SUSY Studies @Hadron Colliders

TT 2006 SUSY course T. Weidberg 1

SUSY Studies @Hadron Colliders

• MSSM and need for models e.g. SUGRA.• Limits from Tevatron. • SUSY studies at LHC:

– inclusive analysis and discovery limits.– exclusive analysis and precision measurements.– Gauge Mediated SB models– R parity violating models.– SUSY Higgs sector

• Conclusions


SUSY Models

• MSSM: – R parity conservation LSP stable – LSP is neutral from cosmology– LSP is weakly interacting (why?)– ~ 100 free parameters! unified models eg

SUGRA, GMSB.

• R Parity violating models.


Minimal SUper GRAvity

• SUSY breaking communicated through flavour-blind gravitational interactions.

• 5 Parameters assuming unified masses & couplings at GUT scale: – Scalars have mass m0, – gauginos and higgsinos m1/2, – trilinear terms A0, – ratio of vacuum expectation values of Higgs doublets β (yields

bilinear couplings and higgsino mass parameter μ2),– sign of the higgs mass term sign(μ).

• Non-minimal: > 100 parameters.• LSP is neutralino or sneutrino.


How to shape our expectation?• Predictions very dependent on SUSY models

and parameters used.• Use different Monte Carlo generators (ISAJET,

SPYTHIA).• Different approximations in the generators

require careful tuning and comparison. • Slight variations can have dramatic change in

behaviour (channels open up or close).• Typically multi-dimensional parameter space,

hard to cover everything by simulation.→ Select benchmark parameter sets (e.g. ‘ATLAS

1-5’) to allow estimate of the search capacity of future experiments.

Masses in SUGRA:

Different parameter sets



Examples of SUSY Searches at Tevatron

• Jets + Missing Et (Why missing Et?)

• Tri-leptons


Tevatron tri-leptons• Final state:

• Leptons are e, μ.• Low SM background: ‘Golden’

SUSY channel • Cuts:

– 2e: pT > 15 GeV/c– 10 < Mee < 70– MT(e,ET)>15– Track isolation– ET > 15 GeV

• D0 Run II (42pb-1): No events observed (0.0±1.4 expected).

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)(~

(~~ 01

01

021

D0


Squarks and Gluinos• Produced through SU(3)C couplings to q and g.• Due to subsequent decays signatures like neutralinos and

charginos, but with more jets. • Final states depend on exact decay channels, but again

typically involve ET and multiplicity of jets and/or leptons.• Cleanest: di-lepton (from chargino/neutralino decays),

especially same-sign (possible in gluino decays as gluino is Majorana particle).

0ig qqc®% % ' ig qq c±®% %*g tt® %% 0

ig gc®% %

D0 limits in m0/m1/2 plane for different SUGRA parameters


SUSY @ LHC

• Discovery: jets+ Missing Et• Precision studies depend on models for SUSY

breaking.– Measure combinations of masses, reconstruct mass

differences or absolute masses.

– Branching ratios.

– Lifetimes.

– Cross-sections.


QCD jets

ttbar

W+jets

Z+jets

4

1

T Misseff i TM E E

S/N > 10:1

Where does SM background come from?


SM Background

• QCD NLO calculation gives much bigger background than Pythia.

• Why?Need to measure

SM background from data.


Reach for SUSY signal at LHC

• Final states:– Jets and missing ET (0l).

– Missing ET and 1 lepton (1l).

– Opposite sign leptons (OS).– Same charge leptons (SS).– Three leptons (3l).

• Reach depends on tan and sign



Supersymmetric Decay Cascades• Heavier supersymmetric particles decay in cascades ending in

LSP.

– Neutralinos & charginos: Typically 2 body decays when kinematically allowed, otherwise 3 body decay ( ) through virtual gauge bosons or sleptons/squarks.

– Charginos (for example from ) can decay through

with an isolated lepton in the final state.

– Long decay chains → several high pT daughters.

– Spherical events.

– Gluino is Majorana fermion → can decay to either ℓ+ or ℓ-. Possibility to have same-charge decay chains on both sides.

• Simplest signatures for SUSY:

– Multiple jets (some of them hard) + missing ET.

– Several leptons + missing ET.

01Wc c± ±®

g qqc±®%

01ffc c®% %


Chargino and Neutralino Production at Hadron Colliders

• Indirectly:– Result of decay chain of heavier sparticles.

• Directly:– Through EW couplings to squarks, , W, Z.

• s-channel gauge boson production• t-channel squark exchange• interference


Precision Measurements• Measurements of sparticle masses.• .

– Select bb with mbb around h mass, add hard jet in event → mbbj, depends on mq.

• – Endpoint of dilepton (same flavour) mass spectrum:

measurement of mass difference.

• Combination allows model independent way to establish sparticle masses.

• After 1y ATLAS (10 fb-1) expect:

0 02 1c c-% %

0 02 1L Rq q q qc c± ± + -® ® ®%% % %l l l l

0 0 02 1 1Lq q hq bbqc c c® ® ®% % % %

0 02 1

3%, 6%, 9%, 12%qL lR

m m m mc c

s s s s±

» » » »% %% %

~


higgs

• hbbar + Etmiss

• Needs b-tagging.


Min M(bbj)=>m(squark)


Lepton Pairs ll0

102

End point M(ll) gives mass difference

)( 01

02 mm


Squark Masses

• End point )~( 0

1 Lqm

llqlqlqq RL02

02

~~

01( )Lm q


Search for MSSM Stop

• 3rd generation left-right mixing → stop can be light (accessible at Tevatron).

• Production rate 10% of rate for t of same mass.• • Signature: Di-lepton• Other possible stop decays:

or with decay signatures bbℓ±jjET and bbjjjjET.

~~~,~~111 bbtttpp

01 1t cc®% % 0

1 1t bWc®% %


High tanβ• For tanβ > 8 final state leptons dominated by . • Large tanβ is theoretically

motivated & favoured by LEP2.

• Tevatron standard trilepton search:

• Improved trigger and reconstruction in Run II.

• ATLAS: reconstruct m (cuts on jet shape, isolation etc.), endpoint gives ∆m. id ???

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(~~ 01

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~~~


Gauge mediated symmetry breaking (GMSB)

• Gauge interactions mediate SUSY breaking.• 6 fundamental parameters:

– Number of equivalent messenger fields N5,– scale factor for gravitino mass CGrav,– tanβ,– sign(μ),– messenger mass Mm,– Ratio of SUSY breaking scale to messenger scale Λ.

• LSP is gravitino with mass «1GeV,Unlike SUGRA

• NLSP either neutralino (small N5) or slepton (large N5).• Small tanβ: slepton masses degenerate, large tanβ: lightest

slepton.• Lifetime model-dependent (c from μm to km).

WGm m»%

Rt%


GMSB with neutralino NLSP

• Phenomenology as for SUGRA, but decay into lightest neutralino is followed by its subsequent decay yielding a photon and ET.

• – Production of pairs provides clear two- signature

(+ET).– SUSY masses can be determined from kinematics

(combine same-flavour, opposite-charge leptons → mℓℓ, then pick smaller mℓℓ, and 2 mℓ distributions give 4 endpoints to determine 3 masses.

– Decay length from Dalitz decays (2% of decays). Can be >1km for large Cgrav.

02c%

0 02 1R Gc c g± ± + - + -® ® ®% %% %l l l l l l

01

cct

%01 Ge ec + -® %%


GSMB search at Tevatron

• Signature (for long lifetime): two non-pointing + missing ET.

• Backgrounds: jets and e faking photons.

Run II:Run I:

TeVGeVm 8.78,105)~( 01

GeVm 75)~( 01

Messenger mass scale


GSMB with slepton NLSP •

– Signature contains final state leptons & missing ET.

– Dilepton mass spectrum has steps given by difference of slepton and neutralino mass.

• N5>1, Cgrav = 5×103: NLSP is stau. Decay length 1km. Low velocity quasi-stable particles resemble muons: measure TOF in μ-detector. Study

0i R Gc ® ®% %% l l l l

ATLAS

01c%

02c%

0 02 1 1Rc c t t® ® ®%% % %l l l l l l

slow


R-parity violation (RPV)

• Can be broken into 3 distinct interaction terms with strengths λ, λ’ and λ”:– λ ≠ 0: Nl violation in

– λ’ ≠ 0: Nl violation in and

– λ” ≠ 0: NB violation in

• To be consistent with proton lifetime: either lepton or baryon number violated.

• Dilutes ET signature but λ and λ’ give multi-jet, multi-lepton events, which are easy to isolate.

• Strategy: completely reconstruct LSP decay.

01c n+ -®% l l

01 qqc n®%

01 qqc ®% l

01 qqqc ®%


SM bounds on RPV opeators

• Charged-current universality

(→e)/(→e)• Bound on the mass of e

• Neutrino-less double-beta decay

• Atomic parity violation• D0-D0 mixing• Rℓ→ had(Z0)/ ℓ(Z0)

• (→e)/( →μ)

• Br(D+→K0*μ+μ)/ Br(D+→K0*e+e)

• μ deep-inelastic scattering

• Br(→ )

• Heavy nucleon decay• n - n oscillations

All in remarkable agreement with SM predictions.


RPV with λ ≠ 0

• • >4ℓ Signature easy to detect. • Mass of neutralino from

dilepton mass spectrum end point (LHC: σm ≈ 180MeV).

• Combining candidates at edge with events in h→bb peak allow reconstruction of (LHC: σm ≈ 5GeV).

Wrong combinations

Correct combinations

End point

01c n + -®% l l

02c%


RPV with λ’ ≠ 0•

– Fully reconstructable with dilepton signature.

• – More diffcult.

– Missing ET is less than in SUGRA.

– Rely on additional leptons from cascade decays and large jet multiplicity.

•

• di-gluinos produce like-sign fermions in 1/8 of time (+2j) CDF Run I: no events

01 qqc ®% l

01 qqc n®%

, ,

,L L R

e e

g c c s s d d

d ec sn n

®¯ ¯ ¯

%% % %

l l


Baryon number violating RPV• Most challenging, as decays like provide no

signatures like missing ET, or special lepton or quark flavour (b) tags.

• Look for dilepton signature from

• Signature: minimum 6 jets (3 jets from other neutralino) + 2 leptons, typically around 12 jets (from cascade involving squarks and gluinos).

• Then: combine triplets of jets and require two combinations/event within 20 GeV. Then combine with leptons and reconstruct decay fully.

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0 02 1

3R

jets

l l l lc c + -® ®]

%% %


3 jet Mass Reconstruction


An indirect evidence for SUSY: H±

• If light enough: produced in t → bH+

• For small tanβ: H+ → cs, large tanβ: H+ →

• CDF:– Direct search: excess over SM of events with leptons– Indirect search: ‘dissappearance’, depletion of SM decay t →

bW (less di-lepton and ℓ+j).


SUSY Higgs

• MSSM depends on tan and m(A).

• Many decay modes important e.g.– h – A enhanced cf SM). mass

reconstruction?


h

Q1: What is the reducible background?

Q2: What is the irreducible background?


A



Summary of current SUSY analyses


Conclusions

• Interesting existing limits from Tevatron• Run II @ Tevatron has chance to discover

SUSY.• LHC will allow for discovery and precision

SUSY PHYSICS.– Discovery or exclusion of low energy SUSY.

– Precision measurements of masses, cross-sections and branching ratios.

– Tests of unified theories (e.g. SUGRA).

Documents

SUSY Studies @Hadron Colliders