analysis of a combined data set, corresponding to 3.6 × 1014

stopped muons on target, in the search for the lepton flavor violating decay μ+ → e+γ is presented. The data collected by the MEG experiment at the Paul Scherrer

Institut show no excess of events compared to background expectations and thus yield a new upper limit on the branching ratio of this decay of 5.7×10−13 @90% CL (Adam J. et al., Phys. Rev. Lett., 110 (2013) 201801). This represents a four times more stringent limit than the previous world limit set by MEG

Ripiccini, E.

English

Scientific Open-access Literature Archive and Repository

DOI 10.1393/ncc/i2014-11691-8Colloquia: IFAE 2013IL NUOVO CIMENTO Vol. 37 C, N. 1 Gennaio-Febbraio 2014New result from the MEG experiment at PSIE. Ripiccini on behalf of the MEG CollaborationUniversita` di Roma “Sapienza” e INFN, Sezione di RomaPiazzale A. Moro, 00185 Roma, Italyricevuto l’1 Ottobre 2013Summary. — The analysis of a combined data set, corresponding to 3.6 × 1014stopped muons on target, in the search for the lepton flavor violating decay μ+ →e+γ is presented. The data collected by the MEG experiment at the Paul ScherrerInstitut show no excess of events compared to background expectations and thusyield a new upper limit on the branching ratio of this decay of 5.7×10−13 @90% CL(Adam J. et al., Phys. Rev. Lett., 110 (2013) 201801). This represents a four timesmore stringent limit than the previous world limit set by MEG.PACS 13.35.Bv – Decays of muons.PACS 14.60.Ef – Muons.PACS 14.60.-z – Leptons.1. – IntroductionThe μ+ → e+γ decay violates the lepton flavor conservation and it is then stronglysuppressed in the Standard Model in the extension to massive neutrino [1], providinga sensitivity on the Branching Ratio experimentally unattainable (O(10−53)). Most ofthe models beyond the Standard Model [2-5] allow the lepton flavor violation with abranching ratio for the μ+ → e+γ decay much larger (O(10−13)). The research of thischannel represents a very good probe for models Beyond the Standard Model (BSM)validation, particulary if we take into account the recent measurement of θ13 at thereactors [6-8] and at accelerators [9]. Moreover a limit on the BR(μ+ → e+γ) providesconstraints on the parameters space of these models, complementary to those obtainableat high energy colliders.The result shown in this paper was set using the data collected in 2009-2010-2011.An updated analysis for 2009 and 2010 with respect to the one presented in [10] is alsoshown.The μ+ → e+γ signature is given by back-to-back, monoenergetic, time coincidentphoton-positron pair. Each event is described by five observables: the photon and posi-tron energy (Eγ , Ee), their relative angles (θeγ , φeγ) [11] and relative emission time (teγ).c© Societa` Italiana di Fisica 202NEW RESULT FROM THE MEG EXPERIMENT AT PSI 203Fig. 1. – The MEG experiment.Two are the main background sources: one coming form the radiative decay (RMD)when the two neutrinos take a small energy amount, one from the accidental (ACC)superposition of energetic positrons from the standard muon Michel decay with photonsfrom RMD, positron-electron annihilation-in-flight or bremsstrahlung. The accidentalbackground is the most important.We use a maximum likelihood analysis technique described in detail in [10].The MEG detector, in fig. 1, is comprised of a positron spectrometer formed by aset of drift chambers (DCH) and scintillation Timing Counter bars, located inside asuperconducting solenoid, with a gradient magnetic field along the beam axis (1.27Tat the centre and 0.49T at both ends), and a photon detector, located outside of thesolenoid, made up of a homogeneous volume (900 l) of liquid xenon, read by 846 UV-sensitive PhotoMultipliers. The MEG detector response, resolutions and stability areconstantly monitored and calibrated with a multi-element calibration system [10,12].Concerning trigger and DAQ system, in 2011 the DAQ efficiency, being the productof DAQ live time fraction and trigger efficiency to select signal event, is at 96% level. Itwas 87% in 2010 and was improved thanks to a new multiple buffer read out scheme.2. – Detector performances and analysis resultConcerning the positron reconstruction analysis some improvements have been intro-duced. Electromagnetic noise were reduced by introducing a new reconstruction algo-rithm based on the fast Fourier filtering techniques, providing a 10% improvement inangular resolution. The positron track is extracted by means of a Kalman filter trackfitting technique [13, 14], completely revised for this analysis, to include a better modelfor the hits and track itself. It also provides per-track error, which is taken into accountin the likelihood analysis. The overall improvement in the positron reconstruction withrespect to the previous data analysis is visible in fig. 2 (left).The energy resolution is evaluated by fitting the kinematic edge and it is well describedby a sum of three Gaussian curves with a resolution of σEe = 305 keV for the core (85%)component. The positron angular resolutions are measured by using the double turnmethod (two turns of the same track are treated as they were a single turn. The resolu-tions are evaluated in the point where the second turn starts), and they are respectivelyσθe = 10.6(10.6)mrad [15] and σφe = 7.5(7.0)mrad. The decay vertex coordinates and204 E. RIPICCINI on behalf of the MEG COLLABORATIONFig. 2. – On the left the positron energy spectrum obtained with old analysis (black line),compared to the one obtained with the new analysis. The same configuration is shown for thephoton on the left with the addiction with the spectrum without pile-up rejection (blue line).the positron direction at the vertex are determined by projecting the reconstructed trackback to the target. The resolution on the decay vertex coordinates are also determinedby the two turn method and are described by a Gaussian curve with σz = 1.9(1.5)mmand, in the vertical direction, by the sum of two Gaussian curves with σy = 1.3(1.2)mmfor the core component (85%).The xenon liquid calorimeter provides the energy, the time and the position of the firstinteraction of the photon. The direction of the photon is defined by the line connectingthe decay vertex to the conversion point in the calorimeter. An important issue inthe photon analysis is to identify and unfold photon pile-up events, since, with a beamrate of 3 × 107 mu/s, the 15% of triggered photons are affected by pile-up. For theprevious analysis pile-up events were identified topologically by the pattern of PMT lightdistribution in both the inner and the outer LXe faces and temporally by the leadingedge time distribution in the time reconstruction, without using any information from thewaveform. In addition to this method, a new algorithm was developed, which analyzeswaveforms after summing up all channels. The pile-up identification and rejection isbased on a template fit. The resulting integrated charge is lower with respect to the oneobtained with the old method, as a consequence the photon energy spectrum has reducedtail fig. 2 (right). The efficiency of the photon reconstruction is also improved from 56%to 67%.Time and energy resolution are evaluated using back to back photons coming fromthe π0 decay. The LXe timing resolution is σtγ = 67ps, while the position-dependentenergy resolution is 1.7(1.9)% and 2.4(2.4)% from the radial depths larger and smallerthan 2 cm respectively. These position-dependent energy resolutions are incorporatedinto the likelihood analysis.By combining photon and positron resolutions, relative resolutions (θeγ , φeγ) are ob-tained. The results are σθeγ = 16.2(15.7) mrad and σφeγ = 8.9(9.0) mrad. The relativetime teγ is derived from the time measurements, one in the LXe detector and the otherin the Timing Counter, after correcting for the length of the track given by the track fit.The teγ resolution is σteγ = 127(135) ps and it is evaluated from the RMD peak observedin Eγ sideband (Eγ < 48MeV, Eγ > 58MeV). The Background PDFs are evaluated inthe blinding teγ and Eγ regions, respectively |teγ | < 1 ns and 48MeV < Eγ < 58MeV.NEW RESULT FROM THE MEG EXPERIMENT AT PSI 205Table I. – Best fit and sensitivity.Dataset BRfit × 1012 BR90 × 1012 Sensitivity × 10122009+2010 0.17 1.7 1.32011 −0.35 0.67 1.12009+2010+2011 −0.06 0.57 0.77A maximum likelihood fit is performed in order to extract the number of backgroundevents in the analysis region defined by 48MeV<Eγ <58MeV, 50MeV < Ee < 56MeV,|teγ | < 0.7 ns, |θeγ | < 50 mrad and |φeγ | < 50 mrad. The definition of the likelihoodfunction is described in detail in [10]. As mentioned before in the new analysis a per-eventerror matrix, estimated with the new Kalman filter, has been introduced in the PDFs.The confidence interval for the number of signal events is calculated by a frequentistmethod with profile likelihood-ratio ordering [10,16,17], where the numbers of RMD andACC events are treated as nuisance parameters.In order to convert the number of signal events into a branching ratio, two independentnormalization method are used: either counting the number of Michel positrons selectedwith a dedicated trigger or the number of RMD events observed in the muon data. Theircombination leads to a 4% in the branching ratio uncertainty.Concerning systematic uncertainties of PDFs parameters and normalization, they aretaken into account in the calculation of the confidence interval by fluctuating the PDFsparameters by the amount of the uncertainties. The systematic uncertainties contributionto the branching ratio is at 1% level. In order to estimate the sensitivity, a number ofpseudo-experiment are performed according to the PDFs in the null-hypothesis, with thevalues of RMD and ACC estimated in the sidebands. The sensitivity is the median of thedistribution of the branching ratio upper limit at 90% C.L. In table I the summary of theobserved upper limits are reported. The profile likelihood as a function of the branchingratio is shown in fig. 3. The upper limit, obtained combining 2009-2011 data taken, is5.7× 10−13 @90% CL. A summary of the previous upper limits is reported in table I. InFig. 3. – Observed prole likelihood ratios λp as a function of the branching ratio for the 2009-2010combined data, the 2011 data alone and the combined 2009-2011 data sample.206 E. RIPICCINI on behalf of the MEG COLLABORATIONFig. 4. – Branching ratio as a function of the data taken weeks with the upgraded detector.After 3 years a sensitivity of 5× 10−14 is expected.order to have a crosscheck of the results, a maximum likelihood fit is repeated withoutthe constrain on the number of background events. We obtain NRMD = 163.1±31.5 andNACC = 2411.1 ± 56.9, in good agreement with the expectation value estimated fromthe sidebands: 〈NRMD〉 = 169.3 ± 17.0 and 〈NACC〉 = 2415.0 ± 25.0. Concerning 2009-2010 analysis, we compared the result obtained with the new analysis to the previousone based on the old analysis, generating simulated experiments. We found that theprobability to found a result equal to or larger than the old one is 31%.3. – ConclusionsIn conclusion the MEG experiment established the most stringent upper limit on thebranching ratio of the μ+ → e+γ decay. We have to analyze more data taken during2012, while the 2013 run is now on going. The final number of muon stopped on target isexpected to be double than the one analyzed so far. With the whole data set (2009-2013)a sensitivity of 5 ·10−13 is expected. At this point the limit decreases as the inverse of √t,where t is the data taking time and the background becomes predominant in the signalregion. Therefore in order to exploit sensitivity at least an order of magnitude lower,a new upgraded MEG experiment is required. An upgrade program was approved inJanuary 2013. The main features of the upgrade are: a higher beam sensitivity, a singlevolume drift chamber with stereo angle wires configuration, a scintillator tile timingcounter with SiPM readout, substitution of the PMTs in the inner face of the calorimeterwith SiPM and a larger liquid Xenon fiducial volume. The expected sensitivity with 3year run with the new detector is 5×10−14 as shown in fig. 4. There is also a possibility toinstall an active target which can provide the decay vertex position improving resolutionsfor the positron and the normalization factor. Further detail concerning the upgrade canbe found in [18].NEW RESULT FROM THE MEG EXPERIMENT AT PSI 207REFERENCES[1] Petcov S. T., Sov. J. Nucl. Phys., 25 (1977) 340.[2] Barbieri R., Hall L. and Strumia A., Nucl. Phys. B, 445 (1995) 219.[3] Hisano J., Nomura D. and Yanagida T., Phys. Lett. B, 437 (1998) 351.[4] Raidal M. et al., Eur. Phys. J. C, 57 (2008) 13.[5] Blankenburg G. et al., Eur. Phys. J. C, 72 (2012) 2126.[6] An F. P. et al., Phys. Rev. Lett., 108 (2012) 171803.[7] Ahn J. K. et al., Phys. Rev. Lett., 108 (2012) 191802.[8] Abe Y. et al., arXiv:1301.2948 [hep-ex].[9] Abe K. et al., Phys. Rev. Lett., 107 (2011) 041801.[10] Adam J. et al., Phys. Rev. Lett., 107 (2011) 171801.[11] θeγ = (π − θe) − θγ and φeγ = (π + φe) − φγ , θ and φ being the polar angle and theazimuthal angle, respectively, taking the z-axis as the beam-axis.[12] Adam J. et al., Eur. Phys. J. C, 73 (2013) 2365.[13] CMS Collaboration, JINST, 5 (2010) T03009.[14] Billoir P., Nucl. Instrum. Methods A, 225 (1984) 352.[15] From here on we will quote in parentheses the value in the 2009–2010 data when differentfrom that in 2011.[16] Beringer J. et al. (Particle Data Group), Phys. Rev. D, 86 (2012) 010001.[17] Feldman G. J. and Cousins R. D., Phys. Rev. D, 57 (1998) 3873.[18] Baldini A. M. et al., arXiv:1301.7225 [physics.ins-det].

New result from the MEG experiment at PSI

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New result from the MEG experiment at PSI

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