A. Papa

P. Schwendimann

English

Archivio della Ricerca - Università di Pisa

Nuclear Inst. and Methods in Physics Research, A 936 (2019) 268–270Contents lists available at ScienceDirectNuclear Inst. and Methods in Physics Research, Ajournal homepage: www.elsevier.com/locate/nimaThe mott based double turn method for the MEG II spectrometercharacterisationA. Papa a,b, P. Schwendimann b,c,∗a Dipartimento di Fisica ‘‘Enrico Fermi’’ University of Pisa and INFN Sezione di Pisa, Largo B. Pontecorvo 3, 56127 Pisa, Italyb Paul Scherrer Institute, 5232 Villigen, Switzerlandc Swiss Federal Institute of Technology ETH, Rämistrasse 101, 8092 Zurich, SwitzerlandA R T I C L E I N F OKeywords:Drift chamber characterisationMEG IIA B S T R A C TThe MEG experiment has set the latest limit of 4.2 × 10−13 at 90% Confidence Level (C.L.) to the branching ratioof the charged Lepton Flavour Violating (cLFV) decay 𝜇+ → 𝑒+𝛾. The MEG II experiment will be more sensitiveby an order of magnitude. High resolutions in terms of the kinematic variables used to identify a 𝜇+ → 𝑒+𝛾 eventare mandatory among other aspects to achieve this sensitivity. Thus careful calibration and monitoring of theexperimental apparatus are required.One of the dedicated calibration methods involves the Mott scattering process of a mono-energetic positronbeam on the MEG II target. As a result positrons with energies around the MEG II signal energy get scatteredinto the drift chamber. The calibration and extraction of resolution can then be done by the double turn methodpresented in this article.Contents1. Introduction .................................................................................................................................................................................................... 2682. The Mott calibration......................................................................................................................................................................................... 2683. The double turn algorithm ................................................................................................................................................................................ 2694. Results of the double turn method...................................................................................................................................................................... 2695. Conclusion ...................................................................................................................................................................................................... 269Acknowledgements .......................................................................................................................................................................................... 269References....................................................................................................................................................................................................... 2701. IntroductionThe MEG II experiment is planned to look for the charged leptonflavour violating decay 𝜇+ → 𝑒+𝛾 with a branching ratio sensitivity of6 × 10−14 [1]. Today’s upper limit was set by the predecessor experimentMEG to 4.2 × 10−13 at 90% C.L.[2].The experiment uses a liquid Xenon calorimeter for the photondetection and a cylindric drift chamber (CDCH) in combination with adedicated timing counter to detect the positrons. A detailed descriptionof the experimental setup can be found in [1]. More information aboutthe CDCH are found in [1,3].The expected signal (Fig. 1a) is characterised by a photon and apositron of 52.8MeV energy each in back to back geometry and incoincidence. A limitation in the sensitivity to this decay channel comesfrom the tremendous amount of background. It consists of radiativemuon decay (RMD, 𝜇+ → 𝑒+𝜈𝑒𝜈𝜇𝛾) events where the two neutrinos∗ Corresponding author at: Paul Scherrer Institute, 5232 Villigen, Switzerland.E-mail addresses: angela.papa@psi.ch (A. Papa), patrick.schwendimann@psi.ch (P. Schwendimann).carry hardly any energy (Fig. 1b) and of positrons emitted at thekinematic endpoint of the Michel decay coinciding with a photon from adifferent source like another RMD or annihilation in flight of a positron(Fig. 1c).2. The Mott calibrationFor the signal sensitivity and background rejection good calibrationsand characterisations of the entire experimental apparatus are needed.One of the available calibration methods is based on a monochromaticpositron beam at the expected signal energy. Due to Mott scattering onthe stopping target, a fraction of the positrons gets scattered into theMEG II spectrometer.By comparing Mott Monte Carlo truth and reconstructed values,one obtains a resolution of 82.4(7) keV in positron energy, 4.71(4)mradhttps://doi.org/10.1016/j.nima.2018.09.137Received 29 June 2018; Received in revised form 17 September 2018; Accepted 28 September 2018Available online 3 October 20180168-9002/© 2018 Elsevier B.V. All rights reserved.A. Papa and P. Schwendimann Nuclear Inst. and Methods in Physics Research, A 936 (2019) 268–270Fig. 1. Different event types.Fig. 2. Basic principle of the double turn algorithm.in polar angle (w.r.t. beam axis) and 4.05(4)mrad in azimuth angle.Software used is described in [4–6].3. The double turn algorithmTo perform a similar calibration on the real detector, one pickspositron tracks with at least two turns. The two parts are analysedseparately and the positron momentum 𝑝𝑖, polar angle 𝛩𝑖 and azimuthangle 𝛷𝑖 are extracted for each part as shown in Fig. 2.As both parts originate from the same positron, they have the sameunderlying kinematic variables. Thus the statistic deviations have theirorigin in the limited resolution in given positron variables. As a resultthe width of the distribution of the differences 𝛥𝑝, 𝛥𝛩 and 𝛥𝛷 containsthe information about the intrinsic resolution of the detector.A similar technique has already been in use during the MEG experi-ment [5,7]. The method was adjusted to the MEG II geometry and wasapplied to various sets of MC simulations.4. Results of the double turn methodIn a first set of studies, the double turn algorithm has been applied todifferent kind of events. The 𝛥𝑝 results are shown in Fig. 3a. Mott events(orange) exhibit a very similar distribution compared to signal events(green). Distributions based on Michel events (violet) are broader.By selecting only Michel events reconstructed close to the kinematicendpoint above 50MeV, the distribution of the differences approachesthe obtained distributions for Mott and signal events. This, however,happens at the cost of a significantly lowered event rate.In Fig. 3b the obtained 𝛥𝑝 distributions for different initial momen-tum beam spread are shown. The results are the same up to statisticalfluctuations. This underlines the advantage of the double turn method:As it is comparing estimated values of the same event, uncertainties dueto the beam quality and details of the scattering process cancel eachother out. What remains is the information about the intrinsic detectorresolution.Comparing the standard deviation from the double turn algorithmwith the results obtained from comparing the MC truth values, a factorof 1.5 to 1.6 can be observed. This deviation is expected as the doubleturn method is based on the difference of two estimates based on fittingthe two parts of a track. The MC truth values are based on the trueFig. 3. 𝛥𝑝 distributions obtained with the double turn algorithm. Standard deviationsrefer to the central peak. (For interpretation of the references to colour in this figurelegend, the reader is referred to the web version of this article.)value at the beginning of the simulation and just one estimate based ona whole track that usually consists of one or one and a half turns. Forthe angular variables very similar results are obtained.5. ConclusionMC simulations show that the double turn method is able to ex-tract the intrinsic CDCH resolutions in terms of the positron variablesmomentum, polar angle and azimuth for MEG II. The use of Mottscattered positrons around the signal energy provides results closest tothe expected behaviour of signal events.The double turn method relies only on reconstructed values ac-cessible during the experiment. The deviations observed between theresolutions obtained from MC truth values and the estimated standarddeviations are as expected and can be explained by the differences inthe two methods.AcknowledgementsThe authors thank the Paul Scherrer Institute in Villigen for hostingthe research facilities. Grants: SNF, Switzerland n. 200020_172706,MIUR, Italy Montalcini D.M. 2014 n. 975.269A. Papa and P. Schwendimann Nuclear Inst. and Methods in Physics Research, A 936 (2019) 268–270References[1] (MEG II Collaboration), A.M. Baldini, et al., Eur. Phys. J. C 78 (2018) 380.[2] (MEG Collaboration), A.M. Baldini, et al., Eur. Phys. J. C 76 (2016) 434.[3] Marco Chiappini, these proceedings.[4] (Geant4 Collaboration), S. Agostinelli, et al., NIMA 506 (2003) 250.[5] (MEG Collaboration), J. Adam, et al., Eur. Phys. J. C 73 (2013) 2365.[6] P.W. Cattaneo, et al., Eur. Phys. J. Plus 126 (2011) 60.[7] G. Rutar, et al., NIMA 824 (2016) 575;G. Rutar, 2017, DISS. ETH No. 24186.270

The mott based double turn method for the MEG II spectrometer characterisation

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The mott based double turn method for the MEG II spectrometer characterisation

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