by Panos Charitos. Published: 22 June 2012

Professor Sudhir Raniwala - University of Rajastanl

A.M. My first question is how long have you been collaborating with the ALICE experiment

S.R. have been associated with the ALICE experiment since the time of Indian team’s participation in it. In more specific terms, we were exploring the measurements, and the subsequent conclusions, that can be made using the completely indigenous photon multiplicity detector (PMD). This was about 15 years ago. At that time what captured my interest was the so-called anisotropic flow. So the question was whether we can measure the anisotropic flow with the photon multiplicity detector and how useful it would be for the ALICE experiment. We investigated this, and demonstrated the feasibility of the measurements of anisotropic flow using the PMD. We used toy model simulations and analyzed them using the latest analysis technique available at that time, the Fourier expansion of azimuthal distributions, now known as the “standard method”. This is what I started working on and this kept me busy for a few years. During this time we also explored little subtleties regarding this technique and the effect of other phenomena on its measurements.

A.M. Have you been working here at CERN or in India? Where did you spend most of your time?

S.R. I spend most of my time in my home institute and I try to spend about a month or a little more at CERN each year to help me be more current with the activities here. For the past some time, I had taken a break for an administrative job, which constrained the time that I could spend pursuing my interest in ALICE. I have now resumed my participation to pursue Physics in ALICE, in an attempt to probe deeper into the interesting Physics that can be discerned here. It is like homecoming, and I am liking it.

A.M Could you please tell us more about this adventure. How did you move on trying to answer your question regarding the detection of anisotropic flows present in the ALICE experiment.

S.R. There are two issues regarding your question. One issue is related to the technicalities of the measurement. This deals with how accurately can one measure the quantity of interest. The other is the physics and the mechanism underlying the phenomena that yields the specific values in the measurement. Over the last more than a decade, various techniques have been developed to measure the anisotropy in particle emission, and most techniques give us ‘similar’ values. This validates the need to describe the observed anisotropy. As the measurements become more precise, and also more accurate, certain differences, beyond the margin of errors, are seen in the values obtained by the different methods. We understand the reason for difference for some of these, and need to investigate the reasons for others. However, what is now widely accepted is that the event shape of a heavy-ion collision can be dominantly expressed as an ellipse, characterising what is termed as the second order anisotropy. The more general wisdom, and the current data indicates validation of this arising from an anisotropic flow of matter. Quantitatively, the parameter that measures this anisotropy, depends on the kinematic region and the particle species and the integrated value is about 8%. The difference in the values measured by different techniques, as established in other experiments, is about 20%. A difference of this order is due to two main reasons: i) firstly because of the presence of other related phenomena and ii) secondly because the different techniques are measuring physical quantities which seem to be slightly different in terms of the phenomena that they quantify. There is a very large group of people working on most of the above to iron out the subtle differences in the measurements, and the techniques.

To summarize, I will like to say that we know that second order anisotropy exists and now we need to move forward and understand the subtleties of measurements. There need to be theoretical developments to explain how these anisotropies are explained based on various kinematic and dynamic properties of the system. Some of these developments will help in comprehending a unique picture for the evolution of colliding system to yield the observed values of anisotropy.

A.M. Do you spend all of your time in the above project or are you pursuing research in different issues as well?

S.R. The other thing that requires more attention, and is of interest, pertains to pursuing one of the goals of the photon multiplicity detector to measure the anisotropy in photons and possibly use PMD as a detector to provide the event plane to look for anisotropies in particles detected in different regions.

A.M. So what do we think today that causes this anisotropy in the observed flows? Could you briefly describe this mechanism to us?

S.R. As we understand it today: to begin with, the nuclei that we collide are spherical. When they collide, because of their extended size, there is an extent of overlap of these nuclei. A projection of the overlap region in the plane perpendicular to the direction of their velocities yields a figure that has a certain almond like shape. The smaller axis of the ellipse is along the direction of the impact parameter. We can talk about a uniform pressure, and a temperature, in this overlap zone of the colliding nuclei, assuming the system is in thermal equilibrium. The difference in pressure between the inner and the outer regions of this zone causes matter to be pushed outwards. Since the shape of the overlapping zone is not isotropic, the pressure gradient is different in different directions, causing unequal push in different directions. Greater push where the pressure gradient is greater, and hence we expect more particles, or more energy to flow into the direction of the impact parameter. This is what causes the emission of particles to be anisotropic. So the initial spatial anisotropy evolves into an anisotropy in momentum space.

Now this is interesting for us because the equilibrium and the subsequent evolution help to determine whether the thermalised system was a system of partons, or a system of hadrons. In order to answer this question one has to define and build and equation of state, an equation that describes the relation between pressure, temperature and the energy density of matter. The equation of state tells us how matter will behave when it experiences external changes in parameters that describe its equilibrium. To give a pedagogical example: consider a racquet ball, and a glass ball. The two balls are struck against a wall with great speed. What happens to the ball in its collision with the wall is decided by the equation of state of the matter that makes up the ball.

The current data points to the fact that the evolution of matter in the overlap zone is akin to evolution of partonic matter.

A.M Does this mean that quark gluon plasma is formed here?

S.R. This is a difficult question to answer. Looking for quark gluon plasma is a little like looking for a needle in the haystack. The only additional problem is that in this case we do not quite know what the ‘needle’ looks like.

A.M. I would also like to ask you about the group that you are working with.

S.R. I lead the small group in the University of Rajastan in Jaipur. Our two member group, and a floating (small) population of students, is a part of the larger Indian group which has contributed the PMD. In all, we are about 30 people in the PMD collaboration working from different cities across the country.

A.M. So what do you think that we should expect in the years to come from the Alice experiment.

S.R. This is a more difficult question to answer. Well, as our understanding evolves, many many changes occur. ALICE has been designed to probe many signatures through various hadronic and electromagnetic signals. I will like to draw attention to two subject areas (which are of greater interest to me). The Letter of Intent of the ALICE experiment, written more than 20 years ago, probably has no mention of studying the equation of state through anisotropic particle emission, nor is there any mention of studying medium properties by measuring jet quenching. As I see it now, major efforts are being made in the ALICE experiment to pursue these topics. With this indicator of caution, I may add that in the next few years, as per existing conventional wisdom, we should be able to iron out any subtle differences in the measurements of anisotropy coefficients, and understand this better. And we should have a much better idea about properties of the medium and the energy loss in the medium as can be discerned from a very wide variety of observables. Given the current plans, the availability of proton nucleus collisions will also provide us with an opportunity to study properties of cold nuclear matter, necessary to understand and appreciate the properties of hot nuclear matter.

The other side that we should appreciate is that our present understanding of the physics of heavy ions collisions is still too model dependent. Models, as against theories, have limited applicability. Different observed quantities, and their systematic behaviour, find completely different underlying explanation, and many of the observed phenomena still defy explanation. While the results from experimentation in this area during the last more than one decade have provided new perspectives to comprehend the observations, it is also true that a universally accepted picture is still missing. This is not to say that there is lack of objectivism, it is just the ponderance of variety of models over a universal fundamental theory. If you allow me, I will like to describe this using an example from outside the domain of science I would like to refer to the story of six men and an elephant in a dark room. What I narrate here may be a little at variance with what was propounded by the Sufi philosopher, Rumi Jalaluddin, while trying to keep the essence.

In a completely dark room, there is an elephant and six men. Each one of the six men is asked to describe the animal and what it looks like. One of them touches the leg of the elephant and says that the elephant is a like a pillar, a strong powerful pillar and nothing more. The second touches the tail of the elephant and announces that the elephant is like a rope. The third the stomach of the elephant and says that the elephant is a strong and sturdy wall and so on... None of the six men could comprehend in totality what the animal looked like. The moral of the story is that each man was right in his own comprehension, which was limited by the limited data that he probed. It is not difficult to surmise that a proper description of the elephant requires probing the right attributes and then reconstructing the elephant.

The story of the 6 blind men trying to describe an elephant

As I see it, this is the present story about understanding the physics of heavy-ion collisions and the search of quark-gluon plasma. Although we see all different things, an overall universal picture is still awaited. I will like to close my eyes, and visualize two nuclei colliding, and seeing the subsequent processes at smaller and smaller space-time scales to smoothly evolve into the final state.

The next decade of experimentation as per the plans should help to understand the scenario better, minimizing the diversity in perspectives. It will be a delight to see some new phenomena, that has not been talked about in the last umpteen years, just to highlight the game that nature plays with us, putting forth new challenges for our comprehension of itself. I will like to quote Einstein here: “the most incomprehensible thing about the universe is that it is comprehensible”.

A.M. Finally, I would like to ask you whether issues scientific new -and particularly news from CERN- are part of the public sphere in India. Do you think that the broader public is interested in what the experiments that are currently taking place at CERN?

S.R. Thanks to information and communication technology, the world is becoming more and more boundary-less. More people know about CERN and its experiments now than 5 years ago. The demography of India is such that one can watch certain patterns, and its fluctuations, about popularity and pursuit of various professions. This is also likely to be seen in popularity of CERN and its experiments. Most people heard about CERN because of the publicity that came associated with the media reports which speculated on the likely productions of mini black holes that may gobble the earth. I think that eventually this was some negative publicity that came from the incident that took place a few months after the first run and because of the size of the experiments in terms of number of people involved and the total cost involved.

There were also fears regarding the new discoveries that would take place and the risk posed to the rest of the humanity. At that time most of my friends who weren't involved with physics kept asking me similar questions. I explained to them that these were just rumors and media hype. On their insistence, I wrote an article for the newspaper so that people would stop worrying about CERN and all these new exciting discoveries. Once published in a newspaper, it was spread all over the country. In a short time-period news from and about CERN were published in most media, and many people from many institutes contributed their bit to keep people informed about this, and also highlighting the Indian contribution. This was also an education for me: the power of the media, the effect seen at close quarters.

At the end of the day, I think it is our duty to enable people in the society to an exposure of good science and help them develop some sensitivity to appreciate the efforts spent towards understanding the laws of nature, at all scales. In a milieu where the value system is dominated by consumerism, a collective effort from our side may contribute very significantly to build a temperament in the society, which may provide an alternate, presumably more desirable, value system.


Flow measurements are unique

Submitted by Bedanga Mohanty (not verified) on

Flow measurements are unique and they are sensitive to equation of state as well as transport properties of QCD matter produced in heavy-ion collisions. We look forward to your further contributions towards these aspects.