TMinuit problem

From: Arthur E. Snyder <>
Date: Mon, 4 Dec 2006 16:50:56 -0800 (PST)

Rene et al.,

I'm having a problem with TMinuit. I have a fit with 5 parameters which work. Then I added a 6th parameter and it fails when I try to fit that parameter.

I found one problem in that I for got to change new TMinuit(5) to new TMinuit(6), but with that fixed the same kind of problem occures.

In the debugger I see that the parameter (which is a background level) when I'm fitting only for it looks ok for the 1st couple of calls, but then minuit inputs par[5]=-20 which makes my PDF negative and my logLike an NaN .. so the fit fails.

Here are 1st three calls.

Breakpoint 2, vvfit::funcToMin (this=0x9de8ef0, npar=@0xbffe0324,

    gin=0x9dfd008, f=@0xbffe03d0, par=0x9dfcae8, flag=4) at 196 f=-logLike;
$4 = 0.10114172087954518
$5 = -132589.34477643354

(gdb) c

Breakpoint 2, vvfit::funcToMin (this=0x9de8ef0, npar=@0xbffe0324,

    gin=0x9dfd008, f=@0xbffe03c8, par=0x9dfcae8, flag=4) at 196 f=-logLike;
$6 = 0.098858279120454834
$7 = -130241.44136714273

(gdb) c

Breakpoint 2, vvfit::funcToMin (this=0x9de8ef0, npar=@0xbffe0194,

    gin=0x9dfd008, f=@0xbffe0268, par=0x9dfcae8, flag=4) at 196 f=-logLike;
$8 = -21.289116679404454
$9 = nan(0x8000000000000)


I don't have debug sysmbols for minuit so I can't see what's genrating this crazy step. The step size a specified in mnparm was small --- 0.01.

I'm a little supprised that 1st call is for 0.10011417 when starting value of fit is exactly 0.1. The values of this parameter are sensible when I fit for other parameters and indeed I get the correct answer for other parameters only when I've set this one to the correct value (0.1).

Doubteless I'm still doing something stupid like TMinuit(5), but I can't see what it is?

Thanks for any help/adivce you can give.

-Art S.

A.E. Snyder, Group EC                        \!c*p?/
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Received on Tue Dec 05 2006 - 01:51:10 MET

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