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NA64 Status Report 2018

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NA64 Status Report 2018 Prepared for submission to SPSC NA64 Status Report 2018 The NA64 Collaboration1 Abstract: The experiment NA64 is aimed at a search for a sub-GeV vector mediator (called dark photon A0) of Dark Matter production at the CERN SPS. The main goal in 2017 was to probe a region of the A0 parameter space of the thermal dark matter model. Ongoing activities on the detector and data analysis are reviewed. The status and results from the NA64 runs in 2016 and 2017 are reported. First results on the search for the X ! e+e− decay of a 17 MeV X boson, which could explain the recently observed excess of e+e− pairs from the excited 8Be nucleus transitions, and A0 ! e+e− decays are also presented. Ongoing analysis on the decays of axion-like particles and plans for further searches beyond LS2 are also discussed. CERN-SPSC-2018-015 / SPSC-SR-231 29/05/2018 1http://webna64.cern.ch Contents Table of Contents2 1 Introduction3 2 NA64 status from the 2017 run4 2.1 The detector and ongoing activities4 2.2 Data sample from 2016-2017 runs7 3 Search for the A0 ! invisible decay8 3.1 Exact tree-level cross section calculations and signal simulation.9 3.2 Data analysis and selection criteria 11 3.3 Background 15 3.4 Results and calculation of limits 22 3.5 Constraints on light thermal Dark Matter 24 3.6 Preliminary results from the 2017 run 27 4 Search for a new 17 MeV gauge boson from the 8Be anomaly and dark photon decays A0 ! e+e− 28 4.1 The search method and the detector 29 4.2 Event selection and background evaluation 30 4.3 First results on the X ! e+e− and A0 ! e+e− decays 33 5 Plans for the 2018 and 2021 runs 35 5.1 Search for the A0 ! invisible decays 35 5.2 Search for the X ! e+e− and A0 ! e+e− decays 37 5.3 Search for the ALP decays a ! γγ and a ! e+e− 40 6 Detector upgrade and location at the H4 line 41 6.1 Detector upgrades for the 2021 run 41 6.2 Detector location at the H4 line 42 7 Summary 43 8 Publications 44 References 45 { 1 { SUMMARY The measurements performed in 2017 are a part of a broad NA64 program which address the most important issues currently accessible to the dark sector: a search for the A0 ! invisible decay of the dark photons, in particular in the A0 parameter space relevant for the possible existence of the sub-GeVThermal Dark Matter (TDM), and a search for the X ! e+e− decay of a new 17 MeV X- boson, which may explain the 8Be anomaly - an excess of e+e− events in the excited 8Be transitions. The upcoming run in 2018, combined with the 2016-17 NA64 runs, provides us with the opportunity to meet and perhaps exceed our original goals for both programs and set a solid base for a possible new physics program for post LS2. During the 2016 run, we collected 4:3 × 1010 electron on target (EOT). Five weeks of running time in 2017 allows us to increase the data sample up to ' 1011 EOT and improve sensitivity to the mixing strength by a factor 6. Our goals for the 2018 run are: (i) to increase the full statistics for the invisible A0 decay by a factor 3; (ii) to perform the search for a new light X boson with the detector configuration similar to the one used in 2017 run, but with the beam energy of 150 GeV. The NA64 collaboration also proposes to carry out further searches of the A0 ! invisible decay and other rare processes with the H4 electron beam after the LS2 in the year 2021 with the main goal to accumulate in total up to ' 5 × 1011 EOT in order to cover much of the remaining (; mA0 ) parameter space and to explore the sub-Gev TDM parameter regions not yet above those currently being constrained by the beam-dump experiments. { 2 { 1 Introduction Despite the intensive searches at the LHC and in non-accelerator experiments Dark Matter (DM) still is a great puzzle. Though stringent constraints obtained on DM coupling to standard model (SM) particles ruled out many DM models, little is known about the origin and dynamics of the dark sector itself. One difficulty so far is that DM can be probed only through its gravitational interaction. An exciting possibility is that in addition to gravity, a new force between the dark sector and visible matter transmitted by a new vector boson A0 (dark photon) might exist. Such A0 could have a mass mA0 . 1 GeV - associated with a spontaneously broken gauged U(1)D symmetry- and couple to the SM through kinetic mixing with the ordinary photon, 1 0µν − 2 FµνA , parametrized by the mixing strength 1. This has motivated a worldwide theoretical and experimental effort towards searches for dark forces and other portals between the visible and dark sectors, shifting the strategy from the high energy to the high intensity frontier, see Refs. [1,2] for a review. An additional motivation for existence of the A0 has been provided by hints on astrophysical signals of dark matter [3], which can be explained by a sub-GeV A0 with the coupling ' 10−3. Such small values of could naturally be obtained from loop effects of particles charged under both the dark and SM U(1) interactions with a typical 1-loop value = eg =16π2 [4], where g is the coupling constant of the D D p U(1)D gauge interactions. A dark photon mass in the sub-GeV range, mA0 ' MZ , can be generated in several physics scenarios. Thus, if U(1)D is embedded in a Grand Unifed Theory (GUT), the mixing can be generated by a one-(two-)loop interaction and naturally results in values of ' 10−3 − 10−1(10−5 − 10−3)[3{5]. If the A0 is the lightest state in the dark sector, then it would decay mainly vis- ibly, i.e., typically to SM leptons l = e; µ or hadrons, which could be used to detect it. Previous beam dump, fixed target, collider, and rare meson decay experiments have already put stringent constraints on the mass mA0 and of such dark photons excluding, in particular, the parameter region favored by the gµ − 2 anomaly [2]. However, in the presence of light dark states, in particular dark matter, with the 0 masses . mA0 , the A would predominantly decay invisibly into those particles pro- 0 vided that gD > e. Models introducing such invisible A offer new intriguing possi- bilities to explain the gµ − 2 and various other anomalies and are subject to different experimental constraints. Compared to the visible decay mode, the invisible decay is constrained in significantly smaller (; mA0 ) parameter space living large area still unexplored. The NA64 was designed as a hermetic general purpose detector to search for dark sector physics in missing energy events from electron, hadron, and muon scattering off nuclei. Because of the higher energy of the incident beam, the centre-of-mass system is boosted relative to the laboratory system. Taking this boost into account results in enhanced hermeticity of the detector providing a nearly full solid angle { 3 { coverage. This NA64 report to the SPSC focuses mostly on the status of the analysis of data accumulated during the runs in 2016 and 2017. The results from these sets of measurements were taken into account also for the detector preparation for the run 2018 which is currently underway as planned. The detector has been completely commissioned. For the success of NA64 the precise identification of the initial state, i.e. the knowledge of the incoming particle ID and its momentum is crucial. The important milestone in development of such tagging system in 2017 was the production and run- ning of the improved Synchrotron Radiation Detector(SRD) consisting of three fully functioning modules. Another important step was the deployment of the completely commissioned Micromegas and GEM chamber tracker. In 2017 data taking was per- formed at high intensities. The results from the run data analysis are as expected confirming that the NA64 is be able to run at high intensities up to ' 5 − 8 × 106 e− / spill. We should underline that at this stage of the experiment in order to probe the previously discussed (, mA0 ) parameter space it will be necessary to accumulate 12 & 5 × 10 EOT with good data quality and efficiency. This requires taking data at the ultimate sustainable rate of ' 107 e− / spill. The document is organized as follows. Sec.2 provides an overview of the NA64 method of search, the summary of the data collected in 2016-17, and the detector performance. Sec.3 presents the status of the A0 ! invisible decay analysis. In Sec.4 the first results on the search for the X ! e+e− decay of a new 17 MeV X- boson, which could explain the 8Be anomaly, as well as on the A0 ! e+e− decays of dark photons are reported. In Sec.5 the continuation of the ongoing program is described. It includes plans for 2018 and beyond the LS2. Detector upgrade and request for its permanent location at the H4 line are presented in Sec.6, while conclusion and finally the status of the NA64 publications are presented in Sect.7,8 respectively. 2 NA64 status from the 2017 run The NA64 experiment first ran five weeks with the H4 electron beam from August 30 to October 2 2017.
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