1. Name, location and approximate number of collaborators of the experiment

Beam Dump eXperiment (BDX), Jefferson Laboratory. The BDX collaboration is formed by 104 members, from 33 collaborating institutions

2. Primary physics goals

The goal of the BDX experiment is to search for light dark matter particles, in the mass range $1 \div 100$ MeV. BDX is an $e^-$ beam, thick-target experiment, expected to run at Jefferson Laboratory. With an electron current up to 65 $\mu$A, and 285 PAC days, BDX will collect $10^{22}$ EOT, ruling out a significant portion of the LDM parameters space.

3. Summary of current and expected physics results, if possible a plot(s) of the expected sensitivity

The expected sensitivity of the BDX experiment has been evaluated for different LDM models: leptophilic LDM, leptophilic inelastic LDM, fermionic thermal relic LDM. For all cases, BDX will be able to explore new territories in the parameters space.


4. Summary of the experimental approach and setup

BDX will detect dark matter particles ($\chi$) produced by the primary $e^-$ beam impinging on the JLab Hall-A beam dump, by measuring their scattering on a detector placed
20 m downstream. The BDX detector will measure $\chi$ scattering on atomic electrons in the detector volume, resulting in a GeV EM shower.
The BDX detector is a segmented homogeneous calorimeter, made by CsI(Tl) crystals formerly used in the end cap BaBar Ecal with an improved SIPM-based readout. The calorimeter is made by 800 crystals, each 30-cm long, for a total active volume of 0.5 $m^3$. Crystals are arranged in a sequence of modules, made by a matrix of 10x10 crystals.
In order to identify and reject cosmogenic and beam-related backgrounds, the calorimeter is surrounded by two active-vetos, made by plastic scintillator paddles read by PMTs and SiPMs. Between the inner and a the outer veto, a 5-cm thick lead vault shields the calorimeter from photons, forcing them to shower and allowing their detection in the inner veto. The BDX experiment will operate at the beam height of the Hall A dump, which is 8 m below grade level, in a new dedicated experimental underground hall with 7 m of overburden. Finally, to shield the detector from any beam-related background, 10 m of concrete and iron will be placed between the beam dump and the new Hall.

5. Status of the experiment (proposal / approved experiment / under construction / taking data /...)

The BDX experiment has been submitted to JLab Program Advisory Commitee 44 (July 2016), and conditionally approved to the results of the beam-on background characterization and detector optimization. To respond to the PAC concern, an on-site campaign of measurements  has been planned for Fall 2017 / Spring 2018. The test will use the current JLab Hall-A beam/dump set-up  - not including new shielding to be installed for the experiment -  to assess the muon background and validate  MonteCarlo simulations. The BDX Collaboration plans to give an update on the status of the proposal to the JLab PAC 45 in July 2017, presenting the plans for on-site measurements and for detector optimization.


6. If applicable, estimated timescale and resources for R&D and construction

R&D activity on the BDX detector started in 2014, with dedicated laboratory measurements on the individual components, later confirmed and extended through the construction and characterization of a small-scale prototype. The prototype response to cosmic rays has been measured, to evaluate the foreseen cosmogenic background in the BDX experiment in a similar overburden configuration. At the moment, the technology employed in the BDX detector has been fully defined, and design of the BDX detector is almost completed: the active volume is made by CsI(Tl) crystals read by SiPM, while the active veto is made by plastic scintillator counters read by SiPM or PMTs. The R&D activity now is mainly focused on detector optimization - in order to enhance the signal efficiency and the background rejection power.

The time and cost estimates for the experiment construction are as follows. The construction of the full BDX detector - refurbishing existing BaBar calorimeter CsI(Tl) crystals with improved SiPM readout, procurement of FE and RO electronics, assembly of the calorimeter modules, and construction of the active veto system would require 1 year and would cost approximately 1 M$. The construction of the civil infrastructure at JLab - the new experimental Hall downstream the existing Hall-A at JLab and the installation of iron shielding between the dump and the hall - would require 2 years and is estimated to cost about 1.5M$. Costs have been estimated with the input of JLab Facilities Management.