What prospects for Supersymmetry at the Large Hadron Collider ? Some of the techniques with which...

What prospects for Supersymmetry at the Large Hadron Collider ?

Some of the techniques with which ATLAS and CMS intend

to constrain Supersymmetry

Christopher.Lester @ cern.ch

July 2005 HCP2005 : SUSY at the LHC : [email protected] 2

What are we going to cover?

• Not a results talk! (WMAP etc)• Briefly look at supersymmetry• Look at RPV / RPC distinction from

point of view of experiment• In no partiular order: look at a few

• inclusive/widely applicable experimental techniques, also

• less general but perhaps more powerful experimental techniques


Supersymmetry – Extra particles

• To stabilise the higgs mass NEED:• A scalar partner for every fermion

• squark, slepton, (stop, sbottom, selectron, smuon, sneutrino, etc)

• A fermion partner for ever boson:• gluino,• photino, wino, zino, higgsino

• (mix to form 4 neutralinos)

• Inexact symmetry – broken somehow


• Two main SUSY scenarios: (RPV/RPC)

• RP-Conserving• RP-Violating

R-Parity: Conservation/Violation

(L.S.P. = “lightest SUSY particle”)

How stable is thelightest SUSYparticle (L.S.P.) ?

Largemissingenergy?

Event can bereconstructedfully?

Sparticleproduction

RPC Stable Yes Usually not Only in pairs

RPV Unstable(decays to leptons or jets)

No Yes Either singly,or in pairs

• R=+1 for Standard Model particles• R= -1 for SUSY particles

sLBR 2)(3)1(

What do events look like?

(Baryon number violating)

RPV RPV

(Lepton number violating)

RPC RPC


So main signatures are:

•Lots of jets•Lots of leptons•Lots of missing energy (RPC)• More on these a little later

• ATLAS Trigger: ETmiss > 70 GeV, 1 jet>80 GeV. (or 4 lower energy jets). Gives 20Hz @ low luminosity.


What do we want to know?

• M.S.S.M.• Squark masses

(12)• The gluino mass (1)• Slepton masses (9)• Neutralino masses (4)• Chargino masses (2)• Spins (?)• Mixing matrices (?)• Phases (?)• ….. (plenty)

Other models:• RP-Violating M.S.S.M.

• RPV couplings (45)

• mSUGRA model• m0, m1/2, A0, tan β, sgn μ (5)

• A.M.S.B. model• m0, m3/2, tan β, sgn μ (4)

• G.M.S.B. model• λ, Mmes, N5, tan β, sgn μ, Cgrav (6)

There is no shortage of parameters which need to be determined!


What can we measure?

• The kinds of measurements which can be made, very much depend on the SUSY model which nature has chosen!

• Two very different approaches: • (1) General techniques• (2) Non general .. specific techniques

• Look at some specific RPV scenarios first

“Lots, but it depends…”


R-Parity Violation RPV

• Easier than RPC?

• The L.S.P. decays!• No missing energy, so reconstruct full event!

• Case 1: Decays into leptons:• Multi-lepton signature

• Case 2: Decays into jets:• Multi-jet signature

• Case 3: Long lifetime:• looks like RPC scenario

• Sparticles may be produced singly!

L.S.P. = lightest SUSY particle


Case 1: Lepton number violating RPV

• λ’ijk couples a slepton to two quarks

• Can have resonant sneutrino production

• Cross section can place lower bound on λ’ijk

• Expect to observe (within 3 years) either

• 900 GeV sneutrino if λ’211>0.05

• 350 GeV sneutrino if λ’211>0.01

• (present limit: )GeV) 100/( 60.0 ~211

'

RdM

λ’ijk =0.09

Reconstructed neutralino mass peak in mjjμ invariant mass distribution


Case 2: Baryon number violating RPV

• Each L.S.P. decays to three quarks (u,d,s) forming three jets (jjj)

• Require 2 leptons and at least 8 jets: (j+jjj)+(j+ll+jjj)

• Look for L.S.P. / chargino peak in mjjj / m jjjll plane

msquark L= 638 ± 5 ±12 GeV

mneutralino 2 = 212 ± 0.3 ± 4 GeV

mslepton R= 155 ± 3 ± 3 GeV

mneutralino 1 = 117 ± 3 ± 3 GeV


R-Parity Conservation RPC

• L.S.P. stable and weakly interacting, and so “goes missing”

• Missing energy signature• Usually incomplete event

reconstruction• Need to rely on long decay chains

and kinematic variables (endpoints and distributions)

• Sparticles are only produced in pairs

• Double the trouble • Missing information in BOTH halves

of event! • More general techniques available!

L.S.P. = lightest SUSY particle

Half an event


Inclusive reach in mSUGRA RPC

(Slide stolen from G.Polesello – SUSY2004)

CMS 100 fb-1 (~3 years)

Squark/gluon mass scale RPC

(GeV)effM

even

ts

Signal

S.M. Background

Peak of Meff distribution correlates well with SUSY scale “as defined above” for mSUGRA and GMSB models. (Tovey)

What you measure:


Kinematic edges: l+l- edge RPC

• EXAMPLE:

• l+l- edge

• The l+l- invariant mass from the decay chain (right) has a kinematic endpoint.

• For 100 fb-1, edge measured at 109.10±0.13(stat) GeV

• Dominant systematic error on lepton energy scale also ~0.1%

• Maximum dilepton invariant mass is related to sparticle masses


Plenty of other kinematic endpoints! RPC

Sequential

Branched

Edge p

osi

tions

Fitted distributionsll

lq high

llq Xq

lq low

llq

Xqllq

lq lowlq high

llqll


Endpoint structure …

What different invariant mass distributions look like for a selection of plausible supersymmetric models.( hep-ph/0410303 )

Note that some edges are not simple!


Coverage of edges / Problems RPC

However ... … different processes can produce the same final state.

• Can the process be identified?• Detailed study of the shape of the

distributions can provide clues

• Likely coverage?• Lepton edge observable over significant

region of m0, m1/2 parameter space (CMS plot left)

• See also hep-ph/0410303 and hep-ph/0501033 for more detailed analysis

• Likely outcome?• Precise sparticle mass differences – 1%

if lucky with which chains are open• When chains are long enough, resolution on

absolute mass scale improves and can measure mass of L.S.P.


The sort of measurement you get


Measuring spins in SUSY

• pp-collider!• Protons have more

quarks than antiquarks• So LHC will make more

squarks than antisquarks!

• Spin-1/2 neutralino can tell the difference between:1. q+l or qbar+lbar, and2. q+lbar or qbar+l

• Look for asymmetry between 1. and 2.

• Asymmetry not washed out (completely) by lepton ambiguity!

5 years’ data

500 fb-1

1.5 years data (HL)

150 fb-1

hep-ph/0405052


Non-edge RPC methods

• For LONG enough decay chains (4 or more 2-body decays) kinematics of decaying system are over-constrained by observed momenta

• So can determine masses from small sample of events

• O(N) events needed to determine N unknown masses

• “Mass Relation Method” -- proof of principle using 1000 events (hep-ph/0402295)

For LONG decay chains. At least 4

decays.

Mass relation method


Mass Relation Method Results

mneutralino (0 to 600 GeV)

mot

her (

0 to

100

0 G

eV)

Gluino

Squark

Neutralino2

SleptonVery good measurements of mass differences < 1%

Correlations still make overall mass scale hard to determine, without input from LC or say some other independent LHC technique

Reconstructed sparticle masses as function of reconstructed LSP mass


Putting it all together!

• Want to make fewer model assumptions

• Huge parameter spaces / model spaces need to be explored

• Have large number of different measurements we can make

• Need Markov Chain techniques to explore likelihood surfaces efficiently


Exploring non-linear experimental constraints upon susy model spaces


LHC & Supersymmetry

• What can the LHC provide if SUSY exists?

• DISCOVERY ? ………………………………. YES!• Excellent prospects• Might even be “easy” !• Largely model-independent

• PRECISE MEASUREMENTS ? …….... Plenty!• but more likely to be model-dependent


The End

• We can expect ATLAS and CMS to • Observe squarks and gluons

below 2.5 TeV andobserve sleptons below 300 GeV in inclusive measurements.

• Accurately measure squark, slepton and neutralino masses using cascade decays (provided chains are sufficiently long and rates are favourable)

• Determine spin of neutralinos

• Success is expected in both RPV and RPC scenarios

• Precise measurements: many can be made in principle, but which of them can measured in practice will depend strongly on the model which nature has chosen

Other areas of completed and ongoing research which there was not time to discuss:

• N.L.S.P. lifetime in G.M.S.B. models • (Non-pointing photons / slow heavy leptons) • A.M.S.B. models• Lepton flavour violation (via slepton mixing)• Measuring the gaugino mixing matrix• Direct slepton production• Non-minimal models• SUSY Higgs sector• Everything else which I have forgotten to mention ...

CMS

Missing energy – early reach RPC

(Slide stolen from G.Polesello – SUSY2004)

ATLAS


Cross sections and rates

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