# High Luminosity LHC: Prospects for New Physics

Marie-Hélène Genest, Greg Landsberg, Marie-Hélène Schune

► To cite this version:

Marie-Hélène Genest, Greg Landsberg, Marie-Hélène Schune. High Luminosity LHC: Prospects for New Physics. O. Brüning; M. Klein; L. Rossi; P. Spagnolo. The Future of the Large Hadron Collider, WORLD SCIENTIFIC, pp.241-255, 2023, 978-981-12-8017-7. 10.1142/9789811280184_0016. in2p3-04239784

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Chapter 16

High Luminosity LHC: Prospects for New Physics

Marie-Hélène Genest*, Greg Landsberg† and Marie-Hélène Schune‡

* *Univ. Grenoble Alpes, CNRS, Grenoble INP, LPSC-IN2P3,*
38000 Grenoble, France

†Brown University, Dept. of Physics,
182 Hope St., Providence, RI 02912, USA

‡Université Paris-Saclay, CNRS/IN2P3, IJCLab, Orsay, France

1. Introduction

While there has been no clear deviation from the Standard Model (SM) seen in the LHC data analysed so far, the data set available at the end of Run 2 corresponds to only about 5% of the full integrated luminosity that will become available by the end of the HL-LHC. Consequently, there are multiple ways in which the searches could benefit from the high-luminosity phase. Considering only the increase in the integrated luminosity (L) of the data samples, the sensitivity will improve as $\sqrt{L}$ and, for processes with no known SM backgrounds, as L, making these searches particularly interesting in the early years of the HL-LHC’s running. However, this will not be the only source of improvement; indeed, increased data sets will also benefit performance studies, reducing systematic uncertainties associated with the searches and further increasing their sensitivity. This will allow the analyses to probe for new physics (NP) at even higher masses, either directly, by, e.g. looking for a resonance in a given mass spectrum, or indirectly, by carefully looking for deviations in the tails of specific kinematic distributions. There could also be new phenomena at low masses, which would have escaped detection due to very weak couplings, and that still could be observed with a larger data set if innovative methods are employed to reduce

<footnote>This is an open access article published by World Scientific Publishing Company. It is
distributed under the terms of the Creative Commons Attribution 4.0 (CC BY) License.</footnote>
the background and to trigger on these events. Production mechanisms or decays, which were not covered in previous analyses due to their rarity, could also become accessible. Finally, going to the high-luminosity phase of the LHC will not imply merely repeating what has already been done before: the upgraded detectors and innovative analysis approaches, notably exploiting fast-developing machine-learning techniques, will most certainly allow the LHC physicists to investigate a much broader range of signatures than originally foreseen, e.g. new types of long-lived particles. Examples of these expected improvements will be discussed in the subsequent sections.

## 2. Where to Look for New Physics at the HL-LHC

### 2.1. Probing Higher Masses

Multiple NP theories predict the existence of new bosons, and the heavier version of the Z boson, the spin-1 $Z'$, is a standard benchmark when it comes to study the potential of colliders. A common benchmark is the $Z'_{SSM}$, for which the fermion couplings of the $Z'$ are taken to be that of the Z boson. Multiple final states can be used to search for this new boson ($e^+e^-, \mu^+\mu^-, q\bar{q},$ ...) by looking for a resonance in the invariant mass spectrum of the decay products over a smoothly falling background. Sensitivity studies for 3 ab$^{-1}$ of data at $\sqrt{s} = 14$ TeV have shown$^{1,2}$ that in the most sensitive channel combining the $e^+e^-$ and the $\mu^+\mu^-$ final states, a discovery of the $Z'_{SSM}$ up to a mass of 6.4 (6.8) TeV would be possible with the ATLAS$^1$ (CMS$^2$) detector, given that the current limits from these experiments are at 5.1 (5.2) TeV with the full Run 2 data set.$^{3,4}$

As in the SM case, in many NP models a $U(1) Z'$ boson would be accompanied by the $SU(2) W^{\pm 1}$ bosons, which could be sought in the leptonic or dijet decay channels. For the case of the SM-like couplings, $W'_{SSM}$ bosons can be discovered (excluded) in CMS with masses up to 6.4 (7.0) TeV based on decays into a $\tau$ lepton and a neutrino, followed by a hadronic $\tau$ lepton decay.¹ For a right-handed $W'$ boson decaying into a top and bottom (anti)quarks, ATLAS can discover (exclude) it up to masses of 4.3 (4.9) TeV.¹

A new vector boson could also act as a portal, allowing the production of weakly interacting massive particles (WIMPs, $\chi$), which can be dark matter candidates, through an s-channel interaction $q\bar{q} \to Z' \to \chi\chi$. The production rate in a simplified model, which only considers the $Z'$ and $\chi$ as new particles, will depend on the couplings of this boson to quarks, $g_q$,
<figure><img src="image_2.png" /><figcaption>Fig. 1. (Upper) ATLAS jet+$\ell^\pm$T search: expected 3$\sigma$ (solid) and 5$\sigma$ (dashed) discovery contours in the $m_\chi$ versus $m_{Z'_A}$ mass plane as described in the text. Three systematic uncertainty scenarios are considered. The regions below the curves would be within reach. (Middle) Projected sensitivity of the CMS mono-Z search in the 2HDM+a model with an extra pseudoscalar, as a function of its mass. (Lower) Projected sensitivity of the CMS LQ search in the plane of the LQ-lepton-quark coupling $\lambda$ versus the leptoquark mass. The areas on the left of the curves are within the reach. Figures taken from Ref. 1.</figcaption></figure>
and to the dark matter particles, $g_\chi$. As $\chi$ interacts only weakly and escape detection, the final state would be completely invisible to the detector (and therefore untriggerable), if it were not for the emission of initial-state QCD or electroweak radiation (ISR), the most sensitive channel being the emission of a gluon by one of the interacting quarks, leading to a jet+$p_T$ final state. A prospective study was performed by the ATLAS experiment, extrapolating the result obtained with 36 fb$^{-1}$ of Run 2 data to 3 ab$^{-1}$ of data at $\sqrt{s} = 14$ TeV. Contrary to the $Z'_{SM}$ search mentioned above, for which the sensitivity is mainly limited by the integrated luminosity, the jet+$p_T$ analysis is impacted by the systematic uncertainties, which will likely be reduced by further studies at the HL-LHC. The impact of different scenarios for the systematic uncertainty (same as before or reduced by factors of 2 or 4) on the sensitivity of the search was thus investigated. The resulting parts of the parameter space that could lead to a 3 (5) $\sigma$ evidence (observation) are shown in Fig. 1(upper) for an axial-vector $Z'$ (here called $Z_A$) with $g_q = 0.25$ and $g_\chi = 1.0$. A CMS projection is based on a complementary $Z+p_T$ (mono-Z) channel, which also offers access to other simplified models of dark matter, such as a two-Higgs doublet model with an extra pseudoscalar particle (2HDM+a),$^5$ via the $H \to Za$ decay, with the pseudoscalar $a$ further decaying into dark-matter particles. The reach within this model, as a function of the $a$ boson mass and the mass of a heavy Higgs boson $H$ (taken to be equal to the mass of a heavy pseudoscalar Higgs boson $A$), expressed in terms of 95% CL limits, is shown in Fig. 1(middle). These conclusions are based on the Run 2 CMS analysis,$^6$ under two assumptions: the same systematic uncertainties as in the Run 2 analysis and reduced by a factor of two, which is expected to be achievable, thanks to larger data samples being used to constrain the experimental uncertainties and improved theoretical calculations.

Mounting evidence for flavour non-universality in $b \to c\ell\bar{\nu}$ and $b \to s\ell^+\ell^-$ transitions has resulted in a renewed interest in searches for leptoquarks (LQs), which have been proposed as a theoretically preferred solution to these anomalies. Direct searches for LQs are therefore complementary to searches for flavour anomalies in $b$ hadron decays discussed in Sec. 2.3. An HL-LHC projection by the CMS experiment for searches for a scalar LQ decaying to a $b$ quark and a $\tau$ lepton, based on the Run 2 analyses,$^{7,8}$ shown in Fig. 1(lower) indicates that the combination of pair and single production would allow to probe these LQs up to a mass of 1.5 TeV,$^1$ which covers a significant range of the parameter space preferred by the theoretical models explaining flavour anomalies$^9$ and corresponds to about 500 GeV improvement on the sensitivity achieved so far.
## 2.2. Probing Lower Masses and Couplings

While HL-LHC offers a sizable improvement to the sensitivity at high masses, it will also allow to significantly extend the reach of searches for NP at lower masses, which could still have evaded the detection at the LHC and earlier machines because of small couplings and/or small production cross section and overwhelming backgrounds.

A significant development during the LHC Run 1 and (especially) Run 2 was the implementation of the trigger-level, or *scouting* analyses by ATLAS and CMS, which effectively increase the bandwidth of specific triggers dramatically by writing out only very compressed, trigger-level information about specific objects, instead of full events. These techniques have been used for searches for low-mass dijet resonances$^{10-13}$ and low-mass dimuon resonances.$^{14}$ While no projections for HL-LHC for analyses using these techniques have been done so far, the upgraded trigger systems of ATLAS and CMS will allow this program to continue at the HL-LHC and likely include additional final states, thus significantly expanding the program of low-mass NP searches. The “triggerless” approach taken by the LHCb experiment for the HL-LHC upgrade would allow for similar analyses in the forward rapidity region to be conducted as well.

Another important new technique pioneered at the LHC was the extensive use of ISR not just for dark matter particle searches (as discussed in Sec. 2.1), but also to look for low-mass dijet resonances (including the Higgs boson decaying to a *b* quark-antiquark pair) recoiling against an ISR jet or photon. The presence of a resonance is inferred from its merged decay products reconstructed as a single large-radius jet with a characteristic two-prong substructure. The jet substructure techniques help to reduce the large background from the QCD production of dijets. Using this approach allowed ATLAS and CMS to probe dijet resonances with masses as low as 10 GeV, and with couplings as low as 10% of the electromagnetic coupling,$^{15-17}$ exploring new territory which had not been accessible to $Sp\bar{p}S$, LEP, or the Tevatron experiments, as those techniques were either not available or not used.

The search for supersymmetry (SUSY), a theory which predicts a new particle (a sparticle) differing by half a unit of spin for every SM particle, has been one of the major axes of NP searches at the LHC. While the focus of the Run 1 LHC analyses was mainly to look for the pair production of strongly produced sparticles; the gluinos and squarks — the superpartners of the gluons and the quarks — the searches soon diversified to a
broader spectrum of signatures. With the limits pushing these states to higher masses, it becomes more and more interesting to probe for the elusive direct production, through electroweak interactions, of charginos ($\tilde{\chi}_1^\pm$) and neutralinos ($\tilde{\chi}_j^0$), the mixed charged and neutral states formed by the superpartners of the electroweak and Higgs bosons, ordered in increasing mass for $i = 1, 2$ and $j = 1, 2, 3, 4$. (In many models, $\tilde{\chi}_1^0$ is stable and can be a dark matter candidate). Indeed, if the masses of the gluinos and squarks are beyond the 3–4 TeV range, charginos and neutralinos may dominate the SUSY production at the HL-LHC energy and, if they exist, could be the first sparticles to be discovered. Prospective studies were performed in various channels.

One such example is the search for the production of $\tilde{\chi}_1^\pm\tilde{\chi}_2^0$, where $\tilde{\chi}_1^\pm \to W^\pm\tilde{\chi}_1^0$ and $\tilde{\chi}_2^0 \to h\tilde{\chi}_1^0$. This analysis channel underlines the speed at which analyses can evolve at the LHC: the once elusive Higgs boson is used here, via its main decay channel ($h \to b\bar{b}$), not as the focus for Higgs-boson studies, but as a tool to search for new particles. As the lightest neutralino is stable and escapes detection, the final state is hence composed of $p_T$, two b-tagged jets whose invariant mass is compatible with the SM-like Higgs boson, and the decay products of the W boson. In ATLAS prospective studies,¹ the leptonic decay of the W is considered, although the fully hadronic decay was later found, in the full Run 2 analysis, to yield even greater sensitivity at higher masses.¹⁸ In the leptonic analysis, in order to better separate this signal from the SM backgrounds, which are dominated by $t\bar{t}$ events, a boosted-decision-tree (BDT) approach is used, taking in input seven kinematic variables such as the transverse mass of lepton and the $p_T$ or the angular separation between the two b-jets. Three different BDTs are trained, each one targeting a different range of mass compression, that is of the mass difference between the produced $\tilde{\chi}_1^\pm/\tilde{\chi}_2^0$ and the final-state $\tilde{\chi}_1^0$. The expected reach at the HL-LHC is shown in Fig. 2: for a massless $\tilde{\chi}_1^0$, $\tilde{\chi}_1^\pm/\tilde{\chi}_2^0$ up to 1.08 TeV could be discovered. However, as mentioned above, this channel would not be expected to be the most sensitive, and this should hence be considered as a conservative reach, but it is still interesting to note that masses at the TeV scale will be within the reach of the HL-LHC even in some scenarios of electroweak production.

## 2.3. Probing Rare b- and c-Hadron Decays

The absence of tree-level Flavour Changing Neutral Current (FCNC) transitions is a feature that is highly specific to the SM. Consequently decays
<figure><img src="image_3.png" /><figcaption>Fig. 2. Expected 95% CL exclusion and 5σ discovery potential of the $W(\to \ell\nu)h(\to b\bar{b})$ analysis in the $\tilde{\chi}_1^0$ versus $\tilde{\chi}_1^{\pm}/\tilde{\chi}_2^0$ mass plane. Figure taken from Ref. 1.</figcaption></figure>

that can only proceed through FCNCs are sensitive NP probes. Since they are only allowed at the one-loop level in the SM, their studies are very well suited as a discovery mode, as any NP effect should be more pronounced.

The leptonic decays $B_s \to \mu^+\mu^-$ and $B_0 \to \mu^+\mu^-$ are helicity suppressed in the SM and, for the latter, further suppressed by the CKM matrix structure. In terms of the SM predictions, this results in tiny branching fractions with a very small relative uncertainty. The ratio between these two branching fractions is a powerful test for SUSY and for NP scenarios with minimal flavour violation. With the Upgrade II data set, LHCb will furthermore have the unique ability to measure the parameters $A_{\Delta\Gamma}^{\mu\mu}$ and $S_{\mu\mu}$ of the $B_s \to \mu^+\mu^-$ decay-time distribution. These are considered as “smoking gun” observables that, if different from their SM expectations of unity and zero respectively, would provide unambiguous evidence for NP.¹⁹ The ratio of branching fractions is expected to be measured with an 11% precision and the $S_{\mu\mu}$ parameter with an uncertainty of ~20%.²⁰

An observation of the $B^0 \to \mu^+\mu^-$ decay and precision measurement of its branching fraction is one of the key goals of the HL-LHC in this sector. The 30% improvement in the dimuon mass resolution offered by the CMS Phase II tracker Upgrade would allow CMS to establish this rare process at a significance of ~7 standard deviations and measure its branching fraction to a precision of 16%.¹ Moreover, the effective lifetime of $B_s$ meson in this decay channel will be measured with a precision of 0.05 ps.¹
Beyond these modes, the study of $b \to s\ell^+\ell^-$ transitions offers a unique opportunity to seek out signs of NP due to their rich phenomenology. The most powerful tests consist of the study of analyses of angular distributions and theoretically clean Lepton Flavour Universality (LFU) violation tests. Indeed, since LFU in the gauge couplings is exact in the SM, and corrections due to mass effects are calculable to good precision, it provides an excellent testing ground for NP theories where the LFU can be violated. The quantity $R_X$,21,22 the ratio of $B \to X\mu^+\mu^-$ and $B \to Xe^+e^-$ decay rates for a specific hadronic system X and in a defined range of $q^2$, has proven to be a particularly interesting test of LFU.23-26 With a 300 fb$^{-1}$ data sample, further LFU tests will be accessible; comparing angular distributions for $b \to s\mu^+\mu^-$ and $b \to se^+e^-$ transitions will allow the axial or vector nature of a potential NP particle to be probed at.27 The expected precision on the $R_K$ and $R_{K^*}$ ratios are 0.007 and 0.009, respectively, in the 1 to 6 GeV$^2$/c$^4$ dilepton invariant mass squared region.20 The expected constraints obtained on the difference between the muon and electron contributions to the Wilson coefficients $C_9$ and $C_{10}$ from such prospective measurements, including angular observables sensitive to the differences between the $b \to se^+e^-$ and $b \to s\mu^+\mu^-$ couplings, are shown in Fig. 3. The precision achievable with the LHCb Upgrade II data set will not only allow NP contributions to be established with an overwhelming significance, but will also provide a characterisation of the NP that will be essential to distinguish between theoretical models.

Due to the respective size of the CKM matrix elements involved, $b \to d\ell^+\ell^-$ transitions are suppressed by a factor of about 20 compared to the corresponding $b \to s\ell^+\ell^-$ processes. The large data sample will therefore provide a unique possibility to study $b \to d\ell^+\ell^-$ transitions with more precision than the one currently obtained for $b \to s\ell^+\ell^-$. It will allow comparisons between transitions of b quarks to the second (s) and first (d) quark generations and thus to discriminate between different models.

## 2.4. Probing CP Violation in b and c Decays

Searches for CP-violating NP require a precise determination of the SM benchmarks which are presented in Sec. 14. The results of the SM benchmark measurements allow prediction of the SM value for the CP-violating weak phase $\phi_s$. This parameter is a particularly sensitive probe for NP models as it is both extremely small and very precisely predicted in the SM. It is accessible though the study of a range of channels mediated by
<figure><img src="image_4.png" /><figcaption>Fig. 3. Projected sensitivity to the difference between the muon and electron mode contributions to the vector, $C_9$, and axial-vector, $C_{10}$, Wilson coefficients for different scenarios: the SM (blue), NP with a pure vector contribution (green), or NP with both vector and axial-vector contributions (red). The blue, red and green filled regions show the $3\sigma$ uncertainty contours for each scenario with the LHCb Upgrade II data set. The grey region shows the current $3\sigma$ uncertainty. Figure taken from Ref. 20.</figcaption></figure>

the $b \to c\bar{c}s$ transition but also from decays to final states that proceed through pure loop processes. Their comparison will allow to probe for the potential existence of NP contributing to the loop processes.

A further theoretically clean approach to search for CP-violating NP is through the parameters of $B_{(s)}^0 - \bar{B}_{(s)}^0$ mixing. These are denoted $a_{\text{sl}}^{d,s}$ since they are typically determined using semileptonic decays, and their tiny values are very precisely predicted in the SM$^{28}$ while being highly sensitive to NP that could enter the mixing loops. Copious signal yields will be available, and the main challenge will be to control potential systematic uncertainties due to production, detection and background asymmetries. These can, however, be determined from control samples. The expected precision compared with the SM predictions is shown in Table 1.

Charm hadrons provide a unique opportunity to measure CP violation associated with processes mediated by up-type quark transitions. These can be affected by NP contributions in fundamentally different ways to the down-type quarks that make up the kaon and beauty systems. Since the amount of CP violation is expected to be very small, $O(10^{-4})$, significant deviations would be clear signs of NP. However, contrary to the b-hadron sector, SM predictions are more challenging and a full understanding
<table><caption>Table 1. Results of the current theoretical and expected experimental determination of the semileptonic asymmetries $a_{sl}^d$ and $a_{sl}^s$. $^{20,29}$</caption><thead><tr><th></th><th>Current theory</th><th>LHCb (300 fb<sup>-1</sup>)</th></tr></thead><tbody><tr><td>$\delta a_{sl}^d$ [$10^{-4}$]</td><td>0.6</td><td>2</td></tr><tr><td>$\delta a_{sl}^s$ [$10^{-4}$]</td><td>0.03</td><td>3</td></tr></tbody></table>

requires the study of a significant number of decay modes. LHCb Upgrade II will have the best sensitivity on the fundamental parameters of CP violation in charm mixing, which has not been observed yet.

## 2.5. Benefiting from Detector Upgrades and Improved Analysis Techniques

The detector upgrades (see Sec. 12) will also improve searches for NP. One sector which may particularly benefit from these upgrades are searches for long-lived particles (LLPs). The LLPs naturally occur in many NP theories, their detector-size lifetimes being due to heavy mediators, small couplings, or compressed mass spectra reducing the available decay phase space. The signatures for such particles involve decays in various parts of the detectors, away from the interaction point. This can lead to, e.g. displaced vertices (DVs) reconstructed in the inner detector. For ATLAS, this part of the detector will be fully replaced in view of the HL-LHC by a full-silicon tracker (ITk) which will significantly impact the DV-based LLP searches. Not only will the ITk cover a larger range in pseudo-rapidity, extending the tracking capabilities from $|\eta| < 2.5$ to $|\eta| < 4.0$, but the tracking reconstruction performances for decays happening at large radii will also be improved, as shown in Fig. 4(upper). In this figure, a SUSY model with long-lived gluinos is considered; these gluinos hadronise into so-called R-hadrons, which eventually decay into a pair of SM quarks and a stable $\tilde{\chi}_1^0$ (masses of 2 TeV and 100 GeV are considered here for the R-hadron and the neutralino respectively). In the current detector, the tracking efficiency for charged particles with $p_T > 1$ GeV worsens drastically beyond a radius of 300 mm, which corresponds to the first layer of the SCT, as a certain number of hits are required in this subdetector to insure the track quality. With the ITk, the larger spacing between the layers provides a high efficiency up to 400 mm instead, with some tracks fulfilling the requirements even at the R-hadron decay radii of 500–600 mm.
<figure><img src="image_5.png" /><figcaption>Fig. 4. (Upper) Probability of a charged particle to be reconstructed as a good-quality track by ATLAS as a function of its radius of origin for the Run 2 and HL-LHC detector configurations (considering or not the effect of material producing hadronic interactions). See the text for details. Figure taken from Ref. 1. (Lower) Distribution of the number of incorrectly associated tracks with or without the use of a $3\sigma$ (where $\sigma = 35$ ps) selection on timing information. The vertical axis is the fraction of primary vertices, which have the number of pile-up tracks shown on the horizontal axis associated with them. Figure taken from Ref. 30.</figcaption></figure>

Another important tool to improve the sensitivity of searches is to use precision timing to help separating particles coming from pile-up
interactions from a signal. In the case of CMS, this will be achieved via a minimum ionizing particle timing detector (MTD) with the time resolution of ~30 ps per track. The precision timing allows separation of the pile-up vertices from the signal hard-scattering vertex not only in space, but also in time, thus allowing for a four-dimensional vertex reconstruction and separation. Figure 4(lower) shows the fraction of primary vertices which have a certain number of tracks from pile-up interactions incorrectly associated with them, with and without the MTD. As can be seen from this plot, at a pile-up of 200, on average there are about 15 tracks incorrectly associated with the primary vertex if no timing information is used. Utilizing this information, the average number of such tracks drops to about 5, which is effectively equivalent to the reduction of pile-up from 200 to 60, similar to the number of pile-up interactions at the beginning of the LHC fills in Run 2. Additionally, the MTD can be used to improve the sensitivity to LLPs by using the time-of-flight information to detect tracks left by massive particles moving with velocities $\beta < 1$.

On the LHCb side, the upgraded detector will be also capable of providing fast-timing information with resolution of few tens of ps per particle. This is an essential attribute for dealing with the very challenging experimental conditions of HL-LHC in the forward direction at very high pile-up.

Novel analyses techniques are also expected to improve the coverage of the possible NP parameter space. While machine-learning techniques have already been used for quite some time in particle physics, from BDTs to deep neural networks, recently there has been a rise in the frequency and breadth of their use.³¹ Some machine-learning techniques can target specific well-defined theories better than simple “cut-and-count” analyses by using training on signal and background samples, especially when the signal extraction from the background relies on minute details in a variety of kinematic distributions. However, the results of these analyses can highly depend on the model under study. An exciting recent area is the search for anomalies in the data themselves, focusing on a search for objects, which would differ significantly from the data-based training sample, without requiring a prior knowledge of the signal characteristics, as, for example, has been done recently in an ATLAS analysis.³² These developments are gaining momentum and they will very likely become one of the standard NP search tools by the time of the HL-LHC.
### 3. Conclusions

In this chapter, we summarised just a handful of projections made in the most active areas of the LHC NP search programme to date, which are inspired either by well-motivated theoretical models or by the existing hints of various anomalies that survived more than a decade of the LHC data. There are many other avenues of NP searches which will be pursued at the HL-LHC, including low-mass NP searches and searches for long-lived particles, where only a few precise projections have been made so far.

One should also keep in mind that the assumptions made in deriving these projections are typically very conservative, as it is very hard to quantify the rapid progress the LHC physics analyses have been continuously achieving. Some of the earlier HL-LHC projections have already been realised or nearly achieved with the existing data sets, thanks to much more elaborated analysis methods, powerful machine-learning techniques used for both particle identification and for refining the analysis strategy, more precise theoretical calculations, and the ingenuity of the LHC physicists tirelessly working on pushing these analyses to the best sensitivity possible. We believe that the enormous progress seen in the last decade will continue in the next one, making the HL-LHC an ultimate place to witness another major discovery, which may change the very way we think about modern physics, bringing us another step closer to solving the remaining mysteries of our universe.

### References

1. X. Cid Vidal et al., Report from Working Group 3: Beyond the Standard Model physics at the HL-LHC and HE-LHC, CERN Yellow Rep. Monogr. **7**, 585-865 (2019). doi: 10.23731/CYRM-2019-007.585.

2. ATLAS and CMS Collaborations. Snowmass White Paper Contribution: Physics with the Phase-2 ATLAS and CMS Detectors. URL https://cds.cern.ch/record/2805993/files/ATL-PHYS-PUB-2022-018.pdf. ATL-PHYS-PUB-2022-018, CMS PAS FTR-22-001 (2022).

3. ATLAS Collaboration, Search for high-mass dilepton resonances using $139 \text{ fb}^{-1}$ of pp collision data collected at $\sqrt{s} = 13$ TeV with the ATLAS detector, Phys. Lett. B. **796**, 68-87 (2019). doi: 10.1016/j.physletb.2019.07.016.

4. CMS Collaboration, Search for resonant and nonresonant new phenomena in high-mass dilepton final states at $\sqrt{s} = 13$ TeV, JHEP. **07**, 208 (2021). doi: 10.1007/JHEP07(2021)208.

5. M. Bauer, U. Haisch, and F. Kahlhoefer, Simplified dark matter models with
two Higgs doublets: I. Pseudoscalar mediators, JHEP. 05, 138 (2017). doi: 10.1007/JHEP05(2017)138.

6. CMS Collaboration, Search for new physics in events with a leptonically decaying Z boson and a large transverse momentum imbalance in proton-proton collisions at $\sqrt{s}$ = 13 TeV, Eur. Phys. J. C. 78 (4), 291 (2018). doi: 10.1140/epjc/s10052-018-5740-1.

7. CMS Collaboration, Search for third-generation scalar leptoquarks decaying to a top quark and a $\tau$ lepton at $\sqrt{s}$ = 13 TeV, Eur. Phys. J. C. 78, 707 (2018). doi: 10.1140/epjc/s10052-018-6143-z.

8. CMS Collaboration, Search for heavy neutrinos and third-generation leptoquarks in hadronic states of two $\tau$ leptons and two jets in proton-proton collisions at $\sqrt{s}$ = 13 TeV, JHEP. 03, 170 (2019). doi: 10.1007/JHEP03(2019)170.

9. D. Buttazzo, A. Greljo, G. Isidori, and D. Marzocca, B-physics anomalies: a guide to combined explanations, JHEP. 11, 044 (2017). doi: 10.1007/JHEP11(2017)044.

10. CMS Collaboration, Search for narrow resonances in dijet final states at $\sqrt{s}$ = 8 TeV with the novel CMS technique of data scouting, Phys. Rev. Lett. 117 (3), 031802 (2016). doi: 10.1103/PhysRevLett.117.031802.

11. ATLAS Collaboration, Search for low-mass dijet resonances using trigger-level jets with the ATLAS detector in pp collisions at $\sqrt{s}$ = 13 TeV, Phys. Rev. Lett. 121 (8), 081801 (2018). doi: 10.1103/PhysRevLett.121.081801.

12. CMS Collaboration, Search for narrow and broad dijet resonances in proton-proton collisions at $\sqrt{s}$ = 13 TeV and constraints on dark matter mediators and other new particles, JHEP. 08, 130 (2018). doi: 10.1007/JHEP08(2018)130.

13. CMS Collaboration, Search for dijet resonances using events with three jets in proton-proton collisions at $s$ = 13 TeV, Phys. Lett. B. 805, 135448 (2020). doi: 10.1016/j.physletb.2020.135448.

14. CMS Collaboration, Search for a Narrow Resonance Lighter than 200 GeV Decaying to a Pair of Muons in Proton-Proton Collisions at $\sqrt{s}$ = TeV, Phys. Rev. Lett. 124 (13), 131802 (2020). doi: 10.1103/PhysRevLett.124.131802.

15. ATLAS Collaboration, Search for low-mass resonances decaying into two jets and produced in association with a photon using pp collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector, Phys. Lett. B. 795, 56-75 (2019). doi: 10.1016/j.physletb.2019.03.067.

16. CMS Collaboration, Search for Low-Mass Quark-Antiquark Resonances Produced in Association with a Photon at $\sqrt{s}$ = 13 TeV, Phys. Rev. Lett. 123 (23), 231803 (2019). doi: 10.1103/PhysRevLett.123.231803.

17. CMS Collaboration, Search for low mass vector resonances decaying into quark-antiquark pairs in proton-proton collisions at $\sqrt{s}$ = 13 TeV, Phys. Rev. D. 100 (11), 112007 (2019). doi: 10.1103/PhysRevD.100.112007.

18. ATLAS Collaboration, Search for charginos and neutralinos in final states with two boosted hadronically decaying bosons and missing transverse momentum in pp collisions at $\sqrt{s}$ = 13 TeV with the ATLAS detector, Phys. Rev. D. 104 (11), 112010 (2021). doi: 10.1103/PhysRevD.104.112010.
19. K. De Bruyn, R. Fleischer, R. Knegjens, P. Koppenburg, M. Merk, A. Pel-legrino, and N. Tuning, Probing new physics via the $B_s^0 \to \mu^+\mu^-$ effective lifetime, Phys. Rev. Lett. **109**, 041801 (2012). doi: 10.1103/PhysRevLett. 109.041801.

20. LHCb Collaboration. Framework TDR for the LHCb Upgrade II Opportunities in flavour physics, and beyond, in the HL-LHC era. Technical report, CERN, Geneva (Jul, 2021). URL http://cds.cern.ch/record/2776420.

21. G. Hiller and F. Krüger, More model-independent analysis of $b \to s$ processes, Phys. Rev. D69, 074020 (2004). doi: 10.1103/PhysRevD.69.074020.

22. Y. Wang and D. Atwood, Rate difference between $b \to s\mu^+\mu^-$ and $b \to se^+e^-$ in supersymmetry with large tan $\beta$, Phys. Rev. D68, 094016 (2003). doi: 10.1103/PhysRevD.68.094016.

23. LHCb Collaboration, Test of lepton universality in $b \to s\ell^+\ell^-$ decays, https://arxiv.org/abs/2212.09152. Accepted for publication in Phys. Rev. Lett.

24. LHCb Collaboration, Test of lepton universality using $\Lambda_b \to pK^-\ell^+\ell^-$ decays, JHEP. 05, 040 (2020). doi: 10.1007/JHEP05(2020)040.

25. LHCb Collaboration, Test of lepton universality in beauty-quark decays. Nat. Phys. 18, 277-282 (2022).

26. LHCb Collaboration, Tests of lepton universality using $B^0 \to K_S^0\ell^+\ell^-$ and $B^+ \to K^{*+}\ell^+\ell^-$ decays (2021). Phys. Rev. Lett. 128 (2022) 191802.

27. B. Capdevila, Assessing lepton-flavour non-universality from $b \to k^*\ell^+\ell^-$ angular analyses, Journal of Physics: Conference Series. 873, 012039 (Jul, 2017). ISSN 1742-6596. doi: 10.1088/1742-6596/873/1/012039. URL http://dx.doi.org/10.1088/1742-6596/873/1/012039.

28. M. Artuso, G. Borissov, and A. Lenz, CP violation in the $B_s^0$ system, Rev. Mod. Phys. 88, 045002 (2016). doi: 10.1103/RevModPhys.88.045002.

29. LHCb Collaboration, Physics case for an LHCb Upgrade II — Opportunities in flavour physics, and beyond, in the HL-LHC era. (CERN-LHCC-2018-027 LHCb-PUB-2018-009) (2018).

30. CMS Collaboration. A MIP Timing Detector for the CMS Phase-2 Upgrade. URL https://cds.cern.ch/record/2667167/files/CMS-TDR-020.pdf. CERN-LHCC-2019-003 (2019).

31. G. Karagiorgi, G. Kasieczka, S. Kravitz et al., Machine learning in the search for new fundamental physics. Nat. Rev. Phys. 4, 399-412 (2022).

32. ATLAS Collaboration, Dijet resonance search with weak supervision using $\sqrt{s} = 13$ TeV pp collisions in the ATLAS detector, Phys. Rev. Lett. 125 (13), 131801 (2020). doi: 10.1103/PhysRevLett.125.131801.