FOCUS double charm baryon search


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

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 (Ξcc++) and 0.2 ps (Ξcc+). 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.

Examples of Λc 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 Λc → pKpi+ signal (19,444 events) while the second plot is a combined plot of D0 → Kπ+, Kπ+ππ+ and D+ → Kπ+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 Λc events is about 5% for Ξcc+ → Λc K pi and 13% for Ξcc++ → Λc K pi pi. Therefore, at 90% CL no more than 2.21/0.05/19,444 = 0.23% of the FOCUS Λc candidates come from Ξcc+ and no more than 2.21/0.13/19,444 = 0.09% of the FOCUS Λc candidates come from Ξcc++. By contrast, SELEX finds 15.8/0.10/1650 = 9.6% of their Λc baryons come from Ξcc+ → Λc K pi and 8/0.05/1650 = 9.7% of their Λc baryons come from Ξcc++ → Λc K pi pi. This implies a production difference between double charm baryons and Λc baryons of greater than a factor of 42 (Ξcc+) to 111 (Ξcc++) 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

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) KS0 p π+ 3.83% 0.69% 0.2
(ccu) D0(K3π) KS0 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+ KS0 p 9.0% 1.03% 0.5
(ccu) Λc+ K π+ π+ 5.0% 5.0% 1.0
(ccu) Λc+ KS0 π+ 5.0% 1.72% 0.4
(ccu) Λc+ KS0 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) KS0 p 3.83% 1.03% 0.1
(ccd) D0(K3π) KS0 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+ KS0 p π 9.0% 0.69% 0.1
(ccd) Λc+ K π+ 5.0% 3.0% 0.3
(ccd) Λc+ K π+ π π+ 5.0% 0.1% 0.0
(ccd) Λc+ KS0 5.0% 0.69% 0.1
(ccd) Λc+ KS0 π π+ 5.0% 0.69% 0.0
Total(ccd) 2.8

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