Searches for stable strangelets in ordinary matter: overview and a recent example

Abstract

Our knowledge on the possible existence in nature of stable exotic particles depends solely upon experimental observation. Guided by this general principle and motivated by theoretical hypotheses on the existence of stable particles of strange quark matter, a variety of experimental searches have been performed. We provide an introduction to the theoretical hypotheses, an overview of the past searches, and a more detailed description of a recent search for helium-like strangelets in the Earth's atmosphere using a sensitive laser spectroscopy method. Is it possible that ordinary nuclear matter that we are familiar with is actually metastable and slowly decaying into quark matter of lower energy? Many hypotheses have been proposed addressing this question. Bodmer first proposed a lower-energy nuclear state, which he called "collapsed nuclei" [1]. Witten investigated the stability of strange quark matter, which consists of approximately equal number of u, d and s quarks in a hadronic state, and reached the intriguing conclusion that it may indeed be stable even at zero external pressure [2]. A particle of strange quark matter is called a "strangelet". Searches for such exotic matter have been performed in astronomical observation, in cosmic rays, in heavy-ion collisions, and in chemical analyses of ordinary matter. Several excellent review articles on this subject are available [3][4][5]. Here we focus on one aspect of the subject: the experimental searches for stable strangelets in ordinary matter. In Section 1, we provide a brief pedagogical introduction to strange quark matter and strangelets; in Section 2, we review the past searches; in Section 3, we present our recent search for helium-like strangelets in the Earth's atmosphere.

Millicharged atomic dark matter

“…We evade the first one because e binds much more strongly (400 eV) to oxygen than to deuterium (3 eV) for ǫ = 0.01, making D[De]O highly unstable to decay into D 2 [Oe]. The second is ameliorated by realizing that He has a lifetime of τ = 10 6 y in the atmosphere [28], reducing the estimated abundance by a factor of τ /(10 Gyr) to 10 −11 . This does not yet take into account the magnetosphere which very effectively shields the earth from slow charged particles, including 3 GeV dark ions with ǫ ∼ 10 −2 , whose gyroradius at the top of the atmosphere is ∼ 0.01 earth radii.…”

Quark stars: features and findings

“…Strangelet searches in terrestrial materials, cosmic rays or as by-products of neutron star-neutron star collisions have thus far yielded negative results [12,13]. Even if strange quark matter is stable in bulk, it may be destabilized by prohibitive surface and Coulomb energy costs so that strangelets do not survive until the present day although they may have existed in the hot and dense epoch of the early universe.…”

Pulsar scintillation patterns and strangelets

“…Our model invokes strangelets [12] (also known as nuclearites), finite droplets of quark matter with non-zero strangeness fraction and slightly charged. They are currently being searched on earth [13] as final products in heavy ion collisions, with the ALICE experiment at the LHC, in the CDMSII, under the form of light ionizing particles, or in the AMS-02 mission. It is expected that these quark droplets can be naturally generated by a series of different astrophysical events where a nucleon-quark deconfinement transition may take place, e.g.…”