MEGEVE package structure

The package MEGEVE contains a list of interesting events which are selected by means of the input variable ITYPE (integer). The implemented events are listed below.

Type of event	ITYPE Code	Routine	Explanation

e⁺ - gamma signal	1	EVESIG	52.8 MeV positron and gamma are generated back to back
e⁺ - gamma radiative decay	2	EVERDE	Radiative decay events; correlated background
e⁺ - gamma accidental background; 1st kind	3	EVEACC	Positron from Michel spectrum; gamma from radiative decay. Uncorrelated background
e⁺ - gamma accidental background; 2nd kind	4	EVEACC	Positron from Michel spectrum; gamma from annihilation in flight. Uncorrelated background

e⁺ only signal	11	EPOMEG	52.8 MeV positron
e⁺ only background	12	EPOMIC	Positron from Michel spectrum

gamma only signal	21	EPHMEG	52.8 MeV gamma
gamma only background; 1st kind	22	EPHRAD	Gamma from radiative decay
gamma only background; 2nd kind	23	EPHANF	Gamma from annihilation in flight
gamma only for studying detector response	24	EPHFLA	Gamma with flat spectrum

The flow chart of the package is shown here.

In addition to the routines used for the simulation of the physical processes, the package includes few auxiliary routines:

EVECAR : this routine reads the cards which define the features of the events to be generated. The cards are described later in detail. Note that THE USE OF EVECAR IS MANDATORY (in other words YOU HAVE TO INCLUDE the statement CALL EVECAR in your UGINIT file !);

EVEPRI: this routine prints the input cards read before. This is a control routine, which can be called after EVECAR to check that you specified all input cards correctly;

EVESTA: this routine initialize the statistics of the simulation. Presently such statistics includes only the number of generated events which were rejected because they did not satisfy the angular cuts. EVESTA can be called within UGINIT if one wants to verify the efficiency of the simulation;

EVPRST: this routine prints the summary of the statistics defined before. It can be included within UGLAST.

MEGEVE Input Cards

The package needs some cards which must be specified in the file CAREVE.DAT and read in EVECAR (see above). We report here the list and the meaning of the input cards (taken from CAREVE.DAT):

                          1   TYPE   See the code for ITYPE reported above
          1   IANG   0 Generation over 4*pi,                                                                1 Generation in the angular range,                                                 2 Generation with fixed direction,                                                  3 Generation in extended angular range
1.5707963 3.1960463 Theta, phi angles in radians (only for IANG = 2)
          1   FORMOP 0 Approximate formulae,                                                              1 Exact formulae
          0. POL    Muon polarization
          1. DX     Maximum allowed difference of e+ energy                                    with respect to m_mu/2
          0.1 DY     Maximum allowed difference of gamma energy                                    with respect to m_mu/2
          1   ITAR   1 Extended Target,                                                                0 Pointlike Target

The card IANG detemines if the positron and/or photon angles must be generated over the whole solid angle (IANG = 0) or in the angular range of interest for the experiment (IANG = 1) or along a fixed direction (IANG = 2) or in the extended angular range (IANG = 3), corresponding to 10 degrees more on both sides for phi and no lower bound for abs(COS(theta)). In the third case, you have to specify the theta and phi angles of the photon initial direction (these two numbers are meaningless for the other cases). The option IANG = 2 is useful for particular tests; for the remaining, IANG = 1 is suggested, since the generation is much more efficient. The angular distributions of the various processes are taken into account in all cases.

The card FORMOP is used to choose between approximate (FORMOP = 0) or exact (FORMOP = 1) formulae. Presently, only the approximate formulae are implemented for pair events (TYPE < 10). Anyway, such formulae are very well suited for studying the background in the energy and angular windows of the expected signal; then the use of the approximate formulae is STRONGLY SUGGESTED if one is interested in studying the background sources for energies close to 52.8 MeV. On the opposite, we suggest the use of the exact formulae for studying the gamma background in wider energy and angular regions (for instance, to define trigger criteria).

The card POL gives the degree of muon polarization (0 < POL < 1). Unpolarized muons corrsespond to POL = 0, fully polarized muons to POL = 1. The degree of muon polarization is a parameter of the positron and gamma angular distributions in the background sources. Since any factor containing POL is energy dependent, the generation of the events is very much slower if POL > 0, because such coefficients must be computed event by event; on the contrary, if POL = 0 the coefficients are computed once at the initialization time. Then, take care of the computer time when using POL > 0 !

The cards DX and DY are defined as the maximum fractional difference of the positron and gamma energies with respect to the kinematical bound m_mu/2. The energy spectra of the particles are generated between m_mu/2*(1 - DX) (or (1 - DY)) e m_mu/2, in agreement with the notations used in the proposal. Then, choose DX = 1 (not 0 !!!) or DY = 1 to generate events in the whole energy spectrum.

The card ITAR is used to choose between extended (ITAR = 1) or pointlike (ITAR = 0) target. The first case is realistic, the second can be used if one wants to disentangle some possible effects on the resolution due to the finite size of the target and to the physical processes inside it.

Use of MEGEVE in a GEANT based program

MEGEVE communicates with any external program using it through the input ITYPE and three output vectors:

CALL MEGEVE(ITYPE, CODE,VSTART,PSTART)

CODE(2) INTEGER

    GEANT code of the particles:
       CODE(1) = 2 positron generated, CODE(1) = 0 positron NOT generated
       CODE(2) = 1 gamma generated,    CODE(2) = 0 gamma    NOT generated

VSTART(2,3) REAL

    Starting position of positron and photon:
       VSTART(1,1:3) carthesian components of the positron position
       VSTART(2,1:3) carthesian components of the photon position

PSTART(2,3) REAL

    Starting momentum of positron and photon:
       PSTART(1,1:3) carthesian components of the positron momentum
       PSTART(2,1:3) carthesian components of the photon momentum

Then, to use MEGEVE in a GEANT-based program you have to perform the following steps:

1) Include the statement:

CALL EVECAR

in your UGINIT routine to read the cards and initialize the package.

2) Include the following lines in your GUKINE routine:

                          INTEGER ITYPE,CODE(2)
                          INTEGER I,J,NVTX,NV,NT
                          REAL VSTART(2,3),PSTART(2,3)
                          REAL PLAB(3),VERTEX(3),UBUF(1)

ITYPE = TYPE
CALL MEGEVE(ITYPE,CODE,VSTART,PSTART)

                          DO I = 1,2
                                 IF (CODE(I).NE.0) THEN
                                     DO J = 1,3
                                           VERTEX(J) = VSTART(I,J)
                                           PLAB (J) = 0.001*PSTART(I,J)
                                     ENDDO
                                     CALL GSVERT(VERTEX,0,0,UBUF,0,NVTX)
                                     NV = NVTX
                                     CALL GSKINE(PLAB,CODE(I),NV,UBUF,0,NT)
                                ENDIF
                          ENDDO

The DO-loop is not mandatory, but it is the most simple and general way to retrieve the starting positions and momenta, remove the useless information (if only one particle is generated) and call the GEANT routines GSVERT and GSKINE with the appropriate parameters. Alternatively, you can select by yourself the information you need from the MEGEVE outputs VSTART and PSTART.

For suggestions and/or gripes, please send a mail to Fabrizio Cei .

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