b and c spectroscopy at LHCb

The LHCb experiment is designed to study the decays and properties of heavy flavoured hadrons produced in the forward region from pp collisions at the CERN Large Hadron Collider. It has recorded the world’s largest data sample of beauty and charm hadrons, enabling precise studies into the spectroscopy of such particles, including discoveries of new states and measurements of their properties such as masses, width and quantum numbers. The latest results in this area are reviewed.


Introduction
The LHCb detector [1] is optimized for measurements in heavy-flavour physics at the Large Hadron Collider LHC, at CERN.As the b b pairs are predominantly produced in the same forward or backward direction at high energies, the LHCb detector is a single forward arm spectrometer with an unique pseudo-rapidity acceptance of 2 < < 5.The LHCb experiment has accumulated 1 fb −1 of data at √ s = 7 TeV during the years 2010-2011, and 2 fb −1 at √ s = 8 TeV during the 2012.

X(3872) quantum numbers
The X(3872) particle (called X in this section) was discovered in 2003 by the Belle experiment [2] and subsequently observed by several other experiments.Despite a large experimental effort, the nature of this new state is still uncertain.Among the open possibilities are conventional charmonium states and loosely bound D * 0 D0 molecules [3], tetra-quark states [4], or their mixture [5].Measurements of the quantum numbers are important to shed light on the nature of the X.
After measuring the mass and the production cross section [6] LHCb has recently measured the X quantum numbers J P C [7], resolving the observed ambiguity between 1 ++ and 2 −+ [8], in favour of the former.With 1 fb −1 collected at √ s = 7 TeV we have performed the first analysis of the complete five-dimensional angular correlations of 313 ± 26 B + → XK + decays, where X → J / ± ∓ and J / → ± ∓ .The selection is optimized on the more abundant B ± → (2S)K ± similar channel, and the signal yield is given by a fit to the data using a Crystal Ball function for the signal and a linear function for the background, as shown in Fig. 1 left.To discriminate between the 1 ++ and 2 −+ assignments, we performed a likelihood ratio test that unambiguously reject the hypotheses J P C = 2 −+ at 8.4 (see Fig. 1

right).
a e-mail: patrizia.desimone@lnf.infn.itThis is an Open Access article distributed under the terms of the Creative Commons Attribution License 4.0, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The results of the fit around the (2S) and the X masses are shown in the inserts.The solid blue, dashed red, and dotted green lines are the total, signal and background components, respectively.Right: distribution of the test statistic for the simulated experiments experiments with J P C = 2 −+ (black circles) and with J P C = 1 ++ (red triangles).A Gaussian fit to the 2 −+ distribution is overlaid (blue solid line).The value of the test statistic for the data, t data , is shown by the solid vertical line.

Studies of D j mesons
The search of the D j mesons has been performed in a data sample of 1 fb −1 collected at √ s = 7 TeV, using the inclusive reactions: where X represents a system composed of any collection of charged and neutral particles [10].
Due to the availability of the helicity angle information, the fit to the D * + − mass spectrum allows a spin analysis of the produced resonances and a separation of the different spin-parity components: we are able to separate our sample in an enhanced unnatural parity sample and a natural parity sample.The D + − and D 0 + mass spectra consist of only natural parity resonances, however these final states are affected by cross-feed from all the resonances that decay to the D * final state. We Where the dominant systematic uncertainty comes from the knowledge of the momentum scale.Our M( − b ) result solves the inconsistency between the CDF and D0 measurements [12] in favour of the former.( B0 ) differences of only a few percent are expected [15].On the experimental side an average of all the available data before 2004 gave 0.786 ± 0.034 [16], while recent measurements at Tevatron and LHC [17] could resolve this discrepancy but the uncertanties are still large.LHCb has recently published the measuremet of the ( 0 b ) ( B0 ) [18] with 1 fb −1 collected at √ s = 7 TeV.The 0 b barions are selected in the J pK − decay channel (note that this is the first

Figure 1 .
Figure1.Left: distribution of m(J / ± ∓ ) − m(J / ) for B ± → J / ± ∓ K ± candidates.The results of the fit around the (2S) and the X masses are shown in the inserts.The solid blue, dashed red, and dotted green lines are the total, signal and background components, respectively.Right: distribution of the test statistic for the simulated experiments experiments with J P C = 2 −+ (black circles) and with J P C = 1 ++ (red triangles).A Gaussian fit to the 2 −+ distribution is overlaid (blue solid line).The value of the test statistic for the data, t data , is shown by the solid vertical line.
observed the already well established resonances D 1 (2420) 0 in the D * + − final state, and D * 2 (2460) in the D * + − , D + − and D 0 + final states, confirming their spin-parity assignments.We also observed two natural parity resonances D * J (2650) 0 and D * J (2760) + in the D * + − mass spectrum.The study of the D + − and D 0 + mass spectra supports the presence of the D * J (2760) + , while the analysis of the D * J (2650) 0 region is inconclusive because of the cross-feed.Finally the analysis of the D * + − final state showes also the presence of two unnatural parity states D J (2580) 0 and D J (2740) 0 .The measured masses and widths of the observed resonances are listed in
Expansion theory supports the hypothesis that the b-hadron lifetimes are quite similar; in the case of the ratio