Early searches for supersymmetry at the LHC in the all-hadronic - - PowerPoint PPT Presentation

Early searches for supersymmetry at the LHC in the all-hadronic channel

Tom Whyntie Imperial College London / CMS experiment

Tom Whyntie, Imperial College London 25th March 2010 2 of 11

Introduction

How do we look for supersymmetry at the LHC?

• What are the challenges of the all-hadronic channel?

How can we guard against mismeasurement?

• Can we use kinematics to constrain fake missing ET

?

How do we account for Standard Model backgrounds?

• What data-driven tools exist to estimate real missing ET

?

Tom Whyntie, Imperial College London 25th March 2010 3 of 11

Introduction

How do we look for supersymmetry at the LHC?

• What are the challenges of the all-hadronic channel?

How can we guard against mismeasurement?

• Can we use kinematics to constrain fake missing ET

?

How do we account for Standard Model backgrounds?

• What data-driven tools exist to estimate real missing ET

?

Tom Whyntie, Imperial College London 25th March 2010 4 of 11

Supersymmetry at the LHC

Supersymmetry is an extension to the Standard Model (SM) that predicts massive, undetectable superpartners to SM particles. These may be produced in LHC proton-proton collisions. Typical experimental signature:

• Large missing transverse energy plus final state objects.

q q q LSP LSP q q q

+ similar

Tom Whyntie, Imperial College London 25th March 2010 5 of 11

The all-hadronic channel

We consider events with only jets in the final state:

• “Jet”: clustered energy deposits in the calorimeters from

the hadronisation of partons.

• Tracking information may also be used for identification

and/or correction of measured jet energy.

Advantage: no isolated leptons, which can indicate SM processes with real missing ET (typically featuring W

l)

• Still leaves Z , W (the

decaying hadronically)

Disadvantage: Large QCD background. Statistically unlikely detector mismeasurements, that produce fake missing ET , start to overwhelm any non-SM signal.

Tom Whyntie, Imperial College London 25th March 2010 6 of 11

Allowing for mismeasurement

QCD-like events will generally conserve transverse momentum. Mismeasurement leads to the observation of “fake” missing ET . The search described here takes the following approach:

• Use only “trusted” physics objects as input to the missing ET

calculations, ignoring unclustered energy and events with “anomalies”.

• Use event observables that compensate for object mismeasurements.

Tom Whyntie, Imperial College London 25th March 2010 7 of 11

The T observable

• 2

2 2 1

cos 1 2

T T j T j T T

H E E M p

• 2

1 j T j T T

E E H

• 2

1 j T j T T

E E H

• Dijets: Randall (2008) proposed

= E j2

T / M(dijet). We use T = E j2 T / MT (dijet):

• T

T j T

H H E

• 2

1 2

• 2

2 ) (

. min 2 1

T T T T N T

H H H p

• Denominator:

Numerator: Conserved event, perfect measurement T = 1/2 Real missing ET : Small denom. large T Mismeasurement: Small num. small T Extension to n-jets: Generalise HT

Tom Whyntie, Imperial College London 25th March 2010 8 of 11

The CMS experiment

Solenoid: 3.8 T magnetic field. Energy measurements:

• Electromagnetic calorimeter (ECAL);
• Hadronic calorimeter (HCAL).

Tracking: All-silicon tracker. Muon chambers outside solenoid.

Tom Whyntie, Imperial College London 25th March 2010 9 of 11

Event selection

Triggers:

• High-Level Trigger (HLT) Single 110 GeV jet.

Pre-selection:

• Jet requirements (also defines event jet multiplicity, N):

» At least two jets with pT > 50 GeV, || < 3.0, EM fraction < 0.9; » Leading jet pT > 100 GeV, || < 2.0; » Second jet pT > 100 GeV;

• Lepton veto: Reject events with isolated e or , pT > 10 GeV;
• Photon veto: Reject events with isolated , pT > 25 GeV;
• “Bad” jet veto: Reject events with pT > 50 GeV jets that fail

||, EM fraction requirements.

Tom Whyntie, Imperial College London 25th March 2010 10 of 11

Applying the kinematic cuts

HT of events passing selection

N = 2 N = 3,4,5,6

Additional cut made on HT > 350 GeV Final cut made on T > 0.55 QCD is heavily suppressed

10 TeV, 100 pb-1

Tom Whyntie, Imperial College London 25th March 2010 11 of 11

Estimating SM contributions

1) Treat the backgrounds as one, exploiting non-SM signal centrality. 2) Estimate individual SM background contributions, e.g. Z + jets Z is statistics limited; use W+jets, +jets. tt + jet(s), W + jet(s), etc. Replace W with W in data using template.

With LM0 signal SM backgrounds only

Tom Whyntie, Imperial College London 25th March 2010 12 of 11

Conclusions

We can look for SUSY in the all-hadronic channel:

• Signature: Large missing ET + final state jets.

Non-SUSY backgrounds can be controlled with kinematics:

• Mismeasured QCD events are the dominant SM background;
• Compensating observables, e.g. T , can suppress these.

Tools exist for estimating real missing ET SM backgrounds. Thanks for listening – any questions?