FAQs – Frequently Asked Questions

What is particle decay?

Particles that are unstable are said to 'decay' into lighter particles. If the decay results from the:

Neutral particle decays produce 'vees' in the bubble chamber and charged particle decays produce 'kinks'.

What is meant by a `particle interaction’?

There are two kinds of interactions:

What are antiparticles?

To every particle that has a non-zero value of some quantity such as electric charge, it is possible to create another particle with the opposite value – this is the antiparticle of the original one. Corresponding to most kinds of particle, there is an associated antiparticle with the same mass and opposite charge. For example, the antiparticle of the electron is the positively charged antielectron (positron) and the antiparticle of the proton is the negatively charged antiproton.

What is a bubble chamber?    

A bubble chamber is a particle detector that records the passage of charged particles in high energy physics experiments.  Bubbles are formed along the path of the particle as it forces its way through a super-heated liquid such as liquid hydrogen.  For more details go to How does a bubble chamber work?

Bubble chambers played an important part in discovering particles whose existence played an important part in establishing the quark model. They are no longer in use at accelerator centres, having been superseded by the faster modern electronic detectors.

Why isn’t a photon track visible in the bubble chamber photographs?

A photon is electrically neutral and therefore cannot exert a Coulomb force on the electrons of the medium (hydrogen) in the bubble chamber.  For additional information refer to FAQ - What is a bubble chamber.

Where are bubble chambers currently being used?

Currently, a bubble chamber is being used during the search for dark matter (WIMPs)).  As recently as 2006, COUPP - The Chicagoland Observatory for Underground Particle Physics, is performing an experiment to demonstrate the performance of a 30 litre, 66 kg, heavy liquid, room temperature, bubble chamber as a Dark Matter (WIMP) detector. The classic bubble chambers, which were built for high energy physics experiments in the 1950s-70s, were too unstable to detect WIMPs.  However, the design of the COUPP detectors employs new techniques to improve stability, making a WIMP search with a bubble chamber feasible for the first time.   

What are WIMPS?

In astrophysics, weakly interacting massive particles, or WIMPs, are hypothetical particles serving as one possible solution to the dark matter problem. These particles interact through the weak nuclear force and gravity, and possibly through other interactions no stronger than the weak force. Because they do not interact with electromagnetism they cannot be seen directly, and because they do not interact with the strong nuclear force they do not react strongly with atomic nuclei. This combination of properties gives WIMPs many of the properties of neutrinos, except that they are far more massive and therefore slower. 

What is a cloud chamber?

          The cloud chamber is used for detecting particles of ionizing radiation. In its basic form, a cloud chamber is a sealed environment containing a super cooled, supersaturated alcohol vapor. When a charged particle interacts with the vapor, the charged particles ionize the vapor. The ions act as condensation nuclei and a mist will be produced and seen as a trail.  For a hands-on experience visit – Build Your Own Cloud Chamber At Home

Why is liquid hydrogen often used in bubble chambers?

The simplest nucleus is found in the hydrogen atom – the proton – which is why hydrogen was a popular bubble chamber liquid. All other nuclei presented problems such as, “Did the beam particle hit a neutron or a proton?”

What is meant by the term `final state particles of a collision’?

This term is usually used for the particles emerging from a collision of 2 particles. If these subsequently decay weakly (such as a π- decaying to - ) these decay products are not referred to as `final state particles’.

What is the purpose of the crosses seen on bubble chamber photographs?

These crosses are known as fiducials and are marked on bubble chamber walls or windows whose positions are accurately surveyed. They are measured along with events and are essential to the process of reconstructing events in 3 dimensions. (From the Latin word ‘fiducia’ meaning truth.)

How do you measure the momentum of a particle in a bubble chamber?

The momentum of a particle is proportional to the radius of curvature of the track in the bubble chamber.  For details click here.

How do you measure the energy of a particle in a bubble chamber?

When a particle has used up its energy making bubbles, it stops. So its range is a measure of its energy. In practice, this is useful for identifying protons that have received only a gentle blow from the beam particle, thus not having enough energy to escape.

Other particles (for example pions) may stop; but if they do, they decay in a characteristic way which tells us that they are pions. In such a case, we would know the mass (m) and its momentum p from the curvature of the track; the energy can then be calculated using: E2 = p2c2 + m2c4.

Why are some tracks thinner or thicker than others?

Often in a given picture, the beam particles are highly relativistic, which means that their speeds are close to that of light. These fast moving particles are called minimum ionizing particles.  Particles moving this fast leave the least number of bubbles per centimetre and therefore have the thinnest tracks.  Particles moving at speeds significantly less than the speed of light leave darker tracks (have more bubbles per centimetre) because they have more time to exert their Coulomb force on the electrons (p= ∫Fdt). For details click here.

Why are some tracks `missing’?

Neutral particles cannot lose energy by ionizing atoms – having no charge they cannot exert a Coulomb force on atomic electrons. However, their existence can be inferred if the total energy and momentum of all the outgoing charged particles is less than the sum of the energy and momentum of the beam and target particles. /p>

Sometimes neutral particles decay into two particles in the bubble chamber: one positive and one negative.  These particles will then produce tracks.  See vees in the glossary.

When were computers used to aid in the identification of particles in a bubble chamber?

From the beginning, computers have been used in controlling the measuring machines and in calculating such things as track momenta (and errors) from co-ordinate measurements on more than one view.

What was BEBC?

The Big European Bubble Chamber (BEBC) was a bubble chamber used at CERN.. BEBC was originally installed at CERN in the early 1970s. It was a stainless-steel vessel with a diameter of 3.7 metres. Charged particles left trails of bubbles as they passed through it. It is no longer in use but is now on display next to CERN's Microcosm museum.

Why is it that when a charged particle passes through an atom (without colliding with the nucleus) it gives most of its energy to the electron and not the proton?

Let us assume that the incoming beam particle passes half-way between an electron and a proton, hence imparting the same momentum p to each particle. Then the electron gains energy  Eelectron = p2 / 2melectron, while the proton gains an energy  Eproton = p2 / 2mproton.

Since mproton is about 2000 times melectron, the electron acquires much more energy than the proton .

Why do electrons spiral into circles in the photographs?

When an electron track spirals inwards, its momentum p (and hence kinetic energy) must be decreasing (p α r, the radius of curvature in the applied magnetic field). The mechanism responsible is bremsstrahlung or `braking radiation’. 

This is a process in which a moving charged particle is accelerated by the electric field of an atomic nucleus. The reason that an electron loses (radiates) more energy than, say, a proton experiencing the same force can be explained with the equation F=ma.  Since they are both subjected to the same force, F, the electron gets a much bigger acceleration because it has a much smaller mass (about 2000 times smaller) compared to that of the proton.  It is due to this greater energy loss by bremsstrahlung that the electron spirals while the proton does not.

Key point: Accelerated charges emit electromagnetic radiation; the greater the acceleration, the greater the radiation.

What is pair production?

When a high energy (E > 2mec2) photon travels through matter, it can, under the influence of the electric field of a nucleus, “materialize” into an electron-positron pair. 

For a BC picture of pair production click here.

What happens in a head-on collision of a positron with an electron? 

According to the laws of quantum mechanics, whatever can happen (does not violate conservation laws) will happen.  The relative probabilities depend on the conditions.

When a positron collides with an electron it can transfer momentum to it; it can also annihilate with it, producing photons.  For BC pictures of collisions of this type click here.

What is an electron-volt? Why are energies of particles measured in eV?

The electron-volt is the kinetic energy that a particle with a charge equal (or opposite) to that of the electron will gain when it is accelerated through a potential difference of 1 volt.  (1eV = 1.602 x 10-19J). 

The electron-volt is a natural unit for energy in atomic physics because, for example, the binding energy of the electron in a hydrogen atom is 13.6 eV – a nice small number.

What are MeV, GeV and TeV?

What is a hadron?

A hadron is a strongly-interacting particle made of quarks and/or anti-quarks held together by gluons, the carriers of the strong force.

What is a baryon?

A baryon/anti-baryon is a hadron made of three quarks/anti-quarks held together by gluons, the carriers of the strong force; ; for example, a proton is made  of 3 quarks (uud); an anti-neutron is made of 3 anti-quarks (u  d  d ).

What is a meson?

A meson is a hadron made of a quark and an antiquark held together by gluons; for example, a π+ is made of (u d). 

What is synchrotron radiation?

Accelerated charges emit electromagnetic radiation.  When this acceleration is produced by an electromagnetic field (in an accelerator for example) this is referred to a synchrotron radiation. 

The effect for a given force is much greater for electrons than protons because a = F/m.

Biologists study complex molecules using synchrotron radiation from electron accelerators.

How do you analyze a bubble chamber picture?

Go through the BC tutorial

What is Dark Matter?

In physical cosmology, dark matter is matter that doesn’t emit electromagnetic radiation but whose presence can be inferred from gravitational effects on visible matter.

What is Dark Energy?

In physical cosmology, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. 

What does LHC stand for?

LHC stands for Large Hadron Collider.  It is a particle accelerator that will collide opposing beams of 7 TeV protons together to explore the validity and limitations of the current theoretical picture for particle physics. It is being built by the European Organization for Nuclear Research (CERN), near Geneva, Switzerland, where it is being cooled down to 1.9 K. The first beams are scheduled to be injected in August 2008.  When operational the LHC will be the world's largest and highest-energy particle accelerator.