FOCUS double charm baryon search

Summary

An extensive search for fully reconstructed doubly-charmed baryon candidates using FOCUS data was conducted in 2000. Using reconstructed D⁰, D⁺, and Lambda_c particles, a search for 21 decay modes of two possible doubly-charmed baryons (ccu and ccd) yielded no evidence for such states. All of the slides shown here can also be obtained in several other formats .

Analysis technique

A single charm candidate (D⁰, D⁺, or Λ_c) is found by reconstructing K⁻π⁺, K⁻π⁺π⁺, K⁻π⁺π⁻π⁺, or pK⁻pi⁺
A double charm candidate (Ξ_cc⁺ or Ξ_cc⁺⁺) is found by combining the reconstructed single charm momentum vector with other tracks/objects
The double charm candidate momentum vector is used to seed the primary vertex. A cartoon of an event is shown below (secondary vertices are labeled in the order found, opposite to the order produced):
Initial cuts required a confidence level better than 1% for all three vertices, all three (between primary & double charm, primary & single charm, double charm & single charm) L/sigma (separation/separation error) greater than 0, and Cerenkov cuts on kaons and protons.

Cut selection

Cuts were chosen by maximizing S/sqrt(B) where S is determined from Monte Carlo simulated events and B is taken from the data over the mass range 3.4 to 4.1 GeV. The double charm baryon states were assumed to have a mass of 3.6 GeV and a lifetime of 1.0 ps (Ξ_cc⁺⁺) and 0.2 ps (Ξ_cc⁺). Since Pythia does not create these states by default, the single charm Cascade_c (csd or csu) with a mass of 3.6 GeV was used to produce the particles.

Final (optimized) cuts

Cuts are listed in approximate order of importance. The cut ranges over all decay modes are shown; the actual cut depends on decay mode.

L/sigma(btw primary & single charm) > 8 − 15
L/sigma(btw primary & double charm) > 1 − 5
L/sigma(btw double charm & single charm) > 0 − 3
no sec1 tracks in primary isolation < 1 − 0.0001
no non-primary tracks in sec1 isolation < 1 − 0.0001
kaon hypothesis favored over pion (kaons) > 0 − 3
proton hypothesis favored over pion (protons) > 2 − 5
proton hypothesis favored over kaon (protons) > −0.01 − 2
Kshort or Lambda0 normalized mass < 2.5 − 3.0
summed transverse momentum of daughter tracks wrt candidate momentum vector > 0 − 0.5
error on separation btw primary and single charm < 0.25 − 0.10 cm
lifetime of single charm < 8 − 1~ps
| normalized D/Λ_c mass | < 3 sigma
confidence level of sec1 vertex > 2 − 5\%
confidence level of sec2 vertex > 2 − 5\%
piconsistency > -8 − -5
primary vertex < 2 sigma out of target

Examples of Λ_c and D meson yields from FOCUS:

The mass plots below are an example of the statistics of singly charmed particles available at FOCUS. The first plot shows a Λ_c → pK⁻pi⁺ signal (19,444 events) while the second plot is a combined plot of D⁰ → K⁻π⁺, K⁻π⁺π⁻π⁺ and D⁺ → K⁻π⁺π⁺ (over 1 million reconstructed charm mesons).

Double charm baryon results

Below is a plot of all double charm baryon candidates which were searched for (shown separately):

Below is a plot of all double charm baryon candidates which were searched for (combined):

Below are plots of all double charm baryon candidates which were searched for as well as just the two decay modes searched for by SELEX, with the SELEX signal areas shaded.

Since the plots are nearly background free and the SELEX ground state signal areas contain zero events we can obtain a Rolke-Lopez 90% upper limit of 2.21 events. The efficiency relative to the 19,444 reconstructed Λ_c events is about 5% for Ξ_cc⁺ → Λ_c K pi and 13% for Ξ_cc⁺⁺ → Λ_c K pi pi. Therefore, at 90% CL no more than 2.21/0.05/19,444 = 0.23% of the FOCUS Λ_c candidates come from Ξ_cc⁺ and no more than 2.21/0.13/19,444 = 0.09% of the FOCUS Λ_c candidates come from Ξ_cc⁺⁺. By contrast, SELEX finds 15.8/0.10/1650 = 9.6% of their Λ_c baryons come from Ξ_cc⁺ → Λ_c K pi and 8/0.05/1650 = 9.7% of their Λ_c baryons come from Ξ_cc⁺⁺ → Λ_c K pi pi. This implies a production difference between double charm baryons and Λ_c baryons of greater than a factor of 42 (Ξ_cc⁺) to 111 (Ξ_cc⁺⁺) between SELEX and FOCUS. For FOCUS, the efficiency numbers are obtained for a double charm baryon produced as described in Cut Selection.

The above information is summarized in the following slide (click for postscript):

Estimating double charm yield from "first" principles

In photoproduction, charm is about 1/100 of hadronic cross section so assume double charm is 1/100 of charm
Assume a factor of 10% for overlapping wave functions (between two charm quarks to form a baryon)
Normalize total produced charm to FOCUS Monte Carlo (using D meson states)
Use double charm branching ratios from Bjorken (plus guesses for ones not considered)
Include efficiencies from MC include acceptance, reconstruction efficiency (of entire decay chain), and cut efficiency.

The table below indicates the expected yield of doubly-charmed baryons given the assumptions above. A postscript file of the table below is also available.

Parent	Decay mode	BR(D or /\c)	BR(ccu)	Expected Signal
(ccu)	D⁰(Kπ) Λ⁰ π⁺ π⁺	3.83%	3.2%	1.0
(ccu)	D⁰(K3π) Λ⁰ π⁺ π⁺	7.5%	3.2%	0.5
(ccu)	D⁰(Kπ) K_S⁰ p π⁺	3.83%	0.69%	0.2
(ccu)	D⁰(K3π) K_S⁰ p π⁺	7.5%	0.69%	0.1
(ccu)	D⁺ K⁻ π⁺ p	9.0%	1.0%	0.2
(ccu)	D⁺ Λ⁰ π⁺	9.0%	1.92%	0.8
(ccu)	D⁺ Λ⁰ π⁺ π⁻ π⁺	9.0%	0.32%	0.1
(ccu)	D⁺ K_S⁰ p	9.0%	1.03%	0.5
(ccu)	Λ_c⁺ K⁻ π⁺ π⁺	5.0%	5.0%	1.0
(ccu)	Λ_c⁺ K_S⁰ π⁺	5.0%	1.72%	0.4
(ccu)	Λ_c⁺ K_S⁰ π⁺ π⁻ π⁺	5.0%	1.72%	0.2
Total(ccu)				5.0

Parent	Decay mode	BR(D or /\c)	BR(ccd)	Expected Signal
(ccd)	D⁰(Kπ) K⁻ p π⁺	3.83%	1.0%	0.1
(ccd)	D⁰(K3π) K⁻ p π⁺	7.5%	1.0%	0.1
(ccd)	D⁰(Kπ) Λ⁰ π⁺	3.83%	4.47%	0.5
(ccd)	D⁰(K3π) Λ⁰ π⁺	7.5%	4.47%	0.4
(ccd)	D⁰(Kπ) Λ⁰ π⁺ π⁻ π⁺	3.83%	0.32%	0.0
(ccd)	D⁰(K3π) Λ⁰ π⁺ π⁻ π⁺	7.5%	0.32%	0.0
(ccd)	D⁰(Kπ) K_S⁰ p	3.83%	1.03%	0.1
(ccd)	D⁰(K3π) K_S⁰ p	7.5%	1.03%	0.1
(ccd)	D⁺ K⁻ p	9.0%	1.0%	0.5
(ccd)	D⁺ Λ⁰	9.0%	1.6%	0.4
(ccd)	D⁺ Λ⁰ π⁺ π⁻	9.0%	0.96%	0.2
(ccd)	D⁺ K_S⁰ p π⁻	9.0%	0.69%	0.1
(ccd)	Λ_c⁺ K⁻ π⁺	5.0%	3.0%	0.3
(ccd)	Λ_c⁺ K⁻ π⁺ π⁻ π⁺	5.0%	0.1%	0.0
(ccd)	Λ_c⁺ K_S⁰	5.0%	0.69%	0.1
(ccd)	Λ_c⁺ K_S⁰ π⁻ π⁺	5.0%	0.69%	0.0
Total(ccd)				2.8

This page was created by Kevin Stenson who can be reached at kevin.stenson@colorado.edu


Security, Privacy, Legal