IL NUOVO CIMENTO DOI 10.1393/ncb/i2008-10528-4 Vol. 123 B, N. 3-4 Marzo-Aprile 2008 Luminosity measurements at LHC S. de Capua(1 ), F. Ferro(2 ), M. De Gruttola(3 ) and M. Villa(4 ) (1 ) (2 ) (3 ) (4 ) CERN, CH-1211 - Gen` eve 23, Switzerland INFN, Sezione di Genova - via Dodecaneso 33, 16146 Genova, Italy INFN, Sezione di Napoli - via Cintia, I-80126, Napoli, Italy INFN, Sezione di Bologna - via Irnerio 46, 40126 Bologna, Italy (ricevuto il 3 Giugno 2008; pubblicato online il 4 Agosto 2008) Summary. — In this paper, we present the various techniques developed for measuring and monitoring the luminosity, both integrated than instantaneous, in p-p collisions at the LHC. The investigated methods range from old and new techniques for instantaneous luminosity measurement during dedicated runs, to the integrated luminosity evaluation via the total and differential forward elastic cross-section measurement, to the offline-integrated measurement using known channels, and finally to the on-line techniques. A brief description of the detectors used for this purposes is given when needed. PACS 13.85.Lg – Total cross sections. PACS 29.27.-a – Beams in particle accelerators. 1. – Introduction A precise luminosity measurement and constant monitoring is essential to guarantee the correct functioning of the accelerator and of course for any physical process crosssection determination. Therefore all the LHC experiments have been devoted many efforts to the construction of sub-detectors essentially dedicated to the luminosity measurement, and to the tuning of various experimental techniques. In the following sections we present a brief discussion of all the experimental techniques to measure both the integrated and the instantaneous luminosity. These techniques can be divided in direct ones, when the luminosity is inferred from the bunch properties, such as the in the well-known Van der Meers scan method or the newest beam gas interaction method, and indirect methods, when offline information of very well-known channels, such as electroweak and electromagnetic “candles” processes, or the total and forward differential elastic cross-section measurement are utilized. A different classification can be made considering that the previous methods need dedicated runs or offline analyses after data collection, hence cannot be used during relevant physical data collection, in contrast with all the on-line measurement monitoring and control techniques, as described in the last section. c Societ` a Italiana di Fisica 423 424 S. DE CAPUA, F. FERRO, M. DE GRUTTOLA and M. VILLA Fig. 1. – (Left) Schematic view of the typical layout of an IP and of the BRAN location. (Right) Simulation of the beam-gas interactions reconstructed inside the Velo detector. 2. – Direct methods for luminosity measurements Various methods have been developed in the past to measure the luminosity. In this section we discuss two methods which measure directly the luminosity: the Van der Meer scan method [1], developed for the LHC experiments, and the beam-gas interactions method, specific to the LHCb experiment [2]. . 2 1. The Van der Meer scan method . – The method consists in moving the beams transversely across each other and recording the relative luminosity (reaction rate). The position for maximum luminosity is found and the absolute luminosity is inferred from the measured beam overlap and the beam currents. This method was successfully applied at the Intersecting Storage Rings (ISR) at CERN. Although the conditions at the LHC will be drastically different (bunched beam versus coasting beams, similar horizontal and vertical beam sizes as opposed to widely different sizes, bunch-to-bunch variations, etc.), the BRAN project [3] aims to adopt the Van der Meer scan method to measure the relative luminosity inside the four experimental detectors: ATLAS, ALICE, CMS and LHCb. In the p-p collisions at the LHC many particles will be produced, including neutral particles like neutrons and photons which will in general follow the trajectories of the protons they descend from. Since the flux of neutral particles is expected to be sufficient to damage the bending magnet D2 in IP1 (ATLAS) and IP5 (CMS), where the luminosity can reach 1034 cm−2 s−1 , an absorber made of copper of several meters of length (TAN), is foreseen in front of D2 at about 140 m from the IP. At that location, the two proton beams are separated by the bending magnets D1 and D2 by about 160 mm and there is therefore a space of about 100 mm between the two vacuum chambers for a detector to be placed. Hence the detector, inserted inside the TAN at the location of the shower maximum, will monitor the rate of the neutral particles coming from the IP (fig. 1, left). In order to monitor the neutral collision rate, the detectors must fulfil two main requirements: i) stand the high radiation dose, about 1 GGy for IP1 (ATLAS) and IP5 (CMS) and ii) perform bunch-by-bunch measurements (i.e. at a measurement speed higher than 40 MHz). For this goal, two different types of detectors are being developed: LBNL is committed to deploy four fast ionization chambers (IC) in the TANs around IP1 and IP5 in the framework of the US-LARP Collaboration [4], while CERN is aiming to install solid-state cadmium telluride (CdTe) detectors [5, 6] developed by LETI laboratories [7] in the remaining two points (where the radiation dose expected is a bit lower). These two technologies have good performances for what concerns one of the two 425 LUMINOSITY MEASUREMENTS AT LHC requirements: the IC is sufficiently radiation hard, but will have difficulties meeting the 40 MHz requirement, while the CdTe can easily comply with the 40 MHz requirement, but can only stand the lower dose expected at IP2 (ALICE) and IP8 (LHCb). . 2 2. The beam-gas interactions method . – With the advent of precise microvertex detectors, a novel method of measuring the absolute luminosity emerges [2]. The method relies on the use of beam-gas interactions for measuring the individual beam shapes and determining the beam overlap integral which enter the luminosity, by means of the vertex locator detector (VeLo) [8] of the LHCb experiment. The method also allows the direct measurement of beam crossing angles or beam offsets. The proposed method is nondestructive: the target gas thickness required is much less than the integrated LHC residual gas density, and the induced radiation is not larger than the one caused by beam-gas collisions. The luminosity L for two counter-rotating bunches with density function ρi (x, t) and uniform velocity vi is given by [9, 10] 2 L = f N1 N2 (v1 × v2 ) × (v1 − v2 ) − c2 2 ρ1 (x, t)ρ2 (x, t)d3 xdt, where f is the revolution frequency and Ni the total number of particles in the bunches. Assuming the velocities and the total number of particles in the bunches can be precisely measured by independent means, the method aims at measuring the densities ρi from beam-gas collisions. An estimation of the rate of inelastic pp collisions, taking xenon as target gas, gives approximately 30 Hz, which will allow mapping the bunch profile within minutes with a statistical precision below 1%. Figure 1 (right) shows the distributions along the z-axis of primary vertices with at least 6 reconstructable tracks. For this plot, 10 k beam-gas inelastic events (for each beam) were generated flat in the range −1.2 m < zpv < 1.2 m, while 20 k beam-beam inelastic events were generated assuming a Gaussian envelope centered on z = 0 and with rms σzlum = 53 mm. Note that the exact distributions of each individual sample can be obtained from separate analysis of a pure beam-gas experiment (for either beam) and pure beam-beam experiments by running with gas but considering only non-colliding bunches (or running with a single beam in the ring) and, respectively, by running without target gas. This allows estimating precisely the contamination of a given sample by the other samples without relying on models or Monte Carlo simulations. Once calibrated by this method, any given reaction can be used later to measure the absolute luminosity in a continuous way without the addition of target gas and at any luminosity. 3. – Measurement of the integrated luminosity . 3 1. Experimental apparatuses. – The TOTEM experiment [11] uses precision detectors (σ ∼ 30 μm) inserted in Roman Pots (movable sections of the vacuum chamber) installed in the machine tunnel, at 147 and 220 m from the IP5, to measure the elastically and diffractively scattered protons close to the beam direction. Each Roman Pot station counts for three Roman Pot units, equipped with ten planes of detectors. Two units approach the beam vertically and one horizontally, providing also an overlap among the detectors. In order to measure the elastic scattering to the smallest |t| values, the detectors should be active as close to their physical edge as possible. In particular the detectors will have to be efficient up to a few tens of microns to their edge. In order to 426 S. DE CAPUA, F. FERRO, M. DE GRUTTOLA and M. VILLA build the edgeless leading proton detectors, planar silicon detectors with current terminating structure, which provide a thin dead area of ∼ 50 μm, have been developed and built at CERN. The telescopes for the detection of the inelastic events (T1 and T2) have a good trigger capability, provide tracking with a good angular resolution and allow the measurement of the trigger efficiency. To discriminate beam-beam from beam-gas events, the telescopes will identify the primary interaction vertex with an accuracy at the level of a cm by reconstructing a few tracks from each side of the interaction point; the knowledge of the full event is not needed. The T1 telescope is made of 5 planes of 6 trapezoidal Cathode Strip Chambers (CSC) [11] and will be placed in the CMS end-caps in the rapidity range 3.1 < |η| < 4.7 with a 2π azimuthal coverage. T2 is made of 20 half circular sectors of GEM [11] (Gas Electron Multiplier) detectors; it will be placed in the shielding behind the CMS Hadronic Forward (HF) calorimeter as a complement of the T1 telescope at larger η. With the present dimension of the vacuum pipe, the T2 telescope will cover with good efficiency the range 5.3 < |η| < 6.5. The ATLAS experiment, in addition to the main detector, will install the so-called ALFA system [12], which consists of scintillating fiber trackers located inside vertical Roman Pots, similar to those developed for TOTEM, at a distance of 240 m from the IP. The main characteristics of this tracker are a spatial resolution of ∼ 30 μm and no significant non-active edge region. . 3 2. Total cross-section. – One of the most precise methods that can be used at the LHC to measure the luminosity is to use the measurement of the total cross-section and of the differential forward elastic cross-section, which are related by the optical theorem. In this approach the measurement of the total cross-section does not depend on the luminosity: σtot = 16π(dNel /dt)t=0 , (1 + ρ2 ) · (Nel + Ninel ) where Nel and Ninel are the observed rates of the elastic and inelastic interactions, respectively, (dNel /dt)t=0 is the extrapolation of the elastic rate to the optical point t = 0 (t is the momentum transfer) and ρ is the ratio of the real and imaginary part of the forward amplitude. The precise measurement of σtot provides an absolute calibration of the machine luminosity: L · σtot = Nel + Ninel . TOTEM needs few one-day runs, with the special running conditions of a high β ∗ = 1540 m (nominal TOTEM optics) and a low luminosity of L ≈ 1028 cm−2 s−1 . The β value at the interaction point (IP) requires zero crossing angle, due to the increase of the beam size (proportional to β), and then a reduced number of bunches which is compatible with the LHC injection scheme. The most likely configuration is 36 proton bunches with 2.5 μs spacing. Almost half of the total cross-section at the LHC is predicted to be from coherent elastic scattering, single, double and central diffractive processes. At β ∗ = 1540 m, the TOTEM experiment efficiently detects protons with −t > 0.004 GeV2 , i.e. 97% of all the diffractively scattered protons, independently of their longitudinal momentum loss in the range of 10−8 < Δp/p < 0.2. With the TOTEM acceptance extending up to the pseudorapidities of 6.6, and with the efficient proton detection capabilities close to the LHC beams, the diffractively excited states with masses higher than 5 GeV/c2 are seen LUMINOSITY MEASUREMENTS AT LHC 427 by the experiment. The leading proton, which will be used as an event trigger, will be detected by the silicon detectors placed inside the Roman Pots, located symmetrically with respect to the IP. The inelastic rate will be measured with T1 and T2 telescopes which provide a fully inclusive trigger with an expected uncertainty on the inelastic rate of the order of 1%, after corrections. Due to the very high cross-sections of the processes involved, a huge statistics can be obtained in a short running time and the main error sources are the systematic uncertainties coming from the beam parameters and from the different theoretical model which can be used to extrapolate the elastic rate to the optical point. For a review of these models the reader can see, for example, [13-18]. . 3 3. Coulomb and nuclear interference. – The different method used by ATLAS to measure the luminosity consists in measuring the elastic scattering down to such small t-values that the cross-section becomes sensitive to the electromagnetic amplitude via the Coulomb interference term [12]. If the Coulomb region can be reached, an additional constraint is available from the well-known electromagnetic amplitude that describes the elastic scattering at small t values: 2 dN 2αEM σtot −b|t|/2 ∼ Lπ − + (i + ρ)e , dt |t| 4π where the first term corresponds to the Coulomb and the second to the strong-interaction amplitude. Using this additional constraint together with the optical theorem allows the determination of both luminosity and the total cross-section without a measurement of the inelastic rate. The ALPHA detectors [12] will allow the measurement of t down to |t|min = 0.0006 GeV2 , inside the region of interference. L will be determined from the fit of the measured rate to the above expression as well as the other parameters ρ, σtot , and the slope parameter b. Simulations show that with special beam optics (β ∗ = 2600 m which corresponds to a luminosity of ∼ 1027 cm−2 s−1 ) more than 10 million events can be achieved with less than 5 days running resulting in a statistical error on the luminosity < 2%. The main source of uncertainty is originating from the alignment, the optical function and the background subtraction which result in a total systematic error of about 2.5%, yielding a total error of ∼ 3% [19]. . 3 4. Relative luminosity measurement. – Due to the limited acceptance of the leading proton detectors with low β ∗ optics (β ∗ = 0.5 m in nominal LHC running conditions) the measurement of the luminosity at L = 1033 –1034 cm−2 s−1 cannot be done directly with the previous methods. As an example, the minimum |t| reached by TOTEM detectors would be 0.4 GeV2 and a precise measurement of the elastic rate as well as its extrapolation at the optical point would be impossible. Moreover, the fit made by the COMPETE [20] Collaboration of the data taken in past experiments results in an error on the prediction of the total cross-section at the LHC energy of 15–20%. The solution to the problem consists in measuring the luminosity with the special optics conditions and consider it as a calibration point. The relative measurement can be achieved by measuring rates of specific processes or using specific trigger conditions; in case of pile-up the zero-event counting approach can be used. 428 S. DE CAPUA, F. FERRO, M. DE GRUTTOLA and M. VILLA Fig. 2. – Feynmann diagram and the kinematical variable definition for the two-photon pp → ppll process. 4. – Integrated luminosity measurement using known channels It will be possible to measure, or at least monitor, the luminosity of the LHC both at the beginning and in regime conditions, using very well understood channels with huge cross-sections and very clear experimental signature. The main idea is to measure the rate of these well-known channels, taking care of background subtraction, and hence to evaluate the luminosity costraining the crosssection to the theoretical value: (1) σ(L) = exp exp Nsig+back − Nback ×L , where the efficiency value takes into account geometry, trigger, reconstruction and selection of signal events. At ATLAS, CMS and LHCb an offline integrated-luminosity determination will be provided with some “candle” channels such as Z/W decaying into leptons, and muons (electrons) from double-γ exchange in proton collision, with the goal of reaching a luminosity measurement precision of ΔL/L ∼ 3% (surely not at the start-up, when the systematic contribution from the detector needs to be investigated). Instead, an online instantaneous luminosity will be provided via inelastic and QCD processes, with the intention of reaching a precision of ΔL/L ∼ 1%. In this section we discuss the method to obtain the offline integrated luminosity, while the following one focuses on online measurements. . 4 1. Rate measurement of two-photon production of leptons pairs. – The process pp → ppl l , where the muons (electrons) are produced both at central and forward rapidities, can be recorded using the standard ATLAS/CMS/LHCb trigger and might provide an attractive method of offline integrated-luminosity determination. In fig. 2 you can see the Feynmann diagram and the kinematical variable definition of this process. The process is very well understood: σ = 1.33 ± 0.01 pb for CMS/ATLAS and σ = 88 ± 1 pb for LHCb (the difference is, naturally, due to the different angular acceptance) and the signature very clear: + − 429 LUMINOSITY MEASUREMENTS AT LHC Fig. 3. – Plots discriminating signal leptons from double-γ exchange (top) from various backgrounds (bottom) at ATLAS (left) and LHCb (right). – very little (few GeV) lepton pair invariant mass, dilepton tranverse momentum ∼ 5 GeV, and acoplanarity angle φ ∼ 2 mrad. ppair T – |η l + − l | < 2.1 for ATLAS/CMS, and 1.6 < |η l + − l | < 5 for LHCb. – Background (mainly Drell-Yan and semileptonic decay of heavy quarks) reduced by offline cut on tranverse momentum and acoplanarity to ∼ 17% of signal for ATLAS/CMS and ∼ 10% for LHCb. In fig. 3 we show the plots discriminating signal from background: the left side shows the distribution of pT and acoplanarity angle φ for signal and background events at ATLAS [21] for 10 fb−1 ; instead in the right one it is plotted the dimuon invariant mass (left) and dimuon tranverse momentum (right) for signal leptons from double-γ exchange (top) and various backgrounds (bottom) at LHCb [22]. All the three experiments expect to measure the integrated luminosity from these electromagnetic channels with a precision of ∼ 2%, but, as the cross-section is not very high (especially at ATLAS and CMS), this will be a very important luminosity monitor only after some fb−1 of data taking. . 4 2. Rate measurement of the Z/W decaying into leptons pairs. – LHC will be a Z/W-factory, so that both at start-up and at higher luminosities, plenty of electroweak bosons will be produced: the Z(W) rate at L = 1034 cm−2 s−1 will be 20(200) Hz. The experimental signature is also very clear: – for Z → l+ l− : two isolated leptons with high transverse momentum (pT > 15, 20 GeV/c) at central(forward) rapidity for ATLAS/CMS (LHCb) with a dilepton invariant mass closer to the nominal peak Z mass. – for W → lν: a lepton with high tranverse momentum (pT > 15 GeV/c) and missing transverse energy (ET > 50 GeV). In these events one evaluates the W transverse 430 S. DE CAPUA, F. FERRO, M. DE GRUTTOLA and M. VILLA Fig. 4. – Left: distribution of reconstructed dimuon invariant mass for signal Z and various background at CMS [23] for 10 fb−1 . Right: distribution of reconstructed W transverse mass for signal muons and various background at CMS [23] for 10 fb−1 . mass from the lepton and the reconstructed neutrino according to the formula W mT = 2plT pνT (1 − cos Δφ), with φ the angle between lepton and neutrino in the tranverse plane. Therefore, it should be clear that a precise measurement of the integrated luminosity can be done starting from Z/W rate at high luminosity when all detector, systematic (lepton reconstruction efficiency, trigger, geometry, missing tranverse energy) and theoretical uncertainties (particle density function, initial state radiation) will be calibrated and better understood (many of these issues will be reached using just these candle channels). Even at the beginning of the data taking the electroweak channels will be very important as a monitor of the luminosity and a cross check with the other luminosity measurement. In fig. 4 we show two plots of the reconstructed Z mass and W tranverse both with muons in the final state, as expected for an integrated luminosity of 10 pb−1 . It can be finally asserted that a precise measurement of the rate of Z/W will be performed at LHC, and hence, assuming the cross-section equal to the theoretical value, an accurate integrated-luminosity measurement can be provided. For example, in the Z → μ+ μ− channel, the expected value (for sure not so obvious at the start-up) of the uncertainty is (2) ΔN (pp → Z/γ ∗ + X → μ+ μ− + X) = 0.005(stat.) + 0.01(ex.syst.) + 0.02(th.syst.) N 5. – Run-time luminosity and experimental techniques The luminosity measurement methods discussed previously are mainly referring to measurements made on special runs (Van-der-Meer or beam gas methods or for σpp measurements), or on already collected data (luminosity calibration on physics channels). During physics data collection a completely different approach has to be taken in order to measure online luminosity. LUMINOSITY MEASUREMENTS AT LHC 431 . 5 1. Instantaneous luminosity. – An essential aspect of a good physics data taking in collider experiments concerns the stability of the beam conditions. Due to several machine aspects actual beam conditions are not constant: the beam has a non-trivial bunch structure with empty and filled bunches (typically out of 3564 bunches 2808 are filled and 756 are empty), the filled bunches can have different levels of filling and after all the beam deteriorates with time (1% loss of luminosity every 10 minutes). Therefore the average numbers of interaction and pile-up are not constant over time and this might undermine the correct evaluation of the trigger, tracking and reconstruction efficiencies, increasing significantly to the total systematic error of cross-sections or branching fractions for the process of interest in LHC. A close monitoring of the number of interactions is thus absolutely mandatory and this is the main (run-time) goal of the instantaneous luminosity measurement. The rate of interactions (R) and the instantaneous luminosity tot tot (L) are related by: Rate = Lσpp , where σpp is the total pp cross-section. A luminosity control system should be able to follow the number of interactions on a bunch-by-bunch basis (every 25 ns). The main goal of the detectors measuring the instantaneous luminosity is to reach an accuracy of about 2–4%. . 5 2. Detectors for instantaneous luminosity: lucid, hadron forward . – Several types of forward detectors are usually employed in the instantaneous luminosity determination. In ATLAS, the forward detectors [12] able to follow the luminosity on a bunch-bybunch basis are the BCM (Beam Condition Monitor, small diamond pixel sensor), LUCID (a Cherenkov detector 5.6 < |η| < 6.2) and a Zero Degree Calorimeter (|η| > 10). The main luminosity detector is the LUCID (LUminosity Cherenkov Integrating Detector): this is composed by two vessels places around the beam-pipe at ±17 m from the interaction point. Each vessel houses 20 aluminum tubes (1.5 m long, 1.8 cm diameter) filled with C4 F10 gas and pointing to the interaction point. When a charged particle coming from the interaction point enters a tube, traveling almost parallel to its axis, it emits Cherenkov photons in the gas. By internal reflections, the photons are conveyed to a photomultiplier placed at a tube end. The signal amplitude is proportional to the number of tracks coming from the IP that entered in the tube. In CMS, the forward detectors [24] devoted to the luminosity measurements are the PLT (Pixel Luminosity Telescope, small diamond pixel sensors), the Hadron Forward calorimeter (HF) [25], CASTOR (a further calorimeter) [26] and the CMS/Totem roman pot system. The main system for the luminosity measurement is the HF calorimeter (3 < |η| < 5) whose active elements are the quartz rods where Cherenkov light from electromagnetic or hadronic showers is produced and piped to photon detectors. The two parts of the calorimeter are segmented in 864 individual towers which provide the deposited energy (or transverse energy) in each tower. The number of towers above a given threshold in transverse energy is some information directly related to the number of interactions. . 5 3. Measurement techniques. – Independently of the actual realization, all the instantaneous luminosity measurements share the same general aspects: – the base quantities come from fast detectors able to provide a digital signal for each beam crossing (BX); the detectors have to be read every 25 ns; – dedicated electronic boards perform the luminosity measurements on-the-fly, collecting statistics in a defined time-slice and providing luminosity information for 432 S. DE CAPUA, F. FERRO, M. DE GRUTTOLA and M. VILLA each bunch of the beam and a total one, i.e. 3564 different luminosity measurements performed on data arriving at a rate of several GByte/s and processed every 25 ns; – the electronic boards provide an “online” luminosity information available during run-time and can be later calibrated, tuned and fine-tuned with other methods. The time-slices are called luminosity blocks and are defined as a time interval for which the luminosity can be considered as constant (≈ few minutes or 106 beam revolutions). The actual measurement of the instantaneous luminosity is performed on inelastic events. At LHC the average number of interactions μ in a given bunch crossing can tot be calculated as: μ = Lσpp Δt, where Δt = 25 ns. Luminosity detectors are sensitive to quantities (N ) proportional to μ as the number of hit tubes or the number of hit towers: N ≈ kμε, where ε is an acceptance and efficiency factor and k is a calibration factor. There are at least three different methods to evaluate the luminosity L from the measurement of N . Direct method : This relies on the fact that N (and μ) is very low. Since for each given BX the actual number of interactions is Poissonian distributed, in the case μ < 0.01, the probability of two or more interactions is negligible. Therefore it is necessary to distinguish between no interaction and an interaction. Any simple definition of an interaction can be used: i.e. a hit tower in CMS or a hit tube in ATLAS. By counting the interactions in a given luminosity block, it is possible to provide a quantity proportional to the luminosity. This method will be used when L < 1032 cm−2 s−1 . Zero counting: When the probability of having two or more interactions become not negligible, the luminosity is evaluated by count events without interactions (zero counting), since their probability is related to N under the assumption of Poissonian distribution of the interactions: P (0) = e−N . This method is usable in a range of 0.01 < N < 4.6 where the statistic uncertainty does not dominate the final measurement. Above this range, less than 1% of events are empty (zero-starvation region). When N > 4.6 with the complete detectors (P (N ) < 0.01), it is possible to consider parts of the detector (halves, quarters, single tubes or single towers) chosen so that for each part N is in the range of indirect measurement (0.01 < N < 4.6). Proportional methods: These are based on quantities roughly proportional to the luminosity, like number of hit tubes, number of hit towers, total transverse energy. Despite the non-linearity problems [27] involved, they can provide ancillary information on time scales longer than 25 ns. The physics measurements discussed in the previous section fall in this category and will provide the absolute calibration of the online luminosity. 6. – Conclusions Many efforts will be devoted during the LHC data taking operation with the aim to obtain a precise luminosity measurement in order to reduce the systematic contribution due to the luminosity on the evaluation of any physics process. This contribution, with the passing of time, and so with the tuning of the experiments will reduce from the initial 10% to hopefully 1%. In this paper many ways of measuring the luminosity, both instantaneous then integrated, have been presented and discussed. Two methods to measure the absolute luminosity directly on bunch properties have been presented. The van-der-Meer scan method aims to monitor the collision rates at the four interaction points with an accuracy of about 10%, by means of two different detector technologies: a fast ionization chamber developed by LBNL and a solid-state LUMINOSITY MEASUREMENTS AT LHC 433 CdTe detector developed by CERN and LETI. A further method relies instead on reconstruction of vertices from beam-gas interactions to measure the colliding bunch profiles and to determine the beam overlap integral. A dedicated one-day-long running time should be sufficient to carry out an absolute luminosity measurement with an accuracy of 1% or better. Measurements of the integrated luminosity have been shown to be feasible via the measurement of the total cross-section and of the elastic scattering, using the optical theorem or measuring directly the elastic cross-section inside the Coulomb-nuclear interference region. These methods can provide a precise measurement of the integrated luminosity (5–10% uncertainty at the very beginning, 1–3% after) but require dedicated runs with special accelerator optics. At ATLAS, CMS and LHCb an offline integrated-luminosity determination will be provided by determining the cross-section of some “candle” channels such as Z/W decaying into leptons, and muons (electrons) from double-γ exchange in proton collision. The latter channel will need a greater integrated luminosity, namely 1 fb−1 , to be used as a tool for luminosity measurement, while the electroweak channels, due to the higher cross-section will be the first measured channels at LHC. Exploiting these channels a luminosity measurement precision of ΔL/L ∼ 3% will be reached at regime condition when the systematic contributions from the detector will be completely understood. 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