First Observation of the Doubly Charmed Baryon Xi_cc^+

et al; M. Mattson; The SELEX Collaboration

REVIEW

First Observation of the Doubly Charmed Baryon Xi_cc^+

The SELEX Collaboration, M. Mattson, et al

High Energy Physics - Experiment · Submitted 12 Aug 2002 · 17 references

Pith's verdict: A real, well-cross-checked excess at 3520 MeV, but the abstract's 6.3σ is a local number and the paper's own global probability is ~3.9σ — read it as evidence, not as an established state.

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arXiv IDhep-ex/0208014 (version 1)

Pith IDJOMIFON6, Pith's permanent record of this paper and its review

Citationet al, M. Mattson, The SELEX Collaboration. "First Observation of the Doubly Charmed Baryon Xi_cc^+." arXiv:hep-ex/0208014 (2002). https://pith.science/paper/JOMIFON6

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The thing to know: SELEX has a 15.9-event excess over 6.1 background in Λc+ K− π+ at 3519 MeV, and the cross-checking is better than I expected for a 22-event sample. Wrong-sign combinations show nothing, every other mass permutation kills the peak, Λc sidebands don't reproduce it, and Fig. 3 shows the significance is stable against everything except the K−π+ vertex separation cut, where the dependence is explained by background rising at small separations. The lifetime likelihood and the beam/target breakdown tables are honest, if statistically thin. Whoever wrote this did the obvious checks and showed them. That counts for something, and it is more than most first-observation letters bother with. The soft spot is the one the stress-test names, and it is real. The abstract leads with 6.3σ, which is a single-bin number computed against a background estimated from sidebands adjacent to a peak whose position was found by looking at the plot. The paper itself gives 1.1×10⁻⁴ for an excess anywhere in the 110-bin, 3.2–4.3 GeV search interval — about 3.9σ. Footnote 7 argues the local probability is the right measure for a discovery claim; I don't buy that argument, and even the authors don't quite: they print the global number in the text. The abstract should have led with it. Secondary concerns, in order of weight. There is no systematic error budget anywhere — the 6.1 ± 0.5 background is counting statistics only, and nothing is said about how the mass scale or the vertex-separation cut uncertainty propagates. The fitted width, 3 ± 1 MeV, is narrower than the ~5 MeV resolution the simulation predicts; the paper calls this consistent and quotes a 20% CL for the fixed-width fit, which is fine but not reassuring. And the claim that ~20% of the Λc in the sample come from Ξcc decay rests entirely on the same 15.9 events, so it inherits every uncertainty above. The production rate being larger than models predict, with FOCUS seeing nothing, is a physics tension the paper flags but doesn't resolve. None of this is fatal; the peak is there and the backgrounds are checked. But a first observation at 3.9σ global, with no systematics, in a channel where the rate is anomalously high, is a claim to hold loosely until someone confirms it. Worth a serious referee — the analysis is careful enough to deserve one — and worth citing as the SELEX result, with the global significance quoted rather than the abstract's number.

Referee report

Significance

If the identification holds, this is the first evidence for a baryon with two valence charm quarks, one of the six states expected in the SU(4) extension of the baryon classification, and it would open a new sector of hadron spectroscopy. The experimental execution has real strengths: a redundant silicon vertex detector with 20 fs proper-time resolution, RICH pi/K separation, and a set of independent background cross-checks (wrong-sign combinations, mass permutations, Lambda_c sidebands) that all fail to reproduce the peak. The result is also falsifiable and was clearly designed to be so, and the paper is candid about the anomalously large production rate and the absence of a signal in FOCUS. The significance of the claim, however, rests on how the excess is quantified, and that is where the manuscript needs work.

Major objections (5)

[Abstract [abstract:s1] and the paragraph 'To determine the combinatoric background...'] The abstract quotes an unqualified '6.3 sigma', but this is the yield significance 15.9/sqrt(6.1 + 0.5^2), and the paper's own footnote [7] states that the measure appropriate for a discovery claim is instead the probability that a background fluctuation accounts for all events in the signal region, explicitly distinguishing it from the yield significance. By that criterion the text itself gives 1.0e-6 locally (about 4.8 sigma one-sided) and 1.1e-4 over the 110-bin search interval (about 3.9 sigma). The abstract should quote the search-interval-corrected number, or at minimum state both, since the headline 'first observation' is being supported by a number the paper's own methodology says is not the discovery measure.
[Whole analysis; background estimate in the paragraph beginning 'To determine the combinatoric background'] No systematic uncertainty budget is presented anywhere in the paper. The background estimate 6.1 +/- 0.5 is purely a counting-statistics error from the sidebands; there is no assessment of the systematic uncertainty from the assumed linear background shape, from the mass-scale calibration, from the RICH identification efficiency, or from the acceptance model. For a first-observation claim this is load-bearing: even a modest systematic on the background or on the peak position would move the global significance, which is already near the conventional threshold. A table separating statistical and systematic contributions to the yield, mass, and width is needed.
[Paragraph beginning 'The yield of this state is larger than most production models predict'] The inference that ~20% of the Lambda_c sample comes from Xi_cc+ decay is built from three loosely constrained multiplicative factors: an 11% acceptance, a factor 1.5 for isospin (to include Lambda_c+ K0 pi0), and Bjorken's factor 1.6 for other Lambda_c-containing modes. None of these carries a quoted uncertainty, and the resulting rate is acknowledged to exceed most model predictions. Because an anomalously large production rate is exactly the kind of claim that needs to be defended quantitatively, the authors should give uncertainties on each factor and state the implied Xi_cc+/Lambda_c+ production ratio with an error band, so that the reader can judge how far the observation sits from expectation.
[Mass determination, paragraph 'This state has a fit mass of 3519 +/- 1 MeV/c^2'] The measured mass of 3519 MeV/c^2 is not compared numerically with the predictions cited in Refs. [2-5], although those references are invoked to motivate the search. Most quark-model estimates for the Xi_cc+ lie near 3600 MeV/c^2, so the observed value sits noticeably low; this bears directly on the identification and should be discussed explicitly rather than left implicit. A short comparison table of predicted versus observed mass, with the model spread, would materially strengthen or qualify the claim.
[Paragraph 'This state has a fit mass of 3519 +/- 1 MeV/c^2' (width discussion)] The fitted Gaussian width of 3 +/- 1 MeV/c^2 is smaller than the expected resolution of ~5 MeV/c^2 quoted from simulation, yet the text describes it as 'consistent with our simulation' and notes only a 20% confidence level for a fit with the width fixed at the expected resolution. An observed width below the detector resolution is unusual and, with 22 events, the test has little power; the manuscript should state plainly that the width is not resolved and avoid implying that the measured 3 MeV confirms the simulation.

Minor objections (6)

[Table II and the sentence 'One sees that the doubly charmed baryon candidates are produced solely by the baryon beams'] The signal region contains 18 Sigma- and 4 proton events and zero meson-beam events, while the beam composition would lead one to expect roughly three meson-beam events in the signal region. With these numbers the 'solely baryon beams' statement is not statistically supported and should be softened to a consistency statement.
[Fig. 2 caption and the surrounding text] The binning changes between panels (5 MeV/c^2 in (a) and (b), 2.5 MeV/c^2 in (c)); the caption should state this explicitly and give the fitted background level and Gaussian normalization so the reader can connect the 22 events and 6.1 background to the plotted curve.
[Wrong-sign control, Fig. 2(b)] The wrong-sign sample is Lambda_c+ K+ pi-, which is not the charge-conjugate of the signal mode. A brief statement of why this particular wrong-sign combination was chosen, and whether a same-charge or charge-conjugate control was examined, would clarify what the control actually tests.
[Table I and the lifetime limit] The likelihood table is computed from the 22 signal-region events and the entries are coarse; the text should state how many events enter the lifetime fit and note that the 33 fs limit is correspondingly weak, so that the limit is not over-interpreted.
[Reference [16]] The FOCUS null result is cited to a conference web page rather than a refereed publication. Since the contrast with FOCUS is used to frame the result, the citation should be updated to a citable source if one exists, or the text should note that the FOCUS result is preliminary.
[Abstract and text, typography] There are several presentation slips: 'fsec' in the abstract versus 'fs' in the text, missing spaces in '3MeV/c^2' and '33fsec', and the phrase '1 sigma track error corridors' in the Fig. 1 caption. These should be cleaned up in the final version.

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The paper

The paper claims

SELEX reports a 6.3-sigma single-bin excess of 15.9 events at 3519 MeV/c^2 in Lambda_c+ K- pi+, interpreted as the first observation of the doubly charmed baryon Xi_cc+.

The reading

Protons and neutrons are baryons: three-quark bundles. The quark model also predicts baryons with two charm quarks, called doubly charmed baryons, but none had been seen. The SELEX experiment at Fermilab fired a 600 GeV beam at thin copper and diamond targets and recorded about 15 billion collisions. The analysis started from 1630 Lambda_c+ baryons decaying to p K- pi+. It then looked for a second vertex made of a K- and a pi+ between the primary vertex and the Lambda_c+ decay point, and plotted the invariant mass of the Lambda_c+ K- pi+ system. A narrow peak appears at 3519 MeV/c^2. The signal region contains 22 events; the sidebands imply 6.1 background events, leaving 15.9 signal events. The paper quotes a 6.3 sigma single-bin significance. Cross-checks: the wrong-sign combination (Lambda_c+ K+ pi-) shows no peak, and reversing the mass assignments makes the peak disappear. The lifetime is bounded at 33 fs at 90% confidence, but a prompt decay cannot be excluded. The production rate is larger than most models predict, and the FOCUS experiment at Fermilab sees no signal. The search-interval-corrected probability is 1.1e-4, about 3.9 sigma, which the abstract does not quote.

Extended reading notes

Core claim

The paper claims: 'We observe a signal for the doubly charmed baryon Xi_cc+ in the charged decay mode Xi_cc+ -> Lambda_c+ K- pi+ ... an excess of 15.9 events over an expected background of 6.1 +/- 0.5 events, a statistical significance of 6.3 sigma.' If correct, this is the first experimental evidence for a baryon containing two valence charm quarks, one of six states predicted by the SU(4) extension of the baryon classification.

Load-bearing premise

The background estimate of 6.1 +/- 0.5 events assumes that the background shape is linear across the signal region and that the 140 sideband events in the 115 MeV/c^2 sidebands scale by 5/115 to the 5 MeV/c^2 signal half-width. The paper justifies linearity with a fit chi2/dof = 0.45, but with only 22 events in the signal region this is a weak test. If the background has any curvature, threshold structure, or reflection near 3520 MeV/c^2, the excess and the 6.3 sigma significance are overestimated. Location: the paragraph beginning 'To determine the combinatoric background under the signal peak we exploit the linearity of the background justified by the fit.'

Original abstract

We observe a signal for the doubly charmed baryon Xi_cc^+ in the charged decay mode Xi_cc^+ --> Lambda_c^+ K- pi+ in data from SELEX, the charm hadro-production experiment at Fermilab. We observe an excess of 15.9 events over an expected background of 6.1 +/- 0.5 events, a statistical significance of 6.3sigma. The observed mass of this state is (3519 +/- 1) MeV/c^2. The Gaussian mass width of this state is 3MeV/c^2, consistent with resolution; its lifetime is less than 33fsec at 90% confidence.

The compiled record

The paper's own sentences, arranged, kept beside the review and separate from it. Each label is drawn from a closed vocabulary; the number beside it is the probability the classifier gave that label. Numbers in the tables are the authors', checked by code. No sentence in this record was written by a model.

Compiled by classifier-1.13-20260917 via OpenRouter, question bank abstract-p1-20260923, 2026-09-23: 1 request, 3,672 input tokens, $0.0002 reported.

What the abstract itself states, as probabilities: claims first 0.72 · states limitation 0.60 · human participants 0.02 · controlled comparison 0.21 · randomized assignment 0.01 · states quantitative result 0.99 · compares with prior results 0.30 · mentions code or data availability 0.02.

The abstract, sentence by sentence

arXiv listing abstract v1. Results and the main claim are underlined; show every role.

We observe a signal for the doubly charmed baryon Xi_cc^+ in the charged decay mode Xi_cc^+ --> Lambda_c^+ K- pi+ in data from SELEX, the charm hadro-production experiment at Fermilab. We observe an excess of 15.9 events over an expected background of 6.1 +/- 0.5 events, a statistical significance of 6.3sigma. The observed mass of this state is (3519 +/- 1) MeV/c^2. The Gaussian mass width of this state is 3MeV/c^2, consistent with resolution; its lifetime is less than 33fsec at 90% confidence.

background contribution method result interpretation main claim

What the classifier reports for each sentence. The number is the probability it gave the label.
#	sentence	role	the authors' own work	states the main conclusion	strength
s0	We observe a signal for the doubly charmed baryon Xi_cc^+ in the charged decay mode Xi_cc^+ --> Lambda_c^+ K- pi+ in data from SELEX, the charm hadro-production experiment at Fermilab.	result 0.88	0.89	0.83	asserted 1.00
s1	We observe an excess of 15.9 events over an expected background of 6.1 +/- 0.5 events, a statistical significance of 6.3sigma.	result 1.00	0.94	0.27	asserted 1.00
s2	The observed mass of this state is (3519 +/- 1) MeV/c^2.	result 1.00	0.94	0.12	asserted 1.00
s3	The Gaussian mass width of this state is 3MeV/c^2, consistent with resolution; its lifetime is less than 33fsec at 90% confidence.	result 1.00	0.94	0.06	asserted 0.79

What kind of paper the classifier reports this is: empirical study 1.00 · evidence from observation or experiment 0.58.

What was examined, and what was not

sources read

arXiv listing abstract v1

not examined

full text
figures and tables
references and citing sentences
relations to other papers

levels run

P abstract

not run

S sections
T body sentences
C citations
R cross-paper relations

Every quoted sentence on this record is the authors'. Every label is from Pith's closed vocabulary, with the probability shown beside it. Every number in the measurements table is the authors', checked by code. Record as JSON

Figures

**Figure 1.** FIG. 1. A candidate event with production target, 1

**Figure 2.** (b). Those events show comparable fluctuations to the sidebands of the signal channel and give no evidence for a significant narrow structure. We have investigated all possible permutations of mass assignments for the non-Λ+ c tracks. The peak at 3520 MeV/c2 disappears for any other mass choice, and no other significant structures are observed. Reconstructions with events from the Λ + c mass sidebands produce relatively few entries and no significant structures in the doubly charmed baryon region. The dependence of the signal significance, as defined above, on several of the…

**Figure 3.** FIG. 3. The signal significance as a function of: (a) the

Figures reproduced from arXiv:hep-ex/0208014 by the authors.

References

Works cited (17)

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SELEX Collaboration, J.S. Russ et al. , in Proceedings of the 29th International Conference on High Energy Physics, 1998, edited by A. Astbury et al. (World Scien- tiﬁc, Singapore, 1998), Vol. II, p. 1259; hep-ex/9812031

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92 later papers cite it

Evidence: substantial. 92 of the citing papers share no author with it, and 34 of those carry a confident label from Pith's citation classifier.

Pith read 18 of the 120 citing sentences, choosing only those whose citation markers point at this paper alone, and wrote the paraphrases from them. The summary, the counts, the evidence level, the links and the quotes come from code; the summary counts the papers listed below. A later paper is listed under a label when Pith's citation classifier gives one of its sentences that label at probability 0.8 or more (0.95 for support), Pith's reading finds that the citing paper itself did it, and a second check confirms the paraphrase against the sentence. This section lists uses only. It does not list criticism or disagreement, so their absence here says nothing about how the paper has been received. Papers that share an author with this one are listed apart, under "By the same authors". A quote is the citing authors' own sentence, shown when their paper's licence is CC BY, CC BY-SA or CC0. JSON

2 later papers by other authors are listed below: 1 used its data or values and 1 compared its own results with it.

Uses its data or values 1 of 20 papers with this label

We use $m_u =m_d =0$, $m_s=96^+8_-4$ MeV, $m_c = 1.28 ±0.03$ GeV, $m_b=4.18^+0.04_-0.03$ GeV, [ref] , $M_Ξ^+_cc=3519 ±1$ MeV [ref] , $M_Ξ_cc^++=3621.40±1.13$ MeV [ref] , $f_3γ=-0.0039$ GeV$^2$ [ref] , $⟨q̅q⟩=(-0.24±0.01)^3$ GeV$^3$ [ref] , $m_0^2 = 0.8 ±0.12$ GeV$^2$ and $χ=-2.85 ±0.5$ GeV$^2$ [ref] .

Magnetic moments of doubly heavy baryons in light-cone QCD (2018) · Numerical analysis and conclusion · 1.00

Compares with it 1 of 8 papers with this label

Compared its own predicted ratio f_Xi_cc/Sigma^- approximately equal to f_Xi_cc/p with this paper's data, finding it comparable.

Production of the Doubly Charmed Baryons at the SELEX experiment -- The double intrinsic charm approach (2016) · 0.81 · paraphrased: the licence does not allow quoting

How the literature uses it

71 arXiv papers cite this in 92 sentences

Each citing sentence is taken from the citing paper's LaTeX source and typed by Pith's classifier. A label appears at probability 0.8 or more (0.95 for support), a contest at 0.7 or more; the quoted words are the citing authors'. How each label was checked · JSON

Papers whose main result relates to this one's

Found among the papers closest to this one by title and main claim. Pith's classifier compared each pair's main results and a second model family confirmed the relation; checked against two further families, shown conflicts agreed 118 times in 120 and shown same results 218 times in 239. The quoted sentence is the other paper's main claim.

States a conflicting result 1 paper

No significant signal is found in the mass range 3300-3800 MeV/c^2.

Search for the doubly charmed baryon Xi_cc^+ · 0.81

Papers citing it · 54 sentences without a confident label

Contests it 6 papers

Because the $Ξ_cc^++$ and $Ξ_cc^+$ are isospin partners whose mass difference should be only a few MeV, the LHCb experiment [ref] did not confirm the SELEX experiment [ref] neither.

An updated review of the new hadron states · Doubly charmed baryons · contest 0.71
The SELEX collaboration reported signals interpreted as the $Ξ_cc^+$ at $3518.7(1.7)$ MeV [ref] , but subsequent searches by FOCUS [ref] , BaBar [ref] , Belle [ref] , and LHCb [ref] did not confirm the existence of this structure.

Charmed bottom baryon spectroscopy from lattice QCD · Conclusions · contest 0.81
However, the existence of $Ξ^+_cc(3519)$, as reported by SELEX [ref] , has not been confirmed by subsequent experiments conducted by FOCUS [ref] , BaBar [ref] , LHCb [ref] , and Belle [ref] .

The strong coupling constants of doubly heavy baryons with heavy mesons · Introduction · contest 0.78

Uses its data or values 16 papers

In early 2000's the SELEX collaboration published the first observation of 15.9 signal over $6.1 ±0.5$ background events of the doubly charmed baryons in the charged decay mode $→K^- π^+$ from $→p K^- π^+$ (1630 events) sample [ref] .

Production properties of the Doubly Charmed Baryons at the large Feynman-X · Introduction · 0.80
We use $m_u =m_d =0$, $m_s=96^+8_-4$ MeV, $m_c = 1.28 ±0.03$ GeV, $m_b=4.18^+0.04_-0.03$ GeV, [ref] , $M_Ξ^+_cc=3519 ±1$ MeV [ref] , $M_Ξ_cc^++=3621.40±1.13$ MeV [ref] , $f_3γ=-0.0039$ GeV$^2$ [ref] , $⟨q̅q⟩=(-0.24±0.01)^3$ GeV$^3$ [ref] , $m_0^2 = 0.8 ±0.12$ GeV$^2$ and $χ=-2.85 ±0.5$ GeV$^2$ [ref] .

Magnetic moments of doubly heavy baryons in light-cone QCD · Numerical analysis and conclusion · 1.00
where $N$ is number of events in the respective sample and the reconstruction efficiencies of $Ξ_cc^+$ and $Ξ_cc^++$ are $ϵ_+ ≃0.11$ [ref] and $1/ϵ_++ ≃3.7$ [ref] respectively.

Production of the Doubly Charmed Baryons at the SELEX experiment -- The double intrinsic charm approach · Introduction · 1.00

Compares with it 6 papers

Integrating Eq. [ref] over the SELEX kinematic region, $x_F > 0.1$, we find $⟨x_F ⟩≈0.33–0.34$ that is in agreement with the SELEX data, $x_F ∼0.33$ for $Ξ_cc ^+→Λ_c^+K^- π^+$ decay [ref] .

Production of the Doubly Charmed Baryons at the SELEX experiment -- The double intrinsic charm approach · The shape of $P_icc (Ξ_cc)$ as a function of $x_F$ · 0.85
It is much heavier than the SELEX's value by approximately $100 MeV$ [ref] , and very closed to the report of LHCb [ref] .

Masses of doubly heavy-quark baryons in an extended chromomagnetic model · Mass spectra of doubly and triply heavy baryons · 0.99
Although, this led to a $m_Ξ_cc$ closer to the experimentally observed value in comparison to our previous work [ref] , our mean value of $m_Ξ_cc$ is still above the mass observed by the SELEX Collaboration by about $76$ MeV [ref] and our combined uncertainty for this particle is $44$ MeV.

Charmed-Baryon Spectroscopy from Lattice QCD with N_f=2+1+1 Flavors · Discussion and Conclusion · 0.86

Cites it as background 6 papers

The SELEX Collaboration [ref] first proposed the existence of $Ξ_cc^+$ in 2002 and 2005.

Production of doubly heavy baryon at the Muon-Ion Collider · Introduction · 0.91
The SELEX Collaboration reported results for $Ξ_cc^+$ in 2002 [ref] , and the LHCb Collaboration reported evidence for $Ξ_cc^++$ in 2017 [ref] .

Chiral extrapolation of the doubly charmed baryons magnetic properties · Introduction · 0.94
In 2002 and 2005, the SELEX Collaboration reported a possible discovery of the $Ξ_cc^+$ baryon [ref] .

Photoproduction of $P$-wave doubly charmed baryon at future $e^+e^-$ collider · Introduction · 0.89

Discussion

Disagree with the review, or know something it missed? Leave a comment, highlight any passage above to comment on it, or challenge this review formally.

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The statistical measure appropriate for a discovery cla im is the probability that a background ﬂuctuation can ac- count for all events in the signal region. This is diﬀerent from the yield signiﬁcance, which goes as S/ √ (S + B), e.g. the area of a Gaussian ﬁt. Applying a yield signiﬁ- cance test to a discovery claim would require ≥ 25 signal events in a...

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