Heavy Photon Search at the Jefferson Laboratory – HPS@JLab

The HPS experiment ultimately relies upon the precision measurement of two quantities: the invariant mass of the heavy photon decay products (an electron-positron paire) and the position of the decay vertex. By placing a tracking and vertexing detector immediately downstream of the target inside an analyzing magnet, the complete kinematic information required for heavy photon reconstruction can be obtained. The proximity to the target naturally maximizes the acceptance of a relatively compact detector and provides excellent momentum and vertexing resolution.

The spectrometer is composed of a silicon tracker and an electromagnetic calorimeter (ECal) made of Lead Tungsten cristals. The role of the ECal in the experiment is twofold, it is the trigger for the data acquisition and it should, at the same time, allow for good electron identification. Indeed, the trigger algorithm is based on energy and position measurements, and requires very short time coincidence in order to suppress orders of magnitude higher background. For a discovery experiment like HPS, the backgrounds need to be strongly suppressed but also well understood, since the trigger can be an important source of experimental bias. Uniformity and stability are achieved with the installation of a gain monitoring system. This system consist in the introduction of light in the front of the crystals with LEDs to test the detector response. It will therefore be sensitive to both transparency losses of the crystal and gain variations from the amplification system. Also, we use the minimum ionizing particles for calibration since their energy deposit is well known. However, in this case the signal produced is very small and we needed to improve the detection threshold of the ECal. The necessary level has been reached by using larger area avalanche photo-diodes (APD) and improving the preamplificators noise level.

The electromagnetic calorimeter of the HPS experiment has been built in the Institut de Physique Nucléaire d'Orsay (Univ. Paris-Sud and CNRS/IN2P3) during the Summer 2014, thanks to the funding of ANR. Based on the prototype calorimeter used during faisability tests in 2011, we focused on the developpment of the following items: new design for mother boards to connect the detector to electronics; large area avalanche photo-diodes (10x10 mm^2) to collect more of the light produced in the calorimeter; faster and lower noise preamplifiers and a live LED monitoring system.

During the fall of 2014, the calorimeter has been tested on beam using real experimental conditions. We have found that the calorimeter function within all our expectations, in term of time resolution as well as energy resolution. In particular, the detector response to the trigger system, which select the events of interest to be recorded, gave results very close to our Monte-Carlo simulations. This commissioning period of the calorimeter has been finished before the end of 2014 on time with the planned schedule.

In the winter of 2015, the silicon vertex tracker has been installed in Jefferson Lab by the Stanford team (SLAC) and has been commissioned during the spring 2015 run. The tracker complied with all requirements, in particular functioning only half a millimeter from the beam without incident. During this run some physics running has even been possible and is now under analysis. The full HPS detector is therefore now ready to take production data and waiting for the next available run period of the JLab.

On the detector technology side, the light monitoring system of the electromagnetic calorimeter has shown excellent first results. Developing such a stable system for short and long term stability studies at the percent level is unique. We want to further test the system and measure precisely its capability to evaluate the radiation damage on the crystals. We also want to test the capability of the system to help regenerate from these radiation damages.

On the physics side, we have good hope that we will be able to publish our first results based solely on the data taken during detector commissionning. We are also waiting for the next run period available at JLab in order to take new data which will allow us to search for a heavy photon on the full domain initially proposed.

Nothing published yet.

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.