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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” 

    Daily Galaxy
    The Daily Galaxy

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    CERN LHC Grand Tunnel
    CERN LHC particles
    LHC at CERN

    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.

    CERN ALICE Icon HUGE
    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.

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    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
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    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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    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.”

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    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.”

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    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

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    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.

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    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.

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  • richardmitnick 4:44 am on February 20, 2015 Permalink | Reply
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    From CERN ALICE: “ALICE contributes to NASA’s Orion mission” 

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    16 February 2015
    Panos Charitos

    On Friday, 5 December, NASA’s new Orion spaceship, a capsule built to take humans farther into space than ever before, made its first test flight.

    NASA Orion Spacecraft
    Orion

    The flight marked the first time, since Apollo 17 was launched to the moon in 1972, that a spacecraft built for humans traveled out of low-Earth orbit. Behind the Orion mission there is a flavour of High Energy physics, following in the tradition of synergies between the two fields. More specifically, following the previous success of the Timepix project on the Inernational Space Station (ISS), CERN scientists worked closely with their colleagues at NASA to integrate Timepix into the Orion spaceship.

    CERN TimePix
    TimePix

    Pivotal for this effort was Lawrence Pinsky, who started his career in heavy-ion physics at the NA48 and later NA49 fixed target experiments, before joining the ALICE collaboration. Later, he became involved with NASA’s APOLLO programme, where he was mainly responsible for heavy particle dosimetry. He worked as a postdoc at NASA’s space programme from 1977 to 1990. At that time, he became interested in the simulation of cosmic rays events with the use of GEANT3. His colleagues were also trying to do the same simulations of heavy particles coming from cosmic rays with FLUKA and he joined them. They developed the first Monte Carlo code to simulate transport phenomena of heavy cosmic rays.

    In 2006, when NASA invited him to give a series of lectures, he met Michael Campbell. Strangely, even though they worked in neighbouring buildings, they never had the chance to interact before and realize the possibility of using some of the technologies developed at CERN in space programmes. Michael showed MEDIPIX2 to Pinsky, who immediately realized its potential and demonstrated the chip to his colleagues in Houston.

    CERN MediPix
    MediPix

    During a workshop held by NASA, they advertised it to experts in space radiation and monitoring from all over the world. Various other projects were presented during the workshop, but it was MEDIPIX2 that had the most advantages and outclassed the rest. For the non-experts, the Medipix2 ASIC is a high spatial, high contrast resolving CMOS pixel read-out chip working in single photon counting mode. It can be combined with different semiconductor sensors which convert the X-rays directly into detectable electric signals. This represents a new solution for various X-ray and gamma-ray imaging applications.

    NASA and Huston joined MEDIPIX2 in 2007 and worked actively in the development of the new chip. In 2010, during the Workshop on Radiation Monitoring for the International Space Station – an annual meeting to discuss the scientific definition of an adequate radiation monitoring package and its use by the scientific community on the International Space Station (ISS) – a sequence of lectures on MEDIPIX took place. As the funding necessary for the project to continue was approved, more institutes joined the MEDIPIX2 collaboration that later developed the Timepix chip. The chip was finally installed on ISS in October, and started collecting data and sending them to physicists for analysis.

    1
    Image of the Timepix USB system in operation on the International Space Station (Image Courtesy of NASA).

    The ISS Timepix detectors gather data to characterize the radiation field as a function of time, taking precise measurements of the spectrum of charge and velocity of particles present inside the spacecraft. These Timepix units are compact USB powered devices, based on Medipix technology and controlled via Flight Software that is deployed on existing ISS Computers. Configuration settings can be modified and uploaded from the ground to adjust data-taking parameters on orbit, and minimal crew time is required for deployment and operation. The flight software displays total dose and dose rate based on LET information compiled from individual particle tracks. In addition, full measurement data is saved and downlinked for further analysis.

    2
    Larry Pinsky and undergraduate physics major Christina Stegemoeller, who worked with the group, display the Timepix detector.

    Timepix technology could improve or replace older devices by helping scientists analyse the particles and energy spectrum and then calculate the risks of exposure to heavy-ion radiation. This first trip was an opportunity to gain experience on the use of detectors in space, contributing to the development of the next generation of Timepix.

    During the test flight, mission controllers extensively checked Orion’s systems. The capsule orbited Earth twice, with its second orbit taking it about 5,793 kilometres away from the planet’s surface — 14 times farther than the orbit of the International Space Station.

    NASA scientists were particularly interested in seeing how the spacecraft behaves during important events, such as separations, once in space. Moreover, they also used the approximately 1,200 sensors aboard Orion to monitor the way the capsule’s computers and other technology behave in the harsh space environment. Orion flew through belts of radiation twice (on the way out, and again on the way back to Earth), allowing scientists to see how the spacecraft’s computers behave in a high-radiation environment.

    NASA has plans for another uncrewed mission in 2017 or 2018, which will be the first flight of Orion with the Space Launch System, a mega rocket, still in development. And in 2021, astronauts will travel with Orion and SLS for the first time to test some of the technologies needed for a trip to Mars. This test flight was just the beginning for Orion.

    See the full article here.

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  • richardmitnick 5:45 pm on August 8, 2014 Permalink | Reply
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    Fromk ALICE at CERN: “Inauguration of the new ALICE Run Control Centre” 

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    12 July 2014
    Federico Ronchetti

    The new ALICE Run Control Centre was inaugurated on the occasion of the collaboration dinner organized at Point 2 during the recent ALICE week. Eight months of restructuring works have reshaped the internal space arrangement of the working areas and fully refurbished all the services such as air conditioning and networking. Almost one hundred collaborators participating to the ALICE week dinner had the chance to enter the ARC for the first time and to get a live experience of the new environment.

    center
    The new ALICE Run Control Centre was inaugurated during the collaboration dinner organized at Point 2.

    In fact the ARC is already being used by several detector groups to carry on the first standalone tests since all the ALICE online systems underwent major improvements in terms of hardware and software requiring now a very intense phase of integration and commissioning. At the same time the ARC was recently used to manage one of the LHC dry runs, in which the machine activity is simulated in order to verify that all the interface systems with the experiment do respond correctly.

    room
    All the ALICE online systems underwent major improvements and the new ARC is getting ready for the second run of the LHC.

    I was personally very happy that our collaborators who signed up for the ALICE dinner could experience the ARC already in an operational phase in addition to appreciating the new ergonomic and neat style. I was also very happy that all the celebration preparation was somewhat kept hidden from me and during a short toast I was “given” as a gift a nice wall handler to hold the beam line technical drawings and that a very stylish and colourful banner with the “Alice Run Control Center” stamp on it appeared from nowhere.

    folks

    I really would like to thank all my colleagues who have helped me in the design of the ARC – Roberto Divià , Gilda Scioli and Ombretta Pinazza and those who followed all the construction and installation phases as Arturo Tauro and once again Roberto Divià. We and the collaboration wanted the ARC so that each of us could contribute to the data taking in the best way, having an efficient and comfortable environment in which for sure we will all spend many hours for the years to come.

    entry
    The entrance to the new ARC; where the journey of discovery begins.

    See the full article here.

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  • richardmitnick 3:44 am on June 12, 2014 Permalink | Reply
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    From CERN/ALICE: “New PMTs for the ALICE V0-C detector” 

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    27 May 2014
    Panos Charitos

    During the 2011 proton-proton run it was observed that the efficiency of the PMTs used in the V0 detectors had started to deteriorate. Gerardo Corral says: “At that time time we still had to take data and we could make no intervention but we continued monitoring the performance of the PMTs. Based on a series of measurements, it was clear in 2012 that the effect was probably due to radiation and ageing effects. Our colleagues from the University of Lyon had the chance to remove a number of PMTs from the C side of the V0 detector and make some more detailed testing and measurements.” He continues: “They took out 6 PMTs out of the 32 that are used in the C side and confirmed that their performance was reduced. We knew that we had to deal with this problem and the LS1 was the perfect opportunity”. For the A side of V0 things were more difficult as this side of the detector lies closer to the region of the beam pipe and one had to be very careful to avoid damaging the pipe.”

    The new PMTs arrived near the end of 2013 and were calibrated to be ready for installation. Gerardo explains that this is not just a replacement but also an upgrade as the new PMTs can operate at lower voltage. Reducing the high voltage means that one is also reducing the after-pulse signal which was one of the problems that V0 faced from the first runs. With the replacement of the PMTs the team is able to tackle these two issues: “We have PMTs with better gain that also work with lower voltage and thus reduce the after-pulse effect”.

    team
    Installing the new ALICE PMTs: Solangel Rojas Torres, Ildefonso Leon Monzon, Gerardo Herrera Corral, Arturo Tauro, Werner Riegler, Pieter Ijzermans and Elisa Laudi.

    The installation of the new PMTs took place during the second week of April. All the PMTs on the A side have been replaced and soon the team will start working on replacing the PMTs on the C side of V0. Gerardo explains: “The A side is much more complicated, though it lies 3.3 m from the IP it is still in the beam-pipe. We had to move the V0-A 30 cm away on the region where a very delicate beryllium pipe sits. It was a very slow process during which we took a lot of precautions”. Gerardo Herrera Corral with Ildefonso Leon Monzon and a PhD student Solangel Rojas Torres worked closely with members of the ALICE Technical Management team, namely Werner Riegler, Arturo Tauro, Corrado Gargiulo, Pieter Ijzermans and Elisa Laudi.

    The installation has been very successful since we had no accidents or other causes of delay. The PMTs have now been tested and a very good signal has been measured in all of them.
    PMTs in V0

    The V0 detector is a disk with 42 cm diameter and 2.5 cm thick. It is segmented in 32 cells with each cell linked to readout with optical fibres. When a charged particle crosses the plastic light is produced by scintillation. The fibres take out the light and shift the wavelength from blue to the green part of the spectrum. So green light arrives to the PMTs since their photocathode is more sensitive to these wavelengths. Through photoelectric effect electrons are emitted from the photo-cathode and travel through an elecromagnetic field; they hit a series of dynodes, amplifying the number of electrons and in that way the signal is strengthened.

    team 2

    V0 is very important for ALICE as it provides level – zero triggering. The new PMTs will give better efficiency but also allow reducing the after-pulsing signal. When you hit the window you have a pulse of electrons coming from the dynodes and a few nanoseconds later you have a second pulse that is not authentic but is created in the PMTs due to the ionization of the gas. The vacuum in the PMTs is not perfect; the gas atoms are ionized by the electrons and go to the opposite direction as they have positive charge, they hit again producing more electrons and give a second pulse. This is very bad for triggering as these signals are fake triggers that we have to suppress. With the new PMTs the after-pulse probability is ten times lower and V0 will be better equipped to play its triggering role in the forthcoming run in 2015.

    See the full article here.

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