Chloé Malbrunot UBC/TRIUMF Measurement of the PIENU branching ratio A sensitive probe in the search for new physics Chloé Malbrunot UBC/TRIUMF For the PIENU collaboration M. Aoki4, M. Blecher9, D. Bryman7,5, S. Chen6, J. Comfort1, L. Doria5, P. Gumplinger5, A. Hussein8, Y.Igarashi3, N. Ito4, S. Kettell2, Y. Kuno4, L. Kurchaninov5, L. Littenberg2, C. Malbrunot7,5, G. Marshall5, T. Numao5, R. Poutissou5, F. Retiere5, A. Sandorfi2, A. Sher5, K. Yamada4, M. Yoshida4 Arizona State University, 2. Brookhaven National Laboratory, 3. KEK, 4. Osaka University, 5. TRIUMF, 6. Tsinghua University 7. University of British Columbia, 8. University of Northern British Columbia, 9. Virginia Tech

Precision measurement Real deviation from the SM  new physics observation Agreement with SM  useful constraints Extreme sensitivity to high mass scales Like the rare or forbidden reactions, precision measurements are an interasting way to look for new physics in the LHC area. Fundamental « new physics» with precision measurement. In the context of teh LHC, why try to continue search for fundamental phsyics whith low energy experiments? Complementary to LHC Intellectually challenging Technology frontier Hadronic effects cancels. positron decay is preferred from phase space, but is helicity suppressed The current world average, unchanged for a decade, gives the ratio [4] Higher order contibution to the process are so well controlled that the ratio can be calculated with the highest accuracy of any allowed meson decay Agrees with SM 1.1sigma Potential 8 sigma sensitivity with the new experiment World average: TRIUMF (1992), PSI (1993) 2 orders of magnitude difference in precision  window for BSM New experiment x5 better precision  < 0.1% 11/11/2018 ACOT Chloé Malbrunot

Universality test / Beyond SM search New pseudoscalar interaction: 0.1% measurement  ΛeP~ 1000 TeV Massive ’s Scalar coupling Leptoquarks Compositeness R-Parity violation SUSY . . . Rapid developments in the neutrino sector in recent years have renewed the interest in lepton universality.Pienu is the Most stringent test of e/ universality No mixing in the charged lepton sector W is equally coupled to leptons. SM: e,, have identical electroweak gauge interaction Unless new physics does not respect universality !!Feynam diagram= New four fermion interaction and charged Higgs Experimental tests of lepton universality provide a useful crosscheck of SM predictions, as well as potentially useful independent limits on masses and couplings of certain particles outside of the SM. In addition to testing lepton universality, precise measurement of the e2 branching ratio can constrain certain other non-standard model scenarios. In particular, if the decay were dominated by a pseudoscalar coupling, then the helicity suppression of the e2 decay would vanish and the branching ratio would be 􀀀( ! e())=􀀀( ! ()) = 5:5. The dierence between the best experimental results and the standard model description of the process provides an estimate of the residual pseudoscalar coupling.Since the pienu decay is helicity suppressed it is extremely sensitive to helicity- unsuppressed couplings such as the pseudo-scalar (PS) coupling.PS contribution comes as interference term with the axial-vector (dominant) term. Contribution is proportional to 1/Λ2 (contrast to LFV decay 1/4) where Λ is the mass of the hypothetical particle. The pseudoscalar nature of the pion determine that only the axial current contributes to the pion decay (unlike tau decay). PiENu : ge/gμ< 0.05% 11/11/2018 ACOT Chloé Malbrunot

Former experiment at TRIUMF E248 Taking the ratio cancels sources of systematics. Tail is 20% of the pienu peak, mostly due to DIF. We think we can reduce to about 4% by tracking. Main source of uncertainties: Low energy tail buried under Michel spectrum Energy dependence of acceptance correction Small acceptance ()  low statistics 11/11/2018 ACOT Chloé Malbrunot

PiENu (E1072): key improvement Larger solid angle (x10) More statistics Lower energy dependent acceptance difference Detect shower leakage (CsI) for low energy tail measurement (biggest systematics) Silicon Strip near target & WC Much improved tracking Detect Decay In Flight  for tail correction High resolution calorimeter BINA resolution 2 times better than TINA Use of fast digitizers Better separation between e and e e+  beam 50 cm Large NaI Calorimeter : 19 X0 Pure CsI ring to contain shower leakage ATLAS silicon strip counters Lower rate to keep a low background arising firm old muons in the target region and to minimize potential distortions in the time spectrum due to pile up 11/11/2018 ACOT Chloé Malbrunot

Detector subsystem PiENu 1 : Beam&Target assembly Annular veto counter (V1) Wire chambers (WC1,WC2) Beam counters (B1, B2) Si-strip detectors (SS1, SS2) Targer counter Si-strip detectors (SS3) Telescope counter (T1) Silicon and WC tracking (determine stop/decay vertex) suppress Decay In Flight Monte Carlo x10 suppression V1 WC1&2 B1 B2 T1 Target  beam Tracking of the decay positron is used for the evauation of shower leakage affects and correction of the path length in the T counters for dE/dx measurments. Scintillators are from Bicron BC-408 cast scintillaotors except V4 which is long extruded scintillators glued together whith 6 holes for wave shifting fibers. Energy resolution of the targer 3% rms at 13 MeV. The two first WCs are monted on the beam pipe itself. They map the direction and position of incoming pions. In combination with the info from Si strip , B1 and B2 can supress dacay in flight. Eliminate triggers from pileup in the target by intersecting this trajectory with the trajectory seen in the silicon strip and wire chamber downstream of the traget t determine which incoming pion produced the downstream particles. Each WC has 3 planes with 60° angle (X, U, V). WC1 and 2 are nested together. Resolution 4 mm. Ecah silicon modules consist of 2 identical hybrids back to back with perpendicular orientation of strips, active area is about 61x61 mm2, strip pitch of 80um. Used in ATLAS central tracker. But for PiENu teh required resolution is 300um.therfore 4 si srips are bounded together to one readout line.->capacitive coupling to reduce ven more the number of readout channels-> 289 in total. Silicon and WC measure also kincut 11/11/2018 ACOT Chloé Malbrunot

Detector subsystem (cont’d) PiENu 2: Positron telescope Telescope counter (T2) Wire chamber (WC3) NaI(Tl) crystal (BINA) Pure CsI crystal ring Veto counters (V2-V4)  beam 25cm The whole crystal assembly is from Brookhaven National Laboratory Better dE/dx improve the tail. Better knowlege of shower leakage reduce the tail also. Very important is the knowledge of the calorimeter line shape. Detachable pienu2 to enable the measurement with e+ beam of the line shape at different angles and entrance position. Two layers of CsI T2 is housed inside BINA flange to make the design compact (the target is as close as possible to BINA and the solid angle is greater) BINA is read out by 19 PMTs and CsI 97 PMTs Housing of CsI: 2 thin inner cylinders of stainless steel In total 142 PMTs. Shielded with u metal.(all of them??) In BINA 3 holes viewed by long fiber opti cables used in conjonction with a light source for gain calibration (Xe lamp) which i sstable to 0.4% for a month. Xe lamp is monitored by temperature controlled phototubes.A special Xe trigger will be generated every 1-10 s for continuous gain minitoring data taking. PienuII is under nitrogen flow. WC3 is used to determine positron trajectories as they enter BINA and for setting the radial acceptance. CsI have a YAlO light pulser with a 1OO Bq 241 Am source that induce a 6 MeV pulse at 50 Hz. Minimal material between Target and BINA to reduce scattering Movable, detachable from PieNu 1 for line shape measurement at various e+ entrance angles 50 cm 11/11/2018 ACOT Chloé Malbrunot

New beamline +  82% +  14% e+ < 2% Suppression of beam positrons Protection against neutral showers F3 Collimator 2mm lucite degrader at F1 e+/+ ~ 1.2% 2.5 cm 5 cm  e F4 Lucite degrader at F1 causes ca. 5cm lateral displacement of + at F3 F1 F2 Total e+ reduction x 100-200 proton detector F1 F4 F3 Collimator F2 +  82% +  14% e+ < 2% Important: Minimize number of beam e+ Minimize number of neutral particle producing pile up Small momentum bite to reduce number of pi+ decay in flight before target We see shilding to reduce the rate of neutral which pushed the pi mu nu decays into the pienu region. If a beam e+ follows a valid pion stop in the time window -300ns +100ns it can be assimilated with a pienu e+. If the beam counters register the e+, the beam e+ background can be eiminated offline but there is a detection inefficiency due to annihilation in flight (order of 10^-4) which gives a 2% background in the pienu spectrum. Positron tail is due to radiative energy loss from bremsstrahung at the degrader. 1cm Carbon production target Beam positron hitting a beam line component generate e shower that can deposit energy in the crystal (neutral pile up) and can contribute to the background of the pienu if E>5MeV (cut is at 55 MeV) . Dangerous background since it has RF structure. Other neutral background come from thermal neutrons and related gamma-rays that do not have strong timing. Adding a third bending magnet help getting less 11/11/2018 ACOT Chloé Malbrunot

Beam test results Y=0.9cm X=1.2cm Good beam spot High resolution calorimeter Y=0.9cm X=1.2cm Good separation in silicon Good tail suppression with CsI at different positron entrance angles High precision experiment: should rely as far as possible on measurement rather than simulation 0.25% uncertainty on the correction in E248 0.03% uncertainty goal in E1072 The tail has to be understood at the 1% level 30 times more statistics gives a factor 5 better on the uncertainty 2x better suppression of  events 5x better statistics in E1072 than E248 Say resolution 2x better than BINA. Show Michel spectrum e+ % DATA: NaI NaI + CsI MC: NaI NaI + CsI + + 11/11/2018 ACOT Chloé Malbrunot angle

The challenge! PiENu schedule : 11/11/2018 ACOT Chloé Malbrunot The low E tail accuracy is mostly better due to statistics 11/11/2018 ACOT Chloé Malbrunot

Conclusion +e+ branching ratio will be measured to <0.1% precision (<0.05% in ge/g) Test of lepton universality High sensitivity to high mass scales ~1000 TeV Complementary to studies at LHC 11/11/2018 ACOT Chloé Malbrunot