From: sichase@csa2.lbl.gov (SCOTT I CHASE)
Newsgroups: sci.physics
Subject: Re: SSC detectors - how??
Date: 7 Apr 92 19:22:50 GMT
Organization: Lawrence Berkeley Laboratory - Berkeley, CA, USA
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In article <8400006@trsvax>, pauls@trsvax.tandy.com writes...
> 
>Just a quick question:
> 
>if the SSC is supposed to find 'new' stuff, how do you detect it?
> 
>Are there any direct (first-order) measurements? Or, are all the
>measurements (excuse me...observations (heh heh)) second-order??
> 
>How do you detect what you don't know beforehand what you'll find?
> 

This is by no means a "quick question".  But here's part of the story, by way
of example:  How do you "find" the top quark?  

One "first-order" way to do it (not the way it will be done) is to build 
an electron-positron collider with enough energy to produce top-antitop pairs.
Such a pair (when bound like a hydrogen atom) is called toponium.  You 
can measure the total cross-section for electron-positron scattering by
counting how many particles are scattered from the beam for each bunch
crossing.  If you vary the machine energy near the toponium resonance, the 
total cross-section goes up and then back down as you sweep past the exact
energy required to make the toponium.  You get a roughly bell-shaped curve
(called a Breit-Wigner resonance).  You can extract the mass of the top quark 
as (roughly) half the energy of the central peak of such a resonance.

This is a very high-precision technique and has been commonly used to do
careful measurements of various resonances, most recently the Z at LEP. 
However, building a sufficiently high energy electron-positron machine is
difficult.  

Typically, the first discovery of a massive resonance is made at a
proton-antiproton collider like the SPS at CERN.  This is because it is much
easier to accelerate protons to high energy.  But then you have a problem.
The fundamental objects which will collide to make your toponium are the quarks
inside the incident proton/anti-proton.  These have a momentum distribution
inside the proton and you don't know the exact energy of the objects which are
actually colliding.  This makes the technique described above (which relies on
the pointlike nature of the electron and positron) useless.

Instead, you simply try to make the toponium and observe its decay products.
The typical top quark decay (according to the Standard Model) is to a b-quark
and a W boson.  The W in turn decays into a hard (high energy) electron or muon
plus a neutrino (which gets lost).  The b-quark becomes a jet of hadrons.
So one possible "signature" for the fleeting existence of the top quark is a 
b-quark jet, a hard lepton, and some missing energy (to account for the
neutrino).  The anti-top quark will also decay in some way (there are a
variety of possibilities). 

There are always backgrounds to such a signature, i.e., other physics processes
which can give a look-alike event topology.  But you can carefully choose what
you look for to minimize the backgrounds in many circumstances.  

By adding up the total energy of the jet and the lepton, and accounting for the
neutrino you didn't see, you can estimate the mass of the top quark, who's
decay you have witnessed.  This method is roughly an order of magnitude poorer
in mass resolution than scanning the electron-positron resonance.  But it's
much better than nothing, and more feasible if you don't yet know what the mass
will be, and so don't know where to scan for the resonance.

Almost all new theories predict some additional new particles which can be
searched for in similar ways.  For example, Supersymmetry predicts
superpartners to every known particle.  There should be photinos and squarks,
sleptons, Winos and Zinos, etc.  Technicolor theories predict Technibosons,
such as the technipion, which can be hunted down.

Other theories predict specific decay modes for known particles.  GUT's
typically predict proton decay into a pion+lepton, or kaon+ other stuff. 
A variety of models predict lepon-violating decays such as muon-> electron +
photon.  

Many theories also predict various otherwise unexpected properties of atoms,
such as CP violating atomic transitions, or an atomic electric-dipole moment
due to an intrinsic dipole moment of the electron.  These are typically looked
for in atomic-beam experiments which are much smaller in scale than the
typical particle physics experiment, but are incredibly high-precision efforts.

Any high-energy physicist could write a book on this subject.  Several have.
A couple I could recommend for interesting reading are "The Higgs Hunter's
Guide", (I don't remember the authors) or "Techniques in Experimental 
High-Energy Phyics" (I think this is the exact title, but I'm not sure)
by Thomas Ferbel.  The first has much the flavor of this post, describing the
various decay modes and signatures according to different models of the 
supposed Higgs particle(s).  The second is more of a detector-builder's guide
to different detectors and the physical principles upon which they are
designed.

-Scott

Newsgroups: sci.physics