Multiparticle Jet Reconstruction

Code Summary

• /home/mvl/core_jet
  
  Data are in
• /home/mvl/core_jet/himicro*
  
  file list
• /home/mvl/core_jet/micro_lists
  
  code
• StRoot/StHighptPool/StHiJet
  
  scripts
• doHiJet.C
• dihijet.csh
  
  e.g.: qsub -j y -m n -o hijet_minb_cent0.o\$JOB_ID dohijet.csh micro_list/year4_minb.lis 0
  

  -j y[es] Specifies whether or not the standard error stream of the job is merged into the standard output stream.

  -m b|e|a|s|n,... Available for qsub, qsh, qrsh, qlogin and qalter only. Defines or redefines under which circumstances mail is to be sent to the job owner or to the users defined with the -M option described below. The option arguments have the following meaning:

  
  `b' Mail is sent at the beginning of the job.
  `e' Mail is sent at the end of the job.
  `a' Mail is sent when the job is aborted or rescheduled.
  `s' Mail is sent when the job is suspended.
  `n' No mail is sent.
  
  NJetPtMax->Draw("colrz")

FYI

• Copied /home/mvl/core_jet to /home/bhaag/core_jet
  
  
• Various existing .root files already produced :
  jethist_pt30_MinBias.root
  jethist_pt35_Cent1.root
  jethist_pt35_Cent0.root
  jethist_pt35_MinBias.root
  
  
• Defined hists:
  NJetPtMax
  NSeed
  PtPt
  dPhiPtMax
  dPhiPtPt
  dPhiSeedAll
  dPhiSeedAssoc
  dPhidEta
  
  
• In addition branches defined:
  jet_tree
  pair_tree

Discussion

Before even discussing full jet reconstruction, we need to explore what we'll call multiparticle jet reconstruction. This is the first step between the correlation studies attempted before (nucl-ex/0604018) and full jet reconstruction. The studies attempted before took 1 trigger particle (defined by pT trigger) and correlated it with other particles above some threshold pT. The multiparticle jet reconstruction will use a similar technique, but define 1+ particles as a trigger (i.e. a pT trigger defined as 10 GeV can either be one 10 GeV hadron or two 5 GeV hadrons, a 2 GeV hadron plus two 4 GeV hadrons, and so on).
There are potential advantages and disadvantages to this method. Two advantages are statistics and better Ejet resolution. You get a more direct understanding of efficiency as a function of partonic pT because you are picking up multiple hadrons in your trigger. You have a better handle on the probability of a given parton fragmenting to multiple hadrons. You also come closer to sampling actual Ejet by clustering multiple hadrons in your trigger.
One of the potential disadvantages is not having any theory for multiparticle fragmentation. The jet energy is connected to the observed hadron energy through a fragmentation function. Adding in additional particles would be considered intra-jet interaction which is not well understood theoretically.
Starting with the description of an actual jet reconstruction in p+p:

• Sum all the track energies
• Sum all the calorimeter hits in a cone around a specified jet axis:
  R = SQRT(Deta^2 + Dphi^2)

You can assume the jet energy is approximately equal to the parton energy. If you do an infrared sum, you can see if theory and experiment are consistent. This is not a perfect schema though as there is "missing" energy in the form of unmeasured neutral jet particles (i.e. neutrons, neutrinos, K0Long).
In addition, all of this assumes partons are fragmenting in vaccuum (pp, dAu). In Au+Au collisions, you can't do this so easily because of all the uncorrelated background. At the LHC, you could potentially do something similar to this though, because the projected jet energies would be so high (> 50 GeV). Jets would pop out far above the uncorrelated background.

Discussion of proposed analysis

Assuming that roughly 2/3 of particles in a jet are charged, and the rest are uncharged. we can start with charged tracks and worry about calorimeter towers later. Considering the interesting measurement is the fragmentation function (D(z)), we consider what has been done previously.
The most recent "fragmentation" function measured the particle yield as a function of zT = pT(associated)/pT(trigger). The correct fragmentation function would measure the yield as a function of z = pT(hadron)/Ejet. A full jet reconstruction would allow one to actually do this measurement. In general you would expect a modification of the fragmentation function due to energy loss, if you plot it as a function of z, since fragmentation should be independent of jet energy. This is due to the fact that your hardest fragments are shifted to a lower pT (scattering in the medium) which subsequently populates the lower z region of the fragmentation function. You would also expect an overall suppression relative to dAu for instance, in the awayside jet, as well as this shape modification.
In the aforementioned recent STAR measured D(zT), the suppression was observed, but not the expected shape modification. PHENIX claims that this could be due to a bias introduced by requiring a second particle, which then biases the trigger particle distribution. This means that D(zT) is mostly sensitive to the number distribution of the parton pT and not to D(z), the actual fragmentation function. (One way to circumvent this is to look at gamma-jet correlations to give a more direct handle on D(z) than a di-hadron measurement).

In summary, the first proposed steps are:

1. Run over the same data as mvl, verifying that I know how to run over the data by generating the same histograms.
2. Adjust various parameters to see how the histograms change;
   const float StHiJet::mMinSeed=5;
   const float StHiJet::mJetR=0.3;
   const float StHiJet::mEtaCut=1;
3. Run over the p+p data for a controlled comparison.
4. Repeat the most recent STAR analysis, using a multi-hadron trigger, such that you form a proto-jet, from which you can derive Ejet(core) -- a better approximation of the jet energy
5. Measure the angular correlations for varied pT triggers and associated hadrons.
6. From this dN/dzT (zT = pT(associated)/Ejet(core)) distributions can be developed for the near and away-side jets.
7. Another observable that can be studied is R(AA), where the ratio of the yield in Au+Au to that in the equivalently scaled p+p collisions is defined. If there are kinematical dependencies the shape will indicate so.
8. Cu+Cu analysis may be more fruitful than Au+Au, since the same hard parton scatterings occur with 2 or 3 times less overall multiplicity.

Sample Histograms