Transcript Document 7298807

Waiting for the LHC: Exploring the
Quantum Universe
Sally Dawson
BNL
2007
What is the Quantum Universe?
 To discover what the
universe is made of and
how it works is the
challenge of particle
physics
A Decade of Discovery
• Electroweak Theory
• Neutrino flavor oscillations
– Three separate neutrino species
•
•
•
•
•
Understanding QCD
Discovery of top quark
B meson decays violate CP
Flat universe dominated by dark matter & energy
Quarks and leptons structureless at TeV scale
Discoveries have us poised for next revolution
Thesis of this talk….
• Particle physics has changed dramatically
in the last 20 years
• And we expect the next few decades to be
just as extraordinary
– Due to new experimental capabilities
– Due to theoretical advances
Einstein’s Dream
• Is there an underlying simplicity
in the laws of nature?
• Einstein dreamed of a unified
picture
• He failed to unify
electromagnetism and gravity
The history of particle physics is the story of the
search for unification
Electromagnetism and Radioactivity
• Maxwell unified Electricity and Magnetism with
his famous equations (1873)
Electromagnetic Theory
• Dirac introduced theory of electron - 1926
• Theoretical work of Feynman, Schwinger,
Tomonga resulted in a theory of electrons
and photons with precise predictive power
• Example: magnetic dipole of the electron
[(g-2)/2]
m = g (eh/2mc) S
• current values of electron (g-2)/2
theory: 0.5 (a/p) - 0.32848 (a/p)2 + 1.19 (a/p)3 +..
= (115965230  10) x 10-11
experiment = (115965218.7  0.4) x 10-11
We can calculate!
Electromagnetism and Radioactivity
• Matter spontaneously emits penetrating radiation
– Becquerel found uranium emissions in 1896
– The Curies find radium emissions by 1898
Could this new interaction
(the weak force) be
related to E&M?
Fermi’s Dream
• Fermi formulated the first
theory of the weak force
(1934)
n  p e- ne
Electroweak Unification
• Glashow, Weinberg, and Salam realized that the field
responsible for the EM force (the photon)
• And the fields responsible for the Weak force
– …the yet undiscovered W+ and W- bosons
• Could be unified if another field existed
– …the then undiscovered heavy neutral boson (Z)
• W and Z bosons discovered at CERN in 1983
Unification is a Guiding Theme
HERA
Experimental evidence for the
unification of the weak and
electromagnetic forces
Model requires Higgs
boson or something like it
for consistency!
The Quest for Unification
•
Figure from H. Murayama
Electroweak Theory is Predictive
Theory has few free parameters
– Mass of the Z boson, MZ=91.1875  .0021 GeV
– Strength of the coupling of the photon to the electron,
a=1/137.0359895(61)
– Strength of the weak interactions (measured in muon
decay) GF=1.16637(1) x 10-5 GeV-2
– Then the W mass is predicted
Tevatron is World’s Highest
Energy Accelerator
Precise measurement of Mw
2007
• CDF has world’s most
precise measurement of W
mass: MW=80.4130.048
GeV
• Predictions of
electroweak theory
Error on MW decreasing
M W
M W
MW
MW
MW
1
80.398GeV
MW
1
80.398GeV
M W
MW
M W
MW
MW
1
80.398GeV
MW
1
80.398 GeV
Standard Model doesn’t explain
the particle spectrum
Top Quark Discovered at
Fermilab
CDF
Why is it so heavy?
Mt=170.91.8 GeV
DØ
Mt (and error) decreasing
M t
Mt
M t
Mt
1
170.9GeV
Mt
M t
Mt
Mt
1
170.9 GeV
Why is Mass a Problem?
• Lagrangian for gauge field (spin 1):
L=-¼ FmnFmn
Fmn=mAn-nAm
• L is invariant under transformation:
Am (x) Am(x)-m(x)
• Gauge invariance is guiding principle
• Mass term for gauge boson: ½ m2 AmAm
• Violates gauge invariance
• Solution requires physical
scalar particle: THE
HIGGS BOSON
Standard Model is Inconsistent
Without a Higgs boson
•Requires physical, scalar particle, h, with unknown mass
•Predictions are infinite without a Higgs boson (or
something like it)
Mh is ONLY unknown parameter of EW sector
No evidence (yet) for existence of Higgs boson
Everything is calculable….testable theory
LEP Looked for the Higgs
• Looked for e+e-  Z h
• Excluded a Higgs boson up to Mh=114 GeV
• This limit assumes a Higgs boson with the
properties predicted by the Standard Model
Higgs at the Tevatron Very Hard!!!
(ggh)1 pb << (bb)
SM Higgs Searches at Tevatron
Tevatron Expected
Tevatron Observed
LP07
Boson
With preciseWmeasurements
of MZ
Mass
and a, we can predict MW:
2
MW =
pa
√2GF (1 - MW2/MZ2)(1 - Dr)
Dr: Quantum corrections dominated by top/bottom
and Higgs loops
DMW  Mt2
DMW  ln (MH/MZ)
2
Mt and MW Limit Higgs Mass
• Direct
observation of
W boson and top quark
(blue)
• Inferred values from
other measurements
(red)
Mt and MW Limit Higgs Mass
2007
• LEP EWWG (July, 2007):
– Mt=170.9  1.8 GeV
– Mh=76+36-24 GeV
– Mh < 144 GeV (one-sided
95% cl)
– Mh < 182 GeV (Precision
measurements plus direct
search limit)
Best fit in region excluded from direct searches
Where is the Higgs ?
• We need to find the Higgs
(Standard Model is
theoretically inconsistent
without it)
– We didn’t find it at LEP
– We haven’t found it at
Fermilab
– The end is in sight…..if we
don’t find it at the LHC, the
Standard Model as it stands
cannot be the whole story
(because precision
measurements would be
inconsistent)
Livingstone Plot—The March of
Progress
• Electron machines access
full energy of collisions
• Quark and gluon interactions
in a hadron machine access
some fraction of total
collision energy
Science Timeline
Tevatron
LHC
LHC Upgrade
2007
2008
2012
Future e+eCollider
Large Hadron Collider (LHC)
• proton-proton collider at
CERN (2008)
• 14 TeV energy
– 7 mph slower than the
speed of light
– cf. 2TeV @ Fermilab
( 307 mph slower than
the speed of light)
Stored Energy of Beams unprecedented
• Ebeam=1.5 Giga Joule
• LHC beams have same
kinetic energy as aircraft
carrier at 15 knots!
• Largest scientific project
ever attempted
Requires Detectors of
Unprecedented Scale
• CMS is 12,000 tons (2
x’s ATLAS)
• ATLAS has 8 times the
volume of CMS
• Collaborations have
~2000 physicists each
ATLAS Experiment at LHC
December 2006
QuickTime™ et un
décompresseur TIFF (non compressé)
sont requis pour visionner cette image.
CMS
ATLAS
LHCb
Particle Physics in the LHC/ILC Era
• Will be data driven
LHC
Willfind
find the
Higgs if it exists
LHC
will
Standard
Model
Higgs
Consistency of SM
REQUIRES a Higgs Boson
or something like it
Needed Ldt per experiment (fb-1)
Higgs discovery at the LHC
Assumes well
understood
detector
ATLAS+CMS
Mh(GeV)
1 fb-1: 95% C.L. exclusion
5 fb-1: 5 discovery
J. Blaising et al, Eur. Strategy Workshop
LHC and the Higgs
• LHC will discover
Higgs boson if it exists
• Sensitive to Mh from
100-1000 GeV
• Higgs signal in just a
few channels
From the Tevatron to the LHC
Tevatron
LHC
High pT QCD Jets
 (nb)
Drell-Yan production of W’s & Z’s
Gluon fusion of 150 GeV Higgs
1 TeV squark/gluino pair
production
s (TeV)
 Large increase in cross sections as
we go from the Tevatron to the LHC
F. Gianotti, Phys. Rep. 403, 379 (2004)
Early Physics at the LHC
 O(100 pb-1) per experiment by early 2009
Channel
Events/100 pb-1
at LHC
Previous # of Events
W→mn
105
104 LEP, 106 Tevatron
Z →e+e-
105
107 LEP, 105 Tevatron
tt  W bW b  mnX
104
104 Tevatron
QCD jets, pT>1
TeV
>103
1 TeV Gluino pairs
50
 Early data used to calibrate detectors
Rediscover SM physics at s=14 TeV: W, Z, top, QCD
Typical Collision Energy at LHC:
1 TeV
l
n
q
q
W+
b
t
t
p
p
W-
b
q
q
n
l
Quantum Corrections Connect
Weak and Planck Scales
M H2  M Pl2
Tevatron/LHC Energy
Weak
103 GeV
Something
new??
GUT
1016
Planck
1019 GeV
Quantum corrections drag weak scale to Planck scale
Quantum Corrections to Higgs Mass
• Higgs mass grows quadratically with scale of new
physics, 



M   
200 GeV 
 0.7 TeV

2
2
h
Mh  200 GeV requires  ~ TeV
Points to 1 TeV as scale of new physics
We expect much at the TeV Scale
• Maybe a Higgs (or something like it)
• Maybe supersymmetry (lots of new particles)
– Supersymmetric models have at least 5 Higgs particles!
• Maybe extra dimensions
• Maybe other new symmetries
We’re not sure what will be
there, but we’re sure there will
be something!
Possibilities galore
•
H. Murayama
More on Unification
Einstein’s Dream of Unification
• Coupling constants change with energy
Supersymmetric
Model
Standard
Model
Supersymmetric Theories
• Predict many new undiscovered particles (>29!)
• Very predictive models
– Can calculate particle masses, interactions, everything you
want in terms of a few parameters
– Solve naturalness problem of Standard Model
• Instead of M h2  2
• Supersymmetric models have M h2  M F2  M S2
Many New Particles in
Supersymmetric Models
Spin ½ quarks  spin 0 squarks
Spin ½ leptons  spin 0 sleptons
Spin 1 gauge bosons spin ½ gauginos
Spin 0 Higgs spin ½ Higgsino
Experimentalists dream….many particles to search
for!
What mass scale?
Supersymmetry is broken….no scalar with mass of
electron
•
•
•
•
(pb)
Supersymmetry at the LHC
M (GeV)
 Huge rates; well defined signatures
M(gluino, squark)  1 TeV gives 100 events with 100 pb-1 at LHC
M1/2 (GeV)
Supersymmetry at the LHC
• LHC will find
SUSY if it is
at 1 TeV
2009
Tevatron
M0(GeV)
Immediate improvements over Tevatron limits
Supersymmetric Theories solve
another problem
2006 Nobel Prize for
COBE: The first survey of
dark matter in the universe
We have a census of the universe
Is Dark Matter a Particle?
The lightest
supersymmetric particle
has the right properties to
be dark matter
Can we produce
dark matter in a
collider and study
all its properties?
Supersymmetry has Dark Matter Candidate
• Supersymmetric models have dark matter candidate
• Lightest supersymmetric particle (LSP) is neutral and
weakly interacting
• On general grounds, LSP contributes correct amount of
dark matter if its mass is 300 GeV-1 TeV
• Supersymmetric particles within reach of the LHC
Particle Dark Matter
• We’d like to detect dark matter particles in
the lab
– To show they’re in the galactic halo …
• And to produce them at an accelerator
– To measure their properties …
WIMP: Weakly Interacting
Massive Particle
aka: dark matter candidate
If LSP is dark matter, LHC will
observe supersymmetric particles
Conclusions