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Discovering New Physics with the LHC

Nadia DavidsonSupervisor: Elisabetta Barberio

EPP

Nobel Prize for Physics in 2010:

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The current model of particle physics – The Standard Model

Fermions Bosons Still Undiscovered

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Finding the Higgs Boson• Higgs mass < 1 TeV for theory

to work.

• The Higgs mass can be constrained by precision electroweak data and direct searches.

• Latest constraints put the favored higgs mass below 182 GeV to 95% confidence level.

• So either the Higgs will be seen at the LHC or something is very wrong with the theory.

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What about Dark Matter?• Evidence

Compelling:– Galactic Rotation

Curves– Gravitational

Lensing

65%

30%

0.5% 4%0.03%

0.03%

Dark energyDark matterFree H and HeStarsNeutrinosHeavy elements

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What about Baryogenesis?• An asymmetry in the baryons

and anti-baryons in the very early universe leads to the matter dominated universe we see today.

• The three Sakharov conditions:– Baryon number violation– C and CP violation– Departure from thermodynamic

equilibrium

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What about neutrino mass?• Neutrinos shown to oscillate from one lepton flavour

into another. (1998 SuperKamiokande)

• Possible if mass eigenstates are not the same as flavour eigenstates.

• Implies neutrinos have a mass.

• No natural mechanism for this in the Standard Model.

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Why don’t the gauge couplings unify at the GUT scale

• If the three forces are manifestations of the same force, their strength should meet when extrapolated to the GUT scale.

• Almost meet at 1016GeV.

• Introduce new physics at the TeV energy scale such as sypersymmetry and they can meet.

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Why is the Higgs mass finely tuned?

• Loop corrections to the mass diverge quadratically.

• Introduce a cut-off scale for new physics.• Corrections are of the order of this scale.

t

t

H H

H H

, , ,H W Z

2 2 20H Hm m m

cut-off2 2

cut-off2 4 2cut-off22 2

0 0

... (...)fH

f

k mm d k

k m

where:

offcutH cmGeVOm 220

2 )100(

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Why is the Higgs mass fine tuned?

• Take the scale of new physics as the GUT scale O(1016GeV)

• Implies that m0 must be of a similar order and cancel to around 15 decimal places in order to give mH a mass O(100GeV).

• An indication of new physics at the TeV energy scale?

offcutH cmGeVOm 220

2 )100(

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Solution: Supersymmetry• Higgs Fine Tuning: Introduce a new symmetry

between fermions and bosons such that contributions from new particles cancel out the loop contributions to the higgs mass.

• Dark Matter: A new conservation law is introduced to prevent proton decay. One consequence is stable lightest supersymmetric particle.

• Gauge Couplings: Can unify at GUT scale.

• Baryogenisis: Some models of supersymmetry allow electroweak baryogenisis if stop lighter than top.

bos

ons

ferm

ion

s

bos

ons

ferm

ion

s

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Supersymmetry

• Not so great:– Supersymmetry has not been observed.– So the new particles must have a mass

much larger than their Standard Model partners.

– The symmetry is broken.– The mechanism for this symmetry

breaking differs from model to model.

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Supersymmetry• Over 100 new free

parameters. Can reduce these to just a few for example m0, m1/2, μ, tan(β).

• Parameter space is limited by:– WMAP constraint on relic

density– Excluded by g-2– Excluded by b →sγ– Excluded because stau is

lightest supersymmetric particle

RG evolution of unified mSUGRA mass parameters

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Solution: Extra Dimensions• Can also solve the higgs mass troubles

by bringing the plank scale down to O(TeV).

• Limit from gravitational tests: R ~ 10–1 cm gravity only tested to R–1 ~ 2 10–4 eV.

• Introduce compactified extra dimensions. If we live on a brane in a subset of space gravity looks weak to use, but could really be strong.

• Particles propagating in the extra dimensions have Kaluza-Klein excitations. Mass difference = 1/R

© Scientific American

• Large Extra Dimensions: only gravitons in the bulk

• Universal Extra Dimensions: all particles in the bulk

• Warped Extra Dimensions: Eg. Randall-Sundrum.

Bulk (y)

Te

V

Pla

nk

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• Large Hadron Collider (LHC)– Proton-proton collider– Collisions at 900 GeV end of this year– Collisions at 14 TeV middle of next year

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How can new physics be found and measured?• New particles are heavy and generally

unstable, decaying quickly into observable Standard Model particles.

• The types of particles, their energies and direction depend on the original particle.

• This can be used to separate a signal from Standard Model background.

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What is the potential of the LHC to see new physics

• Signatures of supersymmetry will be:– A large number of high energy jets– A large amount of missing energy

• Could be seen very soon after data taking begins.

• If supersymmetry is not seen with the LHC we can rule out most of this parameter space.

Parameter space

A few days of data collection

A month of data collectionOne year ofdata collection

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How can new physics be measured.

• Decay products properties can be used to reconstruct the original particles mass and spin.

• Example: Dilepton resonances

• Resonance with mass =1.5TeV• With Standard Model

Background (Red)

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Dilepton resonances continued

• Such a signal could come from a variety of scenarios of new physics including:– Extra gauge bosons

• From GUT theories

• From Kaluza-Klein excitations

– Gravitons in Randall-Sundrum model

• Measure spin with angular distributions of decay products.

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What about new physics with dark matter particles?

• Stable non-interacting particles can be found in:– Supersymmetry (R-Parity conservation).

– Universal Extra Dimensions (KK- Parity conservation).

• Escape detection

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Example: Supersymmetric Decay Chain

Lq~ Lq

02

~

Rl

Rl

Rl

~

01

~

Jet seen in detector

Positron or positive muon seen in detector

Electron or muon seen in detector

Nothing seen by detector

Makes mass and spin reconstructionmore difficult!

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Measuring Mass• Masses are confined by the extremities

or (end-points) of the invariant mass distribution

Lq~ Lq

02

~

Rl

Rl

Rl~

01

~

RlRlRlmmmmmm

ll~0

1~~~0

2~

22222max

2 /))(()(

• Can similarly measure– ml+q max

– ml-q max

– mllq max

• 4 equations and 4 unknown masses

• Solve

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• This method is independent of the type of new physics seen.

• Universal Extra Dimensions may look identical

• Need to know the spins also to distinguish different models

Lq~ Lq

02

~

Rl

Rl

Rl

~

01

~

1Qq

1Z

ll

1l

1

1/2

1/2

1/2

1/2

1/2

0

0

1/2

1/2

1/2

1

1

1/2

1/2

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Measuring Spin - Method by A. Barr hep-ph/0405052

• Recall from the dilepton example: The number of events vs. cos(θ*) depends on the spins of the particles involved in the decay.

• But we don’t know the direction of weakly interacting particle.

• But, the invariant mass distribution depends on cos(θ*) because it can be written as:

*)cos1(max22 mm

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Measuring Spin• m is a function of cosθ*

therefore it is also depends on the spins.

• Looking at the supersymmetric cascade decay again we can use the invariant mass, ml

nearq, to

measure the spin of Rescaled massR

esca

led

num

ber

of e

vent

s

ql nearm

ql nearmNo spin

correlation

Lq~ Lq

02

~)(nearlR

)( farlR

Rl

~

01

~

02

~

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Measuring Spin• Experiments are never so simple:

– There is no way of telling if a jet has come from a quark or anti q!

– It is not clear which is the near and which is the far lepton.

• Still doable:– Asymmetry of squarks anti-squark due to valence quarks in

original collision. This means slightly more quarks.– Far leptons have smaller spin correlation. The far leptons

will smear the results of the near, but not destroy them.

Lq~ Lq

02

~)(nearlR

)( farlR

Rl

~

01

~

Lq~ Lq

02

~)(nearlR

)( farlR

Rl

~

01

~

Lq~ Lq

02

~)(nearlR

)( farlR

Rl

~

01

~

+

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Measuring SpinN

umbe

r of

eve

nts

ll

llA

Cha

rge

asym

met

ryShape indicatesspin 1/2

02

~

• Possible to use this charge asymmetry for other combinations of spins and rule in (out) type of new physics (C. Athanasiou et al. hep-ph/0605286)

Shape indicatesspin 0

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Conclusions• There is theoretical and experimental motivation to

expect new physics.

• Much of this will be accessible with the ATLAS and CMS detectors at the LHC.

• It may be possible to measure the mass and spin of new particles.– Even when one of the decay products is unobservable.

• The battle to be the first to find new physics begins soon…

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CMS ATLAS