Looking forward to test the KOTO anomaly with FASER Felix Kling∗ SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, CA 94025, USA Sebastian Trojanowskiy Consortium for Fundamental Physics, School of Mathematics and Statistics, University of Sheffield, Hounsfield Road, Sheffield, S3 7RH, UK and National Centre for Nuclear Research, Pasteura 7, 02-093 Warsaw, Poland The search for light and long-lived particles at the LHC will be intensified in the upcoming years with a prominent role of the new FASER experiment. In this study, we discuss how FASER could independently probe such scenarios relevant for new physics searches at kaon factories. We put an emphasis on the proposed explanations for the recently observed three anomalous events in the KOTO experiment. The baseline of FASER precisely corresponds to the proposed lifetime solution to the anomaly that avoids the NA62 bounds on charged kaons. As a result, the experiment can start constraining relevant models within the first few weeks of its operation. In some cases, it can confirm a possible discovery with up to 104 spectacular high-energy events in FASER during LHC Run 3. Further complementarities between FASER and kaon factories, which employ FASER capability to study γγ signatures, are illustrated for the model with axion-like particles dominantly coupled to SU(2)W gauge bosons. I. INTRODUCTION or FASER [3{5]. The FASER experiment will search for LLPs abundantly produced in the far-forward direction The quest for beyond the Standard Model (BSM) of the Large Hadron Collider (LHC) during its upcom- physics is undoubtedly among the biggest challenges in ing Run 3. Several features of its setup make it an ideal contemporary physics. It has typically been driven by probe for light LLPs produced in kaon decays: i) Given the lack of answers to some fundamental questions, such a large expected boost factor of light particles traveling as understanding the unification of the forces of nature towards FASER, and the location of the detector, the or the mechanism of baryogenesis, as well as by hints of experiment's sensitivity is optimal for the precise combi- new physics appearing in anomalous experimental obser- nation of the lifetime and mass that are required to fit vations. The latter become especially intriguing when the anomaly. In particular, for high-energy LLPs with the known Standard Model (SM) background (BG) can E ∼ 1 TeV that are produced at the ATLAS IP, the be reduced to negligible levels so that even the observa- sensitivity reach of FASER is maximized for tion of a few events can be attributed to BSM effects. The flavor-changing rare decays of kaons are excep- τLLP 0:1 ns ∼ ; (best FASER reach): (1) tionally promising examples of such discovery channels mLLP 100 MeV given their highly suppressed SM rates. In particular, the KOTO collaboration has recently reported a possible ii) The FASER sensitivity additionally benefits from the observation of three anomalous events in the search for O(150 m) long part of the LHC beam pipe, in which 0 KL ! π νν¯ [1], in which no SM signal was expected at forward-going neutral kaons can travel and decay before the current level of the experimental sensitivity. If taken being absorbed, as well as the presence of additional LLP as a signal, this observation would indicate that the rel- production modes at the LHC. iii) The FASER detector evant branching fraction exceeds the SM prediction by has the capability to not only detect LLP decays into about two orders of magnitude [2]. It has been quickly arXiv:2006.10630v2 [hep-ph] 2 Dec 2020 charged particles but also into photon pairs, due to its noticed that the discrepancy could be resolved by intro- dedicated pre-shower detector. As a result, during Run ducing a new light long-lived particle (LLP) with mass 3 of the LHC, FASER can turn the three currently ob- of order 100 MeV, light enough to be produced in kaon served anomalous events at KOTO into up to O(104) decays. By properly adjusting the lifetime of the LLP, spectacular high-energy LLP decay events with expected one can explain the neutral kaon anomaly while avoid- negligible BG. ing stringent bounds on such models from searches for The rest of this paper is organized as follows. In Sec.II charged kaon decays. and Sec. III we provide more details about the KOTO In this study, we illustrate how such proposed solu- anomaly and the FASER experiment, respectively. The tions to the KOTO anomaly could be thoroughly probed FASER sensitivity reach in several selected BSM scenar- by the recently approved ForwArd Search ExpeRiment, ios is presented in Sec.IV. In Sec.V, we further illustrate the possible interplay between FASER and searches for LLPs in kaon factories. We conclude in Sec.VI. Sev- ∗ [email protected] eral useful expressions for BSM meson decay branching y s.trojanowski@sheffield.ac.uk fractions are listed in AppendixA. 2 II. KOTO EXPERIMENT AND THE with the corresponding branching fraction measured to ANOMALY be at the 68% CL [13], B (K+ ! π+νν¯) = 0:47+0:72 × 10−10; (6) The KOTO experiment [6] has been designed to study NA62 −0:47 decays of kaons into neutral pions and a neutrino/anti- and the upper bound at the 95% CL given by neutrino pair, K ! π0νν¯. The kaons are produced + + −10 L BNA62,bound(K ! π νν¯) < 2:44 × 10 . The mea- in collisions of 30 GeV protons from the Japan Proton surements of the charged kaon decay branching fraction, Accelerator Research Complex [7] (J-PARC) main ring together with the GN bound Eq. (5), lead to (2 − 3)σ accelerator with the target made out of gold. A high- tension with the anomalous observation by the KOTO intensity beam of neutral kaons produced at an angle of collaboration, cf. Eq. (4), depending on how the possible ◦ 16 with respect to the proton beam line is created with impact of new physics is taken into account [2]. A num- the use of dedicated collimators. At a distance of about ber of BSM scenarios have been proposed to explain this 24 m away from the target, the ∼3 m long vacuum cham- discrepancy [2, 14{31]. ber of the KOTO detector begins. Here, kaon decays are Among these scenarios, a prominent role is played by identified by detecting photons produced in prompt neu- models predicting the existence of a new LLP with the tral pion decays. mass mX ∼ 100 MeV, which can be produced in rare The data collected by the KOTO collaboration in 2015 0 kaon decays, KL ! π X [2]. In particular, if such a light allowed them to obtain the leading bound on the branch- BSM particle has a lifetime of order τX ∼ 0:1 ns, it can ing fraction of the aforementioned decay process. At the be effectively stable and invisible in the search performed 90% confidence level (CL) it reads [8] by the KOTO collaboration. At the same time, X will 0 −9 typically decay inside the E949 and NA62 detectors, and BKOTO,bound(KL ! π νν¯) ≤ 3 × 10 : (2) it does not contribute to the measured branching fraction + + of KL ! π νν¯. This leads to an apparent violation of Notably, Eq. (2) is consistent with the SM prediction the GN bound when the results of these experiments are which remains about two orders of magnitude lower, 0 −11 compared with each other, which is further referred to as BSM(KL ! π νν¯) = (3:4 ± 0:6) × 10 [9]. A simi- 0 0 the lifetime solution to the anomaly. lar bound on the two-body decay K ! π X has also A very long-lived X can also explain the anomaly while been obtained, where X is a stable or long-lived BSM avoiding the bounds from past beam-dump searches. In bosonic particle, addition, if its mass is close to the pion mass, it can es- cape detection in E949 and NA62 experiments by hiding B (K ! π0 X) 2:4 × 10−9: (3) KOTO,bound L . in the SM BG [32]. Another distinct possibility is to allow In the subsequent analysis of the data from 2016-18 [1], KL decays into neutral dark states, KL ! X1 X2. Sub- however, a total of four candidate events were found, only sequent Xi decays that produce photons can be detected one of which had the properties of BG. If taken as a in the KOTO electromagnetic calorimeter and resemble signal, the remaining three anomalous events point to a neutral pion signature [26]. In this case, charged kaon the branching fraction which significantly exceeds the SM three-body decays can be suppressed or even kinemati- prediction. At the 95% CL it is given by [2] cally forbidden. Below, we analyze the prospect of probing such se- 0 +4:1 −9 lected BSM scenarios in the FASER experiment at the BKOTO,anom.(KL ! π νν¯) = (2:1 ) × 10 : (4) −1:7 LHC. Importantly, in the SM the aforementioned neutral kaon decay mode is also related by the isospin symme- try to the value of the branching fraction of a charged III. FASER AND FASER 2 EXPERIMENTS kaon decay into a pion π+ and a neutrino/anti-neutrino pair, which proceeds via the same parton level process The FASER experiment [3{5] has been proposed to s ! dνν¯. The relevant so-called Grossman-Nir (GN) search for LLPs [33, 34] produced in the forward region bound [10] reads of the LHC [35{41], as well as to study interactions of high-energy neutrinos [42{44]. It utilizes the fact that 0 + + B(KL ! π νν¯) < 4:3 B(K ! π νν¯): (5) light and high-energy particles produced in pp collisions at the LHC and e.g.