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A dedicated calibration tool for the MEG and MEG II positron spectrometer
Nuclear Instruments and Methods in Physics Research A 824 (2016) 575–577
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A dedicated calibration tool for the MEG and MEG II positron spectrometer G. Rutar a,b,n, C. Bemporad c,d, P.W. Cattaneo e, F. Cei c,d, L. Galli c,d, P.-R. Kettle a, A. Papa a a
Paul Scherrer Institut PSI, CH-5232 Villigen, Switzerland Swiss Federal Institute of Technology ETH, CH-8093 Zürich, Switzerland c INFN Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy d Pisa, Dipartimento di Fisica dell'Università, Largo B. Pontecorvo 3, 56127 Pisa, Italy e INFN Sezione di Pavia, Via Bassi 6, 27100 Pavia, Italy b
art ic l e i nf o
a b s t r a c t
Available online 30 November 2015
The MEG experiment has set the latest limit of 5:7 10 13 (90 % C.L.) on the branching ratio of the charged lepton flavor violating decay μ þ -e þ γ , making use of the most intense continuous surface muon beam in the world at the Paul Scherrer Institut (PSI), Villigen, Switzerland. High resolutions in terms of energy, timing and relative opening angle are needed in the detection of the e þ and gamma, requiring careful calibration and monitoring of the experimental apparatus. A dedicated calibration method involving Mott scattering of a monochromatic positron beam at energies close to the MEG signal energy is presented. & 2015 Elsevier B.V. All rights reserved.
Keywords: Calibration methods Positron beam Spectrometer
1. Introduction The MEG experiment [1,2] has been looking for the μ þ -e þ γ decay, for which the Standard Model predicts an immeasurably small branching ratio of Oð10 54 Þ. Any observation of this decay would be clear evidence for new physics. For muon decay occurring at rest, such an event is characterized by a positron and a gamma being emitted time-coincidently in a back-to-back topology and each carrying away an energy equal to Ee þ ¼ Eγ ¼ Eμ =2 ¼ 52:8 MeV. The spectrometer to measure the positron variables comprises of three components: (1) A drift chamber system together with (2) a gradient magnetic field solenoid to measure the momentum and the e þ vertex on the target, and (3) a scintillation timing counter which provides the e þ time measurement as well as being used for trigger purposes. For a high-sensitivity experiment such as MEG it is imperative to monitor and calibrate its detectors [2]. Long-established calibration tools for the positron spectrometer consist of repeated optical surveys of the drift chamber system and the target, dedicated cosmic ray runs prior to the start of the physics run and the usage of positrons stemming from Michel decays μ þ -e þ νe ν μ , which are abundantly collected during physics data taking. Cosmic n Corresponding author at: Paul Scherrer Institut PSI, CH-5232 Villigen, Switzerland. E-mail address: [email protected] (G. Rutar).
http://dx.doi.org/10.1016/j.nima.2015.11.121 0168-9002/& 2015 Elsevier B.V. All rights reserved.
ray muons, having comparatively large energies, feature straight tracks in the spectrometer and so allow for the alignment of the drift chamber system. However, the MEG spectrometer is optimized for curved trajectories of positive particles with momenta between ca. 40 and 55 MeV/c. Michel positrons provide such curved trajectories, but they are characterized by a broad energy spectrum with a sharp edge around Eμ =2 ¼ 52:8 MeV. The dedicated calibration tool described in the following utilizes a positron beam tuned to a central momentum around 52 MeV/c undergoing Mott scattering, providing positrons with a monochromatic energy spectrum and tracks which resemble the ones exhibited by the signal positron from μ þ -e þ γ .
2. The mott scattering calibration method The πE5 channel at PSI is able to deliver Oð108 Þ μ þ =s of 28 MeV/c momentum. In fact, the beam contains not only muons, but also eight-times as many positrons which are usually eliminated by means of a Wien filter during the MEG physics data taking [2]. Once having selected beam positrons and having tuned the central momentum to around 52 MeV/c (rejecting beamcorrelated backgrounds with the help of the 50 MHz RF-signal of the proton cyclotron) these positrons hit the MEG target (ca. 200 μm of polyethylene at a slant angle of θ 201) and undergo Mott scattering. The Mott cross section is well-known and features a strong dependence on the scattering angle θ [3]. Moreover, it is not sensitive to the e þ polarization. In our case, the outgoing
576
G. Rutar et al. / Nuclear Instruments and Methods in Physics Research A 824 (2016) 575–577
Fig. 2. The double turn method. Each of the two turns is treated as an independent track and propagated to the target plane, where the difference of the observable of interest is computed.
Fig. 1. Energy spectrum (“monochromatic line”) of the Mott scattered positrons (error bars not visible). The total line width measured in the data amounts to σ tot 620 keV=c(RMS).
momentum of the positron is approximately equal to the initial momentum, hence the energy spectrum of the scattered positrons is almost monochromatic, see Fig. 1. The contributions to the energy line's width are twofold: There is (1) a contribution σMott due to the finite detector resolutions, the non-zero target thickness and the fact that the outgoing momentum varies slightly as a function of the scattering angle θ (contribution intrinsic to the method) and (2) a contribution σbeam which is associated to the momentum spread in the positron beam. Accordingly, the total width σtot of the Mott energy line is given by σ 2tot ¼ σ 2Mott þ σ 2beam . From the Monte Carlo simulation we estimate σ Mott 340 keV=c and σ beam 520keV=c (RMS), resulting in a total line width of σ tot 620 keV=c as measured in the data (in the current setup, no direct measurement of σbeam is implemented).
3. Extraction of spectrometer resolutions from double turn tracks How can one extract the spectrometer resolutions while getting rid of any contribution coming from the beam momentum spread? The solution is to use so-called “double turn tracks”, i.e. tracks which feature two turns in the drift chamber system. Each turn is treated as an independent track and propagated towards the target plane (see Fig. 2), where the difference of the observable of interest (e.g. the momentum p) is computed (Δp ¼ pturn2 pturn1 ). Applying this procedure to all available double turn tracks results in a double Gaussian distribution for every observable of interest, namely the momentum, the two angular variables θ and ϕ and the vertex coordinates (x,y) on the target. The standard deviations for the core and the tail fraction of this double Gaussian are considered to be the core and the tail resolutions, respectively. The same method is applied to Michel positrons, requiring a positron energy of 4 50 MeV. For comparison, the momentum resolution obtained with Mott 2012 data amounts to 29075 (stat) keV/c, which is consistent with Michel data.
4. Drift Chamber Alignment The Mott cross section depends on the polar angle θ, but not on the azimuthal angle ϕ (the z-axis points in the direction of the beam). This means that the reconstructed central momentum p^ of
Fig. 3. Reconstructed central momentum p^ vs. ϕ (error bars not visible). Nonaligned drift chambers show a strong dependence of p^ as a function of ϕ.
the positron beam as a function of ϕ should be flat, providing that the drift chambers are aligned. In Fig. 3, chamber displacements of the aligned and non-aligned drift chambers (prior to alignment) ranging up to 1.5 mm give rise to large alterations in the behavior ^ ϕ-curve. This kind of check is also useful when one of the p-vs.wants to compare alignments obtained using different methods. In addition it is possible to perform the drift chamber alignment by applying an iterative procedure which minimizes the residuals of the reconstructed tracks and the actual drift chamber hits left by the positrons up to the point where the average residuals for every chamber drop below 50 μm. The same procedure is performed on Michel positron data, the results are consistent with Mott data.
5. Conclusion The Mott scattering spectrometer calibration method involving a positron beam tuned to the approximate μ þ -e þ γ signal positron energy of 52 MeV/c has proven to be a powerful and independent tool to determine the resolutions of the positron spectrometer, check the alignment of the spectrometer's drift chamber system and to perform the drift chamber alignment.
G. Rutar et al. / Nuclear Instruments and Methods in Physics Research A 824 (2016) 575–577
Moreover, for what concerns the upgraded version of the experiment MEG II [4], the Mott scattering calibration method will constitute an important tool not only to calibrate, but also to explore the new spectrometer system, which will present an even higher degree of complexity.
Acknowledgements We thank PSI as the host laboratory and INFN and PSI for the financial support.
References [1] [2] [3] [4]
J. Adam, et al., Phys. Rev. Lett. 110 (20) (2013) 201801. J. Adam, et al., Eur. Phys. J 4 (C73) (2013) 2365. R. Hofstadter, Rev. Mod. Phys. 28 (3) (1956) 214. A.M. Baldini et al., MEG Upgrade Proposal, arXiv:1301.7225.
577
Contents lists available at ScienceDirect
Nuclear Instruments and Methods in Physics Research A journal homepage: www.elsevier.com/locate/nima
A dedicated calibration tool for the MEG and MEG II positron spectrometer G. Rutar a,b,n, C. Bemporad c,d, P.W. Cattaneo e, F. Cei c,d, L. Galli c,d, P.-R. Kettle a, A. Papa a a
Paul Scherrer Institut PSI, CH-5232 Villigen, Switzerland Swiss Federal Institute of Technology ETH, CH-8093 Zürich, Switzerland c INFN Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italy d Pisa, Dipartimento di Fisica dell'Università, Largo B. Pontecorvo 3, 56127 Pisa, Italy e INFN Sezione di Pavia, Via Bassi 6, 27100 Pavia, Italy b
art ic l e i nf o
a b s t r a c t
Available online 30 November 2015
The MEG experiment has set the latest limit of 5:7 10 13 (90 % C.L.) on the branching ratio of the charged lepton flavor violating decay μ þ -e þ γ , making use of the most intense continuous surface muon beam in the world at the Paul Scherrer Institut (PSI), Villigen, Switzerland. High resolutions in terms of energy, timing and relative opening angle are needed in the detection of the e þ and gamma, requiring careful calibration and monitoring of the experimental apparatus. A dedicated calibration method involving Mott scattering of a monochromatic positron beam at energies close to the MEG signal energy is presented. & 2015 Elsevier B.V. All rights reserved.
Keywords: Calibration methods Positron beam Spectrometer
1. Introduction The MEG experiment [1,2] has been looking for the μ þ -e þ γ decay, for which the Standard Model predicts an immeasurably small branching ratio of Oð10 54 Þ. Any observation of this decay would be clear evidence for new physics. For muon decay occurring at rest, such an event is characterized by a positron and a gamma being emitted time-coincidently in a back-to-back topology and each carrying away an energy equal to Ee þ ¼ Eγ ¼ Eμ =2 ¼ 52:8 MeV. The spectrometer to measure the positron variables comprises of three components: (1) A drift chamber system together with (2) a gradient magnetic field solenoid to measure the momentum and the e þ vertex on the target, and (3) a scintillation timing counter which provides the e þ time measurement as well as being used for trigger purposes. For a high-sensitivity experiment such as MEG it is imperative to monitor and calibrate its detectors [2]. Long-established calibration tools for the positron spectrometer consist of repeated optical surveys of the drift chamber system and the target, dedicated cosmic ray runs prior to the start of the physics run and the usage of positrons stemming from Michel decays μ þ -e þ νe ν μ , which are abundantly collected during physics data taking. Cosmic n Corresponding author at: Paul Scherrer Institut PSI, CH-5232 Villigen, Switzerland. E-mail address: [email protected] (G. Rutar).
http://dx.doi.org/10.1016/j.nima.2015.11.121 0168-9002/& 2015 Elsevier B.V. All rights reserved.
ray muons, having comparatively large energies, feature straight tracks in the spectrometer and so allow for the alignment of the drift chamber system. However, the MEG spectrometer is optimized for curved trajectories of positive particles with momenta between ca. 40 and 55 MeV/c. Michel positrons provide such curved trajectories, but they are characterized by a broad energy spectrum with a sharp edge around Eμ =2 ¼ 52:8 MeV. The dedicated calibration tool described in the following utilizes a positron beam tuned to a central momentum around 52 MeV/c undergoing Mott scattering, providing positrons with a monochromatic energy spectrum and tracks which resemble the ones exhibited by the signal positron from μ þ -e þ γ .
2. The mott scattering calibration method The πE5 channel at PSI is able to deliver Oð108 Þ μ þ =s of 28 MeV/c momentum. In fact, the beam contains not only muons, but also eight-times as many positrons which are usually eliminated by means of a Wien filter during the MEG physics data taking [2]. Once having selected beam positrons and having tuned the central momentum to around 52 MeV/c (rejecting beamcorrelated backgrounds with the help of the 50 MHz RF-signal of the proton cyclotron) these positrons hit the MEG target (ca. 200 μm of polyethylene at a slant angle of θ 201) and undergo Mott scattering. The Mott cross section is well-known and features a strong dependence on the scattering angle θ [3]. Moreover, it is not sensitive to the e þ polarization. In our case, the outgoing
576
G. Rutar et al. / Nuclear Instruments and Methods in Physics Research A 824 (2016) 575–577
Fig. 2. The double turn method. Each of the two turns is treated as an independent track and propagated to the target plane, where the difference of the observable of interest is computed.
Fig. 1. Energy spectrum (“monochromatic line”) of the Mott scattered positrons (error bars not visible). The total line width measured in the data amounts to σ tot 620 keV=c(RMS).
momentum of the positron is approximately equal to the initial momentum, hence the energy spectrum of the scattered positrons is almost monochromatic, see Fig. 1. The contributions to the energy line's width are twofold: There is (1) a contribution σMott due to the finite detector resolutions, the non-zero target thickness and the fact that the outgoing momentum varies slightly as a function of the scattering angle θ (contribution intrinsic to the method) and (2) a contribution σbeam which is associated to the momentum spread in the positron beam. Accordingly, the total width σtot of the Mott energy line is given by σ 2tot ¼ σ 2Mott þ σ 2beam . From the Monte Carlo simulation we estimate σ Mott 340 keV=c and σ beam 520keV=c (RMS), resulting in a total line width of σ tot 620 keV=c as measured in the data (in the current setup, no direct measurement of σbeam is implemented).
3. Extraction of spectrometer resolutions from double turn tracks How can one extract the spectrometer resolutions while getting rid of any contribution coming from the beam momentum spread? The solution is to use so-called “double turn tracks”, i.e. tracks which feature two turns in the drift chamber system. Each turn is treated as an independent track and propagated towards the target plane (see Fig. 2), where the difference of the observable of interest (e.g. the momentum p) is computed (Δp ¼ pturn2 pturn1 ). Applying this procedure to all available double turn tracks results in a double Gaussian distribution for every observable of interest, namely the momentum, the two angular variables θ and ϕ and the vertex coordinates (x,y) on the target. The standard deviations for the core and the tail fraction of this double Gaussian are considered to be the core and the tail resolutions, respectively. The same method is applied to Michel positrons, requiring a positron energy of 4 50 MeV. For comparison, the momentum resolution obtained with Mott 2012 data amounts to 29075 (stat) keV/c, which is consistent with Michel data.
4. Drift Chamber Alignment The Mott cross section depends on the polar angle θ, but not on the azimuthal angle ϕ (the z-axis points in the direction of the beam). This means that the reconstructed central momentum p^ of
Fig. 3. Reconstructed central momentum p^ vs. ϕ (error bars not visible). Nonaligned drift chambers show a strong dependence of p^ as a function of ϕ.
the positron beam as a function of ϕ should be flat, providing that the drift chambers are aligned. In Fig. 3, chamber displacements of the aligned and non-aligned drift chambers (prior to alignment) ranging up to 1.5 mm give rise to large alterations in the behavior ^ ϕ-curve. This kind of check is also useful when one of the p-vs.wants to compare alignments obtained using different methods. In addition it is possible to perform the drift chamber alignment by applying an iterative procedure which minimizes the residuals of the reconstructed tracks and the actual drift chamber hits left by the positrons up to the point where the average residuals for every chamber drop below 50 μm. The same procedure is performed on Michel positron data, the results are consistent with Mott data.
5. Conclusion The Mott scattering spectrometer calibration method involving a positron beam tuned to the approximate μ þ -e þ γ signal positron energy of 52 MeV/c has proven to be a powerful and independent tool to determine the resolutions of the positron spectrometer, check the alignment of the spectrometer's drift chamber system and to perform the drift chamber alignment.
G. Rutar et al. / Nuclear Instruments and Methods in Physics Research A 824 (2016) 575–577
Moreover, for what concerns the upgraded version of the experiment MEG II [4], the Mott scattering calibration method will constitute an important tool not only to calibrate, but also to explore the new spectrometer system, which will present an even higher degree of complexity.
Acknowledgements We thank PSI as the host laboratory and INFN and PSI for the financial support.
References [1] [2] [3] [4]
J. Adam, et al., Phys. Rev. Lett. 110 (20) (2013) 201801. J. Adam, et al., Eur. Phys. J 4 (C73) (2013) 2365. R. Hofstadter, Rev. Mod. Phys. 28 (3) (1956) 214. A.M. Baldini et al., MEG Upgrade Proposal, arXiv:1301.7225.
577