1. Dr. D. Z. LI & Prof. J. GAO Accelerator Center, IHEP 2016.04.09
Studies on Circular Gamma-Gamma Collider
Dr. D. Z. LI & Prof. J. GAO
Accelerator Center, IHEP

2. OUTLINES Introduction to γ-γ Colliders
Conceptual design and parameter choice for Circular γ-γ Colliders
Summaries and Prospects

3. OUTLINES Introduction to γ-γ Colliders
Conceptual design and parameter choice for Circular γ-γ Colliders
Summaries and Prospects

4. What is Gamma-Gamma Collider?
In classical electrodynamics, light doesn’t affect light.
When we make the light more and more intense, at first QED effects occur, and pairs of electrons and positrons are produced. Soon QCD effects occur, and strange particles are produced.
The cross section is extremely small for optical photons (~10-63 cm2), but growing very fast with energy (∝ω6)
Gamma-Gamma Colliders: Making intense beams of gamma rays and having them collide so as to make elementary particles. 4

5. Rich Physics in γ-γ/γ-e Colliders
Perfect Compatible with High Energy Particle Colliders!! 5 5

6. History of Gamma-Gamma Colliders
1930’s, S. Vavilov et al., first attempt to observe the change of the spectrum due to light-light scattering, failed (of course).
, I. Ginzburg, V. Telnov et al., published several papers on the theory of Gamma-Gamma colliders and suggested changing the electrons to photons.
Around 1996, according with the conceptual design of a series large e+e- colliders such as NLC, TESLA, JLC etc., the concept of Gamma-Gamma collider aroused worldwide attention in high energy physics community.
Since 2001, ILC became one of the most important choice of the next HEP facilities. ILC-based Gamma-Gamma collider was put forward.
In 2005, the compatibility with the photon collider was lost in the ILC’s RDR report mainly because of the budget. In 2007, the GDE agreed that the ILC Engineering Design should include the photon collider. Gamma-Gamma collider was re-coupled with ILC.
In the last 10 years, several new kinds of Gamma-Gamma colliders were investigated, such as HFiTT and SAPPhiRE. Photon colliders based on large circular accelerators (FCC, CEPC) were also been discussed. 6

7. Merits and Challenges for Photon Colliders
Rich in Physics
Can be affiliated to the existed e-e+ colliders, with relatively small extra budget to achieve brand new goals. Especially for linear colliders.
Easy to get polarized photons
Have the potential to build a Higgs factory with lower electron energy
Challenges:
Extremely complex conversion & interaction region
Desperately need high average power lasers
Conversion efficiency for eγ
How to manage the spent electron and damp the high power laser?? 7

8. OUTLINES Introduction to γ-γ Colliders
Conceptual design and parameter choice for Circular γ-γ Colliders
Summaries and Prospects

9. Compton Back-scattering for eγ Conversion
High energy e- + Intense low energy photons
High energy gamma photons + Spent e-
X不超过4.8的原因是过高能量的光子有很大几率和激光的光子碰撞，变成正负电子对。
If we choose x=4.8 to increase the gamma photon energy, and take E0=120 GeV, the laser we need is “blue” (λ=475nm).
If we choose Ti: Sapphire lasers (λ=800nm), x=2.85
If we choose YAG lasers or fiber laser (2ω, λ=532nm or 527), x=4.3

10. Principle of the Gamma-Gamma Collider
Conversion Point (CP) to Collision Point (IP): ~1 cm
Even though, e bunch is much CP IP due to final focus
“Crab waist” maybe necessary
The gamma will “focus” itself!!
Also Interaction Point

11. Laser selection for a given Photon Collider
The required laser is dependent on the choice of accelerator parameters
Repetition rate + bunch number/train  Laser frequency

12. Development of Laser Technology

13. Development of Laser Technology

14. Conceptual Design for LC-based Photon Collider
Superconducting RF (ILC like):
thousands bunches (~1ms)/train, 5 Hz repetition
Laser: 15mJ/pulse, 20J/train (mercury Laser system)

15. Conceptual Design for SAPPHire and HFiTT
The recirculating superconducting RF accelerator:
Single bunches at a much larger rate — several to 100 kHz
Laser: ICAN fiber laser system, look forward to 10J*10kHz

16. CEPC-Based Gamma-Gamma Collider (by Prof. Chou)
Accelerator
SAPPHiRE
HFiTT
CEPC-γγ
Beam Energy (GeV) 80
Energy lost per turn (GeV)
3.89
4.53
0.6
Technology
SRF
Pulse repetition rate (kHz)
200
47.7 3
Number of bunches 1
Laser wavelength (nm)
351
Laser pulse energy (J) 5
Average laser power (kW)
2000
240 15
Q1:
18 turns up and down, will the beam quality good enough?
Q2:
Is the energy too low for the removal of the spent electrons?

17. Technical Challenges for (Circular) Photon Colliders
very crowed conversion region
Optics setup and optical elements
High power laser damp also needed
Large crossing angle are needed

18. OUTLINES Introduction to γ-γ Colliders
Conceptual design and parameter choice for Circular γ-γ Colliders
Summaries and Prospects

19. Summary and Prospects
Preliminary studies on gamma-gamma colliders were carried out and basic concepts were reviewed
LC-Based (such as ILC or CLIC) photon colliders seems more realistic.
Rapid development of the laser technology hold the possibility for circular gamma-gamma colliders
Though debatable and immature, Higgs-factory-like type is so far the most practical way to the CEPC-based photon collider. 19

20. Thank you for your attentions!!!