Digital H-CAL at  first

What's  the Digital Hadron  Calorimeter (DHCAL) ? .
In  the  particle experiment, energy  measurement  of  a particle is done by  the calorimeter, beside the momentum measurement is done by the  tracker under the  magnetic field. These two  independent measurements  give us  the mass of a particle and  we can identify the particle. However, in high energy collider experiments, jet production from high energy quarks,  weak bosons and   gluons are  dominating in the events.  Therefore  identifying the source of  the jet is the most important task for  the detector system. To achieve  this task, finer granularity detector  is required not only  in the tracker but also in the  calorimeter so  as  to resolve  each  particle in a  jet. Especially in the calorimeter, dense  bundle  of particles  in a jet makes the  energy  measurement difficult , since the overlapping of the showers spread out in the calorimeter. In order to solve those problems, a novel idea to identify every particle in a jet is proposed. This is called Particle Flow Algorithm (PFA).  To  prove the the PFA, this   project is set up in  the part of the world wide studies of  the calorimeter  for  the  linear  collider.

(This figure shows an   example of  PFA. With PFA one will have  60%  better Z mass resolution with  digital Energy Flow Algorithm in the right figure, although the PFA is not yet tuned well.  ref.:ECFA WS on  Nov./2003at  Montpellier, by V.Zutshi)
‚PDProject
  Most of the important physics processes studied at future high energy linear collider (LC) experiments contain multi-jets  final states,  where  identification of each jet  is required. An idea to clarify the origin of the jet is proposed named Particle Flow algorithm (PFA),  to know all the final state particles interacted in the calorimeter detector. Since the momentum resolution  of the   charged particle is much better than the energy resolution of the calorimeter, the PFA depends on the tracker which determines the momentum of the charged  particles as well as the calorimeter which measures the energy of the neutral particles.

(This figure  is  a result of a digital calorimeter simulation. The solid lines  show tracks by charged particles. The dots are energy  deposited hits due to interactions in the  calorimeter detector, which is express by   digitally. This kind of jet will  have to be analyzed. Since colors of hits correspond to each charged track,  it is rather easy to distinguish  to each other, however, those hits will be mixed  up in  the real  jets. It will be difficult to identify even each track matching. ref.:Snomass WS on  Jul./2001at  Montpellier, by H.Videau,also http://polywww.in2p3.fr/flc/justif/justif-granul.html)
Large fluctuation  is expected to measure  the energy  of the neutral particles, especially for the neutral hadrons, such as KoL  and neutrons. The   sampling  calorimeters  are   devices which  can  identify  those  interactions  by   getting energy   information longitudinally into the shower development direction.   Information of those neutral hadron interactions in the calorimeter may help to  reduce the  fluctuation.  To know such interactions in the detector, one needs very high granularity ever constructed such  as 1cm x 1cm in every layer as well  as  good segmentation   in the  shower  developing direction. To reduce the read out load, from huge number of channels of such detector, digital read out, where on or off of the energy deposit in a small cell is proposed, which  is called Digital Hadron Calorimeter (DHCAL). Intensive simulation works indicate the  PFA works fine with   DHCAL. Our study aim here is to establish the PFA with DHCAL idea by both  simulation  and actual test detector. We can propose such a DHCAL as a LC calorimeter.
  Toward the design of a DHCAL detector, we propose here a three-year program of R&D.  We  develop  the PF Algorithm as  well as to construct a prototype DHCAL  in order to demonstrate the feasibility and to find out the best design of the  detector.
‚QDplan
  In order to construct DHCAL at the LC, the followings are required:
(1) the PFA is proved to be applied to  identify jets in  the simulation.
In  order to achieve this, intensive  developments of the PF  Algorithms are needed.  The members of this project in US have already started the simulation efforts and achieved some results, however, they are not sufficient to  complete PFA yet.  We start from their simulation  study and evolve the  algorithms  in  detail. At this moment, PFA software consists of several finders, which play roles to find a source of the particle after many hits in the DHCAL, since the  detector response is represented  in  hits distributed in  the calorimeter.
In our detector design, the calorimeter consists of two parts, namely electro magnetic calorimeter (ECAL) and hadron calorimeter (HCAL). The ECAL sits in front of the calorimeter to measure the  energy of the photons which  are the neutral components of the jet. The HCAL is situated behind the ECAL and mainly  measures the energy of the neutral hadrons, although the HCAL  responds to the charged particles as  tracks and secondary photons as electromagetic  showers in  hadron shower. Since the  electromagnetic showers  develop  in  the   dense material such as  lead or iron, in locally, one can  find the electromagnetic shower quite efficiently.  This task  is called the photon-finder.
(the figure shows  an example of  the   photon-finder in  an event. The  yellow hit-clusters correspond the photon shower being  found.ref.:Snomass WS on  Jul./2001at  Montpellier, by H.Videau, also http://polywww.in2p3.fr/flc/justif/justif-granul.html)
However, the   other  neutral hadrons  such as KoL or neutrons  interact with the protons or  neutrons  in the  absorber  material, it is  not  easy to  find their strong interactions  in the  detector.   This interaction is characterized  as a  single  track emerges in  the  detector. Finders   for  KoL and neutron  have  to be  implemented and to work efficiently. We  have to   optimize   many parameters   in the  PFA. So as to  distinguish a  track among  the hits  in  the DHCAL, the size of the hit  cell has to  be  small enough.

(The  figure  shows an example of the neutral-kaon finder result.  The neutral kaon being found is marked  in red in this example.ref.:Snomass WS on  Jul./2001at  Montpellier, by H.Videau,  also http://polywww.in2p3.fr/flc/justif/justif-granul.html)

Once the basic interactions in  the DHCAL are classified, a jet can  be recognized  by  taking into   all information such  as   track  momentum and  trajectory.  In this stage one can maintain jet energies can be measured to ~30%/sqrt(E).

(Following picture show a comparison  between different energy resolution detector.  The  figure  shows a scatter plot  in the horizontal axis  of  mass reconstructed  by   a jet and  the vertical axis of mass reconstructed by  another jet in  e+e- ->  ZZnn, WWnn  events. The jet mass-resolution assumed  are 60 %   in the left plot and  30 %  for the  right one. Clearly it indicates better than 30% is needed.ref.:Snomass WS on  Jul./2001at  Montpellier, by H.Videau, also http://polywww.in2p3.fr/flc/justif/justif-granul.html)



     To improve Particle Flow Algorithms on various simulated LC detector calorimeter designs which include :
    a) optimizing the transverse size of DHCAL cells (and the
        longitudinal segmentation as well)
    b) testing different absorber materials and thickness
    c) testing different active material types
    d) including B-field/tracking radius in optimization


(2) we have to demonstrate that the PFA works  well for  the real  experiment.  
In  order to achieve this, we will construct  a  DHCAL of  1m^3 volume which contains hadronic shower  in  it. The  PFA will be  proved under single particle environment.  For the  real test, we will  construct a DHCAL detector. For this  purpose, either RPC ( Resistive Plate Chambers)  or Plastic  scintillator tile  will be employed for the  active  medium  in  the calorimeter.    We develop and   study those devices. The  RPC  is studied by  US group. The  scintillator case  is studied by Japanese group.
The RPC has a simple structure of two parallel plates separated by spacers. The outer surfaces of the plates are coated with layers of graphite paint connected to a high-voltage supply. An insulating film is glued on the graphite to shield external electrodes from the high voltage. The pick-up pads are fixed on a film of a plastic material which is pressed against the detector surface on both sides   which  allow two-dimensional readout of the particle position. It  is  fairly  easy to  have  1m  x 1m glass   plates as  this  detector.  The  major  problem sits on the signal processing  and readout system, since 10,000  channels  per plane  makes 400,000  channels of  total  readout capability. Although the  readout at each  channel   is on  or off ( 1bit  data),  huge number of the readout channel  causes  need for  a  development of special   electronics    dedicated   to this readout which will  be achieved  by ASIC   system.
The   scintillator  is reliable  and well understood material as a particle  detector. However, the  readout  of  the   scintillation lights  need more attention , because  the  number of  those  scintillators are  huge  enough. There we  will employ newly developed silicon  photo-detector (SPD) whose dimensions are small to be  O(mm^2). We can install a  wavelength  shifter fiber (WSF) in   a   scintillator  and  at  the end of  the  WSF, we  put a SPD directory. Here also the readout system needs care. We try  to  optimize  the system  to be used the same ASIC developed for the  RPC system.

(The following  picture shows a conceptual design  of  DHcal scintillator tile  configuration with a small SPD directly coupled. There two kinds  of scintillator configurations are shown.)

In this project, we study  the Digital Hadron Calorimeter both  in the  simulation  and the real experiment and show  the possibility of the  DHCAL for  the LC. Through this study, we would find out whether the Particle  Flow Algorithms  is able to resolve the jet.  After these studies, we can propose a  calorimeter  for  the  future   LC with  the PFA technologies. In order to find out the best design, two types of sensors will be tested. By assigning different types of active  sensors to the US and Japanese  groups without duplicity, the R&D can be carried out efficiently.  Development of ASIC for the multi-channel readout will also be carried out for both sensors.

This  project is part of the world wide Linear Collider detector study. At the end of  the   project, we will carry out  a beam  test experiment including not only US-J collaboration but also european collaborators. The test will  be foreseen in a following picture; (CALICE collaboration, DESY)



 



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