List of Tables
1.1 The Large Hadron Collider
The Large Hadron Collider (LHC) [2] is a 27 km circular synchrotron located 50 -175 m underground on the border between Switzerland and France near the city of Geneva. It is currently the most powerful particle proton-proton (pp) accelerator ever built and is expected to reveal new insights beyond our current understanding of particle physics. Initially, the LHC was intended to be constructed in two stages.
The first stage aimed to build a proton-proton(pp) colliding beam accelerator with a collision energy of √
s =10 TeV followed by an upgrade in stage II that would increase the collision energy to √
s=14 TeV. However, before completion of stage I, adequate funding and contributions from non-member states such as Canada, Japan, India, Russia and the U.S.A allowed for the immediate construction of the 14 TeV machine [3].
The LHC will take particle physics research to a new energy frontier and closer to the maximum limit of achievable accelerator-based collision energies. Since the 1930’s, when Cockcroft and Walton constructed a 800keV accelerator [5], the ef-fectivecollision energy of particle accelerators has increased more than 7 orders of
∗Dark Energy contributes around 72%, while normal matter makes up only 4.6%.
Chapter 1. Introduction to the LHC & CMS detector 3
Figure 1.1: A Livingston plot of e+e− and Hadron colliders in chronological order based on their first year of physics data taking and their center-of-mass,
√scollision energy [4].
magnitude [4]. The limiting factor for the energy of proton accelerators like the LHC is the maximum magnetic field required to bend the particles around a ring of manageable size. The LHC itself incorporates approximately 9300 supercon-ducting magnets including 1232 dipole and 858 quadrupole magnets around its circumference for bending and focusing the beams. The field required to bend the two 7 TeV beams within the bending radius of approximately 2.8 km is 8.33 T.† The collider tunnel contains two pipes enclosed within these superconducting mag-nets cooled by liquid helium, each pipe containing a proton beam during the usual proton runs. The two beams travel in opposite directions around the ring and are brought together into a single pipe for 150 m either side of each collision point.
Sets of Inner Triplet Magnets are used to focus the beams at four intersection points where interactions between them take place.
There are five experiments positioned at the four interaction points (I.P); ALICE (A Large Ion Colliding Experiment)[7], LHCb (Large Hadron Collidor beauty
†The LHC ring is not perfectly circular but has eight long, straight sections (LSS) where the experiments, injectors, services and beam dumps are located.
Table 1.1: Some important characteristics of the LHC [6].
Injection Collision Beam Data
Proton Energy 450GeV 7000GeV
Number of Bunches 2808
Number of Particles per Bunch 1.15 ×1011
Circulating Beam Current 0.582 A
Stored Energy per Beam 23.3 MJ 362 MJ Peak Luminosity in IP1 & IP5 - 1.0 ×1034cm−2s−1
experiment)[8] and TOTEM∗ (Total Cross Section, Elastic Scattering and Diffrac-tion DissociaDiffrac-tion at the LHC)[9] are three specialized experiments. The Compact Muon Solenoid (CMS) [10] and ATLAS (A Toroidal LHC ApparatuS) [11] are the two main, multi-purpose experiments positioned at Point 5 and Point 1 of the LHC ring respectively. A more detailed description of CMS is given in Chapter 2.
1.1.1 LHC Layout
The 27 km ring of the Large Hadron Collider is capable of accelerating pro-ton beams to 7 TeV (√
s = 14 TeV) and lead nuclei (208Pb82+) to an energy of 2.76 TeV/nucleon and a total center of mass energy of 1.15 PeV. [2]. Figure 1.2 shows the complete path taken by the protons towards the LHC ring. Protons are extracted from hydrogen gas in a Duoplasmatron and injected into a linear accelerator (Linac2) where they are accelerated to 50 MeV. They then pass into the Proton Synchrotron Booster (PSB) where they are accelerated to 1.4 GeV.
Next, they are passed into the Proton Synchrotron (PS) itself and again acceler-ated to 26 GeV. Prior to injection into the Super-Proton-Synchrotron (SPS), the proton beam is de-bunched and recaptured in 40 MHz occupied bunch cavities to create the 25 ns bunch spacing used in the LHC (see section 1.1.2). The bunch lengths are shortened to 4 ns and then injected into the SPS ring. Finally, the
∗Totem is situated at Point 5 with CMS.
Chapter 1. Introduction to the LHC & CMS detector 5
Figure 1.2: A schematic of the layout of the injector chain from the LINACs to the LHC at CERN [12].
SPS accelerates these proton bunches from their injection energy of 26 GeV up to 450 GeV before they are injected in groups into the LHC. Within the LHC ring, they are ‘ramped up’ to the desired collision energy which, to date, stands at 4 TeV (√
s =8 TeV.‡)
During physics operations, the LHC circulates two beams of protons or heavy ions in opposite directions. By convention, beam 1 is in the clockwise direction, passing through the experiments from the plus end to the minus end, whilst beam 2 travels in a counter-clockwise direction. The beams are injected at point 2 (Beam 1) and point 8 (Beam 2). There are eight arcs and straight sections around the LHC. The straight sections, which are approximately 530m long, house either LHC machine
‡As of May 2012.
service points (Points 3, 4, 6 and 7) or experimental caverns (Points 1, 2, 5 and 8). The eight ‘points’ contain the following:
Point 1: The ATLAS Experiment.
Point 2: The ALICE Experiment & Beam 1 Injection.
Point 3: Momentum Cleaning.
Point 4: RF Systems.
Point 5: The CMS Experiment.
Point 6: Beam Dump.
Point 7: Betatron Cleaning.
Point 8: The LHCb Experiment & Beam 2 Injection.
LHCb and TOTEM are designed to run at L ≈ 1×1032 cm−2s−1 and
L ≈ 2×1029cm−2s−1 respectively. ALICE, which is dedicated to Heavy Ion (Pb-Pb) collisions has a design luminosity of L = 1027cm−2s−1. The two ‘high lumi-nosity’ experiments at the LHC; namely CMS and ATLAS, are designed around a nominal instantaneous peak luminosity of L = 1034cm−2s−1 during the proton collision program. For comparison, the Tevatron at Fermilab, saw its highest lumi-nosity (at the time of writing) ofL = 4.04 × 1032cm−2s−1§in the CDF experiment [13]. High luminosity is important as it has a direct relation to the number of col-lision events occurring per second in the experiment which in turn increases the probability of finding rare and interesting physics phenomena within an achievable timespan.
1.1.2 LHC Bunch Structure
The large luminosities demanded by the experiments have strong implications for the LHC machine in terms of beam dynamics, injection schemes and the ability to safely dispose of the beams when necessary. The luminosity due to any pair of bunches which collide in the region of any given interaction point is:
§Recorded on April 17, 2010.
Chapter 1. Introduction to the LHC & CMS detector 7
Lbunch = frevNb1Nb2S
4πβ∗ (1.1)
where, Nb1 and Nb2 and the number of protons per bunch, S is a geometry factor based upon the beam profiles and their crossing angles, is the emittance and β∗ is the distance from the I.P at which the width of the beam is double that at the I.P. The revolution frequency frev is governed by the circumference of the LHC and, to some extent, the energy at which the particles are accelerated. Therefore, to obtain the highest possible luminosity, the number of particles per bunch N and the number of bunches circulating the LHC needs to be maximised, running with the minimum, stable and safe emittance and β∗.
To allow for a sufficient number of bunches, the RF system in the LHC runs at a frequency of 400.8 MHz (λ = 0.7479 m; t=2.5ns). Taking the LHC circumference to be≈26659 m, the RF system provides 35640 available ‘buckets’ into which the proton bunches can be injected. However, the readout limitations of the experi-ments’ detectors must be taken into account and it was decided that the minimum space between colliding bunches will be restricted to 25ns or one in every 10 buck-ets. With this restriction in place, an entire LHC orbit can be dissected into 3564 usable buckets. During the early phases of LHC commissioning, only a few of these buckets contained bunches. Gradually, throughout 2010 - 2012, more and more bunches were injected. Presently, the LHC is running with 1374 bunches, 1368 of which are colliding in CMS. The remaining 6 bunches per beam are non-colliding and are used for measuring beam backgrounds. The bunches are spaced by 50 ns.
The experiments are interested in the bunch patterns in terms of the colliding and non-colliding bunches. A LHC-wide nomenclature was created to describe the filling scheme in terms of bunches colliding at each I.P and the total number of bunches present. For example:
informs the experiments that the bunch spacing is 50ns; there are 1374 bunches present in the LHC, 1368 of which are intended to collide at Points 1 and 5 (ATLAS
& CMS), none are to collide at Point 2 (ALICE) and 1262 at Point 8 (LHCb).
Some filling schemes also show the number of bunches per injection (bpi) and the number of injections (inj).