------------------------------------------------------
      HYDRO, fast event generator for simulation of flow effects   
      in central and semi-central heavy ion AA collisions at LHC
      -------------------------------------------------------

      Igor Lokhtin, SINP MSU, Moscow, RU
      e-mail: Igor.Lokhtin@cern.ch 

      References: 
      N.A.Kruglov, I.P.Lokhtin, L.I.Sarycheva, A.M.Snigirev, 
      Z. Phys. C 76 (1997) 99-105;  
      I.P.Lokhtin, L.I.Sarycheva, A.M.Snigirev, 
      Phys. Lett. B 537 (2002) 261-267;   
      I.P.Lokhtin, A.M.Snigirev, Preprint SINP MSU 2004-14/753, hep-ph/0312204. 
 
      Web-page: 
      http://cern.ch/lokhtin/hydro
      -------------------------------------------------------      

      Description of routine
       
CALL hydro(A,ifb,bmin,bmax,bfix,ny)  - generates one event

input parameters to fix 'thermal' hadron event configuration : 
(set in main program by user before hydro call)

  A    - beam and target nucleus atomic weight 
  ifb  - flag of type of centrality generation 
         =0 impact parameter is fixed (bfix)  
	 >0 or <0 impact parameter is generated with standard glauber geometry 
	          between minimum (bmin) and maximum (bmax) values 
  bmin - minimum impact parameter in units of nucleus radius RA
         (i.e. minimum value in [fm] will be bmin*RA), 
	  valid only if ifb not equal to zero   
  bmax - maximum impact parameter in units of nucleus radius RA
         (i.e. maximum value in [fm] will be bmax*RA),
	  valid only if ifb not equal to zero 
  bfix - fixed impact parameter in units of nucleus radius RA 
         (i.e. fixed value in [fm] will be bfix*RA), 
	  valid only if ifb=0
  ny   - baseline mean charged particle multiplicity per unit rapidity at 
         mid-rapidity, <dN(ch)/dy> (y=0), in central Pb-Pb collisions 
	 (total multiplicity for other centralities and atomic numbers 
	  will be calculated automatically)   

strength of transverse flow can be varied by user changing maximum transverse
flow rapidity, ytfl, which is stored in common block hyflow: 
COMMON /hyflow/ ytfl 
(allowed range is 0.01<ytfl<3.0, default value is ytfl=1.)

generated value of impact parameter, bgen, is stored in common block hyfpar:
COMMON /hyfpar/ bgen
(bgen is in units of nucleus radius RA, i.e the value in [fm] will be bgen*RA)

output particle information is stored in JETSET common block lujets
COMMON /lujets/ n,k(150000,5),p(150000,5),v(150000,5)
with extended size of arrays 
(from 4000 in standard version of JETSET to 150000 for heavy ions). 

NOTE! Since hydro uses jetset common block and other jetset elements, main 
      users program should be compiled with jetset_73.f with extended size 
      of common block lujets   
       
  n        - event total multiplicity       
  k(i,1-5) - particle codes (partially used by hydro)      
  p(i,1-5) - particle four-momentum and mass (always used by hydro)
  v(i,1-5) - particle vertex, production time and lifetime (not used by hydro)  

Detailed description of common block lujets and other jetset elements can be
found in PYTHIA and JETSET manual: 
T.Sjostrand, Comput.Phys.Commun. 82 (1994) 74 
      -------------------------------------------------------
      
      Physics validity of the model 
      
1. Internal parameters of the routine for flow were selected as an estimation 
for LHC heavy ion beam energies. The result for other beam energy ranges, 
obtained without additional internal parameters adjusting, is not expected 
to be reasonable. 
2. Hydro-type description of heavy ion collisions is expected to be applicable  
for central and semi-central collisions. The result obtained for very 
peripheral collisions (b~2*RA) can be not adequate. 
3. Hydro flow mechanism in heavy ion collisions is valid for restricted 
kinematic range: mid-rapidity, low and intermediate pt. The model is not 
applicable for very forward rapidity (|y|>3) and very high pt (>> 2-5 GeV/c).