Exclusive displaced hadronic signatures in the LHC forward region

JHEP01(2020)115 Published for SISSA by Springer Received:November 4, 2019 Accepted:December 19, 2019 Published:January 20, 2020 Exclusive displaced hadronic signatures in the LHC forward region Xabier Cid Vidal,aYuhsin Tsaiband Jose Zuritac,d aInstituto Galego de F´ısica de Altas Enerx´ıas (IGFAE), Universidade de Santiago de Compostela, R´ua de Xoaqu´ın D´ıaz de R´abago, s/n, Santiago de Compostela 15782, Spain bMaryland Center for Fundamental Physics, Department of Physics, University of Maryland, Stadium Dr., College Park, MD 20742-4111, U.S.A. cInstitute for Nuclear Physics (IKP), Karlsruhe Institute of Technology, Hermann-von-Helmholtz-Platz 1, Eggenstein-Leopoldshafen D-76344, Germany dInstitute for Theoretical Particle Physics (TTP), Karlsruhe Institute of Technology, Engesserstraße 7, Karlsruhe D-76128, Germany E-mail: [email protected],[email protected],[email protected] Abstract: The LHCb detector provides accurate vertex reconstruction and hadronic particle identification, which make the experiment an ideal place to look for light long-lived particles (LLP) decaying into Standard Model (SM) hadrons. In contrast with the typical search strategy relying on energetic jets and a high multiplicity of tracks from the LLP decay, LHCb can identify LLPs in exclusive, specific hadronic final states. To illustrate the idea, we study the sensitivity of LHCb to an exotic Higgs decay h→SS, followed by the displaced decay of GeV-scale scalars into charged kaons S→K+K−. We show that the reconstruction of kaon vertices in narrow invariant mass windows can efficiently eliminate the combinatorial backgrounds from B-meson decays. While the same signal is extremely difficult to probe in the existing displaced jet searches at ATLAS/CMS, the LHCb search we propose can probe the branching ratio BR(h→SS) down to 0.1% (0.02%) level with 15 (300) fb−1of data. We also apply this projected bound to two scenarios with Higgs portal couplings, where the scalar mediator Seither couples to a) the SM quarks only, or b) to both quarks and leptons in the minimal flavor violation paradigm. In both scenarios we compare the reach of our proposed search with the expected constraints from ATLAS and CMS on the invisible Higgs width and with the constraints from rare B-decays studies at LHCb. We find that for 1 GeV < mS<2 GeV and 0.5 mm .cτ .10 mm our proposed search will be competitive with the ATLAS and CMS projections, while at the same time providing crucial information of the hadronic interactions of S, which can not be obtained from the indirect measurement of the Higgs invisible width. Keywords: Beyond Standard Model, Higgs Physics ArXiv ePrint: 1910.05225 Open Access,c The Authors. Article funded by SCOAP3.https://doi.org/10.1007/JHEP01(2020)115 JHEP01(2020)115 Contents 1 Introduction 1 2 LHCb features 3 3 Displaced Kaon search at LHCb 4 3.1 Search strategy 5 3.2 Branching ratio constraint 8 4 Application to Higgs portal scenarios 10 4.1 The hadrophilic Higgs portal 11 4.2 The Higgs portal through mixing model 11 5 Conclusion and outlook 13 1 Introduction Many important extensions of the Standard Model (SM) feature singlet particles with respect to the SM gauge symmetry that serve as mediators between the dark and visible sectors. Among the four different mechanisms of the mediator couplings: Higgs, vector, axion, and neutrino portals, the mediator coupling to the SM Higgs is especially important both theoretically and experimentally and is the main focus of this article. Here we study the future prospects of a LHCb search for an “exotic” Higgs decay process, h→SS, followed by a displaced decay of the singlet scalar Sinto SM hadrons. The singlet scalar can interact with the SM particles either through the Higgs mixing or through a higher dimensional operator that couples to the SM Yukawa couplings. These portal couplings have been studied in many works in the literature in the context of, e.g., dark matter and neutrino scenarios [1–3], supersymmetric models [4], cosmic inflation [5,6], and cosmological solutions to the Higgs hierarchy problem [7]. The scalar mass mShas no preferred values, and hence it can be much lower than the electroweak scale. Depending on the specific scenario and on the flavor structure of the Scouplings, Scan decay dominantly into hadrons, leptons, or both types of particles. The rate and branching ratio of the S decay is determined by mSand the size of Higgs portal coupling. For the specific case where Shas a GeV scale mass and a Yukawa-type coupling to SM fermions that are suppressed by a mixing angle θ.10−2, the decay length of Sinto SM particles is larger than the millimeter scale. Since our goal is to probe the even smaller Scoupling to SM particles, S shows up as a long-lived particle (LLP) signature in LHC searches.1 In the general purpose ATLAS and CMS detectors, there are two orthogonal probes of the h→SS signal. In the first place, hadronic decays of Sproduce displaced jet signatures. Due to the large hadronic background and a limited ability to identify the SM hadrons, 1For a comprehensive review of LLPs at the LHC see [8]. – 1 – JHEP01(2020)115 the ATLAS/CMS studies rely on various inclusive searches of displaced jet signatures, which are mainly sensitive to signals with LLP mass much larger than O(10) GeV scale or transverse energy HT>O(100) GeV (e.g., [9,10]). Some ATLAS/CMS studies can lower the mass and energy requirements by focusing on decays in the hadronic calorimeter (HCAL) or muon spectrometer (MS) (e.g., [11–13]), but the LLP mass still needs to be &5 GeV. The LHCb study of LLP decaying to jets [14] also performs a similar inclusive search, and the current bounds only apply when mS&10 GeV and track multiplicity &15. However, for the Higgs portal signature we focus on, when mS∼GeV, Sonly decays to few charged tracks with total momentum .60 GeV and decay lengths starting from 0.1 mm, it is extremely difficult to pick up the signal in the existing displaced jet searches. A second route to probe these scenarios, albeit in an indirect way, is to exploit the fact that the additional non-SM final state (“exotic Higgs decays”, see e.g [15] for a review) alter the SM Higgs rates. Hence precision measurements of the SM Higgs set constraints on its “invisible” branching fraction. The latest searches from ATLAS [16] and CMS [17] exclude at the 95% C.L a h→SS branching fraction of 26% and 19%, respectively. The latter result only applies if the Higgs boson production is exactly as in the SM (for exotic Higgs production see e.g [18]). Instead of performing an inclusive search for displaced jets, we propose using an exclusive search of displaced hadrons at LHCb to identify the light Ssignal. Comparing to the ATLAS and CMS detectors, LHCb has a much better hadronic-id thanks to its Rich- Imaging Cherenkov detector (RICH). Moreover, the VELO detector provides a precise reconstruction of charged tracks from the LLP decay, which helps to reduce the combinatorial background from random tracks and also to obtain an accurate measurement of the LLP mass. These two key features allow us to identify the Ssignal based on the specific decay products that show up with different invariant mass and decay location than the SM background. The price to pay for these advantages is a lower instantaneous luminosity. We can therefore trade the requirement of having hard and high multiplicity final states with large rates by a hadronic LLP search with looser, tailored cuts that relate to various details of the final state hadrons.2 In order to demonstrate the power of this type of exclusive searches for displaced hadrons, we focus on the search for a GeV-scale mSthat decays into only two charged tracks S→K+K−, however our strategy applies to other hadronic final states, (e.g.: π+π−, D+D−). We show that the advantages of probing the light and long-lived hadronic decay signals make LHCb being both a discovery machine of a light mediator particle and the ideal setting to study the structure of the portal coupling. The outline of this paper is as follows. In the next section (section 2) we review the important features of the LHCb experiment and explain why the detector is good at looking for the displaced hadronic signal. We describe the parameters for the h→SS(K+K−) search in section 3, including the cuts we design to look for the signal and the Monte Carlo simulation of the signal and background, and estimate the constraints on BR(h→SS) for different Smasses and lifetimes in a potential future LHCb search. The branching 2Also see [19] for the discussion of displaced D-meson signals. – 2 – JHEP01(2020)115 ratio bounds can be applied to different Higgs portal scenarios. In section 4, we translate these branching ratio bounds to the constraint of parameters in two concrete Higgs portal scenarios: one with Scoupling exclusively to the Yukawa coupling of SM quarks, and another one with Scoupling to the SM sector through its mixing with the SM Higgs boson. We discuss in detail the interplay between the searches of S→KK and S→µµ processes that can both exist in the second scenario. Our conclusions are in section 5. 2 LHCb features LHCb [20] is a forward spectrometer situated at the Large Hadron Collider (LHC). Although LHCb was originally designed for flavor physics, its special features can be wellsuited for other physics cases, as we demonstrate in this paper. Concerning exotic decays of the Higgs boson, although the geometrical acceptance and integrated luminosity are significantly reduced compared to ATLAS and CMS, this can be fully compensated by other detector characteristics, allowing LHCb to access unique regions of the parameter space of several BSM physics models. An excellent example of how LHCb capabilities can complement those of other detectors is that of LLPs. LHCb advantages for LLPs include the ability to trigger on soft objects and excellent vertexing. Using Run 1 and 2 data, LHCb has performed searches looking for different types of decays of LLPs, such as muons [21], jets [14], or combinations of these [22]. LHCb is currently undergoing an upgrade that will allow the experiment to collect more statistics without having a hardware trigger [23]. This is specially useful for the hadronic displaced final states, for which the expected trigger efficiencies will be significantly higher than those achieved with the previous detector, used during Runs 1 and 2 of the LHC. The reason for this is that the new readout will make the full reconstruction of the event possible at every bunch crossing, allowing the measurement of the displacement and therefore reducing the transverse momentum thresholds. The upgraded version of LHCb is expected to collect 15 fb−1during Run 3 of the LHC [23], which we will use as baseline for our analysis. Beside this planned upgrade, it is currently under discussion a new run for LHCb [24] in the 2030s decade using a new detector and operating at a much higher instantaneous luminosity, collecting 300 fb−1of data. We will use this as a second benchmark for the prospects, assuming similar conditions to those of the current upgrade. Finally, as baseline for this study, we assume a pp collision energy of √s= 14 TeV for both benchmark integrated luminosities. As stressed in the introduction, one of the most important features of LHCb is its excellent hadronic particle identification (PID). Critically for this search, this provides excellent separation between pions and kaons, not available at other LHC experiments. The discrimination between pions and kaons at LHCb is lead by the RICH detectors [25]. These exploit the Cherenkov radiation emitted by charged particles when traversing a material through which they move faster than light: the emission angle of radiation allows to extract the velocity of these charged particles. Together with the independent measurement of their – 3 – JHEP01(2020)115 momentum, this provides the particles’ mass and therefore allows to recognize them. For the momentum range of interest in this analysis, a rejection of &90% for pions is expected for kaon efficiencies of &90% [26], while the rejection against other hadrons or leptons is expected to be higher. For the analysis, different sets of cuts are applied to the candidates to distinguish signal from background. The cuts, which we describe in the next section, are mainly based on the distance of closest approach (DOCA) between particles, impact parameter (IP) to the pp collision vertex, geometrical distance between vertices and transverse momentum (pT). The cuts applied are based upon the resolution expected at the LHCb upgrade on these quantities for O(GeV) tracks [27]. We also assume the K+K−invariant mass resolution, which is crucial for identifying the Scandidates, to be similar to that achieved at LHCb in Run 1 and 2 searches for various SM resonances decaying to the same final state. From the SM decays φ→KK [28,29] and D0→KK [30], one can estimate an invariant mass resolution of ≈7 MeV. For the background determination, when KK pairs can originate from different physical vertices, a simple pseudo-vertexing is performed based on the closest point to a pair of tracks in the 3D space. To increase the discrimination against the background, we define an isolation criteria for the search [31]. This consist in requiring the absence of charged tracks that have a DOCA with respect to the signal track above 0.1 mm. Only charged tracks associated to kaons, pions, electrons, muons or protons with 2 < η < 5, pT>250 MeV and IP>0.1 mm are considered in the computation of the isolation. Here are few additional remarks before we describe the analysis in the next section. First, although the performance of the upgraded LHCb detector is yet to be measured, it is expected to be the same or better than that of the current detector [32]. In this work we do not simulate the detector response for signals and backgrounds. We simply assume the reconstruction efficiencies of charged tracks to be close to 100% in the fiducial regions defined in the next section. Comparing to the fiducial regions and the simplified geometric cuts we use, a more careful analysis using a simplified description of the upgraded VELO detector geometry [27] can only give an extra inefficiency up to ≈25% for the slowest decays we study. We neglect this effect for simplicity. Other efficiencies and resolutions are obtained on a “softer” QCD environment, namely for a typical transverse momenta below or about a few GeV, while in our case the scalars will have a pTbounded by mh/2∼60 GeV. While it is conceivable that the performance of the detector could be degraded at higher pT, this would be compensated by the fact that our choice of selection cuts (DOCA, IP, etc.) are conservative [33,34]. Finally, concerning trigger, usually the largest source of inefficiency in this type of searches, the upgraded readout renders the efficiency close to 100% assumption plausible, as explained above. 3 Displaced Kaon search at LHCb We study the following exotic Higgs decay at LHCb h→SS, S →K+K−.(3.1) – 4 – JHEP01(2020)115 Here either one or two Sparticles decay at the LHCb VELO and produce displaced K+K− that enter the forward detectors. When simulating events in Pythia 8.1 [35], we force Sto always decay into K+K−and study the signal efficiency according to the analysis proposed in the next subsection. We focus on the scalar mass range 1 GeV ≤mS≤2 GeV, so that Sdominantly decays into KK when the Scoupling to quarks follows the flavor structure of Yukawa coupling [36]. This minimal flavor violation coupling is well motivated by the strong flavor constraints [37], and we will discuss two of the similar scenarios in the next section. Based on the same assumption, the decay S→KK +X, where Xrepresents other light SM hadrons, is also sub-dominant comparing to S→KK. We therefore ignore the complication of having extra hadrons from the Sdecay in our analysis that may worsen the mass reconstruction of S, or reduce the signal efficiency from the track isolation cut (for one of the searches we propose). Our goal is to estimate the bound on BR(h→SS) using the analysis described below. The bounds we set depend on the mass and the proper lifetime of the singlet scalar, (mS, cτS). 3.1 Search strategy The leading background in our search originates from the QCD production of b¯ bpairs [14, 21,22]. Bmeson decays can produce multiple charged kaons, and the invariant mass and location measurements of these kaons can fake the Ssignal. To determine the expected QCD b¯ bbackground yields in our analysis we generate the b¯ bsamples using Pythia 8.1 [35]. Given that the PID at LHCb is very efficient at rejecting other types of particles that could be potentially misidentified as kaons, we simply reconstruct pairs of true kaons that follow the selection cuts presented below. Due to the CKM structure of the SM, Bmeson decays generate fewer pions than kaons, and the subdominant pion background can be further suppressed by the aforementioned PID cuts. The argument also holds for B→Dwith further decay into kaons. Despite this all, a nonnegligible background from misidentified pions should still be present, but we expect it to be smaller than the real kaon background. Furthermore, a real experimental analysis will have handles to discriminate much better against the background, by simply optimizing further the selection cuts or by means of machine learning algorithms. Additional effects due to pile-up are also neglected. Finally, in order to obtain the relation between the b¯ b yield generated with Pythia and the integrated luminosity, we use the b¯ bcross sections as measured by LHCb at √s= 13 TeV and √s= 7 TeV [38], which are then extrapolated linearly with the center-of-mass energy to √s= 14 TeV. Another important background of our displaced signal search comes from hadronic interactions of particles with the detector material, which may fake a displaced vertex decay. LHCb has currently an excellent material map, produced using secondary hadronic interactions [39] that allows to keep this background under superb control. Furthermore, as mentioned below, we will conservatively veto the vertices in the region of VELO with transverse distance to the beam-line ρbetween 10–14 mm following [22], since this is the region where material interactions are more abundant. It should be remarked that this corresponds to a very simplified description of the LHCb VELO material, and excellent – 5 – JHEP01(2020)115 discrimination can be obtained by simply using the map described above. Therefore, we neglect material interactions in this analysis. Other backgrounds that were found to negligible compared to b¯ bQCD production include c¯c,t¯ t, vector boson single and double production, and h→b¯ b. Here we propose cuts for the displaced kaon search that can efficiently suppress the above backgrounds. To perform the analysis using higher quality tracks entering the forward detector, we consider charged tracks with pseudo-rapidity 2 < η < 5, transverse momentum pT>250 MeV, and which have IP to the primary vertex to satisfy IP >0.1 mm. We then identify charged kaons from these tracks and require the signal kaons to have pT>500 MeV to ensure a &90% reconstruction efficiency, as mentioned in section 2. Among all the charged kaon final states, we reconstruct the Scandidates requiring that each contains a K+K−pair, with a DOCA smaller than 1 mm and have the Smomentum pointing back to the primary vertex with IP<0.1 mm. To ensure decays reconstructible by the VELO, we require the Sdecay position to fulfil ρ < 25 mm and z < 400 mm. We also require the Sparticles to have pT>10 GeV.3Since we consider the mass range 1≤mS≤2 GeV that overlaps with several resonances of SM hadrons, we veto Scandidates within the following mass regions under different assumptions of charged track mass from the decay: •480 < mS<520 MeV with the ππ mass hypothesis to avoid the K0 S→ππ background. Although this decay product does not include kaons and therefore will mostly rejected by the PID cuts, we decided to still veto it due to the huge production cross sections of K0 S[40]. •990 < mS<1050 MeV with the KK mass hypothesis to avoid the φ→KK background. Although φmesons are not long-lived, they can be found very often displaced through Bor Dmeson decays. •1110 < mS<1120 MeV with the pπ or πp mass hypothesis to avoid the Λ0→pπ background. Same as with K0 S, following the large Λ0production cross sections [41]. •1850 < mS<1880 MeV with the KK mass hypothesis to avoid the D0→KK background. Depending on the decay location, number of Scandidates reconstructed and track isolation requirements, we perform the search in eight categories (signal regions), listed in table 1, and pick the best bound for each set of parameters. We always consider the cases with and without the isolation cut, to explicitly check its impact on the sensitivity. For categories c1,2and d1,2we require two reconstructed Scandidates with a DOCA <0.1 mm. For a given mass hypothesis mSwe reconstruct both Scandidates and require a mass difference ∆S=|mS1−mS2|<100 MeV. Since the h→SS decay is prompt, we also request the resulting SS vertex to be less than 1 mm away from the primary vertex, and the 3We note that, as mentioned, each individual kaon was selected with a loose pTcut of 500 MeV, yet the kaon pairs are required to satisfy a tighter cut. While the kaon minimum pTcut could be used to optimize the sensitivity, such a detailed study would be beyond the “proof-of-concept” scope of this work. – 6 – JHEP01(2020)115 Signal Region ρrange (mm) Isolation Number of Sbg @ 15 fb−1mS= [1,2] GeV a16<ρ<10 no 1 7.85 ×106 a26<ρ<10 yes 1 2.62 ×105 b114 <ρ<25 no 1 2.01 ×105 b214 <ρ<25 yes 1 3.43 ×103 c1both 6 <ρ<10 no 2 16.8 c2both 6 <ρ<10 yes 2 0.67 d1both 14 <ρ<25 no 2 <10−4 d2both 14 <ρ<25 yes 2 <10−6 Table 1. Description of the different signal regions in terms of the tracker geometry. See main text for details. total invariant mass of the Spairs to fulfill mSS >100 GeV. These cuts are fully efficient for the signal and greatly reduce the bb background. The simple cut mSS >100 GeV that we apply essentially reduces the expected background below 1 event, so that a tighter mass window around the Higgs mass will not affect the expected sensitivity. Finally, we stress that if the new scalars Swhere to come from the decay of a new scalar φwith a mass smaller than 100 GeV, one can always relax the cut at the expense of accepting a few more background events. Along this work we ignore systematic uncertainties on the background or efficiencies, thus estimating our significance as simply Nsig/pNsig +Nbg. The expected systematic uncertainties, following other similar experimental searches [14,21,22], should not significantly distort our estimated sensitivity. Figure 1shows the differential cross section of the expected b¯ bbackground as a function of the KK invariant mass after the cuts applied for two of the signal regions. Enforcing the invariant mass vetoes described above implies the removal of events distributed throughout the whole mKK ∈[1–2] GeV range, which explains the “wiggles” seen in the mKK shape. In the last column of table 1, we show the estimated number of background events in the 1–2 GeV invariant mass window, assuming 15 fb−1of data from this analysis. When calculating the significance of the signal excess, we use a bin size of 50 MeV for the mK+K− distribution. As mentioned in the previous section, the mass resolution of the similar kaon searches at LHCb is about 7 MeV. Hence the invariant mass window we use gives a conservative estimate of the signal significance by including in the same bin background events in the ±3σrange around the central value. The signal production cross-section times efficiency of the cuts can be found in figure 2for some of the categories in table 1 as a function of the scalar lifetime and mass, assuming a h→SS branching fraction of 19% (corresponding to the best current limit on exotic Higgs decays reported by CMS [17]) and a S→KK branching fraction of 100%. The high-luminosity LHC (HL-LHC) with √s= 14 TeV and 3000 fb−1of data is expected to probe an exotic Higgs branching fraction of 2.5% [42]. Our cuts give the best signal efficiencies ∼1% when cτS∼0.1–1 mm, as shown in figure 2. – 7 – JHEP01(2020)115 1.0 1.5 2.0 mK+K−[GeV] 0 10 20 30 40 dσ dmK+K−[pb/GeV] 6<SV <10 mm 1.0 1.5 2.0 mK+K−[GeV] 0.0 0.2 0.4 0.6 14 <SV <25 mm Figure 1. Differential cross section for the b¯ bbackground production after the selection cuts vs. the KK invariant mass. Two of the signal regions, including the selection of a single Sscalar with isolation cuts applied are shown. Left (right) correspond to the a2(b2) selections in table 1. We assume the SM Higgs production cross section at √s= 14 TeV. The wiggles are mainly due to the mass vetos applied. 10−510−310−1101 cτS[m] 100 101 102 103 σS×[fb] mS= 1.2 GeV 1.0 1.5 2.0 mS[GeV] 100 101 102 103cτS= 9 ×10−4m 1S & 6 < SV < 10 mm 1S & 14 < SV < 25 mm 2S & 6 < SV < 10 mm 2S & 14 < SV < 25 mm Figure 2. Signal production cross section times reconstruction efficiency vs. lifetime (left) and mass (right) for a scalar with a mass of 1.2 GeV (left) or lifetime of 0.9 mm (right), assuming a h→SS branching fraction of 19% and a S→KK branching fraction of 100%. The different lines correspond to the reconstruction of 1 or 2 Sscalars with isolation cuts applied, in the same event and with different requirements on their SV decay position (corresponding to categories a2,b2,c2 and d2in table 1). 3.2 Branching ratio constraint The results providing the BR(h→SS) sensitivity are shown in figure 3and figure 4for the 95% CL. 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