3. Trayectoria vital de Cneo Pompeyo Magno
3.5. El viraje conservador y la caída de Pompeyo
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n July 4, 2012, two experiments, working at the CERN Large Hadron Collider, jointly announced a strong evidence of observing a new particle state. The observed particle, approximately 135 times heavier than proton and with a very short life time, could be the elusive Higgs boson which the high energy physicists have been earnestly looking for during the past 25 years. This will close one of the puzzles of the Standard Model of particle physics.During the 1960s, six scientists, G. S. Guralnik, Richard Hagen, T. W. B. Kibble, P. W. Higgs, F. Englert and R. Brout, suggested a mechanism which could rescue the gauge theory approach to explain electroweak interactions. They, as 3 independent groups, published in the same 1964 volume of
Physical Review Letters about
this mechanism, which could give mass to elementary particles, and at the same time retain the basic symmetry in the theory. They wrote from different perspectives and each paper made a distinct contribution in the field of particle
physics. These works enabled S. Glashow, S. Weinberg and A. Salam to formulate the Standard Model of particle physics in the later half of the 1960s. However, this mechanism requires the existence of at least one new particle which is referred to as the Higgs boson.
Many experiments, notably the experiments done at the Large Electron Positron (LEP) collider in CERN, Geneva, and at Tevatron in Fermilab, USA, have tested the predictions of the Standard Model to a high level of accuracy. All these measurements agree with the predictions which start with a few unknown parameters. The Standard Model is a beautiful theory and arguably one that is most precisely tested (some of the measurements have precisions exceeding few parts per million). Unfortunately, the theory could not provide any prediction on the mass of the only missing element, a spin 0 elementary particle, the Higgs boson.
Designing an experiment to look for such an object requires
certain inputs – an idea of the mass could be the first input. It has been shown that the precision measurements from experiments performed at the LEP can be used to predict an estimate of the mass of yet unobserved particles. The LEP measurements provided some idea of the top quark mass before its discovery at the Tevatron collider. Again, the precision measurements from LEP and Tevatron give some indication of mass of the Higgs boson. These measurements indicated that it should be lighter than 200 GeV (billion electron Volt). LEP also provided some lower limit from direct searches and thus the most probable mass range of Higgs boson as required in the Standard Model is between 100 and 200 GeV. Such a heavy object can be produced in very high energy collision and the most economical way of getting to such high energy is through collisions of two beams of protons.
The Large Hadron Collider (LHC) was conceived to provide such a facility (see Fig. 1). It was designed to provide collisions at 14 TeV (tera electron Volt) with beam intensity which could produce thousand million proton proton interactions per seconds. Such high interaction rate is required because probability of producing Higgs boson has been calculated to be rather small (optimistically with such an intensity, one expects to produce one Higgs boson in every 10 second).
The LHC was constructed in an underground tunnel, 27 km circumference, located in the Swiss-French border close to Geneva, Switzerland. It housed
Fig. 2 : Perspective view of the CMS detector showing an all-silicon tracking detector, a crystal electromagnetic calorimeter, a brass-scintillator hadron calorimeter within a superconducting solenoidal magnet and a muon detector system consisting of drift tubes, cathode strip and resistive plate chambers
Fig. 1 : Aerial view of Geneva area which houses the CERN laboratory. The large circle shows approximate location of the LHC ring which is 50-150m underground
several thousand bending magnets operated at very high field and at super cold temperature to contain the beam moving with a speed which is 99.9999991% the speed of light. Proton beams are produced by ionizing hydrogen gas and are accelerated through a series of accelerators before they are injected into the LHC ring at an energy of 450 GeV. The radio frequency (RF) cavities in the LHC ring accelerate protons to 3.5-4.0 TeV using electric field. All the accelerators in the chain use the same basic principle of using RF cavities in energizing the protons. These are a few facts about the LHC. The energy stored in the magnets is 10 Giga Joules – which is equivalent to the energy of an Airbus A380 flight cruising at a speed of 700 km/s. This energy can heat and melt 12 tons of copper. Energy stored in a single beam is 360MJ which is equivalent to 90 kg of TNT. The superconducting magnets require 60 tons or 120 thousand gallons of liquid helium to cool to the required temperature. This makes LHC the coldest place within the solar system with the temperature of 1.9
◦
K. The protons movein vacuum which makes LHC the emptiest place in the solar system with the vacuum in the pipe containing the beams at 10-13
atmosphere.
Protons are produced by ionizing hydrogen and are accelerated in several stages with a set of linear and circular accelerators before they are injected into the LHC ring at an energy of 450 GeV. In the process, protons are bunched with each bunch having on average 1.4 x 1011 protons. 1377
such bunches form the proton
beam within a LHC ring and they are energized to 3.5 or 4 TeV and brought to collision at a few points which houses the detectors. Two detectors, ATLAS and CMS, are designed to look for the Higgs boson and can sustain operation at such a high luminosity.
ATLAS as well as CMS is a high resolution microscope designed to look into the debris of the interaction. Each of these detectors consists of several sub-detector components with several 10s of millions of detector channels. Even the compact detector (see Fig. 2) is 15m in diameter, 22m long weighing 14 ktons. It has a central tracking detector detecting and measuring the charged calorimeter, an electromagnetic and a hadronic calorimeter measuring the energies and directions of electrons/photons and hadrons, respectively. This has more than 76 million detection units which are read out every 50 ns. Such an experiment cannot be made by one individual or by
one institute or even by one country. Some 3,600 scientists, engineers and students from 182 institutes in 38 countries made such an experiment to happen. The detector collects 15 peta (1015)
bytes of data a year. To make sense from these vast amount of data, one needs to distribute the computing task to a network of computers all over the world and this gives birth to a new technology – “Grid Computing”.
One needs a large number of interactions to probe at small cross-section of the production process. The LHC has been generous for that. It has provided excess of ~4 x 1014 interactions
during 2011 at a centre of mass energy of 7 TeV and even larger number of interactions at 8 TeV during the first half of 2012. The experiments collected the provided luminosity with very high efficiency. However, this will produce more than 109
unwanted interactions for each Higgs boson. During the data taking period of 2012, there are on average 30 inelastic proton- proton interactions at every bunch
The first step in the process of finding anything new is to make sure that the detectors are well- understood. This feat is achieved by looking at things which have already been established in the earlier work over the past few decades. Fig. 4 shows a snapshot of particles being discovered from the 1930s till 1995 starting with muons and pions and ending with the top quark. The first proton- proton collision in CMS took place in 2009. Within the next 6 months all these particles were observed in the CMS detector and
Fig. 3 : View of a typical bunch crossing where each line indicates the trajectory of a particle produced in the proton-proton collision within the CMS detetctor
Fig. 4 : Rediscovery of the Standard Model particles by the CMS detector
Fig. 5 : Two typical events seen by the CMS detector. The left picture is a candidate event where two high energetic isolated γs are seen and this is a candidate event of H→γγ. The picture on the right has a pair of electrons and a pair of muons and this is a candidate event for the decay mode H→ZZ(*)→2e+2μ
crossing and each interaction will produce several tens of particles in the final state (see Fig. 3). One has to extract signal from this huge jungle of particles traversing through the detector.
their properties were measured to accuracies better than established knowledge.
ATLAS as well as CMS detectors observed events like the ones in Figure 5. These events carry typical features of a Higgs boson produced in proton-proton collision. However, there are some rare Standard Model processes
which can give similar features. To distinguish signal from background one needs to collect a large number of such events and examine the probability of background Standard Model processes to reproduce the features observed in the data. One key feature will be that a new particle state will have a unique mass. If one sees that reconstructed mass of the (γγ) systems and 4-lepton system shows excess of events at the same mass window, this will be a clear evidence of a new object and in this case a new boson. Figure 6 shows examples of such comparisons from the two LHC experiments. Both the experiments have observed excess of events in γγ and 4-lepton systems at the same mass window (125-126 GeV). ATLAS and CMS examined 3 other decay modes of Higgs boson: H→ WW(*), H → ττ, H → bb-bar.
Combining the results from all these channels, ATLAS and CMS conclude that fluctuation of the Standard Model background can explain the excess at 125-126 GeV with a probability of 1.7x10-9 and
3.0x10-7, respectively.
So the two experiments find a very strong evidence of a new
narrow boson of mass around 125-126 GeV. The search criteria of this object are motivated by Higgs boson within the Standard Model. This feat is achieved by international collaboration of thousands of people working over two decades. The Indian groups have been a part of this from very early days.
If this newly observed boson is indeed the Higgs boson, the mechanism of mass generation is established. If there had been no mechanism for mass generation, the particles produced in the big bang would have never nucleated to make hadrons, nucleus, atoms, molecules, solar system, galaxy – the universe we know today. So the discovery not only solves a problem in particle physics, it also explains one of the most important puzzles of cosmology. The process of experimentation has opened up new technologies by pushing them to the limits: vacuum technology, cryogenics, superconducting magnets, computing, as well as software tools. Some of these new developments finds application in medical physics, notably in diagnostic equipments as well as cancer therapy.
Alumnus (Physics/1968-71)
Fig. 6 : Effective mass of the γγ (left plot) and the 4-lepton (right plot) systems observed by the two experiments, CMS and ATLAS