SLAC B084 R140
11-Dec-2000
Mayda Velasco, Jeff Gronberg,
Dave Asner, Gohan Unel, Adam Para,
Tom Rizzo, Erik Ueggerhoj,
Ulrik Mikkelsen, Steve Mrenna,
Michael Schmitt (minutes)
Monte Carlo and Cross Section programs
======================================
Jeff and Dave explained the generators that have been written for NLC
studies, and which are being maintained.
Pandora = includes Pythia; written in C++ by Michael Peskin.
This program generates events up to the parton level,
including helicity, and passes the event record to
(s)Pythia for hadronization. An arbitrary photon beam
can be implemented by providing a suitable luminosity
object. A limited number of processes is available.
Code and limited documentation are available from the web.
Detector Simulation: There are fast simulation programs, and
more
detailed ones, supported by the American Linear
Collider Collaboration.
They are written in C++ and are supported by Norm
Graf. See, for
example, programs LCD and GIZMO, available from
the web.
We need to have a reliable and standard way of calculating cross
sections and branching ratios. A program such as V2HV
by Michael
Spira would be ideal. There were suggestions to use MADGRAPH
(aka COMPHEP). Steve Mrenna has some codes which he will share,
and will also as Jack Gunion what he might have. Another
possibility
is LOOPTOLLS, written in fortran. Dave has compiled these
programs
but does not yet know whether they produce correct output.
Mayda appointed Michael and Steve to gather these programs together
and make them available to the group.
Machine Design
==============
Mayda wrote down the main quantities required for beam input:
- energy spectrum
- polarization
- multiplicity
- emittance
As Adam pointed out, these quantities typically are correlated
with position in the beam.
Jeff described briefly the typical characteristics of an NLC
electron beam:
- 30 urad divergence
- 4nm x 70nm spot size
- 10**10 electrons per bunch; 95 bunches/train; 120 trains/sec
He explained there will be two energy regimes for the e+e- collider;
the low-E regime extends 90-300 GeV, the high-E 300GeV-1TeV
There are web pages where the official parameter sets are posted;
Dave will tell us where to find these pages.
There are unofficial parameters for a Higgs factory. They are
similar
to the ones above, except that the beam size increases as 1/E.
Photon Beams from Back-Scattered Lasers - Jeff Gronberg
---------------------------------------
(Jeff showed a set of nice transparencies on photons beams from
this technique. These notes do not reproduce his presentation
--
only a few short items which came up during his presentation
are
recorded here.)
* Ref article: NIM RD9 355 (1)
* e-e- would be the best mode, because one can achieve a high degree
of polarization with both beams. Also, backgrounds from
e+e-
annihilation are reduced. The accelerator people don't like
it,
however, because it requires reversing the physical orientation
of all the magnets in one arm.
* Spontaneous production of e+e- pairs limit the laser energy
(x electron energy)
* Nonlinear effects limit the laser intensity.
**** Question for the physics studies: how bad is the "pollution"
from the low-energy electron and photon spray?
* There was a crude optics desgin in the 1996 Snowmass meeting.
* Now there is a real plan, with a 1cm beampipe and 10cm opening.
* There have been breakthroughs in the laser technology, a spin-off
from the need for high power lasers for making clear cuts.
Consequently, the cost has dropped from $1B to $6M! The
current
so-called `mercury' design could be built today.
Photon Beams from Crystals - Mayda Velasco
--------------------------
* The SOS and CM techniques are being confronted with actual
measurements in NA59. Theoretical calculations are confirmed.
* Energy spectra are very similar to the best case LBS technique,
with an advantageous dependence of polarization on photon energy.
* It is known how to grow sufficiently pure diamond crystals, and
how to align them with the incoming beam.
Ulrik: The optimal thickness is about 2.5 mm.
Discussion: A huge amount of energy will be dumped into a very
small volume in a short time. Can the crystals survive?
How
will the heat be dissipated? The space near the IP is constrained,
and bathed in a photon-electron "fog." How will the crystal be
mounted, and aligned in place? Will it generate a lot of "pollution"
by scattering low-E photons and electrons up into the detector?
Mayda has arranged appropriate studies at the FFTB facility to
answer some of these questions. Parameters:
- 25 GeV e- beam, 2*10**10 e/pulse, 120Hz pulse rate.
- spot size within factor of 10 of NLC design.
Future Meetings
===============
The workshop on gamma-gamma colliders will take place in March
at FERMILAB. The exact dates are not yet fixed, as we have to
avoid conflicts with an NLC workshop.
We should meet again (probably a phone conference) on Dec 22,
to make sure that simulation programs are ready.
There will be another meeting in mid-January.
Snowmass
--------
What are the expectations? Answer: we should prepare an "Orange Book"
by mid-April describing briefly plans for the machine and results
of physics studies. Mayda suggests the following areas, in order
of importance:
Higgs physics -at a factory -at a TeV machine
SUSY searches & measurements
two-photon physics
WW production (Note: Tom points out this is the best area
for
extra dimentions!)
Exotics: 4th generation, compositeness, etc.
**********************************************************************
Notes added in proof, from Mayda and Jeff:
The improvements and reduction in cost for the laser
back-scattered technique pertains only with the NLC design.
It is not compatible with the TESLA design because of the
high repetition rate. In that case, you would need to use
95 lasers, which is not realistic.
Concerning photon beams from crystals: Some radiation hardness
tests have already been made on single crystals diamonds. They
survived the equivalent of a few pulses at the NLC. The results
can be found at
http://www.slac.stanford.edu/~pkr/LCLS/T450/T450.html
.
The tests will be continued in January, 2001.
Note: The FFTB facility cnnot deliver 120 Hz if the beam must
be very focussed. A more realistic number, according to
Clive Fields, is 10 Hz.