GUT models - well beyond the value extracted from high energy Bhabhascattering [6]. It can be further shown in the framework of minimal left rightsymmetric and supersymmetric models [11] that lepton number violating muondecay (µ+ → e+ + νµ + νe) is not an option for explaining the excess neutrinocounts in the LSND neutrino experiment at Los Alamos [12].This work was supported by the Bundesminister für Bildung und Forschung(BMBF) of Germany, the Schweizer Nationalfond, the Russian Federation for Fun-damental Research and a NATO research grant.-5-2.502.55-20 -10 0 10 20TOF - TOFexpected [ns]Rdca [cm]-5-2.502.55-20 -10 0 10 20TOF - TOFexpected [ns]Rdca [cm]Rdca [cm]FIGURE 1. The distribution of the distance of closest approach (Rdca) between a track froman energetic particle in the magnetic spectrometer and the back projection of the position onthe MCP detector versus the time of flight (TOF) of the atomic shell particle for a muoniummeasurement (left) and for all data recorded in 1996 while searching for antimuonium (right).One single event falls within 3 standard deviations region of the expected TOF and Rdca whichis indicated by the ellipse. The events concentrated at early times and low Rdcacorrespond to abackground signal from the allowed decay µ → 3e + 2ν.REFERENCES1. Hughes V W and zu Putlitz G, in: Quantum Electrodynamics, Singapore, WorldScientific Kinoshita T (ed.) p. 822 (1990); Jungmann K, in: Atomic Physics 14, NewYork, AIP Press, Wineland D et al. (ed.) p. 102 (1994)2. Herczeg P and Mohapatra R N, Phys.Rev.Lett. 69 2475 (1992)3. Halprin A, Phys.Rev.Lett. 48 1313 (1982)4. Hou W S and Wong G G, Phys.Rev. D53 1537 (1996)5. Mohapatra R N, Z.Phys. C56 117 (1992); Halprin A and Masiero A, Phys.Rev.D48,2987 (1993)6. Horrikawa K and Sasaki K, Phys.Rev. D53 560 (1994)7. Abela R et al., Phys.Rev.Lett. 77 1951 (1996)8. Matthias B et al., Phys.Rev.Lett. 66 2716 (1991)9. Schmidt P, doctoral dissertation, University of Heidelberg, unpublished (1997)10. Wirtz H P, doctoral dissertation, University of Zürich, unpublished (1997)11. Herczeg P, conference ”Beyond the Dessert”, Castle Ringberg, Germany (1997)12. Athanassopoulos C et al. Phys.Rev. C54 2685 (1996)

Crossref

Increased sensitivity to possible muonium to antimuonium conversion

   University of GroningenIncreased Sensitivity to Possible Muonium to Antimuonium ConversionMeyer, V.; Grossmann, A.; Jungmann, K.; Merkel, J.; Putlitz, G. zu; Reinhard, I.; Träger, K.;Schmidt, P. V.; Willmann, L.; Engfer, R .Published in:AIP Conference ProceedingsDOI:10.1063/1.54250IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionFinal author's version (accepted by publisher, after peer review)Publication date:1998Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Meyer, V., Grossmann, A., Jungmann, K., Merkel, J., Putlitz, G. Z., Reinhard, I., Träger, K., Schmidt, P. V.,Willmann, L., Engfer, R. ., Wirtz, H. P., Abela, R., Bertl, W., Renker, D., Walter, H. K., Karpuchin, V., Kisel,I., Korenchenko, A., Korenchenko, S., ... Hughes, V. W. (1998). Increased Sensitivity to Possible Muoniumto Antimuonium Conversion. AIP Conference Proceedings, 412(1). https://doi.org/10.1063/1.54250CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 21-06-2022arXiv:hep-ex/9805012v1  14 May 1998Increased Sensitivity to PossibleMuonium to Antimuonium ConversionV. Meyer1, A. Grossmann1, K. Jungmann1, J. Merkel1, G. zuPutlitz1, I. Reinhard1, K. Träger1, P.V. Schmidt1, L. Willmann1, R .Engfer2, H.P. Wirtz2, R. Abela3, W. Bertl3, D. Renker3, H.K.Walter3, V. Karpuchin4, I. Kisel4, A. Korenchenko4, S.Korenchenko4, N. Kravchuk4, N. Kuchinsky4, A. Moiseenko4, J.Bagaturia5, D. Mzavia5, T. Sakhelashvili5, V.W. Hughes61Physikalisches Institut, Universität Heidelberg, D-69120 Heidelberg, Germany; 2Physik Institut,Universität Zürich, CH-8057 Zürich, Switzerland; 3Paul Scherrer Institut, CH-5232 Villigen,Switzerland; 4Joint Institute of Nuclear Research, RU-141980 Dubna, Russia; 5Tbilisi StateUniversity, GUS-380086 Tbilisi, Georgia; 6Physics Department, Yale University, New HavenCT 06520, USAAbstract. A new experimental search for muonium-antimuonium conversion was con-ducted at the Paul Scherrer Institute, Villigen, Switzerland. The preliminary analysisyielded one event fulfilling all required criteria at an expected background of 1.7(2)events due to accidental coincidences. An upper limit for the conversion probability in0.1 T magnetic field is extracted as 8 · 10−11 (90% CL).The hydrogen like muonium atom (M = µ+e−) consists of two leptons from dif-ferent generations. The close confinement of the bound state offers excellent oppor-tunities to explore precisely fundamental electron-muon interaction. The dominantpart of the binding in this system is electromagnetic and can be calculated to veryhigh accuracy in the framework of quantum electromagnetics (QED). Indeed, pre-cision experiments on electromagnetic transitions in muonium have been employedboth to verify bound state QED calculations and for determining most accuratevalues of fundamental constants [1].Since the effects of all known fundamental forces in muonium are calculable verywell, it renders the possibility to search sensitively for yet unknown interactionsbetween both particles. A conversion of muonium into its antiatom (M = µ−e+)would violate additive lepton family number conservation and is not provided instandard theory. However, muonium-antimuonium conversion appears to be natu-ral in many speculative theories, which try to extend the standard model in orderto explain some of its yet not well understood features like parity violation in weakinteraction and particle mass spectra. The interaction could be mediated by a dou-bly charged Higgs boson [2], heavy Majorana neutrinos [3], a neutral scalar [4], e.g.a supersymmetric τ -sneutrino [5] or a dileptonic gauge boson [6].An experiment had been set up to search for spontaneous muonium-antimuoniumconversion at the Paul Scherrer Institute (PSI) in Villigen, Switzerland [7]. Ituses the powerful signature developed in an experiment at the Los Alamos MesonPhysics Facility (LAMPF) USA, which requires the coincident identification of bothconstituents of the antiatom in its decay [8].Muonium atoms were produced by stopping a beam of surface muons in a SiO2powder target, where a fraction of them forms muonium by electron capture, someof which diffuse through the target surface with thermal energies into vacuum.Energetic electrons from the decay of the µ− in the antiatom can be observed in amagnetic spectrometer at 0.1 T magnetic field consisting of five concentric multiwireproportional chambers and a 64 fold segmented hodoscope. The positron in theatomic shell of the antiatom is left behind after the decay with 13.5 eV averagekinetic energy. It can be electrostatically accelerated to 8 keV and guided in amagnetic transport system onto a position sensitive microchannel plate detector(MCP). Annihilation radiation can be observed in a 12 fold segmented pure CsIcalorimeter surrounding the MCP.The muonium production was monitored regularly by reversing all electric andmagnetic fields of the instrument every five hours for a duration of 20 minutes.Targets had to be replaced twice a week because of observed deterioration of muo-nium production on a one week time scale. In the course of the experiment 5.7 ·1010muonium atoms were observed in the interaction volume for antimuonium decays.There was one event which passed all required criteria, i.e. fell into a 99% con-fidence interval of each relevant distribution. The expected background due toaccidental coincidences is 1.7(2) events.The preliminary combination of all data recorded in the experiment between 1993and 1996 [7,9,10] results in an upper limit for the conversion probability in 0.1 Tmagnetic field of PMM≤ 8 ·10−11 (90 % CL). For an assumed effective (V-A)x(V-A)type four fermion interaction this corresponds to an upper limit for the couplingconstant of GMM≤ 3 · 10−3GF(90 % CL), where GF is the weak interaction Fermicoupling constant [9].This new result allows to rule out definitively a certain Z8 model with morethan three particle generations [4] and to set a new lower limit of 2.6 TeV/c2* g3l (g3l depends on model, of order unity) on the mass of a dileptonic gaugeboson in GUT models - well beyond the value extracted from high energy Bhabhascattering [6]. It can be further shown in the framework of minimal left rightsymmetric and supersymmetric models [11] that lepton number violating muondecay (µ+ → e+ + νµ + νe) is not an option for explaining the excess neutrinocounts in the LSND neutrino experiment at Los Alamos [12].This work was supported by the Bundesminister für Bildung und Forschung(BMBF) of Germany, the Schweizer Nationalfond, the Russian Federation for Fun-damental Research and a NATO research grant.-5-2.502.55-20 -10 0 10 20TOF - TOFexpected [ns]Rdca [cm]-5-2.502.55-20 -10 0 10 20TOF - TOFexpected [ns]Rdca [cm]Rdca [cm]FIGURE 1. The distribution of the distance of closest approach (Rdca) between a track froman energetic particle in the magnetic spectrometer and the back projection of the position onthe MCP detector versus the time of flight (TOF) of the atomic shell particle for a muoniummeasurement (left) and for all data recorded in 1996 while searching for antimuonium (right).One single event falls within 3 standard deviations region of the expected TOF and Rdca whichis indicated by the ellipse. The events concentrated at early times and low Rdcacorrespond to abackground signal from the allowed decay µ → 3e + 2ν.REFERENCES1. Hughes V W and zu Putlitz G, in: Quantum Electrodynamics, Singapore, WorldScientific Kinoshita T (ed.) p. 822 (1990); Jungmann K, in: Atomic Physics 14, NewYork, AIP Press, Wineland D et al. (ed.) p. 102 (1994)2. Herczeg P and Mohapatra R N, Phys.Rev.Lett. 69 2475 (1992)3. Halprin A, Phys.Rev.Lett. 48 1313 (1982)4. Hou W S and Wong G G, Phys.Rev. D53 1537 (1996)5. Mohapatra R N, Z.Phys. C56 117 (1992); Halprin A and Masiero A, Phys.Rev.D48,2987 (1993)6. Horrikawa K and Sasaki K, Phys.Rev. D53 560 (1994)7. Abela R et al., Phys.Rev.Lett. 77 1951 (1996)8. Matthias B et al., Phys.Rev.Lett. 66 2716 (1991)9. Schmidt P, doctoral dissertation, University of Heidelberg, unpublished (1997)10. Wirtz H P, doctoral dissertation, University of Zürich, unpublished (1997)11. Herczeg P, conference ”Beyond the Dessert”, Castle Ringberg, Germany (1997)12. Athanassopoulos C et al. Phys.Rev. C54 2685 (1996)

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   University of GroningenIncreased Sensitivity to Possible Muonium to Antimuonium ConversionMeyer, V.; Grossmann, A.; Jungmann, K.; Merkel, J.; Putlitz, G. zu; Reinhard, I.; Träger, K.;Schmidt, P. V.; Willmann, L.; Engfer, R .Published in:AIP Conference ProceedingsDOI:10.1063/1.54250IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionFinal author's version (accepted by publisher, after peer review)Publication date:1998Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Meyer, V., Grossmann, A., Jungmann, K., Merkel, J., Putlitz, G. Z., Reinhard, I., Träger, K., Schmidt, P. V.,Willmann, L., Engfer, R. ., Wirtz, H. P., Abela, R., Bertl, W., Renker, D., Walter, H. K., Karpuchin, V., Kisel,I., Korenchenko, A., Korenchenko, S., ... Hughes, V. W. (1998). Increased Sensitivity to Possible Muoniumto Antimuonium Conversion. AIP Conference Proceedings, 412(1). https://doi.org/10.1063/1.54250CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 26-12-2020arXiv:hep-ex/9805012v1  14 May 1998Increased Sensitivity to PossibleMuonium to Antimuonium ConversionV. Meyer1, A. Grossmann1, K. Jungmann1, J. Merkel1, G. zuPutlitz1, I. Reinhard1, K. Träger1, P.V. Schmidt1, L. Willmann1, R .Engfer2, H.P. Wirtz2, R. Abela3, W. Bertl3, D. Renker3, H.K.Walter3, V. Karpuchin4, I. Kisel4, A. Korenchenko4, S.Korenchenko4, N. Kravchuk4, N. Kuchinsky4, A. Moiseenko4, J.Bagaturia5, D. Mzavia5, T. Sakhelashvili5, V.W. Hughes61Physikalisches Institut, Universität Heidelberg, D-69120 Heidelberg, Germany; 2Physik Institut,Universität Zürich, CH-8057 Zürich, Switzerland; 3Paul Scherrer Institut, CH-5232 Villigen,Switzerland; 4Joint Institute of Nuclear Research, RU-141980 Dubna, Russia; 5Tbilisi StateUniversity, GUS-380086 Tbilisi, Georgia; 6Physics Department, Yale University, New HavenCT 06520, USAAbstract. A new experimental search for muonium-antimuonium conversion was con-ducted at the Paul Scherrer Institute, Villigen, Switzerland. The preliminary analysisyielded one event fulfilling all required criteria at an expected background of 1.7(2)events due to accidental coincidences. An upper limit for the conversion probability in0.1 T magnetic field is extracted as 8 · 10−11 (90% CL).The hydrogen like muonium atom (M = µ+e−) consists of two leptons from dif-ferent generations. The close confinement of the bound state offers excellent oppor-tunities to explore precisely fundamental electron-muon interaction. The dominantpart of the binding in this system is electromagnetic and can be calculated to veryhigh accuracy in the framework of quantum electromagnetics (QED). Indeed, pre-cision experiments on electromagnetic transitions in muonium have been employedboth to verify bound state QED calculations and for determining most accuratevalues of fundamental constants [1].Since the effects of all known fundamental forces in muonium are calculable verywell, it renders the possibility to search sensitively for yet unknown interactionsbetween both particles. A conversion of muonium into its antiatom (M = µ−e+)would violate additive lepton family number conservation and is not provided instandard theory. However, muonium-antimuonium conversion appears to be natu-ral in many speculative theories, which try to extend the standard model in orderto explain some of its yet not well understood features like parity violation in weakinteraction and particle mass spectra. The interaction could be mediated by a dou-bly charged Higgs boson [2], heavy Majorana neutrinos [3], a neutral scalar [4], e.g.a supersymmetric τ -sneutrino [5] or a dileptonic gauge boson [6].An experiment had been set up to search for spontaneous muonium-antimuoniumconversion at the Paul Scherrer Institute (PSI) in Villigen, Switzerland [7]. Ituses the powerful signature developed in an experiment at the Los Alamos MesonPhysics Facility (LAMPF) USA, which requires the coincident identification of bothconstituents of the antiatom in its decay [8].Muonium atoms were produced by stopping a beam of surface muons in a SiO2powder target, where a fraction of them forms muonium by electron capture, someof which diffuse through the target surface with thermal energies into vacuum.Energetic electrons from the decay of the µ− in the antiatom can be observed in amagnetic spectrometer at 0.1 T magnetic field consisting of five concentric multiwireproportional chambers and a 64 fold segmented hodoscope. The positron in theatomic shell of the antiatom is left behind after the decay with 13.5 eV averagekinetic energy. It can be electrostatically accelerated to 8 keV and guided in amagnetic transport system onto a position sensitive microchannel plate detector(MCP). Annihilation radiation can be observed in a 12 fold segmented pure CsIcalorimeter surrounding the MCP.The muonium production was monitored regularly by reversing all electric andmagnetic fields of the instrument every five hours for a duration of 20 minutes.Targets had to be replaced twice a week because of observed deterioration of muo-nium production on a one week time scale. In the course of the experiment 5.7 ·1010muonium atoms were observed in the interaction volume for antimuonium decays.There was one event which passed all required criteria, i.e. fell into a 99% con-fidence interval of each relevant distribution. The expected background due toaccidental coincidences is 1.7(2) events.The preliminary combination of all data recorded in the experiment between 1993and 1996 [7,9,10] results in an upper limit for the conversion probability in 0.1 Tmagnetic field of PMM≤ 8 ·10−11 (90 % CL). For an assumed effective (V-A)x(V-A)type four fermion interaction this corresponds to an upper limit for the couplingconstant of GMM≤ 3 · 10−3GF(90 % CL), where GF is the weak interaction Fermicoupling constant [9].This new result allows to rule out definitively a certain Z8 model with morethan three particle generations [4] and to set a new lower limit of 2.6 TeV/c2* g3l (g3l depends on model, of order unity) on the mass of a dileptonic gaugeboson in GUT models - well beyond the value extracted from high energy Bhabhascattering [6]. It can be further shown in the framework of minimal left rightsymmetric and supersymmetric models [11] that lepton number violating muondecay (µ+ → e+ + νµ + νe) is not an option for explaining the excess neutrinocounts in the LSND neutrino experiment at Los Alamos [12].This work was supported by the Bundesminister für Bildung und Forschung(BMBF) of Germany, the Schweizer Nationalfond, the Russian Federation for Fun-damental Research and a NATO research grant.-5-2.502.55-20 -10 0 10 20TOF - TOFexpected [ns]Rdca [cm]-5-2.502.55-20 -10 0 10 20TOF - TOFexpected [ns]Rdca [cm]Rdca [cm]FIGURE 1. The distribution of the distance of closest approach (Rdca) between a track froman energetic particle in the magnetic spectrometer and the back projection of the position onthe MCP detector versus the time of flight (TOF) of the atomic shell particle for a muoniummeasurement (left) and for all data recorded in 1996 while searching for antimuonium (right).One single event falls within 3 standard deviations region of the expected TOF and Rdca whichis indicated by the ellipse. The events concentrated at early times and low Rdcacorrespond to abackground signal from the allowed decay µ → 3e + 2ν.REFERENCES1. Hughes V W and zu Putlitz G, in: Quantum Electrodynamics, Singapore, WorldScientific Kinoshita T (ed.) p. 822 (1990); Jungmann K, in: Atomic Physics 14, NewYork, AIP Press, Wineland D et al. (ed.) p. 102 (1994)2. Herczeg P and Mohapatra R N, Phys.Rev.Lett. 69 2475 (1992)3. Halprin A, Phys.Rev.Lett. 48 1313 (1982)4. Hou W S and Wong G G, Phys.Rev. D53 1537 (1996)5. Mohapatra R N, Z.Phys. C56 117 (1992); Halprin A and Masiero A, Phys.Rev.D48,2987 (1993)6. Horrikawa K and Sasaki K, Phys.Rev. D53 560 (1994)7. Abela R et al., Phys.Rev.Lett. 77 1951 (1996)8. Matthias B et al., Phys.Rev.Lett. 66 2716 (1991)9. Schmidt P, doctoral dissertation, University of Heidelberg, unpublished (1997)10. Wirtz H P, doctoral dissertation, University of Zürich, unpublished (1997)11. Herczeg P, conference ”Beyond the Dessert”, Castle Ringberg, Germany (1997)12. Athanassopoulos C et al. Phys.Rev. C54 2685 (1996)

Increased Sensitivity to Possible Muonium to Antimuonium Conversion

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