supersymmetry phenomenology using hadron colliders

TT 2004 SUSY, Phenomenology using Hadron Colliders 1

SupersymmetryPhenomenology using Hadron

Colliders


Where to start looking?

• Many different SUSY models available.• Differ in symmetry breaking mechanisms. • R-parity

– Consequences:• SUSY particles produced in pairs.• Lightest SUSY particle (LSP) must be neutral

and colourless (from cosmological constraints).• LSP is stable.

– No theoretical argument requires R-parity.– Some models conserve, others violate R-parity.

• Concentrate on simplest ones, try to find signatures which are universal.

• But: limits usually only valid in context of specific model.

large missing ET


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.

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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


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.


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


How to establish signal?

• ATLAS: First step: Take four hardest jets and calculate

+ cuts on minimum ET and pT, no isolated leptons.

• 5-10 larger than SM.• Gives first indication of SUSY

mass scale (where SUSY dataexceeds SM backgrounds).

• Simple.

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SM backgrounds

SUSY signal

ttW→ℓ

Z→,QCD jets


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:

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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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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. or or or• 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).

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ig gc®% %

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


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.

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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 jetshape, isolation etc.), endpoint gives ∆m.

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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).


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, where ).

• 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).

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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.

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GSMB search at Tevatron

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

• Backgrounds: jets and e faking photons.

Run II:Run I:

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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

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ATLAS

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Anomaly mediated Supersymmetry (AMSB)

• Conformal anomaly in the auxiliary field of the supergravity multiplet transmits SUSY breaking.

• NLSB ( ) only marginally heavier than → large ET + soft tracks.

• Parameters: m3/2 (gravitino m), m0 (scalar m), tanβ & sign(μ).

• 3 ranges:–

long lived (c≥1m) → track in μ detector, similar to GMSB.–

isolated high pT tracks, which stop in tracker.–

most difficult (short c and soft SM particle, which is hard to distinguish from background).

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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.

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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

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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

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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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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).


CDF eeET event

• One event with SUSY-like signature.

• Probability that SM produced many orders < 1, but very difficult to determine ‘a posteriori’.

• Possibly: or with subsequent decay to and

• But: If model parameters get adjusted to explain event, then very large rate of multijet + multilepton + photon(s) expected.

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Summary of current SUSY analyses


Summary

• SUSY completely uncharted territory.• Still try to find signatures which we understand well, and

which are untypical for SM– Large ET.– High jet multiplicities.– (Like-sign) multi-leptons.

• All limits are highly model-dependent. Comparison of results difficult.

• Almost all the searches introduced in this lecture establish SUSY signal through mass measurements. Beyond these measurements of branching rations, spins etc. will be needed.

• Search so far unsuccessful, but so far luminosity limited. Just entering interesting regions (Tevatron Run II, LHC).

supersymmetry phenomenology using hadron colliders

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