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Alan Barr 7 June 2007
What can we sayabout what we’vefound?
Was it reallySUSY?
How
can
we
disc
over
y
SUSY
at LH
C?
Just find SM Higgs
Alan Barr
Alan Barr 7 June 2007Extended higgs sector(2 doublets)
Your mission…SM SUSY
quarks (L&R)leptons (L&R) neutrinos (L&?)
squarks (L&R)sleptons (L&R)sneutrinos (L&?)
Z0
W±
gluon
BW0
h0
H0
A0
H±
H0
H±
4 x neutralino
2 x chargino
AfterMixing
gluino
Spin-1/2
Spin-1
Spin-0
Spin-1/2
Spin-0
BinoWino0
Wino±
gluino
~
~
Alan Barr 7 June 2007
Features of RP SUSY?
• RPV as a conserved QN:• Events build from blobs
with 2 “exotic legs”• A pair of cascade decays
results• Complicated end result
• RPV as a conserved QN:• Events build from blobs
with 2 “exotic legs”• A pair of cascade decays
results• Complicated end result
Time
standard
2 exotics
Production part
Time
standard
heavyexotic lighter
exotic
Decay part Time
Complete event
= exotic= standard
Alan Barr 7 June 2007
General features
“typical” susy spectrum(mSUGRA)
• 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
LHC Pt5
Alan Barr 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
SIGNAL topology
q
q_
squarkq
LSP
q_
LSP(and similar)
BACKGROUND topology (QCD)
• No unique choice of sensitive topology– Complementary information/sensitivity
• Expect SM backgrounds with similar characteristics to signal– Need to search for excesses
• No unique choice of sensitive topology– Complementary information/sensitivity
• Expect SM backgrounds with similar characteristics to signal– Need to search for excesses
Alan Barr 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
• SM backgrounds are not small
• There are uncertainties in– Cross sections– Kinematical
distributions– Detector response
Lower backgrounds
Higher backgrounds
“Rediscover”
“Discover”
ZZ
WW
Alan Barr 7 June 2007
Just look for jets?
Big QCD background
Scalar sum of transverse energy / GeV
Alan Barr 7 June 2007
Add some missing energyLook for events with jets and missing energy
Cuts
at least two jets with: ET
Jet1,2 > 150,100 GeV
|Jet1,2| < 2.5
Meff = Jets pTi + MET
But with addition of some other cuts…
Missing transverse momentum > 100 GeV
cuts based on i = |(Jet,i)-(MET)|): R1 = (2
2+(-1)2) > 0.5 rad
R2 = (12+ (-2)2) > 0.5 rad
no jet with i < 0.5 rad
Kill events with missing energy from miss-measured jets
Kill events with missing energy from miss-measured jets
QCD dijets
“SUSY”
Alan Barr 7 June 2007
Two-JetTwo-Jet
No MT2
Dijet cuts + MET +
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
• 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
Lesson from the TevatronEt(miss)
Rare occurrences hurt
Alan Barr 7 June 2007
Inclusive reach in mSUGRA parameter space
Map of discovery potential corresponding to a 5σ excess above background in mSUGRA m0 – m1/2 parameter space for the ATLAS experiment.
jets + ETmiss
channel
L = 1033 cm-2 s-
1
~1 year → ~2200 GeV
~1 month → ~1800 GeV
few days (< one week) → ~1300 GeV
Health warning: expecting SUSY discovery in a few days will seriously damage your credibility
Health warning: expecting SUSY discovery in a few days will seriously damage your credibility
Alan Barr 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 • 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 – …
Slide based on Polesello
Alan Barr 7 June 2007
Mapping out the new world
• Some measurements make high demands on:– Statistics (=> time)– Understanding of detector– Clever experimental technique
LHC Measuremen
tSUSY
Extra Dimensions
MassesBreaking
mechanismGeometry &
scale
SpinsDistinguish
from EDDistinguish from SUSY
Mixings,Lifetimes
Gauge unification?Dark matter candidate?
Alan Barr 7 June 2007
Constraining masses• Mass constraints• Invariant masses in
pairs– Missing energy– Kinematic edges
Observable: Depends on:
Limits depend on angles betweensparticle decays
Frequently-studieddecay chain
Alan Barr 7 June 2007
Mass determination
• Basic technique– Measure edges– Try with different SUSY
points– Find likelihood of fitting
data
• Event-by-event likelihood– In progress
Measureedges
Variety of edges/variables
Try variousmasses in equations
C.G. Lester
• Narrow bands in ΔM• Wider in mass scale• Improve using cross- section information
Alan Barr 7 June 2007
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
Tryvariousdecaychains
Tryvariousdecaychains
Look forsensitive variables
(many of them)
Look forsensitive variables
(many of them)
Extractmasses
Extractmasses
Alan Barr 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 cross-
sections– Often have >1 interpretation
• 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?
Alan Barr 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
spinθ
Alan Barr 7 June 2007
Left Squarks-> strongly interacting-> large production-> chiral couplings
mass/G
eV
Revisit “Typical” sparticle spectrum
Some sparticles omitted
10
–> Stable-> weakly interacting
Right slepton(selectron or smuon)-> Production/decayproduce lepton-> chiral couplings
LHC point 5
20 = neutralino2
–> (mostly) partnerof SM W0
10 = neutralino1
–> Stable-> weakly interacting
Right slepton(selectron or smuon)-> Production/decayproduce lepton-> chiral couplings
Alan Barr 7 June 2007
Spin projection factors
Approximate SM particles as massless-> okay since m « p
Lq~Lq
02
~1
0Lq
P
S
Chiral coupling
Alan Barr 7 June 2007
Spin projection factors
Approximate SM particles as massless-> okay since m « p
Lq~Lq
02
~
1
0~ LqP
S
0
1~02 S
Σ=0
Spin-0
Produces polarised neutralino
Alan Barr 7 June 2007
Spin projection factors
Approximate SM particles as massless-> okay since m « p
(near) Rl
θ*p
SLq~
Lq
Rl
~02
~Rl
Scalar
Fermion
Polarisedfermion
Alan Barr 7 June 2007
Spin projection factors
Approximate SM particles as massless-> okay since m « p
(near) Rl
θ*p
S
Lq~Lq
Rl
~02
~Rl
mql – measureinvariant mass
1
0~ LqP
S
Alan Barr 7 June 2007
lnearq invariant mass (1)
m/mmax = sin ½θ*
Back to backin 2
0 frame
θ*
quark
lepton
Phase space -> factor of sin ½θ*Spin projection factor in |M|2: l+q -> sin2 ½θ* l-q -> cos2 ½θ*
l+
l-
Phase space
Pro
bab
ility
Lq~ Lq
Rl
~02
~Rl
Invariant mass
Alan Barr 7 June 2007
After detector simulation (ATLFAST)
l+
l-Change in shapedue to charge-blind cuts
parton-level * 0.6
-> Charge asymmetry survives detector simulation-> Same shape as parton level (but with BG and smearing)
detector-levelInvariant mass
Ch
arg
e a
sym
metr
y,
spin-0
Even
ts
detector effects cuts to greatly reduce SM
SUSY
Alan Barr 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 cross-
sections?• Can we confirm spin(s)?
Alan Barr 7 June 2007
Extras
Alan Barr 7 June 2007
Standard Model backgrounds: measure from LHC DATA
• Example: SUSY BG– Missing energy + jets
from Z0 to neutrinos– Measure in Z -> μμ– Use for Z ->
• Good match– Useful technique
• Statistics limited– Go on to use W => μ
to improve
Measure in Z -> μμ
Use in Z -> νν R: Z
B: Estimated
R: Z
B: Estimated
Alan Barr 7 June 2007
W contribution to no-lepton BG
• Use visible leptons from W’s to estimate background to no-lepton SUSY search
Oe, Okawa,Asai
Alan Barr 7 June 2007
Normalising not necessarily good enough
Distributions arebiased by lepton selection
Distributions arebiased 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 selectionSimilar 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
q
q_
e+
e-
Z/γ0
1~
01
~
e+
e-~
~
l-
l+
θ*q q_
~
~
Alan Barr 7 June 2007
Distributions @ parton level
black=SUSY
Parallel ParallelPerpendicularto beam
• Spin-0– SUSY– Sleptons– “perpendicular”
to beam
• Spin-½– UED– KK leptons– “parallel” to
beam
l-
l+
θq q_
~
~
red=UED
blue=PS
σtotal not to scale
Alan Barr 7 June 2007
Sensitive variables?• cos θlab
– Good for linear collider– Not boost invariant
• Missing energy means Z boost not known @ LHC
• Not sensitive @ LHC
• Δη– Boost invariant– Sensitive– Not easy to compare with
theory
• cos θll*– 1-D function of Δη:
– All benefits of Δη– Interpretation as angle in
boosted frame– Easier to compare with
theory
l1l2
θ2lab
θ1lab
l1l2η2
lab
η1lab
Δη
l1Δη
l2
θl*θl
*
(A)
(B)
(C)
N.B. ignores azimuthal angle
boos
t
)tanh()tan2cos(cos 211* 2
1
ell
Alan Barr 7 June 2007
“Data” = inclusive SUSY after cuts
• “SPS5” point– Below: spectrum– Right: results– Good stat.
discrimination
Some results