present:
Mayda Velasco
Stefan Soeldner-Rembolt
Heather Logan
Marcela Carena
Steve Mrenna
David Asner
Michel Schmitt

======================================================================
Heather Logan: charginos
               ---------

e+e- overview:
--------------
Heather reminded us of the currently allowed range of charginos
masses.  From LEP, one has generally Mcha > 104 GeV, though there
are conditions under which this bound can be weakened.

The mass matrix depends on M2, mu and beta.  One can arrange to
put all complex phases into mu.

The chargino can decay through a W*, or virtual sleptons and
sneutrinos.  They are produced in the s-channel by photon
and Z exchange, and there is also a t-channel electron-sneutrino
exchange.  Cross sections can be as large as 300 fb.

For a review on how e+e- measurements can constrain the chargino
parameters, see S.Y.Choi et al:
 http://lanl.arXiv.org/abs/hep-ph/0002033
The authors show that with all three production channels
(cha1cha1, cha2cha2, cha1cha2) one can deduce the masses
and two mixing angles.  This is enough to determine the
complete mass matrix including complex phase.

For a discussion, see, for example:
  SUSY Parameters from Charginos,
  Jan Kalinowski & Gudrid Moortgat--Pick
  http://lanl.arXiv.org/abs/hep-ph/0202084
which show very good prospects for 1 ab-2 and sqrt(s) = 800 GeV.
Specifically, they estimate that the errors on the masses
will be on the order of 0.1 GeV.  The constraints on tanbeta
(as it appears in the mass matrix) will be quite weak, however.

Marcela commented that one has to keep SUSY radiative
corrections in mind.  Without them, a measurement is not
very meaningful.  There was a discussion on whether one can
separate the discussion of measurement errors and the `meaning'
of a given theoretical parameter such as tanbeta.  At issue
is the process-INdependence of any given definition of tanbeta.
The theoretical work has not progressed far enough to make any
definitions analogous to, for example, sin^2(theta_eff^lep).
But Marcela points out that the radiative corrections are
known to be large.  For example, see
  A.B. Lahanas, K. Tamvakis and N.D.Tracas,
  Phys Lett B324 (1994) 387

  D.Pierce and A.Papadopoulos, PRD50(1994)565 and
  Nucl Phys B430 (1994) 278

  D.M.Pierce J.A.Bagger K.Matchev  R-J Zhang,
  Nucl Phys B491 (1997) 3.
 

charginos at a gamma gamma collider:
------------------------------------
According to Heather, the
challenge for gamma-gamma collider will be to measure masses
at least as well as can be done in e+e-.  The cross section
shows a sharp turn-on at threshold, at lease when computed
with delta-function
photon beams.  Cross sections are large -- about 4000 fb,
and there is only a modest rise as |cos(theta)| -> 1.

What could we measure?
 - masses: perhaps we can do better than e+e- -- need a study
 - forget about mixing angles
 - Perhaps we can measure decay branching ratios.

Note: gamma-gamma cross sections are not susceptible to sneutrino
exchange, and in this sense are cleaner.

======================================================================
Heather: charged higgs
         -------------

First, we should consider measuring the mass, even though the
cross section proceeds through a p-wave.  One virtue of this
process is that radiative corrections to M(H+) are relatively small.

Information on mass measurement at e+e- colliders can be found in
   Study of e+e- -> H+H- at a 800 GeV Linear Collider
   M. Battaglia, A. Kiiskinen, P. Poyhonen
   http://lanl.arXiv.org/abs/hep-ph/0101239
The authors show that, given 500 fb-1 at 800 GeV, the masses can
be reconstructed to within 1 GeV.

At gamma-gamma colliders, the lineshape will depend on the width,
which depends on tanbeta.  This might allow a measurement of
tanbeta at high tanbeta, which would nicely complement the
tanbeta measurements from chargino processes.  (Not, however,
Marcela's caution above.)

Radiative corrections to the cross section are about 10%.
See hep-ph/0112334.

We should compare this measurement to the measurement of mA,
in the interest of pushing m(H+) as the benchmark parameter
instead of m(A).

Also, we should see how well we can distinguish the MSSM from a
general 2HDM on the basis of the mass differences m(A)-m(H+).
======================================================================
Heather (+ Dave): Higgs Trilinear Couplings
                  -------------------------

Heather presented some considerations on measuring the Higgs
self-coupling at a gamma-gamma collider.  She showed that the
cross section is quite small: 0.2fb, for m(h) = 115 GeV.
There is an interesting costheta dependence, due to the contributions
from J = 0 and J = 2.   The J = 2 contribution i strongly suppressed
near threshold.

It was felt that this analysis would be hard, requiring a large
integrated luminosity.  But this is also true for e+e- analyses
with the same goal. And it may turn out the S/B is better at
a gamma-gamma collider -- a detailed study is needed.

Steve pointed out that, with a gamma-gamma collider, we can
use the photon polarization to turn on and off the true
Trilinear graph.  This could play an important role in
demonstrating the validity of any measurement for this process.
 

======================================================================
Marcela Carena: BR(h->mumu)
                -----------

Marcela and Heather had looked into the branching ratio for
h --> mumu in a region of MSSM parameter space where h -> bbbar
is highly suppressed.  The mumu branching ratio, like several
others, is enhanced by a factor 4 to 4.5.  This may not be
enough to allow this channel to become visible: a study is needed
to tell.

======================================================================
Steve Mrenna:  Four-fermion Generators
               -----------------------

Steve reported that we can use Wizard (sp?) to do
gamma gamma -> four fermions.  He has been held up by the lack
of an F90 compiler -- today he has one!

======================================================================
Stefan: News from DESY
        --------------

Stefan was at DESY (both Hamburg and Zeuthen).  Klaus Moenig has a
new postdoc who will work on higgs and gamma-gamma studies.  They
want to concentrate on a more realistic b-tagging, with a serious
algorithm rather than just parameterizations.

They also recognize the need to work on the bbbar mass resolution.

Stefan stated three crucial questions in this context:
 1- How well can we suppress the charm background?
 2- How close can the inner silicon layers be placed to the beampipe?
 3- What is contribution to the underlying event from resolved
    photon processes?
 
 

======================================================================
David: MC News
       -------

Tony Hill has put a Cain-Pythia interface together, but David has
had a chancnot e to use it very much.  He explained that there are
many messes to be cleaned up in order to proceed in a more
professional fashion.  For example, CAIN is not yet officially
interfaced to PANDORA.  Jeff Gronberg is working on this together
with Michael Peskin.  David is working with Norman Graf to put
together the CAIN+(Pandora)+Pythia package to be released on the next
LC tools CD.

David suggested we might bypass PANDORA, and work to implement
spin-dependent cross sections in PYTHIA.  Steve has already done
some important steps in this direction.  (Next, perhaps, would
be the W pair production.)

David also pointed out that we should use PYTHIA in a mode
to take automatically into account all beam species: electrons,
positrons, photons, hadronic photons, and QCD photons.  Tony Hill's
version, with the Cain interface, does this.  David is testing
this now, and has been using it for quite a while.

Stefan pointed out that a lot of work has already been invested in
PHOJETS, and we should be sure to take advantage of that.  Ralph
Engel might be able to help.

======================================================================
amended 21-Feb-2002.  M.S.
 

tentative next meeting date:  Feb 28  or   Mar 7