Transcript Kara Hoffman The University of Chicago Enrico Fermi Institute On behalf of the Muon Collaboration.

Kara Hoffman
The University of Chicago
Enrico Fermi Institute
On behalf of the Muon Collaboration
From Neutrino Factory to Muon Collider…
You have just been given the highlights of the considerable recent
progress in ongoing efforts to design a realizable neutrino factory,
and to experimentally demonstrate “ionization cooling”, a concept
central to coalescing a muon beam.
•Actually…muon colliders have been the subject of study for much longer.
•The specifications for a muon collider were quite ambitious-it seemed doomed to
the realm of science fiction.
•However it was realized that each stage in the construction of a muon collider
could yield interesting physics ( using proton drivers, cold muon beams, neutrino
beams…) and attention turned toward the less challenging neutrino factories.
•Of course the progress made in building a neutrino factory also brings us closer
to a muon collider…
•In the meantime conceptual progress has been made to make the muon collider
appear that it may be realizable.
Why should we be interested in pursuing muon colliders?
Low Energy Higgs Factory
Reason 1: muons are massive
•only scenario where s-channel resonance can
be observed
•the Higgs width can be measured directly
•hgmm coupling is a direct test of the fermion
mass generation mechanism. It can be
measured to +/-4% with L = 0.2 fb-1 if the beam
energy resolution R=0.003%
Reason 2: because of Reason 1, you
can get a narrow beam energy spread
m
mm   6 1010
4
e
Muon collider can provide the most precise
measurement of the mass of a light Higgs using
a beam energy scan of the resonance
L
500 fb
-1
0.2 fb
-1
Exceeds precision
of theoretical
predictions?
SUSY Higgs Factory
Reason 3: if nature happens to
be supersymmetric, there are
some regions of parameter space
other machines just can’t probe
…but must first raise
s!
Note dependence on
beam energy resolution!
The infamous LHC and LC “blind spot”
mH-mA: an important SUSY constraint
If masses are degenerate, they
can only be resolved by exploiting
the narrow beam energy spread at
a muon collider using a scan.
Energy Frontier Machines
•If nature is supersymmetric, heavy scale for SUSY
particles (squarks, sleptons) may be preferred.
Reason 4: we don’t
know what lies beyond
the electroweak scale.
•No SUSY? No Higgs?? Then we should see strong WW
scattering.
•None of the above?? Who knows what surprises are in
store? Technicolor, Z’, large extra dimensions…all need
energy frontier machines for discovery.
Muon colliders are smaller than
other machines for a given energy
less real estate
High energy muon colliders
retain the possibility of
narrow beam energy spread
High energy machine chosen will
be the one that optimizes the
cost/luminosity/energy equation
Ultimately the physics possibilities are determined by the
machine parameters…
orders of magnitude
more cooling than
for a v factory!
  5 104 fb
Also, high luminosity means fewer bunches (1-4)!
Snowmass ‘96
Evolution of Muon Collider Designs
A more contemporary view
Note new features which
compactify the design
6D Cooling
strong
focusing
 N ,eq 
  Es2
2 g x mm c 2 LR
small multiple
scattering
dE
ds
Sg = (gx+gy+gL) =constant
Introduces transverse-longitudinal
coupling “emittance exchange”
Bent Solenoid drift proportional
to particle’s momentum, introduces
dispersion, h
x g xo + h dp/p
Ring Coolers
dipole introduces dispersion, h
dipole
They differ primarily in the technique
used to contain the beam.
+dp
p
-dp
Several different designs for ring
coolers are being studied.
wedge
absorber
To compare their performance, we
define a “merit factor”:
M
Recirculation reduces total length and
cost.
( x y z )initial
( x y z ) final
 transmission
which includes muon decay rates.
BEFOR
transverse
E
longitudinal
Solenoidal Coolers
•provides same transverse
cooling as sFOFO linear channel
considered in neutrino factory
Study II and transverse cooling
as well
•heat dissipation in absorber
could be challenging
•injection and extraction is
difficult-no space
AFTER
GEANT
transverse
beam smallest in
absorber where
field is largest
transmission
losses most likely
in dispersion
region
longitudinal
Merit Factor=38 after 15 turns
Bunch Compression
Muon from decay are diffuse. Cooling
channel has high gradient (short
wavelength) cavities.
For a given number of particles, the
luminosity is inversely proportional
to the number of bunches.
Beam has to be bunched for
cooling channel acceptance.
Delay or stacking ring
required.
The solenoidal cooler suggests another way to
achieve bunch compression…
different
wedge
absorbers
•similar but larger
ring
•weaker solenoidal
field
•lower frequency
lower gradient
cavities
Lattice cooling rings
dipole only
Use only convention quadrupole
and/or dipole magnets to contain
beam.
Quadrupole/dipole ring
Merit Factor=15
after 15 turns
Merit Factor=80
after 15 turns
Performance improves for more compact lattices-could be a problem for
injection/extraction
Alternating Solenoid Ring
Injection/extraction
Vertical kicker
Solenoids flip polarity at the center
of a cell. All cells are identical.
200 MHz
rf 12MV/m
hydrogen
absorbers
alternating
solenoid
tilted solenoids
RF cavities
H2 absorber
Bending generated
by alternately tilting
the solenoids.
30
Merit factor decreases by ~30%
after accounting for
injection/extraction.
High Gradient Cavities: a gas filled approach
For gaseous hydrogen:
Paschen’s Law the breakdown voltage for
Vbreakdown  0.448nd  0.6 nd
a discharge between electrodes in gases isGradient
a
vs Pressure for GH2 at 77K
density, n, is figure of merit E pressure
function of the product of pressure and distance.
required decreases with temperature
60
H2 gas @77K
Gradient (MV/m)
50
805 MHz
40
30
20
Muon Collaboration
results (2003)
10
Felici (1948)
0
0
50
•suppresses breakdown, allowing higher gradients
•absorbs dark current radiation
100
150
200
P re s s u re ( P S IA )
T his Experiment
Felici (1948)
•gas with high heat capacity cools RF windows and increases electrical efficiency
•gas can even act as a homogeneous absorber to provide ionization cooling!
250
Cooling in a gas filled RF cavity
•To achieve the same cooling power :
X
dE
0

dx
2
for transverse cooling as in current LH2 cooling channels
requires a GH2 pressure above that needed to suppress
breakdown
•Works best in beam with constant 
6D Cooling Channel: a gas–filled cavity in a
solenoid plus transverse helical dipole fields
m beam
evacuated
dipole magnet
-dp
Longer path length in
gas filled magnet slows
higher p particles
m beam
H2 gas filled
dipole magnet
p
+dp
wedge absorber
+dp
Calculations show 10 -6 phase
space reduction for a 150m channel
with an energy reduction of 1/3
-dp
•Muon colliders would be the final stage in an ongoing program which
could prove to be a cornucopia of physics results as well as technological
innovation.
•The muon collider R&D program benefits from recent ideological
breakthroughs made in neutrino factory studies (i.e. FFAG’s), and in turn,
work in realization of a muon collider has lead to innovations (i.e. ring
coolers) which could reduce the cost of a neutrino factory.
•More realistic ring coolers with injection/extraction are being developed,
however, the calculated 6D emittance reduction exceeds those achieved
for straight cooling channels used in Neutrino Factory Study II, perhaps
even making the specification for a muon collider within short reach.
•GH2 has been demonstrated to inhibit cavity breakdown and may
provide 6D cooling while avoiding the injection/extraction and absorber
heating problems facing ring coolers.
•The Muon Collaboration has done a lot with very little. Ask me about our
creative approach to funding!
Extra plot: the path to a muon collider
Machine details