The Mu2e experiment at Fermilab Ivano Sarra, LNF - INFN on behalf of the Mu2e collaboration PASCOS 2016 XIIth Rencontres du Vietnam, ICISE, Quy Nhon, Vietnam July 12, 2016

The Mu2e experiment Mu2e searches for charged-lepton flavor violation (CLFV) with muons in the presence of a nucleus: Measure ratio of conversions (CLFV) to the number of µ captures. Assuming 3.6 x 1020 proton on target collected over 3 years of physics data-taking: Single-event-sensitivity (SES): 2.87 × 10-17 Upper limit (assuming 0 signal ) 7 × 10-17 Provides >5s discovery sensitivity for all Rme(Al) > few 10-16 Mu2e goals: The recently proposed Mu2e experiment at Fermilab is a search for the Charged Lepton Flavor Violating (CLFV) process μ- + N  e- + N, which is the coherent conversion of a muon into an electron in the field of a nucleus. In the process of muon to electron conversion the initial state is a muonic atom and the final state consists of a mono-energetic electron recoiling against an intact atomic nucleus. There are no neutrinos in the final state. The nucleus is required to conserve energy and momentum! The recoiling nucleus is not observed, leaving an observed final state of just the electron, which has the energy of the muon rest mass less corrections for the nuclear recoil and the K-shell binding energy of the muon. The result of the experiment will be expressed by the ratio: Muons will be stopped in a thin aluminum target where they can interact with a nucleus and convert into electrons. The search will have a single event sensitivity (SES) of about 2 x10^-17, and will be able to set a limit of about 6 x10^-17 at 90% CL. The calorimeter will be used to confirm that a reconstructed track is well measured and was not created by a spurious combination of hits in the tracker. Diagram of the Mu2e muon beam-line and detector. The proton beam enters from the left. A back-scattered muon beam is captured by the Production Solenoid and transported through the S-bend Transport Solenoid to the stopping targets. Conversion electrons, produced in the stopping target are captured by the magnetic field in the Detector Solenoid and transported through the Tracker, which makes a precision measurement of the momentum. The conversion electrons strike the Electromagnetic Calorimeter, which produces an event trigger algorithm under improvement I. Sarra @ PASCOS 2016 July 12, 2016

Muonic Atoms – a primer Bound muon cascades quickly to the 1s ground state (emits X-rays) Bohr radius of ground state: 8 fm 4000 fm / / e- µ- 20 fm I. Sarra @ PASCOS 2016 July 12, 2016

Muonic Atoms Nuclear capture (61% of bound muons on Al) γ p n µ- νμ I. Sarra @ PASCOS 2016 July 12, 2016

Muonic Atoms Decay-in-orbit (39% of bound muons on Al) Rest of talk: DIO µ- e- νe νμ I. Sarra @ PASCOS 2016 July 12, 2016

Muonic Atoms Experimental signature is a Muon to electron conversion µ- e- Experimental signature is a mono-energetic electron of energy We know exactly where to look. I. Sarra @ PASCOS 2016 July 12, 2016

Charged lepton flavor violation In principle, CLFV is not forbidden by massive-ν SM due to neutrino oscillations In practice, we will never see the SM process! Transition rate < 10-50 Various NP models allow CLFV at levels just beyond current CLFV upper limits. Some of these: SO(10) SUSY L. Calibbi et al., Phys. Rev. D 74, 116002 (2006); L. Calibbi et al., JHEP 1211, 40 (2012). Scalar leptoquarks J.M. Arnold et al., Phys. Rev D 88, 035009 (2013). Left-right symmetric model C.-H. Lee et al., Phys. ReV D 88, 093010 (2013). I. Sarra @ PASCOS 2016 July 12, 2016

History of CLFV limits with muons R. Bernstein and P.S. Cooper, Phys Rept C (1307.5787) Best limit: Rµe < 7x10-13 by SINDRUM II at PSI [Eur.Phys.J C47(2006)] Mu2e will improve by a factor 104 I. Sarra @ PASCOS 2016 July 12, 2016

The Mu2e collaboration I. Sarra @ PASCOS 2016 July 12, 2016

Experimental setup Production Solenoid: Transport Solenoid: Proton beam strikes target, producing mostly pions Graded magnetic field contains backwards pions/muons and reflects slow forward pions/muons Detector Solenoid: Capture muons on Al target Measure momentum in tracker and energy in calorimeter Graded field “reflects” downstream conversion electrons emitted upstream Transport Solenoid: Select low momentum, negative muons Antiproton absorber in the mid-section Using 8 GeV protons from the Fermilab accelerator complex, a beam of low momentum negative muons (<60 MeV/c) is produced via upstream charged pion decays. The beam is stopped on a set of thin Al target foils, where the muons drop to the K-shell, forming a muonic atom. The Bohr radius of the K-shell of muonic Al is about 20 fm and the nuclear radius of Al is about 4 fm, which yields a large overlap between the muon wavefunction and that of the nucleus. The stopped muons have a characteristic decay time, τ_μ^N = 864 ns. Wait ~700 ns to start measurement, pion stopping rate is reduced by ~1011  ~0.0007 events background, compared to ~4 events signal at Rme=10-16 Extinction (=between-pulse proton rate) < 10-9 gives ~0.07 counts Recognized and studied by time dependence, presence of e+ The experiment must therefore examine about 10^18 stopped muons, requiring the most intense muon beam ever constructed, and achieve background rejection to an appropriate, and unprecedented, level. Diagram of the Mu2e muon beam-line and detector. The proton beam enters from the left. A back-scattered muon beam is captured by the Production Solenoid and transported through the S-bend Transport Solenoid to the stopping targets. Conversion electrons, produced in the stopping target are captured by the magnetic field in the Detector Solenoid and transported through the Tracker, which makes a precision measurement of the momentum. The conversion electrons strike the Electromagnetic Calorimeter, which produces an event trigger. stopped muon ~ 40 MeV/c I. Sarra @ PASCOS 2016 July 12, 2016

Physics backgrounds The Mu2e experiment could be affected by many physics backgrounds. Below the most relevant: Radiative π capture; µ decay in orbit; Cosmic-induced background. I. Sarra @ PASCOS 2016 July 12, 2016

Radiative π capture -1- What if a pion doesn’t decay but survives and stops in the Al target?  A PULSED BEAM pulse separation is ~ 1 thousand 7 hundred ns I. Sarra @ PASCOS 2016 July 12, 2016

Radiative π capture -2- Signal window Pion capture can produce a significant background: Can produce electron at same energy as the signal electron! Trick: Muon decays from Al are slow; pion captures are fast. Wait out the pion captures before starting the signal window. Signal window I. Sarra @ PASCOS 2016 July 12, 2016

Physics backgrounds Radiative π capture:  The 1695 ns proton pulse separation allows various backgrounds to significantly dissipate before we start the live gate. µ decay in orbit; Cosmic-induced background. I. Sarra @ PASCOS 2016 July 12, 2016

Important design consideration! µ decay in orbit The energy distribution of electrons from muon decay is given by a (modified) Michel spectrum: Michel spectrum endpoint: 52.8 MeV; Presence of atomic nucleus  momentum transfer;  DIO electron energies up to signal energy Eμe . Important design consideration! I. Sarra @ PASCOS 2016 July 12, 2016

Mu2e Detectors Targets: 34 Al foils; Aluminum was selected mainly for the muon lifetime in capture events (864 ns) that matches nicely the need of prompt separation in the Mu2e beam structure. Tracker: ~20k straw tubes arranged in planes on stations, the tracker has 18 stations Expected momentum resolution < 200 keV/c Polietilin Calorimeter: 2 disks composed of undoped CsI crystals Muon beam stop: made of several cylinders of different materials: stainless steel, lead and high density polyethylene I. Sarra @ PASCOS 2016 July 12, 2016

Tracker - Expected σp < 200 keV/c. - 18 stations with straws transverse to the beam; - Straw technology employed: 5 mm diameter, 15 µm Mylar walls 25 µm Au-plated W sense wire 80/20 Ar/CO2 with HV ~ 1500 V - Inner 38 cm un-instrumented: blind to beam flash blind to low pT charged particles coming from the Al target - Expected σp < 200 keV/c. Straw Tube Conversion Electron low pT electrons Station 3.2 m FEE x18 I. Sarra @ PASCOS 2016 July 12, 2016

Beam test @ BTF in Frascati Calorimeter 2 disks; each disk contains 674 undoped CsI crystals 20 x 3.4 x 3.4 cm3; Readout by 2 large area MPPC each + waveform digitizer boards @ 250 MHz; Allows to measure: Electron/muon discrimination from cosmic rays; Improve track search via a calorimeter-seed pattern recognition; Time resolution σt < 200 ps @ 100 MeV measured @ BTF in Frascati. Beam test @ BTF in Frascati Undoped CsI I. Sarra @ PASCOS 2016 July 12, 2016

Physics backgrounds Radiative π capture; µ decay in orbit:  low-mass tracker with high performance and fast calorimeter with timing information for background reduction. Cosmic-induced background. I. Sarra @ PASCOS 2016 July 12, 2016

Cosmic Ray Veto - Inefficiency < 10-4 - Veto system covers entire DS and half TS; - 4 layers of scintillator: each bar is 5x2x(~450) cm3 2 WLS fibers/bar read out with SiPM - Inefficiency < 10-4 µ mimicking the signal Al Target Tracker Calo I. Sarra @ PASCOS 2016 July 12, 2016

Physics backgrounds Radiative π capture; µ decay in orbit; Cosmic-induced background:  cosmic ray veto and PID with the Calorimeter I. Sarra @ PASCOS 2016 July 12, 2016

The full simulation Total background < 0.5 events 3.6 x 1020 POT 6.7 x 1017 stop-m Rme = 1 x 10-16 NDIO : 0.20 +/- 0.02 NSignal : 3.72 +/- 0.01 (stat. uncertainties only) No PID selection applied Total background < 0.5 events I. Sarra @ PASCOS 2016 July 12, 2016

Summary Mu2e is an experiment to search for CLFV in µ coherent conversion aims 4 orders of magnitude improvement Civil construction and magnets procurement already started R&D mature with data taking scheduled on 2021 More info: http://mu2e.fnal.gov FERMILAB Mu2e building I. Sarra @ PASCOS 2016 July 12, 2016

SPARES I. Sarra @ PASCOS 2016 July 12, 2016

Mu2e and MEG Mu2e is a potential discovery experiment that is relevant in all possible scenarios TeV)  Loop dominated Contact Courtesy A. de Gouvea , R. Bernstein, D. Hitlin Mu2e Mu2e should see a signal. Combination of results a powerful discriminator Mu2e could still see a signal. Sensitive to processes that MEG is not. MEG Signal? Yes No I. Sarra @ PASCOS 2016 July 12, 2016

Mu2e and the LHC Mu2e is a potential discovery experiment that is relevant in all possible scenarios Mu2e signal? Mu2e a powerful discriminator that constrains parameter space Null Mu2e result severely constrains possible new physics mechanisms Mu2e could still see a signal. Sensitivity to mass scales well beyond LHC reach. New Physics at LHC? Yes No I. Sarra @ PASCOS 2016 July 12, 2016

Next Generation Mu2e Precision Measurement if necessary Measure conversion rate as a function of Z Higher Sensitivity search Mu2e Signal? Yes No Accelerator Upgrade A next-generation Mu2e experiment makes sense in all scenarios Push sensitivity or Study underlying new physics Will need more protons  upgrade accelerator Snowmass white paper, arXiv:1307.1168 X10 improvement in sensitivity plausible with modest upgrades of current design I. Sarra @ PASCOS 2016 July 12, 2016

COMET Phase 1: scheduled to begin 2019, x100 improvement Phase I Phase II Design beam power: 56 kW (8kW for Mu2e) Path length of solenoids: ~38m (28m for Mu2e) Phase 1: scheduled to begin 2019, x100 improvement Phase 2: aim to begin so that they’re competitive with Mu2e, another x100 improvement I. Sarra @ PASCOS 2016 July 12, 2016

Why aluminum? Signal window Since pion capture process happens very quickly, need a target where the muon decay/capture happens slowly, so we can collect as many muon decays as possible Al Ti Au Stopped muons that decay 39% 15% 3% Stopped muon decays in sig. window 50% 30% 1% Time constant for muon decay 864 ns 329 ns 75 ns Aluminum is best option Signal window I. Sarra @ PASCOS 2016 July 12, 2016

Updated Background Estimate Process TDR (stat+syst) Decay in Orbit: 0.199 +/- 0.092 Radiative m Capture: 0.000 +0.004-0.000 p Capture: 0.023 +/- 0.006 m DIF: < 0.003 p DIF: 0.001 +/- <0.001 Beam electrons: 0.003 +/- 0.001 Anti-protons: 0.047 +/- 0.024 Cosmic ray induced: 0.082 +/- 0.018 Total Background: 0.36 +/- 0.10 SES for signal (10-17): (2.87 +/-0.32) Nb. TDR numbers include a 5% correction for CRV deadtime from false vetos, CD-3 does not. The CD-3c numbers come from doc-db-7464-v2 with the cosmic-ray background updated from doc-db-7479-v6. The SES for CD-3c comes from doc-db-7549-v11. For 3.6 x 1020 POT, extinction of 10-10, and a CRV inefficiency of 10-4 Momentum window tuned to give ~0.20 DIO events. After all selection requirements. I. Sarra @ PASCOS 2016 July 12, 2016

Why Particle Identification is needed Cosmic ray and antiproton induced background can be divided into 2 main categories: e- generated via interactions producing a track mimicking the CE non-electron particles (µ and π) that are reconstructed as an “electron-like” track mimicking the CE (1) represents the irreducible background, while (2) can be suppressed using a PID method Mu2e PID method: Information from reconstructed tracks and calorimeter clusters are combined for identifying group (2) Stringent requirement from Cosmic: µ-rejection factor ≥ 200 I. Sarra @ PASCOS 2016 July 12, 2016

Cosmic µ rejection 105 MeV/c e- are ultra-relativistic, while 105 MeV/c µ have β ~ 0.7 and a kinetic energy of ~ 40 MeV; Likelihood rejection combines 𝚫t = ttrack - tcluster and E/p: I. Sarra @ PASCOS 2016 July 12, 2016

PID performance A muon-rejection of 200 corresponds to a cut at ln Le/µ > 1.5 and an e- efficiency of ~ 96% I. Sarra @ PASCOS 2016 July 12, 2016

Mu2e Schedule CD-2/3b CD-3c CD-4 Project Complete Critical Path CD-2/3b CD-3c CD-4 Project Complete Fabricate and QA Superconductor PS/DS Final Design PS Fabrication and QA PS Installation PO issued for TS Module Fabrication Fabricate and QA TS Modules, Assemble TS TS Installation 22 months of float DS Fabrication and QA DS Installation Detector Hall Construction Solenoid Infrastructure Solenoid Checkout and Commissioning KPPs Satisfied Detector Construction Cosmic Ray System Test Q3 Q4 Q1 Q2   Accelerator and Beamline Construction FY15 FY16 FY17 FY18 FY19 FY20 FY21 FY22 I. Sarra @ PASCOS 2016 July 12, 2016