The Large Hadron Collider at CERN: Entering a new era in Particle Physics

Günther Dissertori, Institute for Particle Physics, ETH Zürich, CH-8093 Zürich

 

1. Introduction

We are eagerly awaiting the re-start of the world's most powerful particle accelerator ever built. Later this year, CERN's Large Hadron Collider (LHC) is expected to start operations again. Thanks to the unprecedented energies and luminosities, it will give particle physicists the possibility to explore in detail the Tera electron Volt (TeV) energy range and hopefully discover new phenomena, which go beyond the so successful Standard Model (SM). The ability to compute and experimentally test this theory at the level of quantum corrections was the crucial requirement to establish it as a corner stone of our understanding of the Universe. Despite the impressive successes of the SM, it is generally accepted that it cannot be the ultimate theory, amongst several reasons also due to the fact that gravity is not included in the SM – which represents one of the fundamental limitations.

In this short article I will first recall the Physics motivations for the experiments at the LHC and then summarize the events prior, during and after the September days in 2008, namely LHC commissioning, first beams and a subsequent incident, which abruptly stopped operations.

2. Physics at the LHC

If we step back for a moment, we should remind ourselves that our ultimate goal as physicists is to obtain a consistent understanding of all observed phenomena in the Universe, from the largest distance scales (~1026 m) studied by Astronomy, Astrophysics and Cosmology, to the smallest scales (~10-18 m) explored by Particle Physics. The emphasis in the previous sentence lies on “consistent”. With the SM we have a quantum field theory, which gives an excellent description of the phenomena at the sub-atomic scales. On the other hand, during the last decade or so a standard cosmological model has emerged, which synthesizes the theoretical understanding of astrophysical and cosmological phenomena. The key question now and for the future is how to relate to each other the various observations in this different domains and obtain answers to questions such as “What is the cosmic dark matter? Is it related to Supersymmetry? Or something else? Is the universe filled with a Higgs field? How does this relate to dark energy? What is the structure of space-time? Are there extra dimensions? What is at the origin of the matter-antimatter asymmetry in the Universe? Can we learn something about the phase of the early Universe by studying a quark-gluon plasma?” [1]. Obviously, we can make progress on these questions only by experimental and theoretical advances in all directions. Here I emphasize what a hadron collider such as the LHC can contribute.

Above all, it is believed that the origin of electro-weak symmetry breaking will be elucidated by the LHC experiments, ie., allow for a deep understanding of why the photon is massless but the W and Z bosons are so heavy. Concretely speaking, the LHC contribution might consist in the discovery of one or more Higgs bosons and thus confirm the prediction that there is spontaneous symmetry breaking via the Higgs mechanism. Such a discovery would be very fundamental in the sense that it would give us an unprecedented insight into the structure of the vacuum. A straightforward (or maybe bold) assumption could be that this insight might also lead to an insight into the origin of dark energy, which is postulated to explain the accelerated expansion of the Universe. Unfortunately, the energy density of the Higgs field seems to be at least 1060 times greater than what is expected for dark energy. Thus we couldn’t be farer away from a consistent (!) understanding. In any case, the role of the LHC is to discover or exclude the existence of such a Higgs field. Indeed, if no evidence is obtained for a Higgs mechanism, we nevertheless expect new phenomena to show up in the TeV energy range, which after all have to ensure the conservation of unitarity. The latter is known to be violated, for example in the scattering of the longitudinal components of two W bosons, if no new phenomena set in at the TeV scale. In recent years, a number of alternative models to the standard Higgs mechanism have been developed to explain electro-weak symmetry breaking. Many of them involve the appearance of new strong interactions in the TeV energy range and the related production of heavy resonances and/or additional heavy quarks with unconventional electrical charges. The LHC experiments are well prepared for the search of such new particles.

The other main field of activity will be the search for new types of symmetries and particles, most notably Supersymmetry (SUSY). SUSY is the most prominent and extensively studied model of all proposed extensions of the SM. It postulates a symmetry between fermions and bosons and introduces a rich new spectrum of particles. This theory has several strong motivations. For example, it proposes a rather natural solution of the hierarchy problem (the apparent enormous difference between fundamental energy scales such as the electro-weak, the Grand Unification and the Planck scales), if supersymmetric partners of the SM particles appear with masses below or around the TeV scale. This would prevent the Higgs mass to acquire enormously large radiative corrections (from vacuum fluctuations of all possible quantum states) and eliminate the need to have an unnatural fine-tuning in order to explain the apparently small Higgs mass. The appearance of SUSY particles would also lead to the convergence of the electro-weak and strong coupling constants at an energy of about 1016 GeV, as generally foreseen in scenarios of Grand Unified Theories. Finally, some implementations of SUSY provide an excellent candidate for the dark matter observed in our Universe, namely the weakly interacting and stable lightest neutralino. Thus any evidence for the existence of a neutralino, possibly corroborated by direct dark matter searches, would constitute a fantastic step towards a consistent understanding of phenomena at the smallest and largest scales.

Other solutions for the hierarchy problem have been put forward, which postulate the existence of Extra Dimensions (ED). Some of these models conjecture that the fundamental scale of gravity could be as low as the TeV scale. The Planck scale only appears as a derived scale, because of the large volume of the EDs and the fact that only gravity can propagate there, whereas all SM fields are confined to a four-dimensional brane. Thus we see only a small part of the total gravitational flux, which explains the relative weakness of gravity compared to the other SM interactions. Other models exist which try to explain the hierarchy problem by a very strong curvature of the EDs. In general, the phenomenology of EDs foresees the production of gravitons via parton scattering at the LHC. These gravitons could escape into the EDs, leading to an apparent violation of energy-momentum conservation in our four-dimensional world, or decay to SM particles in a resonant-like behaviour. It goes without saying that any such discovery would revolutionize our understanding of space-time.

Besides the direct searches for physics beyond the SM, precision studies of the heavy flavour sector (most notably the physics of bottom and top quarks) could lead to indirect evidence for new physics, for example via an enhancement of otherwise very rare decays. The flavour mixing parameters which appear in the Cabbibo-Kobayashi-Maskawa (CKM) matrix will be measured using several different decay channels, complementing the very rich physics output of the so-called B-factories (e+e- colliders in the US and Japan) and hopefully leading to a better understanding of CP-violation. Regarding the question of the matter-antimatter asymmetry in the Universe, currently we don’t have a consistent understanding, since the observed CP violation in the quark sector is insufficient for this to work out in practice.

Finally, the LHC will also allow colliding heavy ions. The unprecedented energy densities achieved in these collisions are expected to lead to the formation of new forms of partonic matter, most notably a quark-gluon plasma, which should have been the state of matter in the very early Universe. The properties of this new state of matter, as well as the phase transition to hadronic matter will be the subject of an intense research.

This very rich physics programme will be pursued at CERN by observing proton-proton collisions (as well as heavy ion collisions) at four experimental sites around the LHC ring. It is installed in the former LEP tunnel, about 100 m underground with a circumference of approximately 27 km. Two of the four experiments, ATLAS and CMS, will be large general purpose detectors designed to cover practically the whole range of physics questions outlined above. The other two experiments are optimized for the study of heavy flavour physics (LHCb) and heavy ion collisions (ALICE). For a further review of the physics plans and the detectors see, eg., Ref. [2].

3. The LHC commissioning and current status

The LHC is a proton-proton collider with a design energy per beam of 7 TeV, a factor of seven larger than the currently highest energy achieved in the world, namely with the TEVATRON at FERMILAB. The LHC magnet system consists of about 10’000 superconducting magnets of various size and energy and of 154 resistive magnets [3]. Its main components are 1232 superconducting dipoles, each 14.2 m long (magnetic length). The nominal field in these dipoles, 8.3 Tesla, is much higher than that of previous superconducting accelerators. This required the use of superfluid helium, at 1.9 K, to boost the performance of the superconductors. The superconducting dipoles are of the "2 in 1" type, meaning that the apertures (56 mm) for both beams have a common mechanical structure and cryostat (Fig. 1). For a recent review of the performance of the LHC magnet system we refer to [4].

In total 2808 bunches with a nominal intensity of 1.1x1011 protons/bunch and a bunch-spacing of 24.95 ns will circulate in the ring, leading to a nominal luminosity of 1034 cm-2 s-1. The total energy stored in the beam will reach a macroscopic value of 350 MJ, which imposes severe constraints and requirements on its safe operation, since an uncontrolled beam loss unavoidably would damage the equipment. It is worth noting that in terms of stored energy per beam, the LHC exceeds all previous and existing machines by a factor of 200, thus we enter unexplored territory, indeed.

Fig. 1: Left: Interconnection work between two superconducting LHC magnets. Next to the magnets is the cryogenics distribution line;
Right: Illustration of the dipole magnetic flux, showing the “2 in 1” concept.

During spring and summer 2008 the eight LHC sectors were cooled down one after the other to 1.9 K. During the same period, the Hardware Commissioning of the LHC superconducting magnets and electrical circuits was performed, which corresponds to the testing of about 11’000 circuits. The success of the first beams day on September 10, 2008, demonstrated the very good field quality and geometry of the magnets, their precise alignment and very good stability, the accuracy of the power supply and the successful operation of the highly complex 1.9 K cryogenic system. The thermal performance of the magnet cryostats was even better than specified [4].

In the month preceding the official LHC start-up on September 10, first successful injection tests were performed, probing with beam three sectors (1 sector corresponds to 1/8th of the ring) of ring 1 and two sectors of ring 2. This allowed testing the optics, the quality of the beam position monitors and the polarity of dipole correctors, as well as to measure the aperture in the injection regions and the arcs. Then, on the morning of September 10, 2008, beam threading started with ring 1 and continued with ring 2 after having established the first turn for beam 1. Within one shift the first turn was achieved for both beams [5]. During threading the collimators were closed wherever possible to stop the beam and give the possibility of optimizing the trajectory steering before sending the beam into the next sector(s). This strategy proved to be very effective and both beams went through all sectors at the first shot with small losses. When the beams reached the experimental sites, several shots were intentionally dumped on tertiary collimators upstream of the nominal interaction points to provide the ATLAS and CMS experiments the first chance to record “beam-induced” events in their detectors. Within a few shots both detectors were timed in on the beam signals and recorded first events as shown in Fig. 2.

A few days after this major achievement, which caught world-wide attention, one of the beams could be captured by the RF-system, which is a necessary step towards later acceleration and bunching of the proton beams, followed by further commissioning of the whole complex (such as beam instrumentation, beam dump system, beam position monitors, beam optics measurements). Beam commissioning had to be stopped in the evening of September 12 due to a high voltage transformer failure. In the following days the powering tests of the few circuits that had not been fully commissioned for September 10 were resumed. Most of those circuits concerned magnets in sector 34 of the LHC, including the main dipole circuit.

In the morning of September 19 the last commissioning step of the main dipole circuit (154 magnets) of sector 34 was started, a ramp to 9.3 kA which corresponds to a beam energy of 5.5 TeV. During the ramp an electrical fault developed in the powering bus-bar between a dipole and a quadrupole at a current of 8.7 kA. A resistive voltage appeared and increased to 1 V after less than 0.5 s, leading to the power converter trip. Within the first second, an electrical arc developed and punctured the helium enclosure, leading to release of helium into the insulation vacuum of the cryostat. Immediately afterwards also the beam vacuum degraded. The spring-loaded relief discs on the insulation vacuum enclosure opened when the pressure exceeded atmospheric, thus relieving the helium into the tunnel. However, they were unable to maintain the pressure rise (nominal 0.15 MPa absolute), thus resulting in large pressure forces acting on the vacuum barriers separating neighboring sub-sectors (a sub-sector corresponds to two 107m long cells), which damaged them as can be seen in Fig. 3, cf. Ref. [5]. As a consequence, 39 dipoles and 14 quadrupoles had to be removed from the tunnel, repaired or replaced by spares (cf. Ref. [4])

Fig. 2: a): Snapshot of the beam screen that illustrates the very first turn of beam 1. This screen could record the image of the injected beam as well as the image of the beam after one turn, 89 μs later;
b): Image of the signals recorded by the CMS detector, induced by the splash of particles created by the beam hitting the collimators upstream;
c): First turn of beam 2. Shown are the horizontal and vertical positions (in mm) w.r.t. the nominal orbit of the beam around the full ring.

 

A post-mortem analysis of cryogenic temperature data revealed a significant temperature anomaly in sector 34 during a powering step to 7 kA performed a few days before the incident. The excess power in the incident cell corresponds to an unaccounted resistance of around 220 nΩ. Given the location of the primary electrical arc, the most likely hypothesis for the cause of the incident is a problem of the bus-bar joint (cf. Fig. 3). The joints are brazed but not clamped, and the nominal joint resistance is 0.35 nΩ. The incident could be reproduced in simulation assuming a bad electrical and thermal contact of the copper stabilizer at the joint due to lack of solder or poor quality brazing [6]. Following the incident, extensive calorimetric and electrical measurement campaigns were applied to all available sectors, revealing some smaller anomalies in other sectors. The currently ongoing consolidation of the machine includes a major upgrade of the quench protection system, which will provide online diagnostics of all joint resistances of the main dipoles and quadrupoles of the LHC arc sectors, as well as an upgrade of the pressure relief system. As it is not possible to fully implement the remedies in half of the accelerator (half of the sectors were kept cold), it is imperative to proceed carefully, in order to exploit all diagnostics available and to minimize the damage in case of faults. The next run might include several steps in energy, such as a run at 4 TeV per beam (slightly below 7 kA) for a certain period, followed by pushing the machine up to a maximum of 5 TeV (8.6 kA in the dipoles) [4].

The current schedule foresees the re-start of LHC with beam commissioning in October 2009, followed by a long running period until fall 2010, with the goal to reach a centre-of-mass energy of about 10 TeV and to collect data corresponding to an integrated luminosity of up to 300 pb-1. This should allow to reach and even surpass the physics reach of the TEVATRON in several areas. In fall 2010 also a short lead ion run is foreseen. Then follows a longer shutdown until late spring 2011 for further consolidation work on the machine, which will allow to increase the centre-of-mass energy to 14 TeV for the 2011 run and progress gradually towards the nominal luminosity. As the experience of previous colliders has shown, this may take several years.

The four detectors, which have not been discussed in this article, currently undergo extensive commissioning and calibration campaigns, mostly using cosmic rays. They have proven to be ready already last September during first beam operations, and they will be up to the challenge later this year, when first collisions will be recorded and the real data taking starts.

 

Fig. 3: Left: Schematic of the main dipole bus-bar joint;
Right: A damaged interconnect between an LHC quadrupole and a dipole.

References
[1] J. Womersley, “Experimental summary and perspectives”, Proceedings of the Hadron Collider Physics Symposium 2005, Les Diablerets, Switzerland, 2005.
[2] G. Dissertori, “LHC expectations (machine, detectors and physics)”, invited plenary talk, in: Proceedings of the International Europhysics Conference on High Energy Physics (HEP-EPS 2005), Lisbon, Portugal, 21-27 Jul 2005; PoS HEP2005:401, 2006; also HEP preprint arXiv:hep-ex/0512007.
[3] LHC Design Report, Vol. 1, CERN-2004-003, ab-div.web.cern.ch/ab-div/Publications/LHC-DesignReport.html
[4] V. Parma and L. Rossi, “Performance of the LHC Magnet System”, Contribution to the PAC 2009 conference, Vancouver, 2009.
[5] J. Wenninger, “Status of LHC Commissioning”, Contribution to the PAC 2009 conference, Vancouver, 2009.
[6] M. Bajko et al., “Report of the Task Force on the Incident of 19th September 2008 at the LHC”, CERN-LHC-PROJECT-Report-1168. [Released: July 2009]