First evidence for the two-body charmless baryonic decay $B^0 \to p \bar{p}$

The results of a search for the rare two-body charmless baryonic decays $B^0 \to p \bar{p}$ and $B_s^0 \to p \bar{p}$ are reported. The analysis uses a data sample, corresponding to an integrated luminosity of 0.9 fb$^{-1}$, of $pp$ collision data collected by the LHCb experiment at a centre-of-mass energy of 7 TeV. An excess of $B^0 \to p \bar{p}$ candidates with respect to background expectations is seen with a statistical significance of 3.3 standard deviations. This is the first evidence for a two-body charmless baryonic $B^0$ decay. No significant $B_s^0 \to p \bar{p}$ signal is observed, leading to an improvement of three orders of magnitude over previous bounds. If the excess events are interpreted as signal, the 68.3% confidence level intervals on the branching fractions are {eqnarray} \cal{B}(B^0 \to p \bar{p})&=&(1.47 \,^{+0.62}_{-0.51} \,^{+0.35}_{-0.14}) \times 10^{-8} \,, *{0.3cm} \cal{B}(B_s^0 \to p \bar{p})&=&(2.84 \,^{+2.03}_{-1.68} \,^{+0.85}_{-0.18}) \times 10^{-8} \,, {eqnarray} where the first uncertainty is statistical and the second is systematic.

The observation of B meson decays into two charmless mesons has been reported in several 2 decay modes [1]. Despite various searches at e + e − colliders [2][3][4][5], it is only recently that 3 the LHCb collaboration reported the first observation of a two-body charmless baryonic B 4 decay, the B + → pΛ(1520) mode [6]. This situation is in contrast with the observation 5 of a multitude of three-body charmless baryonic B decays whose branching fractions are 6 known to be larger than those of the two-body modes; the former exhibit a so-called 7 threshold enhancement, with the baryon-antibaryon pair being preferentially produced 8 at low invariant mass, while the suppression of the latter may be related to the same 9 effect [7]. 10 In this paper, a search for the B 0 → pp and B 0 s → pp rare decay modes at LHCb is 11 presented. Both branching fractions are measured with respect to that of the B 0 → K + π − 12 decay mode. The inclusion of charge-conjugate processes is implied throughout this paper.

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In the Standard Model (SM), the B 0 → pp mode decays via the b → u tree-level process 14 whereas the penguin-dominated decay B 0 s → pp is expected to be further suppressed.    The B candidate variables are: the vertex χ 2 per number of degrees of freedom; the vertex 86 χ 2 IP ; the direction angle; the distance in z (the direction of the interacting proton beams) 87 between its decay vertex and the related PV; and the p T asymmetry within a cone around where BDT is the efficiency of the BDT selection on the B 0 (s) → pp signal candidates, which 104 is determined from simulation, B BDT is the expected number of background events within 105 the (initially excluded) signal region, estimated from the data sidebands, and the term Specific PID criteria are separately defined for the two signal modes and the normali-123 sation channel. The PID efficiencies are found to be approximately 56% for the B 0 (s) → pp 124 signals and 42% for B 0 → K + π − decays.

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The ratio of efficiencies of B 0 (s) → pp with respect to The K + π − mass spectrum of the normalisation mode is described with a series of spectrum. The B 0 s → π + K − signal width is constrained to the fitted B 0 → K + π − width 150 such that the ratio of the widths is identical to that obtained in simulation.

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The invariant mass distributions of the misidentified B 0 where erfc(x) = 1 − erf(x) is the complementary error function. The signs of the variable 165 x and parameter µ are reversed compared to the standard definition of an EMG function.

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The parameters defining the shape of the two EMG functions and their relative weight simulation. The mass distribution of the combinatorial background is found to be well 170 described by a linear function whose gradient is determined by the fit.

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The fit to the K + π − spectrum, presented in Fig. 1, determines seven parameters, and  The sources of systematic uncertainty are minimised by performing the branching fraction 213 measurement relative to a decay mode topologically identical to the decays of interest.

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They are summarised in Table 1.

Source
Value (%)   The branching fractions are determined relative to the B 0 → K + π − normalisation channel 247 according to where the first uncertainties are statistical and the second are systematic.

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In summary, a search has been performed for the rare two-body charmless baryonic