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LHC-ATLAS-CMS Primer October 29, 2008 8:33 WSPC/139-IJMPA 04234 International Journal of Modern Physics A Vol. 23, No. 25 (2008) 4081–4105 c World Scientific Publishing Company ! LHC’s ATLAS AND CMS DETECTORS∗ , , , MARIA SPIROPULU∗ ‡ and STEINAR STAPNES∗ † § ∗Physics Department, CERN, CH-1211 Geneva, Switzerland and †Department of Physics, University of Oslo, 0316 Blindern, Oslo, Norway ‡[email protected] §[email protected] We describe the design of the ATLAS and CMS detectors as they are being prepared to commence data-taking at CERN’s Large Hadron Collider (LHC). The very high energy proton–proton collisions are meant to dissect matter and space–time itself into its primary elements and generators. The detectors by synthesizing the information from the debris of the collisions are reconstituting the interactions that took place. LHC’s ATLAS and CMS experiments (and not only these) are at the closest point of answering in the lab some of the most puzzling fundamental observations in nature today. Keywords: LHC; ATLAS and CMS detectors. 1. Introduction The Large Hadron Collider (LHC)1 is the largest and most complex scientific under- taking ever attempted. Its results will determine the future of the full discipline of high energy physics. The LHC will produce 14 TeV proton–proton collisions in the next year and we expect it will discover a new sector of particles/fields associated with electroweak symmetry breaking and dark matter. The two major experimental observations behind this expectation are: (1) the masses of the W and Z vector bosons; (2) the dark matter in the universe. These, in concert with the theoretical considerations are corroborative evidence for physics mechanisms that broaden the Standard Model (SM). Both the ATLAS (A large ToroidaL ApparatuS)2,3 and the CMS (Compact Muon Solenoid4–6) experiments are in the stage of commissioning and integration. ∗This paper is also published in Perspectives on LHC Physics, edited by G. Kane and A. Pierce (World Scientific, 2008). 4081 October 29, 2008 8:33 WSPC/139-IJMPA 04234 4082 M. Spiropulu & S. Stapnes Fig. 1. This “gold plated” event going through all central detectors — the tracker, the hadronic calorimeter, HCAL (top and bottom), the electromagnetic calorimeter ECAL and the muon Drift Tubes — it was recorded in August 2006 in the CMS assembly hall at Point 5; Run No. 2378, event 123 at a magnetic field of 3.8 T. Fig. 2. This cosmic muon event recorded with ATLAS is going through all barrel detectors. The tracker (silicon strip and straw tracker), hadronic calorimeter and muon detectors were read out, while the barrel liquid argon calorimeter (been operated in several periods earlier) and pixel system (being cabled) were not read out. The event was recorded in March 2008 at Point 1. October 29, 2008 8:33 WSPC/139-IJMPA 04234 LHC’s ATLAS and CMS Detectors 4083 The experiments have been collecting cosmic data with the major detectors in place already since 2006 (see for example an event display of a cosmic muon observed at the CMS Magnet and Cosmic Test runs in Fig. 1 and a recent one at ATLAS in Fig. 2) and finalizing the analysis of beam tests of most-all detector elements. The experiments are also preparing the strategies for the careful understanding and use of the SM data at 14 TeV. 1.1. LHC : The machine The LHC is built in a circular tunnel 27 km in circumference. The tunnel is buried around 50 to 175 m underground and straddles the Swiss and French borders on the outskirts of Geneva as shown in Fig. 4. It will circulate the first beams in the summer of 2008 and provide first collisions at high energy soon after. The LHC is at the edge of the accelerator based energy frontier as illustrated in the “Livingston plot” shown in Fig. 3. Note that since the 30’s (with the 600 MeV Cockcroft-Walton — not shown here) the effective energy increase is more than 11 orders of magnitude while the effective cost per GeV of a typical accelerator is drastically reduced (see also Ref. 7). In accelerator based high energy physics experiments, the type and number of particles brought into collision and their center-of-mass energy characterize an interaction. 4E6 (ADRON#OLLIDERS E En #OLLIDERS ,(# 4E6 #%2. Y .,# G 4%6!42/. R E &ERMILAB N % ,%0)) n S 3003 S A #%2. 3,# ,%0 - F 'E6 3,!# #%2. O R 42)34!. E T +%+ N E 0%42! 0%0 # T $%39 3,!# N E U )32 #%32#ORNELL T I T #%2. S 'E6 6%00)6.OVOSIBIRSK N O 30%!2)) # 30%!2 $/2)3 6%00))) 3,!# $%39 .OVOSIBIRSK !$/.% )TALY 'E6 02). 34!. 6%00)) !#/ 3TANFORD .OVOSIBIRSK &RANCE 9EAROF&IRST0HYSICS Fig. 3. (Left) A variant of the “Livingston plot” showing the energy in the constituent frame of electron–positron and hadron colliders constructed (filled circles and squares) or planned. The energy of hadron colliders has here been derated by factors of 6–10 in accordance with the fact that the incident proton energy is shared among its quark and gluon constituents. (Right) Phases of the hadron–hadron interaction; beads represent hadrons, straight lines quarks and springs gluons. HP indicates the hard process, and UE the underlying event, i.e. the remnants of the protons. October 29, 2008 8:33 WSPC/139-IJMPA 04234 4084 M. Spiropulu & S. Stapnes Fig. 4. Overall layout of the LHC machine (left) and geographic location (right). The limitation on the energy for a proton accelerator storage ring is the maxi- mum magnetic field to bend the particles (Rring(m) = P (GeV)/0.3B(Tesla)). For an electron storage rings it is the energy lost to synchrotron radiation per 4 1 revolution (U 0.0885E(GeV) R(m)− ). For relativistic proton beams this is 15 ≈ 4 1 7.79− E(GeV) R(m)− . At the LHC this amounts to about 3.6 kW per beam ≈ and is absorbed by the cryosystem. The largest electron–positron storage ring was the 27 km LEP ring at CERN in Geneva. It is expected that all future electron– positron machines at higher energy than LEP will be linear. The LHC is colliding proton beams. A monoenergetic proton beam is equivalent to a wide-band parton beam (where parton quarks, antiquarks, gluons), described ≡ by momentum distribution dni/dp (also referred to as structure function) where i specifies the parton type, i.e. u, d, g,¯u, d¯. Proton structure functions are mea- sured in deep inelastic scattering experiments. Some of the advantages of hadron collisions include the simultaneous study of a wide energy interval, therefore there is no requirement for precise tuning of the machine energy; the greater variety of + initial state quantum numbers, e.g. u + d¯ W ,¯u + d W −; the fact that the → → + maximum energy is much higher than the maximum energy of e e− machines; and finally that hadron collisions are the only way to study parton–parton collisions, including gluon–gluon. Some of the disadvantages are the huge cross-sections for uninteresting events; the multiple parton collisions in the same hadron collision that result in complicated final states; and that the center-of-mass frame of the colliding partons is not at rest at the lab frame. Figure 3(right),8 illustrates the phases and complexity of a hadron–hadron interaction. In this example HP is the hard process in which a gluon (d) form the left collides with a quark (e) from the right. This process can be approximately described as an interaction among fundamental freely October 29, 2008 8:33 WSPC/139-IJMPA 04234 LHC’s ATLAS and CMS Detectors 4085 moving constituents of the proton. The boundary region denoted with a dotted line (H) includes the radiation process that has a “memory” of the hard scattering. The phases of the collision outside the line have no (distinct) memory of the hard scat- tering. The final state partons radiate and close-by ones merge into color-singlet clusters that are then decaying to physical hadrons (beads around the dotted line). The overall interaction of the remaining hadron fragments is represented with the UE bubble (Underlying Event); initial state gluons such as (f) and (g) are splitting in qq¯ pairs that are shared with the underlying event. An impressive technological challenge of the LHC and the main budget item of the machine are the 1232 superconducting dipoles operating at temperature 1.9 K that bend the two proton beams around the 27 km circumference tunnel. At 7 TeV these magnets have to produce a field of 8.33 Tesla at a current of around 11,700 A. The magnets have two side-by-side apertures (dual-core or “two-in-one” design), one for each of the counterrotating proton beams.9–11 Each dipole is 14.3 meters long. The manufacture of the coil that contain the superconducting cable to pro- vide the all-important 8.33 T magnetic field, represents 60% of the magnet produc- tion work. The niobium-titanium coils create the magnetic fields to guide the two counterrotating proton beams in separate magnetic channels, but within the same physical structure. The coils are surrounded by non-magnetic “collars” of austenitic steel, a material that combines the required properties of good thermal contraction and magnetic permeability. The collars hold the coils in place against the strong magnetic forces that arise when the coils are at full field — the force loading 1 m of dipole is about 400 tons. The LHC dipoles comprise 7600 Km superconducting cable that weighs 1200 tons. Each cable is made up of 36 strands of superconducting wire while each strand houses 6300 superconducting filaments of Niobium-titanium (NbTi) — the total length of the filaments is astronomical, more than 10 AU.
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