From ALICE at CERN: “What the LHC upgrade brings to CERN”

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From From ALICE at CERN

25 September 2018
Rashmi Raniwala
Sudhir Raniwala

Six years after discovery, Higgs boson validates a prediction. Soon, an upgrade to Large Hadron Collider will allow CERN scientists to produce more of these particles for testing Standard Model of physics.

FNAL magnets such as this one, which is mounted on a test stand at Fermilab, for the High-Luminosity LHC Photo Reidar Hahn

Six years after the Higgs boson was discovered at the CERN Large Hadron Collider (LHC), particle physicists announced last week that they have observed how the elusive particle decays.

CERN CMS Higgs Event

CERN ATLAS Higgs Event

The finding, presented by ATLAS and CMS collaborations, observed the Higgs boson decaying to fundamental particles known as bottom quarks.

In 2012, the Nobel-winning discovery of the Higgs boson validated the Standard Model of physics, which also predicts that about 60% of the time a Higgs boson will decay to a pair of bottom quarks.

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.

Standard Model of Particle Physics from Symmetry Magazine

According to CERN, “testing this prediction is crucial because the result will either lend support to the Standard Model — which is built upon the idea that the Higgs field endows quarks and other fundamental particles with mass — or rock its foundations and point to new physics”.

The Higgs boson was detected by studying collisions of particles at different energies. But they last only for one zeptosecond, which is 0.000000000000000000001 seconds, so detecting and studying their properties requires an incredible amount of energy and advanced detectors. CERN announced earlier this year that it is getting a massive upgrade, which will be completed by 2026.

Why study particles?

Particle physics probes nature at extreme scales, to understand the fundamental constituents of matter. Just like grammar and vocabulary guide (and constrain) our communication, particles communicate with each other in accordance with certain rules which are embedded in what are known as the ‘four fundamental interactions’. The particles and three of these interactions are successfully described by a unified approach known as the Standard Model. The SM is a framework that required the existence of a particle called the Higgs boson, and one of the major aims of the LHC was to search for the Higgs boson.

How are such tiny particles studied?

Protons are collected in bunches, accelerated to nearly the speed of light and made to collide. Many particles emerge from such a collision, termed as an event. The emergent particles exhibit an apparently random pattern but follow underlying laws that govern part of their behaviour. Studying the patterns in the emission of these particles help us understand the properties and structure of particles.

Initially, the LHC provided collisions at unprecedented energies allowing us to focus on studying new territories. But, it is now time to increase the discovery potential of the LHC by recording a larger number of events.

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So, what will an upgrade mean?

After discovering the Higgs boson, it is imperative to study the properties of the newly discovered particle and its effect on all other particles. This requires a large number of Higgs bosons. The SM has its shortcomings, and there are alternative models that fill these gaps. The validity of these and other models that provide an alternative to SM can be tested by experimenting to check their predictions. Some of these predictions, including signals for “dark matter”, “supersymmetric particles” and other deep mysteries of nature are very rare, and hence difficult to observe, further necessitating the need of a High Luminosity LHC (HL-LHC).

Imagine trying to find a rare variety of diamond amongst a very large number of apparently similar looking pieces. The time taken to find the coveted diamond will depend on the number of pieces provided per unit time for inspection, and the time taken in inspection. To complete this task faster, we need to increase the number of pieces provided and inspect faster. In the process, some new pieces of diamond, hitherto unobserved and unknown, may be discovered, changing our perspective about rare varieties of diamonds.

Once upgraded, the rate of collisions will increase and so will the probability of most rare events. In addition, discerning the properties of the Higgs boson will require their copious supply. After the upgrade, the total number of Higgs bosons produced in one year may be about 5 times the number produced currently; and in the same duration, the total data recorded may be more than 20 times.

With the proposed luminosity (a measure of the number of protons crossing per unit area per unit time) of the HL-LHC, the experiments will be able to record about 25 times more data in the same period as for LHC running. The beam in the LHC has about 2,800 bunches, each of which contains about 115 billion protons. The HL- LHC will have about 170 billion protons in each bunch, contributing to an increase in luminosity by a factor of 1.5.

How will it be upgraded?

The protons are kept together in the bunch using strong magnetic fields of special kinds, formed using quadrupole magnets. Focusing the bunch into a smaller size requires stronger fields, and therefore greater currents, necessitating the use of superconducting cables. Newer technologies and new material (Niobium-tin) will be used to produce the required strong magnetic fields that are 1.5 times the present fields (8-12 tesla).

The creation of long coils for such fields is being tested. New equipment will be installed over 1.2 km of the 27-km LHC ring close to the two major experiments (ATLAS and CMS), for focusing and squeezing the bunches just before they cross.

CERN crab cavities that will be used in the HL-LHC

FNAL Crab cavities for the HL-LHC

Hundred-metre cables of superconducting material (superconducting links) with the capacity to carry up to 100,000 amperes will be used to connect the power converters to the accelerator. The LHC gets the protons from an accelerator chain, which will also need to be upgraded to meet the requirements of the high luminosity.

Since the length of each bunch is a few cm, to increase the number of collisions a slight tilt is being produced in the bunches just before the collisions to increase the effective area of overlap. This is being done using ‘crab cavities’.

The experimental particle physics community in India has actively participated in the experiments ALICE and CMS. The HL-LHC will require an upgrade of these too. Both the design and the fabrication of the new detectors, and the ensuing data analysis will have a significant contribution from the Indian scientists.

See the full article here .

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