1 gaskelld 1.1 March 9, 2004
 2              
 3              I have implemented some new flags to help calculate the "central" cross
 4              section in simc_semi.
 5              
 6              The basic idea is that, starting with a model cross section that describes
 7              the data well, we can extract the experimental cross section using:
 8              
 9              sigma_exp = Ndata/Nsimc * sigma_model
10              
11              In simc_semi, we want the cross section at some fixed Q2 and xbj and binning
12              in either z or pt2.  The modifications to simc_semi will allow you to dump
13              the "central" cross section event-by-event to the ntuple. The central cross
14              section is given in terms of
15              
16              	sigcent = dSigma/( dOmega_e dE_e dOmega_had dE_had) 
17              
18              in units of microbarns/MeV**2/sr**2.  The fixed Q2 and xbj are taken from the
19              beam energy and central spectrometer settings.
20              
21              If you are binning in z, pt2 is fixed at some constant value that you control 
22 gaskelld 1.1 in the input file.  The z-binning is also determined in the input file where 
23              you control z_min, z_max, and the number of bins.  Using the same input 
24              parameters, you can likewise bin in pt2 and pick some constant z.  
25              
26              The new flags of interest are:
27              
28              sigc_flag = 0  ;  Set this to 0 to bin in z, or 1 to bin in pt2
29              sigc_nbin = 10 ; number of bins in z or pt2 for "central" cross section calc
30              sigc_kin_min = 0.31  ; minumum z (or pt2) for central cross section calc 
31              sigc_kin_max = 0.43  ; maximum z (or pt2) for central cross section calc
32              sigc_kin_ind = 0.005 ; value for 'independent' variable (pt2 in GeV2 if 
33              	             ; binning in z)
34              
35              The above example is for binning in z.  One might use the settings below to bin
36              in pt2.
37              
38              
39              sigc_flag = 1         ; bin in pt2
40              sigc_nbin = 10        ; number of bins for "central" cross section calc
41              sigc_kin_min = 0.0    ; minumum pt2  for central cross section calc 
42              sigc_kin_max = 0.02   ; maximum  pt2 for central cross section calc
43 gaskelld 1.1 sigc_kin_ind = 0.37  ; value for 'independent' variable, z in this case
44              
45              
46              Some new variables in the ntuple are:
47              
48              sigcent - already defined above. the lab cross section
49              centjac - the "central" jacobian that converts dz dpt2 dphi -> dE_h dOmega_h
50              	you can get dSigma/(dOmega_e dE_e dz dPt2 dphi) by taking 
51              	-> sigcent/centjac
52              zi = E_hadron/nu at the vertex.  This is what's used in the model
53              pt2i + the transverse momentum at the vertex
54              
55              If you plot, for example, sigcent vs. zi after running simc in the mode that
56              calculates the z-binned central cross section, you should see a step-like
57              pattern, i.e., a constant cross section for a range in z equal to the bin 
58              width.
59              
60              Something like:
61              
62              	   ---
63              	      ---
64 gaskelld 1.1 	         ---
65              		    ---
66              		       ---
67              
68              
69              Note that if you plot vs. the reconstructed z, the pattern will be smeared 
70              around because of multiple scattering, energy loss, etc.
71              
72              
73              To get the cross section as a function of z, one can then define the normalized
74              yields per z-bin of data and Monte Carlo multiplied by the central cross 
75              section in each zi bin or
76              
77              sig_exp (z-vertex) = Ndata (z-reconstructed) * sig_model(z-vertex)
78              		     -----------------------
79                                   Nsimc (z-reconstructed)
80              
81              
82              
83              
84              
85 gaskelld 1.1 
86              
87