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Hi Bob,

I'm not following your discourse very well. Can you clarify what you mean when you say in point number 1:"All particles are assumed massless in the jet mass calculations".

You can pass the jet finder either a set of three vectors (which, of course, have no rest mass) or a set of four vectors which may imply a non-zero rest mass. If you pass four vectors with a non-zero mass then the jet mass calculation includes those masses. If you pass three vectors the jet finder immediately turns them into four vectors and then does all the rest of the processing based on four vectors. If you pass three vectors the jet finder will convert them to four vectors with a mass of zero unless you use the option to specify the same default mass for all the vectors. You have correctly identified two errors about this process of using an assumed mass for the three vector case, namely the spelling error and the calculation error where the default mass should have been squared.

The other thing I need clarified is your comment in several places to the effect that in some places "mass means mass-squared". Can you tell me exactly where you see that happening and I will try to make it read consistently to be "mass" when it is mass and "mass-squared" when it is mass*mass (java does not have mass**2 like fortran.)

The masscut and the mass for Geneva are not in the "correct" units as you point out but the only effect the two parameters have is by working together where the difference in units is consistent. Perhaps Geneva can be recoded to be consistent with the other jet finders but then it probably won't be consistent with the original authors discription of their jet finder.

-----Original Message-----
From: Robert J. Wilson [mailto:[log in to unmask]]
Sent: Thursday, October 12, 2000 2:35 PM
To: lcd-sim
Subject: AbstractJetFinder -- observations (+bugs?)

A few observations on AbstractJetFinder ... please excuse me if these are common knowledege, hopefully they will

be useful to newbies at least (especially if experts correct my misunderstandings!).

1. All particles are assumed massless in the jet mass calculations. This wouldn't be important for high energy

particles but may be for events with high jet multiplicity and with significant strange or baryon content.

2. There is a method to set a defaultMass:

        /**
         * Set the mass to use when converting 3-vectors to 4-vectors
         * This is a static method so the mass applies to all instances
         * of AbstractJetFinder. The default is 0.
         * @param mass The new value to use
         */
        public static void setAssumedMassFor3Vecors(double mass)
        {
                defaultMass = mass;
        }

Note the unexpected spelling of "Vecors" :). This mass is only used to calculate the total energy of

the enumeration passed to the finder if it consists of 3-vectors (the .t() method of LorentzVector is

used otherwise). This is used for some masscut methods.

There is a bug (?) in the current version so that you need to pass the mass**2 value (or it is just following

the scheme elsewhere in the class that "mass" often means mass*2 - then just a slight change to the documentation

would be enough). (I have notified to Gary about this).

Note that the mass is NOT used to upgrade the particle/track 3-vectors, the energy component of the

4-vector created only uses the magnitude of the 3-vector (i.e. effectively mass=0.). This doesn't

seem consistent to me, but perhaps there is a good reason for it.

Currently, there is a bug in the JAS version that prevents the class from accepting LorentzVectors so

this bug/perculiarity of the 3-vector input might catch a few people. (Tony knows about this problem,

as he mentioned in his last posting).

3. As I mentioned above, for most of the calculations, "mass" often means mass**2. However, there are a

few places where it may be confusing. For example, the energy sum passed as a parameter to the masscut is

in linear units, but the masscut value returned is in units squared for all the methods except geneva.

I think it is used consistently, but you might want to be careful if you reuse parts parts of the code since

the naming convention may confuse you too(okay, so maybe I am easily confused!).

Bob

-- 
Robert J. Wilson
Professor of Physics
Department of Physics
Colorado State University
Fort Collins, CO 80523
Phone:  (970) 491 5033
Fax:     (970) 491 7947
Email: [log in to unmask]

August 2000 - July 2001:
========================
Institut de Fisica d'Altes Energies
Universitat Autonoma de Barcelona
E-08193 Bellaterra (Barcelona)
SPAIN
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