Today a very special event took place in Barcelona: the LatorreFest, a celebration of Jose Ignacio Latorre’s 60 birthday. Jose Ignacio is of course a very important person in both my scientific and personal history, having been my PhD supervisor and then collaborator and friend for almost 20 years now. So together with two other former PhD students of Jose Ignacio, Antonio Acin from ICFO and Roman Orus from the Donostia International Physics Center from the Donostia International Physics Center, we decided to invite a number of the many friends and collaborators of Jose Ignacio to celebrate together not just an anniversary, which is just an excuse, but more a friendship and an adventure in science.
The talks covered a very wide range of topics, which are just a small but representative sample of Jose Ignacio’s ample interests. We started with Pedro Echenique, the president of the Donostia International Physics Center Foundation and Professor of Physics at the University of the Basque Country, and one of the founding fathers of the Spanish physics community. Pedro gave a beautiful and inspiring talk about the role of beauty in science, highlighting how for example Maxwell’s equations represent one of the pinnacles of the human endeavour. He was also careful to emphasise that while beauty can be a guiding principle no theory can be so beautiful that it deserves to the true, and that in science it is experiment the ultimate referee to decide whether or not a scientific theory, either beautiful or ugly, describes our natural world.
Pedro Echenique and Maxwell’s equations.
Pedro’s talk was followed by Luis Alvarez-Gaume, a former staff member of the Theory group at CERN and since a few years the director of the Simons Center for Geometry and Physics at Stony Brook. Then we had a superb talk by Ignacio Cirac from the Max Plank Institute, who provide an extensive and hype-less overview of the present status and future challenges in quantum information and computation. Cirac, who many predict will receive a Nobel Prize for his foundational work in quantum computation, highlighted the many potentialities of quantum computation, and that while we are still far from truly groundbreaking quantum computers we are already in the position to attack many non-trivial problems, many of which with direct societal and commercial applications.
Ignacio Cirac and the many problems that a quantum computer could attack.
Other speakers of the LatorreFest included Stefano Forte from Milan, who emphasized the rile of Jose Ignacio as a visionary, in particular suggesting the crucial role that neural networks and machine learning tools could have in high energy physics well before this techniques were as commonplace as they are now; German Sierra from IFT Madrid, who discussed another of Jose Ignacio’s passions which is number theory and in particular what we can learn about the properties of prime numbers using quantum computers; and Manuel Asorey from the University of Zaragoza, who presented another of Jose Ignacio’s main achievements and that has been a driver for excellent science both in Spain and worldwide: the now-famous Benasque Center for Science.
The last talk of this excellent event was given by Juan Fuster from IFIC in Valencia, who discussed the future of high-energy physics and particle accelerators. And he also presented another of Jose Ignacio’s many passions, namely wine-making! Quite possibly the wine that Jose Ignacio, Juan, and their collaborators produce every year is the most scientific one ever made, and is arguably the only wine I am aware of that is directly inspired in quantum mechanics. Juan presented a strong case for a future high energy particle collider, emphasising that the exploration of the energy frontier is far from a job done and the role of a global approach to built such machine as soon as possible, ideally to ensure a smooth transition with the operations of the high-luminosity LHC.
Quantum is arguably the only brand of wine ever made that is directly inspired by the principles of quantum mechanics.
It was a most enjoyable day and a beautiful opportunity to celebrate together a prolific friendship. In a time where toxic dynamics of power, harassment, and exploitation in the scientific world are so in the spotlight, I feel truly privileged by having had such a selfless, devoted, and inspired PhD advisor as Jose Ignacio. Many congratulations, and remember that the best is yet to come!
Amazing lineup of speakers at the LatorreFest60, almost modern version of the famous Solvay conference picture ….
Since I am still exploring the potentialities of the blog, I will try next to summarise one of my recent research publications, which addresses the rather non-trivial point of whether or not possible new physics beyond the Standard Model can be confounded by effects of the strong interaction. Let’s see how this works out!
At hadron colliders such as the Large Hadron Collider (LHC), one accelerates protons up to almost the speed of light and then makes them collide. By reconstructing the debris of such very energetic collisions, we can have access to the laws of nature at the smallest of the distances, well below the atomic or the nuclear radius. Since protons are not fundamental particles, the LHC is more of a Large Quark and Gluon Collider (LQGC), though it is unlikely that this acronym will catch up. Therefore, to be able to make predictions about what will happen at the LHC, for example how many Higgs bosons will be produced within a year, it is not enough to know the energy of the colliding protons: we also need to determine how this energy is distributed among the proton’s constituents, namely the quarks and gluons.
To be able to predict the event rates of processes such as W boson production, we need to understand the quark and gluon content of the proton first.
The information about the energy distribution of the quark and gluons in the proton is encoded by quantities called the Parton Distribution Functions (PDFs), see here for a rather extensive recent review. We cannot compute these PDFs from first principles, at least with current technology, so we need to extract them from experimental data using a global QCD analysis – basically a big fitting machinery where the parameters that define the PDFs are adjusted to maximise the agreement with the input experimental data.
A recent development in this context has been the availability of LHC measurements themselves to constrain the PDFs. For example, one can use the production of top quark pairs to obtain information on the gluon content of the proton. As you can see from the Feynman diagram below, the production of top quark pairs provides direct information on the gluon content of the proton. The same holds for many other processes, for example, the production of a lepton-antilepton pair, the so call Drell-Yan process, allows us to separate the quark and antiquark content, and disentangle different quark flavours among them.
LHC processes provide valuable information on the quark and gluon content of the proton.
As the LHC accumulates more and more data, these PDF-sensitive measurements start to probe higher and higher scales, well above the Higgs boson mass. While in our determination of the PDFs we always assume that the Standard Model calculations are valid everywhere, if new physics Beyond the Standard Model are present within the LHC reach (perhaps in the form of subtle deviations with respect the SM predictions) there is the real risk that they would be “fitted away” into the PDFs. In other words, you would be missing out a unique opportunity to identify deviations with respect to the Standard Model since you have reabsorbed them into the PDFs!
One possible way to eliminate this risk is by determining simultaneously the PDFs together with this possible New Physics effects. So in our paper we performed the first join extraction of the PDF parameters and of the coefficients of the Standard Model Effective Field Theory (SMEFT). The SMEFT is a powerful theoretical framework that encodes in a model-independent way the effects of any new physics scenario at high energies that reduces to the SM at low energies. The question we want to address is to which extent these SMEFT effects can be reabsorbed into the PDFs. In other words, is new physics hiding in plain sight and we are just fooling ourselves since we are fitting it away into our QCD parameters?
One first check is to perform a sampling of the SMEFT parameter space and run PDF fits for a number of benchmark points. Does the fit quality improve once these additional New Physics parameters are added? The answer is that it does, though not dramatically. As you can see below, the fit quality (quantified by the ) improves at the post-fit level, that is, once the PDFs are readjusted to match the change in underlying theory as compared to the SM assumption. The effect is not dramatic but noticeable, showing that yes, indeed, partially new physics effects can be absorbed into the PDFs.
The fit quality improves once PDFs are fitted with the SMEFT calculations, showing that the latter effects can be partially reabsorbed into the PDFs.
If this is the case, how we can tell SM effects from possible new physics ones? One can exploit kinematical differences between effects that arise from the strong interaction (one of the three fundamental forces that compose the Standard Model) and those related to physics beyond the SM. They key aspect here is the different dependence on the energy of these effects. In the case of the strong force, as we go towards higher energies we expect differences that scale as , while in the case of SMEFT corrections, they scale as . In the previous equations, is the typical mass scale of the strong interactions (basically one third of the proton mass) while is the (unknown) energy scale where new physics beyond the SM appear. And indeed, if one plots the fit quality as a function of the energy of the process, one observes a very different trend in the case of the Standard Model (feeble dependence with the energy) and in the SMEFT case (strong sensitivity to changes in the energy). Such trend would be the smoking gun to disentangle QCD and new physics effects within the global fit.
The fit quality exhibits a different qualitative trend in the SM case and in the case where there are deviations with respect to the SM theory.
To summarise, in this work we have successfully demonstrated how one can disentangle possible BSM effects within the global PDF analysis. The next step will be to carry out a more extensive interpretation of LHC measurements as well as to explore a basis as wide as possible in the SMEFT parameter space.
Disclaimer: I write a first version of this article as part of my guest blogging in the Particle Physics People blog at Interactions.org. Given the intense discussions about the future of high energy physics that are taking place in the last months, including the update of the European Strategy, I though it would be nice to extend a bit this entry and post it this time in my new blog. Hopefully this will also allow testing the comments functionalities!
It is a well known fact within the community that high energy Physics finds itself at a crossroads. Paradoxically, the main reason for this state of affairs is none other than the extreme success of both our theoretical framework and our experimental programs. Indeed, our current understanding of elementary particles, as encapsulated by the Standard Model, has so far been confirmed with exquisite precision by countless experiments, except for a handful of anomalies that might or might not lead to something deeper. Even then, there are plenty enough urgent fundamental questions that are so far left unanswered! Indeed, while the discovery of the Higgs boson in 2012 by the ATLAS and CMS collaborations implies that there are no obvious targets of where the next layer of complexity in our understanding of physical reality can be identified, we should not be idle and keep pushing our efforts towards addressing these pressing questions.
To begin with, the Standard Model (SM) does not provide a candidate for dark matter, the mysterious non-luminous form of matter five times more abundant than normal matter and whose existence we infer from astronomical observations. It does not provide either a microscopic mechanism for the dark energy accelerating the expansion of the universe. Neither does the SM explain how the observed asymmetry between matter and antimatter was generated in the early universe, nor the fact that neutrinos have non-zero masses. While some of these puzzles have different levels of relevance (for example, it is not complicated to augment the Standard Model to include neutrino masses), all of them are truly fundamental questions which we should work hard to answer if our goal is to understand Nature at its deepest levels.
In addition to these ’observational’ conundrums, the SM also contains several puzzles of a more theoretical nature. To begin with, we still don’t know for sure if the scalar boson observed at the LHC is really the SM Higgs boson, or if it is instead a more complicated creature. For example, it could very well be that the Higgs is a composite particle itself, in the same way as how protons are not fundamental but rather composed by quarks and gluons. In addition, in the SM the mass of the Higgs boson is not protected by any symmetry, and for this reason it will tend to grow up to the highest energies at which the theory is valid. In this respect, we do not really understand the unbearable lightness of the Higgs particle. We also have no clue whatsoever of the origin of the flavour structure in the SM, for instance why there are three generations and not 27, and what mechanism determines the observed values of the masses of the SM particles. Moreover, the Higgs boson interactions are mediated by a force completely different from any of the forces we know (such as electromagnetism, drive by the gauge principle) so if there are any hidden sectors beyond the SM the Higgs offers a unique portal to access therm. So there is definitely no lack of fascinating problems to be tackled! Whether these theoretical puzzles are real conceptual issues or not is hotly debated within the community. For example, perhaps the flavour puzzle is merely a consequence of anthropic selection, as speculative as the latter is. In any case is clear that there is still a lot to learn about fundamental physics at the higher energies and smallest distances.
Going even deeper into the foundations of high-energy physics, we don’t know how to marry the two most arguably successful physical theories ever formulated, quantum mechanics and general relativity. Indeed, the ongoing quest for quantum gravity has turned out to be a formidable challenge attacked without success by some of the most brilliant physicists of the last decades. The fact that the experimental signatures of quantum gravity are in most cases orders of magnitude beyond our foreseeable experimental reach does for sure not help in this context. Quantum gravity has been so far the playground of mostly theoretical speculations, though there are hopes that its effects can be probed experimentally in the near future either from cosmological observations or from ultra-high precision measurements of quantum systems. Formulating a theory of quantum gravity would be a massive breakthrough, comparable or even more important than the formulation of quantum mechanics and general relativity themselves.
I encourage the interested reader to take an interactive look at the various mysteries of the Standard Model and the various “Theories of Everything” that have been proposed in this infographic by Quanta Magazine.
Image credit: Quanta Magazine
In addition, particle physics is much, but really much, broader than just searching for new theories beyond the Standard Model or trying to formulate the theory of quantum gravity. Particle physics is also about achieving an improved understanding of pressing questions within the strong interaction, including how the masses and spin of the hadrons are generated in terms of its constituents, whether or not heavy quarks are part of the proton wave function, the possible onset of extreme dynamical regimes of gluon-dominated matter, or pinning down the properties of the Quark-Gluon Plasma, the hot and dense medium created in the collisions between heavy ions. Anyone who reduces high energy physics to model building is deliberately misleading, and leaving out crucial (and thriving) areas of our field.
One of the main hopes to unravel the next layer of the physical reality is that the thorough exploration of the Higgs boson properties can shed some light on the SM mysteries. For instance, we are now only starting to scratch the surface of the Higgs particle, and current and future measurements at the LHC will tell us more about its underlying nature. Indeed, one of the main goals of the High-Luminosity upgrade of the LHC (HL-LHC), which will deliver up to a factor 10 more collisions, is the accurate profiling of the properties of the Higgs boson, where any deviation with respect to the tightly fixed properties of the SM would represent a “smoking gun” for new physics beyond it. Crucial in this context is the measurement of its self-coupling: not only we have never observed a fundamental scalar particle interacting with itself, it could play a role to explain the matter-antimatter asymmetry in the universe by means of electroweak baryogenesis.
While the HEP community is certainly together in its support for the full exploitation of the physics potential of the HL-LHC as a major priority, it’s less clear what should come next. Should we build yet a bigger particle collider? A different type of collider? Perhaps the key is in the intensity, high-precision frontier? Should we focus on completely different types of experiments, perhaps more weighted towards astrophysics and cosmology? Something else that no one has even thought of before?
In this context, one particularly attractive proposal goes under the name of Future Circular Collider (FCC). The FCC would be a gargantuan particle collider with a radius of around 100 kilometers, dwarfing the already pretty huge LHC. This collider could accelerate protons up to the extreme energies of 100 TeV, about 7 times more powerful than those available at the LHC. In addition, this machine could also accommodate the collisions between electrons and positrons at high energy and luminosity, which would make extremely high precision characterization of SM particles possible, such as the Higgs boson, the W and Z gauge bosons, and the top quarks. Similar machines are under active study by the Chinese HEP community. Another proposal for the next collider is the International Linear Collider (ILC), a high energy linear accelerator of electrons and positrons, to be hosted by Japan.
While it would be amazing if we had machines like those at our disposal, they will come with a hefty price tag, and it is obviously not a decision that can be taken lightly, and the science case in each option must the weighted carefully. One particularly challenging aspect of the current situation for high-energy physics is that there is no machine that can guarantee discoveries, such as new particles or novel fundamental interactions. This was not the case in the past: at the LHC for instance there was a “no-lose” theorem guaranteeing that it would either discover the Higgs boson or instead an altogether novel force of nature. It is worth emphasizing that this is true also for many other fields, such as cosmology, where there is no current or planned experiment that can lead to guaranteed breakthroughs such as evidence for inflation or pinning down the nature of dark energy. The pros and cons of the various proposals for future collider are now being discussed in detail within the community, see for example the recent EPPSU open meeting in Granada.
The bottom line of all this lengthy disquisition is that future progress in HEP should be driven by exploration, rather than by theoretical prejudice, see also Nima Arkani-Hamed’s reflections. For many years (better said, decades) HEP was driven by theoretical efforts, with experiments successfully confirming prediction after prediction. But now our field is experiencing a U-turn, where we should think outside the box and be ready for the unexpected. A nice example of the latter is provided by the recent anomalous in the b-quark sector presented by LHCb. These anomalies seem to indicate the violation of one of the cornerstones of the Standard Model, namely the symmetry telling us that leptons of different families (say muons and electrons) interact with other particles in exactly the same way. Only time will tell the fate of these anomalies, but if confirmed they would represent an arguably more important discovery than that of the Higgs itself!
With the same motivation, and in order to make sure that no stone is left unturned, it is healthy for our field to develop a varied program of experiments that are not limited to high-energy colliders. For instance, CERN has recently set up a Working Group focusing on the potential of ’Physics Beyond Colliders’ (PBC). The idea underlying this approach is that high-precision measurements of specific properties of known particles can reveal the presence of new, heavy particles beyond the direct reach of future colliders. This is possible by means of quantum effects, where heavy virtual particles pop up from the vacuum for a fleeting moment, leaving a measurable imprint in the SM particles.
A prime example of this precision program is shown above: the muon storage ring at Fermilab. There the “muon g-2” experiment aims to measure with exquisite precision the internal magnet of the muons, its so-called magnetic moment. The hope is to resolve a long-standing discrepancy between similar measurements and the SM predictions, which could unveil new physics beyond the SM.
Can we now summarise what the best option is for the future of HEP? Well, not really, this is precisely the million dollar question! Every member of the HEP community, including several popular bloggers within the field, has something important to say there. I think that irrespective of the exact path that our field chooses for the next years, the future is certainly bright for particle physics and everyone should certainly stay tuned for news from the high-energy frontier!
As part of an academic training course that I recently completed (the Senior Kwalificatie Onderwijs aka Senior Teaching Qualification or STQ), one of the assignments was to reflect about one problem that affects the Dutch higher education system and how would I tackle it if I were the rector of my university. I though that an abridged version of this assignment could be a nice first entry of this blog, so here it is.
The marked increase in student numbers in the Dutch higher education system in the last two decades have lead to an (non proportional) increase in the number of faculty positions, that is, in the number of staff that in principle are expected to carry out a research program. Note that most TT positions have as condition for tenure applying and securing external funding, so this is really not an option. Unfortunately, the fact that the figures for government-funded research have not changed much in the same period, coupled with the situation that in most universities there are no structural funds to carry out research programs, means that the success rates of competitive funding applications is plummeting. Currently, the situation is becoming ridiculous: in the last call for the NWO Physics Projectruimte, the success rate was lower than for the already very competitive European-wide ERC Consolidator grant. In other words, even projects where one asks for one PhD student plus some running budgets have now success rates at the 10% level. Under these conditions, there is growing evidence that currently researchers spend similar or even more time writing grant applications that carrying out the actual research, resulting in a situation where universities are effectively throwing away precious resources while receiving very little in return.
As the number of students that join higher education in the Netherlands has markedly increased in the last 20 years, the total contribution that the universities receive per student has went down by around 25%.
While these low success rates in competitive funding applications represent a global problem in higher education, the fact that the Dutch university system has limited or no structural research funds only exacerbates this situation. In other European countries this is not the case, for example, in the United Kingdom there are “rolling grants” that are renewed every few years that that provide structural funding for PhD students and postdocs within given research group. Moreover, within the Dutch research ecosystem, university staff are in marked disadvantage as compared to for example group leaders at NWO institutes: since the latter do not have (or have much less) teaching and management responsibilities, they can devote more quality time to write competitive grant applications. All these factors pile up, and combined with the already high work pressure are leading the system to a position which might not be sustainable in the medium and long term, at least if the goal is to maintain or even improve the outstanding quality level of the Dutch scientific research.
In these circumstances, how can overworked university staff find time and energy to, in addition of their various other duties, also aim to innovate and excel in education? For instance, my own university, the VU Amsterdam, in their vision document states that they have in mind “a vision of education which places a strong emphasis upon investigative learning. Asking the right questions is at least as important as giving the right answers. The close links between our education and research activities keep the quality of education high and ensure that students are constantly being challenged intellectually.” While I fully agree with these ambitions, however I believe that both the high work pressure in general and the (ever-increasing) need to apply for competitive funding with (ever-decreasing) success rates in particular hamper seriously this vision. For example, developing and implementing innovative education methods that succeed in activating the students and promoting them to become independent learners takes time and effort, commodities that are scarce in the current environment of higher education.
How to move forward? This is a thorny problem without easy solutions, and clearly requires coordinated action among all the Dutch universities. Let me however suggest some possible strategies in order to at least reduce the severity of this problem.
Universities, either at the faculty or at department level, could allocate some structural funding for research, for example from the first money stream (in the case of successful education programs) or from the overheads coming from large NWO and ERC projects. This structural funding could be distributed among departments following some model, where for example the management teams distribute PhD positions following an internal, low-hassle, application process. This seems to be one of the suggestions of the infamous Van Rijn report, namely to transfer fundings from the second money stream (now allocated purely on the basis of competitive application) to the first money stream (more structural, in some sense).
Provided the involved researchers agree, another option could be the departments could also allocate a fraction of personal grant funding to other researchers who have not been successful in similar grant application but whose research project has been evaluated very positively: a model that fosters cooperation between academics, in particular colleagues, rather than competition.
Moreover, In the same way that in a company the people that work in the R&D department are different than those that work on sales or in communication, not everyone in the university should excel (and devote the same amount of time) both in teaching, research and management. In this respect, the creation of prestigious, teaching-oriented professorships, as well as a well-defined career track for those academics that aim to focus in teaching excellence would achieve two important goals. First, to reduce the large teaching load that affects most departments: someone working say 90% of her/his time in teaching is going to me more efficient than three academics spending each 30% of their time (due to scalability, more time for planning and implementing feedback, better incentives to develop innovative teaching methods, …). Second, to reduce the competition in research funding applications and increase success rates even for fixed resources. This is also something that has been advocated by the VSNU, and the UvA Science faculty is also moving towards this system.
Thirdly, both systematic studies as well as ample anecdotal evidence demonstrate that the current model for research funding allocation has a very strong stochastic component (sheer luck!) as well as a number of biases (against women, in favour of prominent institutions against PIs that have not obtained big grants before, …), so that they are not necessarily a proxy to identify the most promising research programs. The Dutch universities and NWO should work towards modify the model in a way that is much less burdensome for researchers (as well as for external referees and civil servants) and that it reflects better its intrinsic limitations (for example, by randomising grant funding allocation for research projects that satisfy all the requirements described in the call for proposals and satisfy some given quality threshold). Increasing the success rate in competitive grant funding and ensuring a most unbiased selection will be extremely helpful in reducing the work pressure of the staff from our universities.
I believe that this is a genuine problem that seriously hampers the long-term excellence and viability of Dutch research and higher education system. If we keep fostering aggressive internal competition between university staff and researchers, rather than promoting collaboration, we are clearly shooting ourselves in the foot. While we are not going to solve these complex problems any time soon, starting to take actions in some of the directions outlined above could already improve the general climate and show that we are taking this severe problem into consideration.
This will hopefully become soon a full fledged personal website and blog, so stay tuned! In the meantime you can contact me via email or find me on Twitter. Some information about myself as well as my contact coordinates can be found here. A brief snapshot of my research interests is also provided, as well as a list of selected talks at seminars, conferences, and workshops.
) improves at the post-fit level, that is, once the PDFs are readjusted to match the change in underlying theory as compared to the SM assumption. The effect is not dramatic but noticeable, showing that yes, indeed, partially new physics effects can be absorbed into the PDFs.
we expect differences that scale as 
, while in the case of SMEFT corrections, they scale as 
. In the previous equations, 
is the typical mass scale of the strong interactions (basically one third of the proton mass) while 
is the (unknown) energy scale where new physics beyond the SM appear. And indeed, if one plots the fit quality as a function of the energy of the process, one observes a very different trend in the case of the Standard Model (feeble dependence with the energy) and in the SMEFT case (strong sensitivity to changes in the energy). Such trend would be the smoking gun to disentangle QCD and new physics effects within the global fit.
Juan Rojo's Website 