Transcript SUSY at the LHC - University of Oxford

Alan Barr
Just find SM Higgs
Map
Anything unusual out there
Was it really
SUSY?
Why look for SUSY?
What can we say
about what we’ve
found?
Dark Matter
Visible mass
Invisible mass
• Atoms ~ 4%
• Evidence for Dark Matter from
– Rotation curves of galaxies
– Microwave background radiation
– Galaxy cluster collision
Particle physicists should hunt:
Weakly Interacting, Stable, Massive Particles
Producing exotics?
standard
exotic
exotic
Time
standard
• If exotics can
be produced
standard
singly they can
decay
Time
exotics
exotics
Time
• If they can only
be pairproduced they
standard
are stable
Time
Require an even number of exotic legs to/from blobs
(Conserved multiplicative quantum number)
– No good for
Dark Matter
candidate
– Only
disappear on
collision
(rare)
How do they then behave?
Production part
Complete event
standard
2 exotics
Time
Decay part
heavy
exotic
Time
lighter
exotic
standard
• Events build from blobs with
2 “exotic legs”
• A pair of cascade decays
results
• Complicated end result
Time
= exotic
= standard
Candidates?
• New particles by a
symmetry:
electron
…?
– Supersymmetry
• Relationship
between particles
with spins differing
by ½h
_
– Spatial symmetry
neutrino
quarks
x3x2
…?
Force-carriers
• With extra
dimensions
– Gauge symmetry
• Extra force
interactions (and often
…?
Already
observed
exotic
partners?
matter particles)
Related by
symmetry
What is supersymmetry?
• Nature permits
various only types of
symmetry:
– Space & time
• Lorentz transforms
• Rotations and
translations
– Gauge symmetry
• SU(3)c x SU(2)L x U(1)
– Supersymmetry
• Anti-commuting
(Fermionic) generators
• Relationship with spacetime
{Q,Q†} = -2γμPμ
• Consequences:
– Q(fermion)=boson
– Q(boson)=fermion
• Equal fermionic and
bosonic DF
– Double particle
content of theory
– Partners not yet
observed
– Must be broken!
• Otherwise we’d
have seen it
Why SUSY?
• Higgs
mass2
top
λ
λ
higgs
higgs
– Quadratic loop
corrections
Δm2(h)  Λ2cutoff
– In SM natural scale
• Enter SUSY
• Λcutoff ~ Mplanck
• v. high!
– Need m(h) near
electroweak scale
• Fine tuning
• Many orders of
magnitude
λ
stop
λ
higgs
– 2 x Stop quarks
– Factor of -1 from
Feynman rules
– Same coupling, λ
– Quadratic
corrections cancel
higgs
What does SUSY do for us?
stop
• Coupling of stop to Higgs
– RGE corrections
– Make mHH coupling
negative
– Drives electro-weak
symmetry breaking
higgs
1/α
• Predicts gauge unification
– Modifies RGE’s
– Step towards “higher
things”
+SUSY
Hit!
Log10 (μ / GeV)
higgs
(S)particles
SM
Spin-1/2
Spin-1
Spin-0
SUSY
squarks (L&R)
quarks (L&R)
sleptons (L&R)
leptons (L&R)
neutrinos (L&?) sneutrinos (L&?)

Z0
W±
gluon
h0
H0
A0
H±
Extended higgs sector
(2 doublets)
B
W0
Bino
Wino0
Wino±
gluino
~
H0
~
H±
Spin-0
After
Mixing
4 x neutralino
Spin-1/2
gluino
2 x chargino
Proton on Proton at 14 TeV
40 million bunch crossings/minute
Something to see it with
General features
Mass/GeV
• Complicated cascade
decays
– Many intermediates
• Typical signal
– Jets
• Squarks and Gluinos
– Leptons
• Sleptons and weak
gauginos
– Missing energy
• Undetected LSP
• Model dependent
“typical” SUSY spectrum
(mSUGRA)
– Various ways of
transmitting SUSY
breaking from a
hidden sector
SUSY event
Missing transverse momentum
Jets
Leptons
Heavy quarks
Cross-sections etc
“Rediscover”
Lower backgrounds
WW
ZZ
“Discover”
Higher backgrounds
Discovering SUSY with jets
SIGNAL topology
• Select a small number of high PT jets
– Large signal cross-section
– Large control statistics
– Relatively well known SM
backgrounds
• Relatively “model independent”
– Does not rely on leptonic cascades
– Does not rely on hadronic cascades
BACKGROUND
topology (QCD)
Importance of detailed
detector understanding
Et(miss)
Lesson from the Tevatron
Geant simulation showing
fake missing energy
Suppressing backgrounds
QCD
SUSY
Jet
Jet
Remove events with
missing energy back-to-back
with leading jets
Measuring Backgrounds
m
m
Measure in
Z -> μμ


Use in
Z -> νν
• Example: SUSY BG
– Missing energy + jets from
Z0 to neutrinos
– Measure in Z -> μμ
– Use for Z -> 
R: Z -> 
B: Estimated
• Good match
– Useful technique
• Statistics limited
– Go on to use W => μ to
improve
Di-jets + MET measurement
Dijet inclusive:
- No lepton veto
- No b-jet veto
- No multi-jet veto
• Keeping it simple
– >=2 jets
– ET (J1,2) > 150 GeV;
|η1,2| < 2.5
M T2 
min
p p p T
(1)
( 2)
max m p
T
j(1)
T
 
, p (1) , mT p Tj(2) , p ( 2)
Cambridge “Stransverse mass”

Discovering SUSY with leptons
Small Standard Model
Backgrounds
Golden channel @ Tevatron
• Particularly important if strongly interacting
particles are heavy
Top pair backgrounds
m
Leptons from b-decays
contribute to background
Use track isolation to
reduce these
e
Again: measure the background
Measure this background
in same-sign leptons in
semi-leptonic b-decays
signal
After 10
-1
fb
signal
“Standard” SUSY point
Very light SUSY point
• Great discovery potential here…
• Lots of other channels:
– M jets + N leptons + missing transverse energy
Reach in cMSSM?
‘Focus point’ region:
annihilation to gauge
bosons
mSUGRA A0=0,
tan(b) = 10, m>0
Slepton Coannihilation
region
Rule out
with 1fb-1
WMAP constraints
‘Bulk’ region: tchannel slepton
exchange
Mass scale?
Spectrum
SUSY
kinematic
variable
“MTGEN”
ET sum / 2
What might we then know?
• “Discovered supersymmetry?”
• Can say:
– Undetected particles produced
• missing energy
– Some particles have mass ~ 600 GeV,
with couplings similar to QCD
–
–
–
–
–
• MTGEN & cross-section
Some of the particles are coloured
Perhaps not
• jets
what we think!
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
Mapping out the new world
LHC
Measurement
SUSY
Extra
Dimensions
Masses
Breaking
mechanism
Geometry &
scale
Spins
Distinguish
from ED
Distinguish
from SUSY
Mixings,
Lifetimes
Gauge unification?
Dark matter candidate?
• Some measurements make high
demands on:
– Statistics (=> time)
– Understanding of detector
– Clever experimental technique
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
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
• 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
Try
various
decay
chains
Look for
sensitive variables
(many of them)
Extract
masses
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 scale
[O(10%) from direct measurements]
Mass scale possibly best “measured” from crosssections
– 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?
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…
Universal Extra Dimensions
• TeV-scale universal extra
dimension model
R
• Kaluza-Klein states of SM
particles
KK tower of
Radius of extra
– same QN’s as SM
masses n=0,1,…
dimension ~ TeV-1
S /Z
– mn2 ≈ m02 + n2/R2
[+ boundary terms]
• First KK level looks a lot like
– KK parity:
SUSY
1
• From P conservation in extra
dimension
• 1st KK mode pair-produced
• Lightest KK state stable, and
weakly interacting
Cheng, Matchev
hep-ph/0205314
2
• BUT same spin as SM
Dubbed “Bosonic Supersymmetry”
SPIN 2
Spin 2 particle: looks same after 180° rotation
SPIN 1
Spin 1 particle : looks same after 360° rotation
SPIN ½
Spin ½ particle : looks different after 360° rotation
indistinguishable after 720° rotation
Measuring spins of particles
• Basic recipe:
– Produce polarised particle
– Look at angular distributions in its decay
spin
θ
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
Stable
-> weakly
weakly interacting
interacting
Some sparticles omitted
Spin projection factors
q~L
qL
~20
Chiral coupling
Approximate SM particles as massless
-> okay since m « p
qL
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
-> okay since m « p
Spin projection factors
Fermion
q~L
θ*
qL
~ 0
2
Polarised
fermion
l
~lR

R
Scalar
Approximate SM particles as massless
-> okay since m « p
l
 (near)
R
p
S
Spin projection factors
0
~
qL 
1
q~L
P
mql – measure
invariant mass
S
qL
~ 0
2
l

R
~lR
Approximate SM particles as massless
-> okay since m « p
lR (near)
θ*
p
S
lnearq invariant mass (1)
Probability
quark
θ*
q~L
lepton
qL
0
~

2
Back to back
in 20 frame
Invariant mass
l
~lR

R
Phase space -> factor of sin ½θ*
Spin projection factor in |M|2:
l+q -> sin2 ½θ*
l-q -> cos2 ½θ*
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
spin-0
detector-level
Invariant mass
-> Charge asymmetry survives detector simulation
-> Same shape as parton level (but with BG and smearing)
ql- or ql+
Universal
Extra Dim.
Sin (θ*/2)
dP/dSin (θ*/2)
dP/dSin (θ*/2)
Distinguishing between models
_
ql+ or qlUniversal
Extra Dim.
Sin (θ*/2)
As expected, UED differs
from all-scalar (no-spin)
and from SUSY
Smillie et al.
What else can we do?
Measure the invisible particle mass
(WIMP mass)
Measure couplings from
rates and branching ratios
Predict WIMP relic density
Summary
• Discovering something new is an important step
– Need to understand backgrounds and detector very
well
• Finding out what we have discovered is even more
interesting!
– Masses
Spins
• These tell us about
–
–
–
–
SUSY vs Extra Dimensions
Dark Matter
Unification
SUSY breaking
Branching Ratios
Extras
How is SUSY broken?
Weak
coupling
(mediation)
• Direct breaking in
visible sector not
possible
– Would require
squarks/sleptons with
mass < mSM
– Not observed!
• Must be strongly
broken “elsewhere”
and then mediated
– Soft breaking terms
enter in visible sector
– (>100 parameters)
Soft SUSYbreaking terms
enter lagrangian
in visible sector
Various models
offer different
mediation
Strongly
broken
sector
mSUGRA – “super gravity”
• A.K.A. cMSSM
• Gravity mediated SUSY
breaking
1016 GeV Unification of couplings
– Flavour-blind (no FCNCs)
Iterate using
Renormalisation
Group
Equations
• Strong expt. limits
– Unification at high scales
• Reduce SUSY parameter
space
– Common scalar mass M0
• squarks, sleptons
– Common fermionic mass M½
• Gauginos
– Common trilinear couplings A0
• Susy equivalent of Yukawas
EW scale
Correct MZ, MW, …
Programs include
e.g. ISASUSY,
SOFTSUSY
Production Asymmetry
Twice as much squark as
anti-squark pp collider
 Good news!
Squark
Anti-squark
Note opposite shapes in distributions
Other suggestions
• Gauge mediation
– Gauge (SM) fields in extra dimensions mediate SUSY breaking
• Automatic diagonal couplings  no EWSB
– No direct gravitino mass until Mpl
• Lightest SUSY particle is gravitino
• Next-to-lightest can be long-lived (e.g. stau or neutralino)
• Anomaly mediation
– Sequestered sector (via extra dimension)
• Loop diagram in scalar part of graviton mediates SUSY breaking
• Dominates in absence of direct couplings
– Leads to SUSY breaking  RGE β-functions
• Neutral Wino LSP
• Charged Wino near-degenerate with LSP  lifetime
• Interesting track signatures
Not exhaustive!
R-Parity
General soft
breaking terms
include:
WRPV  12 (LLE LQD  UDD)  mLH
L-violating
B-violating
L-violating
u
u
_
s~
_
u
d
Λ”112
Λ’112
Pion
Proton
• Unrestricted couplings lead to proton decay:
e-
Unacceptably high rate compared
to experimental limits
(proton lifetime > 1033 years)
Strong limits on products of
couplings
• Impose RP = (-1)3B+L+2S (by hand)
– Distinguishes SM from SUSY partners
– Leads to stable LSP
• Required for dark matter
– Sparticles produced in pairs
Gauge Mediated SUSY Breaking
• Signature depends on
Next to Lightest SUSY
Particle (NLSP)
lifetime
• Interesting cases:
–
–
Non-pointing photons
Long lived staus
• Extraction of masses
possible from full
event reconstruction
• More detailed studies
in progress by both
detectors
R-hadrons
• Motivated by e.g. “split
SUSY”
– Heavy scalars
– Gluino decay through
heavy virtual squark very
suppressed
– R-parity conserved
– Gluinos long-lived
• Lots of interesting
nuclear physics in
interactions
– Charge flipping, mass
degeneracy, …
• Importance here is that
signal is very different
from standard SUSY
R-hadrons in detectors
• Signatures:
1. High energy tracks
(charged hadrons)
2. High ionisation in tracker
(slow, charged)
3. Characteristic energy
deposition in calorimeters
4. Large time-of-flight (muon
chambers)
5. Charge may flip
• Trigger:
1. Calorimeter: etsum or
etmiss
2. Time-of-flight in muon
system
GEANT simulation of
pair of R-hadrons
(gluino pair production)
– Overall high selection
efficiency
– Reach up to mass of 1.8
TeV at 30 fb-1
Method 2: Angular distributions in direct
slepton pair production
Normalised cross-sections
SUSY : qq  slepton pair
UED : qq  KK lepton pair
Phase Space :
AJB hep-ph/0511115
Sensitive variables?
AJB hep-ph/0511115
• cos θlab
– Good for linear e+ecollider
– Not boost invariant
θ1lab
θ2lab
• Missing energy means Z
boost not known @ LHC
• Not sensitive @ LHC
l1
cos θlab
l2
• cos θll*
η1lab
– 1-D function of Δη:
-1
- 12 
)  tanh(  )
1
2
Δη
– Boost invariant
– Interpretation as angle in
boosted frame
– Easier to compare with
theory
cos θ*ll
N.B. ignore azimuthal angle
η2lab
l2
Δη
θl*
θl*
l2
l1
Δη
boost
cos  cos(2 tan e
*
ll
l1
Slepton spin
AJB hep-ph/0511115
Slepton spin – LHC pt 5
• Statistically
measurable
• Relatively large
luminosity required
• Study of systematics
in progress
– SM background
determination
– SUSY BG
determination
– Experimental
systematics
“Data” = inclusive SUSY after cuts
Slepton spin
Snowmass points
AJB hep-ph/0511115
SPS1a, SPS1b, SPS5
mSUGRA “Bulk” points
Good sensitivity
SPS3 sensitive
Co-annihilation point
(stau-1 close to LSP)
Signal from left-sleptons
SPS4 – non-universal cMSSM
Larger mass LSP
Softer leptons
Signal lost in WW background
Statistical significance of spin measurement
LHC design luminosity ≈ 100 fb-1 / year
Analysis fails in “focus point”
region (SPS2). No surprise:
Sleptons > 1TeV  no xsection
Neutralino spin
SUSY vs UED:
Helicity structure
Smillie, Webber
hep-ph/0507170
See also:
Battaglia, Datta,
De Roeck,
Kong, Matchev
hep-ph/0507284
• Both prefer quark
and lepton back-toback
SUSY case
– Both favour large
(ql-) invariant mass
• Shape of
asymmetry plots
similar
UED case
Neutralino spin
• For UED masses not measureable
– Near-degenerate masses  little asymmetry
• For SUSY masses, measurable @ SPS1a
– but shape is similar
– need to measure size as well as shape of asymmetry
Smillie, Webber
hep-ph/0507170
Neutralino spin
Goto, Kawagoe, Nojiri
hep-ph/0406317
Lepton non-universality
• Lepton Yukawa’s
lead to differences
in slepton mixing
– Mixing measurable
in this decay chain
• Not easy, but there
is sensitivity at e.g.
SPS1a
– Biggest effect for
taus – but they are
the most difficult
experimentally
Neutralino spin
Range of Validity
Allanach & Mahmoudi
To appear in proceedings
Les Houches 05
• Limits:
– Decay chain
must exist
– Sparticles must
be fairly light
• Relatively small
area of validity
Decay chain
kinematically
forbidden
– ~ red + orange
areas in plot
after cuts
Spin Significance at the parton level – no cuts etc
Precise measurement of SM
backgrounds: the problem
• SM backgrounds are
not small
• There are
uncertainties in
– Cross sections
– Kinematical
distributions
– Detector response
W contribution to no-lepton BG
Oe, Okawa,
Asai
• Use visible leptons from W’s to estimate
background to no-lepton SUSY search
Normalising not necessarily
good enough
Distributions are
biased by lepton
selection 
Need to isolate individual
components…
Then possible to get it right…
Similar story for other backgrounds – control needs careful selection
Dark matter relic density consistency?
• Use LHC measurements to predict
relic density of observed LSPs
• Caveats:
– Can’t tell about lifetimes beyond
detector
• To remove mSUGRA assumption
need extra constraints:
1. All neutralino masses
mSUGRA
assumed
• Use as inputs to gaugino & higgsino
content of LSP
2. Lightest stau mass
• Is stau-coannihilation important?
3. Heavy Higgs boson mass
• Is Higgs co-annihilation important?
• More work is in progress
–
–
Probably not all achievable at LHC
ILC would help lots (if in reach)