Searches for Fractionally Charged Particles: What Should Be Done Next?

Abstract

Since the initial measurements of the electron charge a century ago, experimenters have faced the persistent question as to whether elementary particles exist that have charges that are fractional multiples of the electron charge. I concisely review the results of the last 50 years of searching for fractional charge particles with no confirmed positive results. I discuss the question of whether more searching is worthwhile? THE PUZZLE OF UNIT ELECTRIC CHARGEWe have no explanation why the electric charges of all the known elementary particles are either zero or q or ± 1 3 q or ± 2 3 q or ±q where q is the magnitude of the electrons charge, 1.602 × 10 −19 coulombs. We call q the unit electric charge.There are no confirmed observations of elementary or composite particles with charge Q = rq where r is a fraction such as 2 7 or an irrational or transcendental number. We call these hypothetical particles fractional electric charge particles, even though the fraction Q q might be greater than 1; for example a particle with charge Q = πq. We use F to mean a fractional electric charge particle.My colleagues, Dinesh Loomba and Eric Lee, and I are publishing a detailed review of fractional charge searches [1]. Therefore, I give a few references in this paper and refer the reader to Ref. 1 for details and full references. This paper will be published in the Proceedings of the X INTERNATIONAL WORKSHOP ON TAU LEPTON PHYSICS held at the Budker Institute of Nuclear Physics, Novosibirsk, Russia, September 22 -25, 2008. QUARKS AND FREE QUARK SEARCHES About 1910 Robert Millikan and HarveyFletcher elucidated the magnitude of the electron charge q [2]. And by the early 1920s there was consensus that q was the smallest electric charge. This was not challenged until the 1960s when physicists adopted the view of quarks as real elementary particles. This view of quarks and the increasing use of particle accelerators led to many searches for particles with charge SEARCH METHODS AND RE-MARKS ON SEARCHESThere are five types of searches for fractional charge particles: searches using particle accelerators and fixed targets, searches using particle colliders, searches in cosmic rays, searches in bulk matter, and special search methods for particles with Q very close to 0 called millicharged or minicharged particles.We do not know how fractional charge particles interact with ordinary particles: is the interaction strong or electromagnetic or weak or a not yet discovered force?Since we do not know the F mass, m F , searches using accelerators, colliders, or cosmic rays are broader as the energy increases. On the other hand, the sensitivity of searches in bulk matter is independent of m F .

A Simple Model of Gauged Lepton and Baryon Charges

“…However, our proposal is different from the cases of the charged, next-to-lightest supersymmetric particles which can decay (Dimopoulos et al, 1997;Giudice and Rattazzi, 1999). If the stable scalar χ is produced at some collider or coming from cosmic rays, halo particles, or bulk matter (Perl, 2008;Perl et al, 2009;Ambrosio et al, 2000;Kim et al, 2007;Lee et al, 2002), it can easily escape from the detectors. Indeed, the general collider detectors cannot be used to find particles with an electric charge smaller than 1/3 due to the uncertainty in trace reconstruction (Perl, 2008;Perl et al, 2009).…”

“…The bulk matter and sea water searches (only with integer charges) (Smith and Bennett, 1979; and so forth, up to Chuzhoy and Kolb, 2009) have a similar sensitivity (Kim et al, 2007;Lee et al, 2002;Perl et al, 2000;. There are also searches for millicharge particles with charges smaller than 0.1, which are irrelevant (see, for a review, Perl, 2008). Note that the searches for the charged, next-to-lightest supersymmetric particles take the same status (Asaka et al, 2000;Heister et al, 2002;Aaltonen et al, 2009;Chen and Adams, 2010; LEP SUSY Working Group).…”

“…If the stable scalar χ is produced at some collider or coming from cosmic rays, halo particles, or bulk matter (Perl, 2008;Perl et al, 2009;Ambrosio et al, 2000;Kim et al, 2007;Lee et al, 2002), it can easily escape from the detectors. Indeed, the general collider detectors cannot be used to find particles with an electric charge smaller than 1/3 due to the uncertainty in trace reconstruction (Perl, 2008;Perl et al, 2009). The searches for its impinging on the earth due to the cosmic rays or halo particles have been sensitive only to the smallest charges of 1/5 (Perl et al, 2001;Ambrosio et al, 2000).…”