Production and spectroscopy at LHCb

Abstract. During 2011, the LHCb experiment accumulated ∼ 1 fb−1 of integrated luminosity in proton-proton collisions data at 7 TeV, collecting a sample rich in b mesons and baryons. The data provide a wealth of new measurements that probes of QCD theory predictions. We present recent results in quarkonium and b− and c− hadron production, as well as studies of these states’ properties such as masses and kinematics.


Introduction
The Large Hadron Collider Beauty experiment (LHCb) at CERN is a single-arm forward spectrometer for precision measurements of CP violation and rare decays in the b− and c− hadron sector [1]. The resolution for primary (secondary) vertices is ∼ 50(100) μm and the momentum resolution for tracks having hits in both the VELO and the tracking stations is around 5%. Two Ring Imaging Cherenkov (RICH) detectors provide excellent charged particle identification capabilities over a wide momentum range of 2 − 100 GeV/c. The tracking system is composed by a precision Vertex Locator (VELO) surrounding the interaction region, a dipole magnet and three downstream tracking stations.
Heavy quark physics studies at LHCb provide a valuable test of both perturbative and nonperturbative Quantum Chromodynamics (QCD) and of the underlying event modelling in a region complementary to other general purpose detectors. The reconstruction of ∼ 40% of the total heavy quark production is possible thanks to the unique acceptance of LHCb (2 < η < 5). Moreover, the large cc and bb production cross sections in the forward region (2 < y < 6) in pp collisions, measured to be 1742 ± 267 μb [3] and 75.3 ± 5.4 stat ± 13.0 syst μb [2] respectively, combined with the excellent detector capabilities allow for data samples of unprecedented statistical content and purity.
The results presented below are based on 1.1 fb −1 of data collected by the LHCb detector during the 2010 and 2011 LHC runs.

Heavy meson production and spectroscopy 2.1 B + production cross-section
The B + cross-section measurement performed at LHCb [4] is the first in the forward region and is a powerful test of QCD@NLO. Candidate B ± → J/ψ(μμ)K ± decays are selected within a fiducial region in rapidity, 2 < y < 4.5, and transverse momentum, 0 < p T < 15 GeV/c, in which a measurement of total and differential cross-section (dσ/dp T ) is performed. The mass distribution of the a e-mail: andrea.contu@ca.infn.it DOI: 10.1051/ C Owned by the authors, published by EDP Sciences, 2014 , / selected candidates is shown in Figure 1. The main systematic uncertainties are the tracking and the muon identification efficiencies. The differential cross-section, dσ/dp T , when compared with FONLL predictions [5] (see Figure 2), shows good agreement.

B + c to B + production ratio
Measuring the properties of the B + c mesons, such as mass, lifetime and production helps to constrain QCD calculations. The B + c meson has been first observed at CDF [6]. Since no B + c absolute branching fraction has been measured yet, the analysis performed at LHCb [7] uses B ± → J/ψK ± and B ± c → J/ψπ ± candidates (see Figure 3) to perform a measurement of the production cross-section times the branching fraction of the B + c → J/ψπ + decay mode. We measure the ratio where σ(B + (c) ) is the production cross-section for the B + (c) meson. The measurement, performed for B mesons with p T > 4 GeV/c and 2.4 < η < 4.5, is R + c = 2.2 ± 0.8 ± 0.2 where the first uncertainty is statistical and the second is systematic.

First observation of B
The first observation of B + c → J/ψπ + π − π + decay mode has been made by the LHCb collaboration [8]. Invariant mass distributions for selected B + c → J/ψπ + π − π + and B + c → J/ψπ + (used as a normalisation mode) candidates are shown in Figure 4. The analysis of the resonant structures show that the domi- predictions from the BLL model described in Ref. [9]. The relative branching fraction with respect to the single pion mode has been measured to be

Orbitally exited B * * mesons observation
Orbitally excited B (s) mesons containing a light quark, hereafter collectively referred to as B * * mesons, are predicted by the Heavy Quark Effective Theory (HQET) [10] in the limit of infinite b-quark mass. Several of these states have been already observed at the Tevatron by the CDF [11] and D0 [12,13] experiments. A search for B * * → Bh and B * * → B * γ has been performed using 336 pb −1 of data collected in 2011. Soft photons from the B * 0 decay are not reconstructed, therefore, signal peaks are expected to appear in the relative to threshold invariant mass Distributions for Q, where signal resonance distributions are fitted using relativistic Breit-Wigner shapes, are shown in Figure Figure 6. Q(B + π − ) distribution. LHCb data (black markers) and the total fit are superimposed. Individual components contributing to the total fit are combinatorial background and associated production (red, green for combinatorial background only), B 0   Figure 7. Q(B 0 π + ) distribution. LHCb data (black markers) and the total fit are superimposed. Individual components contributing to the total fit are combinatorial background and associated production (red, green for combinatorial background only), arise from the variation of selection requirements and the uncertainty on the B 0 mass. The measured Q values can be translated into masses to give the following results The B * + 2 and B + 1 states are observed for the first time with a signal significance of 4.0σ and 9.9σ respectively. It is also important to notice that measured masses are in agreement with HQET prediction and that isospin partners have compatible masses.

Heavy baryon spectroscopy
While b-mesons are relatively well known objects, very little is known about b-baryons. There are seven ground-state (J P = 1/2 + ) baryons involving a b quark and two light quarks (u, d, s). These states can be ordered in a triangular scheme as shown in Figure 8. In the following section we will investigate properties of these baryons and their orbitally excited states.
3.1 Beauty baryons decaying to D 0 pK − and D 0 pπ − Decays of Λ 0 b and Ξ 0 b into the D 0 pK − and D 0 pπ − final states are potentially useful to improve the precision on the CKM angle γ [18,19]. We selected Λ 0 b and Ξ 0 b candidates undergoing the above decay chain using decay topology and particle identification in a data sample of 330 fb −1 [17]. We performed several measurements involving these decays. In particular, we measured the ratio

ICFP 2012
00037-p.5 and report the first observation of the decay Λ 0 b → D 0 pK − , for which a mass peak is clearly visible in Figure 9. The Ξ 0 b has already been observed at CDF [16] and the mass measured to be 5787.8±5.5 stat ±  Previous measurements of the Ξ − b and Ω − b masses were performed at the Tevatron by the CDF [22] and D0 [23] experiments. The same measurement has been repeated at LHCb [20] using reconstructed Ξ − b and Ω − b candidates decaying into J/ψΞ − (Λ 0 K − ) and J/ψΩ − (Λ 0 π − ) respectively, with J/ψ → μ + μ − and Λ 0 → pπ − . Having three displaced vertices, the decay topology is rich, as outlined in Figure 10. The invariant mass distributions for the selected candidates are shown in Figure 11

First observation of
Orbitally excited Λ 0 b states having J P = 1/2 − and J P = 3/2 − are predicted in several scenarios [15]. These states are expected to decay into Λ 0 via radiative or dipion decay. We performed a search for new states in the Λ 0 b π + π − invariant mass using 1 fb −1 of data from the 2011 run [14]. The invariant mass distributions for the Λ 0 b and Λ 0 * b candidates are shown in Figure 13 and Figure 14 respectively. The two mass peaks in the Λ b π + π − invariant mass have been described using a Gaussian distribution. The background model is extracted from the invariant mass distribution of wrong-sign Λ 0 b π + π + combinations. The mass values have been fitted to be 5911.95 ± 0.12 stat ± 0.03 syst ± 0.66 Λ 0 b mass , 5919.76 ± 0.07 stat ± 0.02 syst ± 0.66 Λ 0 b mass . Therefore, these states can be interpreted as the states Λ * 0 b (5912) and Λ * 0 b (5920).

Double charm production
Studies of double charmonium and charmonium with associated open charm production can be a useful probe of the quarkonium production mechanism. In addition, contributions from other mechanisms, such as Double Parton Scattering (DPS) may be investigated [28,29]. A search for the production of an open charm hadron (D 0 , D + , D + s , Λ + c ), hereafter generically referred to as C, in association with a J/ψ meson or another open charm hadron has been performed at LHCb using 355 pb −1 of data collected in 2011 [24]. Examples two-dimensional mass distribution are shown in Figure 15 for the selected candidates.
Absolute cross-sections and cross-section ratios have been measured and compared with theoretical expectations, as shown in Figure 16-17. In addition, the properties of J/ψC, CC, and CC events, such as transverse momentum spectra, have been studied (see an example plot in Figure 18).

χ c production
Radiative decays of the P-wave charmonia χ cJ (J = 1, 2, 3) give a substantial feed-down contribution to J/ψ production and have a major impact on the J/ψ polarisation. We present a measurement of the production cross-section ratio of χ c2 to χ c1 [30,31], which is sensitive to colour-singlet and colouroctet production mechanisms. The identification of the χ c is obtained via the reconstruction of its radiative decay into J/ψγ. The analysis has been performed separately for converted and unconverted photons. Converted photons interacted with the detector material late in the tracking system and converted into an e + e − pair. Both converted and unconverted categories are measured using calorimeter information alone.
Mass distribution are shown separately when using converted and unconverted photons in Figure 19. The results are shown in Figure 20 and compared to CDF [32] results (at √ s = 1.96 TeV in pp collisions and in the J/ψ pseudo-rapidity range |η J/ψ | < 1.0), NLO NRQCD [33] and ChiGen  Monte Carlo generator [34] predictions. It can be noted that the ChiGen generator describes the shape of the distribution reasonably well, although the data lie consistently above the model prediction. The results are in agreement with NLO NRQCD predictions only for p J/ψ T > 8 GeV/c.

Conclusions
We presented selected measurement in the heavy flavour sector performed by the LHCb collaboration. The outstanding capabilities and statistical power of the LHCb detector produced competitive results on heavy baryon production and spectroscopy. New results are expected from the analysis of the full 2011 sample and the 2012 data.