Transcript HEP2012: Recent Developments in High Energy Physics and Cosmology Ioannina, Greece , April 5-8 2012 LEP3: A high Luminosity e+e– Collider in the LHC.

HEP2012: Recent Developments in High Energy
Physics and Cosmology
Ioannina, Greece , April 5-8 2012
LEP3: A high Luminosity e+e– Collider
in the LHC tunnel to study the Higgs
Boson
M. Koratzinos
On behalf of the
LEP3 proto-working group
Introduction
• What is LEP3?
– LEP3 is an idea for a future CERN project of a high
luminosity e+e– collider in the LHC tunnel, to study the
Higgs Boson
– The idea is that by re-using existing infrastructure, the
project would be low-cost
• The LEP3 project requires as a prerequisite
– The existence of the Higgs boson
– That the Higgs boson is light (around 125GeV)
This is not an original idea; many people have been toying with the idea since the
demise of LEP2 until today. The indications of a low-mass Higgs gave the idea new
impetus. A good reading on the idea can be found in: A High Luminosity e+e- Collider
in the LHC tunnel to study the Higgs Boson, A. Blondel and F. Zimmermann,
arXiv:1112.2518v2 [hep-ex] (submitted to Phys. Lett. B)
Higgs mass
• From Moriond 2012:
A. Blondel, experimental summary talk:
Both ATLAS and CMS exclude a SM scalar boson
up to ~550 GeV
except in range (117-128 GeV): excess 2.5-2.9 
at 125-126 GeV/c2 (consistent)
ATLAS :  and ZZ
CMS : 
CDF+ D0 mostlybb&WW
Too soon to claim even evidence, but…
‘Who would bet against Higgs boson @125
GeV?’
My guess: Look Elsewhere + Look There
 CL probably >~ local significance of 2d experiment
More data in 2012 5 and more channels!
Higgs production mechanism
• Assuming that the Higgs is light, in an e+e– machine it is produced by the
“higgstrahlung” process
• Production xsection actually peaks at relatively low centre-of-mass
energies
e-
H
Z*
e+
Z
For a Higgs of 125GeV, a centre of mass
energy of 240GeV is sufficient
120GeV per beam
• This is “only” 15% higher than the final energy reached by
LEP2
• Although synchrotron radiation goes with the fourth power
of beam energy, the increase in SR power lost would be less
than a factor of two more than LEP (provided we use a ring
with the same bending ratio and beam current)
• The need for accelerating cavities would also be (a bit less
than) a factor of two higher than that of LEP2
• So, the question presents itself: could an e+e– accelerator
be designed so that it fits in the LHC tunnel that would lead
to acceptable luminosity and not require too much cooling
power?
The LHeC synergy
• As it turns out, there exists a design for an electron ring
in the LHC tunnel, provided by the LHeC working group
• In the LHeC design, the electron energy is 60GeV but
the beam current is rather high, resulting to a 44MW
SR power dissipation.
• The lattice of the electron ring in the LHeC design has
been developed with the primary goal that the ring fits
on top of the LHC magnets.
• The dipole filling factor is low (75%) resulting in a
bending radius of 2620m (compared to 3096m for
LEP2)
The major assumptions
• For the LEP3 preliminary study, we started first by limiting
the total SR power dissipation to 100MW (50MW per
beam).
– If we assume a wall-to-beam energy conversion efficiency of
50%, we end up with an RF system that consumes 200MW.
– This is a high power consumption, but not extremely high
(current CERN contract with EDF is 200MW for the whole of
CERN during LHC operation)
• We have used as baseline the lattice calculations of the
LHeC study (it provides horizontal emittance significantly
smaller than for LEP). As this was not optimised for the
LEP3 requirements, we will soon have our own lattice.
The RF system
• We can profit of 20 years of development in RF accelerating technology
to assume an RF gradient of 18MV/m, almost 2.5 times higher that of
LEP.
• The energy loss per turn of a single electron at 120GeV is 7GeV
• The total length of the RF system is therefore around 500m, similar to
that of LEP2.
• A good candidate for the RF system would be ILC-developed SC
accelerating cavities at a frequency of 1.3 GHz that help to reduce the
bunch length, thus enabling a smaller by*. Cryo power needed is less
than half that of the LHC.
LHeC space considerations
: LHeC
LHeC
: Space
reserved for
future e+e–
machine
The LHeC ring
is displaced
due to the
requirement
of keeping
the same
circumference
as the LHC
ring. LEP3 has
no such
requirement
The low field dipoles
• Another synergy with LHeC, although LEP3
would require a “double decker” magnet
LEP3 Artist’s impression
BINP short model
CERN 400 mm long
model
Prototypes of LHeC
designs: Compact
and lightweight to fit
in the existing tunnel,
yet mechanically
stable
Emittances
• Horizontal: The unnormalized horizontal emittance is
determined by the optics and varies with the square of
the beam energy. We simply scale it from the 60-GeV
LHeC value.
• Vertical: The vertical emittance depends on the quality
of vertical dispersion and coupling correction. We
assume the vertical to horizontal emittance ratio to be
similar to the one for LEP.
• [The ultimate limit on the vertical emittance is set by
the opening angle effect (emission angle of the SR
photons), which is negligible (below 1fm) for our field
(0.15T) and energy (120GeV)]
Bunch length
• The bunch length scales linearly with the
momentum spread, with the momentum
compaction factor and with the inverse
synchrotron frequency.
• The bunch length of LEP3 (0.3cm) is smaller
than for LEP (1.6cm), despite the higher beam
energy. This is due to the smaller momentum
compaction factor, the larger RF voltage, and
the higher synchrotron frequency.
Beam current/# bunches
• The limit on SR power defines the beam
current, which is 7.2mA or 4x1012 particles per
beam.
• Distributing this over three bunches gives a
beam-beam tune shift of 0.13, similar to the
max. beam-beam tune shift reached at KEKB.
• This requires further studies
Vertical β*
• We are aiming for a value of 1.2mm. This can
be achieved if the final focusing quad is inside
the experiments.
• With a free length between the IP and the
entrance face of the first quadrupole of 4 m,
plus a quadrupole length of 4 m, the
quadrupole field gradient needs to be about
17 T/m and an aperture (radius) of 5 cm would
correspond to more than 20sy.
Luminosity/Beam lifetime
• With the β* values mentioned, luminosity for
LEP3 is projected to be 1.3x1034cm-2s-1
• At LEP The lifetime of colliding beams was
determined by radiative Bhahba scattering with a
cross section of 0.215 barn.
• At top energy in LEP2, the lifetime was
dominated by the loss of particles in collisions.
• For LEP3, we find a beam lifetime teff of 12
minutes – LEP3 would be “burning” the beams to
produce physics very efficiently.
Two-ring design
• Due to low beam lifetime, it is more efficient to use a
second ring to continuously top up the main ring that
would remain at an energy of 120GeV. If the top-up interval
is short compared with the beam lifetime, this would
provide an average luminosity very close to the peak
luminosity.
• For the top-up we need to produce about 4x1012 positrons
every few minutes, or of order 2x1010 positrons per second.
• For comparison, the LEP injector complex delivered
positrons at a rate of order 1011 per second.
Injection scheme
• The LEP injector complex is no more.
• We would need to either revive a scheme a-la
LEP, or design a completely new injector
• Injecting at 20GeV would be ideal, injecting at
10GeV would be tolerable
• [A CLIC demonstrator of about 1km long
would be a perfect injector (at a cost)]
Yearly statistics
• Operation at a luminosity of 1034 /cm2/s for 107s/year leads
to an integrated luminosity of 100 fb-1/yr.
• For an e+e- → HZ cross section of 200 fb, this yields 2x104
events per year in each experiment (we have assumed 2),
allowing precise measurements of the Higgs Boson mass,
cross-section and decay modes, even invisible ones.
• It would also provide more than 106 WW events per year in
each IP. This machine would have similar or better
performance than a linear collider operating at the same
energy, and would reach it more economically.
• Such a machine can also revisit the Z peak, possibly with
polarized beams, and obliterate the LEP results.
Possible showstoppers
• The LEP3 design is at its infancy and a much
deeper look is needed before we can
determine if the project is viable or not.
• Beamstrahlung problems would need to be
looked at first (V. I. Telnov, arXiv:1203.6563v1
[physics.acc-ph] 29 Mar 2012 – reduces the
LEP3 luminosity by a factor of 4)
Similar ideas
• Katsunobu Oide (KEK Director, Accelerator Laboratory) gave
a talk about a possible LEP3- like machine in Japan:
Very rough estimates of cost
K. Oide attemts a very rough cost exercise for a 40 and 60km ring:
LEP3 does not
have (most of )
the cost of the ring
and the detector
Comparison of ring to linear
...He also attempts to compare ring and linear accelerators of energies
240, 400 and 500GeV – a ring design appears more economical
Main parameters
Eb beam energy
beam current
total #e- / beam
horizontal emittance
vertical emittance
rb dipole bending radius
partition number Je
momentum compaction
SR power / beam
bx,y*
rms IP beam size
hourglass loss factor
energy loss per turn
total RF voltage
beam-beam tune shift (/IP)
synchrotron frequency
average acc.field
effective RF length
RF frequency
rms energy spread
rms bunch length
peak luminosity / IP
number of IPs
beam lifetime
LEP
104.5 GeV
4 mA (4 bunches)
2.3e12
48 nm
0.25 nm
3096 m
1.1
1.85x10-4
11 MW
1.5, 0.05 m
270, 3.5 micron
0.98
3.408 GeV
3641 MV
0.025, 0.065
1.6 kHz
7.5 MV/m
485 m
352 MHz
0.22%
1.61 cm
1.25x1032 cm-2s-1
4
6.0 h
LHeC ring design
60 GeV
100 mA (2808 bunches)
5.6e13
5 nm
2.5 nm
2620 m
1.5
8.1x10-5
44 MW
0.18, 0.10 m
30, 16 micron
0.99
0.44 GeV
500 MV
N/A
0.65 kHz
11.9 MV/m
42 m
721 MHz
0.116%
0.688 cm
N/A
1
N/A
LEP3
120 GeV
7.2 mA (3 bunches)
4.0e12
20 nm
0.15 nm
2620 m
1.5
8.1x10-5
50 MW
0.15 0.0012 m
55, 0.4 micron
0.65
6.99 GeV
9000 MV
0.126, 0.130
2.98 kHz
18 MV/m
505 m
1300 MHz
0.232%
0.30 cm
1.33x1034 cm-2s-1
2
12 minutes
Conclusions
• The LEP3 idea might be a viable alternative as
a future HEP project.
• This would depend firstly (but not solely) on
the physics output coming out of LHC.
• A working group to study viability/ challenges/
prospects should be formed in the near
future, so that it can report at the open
symposium of the European Strategy for
Particle Physics.