Hi Rooters,
I found two troubling things using version 2.23/12 of root.
First, it turns out that Axis_t happens to be a float. For a precision
experiment such as we are involved in (Muon (g-2) at BNL), float
precission is not sufficient. We intend to measure a modulation frequency
down to 0.35 parts-per-million over approximately 1.4ms and obviously need
an accuracy (much) better than 0.35ppm on the bin boundaries. I find a
typical (relative) precision of 6e-8 (see attached ps-file), which is
small, but not conveniently so. I realise that changing the axis-type will
create backwards-compatibility problems, but would still like to urge you
to change this type to a double-precision float (why did you guys start
with something inferior in the first place ????).
Second, it turns out that it is impossible to fit the spectrum in the
attached file "demo.root" to a pol4 (fitting option "W"; the errors are
not really errors). The problem is not so much that the fit fails (tough
problem), but more that Minuit says that the minimization has converged:
root [3] fscan->Fit("pol4","W")
FCN=0.0517549 FROM MIGRAD STATUS=CONVERGED 264 CALLS 265
TOTAL
EDM=3.83883e-08 STRATEGY= 1 ERROR MATRIX
ACCURATE
EXT PARAMETER STEP FIRST
NO. NAME VALUE ERROR SIZE DERIVATIVE
1 p0 3.49545e+01 1.40721e-01 2.48411e-04 1.96904e-03
2 p1 -3.60430e+04 1.41421e+00 5.25577e-01 5.33101e-12
3 p2 -7.66045e+07 1.41421e+00 3.65279e+01 -4.35899e-10
4 p3 -1.97077e+04 1.41421e+00 2.90048e+00 -4.10165e-13
5 p4 2.54398e-12 1.41421e+00 7.66554e+05 -3.02779e-16
Clearly, the fit is beyond lousy. Is the problem simply that I gave Minuit
a problem that is too difficult, or is there something wrong with it?
Greetings,
-- Gerco
Dr. C.J.G. Onderwater
Nuclear Physics Laboratory
312 Loomis Laboratory of Physics
University of Illinois at Urbana-Champaign
1110 West Green Street
Urbana, IL 61801-3080
Phone : (217) 244-7363
Fax : (217) 333-1215
E-mail: onderwat@uiuc.edu
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