Transcript lc_madison

P. Grannis/PHENO 2001
May 7, 2001
What can an e+ e- Linear
Collider teach us ?
 Understanding the source of electroweak
symmetry breaking is the most pressing issue in high
energy physics for the coming decade.
 The LHC (or the Tevatron) seems assured of
discovering new phenomena related to EWSB but will
leave critical questions unanswered.
 An e+e- linear collider at ~500 GeV can discover
new phenomena and make precision measurements
that illuminate the nature of EWSB, and point the
way toward higher energy phenomena. There is likely
to be a need for evolution of the collider.
 The e+ e- linear colliders are well developed
technically and it is likely that we will make decisions
a linear collider within the next few years. It is
imperative that we understand the physics case as
clearly as possible.
2
Linear ee Colliders
TESLA
Ldesign
ECM
(1034)
(GeV)
Eff. Gradient (MV/m)
RF freq.
(GHz)
Dtbunch
(ns)
#bunch/train
Beamsstrahlung (%)
L
3.4
JLC-C
5.8
0.43
NLC/JLC-X *
2.2
500
800
500
23.4
35
34
70
5.7
11.4
1.3
500
337
176
2.8
1.4
2820
4886
72
190
3.2
4.4
= 1x1034 cm-2s-1 for 107 sec. year
gives 100 fb-1 per year
4.6
3.4
1000
8.8
* US and Japanese X-band R&D
cooperation, but machine
parameters may differ
TESLA : Design report March 2001; German
Science Council recommendation mid-2002
NLC : aim complete R&D for Design Rept 2003
JLC : set milestones end 2000: Design Rept ~2003
CLIC : multi-TeV, 30 GHz, 150 MV/m gradient with
drive beam power source; in R&D phase
NLC - 2001
NLC baseline 2001: 26 km site (2 in CA, 2 in IL). Two 10
km linacs sized for 1 TeV; final focus, injector for 1.5 TeV.
Two IRs; ‘Hi E’ IR with no bend (crossing angle 20 mrad)
can work at multi-TeV; ‘Lo E IR requires bend; maximum
energy 500 GeV ( 1 TeV?)
Recent work:
Improved klystrons and SS
modulators give x3-4
efficiency gain.
New compact final focus
region.
Optimum cost for gradient
70 MV/m but deterioration
of accelerating structure
surfaces seen (at high group
velocity). Active R&D this
year to understand. If need
to reduce to 50 MV/m, cost
penalty is 5-10%.
Cost reduction to date: from
$5.1B in Lehman review ($FY00, no
escalation, contingency, detectors)
to $3.7B (30% reduction).
Another 10 – 15% from possible
scope reductions.
Injectors: 19%;
global costs: 17%;
Linacs: 39%;
beam delivery: 11%;
management/business: 14%
3
TESLA
4
TESLA site length = 33 km (15 km
linacs). Operates with superconducting
RF cavities; design for 500 GeV is
22 MV/m. Bunches are separated by
337 ns, allowing for head-on collisions
without satellite crossings.
spec
Cost: $3.16B (using 0.93$/Euro). Includes 1 IR, 1 detector
($233M). xFEL added cost is $495M.
Cost in 2000 prices; no contingency (HERA was on budget);
no escalation; no second detector/IR; exclusive of manpower
at collaborating institutes (6933 man-yrs estimated: ~$700M)
5
There are two fundamental questions before
experimental high energy physics at present:
•
What is the origin of the symmetry breaking
observed in the electroweak interaction?
What gives the W/Z (and fermions) their mass?
Is there unification of forces, and if so, at what
scale? Can gravity be incorporated?
Are there new phenomena or new particles
associated with the physics responsible for EWSB?
•
What is the origin of flavors?
Why three generations and the peculiar fermion
mass patterns?
Why is there CP violation, and why is it insufficient
to give the matter/antimatter asymmetry in the
universe?
Does flavor physics (neutrino mass) imply something
about physics at the GUT scale?
Main themes for the Linear
Collider physics program :
Experiments in the past decade (LEP, SLC,
Tevatron, n scattering) have made precision
measurements that clearly indicate the need for
something like the Higgs boson. LEP has
indication of ~115 GeV state (2.9s ).
Study the `Higgs boson’ (or its surrogate)
and measure its characteristics.
The SM Higgs mechanism is unstable; vacuum
polarization contributions from the known
particles should drive its mass to the force
unification scale. We expect some new physics
entering at the TeV scale. BNL (g-2 )
experiment comparison with theory suggests
new physics (2.6s ).
Find and explore this new physics sector.
6
Where is the Higgs ??
7
(The Higgs is what the
measurements tell us it is! )
All Expts: Bknd
4 jet
0.93
missing ET 0.30
leptons
0.35
taus
0.14
ALL chan.
1.72
Signal exp. Evnts
1.60
3
0.46
1
0.68
0
0.29
0
3.03
4
(SM) Mhiggs < 190 GeV at 90% CL.
LEP2 limit Mhiggs > 113.5 GeV.
LEP Higgs search – Maximum Likelihood
Tevatron can discover up to 180 GeV for Higgs signal at m = 115.0 GeV with
H
overall significance (4 exp’ts) = 2.9s
Higgs self-coupling
diverges
Higgs potential has 2nd min.
If LEP indication correct,
there must be physics beyond
the SM before the GUT scale
LHC (Tevatron) will discover > 1 Higgs; LHC will get mass
accurately; total width, couplings poorly; likely not the
quantum numbers; will not do self couplings; and may well not
see heavy Susy Higgs.
Higgs studies -- Mass
8
The key discovery question for LC is What is the nature of the
`Higgs’ ? -- revealed by its quantum no’s, couplings, total width.
The LHC is unlikely to do these. LC produces Higgs in association
with Z allowing study of its decays without bias -- even invisible
decays of Higgs are possible using the recoil Z (in ee, mm, qq modes).
For 120 GeV Higgs, ZH
production mode:
Width
~30K evts/yr at 350 GeV;
~15K evts/yr at 500 GeV
dM/M ~ 1.2x10-3
GTOT : Measuring the lightest Higgs
coupling tests whether there are
additional higher mass Higgs.
In MSSM or 2 doublet models:
S g2(h ZZ)i = (MZ gEW / cos qW)2
GTOT to few% at LC for mass < 150
GeV, using measured GWW from WW
fusion or sZH , & BR(H WW).
LHC can do GTOT to 10-20% in this
range.
Higgs spin parity
q = cm production angle;
f = fermion decay angle in Z frame
JP = 0+
JP = 0-
ds/dcosq
sin2q
(1 - sin2q )
ds/dcosf
sin2f
(1 +/- cosf )2
 and angular dependence near threshold permits unambiguous
determination of spin-parity
Can produce CP even and odd states separately using polarized
gg collisions. gg
H or A (can reach higher masses than e+e-)
Higgs Couplings
We need to determine experimentally 9
that Higgs couplings are indeed
proportional to mass.
Use vertex meas., jet mass,
topology in likelihood to get
BRs for 500 fb-1 , 300 GeV LC
H
H
H
H
H
bb
cc
gg
tt
WW
2.4%
8.3%
5.5 %
6.0%
5.4%
approx.
errors
(Mh = 120 GeV)
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.)
Higgs self couplings
SM value (decoupling limit)
Study ZHH production and decay
to 6 jets (4 b’s). Cross section is
small; premium on very good jet
energy resolution. Can enhance XS
with positron polarization.
Dl/l ~ 20% with 1000 fb-1.
Physics beyond the Standard Model
10
The defects of the SM are widely known:
No gauge interaction unification occurs
Higgs mass is unstable to loop corrections
Many possible new theories proposed to cure these
ills and embed the SM in a larger framework
Supersymmetry -- fermion/boson partners, extending
the Poincare group to include fermionic dimensions.
Susy models come in many variants with different
mechanisms and scales of Susy breaking (supergravity,
gauge mediation, anomaly mediation …) Each has a
different spectrum of particles, underlying parameters.
A new gauge interaction like QCD with `mesons’ at
larger masses. (Techicolor/topcolor) These
interactions avoid introducing a fundamental scalar.
`technipions’ play the role of Higgs; new particles to be
observed, and modifications to WW scattering.
String-inspired models with some extra dimensions
compactified at millimeter to femtometer scales. These
yield anomalous mono-photon or mono-jet production,
heavy Z/W states, modification to ee/gg production.
LC must be able to sort out which is at
work, and make precision measurements.
Examples exist where LHC sees new phenomena, but
mis-understands the source
Supersymmetry
11
Fermion/boson symmetry stabilizes the Higgs mass -- scale
of new Susy particles is O (1 TeV). Lightest higgs state
< 130 GeV.
The main issue is to measure the underlying model parameters and
deduce the character of the supersymmetry, energy scale for
supersymmetry breaking. There are ~ 105 unknown parameters, all
of which need to be measured, and used to fix models.
This can be done through measurement of the masses, quantum
numbers, branching ratios, asymmetries -- and in particular
the pattern of mixing of states with similar quantum numbers -the 2 stops, sbottoms, staus, and the 2 chargino and 4 neutralino
states (partners of the g/Z/W and supersymmetric Higgs states).
Susy may well be the next frontier for flavor physics –
FCNC, CP violation in the sparticles, generation
patterns, etc.
The LHC should discover Susy if it exists.
But disentangling the information on the full
mass spectrum and particle quantum no’s/couplings
and the mixings will be very difficult at LHC.
The LC can make these crucial measurements, (e.g.
sparticle masses to 0.1 – few % level) benefitting
from -Polarization of electron (positron) beam
Known partonic cm energy
Known initial state (JP = 1- )
Supersymmetry studies
12
at the Linear Collider
An example: production of selectron pairs -- have two
diagrams; typically the t-channel dominates and allows
measurement of neutralino couplings to lepton/slepton.
e+
e-
~
e
g,Z
e c1
0
~+
e
~
e-
~+
e
e+
c0
e-
~
e-
Upper & lower edges of decay
electron energy distribution from
~
eL,R
e c10
gives masses of left and right
handed selectrons.
Angular distribution of decay
electrons, using both polarization
states of beam e-, tell us about
quantum numbers, coupling of
exchanged neutralino and give
information on neutralino mixing,
hence the underlying Susy mass
parameters.
Similar studies for neutralino, chargino, stau etc.
production lead to independent measures of similar
parameters and enable constrained fit to Susy model.
Linear Collider Supersymmetry
13
The Linear Collider can determine the Susy model, and make
progress to understand the higher energy supersymmetry
breaking scale. To do this, one would like to see the full
spectrum of sleptons, gaugino/higgsino states.
Thresholds for selected sparticle pair productions
-- at LHC mSUGRA model points.
Point 1
reaction
GeV
2
GeV
3
GeV
4
GeV
5
GeV
6
GeV
c10 c10
336
336
90
160
244
92
c1 0 c 2 0
494
489
142
228
355
233
c1 + c 1 -
650
642
192
294
464
304
1089
858
368
462
750
459
920
922
422
1620
396
470
860
850
412
1594
314
264
186
207
160
203
184
203
Z H/A
1137
828
466
950
727
248
H+ H ~ ~
q q
2092
1482
756
1724
1276
364
1882
1896
630
1828
1352
1010
c1 + c 2 ~ ~ ~ ~
e e/ m m
~ ~
t t
Zh
RED:
Accessible at
500 GeV
GREEN: added
at 1 TeV
Operation in eg mode can increase mass reach:
~~
e.g.
e- g
e c10
(g-2) result suggests relatively light sfermions or charginos
It is likely that, in the case that supersymmetry exists,
one will want upgrades of energy to at least 1 TeV.
Susy breaking mechanism
14
The LC complements the LHC ( LC will do sleptons,
sneutrinos, gauginos well). LHC will see those particles
coupling to color, some Higgs, lighter gauginos if present in
cascade decays of squarks and gluinos. Electron
polarization (positron?) is essential for disentangling states
and processes at LC.
We really want understand the
origin of Susy -- determine the
105 soft parameters from
experiment without assuming the
model. (mSUGRA, GMSB, anomaly,
gaugino … ) mediation. We want
to understand Susy breaking, gain
insight into the unification scale
and illuminate string theory.
Mass spectra give some indications
of the model class.
LC mass, cross sect. as input
to RGE evolution of
couplings reveal the model
class without assumptions.
This study for ~ 1000 fb-1
LC operation and LHC meas.
of gluinos and squarks show
dramatically different
patterns of mSUGRA and
GMSB.
Precision studies constrain ANY any new
physics generating the Higgs mechanism
15
Standard Model (SU(2) x U(1) S=T = 0) agrees with data
sin2 qW
S & T measure effect on W/Z
vacuum polarization amplitudes.
S for wk isoscalar and T for
isotriplet
All EW observables are linear
functions of S & T and are
presently measured to 0.01.
Giga-Z samples at LC (20 fb-1) would improve sin2qW by x10,
WW threshold run improves dMW to 6 MeV, etc. Factor 8
improvement on S,T
The chevron shows the
change in S & T as the Higgs
mass increases from 100 to
1000 GeV, given the current
top mass constraint. If the
Higgs is heavy (> 200 GeV),
need some compensating
effects from new physics.
Need a positive DT or
negative DS. Several classes
of models to do this – all have
observable consequences at
LC.
Present 68%
S,T limits
68% S,T limits
at LC
The precision measurement of
S&T at a linear collider could
crucial to understand the nature
of the new physics.
Strong Coupling Gauge Models
16
For many, fundamental scalars are unnatural. We have a
theory (QCD) in which pseudoscalars (pions) arise as bound
states of fundamental fermions (quarks).
Analogs of SU(3) color are postulated with `technicolor’ degrees of
freedom, but fermions at higher mass scale. The ‘technipions’
generate the Higgs mechanism. Though inspired by QCD, the new
model must differ quantitatively (slow evolution of coupling)
In ‘topcolor’, the 3rd generation SM quarks (top in particular) are
singled out as being strongly coupled to the new sector. Fermion
pair (tt) condensates play the role of `higgs’.
Observables in Strong coupling models:
New `technicolor’ particles should occur on the TeV scale. Since
the longitudinal components of W/Z are primordial higgs particles,
WW (ZZ) scattering is modified. Also expect modifications to
WWg coupling and top V&A form factors, seen at LC (tough at LHC).
Envisioned S&T constraints suggest that composite Higgs state(s)
should have mass < ~500 GeV.
Allowed regions in Higgs mass and
DT for W mass error of 30 MeV
and top mass error of 2 GeV
Chivukula, Holbling, hep-ph/0002022
Seeing strong coupling
effects may require LC
energy above 500 GeV.
Better S&T precision will
be crucial.
Large Extra Dimensions
17
String theories represent the only known avenue for
incorporating gravity and the microscopic forces. Until
recently, hope for any observable effects from the
compactification of extra dimensions was dim.
Recent suggestions that the extra dimensions might be
compactified on scales larger than Planck length lead to
observable consequences for experiment – and could explain
the heirarchy problem.
If the effective Planck mass ~TeV, gravity is modified at
mm scale, leading to anomalous g production (with missing ET
from gravitons), or modified cross sections for fermion pairs.
For compactification scale O (TeV), Kaluza-Klein excitations
of graviton or gauge bosons should exist at TeV scale, and
observable as excited states at the LHC or LC. If Susy in
extra dimensions, gravitino towers modify XS’s.
For compactification at the GUT scale, new states are
unobservable, but the characteristic Susy pattern of these
models should remain, and the unification pattern of the
couplings should provide information.
Models with SM gauge field propagation in TeV sized extra
dimensions, can get scalars (Higgs) with SM properties and
EWSB. MH = 165 – 230 GeV (and other scalar composites).
(Arkani-Hamed, Cheng, Dobrescu, Hall hep-ph/0006238)
Large extra dimension models still being
developed. If this is our world, it is likely that
higher Linear Collider energies will be desired.
Scenarios for New Physics
18
Although our experiments point to the Standard Model,
the Linear Collider should be capable of illuminating the
nature of physics beyond the SM. We believe that some
manifestation of the equivalent to the Higgs mechanism
should be seen at 500 GeV or less.
Some Scenarios for Physics after few yrs LHC:
1. Higgs-like state<150 GeV and evidence for Susy:
LHC/Tevatron discover: Linear Collider program is assured,
exploring the Higgs and Susy spectrum and determining their
detailed structures.
2. Higgs candidate seen but nothing else:
LC studies all aspects of the Higgs (accessible couplings, width
etc.) to compare with SM. Revisiting the Z-pole to refine the
precision measurements will be essential. Seek Z’ at LHC/LC,
anomalous VVV couplings, strong WW scattering, etc.
3. No Higgs, No Susy seen:
Verify that no Higgs to invisible modes. Measure anomalous
W/Z couplings and top anomalous form factors. Increase the
energy to seek new strong-coupling or extra dimension physics.
Return to the Z-pole for precision S and T.
4. Multiple kinds of new phenomena seen at LHC/TeV:
A wealth of new physics that needs untangling -- Linear Collider
has a field day!
Options for beams / energies
19
There are several special operating conditions for the
Linear Collider that may add important physics
capabilities, but also create extra complexity or costs.
How should we view these options?
 Positron polarization:
Polarized e+ probably required for improved precision EW
measurements (S&T) with Giga-Z; provide increased XS for H
iggs, useful for self-couplings; allow improved measurements of
Susy couplings/mixings. Obtain polarized e+ from intense
polarized g beams (TESLA requires these anyway).
 gg , e-g, e-e- collisions:
Larger cross sections in gg offsets lower luminosity; can
separate Susy H/A, complementary triple gauge coupling info,
lower threshold for selectrons in eg; e-e- allows clean
environment, high polarization, only one subprocess, good probes
for new physics (KK towers, some Susy states …)
 Low energy collisions (MZ , WW threshold, ZH cross
section maximum)
For any new physics whose origin is not immediately
understood, return to the Z, WW threshold will greatly aid
understanding. Operation at the maximum of the ZH cross
section gives largest rates. Ideas exist to permit simultaneous
operation at low (<500 GeV) and high energy for NLC.
 X-ray Free Electron Laser
Structural biology, plasma physics, materials science,
chemical kinetics, surface science all benefit from short pulse
angstrom level sources. There can be synergy between HEP and
these communities through use of LC as XFEL.
How does the world
community proceed?
20
(a personal point of view)
1. Linear Collider Timelines:
o
Tesla design report in spring 2001; decision 2002
o
Japan JLC proposal in few years
o
US NLC R&D over next 3 years leading to proposal
All 3 regions conducting studies of physics priorities
for next ~20 years during coming year.
LC decision likely in next few years!
Other new projects (HEPAP whitepaper timelines):

m Storage Ring might be ready for decision ~2010;
nature of physics questions fairly clear (n matrix/CP)
Next generation expts will probably teach us much.

m Collider or multi-TeV ee collider > 2010;

VLHC after 2010
Physics case for the multi-TeV colliders is not yet clear to
me; Higher energy than LHC/LC may not be highest priority
if there is rich TeV scale physics.

Very large underground laboratories (proton decay,
solar neutrinos, neutrino oscillations, supernova watch)
CERN is evaluating its future beyond LHC
How do we proceed?
21
2. Should the LC be the next world HE machine?

Inevitable that the LC decision is the next that will be taken
by the worldwide community. Real proposals exist; potential
alternatives much further off. Not all regions may propose a
LC in their region, but we will make decisions soon.

Worldwide support for the LC (somewhere) will be essential
if it is to succeed. Arguing against the LC will not enhance
the prospect for other projects. LC should not be the last
frontier accelerator.

Particularly in the US community, we must engage the LC
question, and consequence of opting for other paths.
Snowmass 2001 and HEPAP subpanel affords the chance to
confront these issues as a community.
3. Is the Linear Collider too expensive?

One hears, particularly in the US, that the likely cost of the
LC is too large to sell to the government. But ANY future
collider discussed is at least comparable cost. An endemic
problem! We need to be optimistic enough to expect to
succeed in arguing for a next project if it has a clear
scientific justification!

Cost of the LC seen by some as the primary driver toward
the initial stage at ~500 GeV. “Will such a stage address the
crucial next questions? ” Physics arguments above show clear
role for 500 GeV program. But expect that upgrades will be
needed.

Cost is a factor, and we must press all ways to control it.
But we must not lose sight of the probable need for future
evolution in the design.
How do we proceed?
22
4. Steps toward Internationalization:
The LC (or any other frontier project) should be fundamentally
international. Each region needs strong involvement at the
frontier to retain health of HEP and accelerator physics in that
region. With LHC, Europe takes the energy frontier; Asia and
North America will need frontier facilities to remain healthy.

Need a collective decision on the right sequence of next
steps (for the US, Snowmass 2001 and HEPAP subpanel are
critical activities). Proceed to a world view on the
preferred next step.

Site is likely chosen by funding – which region will put up
~2/3(??) of the cost?

For LC (or other projects), allocate spheres of responsibility;
empower all regions to take primary responsibility for major
systems from leadership of design, R&D, construction,
commissioning, operations. e.g. for LC, might assign
injector/damping rings; rf/linacs; final focus/beam
delivery/monitors to separate regions.

Need international advocacy for the project; ICFA, Physical
Societies, Global Science Forum involvement. International
cooperation in presenting proposals to national governments
stressing the joint ownership of project, shared access and
continuity of world wide effort.

Connect governmental science policy officials through forums
on how to forge international science project structures.

International review – comparative cost, performance
upgradability, technical risks assessment (being done for LC).
International oversight of accelerator, detectors, scientific
program.
Conclusion
The physics case for the LC with a 1st stage at
~ 500 GeV is very strong. We need a linear
collider to understand EWSB in any scenario.
We know enough to make the choice now.
Community engagement through Snowmass,
HEPAP is essential if we are to reach a
consensus.
With present lack of understanding of how
EWSB is manifested, flexibility of Linear
Collider design (energy, L , beam particles) is
essential. The LC will be an evolving facility.
The cost will be high. Unless we
internationalize so as to satisfy the needs of
all regions and allow productive collaboration,
we jeopardize the prospect of the LC, or any
other new frontier facility anywhere.