1. 120 GeV Proton beam FMAG: Focusing Magnet STATION 1: Wire chamber Hodoscopes STATION 4 Prop tubes Hodoscopes KMAG: Tracking magnet STATION 2: Wire chamber Hodoscopes STATION 3 (+ & -): Wire chambers Hodoscopes Iron dump SEAQUEST SPECTROMETER 25 m Targets -Liquid H2 & D2 -C, Fe, W This work is supported in part by the U.S. Department of Energy, Office of Nuclear Physics. My work is supported by the National Science Foundation. ACKNOWLEDGEMENTS DARK PHOTONS AT EXPERIMENT Arun Tadepalli 1, Roy Holt 2, Ron Gilman 1, Evan McClellan 3 (for the SeaQuest E906 Collaboration) Rutgers, The State University of New Jersey 1 Argonne National Laboratory 2 University of Illinois, Urbana-Champaign 3 SeaQuest is a fixed target Drell-Yan experiment that uses the 120-GeV proton beam extracted from the Main Injector at Fermilab. The experiment measures cross section ratios of the reactions of p-p, p-d and p-A collisions to study flavor asymmetry in the nucleon sea. The aerial view to the right shows the path of the 120 GeV beam as a yellow line. THE SEAQUEST EXPERIMENT Fixed Target Beam lines Tevatron 800 GeV Main Injector 120 GeV WHAT’S THE MATTER? DARK SECTOR AND STANDARD MODEL COUPLING DARK PHOTON PRODUCTION MECHANISM SEAQUEST SENSITIVITY REGION SEAQUEST DARK PHOTON SEARCH STRATEGY In 1930, Fritz Zwicky carefully studied the luminosities and dynamics of individual galaxies in clusters. He noticed that galaxies rotating around each other needed to be far more massive than observed to maintain their paths in their respective orbits. He coined the term “dark matter” to explain the non luminous matter that enveloped these individual galaxies that could account for their properties. Today, there is overwhelming evidence from detailed studies of rotational curves of galaxies, gravitational lensing, colliding clusters of galaxies, and surveys of cosmic microwave background that dark matter not only exists, but also constitutes 27% of the energy density of the entire universe. Dark energy Dark matter Ordinary matter Indirect evidence for dark matter has been observed through its gravitational effects on baryonic matter in galaxies. However, the challenge undertaken by many experiments around the world is to find out if particles from the dark sector couple to those in ordinary matter. From a theoretical point of view, analogous to how the photons couple to the electromagnetic field (with a U(1) gauge symmetry), the dark sector could also interact with ordinary matter via a similar mechanism. Under such circumstances, a hidden gauge boson A’ (dark photon) would couple to ordinary matter with a small coupling constant ε. Careful measurements on the decay channels of a dark photon will provide an insight into the dark sector. Recently, satellite data from Fermi, AMS and PAMELA confirmed an excess of positrons in high energy cosmic rays that has been hypothesized to be from dark matter annihilation. The nature of dark matter remains a mystery and scientists are more determined than ever to gain a deeper understanding of dark matter. 26.8% 4.9% 68.3% An A’ can be produced in collisions of charged particles with nuclei and can decay into a lepton pair. SeaQuest looks at three such processes. The Feynman diagrams along with a brief description of these processes are shown below. The equation below shows the Lagrangian of the interaction between particles from the dark or hidden sector and the Standard model. Maxwell’s equations for Standard model particles Analogous counterpart in the hidden sector Coupling of the two sectors Mass term in the dark sector In a Drell-Yan process, a quark from one hadron annihilates with a sea anti-quark from another hadron producing a virtual photon. Instead, they could annihilate into a dark photon which then decays into a lepton pair. Many pseudoscalar mesons decay by emitting two photons. Instead of emitting two photons, they can decay into a photon and a dark photon which can then decay into a lepton pair. When protons traverse a nuclear medium, they decelerate due to interactions with the medium and as a consequence, emit electromagnetic radiation. This phenomenon is known as “Proton Bremsstrahlung”. Instead of emitting photons, the proton could emit a dark photon. This dark photon could decay into a lepton pair.. The figure above shows what a simulated dark photon event would look like in the SeaQuest event display software. The cylindrical object to the left shows the 120 GeV beam line, the target position is between the beam line and the origin, the origin of the coordinate axis is at the front face of FMAG, the lines in red are If A’ decays into Standard model particles, its decay length l o is given by where N eff is the number of available decay products, E o is the total energy of the incoming proton, ε is the coupling constant between the dark sector and the Standard model particles and m A’ is the mass of A’ in MeV. For the ranges of ε and m A’ covered by SeaQuest, N eff = 2, fiducial region is 1.95 (2.95) m, the decay length is assumed to be between 1 m (2 m) into the FMAG for η decay (Proton Bremsstrahlung). The plot below shows a preliminary contour of 100 events in 200 days of 2E12 protons per pulse beam intensity at 100% efficiency, in mass and coupling constant parameter space. It also shows the regions covered by several other experiments around the world. SeaQuest covers a very unique parameter space that is not probed by others.  Overwhelming evidence of dark matter has been observed through its gravitational interactions by many detailed studies around the world.  SeaQuest is a fixed target Drell-Yan experiment that uses the 120 GeV proton beam from the Main Injector at Fermilab.  SeaQuest takes advantage of the η decay, proton bremsstrahlung and Drell-Yan processes to search for dark photons.  The 5m long Iron beam dump significantly reduces the background compared to the enormous Bethe-Heitler background in other experiments.  A preliminary estimate has been made of the range of dark photon ε and mass parameters to which E906 is sensitive.  SeaQuest is currently taking data with a trigger that has some acceptance to dark photon decays.  Ongoing investigations aim to increase the sensitivity to dark photon decays. Prop tube hits Reconstructed dimuon tracks Coordinate axes St 1 wire chambers hits St 2 wire chambers hits Beam line St 3 wire chamber hits Hodoscope hits hits on wire chambers, the blue horizontal and vertical strips show hits on hodoscope paddles, the green horizontal and vertical strips are hits on prop tubes and the dotted lines in blue and green show the reconstructed tracks of the di- muons. The reconstructed vertex is displaced from the target position. SUMMARY AND OUTLOOK Since the SeaQuest spectrometer is optimized for detecting high rate dimuons, the lower limit on the mass range of A’ is 220 MeV. A’ dimuons traverse the whole spectrometer with a displaced vertex, unlike other Drell-Yan events arising from the target and upstream side of FMAG. The trigger road sets and track reconstruction software are modified accordingly to look for such events. After calculating the four momenta of the muons, an invariant mass spectrum is constructed for all such events and scrutinized for a peak in 220 to 700 MeV mass region. To carry out this search, we assume that the 120 GeV proton beam interacts with various nuclear targets and produces a dark photon. This dark photon traverses the FMAG without being affected by the magnetic field and decays in the last meter.