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  • richardmitnick 11:57 am on December 16, 2016 Permalink | Reply
    Tags: , CERN ALICE, , , , , Reinhard Stock   

    From CERN ALICE: “The entry of heavy-ion physics in the high-energy sanctuary” 

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    06 December 2016
    Reinhard Stock

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    Reinhard Stock
    After the welcome address, the first who came on stage at the 30 Years of Heavy Ions celebration at CERN was Reinhard Stock, who told the story of the birth of heavy-ion physics and of its “entry in the high-energy sanctuary”, i.e. CERN.

    At that time – the late 1970’s and early 1980’s – Stock was working on the Streamer Chamber experiment, set at the Laurence Berkley National Laboratory (LBNL) in US, in which high energy heavy-ion reactions were studied and collision events were analyzed by eye on scan-tables (“It took two or three hours to analyze just an event!” Stock commented). In 1980 the Bevatron accelerator at LBNL had been upgraded to work with heavy-ion injection from the the SuperHILAC linear accelerator, and the complex of the two had been called Bevalac.

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    Inside the Bevatron. Credit: Lawrence Berkeley National Lab.

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    Inside the Super HILAC
    Super HILAC (Super Heavy Ion Linear Accelerator) was one of the first particle accelerators that could accelerate heavier elements to “atom-smashing” speeds. The device was built in 1972 and played a significant role in four decades of scientific research at Lawrence Berkeley National Laboratory. In addition to being the launchpad for a variety of major experiments, the Super HILAC was crucial in the discovery of five superheavy elements. In this photo, Lawrence Berkeley National Laboratory’s Bob Stevenson and Frank Grobelch are sitting inside the Super HILAC’s poststripper. The maze of piping behind them is meant to circulate cooling water through the accelerator. | Photo courtesy of Lawrence Berkeley National Laboratory. https://energy.gov.

    Under the direction of Hermann Grunder, it was the first universal facility to study relativistic collisions of light or heavy nuclei. Its research programme was oriented to studying the Nuclear Matter Equation of State, which – according to Stock – was “the holy grail of Bevalac’s physics” and was of key importance to understand the structure of neutron stars and supernova dynamics.

    At the beginning of the 1980’s the idea rose to to search for the plasma state of QCD with heavy-ion collisions at the higher CERN energies, so a group of researchers in nuclear and heavy-ion physics (mainly of GSI and LBL, but also of Heidelberg, Marburg and Warsaw) in 1982 wrote a proposal and submitted it to the CERN Proton-Synchrotron Committee, principally under initiative of Bock, Stock and Gutbrod from GSI, and Pugh and Poskanzer from LBNL.

    When and how was heavy-ion physics born?

    The transition from the goal of the nuclear (or hadronic) matter Equation of State to the investigation of deconfined QCD matter with high-energy heavy-ion collisions happened when physicists, both in the environment of the Bevalac experiments and among particle physics groups at CERN and BNL, began to address the question of whether a deconfined (colour conducting plasma) QCD state might be created in very high energy collisions of nuclei. In this situation our 1982 proposal to CERN, which included construction of heavy-ion injection instrumentation at the Proton Synchrotron (PS), set a lot of things rolling.

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    Proton Synchrotron. CERN

    Why did you and your colleagues decide to write this proposal? Why going to CERN?

    The idea to investigate a deconfined state of QCD in very high energy collisions was somehow irresistible to us. The Quark-Gluon Plasma (QGP) state had been postulated theoretically to arise from the newly discovered so-called asymptotic freedom limit of a system of quarks and gluons. This QGP is a much more elementary state than that of the matter we are made of and plays an important role in the early evolution of the Universe because such a state has apparently dominated the attosecond to microsecond era of the Big Bang expansion. Of course, in the early 1980’s the properties of the QGP were not yet known. It has turned out to be fundamental quantum liquid with unexpected properties, very far away indeed from QCD asymptotic freedom. This is the key result of the research we are talking about here.

    Although the idea was already there since the late 1970’s, iit took time to set up the EOS Bevalac experiments, at a more modest energy, and gain experience with the new physics, as well as instrumentation of substantial cost (including providing for funding) novel to the Nuclear Physics community. Most importantly, it soon turned out that even the maximal Bevalac energy did not suffice to create the QCD deconfinement energy. So it was natural to turn to CERN where about tenfold higher center of mass energy was available.

    The proposal was for an experiment at the PS, but it actually ended up at the SPS…

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    The Super Proton Synchrotron. CERN

    Yes and that was a terribly fortunate development of the events. My colleague from MPI Munich, Volker Eckardt, had ignited my interest in QCD deconfinement by suggesting that there was a heritage of potentially useful instrumentation and infrastructure, from former SPS experiments NA5 and NA24, that could be adaptable to QGP research. But these experiments were located in the North extraction area of the SPS. There was a modern Streamer Chamber, as well as a host of calorimetry, plus beam line and other structures, fully intact. Actually it was this prospect that had prompted the 1982 proposal, but we were outsiders to CERN and did not dare to propose an SPS experiment, with much more weightful implication to CERN’s programme, so the experiment called for two PS experiments, one based on the LBL Streamer Chamber experiences, the other on an extended scheme of the GSI – LBL Plastic Ball experiment.

    But the historically most shaking element of our proposal was that GSI and LBL offered to procure a pre-installed and tested new injector complex for heavy ions, consisting of a Gellert-Grenoble ECR ion source bought by GSI, and an LBL-built RFQ preaccelerator. This latter facility proved irresistible to CERN as it invited the idea that other groups at CERN, coming from particle physics background, could also articulate their intentions to participate in QCD plasma research. The combined impact of our interest in inheriting the NA5/24 setup in the North SPS extraction hall and the push of the (North area based) former experiments now preparing for QCD matter physics finally convinced R.Klapisch (then CERN research director) to initiate the launch of a full-fledged SPS programme. In retrospect this was an overwhelmingly constructive development because the CERN SPS nuclear collision research facility provided for the first, crucial inroad into QCD matter physics.

    Oddly enough, though, your proposal was never formally approved, was it?

    It is true, indeed. But from the above you see that this, our initial proposal for PS experiments, was swiftly “washed away” by a complex set of ideas and decision-making at CERN, which also entailed a formal CERN-GSI-LBL agreement to build the new heavy ion injector system. As a personal note – as my talk was about entering the CERN sanctuary from the outside Nuclear Physics community – I included the recollection that the initial young physicists proposing this research to CERN, notably H. Gutbrod and myself, were obviously mostly regarded as “catalysts” in this development, somehow “idiots utiles” for the major good, as was the object of CERN internal deliberations. Indeed we were never received in committee hearings about our planned experiments. However, putting this aside, of course the transformations of our initial proposal were then well taken care of in the ensuing discussion of the resulting SPS experiment programme, and our experiments ended up as NA35 and WA80. As I said, NA35 was the result of MPI Munich joining and providing all the invaluable instrumentation in the North hall, without which I do not know how we would have managed to set up NA35 (if based on Nuclear Physics funding only).

    Following these events, a first generation of experiments was installed at CERN’s SPS and measurements were performed with beams of fully stripped oxygen first and sulphur later. Stock was the leader of the NA35 experiment.

    The second act of this story began with another proposal, dated 1986. In this case the idea was to study lead collisions in the SPS and the proposal went through all the formalities up to reaching approval. It was decided that the upgrade of the infrastructure would be paid with “in kind” contribution by CERN and other eight European agencies.

    Thus, a second generation of experiments was established at the SPS. The follow-up of NA35 was NA49, in which streamer chambers were replaced by Time Projection Chambers (TPC), based on a more recent technology. In addition, an automatic system to analyze the data was developed, since the eye scanning method could not cope with the increased number of collisions.

    How do you feel, now that we are celebrating 30 years of heavy ions, being one of the beginners of this kind of physics?

    I think that intellectually it was a very interesting voyage. At SPS we have learnt a lot both about QCD and about detector technology. Great things came also from RHIC, which has been the world leader in this field in particular in the period from 2000 to 2005.

    BNL RHIC Campus
    BNL/RHIC
    BNL/RHIC

    Then LHC followed in the footsteps of RHIC. At first, remarkably, the QCD matter research was a shared initiative of the two – hitherto essentially separately marching – communities of nuclear and particle physicists. A landmark accomplishment of the open horizon of CERN research! LHC brought a transition from qualitative to quantitative results, much closer to the fundamental goals of determining the transport coefficients of quark matter. And, thus, radiating a sharp stimulus to our theoretical colleagues.

    What can we expect from the next lead-ion runs at LHC?

    First of all, more statistics to have higher confidence levels for already performed measurements. Then, hopefully, something new that could trigger a different theoretical view. It could be something like a generalization of matter under the government of QCD, or going back earlier in time in the reconstruction of the development of matter after the Big Bang, or finding a more fundamental theory of which QCD is some sort of low energy limit. These are our hopes, beyond accumulating more statistics. There is actually a need for a new theoretical paradigm.

    See the full article here .

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  • richardmitnick 4:25 pm on December 6, 2016 Permalink | Reply
    Tags: 2016: an exceptional year for the LHC, , , CERN ALICE, , , ,   

    From CERN: “2016: an exceptional year for the LHC” 

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    6 Dec 2016
    Corinne Pralavorio

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    This proton-lead ion collision in the ATLAS detector produced a top quark – the heaviest quark – and its antiquark (Image: ATLAS)

    It’s the particles’ last lap of the ring. On 5 December 2016, protons and lead ions circulated in the Large Hadron Collider (LHC) for the last time. At exactly 6.02am, the experiments recorded their last collisions (also known as ‘events’).

    When the machines are turned off, the LHC operators take stock, and the resulting figures are astonishing.

    The number of collisions recorded by ATLAS and CMS during the proton run from April to the end of October was 60% higher than anticipated. Overall, all of the LHC experiments observed more than 6.5 million billion (6.5 x 1015) collisions, at an energy of 13 TeV. That equates to more data than had been collected in the previous three runs combined.

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    One of the first proton-lead ion collisions at 8.16 TeV recorded by the ALICE experiment. (Image: ALICE/CERN)

    In technical terms, the integrated luminosity received by ATLAS and CMS reached 40 inverse femtobarns (fb−1), compared with the 25fb−1 originally planned. Luminosity, which measures the number of potential collisions in a given time, is a crucial indicator of an accelerator’s performance.

    “One of the key factors contributing to this success was the remarkable availability of the LHC and its injectors,” explains Mike Lamont, who leads the team that operates the accelerators. The LHC’s overall availability in 2016 was just shy of 50%, which means the accelerator was in ‘collision mode’ 50% of the time: a very impressive achievement for the operators. “It’s the result of an ongoing programme of work over the last few years to consolidate and upgrade the machines and procedures,” Lamont continues.

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    An event recorded by the CMS experiment during the LHC’s proton-lead ion run for which no fewer than 449 particles tracks were reconstructed. (Image: CMS/CERN)

    For the last four weeks, the machine has turned to a different type of collision, where lead ions have been colliding with protons. “This is a new and complex operating mode, but the excellent functioning of the accelerators and the competence of the teams involved has allowed us to surpass our performance expectations,” says John Jowett, who is in charge of heavy-ion runs.

    With the machine running at an energy of 8.16 TeV, a record for this assymetric type of collision, the experiments have recorded more than 380 billion collisions. The machine achieved a peak luminosity over seven times higher than initially expected, as well as exceptional beam lifetimes. The performance is even more remarkable considering that colliding protons with lead ions, which have a mass 206 times greater and a charge 82 times higher, requires numerous painstaking adjustments to the machine.

    The physicists are now analysing the enormous amounts of data that have been collected, in preparation for presenting their results at the winter conferences.

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    A proton-lead ion collision recorded by the LHCb experiment in the last few days of the LHC’s 2016 run. (Image: LHCb)

    Meanwhile, CERN’s accelerators will take a long break, called the Extended Year End Technical Stop (EYETS) until the end of March 2017. But, while the accelerators might be on holiday, the technical teams certainly aren’t. The winter stop is an opportunity to carry out maintenance on these extremely complex machines, which are made up of thousands of components. The annual stop for the LHC is being extended by two months in 2017 to allow more major renovation work on the accelerator complex and its 35 kilometres of machines to take place. Particles will return to the LHC in spring 2017.

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    The integrated luminosity of the LHC with proton-proton collisions in 2016 compared to previous years. Luminosity is a measure of a collider’s efficiency and is proportional to the number of collisions. The integrated luminosity achieved by the LHC in 2016 far surpassed expectations and is double that achieved at a lower energy in 2012. (Image : CERN)

    See the full article here.

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  • richardmitnick 5:09 pm on November 14, 2016 Permalink | Reply
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    From Alice at CERN: “Proton-lead collision at 5.02 TeV as seen by ALICE” 

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    ALICE-EVENTDISPLAY-2016-011-1

    One of the first proton-lead events at 5.02 TeV as seen by ALICE in November 2016. The event comes from fill 5506 with 189 colliding bunches at an interaction rate of 17 kHz.

    Date: 11-11-2016

    See the full article here .

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  • richardmitnick 3:21 pm on April 10, 2016 Permalink | Reply
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    From The Daily Galaxy: “CERN LHC Reveals: “The Universe a Billionth of a Second After the Big Bang” 

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    The Daily Galaxy

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    April 09, 2016
    No writer credit found

    “It is remarkable that we are able to carry out such detailed measurements on a drop of ‘early universe’, that only has a radius of about one millionth of a billionth of a meter. The results are fully consistent with the physical laws of hydrodynamics, i.e. the theory of flowing liquids and it shows that the quark-gluon plasma behaves like a fluid.

    It is however a very special liquid, as it does not consist of molecules like water, but of the fundamental particles quarks and gluons,” explained Jens Jørgen Gaardhøje, professor and head of the ALICE group at the Niels Bohr Institute at the University of Copenhagen.

    A few billionths of a second after the Big Bang, the universe was made up of a kind of extremely hot and dense primordial soup of the most fundamental particles, especially quarks and gluons. This state is called quark-gluon plasma. By colliding lead nuclei at a record-high energy of 5.02 TeV in the world’s most powerful particle accelerator, the 27 km long Large Hadron Collider, LHC at CERN in Geneva, it has been possible to recreate this state in the ALICE experiment’s detector and measure its properties.

    Quark gluon plasma. Duke University
    Quark-gluon plasma. Duke University

    CERN researchers recreated the universe’s primordial soup in miniature format by colliding lead atoms with extremely high energy in the 27 km long particle accelerator, the LHC in Geneva. The primordial soup is a so-called quark-gluon plasma and researchers from the Niels Bohr Institute, among others, have measured its liquid properties with great accuracy at the LHC’s top energy. The results were submitted to Physical Review Letters, which is the top scientific journal for nuclear and particle physics.

    “The analyses of the collisions make it possible, for the first time, to measure the precise characteristics of a quark-gluon plasma at the highest energy ever and to determine how it flows,” explains You Zhou, who is a postdoc in the ALICE research group at the Niels Bohr Institute.

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    ALICE Run Control Center
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    CERN ALICE and the Control Room

    You Zhou, together with a small, fast-working team of international collaboration partners, led the analysis of the new data and measured how the quark-gluon plasma flows and fluctuates after it is formed by the collisions between lead ions.

    The focus has been on the quark-gluon plasma’s collective properties, which show that this state of matter behaves more like a liquid than a gas, even at the very highest energy densities. The new measurements, which uses new methods to study the correlation between many particles, make it possible to determine the viscosity of this exotic fluid with great precision.

    You Zhou explains that the experimental method is very advanced and is based on the fact that when two spherical atomic nuclei are shot at each other and hit each other a bit off center, a quark-gluon plasma is formed with a slightly elongated shape somewhat like an American football. This means that the pressure difference between the centre of this extremely hot ‘droplet’ and the surface varies along the different axes. The pressure differential drives the expansion and flow and consequently one can measure a characteristic variation in the number of particles produced in the collisions as a function of the angle.

    Jens Jørgen Gaardhøje adds that they are now in the process of mapping this state with ever increasing precision — and even further back in time.

    See the full article here .

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  • richardmitnick 4:09 pm on April 3, 2016 Permalink | Reply
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    From ALICE at CERN: “ALICE data visualisation: How it works” 

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    16 December 2015 [Just posted on ALICE Matters]
    Iva Raynova

    Lead Ions collision at ALICE
    Lead Ions collision at ALICE

    The process behind ALICE data visualisation is long and complex, just like every other process included in the Large Hadron Collider operation. We met with Jeremi Niedziela, one of the people behind the development of the Event Display system, and asked him to reveal the path from a collision inside ALICE to its appearance on our monitors.

    Hello, Jeremi, what is your involvement in the process of data visualisation?

    My aim is to make sure that the visualisation for ALICE works properly. It mainly concerns online visualisation. When we have collisions, when we gather data, we want to see the outcome immediately. This is the main part of my job, to make all systems work.

    Would you describe the whole process?

    Everything starts inside ALICE. When we have collisions, new particles are created, which go through the detector and interact with it. As a result electric signals are generated, giving information about the particles. These are transformed into numbers (they are digitized) and they form the raw data, which are then sent to our computing rooms. These are filled with hundreds of computers recording and processing the information.

    Inside one of the computing rooms there is a machine dedicated to perform online reconstruction. Let’s take for example the Time Projection Chamber (TPC), which is filled with a gas mixture. When a particle goes through the TPC, it ionises the gas in several points in space. But we don’t have a continuous line, instead we have many, many points of interaction. This is valid not only for the TPC, but for other detectors as well.

    From this information we have to extract physical quantities like the momentum, the mass, the charge and the energy of the particle. The reconstruction takes the raw data, which are simply numbers, corresponding to an electronic output, and translates it into the language of physics. A part of this process is to fit tracks representing particles’ trajectories to those points of interaction with the detectors’ material.

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    When the run starts, a process called Visualisation Manager receives a signal and starts the reconstruction. It begins gathering raw data and producing reconstructed events, which are sent to the Event Display, running on one of the big screens in the Run Control Centre. The Event Display draws the tracks, the geometry of the detector and the calorimeter towers. It also produces a screenshot for each event and uploads it to a website called ALICE Live.

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    ALICE Live image

    This way we can only observe the last collision. If we want to see an event which happened for example two days ago, we send a query to the Visualisation Manager where the last few thousands of collisions are kept in a dedicated storage.

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    Do you know who uses the Event Display the most?

    Yes, one year ago I made a survey to find out who uses it and for what purposes. First of all, physicists and detector experts benefit from it. For example, someone working for the Inner Tracking System (ITS) replied that they use it to check hits position on the Silicon Drift Detector (SDD) for geometry check. Others use it for browsing events to understand what situations can be met during the analysis.

    It could also be used to create images or videos for conferences or for the general public. It’s useful in the outreach activities as well. You can display events for physics, fun and education of students. Talking about outreach, CERN MediaLab has a project, which is called Total Event Display. It is meant to be a common visualisation environment for all the experiments. ALICE also takes part in it, so I developed the code, which is needed for it.

    Another interesting project we’re working on is the Magic Window. There is a window between the ALICE Run Control Centre and the entrance to the elevator.

    ALICE Run Control Center
    ALICE Run Control Center

    It will be turned into a magic window with the help of a polarisation filter and a projector. It will also be a touchscreen, so we could display an interactive presentation about ALICE with Total Event Display. That means that visitors could see and explore collisions happening in real time.

    The Event Display in the ALICE Run Control Centre is also a part of the Data Quality Monitoring. It serves not so much for the experts to understand the physics process, but for us to be able to see if everything is working properly. If we see that tracks are drawn incorrectly, or if we don’t see tracks at all, then we immediately know that something went wrong along the way and we need to fix it. Sometimes it reveals problems which one would not associate with visualisation, but as we need to reconstruct events on the fly, it is a great way to control the whole system.

    See the full article here .

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  • richardmitnick 1:23 pm on February 2, 2016 Permalink | Reply
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    From CERN: “The story of ALICE” 

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    01 February 2016
    Iva Raynova

    The discussions about the future of heavy-ion physics at CERN started in 1986, even before the building of the Large Electron-Positron Collider (LEP) had been completed. Four years later the idea of creating a dedicated heavy-ion detector for the Large Hadron Collider was born and on 13 December 1990 the Heavy Ion Proto Collaboration (HIPC) held its first meeting. Later on, during the historic Evian meeting Towards the LHC experimental programme in 1992, the expression of interest to create ALICE was submitted, followed by the letter of intent in 1994 and by the technical proposal in 1995.

    CERN ALICE Icon HUGE

    One of the people, responsible for the creation of ALICE, is Jurgen Schukraft. Spokesperson of the collaboration for the first 20 years of its existence, he is also the person who organised the initial meeting of HIPC. In the following interview we will try to show you the evolution of ALICE through his eyes.

    Has ALICE changed much since the beginning?

    Jurgen: In the beginning, the plan for the experiment was different from what it eventually turned out to be. We had a big TPC, we had a silicon vertex detector, we had time of flight, but the magnet was completely different. Ever since we sent the letter of intent, we had many different ideas. All the details were missing and we made a lot of additions afterwards, but the essential part of the detector was already decided by 1992.

    AliceDetectorLarge
    ALICE Detector

    In terms of the collaboration, it was very different at the time, because most of the people at CERN were doing experiments at low energies – the LEP programme at CERN. The Large Hadron Collider was still far in the future. It was after the approval of the technical proposal in 1994 when we started some serious research and development. In 1998, when the SPS experiment stopped, more people joined our collaboration.

    Which are the most interesting discoveries, made in ALICE?

    Jurgen: We have made many discoveries so far, but one thing which we did not expect is that each of these little “big bangs” has its own character. These explosions are so strong that every one of them is different and individual. This couldn’t be observed in the other types of collisions, where we only look at the average properties of the particles.

    The other very interesting thing for me is the discovery that there is a much deeper connection between all the QCD processes – everything which involves strong interaction – they are much [more] deeply connected than we originally thought.

    I think it would be very good if in the next 10 or 15 years we manage to embed what we have learned from the heavy-ion physics into the bigger context of the standard model.

    Standard model with Higgs New
    The Standard Model of elementary particles (more schematic depiction), with the three generations of matter, gauge bosons in the fourth column, and the Higgs boson in the fifth.

    Are you happy with how the experiment developed?

    Jurgen: I think overall it worked out as well as we could have hoped. The physics at the LHC turned out to be extremely interesting. Even more interesting than we initially thought. Also, the experiment worked very well. There are always things that could be done better, but we constantly learn. That is why ALICE is going to be upgraded during the next long shutdown.

    In addition, more people came to the collaboration than we thought would join. There are currently about 1500 members. In these terms we developed even better than I hoped. I am pleased and also proud of our community and of the fact that we managed to create such a huge experiment.

    We were a bit naive in the beginning, thinking that 10-12 years were going to be enough to do what eventually took us 20 years. A bit naive, but also very enthusiastic. What I am happy about is that we didn’t have big disappointments along the way. On the contrary – we had a very satisfactory development. This project was more complicated, more expensive and much bigger than what we had done before. It was a big mountain to climb and I am proud that we managed to get to the top.

    See the full article here.

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  • richardmitnick 2:28 pm on January 20, 2016 Permalink | Reply
    Tags: , CERN ALICE, Heavy-Ion run, ,   

    From CERN: “LHC surpasses design luminosity with heavy ions” 

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    Jan 15, 2016
    John Jowett

    The LHC has finished 2015 with a successful heavy-ion run. For the first time, the lead nuclei have collided with an average centre-of-mass energy per pair of nucleons of 5.02 TeV.

    Temp 1
    First events

    The extensive modifications made to the LHC during its first long shutdown allowed the energy of the proton beams to be increased from 4 TeV in 2012 to 6.5 TeV, enabling proton–proton collisions at a centre-of-mass energy of 13 TeV, in 2015. As usual, a one-month heavy-ion run was scheduled at the end of the year. With lead nuclei colliding, the same fields in the LHC’s bending magnets would have allowed 5.13 TeV per colliding nucleon pair. However, it was decided to forego the last whisker of this increase to match the equivalent energy of the proton–lead collisions that took place in 2013, namely 5.02 TeV. Furthermore, the first week of the run was devoted to colliding protons at 2.51 TeV per beam. This will allow the LHC experiments to make precise comparisons of three different combinations of colliding particles, p–p, p–Pb and Pb–Pb, at the same effective energy of 5.02 TeV. This is crucial to disentangling the ascending complexity of the observed phenomena (CERN Courier March 2014 p17).

    The first (and last, until 2018) Pb–Pb operation close to the full energy of the LHC was also the opportunity to finally assess some of its ultimate performance limits as a heavy-ion collider. A carefully targeted set of accelerator-physics studies also had to be scheduled within the tight time frame.

    Delivering luminosity

    The chain of specialised heavy-ion injectors, comprising the electron cyclotron resonance ion source, Linac3 and the LEIR ring, with its elaborate bunch-forming and cooling, were recommissioned to provide intense and dense lead bunches in the weeks preceding the run. Through a series of elaborate RF gymnastics, the PS and SPS assemble these into 24-bunch trains for injection into the LHC. The beam intensity delivered by the injectors is a crucial determinant of the luminosity of the collider.

    Planning for the recommissioning of the LHC to run in two different operational conditions after the November technical stop resembled a temporal jigsaw puzzle, with alternating phases of proton and heavy-ion set-up (the latter using proton beams at first) continually readapted to the manifold constraints imposed by other activities in the injector complex, the strictures of machine protection, and the unexpected. For Pb–Pb operation, a new heavy-ion magnetic cycle was implemented in the LHC, including a squeeze to β* = 0.8 m, together with manipulations of the crossing angle and interaction-point position at the ALICE experiment. First test collisions occurred early in the morning of 17 November, some 10 hours after first injection of lead.

    The new Pb–Pb energy was almost twice that of the previous Pb–Pb run in 2011, and some 25 times that of RHIC at Brookhaven, extending the study of the quark–gluon plasma to still-higher energy density and temperature. Although the energy per colliding nucleon pair characterises the physical processes, it is worth noting that the total energy packed into a volume on the few-fm scale exceeded 1 PeV for the first time in the laboratory.

    After the successful collection of the required number of p–p reference collisions, the Pb–Pb configuration was validated through an extensive series of aperture measurements and collimation-loss maps. Only then could “stable beams” for physics be declared at 10.59 a.m. on 25 November, and spectacular event displays started to flow from the experiments.

    Temp 2
    Beam-loss monitor signals

    In the next few days, the number of colliding bunches in each beam was stepped up to the anticipated value of 426 and the intensity delivered by the injectors was boosted to its highest-ever values. The LHC passed a historic milestone by exceeding the luminosity of 1027 cm–2 s–1, the value advertised in its official design report in 2004.

    This allowed the ALICE experiment to run in its long-awaited saturated mode with the luminosity levelled at this value for the first few hours of each fill.

    Soon afterwards, an unexpected bonus came from the SPS injection team, who pulled off the feat of shortening the rise time of the SPS injection kicker array, first to 175 ns then to 150 ns, allowing 474, then 518, bunches to be stored in the LHC. The ATLAS and CMS experiments were able to benefit from luminosities over three times the design value. A small fraction of the luminosity in this run was delivered to the LHCb experiment, a newcomer to Pb–Pb collisions.
    Nuclear beam physics

    The electromagnetic fields surrounding highly charged ultrarelativistic nuclei are strongly Lorentz-contracted into a flat “pancake”. According to the original insight of Fermi, Weizsäcker and Williams, these fields can be represented as a flash of quasi-real photons. At LHC energies, their spectrum extends up to hundreds of GeV. In a very real sense, the LHC is a photon–photon and photon–nucleus collider (CERN Courier November 2012 p9). The study of such ultraperipheral (or “near-miss”) interactions, in which the two nuclei do not overlap, is an important subfield of the LHC experimental programme, alongside its main focus on the study of truly nuclear collisions.

    From the point of view of accelerator physics, the ultraperipheral interactions with their much higher cross-sections loom still larger in importance. They dominate the luminosity “burn-off”, or rate at which particles are removed from colliding beams, leading to short beam and luminosity lifetimes. Furthermore, they do so in a way that is qualitatively different from the spray of a few watts of “luminosity debris” by hadronic interactions. Rather, the removed nuclei are slightly modified in charge and/or mass, and emerge as new, well-focussed, secondary beams. These travel along the interaction region just like the main beam but, as soon as they encounter the bending magnets of the dispersion-suppressor section, their trajectories deviate, as in a spectrometer.

    The largest contribution to the burn-off cross-section comes from the so-called bound-free pair-production (BFPP) in which the colliding photons create electron–positron pairs with the electron in a bound-state of one nucleus. A beam of these one-electron ions, carrying a power of some tens of watts, emerges from the interaction point and is eventually lost on the outer side of the beam pipe.
    Controlled quench

    The LHC operators have become used to holding their breath as the BFPP loss peaks on the beam-loss monitors rise towards the threshold for dumping the beams (figure). There has long been a concern that the energy deposited into superconducting magnet coils may cause them to quench, bringing the run to an immediate halt and imposing a limit on luminosity. In line with recent re-evaluations of the magnet-quench limits, this did not happen during physics operation in 2015 but may happen in future operation at still-higher luminosity. During this run, mitigation strategies to move the losses out of the magnets were successfully implemented. Later, in a special experiment, one of these bumps was removed and the luminosity slowly increased. This led to the first controlled steady-state quench of an LHC dipole magnet with beam, providing long-sought data on their propensity to quench. On the last night of the run, another magnet quench was deliberately induced by exciting the beam to create losses on the primary collimators.

    Photonuclear interactions also occur at comparable rates in the collisions and in the interactions with the graphite of the LHC collimator jaws. Nuclei of 207Pb, created by the electromagnetic dissociation of a neutron from the original 208Pb at the primary collimators, were identified as a source of background after traversing more than a quarter of the ring to the tertiary collimators near ALICE.

    These, and other phenomena peculiar to heavy-ion operation, must be tackled in the quest for still-higher performance in future years.

    See the full article here.

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  • richardmitnick 5:16 pm on November 25, 2015 Permalink | Reply
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    From ALICE at CERN: “LHC collides ions at new record energy” 

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    1
    Lead ions collide in the CMS detector (Image:CMS)

    After the successful restart of the Large Hadron Collider (LHC) and its first months of data taking with proton collisions at a new energy frontier, the LHC is moving to a new phase, with the first lead-ion collisions of season 2 at an energy about twice as high as that of any previous collider experiment. Following a period of intense activity to re-configure the LHC and its chain of accelerators for heavy-ion beams, CERN’s accelerator specialists put the beams into collision for the first time in the early morning of 17 November 2015 and ‘stable beams’ were declared at 10.59am today, marking the start of a one-month run with positively charged lead ions: lead atoms stripped of electrons. The four large LHC experiments will all take data over this campaign, including LHCb, which will record this kind of collision for the first time. Colliding lead ions allows the LHC experiments to study a state of matter that existed shortly after the big bang, reaching a temperature of several trillion degrees.

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    Lead ions collide in the ALICE detector (Image:ALICE)

    “It is a tradition to collide ions over one month every year as part of our diverse research programme at the LHC,” said CERN Director-General Rolf Heuer. “This year however is special as we reach a new energy and will explore matter at an even earlier stage of our universe.”

    Early in the life of our universe, for a few millionths of a second, matter was a very hot and very dense medium – a kind of primordial ‘soup’ of particles, mainly composed of fundamental particles known as quarks and gluons. In today’s cold Universe, the gluons “glue” quarks together into the protons and neutrons that form bulk matter, including us, as well as other kinds of particles.

    “There are many very dense and very hot questions to be addressed with the ion run for which our experiment was specifically designed and further improved during the shutdown,” said ALICE collaboration spokesperson Paolo Giubellino. “For instance, we are eager to learn how the increase in energy will affect charmonium production, and to probe heavy flavour and jet quenching with higher statistics. The whole collaboration is enthusiastically preparing for a new journey of discovery.”

    3
    Lead ions collide in the LHCb detector (Image: LHCb)

    Increasing the energy of collisions will increase the volume and the temperature of the quark and gluon plasma, allowing for significant advances in understanding the strongly-interacting medium formed in lead-ion collisions at the LHC. As an example, in season 1 the LHC experiments confirmed the perfect liquid nature of the quark-gluon plasma and the existence of “jet quenching” in ion collisions, a phenomenon in which generated particles lose energy through the quark-gluon plasma. The high abundance of such phenomena will provide the experiments with tools to characterize the behaviour of this quark-gluon plasma. Measurements to higher jet energies will thus allow new and more detailed characterization of this very interesting state of matter.

    “The heavy-ion run will provide a great complement to the proton-proton data we’ve taken this year,” said ATLAS collaboration spokesperson Dave Charlton. “We are looking forward to extending ATLAS’ studies of how energetic objects such as jets and W and Z bosons behave in the quark gluon plasma.”

    4
    Lead ions collide in the ATLAS dectector (Image: ATLAS)

    The LHC detectors were substantially improved during the LHC’s first long shutdown. With higher statistics expected, physicists will be able to look deeper at the tantalising signals observed in season 1.

    “Heavy flavour particles will be produced at high rate in Season 2, opening up unprecedented opportunities to study hadronic matter in extreme conditions,” said CMS collaboration spokesperson Tiziano Camporesi. « CMS is ideally suited to trigger on these rare probes and to measure them with high precision. »

    For the very first time, the LHCb collaboration will join the club of experiments taking data with ion-ion collisions.

    “This is an exciting step into the unknown for LHCb, which has very precise particle identification capabilities. Our detector will enable us to perform measurements that are highly complementary to those of our friends elsewhere around the ring,” said LHCb collaboration spokesperson Guy Wilkinson.

    See the full article here .

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  • richardmitnick 7:55 pm on September 21, 2015 Permalink | Reply
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    From CERN ALICE: “ALICE precisely compares light nuclei and antinuclei” 

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    17 Aug 2015
    Cian O’Luanaigh

    1
    The ALICE detector on CERN’s Large Hadron Collider (Image: A Saba/CERN)

    The ALICE experiment at the Large Hadron Collider (LHC) at CERN has made a precise measurement of the difference between ratios of the mass and electric charge of light nuclei and antinuclei. The result, published today in Nature Physics (10.1038/nphys3432), confirms a fundamental symmetry of nature to an unprecedented precision for light nuclei. The measurements are based on the ALICE experiment’s abilities to track and identify particles produced in high-energy heavy-ion collisions at the LHC.

    The ALICE collaboration has measured the difference between mass-to-charge ratios for deuterons (a proton, or hydrogen nucleus, with an additional neutron) and antideuterons, as well as for helium-3 nuclei (two protons plus a neutron) and antihelium-3 nuclei. Measurements at CERN, most recently by the BASE experiment, have already compared the same properties of protons and antiprotons to high precision. The study by ALICE takes this research further as it probes the possibility of subtle differences between the way that protons and neutrons bind together in nuclei compared with how their antiparticle counterparts form antinuclei.

    “The measurements by ALICE and by BASE have taken place at the highest and lowest energies available at CERN, at the LHC and the Antiproton Decelerator, respectively,” says CERN Director-General Rolf Heuer. “This is a perfect illustration of the diversity in the laboratory’s research programme.”

    The measurement by ALICE comparing the mass-to-charge ratios in deuterons/antideuterons and in helium-3/antihelium-3 confirms the fundamental symmetry known as CPT in these light nuclei. This symmetry of nature implies that all of the laws of physics are the same under the simultaneous reversal of charges (charge conjugation C), reflection of spatial coordinates (parity transformation P) and time inversion (T). The new result, which comes exactly 50 years after the discovery of the antideuteron at CERN and in the US, improves on existing measurements by a factor of 10-100.

    2
    Measurements of energy loss in the time-projection chamber enable the ALICE experiment to identify antinuclei (upper curves on the left) and nuclei (upper curves on the right) produced in the lead-ion collisions at the LHC (Image: ALICE)

    The ALICE experiment records high-energy collisions of lead ions at the LHC, enabling it to study matter at extremely high temperatures and densities. The lead-ion collisions provide a copious source of particles and antiparticles, and nuclei and the corresponding antinuclei are produced at nearly equal rates. This allows ALICE to make a detailed comparison of the properties of the nuclei and antinuclei that are most abundantly produced. The experiment makes precise measurements of the curvature of particle tracks in the detector’s magnetic field and of the particles’ time of flight, and uses this information to determine the mass-to-charge ratios for the nuclei and antinuclei.

    “The high precision of our time-of-flight detector, which determines the arrival time of particles and antiparticles with a resolution of 80 picoseconds, associated with the energy-loss measurement provided by our time-projection chamber, allows us to measure a clear signal for deuterons/antideuterons and helium-3/antihelium-3 over a wide range of momentum”, says ALICE spokesperson Paolo Giubellino.

    The measured differences in the mass-to-charge ratios are compatible with zero within the estimated uncertainties, in agreement with expectations for CPT symmetry. These measurements, as well as those that compare the protons with antiprotons, may further constrain theories that go beyond the existing Standard Model of particles and the forces through which they interact.

    See the full article here.

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  • richardmitnick 10:48 am on March 31, 2015 Permalink | Reply
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    From ALICE at CERN: “Interview with Savas Dimopoulos” 

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    24 March 2015
    Panos Charitos

    1
    Savas Dimopoulos

    Savas Dimopoulos, professor at Stanford University, is searching for answers to some of the most profound mysteries of nature. In this interview we discuss the recent findings of the LHC and his expectations from future HEP experiments, the quest for “truth” that drives our scientific endeavours, as well as the relation between science and art.

    P.C. Why did you decide to become a physicist?

    S.D. What attracted me to physics and mathematics was the truth of the statements made in these disciplines. This dates back to my childhood. I was born in Constantinople, and my family moved to Athens when I was twelve. It was a time of great turmoil and I witnessed political tensions, people on the left and on the right were expressing opposing arguments that both seemed reasonable to me.

    I decided to go into a discipline that seeks the absolute truth: a truth that does not depend on the eloquence of the speaker. That limited my choices to mathematics and physics. I finally decided to study physics, as I had doubts about the certainty of truth in mathematics; in physics, in addition to mathematical proofs, the experiments add an extra layer of certainty that brings us closer to the truth.

    I was enamoured of the fact that through physics we can explain all phenomena from very few principles, as nature turns out to be exceedingly simple in principles and exceedingly complex in phenomena. The laws of nature can be written down on a single piece of paper and explain everything that we have seen so far in the universe. This is the magic of theory: it compactifies facts and reduces them to a handful of principles from which everything can be derived.

    P.C. You referred to the balance between experiment and theory, but it somehow seems that you were more intrigued by the latter. What attracted you to what is now called theoretical physics?

    S.D. In the beginning, I had not decided whether I was going to be a theorist or an experimentalist. I went to a high-school without laboratories in Greece. The first time I had the chance to work in a laboratory was as a student at the university. That’s when I realised that I lacked the talent to be an experimentalist and felt that I was better in theory.

    At the time, I thought that the truth of mathematics exists only in our human brains, whereas physics is independent of human existence and therefore the ultimate discipline for the search for the absolute and most important truths. Plato believed that mathematical reality in some sense exists in the so-called platonic world of ideas, where objects on earth have their idealized counterparts. A sphere, for example, is never perfect in real life but in the platonic world, which we call mathematics, perfectly round spheres exist. As mathematical entities are not necessarily realized in nature, I felt uncomfortable as a child to just focus on mathematics. However, I think that it is an amazing language. The rules are well defined and once you pose the right question anybody can follow the steps to find the correct answer, even computers.

    P.C. Do you think that, besides experiments, mathematics is also another way to control our theories?

    S.D. You are absolutely right. Mathematics is crucial for controlling the truth because it is not a random game. You start with a few axioms, and, as long as they are self-consistent, you can produce theorems and derive truths that follow from them. In that sense, mathematics is very important to theoreticians, as mathematical consistency is a huge constraint on our theoretical ideas.

    P.C. What is the situation today in theoretical physics, following the recent results of the LHC?

    S.D. We are now standing at a crossroads, with one path leading to naturalness and the other to the multiverse or something else. It is very exciting, we are testing if the idea of naturalness can be applied to the hierarchy problem – which is the disparity between the weak and gravitational forces. In the next several years, the LHC will be the epicentre of excitement, because it is testing such a fundamental principle and such a dichotomy in physics.

    In the light of these data, physicists react in different ways. As I often emphasize in my recent talks, the state of beyond Standard Model physics after the LHC8 can be compared to headless chickens running in all possible directions.

    4
    Standard Model of Particle Physics. The diagram shows the elementary particles of the Standard Model (the Higgs boson, the three generations of quarks and leptons, and the gauge bosons), including their names, masses, spins, charges, chiralities, and interactions with the strong, weak and electromagnetic forces. It also depicts the crucial role of the Higgs boson in electroweak symmetry breaking, and shows how the properties of the various particles differ in the (high-energy) symmetric phase (top) and the (low-energy) broken-symmetry phase (bottom)

    What is more interesting, headless chickens can live for up to two years; that is also the timescale which we need to get more results from the second run of the LHC. This run will indicate the research that we will pursue in the coming decades.

    2
    Mixed reactions should not frighten us, as they characterize every scientific revolution.

    P.C. Why do you believe there is so much enthusiasm for the search of supersymmetry at the LHC?

    Supersymmetry standard model
    Standard Model of Supersymmetry

    S.D. People are enthusiastic about the possibility of discovering supersymmetry for a number of reasons. In the early 1990s, LEP measured the strengths of the strong and weak electromagnetic interactions and discovered that supersymmetric grand unification is favoured over the non-supersymmetric one. That was a great source of excitement, and theorists looked forward to discovering the super-partners at LEP, LEP 2, or at the LHC. However, no hint of supersymmetry was found after the first collisions at the LHC8 energies.

    CERN LEP
    CERN/LEP

    This story reminds me one of Sherlock Holmes’ stories where he points out “the curious incident of the dog in the night-time”, the incident being that the dog did nothing. In the same way, the absence of supersymmetry at the energies explored so far at the LHC can teach us many things. Supersymmetry is one of the rare ideas that is so important, that even its absence is worth knowing about.

    In addition to that, there is also a sociological aspect to the popularity of supersymmetry: it is an easy theory to work with, and, as a result, it can be tested experimentally in great detail, unlike other alternatives to the hierarchy problem.

    Because of these reasons, the search for supersymmetry is the [?]primary aim of the LHC. In the next years we should have a better idea of the path chosen by nature and we may be talking with enthusiasm about the discovery of the first supersymmetric particle. In the best case scenario, though, our theories will be proven wrong, and we will discover something unanticipated, something truly revolutionary as was the case with quantum mechanics.

    P.C. How did it feel to have your prediction of unification of couplings confirmed by experiment?

    S.D. Having your theory confirmed by experiment feels like a present that you didn’t deserve. When we do science on a day-to-day basis, it’s sort of like a puzzle – this very intricate game with strict rules. It’s like nature is a giant puzzle and mathematics is the language of nature. When a mathematical theory is verified by experiment, you feel awe. It somehow becomes real. You get amazed when you realize that all these games you have been playing are not just games but actually describe nature.

    P.C. Do you think LHC will have the last word or will we also need to design new experiments?

    S.D. There are two directions that we should pursue vigorously. One is to continue with colliders, and go to much higher energy. The other is to design new experiments, as there are some great theoretical ideas that cannot be tested in colliders. For example, some very weakly interacting new particles such as the axion can only be discovered in low energy, tabletop, small-scale experiments.

    There can be forces that are too weak to be discovered in colliders but can nevertheless be observed by testing gravity-like forces in small scale. For example, one can look for deviations from Newton’s law at short distances. In addition to the theoretical importance, many fields (i.e. condensed matter physics, atomic physics, quantum information) have made great progress in precision studies, and these new techniques are begging to be used for fundamental discoveries. They also have the sociological advantage of shorter timescales, typically less than five years, compared to those between two consecutive colliders, which can be decades.

    Another interesting point is that you can roughly separate physics to two periods. Before WWII a number of techniques were used to explore the truth, and the job of theoreticians was both to come up with theoretical ideas and to design experiments to test them. Enrico Fermi and Felix Bloch, for example, did not just do theory; they came up with experiments and, in some cases, even conducted them themselves. After the War, fundamental physics started focusing increasingly on the high energy frontier. This has been a golden road, as the recent discovery of the Higgs shows. Nevertheless, in the long timescale between consecutive colliders, it will be exciting to look for new physics using low energy experiments.

    P.C. Do you think that we still learn something, even when our theories are proven wrong? Is this another step bringing us closer to truth?

    S.D. Absolutely. Truth is both discovering new things and proving that some preconceptions, speculations, or theories are wrong. For example, the idea of the aether seemed plausible at a time, as it was logical to assume that electromagnetic waves need a medium, but it was disproved by the Michelson–Morley experiment. In this case, it was the non-discovery of something that created the big earthquake that led to relativity. Knowing what is false can be as important as knowing what is true.

    3

    P.C. What drives people to formulate new theories and models?

    S.D. One obvious reason is the inconsistency of an existing theory with data. The Standard Model has survived every laboratory test so far and in some cases the validity of its predictions has been tested to 12 decimal precision. It nevertheless fails to explain roughly 95% of our Cosmos. It does not explain Dark Matter or what the origin of Dark Energy is. For the latter, the SM prediction is at least 60 orders of magnitude larger than what we observe it to be. In addition to all this, we eventually run into theoretical problems once we extrapolate the theory to high energies.

    The other motivations are beauty and economy. In the context of physics, the idea of beauty has a relatively precise meaning: it involves symmetry, i.e. the idea that one object appears the same from different perspectives. Economy refers to economy of structure, particles, and parameters. Ideally, there are as few “moving parts” postulated into the theory as possible. In that sense, it is hard to believe that the Standard Model, despite being an amazing theory, is fundamental, because it has over twenty parameters and tens of particles. There must be a more economic version.

    A philosophical, more reflective reason for doing theory is our love of patterns. We are pattern junkies. In our effort to find harmony and conceptually beautiful ways to understand everything at the deepest possible level we do science or create art. Neither of them directly enhances or contributes to our survival probability, but the least important things for our survival are the very things that make us human. For me, art and science are equally important; after a hard day of research I listen to music and find these patterns very relaxing because they are beautiful, and also because I don’t have to actively scrutinise them.

    P.C. Is it possible that at some point we will have answered all the fundamental questions and the scientific endeavours will come to an end?

    S.D. Humans tend to be quite dismissive of the things they learn. There is a famous saying: “Yesterday’s sensation, today’s calibration, tomorrow’s background”. We get bored, and want to move immediately to the next level. For many decades, if not centuries, we have been trying to find a model that explains all the interactions to any conceivable energy that we have experimented with so far. We came up with the Standard Model that may describe almost all known phenomena, but now we want to effectively build a meta-theory that explains the theory itself. However, I am sure that even if we find this meta-theory, we will still come up with more questions. That’s what makes us, as humans, a progressive species: we get excited, we investigate, we discover, and then we get bored and want to get excited again by moving to the next questions.

    P.C. Do you think that the social context is still in favour of researching particle physics and fundamental questions?

    S.D. I think that the public is very interested in fundamental physics. Physics enrollment at universities like Stanford has been going steadily up for the last 15 years at undergraduate and graduate level, despite the fact that there are more competing disciplines, such as biology and information technology. I have also received a lot of positive feedback from the movie Particle Fever.

    However, when the producer approached me ten years ago and told me that he wanted to make a movie about particle physics, I said: “That sounds boring. Who cares about particle physics? You are wasting your time”. “It’s not about particle physics,” he replied, “it’s about particle physicists”. I said: “This is even worse. They are the plainest people on the planet”. I was proven blatantly wrong. And it’s not just Particle Fever. This year there are several movies about science: Gravity, Interstellar, the Imitation Game that is about Alan Turing, and The Theory of Everything about Steven Hawking.

    I think that part of the reason why many more young people don’t go into physics in general and particle physics in particular is that we are not very good at communicating the sense of excitement or even the practical importance of our discoveries to the public. If more effort is put in that direction, it will do wonders to attract bright young people.

    Outreach is a little easier for astrophysicists and cosmologists, because people can lift their eyes to the sky and see what they talk about. Our job, however, is to explain that big entities consist of small parts, which, in a sense, are more fundamental.

    In my experience, two books that I read when I was twelve played a big role in my choosing to be a physicist. One was by Einstein and Infeld and the other was a biography of Einstein by Philipp Frank.

    P.C. Maybe this is the right time to ask you, as a teacher now, what’s your main advice to your students?

    S.D. Enjoy yourself and work on the biggest problems that you can tackle.

    See the full article here.

    Please help promote STEM in your local schools.

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    THE FOUR MAJOR PROJECT COLLABORATIONS

    ATLAS
    CERN ATLAS New
    ALICE
    CERN ALICE New

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    CERN CMS New

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    LHC

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