How do we know QCD is right?
Result of Collisions at LEP:
Event 1 (E1)
Event 2 (E2)
P(E1) = P(E2) = 0.5
Experiment
is the ultimate arbiter of scientific truth. Since quarks and gluons cannot be
seen through naked eyes, we have to rely on experiments.
Let us see what happens in a LEP
A funda (wich is very essential for making any sense at all
of what happens) à
“ACCORDING TO THE PRINCIPLES OF QUANTUM MECHANICS, THE RESULT OF AN INDIVIDUAL
COLLISION IS UNPREDICTABLE”.
Thus different results emerge. By making many repetitions,
we can determine the probabilities for different outcomes. These probabilities
encode basic information about the underlying fundamental interactions;
according to Quantum Mechanics, they contain all the meaningful information.
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E1 à
QED eventsàlepton
+ antilepton
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E2 à
QCD events à
quark + antiquark
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Particle* Antiparticle**
#electron #positron
#muon #antimuon
#tau #antitau
(1 unit of –ve charge)
(1 unit of +ve
charge)
* & ** are similar in properties and are called as leptons.
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These particles are made up of quarks and gluons which in
other circumstances strongly interact with each other.
(Further explanation about E2 is given below)
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E1 : The final states are very simple states.
Once produced, any of these particles could
-- in the language of elementary acts – attach a photon using a QED hub,
or alternatively, in physical terms, radiate a photon. The basic coupling of
photon to a unit charge is fairly weak. Hence, each additional attachment is
predicted to decrease the probability of the process being described, and so
the most usual case is no attachment. Infact, the final state that includes a
photon does occur, with about 1% of the rate of the particles simply scattering
off each other ( and similarly for the other leptons). By studying the details
of these 3 particle events, such as the probability for the photon to be
emitted in different directions (the “antenna pattern”) and with different
energy, we can check all aspects of our hypothesis for the elementary act. Let
us call this first class of outcomes “QED events”
E2 : Here, they make a smorgasbord of the Greek
and Latin alphabet. It is such a mess that physicists have given up on trying
to describe all the possibilities and their probabilities in detail.
Some simple
patterns emerge if we focus on the overall flow of energy and momentum.
90% of the
cases ----- particles always emerge moving out in opposite directions to each
other. We say these are back-to-back jets.
9% of the
cases ------ particles flow in three directions.
0.9% of the
cases ----- particles flow in four directions.
The
remaining broad class of outcomes are called as “QCD Events”.
The QCD events and the QED events begin to look familiar.
Indeed, the pattern of energy flow is qualitatively the same in both cases,
that is, heavily concentrated in a few narrow jets.
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QCD@
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QED@@
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Multiple jets are more common.
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Multiple jets are less common.
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The jets are sprays of several particles.
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Here the jets are made up of a single particle.
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Pre-requisites for understanding @ and @@ is “ASYMPTOTIC FREEDOM”.
ASYMPTOTIC FREEDOM:
Basic Concept: The probability for a fast-moving quark or
gluon to radiate away some of its energy in the form of other quarks &
gluons depends on whether this radiation is “hard” or “soft”.
Hard radiation is radiation that involves a substantial
deflection of the particle doing the radiating, while soft radiation is
radiation that does not cause such a deflection. Thus, hard radiation changes
the flow of energy and momentum, while soft radiation merely distributes it
among additional particles, all moving together. Asymptotic freedom says that
hard radiation is rare, but soft soft radiation is not.
This distinction explains why on the one hand there are
jets, and on the other hand why the jets are not single particles. A QCD event
begins as the materialization of quarks and anti quarks, similar to how a QED
event begins as the materialization of lepton-antilepton.
By studying the antenna patterns of the multi-jet QCD events
we can check all aspects of our hypotheses for the underlying hubs. Just as for
QED, such antenna patterns provide a wonderfully direct and incisive way to
check the soundness of the elementary acts from which we construct QCD. “Testing
QCD” is also known as “Calculating backgrounds”.
Result of Collisions at LEP:
Fig (1) A three-jet event : The tracks of particles emerging from this high-energy collision at the LEP mark the direction set by the quark, an antiquark and a gluon. The probablility that a given jet pattern emerges depends on the relative angles between the jets and the total energies they carry in an intricate manner. QCD, the fundamental theory of these particles, allows us to predict this dependence precisely.
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