1. Ivan Logashenko for Mu2e Collaboration
Workshop on e+e- collisions from Phi to Psi, Sep.19-22, 2011
Novosibirsk, Russia

2. Search for cLFV
We had several talks on cLFV already – MEG, COMET, BELLE,…
Why is it interesting?
Charged Lepton Flavour Violation in SM is extremely small. Any detectable signal is a sign of New Physics.
Why muons?
Copiusly produced
We are trying to measure
Standard Model predicts extremely small value:
e+e- collisions from Phi to Psi
Novosibirsk, 2011

3. History of Mu2e Mu2e goal:
The current best limit was obtained at SINDRUM II experiment (90’s-2000)
Mu2e is the successor of two earlier proposals:
MELC at Moscow, early 90’s
The main ideas of the experimental design were proposed
MECO at Brookhaven, early 2000’s
Part of RSVP program, along with KOPIO, search for K->pinunu
At lof of design work and R&D were performed before the program was terminated
Mu2e goal:
e+e- collisions from Phi to Psi
Novosibirsk, 2011

4. Mu2e vs MEG μ → eγ decay and μN → eN conversion are complementary
“Model-independent” lagrangian:
Mu2e
k << 1
magnetic moment type
m → eg rate ~300X
mN → eN rate
MEG
MEG 2011
k >> 1
four-fermion interation
almost no contribution to m → eg
MEGA
SINDRUM II
e+e- collisions from Phi to Psi
Novosibirsk, 2011

5. Overview of the technique
Stop muons in aluminum
Muons quickly get to 1S orbit
Lifetime of muonic atom is 864 ns
Look for 105 MeV electron
Hydrogen-like atom
Bohr radius ≈20 fm
Nucleus radius ≈4 fm
Use X-rays to monitor number of muons
Most of the background is well below this energy
e+e- collisions from Phi to Psi
Novosibirsk, 2011

6. Background: Muon decay-in-orbit (DIO)
For decay-in-orbit , the maximum energy of electron is equal to the energy of conversion electron (≈105 MeV).
The high-energy tail
Need good energy resolution to beat this background!
Free muon
In-orbit
Long tail!
e+e- collisions from Phi to Psi
Novosibirsk, 2011

7. Background: Radiative muon capture (RMC)
Muon capture is the source of many background particles:
This background is mostly low energy, but:
Radiative Muon Capture:
with small BR≈10-5, γ’s from muon captures can have high energy up to
source of high energy electrons, comparable with DIO at the tail: γ can convert in target to e+e- pair, and electron can get almost all energy
With good energy resolution and proper choice of target (MZ-1>MZ) this source of background is under control
e+e- collisions from Phi to Psi
Novosibirsk, 2011

8. Background: Radiative pion capture (RPC)
Muon are produced from pion decays, therefore there are pions in the muon beam.
Radiative Pion Capture:
pions stop at the target and promptly annihilate on the nucleus
with BR~10-2, γ is produced with energy up to 137 MeV (for Al)
source of high energy electrons: γ can convert in target to e+e- pair, and electron can enough energy to mimic conversion electron
This is prompt background, it quickly dies away. Solution: pulsed beam.
Time distribution of stopped pions in the target
e+e- collisions from Phi to Psi
Novosibirsk, 2011

9. Overview of the design Production Solenoid 4 m long
Transport Solenoid 13 m long
Detector Solenoid 12 m long
e+e- collisions from Phi to Psi
Novosibirsk, 2011

10. Historical perspective: MELC proposal (1992)
e+e- collisions from Phi to Psi
Novosibirsk, 2011

11. Production Solenoid
e+e- collisions from Phi to Psi
Novosibirsk, 2011

12. Extinction To avoid prompt background, we use pulsed beam
The signal is extremely rare
Need extinction factor 10-10!
…and it has to be measured
Use combination of techniques to achieve this factor
e+e- collisions from Phi to Psi
Novosibirsk, 2011

13. Accelerator
e+e- collisions from Phi to Psi
Novosibirsk, 2011

14. Mu2e vs NOvA vs g-2
Mu2e can operate concurrently with FNAL neutrino experiments (NOvA)
we use 6 out of 18 Booster batches, which NOvA can’t use during Main Injector cycle
G-2 and Mu2e use the same beam, but in a different manner
cannot run concurrently, but there is effort to make it possible to switch between the experiments
e+e- collisions from Phi to Psi
Novosibirsk, 2011

15. Muon Beam Line Negative gradient magnetic mirrow no trapped particles
Uniform field in tracker region
good resolution
Torroidal field in the transport solenoid
separation
e+e- collisions from Phi to Psi
Novosibirsk, 2011

16. Transport Solenoid
e+e- collisions from Phi to Psi
Novosibirsk, 2011

17. Stopping target 17 Al disks, 200 mkm thickness
Why Aluminum? Perfect for this experiment
higher Z:
larger conversion rate
different contributions to the rate
lower Z:
larger lifetime
higher conversion energy
e+e- collisions from Phi to Psi
Novosibirsk, 2011

18. T-tracker 18 stations x 12 panels x 2x50 straws =21600 straws
5 mm dia. Mylar straws,
15 μm walls, 20 μm Au-plated W wire
200 μm position resolution
e+e- collisions from Phi to Psi
Novosibirsk, 2011

19. Measuring the momentum
Low energy e- pass undetected
For high energy e- we detect several points on the helix
e+e- collisions from Phi to Psi
Novosibirsk, 2011

20. Calorimeter Located downstream of tracker
Main purpose: to provide trigger (1 kHz for E>80 MeV)
Provides redundant position, timing and energy data
Possible design:
2112 LYSO crystals,
3x3x13 cm3 arranged in 4 vanes. Density 7.3g/cm3, Moliere radius 2.07 cm, Rad. Length 1.14 cm, decay time 40 ns. Light yield relative to NaI 85%.
Two APDs per crystal
e+e- collisions from Phi to Psi
Novosibirsk, 2011

21. Cosmic rays can generate high energy electrons
Cosmic Ray Shield
Cosmic rays can generate high energy electrons
Need high suppression factor – a lot of cosmic ray events over 2 year running period
Combination of passive (magnetized iron + dirt) and active (scintillation counters) shielding
99.99% coverage
e+e- collisions from Phi to Psi
Novosibirsk, 2011

22. Expected sensitivity 1.8x1013 p/s Proton flux 2x107 s Running time
Total protons
3.6x1020 p
Stopped μ per proton
0.0025
μ capture probability
0.61
Time window
0.49
Trigger efficiency
0.80
Selection efficiency
0.19
Sensitivity (90% CL)
6x10-17
Events for Rμe=10-16 4
Estimated background
0.18
e+e- collisions from Phi to Psi
Novosibirsk, 2011

23. What we expect to see e+e- collisions from Phi to Psi
Novosibirsk, 2011

24. Project schedule
July 2005
cancelled for financial reasons not related to MECO
October 2007
Mu2e LOI submitted to Fermilab
Fall 2008
Proposal submitted to Fermilab and receives Stage I approval
November 2009
CD0 status granted
R&D and design, preparation of CDR
2012
CD1
~2013
CD2, Start of Solenoid Construction ~
Start of data taking
e+e- collisions from Phi to Psi
Novosibirsk, 2011

25. Project X
There is proposal to replace FNAL’s Booster with new 8 GeV high-intensity linear accelerator, based on ILC technologies – “Project X”
Project X would feed neutrino and rare decay/high precision experiments
Potential to upgrade Mu2e by factor x100!
set much stronger limits (if there is no signal)
or use different target materials to study nature of new physics (if there is signal)
Serious upgrade of the detector is required
e+e- collisions from Phi to Psi
Novosibirsk, 2011

26. Conclusion
In May 2008 P5 (US Particle Physics Project Prioritization Panel) Report stated:
“recommends pursuing the muon-to-electron conversion experiment under all budget scenarios considered by the panel.”
Why?
Mu2e experiment will allow to improve the current limit by 4 orders of magnitude
if signal is found, it will be an undeniable proof of the New Physics and it will provide data, complementary to LHC
if no signal is found, it will set constrains much stronger than LHC (up to 1000 TeV mass scale)
with better muon source and other upgrades, the limits can be improved by another 2 orders of magnitude
e+e- collisions from Phi to Psi
Novosibirsk, 2011