Proposal for: A Precision Measurement of the Positive Muon Lifetime Using a Pulsed Muon Beam and the µLan Detector

R. M. Carey,$^{a,\dag }$ P. Cushman,^c P. T. Debevec,^b W. Earle,^a F. E. Gray,^b
M. Hare,^a E. Hazen,^a D.W. Hertzog,$^{b,\dag }$ J. P. Miller,^a
O. Rind,^a B. L. Roberts,^a C. J. G. Onderwater,^b C. C. Polly,^b
M. Sossong,^b D. C. Urner,^b S. Williamson,^b
aBoston University
^bUniversity of Illinois at Urbana-Champaign
^cUniversity of Minnesota

$^{\dag }$Co-Spokespersons

May 25, 1999

1.51.0Contact Information:
Prof. David W.Hertzog
Loomis Laboratory of Physics
University of Illinois at Urbana-Champaign
1110 W. Green St.
Urbana, IL 61801, USA
Phone: 1-217-333-3988
FAX: 1-217-333-1215
Email: hertzog@uiuc.edu

Proposal Summary

1.51.0We propose to measure the positive muon lifetime to a precision of 1 part in 10⁶ giving an absolute uncertainty on the level of 2 ps. This represents more than an order of magnitude increase in precision beyond the current world average. The muon lifetime is used to determine the Fermi coupling constant, G_F, which is the fundamental quantity governing the strength of any electroweak process. Recent theoretical work [1] enables a clean extraction of G_F from $\tau_{\mu}$ and paves the way for a new high-precision measurement. Such an undertaking is challenging and in keeping with a recent trend to improve the precision on the knowledge of the fundamental parameters of the Standard Model to the extent that modern technology permits.

A 1 ppm determination of $\tau_{\mu}$ requires 10¹² muon decays. An intense muon source and a detector system capable of handling multiple events at once are required in order to acquire the data in a relatively short running period (e.g., one month). Our collaboration has promoted [2] the concept of using a pulsed muon beam combined with a segmented, fast timing detector. The approach distinguishes a muon ``accumulation period" of duration $1~\mu$s from the decay ``measuring period" of approximately $11~\mu$s. In the past year, discussion of time-structured pion and muon beams at PSI has been led by Kammel et al. [3]. Our experiment, having been designed from the start to utilize a pulsed beam, is ideally suited for a chopped surface muon beam at PSI.

The experiment is designed to address specifically the known critical sources of systematic error; each is discussed in detail in the body of the proposal. A depolarizing sulfur target is used to stop the $\mu^{+}$ and to reduce the residual polarization of the ensemble to a few percent. Individual muon spins are dephased during the accumulation period by the inclusion of a 75 G transverse magnetic field. Decay positrons are registered in the µLan (Muon Lifetime Analysis) Detector which consists of 180 triangular timing tiles distributed uniformly within the 20 SuperTriangles of an icosahedral geometry centered on the target. Each tile consists of an inner and an outer scintillator coupled to independent photomultiplier tubes (PMT). Waveform digitizers are used to read out each PMT. This enables both timing and energy deposition to be recorded for each event. The high segmentation and the double-pulse rejection capabilities reduce the effect of pileup on the measured lifetime to a level below 1 ppm. The geometry features 90 point-like symmetric tile pairs; the sum of any pair is used in the lifetime analysis. The difference in rate versus time in any tile pair highlights sources of asymmetry.

Our plan evolves from the considerable experience we have gained in the present muon 2 experiment [4] at BNL. Our collaboration has been responsible for detector, electronics, data acquisition and analysis software development there. We have experience in building counting systems with a high level of timing and gain stability. Many of the systematic issues critical to the measurement of 2 apply to this experiment as well.

We plan to build and stage the µLan Experiment over a three year period as outlined in this proposal.

Beam Requirements

Our overall beamtime request follows a three-step approach toward the final production data. For purposes of illustration, we outline these as occurring over a three year period. The actual schedule will depend on the development of the chopped beam facility at PSI and on the progress made in detector and electronics tests.

In this plan, the first year following approval will be used to finalize the design and construct a significant prototype, namely two full SuperTriangle systems. We will take these to an ordinary continuous surface muon beam at PSI in order to test their response and to test polarization issues related to the target.

The second year will be used to complete the detector and electronics production and to build the mechanical support structure. The data acquisition system will be finalized. This system will be tested at PSI using a low-intensity beam in order to measure detector response and to debug the system. If the chopped beam is available, we would make use of it to do a short commissioning run.

We envision the third year as being slated for a full production run, following a commissioning run with the chopped beam in which we will measure the extinction factor of the chopping system, the dephasing factor, and the level of symmetry in the detector tile pairs. The production run will require approximately 1 month of beam time.

A compact list of steps is given in the itemization which follows:

1.

Year 1: Use beam line $\pi$E1 with separator tuned for surface muons. Use intensity of up to 10⁶ Hz. Setup includes (1) installation and tests of the beam counter in vacuum, (2) installation of helium bag with target assembly, (2) installation of two prototype SuperTriangles (scintillator assemblies), (3) installation of laser calibration system, (4) installation of cabling to counting room, (5) installation of electronics and data acquisition system in counting area. Estimated setup time 1 week. Run with depolarizing and non-depolarizing targets. Measure backgrounds. Test ability to cancel forward to backward asymmetry. Request 1 week of beam time.

2.

Year 2: Use beam line $\pi$E1 with separator tuned for surface muons. Use intensity of up to 10⁶ Hz. Install and commission at least half of the µLan Detector. Install ancillary equipment as above. Estimated setup time 2 weeks. Measure $\tau_{\mu}$ to 20 ppm. Request 3 weeks of beam time.

3.

Year 3: Use chopped muon bean line. Use intensity of 1.5 x 10⁷ Hz with a $1~\mu$s on, $11~\mu$s off cycle. Install µLan Detector and ancillary equipment as above. Estimated setup time 2 weeks. Commission chopped beam with the µLan Detector. Estimated beam time 1 week. Do production run for lifetime. Estimated beam time 4 weeks. Further steps depend on progress.

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