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PAC Presentation 10/17/97
Basic Experimental Arrangement
Results of Detailed Simulation
Centauros/Disoriented Chiral Condensate
Motivation is overwhelming.
This region is completely unexplored above fixed target energies. Many very basic questions are unanswered.
Not just theoretical. It is really cultural.
Some hints of general features from handful of VHE cosmic ray events; yet these same data indicate that something funny is going on.
What we think we know about very forward region is mostly folklore.
We consult PYTHIA to learn what "minimum bias" events look like. This is based on extrapolating low energy data by two orders of magnitude. General features may be correct but details are probably quite different.
It is impossible to understand composition (and source) of highest energy cosmic rays without data on the inelasticity of N-N collisions (see Auger proposal).
This experiment really will open a new window in physics
and surprises are likely.
The basic "trick" is to have collisions take place in bending magnets adjacent to a straight section, rather than the center of the straight section as normally done.
This gives a clear shot at 0° for both neutral and charged particles. For charged, the momentum range can be tuned by moving the interaction point.
Because of the new experimental area now under construction, the C0 straight section is the natural choice.
The proposal showed results of a simple simulation which was designed to study the background from sources other than beam-beam collisions and to show that the p0, L0, and K0 signals could be extracted.
We now have a detailed simulation with the cold magnets, the C-magnets, and the beam pipe. The results for the backgrounds are very similar to the simple simulation.
The magnetic field in the iron is ignored, so these may
be a bit pessimistic.
Bottom set are from complete simulation. Both sets assume a g separation resolution of 20 mm and energy resolution of 0.15 E-1/2.
Also a cut to remove g pairs
with smeared separation <2 cm has been added in the second set. Files
contain 200 K events.
Complete MC, 200K sample, 0.5<E2/E1<2, angular res. = 0.1 mrad.
The simple simulation was primarily designed to estimate
backgrounds from particles which do not come from the
collision. The complete simulation shows that these backgrounds are, if
anything, smaller than predicted by the simple simulation. The acceptance
for 2g and K0 Æ2p
events is considerably smaller in the complete simulation due to
the restricted vertical aperture of the C-magnet. Rates are high enough
that this is not a problem, but we are looking at alternatives to increase
We conclude that the backgrounds due to particles from
sources other than the collision are very small,
even with no pointing cuts applied to the tracks.
Centauros/Disoriented Chiral Condensate
The "Centauro" events are basically VHE cosmic ray events seen in emulsion stacks which show an abnormal ratio of gammas to charged hadrons. Note that these appear in the very forward region.
Only a few of these have been seen, but they appear in ~1% of the events above ~1000 TeV ( = 1 TeV).
Thus the cross section to produce them must be ~1 mb.
JACEE event showing the leading particle region h
> 6.5. At lower rapidities the photon detection efficiency becomes small.
The leading cluster, indicated by the circle, consists of about 32 g's
with only one accompanying charged particle.
One possible explanation of large fluctuations in the gamma/charged ratio is that they are due to the formation of a disoriented chiral condensate. (See www.cern.ch/WA98/DCC/ dcc_top.html with 100+ refs.)
Bj defines DCC as a cluster of pions with near-identical momentum, coherently produced, with anomalously large fluctuations in the neutral fraction.
A convenient quantity to characterize DCC is the fraction of neutral pions per event. We define
In DCC the probability of finding a given neutral fraction is
There is also a significant low pT enhancement.
From "low energy" experiments, the charge distribution
is found generically to be a binomial distribution. In both cases,
= 1/3, but the width of the distribution is much different in the two cases.
Thus we can search for DCC by looking at the distribution in the neutral
fraction on an event-by-event basis.
Another way to look for DCC is to use the "robust" moments of the charge distribution, as defined by Cyrus Taylor et al. One of these is
This has the important property of being relatively independent of the detection efficiencies.
For various simulations we find for the gammas and charged
particles in the detector (2-detector setup):
|Normal minimum bias (PYTHIA)||1.068|
|DCC (primary pions only)||0.892|
|All charged pion/all neutral pion mix||0.702|
Thus there is considerable leverage for determining whether
in fact the charged/neutral pion mix is different than the generic prediction.
The geometry of the experiment is close to ideal, so the detector is not technically challenging, but it has to be done carefully.
From the Monte Carlo simulation, the main requirement for the calorimeter is good shower resolution capability. For p0s, for example, we need to resolve 2 g's within a few cm of each other. We also need reasonably good energy resolution.
These goals can be achieved with a lead plate calorimeter and a combination of proportional tube chambers and scintillators.
The electromagnetic section of the calorimeter is similar to that used in UA6. UA6 reports a shower localization of 1.7 mm at 70 GeV and energy resolution dE/E=0.28 E-1/2 based on proportional tubes alone.
The hadronic section is similar to the ZEUS forward neutron
calorimeter which has an energy resolution for hadrons of dE/E=0.45
Because of the unusual location of the Interaction Point, the experiment requires dedicated Collider time.
However, the cross sections are large, so luminosity requirements are modest. A total running time < 1 week is anticipated. [CDF and D0 can tune up with beam-gas interactions.] Beam-gas backgrounds are expected to be very small as explained in the proposal, so no special requirements on vacuum.
We can use the two C-magnets now in the C0 straight section. However, we are looking at other possibilities to give us a larger acceptance.
It would be highly desirable to remove the Lambertson magnets now there, and replace them with additional cold dipoles as planned for BTeV. One or two very small correction magnets may also be required to close the lattice. These would be comparable to half a C-magnet.
We need a clear path between the IP and the detectors, and a beam pipe =2.5" D. The "thin" window is fairly small and can be ~1 mm thick.
Data taking rates will be ~1 kHz. Most of the events are good, so a very simple one-level trigger is adequate.
PREP and computing requirements are modest.
This is an experiment that cries out to be done! It is a basic measurement in an almost completely unexplored region of particle physics. It will fill a gaping hole in our knowledge of very high energy interactions.
We know how to do it!