Transcript SUSY at the LHC

Alan Barr
Just find SM Higgs
Was it really
SUSY?
What can we say
about what we’ve
found?
Alan Barr
7 June 2007
Your mission…
SM
Spin-1/2
Spin-1
Spin-0
SUSY
quarks (L&R)
leptons (L&R)
neutrinos (L&?)

Z0
W±
gluon
h0
H0
A0
H±
Extended higgs sector
Alan Barr
(2 doublets)
B
W0
squarks (L&R)
sleptons (L&R)
sneutrinos (L&?)
Bino
Wino0
Wino±
gluino
Spin-0
After
Mixing
4 x neutralino
Spin-1/2
~
H0
~
H±
gluino
2 x chargino
7 June 2007
Features of RP SUSY?
Production part
Complete event
standard
2 exotics
Time
Decay part
heavy
exotic
Time
lighter
exotic
standard
Time
Alan Barr
•
•
•
•
RPV as a conserved QN:
Events build from blobs
with 2 “exotic legs”
A pair of cascade decays
results
Complicated end result
7 June 2007
= exotic
= standard
General features
• Complicated cascade
decays
– Many intermediates
• Typical signal
– Jets
• Squarks and Gluinos
– Leptons
• Sleptons and weak
gauginos
– Missing energy
• Undetected LSP
• Model dependent
– Various ways of
transmitting SUSY
breaking from a
hidden sector
“typical” susy spectrum
(mSUGRA)
Alan Barr
LHC Pt5
7 June 2007
What do we see?
Lifetimes short -> look for Standard Model decay relics + missing energy
Alan Barr
7 June 2007
Example of a search topology
LSP
q
squark
q
_
q
_
q
(and similar)
BACKGROUND
topology (QCD)
LSP
• No unique choice of sensitive topology
– Complementary information/sensitivity
• Expect SM backgrounds with similar
characteristics to signal
SIGNAL topology
Alan Barr
– Need to search for excesses
7 June 2007
Practical Problems
• See only SM decay products
– Expect short lifetimes
• Lose information about order of decays
– Jets (other than b and t) indistinguishable
• Loose flavour information for other squarks
• “Missing momentum” from neutralinos only
determined perpendicular to beam
– Individual LSP momenta not individually
measurable
– Z-momentum of initial state unknown (PDFs)
– Can’t reconstruct from final state
• Forward jets lost down beam pipe
– Can’t form “invariant masses” of sparticles
• No “clean” mass peaks for resonances
Alan Barr
7 June 2007
Precise
measurement of
SM backgrounds:
the problem
“Rediscover”
Lower backgrounds
• SM backgrounds
are not small
• There are
uncertainties in
– Cross sections
– Kinematical
distributions
– Detector response
Alan Barr
WW
ZZ
“Discover”
Higher backgrounds
7 June 2007
Just look for jets?
Big QCD background
Scalar sum of transverse energy / GeV
Alan Barr
7 June 2007
Add some missing energy
Look for events with jets and missing energy
Cuts
 at least two jets with:
Meff = Jets pTi + MET
 ETJet1,2 > 150,100 GeV
 |Jet1,2| < 2.5
But with addition of some other cuts…
 Missing transverse momentum
> 100 GeV
QCD dijets
 cuts based on i = |(Jet,i)-(MET)|):
 R1 = (22+(-1)2) > 0.5 rad
 R2 = (12+ (-2)2) > 0.5 rad
 no jet with i < 0.5 rad
Alan Barr
Kill events with
missing energy from
miss-measured 7jets
June 2007
“SUSY”
No MT2
Dijet cuts + MET + 
Two-Jet
Scalar sum of transverse energy / GeV
Expect discovery distribution to be of something like this form:
Excess of “some sort” of new physics about SM backgrounds.
Alan Barr
7 June 2007
Importance of detailed detector
understanding
Et(miss)
Lesson from the Tevatron
•
•
•
GEANT simulation already
shows events with large
missing energy
– Jets falling in “crack” region
– Calorimeter punch-through
Vital to remove these in
missing energy tails
Large effort in physics
commissioning
Alan Barr
Rare occurrences hurt
7 June 2007
Inclusive reach in mSUGRA
parameter space
 Map of discovery potential
L = 1033 cm-2 s-1
corresponding to a 5σ excess above
background in mSUGRA m0 – m1/2
parameter space for the ATLAS
experiment.
jets + ETmiss
channel
~1 year → ~2200 GeV
~1 month → ~1800 GeV
few days (< one week) → ~1300 GeV
Alan Barr
Health warning: expecting SUSY
discovery in a few days will
seriously damage your credibility
7 June 2007
Different searches
• We will be looking in many different channels
–
–
–
–
–
–
–
–
n jets + m leptons + missing energy
+- b-jets (common at large tan β)
+- tau-jets (“
“ “)
Charged stable particles
NLSP -> photon gravitino (GMSB)
R-parity violating modes
R-hadrons
…
Alan Barr
7 June 2007
What might we then know?
•
•
•
Assume we have MSSM-like
SUSY with
m(squark)~m(gluino)~600 GeV
See excesses in these
distributions
Can’t say “we have discovered
SUSY”
• Can say some things:
– Undetected particles produced
• missing energy
– Some particles have mass ~ 600 GeV,
with couplings similar to QCD
–
–
–
–
–
–
Alan Barr
• Meff & cross-section
Some of the particles are coloured
• jets
Some of the particles are Majorana
• excess of like-sign lepton pairs
Lepton flavour ~ conserved in first two
generations
• e vs mu numbers
Possibly Yukawa-like couplings
• excess of third generation
Some particles contain lepton quantum
numbers
• opposite sign, same family dileptons
…
7 June 2007
Slide based on Polesello
Mapping out the new world
LHC
Measurement
SUSY
Extra
Dimensions
Masses
Breaking
mechanism
Geometry &
scale
Spins
Distinguish
from ED
Distinguish
from SUSY
Mixings,
Gauge unification?
Lifetimes
Dark matter candidate?
• Some measurements make high demands on:
– Statistics (=> time)
– Understanding of detector
– Clever experimental technique
Alan Barr
7 June 2007
Constraining masses
• Mass constraints
• Invariant masses in pairs
Frequentlystudied
decay chain
– Missing energy
– Kinematic edges
Observable:
Depends on:
Limits depend on
angles between
sparticle decays
Alan Barr
7 June 2007
Measure
edges
Mass determination
Try various
masses in
equations
Variety of edges/variables
• Basic technique
– Measure edges
– Try with different SUSY
points
– Find likelihood of fitting
data
• Event-by-event likelihood
– In progress
Alan Barr
7 June 2007
C.G. Lester
• Narrow bands in ΔM
• Wider in mass scale
• Improve using crosssection information
SUSY mass measurements
• Extracting
parameters of
interest
– Difficult problem
– Lots of competing
channels
– Can be difficult to
disentangle
– Ambiguities in
interpretation
– Lots of effort has
been made to find
good techniques
Alan Barr
Try
various
decay
chains
Look for
sensitive variables
(many of them)
Extract
masses
7 June 2007
SUSY mass measurements:
• LHC clearly cannot fully constrain all
parameters of mSUGRA
– However it makes good constraints
•
•
•
•
Particularly good at mass differences [O(1%)]
Not so good at mass scales
[O(10%) from direct measurements]
Mass scale possibly best “measured” from crosssections
– Often have >1 interpretation
•
•
•
•
Alan Barr
What solution to end-point formula is relevant?
Which neutralino was in this decay chain?
What was the “chirality” of the slepton “ “ “ ?
Was it a 2-body or 3-body decay?
7 June 2007
SUSY spin measurements
• The defining property of
supersymmetry
– Distinguish from e.g. similar-looking
Universal Extra Dimensions
• Difficult to measure @ LHC
– No polarised beams
– Missing energy
– Indeterminate initial state from pp
collision
• Nevertheless, we have some very good
chances…
Alan Barr
7 June 2007
Measuring spins of particles
• Basic recipe:
– Produce polarised particle
– Look at angular distributions in its decay
spin
Alan Barr
θ
7 June 2007
Revisit “Typical” sparticle spectrum
Left Squarks
-> strongly interacting
-> large production
-> chiral couplings
LHC point 5
mass/GeV
20 = neutralino2
–> (mostly) partner
of SM W0
Right slepton
(selectron or smuon)
-> Production/decay
produce lepton
-> chiral couplings
10 = neutralino1
–> Stable
-> weakly interacting
Alan Barr
Some
sparticles
omitted
7 June
2007
Spin projection factors
q~L
qL
qL 
~20
Chiral coupling
Approximate SM particles as massless
Alansince
Barr m « p
-> okay
7 June 2007
0
1
P
S
Spin projection factors
0
~
qL 
1
q~L
qL
~20
Spin-0
P
S
Σ=0
1
0
~
2 
0
S
Produces polarised
neutralino
Approximate SM particles as massless
Alansince
Barr m « p
-> okay
7 June 2007
Spin projection factors
Fermion
q~L
θ*
qL
~ 0
2
Polarised
fermion
l
~
lR

R
Scalar
Approximate SM particles as massless
Alansince
Barr m « p
-> okay
l
7 June 2007
 (near)
R
p
S
Spin projection factors
0
~
qL 
1
q~L
P
mql – measure
invariant mass
S
qL
~ 0
2
l

R
l R (near)
~
lR
θ*
Approximate SM particles as massless
Alansince
Barr m « p
-> okay
7 June 2007
p
S
invariant mass (1)
quark
θ*
q~L
lepton
qL
~ 0
2
Back to back
in 20 frame
Probability
near
l q
Invariant mass
l
~
lR

R
Phase space -> factor of sin ½θ*
Spin projection factor in |M|2:
l+q -> sin2 ½θ*
-q -> cos2 ½θ*
lAlan
7 June 2007
Barr
l+
Phase space
l-
m/mmax = sin ½θ*
l-
Change in shape
due to chargeblind cuts
l+
parton-level * 0.6
Charge asymmetry,
Events
After detector simulation (ATLFAST)
Invariant mass
spin-0
detector-level
-> Charge asymmetry survives detector simulation
-> Same shape as parton level (but with BG and smearing)
 detector effects
 cuts to greatly
Alan Barr SM
reduce
7 June 2007
Interesting questions
• Can we test gaugino universality?
– Can we constrain the neutralino mass mixing
matrix?
• Can we measure sparticle splittings?
– JMR: Htt coupling interesting
• Can we “predict”/confirm dark matter
density?
• Can we measure mass scale to better than
~10%
– Precision measurement/prediction for crosssections?
• Can we confirm spin(s)?
Alan Barr
7 June 2007
Extras
Alan Barr
7 June 2007
Standard Model backgrounds:
measure from LHC DATA
m
m
Measure in
Z -> μμ


Use in
Z -> νν
R: Z > 
B: Estimated
• Example: SUSY BG
– Missing energy + jets
from Z0 to neutrinos
– Measure in Z -> μμ
– Use for Z -> 
Alan Barr
• Good match
– Useful technique
• Statistics limited
– Go on to use W => μ to
improve
7 June 2007
W contribution to no-lepton BG
Oe, Okawa,
Asai
• Use visible leptons from W’s to estimate
background to no-lepton SUSY search
Alan Barr
7 June 2007
Normalising not necessarily
good enough
Distributions are
biased by lepton
selection 
Alan Barr
7 June 2007
Need to isolate individual
components…
Alan Barr
7 June 2007
Then possible to get it right…
Similar story for other backgrounds – control needs careful selection
Alan Barr
7 June 2007
Direct slepton spin determination
• Spin important in
slepton production
– Occurs through
s-channel spin-1
process only
– Characteristic
angular
distribution in
production
Alan Barr
q
e~+0
1
~
e+
Z/γ
_
q
~10
~
e-
e-
~
l+
q
7 June 2007
θ*
~
l-
_
q
Distributions @ parton level
~
l+
q
Parallel
θ
• Spin-0
~-
l
blue=PS
σtotal not to scale
black=SUSY
– UED
– KK leptons
– “parallel” to beam
Alan Barr
Parallel
red=UED
_
q
– SUSY
– Sleptons
– “perpendicular”
to beam
• Spin-½
Perpendicular
to beam
7 June 2007
• cos θlab
Sensitive variables?
– Good for linear collider
– Not boost invariant
θ2
θ1lab
lab
l1
l2
– Boost invariant
– Sensitive
– Not easy to compare with
theory
η1lab
l1
• cos θll*
η2lab
– 1-D function of Δη:
cos   cos(2 tan e
*
ll
1
 12 
l2
Δη
)  tanh( 12  )
θl*
– All benefits of Δη
– Interpretation as angle in
boosted frame
– Easier to compare with theory
Alan Barr
(A)
l2
θl*
Δη
l1
N.B. ignores azimuthal angle
7 June 2007
(B)
boost
• Δη
• Missing energy means Z boost
not known @ LHC
• Not sensitive @ LHC
(C)
• “SPS5” point
Some results
– Below: spectrum
– Right: results
– Good stat. discrimination
“Data” = inclusive SUSY after cuts
Alan Barr
7 June 2007