Minutes, DØ Meeting, 4/17/98

Ela Barberis, WH to lnubbar

I have started studies at the generator level, using Pythia events generated at
MH=90GeV (sigma*Br=64fb).  The output of Pyhtia is in STDHEP format and a PJET
algorithm is run on this output.  The principal backgrounds to this analysis,
since a lepton+missing Et signature is required, are those containing at least
one W.  The W+jets background is suppressed by the b-tagging requirement.
Assuming a tagging efficiency of 50% for 1 b, with a fake rate of 0.5% (CDF
RunI estimates), augmented by an additional 1.2 factor which takes into account
soft lepton tagging (from D0 RunI numbers) the S/sqrt(B_Wjets) improvement is >
100, and >10000 for 2 tagged b's.  Other backgrounds which do contain two b's
are not reduced by tagging.  The background with larger cross section
(sigma*Br=16133) is Wbbar production. I plan to generate Wbbar events with the
CompHEP generator, which has been coupled to JETSET/PYTHIA for fragmentation
and FSR (ISR are not included), by the single top group. I have looked at some
existing files, the background distribution is peaked at low values of the
dijet invariant mass Mjj, with a long to higher Mjj. Some topological cuts
might be investigated in order to reduce this particular background.  An
irreducible background is WZ production, with Z->bbar; this reconstructs to Mjj
equal to the Z mass and has a cross section (sigma*Br=91fb) comparable to the
signal.  Other backgrounds are ttbar production, sigma*br=1440fb (it
distributes at high values of Mjj), and single top, sigma*Br=533fb (low Mjj
values).  All of these have been generated with PYTHIA and the cross sections
quoted in here, except for Wbbar, are from PYTHIA.  The S/sqrt(B), taking in
consideration all of the above backgrounds, prior to kinematic cuts, is 0.5 for
1fb-1.  Further studies involve, of course, more Monte Carlo generation, fast
reconstruction (using either MCFAST or SHW), topological studies, tagging and
studies on how to improve the dijet mass resolution.

John Hobbs, WH to jjbb

Vladimir Sirotenko

                                                                April 17, 1998

                    HZ Channel ( H->bb, Z->nunu )

  Higgs Mass = 100 GeV

  Channel       HZ        tt       ZZ       ZW      WW       Z->nunu + X
  ------------------------------------------------------------------------
  sigma*br    .022 pb    5.6 pb   .9 pb    2.3 pb   8.2 pb    1.1 nb
               (pythia)  (D0)    (pythia) (pythia) (pythia)    (D0)

  no. events    2000    20000     10000    10000   10000      10^6
  .ge.2 jets20
   (with b's)   1394    19680      7832     7199    7401      2202 (with b,c)
  met>35         978     7691      1413     1448    1699      1387
  dphi>.5        905     7048      1303     1323    1575      1280
  HT<200         739      232      1114     1133    1312      1036
  isole,mu       735      104      1040      678     480      1035
  2 b-jets15     609       20       143       47       0        91
  2 b-jets20     562       17       128       43       0        76

  accepted sig    6 fb    4.8      11.5      9.9       0      84.0

  in mass 
  window 80-120   5.2fb   1.4       6.6      5.8       0       4.2
                                                           (1.89 both b's)    

  In bb mass histogramms all chanells are weighted according to x-section


  notes: once the HT cut is made, making a cut on .le.3 jets doesn't
  improve the sig/noise much. cut at =2 not explored but undesired

Gordon Watts, Mass analyses

---

John Hobbs

Last modified: Sun Apr 19 16:24:03 CDT 1998