Discovery machine

PROFESSOR Tejinder (Jim) Virdee of Imperial College, London, has played an extremely prominent role in the design and implementation of the Compact Muon Solenoid (CMS) experiment at the Large Hadron Collider (LHC). He was involved in the development of the CMS detector concept from the earliest days and has been influential in many areas of the detector design. The innovative concepts in CMS are likely to influence the next generation of high-energy physics experiments. He proposed the idea of discovering the elusive Higgs boson via its decay into two photons, which is central to the concept of the high resolution lead-tungstate crystal calorimeter, one of the major components of the CMS design. Having served for many years as CMS deputy spokesperson, he was elected CMS spokesperson in January 2007. He will thus be the scientific leader of the experiment during first LHC data gathering and analyses. Experts from an interview with him shortly before the restart of the LHC at CERN in Geneva:

Are there any changes in your plans at CMS now that the machine is slated to begin its physics run only at 3.5 TeV per beam initially instead of the original 5 TeV?

Well, it has taken us a long time to build the experiment and we have also been commissioning the experiment. The commissioning of such an experiment actually has several phases in it. Straight after finishing construction we were ready to take beam last year and we took some good data, but then the incident occurred. Nevertheless we continued. We ran the experiment for about six weeks round the clock for several reasons. One was to see whether this very complicated piece of scientific equipment actually can sustain long operation. We also have to figure out if it is working as expected. What do I mean by that? We have documents written down ten years ago, which run into a stack three-quarters of a metre thick, which explain what performance of these would be to get to the physics at the LHC. So we have to check whether the instrument is actually working as we had written down. For that we ran the detector for six weeks round the clock, recording cosmic rays.

Cosmic rays actually manifest themselves at sea level as muons. And some of these muons have very high energy. They can penetrate 100 metres of earth and still have a lot of energy. These are the particles that go through the detector and we could actually check whether the detector was working: Does it have the expected efficiency close to 95-99 per cent? Does it have the position resolution in some cases 15 microns and in other cases 200 microns? And does it have the time resolution order of nanoseconds? And does it work as a whole system? Does it have the [expected] performance in kinematics like momentum resolution, energy resolution and so on? And all in all we found that the amount of the detector that was working was very high order of a per cent not functioning of this complex instrument that has 100 million electronic channels in all. So everything was very encouraging. There were a few issues that cropped up which were related actually to cooling. A few detectors had to be replaced. And some refurbishment in terms of the infrastructure, like the cooling system and so on. We had to install one detector and we did that.

What are the special characteristics of the detector and what are the various steps involved in your checks?

We have a design which is entirely based on a single magnet, a high field solenoid. The first thing one actually does in the design of the experiment is actually to figure out the magnetic field configuration for the measurement of muons. That then determines the rest of the design. The detector is built such that [see diagram] the first layer is within a tracker, which is all silicon. It has a volume of a cylinder of 6 m in length and 2.5 m in diameter. The next layer is the electromagnetic calorimeter, which uses lead-tungstate crystals (in green). Then the hadron calorimeter, which is a brass scintillator. The fourth layer is the muon system, which is dominated by the coil. It is the most powerful coil ever built. It is 13 metres long and 6 m in diameter and it has a very high magnetic field which supplies the field for this detector and when the field returns in the iron yoke (shown in red), we use it to analyse the momenta of the muons.

We had also taken data when the beams were stopped at the collimator and that actually allowed us to check some of the timing characteristics. With this large number of channels they also have to sing in tune to hit the right note to see the track. If they are out of time you see a jagged track. From there we can actually figure out if the detector is functioning well. After we finished the maintenance part, we started taking the data again about three or four weeks ago. The detector seems to work well and we are analysing that data so as to be as ready as we can when the beam comes. Thats the first part commissioning of the experiment without beam.

The next thing to do is to commission it with beam. When the beam comes, particles actually go outwards and time synchronisation is quite different. So we have to check that we actually reproduce or have performance that this set of data are indicating. This is independent of energy. The next step is what we call Rediscover the Standard Model. So this is known physics. We know how some of these standard model processes are: things like W and Z production, particles discovered in the early 1980s. These particles are used to check whether the experiment is working properly or not. By checking it means: are the rate of the W and Z production and the way they decay as we expect from the Standard Model? What it is at [Fermilabs] Tevatron energies [2 TeV] we know. And the standard models predictions are precise to a few per cent. It will be a first challenge for us to make measurements at that precision. We know what the answer should be. And so that again is also, roughly speaking, independent of energy because W and Z production cross-sections are not changing very fast with energy. So sitting at 7 TeV for some period of the time, collecting these data will take a little bit longer than at higher energy but not that much longer. So thats the next step, what I would call Physics Commissioning the detector.

Wont some new channels open up even at 7 TeV?

At 7 TeV if you start picking up a lot of data then you would go beyond what Fermilab is doing currently. What we are talking about is something we do in orders of six months to one year at most, to compare with something thats been going on since 1989. Thats the first thing. If we get confidence there and if we understand Ws and Zs production, the top-antitop quark production, things like J/psi and upsilon particles these are what we call the standard candles we will clearly be looking for new things. Then after a certain time at 7 TeV when we understand the detector, when we get familiar with the machine and start playing with the beam we will go up in energy and there the aim is to take sizable data sets. And it is clear that as we go above 7 TeV, closer to 10 TeV, each unit of data brings us beyond Fermilab all the more faster, go to new territory, make a significant step beyond Fermilab, in terms of new physics like supersymmetry, extra dimensions, heavy Ws and Zs. Higgs will take time.

Why do you say Higgs will take time?

Because, first, the amount of time that something takes depends on two or three different things. One clearly is the centre-of-mass energy. Second is how heavy it is, and how easily it is produced. So, for example, in supersymmetric particles, they have high cross-sections so they are easily produced, but their masses are high. When I say beyond the Fermilab territory, it is some number like 450 GeV and when I say go significantly beyond, it will be a few hundred GeV above that. The higher the mass, the rarer they are and the more the luminosity you need. Luminosity at higher energy is significantly more. The Higgs is a rare beast to be produced and at the LHC, depending on its mass, its signatures that are easily visible are rare as well. So we need to take a lot more collisions to see it. We have to accumulate a lot more proton-proton collisions, preferably at higher energies than at lower energies. Thats why it will take a bit of time.

But as soon as you have a 3.5 TeV beam thats 7 TeV are there are any particular things that you would be looking for?

We would be looking for everything.

Thats true. But are there any specific things because of their higher cross-sections and things like that?

Supersymmetric particlesAt 7 TeV itself?

If we stay there for some period of time, because the idea is to go up in energy after a reasonable time but not too long a time either. Higher energy gives you more reach, but even at that energy if you stay there for significant length of time we would be exploring territories beyond what Fermilab is doing because, after all, it is 3.5 times that energy.

As you continuously ramp up the energy beyond 7 TeV, would you progressively be setting lower limits on Higgs mass? When will you be able to say with confidence that there is no Higgs below such and such energy?

Well, the LHC is a discovery machine. We are actually looking to make discoveries. So, I think thats the name of the game. As the data come in, the emphasis will be on the high statistical significance of the statements that we make at the end of the programme we are half way through the LHC programme today and so [there is] another 10-15 years to go. The name of the game is actually to retrieve all the physics that is at this special energy scale of the LHC. There is some magic, I think, about this energy scale. We expect things to happen because we know that the standard model doesnt work at this energy. There are some parts of it which fail. And so there is something which nature has done to give mass and also to control some other things which give nonsensical answers as we extrapolate standard model to these energies. So what is it? Thats what we want to find out. When we find it out we need to know how nature has done this. What is the underlying theory and that will take time as well and thats why the programme is longish. Discovering is one thing, getting into a deep understanding is another game.

One of the great things about Higgs is that, when we started, its mass was not known. It is still not known. It can actually range from 115 GeV to 1 TeV although the measurements indicate that it is closer to 115 GeV. The good thing about Higgs is that depending on the mass it actually manifests itself inside the detector in completely different ways. And many different ways depending on the mass, and we have to cover all the different ways and, in fact, when you have done you find that detector can do anything that the nature has in store for us. Anything.

So I think it will be very exciting time coming up.