diff -N RKDeDxFixed.java
--- /dev/null	1 Jan 1970 00:00:00 -0000
+++ RKDeDxFixed.java	8 Jun 2009 05:58:29 -0000	1.1
@@ -0,0 +1,103 @@

+package org.lcsim.contrib.RobKutschke.TRF.trfeloss;
+
+import org.lcsim.recon.tracking.trfeloss.DeDx;
+
+/**
+ *  DeDx code for testing the fitter performance:
+ *     - constant energy loss
+ *     - gaussian straggling.
+ *
+ *  The amount of energy loss and the sigma of the gaussian are
+ *  controlled c'tor arguments.  
+
+ *  For a large enough sigma, this code will allow "negative" energy 
+ *  loss in the tail of the gaussian. This is the desired behaviour to
+ *  get a zero offset in the track parameter pull distributions.
+ *
+ *@author $author$
+ *@version $Id: RKDeDxFixed.java,v 1.1 2009/06/08 05:58:29 kutschke Exp $
+ *
+ */
+public class RKDeDxFixed extends DeDx
+{
+    
+    private double _density;
+
+    // Scale the nominal energy loss by this factor.
+    private double _scale = 1.0;
+
+    // Sigma of the gaussian smearing.
+    private double _sigma   = 0.15;
+
+    /**
+     *Construct an instance given a material density.
+     *
+     * @param   density The density of the material.
+     */
+    public RKDeDxFixed(double density)
+    {
+        _density = density;
+    }
+
+    public RKDeDxFixed(double density, double scale, double sigma)
+    {
+        _density = density;
+	_scale   = scale;
+	_sigma   = sigma;
+    }
+
+    
+    /**
+     *Return energy loss (dE/dx) for a given energy.
+     *
+     * @param   energy The energy of the particle.
+     * @return The average energy lost by a particle of this energy.
+     */
+    public double dEdX(double energy)
+    {
+        double mip = 1.665; // this is an average mip
+        double de_dx = mip*_density;   // MeV/cm
+        de_dx /= 1000.; // GeV/cm
+
+	de_dx *= _scale;
+        return de_dx;
+    }
+    
+    /**
+     *Return the uncertainty in the energy lost sigma(E).
+     *
+     * @param   energy The energy of the particle.
+     * @param   x The amount of material.
+     * @return The uncertainty in the energy lost sigma(E).
+     */
+    public double sigmaEnergy(double energy, double x)
+    {
+	double sigma_e = _sigma*dEdX(energy)*x;
+        return sigma_e;
+    }
+    
+    /**
+     *Return new energy for a given path distance.
+     * Energy increases if x < 0.
+     *
+     * @param   energy The energy of the particle.
+     * @param   x The amount of material.
+     * @return New energy for a given path distance.
+     */
+    public double loseEnergy(double energy, double x)
+    {
+        double deltaE = dEdX(energy)*x;
+        return energy-deltaE;
+    }
+    
+    
+    /**
+     *output stream
+     *
+     * @return A String representation of this instance.
+     */
+    public String toString()
+    {
+        return "RKDeDxFixed with density "+_density;
+    }
+}