First Results from the Large Hadron Collider
Hans Peter Beck, Albert Einstein Center for Fundamental Physics, Laboratory for High Energy Physics, University of Bern, CH-3012 Bern
Introduction
With the Large Hadron Collider (LHC), the 27 km circumference underground particle accelerator at CERN, straddling the Franco-Swiss border between the Jura-mountains and lake Geneva, the physics of particle collisions is probed at the Teraelectron volt scale (tera-scale), the highest accessible energies in a well-defined laboratory set-up.
Probing physics at the tera-scale scale will allow to experimentally conclude on the existence or non-existence of the Higgs particle and this way is shedding light on the related question of why particles do have mass. A rich search program for new phenomena not contained in the up to now so extremely successful Standard Model of particle physics (SM) are pursued in vigor to further advance our understanding of the world at the quantum level. Among the most important questions are whether quarks and leptons do have a substructure and therefore would be composed from constituents not yet known; whether there are more than three spatial dimensions to build up the space-time fabric; whether there are further fundamental symmetries in nature like e.g. supersymmetry; whether the dark matter prevailing in the universe can be explained by a new to be identified elementary particle; and whether gravity plays a role at the quantum level at the now accessible tera-scale.
After almost twenty years of planning and construction, the LHC took off in Spring 2010 to its first full data-taking year delivering proton-proton collisions at √s=7 TeV, followed by a one month period of Pb-Pb collisions during November 2010 at √sNN=2.76 TeV, the by the neutron to proton ratio of the Pb-ion reduced centre of mass energy per nucleon pair. With this, the LHC is taking over as the world most powerful particle accelerator from the Tevatron, a proton anti-proton collider near Chicago, USA. First results have been published by the ALICE, ATLAS, CMS, and LHCb collaborations comprising a total of about 10’000 physicists from few hundred institutes placed all over the world. In this article, I will present a few highlights of this first year of data taking and will give a brief outlook of the near future program with the LHC.
Four large detectors at the Large Hadron Collider
At four intersection points along the 27 km orbit, the clockwise and the anti-clockwise beam trajectories cross and it is at these four points where four large sophisticated detectors have been built and are now operated by correspondingly four distinct international collaborations. Figure 1 shows the layout of the LHC and the underground caverns that house the experiments.
The Swiss participation lies with the ATLAS, CMS and LHCb experiments with the Universities of Bern and Geneva participating in ATLAS, the University of Zürich and ETH Zürich participating in CMS and the University of Zürich and EPF Lausanne participating in LHCb. ATLAS and CMS are multi-purpose experiments, with the aim to cover range as wide as possible the physics potential of proton-proton collisions. The LHCb detector is specialized measuring rare decays of mesons containing b-quarks, with the aim to explore tiny differences in the symmetry between matter and anti-matter [1]. ALICE is dedicated to measure heavy-ion collisions and in particular to explore the properties of the quark-gluon plasma, a state of matter thought to have predominated during a brief moment of the early universe [2].
Figure 1: Layout of the Large Hadron Collider and the underground caverns that house the ATLAS, ALICE, CMS, and LHCb experiment. The accelerator is cooled down to 1.9 K all along its 27 km circumference.
Figure 2: Candidate for a WZ → eνμμ decay, collected on 7 October 2010. The invariant mass of the two muons is 96 GeV/c². The transverse mass of the potential W boson is 57 GeV/c².
Figure 3: Invariant mass spectrum of opposite-sign muon pairs measured with the CMS detector using the full 2010 data sample. Standard Model particles are clearly seen as resonances above a continuum background of Drell-Yan produced muon-pairs [1].
Initial results from the 2010 data taking
First physics runs with proton-proton collisions at √s = 7 TeV took place on 27 March 2010 with an initial peak luminosity of 1027 cm-2s-1. All experiments were ready to profit from these early collisions and operation continued without interruption, except for a few technical stops, until 31 October 2010 where peak luminosities of 2×1032 cm-2s-1 have been achieved, corresponding to an increase in collision rates of five orders of magnitude with respect to the start of operation. A total of 45 pb-1 of collision data has been produced by the LHC at the three collision points for the ATLAS, CMS and LHCb experiment, corresponding to about three trillion proton-proton collisions for each of these three experiments to record, and about 0.5 pb-1 for the ALICE experiment. The ALICE detector, optimized to record Pb-Pb collision data, needs the proton-proton collision rate to be limited, therefore, the two proton beams are slightly separated at the ALICE intetraction point to control the collision rate. Only a small fraction of these collisions are sufficiently hard collisions to be of interest for a detailed analysis. Every experiment contains a complex trigger system to select in real-time only those collisions that contain interesting signatures such as particle jets; photons, electrons or muons; or missing transverse energies. Sufficiently high thresholds in the transverse momentum and further topological constraints are applied to select about a few hundred collision events per second for permanent storage and offline analysis. Examples for topological constraints are isolation criteria, where no significant detector activity is allowed within a narrow cone around an electron or a muon track; the occurrence of multiple leptons or jets with or without extra missing transverse energy; or constraints in the separation of jets and leptons in polar and azimuthal coordinates. Figure 2 shows a spectacular event measured with the ATLAS detector with one electron, two muons, and missing transverse energy.
Exploring the physics at the tera-scale started with the proper calibration and alignment of the detector subsystems. Known signatures from the Standard Model are used as standard candles for this task. Figure 3 shows as example the performance of the CMS muon system with the invariant mass distribution of muon pairs of opposite electric charges. Standard Model particles like the J/Ψ, which is the charmonium bound state of a 'c' and an 'anti-c' quark pair at an invariant mass of 3.1 GeV/c2, the bottonium bound state Υ of correspondingly a 'b' and an 'anti-b' quark at an invariant mass of 9.46 GeV/c2 or the Z boson, a mediator of the electro-weak force at 91.2 GeV/c2 have been used for calibrating the detectors.
All detectors perform up to their expectations and first results have been obtained measuring production rates of charged particle tracks and jets of particles. In average about 30 charged tracks with at least 100 MeV/c of transverse momentum pT are produced with every collision at √s = 7 TeV [3] and jets with transverse momenta of up to 1.5 TeV have been recorded [5] probing a new kinematic domain far above the reach of previous experiments.
Probing for new physics beyond the Standard Model with 2010 data
The 45 pb-1 delivered by the LHC during the 2010 data-taking period open a new window in the search for new physics beyond the Standard Model.
Probing a potential substructure of quarks is done by looking for the invariant mass spectrum of the two hardest jets in events containing a pair of back-to-back oriented jets, so called di-jet events. No resonance structure has been found up to di-jet masses of 2.64 TeV/c2 and, in conclusion, leaving the possible dimension of a quark structure to be smaller than 10-19 m [6].
The existence of new fundamental forces is predicted by many theorized extensions to the Standard Model. Experimental signatures for these would be the existence of new force mediating particles such as the Z’ or W’ with masses in the range one to many TeV/c2. A Z’ would be seen as a resonance structure in the invariant mass spectrum of lepton pairs with opposite electric charge and a W’ could be identified in the transverse mass distribution of events with one lepton and missing transverse momentum due to the escaping neutrino. Both, the ATLAS and the CMS collaborations have been looking extensively for such signatures and the existence of Z’ and W’ bosons could be excluded at a 95% confidence level for masses below 1.14 TeV/c2 for the Z’ [7] and 1.58 TeV/c2 for the W’ [8] assuming Standard Model like couplings.
Supersymmetry relates matter particles (the fermions) with force mediating particles (the bosons) via a new hypothetical symmetry and predicts the existence of new superpartner to all Standard Model particles. A spin-½ fermion, such as e.g. the electron, would then be complemented with a spin-0 scalar superpartner, the selectron, with the same charge and couplings as the well-known electron. The superpartners of the spin-1 gauge bosons are the spin-½ gauginos; i.e. the gluino is the superpartner of the gluon. An attractive feature of supersymmetry is that it also predicts a candidate particle capable of explaining the dark matter content in the Universe: the lightest supersymmetric particle is thought to be stable, and only weakly interacting with Standard Model particles. If produced in proton-proton collisions at the LHC it would be detectable via the large missing transverse energy produced when escaping from the detector systems. ATLAS and CMS have been looking for signatures of supersymmetry pursuing different strategies and new limits have been set [9], [10]. This is shown in Figure 4 which depicts the fundamental mass-scales of the scalar and spin-½ supersymmetric particles for a whole set of supersymmetric models in events containing zero or one lepton and particle jets.
Similar, searches for events containing black holes or signatures from extra dimensions have been conducted and no evidence for physics beyond the Standard Model has been found.
Figure 4: Exclusion at the 95% confidence level of MSUGRA and CMSSM type supersymmetric models in the zero lepton and one lepton channels. No evidence for supersymmetry has been found for gluino masses below 500 GeV/c² and a limit of 775 GeV/c² has been found under the assumption of equal gluino and squark masses.
What about the Higgs?
Finding the Higgs particle or proving its non-existence is one of the main goals of the LHC experiments. The Higgs-mechanism allows the Standard Model bosons to be massive via the spontaneous symmetry breaking of the ground state of the Higgs-potential and for the fermions to acquire mass via their Yukawa-coupling to the all-pervading Higgs-field. The Standard Model precisely predicts all properties of the Higgs particle except its mass. With this, the production mechanisms and rates, and decay modes of the Higgs particle are known in theory predictions as a function of the unknown Higgs mass. Taking into account past searches for the Higgs particle at the LEP and Tevatron accelerators, as well as precision measurements of the top-quark mass and the W-boson mass, the allowed range for the Higgs mass is constraint to be larger than 114 GeV/c2 and lighter than 186 GeV/c2 at a 95% confidence level, in addition, the range between 158 GeV/c2 and 175 GeV/c2 is also excluded by recent results from the Tevatron accelerator experiments [11]. With the 2010 data, ATLAS and CMS have performed Higgs searches and both experiments have been publishing first limits but these are not yet competitive [12]. Finding the Higgs is particularly difficult in the low mass region of its allowed mass range. If the Higgs mass is less than 130 GeV/c2 it predominantly decays into a pair of b-quarks producing a signature almost indistinguishable from genuine proton-proton collision events with b-quark content. Therefore, rare decay modes of the Higgs particle need to be looked for, such as e.g. H→γγ, to clearly identify a Higgs signal over background. Looking for rare decay modes in turn requires larger data-samples to be collected during the coming two years of LHC operation. ATLAS and CMS are planning to combine their individual results to reach faster conclusion on the Higgs particle. If it is not found by end of 2012 then something evidently is wrong with the Higgs-mechanism and in the way particles are thought to acquire their masses. Physics would be in an equivalent state as it was in the late 19th century, where Michelson and Morley have proven the non-existence of the lumiferous ether. New revolutionary models would then be needed.
Conclusion and Outlook
The LHC and its four experiments have marvelously mastered their first year of operation. 45 pb-1 of collision data have been delivered by the LHC in 2010, collected with high efficiency by the experiments, and carefully analyzed by the collaborations. The Standard Model of particle physics has proven to remain a strong and solid theory up to the tera-scale and no immediate hints of new physics beyond it have been found yet.
For 2011, the LHC foresees to deliver about 1-3 fb-1 per experiment, corresponding to a 20-50-fold increase in statistical power over the 2010 data set. With this amount, the window for searching new physics beyond the Standard Model opens much wider than the narrow glimpse we had with the initial 45 pb-1. By combining the individual results of the ATLAS and CMS collaborations, the existence or otherwise of the Higgs particle will be concluded latest by end of 2012, and with some luck even earlier.
The LHC will undergo a long shutdown in 2013 and part of 2014 to get ready to double its energy to provide collisions at the originally foreseen 14 TeV. With this, new physics beyond the Standard Model potentially found during the 7 TeV data taking will be measured with higher precision and searches for new phenomena will continue.
References
[1] [LHCb Collaboration] to be published in Phys Lett B; Andrey Golutvin: Status and Results from LHCb, La Thuile 2011
[2] [ALICE Collaboration] arxiv.org/abs/1012.1657; Phys. Rev. Lett. 106, 032301 (2011)
[3] [CMS Collaboration] https://twiki.cern.ch/twiki/bin/view/CMSPublic/PhysicsResultsMUO#Full_invariant_mass_spectrum_of
[4] [CMS Collaboration] CMS-PAS-QCD-10-004
[5] [ATLAS Collaboration] ATLAS-CONF-2011-047
[6] [ATLAS Collaboration] accepted by NJP
[7] [CMS Collaboration] to be published in JHEP
[8] [CMS Collaboration] to be published in Phys. Lett. B; [ATLAS Collaboration] to be published in Phys. Lett. B
[9] [ATLAS Collaboration] to be published in Phys. Lett. B
[10] [ATLAS Collaboration] arxiv.org/abs/1102.2357; Phys. Rev. Lett. 106, 131802 (2011)
[11] [CDF and D0 Collaborations] talk given by M. Cooke at the 46th rencentre de Moriond, 21 March, 2011
[12] [ATLAS and CMS Collaborations] talk given by W. Quayle at the 46th rencentre de Moriond, 21 March, 2011
Hans Peter Beck is lecturer at University of Bern and member of the ATLAS collaboration since 1997. He is deputy group leader of the ATLAS activities of the University of Bern group and is interested in the search for new physics beyond the Standard Model, in particular in searches for supersymmetry. The on-line selection of candidate events poses a major challenge at the LHC and only about one in 105 collision events, reducing from an initial rate of 40 MHz down to a few hundred Hertz, can be recorded for final analysis. Hans Peter Beck played and plays a pivotal role in the design, implementation and optimization of the ATLAS trigger and data acquisition system. In a three-level scheme, built from special purpose digital signal processors, large networking infrastructure and big computing farms, only those collisions are selected which are of interest for off-line analysis, especially those promising to unveil new physics beyond the Standard Model.
[Released: September 2011]