1. 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

2. 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%
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ACOT Chloé Malbrunot

3. 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%
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ACOT Chloé Malbrunot

4. 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
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ACOT Chloé Malbrunot

5. 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
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ACOT Chloé Malbrunot

6. 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
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ACOT Chloé Malbrunot

7. 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
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ACOT Chloé Malbrunot

8. 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
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
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ACOT Chloé Malbrunot

9. 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 + +
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ACOT Chloé Malbrunot
angle

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

11. 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