Heavy ion results from LHCb

The LHCb detector is a single-arm spectrometer fully instrumented in the forward rapidity at the LHC. In this contribution we introduce some of the most recent results in heavy ion collisions, the upgrades for LHC Run3 and the future perspectives of the heavy ion program from the LHCb experiment.

covers 2.0 < η < 4.8 for pp, 1.5 < η < 4.3 for pPb and −5.2 < η < −2.5 for Pbp, and is divided into 6 η bins in this measurement. The prompt charged hadrons are measured with the LHCb tracking system. Their yields are corrected for the reconstruction and selection efficiency, as well as contamination from ghost tracks and non-prompt particles. The total uncertainty of the production cross-section is about 3% for most (p T , η) bins. The nuclear modification factor R pPb is presented in Fig. 2. The five upper (lower) panels show R pPb as a function of p T in the pPb (Pbp) configuration. A significant suppression is observed in the forward region. The R pPb value reaches ∼ 0.3 at lowest p T in the most forward pseudorapidity bin. In the backward region, an enhancement is found for p T > 1.5 GeV/c. The measurement is compared to calculations from three phenomenological models in the region p T ≥ 1.5 GeV/c. The calculation using nuclear PDFs [6] agrees with the forward data within large nPDF uncertainties, but does not reproduce the enhancement in the backward region. The gluon saturation model [7] qualitatively describes the increasing trend in the forward rapidity. In the backward rapidity, the data are not well described by the pQCD calculation considering parton multiple scattering [8].

Prompt production ratio of χ c2 /χ c1 in pPb collisions at 8.16 TeV
The χ c2 and χ c1 charmonium production in nuclear collisions at LHC energies is first measured by the LHCb with pPb collisions at 8.16 TeV [9]. The χ cJ states are reconstructed via the decay χ c → J/ψ(→ µ + µ − )γ The J/ψ candidates are reconstructed from a pair of oppositely charged muons. They are combined with a photon candidate to form a χ c candidate. Two types of photons are used in this analysis: photons converted into electron positron pairs in the detector material and reconstructed by the tracking system (converted photons), and photons from their energy deposits in the electromagnetic calorimeter (calorimetric photons). The χ c1 and χ c2 signals reconstructed via converted photons are shown in the left panel of Fig. 3, while those reconstructed via calorimetric photons are shown in the right. The converted photon technique provides better mass resolution with well separated χ c1 and χ c2 peaks, whereas the calorimetric photon technique provides ∼ 20 times more statistics. An upper limit of the pseudo-decay time, t z < 0.1 ps, is imposed to remove non-prompt χ c production from b-hadron decays.  As the χ c1 and χ c2 decays share nearly identical kinematics, various detector effects in the ratio of σχ c2 /σχ c1 , such as tracking and particle identification efficiencies, are cancelled, reducing the systematic uncertainty of the measurement. The measured ratio is presented in Fig. 4. The σχ c2 /σχ c1 ratios measured with the two photon types are consistent with each other, and with unity in both the forward and backward rapidity regions. The significantly larger calorimetric photon sample allows for a more precise measurement. The cross-section ratio in pPb data is also compared with a similar measurement from pp collisions at √ s = 7 TeV [10]. The results are consistent within two standard deviations. This implies that nuclear effects, if any, have the similar impacts on both states.    Figure 6 presents the measured production yield of photo-produced J/ψ mesons as a function of p T (left), and rapidity, y, (right), determined for the first time at the LHC. The mean p T of the coherent J/ψ is estimated to be p T = 64.9 ± 2.4 MeV/c. Theoretical calculations [12,13] are drawn in open circles, and are qualitatively in agreement with the data in the p T and y shape.  Figure 6. Invariant yields of photo-produced J/ψ mesons as a function of p T (left) and rapidity (right), for the centrality interval N part = 19.7 ± 9.2. The blue error bars represent the statistical uncertainty and the red error bars the total uncertainty. Theoretical model predictions [12,13] are shown in open circles in black and green.

Upgrade and future plans
During the LHC Long Shutdown 2 the LHCb is undergoing a major upgrade [14] to meet the challenge of the increased luminosity in the LHC Run3. A new software-only trigger system with 40 MHz data acquisition and real time data reconstruction will replace the current trigger system using a hardware-based trigger step. The detector readout scheme is being upgraded to allow event processing at 40 MHz rate. Upgrades of the sub-detectors include a new Vertex Locator (VELO) with pixel segmentation, new tracking stations and new RICH optics and PMTs. The upgrade will reduce the occupancy limitation in PbPb collisions, and allow access to mid-central PbPb collisions up to 30% in centrality.
The upgrade of the LHCb's fixed target program, SMOG2, is the most important upgrade for heavy-ion physics [15]. Depending on the beam energy, the centre-of-mass energies of the fixed-target collisions range from 40 to 115 GeV, between the SPS and RHIC energies. The forward acceptance of the LHCb geometry covers the middle and backward rapidity regions, providing access to high Bjorken-x region in the target nuclei. The installation of a gas storage cell upstream of the interaction point together with a new Gas Feed System for precise luminosity determination opens the door to many future measurements of great interest. The design of the SMOG2 storage cell is shown in the left panel of Fig. 7, the right panel shows a photo of the installed storage cell inside the LHCb detector together with the vertex detector VELO. With the new storage cell, more gas species such as H 2 , D 2 , N 2 , O 2 . Kr and Xe can be injected in addition to He, Ne and Ar. A well-defined interaction region of beam-gas collisions allows the possibility to record beam-gas and beam-beam collisions simultaneously without affecting the performance in the collider mode. The gas target areal density can be increased by up to 100 times, leading to higher luminosity for fixed-target collisions.

Conclusion and outlook
LHCb has a rich heavy ion program with excellent detector performance and unique kinematic coverage. The high statistical pPb datasets offer valuable opportunities to perform precision measurements, study rare probes and investigate small Bjorken-x physics. First results from the PbPb datasets also demonstrate the capabilities of LHCb in studying nuclear effects in different systems. With the current upgrade, significantly increased statistics of pPb and PbPb data samples in Run3 will allow for more precision measurements and open the possibilities for many new analyses. The upgrade will also unlock PbPb collisions up to mid-central events, enabling QGP studies with the LHCb detector. SMOG2 will bring the current fixed-target system to a new level with high statistics without detector saturation in unexplored energy and kinematic regions.