pp physics, RWTH, WS 2003/04, T.Hebbeker

Transcription

1 1. PP TH 03/04 Accelerators and Detectors 1 hysics, RWTH, WS 2003/04, T.Hebbeker Accelerators and Detectors In the following, we concentrate on the three machines SPS, Tevatron and LHC with the exeriments UA1, UA2, CDF, D0, ATLAS and CMS, reresenting the ast, resent and future of high energy hysics 1. The accelerators are treated as black boxes : the arameters relevant for the exerimenters are reviewed, but the hysics and technical asects are not covered. The detector design is discussed first in general terms, then the six detectors are resented and comared in some detail Accelerators arameter SPS Tevatron LHC time articles c.m. energy / GeV circumference l/km eak lumi / /cm 2 /s average lumi / fb 1 /year number of bunches articles (, ) / bunch / , 8 25, 3 11 bunch searation t/ns beam crossing angle µrad inelastic collisions / crossing inelastic collisions / s article roduction / s bunch size σ x, σ y /µm bunch length σ z /cm Note: The figures for the SPS refer to the best erformance reached; the Tevatron arameters are those exected for the data taking eriod 2004 of run IIa ; the LHC numbers are design values which will robably not be reached before the end of the decade. Reference: Review of Particle Physics 2002 (Phys. Rev. D 66 (2002) 1). Some comments: i) The beam crossing angle in the LHC (in which two searate beams circulate!) avoids unwanted arasitic collisions: 1 the generic exression hysics covers both and collisions

2 1. PP TH 03/04 Accelerators and Detectors 2 Note: 25 ns = 7.5 m. ii) The number ñ of inelastic collisions er bunch crossing can be calulated this way: ñ = Ṅ t = L σ inel t σ inel 50 mb (1) where t denotes the (average) bunch sacing. iii) The article roduction rate is given by the average article multilicity er inelastic collision (increases from 40 at SPS to 140 at LHC 2 ) and the event roduction rate. iv) In general the bunch sacing is not uniform in time: Examle Tevatron: Radiofrequency = RF = 53 MHz. This corresonds to τ = 1/RF = 19 ns and d = τ c = 5.7 m. But not all buckets can be filled in this accelerator scheme, only every seventh, resulting in a smallest bunch searation of 7 19 ns = 132 ns. Every 132 ns-clock-signal is called a tick. This oerating mode was originally foreseen for Run IIb, but it might never be realised. It would also require a nonzero beam crossing angle for the reasons exlained above. Currently, in run IIa, only every third tick is filled, that is t = 396 ns. However, the timing is more comlicated: Along the Tevatron circumference of about l = 6.3 km one can fit exactly 153 ticks, which are divided into 3 grous of 53 ticks, since the Tevatron has a threefold symmetry 3. But 53 is not a multile of 3, so that some adjustment is necessary, resulting in 12 bunches (= 36 ticks): 2 Pythia simulation: all stable articles 3 so that CDF and D0, at a distance of l/3, get the same luminosity

3 1. PP TH 03/04 Accelerators and Detectors 3 Examle: LHC 4 : Radiofrequency = RF = MHz. This corresonds to τ = 1/RF = 2.5 ns and d = τ c = 75 cm. Only every 10th bucket is used, resulting in a bunch sacing of 25 ns: Here glitches are unavoidable due to the transfers between the different accelerators: PS SPS LHC. Sometimes bunches remain emty, u to 119 in a row! This results in a total of 2808 bunches 4 for oeration; in heavy ion mode the bunch searation will be even smaller

4 1. PP TH 03/04 Accelerators and Detectors 4 er LHC ring/beam. The emty crossings can be used to measure backgrounds (electronic noise, cosmics). v) The transverse beam size is given by the squeezing in the nearest quadruoles. The total longitudinal beam size can not exceed half the wavelength d; here we quote the r.m.s of the distribution, σ z, which is then limited by d/(4 3). The bunch length might be substantially shorter than this limit, if stable beam acceleration is limited to a smaller hase interval. Beyond the arameters listed in the table, the exerimenter is interested in background rates (e.g. roton halo scraing collimators ustream 5 ) and - related - the mimimum radius of the beam tube (in order to lace vertex detectors as close to the interaction region as ossible) Detectors Since the magnet choice has a major imact on the design of all the other detector arts, the different tyes of magnets are described first in this chater Magnets A good momentum measurement requires recise detectors and thick magnetic fields with a large B field. The track curvature radius grows with article energy/momentum, so that at high energy colliders this task becomes articularly difficult. Some formulae: i) Trajectory inside magnetic field A charged article describes a helix inside a homogeneous magnetic field; the radius of curvature for a article with charge ±1 is given by R = B e B = 3.3 m B/GeV B/T (2) where B denotes the comonent of erendicular to B. Examle: The Tevatron suerconducting diole magnets roduce B = 4.4 T. For 1 TeV articles this imlies a radius of R = 750 m. This number is smaller than the geometrical Tevatron radius of 1 km, since the magnets do not cover all the circumference! 5 this is not the most imortant background source at the LHC, see section 1.2.2

5 1. PP TH 03/04 Accelerators and Detectors 5 ii) Multile scattering Multile deflections in the Coulomb field of nuclei result in both a satial and an angular dislacement: The overall scattering angle is Gaussian distributed. In one rojection the width is given by 13.6 MeV L θ MS = ( ln L ) (3) X 0 X 0 for relativistic articles with unit charge. The last factor is close to 1 and can often be ignored. L is the traversed matter thickness and X 0 the radiation length of the material. The track dislacement is (aroximately) y MS = 1 3 L θ MS (4) and s MS = L θ MS (5) Reference: Review of Particle Physics 2002 (Phys. Rev. D 66 (2002) 1). Examle: A muon of = 1 GeV is deflected by tyically θ MS assing through L = 1 cm of iron (X 0 = 1.8 cm). = (in one rojection) when iii) Curvature measurement The momentum resolution deends on the detector recision, on the amount of material the article has to traverse, and on the measurement rincile. In collider exeriments often the sagitta s is measured inside the magnet region:

6 1. PP TH 03/04 Accelerators and Detectors 6 At least three coordinate measurements are necessary. It is also ossible to measure the track direction inside and/or outside the magnet area and to include the collision oint as a constraint in the momentum determination - but we will not discuss these oints here. The sagitta is given by ( tutorial) s = e B L2 8 B = 0.3 m B/T (L/m)2 8 B /GeV (6) Obviously, the more recise the s measurement, the better the momentum resolution. The quadratic deendence on L is very imortant! Some comments on this deendence: - intuitive exlanation: trajectory in homogeneous field (angle B arox. constant) = arabola: transverse momentum L, satial deviation L 2. - doubling the radius of an inner tracking detector = fourfold imrovement in momentum resolution! Difficult to reach by increasing B or by imroving detector resolution! - If a detector is subdivided into two indeendent identical halves, each of thickness L/2: Measuring the sagitta twice and combining the momentum measurements yields a resolution which is worse by a factor 4/ 2 comared to the full detector s resolution. To avoid this degradation a common track fit through both detector arts is mandatory, imlying: no material (MS) in between, negligible alignment uncertainties. Examle: A muon of = 100 GeV traversing a magnetic field B = 1 T (at right angles) of thickness L = 1 m results in a sagitta of only s = 0.4 mm. A sagitta measurement requires three coordinate measurements, for examle the measurement of y 1, y 2 and y 3 at the coordinates x 1, x 2 = x 1 + L/2, x 3 = x 1 + L along the trajectory: s = y 2 y 1 + y 3. (7) 2 The formula imlies that the central oint should be measured more recisely than the outer ones. This can be realized by using four identical coordinate detectors, which measure indeendently with a resolution of y at x = 0, x = L/2, x = L/2 (!) and x = L: In this case the sagitta error is s = y 2a + y 2b 2 y 1 + y 3. (8) 2 s = y 1/4 + 1/4 + 1/4 + 1/4 = y (9)

7 1. PP TH 03/04 Accelerators and Detectors 7 Examle: A detector resolution of y = 0.1 mm yields s = 1.5 y = 0.12 mm using three detectors and s = y = 0.1 mm with four detectors. iv) Momentum resolution In many cases the relative momentum resolution can be arametrised as follows: = det where a b stands for a 2 + b 2. MS = c det GeV c MS (10) Here B was assumed, the more general case is discussed below. This formula alies for examle when a muon track through an iron yoke is measured several times in holes inside the material along the trajectory. Note that the detector resolution contribution is growing with, while the MS art is indeendent of momentum: From the sagitta formula (6) we can calculate the detector term c det : assuming s = y, thus = s s = 8 y el 2 B = 26.4 y/m L 2 /m 2 B/T σ/mm c det = L 2 /m 2 B/T /GeV (11) (12) All these formulae assume B! The multile scattering term can be calculated in a similar way from the fake sagitta (equation 5): c MS = = s MS = 1 s MeV L 1 16 MeV L L 8 = (13) 3 X 0 L 2 B L B X L = (14) L/m B/T X 0

8 1. PP TH 03/04 Accelerators and Detectors 8 Examle: We assume a magnetic field of 2 T and an iron layer of L = 1 m thickness. For the 4 chamber model with y = 0.1 mm we get c det = 0.13% c MS = 19% (15) Thus the detector contribution becomes relevant only for momenta above 100 GeV. v) Magnetic field configurations Since the beam must not be disturbed by the detector s magnetic field only two field configurations seem ossible: a) Solenoid The symmetry axis of the solenoid coincides with the beam line, so v B = 0. The cross section of the coil can be circular, but also a rectangular design will work. The field lines extend far outside the magnet; to avoid related roblems one can cature the field in a cylindrical magnet yoke outside the solenoid and feed it back. Another advantage of the yoke is the extra field that can be used to (re)measure track momenta.

9 1. PP TH 03/04 Accelerators and Detectors 9 b) Toroid Toroids contain closed B field lines, thus there is no need for extra yokes, avoiding the resulting multile scattering! In the endca regions often iron toroids are used: c) There is a notable excetion to the rule v B = 0: the UA1 detector used a horizontal diole field (generated by a rectangular solenoid ) which was oriented erendicular to the beam:

10 1. PP TH 03/04 Accelerators and Detectors 10 Of course this requires another magnetic field outside the detector to comensate. A diole magnet can never be used in electron accelerators, due to the induced synchrotron radiation. Some general comments on magnet technology: - The magnetic fields can be generated by conventional or by suerconducting coils, the latter tye is used in all modern exeriments. The resulting field strengths are in the range 0.7 T (UA1 diole) to 4 T (CMS sc. solenoid). These values are limited by the ower dissiation (conventional) and the breakdown of suerconductivity for high B fields. The field strength inside an iron magnet is normally close to the saturation value, i.e. B = T. - Suerconducting magnets need a cryostat, thus comlicating the detector design. - Mechanical forces can be imortant. Examle: the CMS iron yoke can move by O(cm) when the coil is turned on/off. - The energy stored is huge. Quenches must be avoided. Examle: ATLAS toroid = 1500 MJ. - An iron filled magnet needs substantially lower currents than an air magnet. Examles: D0 central toroid: each coil: n I = 25 ka, ATLAS barrel toroid: each coil = 3 MA. Comarison of hysics erformance : The two main asects are: * multile scattering: dominated by iron yoke (if existing); severe limitation for momentum resolution, see above. * field geometry: a) barrel toroid: bending in lane containing beam ~ in barrel toroid B L2 largest at small angles! + ~ B, - no vertex constraint (long collision region) - no field in central detector art b) barrel solenoid: axial field, bending in lane erendicular to beam o ~ is measured, resolution inde. of angle (without MS) + vertex constraint (small beam diameter) + B field inside and close to beam ie c) barrel diole: bending in lane containing beam, φ deendent!

11 1. PP TH 03/04 Accelerators and Detectors 11 + B L 2 largest at small angles, works even for η ±. - no vertex constraint (long collision region) - no bending for tracks B - needs comensation In the forward region there is less choice: only toroids make sense ( P B). Above the momentum resolution formulae were given for barrel magnets at θ = 90 0, η = 0. For other olar angles the following modifications have to be taken into account for cylindrical magnets: - solenoids measure T ( = T / sin θ). - toroids measure, effective track length L = L 0 / sin θ, resolution 1/L 2 (sagitta method). - similar for diole, but additional φ deendence. - multile scattering increases: effective thickness = L 0 / sin θ. Summary of the magnet configurations of the six detectors: exeriment UA1 UA2 CDF D0 ATLAS CMS solenoid T sc 2.0 T sc 2.0 T sc 4.0 T sc solenoid yoke - - iron - yes 1.6 T iron barrel toroid T iron 4 T sc air - endca toroid - yes iron 2.0 T iron 4 T sc air - endca yoke - yes! T iron diole 0.7 T diole yoke 1.8 T iron Some (exected) erformance figures (deend of course on osition resolution) for η = 0: CMS central tracker: = 0.012% /GeV 0.5% (16) ATLAS toroid: = 0.01% /GeV 1.5% (17) D0 toroid: = 0.3% /GeV 18% (18) Finally a few ictures: UA2 detector:

12 1. PP TH 03/04 Accelerators and Detectors 12 D0 toroid:

13 1. PP TH 03/04 Accelerators and Detectors 13 Note: here no sagitta measurement, but determination of track direction before and after traversing magnet. Atlas toroid coil: CMS magnetic field: