ExperimentFAQ

Mu2e Newcomer FAQ: Physics and the Experiment

The FAQ section is organized so that questions related to a specific subsystem are grouped together. For example, questions about the tracker and its many parts are found in a section called tracker.

Mu2e Physics

What is Mu2e trying to measure?
Mu2e is an experiment designed to measure the conversion of a muon into an electron (in the field of the nucleus) without the emission of the neutrinos. In the standard model, lepton conservation tells us that when a muon decays to an electron there should be an accompanying muon neutrino to conserve muon number in the decay and an anti-electron neutrino to conserve electron number.

What physics beyond the Standard Model could the experiment be sensitive to?
Some of the models that predict large effects at the sensitivity mu2e are LHT (higgs with T parity), flavor blind minimal supersymmetric standard model (FBMSSM). The following table lists some of the rest. (Link to paper)File:Table of models.jpg

Where did the idea originate? -- answer needed
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What competing and complementary measurements are being done by other experiments and what is the current best limit?
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Where do the muons come from?
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How can we determine the type of interaction that produces muon to electron conversion?
A given interaction has different rates for different nuclei. Once we measure the rate for aluminum and get sufficiently high number of events, we could change the target and measure the rate again. How the rate changes with different targets reveals information about the type of interaction. (link to paper)

how sensitive is mu2e to the decay of a negative muon to a positron (LVF) (µ- → e+ )?
Given the intensity of the muon beam, mu2e will have the most sensitive result yet after just one month of operation. The current most sensitive result from TRIUMF on 48Ti is 1.7 10^-12 with CL. 90%. Mu2e, after one month of operation will have an exclusion power of 1.6 × 10-14 and 2.7 × 10-15 over the whole run period.

In case of a signal, will mu2e be able to exclude certain physics models?

Mu2e is complementary to other experiments looking for CLVF, alone it can’t exclude any models that the collaboration is aware of (needs checking). Mu2e’s measurement doesn’t give information about the type of decay, does the decay occur through a contact interaction, a loop interaction or a combination of both or something else. Experiments that look for CLFV through the loop type only, for example, will help determine what kind of interaction.

Why use muons and not taus, don’t taus have lower rates than muons in many new physics models?

Yes, the taus have much lower rates, however, • They are more difficult to produce and harder to control. In mu2e we produce 1011 muon per second, taus are produced ~ 1010 per year. • Also, with muon to electron conversion, the standard model background rate is ~ 10-54 while the background for taus is ~10-14. Background for mu2e is over 35 orders of magnitude away while taus are only 4-5 orders of magnitude away from the reach of the next generation of experiments (Belle II).

What is the DIO end point. Does it include radiative corrections?

104.971 (1) MeV. This Number includes interaction of the incoming muon and the outgoing electron with the nucleus' coulomb field. Link to research paper https://journals-aps-org.ezp2.lib.umn.edu/prd/abstract/10.1103/PhysRevD.94.051301 Link to archive https://arxiv.org/abs/1608.05447 Link to docdb file (internal file) https://mu2e-docdb.fnal.gov/cgi-bin/private/RetrieveFile?docid=17910&filename=2018-06-07-rad-corr.pdf

Mu2e Challenges

What are the primary experimental challenges?
To get to the desired sensitivity, we need 1018 stopped muons. There hasn't been a moun source intense enough at the time so we had to build one. Also, Given the sensitivity, we need to reduce the background from cosmic rays to acceptable level. A cosmic ray veto (CRV) system with 99.99% efficiency had to be built. More details on this point are in (link to document)

Production Target

Why does the production target have fins?
The target heats up because of the proton beam. The temperature to which the target reaches is high enough to make the target sag. If part of the target is not on axis due to sagging, our pion and thereby muon yield is reduced. The fins add to rigidity (structural integrity) and increase radiative cooling. (link to docdc-doc)

Transport Solenoid

Why does the TS have two bends instead of one
The first bend separates positive and negative particles, negative goes up and positive goes down. In the middle section between the bends, upper part of beam goes through while lower part gets absorbed. Now the beam is primarily negative particles and it is slightly high. The second bend, realigns the beam to axis.

If the field is only along the axis of the transport solenoid (TS) what makes the muons direction of motion follow the bends of transport solenoid (TS)?
From Lorentz Force law, we can see that the plane of rotation is always perpendicular to the magnetic field direction. As the B-field changes orientation, so does the plane of rotation and hence forth the direction of the muon as is goes through the TS.

Detector Solenoid

what is the magnetic field strength in the tracker region?
1 Tesla.

Stopping Target

Was use 34 disks of aluminum for the stopping target? Why not a continuous aluminum disk of the same mass?
The design of the stopping target needs to minimise the energy loss of the electrons as it leaves the target. A solid block will produce a large energy loss, whereas disks give a smaller energy loss.

Why Aluminum for the stopping target? Is there any other choice?
Yes, there are many other choices. Al was chosen for: • The bound muon lifetime for Al is longer than other choices which allows for better pion background suppression. The other candidate target is Ti, which also has a relatively long muonic atom lifetime (although it's shorter than Al's). Also, due to its larger atomic number one would expect the conversion rate to be larger than that in Al.

Tracker

Why put gold on the inside of the drift tubes and not copper?

Our first prototype tubes had copper on the inside but oxidation was quicker than previously thought. Copper oxides are generally insulating. An accumulation of charge on the tube inner walls will cause a spark and destroy that tube and probably neighboring tubes. Gold doesn’t oxidize and is very conductive. It should be noted that gold and aluminum from a compound at room temp., Au5Al2 , which is ~10 times less conductive than gold. A study showed that Au5Al2 is also okay.

What is mu2e using for measuring electron energy loss?

There are many ways in which we could measure energy loss, two of which are being studied. • Protons from stopping target: muon capture on Aluminum rate is ~61%. The resulting Mg nucleus then decays to Na and emits a proton (µ+27Al → Mg* + νµ → Na + p + ν + a n. Where "a" is 1,2 or 3) 5 % of the time so we get a few protons per pulse. Protons are very ionizing, they lose their energy faster as they go through the tracker. From that we calculate dE/dx. • Cosmic rays: This method uses background CR muons. Cosmic ray muons that are tagged by the calorimeter first then propagate backwards through the tracker can be used for energy loss measurement. As the electron propagates upstream through the tracker, it loses energy. When it gets reflected by the magnetic mirror at one end of the DS, it propagates again through the tracker downstream. The energy loss can be measured using the reconstructed momentum of the electron.

Why are the tubes 5 mm in diameter? Was this optimized based on calculation or simulation to get the best momentum resolution?
No. CKM experiment was cancelled and they used 5 mm for their tubes, we wanted to use some of the available material for prototyping our tubes.

How was the muon lifetime calculated? Was it measured?
~61% of the time muons get captured on the AL nucleus and you are left with ~39% of muons. Free muon lifetime is ~2200 ns. 2200* (~0.39) = 858 ns. Muon lifetime has been measured (link to paper) and it is 864(4) ns.

How many tracks do we expect to have over the entire run period?
Less than 1 million tracks. About 100K reconstruct-able tracks.

What is a station, a plane and a panel in the tracker?
a panel is 120 degrees arc that holds 96 straws and the electronics (see fig.1). This is our 2nd smallest tracker block (the first being a straw). A plane is made of 6 panels (see fig.2). a station is made of 2 planes, meaning that a station is made of 12 panels (see fig.3)

What would the pressure be in tracker? What is the pressure of the gas in the straws?
To reduce Multiple scattering, the tracker is in near vacuum with a pressure of 10-4 Torr. The gas is at 1 atmospheric pressure, equivalent to 51.7149 Torr.

What is the drift gas gain?
3-5 × 104. waiting for confirmation on exact value.

What is the drift gas gain?
3-5 × 104 (don't quote because other documents assume 7 × 104 for simulation. waiting for confirmation on exact value.)

Cosmic Ray Veto (CRV)

What is the dead time of the CRV?
The dead time is about 3% (As of July 11, 2018). (link to doc)

Stopping Target Monitor

Why not use characteristic X rays from muons kicking K-shell electrons out to count stopped muons?
That the original plan but there were concerns that the X ray detectors will degrade fast in the intense radiation environment. We are now planning on using gammas from excited Mg nucleus. ~61% of muons get captured by Al. the result being an excited Mg and one of its emissions (~13%) are delayed gammas. The detector can take the intensity of those delayed emissions . (internal link - [1])

Miscellaneous FAQs

How can I learn more about the experiment?
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Mu2e Newcomer FAQ: Organization and Logistics

HELP! What does that acronym mean?
Muon to electron

How is the Project organized?
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How is the Collaboration Organized?
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How do collaborators communicate?
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Where can documentation be found?
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