An odd interview about the odderon
with Tamás Csörgő by Georgina Anna Zsóri
- Dear professor Csörgő, could you please introduce yourself a bit?
-I am a nuclear and particle physicist, member of the Academia Europea, an experienced outreach speaker and inventor, as well as a novice translator and poet.
- Please, tell us a little about your recent discovery of the odderon!
-It was an odd discovery of the odderon: 48 years after the first prediction, after a long-lasting and intense international competition, our Hungarian — Swedish team of five physicists published, for the first time, decisive evidence for odderon exchange. This result implies the existence of several, new kinds of strongly interacting particle states, too. This discovery of the odderon has just been based on a meta-analysis of public domain data of the D0 experiment at Tevatron, Fermi National Laboratory in the US and public domain data of the TOTEM collaboration at CERN LHC. Our discovery was published in the respected European Physical Journal, EPJ C in February 2021. This meta-analysis of public domain data was followed in July 2021 by a theoretical analysis by me and my PhD student István Szanyi, which raised the statistical significance of odderon observation to at least 7.08 σ. The D0 and TOTEM Collaborations published an experimental paper on a statistically significant observation of odderon-exchange in August 2021. D0 and TOTEM extrapolated the TOTEM proton-proton data in the region of the diffractive minimum and maximum from 13, 8, 7 and 2.76 TeV to 1.96 TeV and compared this to D0 data at 1.96 TeV in the same t-range. This analysis found an odderon significance of at least 3.4 σ. When combined with the 3.4–4.6 σ odderon signal based on TOTEM experimental data and theoretical models at 13 TeV at t=0, this D0-TOTEM analysis also resulted in a discovery level odderon significance of at least 5.2 σ. So we stand confirmed, and the papers that solved this 48-year-old mystery were published nearly simultaneously, in a short time interval of about 6 months. Solving a 48-year-old issue in high energy particle physics is a rare achievement as far as I know.
-Could you please define what the odderon is?
-According to modern physics, all interactions are mediated by particle exchanges. The well-known electromagnetism can be described by the exchanges of photons, quanta of light. Strong interactions are mediated by the exchange of gluons. In elastic proton-proton and proton-antiproton scattering, odderon exchange corresponds to the exchange of three or more, odd numbers of gluons, while pomeron exchange corresponds to the exchange of two or more, even numbers of gluons. Pomeron exchange leads to similar interaction between elastic proton-proton and proton-antiproton collisions. Odderon exchange leads to a small difference between elastic proton-proton and elastic proton-antiproton collisions. Due to the smallness of this difference, it was very difficult to find evidence for odderon exchange. In popular terms, one may say that finding the odderon is something like finding a set of odd numbers, without actually identifying all the odd numbers that belong to this set. Or, one may compare odderon to an odd forest: you see the forest, but you are not yet close enough to see each tree individually.
Actually, in our paper, we used the following odderon definition: elastic proton-proton and proton-antiproton scatterings are both characterized by their scattering amplitudes. If the energy of the collisions is high enough, in the TeV energy range, only two terms, the pomeron and the odderon exchange contribute to these amplitudes. Thus we defined pomeron exchange as the average of the proton-proton and proton-antiproton scattering amplitudes, while our definition of odderon exchange was the difference of the proton-antiproton and proton-proton elastic scattering amplitudes for high, at least 1 TeV colliding energies.
- Did you accidentally come across the odderon problem, or has it been researched for a long time?
- Odderon has been searched for a long-long time, close to 48 years. The first published proposal for a possible odderon exchange was written by B. Nicolescu and L. Lukaszuk already in 1973. The topic of odderon search at the CERN accelerator LHC was proposed to me by professor László Jenkovszky, an external member of the Hungarian Academy of Sciences from Kyiv, Ukraine. Professor Jenkovszky is a well-known expert in this field, who spent his life studying similar processes. We published a paper together with A. Ster and L. Jenkovszky in 2015, more than 6 years ago. In this paper, we proposed that the LHC accelerator is the ideal place for looking for the effects of odderon exchange.
Of course, 48 years in high energy physics is a very long time to search for a new family of particles, it can be considered as extremely long.
- What made it difficult to find the Odderon?
- Its elusiveness. The signal is very small and also it mixes with other participant signals at lower energies so the key thing is that you have to go to very high energies. And then only two exchanges contribute, the pomeron and the odderon exchange. Pomeron exchange is an interaction exchanging an even number of gluons while odderon exchange is exchanging an odd number of gluons. If you have an exchange of massive particles which we know for example quarks, electrons or Z0 bosons, anything which does not carry a charge and has a mass, is suppressed as a negative power of the colliding energy, so the suppression is increasing with energy. However, the odderon and the pomeron exchange corresponds to the exchange of gluons. Gluons are massless and their exchange is not suppressed. So at the LHC energies, all the other exchanges are less than the experimental error bars. It follows that it was very difficult to find it because the signal is very small and it seems to grow very fast in the LHC energy region. An experimental paper from 1985 by Breakstone et al, published in the Physical Review Letters, had a signal for the odderon exchange of 3.35 sigma which means about 99.9 % probability of an odderon signal. However, this probability in high energy physics is not acceptable for evidence or proof of existence: we consider as a strong indication but we require that the signal must be above 5 sigmas. This level of certainty was only possible to get in 2021, first by our paper, then confirmed by two other papers later in 2021. The 5 sigma discovery threshold means that the probability of the discovery is at least 99.9999 % or larger. Our discovery paper found a more than 6 sigma effect, corresponding to at least 99.9999999 % probability. In a subsequent paper, we were able to increase the number of 9-s to more than 300, so in practical terms, this is a certainty, a probability that is essentially 1.
We had to face several difficulties to reach such a level of certainty. The main difficulty was that we did not have the accelerators for proton-proton and proton-antiproton in the TeV range that were operated at the same colliding energies. The Tevatron accelerator broke this TeV threshold at Fermi National Laboratory in the US so they have published measurements at 1.8 and 1.96 TeV for proton-antiproton data, while the LHC accelerator provided the proton-proton data. Nevertheless, for an odderon search, you should have the two sets exactly at the same energies and that was not possible. The TOTEM data had to be extrapolated down to the Tevatron energies from the measured energies of 2.76, 7, 8 and 13 TeV and this extrapolation requires not only experimental but theoretical skills too. And this was where we had some advantage.
- How long did you work on the project before you came up with any results?
- Well, we started to work on this project in about 2016. We solved several other problems and published other papers but actually, the real missing piece was a good idea to solve the problem. The data collapsing behaviour was the key to our discovery. We have seen that the proton-proton data at 3 different energies, 2.76, 7 and 8 TeV at LHC collapse to a simple scaling curve, if you find the good scaling variables and this data collapsing behaviour made it possible for us to go down to the Tevatron energies. This idea was very difficult to find because nobody expected such data collapsing behaviour before. However, once we have the idea from the concession of the idea to get to the correct result was a short time. We have published the first result in the proceedings of a conference which is the so-called International Symposium on Multiparticle Dynamics in 2019 in Santa Fe, New Mexico, US. I remember very well that the idea that solved the problem was born on the 9th of September, the first day of the conference. I gave a talk on the 13th of September 2019, the last day of the conference, and that time giving the talk we just presented the idea but the significance was evaluated a few days after the meeting. By October we had the correct results and then by April next year we had in our conference paper the correct result. We submitted the paper form for publication in December 2019 already so that was the first time we placed our result in the public domain. The conference paper came only in April 2020 so basically, you can see that from September to December we managed to get the result but we had to wait more than one year for publishing the result. The most difficult part of the process was to go through the anonymous review process of publishing the paper.
- Who contributed largely to this discovery?
- First of all, let me mention that the discovery has several meanings and we use the discovery in the sense of Albert Szent-Györgyi, the Nobel laureate Hungarian scientist who discovered vitamin C and the Szentgyörgyi- Krebs cycle. His view about the discovery is very famous. He said something like that a discovery consists of seeing that everybody has seen but thinking that nobody has thought before. Well, his view is very relevant in our case because we have somehow seen the same data, like everybody else: data that have been in the public domain published by the TOTEM Collaboration at LHC and by the D0 Collaboration at Tevatron, but nobody imagined that the TOTEM data have a certain kind of data collapsing behaviour.
When we realized that it does have such a scaling or data collapsing behaviour, then we were making the discovery. This new kind of view was the key question to reach and to publish the result for the first time.
Our team consisted of 4 Hungarian and a Swedish colleague. I had the idea of the data collapsing behaviour, we discussed it and my Hungarian colleague András Ster implemented it. Then two of my colleagues, Tamás Novák and István Szanyi, both Hungarians, joined our team and they did several data and cross-checks. Tamás Novák and András Ster implemented the idea in two different ways, in independent software, different languages and István Szanyi worked with me to have the theoretical interpretation and to understand the domain of validity of the scaling. Our Swedish colleague Roman Pasechnik was our theory expert and provided a lot of inputs and discussions and made sure that the theoretical concepts from QCD are correctly implemented. Overall, all of us contributed equally and the discovery would have been impossible without the public domain data of the D0 and the TOTEM Collaborations.
- How should we laypeople imagine the research process?
-I think it is like an upward going helix, or, an ever-increasing spiral.
-You have some data, then you try to put it into some context and interpret it. When you have an interpretation, you make predictions and then cross-check the predictions against the data. Some things work but then you will see some more details and deviations from the predictions so you have to improve the model and so the cycle starts again. This is an interplay between our understanding and our observations and this interplay results in new questions. We ask Mother Nature for answers. It is kind of a process that walks upon an increasing spiral but you need two legs, one is the experimental observations and the other leg is the theoretical understanding. The process without any of these can not progress. In my opinion, what is very attractive in this is the intellectual challenge. I think it is a very nice present from life that you can solve a scientific problem that no one else has been able to solve for 48 years.
- Did you have to face any difficulties during the research process? If yes, please tell me some details about it!
- Yes, of course, we had several difficulties. I would like to forget them, so I prefer not to detail them this time. In Hungarian history, one can find certain periods, when we Hungarians had to fight for our freedom. These periods were incomparably more difficult than the search for odderon — although I do not remember any Hungarian fight for freedom that lasted for 48 years …
- Was the coronavirus an obstacle to the work?
- No, it was not. COVID was supporting our case in several ways. Due to the home office and the decreased or vanishing travels, I could learn new skills and my life was calmer. I was happy because I could focus on this problem, and I could work on it without interruption. We had some financial problems because we had a grant, we had to pay some membership fees in the big collaborations and we were short of money. However, because of the coronavirus, we had sufficient money left over from the cancelled conferences and other scientific travels so we managed to solve the funding problem and we had enough time to work on this problem. Collaboration with my colleagues was smooth due to the good internet connectivity.
- What locations did most of the work take place in?
- Most of my work has taken place in Gyöngyös Hungary, in my offices at the local Károly Róbert Campus of the local University. I spent a whole week there, without going home, because we wanted to finish the paper in 2019, before the end of the year. That was an incredible amount of work. My colleagues worked very hard, András Ster and István Szanyi in Budapest, Tamás Novák also in Gyöngyös and Roman Pasechnik in Lund, Sweden, and we interacted smoothly with the help of internet connections.
- What was the approximate budget for the research?
- It is very difficult to evaluate the specific cost of this discovery because it was more like a question of the quality of people and not a question of money. So the basic information is that to be able to discover the odderon the LHC accelerator and the Tevatron accelerator both were necessary and if you look for the construction cost of the LHC then actually you can see that it is one of the most expensive scientific equipment ever built. And this is just one of the two accelerators. The LHC and the Tevatron accelerators were not constructed to find the odderon but without them, you could not find the odderon.
Although I cannot give you a full answer to this question, I can give you an order of magnitude estimate of our annual research budget, available based on public documents and funds. Of course, our basic salaries were covered by our home institutions, both in Hungary and in Sweden. In Hungary, our current research grant covers about 12 MHUF annually, before taxation. After taxation, the amount was just sufficient to pay for the annual membership fees of my group in TOTEM and CMS experiments or about 26 kCHF, but not much was left after this step. We had some other support for research visits between Hungary and Sweden, but by 2020 these resources were not usable due to COVID travel limitations. The annual budget of the TOTEM experiment at CERN was of the order of 550 kCHF, but it covers all the experimental and personnel costs of running the TOTEM experiment.
Thus it is my order of magnitude, inofficial estimate, that the funding available for completing the D0-TOTEM experimental odderon discovery paper was at least a factor 20 larger than the funding resources available for the Hungarian group and the Hungarian — Swedish collaboration for our odderon discovery papers. I have to stress that people from my Hungarian group played important roles in all the three odderon discovery papers so far, and I am very pleased about this fact.
- Were there any supporters of the research?
-We had several supporters and the full list of supporters is included in the acknowledgement section of our odderon discovery paper from February 2021. Our supporters included the Swedish Research Council, the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme, as well as by the NKFI Grants K133046, FK-123842 and FK-123959 as well as the EFOP 3.6.1–16–2016–00001 grants in Hungary. Our collaboration was supported by the framework of EU COST Action CA15213 THOR. The odderon discovery paper of the DO-TOTEM collaboration from August 2021 (where we have also contributed as co-authors) was supported by all the funding agencies that support D0 and TOTEM. This list includes about 50 funding agencies from all over the world, including two more funding agencies from Hungary as well.
The cost of the Hungarian participation in the TOTEM experiment so far totals up to 330 000 CHF from 2008 to 2021 and we had to apply for parts of this money from various funding resources. You can also ask what is the funding level of our Hungarian Swedish collaboration and I can just roughly tell that we have now a research grant, the funding level is about 12 million HUF per year but compared to the available resources for the large experiments this is a very small fraction. On the other hand, it is very important for us, because it makes it possible for us to collaborate and in some limited sense also to compete with the big LHC experiments.
On this occasion, I would like to thank all the supporters and funding agencies, that sponsor our research, which has been supported by several organizations including NKFIH, the Hungarian National Office for Innovation and Development. One of our supporters was also Circles of Knowledge Club (Hungary) and we had support from the European level of funding for the Hungarian — Swedish collaboration and all the supporters we have listed at the end of our papers in the Acknowledgements section. We are very grateful for the Hungarian taxpayers and the international taxpayers because without their funding we could not perform this research.
Some reasonable level of funding is thus necessary, but it is not a key element of a successful research project. Scientific success is more about having the correct vision or scientific foresight: if you have the right idea and you can implement it, you can get the correct answer. With a reasonable amount of funding, the answer is coming earlier, with lack of resources, the answer is delayed but still, it can be obtained if the researchers have sufficient perseverance. On the other hand, if you do not have the right scientific questions and the right scientific foresight, you will not get the correct answers, regardless of how much financial resources you may have. Let me paraphrase Euclid: There is no royal road, not only to geometry but also to other fields of science.
- What is the significance of your discovery of odderon?
- There are very few puzzles in particle physics that are solved only after 48 long years of research. Now, in 2022, we do not know the answer to your question yet. We had to wait for the discovery of the odderon for 48 years. So I recommend asking this question after another 48 years, around 2070.
- What other new research directions are opened by the odderon discovery?
- The existence of the odderon implies the existence of several other gluonic excitations or new strongly interacting gluonic bound states, namely the so-called glueballs. We expect that there will be little rings of gluons and they will have 3, 5, 7 or other odd numbers of gluons inside and the research for their discovery is intensified. According to the so-called Regge theory, pomeron exchange implies the existence of glueballs formed from an even number of gluons, while odderon exchange implies the existence of glueballs from an odd number of gluons. These states may be produced in the so-called central exclusive production mode and then their decay properties can be experimentally observed.
Now we have discovered the evidence for odderon exchange, the exchange of a family of such gluonic rings inelastically scattered protons and antiprotons. So these new gluonic states are very exciting because they do not contain quarks. So far, the LHC experiments discovered 59 new hadrons: strongly interacting particles which are all composite particles, consisting of known quarks and antiquarks so these states are not fundamentally new. In contrast, the odderon is a new family (a Regge trajectory in technical terms) of strongly interacting composite particles called glueballs. As these glueballs do not contain quarks, they are new kinds of strongly interacting particles and they have not been observed before in experiments.
So far all the known strongly interacting particles contain quarks and antiquarks. They were classified either as baryons or mesons. (Anti)baryons are bound states of three (anti)quarks. There are some recently found exotic hadronic molecules like the X, Y and Z states too, that may contain 4, 5 or 6 quarks. The odderon and the pomeron do not belong to this already familiar family of hadrons.
The other, trivial new research direction is to determine the properties of the odderon. This research is ongoing and is very interesting too. I think we need a little bit more time to see the long-term implications of the discovery of odderon-exchange.
We thus have good reasons to hope that the discovery of the odderon exchange in high energy proton-proton and proton-antiproton elastic collisions may open a new chapter within the standard model of particle physics.
