Hello. I just started working with Root, and I’m writing a script to take a histogram and fit it to a landau function using Pyroot. To my knowledge, the first parameter of the landau function is the location of its peak, so I set it to be the location of the peak of my plot. However, the landau function is shifted rightward, and its peak is not placed at the value which I set it to.

Here’s the relevant portion of my code:

```
for E, file in files.iteritems():
hists[E].Rebin(50)
xAxis = hists[E].GetXaxis()
nBins = xAxis.GetNbins()
peakN = hists[E].GetMaximumBin()
peakX = hists[E].GetXaxis().GetBinCenter(peakN)
peak = hists[E].GetBinContent(peakN)
print('final peak:', peak)
print('final peak X:', peakX)
fit = r.TF1("landau","landau(0)") # Define the fit object to be landau
fit.SetParameter(0,peakX) # Initialize the fit object to the mean of the histogram...
hists[E].Draw("Same PLC PMC") # Draw the histogram, the "Same" argument keeps previous content on the Canvas
hists[E].Fit(fit, "B") # Fit the curve
fit.Draw("Same") # Draw the fit onto the histogram
## SAVE THE HISTOGRAM
canvas.SaveAs("output.pdf");
```

I get an output of peakX = 252.5. This is the resulting plot and fitted function.

I would prefer that the landau function be fit so that its peak matches up with the peak of my histogram plot. How would I achieve this? Thanks a lot in advance.

I’m using Root 6.26.04 and Python 3.10.5