The measurement of SUSY masses in cascade decays at the LHC - PowerPoint PPT Presentation

Loading...

PPT – The measurement of SUSY masses in cascade decays at the LHC PowerPoint presentation | free to download - id: 173228-ZDc1Z



Loading


The Adobe Flash plugin is needed to view this content

Get the plugin now

View by Category
About This Presentation
Title:

The measurement of SUSY masses in cascade decays at the LHC

Description:

B. K. Gjelsten, D. J. Miller, P. Osland. ATL-PHYS-2004-029. hep-ph/0410303 ... PYTHIA forgets' spin. This could be a problem for mql. November 10, 2004. D.J. Miller ... – PowerPoint PPT presentation

Number of Views:15
Avg rating:3.0/5.0
Slides: 41
Provided by: davidj123
Learn more at: http://www.physics.gla.ac.uk
Category:

less

Write a Comment
User Comments (0)
Transcript and Presenter's Notes

Title: The measurement of SUSY masses in cascade decays at the LHC


1
The measurement of SUSY masses in cascade decays
at the LHC
D. J. Miller
  • Based on
  • B. K. Gjelsten, D. J. Miller, P. Osland
  • ATL-PHYS-2004-029
  • hep-ph/0410303
  • B.K. Gjelsten, E. Lytken, D.J. Miller, P. Osland,
    G. Polesello,
  • LHC/LC Study Group Working Document.
  • ATL-PHYS-2004-007

2
Contents
  • Introduction
  • How applicable is this method?
  • The SPS 1a point(s) and slope
  • Cascade decays at ATLAS
  • Summary and conclusions

3
Introduction
  • Low energy supersymmetry presents an exciting and
    plausible extension to the Standard Model.
  • It has many advantages
  • Extends the Poincarré algebra of space-time
  • Solves the Hierarchy Problem
  • More amenable to gauge unification
  • Provides a natural mechanism for generating the
    Higgs potential
  • Provides a good Dark Matter candidate (? )
  • Supersymmetry may be discovered at the LHC
    (switch on in 2007)


0 1
4
Supersymmetry predicts many new
particles Scalars squarks sleptons Spin
½ gauginos higgsinos (neutralinos) Predicts
SUSY particles have same mass as SM partners
wrong!
SUSY must be broken, but how is not clear
MSSM break supersymmetry by hand by adding
masses for each SUSY particle
Supergravity break SUSY via gravity GMSB SUSY
is broken by new gauge interactions AMSB SUSY
is broken by anomalies
Which, if any, of these is true?
5
SUSY breaking models predict masses at high
energy Evolved to EW scale using (logarithmic)
Renormalisation Group Equations
Uncertainties in masses at low energy magnified
by RGE running
Zerwas et al, hep-ph/0211076
Need very accurate measurements of SUSY masses
6
  • 2 problems with measuring masses at the LHC
  • Dont know centre of mass energy of collision vs
  • R-parity conserved (to prevent proton decay)

SM particles have P 1
R
SUSY partners have P - 1
R
R-parity ? Lightest SUSY Particle (LSP) does not
decay
All decays of SUSY particle have missing
energy/momentum This cannot be recovered by
using conservation of momentum
7
Measure masses using endpoints of invariant mass
distributions
e.g. consider the decay
mll is maximised when leptons are back-to-back in
slepton rest frame
angle between leptons
8
3 unknown masses, but only 1 observable, mll
extend chain further to include squark parent
now have mll, mql, mql-, mqll 4 unknown
masses, but now have 4 observables
Hinchliffe et al, Phys. Rev D 55 (1997) 5520,
and many others
9
How applicable is this method?
  • To make this work we need
  • The correct mass hierarchy to allow
  • i.e.
  • A large enough cross-section and branching ratio

Examine mSUGRA scenarios to see if this is
likely (if it isnt
we would have to study a different decay)
10
In mSUGRA models have universal boundary
conditions at GUT scale (1016 GeV)
SUSY scalar mass m0 SUSY fermion mass
m1/2 Common triple coupling A0 Higgs vacuum
expectation values tan?, ?gt0
Also
11
Snowmass benchmark model slope SPS 1a A0
-m0, tan? 10, ?gt0
12
(?gt0)
A0 -m0, tan? 10
13
Squark decay branching ratios
(¼ SU(2) singlet)
14
bottom squarks are mixtures of left and right
handed states
15
(No Transcript)
16
Constraints from WMAP
2? exclusion
Ellis et al, hep-ph/0303043
A large part of interesting parameter space has
the decay
17
The SPS 1a slope and point(s)
Snowmass points and slopes are benchmark
scenarios for SUSY studies
See Allanach et al, Eur.Phys.J.C25 (2002) 113,
hep-ph/0202233
Defined as low energy (TeV scale) parameters
(masses, couplings etc) as evolved by version
7.58 of the program ISAJET from the GUT scale
parameters
SPS 1a slope
18
widths
masses
a
a
ß
ß
NB instabilities due to inaccuracy in ISAJET,
and thus inherent to definition
19
Parent gluino/squark production cross-sections in
pb
a
ß
not useful
These are not yet the relevant numbers for our
analysis it doesnt matter where the parent
squark comes from
20

?20 branching ratios
21
Some extra difficulties
Cannot normally distinguish the two leptons
Endpoints are not always linearly independent
? Four endpoints not always sufficient to find
the masses
It is the minimum of this distribution which is
interesting
22
Spin correlations
PYTHIA does not include spin correlations
(HERWIG does!) OK for decays of scalars, but may
give wrong results for fermions
This could be a problem for mql
23
Without spin correlations
With spin correlations
  • Recall, cannot distinguish ql and ql-
  • must average over them
  • Spin correlations cancel when we
  • sum over lepton charges
  • Pythia OK

Barr, Phys.Lett. B596 (2004) 205
24
Cascade decays at ATLAS
25
Generate simulated data using PYTHIA 6.2 (with
CTEQ 5L) Pass events through ATLFAST 2.53, a
fast simulation of ATLAS.
  • Acceptance requirements
  • ATLFAST has no lepton identification efficiency

  • include 90 efficiency per lepton by hand
  • ATLFAST has no pile-up, or jets misidentified as
    leptons

  • not included here

26
  • Initial (untuned) cuts to remove backgrounds
  • 3 jets, with pT gt 150, 100, 50 GeV
  • ET, miss gt max(100 GeV, 0.2 Meff) with
  • 2 isolated opposite-sign same-flavour leptons
    (e,?) with pT gt 20,10 GeV
  • Split remaining background into two categories
  • Correlated leptons (e.g. Z ? ee-)
  • - processes where the
    leptons are of the Same Flavour (SF)
  • Uncorrelated leptons (e.g. leptons from
    different decay branches)
  • - processes where the
    leptons need not be SF

27
Uncorrelated backgrounds have the same number of
events with SF leptons (a background to the
signal) as events with Different Flavour (DF)
leptons Can remove SF events by Different
Flavour (DF) subtraction
End result of DF subtraction
Theory curve
Z peak (correlated leptons)
28
When distribution includes a quark have an extra
problem - which quark to pick?
This will give a combinotoric background
Estimate this background with mixed
events Combine the lepton pair with a jet from
a different event to mimic choosing the wrong
jet gives dashed curve
Here we have chosen the jet (from the two
highest pT jets) which minimises mqll
29
Fit mll endpoint to Gaussian smeared
triangle Fit other distributions to a Gaussian
smeared straight line where indicated It is not
clear that this is the best thing to do!
30
Theory curves
can we really trust a linear fit? something to
improve in the future?
  • notice the foot here
  • this can be easily
  • hidden by backgrounds

31
Point ß much more difficult due to lower
cross-sections
32
Energy scale error 1 for jets, 0.1 for leptons
33
From endpoints to masses
  • Can (in principle) extract the masses in two
    ways
  • Analytically invert endpoint formulae for masses
  • Endpoints in terms of masses are already
    complicated,
  • with 9 different physical mass regions.
  • mqll(?gt?/2) particularly complicated to invert
  • Not very flexible
  • Not all endpoints should be given the same
    weight,
  • e.g. mll is much better measured.

see over
34
(No Transcript)
35
2. Fit masses to these endpoints using method of
least squares
Use analytic expressions to find a starting point
for the fit
Problem the multi-region nature of the endpoint
formulae often lead to 2 consistent solutions
for the masses. Usually these are sufficiently
different that we can distinguish them from the
real masses by some other means and/or the
wrong mass spectrum has a much lower
likelihood.
36
SPS 1a (a) results
37
second solution
38
SPS 1a (ß) results
39
Conclusions and summary
It will be important to accurately measure SUSY
masses at the LHC R-parity conservation and
unknown CME makes measuring masses
difficult Can measure masses using endpoints of
invariant mass distributions in cascade
decays We have studied the decay
at ATLAS
for the Snowmass benchmark SPS 1a This decay is
applicable over much of the allowed parameter
space as long as m0 is not too large compared
with m1/2 We examined a second point on the SPS
1a line which has less optimistic cross-sections
40
Simulated data using PYTHIA and ATLFAST Remove
real and combinotoric backgrounds using DF
subtraction and mixed events Fit straight
lines to edges of distributions to find
endpoints it is not clear whether this is a
good idea Use method of least squares to fit for
the masses Often find multiple solution (though
correct solution is always favoured) This method
provides reasonable mass measurements, but even
better measurements of mass differences
About PowerShow.com