summer95

	Finding Heavy Neutrino Signatures at LEP2

The LEP (Large Electron Positron) collider operated by CERN in Geneva, 
Switzerland has just completed its studies of precision electroweak 
physics in the Z0 resonance region, and searches for new particles up 
to the 60 GeV mass range. Starting this Spring, it will progressively
upgrade its maximum energy to 192 GeV (LEP2), making it possible to produce 
and discover new particles up to approximately 100 GeV in mass, as
well as new physics processes beyond the Standard Model of electroweak 
interactions. L3 is the largest and most precise of the LEP detectors.
It combines high energy resolution for electrons and photons, measured
in a large array of Bismuth Germanate (BGO) crystals, with good resolution 
for the jets of hadrons that result from quark, antiquark and gluon 
production.

My goal is to develop a strategy for detecting (previously unseen) heavy 
neutrinos in the L3 detector at LEP.  The key is to extract a sample of 
events with distinctive topologies involving high momentum leptons 
(electrons, muons and tau particles) as well as jets, and in some cases 
a large fraction of missing energy due to escaping neutrinos. Extracting 
the sample means separating the "signal" for a new heavy neutrino from 
standard electroweak processes which constitute the background. Two examples
of such backgrounds are (1) radiative events, where an initial-state energetic 
photon is emitted down the beamline, leading to a final state with a large
missing energy and (2) WW production, where one or both W bosons decay
to a lepton and a (standard) neutrino.

During this SURF, and based on earlier work on searches for heavy neutrinos
at LEP 1 (up to 92 GeV center of mass energy) and LEP 1.5 (up to 140 GeV
center of mass energy) I will develop strategies to optimize the 
signal (S) for the heavy neutrinos, and to suppress the background (B),
in order to achieve the highest ratio of signal to background S/B. 
Some of the signatures and event selection criteria I will have to use 
are new, since unlike LEP1 the probability for radiative events is enhanced, 
and the WW production rate is substantial. Another interesting possibility
is that other new particles such as Supersymmetric particles may appear,
and distinguishing the two would be complex (and exciting !). 
 
Whether or not I manage to find the heavy neutrinos (which could be 
part of the "Dark Matter" in our universe, since they are heavy and
interact weakly), the appearance of any heavy-neutrino-like signatures
in the LEP2 data would signal the long-awaited breakdown of the Standard
Model. This theory of the unified electroweak interactions has been
incredibly successful but now, I am told, has begun to show serious flaws 
-- both in its theoretical self-consistency and inconsistencies with 
Grand Unified Theories, and perhaps experimentally in its failure to 
predict the rate of Z0 decay into heavy quarks. The breakdown of the
Standard Model would be a landmark in the history of physics, like 
the discovery of quantum electrodynamics by Feynman 50 years ago.

I intend to accomplish my SURF by building and running software which uses a
Monte Carlo method for predicting the signatures.  A random event generator
will produce particles with different 4-momenta, and allow the unstable 
ones to decay into the final state particles that will be seen in the L3 
detector.  Operations will be applied to these particles stochastically 
to produce a "signature" consisting of the pattern of energies and angles 
of the individual particles, the amount and direction of missing energy, 
and the invariant masses of various particle or jet-pair combinations.  
Different selection criteria will be applied to simulated events, and the 
resultant efficiency for selecting heavy neutrino events, and for picking 
up background events, will be computed.  By studying many of the  
probability distributions (histograms and scatterplots) in the selcted signal 
and background samples, I will optimize the selection criteria by 
maximizing the signal/background ratio. As a result I will also understand 
the likelihood of detecting a heavy neutrino in a given mass range, as a 
function of the number of signal and background events produced by LEP. 
 
In the course of this SURF, and during preparatory work in the Spring 
term, I will: 

1. Learn about high energy physics, especially the Standard Model 
   and the signatures used in previous heavy neutrino searches at
   lower energies.
2. Construct one or more generator programs, to produce simulated events;
   use existing generators to simulate the background processes.
3. Use the generators to extract a simulated heavy neutrino signal.
4. If enough real LEP2 data is available this Summer, test my ideas
   on real LEP events, and hope for the best ! 

In order to assure that my SURF project will lead to a significant 
study of the problem, if not a discovery, I will prepare for the 
steps listed above as follows:
 
1. I intend to complete a thorough program of self-study of the physics 
   concepts and methods. I will start with 1 week on my own over spring break,
   and continue by taking Ph 172 with Professor Newman and members of his
   high energy physics group 3rd term.  Aids: there are many books available 
   and I have been given many references to relevant material, both on the
   physics concepts and the programs. 
 
2. The writing and verification of the generators will be the largest 
   active part of my SURF.  Aids: Many event generators for other particles 
   exist. I am not afraid to scavenge code (apart from the key sectors
   related to heavy neutrinos) liberally. Combined with my experience 
   in programming C, C++, Pascal, and Fortran, I expect to get through
   this part in a matter of a few (up to several) weeks, during and
   following my training and preparation in Ph 172.
 
3. Searching for simulated heavy neutrinos, and optimizing the strategy.
   Aids: My advisor has access to a large number of fast processors / parallel
   processors, and he and his colleagues (especially Dr. Sergey Shevchenko
   based at CERN who will visit Caltech) have a lot of experience in previous
   searches.

4. Actual searches for heavy neutrinos or other new physics: this is the
   exciting part !