A search for the doubly charmed baryon Xi_cc^+ in the decay mode Xi_cc^+ > Lambda_c^+ K^ pi^+ is performed with a data sample, corresponding to an integrated luminosity of 0.65/fb, of pp collisions recorded at a centreofmass energy of 7 TeV. No significant signal is found in the mass range 33003800 MeV/c^2. Upper limits at the 95 confidence level on the ratio of the Xi_cc^+ production crosssection times branching fraction to that of the Lambda_c^+, R, are given as a function of the Xi_cc^+ mass and lifetime. The largest upper limits range from R < 1.5 x 10^2 for a lifetime of 100 fs to R < 3.9 x 10^4 for a lifetime of 400 fs.
Invariant mass spectrum of $\Lambda _ c ^+ \rightarrow p K ^ \pi ^+ $ candidates for 5$\%$ of the data, with events chosen at random during preselection (due to bandwidth limits for the normalisation mode). The dashed line shows the fitted background contribution, and the solid line the sum of $\Lambda _ c ^+$ signal and background. 
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The distribution of the invariant mass difference $\delta m$, defined in Eq. 3, for simulated $\Xi_{ c c }^+$ events with a $\Xi_{ c c }^+$ mass of 3500 $ {\mathrm{ Me V /}c^2}$ . The solid line shows the fitted signal shape. In order to increase the available statistics, the trigger and ANN requirements are not applied in this plot. 
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Spectrum of $\delta m$ requiring $2273 < m([ p K ^ \pi ^+ ]_{\Lambda _ c }) < 2303$ $ {\mathrm{ Me V /}c^2}$ . Both plots show the same data sample, but with different $\delta m$ ranges and binnings. The wide signal region is shown in the right plot and indicated by the dotted vertical lines in the left plot. 
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Measured signal yields as a function of $\delta m$. The upper two plots show the estimated signal yield as a dark line and the $\pm 1\sigma$ statistical error bands as light grey lines for (upper left) the baseline method and (upper right) the crosscheck method. The central values of the two methods are compared in the lower plot and found to agree well. 
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Upper limits on $R$ at the 95% CL as a function of $\delta m$, for five $\Xi_{ c c }^+$ lifetime hypotheses. 
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Mass difference spectrum requiring $2273 < m([ p K ^ \pi ^+ ]_{\Lambda _ c }) < 2303$ $ {\mathrm{ Me V /}c^2}$ . Candidates are also required to be consistent with (left) an intermediate $\Sigma_{ c }(2455)^{++}$, (right) an intermediate $\Sigma_{ c }(2520)^{++}$. 
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Animated gif made out of all figures. 
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Systematic uncertainties on the singleevent sensitivity $\alpha$. 
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Singleevent sensitivity $\alpha$ for different lifetime hypotheses $\tau$, assuming $m(\Xi_{ c c }^+ )=3500$ $ {\mathrm{ Me V /}c^2}$ . The uncertainties quoted include statistical and systematic effects, and are correlated between different lifetime hypotheses. 
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Variation in singleevent sensitivity for different mass hypotheses $m(\Xi_{ c c }^+ )$, assuming $\tau=333$ $\rm fs$ . The uncertainties quoted include statistical and systematic effects, and are correlated between different mass hypotheses. The variation is shown separately for all effects other than the efficiency of the $\delta m$ window ($\alpha_u$), for the correction due to the massdependent resolution, and for the combination ($\alpha$). 
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Expected value of the signal yield $N_{\text{sig}}$ for different values of $R$ and lifetime hypotheses, assuming $m(\Xi_{ c c }^+ )=3500$ $ {\mathrm{ Me V /}c^2}$ . The uncertainties quoted are due to the systematic uncertainty on $\alpha$. 
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Largest values of the upper limits (UL) on $R$ at the 95% CL in blocks of $\delta m$ for a range of lifetime hypotheses, given in units of $10^{3}$. The largest values across the entire 500 $ {\mathrm{ Me V /}c^2}$ range are also shown. 
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Created on 11 July 2020.