A Search for a State which Decays to a Charged D* and Proton at FOCUS

April 28, 2004


Recently, the H1 collaboration announced an observation of a narrow resonance at 3.099 GeV decaying to D* - proton and D*+ pbar, which can be interpreted as a pentaquark {u u d d cbar} and its charge conjugate. The signal is shown in the left-hand plot below. On the right is shown the corresponding plot of the D* - D mass difference of the charged D* sample. (More details are in the H1 preprint, hep-ex/0403017.)


     


Since the FOCUS/E831 experiment has accumulated a large, clean sample of D*'s and D's, we searched for the H1 state to try to confirm its existence. As shown in the plots below, our analysis DOES NOT corroborate the H1 state at a mass of 3.099 GeV/c^2. The FOCUS D* sample is about 5 times larger than the one presented by the H1 experiment. In the following text, charge conjugate modes are included implicitly in the candidate channels.

Search for Pentaquark Decays to D* - and Proton

Click here for the original postscript file.

Selection of the charged D* candidates

We applied a tight selection cut to get a very clean sample of charged D* events, where we require a D* decays into a neutral D and a slow charged pion, and the neutral D subsequently decays into either Kpi or K3pi. For the neutral D (Kpi/K3pi), we asked
  • Cerenkov particle identification cuts for kaons and pions.
  • Quality cuts for the D production/decay (primary/secondary) vertices.
  • D decay distance to be more than 5 sigmas.
  • The location of the D decay vertex is required to be out of the target matter.
  • The decay tracks from the D are required to be inconsistent with being in the primary vertex.
  • Normalized mass of the D candidates should be within 2 sigmas from the D mass peak. (Normalized mass = reconstructed mass/error on mass)
The slow pion candidates are required to be from the primary vertex. In the plot, the background level in the sample is represented by a red dotted histogram, which shows events with a wrong sign charge correlation between the neutral D and the slow pion.
Click here for the original postscript file.

Pentaquark -> D* - proton candidates


The proton candidates are required to be tracks from the primary vertex and to have the Cerenkov response of likelihood(kaon)-likelihood(proton) >1 unit.

The D* candidates within the D* - D mass difference range of 0.142 GeV and 0.149 GeV are used for the resonance search.

The D* - proton invariant mass of the pentaquark candidates are shown in the left plot. The charge conjugates are included in the histogram. We do not see the resonance structure at 3.099 GeV or any other prominent structure under 3.7 GeV.


Search for Pentaquark Decays to D - and Proton

Click here for the original postscript file.

Selection of the charged D candidates

We decided to look for a penta quark candidate which may decay into a charged D and a proton. For the charged D, we used the K-pi+pi+ decay mode. As in the case of the D* sample, we applied tight selection criteria to obtain a very clean sample. For the K-pi+pi+ selection, we require
  • Cerenkov particle identification cuts for kaons and pions.
  • Quality cuts for the D production/decay (primary/secondary) vertices.
  • D decay distance to be more than 6 sigma.
  • Any orphaned track in the event (neither from the primary nor from the secondary vertices) is required to be inconsistent with being in the secondary vertex.
  • The location of the D decay vertex is required to be out of the target material.
  • The decay tracks from the D are required to be inconsistent with being in the primary vertex.
  • Normalized mass of the D candidates should be within 2 sigma of the D mass peak.
Click here for the original postscript file.

Pentaquark -> D - proton candidates

The proton candidate requirements are the same as used in the D* - proton study above.

The D - proton invariant mass of the pentaquark candidates are shown in the plot to the left. Their charge conjugates are also included in the histogram. We do not see the resonance structure at 3.099 GeV or any other prominent structure under 3.5 GeV.

Questions about the analysis or problems with the web page should be addressed to Doris Kim at doriskim@uiuc.edu.
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