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SLAC-PUB-15727
The Heavy Photon Search Experiment
Per Hansson Adrian1
'SLAC National Accelerator Laboratory, Menlo Park, CA, USA
DOI: will be assigned
Interest in new physics models including so-called hidden sectors has increased in recent
years as a result of anomalies from astrophysical observations. The Heavy Photon Search
(HPS) experiment proposed at Jefferson Lab will look for a mediator of a new force, a GeV-
scale massive U(1) vector boson, the Heavy Photon, which acquires a weak coupling to
electrically charged matter through kinetic mixing. The HPS detector, a large acceptance
forward spectrometer based on a dipole magnet, consists of a silicon tracker-vertexer, a
lead-tungstate electromagnetic calorimeter, and a muon detector. HPS will search for the
e+e-- or p+ decay of the Heavy Photon produced in the interaction of high energy
electrons with a high Z target, possibly with a displaced decay vertex. In this article, the
description of the detector and its sensitivity are presented.
1 The physics of the Heavy Photon Search experiment
The nature of dark matter is one of the most important questions in particle physics. Re-
cently, an excess in the cosmic ray electron and positron spectra reported by the PAMELA
experiment [1] has been confirmed and extended by other experiments [2]. One interesting
possibility [3] is that the signal can be explained by the existence of a new force, mediated by a
massive, sub-GeV scale, U(1) gauge boson (the Heavy Photon or A') that couples very weakly
to ordinary matter through "kinetic mixing" [4]. TeV-scale dark matter could annihilate via
an A' boson which decay pre-dominantly into an e-- pair. This explanation is in accord with
the dark matter relic abundance, the relatively large cross-section and the lack of excess in
the baryon spectra for vector boson masses < 2mp. This weak coupling to the electric charge
could be the only non-gravitational window into the existence of the hidden sector consisting
of particles that do not couple to any of the known forces. Such hidden sectors are common in
many new physics scenarios, see Ref. [5] for a recent review.
Despite many existing constraints [6], there is a surprisingly large allowed parameter space
to be examined by planned and proposed experiments. In the simplest scenarios, there are
two main parameters that determine the characteristics of the A' and thus the experimental
search strategies: the kinetic mixing parameter E ~ 10-12 -10-2 and the mass of the A'. While
a huge range of mixing parameters and masses are possible, it is natural that E be around
10-3, and necessary that masses be around GeV if the positron excess is to be explained [5].
Experimentally, a very important aspect is that in large parts of this parameter space the A' can
be long-lived with a lifetime proportional to ~ 22 , and proper decay lengths varying from
prompt to hundreds of meters [7]. The Heavy Photon Search experiment (HPS) is a proposed
fixed-target experiment [8] specifically designed to discover an A' with mA' 10 - 1000 MeV,
Published in arXiv:1301.1103.
Work supported in part by US Department of Energy under contract DE-ACO2-76SF00515.
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Hansson Adrian, Per. The Heavy Photon Search Experiment, article, September 4, 2013; United States. (https://digital.library.unt.edu/ark:/67531/metadc843963/m1/1/: accessed April 25, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.