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 2016.04.09

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

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

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

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

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). 1980-1983, 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

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

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

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

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

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

Development of Laser Technology

Development of Laser Technology

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)

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

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?

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

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

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

Thank you for your attentions!!!