### Interview: Nobel Laureate Sheldon Glashow Discussing Future High Energy Colliders

##### -- Sheldon Lee Glashow、Hong-Jian He

[This article will be published in ICCM Notices, and the Chinese translation had been published in Math. Sci. History & Culture (数理人文) magazine (Wechat version)].

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.

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), webpage. 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, webpage, 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. webpage. For Chinese edition of this article (translated by Zhong-Zhi Xianyu), see: webpage

[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, “Sufficient Reasons for Nation to Build a Supercollider”, see webpage

[7] David Gross and Edward Witten, "China's Great Scientific Leap Forward", in The Wall Street Journal, September, 2015. webpage For Chinese translation of this article, see: webpage.

#### China Should Not Build a Super-Collider Now

-- C. N. Yang

In an article published in WeChat public account “Mr. Gu on Geometry”, titled “S. T. Yau: A few comments on the construction of high energy collider in China, and answer to questions from the media”, he said he support the construction of super collider in China, but I (Yang) am against, he can’t believe this. This is specifically mention in the following paragraph:

“The theoretical basis of all these experiments used the theory invented by Mr. Yang. My respect for him grew after each breakthrough. Therefore, it is hard to understand why Mr. Yang opposes the idea that high energy physics needs further development”.

Professor Yau has misunderstood me! I am definitely not against the further development of high energy physics. Instead, I oppose we begin to construct super big collider in China now, for the following reasons:

1) Construction of big collider has been a painful experience for the US: In 1989, it started to build the world’s largest collider. The initial budget was estimated to be 3 billion dollars, but it was added upon for several times later, reaching 8 billion dollars, which triggered a lot of opposing voice. Eventually, the congress painfully cancelled this project in 1992, wasting about 3 billion dollars. This experience led many people to believe that the construction of large collider is a bottomless money sink.

Currently, the largest collider is the LHC at CERN. In 2012, a collaboration of about 6000 physicists discovered the Higgs boson at the LHC. This is a major achievement of particle physics, verified the Standard Model. It has taken many years to construct the LHC. The total cost is no less than 10 billion dollars, including the machine and the detectors. The budget of the super big collider proposed by IHEP is certainly more than 20 billion dollars.

2) IHEP proposed to construct the super big collider in China, sharing the cost among many countries. However, China must shoulder a significant portion of it. Today, to the amazement of the world, the GDP of China has jumped to the second place in the world. However, China is still a developing country, its GDP per capita is still less than that of Brazil, Mexico or Malaysia. There are still a couple hundred millions of farmers and migrant workers. There are urgent problems to deal with, including environment, education, health and medicine, etc. The construction of the super big collider will be very costly, and hamper the solution of those urgent problems. I think we should not consider it now.

3) The construction of the super big collider will significantly squeeze the funding for other basic science, including biological science, condensed matter physics, astrophysics, etc.

4) Why is the construction of super big collider enthusiastically supported by many high energy physicists? The reasons are:

a. High energy physics is a new area developed after the World War II. It has had spectacular success in the past 70 years, verified the Standard Model, enabling deep understanding of the three fundamental interactions in nature. However, there are still two unsolved big problems:

i. Deeper understanding of the remaining fourth fundamental interaction, gravity, have encountered fundamental difficulties.

ii. It has been not possible to unify force and mass. It is certain the hope of all physicists to solve these two problems.

b. Some high energy physicists hope that the super big collider can discover “supersymmetric particles”, and therefore point to the way of the solution for these two problems.

However, the search for supersymmetry has been going on for many years, with no discovery. The supersymmetric particles which they hope to find at the super big collider is only a hypothesis by a subset of high energy physicists. Most of the physicists, myself included, think the existence of supersymmetric particles is just a hypothesis without any experimental evidence. The hope to find them at the super big collider is only a hypothesis about a hypothesis.

5) What are tangible benefits to people’s life from the big achievement of the high energy physics in the past 70 years? Nothing. What are tangible benefits to people’s life if the super big collider proposed by IHEP can be built, and it can succeed in significantly advancing high energy physics? I think it is impossible in the short term, impossible in 30 or 50 years. Moreover, I know my point of view is agreed upon by the absolute majority of the physicists.

6) IHEP has been established for 30 years. How should we evaluate its achievements in this period? Among the important high energy physicists in the world, only less than 1 to 2 per cent are in China. For the super big collider, its design, construction, and the operation and data analysis after its completion, will definitely be led 90% by foreigners. If there will be Nobel prize, would it be awarded to Chinese?

7) Without the construction of the super collider, is there no future for high energy physics? No. I think there are at least two directions worth pursuing: A. Search for new accelerator design principles. B. Search for beautiful geometrical structure, such as in the research of string theory. The research in both of these directions do not cost as much money. It is suitable under the current trend of economic development of the world.

#### Attachement-2

Note: The English translation of Yifang Wang’s original Reply (“China Should Build a Super-Collider Now”) is not available yet. Below is a summary report of Wang’s refutation against Yang, by “China Daily”, which can be a useful reference.

[Published in China Daily, Updated: 2016-09-10 07:10]

#### Sufficient Reasons for Nation to Build a Supercollider

Editor's Note: Chen-Ning Yang, a Chinese-American Nobel Prize-winning physicist, recently advised China publically not to proceed to build a super collider. One major argument is that the project involves huge investment, which can be used for social programs and other research projects. Responding to this, two physicists expressed their views:

Progress will be worth the investment

Professor Yang's first argument in opposing the supercollider is the high cost. Citing the United States' Superconducting Super Collider, which had initially been budgeted at $3 billion in 1989, but was abandoned three years later because of rapidly rising cost. Yang said China should not repeat the mistake made by the US. But the SSC failed because of multiple reasons, among them are US federal government budget deficit, political struggles between the Democratic and Republican parties, as well as regional competition between Texas and other states. It would not be the case in China. The Institute of High Energy Physics of the Chinese Academy of Sciences has built a number of large scale scientific facilities, such as the Beijing Electron-Positron Colliders (BEPC, BEPCII) and the ADS injector. All the costs have been well controlled within budget range. There is no reason to expect China's supercollider program will exceed the budget as the case of SSC. Yang also said that the investment in such a huge project would mean less funding for programs to improve people's livelihoods and social welfare. Of course China needs to fund programs to improve social welfare, but it also has to spend on scientific research to facilitate technological breakthroughs. Furthermore, progresses made in high energy physics help improve people's livelihoods, too. Today we have Magnetic Resonance Imaging devices to diagnose diseases in hospitals, we have touch screens that allow us to conveniently use our smartphones, and we have the World Wide Web to share information. Without pursuing research in high energy physics for the past 70 years and the technology innovations associated with the research, all these would have been impossible, or significantly delayed in time. For China, a supercollider will allow its scientists to keep abreast of the latest developments in physics, even enabling the country to become a global research center. That is irreplaceable to its development in the long run. The budget for building a supercollider will not dry up funds for other research programs, either. Basic research accounts for only 5 percent of R&D funding in China, which only represents one third of developed countries. As the top Chinese leadership vows to increase investment in basic research, the fund for basic research is expected to increase by 100 billion yuan ($15 billion) annually; as a comparison, the planned supercollider would require 3 billion yuan a year from 2022 to 2032 in its first stage. Therefore, the supercollider will not squeeze the funds for basic research programs.

Yang was also pessimistic about building Chinese particle accelerators in the 1970s, yet the returns from the program were well worth the investment: the science came out of the BEPC, BEPCII colliders, the expertise acquired through the associated experimental programs was vital in building the successful Daya Bay experiment, and the synchrotron radiation facilities and the neutron spallation sources constructed and operated to serve the broad scientific community are all examples of good investment returns.

For the future we should listen more to the young scientists at the front line of research, who will carry China's science programs to the next level. We believe a supercollider in China will help in a big way.

Public discussion on program welcome

That Yang has used the mass media, instead of a professional journal, to disseminate his idea is welcome because the issue should be discussed not only by physicists, but also taxpayers, because taxpayers' money will fund the supercollider program.

Also, this will allow scientists to share their views with the public, which rarely happens. Physicists believe supercolliders are necessary for the advancement of physics but to build one, a country must have enough money, thoroughly discuss the issue, and cooperate with other countries in technology. China meets all these requirements.

There are fears that the supercollider program might take away a huge percentage of China's budget for basic research. But the cost of 3 billion yuan (\$449 million) a year from 2022 to 2032 is not very high considering the total research budget. Besides, a supercollider will help China catch up with the advanced world in science.

More importantly, particle physics advances on the basis of repeated experiments. Only when China has a supercollider to conduct experiments will its scientists be able to improve their research. Taxpayers should realize that the investment in a supercollider will yield promising returns because it will help upgrade technologies as a whole.