HPS@JLab

The Heavy Photon Search (HPS) is a fixed target experiment planned to run intermittently in 2014-2015 at the Jefferson Laboratory (JLab), based in Virginia (USA). We will be using the high intensity electron accelerator facility available there to search for a new vector boson, called "heavy photon" in the mass range from 20 to 1000 MeV. The search for such a boson is motivated by the introduction of a new interaction, mediated by such gauge boson, which could help to solve several important puzzles of contemporary physics. For instance are concerned the (g-2)µ anomaly, the muonic hydrogen Lamb shift in atomic physics and the excess of electrons and positrons observed in cosmic ray data. Moreover, by coupling to dark matter, this force could also explain the discrepancies observed between the various direct dark matter searches (DAMA/LIBRA and CDMS for instance). This project seeks to produce the heavy photons by Bremsstrahlung-like emission of 2.2 and 6.6 GeV electrons sent on a high Z target. The heavy photon would decay into charged lepton pairs (mainly e+-e-) and give a twofold signature: a sharp peak in the invariant mass spectra of the e+-e- system above the QED (Quantum Electro-Dynamics) background and a secondary decay vertex distinct from the interaction point; the latter being possible when the coupling is weak enough to generate detectable life time effects. By removing most of the QED background, this latter method provides unparalleled sensitivity in the mass range 20-250 MeV making HPS unique among other experiments searching for a heavy photon. The HPS is also the occasion to discover true muonium (µ+-µ-) a predicted bound QED state that was never observed. The very compact structure of the true muonium makes this measurement an important precision test for QED calculations involving muons. The project is led in IPN Orsay by a group of young scientists, fitting particularly well with the objectives of the JCJC program. Indeed, with this project R. Dupré will take first responsibilities in a new and original field for the laboratory with the support of slightly more experienced colleagues. The HPS project also permits important interdisciplinary collaborations and raises multidisciplinary interests (atomic, nuclear, particle and astroparticle physics communities). Negative result for the search would also give important insight in these domains by reducing drastically the available phase space for the existence of a new gauge boson. Therefore, it will be an important constraint for the models including new forces in order to solve the physics problems highlighted previously. The HPS experiment has been approved by the JLab PAC (Program Advisory Committee) in 2012 with the highest rating, "A". In the very positive report the committee stated that "This experiment has the potential to make a revolutionary discovery if carried out in a timely manner". The IPN Orsay seeks the financial help of the ANR to take responsibility in the collaboration with a contribution to the electromagnetic calorimeter construction, an essential piece of equipment of the experiment. We request in this grant 257 kEuros to finance equipment on the calorimeter (for front end electronics and a monitoring system), a 2-year postdoc (for the development, construction and installation of the monitoring system) and travel money needed for this project. Although the experiment takes place in the US, because JLab is the only facility in the world offering the appropriate beam characteristics, the funding will be invested in France. The construction and assembly of the equipment as well as our part of the data analysis will be carried out in IPN Orsay.