Edges and things, and what experimenters do in HEP.

Edges and things, and what experimenters do in HEP.

Christopher Lester

Christopher Lester, November 2004 Damtp Pheno

What’s this all about?


Decay chains used

Sequential

Branched

Edge p

osi

tions

Fitted distributionsll

lq high

llq Xq

lq low

llq

Xqllq

lq lowlq high

llqll

What the different invariant mass distributions look like for a selection of plausible supersymmetric models.

From Miller et. al.

hep-ph/0410303


ΔM = MT2(χ) - χ

01

~

01

~Rl~

Rl~

l

l

Given: the lepton momenta the missing transverse

momentum an estimate “Χ” of the

neutralino mass

Deduce: lower bound MT2(Χ) on

slepton mass slepton-neutralino mass

difference ΔM


How does this mT2 work?

For each event you want a lower bound for mslepton

You get this by trying ALL POSSIBLE neutralino momenta, k, consistent with

observed missing momentum, identical (unknown) slepton masses, Hypothesised neutralino mass.

For each of these momenta, k, “that might have been” there is an “mslepton(k) that might have been”.

There is a least such mslepton(k) (call it mT2) which cannot be bigger than the true value of mslepton, because one of the k’s is acutally right!

Hence mT2 is a lower bound for mslepton.


So summarising:

In each event: mT2<=mslepton.

Can show that: there exist events for which mT2 = mslepton.

So: the endpoint of the mT2 distribution is

mslepton!


Example mT2 distributions

(This old plot underestimates SM backgrounds)


No mqll-thresh, no mlq-low, no mT2


No mlq-low, no mT2


No mT2


Everything!


A long cascade decay from pp


Mass Relation Method Results

mneutralino (0 to 600 GeV)

mot

hers (

0 to

100

0 G

eV)

Gluon

Squark

Neutralino2

Slepton

Neutralino1

Edges and things, and what experimenters do in HEP.

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