Transcript News from ATLAS physics studies

Can we tell SUSY
(from a hole in the
ground)?
Alan Barr
UCL
YETI06 11 Jan 2006
Scenario:
• End of first year of data taking
– After careful calibration …
– ATLAS observes excess of events with:
• Missing transverse energy
• Leptons
• Jets
– Produces letter
• Definitely new physics!
• But what sort of new physics?!?
2006: New year’s resolutions:
1. Give up assumptions
2. Exercise more caution
11 Jan 2005
Alan Barr, UCL
2
What will we find at the LHC
• We expect to find:
– All the Standard
Model particles
Susy in the lead …
• W, Z, top, …
– Including a Higgs
boson
• We hope to also
find:
– Explanation for the
scale of the Higgs
mass
… but not a one-horse race.
• Supersymmetry?
• Extra gauge groups?
• Extra dimensions?
11 Jan 2005
Alan Barr, UCL
3
What is supersymmetry?
• Nature permits only • Consequences:
various types of
– Q(fermion)=boson
symmetry:
– Q(boson)=fermion
– Space & time
• Lorentz transforms
• Rotations and
translations
– Gauge symmetry
• e.g. SM: SU(3)c x
SU(2)L x U(1)
– Supersymmetry
• Anti-commuting
(Fermionic)
generators
• Relationship with
space-time
• Equal fermionic and
bosonic DF
– Double particle
content of theory!
– Partners not yet
observed
– Must be broken!
• Otherwise we’d have
seen it!
– {Q,Q†} = -2γμPμ
11 Jan 2005
Alan Barr, UCL
4
(S)particles
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
(2
11 doublets)
Jan 2005
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
Alan Barr, UCL
5
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
– Various ways of
transmitting SUSY
breaking from a
hidden sector
“typical” susy spectrum
(mSUGRA)
11 Jan 2005
LHC Pt5
Alan Barr, UCL
6
Constraining masses
• Mass constraints
• Invariant masses in pairs
Frequentlystudied
decay chain
– Missing energy
– Kinematic edges
Observable:
Depends on:
Limits depend on
angles between
Extractsparticle
information
decays
about masses from
edges in kinematical
distributions
Di-lepton invariant mass
11 Jan 2005
Alan Barr, UCL
7
What particles are we seeing?
•
End points tell us
something about
masses but not:
1.
2.
Are these
supersymmetric
particles?
If so, which particles
are participating?
•
•
3.
What sort of decays
were involved?
•
11 Jan 2005
Which neutralino?
Left or right
slepton?
Two or three body?
•
Ambiguities in SUSY
parameters from (2)
and (3)
Alan Barr, UCL
Lester, Parker, White
hep-ph/0508143 8
What particles are we seeing?
•
End points tell us
something about
masses but not:
1.
2.
3.
11 Jan 2005
Are these
supersymmetric
particles?
If so, which particles
are participating?
•
•
Which neutralino?
Left or right slepton?
•
Two or three body?
What sort of decays
were involved?
• SUSY predicts
particles with
particular
properties:
– Couplings are SAME
as SM partners
– Spins differ by ½
– Unbroken SUSY
masses equal to SM
partners
• SUSY breaking terms
dominate masses
Does anything else look like SUSY?
Alan Barr, UCL
Lester, Parker, White
hep-ph/0508143 9
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
11hep-ph/0205314
Jan 2005
2
• BUT same spin as SM
Dubbed “Bosonic Supersymmetry”
Alan Barr, UCL
10
Cheng, Matchev
UED level-1 spectrum
hep-ph/0205314
1st excited KK level
(example masses)
After adding boundary terms
-> SUSY-like spectrum
11 Jan 2005
Example decay
spectrum
Stable lightest
KK particle
(weakly interacting)
Alan Barr, UCL
11
Similarities and differences
SUSY
UED
One extra particle for each
SM particle
Tower of extra particles for
each SM particle
Spins differ by half unit
Same spins
Same couplings
Same couplings
R-parity  particles produced KK parity  n=1 level
in pairs
particles produced in pairs
Weakly interacting Lightest
SUSY particle
Weakly interacting Lightest
KK particle
Can have good WIMP relic
density
Can have good WIMP relic
density
Mass spectrum depends on
method of SUSY breaking
Typically rather degenerate
mass spectrum
11 Jan 2005
Alan Barr, UCL
12
2nd
KK mode?
Datta, Kong, Matchev
hep-ph/0509246
• If 2nd KK mode light enough we can see it at LHC
– Can even separate 2 and Z2
– Good evidence in favour of UED
• Problems:
– May not be seen
– Could have other interpretations (new gauge bosons?)
11 Jan 2005
Alan Barr, UCL
13
Similarities and differences
SUSY
UED
One extra particle for each
SM particle
Tower of extra particles for
each SM particle
Spins differ by half unit
Same spins
Same couplings
Same couplings
R-parity  particles produced KK parity  n=1 level
in pairs
particles produced in pairs
Weakly interacting Lightest
SUSY particle
Weakly interacting Lightest
KK particle
Can have good WIMP relic
density
Can have good WIMP relic
density
Mass spectrum depends on
method of SUSY breaking
Typically rather degenerate
mass spectrum
11 Jan 2005
Alan Barr, UCL
14
SPIN 2
Spin 2 particle : looks same after 180° rotation
11 Jan 2005
Alan Barr, UCL
15
SPIN 1
Spin 1 particle : looks same after 360° rotation
11 Jan 2005
Alan Barr, UCL
16
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
Two methods for measuring SUSY particle spins @LHC so far…
11 Jan 2005
Alan Barr, UCL
18
AJB hep-ph/0405052
Method 1 : 20 spin
• Use particular decay
chain (right)
• Make use of chiral
couplings of Wino
• Consider invariant
mass of quark with
lepton
Measure Angle
qL
q~ L
0
~

2
Mostly Wino:
Polarised
l

R
~
lR
~0

l

R
1
11 Jan 2005
Alan Barr, UCL
19
Spin projection factors
qL
q~ L
qL 
0
1
P
S
~ 2
0
Chiral coupling
Approximate SM particles as massless
->
since m « p
11 okay
Jan 2005
Alan Barr, UCL
20
Spin projection factors
0
~
qL 
1
qL
q~ L
~ 2
0
Spin-0
P
S
Σ=0
1
0
~
2 
0
S
Produces polarised
neutralino
Approximate SM particles as massless
->
since m « p
11 okay
Jan 2005
Alan Barr, UCL
21
Spin projection factors
Fermion
θ*
qL
q~ L
0
~
2
Polarised
fermion
l
l
~
lR

R
 (near)
R
p
S
Scalar
Approximate SM particles as massless
->
since m « p
11 okay
Jan 2005
Alan Barr, UCL
22
Spin projection factors
0
~
qL 
1
P
mql – measure
invariant mass
S
qL
q~ L
0
~
2
l

R
 (near)
lR
~
lR
θ*
p
S
Approximate SM particles as massless
->
since m « p
11 okay
Jan 2005
Alan Barr, UCL
23
invariant mass (1)
quark
θ*
lepton
qL
q~ L
0
~

2
Invariant mass
l
~
lR

R
l+
Phase space
l-
Phase space -> factor of sin ½θ*
Spin projection factor in |M|2:
l+q -> sin2 ½θ*
l-q -> cos2 ½θ*
11 Jan 2005
Back to back
in 20 frame
Probability
near
l q
Alan Barr, UCL
m/mmax = sin ½θ*
24
Experimental issues?
Measure Inv Mass
qL
q~ L
~
l
0
2
– Cant – becomes a
combinatorial
background

R
~
lR
0
~

l
1
A


 ql   ql

 ql   ql

11 Jan 2005
• How to distinguish
between leptons?



R
• What about antisquarks?
– Asymmetry is
reversed
– Effect would cancel
– However at pp
collider more squark
than anti-squark
produced!
Alan Barr, UCL
25
AJB hep-ph/0405052
Charge asymmetry
• Demonstration
that spin
determination is
possible @ LHC
Charge asymmetry,
SUSY “data”
spin-0
• Encouraging… but
ATLFAST-level
After cuts, detector simulation etc…
11 Jan 2005
Alan Barr, UCL
– Relies on presence
of particular chain
– Not a general
technique.
– UED has similar
shape for
asymmetry…
26
Neutralino spin
SUSY case
UED case
11 Jan 2005
SUSY vs UED:
Helicity structure
Smillie, Webber
hep-ph/0507170
Battaglia, Datta,
De Roeck,
Kong, Matchev
hep-ph/0507284
• Both prefer quark
and lepton backto-back
– Both favour large
(ql-) invariant mass
• Shape of
asymmetry plots
similar
Alan Barr, UCL
27
Neutralino spin
Smillie, Webber
hep-ph/0507170
• 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
11 Jan 2005
Alan Barr, UCL
28
Neutralino spin
Range of Validity
Allanach & Mahmoudi
To appear in proceedings
Les Houches 05
• Relatively
small area of
validity
– ~ red +
orange areas
in plot after
cuts
11 Jan 2005
LEP excluded
– Decay chain
must exist
– Sparticles
must be
fairly light
Universal SUSY scalar mass, m0
• Limits:
Sleptons too
heavy
Universal SUSY fermion mass, m½
Alan Barr, UCL
29
Summary of spin method 1
• Sensitive to neutralino-2 spin
• Workable in some regions of parameter space
– But those regions are not very large
• Can give slepton mixing information
– Sensitive to more than just spin
• Works best when sparticles non-degenerate
(SUSY-like)
– Not workable when masses are near-degenerate
(UED-like)
• Similar shape for UED and SUSY
– Size of asymmetry must be experimentally
measured, not simply shape
11 Jan 2005
Alan Barr, UCL
30
Method 2: Angular distributions in direct
slepton pair production
Slepton
θ
Beam
Beam
Slepton
Incoming (anti-)quark spins?
+
or
+
Z/ spin?
• Z/ is polarised
along the ±z axis
• For slepton
production
– Measure m=0 at
angle θ from
direction where l=1,
m=±1
• Average over initial
states
• Sum over final states
or
Final state spin?
No component along
slepton axis (m=0)
11 Jan 2005
– P ~ 1 – cos2 θ
– Favours θ ~ 90°
Alan Barr, UCL
31
KK lepton pair production
KK lepton
Relativistic limit:
θ
Beam
or
Beam
m=±1
KK lepton
Incoming (anti-)quark spins?
+
or
With KK-lepton masses:
+
or
Z/ spin?
or
Final state spin?
…
11 Jan 2005
m=0
 E2  m2 
2

P  1   2
cos

2 
E m 
Alan Barr, UCL
32
Angular distributions in direct slepton
pair production
Normalised angular distributions
SUSY : qq  slepton pair :
“Perpendicular”
UED : qq  KK lepton pair :
Phase Space :
“Parallel”
“Flat”
Slepton
Beam
Slepton
11 Jan 2005
Alan Barr, UCL
θ
Beam
33
What we see experimentally
Invisible particle
LSP or LKP
Beam
θ
+ve lepton
Slepton or KK lepton
(very short-lived)
Beam
Invisible particle
LSP or LKP
-ve lepton
Leptons “inherit” some knowledge of θ
from their slepton or KK lepton mother
11 Jan 2005
Alan Barr, UCL
Observables:
• Pair of opposite sign,
same family leptons
• Sum of PT of invisibles:
PTMISS
34
– 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:
cos   cos( 2 tan
*
ll
1
e
 12  
)  tanh(
l1
1
2
 )
– Boost invariant
– Interpretation as angle in
boosted frame
– Easier to compare with
theory
η2lab
Δη
cos θ*ll
l2
Δη
θl*
θl*
l2
l1
Δη
boost
• cos θlab
How to measure it?
N.B. ignore azimuthal angle
11 Jan 2005
Alan Barr, UCL
35
slepton  lepton correlations
Observable angle
Production angle
• Slepton/KK lepton production angle not measurable
• Lepton inherits from boost of slepton parent
– Good correlation in plots above
• Observable cos θ*ll smaller for SUSY than UED
11 Jan 2005
Alan Barr, UCL
36
Cuts and acceptance
• Need to beat down
SM backgrounds
• l+l- + missing energy
signature
• Z0Z0 background
– Cut on l+l- invariant
mass (not near Z)
• W+W- background
– Use mT2 variable to
reduce this
11 Jan 2005
Pseudorapidity of leptons
Detector acceptance: |η| < 2.5
Alan Barr, UCL
37
Slepton spin – LHC pt 5
• Statistically
measurable
• Relatively large
luminosity required
• Study of
systematics in
progress
– SM background
determination
– SUSY BG
determination
– Experimental
systematics
• No show-stoppers
so far
11 Jan 2005
“Data” = inclusive SUSY after cuts
Alan Barr, UCL
38
AJB hep-ph/0511115
SPS1a
SPS1b
Similar results
at various SPS
benchmark
points
200-300 fb-1
(2-3 years at
design lumi)
SPS3
SPS5
Includes stat
error from SM
and SUSY BG
subtraction
No systematic
uncertainty in
backgrounds
11 Jan 2005
Alan Barr, UCL
39
Required luminosity?
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
11 Jan 2005
Analysis fails in “focus point”
region (SPS2). No surprise:
Sleptons > 1TeV  no xsection
Alan Barr, UCL
40
Summary of spin method 2
•
•
•
–
Sensitive to slepton spin
A more general method than method 1
Works at various SPS points
Sensitive when both:
1.
sleptons are light
1.
 reasonable x-sec
2. slepton-LSP mass difference is > mW
•
(for either slepton)  separate from WW
–
Possible extensions
Clean environment for measuring slepton
pair production cross-section
•
11 Jan 2005
Very useful constraint esp. if mass scale can be
independently measured
Alan Barr, UCL
41
So what can we tell from a hole in
the ground?
LHC Observation
Importance for (SUSY) Phenomenology
New particles
“Death” of the Standard Model
Spin
SUSY or not SUSY?
Kinematical end points
Masses  How is SUSY broken?
Missing energy
Possible dark matter candidate
Branching ratios
Detailed mass studies
Is LSP the major contribution to dark
matter? Etc…
• Extra excavations can extract:
– Direct dark matter searches
• Cosmologically stable LSP?
– ILC measurements (when available)
• Detailed properties of SUSY breaking, masses etc
11 Jan 2005
Alan Barr, UCL
42
Questions?
11 Jan 2005
Alan Barr, UCL
43
Paper trail
•Phys.Rev. D64 (2001) 035002 Bounds On Universal Extra Dimensions
•hep-ph/0110108 Spin Correlations In Monte Carlo Simulations
•hep-ph/0205314 Bosonic Supersymmetry? Getting Fooled At The LHC
•Phys.Lett. B596 (2004) 205 Determining the Spin of Supersymmetric
Particles at the LHC Using Lepton Charge Asymmetry
•hep-ph/0406317 Study of the slepton non-universality at the CERN
Large Hadronic Collider
•hep-ph/0502031 Probing Universal Extra Dimension at the
International Linear Collider
•JHEP 0507 (2005) 033 Contrasting Supersymmetry and
Universal Extra Dimensions at the CLIC Multi-TeV e+ e- Collider
•hep-ph/0507170 Distinguishing Spins In Supersymmetric and
Universal Extra Dimension Models at the Large Hadron Collider
•hep-ph/0509246 Discrimination of Supersymmetry and
Universal Extra Dimensions at Hadron Colliders
•hep-ph/0511115 Measuring Slepton Spin at the LHC
11 Jan 2005
Alan Barr, UCL
44
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
11 Jan 2005
Alan Barr, UCL
45