Martin L. Perl Stanford Linear Accelerator Center Kavli Institute for Particle Astrophysics and Cosmology Talk presente

1. Searches for Fractionally Charged Particles: What Should Be Done Next ? Martin L. Perl Stanford Linear Accelerator Center Kavli Institute for Particle Astrophysics and Cosmology Talk presented at TAU08 Workshop September, 2008

2. 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 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 while more searching can be done, is it worthwhile?

3. THE PUZZLE OF UNIT ELECTRIC CHARGE We have no explanation why the electric charges of all the known elementary particles are either zero or q or ± q/3 or ±2q/3 or ±q where q is the magnitude of the electron’s charge, 1.6 x 10-19coulombs. 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=pq. We use F to mean a fractional electric charge particle. ´´

4. Particle (units of q) Charge Charged leptons: e, m, t ±1 Neutrinos 0 Quarks: u, c, t ±2/3 Quark: d, s, b ±1/3 Photon 0 Z0 0 W ±1 Graviton ? 0? Dark matter particle ? 0?

5. A Bit of History • About 1910 Robert Millikan and Harvey Fletcher elucidated the magnitude of the electron charge q. 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 q/3 or 2q/3 q or higher fractions such as 4/3q.

6. Quarks and Free Quark Searches • Searches for fractional charge particles beginning in the 1960s emphasized searching for free quarks. • The incentive was the possibility that once in a while isolated quarks might break free in a high-energy interaction. This possibility has not been realized, and the absence of free quarks has become enshrined in the theory of quark confinement inside quantum chromodynamics • The acceptance of quark confinement has led searchers for fractional charge particles to look more broadly for fractional charge particles with any charge.

7. Search interest is in free = isolated elementary particles with fractional electric charge. .45 q Examples: -1.5 q 1/3 q 1.0001 q p q

8. Search Methods • Searches using particle accelerators and fixed targets. • Searches using particle colliders. • Searches in cosmic rays. • Searches in bulk matter. • Special search methods for particles with Q very close to 0. called millicharged or minicharged particles.

9. Remarks on Fractional Charge Particles searches • 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, mF, searches using accelerators, colliders, or cosmic rays are broader as the energy increases. On the other hand, sensitivity of searches in bulk matter are independent of mF.

10. More Remarks on Fractional Charge Particles searches • All past and present searches are limited to particles, that are or can be isolated at the elementary particle level from their antiparticles or other related particles. • General collider detectors cannot be used to find particles with Q/q <1/3 because track reconstruction is uncertain.

11. SEARCHES USING PARTICLE ACCELERATORS AND FIXED TARGETS proton or antiproton + nucleon ® F+Q + X e + nucleon ® F+Q + X m + nucleon ® F+Q + X n + nucleon ® F+Q + X where X = F-Q + other known particles All searches null.

12. Nucleus-Nucleus Collisions Possibility that fractional charge particles could be produced in high-energy nucleus-nucleus collisions where quark confinement might not hold perfectly. All searches null. No definitive data from RHIC.

13. Search in Electron-Positron Colliders • e+ + e- ® F+Q + F-Q • = (2pa2) b(3 - b2)/3s. • e+ + e-® Z0 ®F+Q + F-Q • (From Opal & ALEPH) • Etotal (GeV) Charges sought (q units) • 130—209 2/3, 4/3, 5/3 • 130--136, 161, 172 2/3 • Z0 2/3, 4/3 • Z0 4/3 • All searches null.

14. Searches in Proton-Antiproton Colliders (From D0 & CDF) Etotal (TeV) Charges sought (q units) 1.8 2/3, 4/3 1.8 & 2/3 All searches null. Searches in Proton-Proton Colliders Wait for L H C

15. SEARCHES FOR FRACTIONAL CHARGE PARTICLES COMING FROM OUTSIDE THE EARTH Possible sources: (a) The particles may have been produced in the early universe and be a stable component of the present material in the universe. (b) The particles may be produced in the present era in violent astrophysical processes . (c) The particles may be produced in the interaction of ordinary cosmic rays with the Earth’s atmosphere . Search sensitivity is given in terms of the incoming flux with units of cm-2sr-1s-1

16. .17 .20 .25 .33 .50 1.0 Q/q

17. SEARCHES FOR FRACTIONAL CHARGE PARTICLES IN BULK MATTER • Levitometer method • Millikan liquid drop method

18. From Early Universe to Solar System f f f f f f early universe f f star fX fY f space solar system

19. Searches have been carried out in the following materials: sea water silicone oil mercury iron niobium meteorites Many of these materials were chosen for ease of use in the search technology.

20. Levitometer Method Smith-Jones Group, England Morpurgo Group, Italy LaRue-Fairbank Group, USA

21. 9 The Millikan Liquid Drop Method Using Stokes Law S. F. State Group, USA SLAC Group, USA E Q terminal velocity charge on drop QE=Electric Force = 6phrVterm radius of drop viscosity of air electric field

22. The Millikan Liquid Drop Method Using Stokes Law

23. Searches in Bulk Material Method Material Sample(mg) Nucleons ferromagnetic lev. steel 3.7 2.4x1021 ferromagnetic lev. tungsten 3.0 1.4x1021 ferromagnetic lev. niobium 6.5 4.2x1021 ferromagnetic lev. meteorite 2.8 1.8x1021 superconducting lev. niobium 1.1 7x l020 liquid drop mercury 2.0 1.3x1021 liquid drop silicone oil 259 1.7x1023 liquid drop meteorite 3.9 2.5x1021 All null except superconducting levitometer using niobium, but…

24. Searches for Millicharged Particles • Q/q < 0.1 • Q/q as small as 10-15

25. Only Experimental Millicharged Particle Search Prinz-Jaros (SLAC) PRL 81, 1175 (1998)

26. Summary of millicharged particle searches Davidson et al. JHEP05 (2000) 003 e =Q/q RG: decay in red giants WD: decay in white dwarfs BBN: nucleosynthesis

27. Summary • Since no evidence for fractional charge particles • Extend the millicharged particle search of Prinz-Jaros to higher energies and larger statistics • Search for fractional charged particles at he Large Hadron Collider. Not easy because most events have large multiplicity. • 3. Perhaps extend searches in bulk matter using the ferromagnetic levitometer method ?? (Meteoritic material from asteroids is most appropriate for future examination.)