This document was prepared by Mu2e collaboration using the resources of the Fermi National FERMILAB-SLIDES-19-078-ND Accelerator Laboratory (Fermilab), a U.S. Department of Energy, Office of Science, HEP User Facility. Fermilab is managed by Fermi Research Alliance, LLC (FRA), acting under Contract No. DE-AC02-07CH11359. Searching for Muon to electron conversion: The Mu2e experiment at Fermilab Richie Bonventre NuFACT 2019 Lawrence Berkeley National Lab
Charged Lepton Flavor Violation (CLFV) has never been ob- served • Standard model CLFV contribution is undetectably small ( < 10 − 50 ) • Any detection of charged lepton flavor violation would be an unambiguous sign of new physics! 1 / 100
Muon searches reach the smallest branching ratio limits on CLFV processes Process Current Limit Next Generation exp. 10 − 9 - 10 − 10 (Belle II, LHCb) BR < 6.5 × 10 − 8 τ → µη BR < 6.8 × 10 − 8 τ → µγ BR < 3.2 × 10 − 8 τ → µµµ BR < 3.6 × 10 − 8 τ → eee BR < 4.7 × 10 − 12 K L → e µ K + → π + e − µ + BR < 1.3 × 10 − 11 B 0 → e µ BR < 7.8 × 10 − 8 B + → K + e µ BR < 9.1 × 10 − 8 µ + → e + γ BR < 4 . 2 × 10 − 13 10 − 14 (MEG) µ + → e + e + e − BR < 1 . 0 × 10 − 12 10 − 16 (Mu3e) R µ e < 7 . 0 × 10 − 13 10 − 17 (Mu2e, COMET) µ − N → e − N 2 / 100
Muon conversion can probe mass scales up to 10 4 TeV (assum- ing unit coupling) (1+ κ )Λ 2 µ R σ µν e L F µν + �� � m µ κ q = u , d q L γ µ q L L CLFV = (1+ κ )Λ 2 µ L γ µ e L • loop: κ ≪ 1, µ N → e N and µ → e γ • contact: κ ≫ 1, µ N → e N only • Mass scale reach makes these measurements complementary to LHC 3 / 100 Derived from A. de Gouvea, P. Vogl, Prog. Part. Nucl. Phys. 71 (2013) 75
Basics of a Muon conversion experiment Measure the ratio of conversions to muon nuclear captures: µ − + A ( Z , N ) → e − + A ( Z , N ) R µ e = µ − + A ( Z , N ) → ν µ + A ( Z − 1 , N ) • Signal of CLFV conversion is single monoenergetic electron 4 / 100
Anything that can produce a ∼ 105 MeV electron is a back- ground to a µ to e conversion search • Muon Decay in orbit: µ − N → e − N ν µ ν e • Beam related: π − N → γ N ′ , γ → e + e − • Cosmic rays: µ − → e − ν µ ν e 5 / 100
Anything that can produce a ∼ 105 MeV electron is a back- ground to a µ to e conversion search Robert Szafron and Andrzej Czarnecki, Phys. Rev. D 94, 051301 (2016) • Muon Decay in orbit: µ − N → e − N ν µ ν e • Beam related: π − N → γ N ′ , γ → e + e − • Cosmic rays: µ − → e − ν µ ν e 5 / 100
Anything that can produce a ∼ 105 MeV electron is a back- ground to a µ to e conversion search • Muon Decay in orbit: µ − N → e − N ν µ ν e • Beam related: π − N → γ N ′ , γ → e + e − • Cosmic rays: µ − → e − ν µ ν e 5 / 100
Anything that can produce a ∼ 105 MeV electron is a back- ground to a µ to e conversion search • Muon Decay in orbit: µ − N → e − N ν µ ν e (Momentum resolution) • Beam related: π − N → γ N ′ , γ → e + e − • Cosmic rays: µ − → e − ν µ ν e 5 / 100
Anything that can produce a ∼ 105 MeV electron is a back- ground to a µ to e conversion search • Muon Decay in orbit: µ − N → e − N ν µ ν e (Momentum resolution) • Beam related: π − N → γ N ′ , γ → e + e − (Delayed event window) • Cosmic rays: µ − → e − ν µ ν e 5 / 100
Anything that can produce a ∼ 105 MeV electron is a back- ground to a µ to e conversion search • Muon Decay in orbit: µ − N → e − N ν µ ν e (Momentum resolution) • Beam related: π − N → γ N ′ , γ → e + e − (Delayed event window) • Cosmic rays: µ − → e − ν µ ν e (Active veto) 5 / 100
The Mu2e Experiment at Fermilab • Aim is 10 4 improvement in sensitivity • Greatly increase muon production • Reduce backgrounds • High resolution detector that can survive event rate 6 / 100
Pulsed proton beam allows us to reject radiative pion capture events ( π − + Al → Mg ⋆ + γ ) • 8 GeV 8 kW proton beam from Fermilab booster • Resonantly extracted to get pulses of 4x10 7 protons separated by 1.7 µ s • 700 ns delay followed by 1 µ s livegate • Must have very few protons outside of pulse (ratio to in-pulse < 10 − 10 ) Proton%pulse% Proton%pulse% Prompt%background% Live Window Signal% 7 / 100
Mu2e experimental setup • Consists of three superconducting solenoids: • Production Solenoid (PS) • Transport Solenoid (TS) • Detector Solenoid (DS) 8 / 100
Mu2e experimental setup • Consists of three superconducting solenoids: • Production Solenoid (PS) • Transport Solenoid (TS) • Detector Solenoid (DS) 8 / 100
Mu2e experimental setup • Consists of three superconducting solenoids: • Production Solenoid (PS) • Transport Solenoid (TS) • Detector Solenoid (DS) 8 / 100
Mu2e experimental setup • Consists of three superconducting solenoids: • Production Solenoid (PS) • Transport Solenoid (TS) • Detector Solenoid (DS) 8 / 100
Production Target and Solenoid produce slow muon beam in the reverse direction of the proton beam • Tungsten production target • Magnetic mirror traps and redirects back to TS 9 / 100
Transport Solenoid sign selects charged particles 10 / 100
Detector solenoid directs electrons to detector elements • Muons stopped on thin aluminum foils, again graded field for magnetic mirror • Constant field in tracking volume • High precision straw tracker in vacuum • Electromagnetic calorimeter for PID 11 / 100
Straw Tracker designed to survive beam flash while providing resolution better than 200 keV/c • 18 stations, each containing 12 × 120 ◦ panels for stereo measurement 12 / 100
Straw Tracker designed to survive beam flash while providing resolution better than 200 keV/c • Blind to DIO electron momentum peak and beam flash 13 / 100
Straw Tracker designed to survive beam flash while providing resolution better than 200 keV/c • ∼ 21,000 low mass straw tubes in vacuum • 5 mm diameter, 15 µ m thick mylar walls • 25 µ m tungsten wire at 1425V • 80:20 ArCO 2 14 / 100
8 straw tracker prototype used to tune simulation and verify expected resolution 15 / 100
Reconstruction using tuned simulation shows we expect tracker to meet momentum resolution requirements momentum resolution at start of tracker (simulation) Entries / 0.010 MeV/c 4 10 3 10 Core width = 159 keV/c 2 10 1 µ s selection window after beam flash 10 1 4 3 2 1 0 1 2 3 4 − − − − p -p (MeV/c) true measured • Helix fit followed by iterative Kalman Filter track fit Hits selected by track finder within ± 50 ns selection window 16 / 100
Calorimeter provides particle ID for track rejection • Two annular disks separated by half a “wavelength” (70cm) of electron’s helical path • Maximize probability to hit at least one disk • Each disk contains 674 undoped CsI 34x34x200 mm 3 crystals read out by SiPMs • 0.5 ns time, 5% energy, 1 cm position measurement independent of straw tracker • Seed for tracking algorithm 17 / 100
Large calorimeter prototype tested in electron beam at BTF in Frascati • Prototype has 51 crystals, 102 SiPMs, 102 FEE boards • Demonstrates energy and time resolution 0.5 DATA: Orthogonal Beam [ns] 10 [%] DATA: Beam @ 50 ° o 0.45 Beam at 0 - Hamamatsu 9 MC: Orthogonal Beam T dep σ Cosmic Rays - Hamamatsu MC: Beam @ 50 ° 0.4 /E 8 o Beam at 50 - SensL σ 0.35 Cosmic Rays - SensL 7 0.3 6 0.25 5 0.2 4 χ χ 2 2 / ndf / ndf 3.142 / 3 3.142 / 3 0.15 χ χ 2 2 / ndf / ndf 0.8783 / 2 0.8783 / 2 3 a a a a 0.6 0.6 ± ± 0 0 0.6 0.6 ± ± 0 0 0.1 2 2 / ndf / ndf 2 2 / ndf / ndf χ χ 3.081 / 5 3.081 / 5 χ χ 5.155 / 3 5.155 / 3 2 b b 0.3747 0.3747 ± ± 0.045 0.045 b b 0.2732 0.2732 ± ± 0.02913 0.02913 a a a a 8.906 8.906 ± ± 0.2043 0.2043 6.858 6.858 ± ± 0.1425 0.1425 0.05 c c 5.863 5.863 ± ± 0.3911 0.3911 1 c c 4.05 4.05 ± ± 0.2705 0.2705 b b 0.118 0.118 ± ± 0.007005 0.007005 b b 0.0911 0.0911 ± ± 0.004349 0.004349 0 10 20 30 40 50 60 70 80 90 100 0 0.05 0.06 0.07 0.08 0.09 0.1 0.11 Energy [MeV] E [GeV] dep 18 / 100
Cosmic rays can produce dangerous background events • Cosmic muon track can look like 105 MeV/c electron (mitigated by Calorimeter PID) • Or - cosmic muon can decay inside the detector volume or knock out electron from stopping target → indistinguishable from signal • Expect 1 such event per day • Need highly efficient cosmic ray veto 19 / 100
Reject with efficiency cosmic ray veto • 4 overlapping layers of scintillator, read out on both ends with SiPMs • Veto on 3-fold coincidence • Covers entire DS, half of TS, better than 10 − 4 inefficiency 20 / 100
CRV prototype counters tested with 120 GeV protons in Fer- milab test beam 21 / 100
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