A proposal for a test of Weak Equivalence Principle with improved accuracy using a cryogenic differential accelerometer installed on a pendulum q V.A. Iafolla a,⇑ , E. Fiorenza a , C. Lefevre a , D.M. Lucchesi a , M. Lucente a , C. Magnafico a , S. Nozzoli a , R. Peron a , F. Santoli a , E.C. Lorenzini b , V. Milyukov c , I.I. Shapiro d , S. Glashow e a Istituto di Astrofisica e Planetologia Spaziali (IAPS-INAF), Via del Fosso del Cavaliere, 00133 Roma, Italy b Universita ` di Padova, Dipartimento di Ingegneria Industriale, Via Venezia 1, Padova, Italy c Sternberg Astronomical Institute, Moscow University, Universitetskij prospect 13, Moscow 119992, Russia d Harvard-Smithsonian Center for Astrophysics (CfA), Cambridge, MA, USA e Boston University, Boston, MA, USA Received 19 November 2014; received in revised form 12 November 2015; accepted 18 November 2015 Available online 2 December 2015 Abstract We present here the concept for a new experimental test of the Weak Equivalence Principle (WEP) carried out in the gravity field of the Sun. The WEP, stating the independence of the gravitational acceleration a body is subject to from its mass and composition, is at the basis of general relativity theory and more in general of metric theories of gravitation. It is therefore very important to test it to the pre- cision allowable by current technology. The experiment here proposed aims at measuring the relative acceleration of two test masses in free fall, searching for a possible violation of the WEP, which would show up as a non-zero acceleration signal. The core of the exper- iment is constituted by a differential accelerometer with zero baseline, whose central elements are two test masses of different materials. This differential accelerometer is placed on a pendulum, in such a way as to make the common center of mass coincident with the center of mass of the pendulum itself. Ensuring a very precise centering, such a system should provide a high degree of attenuation of the local seismic noise, which — together with an integration time of the order of tens of days — would allow a test of the WEP with an accuracy improved by at least an order of magnitude with respect to the best measurements achieved so far. One of the strengths of this proposal is the know-how acquired from a previous study and technology development (GReAT: General Relativity Accuracy Test) that involved a test of the WEP in the gravity field of the Earth, in free fall inside a co-moving capsule released from a stratospheric balloon. The concept of the experiment is introduced, with particular attention to the differential accelerometer and its accommodation on the pendulum. A preliminary estimate of the attainable precision is given, along with a critical analysis of the associated challenges. Ó 2015 COSPAR. Published by Elsevier Ltd. All rights reserved. Keywords: Weak Equivalence Principle; Differential accelerometer; GReAT; GReAT_G 1. Introduction The universality of free fall, postulated by G. Galilei and expressed in terms of equality between inertial and gravita- tional mass by I. Newton, is at the basis of A. Einstein’s general relativity, in terms of the local identity of the http://dx.doi.org/10.1016/j.asr.2015.11.013 0273-1177/Ó 2015 COSPAR. Published by Elsevier Ltd. All rights reserved. q The first idea of this experiment has been elaborated with the late Franco Fuligni. ⇑ Corresponding author. E-mail address: valerio.iafolla@iaps.inaf.it (V.A. Iafolla). www.elsevier.com/locate/asr Available online at www.sciencedirect.com ScienceDirect Advances in Space Research 57 (2016) 715–723