The LHC & the Future of Particle Physics

TeV Particle Astrophysics· Venice· 29 August 2007

Chris QuiggFermi National Accelerator Laboratory

A Decade of Discovery Past

! Electroweak theory → law of nature [Z, e+e−, p̄p, !N , (g − 2)µ, . . . ]

! Higgs-boson influence observed in the vacuum [EW experiments]

! Neutrino flavor oscillations: !µ → !τ , !e → !µ/!τ [!", !atm]

! Understanding QCD [heavy flavor, Z0, p̄p, !N , ep, lattice]

! Discovery of top quark [p̄p]

! Direct CP violation in K → "" decay [fixed-target]

! B-meson decays violate CP [e+e− → BB̄]

! Flat universe dominated by dark matter & energy [SN Ia, CMB, LSS]

! Detection of !τ interactions [fixed-target]

! Quarks & leptons structureless at TeV scale [mainly colliders]

2

Our Picture of Matter (the revolution just past)

Pointlike (r ≤ 10!18 m) quarks and leptons

Interactions: SU(3)c ⊗ SU(2)L ⊗ U(1)Y gauge symmetries

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The World’s Most Powerful Microscopesnanonanophysics

Tevatron Collider & DetectorsEcm = 1.96 TeV, FTE: 2×1015 eV; c - 495 km/h

107 collisions/s

Large Hadron Collider & DetectorsEcm = 14 TeV, FTE: 1017 eV; c - 10 km/h

↝109 collisions/s

4

5

CDF dijet event (√

s = 1.96 TeV): ET = 1.364 TeV qq̄ → jet + jet

The World’s Most Powerful Microscopesnanonanophysics

6

Gauge symmetry (group-theory structure) tested in

e+e− → W+W−

(a) (b)

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7

Gauge symmetry (group-theory structure) tested in

e+e− → W+W−

0

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LEPPRELIMINARY

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8

Gauge symmetry (group-theory structure) tested in

e+e− → W+W−

(a) (b)

(c)(d)

e+e–

e– e–

e–

e+ e+

e+

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Z

H

Massive weak bosons:

Higgs boson

Meissner effect

9

Not to replicate the early universe,but to create conditions that allow us to discover

something of the laws that prevailedwhen the universe was smaller & hotter.

Particle acceleratorsare time machines …

(now back to 1 picosecond)

10

The Importance of the 1-TeV Scale

EW theory does not predict Higgs-boson massThought experiment: conditional upper bound

W+L W!

L , Z0LZ0

L,HH,HZ0L satisfy s-wave unitarity,

provided MH ≤(8π√

2/3GF

)1/2= 1 TeV

• If bound is respected, perturbation theory is everywhere reliable

• If not, weak interactions among W±, Z, H become strong on 1-TeV scale (within standard EW theory)

New phenomena are to be found around 1 TeV11

Precision Measurements Test the Theory …

LEP EWWG

Measurement Fit |Omeas−Ofit|/σmeas

0 1 2 3

0 1 2 3

∆αhad(mZ)∆α(5) 0.02758 ± 0.00035 0.02768mZ [GeV]mZ [GeV] 91.1875 ± 0.0021 91.1875ΓZ [GeV]ΓZ [GeV] 2.4952 ± 0.0023 2.4957σhad [nb]σ0 41.540 ± 0.037 41.477RlRl 20.767 ± 0.025 20.744AfbA0,l 0.01714 ± 0.00095 0.01645Al(Pτ)Al(Pτ) 0.1465 ± 0.0032 0.1481RbRb 0.21629 ± 0.00066 0.21586RcRc 0.1721 ± 0.0030 0.1722AfbA0,b 0.0992 ± 0.0016 0.1038AfbA0,c 0.0707 ± 0.0035 0.0742AbAb 0.923 ± 0.020 0.935AcAc 0.670 ± 0.027 0.668Al(SLD)Al(SLD) 0.1513 ± 0.0021 0.1481sin2θeffsin2θlept(Qfb) 0.2324 ± 0.0012 0.2314mW [GeV]mW [GeV] 80.398 ± 0.025 80.374ΓW [GeV]ΓW [GeV] 2.140 ± 0.060 2.091mt [GeV]mt [GeV] 170.9 ± 1.8 171.3

12

… and determine unknown parameters

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… and determine unknown parameters

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15

Young-Kee 62

SM Higgs Searches at Tevatron

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16

What the LHC is not really for …1. Find the Higgs boson,the Holy Grail of particle physics,the source of all mass in the Universe.

2. Celebrate!

3. Then particle physics will be over.

We are not ticking off items on a shopping list …

We are exploring a vast new terrain… and reaching the Fermi scale

17

Imagine a world without a Higgs mechanism

18

If electroweak symmetry were not hidden …

•Massless quarks and leptons•QCD confines quarks into color-singlet hadrons•Nucleon mass little changed•QCD breaks EW symmetry, gives tiny W, Z masses;weak-isospin force doesn’t confine

•p outweighs n: rapid β-decay ⇒ lightest nucleus is n … no hydrogen atom

•Some light elements from BBN, but ∞ Bohr radius

•No atoms means no chemistry, no stable composite structures like liquids, solids, …

… character of the physical worldwould be profoundly changed

19

What is the nature of the mysterious new force that hides electroweak symmetry?

✴A force of a new character, based on interactions of an elementary scalar

✴A new gauge force, perhaps acting on undiscovered constituents

✴A residual force that emerges from strong dynamics among electroweak gauge bosons

✴An echo of extra spacetime dimensions

Which path has Nature taken?20

21

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CMS

23

CMS

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ATLAS

ATLAS

27

ATLAS

28

ALICE

29

New Physics on the Fermi ScaleMore

Does MH < 1 TeV make sense?The peril of quantum corrections – hierarchy problem

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30

New Physics on the Fermi ScaleMore

Does MH < 1 TeV make sense?The peril of quantum corrections – hierarchy problem

?

Responses: extend electroweak theory

Supersymmetry

Technicolor

Extra spacetime dimensions

“Little Higgs” modelsBring new physics down to 1 TeV

Opinion: Fermi scale holds Higgs boson + other new physics31

New Physics on the Fermi ScaleMore?

If dark matter interacts weakly …

… its likely mass is 0.1 to 1 TeV: Fermi scale

COSMOS

32

The Meaning of Identity

• What sets masses, mixings of quarks & leptons?

• What is CP violation, the subtle difference between matter and antimatter, telling us?

Fermion masses: physics beyond standard modelHiggs mechanism doesn’t predict

• Will new kinds of matter help us see pattern? Mendele’ev didn’t know about noble gases

What makes a top quark a top quark,an electron an electron, a neutrino a neutrino?

Fermi-scale discoveries have implications for flavor physics33

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Quark family patterns: generations

Veltman: Higgs boson knows something we don’t know!35

Neutrino family patterns (an example)

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The Unity of Quarks and Leptons

• What do quarks and leptons have in common?

• Why are atoms so remarkably neutral?

• Which quarks go with which leptons?

• Quark-lepton extended family: proton decay

• Unified theories: coupling constant unification

• Rational mass pattern at high energies?

Project from Fermi scale to higher energies

37

SU(3)c

SU(2)L

U(1)Y

log10

(E

1 GeV

)

1/α

60

40

20

0 5 10 15

Fermi scale + supersymmetry : unification?

coupling constants

Why is empty space so nearly massless?

Higgs field fills all of space with energy density >1024 g/cm3

But empty space weighs next to nothing: < few ×10-29 g/cm3 (Ωcrit)

Accelerating expansion of universe: vacuum energy is present … recasts problem

Solution from particle physics?

39

A New Conception of Spacetime?

• Could there be more space dimensions than we have perceived?

• What is their size? their shape?

• How do they influence the world?

• How can we map them?

Key to understanding why gravity is so weak?

40

V (r) = !∫

dr1

∫dr2

GNewton!(r1)!(r2)r12

[1 + "G exp(!r12/#G)]

Gravity follows Newtonian force law down to ≲ 1 mm

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E (meV)

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EW Theory is a low-energy effective theory

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MH, mt, and Human Destiny

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43

QCD@LHC

No reason to think QCD a low-energy theory(but strong CP problem)

Larger symmetry, broken to SU(3)c?New colored objects?

Unrecognized consequences of QCD?Novel event structure?

Quark configurations in impact-parameter space

“Engineering” information for LHC, Auger, …underlying event, cross section, multiplicity

44

When might we be sensitive to … ?

F. Gianotti, CERN TH Institute, 23/8/2007 67

With more time and more data, LHC can discover:

Large number of scenarios studiedMain conclusions: ! LHC direct discovery reach up to m ~ 5-6 TeV ! demonstrated detector sensitivity to many signatures " robustness, ability to cope with unexpected scenarios

Discoveries vs time: our guess …

L=1035

Excited quarks q*" #q: up to m $ 6 TeVLeptoquarks: up to m $ 1.5 TeVMonopoles pp " ##pp: up to m $ 20 TeVCompositeness: up to % $ 40 TeVZ’ " ll, jj: up to m $ 5 TeVW’ " l! : up to m $ 6 TeVetc.... etc….

(AT

LAS)

45

Questions …• What is the agent that hides the electroweak symmetry?

• Is there a Higgs boson? Might there be several?

• Does H give mass to fermions, or only to the weak bosons? What sets the masses and mixings of the quarks and leptons?

• How does H interact with itself? What shapes the Higgs potential?

• Could we be living in a false (metastable) vacuum?

• Is the Higgs boson elementary or composite?

• Does the pattern of Higgs-boson decays imply new physics? Will unexpected or rare decays of H reveal new kinds of matter?

• What stabilizes the Higgs-boson mass on the Fermi scale? Is Nature supersymmetric? Is EWSB an emergent phenomenon connected with strong dynamics? … related to gravity through extra dimensions?

• How can a light H coexist with the absence of signals for new physics?

• What resolves the vacuum energy problem?

• What lessons does EWSB hold for unified theories of the strong, weak, and EM interactions? for the inflationary Universe? for dark energy?

46

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