Indian presence

Print edition : January 29, 2010

THE PHOTON MULTIPLICITY Detector, an important component of the ALICE detector. Its fabrication was entirely India's responsibility.-CERN

THE six experiments that will record events at the Large Hadron Collider (LHC) are: A Large Ion Collider Experiment (ALICE); ATLAS; the Compact Muon Solenoid (CMS); the LHC beauty (LHCb) experiment; the LHC forward (LHCf) experiment; and the TOTal Elastic and diffractive cross section Measurement (TOTEM) experiment. ALICE, ATLAS, CMS and LHCb are installed in four huge underground caverns built around the four collision points of the LHC beams. TOTEM will be installed close to the CMS interaction point and LHCf near ATLAS. There is significant Indian participation in ALICE and CMS.

The participating institutions in ALICE are the Variable Energy Cyclotron Centre (VECC), Kolkata; the Saha Institute of Nuclear Physics (SINP), Kolkata; the Institute of Physics, Bhubaneswar; the Indian Institute of Technology Bombay; Aligarh Muslim University (AMU); Punjab University, Chandigarh; Jammu University; and Rajasthan University, Jaipur. Participating in CMS are Punjab University; University of Delhi; the Bhabha Atomic Research Centre (BARC), Mumbai; and the Tata Institute of Fundamental Research (TIFR), Mumbai. While participation in ALICE is led by Bikash Sinha, former Director of the VECC, that in CMS is led by Atul Gurtu of the TIFR.

ALICE is a detector specially designed to analyse lead-ion collisions. When LHC delivers its peak energy of 7 TeV, a lead ion (with 82 protons in its nucleus) will have a total per beam energy of 574 TeV, which means a total energy of 1,150 TeV will be available for ion-ion collisions. At such extreme energies, lead-ion collisions are expected to recreate conditions of extreme temperature and density just after the Big Bang under laboratory conditions. Under these conditions it is believed that the bound constituents of protons and neutrons quarks and gluons will break free for a very short time, creating a soup of quarks and gluons. This state of matter is called Quark-Gluon Plasma (QGP). The data obtained will allow the study of QGP as it expands and cools and help understand how progressively particles that constitute the matter of our universe arise.

For this purpose, ALICE will carry out a comprehensive study of the hadrons, electrons, muons and photons produced in the collision of heavy nuclei for which its detectors are appropriately designed and tuned. An important component of the ALICE detector is the Photon Multiplicity Detector (PMD), whose fabrication was entirely Indias responsibility. The PMD is a unique detector based on highly granular, honeycomb gas cell detectors consisting of 220,000 cells. India was also responsible for making some parts of the Muon Chamber Arm. The most significant element in the contribution to the muon arm is the design of a pre-amplifier ASIC chip (application-specific integrated circuit chip) called MANAS by the VECC, which was fabricated by Semiconductor Complex Ltd. (SCL), Chandigarh.

Each chip reads data from 16 electronic channels. India has supplied 100,000 MANAS chips for the 1.6 million channels in the muon arm as well as 14,000 chips for all the PMD channels. MANAS being a generic ASIC for high-energy experiments, these chips are also being used in the ongoing STAR experiment at Brookhaven National Laboratory (BNL).

Though ions will be introduced into the LHC only in a couple of years from now, until such time ALICE will not sit idle. While the primary objective of ALICE is to study strongly interacting matter at extreme energy densities where QGP is expected to form, it will also study proton-proton collisions both as a comparison with lead-lead collisions when they happen and in physics areas where ALICE is competitive with other LHC experiments. Interestingly, of the six experiments, ALICE has come out with the first physics paper using the limited data gathered from the first 284 collision events at 900 GeV that were observed over a period of one hour on November 23. (When the LHC is running at its peak energy and intensity, it will produce 600 million collisions per second.)

ALICE scientists measured the multiplicity of charged particles produced in these proton-proton collisions. Since data of such (low) energy collisions were available from other earlier experiments, ALICE data served to confirm the same as well as validate the model calculations made for the experiment.

CMS is an advanced detector comprising many layers. Consisting of 100 million individual detecting elements, each looking for signatures of new particles and phenomena at 40 million times a second, it is one of the most complex scientific instruments ever constructed. It derives its name from the fact that it is small and compact for its enormous weight of 12,500 tonnes. It is designed specially to detect and measure muon energies and it has a large 13 m x 7 m solenoid coil, the largest and the most powerful ever built, for its huge superconducting magnet around which the detector is built. The magnet has a field of 4 tesla, which is 100,000 times stronger than that of the earth. CMS is designed to see a wide range of particles and phenomena resulting from high-energy collisions at the LHC. The 21 m x 15 m x 15 m detector is like a giant filter, a cylindrical onion, each layer of which is designed to stop, track or measure different particles emerging from proton-proton collisions. Particles emerging from collisions first meet a tracker, made entirely of silicon, which traces their positions as they move through the detector, allowing a measurement of their momenta. While the silicon tracker interferes with the particles as little as possible, the calorimeters in the outer layers are specifically designed to stop the particles in their tracks and provide a measure of their energies.

The next layer is the Electromagnetic Calorimeter (ECAL) made of lead tungstate crystals, a very dense material that produces light when struck which measures the energy of photons and hadrons. The Hadron Calorimeter (HCAL), which is the next layer, is designed mainly to detect particles made up of quarks. The size of the magnet allows the tracker and calorimeters to be placed inside its coil, thus resulting in an overall compact detector. For the measurement of muon energies, the outer part of the detector, which is the iron magnet return yoke, is utilised. It stops all particles except muons and other weakly interacting particles, such as neutrinos, from reaching the muon detectors.

The Indian contribution to CMS includes the fabrication of 1,000 of the 4,300 pre-shower silicon strip detector modules that are attached to the end caps of ECAL and HCAL for discriminating against pions and photons before they deposit energy in the calorimeters. These detectors were developed by BARC and fabricated and tested by Bharat Electronics Ltd (BEL). Like MANAS, these strip detectors are generic devices for use in other high-energy experiments as well. Another important contribution is the complete fabrication, installation and commissioning of the Outer Hadron Calorimeter, a supplementary system outside the magnet (but just before the muon detector) to enable total containment of hadronic energy from particle showers in HCAL. It consists of 72 honeycomb panels of scintillation detectors (each 2.5 m x 2 m) with a data read-out system using wavelength shift fibre and hybrid photo diodes. K. Sudhakar of the TIFR was responsible for this part.

A unique method was employed in the construction of the CMS detector. It was designed in 15 separate sections or slices that were built on the surface and lowered down ready-made into the cavern 100 m below. This enabled saving valuable time as excavating the cavern and building the detector could go on in parallel. Lowering CMS by means of heavy lifting was a decision that was taken right in the beginning, some 16 years ago, inspired by experiences with Large Electron Positron Collider (LEP). The first lowering took place in November 2006 and the last in January 2008. According to Alain Herve, CMSs original technical coordinator, the concept of building large objects on the surface and transferring them underground as completed elements is the clear way to go in the future.

CMS has the same physics goals as the other general purpose detector ATLAS, but the two have quite different technical solutions and designs. In a sense, the two are complementary. The chief difference between the two is that while CMS is built around a huge superconducting solenoid, ATLAS has a toroidal configuration. A toroidal configuration can either have an air core or an iron core, and ATLAS has chosen an air core. The tracker in CMS is all silicon whereas in ATLAS it is half silicon and half gaseous detectors. Similarly, while CMS has a solid lead-tungstate calorimeter, ATLAS has a liquid argon calorimeter. The first thing one actually does in the design of the experiment, points out Tejinder Virdee of CMS, is actually to figure out the magnetic field configuration for the measurement of muons. That then determines the rest of the design. We start from the physics and then we have to build these complicated experiments that allow you to get to the physics. Physics determines the performance that you require.

The data thus gathered by CMS and ATLAS will be used to answer questions such as: What is the universe really made of and what forces act within it? And what gives everything substance or mass? CMS is tuned to measure the properties of previously discovered particles with unprecedented precision as well as discover new particles, such as the Higgs boson, supersymmetric particles and gravitons, and completely new phenomena. The experiments are also expected to throw light on what constitutes dark matter and whether there are more than three dimensions of space. Indeed, CMS and ATLAS will be the two key experiments that will be keenly followed by physicists for their results on the elusive Higgs boson.

R. Ramachandran

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