1

Martin L. Perl

Stanford Linear Accelerator Center

Kavli Institute for Particle Astrophysics and Cosmology

Talk presented at TAU08 WorkshopSeptember, 2008

Searches forFractionally Charged Particles:What Should Be Done Next ?

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 Qq. We use F to mean a fractional electric charge particle.

´´

4

Particle (units of q) Charge

Charged leptons: e, ±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 = isolatedelementary particles withfractional electric charge.

Examples:1/3 q

q

-1.5 q

.45 q

1.0001 q

8

Search Methods

1. Searches using particle accelerators and fixed targets.

2. Searches using particle colliders.

3 Searches in cosmic rays.

4 Searches in bulk matter.

5 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

+ nucleon F+Q + X

+ 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

233s

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

1. Levitometer method

2. Millikan liquid drop method

18

From Early Universeto Solar System

space

solar system

early universe

star

ff f

f

ff f

f

fXf fY

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 MethodSmith-Jones Group, EnglandMorpurgo Group, ItalyLaRue-Fairbank Group, USA

21

QE=Electric Force = 6rVterm

viscosityof air

terminalvelocity

radiusof drop

The Millikan Liquid DropMethod Using Stokes LawS. F. State Group, USASLAC Group, USA

electricfield

chargeon drop

QE

22

The Millikan Liquid DropMethod Using Stokes Law

23

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

Searches in Bulk Material

All null except superconductinglevitometer using niobium, but…

24

Searches for Millicharged Particles

•Q/q < 0.1

•Q/q as small as 10-15

25

Only ExperimentalMillicharged Particle Search

Prinz-Jaros (SLAC)PRL 81, 1175 (1998)

26

Summary of millicharged particle searchesDavidson et al. JHEP05 (2000) 003

Q/qRG: decay in red giantsWD: decay in white dwarfsBBN: nucleosynthesis

27

Summary

Since no evidence for fractional charge particles

1. Extend the millicharged particle search of Prinz-Jaros to higher energies and larger statistics

2. 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.)