Collaborations on the starting blocks

The detectors are finished, the caverns are closed, and the experiments are taking their first data (courtesy of the cosmos). Together, the six experiments of the LHC represent a culmination of an immense international journey, one that has depended on technology and innovation that could barely have been imagined when the experiments were first proposed, some as long ago as 1992. Here we take a brief look at how they have each evolved since those early days.

ATLAS

In terms of volume, ATLAS is the largest detector ever constructed. Created after the merging of two existing collaborations, ASCOT and EAGLE, the designs for the multi-purpose detector have not changed significantly since their inception.

"When, in December 1994, we submitted the technical proposal, all the big decisions such as which type of calorimeter and magnetic field had already been taken," says Peter Jenni, the ATLAS spokesperson.

There have been just two main turning points for the collaboration. The design of the precision inner detectors was not established until 1997, when silicon strips were finally favoured over Micro Strip Gas Chambers (MSGC) for the outer layers. Then, in 2002, the detector underwent a financial audit. The main impact of this was on the high-level trigger and data acquisition, with some review of the inner detector, muon system, calorimeter electronics and shielding system. Overall, the main design was not affected.

Having always aimed at being able to detect the Higgs boson and super-symmetric particles at high luminosities, what will the detector be up to in the initial, low energy stages? "Ten TeV at low luminosity will already give us a lot of data to calibrate… before any discovery can be claimed we have to show that the known physics can be reproduced and the detector is performing well."

ALICE

The first ALICE proposal in 1992 was a work of impressive extrapolation, as the energy of the LHC was a factor-of-300 increase on the ion colliders that existed at the time. Sixteen years on ALICE has a little more to go on, with results from heavy ion programmes at SPS and the Brookhaven National Laboratory’s RHIC.

ALICE will study quark-gluon plasma, a very early state of matter. "Back in 1993, we were imagining what the quark-gluon plasma would look like and we expected it to behave like a gas or like weakly interacting particles, but what we found is that it behaves like an ideal fluid, so it is completely different," explains Jurgen Schukraft, the ALICE spokesperson. Armed with this knowledge, the ALICE collaboration is now also considering the possibility of another initial state of matter called colour glass condensate, which possibly forms at very high gluon densities in heavy nuclei.

As the field of heavy ions has developed, the ALICE collaborators have been very flexible with their detector design. This includes adding the muon spectrometer, a transition radiation detector and the electromagnetic calorimeter (scheduled installation 2010/11).

ALICE is expecting to receive 1.25 GB/s of data for the one month out of the year the LHC will be in heavy-ion operation. "There were many discussions on how to handle this huge amount of data and today within a factor 2-3 it’s quite common. However, 15 years ago one could not dream of handling such a large amount of data at such a rapid rate," says Schukraft. By 2009 he thinks the experiment will start to produce some interesting results.

LHCb

Unlike the general-purpose detectors, LHCb is asymmetric in the forward direction, optimised for the study of B-mesons.

Consisting of a beauty quark and another of a different flavour, B-mesons have gained the attention of physicists. Andrei Golutivin, who recently took over from Tatsuya Nakada as LHCb spokesperson, explains, "We focus on the study of B-mesons, where some of their behaviours are very precisely predicted by the Standard Model. However small, a deviation from these predictions would indicate the existence of new phenomena." Investigating B-systems should also help physicists understand the asymmetry between matter and antimatter in the Universe (CP violation).

There has been one major layout change in LHCb’s history. Known as the ‘LHCb light’ option, the number of layers of detectors that the particles will cross was significantly reduced, lowering interaction between primary particles and the material of the detectors.

LHCb is designed to run at a luminosity much smaller than the maximum LHC luminosity, achieved by simply focusing the beams less at the interaction point. However, the collaboration is considering a major upgrade after the initial physics programme in 5-6 years. "The original LHC will be able to deliver more than enough luminosity for us anyway. But an upgraded LHCb would be compatible with the Super-LHC also" says Golutvin.

CMS

Also a multi-purpose detector, CMS has not changed in terms of its main objectives since it was first dreamt up 13 years ago. "We had to build a detector that is capable of finding the Higgs at any mass, covering almost everything that it could be possible to discover, including supersymmetry, extra dimensions and all sorts of sub-structures," says the collaboration’s spokesperson, Jim Virdee.

From the start it was planned that the detector should be built above ground and then lowered down, piece by piece, into the experimental cavern. This method of construction had never been used before, but was ground-breaking in the sense that the teams could work on various elements in parallel.

Over time the collaboration has also incorporated changes to the design to take advantage of developments in technology. For example, in the late 1990s, the radiation- hard electronics, vital to many different sub-detectors, had to be entirely re-designed by CERN scientists after an outside contractor could no longer deliver the goods. Other challenges, such as the production of 75,000 lead tungstate crystals in Russia and China, and the eventual decision to use only silicon in the inner tracker, hinged on the availability of new, affordable technologies.

CMS has some upgrades planned, for example four more muon stations and eventually the compete replacement of the inner tracker. "There are plans to extend the life of the experiment to the middle of the 2020s," concludes Jim Virdee.

The above text is based on material that will be published in a special issue of the CERN Courier dedicated to the LHC.

TOTEM

With detectors positioned at distances of 147 and 220 metres from the CMS interaction point (see Bulletin 49 & 50/2006) and others inside CMS, TOTEM (TOTal Elastic and Diffractive Cross Section Measurement) will measure the total interaction cross-section of protons at the LHC.

Specific to the TOTEM experiment are the "Roman pots". Veritable marvels of technology, these cylindrical vessels can be moved to within one millimetre of the beam centre. They contain detectors that will measure very forward protons, only a few microradians away from the beams, that arise from elastic scattering and diffractive processes. Inelastic interactions between protons will be studied by the detectors installed inside CMS.

The data collected by the experiment will help to further improve our knowledge of the internal structure of the proton and the principles that determine their shape and form as a function of their energy. Furthermore, TOTEM will also allow very precise measurements of the LHC luminosity and individual cross-sections to be used by the other LHC experiments.

TOTEM has installed all the Roman pots and has equipped a few of them with detectors. This will allow the collaboration to test the movement of the Roman pots with respect to the beams at the LHC start-up, and to take some first data. Some detectors have also been installed within CMS. After having gained experience this year, the collaboration will install the rest of the detectors during the winter shut-down to make the experiment fully operational for next year’s runs.

A view of TOTEM’s silicon detectors.

LHCf

Positioned 140 metres from the ATLAS interaction point, the LHCf experiment will attempt to improve the models that describe the disintegration of ultra-high-energy cosmic rays as they enter the atmosphere. This will allow their energies to be determined more accurately and their composition to be analysed with greater precision. This information will help to support the hypotheses on the mysterious origins of cosmic rays.

The LHCf detectors are placed along the beam pipe, at the point where the pipe splits into two, just beyond the experiment cavern. This location allows them to detect the neutral particles (or their decay products) that are emitted in the forward region and are not bent off course by the magnetic fields of ATLAS and the LHC magnets.

While the old generation of accelerators allowed researchers to verify the cosmic ray disintegration models up to energies in the region of 10¹⁵ eV, LHCf will test them at energies of up to 10¹⁹ eV. Even if this year’s data is generated by lower-energy collisions, it will still be important as it will lie in the topmost region of data collected from previous experiments.

For further information on LHCf, see Bulletin 41 & 42/2006.

CMS launches new website

CMS today launches its new public website in advance of the LHC first beam event. The website is an extensive resource where visitors can learn about the detector and its design, the physics behind the experiment, see information about its 3000-strong international collaboration and access a range of multimedia resources.

A view of the LHCf experiment.

"We would like people to be able to share in the excitement of CMS as much as possible," explains CMS spokesperson, Jim Virdee. "We hope people will enjoy the new website and find it a valuable resource; in the future they will also be able to use it to keep up to date with the latest news on physics results.”

The new website can be found at:
http://cmsinfo.cern.ch