CERN Accelerating science

LHCb physics looks forward to Run 3

The first high energy beams of the LHC on July 5th 2022 marked the start of the third Run of the LHC. While still harvesting and exploring in depth the physics of the first two runs, the LHCb experiment has undergone a major upgrade [1]. Ninety percent of the detector components have been replaced including the entire readout system, and the way that the data are selected and processed has been redesigned. An example of a displaced particle reconstructed with the very first Run 3 data in the first high-level trigger is shown in Figure 1.

The summer and autumn of 2022 have been dedicated to the installation and commissioning of the apparatus. A number of important milestones have been crossed. For instance, the ability to operate and read out the full LHCb detector at the nominal LHC bunch-crossing rate of 40 MHz has been demonstrated [2]. However, what one learns with time is that particle-physics experiments are never an easy ride. At the start of 2023 the collaboration faced an unforeseen “detour” in its journey towards its goals with an incident of the LHC vacuum deforming the shape of the RF foil of the vertex detector, known as the VELO. In agreement with all relevant parties and given the time constraints, it was decided to replace this foil during the upcoming year-end technical stop. The upcoming period is crucial to establish how close the VELO can be positioned to the beamline and the optimal point at which to operate the detector for the rest of the year, to maximise physics potential [3]. It goes without saying that all of this makes 2023 a very special year for the collaboration.

Throughout the years the LHCb collaboration, which was initially thought of as flavour experiment, expanded its physics programme beyond design goals. It now covers precision CP violation in beauty and charm hadrons, the study of both flavour-changing neutral and charged currents, conven- tional and exotic hadron spectroscopy, electroweak physics, kaon physics, direct searches for new particles,fixed target and heavy-ion physics. Out of this myriad of physics opportunities, a few will be cherry-picked below.

Figure 1: Invariant mass for K0π+π candidates collected by the first high-level trigger.

For Runs 1 and 2 the LHCb detector employed a hardware first-level trigger. It operated based on the responses of the electromagnetic and hadronic calorimeters as well as the muon stations. This negatively affected, in particular, fully hadronic decays, for which the hardware trigger was at best 50% efficient. The upgraded LHCb detector is now entirely read out at the LHC interaction rate and data are processed using a flexible software trigger, which removes the forementioned limitation. The new trigger will significantly improve the efficiency for hadronic modes, as for example charm decays or beauty decays which give access to the CKM phase γ. LHCb has become the leading experiment in measurements of this fundamental pa- rameter of the Standard Model (SM) and the sensitivity to γ is expected to improve with Run 3 and beyond.

The charm sector is unique for CP violation studies and searches for physics beyond the Standard Model (BSM) [4]. The LHCb experiment ob- served CP violation in charm decays for the first time with the data collected during Run 1 and Run 2 in two-body D0 decays, reaching a milestone of 10−4 level of precision. The direct CP asymmetry that was measured puts the effect at the higher end of expectations based on the SM. Comprehensive studies of two-, three- and four-body modes are fundamental to understand whether the emerging CP violation picture in charm can be explained within the SM or not. It has to be noted that CP violation in charm mixing is not yet observed; SM predictions are very small (CP asymmetries of O(10−4) or less), hence these processes are excellent probes for BSM physics. Some key observables are expected to prove CP violation in charm mixing at the level of ∼ 3 × 10−5; for a central value of 10−4 this could lead to evidence for nonzero value. Figure 2 shows a three-body charm-meson decay recon- structed with 2022 data.

Figure 2: Invariant mass for Ds+K+Kπ+ candidates collected in pp 2022 data selected by the second high-level trigger.

Flavour oscillations in the B sector provide many interesting observables. One of the most important is the so-called ϕs weak phase, accessible via B0 and Bs0 decays to J/ψK+K, for example.  The SM has a very precise prediction of ϕs = −36.4 ± 1.2 mrad. The most recent measurement from LHCb dominates the world average. At the end of nominal Run 3, the LHCb precision on ϕs is estimated to be about 10 mrad.

In the last decade both the charged- and neutral-current flavour-changing measurements, b cℓν and b sℓ+, have been somewhat of a roller-coaster for flavour-physics enthusiasts. In a seminar held at CERN in December 2022 LHCb announced a new combined test of lepton universality in b sℓ+ transitions [5], namely measurements of the RK and RK∗ ratios, using the legacy Run-1 and Run-2 datasets. One of the strengths of such observables is that they are “theoretically clean”, as the uncertainties that arise due to hadronic interactions in the decays cancel in the ratio. It is worth noting that despite the “cold shower” effect [6], to quote a colleague that followed the announcement of these results washing away hints of significant deviations from the SM expectation, the precision of these measurements is still statistically dominated. Figure 3 shows the invariant mass distribution of a B+J/ψK+ decay where the J/ψ is reconstructed from two oppositely charged electrons in the Run-3 data collected in 2022. This demonstrates the ability to reconstruct final states with electrons and their associated photons from bremsstrahlung emission with the new detector.

Angular analyses of b → sµ+µ also indicate tensions with the SM in certain observables, though in this case debates exist in the community about the actual values of SM predictions.  A way forward is to follow the path of the Belle experiment which conducted tests of the difference of angular observables using muons and electrons in the final state. In the future LHCb will also be able to contribute to this kind of measurements.

Figure 3: Invariant-mass distribution of K+e+e candidates, reconstructed by constraining the invariant mass of the electron pair to the known mass of the J/ψ meson, from 2022 pp data.

Since 2013 and the recording of a pilot pPb Run, the LHCb detector has developed a fruitful heavy-ion programme. LHCb has been able to inject noble gases into the LHC vacuum pipe, offering a unique opportunity to study beam-gas collisions at the LHC. The device, known as SMOG, has also been upgraded for Run 3, enabling gas injection with higher pressure: Neon, Argon, Hydrogen, Xenon and possibly further species.

This fixed-target configuration is also a bridge between collider and cosmic-ray physics, as demonstrated by the antiproton cross-section measurement, which constrained theory models in the search for dark matter in cosmic rays. A first look at the performance of SMOG in Run 3 using the 2022 data looks promising, as illustrated in Figure 4, which shows a reconstructed dimuon decay in pAr collisions.

The design of the new scintillating fibre detector will expand the centrality range accessible in PbPb collisions, allowing large volumes of quark-gluon plasma to be probed for the first time at LHCb. e.g., searches for gluon-saturation effects and studies of Drell-Yan processes. Such so-called small systems are very important to fill the gaps among pp, pPb and PbPb, and these studies will put new constraints on the conditions which allow quarks to become deconfined in a plasma phase.

Due to its non-perturbative nature, quantum chromodynamics remains a very active and fruitful field that could be explored in multiple ways. An example is the spectroscopy of conventional and exotic hadrons. In the conventional quark model, hadrons are formed either from quark-antiquark pairs (mesons) or three quarks (baryons). Particles which cannot be classified within this scheme are referred to as exotic hadrons.

Figure 4: Invariant-mass spectrum of the µ+µ final-state in pAr 2022 data.

Seventy-two new hadrons have been discovered at the LHC and a large fraction of them comes from LHCb, as shown in Figure 5. As of today only one doubly heavy baryon, i.e., a baryon with two heavy quarks known as the Ξ++, has been observed. With Run-3 data, LHCb will be also able to search for its isospin partner, the Ξbc+ , and potentially also the Ωbc+.

More broadly speaking, the sensitivity to beauty-charm, Ξbc, baryons will also be significantly improved, with good prospects for discovery. A similar picture can be drawn for B+- meson physics.

The overwhelming rate of discoveries of exotic hadrons allows us to be confident that more discoveries are within our horizon. Until now, the majority of the observations of exotic hadrons relied on the reconstruction of J/ψ into two muons. In the future, key aspects such as the decay rate of the pentaquarks (a three-quark plus two-antiquark state) may be addressed thanks to the study of fully hadronic final states such as D(∗)Λc and D(∗)Σc(∗).

Finally, it is worth emphasising that not all of the topics and possibilities have been covered in this letter. If experience taught us one thing, it is that the future will come with surprises. Some may be good, some may be not as good. However, one can only look forward to seeing results from a collaboration that has demonstrated multiple times its capacity to re-invent itself and explore unforeseen corners of particle physics.

Figure 5: The 72 new hadrons discovered at the LHC.



  1.…- record-energy-for-a-brand-new-lhcb-detector
  2.…; data-processing
  4.…; and-mixing-beauty-and-charm-decays-lhcb
  5.…- measurements-show-agreement-with-the-standard-model