Transcript Particles, Mysteries, and Linear Colliders Jim Brau

Particles,
Mysteries,
and Linear Colliders
Jim Brau
APSNW 2003
Reed College
May 30, 2003
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J. Brau, APS NW, May 30, 2003
Particles
• Particle Physics left the 20th Century with a
triumphant model of the fundamental particles and
interactions - the Standard Model
– quarks, leptons, gauge bosons mediating gauge interactions
– thoroughly tested
– no confirmed violations*
e+e-  W+W- (LEP)
–
* neglecting neutrino mixing
J. Brau, APS NW, May 30, 2003
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Mysteries
• We enter the 21st Century with mysteries of
monumental significance in particle physics:
– Physics behind Electroweak unification?
• The top quark mass
– Ultimate unification of the interactions?
– Hidden spacetime dimensions?
– Supersymmetry? - doubling Nature’s fundamental particles
– Cosmological discoveries?
• What is dark matter?
selectron
20th Century
electron
spin = 0
• What is dark energy?
spin =1/2
Particle Physics
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Linear Colliders
• The electron-positron Linear Collider is a critical tool
in our probe of these secrets of Nature
The First Linear Collider
SLAC Linear Collider (SLC)
built on existing SLAC linac
Operated 1989-98
3 KM
future Linear Collider
500 GeV
- 1 TeV
and beyond
could operate
by 2015
25 KM
expanded horizontal scale
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Electroweak Symmetry Breaking
• One of the major mysteries in particle physics today is the origin of
Electroweak Symmetry Breaking
– Weak nuclear force and the electromagnetic force unified
• SU(2) x U(1)Y with massless gauge fields
– Why is this symmetry hidden?
• The physical bosons are not all massless
• Mg = 0
MW = 80. 4260.034 GeV/c2 MZ = 91.1880.002 GeV/c2
– The full understanding of this mystery appears to promise deep
understanding of fundamental physics
• the origin of mass
• supersymmetry and possibly the origin of dark matter
• additional unification (strong force, gravity) and possibly hidden
space-time dimensions
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Electroweak Unification
• The massless gauge fields are mixed by the Higgs mechanism
into the massive gauge bosons
Massless gauge
bosons in
symmetry limit
(
)
( )
b
w- w0 w+
Complex spin 0
Higgs doublet
f1
f2
(
g
W- W+
Z0
)
Physical
bosons
Electroweak
symmetry breaking
tan qW = g’/g
sin2qW=g’2/(g’2+g2)
e Jm(em) Am
e = g sin qW = g’ cos qW
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The Higgs Boson
• The quantum(a) of the Higgs Mechanism is(are)
the Higgs Boson or Bosons
– Minimal model - one complex doublet  4 fields
– 3 “eaten” by W+, W-, Z to give mass
– 1 left as physical Higgs
• This spontaneously broken local gauge
theory is renormalizable - t’Hooft (1971)
• The Higgs boson properties
– Mass < ~ 800 GeV/c2 (unitarity arguments)
– Strength of Higgs coupling increases with mass
• fermions: gffh = mf / v
v = 246 GeV
• gauge boson: gwwh = 2 mZ2/v
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Standard Model
Fit
• MH = 91
+58
-37
GeV/c2
LEPEWWG
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Indications for a Light
Standard Model-like Higgs
(SM) Mhiggs < 211 GeV at 95% CL.
LEP2 limit Mhiggs > 114.4 GeV.
Tevatron discovery reach ~180 GeV
W mass ( 80.426  0.034 MeV)
and top mass ( 174.3  5.1 GeV)
agree with precision measures
and indicate low SM Higgs mass
LEP Higgs search – Maximum Likelihood for Higgs signal at
mH = 115.6 GeV with overall significance (4 experiments) ~ 2s
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History of Anticipated Particles
Positron
Pi meson
Neutrino
Quark
Charmed quark
Bottom quark
W boson
Z boson
Top quark
1932 - Dirac theory of the electron
1947 - Yukawa’s theory of strong interaction
1955 - missing energy in beta decay
1968 - patterns of observed particles
1974 - absence of flavor changing neutral currents
1977 - Kobayashi-Maskawa theory of CP violation
1983 - Weinberg-Salam electroweak theory
1984 -
“
“
1997 - Expected once Bottom was discovered
Mass predicted by precision Z0 measurements
Higgs boson
???? - Electroweak theory and experiments
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The Search for the Higgs Boson
• Tevatron at Fermilab
– Proton/anti-proton collisions
at Ecm=2000 GeV
– Collecting data now
2 km
• discovery possible by ~2008?
• LHC at CERN
– Proton/proton collisions at
Ecm=14,000 GeV
– Begins operation ~2007
8.6 km
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Establishing Standard Model Higgs
precision studies of the Higgs boson will be required
to understand Electroweak Symmetry Breaking;
just finding the Higgs is of limited value
We expect the Higgs to be discovered at LHC
(or Tevatron) and the measurement of its
properties will begin at the LHC
We need to measure the full nature of the
Higgs
to understand EWSB
The 500 GeV (and beyond) Linear Collider is the tool
needed to complete these precision studies
References:
TESLA Technical Design Report
Linear Collider Physics Resource Book for Snowmass
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2001
(contain
references
J. Brau,
APS NW, Mayto
30,many
2003 studies)
Some Candidate Models for
Electroweak Symmetry Breaking
Standard Model Higgs
excellent agreement with EW precision measurements
implies MH < 200 GeV (but theoretically ugly - h’archy prob.)
Minimal Supersymmetric Standard Model (MSSM) Higgs
expect Mh< ~135 GeV
light Higgs boson (h) may be very “SM Higgs-like”
with small differences (de-coupling limit)
Non-exotic extended Higgs sector
eg. 2HDM
Strong Coupling Models
New strong interaction
The LC will provide critical data to resolve these possibilities
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The Next Linear Collider
• Acceleration of electrons in a circular accelerator is
plagued by Nature’s resistance to acceleration
– Synchrotron radiation
– DE = 4p/3 (e2b3g4 / R) per turn (recall g = E/m, so DE ~ E4/m4)
– eg. LEP2
DE = 4 GeV
Power ~ 20 MW
• For this reason, at very high energy it is preferable to
accelerate electrons in a linear accelerator, rather
than a circular accelerator
electrons
positrons
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Cost Advantage of Linear Colliders
• Synchrotron radiation
– DE ~ (E4 /m4 R)
m,E
R
• Therefore
– Cost (circular)
~ a R + b DE
• Optimization R ~ E2
~ a R + b (E4 /m4 R)
 Cost ~ c E2
cost
– Cost (linear) ~ a L, where L ~ E
Circular
Collider
Linear Collider
Energy
At high energy,
linear collider is
more cost effective
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The Linear Collider
• A plan for a high-energy, highluminosity, electron-positron
collider (international project)
– Ecm = 500 - 1000 GeV
– Length ~25 km
• Physics Motivation for the LC
25 KM
– Elucidate Electroweak Interaction
• particular symmetry breaking
• This includes
– Higgs bosons
– supersymmetric particles
– extra dimensions
• Construction could begin around 2009
following intensive pre-construction R&D
with operation beginning around 2015
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expanded horizontal scale
Warm X-band version
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The First Linear Collider
• This concept was demonstrated at SLAC in a linear
collider prototype operating at ~91 GeV (the SLC)
• SLC was built in the
80’s within the existing
SLAC linear accelerator
• Operated 1989-98
– precision Z0 measurements
• ALR = 0.1513  0.0021 (SLD)
asymmetry in Z0 production
with L and R electrons
– established Linear Collider concepts
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The “next” Linear Collider
The next Linear Collider proposals include plans to
deliver a few hundred fb-1 of integrated lum. per year
TESLA
(DESY-Germany)
JLC-C
(Japan)
Superconducting
Room T
RF cavities
structures
Ldesign
ECM
NLC/JLC-X *
(SLAC/KEK-Japan)
Room T
structures
(1034)
3.4  5.8
0.43
2.2  3.4
(GeV)
500  800
500
500  1000
23.4  35
1.3
337  176
34
5.7
2.8
65
11.4
1.4
72
190
Eff. Gradient (MV/m)
RF freq.
(GHz)
Dtbunch
(ns)
#bunch/train
Beamstrahlung (%)
2820  4886
3.2  4.4
There will only be one in the world
ILC-Technical Review Committee
4.6  8.8
* US and Japanese X-band R&D
cooperation, but machine
parameters may differ
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The “next” Linear Collider
Standard Package:
e+ e- Collisions
Initially at 500 GeV
Electron Polarization  80%
Options:
Energy upgrades to ~ 1.0 -1.5 TeV
Positron Polarization (~ 40 - 60% ?)
gg Collisions
e- e- and e-g Collisions
Giga-Z (precision measurements)
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Special Advantages of Experiments
at the Linear Collider
Elementary interactions at known Ecm*
eg. e+e-  Z H
Democratic Cross sections
eg. s (e+e -  ZH) ~ 1/2 s(e+e -  d d)
Inclusive Trigger
total cross-section
Highly Polarized Electron Beam
~ 80%
Exquisite vertex detection
eg. Rbeampipe ~ 1 cm and s hit ~ 3 mm
Calorimetry with Jet Energy Flow
sE/E ~ 30-40%/E
* beamstrahlung must be dealt with, but it’s manageable
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Linear Collider Detectors
The Linear Collider provides very special experimental
conditions (eg. superb vertexing and jet calorimetry)
CCD Vertex Detectors
Silicon/Tungsten Calorimetry
SLD Lum (1990)
Aleph Lum (1993)
Opal Lum (1993)
NLC a TESLA
Snowmass - 96 Proceedings
NLC Detector - fine gran. Si/W
NLC
Now TESLA & NLD
have proposed Si/W
as central elements in
jet flow measurement
SLD’s VXD3
TESLA
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Example of Precision of Higgs Measurements at the
Next Linear Collider
For MH = 140 GeV, 500 fb-1 @ 500 GeV
Mass Measurement
Total width
Particle couplings
tt
 MH  60 MeV  5 x 10-4 MH
 H / H  3 %
(needs higher s for 140 GeV,
except through H  gg)
bb
 gHbb / gHbb  2 %
cc
 gHcc / gHcc  22.5 %
+

 gH  / gH    5 %
*
WW
 gHww/ gHww  2 %
ZZ
 gHZZ/ gHZZ  6 %
gg
 gHgg / gHgg  12.5 %
gg
 gHgg / gHgg  10 %
Spin-parity-charge conjugation
establish JPC = 0++
Self-coupling
HHH / HHH  32 %
(statistics limited)
If Higgs is lighter, precision is often better
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Higgs Studies
- the Power of Simple Reactions
Higgs recoiling from a Z0 (nothing else)
known CM energy (powerful for unbiassed tagging of Higgs decays)
measurement even of invisible decays
•Tag Zl+ l•Select Mrecoil = MHiggs
> 10,000 ZH events/year
for mH = 120 GeV
at 500 GeV
Invisible decays are included
( - some beamstrahlung)
500 fb-1 @ 500 GeV, TESLA TDR, Fig 2.1.4
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Higgs Couplings - the Branching Ratios
SM predictions
bb
cc
+WW*
ZZ
gg
gg
 gHbb / gHbb  2 %
 gHcc / gHcc  22.5 %
 gH  / gH    5 %
 gHww/ gHww  2 %
 gHZZ/ gHZZ  6 %
 gHgg / gHgg  12.5 %
 gHgg / gHgg  10 %
Measurement of BR’s is powerful indicator of new physics
e.g. in MSSM, these differ from the SM in a characteristic way.
Higgs BR must agree with MSSM parameters from many other measurements.
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Higgs Spin Parity and Charge Conjugation (JPC = 0++)
H gg or gg  H rules out J=1 and indicates C=+1
Threshold cross section ( e+ e-  Z H) for J=0
s ~ b , while for J > 0, generally higher
power of b (assuming n = (-1)J P)
Production angle (q) and Z decay angle in Higgs-strahlung
reveals JP (e+ e-  Z H  ffH)
JP = 0+
JP = 0-
ds/dcosq
sin2q
(1 - sin2q )
ds/dcosf
sin2f
(1 +/- cosf )2
LC Physics Resource Book,
Fig 3.23(a)
f is angle of the fermion,
relative to the Z direction
of flight, in Z rest frame
Also e+e-  e+e-Z
Han, Jiang
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TESLA TDR, Fig 2.2.8
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Is This the Standard Model Higgs?
Z vs. W
b vs. c
Arrows at:
MA = 200-400
MA = 400-600
MA = 600-800
MA = 800-1000
b vs. W
b vs. tau
HFITTER output
conclusion:
for MA < 600,
likely distinguish
J. Brau, APS NW, May 30, 2003TESLA TDR, Fig 2.2.6
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Supersymmetry
• Supersymmetry
– all particles matched by super-partners
• super-partners of fermions are bosons
• super-partners of bosons are fermions
– inspired by string theory
Arkani-Hamed, Dimopoulos, Dvali
– high energy cancellation of divergences
– could play role in dark matter (stable neutral particle)
– many new particles (detailed properties at LC)
eg. selectron pair
production
~+~
e+e-  e
e-
LC Resource Book,
Fig 4.3
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Extra Dimensions
g
• Extra Dimensions
– string theory predicts them
– solves hierarchy problem (Mplanck >> MEW) if extra
dimensions are large (or why gravity is so weak)
– large extra dimensions would be observable at LC
e+e-  m+ m-
e+
e-
Gn
e+e-  g Gn
LC Resource Book,
Fig 5.17
LC Resource Book,
Fig 5.13
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Adding Value to LHC measurements
The Linear Collider will enhance the LHC
measurements (“enabling technology”)
How this happens depends on the Physics:
•Add precision to the discoveries of LHC
•eg. light higgs measurements
•Measure superpartner masses
•SUSY parameters may fall in the tan b /MA wedge.
•Directly observed strong WW/ZZ resonances at LHC
are understood from asymmetries at Linear Collider
•Analyze extra neutral gauge bosons
•Giga-Z constraints
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Political Developments
In US, HEPAP subpanel endorsed LC
as the highest priority future
project for the US program (2002)
DOE Off. of Science 20 year
Roadmap for new facilities(2003)
• HEP Facilities Committee
•LC gets top marks for
science and feasibility
•Recommended
Japanese Roadmap Report (2003)
German Science Council (2001)
requests gov’t consent
German Government (2003)
formal statement
- supports project
- wait for int’l plan
OECD Global Science Forum Report strongly endorses LC
project as next facility in Elem. Particle Physics (2002)
Discussions between governmental representatives from
countries interested in the LC have been active recently
and are continuing . . . . . .
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Conclusion
The Linear Collider will be a powerful tool for studying
Electroweak Symmetry Breaking and the Higgs
Mechanism, as well as the other possible scenarios for
TeV physics
Current status of Electroweak Precision measurements
strongly suggests that the physics at the LC will be rich.
We can expect these studies to further our knowledge
of fundamental physics in unanticipated ways
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