Interview: Nobel Laureate Sheldon Glashow Discussing Future High Energy Colliders

By Sheldon Lee Glashow and Hong Jian He

About the Interviewee: Sheldon Lee Glashow is a renowned theoretical physicist, the Higgins Professor of Physics at Harvard University until 2000, and currently the Metcalf Professor of Mathematics and Physics at Boston University. He is a major founder of the Standard Model of particle physics, and one of the masters in modern physics. He was awarded the Nobel Prize in Physics in 1979 (together with Abdus Salam and Steven Weinberg) “for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including, inter alia, the prediction of the weak neutral current”. He made many other profound contributions to particle physics, including the GIM (Glashow-Iliopoulos-Maiani) mechanism, the prediction of the 4th quark — the Charm Quark, and the Georgi-Glashow SU(5) Grand Unification Model, to just name a few.

About the Interviewer: Hong-Jian He, Professor of Physics at Tsinghua University, working in particle physics, cosmology, quantum gravity and their interface.

The Interview

Below are our interview questions (Q) and the answers (A) of Professor Glashow.

Q1: Professor Glashow, it is our great pleasure to have this interview with you, and to discuss with you about the future plans of particle physics, especially, the recent Chinese proposal of CEPC/SPPC and related public debate which you may have heard. Would you be glad to share your insights with Chinese people?

A1: I am astounded by the surprising and outspoken opposition expressed by our much-respected colleague Yang to China’s ambitious and thrilling proposal to initiate its CEPC/SPPC project. Perhaps Planck was right when he said that science progresses one funeral at a time [1].

Q2: Lately I re-read your article “Particle Physics in The United States, A Personal View” [2]. It is very thoughtful, though it was intended for the prospects of the high energy physics (HEP) in USA. You said that you hope your country (and your university) will continue its active and effective engagement in the Large Hadron Collider (LHC) at CERN. This is also our plan in China. The LHC Run-2 has been performing well to collide proton-proton beams at an energy of 13TeV. It has collected about 28/fb integrated luminosity in each detector so far, which amounts to about 10% of the planned full data collection at the Run-2. Although no new physics was announced at the ICHEP conference in August, would you like to share with us about your views on possible new findings (or not) at the on-going LHC?

A2: Very disappointingly and to my great surprise, LHC as yet finds no indication, nor even a plausible hint, of physics beyond the standard model. Yet, the enthusiasm and commitment of high-energy experimenters and particle theorists have not been impacted. With only 10% of run-2 data in hand, future surprises remain plausible to anticipate, if not in run-2, then in future work at higher luminosity and/or energy.  LHC will remain the front of discovery potential in particle physics for decades to come, for Chinese and American scientists as well as those of CERN’s member states. The very big problem is what happens afterward.

Q3: We are glad that you stressed the importance of precision measurements in your article [2]. You also said [2], “Although the world’s next great collider is unlikely to be built in the US, I hope that we will be eager participants in any sensible future multinational efforts.” Given the fact that the LHC with pp collisions is unable to precisely measure the Higgs boson properties, would you feel crucial to build up a Higgs factory (e+e

collider) such as the CEPC [3] proposed in China? We recall that you visited China a number of times before, including one in this fall. What is your viewpoints on the Chinese proposal CEPC?

A3: I have visited China several times, and experienced its very rapid progress. Last month I was in Chengdu: a pleasant town 25 years ago when my children saw it, but now an immense city of 14 million! I was delighted by the panda breeding station, but wasn’t then aware of the very promising PandaX project that I should have seen. YES! We need a Higgs factory to verify that the properties of the particle found at LHC are just those expected in a one-Higgs standard model. The Chinese hosted CEPC and the Japanese hosted ILC would have similar energies and luminosities: at least one such machine is truly necessary for the health and survival of particle physics.  Each of these machines could also provide useful precision data about Z decay modes, and at higher energy, the WZ coupling.  Perhaps new particles would be found that have eluded discovery at LHC… But, most importantly, CEPC is an obligatory precursor to the magnificent SPPC project.

Q4: Regarding the lessons of Superconducting Super Collider (SSC) in USA, perhaps, may you have seen an article “The Crisis of Big Science” [4] written by your colleague Steven Weinberg in 2012 for the New York Review of Books? Early this month, we recommended the Chinese edition of this article [4] to the Chinese publics. The cancellation of SSC by US congress in 1993 was a great loss for the high energy physics (HEP) community in USA and worldwide, although the proposed Space Station Project in the same state Texas (with about ten times more cost) got approved at the same time [4]. It seems to have made vital negative impacts on American HEP in particular and in its whole fundamental science in general. On the one hand, SSC was designed to collide proton-proton beams at a center of mass energy of 40TeV, which is a factor 3 larger than the current Run-2 energy (13TeV) of the LHC at CERN, Geneva. It is thus not so unexpected and disappointed that the LHC Run-2 has not found any new physics beyond the Standard Model (SM) so far, because we all know that the SSC with 40TeV colliding energy was designed to ensure a much more solid new physics discovery reach at TeV scales. As many physicists expected, if the SSC had not been canceled in 1993, it would probably have already made a revolutionary new physics discovery pointing to a new direction of the fundamental physics in the 21st century. Since you witnessed the history of the SSC and the subsequent developments of the LHC, would you like to share your thoughts with the Chinese community regarding the lessons of the SSC and LHC?

A4: The energy of the SSC was carefully considered by many outstanding physicists, both experimenters and theorists.  The center of mass energy was agreed to be at least 40 TeV so as to guarantee post-standard model discoveries. As I recall, when SSC was aborted by Congress, CERN soon and very fortunately came up with an initially proposed 20 TeV LHC, which gradually fell from 1/2 to 1/3 of the SSC dream energy. Not to diminish the triumph of CERN’s Higgs discovery, it is not all that surprising that new physics has so far escaped detection at LHC: It simply was not a machine designed to push beyond the standard model envelope. The consequences of SSC termination for American high-energy physics have been disastrous. We had dominated high-energy physics from 1953 (with the commissioning of the Cosmotron) to 2011 (with the shutdown of the Tevatron). We have no plan to construct a new forefront particle accelerator at any time in the foreseeable future. I hope that the capricious and unpredictable nature of our Congress will not preclude American participation in large and long-term multinational scientific projects, such as the much desired CEPC and SPPC.

Q5: Perhaps, you already heard about the current Chinese plan of the “Great Collider” project [3], whose first phase is called CEPC, an electron-positron collider of energy 250GeV, running in a circular tunnel of circumference about 100km long. It has a potential second phase for a proton-proton collider with energy up to 100TeV (called SPPC). We are glad to tell you that this proposal has been officially ranked as the “First Priority HEP Project” at the recent “Strategy Plan Meeting for Future High Energy Physics” of the Chinese Particle Physics Association, held last month on August 20-21. In fact, CERN is also taking active studies on a similar proposal, called FCC (Future Circular Colliders), despite that CERN has been mainly occupied by the LHC Run-2 and the subsequent LHC Upgrade over the next 15-20 years. Most colleagues worldwide think that this is a truly promising direction for the next step forward in the high energy physics. — Would you like to share your views on the CEPC Project with the Chinese community? Also, please feel free to comment on the status and achievements of the Chinese HEP developments, including the past and on-going major experiments, such as the BEPC e+e

collider, the neutrino experiments Daya Bay and JUNO, and the dark matter experiment PandaX at Jinping deep underground lab in south China.

A5: I am reasonably well informed about the Chinese Great Collider project, and I am delighted that the project is the First Priority of Chinese particle physicists. I strongly support the first stage, the CEPC Project, and even more, the culminating deployment of the Great SPPC Collider. I am also aware that CERN has also been considering future circular colliders. I believe that China is in a much better financial situation than Europe to initiate such a project. I would hope that many countries, including but not limited to the EU and America, would partner with China to complete and exploit the CEPC and SPPC endeavors.

China has been taking giant steps in particle physics! The multinational Daya Bay experiment was the first to measure θ13 despite many unsuccessful prior attempts elsewhere, such as by the French. This was a truly important discovery. Further precision studies of neutrino oscillations will soon be carried out at the JUNO facility.  These are expected to resolve the important question of neutrino hierarchy. China has also entered the dark matter sweepstakes with Panda-X, a series of Xenon detectors with increasing sensitivity at the world’s deepest underground lab. As of this year (2016), PandaX-II has established the world’s most stringent dark matter constraint [5]. Significant future increases in sensitivity are planned. Lastly, BEPC II has achieved world record luminosity for e+e

collisions in the energy range 2–4 GeV. With its new BES III detector, it has obtained several exciting results, such as the discovery of the Z_c (3900) particle, with more new states soon to follow. All in all, Chinese particle physics has experienced a remarkable growth spurt, as is both befitting and essential if China is to host the Great Collider.

Q6: You may have heard about the on-going public debate [6] in the Chinese community on whether such a new collider should be built in China at all. This debate was provoked early this month by the 94-year-old Chinese-American theoretical physicist C. N. Yang (one of the Nobel laureates in 1957, retired in 1997), who has been strongly against any collider project in China, including the current CEPC-SPPC project [3] led by IHEP director Yifang Wang. Attached below is an English translation of Yang’s recent public article, and a summary of the debate between Yang and Wang (published in “China Daily”). It’s clear that Yang’s major objection is that this collider would cost too much for China. (CEPC was actually estimated by the Chinese team at IHEP to be about 6 billion US dollar over a ten-year period of construction). A misconception of Yang was to stress the cost of the potential second phase of pp collider SPPC that would be built from 2040s. (As anyone may recall, the funds of the LEP and LHC at CERN were approved separately and in sequence.) – It will be extremely helpful for the Chinese community to learn your opinions and advice from the international side.  Would you think that the fund invested for CEPC worthwhile? and what would this contribute to the world through the international collaborations? and to the society of China?

A6: Needless to say, I disagree with Yang:

1) China can easily afford to build and operate the proposed facilities.

2) China has won only one Nobel Prize is science. It wants more. Many Nobels have gone to particle physicists in the past, as they will in the future. CEPC and SPPC will make China the world hub of particle physics.

3) It is our duty to try to understand the world we are born to. Others have become reluctant, so it is now the opportunity and the obligation of the Chinese people to take up the challenge.

4) Fundamental physics affects society: One third of the global GDP depends on quantum mechanics; the world-wide-web was developed by and for particle physicists. Of medical scanners: CAT won Nobels for two particle physicists, MRI (as well as nuclear power) spun off nuclear physics, PET uses positrons. Industrial and medical accelerators are a multi-billion dollar business… need I go on?

5) Science has always been international. Both CEPC and SPPC, as multinational efforts hosted by China would continue the tradition of such international efforts as CERN, LIGO, HUBBLE, etc.

References

[1] Note: Max Planck’s original statement was in German, “Eine neue wissenschaftliche Wahrheit pflegt sich nicht in der Weise durchzusetzen, daß ihre Gegner überzeugt werden und sich als belehrt erklären, sondern vielmehr dadurch, daß ihre Gegner allmählich aussterben und daß die heranwachsende Generation von vornherein mit der Wahrheit vertraut gemacht ist.” (English translation: A new scientific truth does not triumph by convincing  its opponents and making them see the light, but rather because its  opponents eventually die, and a new generation grows up who is  familiar with it.) Professor Glashow’s translation has highlighted its essence.

[2] Sheldon Lee Glashow, “Particle Physics in The United States, A Personal View”, arXiv:1305.5482, in the Proceedings of Community Summer Study 2013: Snowmass on the Mississippi (CSS2013), Minneapolis, MN, USA, July 29-August 6, 2013.

[3] Circular Electron Positron Collider (CEPC) and Super pp Collider (SPPC), http://cepc.ihep.ac.cn/. See the recent science book for introducing this subject: S. Nadis and S. T. Yau, “From the Great Wall to the Great Collider — China and the Quest to Uncover the Inner Workings of the Universe”, 2015,  International Press of Boston, Inc., MA, USA.

[4] Steven Weinberg, “The Crisis of Big Science”, in The New York Review of Books, May 10, 2012. For Chinese edition of this article (translated by Zhong-Zhi Xianyu), see: http://chuansong.me/n/678905846230

[5] A. Tan et al. [PandaX-II Collaboration], Phys. Rev. Lett. 117 (2016) 121303 [arXiv:1607.07400 [hep-ex]], which is highlighted as the Cover Article of the issue no.12.

[6] For a summary report of Yifang Wang’s refutation against C. N. Yang, by China Daily, “It is Suitable Now for China to Build Large Collider”.

[7] David Gross and Edward Witten, “China’s Great Scientific Leap Forward”, in The Wall Street Journal, September, 2015. For Chinese translation of this article, see: http://www.ihep.cas.cn/xwdt/cmsm/2015/201509/t20150926_4431136.html

 

(2016/10/02)

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