1. 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 HEP2012: Recent Developments in High Energy Physics and Cosmology Ioannina, Greece, April 5-8 2012

2. 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) arXiv:1112.2518v2 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) arXiv:1112.2518v2

3. Higgs mass From Moriond 2012: 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/c 2 (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! 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/c 2 (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! A. Blondel, experimental summary talk:

4. 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+e+ e-e- Z* Z H For a Higgs of 125GeV, a centre of mass energy of 240GeV is sufficient

5. 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?

6. 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)

7. 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.

8. 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 b y *. Cryo power needed is less than half that of the LHC.

9. LHeC space considerations : 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

10. The low field dipoles Another synergy with LHeC, although LEP3 would require a “double decker” magnet BINP short model Prototypes of LHeC designs: Compact and lightweight to fit in the existing tunnel, yet mechanically stable CERN 400 mm long model LEP3 Artist’s impression

11. 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)]

12. 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.

13. Beam current/# bunches The limit on SR power defines the beam current, which is 7.2mA or 4x10 12 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

14. 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 20s y.

15. Luminosity/Beam lifetime With the β* values mentioned, luminosity for LEP3 is projected to be 1.3x10 34 cm -2 s -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 t eff of 12 minutes – LEP3 would be “burning” the beams to produce physics very efficiently.

16. 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 4x10 12 positrons every few minutes, or of order 2x10 10 positrons per second. For comparison, the LEP injector complex delivered positrons at a rate of order 10 11 per second.

17. 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)]

18. Yearly statistics Operation at a luminosity of 10 34 /cm 2 /s for 10 7 s/year leads to an integrated luminosity of 100 fb -1 /yr. For an e + e - → HZ cross section of 200 fb, this yields 2x10 4 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 10 6 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.

19. 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)

20. Similar ideas Katsunobu Oide (KEK Director, Accelerator Laboratory) gave a talk about a possible LEP3- like machine in Japan:

21. 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

22. 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

23. Main parameters LEPLHeC ring designLEP3 E b beam energy104.5 GeV60 GeV120 GeV beam current4 mA (4 bunches)100 mA (2808 bunches)7.2 mA (3 bunches) total #e- / beam2.3e125.6e134.0e12 horizontal emittance48 nm5 nm20 nm vertical emittance0.25 nm2.5 nm0.15 nm  b dipole bending radius 3096 m2620 m partition number J  1.11.5 momentum compaction1.85x10 -4 8.1x10 -5 SR power / beam11 MW44 MW50 MW  x,y * 1.5, 0.05 m0.18, 0.10 m0.15 0.0012 m rms IP beam size270, 3.5 micron30, 16 micron55, 0.4 micron hourglass loss factor0.980.990.65 energy loss per turn3.408 GeV0.44 GeV6.99 GeV total RF voltage3641 MV500 MV9000 MV beam-beam tune shift (/IP)0.025, 0.065N/A0.126, 0.130 synchrotron frequency1.6 kHz0.65 kHz2.98 kHz average acc.field7.5 MV/m11.9 MV/m18 MV/m effective RF length485 m42 m505 m RF frequency352 MHz721 MHz1300 MHz rms energy spread0.22%0.116%0.232% rms bunch length1.61 cm0.688 cm0.30 cm peak luminosity / IP1.25x10 32 cm -2 s -1 N/A1.33x10 34 cm -2 s -1 number of IPs412 beam lifetime6.0 hN/A12 minutes

24. 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.